digital hydraulics in aircraft control surface...
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Linköping University | Fluid and Mechatronic SystemsMaster Thesis, 30 hp | Mechanical enginering- Fluid Power
Spring 2017| LIU-IEI-TEK-A--17/02819–SE
Digital hydraulics in aircraft controlsurface actuation– Modelling and evaluation of digital hydraulic systemswith focus on performance and energy efficiency
Simon Ward
Supervisors: Dr. Liselott Ericson at IEI, Linköping UniversityDr. Alessandro Dell’Amico at Saab Aeronautics, Saab AB
Examiner: Prof. Petter Krus at Linköping University
Linköpings UniversitetSE-581 83 Linköping
013-28 10 00, www.liu.se
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c© Simon Ward
AbstractThe purpose of this thesis has been to compare and analyse the use of digital hydraulic actuatorsin place of traditional actuators in aircraft control surface manipulation. Digital hydraulic actuatorreferring to a hydraulic actuator where the power has been discretized using discrete on/off-valves.For this purpose three simulation models have been used. The first model consists of a benchmarkmodel, designed to represent a digital hydraulic actuator acting on a mass under the influence ofan external spring load. The discretization in this case comes from the fact that three separatepressure levels have been used to power a four-chambered tandem piston, resulting in 81 possibleforce combinations.
The second simulation model represents a 6 degrees of freedom aircraft model parametrisedto behave like a F16 fighter aircraft. The purpose of this model has been to serve as a means toimplement the digital actuator in an aircraft. The third model has been heavily based on the F16model but re-parametrised such that it represents a delta canard aircraft. The actuators in theaircraft models was initially mounted on the control surface primarily dedicated for the manoeuvrewhich was simulated, in this case a step in altitude, meaning that the control surface was theelevon.
As it would turn out the digital actuator had trouble achieving the precision required in orderto adequately fly the aircraft at a low enough energy consumption. As such the idea took formto implement a hybrid design where the digital actuator would be paired with a proportionalactuator on a separate control surface, flaperons. The digital actuator would then only requireto be positioned in a close enough position and once there lock in place, leaving the proportionalactuator to handle the fine tuning and trim of the aircraft.
It would appear that by using the hybrid actuator design the energy consumption during theright circumstances could be reduced by as much as 40% for the delta canard configuration and30% for the F16 case.
iii
Acknowledgments
The work presented in this thesis has been conducted at Saab Aeronautics, Saab AB. Examiningthe thesis is Professor Petter Krus at Linköping University. The supervisor at the university hasbeen Dr. Liselott Ericson. At Saab, the supervisor has been Dr. Alessandro Dell’Amico. A specialthank is sent out to Vincent and Erik, my two roommates during the course of this thesis, whomhave been the source of many interesting discussions as well as a source for ideas when I got stuck.I would also like to thank the co-workers with whom I have been sharing an office at Saab for thefriendly receiving I have gotten there.
Linköping, June, 2017
Simon Ward
v
Contents
1 Introduction 11.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.3 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.4 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21.5 Delimitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.6 Outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Literature study 52.1 Hydraulics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Hydraulic system energy efficiency . . . . . . . . . . . . . . . . . . . . . . . 72.1.2 Servo valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.3 On/off valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.4 Secondary control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.2 Aviation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.1 Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.2.2 Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.2.3 Stable and unstable aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.3 Aircraft hydraulics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.4 Requirements on aircraft actuation systems . . . . . . . . . . . . . . . . . . . . . . 12
2.4.1 Stall load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.4.2 Maximum rate capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.4.3 Frequency response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.4.4 Requirements conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.5 Digital hydraulics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.6 Hopsan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
3 Model 173.1 DHA layout concept . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
3.1.1 PID . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.1.2 Digital Hydraulic Quantisation . . . . . . . . . . . . . . . . . . . . . . . . . 193.1.3 Delay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.1.4 Lock . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.1.5 Act . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.1.6 Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
3.2 Simulation models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.2.1 Baseline DHA model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233.2.2 Aircraft model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
3.3 Actuator dimensioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
4 Simulation procedure 334.1 Baseline DHA model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
4.1.1 Frequency response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334.2 F16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
4.2.1 Hybrid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
vii
4.3 Delta canard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
5 Results 375.1 Baseline DHA model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375.2 Aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.2.1 F16 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.2.2 Hybrid configuration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405.2.3 Delta canard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
6 Discussion 476.1 Simulation models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
6.2.1 Baseline model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 476.2.2 Aircraft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
6.3 Initial objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486.4 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 496.5 Ethics and morality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
7 Conclusion 517.1 Suggestions for further studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Nomenclature 53
Bibliography 55
Contents ix
List of Figures2.1 Illustration of hydraulic force transfer where a large force acting on a large surface
can be lifted by a smaller force acting on a smaller surface. . . . . . . . . . . . . . 52.2 Nomenclature of the continuum principle. . . . . . . . . . . . . . . . . . . . . . . . 52.3 Linear, a, and rotating, b, hydraulic actuator. . . . . . . . . . . . . . . . . . . . . . 62.4 Illustration of a proportional valve. . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.5 Illustration of the air velocity difference over and under the wing surface. . . . . . 82.6 Illustration of the rotation angles of an aircraft. . . . . . . . . . . . . . . . . . . . . 92.7 List and location of the primary and secondary control surfaces. . . . . . . . . . . 102.8 Illustration of stable and unstable aircraft. . . . . . . . . . . . . . . . . . . . . . . . 102.9 Illustration of redundant hydraulic system design in aviation. . . . . . . . . . . . . 112.10 Common aircraft actuator system. . . . . . . . . . . . . . . . . . . . . . . . . . . . 122.11 Typical hydraulic actuator max rate plot. . . . . . . . . . . . . . . . . . . . . . . . 132.12 Illustration of bang bang control. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
3.1 DHA system layout. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.2 DHA block diagram. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183.3 Resulting force spectrum at the chosen pressure levels. . . . . . . . . . . . . . . . . 203.4 DHQ penalty principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203.5 Comparison of the reference on/off-valve and the Hopsan simulated one. . . . . . . 213.6 Illustration of the aerodynamic spring effect. . . . . . . . . . . . . . . . . . . . . . 223.7 Control surfaces in the Hopsan 6DOFSS aircraft model. . . . . . . . . . . . . . . . 243.8 Illustration of the Hopsan aircraft hydraulic system. . . . . . . . . . . . . . . . . . 273.9 Content of the control surface actuator blocks of the Hopsan aircraft hydraulic system. 283.10 Magnitude of Kv as a function of increasing aircraft velocity. . . . . . . . . . . . . 283.11 Illustration of the control loops of the aircraft model. . . . . . . . . . . . . . . . . . 283.12 Graphical representation showing the general position of the control surfaces on an
F16 fighter aircraft. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.13 Illustration of the changes to the controls when utilizing hybrid hydraulics to ma-
nipulate the control surfaces. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.14 Illustration of the change in leverage between traditional and delta canard wing
placement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293.15 Nomenclature for the parameters used in table 3.3. . . . . . . . . . . . . . . . . . . 30
4.1 Benchmark specification nomenclature. . . . . . . . . . . . . . . . . . . . . . . . . . 344.2 Control surface geometry. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.1 Simulation results for the benchmark model. . . . . . . . . . . . . . . . . . . . . . . 375.2 Force plot for the benchmark DHA model. . . . . . . . . . . . . . . . . . . . . . . . 385.3 5% amplitude frequency responce for the benchmark model. . . . . . . . . . . . . . 395.4 0.5% amplitude frequency responce for the benchmark model. . . . . . . . . . . . . 395.5 Initial simulations of the F16 aircraft with DHA. . . . . . . . . . . . . . . . . . . . 405.6 Flight performance of the stock F16 aircraft during the three flight cases. . . . . . 425.7 Hydraulic energy consumption for the stock F16 aircraft during the three flight cases. 435.8 Control surface angles of the stock F16 aircraft. . . . . . . . . . . . . . . . . . . . . 435.9 Control surface angles of the F16 aircraft with DHA. . . . . . . . . . . . . . . . . . 435.10 F16 DHA flight performance and energy consumption. . . . . . . . . . . . . . . . . 445.11 Hydraulic energy consumption for the F16 and delta canard simulation models with
DHA during the three flight cases. . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.12 Delta canard DHA flight performance and energy consumption. . . . . . . . . . . . 46
x Contents
List of Tables2.1 Aircraft actuator requirements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142.2 Comparison between digital hydraulic concepts. . . . . . . . . . . . . . . . . . . . . 16
3.1 Hopsan on/off-valve parameter values. . . . . . . . . . . . . . . . . . . . . . . . . . 213.2 Proportional force levels required during different manoeuvres and flight stages. . . 233.3 Parameters and results from calculating DHA force requirements. . . . . . . . . . . 313.4 DHA areas and resulting force limits. . . . . . . . . . . . . . . . . . . . . . . . . . . 31
4.1 Specifications for the parameters used in the baseline model simulations. . . . . . . 334.2 Case specific differences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 344.3 Flight cases used to test the aircraft. . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Chapter 1
Introduction
1.1 Background
In the modern world aviation plays a fundamental part. By using aircraft, it is possible to travelthe skies for great distances in short times enabling amongst others travel and comers across theglobe. In the world of aircraft there are two major classes: rotary wing aircraft and fixed wingaircraft. The difference between the two lies in the way lift is generated. Rotary wing aircraft useswings which rotate around a shaft. An example of a rotary wing aircraft is the helicopter, whichdue to the wings rotation can achieve flight without the main body being in motion, known ashovering. For a fixed wing aircraft, the wings are fixed to the body of the aircraft and thus theentire body needs to move in order to achieve lift.
The wings are fundamental to the flight of the aircraft, giving the aircraft the lift it needs. Byonly using solid wings however it is not possible to control the aircraft in flight. Due to this one ofthe most important enablers for fixed wing aviation is the implementation of control surfaces onthe wings, used to alter the wing geometry. The manipulation of these control surfaces is criticalto flight [1]. Historically manipulation of control surfaces has been accomplished through the useof mechanical connections, i.e. wires, springs, and linkages, which have been used to connect thecontrol inputs to the control surfaces of the aircraft. However, with increasing size and speed ofthe aircraft the required actuation forces needed in order to manipulate the control surfaces havegrown larger. This has led to the introduction of powered actuators, often in the form of poweredhydraulics. By using hydraulically powered servo valves and actuators when manipulating thecontrol surfaces of an aircraft it is possible to achieve great stability, force, and response timeswhilst limiting the control inputs.
The traditional means of controlling a hydraulic actuator is to have a constant pressure sourceconnected via a flow regulating valve to the actuator. The valve is used to throttle the amountof fluid going from the source to the actuator and thus generating force and movement. By usingtraditional proportional valves a lot of the power is converted to heat when throttling the hydraulicfluid which means that more energy is required. Another approach to the actuator control is touse digital hydraulics [2–8]. Digital hydraulics means that, instead of throttling the hydraulicpower, a number of discrete on/off valves are used to connect one or more pressure sources withvarying pressure levels to the actuator/s. The benefit to this is that the hydraulic fluid is eitherstanding still or going unrestricted, causing small pressure differences over the control valves andthus minimising the amount of power lost to heat. Additionally, if a parallel valve approach is usedthere is also potential for increased system safety due to the fact that multiple parallel valves areused to control a cylinder with multiple chambers, resulting in an inherently redundant system.Digital hydraulics has been the subject of recent research between LiU, SAAB, and the FederalUniversity of Santa Catarina in Brazil.
1
2 Introduction
1.2 PurposeWhenever new technology is being researched it is important to have a solid understanding andknowledge of how it works. One of the ways that this knowledge can be achieved is through sim-ulations, where the expected behaviour is calculated. In the initial stages of project development,the simulations are most often solely based on mathematical relations. The reason for this is thatin the early stages there may not be a product or prototype to take measurements on. However,as more research is done and an eventual prototype is made it is possible to complement the simu-lations with measurement data, increasing the validity of the simulation model. Simulations suchas these are fundamental in achieving an understanding of the technology and in most cases, theycan be used to foresee the behaviour of the system. One of the properties which is usually analysedis the energy efficiency. In avionics energy efficiency plays a key part since an aircraft which usesmore energy efficient systems uses less fuel, which leads to lower fuel costs for the aircraft. Anotherbenefit of high energy efficiency is that, as less fuel is required to power the system, the amount ofairtime which is possible to get from a fixed amount of fuel can be increased. The system energyefficiency is highly dependent on the system design and on the choice of components. In aircraft,there are some systems which are generally more energy consuming than the other systems sincethey are constantly active. The main energy consumer is the engines which are constantly con-verting fuel to thrust which in turn gives the aircraft its speed. Other non-insignificant consumersof energy are the actuators, specifically the ones responsible for manipulating the control surfaces.
The basis for this project lies in the work previously made in a research project between LiU,SAAB and, the Federal University of Santa Catarina in Brazil. These projects have been aimedat evaluating the feasibility of using digital hydraulics as a more energy efficient substitute to thestandard hydraulic servo technology when manipulating the control surfaces of aircraft [5, 7, 8].This thesis will be a continuation of these projects, specifically focusing on analysing the strengthsand weaknesses of using digital hydraulics when manipulating the control surfaces on aircraft.As a part of this analysis the system will be compared to the governing regulations and currentstandards regarding energy efficiency, performance, accuracy, and potentially also safety duringauthentic flight conditions.
1.3 ObjectivesThis thesis is intended to compare a digital hydraulic system to a traditional proportionally con-trolled hydraulic system for use in manipulation of aircraft control surfaces. The main focusesof these comparisons will be on whether the energy requirements of a digital hydraulic system,combined with the performance of the system, can compare to that of a proportionally controlledsystem. Based on this the following list of main objectives for this thesis can be specified:
Compare and analyse digital hydraulics1. ...in relation to proportional hydraulics. How does a digital hydraulic system compare to a
traditional proportionally controlled one in regards to performance and energy efficiency?
2. ...as a replacement to proportional hydraulics in aircraft. Is it possible to replace the tradi-tional actuators used in aircraft control surface actuation with digital actuators and achievesufficient performance? If so, during what circumstances is this possible and what are therequirements for doing so?
3. ...as a hybrid addition to proportional hydraulics in aircraft. If the digital and proportionalactuators are combined in a hybrid configuration, how does this affect the performance andenergy consumption of the aircraft? Is this approach maybe to prefer over using pure digitalcontrol?
1.4 MethodIn order to analyse whether digital hydraulics is a viable alternative in aviation multiple simulationswill be conducted. These simulations will be made using Hopsan, a hydraulic simulation tool. The
1.5 Delimitations 3
simulations will be made using three distinct system set-ups. The first of these set-ups will bea benchmark model of the digital hydraulic system. This model will be used in order to analysefundamental behaviours as well as serve as a means for parameter tuning. The other two set-upswill be used to analyse the digital system implemented in simulated aircraft. Of these two modelsthe first one will be based on an F16 fighter aircraft and is included as a standard system in Hopsan.The other model is an altered version of the F16 model where the wing geometry is changed toresemble a delta canard aircraft. For comparison and validation purposes all of these models willhave a comparison model which will be using traditional proportional control.
1.5 DelimitationsFor this thesis, some delimitations have been made. These are summarised in the following list.
• No consideration will be taken in regards to sensors, all sensors will be treated as ideal.
• Only the force discretization digital system design will be considered.
• When testing the system against an aircraft model, only the pitching motion will be analysed.This means that the actuators manipulating the control surfaces responsible for these typesof manoeuvres will be replaced with the DHA whilst the rest of the actuators will be left asproportional.
• No analysis will be made in regards to
– the accuracy and validity of the aircraft models used in this thesis.– the hydraulic supply system– safety and redundancy of the hydraulic system.
1.6 OutlineFollowing is an outline of this thesis and a brief description of the content of each chapter.
2 Literature study
The literature study chapter contains most of the pre-required information which might be neededin order to fully understand the thesis. Also included in this chapter are a lot of the relevantequations and physical relationships which are used in the modelling and analysis of the thesis aswell as some of the requirements on the actuator.
3 Model
In the model chapter the simulation models which have been used is described in detail. Thisincludes the layout of the core digital actuator, the three simulated systems, and some of thedimensioning of the models.
4 Simulation procedure
The simulation procedure chapter contains descriptions of the simulations intended for the val-idation purposes. Some of the parameters used in the different simulations can also be foundhere.
5 Results
The results chapter displays the results from running the simulations. A description of the resultscan also be found here.
4 Introduction
6 Discussion
In the discussion chapter the content of the results chapter is analysed and related to the questioningstated in the introduction chapter. In addition to this some reflection on the thesis work is presentedas well as a brief section on the ethics of the thesis.
7 Conclusion
The conclusion chapter contains a concluding statement regarding the results of the thesis. Thischapter also contains a few suggestions for further studies.
Chapter 2
Literature study
2.1 HydraulicsA hydraulic system can be compared to a transmission where a pressurised fluid, hydraulic oil, isused as the means of transferring force and motion instead of solid mechanical elements. One ofthe most common illustrations of this is when a small force acting on a small surface is able tocounter a large force acting on a larger surface, figure 2.1. Besides scaling forces through pressureand areas the fluid is also able to transfer motion through the same principle of differently sizedareas. Both of these phenomena obey the laws of the continuum principle, equation (2.1) andfigure 2.2.
Heavy LightShort movement Long movement
Flow
Figure 2.1: Illustration of hydraulic force transfer where a large force acting on a large surface canbe lifted by a smaller force acting on a smaller surface.
∑qin = dV
dt+ V
βe
dp
dt(2.1)
p, V
qindV
Figure 2.2: Nomenclature of the continuum principle [9].
This ability to move and amplify forces is applied in one of two ways. For linear forces andmovements, the most common means of power transformation is through the use of a piston,figure 2.3 a. The piston has the ability to be used both as a pump and actuator depending onwhether the power input is applied through the oil or through the piston rod. For rotating motions
5
6 Literature study
and torque a rotating displacement machine is used. These work by having the fluid work on asurface which by some way is attached as a lever on a rotating shaft, figure 2.3 b. These machinesalso have the ability to work as both pumps and motors depending on the source of power.
Figure 2.3: Linear, a, and rotating, b, hydraulic actuator.
In order to regulate the flow of fluid, and thus the behaviour of the system, valves are used,figure 2.4. A valve is basically a variable restriction which, through variation of the opening, couldbe used to alter the amount of fluid passing through it. This can be used to regulate both forceand velocity of the controlled actuators which are attached to the system and also entirely shuttingof the flow to them, locking them in place. Valves come in a variation of configurations dependingon the purpose of the valves. The simplest valves work as a pure restriction of the fluid. Othervalves are used to redirect the flow of the fluid between different ports in the valve block and somevalves are used to achieve more advanced behaviours such as for example maintaining a constantflow regardless of the pressure levels. The flow through a valve is proportional to the square rootof the pressure difference and the opening area, see equation (2.2).
qh = Cqwxv
√2ρ
(p1 − p2) (2.2)
xv
p1, qh
p2
xv,max
dv(1− f) dv
Figure 2.4: Illustration of a proportional valve. p and q corresponds to the pressure and flowthrough the valve. additionally xv is the spool position, dv is the diameter of the spool, and f isthe fraction of the diameter corresponding to the central rods diameter.
2.1 Hydraulics 7
2.1.1 Hydraulic system energy efficiencyIn order to adequately compare two hydraulic systems some way of calculating the efficiency ofa hydraulic system is required. For this end the power loss over individual components can becalculated according to equation (2.3). In this equation Ph,loss is the power which is lost, mainlythrough heat, in the component. Additionally ∆p is the difference in pressure from the inlet tothe outlet of the component and qh is the volume flow of the fluid.
Ph,loss = qh∆ph (2.3)
In hydraulics there are two major types of losses, volumetric and hydro-mechanical losses.Volumetric losses, are losses in regards to changes in volume flow in and out of a system. Someexamples of this type of losses are internal and external leakages, leading to a higher input of fluidbeing required for the same output velocity, and also compressibility losses, changes in volumewhen under high pressures. Compressibility losses could be simplified as having the same effectas leakages, more fluid is required to be pumped in than might actually be required. The othertypes of losses, hydro-mechanical, are responsible for pressure decreases in a system. These lossesconsist of friction in components, throttling through orifices, and flow resistances in hoses andlines. Hydro mechanical losses decide the amount of input pressure required for a system in orderto be able to deliver sufficient force as output.
In order to calculate the amount of energy used by a system the power usage is integrated overtime, equation (2.4). In the case of a digital hydraulic tandem piston utilizing three input pressurelevels the input power can be expressed according to equation (2.5). Similarly equation (2.6) canbe used to measure the mechanical output power.
Wloss = Win −Wout =∫T
(Pin − Pout)dt (2.4)
Ph,in = phighqhigh + pmidqmid + plowqlow (2.5)
Pm,out = Factvact (2.6)
2.1.2 Servo valveThe traditional hydraulic servo valve has very good properties when it comes to response andstability. A traditional servo valve also has beneficial characteristics when it comes to controllabilityand can deliver a continuously varying power level to the actuator. Unfortunately servo valves areinherently inefficient due to the fact that they control the flow via restrictions. Another problemcommon in radial slide servo valves is the fact that they have leakage problems. Further downsidesof this way of control is that the system needs to be dimensioned to handle any situation. Thismeans that during normal operating conditions the system is operating at well below the designspecification. Because of this, there is a great reduction in power from the supply to the actuatorwhich needs to be throttled, resulting in a loss in efficiency. For a given actuator speed the loss inpower is proportional to the pressure reduction over the valve, equation (2.3), this means that ifthe pressure drop could be minimized the energy efficiency could be increased.
2.1.3 On/off valvesTraditionally the amount of power conveyed to the actuators is regulated by throttling the fluidthrough valves. This method is capable of high accuracy in control due to its ability to propor-tionally scale the opening area. However since it relies on throttling of the fluid it is a highlyinefficient way of regulating regarding energy efficiency, since a lot of power is wasted as heat[2,10,11]. One way of getting around this low energy efficiency is by using on/off valves to regulatethe flow. On/off valves have a much higher efficiency ratio due to the fact that they minimise thepressure drop over the valve when it is open, combined with a much tighter seal when closed which
8 Literature study
minimises the leakage flow. This means that the power loss over the valve is lower and that theefficiency increases.
On/off-valves are however not without problems them self. Due to the discrete state, open orclosed, they are not in them self capable of power control in a hydraulic system for anything otherthan none or full power deliveries. The use of discrete on/off valves in place of proportional valvesis sometimes referred to as digital hydraulics due to the likeness with digital electronics wheretransistors are used to turn the electric power on and off.
2.1.4 Secondary controlIn hydraulics there are two major ways of controlling a system. The separating factor between thedifferent control ways is mainly at which stage of the system control is applied.
In primary control the power source is the controlled part. This means that the system be-haviour is controlled by varying, for example, the speed or displacement setting of the pump.Primary controlled systems uses the flow level out of the pump to control the behaviour of theactuator.
Another way of controlling the system is by secondary control. With secondary control thepower input to individual actuators is controlled [12]. This is commonly achieved by having aconstant pressure system where valves are used to control the amount of fluid going to the actuator.In the case of a rotating actuator, motor, with variable displacement it is also possible to controlthe power input via the displacement setting, this is also a form of secondary control. Since thisproject is entirely focused on the use of valves to vary the power input to a tandem piston all ofthe control will be of the secondary kind.
2.2 Aviation2.2.1 FundamentalsAviation is the name given to the practical application of aeronautics and the term was first coinedby French writer and former naval officer Gabriel La Landelle in 1863 [13]. In aviation thereare two largely distinct ways to achieve flight. The first of these is to reduce the density of theaircraft, from here referred to as aircraft, low enough that it becomes lighter than air. Lighterthan air aircraft uses buoyancy to float in the air, much like a ship floating in water. Thesetypes of aircraft rely on large volumes and low mass in order to reduce the density enough tofly. The first piloted use of an untethered balloon to achieve flight was made on November 21,1783, using a balloon designed by the Montgolfier brothers [14]. Since then balloons have seen usein multiple applications, both civilian and military. Some famous balloon examples include themighty Zeppelins and the high-altitude balloons used by NASA amongst others [15].
The other way of achieving flight is through the use of a heavier than air aircraft. Heavier thanair aircraft are generally able to fly due to a pressure difference over a wing surface. This pressuredifference comes from speed variation on the top and bottom side of the wing where the air goingover the wing has a higher relative velocity than the air going under the wing, see figure 2.5. Thisspeed difference causes a difference in the dynamic pressure of the air acting on either side of thewing which in turn causes a lifting force proportional to the wing surface area on the aircraft,generating a lifting force. In equation (2.7) the Bernoulli equation, partially responsible for thiseffect, can be seen [16]. By rewriting this equation, (2.8), it can be shown that the pressuredifference over the wing is proportional to the difference in air speed over the top and bottom sideof the wing. It can be noted that index 1 indicates the top surface of the wing and index 2 thebottom surface.
Figure 2.5: Illustration of the air velocity difference over and under the wing surface.
2.2 Aviation 9
p1,stat +ρairv
21,air
2 + ρairgh1 = p2,stat +ρairv
22,air
2 + ρairgh2 (2.7)
h1 ≈ h2 → p1,stat = p2,stat + ρair2 (v2
2,air − v21,air) (2.8)
2.2.2 ControlIn order to be able to manipulate the aircraft whilst airborne a number of control surfaces are usedto alter the geometry of the wings, this alteration leads to changes in the lifting force coming fromthe pressure. The deflection of the control surfaces also induces some aerodynamic drag whichcontributes to the manipulation of the airborne aircraft through a braking force.
The control surfaces can be divided into primary and secondary control groups. In primarycontrol the surfaces responsible for control of the pitch, yaw, and roll are included. The axis ofpitch, yaw and roll can be seen in figure 2.6, as well as the general location of their primary controlsurfaces. These surfaces are responsible for the most important behaviours of the aircraft. Thesecondary control surfaces are responsible for non-critical manipulations of the aircraft. Theseinclude fine adjustments of the aircraft such as air braking, trim, and fine tuning of the winggeometry. A general list containing the most common primary and secondary control surfaces,including their general location on an aircraft, can be seen in figure 2.7. Also included in thisfigure is a list of some utility functions which are also generally powered by the hydraulic system.
Figure 2.6: Illustration of the rotation angles of an aircraft.
2.2.3 Stable and unstable aircraftWhen designing the shape of an aircraft there are some general guidelines which should be con-sidered in order to allow for stable flight. One of these guidelines relates to the positioning of thewings in relation to the centre of mass, as illustrated in figure 2.8. In order to increase the stabilityof the aircraft the wings should be positioned such that the centre of lift is located behind thecentre of mass. This leads to a stabilising effect on the aircraft similar to the stabilizing fins of adart or an arrow.
Due to the self-stabilising properties of a stable aircraft the dynamic properties are somewhatdecreased since the aircraft is prone to resist state changes. In some applications, such as whendesigning a nimble fighter aircraft, where manoeuvrability is vital this behaviour is not desired.For these applications a statically unstable aircraft might be preferred. By positioning the wingssuch that the centre of mass is positioned behind the centre of lift the static stability of the aircraftbecomes drastically reduced to the point where it becomes statically unstable. In these types ofaircraft all directional state deviations will be amplified, putting large requirements on the controlsoftware to be able to fly.
10 Literature study
(1)
(2)(3)
(3)
(4)
(5)
(5)
(6)
(6)
(7),(8)
(9)
Primary Flight Controls:– Elevators – (1)– All-moving tail surfaces (military)– Rudders – (2)– Ailerons – (3)– Flaperons – (4)– Canards
Secondary Flight Controls:– Flaps – (5)– Slats – (6)– Spoilers – (7)– Airbrakes – (8)– Stabilizer trim – (9)
Utilities:– Landing gear– Brakes– Gear steering– Aerial refueling probes (military)– Cargo doors– Loading ramp (military)– Passenger stairs
Figure 2.7: List and location of the primary and secondary control surfaces [17]. Also included isa list of generic utility systems which tends to be powered by the hydraulic system.
Figure 2.8: Illustration of unstable and unstable aircraft. The left aircraft illustrates the AJS 37Viggen aircraft and the right one is the JAS 39 Gripen. Both are manufactured by Saab.
2.3 Aircraft hydraulics 11
2.3 Aircraft hydraulics
Most aircraft, both civilian and military, have requirements on redundancy in all systems [1]. Theredundancy manifests itself in the hydraulic system in the way that there are multiple separatesystems working in parallel to drive the control surfaces. These separate systems usually have acombination of individual and shared actuators under their control. An example of this can be seenin figure 2.9. In this figure a layout with two hydraulic systems is depicted. Each of the two systemscontains the same set up of supply, tank, and auxiliary components and each system is the solesupply to two actuators. The final actuator however is jointly powered by the systems, meaningthat should either of the two systems fail it would still be possible to power this actuator, atreduced maximum power. In order to maintain a high redundancy the separate hydraulic systemsusually work on a no fluid exchange policy, meaning that there is no fluid exchange in betweenthem. The reason comes back to the redundancy, should one system fail, either due to mechanicalfailure of a component or from a hose burst, the other system should not be affected. Another wayof increasing the redundancy of the actuators is to make sure that most of the control surfaces areequipped with multiple actuators, each of which are connected to a separate system. Actuatorscontrolling highly critical surfaces should additionally be powered by multiple systems.
P1
P2
Figure 2.9: Illustration of redundant hydraulic system design in aviation [1]. In this figure twoseparated supply systems are shown. Both of the two systems are supplying the tandem actuator.Note that the flow control valves have not been depicted.
12 Literature study
A simplified view of an aircraft hydraulic system can be seen in figure 2.10. This system showsthe way two systems are usually connected to power a shared actuator. In this case it consists oftwo separated supply sources each controlling a separate section of a tandem piston which in turnis attached to a control surface. Control of the actuator is achieved through the use of a directionalcontrol valve. In the figure this valve is depicted as two separate 4/3 proportional valves driven bythe same reference signal. However, in reality this is usually done using an 8/3 valve. Additionallyin this set up bypass valves are used. The bypass valve serves as a means of making sure that ifthe hydraulic lines should get blocked, halting the flow, the actuator should not be locked in placeby the incompressibility of the hydraulic fluid but instead the affected chambers will simply pushthe oil back and forth between them. In practice, this would convert the actuator to a traditionaltwo-chamber hydraulic piston, powered by one supply system. Should both systems fail the controlsurface will be put in free-flailing mode, allowing it to freely follow the aerodynamic loads and thusminimising the extra drag and lift which could be caused by a locked control surface.
Figure 2.10: Common aircraft actuator system [7]. The four colours corresponds to actuator (red),bypass valves (blue), directional control valve (yellow), and supply source (green).
2.4 Requirements on aircraft actuation systems [18]In order to maintain the safety of the aircraft, as well as the feeling of the controls as experienced bythe pilot, the actuators used in aviation are subject to high requirements. The specification of theserequirements depends on the type and purpose of the aircraft but the fundamental requirementsare the same. The requirements exist in order to ensure that the aircraft can be handled correctlyunder all possible circumstances. Following is a brief list of the various requirements.
2.4.1 Stall loadSince the pilot needs to have control of the aircraft under all circumstances, the actuators mustbe designed to be able to withstand the load applied on the actuator during all cases. This meansthat the actuators need to be chosen in relation to the available supply pressure such that thesestall loads in both tension and compression are possible to be met. In short, the diameters of theactuator need to be sufficient to generate the required forces in the range of pressures available. Ifthese two parameters, pressure and actuator diameters, are not sufficiently designed compared tothe load conditions the actuator will not be able to maintain force equilibrium and thus either befrozen in place or potentially even start to sink.
2.4 Requirements on aircraft actuation systems 13
There are three limiting factors to the force requirement of the actuators.
• Any individual actuator has to be able to withstand the highest force which it could bepresented with.
• All subsystems of actuators, for example the actuators on one of the flaps, has to be able toprovide enough force to handle any possible load situation.
• The force of the actuators cannot be so high that it, under full static pressure, is powerfulenough to damage the structure of the aircraft.
The magnitude of the stall load is determined by the geometry of the hinges and the highestaerodynamic load which the aircraft could be presented with. If the requirements for availableactuator force in relation to maximum required force are not met the actuator could stall andpotentially even begin to advance/retract, depending on force direction, uncontrollably. Thisimplies that the fluid will flow in reverse direction unless the system is designed to prevent this.In any case the fluid will be put under high pressure, potentially exceeding the maximum safetyor burst pressure levels.
2.4.2 Maximum rate capabilityAnother important factor when designing a hydraulic system is the rate at which the actuatorhas to be able to move. The maximum rate needs to be specified for both actuator directions,extension and retraction, and this rate should be defined for a given load and pressure differenceover the actuator. Usually the rates are defined at no-load condition and at 60 to 70 per cent ofthe stall load. The maximum rate corresponds to the maximum opening of the control valve andthe size of the actuator. Since the size of the actuator is decided by the load however the onlyparameter left is the size of the control valves. In figure 2.11 a typical plot of maximum rate forsteady load cases can be seen.
Figure 2.11: Typical hydraulic actuator max rate plot[18].
2.4.3 Frequency responseFrequency response is an effective way of measuring expected behaviour, and by defining clearboundaries on the gain and phase of the system certain behaviours can be ensured. These bound-aries are usually defined under specific test conditions such as under a certain load and amplitude.
14 Literature study
The boundaries restrict the gain and phase of the response from the input signal to the displace-ment of the piston. Because of this the gain should approach 0 dB and the phase-lag 0 deg as thefrequency approaches 0. For the rest of the frequencies the maximum and minimum gain is limitedand the phase shift cannot be too high. Usually the load inertia is thoroughly modelled since theboundaries are to be respected even when the control surfaces are mounted.
2.4.4 Requirements conclusionBy combining the separate requirements from the previous section a list of fundamental require-ments can be specified. Data values are taken from specifications for a F-18 systems researchaircraft used by NASA [19]. The can be seen in table 2.1 list will be used as a gauge in the analysisof the end results.
Table 2.1: Aircraft actuator requirements. Values are gathered from [19].
Parameter Unit RequirementOutput stall force kN 58.3± 2.2Steady load kN 22± 2.2Ram output stroke mm ±57.15No load ram velocity mm/s 170.18 minimumLoad vs rateload kN 28.2velocity mm/s 123.95 minimum
Frequency response±0.5% response Hz/dB/deg 7/− 7.25/92 maximum±5.0% response Hz/dB/deg 7/− 7.25/92 maximum
2.5 Digital hydraulicsThere are three major approaches to consider when designing a digital hydraulic system. Thefirst way is also the most basic one consists only of a single on/off valve. By either opening orclosing this valve the actuator is either moving or locked in place. See figure 2.12. This method ofcontrolling the actuator, sometimes referred to as bang bang control, is suitable for systems whereprecision and smoothness is not essential. The reason for this is that the actuator will move at fullspeed and force and then come to a sudden stop when it gets close enough to the target, whetherthe target is a set open/closed time or a reference position of the actuator. This approach is notvery common in hydraulic systems, partly due to the stiffness of the hydraulic fluid, but can oftenbe found in both pneumatic systems and also in some automobile applications such as in the fuelinjection [11].
Another way to implement a digital hydraulic system is to use the same system as with the bangbang control, using an on/off valve, but instead of using a simple on/off control the state of theposition, or state, of the valve is set using a PWM signal [2,20–22]. This gives an almost infinitelyvariable flow or pressure level depending on the resolution of the PWM signal and the responsetime of the valve. Due to the stiffness of hydraulic fluids this approach generates pulsations in thesystem, causing vibrations in the structure and in the surrounding air. One way of mitigating thisis by introducing inertia into the system [2]. Some simple ways of increasing the inertia includes:increasing the length of the hydraulic lines, increasing the inertia of the load or connecting ahydraulic motor in series with the load and attaching a high inertia disc to it. This approach ishighly promising due to the high efficiency of the on/off valve and the continuously variable flowand pressure output. However due to the pulsations in the system, the large mass from the inertia,and the limitation in settling time for on/off valves it has not seen much use outside of laboratoryenvironments [2, 6, 11].
The final approach is to use a grid of on/off-valves to relay discretized flow or pressure levels.This approach shares some likeness with the boolean logic used in digital electronics, based on theon/off combinations relayed to the grid the system will behave differently. This can be used in
2.5 Digital hydraulics 15
α
topen tclose-1
0
1
2
3
4
5
Time, t---Sh
aftan
gle,α
—–Va
lvesig
nal[-]
1
0
Figure 2.12: Illustration of bang bang control.
one of two ways. By combining on/off-valves in parallel and/or series it is possible to either alterthe maximum opening areas, allowing for discretely variable flow, or redistributing pressure levelsto different chambers in an actuator, discretizing the force output. In practice this means that itis possible to discretize the power output of the actuator. There are, as stated above, two majorways of doing this, each of which focuses on a separate part of the power.
The first way is to discretize the flow [4, 11]. This is achieved by leading the fluid throughseveral parallel on/off-valves, each being dimensioned with a smaller opening area than that ofthe hydraulic line thus allowing the effective hydraulic opening to be decided by the valves. Byvarying which valves are open or closed at any given time the amount of oil passing through canbe regulated in discrete levels according to equations (2.9) and (2.10). If the valves have differingopening areas in between them the number of valves required for high resolution is not necessarilythat high either. One of the ways the opening areas can be chosen is by following the Fibonaccisequence which gives high resolution in the resulting opening whilst still limiting the requiredamount of valves. A benefit of scaling the valves based on the Fibonacci sequence is that therewill exist two valve combinations for each effective opening area. The flow discretization approachis good if high precision in speed is desired since the flow levels are being controlled, flow beingproportional to the speed of the actuator. The grid of on/off valves used in these two methods iscommonly known as a Digital Flow Control Unit (DFCU).
qh = CqAtot
√2ρh
∆ph (2.9)
Atot =nv∑i=1
Ai (2.10)
The other way to discretize the power output, which is the method used in this thesis, is todiscretize the force levels applied through the actuator. This can be achieved by having differingsupply pressures, by having varying diameters in the actuator chambers, or a combination of thetwo [3,5,23]. By connecting each of the pressure sources to each of the piston chambers via discreteon/off-valves it is possible to vary the effective force being generated in the piston. This meansthat the piston will have a set level of discrete forces which it is capable to generate. The resolutionof the force steps is however almost infinitely improvable. The limiting factor to the number ofsteps is the amount of available pressure levels and piston chambers, compared to the number ofvalves being the limiting factor in the flow control method. A downside to this method of force
16 Literature study
Table 2.2: Comparison between digital hydraulic concepts [11].
Bang bang PWM DFCUPros
Simple design Infinitely variable RedundantBlunt control easy
ConsPrecise control hard Requires high carrier frequency Large on/off-valve blocks
Might require high inertia Heavy
discretization is that, similarly to the DFCU, the valve block needs to be rather large and heavy[11]. The reason for this is the fact that the amount of individual on/off-valves needs to be at leastequal to the product of the number of forces and chambers available to the actuator. Anotherdownside is that besides through switching between discrete acceleration levels there is no controlof the speed. Because of this fine control of position is hard and requires either a relatively highswitch frequency or a precisely tuned controller. The valve block, combined with the tandemactuator, used in this approach will be referenced to as DHA, digital hydraulic actuator.
Due to the DHA operating using discrete on/off valves there are some fundamental benefitswhich can be associated with it compared to the traditional proportionally controlled actuators.The first of these benefits comes in the form of energy. By using on/off-valves the throttling lossesassociated with traditional hydraulic control can be minimized, enabling the DHA to achieve higherenergy efficiency than the proportional actuator. Another benefit can be found in the fact thatdue to control being conducted through switching of individual valves the DHA can switch frommaximum power in one direction to maximum power in the other in the same amount of time asbetween two adjacent force levels. This can be compared to the proportional valve having to movethe entire span of positions to achieve the same. The final benefit has to do with redundancy. Dueto the DHA utilizing multiple valves to achieve motion the failure of an individual valve does notcause the entire DHA to go down.
Some of the fundamental pros and cons of the three approaches of designing a digital hy-draulic system can be seen in table 2.2. Additionally in this table some comparisons between theapproaches are included.
2.6 HopsanIn order to create the simulation models required for this project the simulation software Hopsanwill be used. Hopsan is a hydraulic modelling software, developed at Linköping University, designedto be able to accurately model the dynamic behaviour of hydraulic systems [24]. Hopsan doesthis through the use of Transmission Line Modelling, TLM. The purpose of TLM is to use wavecharacteristics to calculate the propagation of flow and pressure.
Chapter 3
Model
3.1 DHA layout conceptThe core DHA layout consists of three supply sources, of separate pressure levels, connected toa tandem piston with four chambers of varying size. Connecting the supply and the actuator isa DFCU-block consisting of 12 on/off valves. A simplified schematic of the DHA can be seen infigure 3.1. Due to the number of pressure levels and chambers 81 pressure-area combinations arepossible, in turn resulting in up to 81 available distinct force levels. This relationship can be seenin equation (3.1) where nF is the amount of distinct forces, np is the number of separate pressurelevels, and nc is the number of chambers. If the relations between pressure levels and chambersizes allow for more than one combination generating the same effective force from the actuatorthe number of unique force levels will go down.
nF ≤ nncp (3.1)
plow
pmid
phigh
DFCU
Figure 3.1: Simplified view of the DHA system used in this thesis.
17
18 Model
xref + e∑PID
FrefDHQ
udisc Delayudisc
ActFact
Loadx, x
x− x
Lock
x
I/O
DHA
Figure 3.2: Block diagram depicting the control loops of the DHA.
In figure 3.2 a block diagram containing the internal control loops of the DHA are shown. Themodel uses a PID controller to set a desired actuator force, Fref , based on the current actuatorposition in relation to a reference and the piston velocity. This desired actuator force is then usedas a reference in a look-up table, containing all of the available force levels and their respectivecontrol valve combinations. When a suitable valve combination has been decided it is then for-warded through a delay function to the control valves. In this diagram xref corresponds to therequested actuator position, generated either from the flight controller or from a manually spec-ified signal. The e signal is the error between the reference and the current position, Fref is thecontinuous reference force generated by the PID. The DHQ block converts the reference force tothe combination of on/off-valve states, udisc, which generates the closest available actuator forceto the one requested. udisc represents a binary matrix of size np × nc stating which valves shouldbe open or closed. The purpose of the delay block is to make sure that all valves are closed for ashort period between any valve state changes, this is to avoid short circuiting the hydraulic sourcesby opening more than one to the same chamber. The Act block represents the combined valve andactuator combination seen in figure 3.1. Load represents the dynamics of the load. At the end xand x is the physical state of the load.
To conclude the DHA receives a force reference which is then translated to a DFCU combinationsignal which in turn causes the piston to deliver a force approximately equalling the required force.In order to further understand the functionality of the DHA system the separate components willbe described.
3.1.1 PIDThe PID block contains the controller for the actuator. The PID receives the error betweenreference position and current position, as well as the actuator velocity and generates a controlsignal intended to minimise the error according to equation (3.2). In this equation KP , KI , andKD represents the PID gains, e represents the error i.e. the difference between a reference valueand the current value, and u represents the resulting control signal. Also included in the PID isan anti wind-up parameter KAW . The purpose of the anti wind-up parameter is to prevent theintegrator from counting up or down if the system hits a saturation level in either rate or limitingvalues. In this specific system there is a limit in available force output which, if reached, wouldcause the integrator to increase its value. Another functionality in this system which needs toactivate the anti wind-up is the lock function which will be described later. In equation (3.3) thecriteria for the anti wind-up can be seen.
u(t) = KP e(t) +KI
∫T
KAW e(τ)dτ +KDde(t)dt
(3.2)
KAW ={
1, u(t) ∈ [umin, umax] and KI/O = 10, else
(3.3)
3.1 DHA layout concept 19
3.1.2 Digital Hydraulic QuantisationThe digital hydraulic quantisation block, or DHQ, is the core component of the DHA. This blockis responsible for taking a reference force, convert it to the closest available force output and thenoutput the valve combination which enables the piston to deliver this force. For this purpose theDHQ needs to be aware of the available pressure levels and also the chamber areas in order tocalculate the force levels which are available.
In order to calculate the available force spectrum the DHQ sums every combination of pressurelevels and chamber size according to equation (3.4). This results in a total of 81 calculationsaccording to equation (3.1). For each of these force levels the valve combination used for eachchamber is stored in a separate vector. When the force levels have been calculated the force vectoris put through a sorting algorithm to order them according to magnitude, lowest to highest forceoutput. The pressure combination vector also gets updated in order to correctly map the valvecombination to the force requirements. The sorted force spectrum achieved with the parametersused in this thesis can be seen in figure 3.3. The force level calculation and sorting takes place inthe initialisation phase of each simulation in order to make sure that the forces are accurate basedon the prescribed actuator dimensions and pressure levels. The high and low pressure levels usedto achieve this was chosen such that the pressure levels in the stock F16 could still be used withoutmodifications. The middle pressure was chosen such that the resulting force spectrum had as smalldifferences in magnitude between adjoining force levels as possible, whilst being slightly heavier inresolution at small force levels. The resulting pressure levels used based on this were: 28, 7.6, and0.75 MPa.
F = piAA − pjAB + pkAC − plAD, p =
phighpmidplow
, i, j, k, l ∈ {1..3} (3.4)
In order to avoid high frequency shifting in force levels the DHQ includes a functionalityintended to prevent unnecessary switching of desired force levels. This switch prevention is imple-mented in the form of a penalty factor, as can be seen in equation (3.5).
Fcmd = min(|Fref − Fi|+ jP ), j ={
0, i = iprevious
1, i 6= iprevious(3.5)
At each time step the reference force specified by the PID is compared to the available setof forces in the force spectrum. In order to prevent switching force levels between time steps apenalty is added to the difference if the force is not the same as in the previous time step. Whenthe force which minimizes the penalty equation is found the corresponding valve combination isforwarded to the delay block. In figure 3.4 the principle of the penalty function is shown for somepenalty values. In this figure the reference force level is shown as the blue line. The dashed linesindicate the selected force level depending at various penalty setting.
3.1.3 DelayThe on/off-valves used in the DFCU have a limitation in state change frequency due to the inertiaof the mechanism in the valve [11]. This limitation in frequency means that for a high enoughfrequency of the reference signal the valve, despite being discrete, could be put in a partially openstate which would eliminate the purpose of a discrete on/off-valve. The consequences of this wouldresult in a short circuit between pressure sources. If a short circuit would occur it could compromisethe control of the system since the pressure levels would not be correct and potentially the systemmight even take damage. In order to prevent this from happening a delay function is included.
The functionality of the delay block can be seen in equation (3.6). Whenever the control signal,u, to a chamber changes the delay block will make sure that all of the valves connected to thatchamber will be closed for a short time period before receiving the new states. The time periodof this delay, Tdelay, is set as low as possible in order to minimise the phase shift of the controlsystem whilst still being high enough to allow for the valves to change their state properly.
20 Model
10 20 30 40 50 60 70 80
−60
−40
−20
0
20
40
60
Sorted index
Force[kN]
Force spectrum
Figure 3.3: Resulting force spectrum at the chosen pressure levels.
100 200 300 400 500 600 700 800 900 1,000−6
−4
−2
0
2
4
6
Time [s]
Force[kN]
Penalty visualization
ReferenceP = 0P = 500P = 1000
Figure 3.4: Basic principle of the penalty function. In order for a force level switch to occur therequired force needs pass a minimum value check compared to the current value plus a penaltyfactor.
uout(t) ={uin(t), uin(t) = uin(t− Tdelay)0, else
(3.6)
3.1.4 LockDue to the nature of the DHA where only discrete force outputs are available some sort of lockingfunction is required, referred to as the lock. Without this lock the controller needs to constantly
3.1 DHA layout concept 21
jump between two force levels whenever the actuator is in a position where the load force lies inbetween two available force levels. This will in turn result in a pulsation in the actuator wherethe load position will oscillate around the reference point. By introducing two comparators whichcompares the absolute value of the position error and the velocity of the load a simple lock is created.When both of these comparators are activated the control signal to the DFCU is overridden andall of the on/off-valves are forced to close. This will effectively lock the piston in place, ideallyforcing the load to remain in a static position. In reality however the piston will not be staticunless positioned in a minimal load condition since there will be internal leakages in the pistonchambers when there is a force applied via the load. The lock also sets the input to the integratingpart of the PID to zero in order to prevent it from winding up the control signal when the actuatoris locked.
The reason for the lock signal being implemented before the delay block has to do with theinitial requirement for the delay to be implemented in the first place. If the lock signal shouldnot be sent through the delay a situation could occur where the control signals comes in a higherfrequency than the valves can respond to, thus possibly forcing the valve to oscillate in a half openstate which would increase the risk of short circuiting the pressure sources.
3.1.5 ActThe Act block represents the combined DFCU and the actuator. The DFCU part represents thearray of twelve on/off-valves used to connect the three pressure sources to the four chambers ofthe piston, as seen in figure 3.1. The DFCU block takes in the control signal from the delay blockand uses this signal to open and close the valves in the array. The valves have been dimensionedso that they mimic the characteristics of a real on/off-valve [25]. The parameters used in orderto achieve this can be seen in table 3.1. For each individual valve a flow-meter is used to checkthe direction of flow through the valves. This signal is then used to alter the value of the flowcoefficient, Cq. This alteration allows for the valve to have different characteristics based on thedirection of flow, similarly to the actual characteristics which it is based on. The characteristics ofthe reference and simulated on/off-valves can be seen in figure 3.5.
Table 3.1: Hopsan on/off-valve parameter values. Location of parameters can be seen in figure 2.4
Description Parameter Value UnitMaximum spool position xv,max 2 [mm]Fluid density ρ 850 [kg/m3]Spool diameter dv 5 [mm]Spool fraction of diameter f 0.675 [−]
0 10 20 30 40
Flow, q [m3/s]
0
5
10
15
20
Pre
ssu
re d
rop
, p
[b
ar]
Positive flow
Negative flow
(a) On/off-valve characteristics from simulation. (b) Characteristics of the reference poppet valve [25].
Figure 3.5: Comparison of the reference on/off-valve and the Hopsan simulated one.
The actuator in the Act block is a tandem piston. A tandem piston works similarly to anordinary piston with the difference that there are two separate chambers acting in extension and
22 Model
Figure 3.6: Illustration of the aerodynamic spring effect.
two chambers acting in retraction. The tandem piston used in the simulation models is based ona modified standard piston included in Hopsan. Initially the piston in the system was modelledwith internal volumes in the four chambers. Unfortunately this lead to the simulation time beingrequired to be in the micro seconds range in order to provide stable simulation results. In orderto be able to increase the step time of the simulation the number of volumes in the system wasreduced. As a result of this a piston model which does not use internal volumes was adapted suchthat it represented a standard piston. The equations of motion represented in this actuator isbased on Newtons laws of motions and depicts a damped, spring loaded mass system with externalforces, equation (3.7).
xpmp + xpBp + xpkp = FL − Ffluid (3.7)
Another modification to the piston model is that the lever used to transfer linear motion torotational is included in the piston, meaning that the output is rotational. The actuator used hasthe possibility of individually sized surface areas in the chambers, this means that the four separateareas required for the decided force spectrum could be used. The piston also includes a model forinternal leakages, equation (3.8). In this equation qi represents the effective flow into chamber i, q′
i
represents the flow generated from the continuity equation as a function of pressure and actuatorposition. Cl∆p represents the leakage flow from the adjoining chambers, i ± 1 when applicable,with the pressure difference deciding the sign of the flow.
qi = q′i +
∑Cl∆p (3.8)
3.1.6 LoadThe load block in the simple model is intended to loosely mimic the behaviour of an aircraft controlsurface. In order to do this an approximation of the characteristics of control surfaces is required.The control surface can in its most basic form be seen as a spring acting on the mass of the surface.This is based on the idea that in its centred position the surface will experience a minimum ofaerodynamic loads from the air sweeping over it. Any disturbance from this central position causesan increased aerodynamic force, acting towards the central position. The relation between the forceacting on the control surface and the deflection angle can be seen in equation (3.9).
FL ≈Asin(δ)ρv2Cd
2 (3.9)
In this equation FL represents the aerodynamic load applied under the deflection. A is thesurface area of the control surface, δ is the deflection from the centralised position, ρ is the densityof the air, v is the speed of the air relative to the control surface, and Cd is the drag coefficient.An illustration of this centring effect can be seen in figure 3.6.
Approximate force usage during different parts of the flight can be seen in table 3.2. For anaircraft in flight the majority of time is spent doing ferry flight, going from point A to B, andonly a small amount of time the aircraft is engaged in dogfight/turbulent flying. Take-off andlanding only occurs once per flight. This means that for the majority of time the actuators willonly experience around 10% of their maximum load capability.
3.2 Simulation models 23
Table 3.2: Proportional force levels required during different manoeuvres and flight stages [7].
Action Takeoff/landing Ferry flight Dogfight/turbulent flyingMilitary aircraft Pitch 20% 10% 60-100%
Roll 20% 10% 60-100%Yaw 10% 5% 60-100%
Civilian aircraft Pitch 40% 20% 60-100%Roll 40% 20% 60-100%Yaw 10% 10% 60-100%
3.2 Simulation modelsFor this project three simulation models was to be used. For the initial tests on the system asimplified baseline model was used. The purpose of this model was to give an understanding ofthe system and for tuning of the controller.
When the controls have been tuned for the simplified model the system will be modified forimplementation in aircraft models. For this purpose two readily supplied models have been utilised.The initial aircraft simulations will be made using an aircraft model available in Hopsan which isbased on the F16 fighter aircraft. When the results from the F16 simulations are satisfactory asecond aircraft model will be used for further validation. This final model is an altered version ofthe F16 model where the parameters of the wings have been altered such that it represents a deltacanard aircraft. In the following sections the different models will be described.
3.2.1 Baseline DHA model
The baseline model represents one of the most basic setup of a hydraulic system, a piston movinga mass under the influence of an external load. This setup allows for the system to be analysedwithout influence from other components such as the aircraft model itself or the outer loop controlsystem responsible thereof. The fundamental idea behind the system set up and layout can be seenin figure 3.1. In figure 3.2 the block diagram of the DHA is shown, indicating the functionality ofthe system.
3.2.2 Aircraft model
In order to validate the DHA for aircraft purposes an aircraft model has been used. The modelis developed in Hopsan and is included in the standard component library. The aircraft modelincorporates a 6 Degrees Of Freedom State Space, 6DOFSS, representation of an aircraft with sevenmovable control surfaces, figure 3.7. One of the main benefits of using this model is that the controlsurfaces are modelled in a way which allows for them to be driven by hydraulic components. Thismeans that the DHA can be easily implemented in place of an ordinary proportionally controlledactuator.
In order to simulate the aircraft it has to be included in a control system. The system whichhas been used for this is also included in Hopsan and is used to simulate the aircraft during aspecified set of manoeuvres. The main benefit of using this system is that most parts of an aircraftis included, meaning that a full hydraulic system complete with dual system redundancy, pumps,accumulators, and pressurised tanks is already modelled. This simplifies the implementation ofthe DHA since it will only need to be modified to take hydraulics, load, and control signal from anexternal source, leaving the rest of the DHA untouched. A simplification of the hydraulic systemincluded can be seen in figure 3.8.
Also included in the aircraft system is a number of other models responsible for flight control,thrust, and atmosphere modelling. The system and many of the specialised models, including the6DOFSS model, have been created by Prof. Petter Krus and comes included as standard examplemodels in Hopsan [24]. A list of specialized auxiliary components used in the simulation, as wellas a brief description of their functionality, can be seen below.
24 Model
Flaperon Aileron
Elevon
Rudder
FlaperonAileron
Elevon
Rudder
Figure 3.7: Control surfaces in the Hopsan 6DOFSS aircraft model. In the stock model theflaperons are unused, receiving a 0 degree angle reference at all times.
• Atmospheric properties calculator. This model generates appropriate atmospheric data basedon the current altitude of the aircraft.
• Jet engine. This model generates thrust based on the atmospheric data and a reference basedcontrol signal.
• Fuel tank. The fuel tank model simulates the mass addition from the fuel in the aircraft fueltankbased on initial conditions and the fuel consumption in the engine as it runs.
• Attitude control. This is one of the most important parts since this is the control whichtranslates a desired change in aircraft attitude to deflections of the control surfaces. All ofthe control allocation for which control surface will be used at any given time is also decidedin this block. In short this control works as a PD-controller where the deflection of a controlsurface is proportional to a speed of, and errors in, the angles of the aircraft relative to areference. Equations (3.10) through (3.13) shows which control surfaces of the aircraft areused to correct which angle error.
δref,L,aileron = KϕKv∆ϕ+Kϕψ∆ψ (3.10)
δref,R,aileron = −δref,L,aileron (3.11)
δref,elevon = Kv (Kdelevq −Kelev∆θ) (3.12)
δref,rudder = −Kv (βKrud +Kdrudr) (3.13)
Kv = v20
v20 + v2
aircraft
, β ∝ ψ, q = θ, r = ψ
In these equations δref corresponds to the reference angle of the respective control surface, Kϕ,Kϕψ, Kelev, and Krud each correspond to the proportional gain of these surfaces, and Kdelev and
3.2 Simulation models 25
Kdrud corresponds to derivative gains. Kv is a velocity factor which lowers the gain as the velocity ofthe aircraft increases, see figure 3.10. The model also contains a couple of PID controllers includedto generate appropriate control signals. A block diagram showing the full model, including themost important control signals, can be seen in figure 3.11.
3.2.2.1 F16
In its base configuration the aircraft model is preloaded with parameters intended to represent aF16 fighter aircraft, figure 3.12. The wing design of an F16 is a cropped delta wing with all-movinghorizontal stabilator tailplanes, meaning that the entire rear wing is a big control surface. Thetailplane stabilators, hereforth referred to as elevons, are the ones which were initially intended tobe controlled with DHA. The reason for this is that the pitching motion is the focus of this thesisand these wings are the primary source for pitching motion. The purposes of the F16 version willbe to further tune the controllers, comparing the energy efficiency in the same manoeuvre betweenthe proportional actuator and the DHA, and ultimately checking whether the DHA is a viablealternative for controlling the control surfaces of an aircraft.
3.2.2.2 F16 - Hybrid alterations
During the simulations it was realized that the DHA on its own has a hard time controlling thepitch angle of the aircraft sufficiently. As it would seem the pitch rate is highly sensitive to thedeflection angle of the elevons, meaning that high accuracy and smoothness during small motionsare required in order to smoothly fly the aircraft. This is a problem since for the DHA to achievehigh smoothness and accuracy, especially when following a dynamic reference signal, it requireshigh frequency switching. As will be shown in the simulation results there is a correlation betweenthe switch frequency and the energy consumption, meaning that the high frequency switchingis preferably avoided. This lead to the idea of implementing a hybrid system instead of thepure DHA controlled one. The basic idea of the hybrid system is that the DHA will only movewhen a large enough request in angle deflection is received. For small deflections a traditionalproportionally controlled piston, capable of small precise position adjustments but inefficient dueto valve leakage and throttling, will be used. As long as the reference is fairly stable the DHA willmove to a sufficiently close position and then lock in place through the lock mechanism, leavingthe proportional to maintain stable flight. Similar ideas have been explored in [26]
Due to the design of the aircraft model there are two main ways of implementing the hybridsystem in Hopsan. The first way is to use a combination of DHA and proportional actuators actingon a common lever which in turn is connected to the control surface of the aircraft. This approachis similar to the original actuator layout of the aircraft as can be seen in figure 3.9. However, thisapproach has some drawbacks in the hybrid case. The reason that this works in the proportionalcase is that there are two identical actuators being controlled with the same signal, meaning thatthey work in unison. In the hybrid case the two actuators will be required to move independentlyof each other so that they can handle their individual references unhindered by the stiffness andmotion of the other actuator.
The other way of implementing the hybrid system is to utilize the up until now unused flaperonsurfaces for small movements and trimming purposes and using the sensitive elevons for highamplitude control signals. This allows the DHA and proportional actuators to be placed on separatesurfaces, eliminating the interference between them. This approach is realized in two versions: DHAon the elevons and proportional on the flaperons, and in the reverse order, referred to as reverse.
In order to include the flaps for trimming purposes some alterations to the controller are requiredsince the flaperons have no controls in the original configuration. Since the flaperons are supposedto more or less fill the same purpose as the elevons the base design of the control is almost thesame. In figure 3.13 the implementation of the flaperon control is indicated.
3.2.2.3 Delta canard
As the decision was made to implement a hybrid system some limitations in the aircraft design ofthe F16 model became apparent. These limitations have to do with the lifting forces generated onthe aircraft by the wings and control surfaces and the spacing of them in relation to the centre of
26 Model
gravity, figure 3.14 Traditional. Simplified it can be stated that all rotation of an aircraft is centredin the centre of gravity, black and white circle. As such, according to Newtons second law, theinfluence from a control surface on the rotation of the aircraft is proportional to the force exertedtimes the distance to the centre of gravity. With the traditional wing placement this means thatthe flaps needs to be extended further than the elevons to generate a similar torque. By switchingto a delta canard wing profile instead, figure 3.14 DC, the difference in leverage between the twopairs of control surfaces becomes much smaller, resulting in them having a more equal torquecontribution. This is beneficial for the hybrid approach since both of the actuator types will havesimilar say in the pitch rate.
3.2 Simulation models 27
Nose wheelsteering
Landinggear
Rudder
Rightelevon
Leftelevon
Rightflapperon
Leftflapperon
Rightaileron
Leftaileron
System A System B
Figure 3.8: Illustration of the Hopsan aircraft hydraulic system. The contents of the control surfaceactuator blocks can be seen in figure 3.9. Figure 3.7 indicates the location of the control surfaces.
28 Model
AP
AT
BP
BT
δ
Figure 3.9: Content of the control surface actuator blocks of the Hopsan aircraft hydraulic system
0 50 100 150 200 250 300 350 400
Aircraft velocity, vaircraft
[m/s]
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Kv v
alu
e [-]
Figure 3.10: Magnitude of Kv as a function of increasing aircraft velocity.
hrefPID
θref Attitudecontrol
∑δref δ
−
Actuator
×7
Aircraftmodel
ϕ, θ, ψ, β, q, r, vaircraft
h
Atmosphere
Engine
Fuel tank
h
ρair ρair, p0
Fthrust
qfuelmfuel
Figure 3.11: Illustration of the control loops of the aircraft model.
3.2 Simulation models 29
Figure 3.12: Graphical representation showing the wing layout and general position of the controlsurfaces on an F16 aircraft.
eθ
q
KP1
KD1
KP2
KI2
KD2
∫
∑
∑
δelev,ref
δflap,ref
Figure 3.13: Illustration of the changes to the controls when utilizing hybrid hydraulics to manip-ulate the control surfaces.
FL,flapsFL,elevon
lelevon
lflap
Traditional
FL,elevonFL,canard
lcanard lelevon
DC
Figure 3.14: Illustration of the change in leverage between traditional and delta canard wingplacement.
30 Model
3.3 Actuator dimensioningIn order to set relevant values for the size and pressures used in the DHA system some comparativereference data is needed, apart from the requirements listed earlier in table 2.1. These requirementson the actuator are extended to also contain the following:
• The F16 models actuators will be used as reference for these requirements in order to allowfor the results to be comparable.
– The largest diameter of the chamber areas should not be more than 50% larger thanthe diameter used in the replaced actuator of the F16 model. In addition to this themaximum stroke length should be equal to the standard value. This is based on thefact that installation space in an aircraft wing is limited and increases in actuator sizeshould be minimised or preferably avoided.
– The DHA output forces must span at least the range of the original actuators in orderto not limit the available performance.
– The supply pressure levels will be such that the maximum pressure difference availablein the F16 model is not exceeded i.e. the pressures must lie in the span of 7.5 bar (tank)to 280 bar (pump).
• These requirements are not actuator specific and will therefore be based on the performanceof the individual aircraft model.
– The aircraft has to be able to perform a manoeuvre and then maintain level flight.– During flight the performance of the aircraft should not be impaired through the use of
the DHA. The flying properties should also not be noticeably affected by the introduc-tion of the DHA.
In order to get a suitable reference point for the required force levels for the actuator, theelevon actuators of the F16 model has been analysed. The elevons in this model are designed astwo proportionally controlled pistons connected in parallel to a lever, see figure 3.9. This levertransforms the linear displacement of the pistons to a rotational motion which is then relayed tothe control surfaces of the aircraft model. The pistons are connected to the A and B pressuresystems as indicated in figure 3.8 and receive the same valve reference opening signal, which isgenerated proportionally from the error in lever angle relative to a reference angle. This leadsto the conclusion that the required force span could be calculated from the sum of the systempressures and piston areas. By using the standard equation for hydraulic force, equation (3.14),the maximum static forces in tension and compression could be calculated. The data used here aswell as the resulting forces can be seen in table 3.3.
Fp = A1p1 −A2p2 (3.14)
AA1/B1 AA2/B2
pAP/BP /pAT/BT pAT/BT /pAP/BP
FA/B
Figure 3.15: Nomenclature for the parameters used in table 3.3.
With the maximum force span calculated it is possible to calculate the piston areas requiredin the DHA in order to satisfy the force requirements. The force output for the tandem pistoncan be calculated through the use of an altered version of equation (3.14) which can be seen inequation (3.15). From this equation it is clear that for the maximum positive force output to bedelivered the first and third chamber should get the highest possible pressure whilst the second
3.3 Actuator dimensioning 31
Table 3.3: Parameters and results from calculating DHA force requirements. Location of parame-ters can be seen in figure 3.15.
Entity Parameter Value UnitElevon piston area AA1 10 cm2
AA2 10 cm2
AB1 10 cm2
AB2 10 cm2
Supply pressures pAP 28 MPapAT 0.75 MPapBP 28 MPapBT 0.75 MPa
Resulting maximum force FA,max ±27.25 kNFB,max ±27.25 kNFmax,tot ±54.5 kNFspan,tot 109 kN
and fourth chambers should get the lowest pressure. For the highest negative force the oppositeholds. The resulting forces from implementing the chamber sizes used from [5] into equation (3.15)can be seen in table 3.3. It can be seen that the force requirements stated in table 3.3 are fulfilled.
FDHA = A1p1 −A2p2 +A3p3 −A4p4 (3.15)
Table 3.4: Areas and resulting force limits when using the areas from [5].
Entity Parameter Value UnitTandem piston area A1 12.1 cm2
A2 10.6 cm2
A3 10.0 cm2
A4 10.1 cm2
Supply pressures pAP 28 MPapAT 0.75 MPapBP 28 MPapBT 0.75 MPa
Resulting maximum force Fmax 60.3 kNFmin -56.3 kN
Ftot,span 116.6 kN
32 Model
Chapter 4
Simulation procedure
4.1 Baseline DHA modelIn order to test the baseline DHA and compare it to a proportional actuator they should beput through similar test cases and conditions. The intention for these simulations have been tocompare the DHA to a similarly sized proportionally controlled hydraulic system following the samereference signal as well as comparing them to the specified actuator requirements. The commonspecifications for the baseline model can be seen in table 4.1 and case specific differences can beseen in table 4.2. The mass of the control surface has been approximated and the correspondingload mass has then been calculated using Steiners theorem.The value for the spring stiffness is setsuch that the piston should barely be able to reach the end position during a full stroke. Theinternal leakage of the pistons have been set to zero. In order to calculate a comparable energyefficiency the three cases will have the same pump and accumulator set up, as taken from the F16model, and the output power of the pump/s will be measured.
The reference used in the baseline system simulations is the same as in [5] with a small variation.At the end of the simulation an additional ten seconds have been added where a static referenceposition is hold. By doing this one of the benefits in energy consumption of using DHA is shown.In order to highlight some strengths and weaknesses during different circumstances the simulationsare run at a high and a low amplitude.
Table 4.1: Specifications for the parameters used in the baseline model simulations. A nomenclaturefor the parameters can be seen in figures 4.1 and 4.2.
Description Parameter Value UnitControl surface mass msurf 50 [kg]Load mass mload 249 [kg]Spring stiffness kload 139 [kN/m]Lever length l 65 [mm]Maximum stroke S 200 [mm]Leakage coefficient Cl 0 [m3/sPa]Delay time Tdelay 0 [s]
4.1.1 Frequency responseIn addition to testing the energy efficiency and reference following the DHA also needs to fulfil somerequirements in frequency response. The requirements have been stated for two cases. The firstrequirement is stated for an amplitude of 0.5% of maximum stroke and the second requirement for5%. Because the DHA actuator has been modified such that the lever is included in the actuatorand the rotational angle is used in the feedback some calculations are required in order to get aangular amplitude corresponding to the two strokes. By using equation (4.1) it can be calculatedthat an angle of 0.15 rad and 0.015 rad corresponds to a stroke of 5 and 0.5%. Since the resulting
33
34 Simulation procedure
Table 4.2: Case specific differences
Case 1 2 3 UnitActuator type Digital Digital Proportional [-]Focus Performance Energy Performance [-]Chamber areas {12.1, 10.6. 10.0, 10.1} {12.1, 10.6. 10.0, 10.1} 4× 10.7 [cm2]Mean chamber area 10.7 10.7 10.7 [cm2]Lock speed threshold 0.01 1 N/A [rad/s]Lock position threshold 0.005 0.015 N/A [rad]Penalty factor, P 0.25 1 N/A [kN ]
mload
xp, xp
δ, δ
l
kload
Figure 4.1: Benchmark specification nomenclature.
angles are small the error from not using trigonometric functions could be considered small enoughthat the calculated values are valid. In order to test the frequency response a step with exponentialdecay is used as a reference. By using this reference the results will contain a sufficient range offrequencies that a frequency analysis could be made from the results. To calculate the frequencyresponse the built in transfer function analysis tool in Hopsan has been utilized.
xp ≈ lδ (4.1)
4.2 F16 35
δ, δ
xpxp
l
Figure 4.2: Control surface geometry.
4.2 F16The initial aircraft simulations are intended to give an indication of the level of control requiredfor the DHA to be able to, with sufficient performance, control an aircraft. For this purposea simulation case consisting of the F16 aircraft model, equipped with DHA on the elevons andhooked in to the aircraft hydraulic supply system, performing a step in elevation was used. Duringthe simulations the aircraft is to fly at a set velocity for 100 seconds after which a step in altitudeof one kilometre will be made. The new altitude will then be held for another 100 seconds afterwhich another step will take place, returning the aircraft to the initial altitude. This manoeuvrewill allow for the DHA to perform both positive and negative motion, as well as steady stateperformance. This simulation will be run with different gains in the DHA controller in order tofind out if sufficient flight performance is possible at a lower energy cost than for the proportionallycontrolled reference aircraft. Similarly as in the baseline model the power outputs of the pumpswill be measured in order to compare the different cases.
In order to get an idea of the performance in different parts of the flight envelope three caseshave been used. These cases have been chosen such that they provide different characteristics tothe behaviour of the aircraft. In table 4.3 the cases can be seen. In case 1 the aircraft will be flyingin the middle of the speed range and at low altitude where the air density is high. In this case thecontrol surface deflections should be small and relatively effortless due to the low speed and highdensity. The second case is intended to show the performance when sensitivity is high and therequirement in response time is critical. The final case is intended to show the performance duringthe opposite conditions. At high altitude and low speed the actuators are required to make largedeflections to manipulate the aircraft since the air density and velocity is low. At this altitude thesettings for the pitch reference controller proved insufficient, requiring alterations of the controllerparameters in order to be able to fly.
Table 4.3: Flight cases used to test the aircraft.
Case 1 2 3 UnitInitial altitude, y0 1 5 10 [km]Reference velocity, v0 200 300 180 [m/s]Pitch control parametersBreak frequency, ωA 0.115 0.115 0.09 [rad/s]Relative damping, δA 1.01 1.01 1.1 [−]
4.2.1 HybridFor the simulations of the hybrid aircraft configuration the same flight cases as in the pure DHA setup has been used. During the initial tests these simulations have been run using only the standard
36 Simulation procedure
proportional actuators. The intention of this has simply been to provide a proof of concept forthe hybrid design, testing whether the flaperons can be used for trim.When the hybrid control wasimplemented and functional the DHA could be implemented. This implementation was made intwo ways. The first implementation followed the initial design idea with the DHA controlling theelevons and the flaperons being controlled proportionally. The second implementation utilized thereverse layout, DHA on the flaperons. With the DHA implemented the three cases in table 4.3was simulated.
4.3 Delta canardIn order to simulate the delta canard aircraft some internal design parameters had to be altered.These parameters correspond to the positioning of the wings so that they resemble a delta canardaircraft and a change in the positive direction of the canard, former elevon, control surface. Forthe sake of comparison the delta canard aircraft is simulated as entirely controlled by proportionalactuators as well as DHA. The simulation with proportional actuators will then be used to analysethe results from the DHA simulations. Regarding the control configuration of the delta canardaircraft the standard DHA layout uses the DHA for the canard control surface and the reverselayout uses the DHA for the elevons, former flaperons. Finally, in order to be consistent, the deltacanard aircraft is put through the same three test cases as the F16 stated earlier.
Chapter 5
Results
5.1 Baseline DHA modelIn the baseline comparison the DHA was compared to a proportional actuator during two cases, ahigh amplitude and a low amplitude case. In figure 5.1 the results from these simulations can beseen. As well as the difference in performance of the DHA during different controller specificationsthe differences compared to a proportional actuator can also bee seen here. From these results itcan be seen that in general, the proportional actuator has a greater ability to precisely follow thereference position than the DHA. In the case of the energy efficiency tuned DHA the referencefollowing is shown to be especially lacking as a result of the controls being tuned for low energyconsumption.
0 2 4 6 8 10 12 14 16 18 20
Time [s]
-40
-30
-20
-10
0
10
20
30
40
50
Angle
[deg
]
-2
0
2
4
6
8
10
12
Energ
y [
kJ]
Reference
Proportional
DHA performance
DHA energy
(a) High stroke amplitude.
0 2 4 6 8 10 12 14 16 18 20
Time [s]
-8
-6
-4
-2
0
2
4
6
8
10
12
Angle
[deg
]
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Energ
y [
kJ]
Reference
Proportional
DHA performance
DHA energy
(b) Low stroke amplitude.
Figure 5.1: Simulation results for the benchmark model. Solid lines shows the angular position anddashed lines indicate consumed energy. DHA performance and DHA energy indicates the tuningfocus for the DHA controller. In figure 5.2 the reference and commanded force for each case canbe seen.
In figure 5.2 it can be seen that for the energy tuning the amount of switches performed by thevalves are significantly lower than in the case of performance tuning. This results in the DHA beinghighly energy efficient at the cost of accuracy, 9.5% of the energy consumption of the proportionalactuator at 20s. In the low amplitude case this becomes especially obvious as the DHA has ahard time following the small references but is much more energy efficient, 6.8% of the energyconsumption.
When simulating the performance tuned DHA the controllers receive an increase in power. Thisallows for the DHA to be faster to respond to small errors and allows for more precise control ofthe position. This allows for a much improved ability to follow the reference. The performance
37
38 Results
0 5 10 15 20
Time [s]
-60
-50
-40
-30
-20
-10
0
10
Fo
rce
[kN
]
High amplitude performance
Fref
Fchosen
0 5 10 15 20
Time [s]
-6
-4
-2
0
2
4
6
Fo
rce
[kN
]
High amplitude energy
Fref
Fchosen
0 5 10 15 20
Time [s]
-8
-6
-4
-2
0
2
4
Fo
rce
[kN
]
Low amplitude performance
Fref
Fchosen
0 5 10 15 20
Time [s]
-2
-1
0
1
2
Fo
rce
[kN
]
Low amplitude energy
Fref
Fchosen
Figure 5.2: Force plot for the benchmark DHA model. The amount of switches performed for eachcase is: 77 (TL), 19 (TR), 18 (BL), 10 (BR).
tuned controller however generates a drastic increase in switching frequency, leading to a decreasein energy efficiency. Even though the energy efficiency is worse than for the energy tuned controllerthe performance tuning is still much more energy efficient than the proportional actuator, 43.8% ofthe energy consumption during the high amplitude case. Also notable is that with the performancefocused controller the DHA is able to almost be as precise as the proportionally controlled actuator.
In figures 5.3 and 5.4 the high performance DHA is subjected to a step response with exponentialdecay and an amplitude corresponding to 5% and 0.5% of the maximum stroke of the actuator.The purpose of this is to be able to generate a frequency response analysis and see whether therequirements stated are fulfilled. At a frequency of 7Hz the amplitude is required to not be lowerthan -7.25 dB and the phase shift can be maximum 92 degrees. From these results it can be seenthat for the case of the high amplitude response the DHA is able to pass the requirements statedwith a amplitude of 0.89dB and a phase shift of 72.7 degrees.
During the low amplitude test the DHA still managed to fulfil the stated requirements, achiev-ing a -1.83dB amplification and a phase shift of 48.5 degrees. This was however only possibleafter lowering the tolerance for the position locking mechanism as far as possible without causinginstability. Still, after this the static error was still large in comparison to the amplitude of thestep, leading to the amplification moving towards negative infinity dB for the low amplitude case,similarly with the phase. Additionally it can be noted that there is an amplification peak at 16Hzof 8.54dB. This peak is most likely a result of the limited ability for fine speed control whichbecomes more apparent when trying perform fine control at low amplitudes.
The fact that both of these impulse tests shows bad behaviour in the low frequency range comesfrom the lack of precision in the DHA. Because there is a lingering static error after the impulseboth the amplitude and phase in this region gets reduced in the corresponding Bode diagrams.While this is especially obvious in the low amplitude case where the relative error is rather largeit is also noticeable in the high amplitude case.
5.1 Baseline DHA model 39
0 1 2 3 4 5 6 7 8 9 10
Time [s]
-0.02
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le [
rad]
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(a) Simulation results.
10-1 100 101
Frequency [Hz]
-30
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nitud
e [d
B]
10-1 100 101
Frequency [Hz]
-1500
-1000
-500
0
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se
[de
g]
(b) Bode diagram.
Figure 5.3: 5% amplitude frequency response for the benchmark model. Both the simulation resultsand corresponding bode plot, as calculated using Hopsans built in transfer function analysis tool,is shown. At 7 Hz the magnitude is 0.89 dB and the phase is -72.7 degrees. The magnitude at 0.1Hz is -2.1 dB, this can be traced to the error in final position after the impulse.
0 1 2 3 4 5 6 7 8 9 10
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d]
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0
10
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B]
10-1 100 101
Frequency [Hz]
-300
-200
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0
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(b) Bode diagram.
Figure 5.4: 0.5% amplitude frequency response for the benchmark model. Both the simulationresults and corresponding bode plot, as calculated using Hopsans built in transfer function analysistool, is shown. At 7 Hz the magnitude is -1.83 dB and the phase is -48.5 degrees. At 16 Hz themagnitude peaks at 8.54 dB.
40 Results
5.2 Aircraft5.2.1 F16As can be seen in figure 5.5 there appears to be a trade off between the performance and theenergy consumption of the pure DHA controlled F16 aircraft. With the controller set to deliverthe highest performance the aircraft is capable of flying with near identical performance as withthe stock actuators. This does however demand high frequency in the switching of the force levels,leading to high energy consumption from the actuator being in near constant motion.
By altering some of the parameters in the controller of the DHA the energy consumption couldbe drastically lowered. This comes at a cost of performance though as the control surfaces are onlypositioned close enough to the requested position. This manifests itself in the performance of theaircraft in that it has trouble maintaining level flight. The manoeuvres them self are however notnoticeably affected by this.
The mid performance line in the figure is intended to show a middle ground where neither theenergy consumption or the flight performance is optimal but the trade of between them is fairlyreasonable.
(a) Altitude for three controller setting duringthe initial F16 simulations.
0 50 100 150 200 250 300Time [s]
-30
-20
-10
0
10
20
30
40
Tip
ang
le,
[deg
]
Tip angle
Low energyMid energyHigh energy
(b) Pitch angles of the F16 aircraft during the initialflight simulations.
Figure 5.5: Initial simulations of the F16 aircraft with DHA. In the left figure the altitude of theaircraft is shown during the entire manoeuvre. The right figure shows the corresponding pitchangle of the aircraft during the three cases. Note that in the low energy case the pitch angle hastrouble achieving the required value during steady state.
5.2.2 Hybrid configurationIn figure 5.6 additional simulation results from simulating the stock aircraft can be seen. Thepurpose of these results is to show the similarities in flight performance between using only theelevons and using a combination of elevons and flaperons to control the pitch angle during amanoeuvre. What can be seen in these results is that although the differences are small the additionof the flaperons contributes slightly to the response of the aircraft. The major behaviour of theaircraft are similar with minimal differences in altitude rise and response. The main difference liesin the acceleration of the pitch angle of the aircraft caused by the force applied by the additionalcontrol surfaces. When using flaps it can be seen that the added control enables the aircraft tostabilize at a higher rate than the pure aircraft. Because the flaps used in the hybrid configurationare no longer unused and centralized the hydraulic energy consumption is increased slightly in thiscase, see the comparison in figure 5.7. Here it can be seen that the improved manoeuvrabilitycarries with it a mean increase in energy consumption of approximately 5%.
In figure 5.8 the deflection angles of the elevon and flaperon control surface for case 1 can beseen. The angles are shown with the hybrid flaperon control both enabled and disabled. The
5.2 Aircraft 41
deflection of the flaperons in the disabled case comes from the external load acting on the controlsurfaces in the manoeuvres. With the flaperon control enabled it can be noted that the amplitudeof the flaperons deflection is much higher than that of the elevons. Since the elevons have a muchgreater leverage to the centre of gravity than the flaperons in the F16 configuration this differenceis to be expected. This difference is also the reason for the reverse DHA configuration.
In figure 5.9 the deflection of the elevons and flaperons can be seen again. In this figure howeverthey are shown for the DHA controlled aircraft, both in the original and reverse configuration. Ascan be seen in the original configuration, the DHA is required to work hard in order to maintain astable flight when mounted on the sensitive elevons. The high frequency oscillations of the DHAas it tries to fine tune the position resonates to the tip angle and rate of the aircraft, forcing theproportionally controlled flaperons to counter the oscillations.
In the reverse configuration however, when the DHA controls the flaperons, the precise controlof the elevons is handled by a proportional actuator. Since the DHA is no longer required toperform the precision low amplitude control the tolerances for the position lock script could beadjusted such that the DHA is only required to get close enough and then lock in place. When themanoeuvres come, the DHA is then released and contributes to the control of the aircraft again.By locking the actuator the energy consumption is reduced significantly.
In figure 5.10 the flight performance and energy consumption of the F16 for every test caseand configuration can be seen. As can be seen in this figure the original configuration of the DHAhas the highest energy consumption. The reason for this can be traced back to the differencein behaviour of the DHA between the original and reverse configuration, where the oscillationsof the DHA resonates to the aircraft in a much greater amplitude than when it is mounted in itsreverse configuration. In the reverse configuration it can be seen that there is a reduction in energyconsumption compared to the unmodified aircraft.
In the third case the differences in energy efficiency is practically equal between the reverseDHA and the stock actuator. This indicates that at this altitude, when the air density and pressureis low, the actuators does not need to work as hard, limiting the power consumption of them.
5.2.3 Delta canardWhen simulating the delta canard aircraft it was noted that both the canard and the elevons areconstantly working to control the now unstable aircraft. Due to this the overall energy consumptionwas increased, see figure 5.11. This figure shows the energy consumption of the F16 and the deltacanard configured aircraft. It can clearly be seen that the delta canard configuration consumesmore energy than the F16 in all actuator configurations. Besides for the delta canard with theDHA configuration in case 3 the distribution in energy consumption between the different actuatorconfigurations are relatively consistent.
The reason for the Delta canard with DHA in case 3 breaking the trend appears to be the resultof the DHA not being able to place the control surface in a sufficiently precise position for the lockfunction to kick in. As a result of this the DHA is constantly adjusting its position, leading to ahigh number of switches and thus high energy consumption.
Figure 5.12 shows the results from simulating the aircraft in its delta canard configuration. Inthis figure it can be seen that the there is a slight increase in the difference between the responsesof the different actuator configurations of the aircraft. The difference shown up until the 50 secondmark for the DHA derives from the fact that the DHA and the proportional actuators have slightlydifferent initialisation positions. As a result of this the aircraft receives a slight difference in initialbehaviour. After it the 50 second mark however the aircraft can be seen to show a similar behaviouras the other references.
Similarly to the F16 results, the delta canard shows a reduction in energy efficiency whenmounted on the more sensitive control surface, canard wing in this case. This can be tracedback to the same reasons as in the F16. This control surface resonates in a high degree to thebehaviour of the aircraft, demanding that the actuator responsible for its control to be precisein its positioning and motions. In the reverse case however the DHA shows an increased energyefficiency compared to the stock actuators.
42 Results
0 50 100 150 200 250
Time [s]
0.5
1
1.5
2
2.5
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de
[km
]
Case 1 altitude
href
h
hflaps
0 50 100 150 200 250
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-40
-20
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20
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60
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an
gle
[d
eg
]
Case 1 tip angle
ref
flaps
0 50 100 150 200 250
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4.5
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5.5
6
6.5
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de
[km
]
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href
h
hflaps
0 50 100 150 200 250
Time [s]
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-20
-10
0
10
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an
gle
[d
eg
]Case 2 tip angle
ref
flaps
0 50 100 150 200 250
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10
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11
11.5
Altitu
de
[km
]
Case 3 altitude
href
h
hflaps
0 50 100 150 200 250
Time [s]
-20
0
20
40
60
Tip
an
gle
[d
eg
]
Case 3 tip angle
ref
flaps
Figure 5.6: Flight performance of the stock F16 aircraft during the three flight cases. The graphdisplays the simulation results both with and without addition of the flaps used in the hybrid DHAversions. The corresponding energy consumption can be seen in figure 5.7.
5.2 Aircraft 43
0 50 100 150 200 250 300
Time [s]
0
100
200
300
400
500
600
700
800
900
Hydra
ulic
energ
y c
on
sum
ption
[kJ]
Stock
625.466 kJ
706.566 kJ
778.934 kJ
Case: 1
Case: 2
Case: 3
0 50 100 150 200 250 300
Time [s]
0
100
200
300
400
500
600
700
800
900
Hydra
ulic
energ
y c
on
sum
ption
[kJ]
Hybrid
661.798 kJ
766.812 kJ
790.598 kJ
Case: 1
Case: 2
Case: 3
Figure 5.7: Hydraulic energy consumption for the stock F16 aircraft during the three flight cases.The graph displays the simulation results without addition of the flaps used in the hybrid DHAversions.
Time [s]0 50 100 150 200 250 300
Def
lect
ion
angl
e [d
eg]
-0.14
-0.12
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0
0.02Flaperons off
ElevonFlaperon
Time [s]0 50 100 150 200 250 300
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lect
ion
angl
e [d
eg]
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6Flaperons on
ElevonFlaperon
Figure 5.8: Control surface angles of the stock F16 aircraft. To the left the angles can be seenwhen the flap reference is zero and to the right the hybrid case can be seen.
Time [s]0 50 100 150 200 250 300
Def
lect
ion
angl
e [d
eg]
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6Flaperons on
ElevonFlaperon
Time [s]0 50 100 150 200 250 300
Def
lect
ion
angl
e [d
eg]
-0.3
-0.25
-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2Flaperons off
ElevonFlaperon
Figure 5.9: Control surface angles of the stock F16 aircraft with DHA. To the left the angles canbe seen when the flap reference is zero and to the right the hybrid case can be seen.
44 Results
Figure 5.10: F16 flight performance and energy consumption. The solid lines represents the refer-ence altitude, black, actual altitude of the unmodified F16, blue, the altitude with DHA mountedon the elevons, red, and reverse configuration, magenta. On the right axis the energy consumptioncan be read, dashed lines. The graph displays the simulation results from all three flight cases andall cases have been simulated with flaperons on.
5.2 Aircraft 45
Figure 5.11: Hydraulic energy consumption for the F16 and delta canard, DC, simulation modelswith DHA during the three flight cases. In this graph the difference in energy consumption betweenthe different actuator configurations can be seen to be similar, despite case or aircraft model. Anexception can be seen in case 3 for the delta canard, where the DHA configuration shows a muchhigher energy consumption. The reason for this can be traced to the fact that in this case the DHAis unable to achieve the requirements for locking it in place, leading to high energy consumptionfrom the near constant motion.
46 Results
Figure 5.12: Delta canard flight performance and energy consumption. The solid lines representsthe reference altitude, black, actual altitude of the unmodified delta canard, blue, the altitudewith DHA mounted on the elevons, red, and reverse configuration, magenta. On the right axis theenergy consumption can be read, dashed lines. The graph displays the simulation results from allthree flight cases and all cases have been simulated with flaperons on.
Chapter 6
Discussion
6.1 Simulation modelsIn order to be able to compare the energy consumption between different aircraft configurations,as well as between the two benchmark configurations, the energy source needs to be the samefor each of the systems. The initial idea to achieve this was to just use ideal pressure sources inHopsan, set to the desired pressure levels, and measure the power consumption of the actuator andcontrol valves. As it turned out this carried with it some complications in the case of the DHA.These complications comes from the combination of the on/off-valves only having the possibilityto toggle between fully opened and fully closed, and the pressure sources having no limitation inoutput flow levels. This combination enabled extremely high accelerations of the actuator which,besides making it hard to control, caused the energy consumption to become unreasonably high,not due to high pressure drops but due to high flow spikes.
In order to mitigate this behaviour the hydraulic supply system of the non-modified F16 wasadapted for use in all of the systems. Introducing the pressure controlled pump to the systemmeant that the supply pressure would drop at the opening of the valves during the time it takes forthe displacement of the pump to respond. This ensured that the flow spikes would be significantlysmaller. By using the same sized pump, accumulator, tank, and pressure levels the conditionsfor each of the systems could be considered to be equal. The use of pumps also meant that thepower consumption for the entire system could be measured as the hydraulic power output fromthe pumps.
In order to handle three pressure levels required for the DHA two pumps was used. Themotivation for this supply set up is that the F16 model has two hydraulic systems, each with itsown pump. By redesigning the hydraulic circuit slightly such that the A side pump, doubled insize, powered all of the high pressure actuators, both from the A and B side, the B pump could bereconfigured such that it supplied the mid-range pressure required by the DHA. The B pump wasalso reduced to half its original size. After the redesign the A and B pumps shared tank, effectivelyremoving every trace of dual system redundancy. This is not necessarily the best way of modellingthe system from a redundancy viewpoint but for the sake of this thesis it was considered adequateas it provided a good way of generating the middle pressure.
6.2 Results6.2.1 Baseline modelThe requirements for the actuator as stated in table 2.1 are used to analyse the results of thesimulations. These requirements have been specified for the control surface actuators of a F18research aircraft. The F18 fighter aircraft is similar in design, size, and weight to the F16 meaningthat the requirements posed to the F18 should be transferable to the F16. At the very least itrepresents a quantitative way of comparing the DHA to authentic aircraft control surface actuators.
The results shown in this thesis shows that the requirements stated are possible to fulfil throughthe use of the DHA. However for some of the requirements there was a need to adjust certain aspects
47
48 Discussion
of the control in order to sufficiently pass the requirements. In the end, the DHA, when comparedto a similar proportional actuator, managed to get comparable results in regards to response.
By using a more advanced control strategy to decide the reference force than the PID used inthis thesis the results shown here could possibly be improved. When looking at the impulse resultsit could be assumed that by using a controller which could better predict the behaviour of theDHA the error in final positioning could potentially be more or less entirely removed. This wouldthen lead to increased precision of the actuator which in turn could mean that even the precisionrequirements could be met.
6.2.2 AircraftFor the simulations of the aircraft it proved to be difficult to find good and measurable requirementsto compare and analyse the results against. As such the results in the aircraft simulations areanalysed through a combination of the requirements stated for the benchmark model, the propertiesof the hydraulic system, and an intuitive sense for what is acceptable flight performance.
However, as all of the simulations have had the same conditions in regards to starting condi-tion of the aircraft and the manoeuvre to be performed it could be considered that from a purecomparison viewpoint the conclusions drawn from the results are valid. This is especially true ifit is also taken into consideration that the core layout of the hydraulic system as well as the waythat energy consumption have been measured is the same for every simulation system.
When looking at the results in energy consumption, figure 5.11, the delta canard with DHAconfiguration in case 3 separated itself from the other results significantly. This would appearto be as a result of the fact that the DHA is unable to achieve a position which satisfies thelocking conditions. Additionally from these results the difference in energy consumption betweenthe traditional and the digitally controlled aircraft is indicated. This difference can be seen as thedifference in slope angle of the energy consumption for the DHA simulations. It can clearly be seenthat while the DHA is under motion the energy consumption increases significantly The reason forthis is the fact that the actuator is under near constant motion during the unlocked state. Thisindicates that the key to achieving better energy consumptions with the DHA lies in locking theactuator and effectively turning of the energy consumption in it.
By having a more advanced controller for the DHA, as discussed for the baseline model, itcould be possible to achieve better motion control of the actuator. This could in turn lead to asituation where it could lock in place more often whilst maintaining the flight performance, whichwould additionally lower the energy consumption.
6.3 Initial objectivesIn this section the objectives stated in the beginning of the thesis will be returned to and answered.
1. Compare and analyse digital hydraulics in relation to proportionalhydraulicsAs has been shown there are some fundamental differences between the DHA and the proportionalhydraulics. Most significant of these differences is the energy efficiency and ability for precisecontrol. The proportionally controlled actuator has a greater ability for precise control due toit being controlled by a proportional valve. If high precision is a requirement the proportionalactuator is most likely the better choice. However, the gains in precision must be compared to thespeed and energy efficiency of digital control. Speed here referring to the step between full force inthe opposite direction taking the same time as a step between two adjacent force steps, comparedto the proportional control valve where the spool needs to traverse the entire stroke to achieve thesame span. In regards to the energy, due to the on/off-valves having a low pressure drop over them,resulting in minimal throttling losses, the energy consumption gets drastically reduced comparedto the traditional actuator. If the possibility to close all of the on/off-valves when no movement isrequired the energy consumption of the DHA can be entirely removed, at least momentarily.
Additional differences between the two schools of controlling a hydraulic actuator which re-quires consideration is the size and weight of the control units. Since the DHA requires a grid
6.4 Method 49
of on/off-valves the weight of such a system risks becoming rather large. In the system layout ofthis thesis the DHA requires 12 valves in order to be fully operational. Compared to the singlecontrol valve of the traditional actuator this could potentially result in a rather significant weightincrease. This property has however some benefits when it comes to redundancy since a single valvemalfunctioning will not cause the DHA to fail in the same way as the proportionally controlledactuator would.
2. Compare and analyse digital hydraulics as a replacement to propor-tional hydraulics in aircraftAs suggested by the simulation results the replacement of proportional hydraulics in aircraft byDHA has some great benefits. By inserting the DHA to the control surface of an aircraft it could beshown that a performance equalling the performance of the stock actuators was easily achievable.However due to a high precision requirement a compromise had to be made between performanceand energy efficiency. On one hand there was the case of high precision and performance. Dueto the nature of the DHA this carries with it high frequency switching between force levels. Ahigh switch frequency in turn causes the energy consumption to rise, meaning that the energyconsumption at this level of performance becomes high from the actuator being in near constantmotion. By altering the controller gains, the switch penalty value, and the position tolerances forthe lock function the amount of switches, and thus the energy consumption, could be reduced.This happened at the cost of performance of the aircraft though.
3. Compare and analyse digital hydraulics as a hybrid addition to pro-portional hydraulics in aircraftFrom the hybrid aircraft simulations it can be shown that by combining the energy efficiency of theDHA with the precision of a proportional actuator the results from using only a DHA is improvedgreatly. This set up achieves a flight behaviour which performs similarly to the stock aircraft butat a lower energy consumption, up to a 40% reduction in these results. This is due to the DHAbeing able to lock, letting the proportional actuator handle the fine adjustments and trim.
A potential downside to using this form of hybrid system is that the weight and the size of thecontrol units might become greater than those of a traditional valve. That means that for thissystem to be implemented there might arise a need for major re-design of the aircraft in order tofit the control valves. This might show to be a problem for implementation in existing aircraft butshould be possible in a newly designed aircraft.
6.4 MethodThis thesis work has been intended to be more of a comparison and analysis than a means to geta specific result. As such the working plan has not been a straight line but instead been a livingobject and subject to many alterations in order to adapt to the progress and results.
Some examples of this can be seen when comparing the initial methodology proposal to theend results. Initially all of the simulations were intended to be made using the Modelica basedmodelling software Dymola. However due to a lack of licences the change was made to use theopen source hydraulic simulation software Hopsan instead. This had some clear benefits howeversince experience with Hopsan was already had and work could be commenced instantly withouthaving to put time into learning a new simulation software.
Similar changes were made to the validation process since the initial idea was to export the DHAmodel and implement it in external aircraft simulators. However due to the change in simulationsoftware from Dymola to Hopsan the decision was made to forego use of the intended simulators.The reason for this lies in the time consumption of porting the models to the simulation software.
In the initial time plan of the thesis it was believed that a great amount of time would be spentlearning new simulation software and porting simulation models from these software into externalsimulators where they would be tested. As it would turn out this would not be the case since acombination of results, time, and difficulties in getting the simulation model to work caused for
50 Discussion
an alteration in these plans. This alteration meant that instead of using external simulators totest the validity of the DHA the Hopsan aircraft model used for implementation and testing of theDHA as an aircraft actuator was to be used for final validation. This new method of validation hadhowever its own benefits as the adaptation of the DHA model was minimal in order to implementit in the aircraft model.
6.5 Ethics and moralityThis thesis has focused on control of military aircraft. Due to this some considerations on theethics and morality of both the subject itself and of the corporation behind the thesis proposalshould be considered. First and foremost, the actuators and controls considered in this thesis arenot in any way limited to military use. The system used and most of the results could be applicableon civilian aircraft. As such it should not be considered as harmful, but instead as a study onaircraft actuators in general. Secondly, this thesis has been conducted at the request of Saab. Saabis a manufacturer of military equipment but with a focus on defence. As such, in alignment withSwedish export laws, equipment manufactured by Saab can not be directly sold to nations wagingoffensive wars.
Chapter 7
Conclusion
Based on the results shown in this thesis it would appear that digital hydraulics is a feasiblealternative to traditional hydraulics in aircraft control. Due to the low pressure drops over thevalves in its opened state and the near perfect seal when closed the energy consumption can beminimized. This is especially true if the actuator is allowed to move to a position close enoughto the required position and once there shut the control valves, effectively locking it in place andreducing the power consumption to zero. Through the hybrid combination of digital and traditionalactuators this is especially achievable.
There are some drawbacks however. If the precision requirements for the actuator is set to highthe actuator might be unable to find a position where the requirements for the locking mechanismare fulfilled. Should this occur the actuator will be put in a state of high frequency switchingbetween different force levels, trying to correct the error in positioning. Although this is beneficialto the positioning of the actuator in that the mean position will be rather good, this also meansthat the piston is in more or less constant motion. This constant motion will cause the actuatorto consume lots of energy due to the near unrestricted flow to the actuator.
Additionally, due to the digital actuator requiring a large number of valves in order to function,the size and weight of the system might become higher than that of a traditional servo valvecontrolled actuator. In static applications this is not a big problem. In an aircraft however boththe weight and space is limited, meaning that the energy saving of using digital hydraulics mightnot be enough to counter the added weight and volume required by the twelve on/off-valves. Itshould also be considered that there is a possibility that the potential weight increase could benegated entirely by existing components not being required in a DHA controlled system.
Having said that. If the weight and size were to be acceptable and the high precision requirementcould be relaxed, then digital actuators would appear to be a future challenger of the traditionalhydraulics which are in use today.
7.1 Suggestions for further studiesIn order to continue studying this subject the following suggestions for further studies will bementioned.
• Control design - By using a more advanced control system for the actuator it is possiblethat a higher level of precision could be achieved at the same level of energy consumption.
• More detailed aircraft model - The hybrid design used in this thesis relies on the use ofauxiliary control surfaces to perform the fine tuning of the flight of the aircraft. By using anaircraft model with dedicated trim surfaces the overall performance of the hybrid controlledaircraft might be improved.
• Other digital actuator versions - This thesis has been limited to the use of the forcediscretized digital actuator design. It would be interesting to see whether the results wouldbe the same with another design of DHA, such as the flow discretized or PWM controlledDHA.
51
52 Conclusion
• More detailed load model - In this thesis the load in the benchmark system has beenmodelled as a spring. A suggestion for further studies is to develop a more detailed loadmodel for the benchmark system such that the load is more dynamic such that it mightbetter represent the loads on an aircraft.
Nomenclature
Abbreviations6DOFSS 6 Degrees Of Freedom State Space
DFCU Digital Flow Control Unit
DHA Digital Hydraulic Actuator
DHQ Digital Hydraulic Quantisation
PID Proportional Integral Derivative controller
PWM Pulse Width Modulation
TLM Transmission Line Modelling
Parametersβe Effective bulk modulus Pa
A Area m2
Bp Piston damping Ns/m
Cd Drag coefficient
Cl Leakage coefficient m3/sPa
dv Valve spool diameter m
f Central rod fraction of valve spool diameter
KD PID derivative gain
KI PID integrative gain
KP PID proportional gain
kp Spring constant N/m
KAW Anti wind-up parameter
KI/O Lock function state
P Penalty parameter N
Tdelay Time delay s
v0 Reference veolcity m/s
w Valve area gradient m
xv,max Maximum valve spool displacement m
y0 Reference altitude m
53
54 Conclusion
ρ Density kg/m3
Cq Flow coefficient
nF Number of distinct forces
np Number of pressure levels
nv Number of on/off-valves
nc Number of chambers
T Time period s
Variablesδ Control surface deflection angle deg
φ Roll angle deg
ψ Yaw angle deg
θ Pitch angle deg
Fact Force applied throgh th actuator N
p1 Uppstream pressure Pa
p2 Downstream pressure Pa
q Flow m3/s
q Pitch rotational velocity deg/s
r Roll rotational velocity deg/s
t Time s
udisc Discrete valve state matrix
V Volume m3
vact Velocity of the actuator m/s
xp Piston position m
xref Reference position m
ph Hydraulic pressure Pa
Pm Mechanical power J/s
Ph Hydraulic power J/s
qh Hydraulic flow m3/s
W Energy consumption J
p Pressure Pa
Bibliography
[1] S. Wang, M. Tomovic, and H. Liu, Commercial Aircraft Hydraulic Systems. Elsevier Scienceand Technology Books, 2015.
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