optimal gear shifting strategy for a seven-speed automatic transmission used on a hydraulic hybrid...
TRANSCRIPT
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A Thesis
entitled
Optimal Gear Shifting Strategy for a Seven-speed Automatic Transmission
Used on a Hydraulic Hybrid Vehicle
by
Yaoying Wang
Submitted to the Graduate Faculty as partial fulfillment of the requirements for the
Master of Science Degree in Mechanical Engineering
__________________________________________
Dr. Walter W. Olson, Committee Chair
__________________________________________
Dr. Yong Gan, Committee Member
__________________________________________
Dr. Mohammad Elahinia, Committee Member
__________________________________________
Dr. Patricia Komuniecki, Dean
College of Graduate Studies
The University of Toledo
March 2012
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Copyright 2012, Yaoying Wang
This document is copyrighted material. Under copyright law, no parts of this
document may be reproduced without the expressed permission of the author.
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iii
An Abstract of
Optimal Gear Shifting Strategy for a Seven-speed Automatic Transmission
Used on a Hydraulic Hybrid Vehicle
by
Yaoying Wang
Submitted to the Graduate Faculty as partial fulfillment of the requirements for
The Master of Science in Mechanical Engineering
The University of Toledo
May 2011
Hydraulic technology can be used to capture and transfer high levels of
energy extremely quickly and have a longer operating life compared with similarly
sized electric systems. The hydraulic hybrid vehicle (HHV) includes two power
sources that propel the vehicle: a fuel-efficient diesel combustion engine and
hydraulic components. This technology replaces a conventional drive train with a
hydraulic one, which eliminates the need for a mechanical transmission and driveline.
To explore an optimal gear shifting strategy with best fuel economy for a
seven-speed automatic transmission used on a hydraulic hybrid vehicle, a strategy is
designed with a highest possible gear criterion as long as the torque requirement can
be satisfied, except for braking process and torque demanding situations. The
optimization strategy takes several other criteria into consideration, such as high
motor displacement criterion, to improve efficiency and fuel economy. Then the
optimization strategy is developed on the basis of these criteria from two main aspects
of the existing SIMULINK truck model. One approach is based on the hydraulic
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motor working conditions, such as motor displacement, and the other is based on the
drivers intention, which is interpreted as the driver pedal position. This controller is
able to recognize the drivers intention to change the speed and incorporate it into
gear shifting decision making.
Then a SIMULINK controller model is developed based on the optimal gear
shifting strategy and criteria and validated both in fuel economy and power
performance by analyzing the simulation results in the Federal Urban Driving Cycle.
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Acknowledgments
First, I would like to express my sincerest gratitude to my advisor, Dr.
Walter W. Olson, for his guidance on my graduate study and research. It is very
lucky and a great honor to be his student and have the opportunity to work with him
in such an interesting research area of hydraulic power. His academic enthusiasm
and persistence do inspire my motivation for further study and future research work.
Without his tremendous support and patience, I would not be able to complete this
thesis.
Special thanks to my committee members Dr. Yong Gan and Dr.
Mohammad Elahinia, who give constructive advices and comments on my thesis. I
would specifically like to give a thank you to the help and friendship of my partners,
Andrew Sulzer and James Sweetman, who spare their precious time to review and
correct my writing while they are busy working on their theses.
Many thanks to the financial and technical support from Southwest
Research Institute (SwRI), Bapiraju Surampudi, Joe B. Redfield and Glenn R.
Wendel.
Most thankful for my family and friends who always give their
unconditional love and support to me wherever I am and whatever I do. Thank you
all for accompanying me in my heart.
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Table of Contents
Acknowledgments..................................................................................................... v
Table of Contents ..................................................................................................... vi
List of Tables ............................................................................................................. ix
List of Figures ............................................................................................................ x
Nomenclature ........................................................................................................... xii
Chapter 1 Introduction .......................................................................................... 1
1.1 Background of Research ................................................................................. 1
1.2 Problem Statement ........................................................................................... 3
1.3 Work Outline ................................................................................................... 3
Chapter 2 Literature Review ............................................................................... 5
2.1 Hydraulic Hybrid Vehicles ............................................................................ 5
2.1.1. Parallel ..................................................................................................... 7
2.1.2 Series ......................................................................................................... 8
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2.2 Optimal Transmission Strategies ..................................................................... 9
Chapter 3 Control Criteria and Shifting Strategy ........................................ 18
3.1 Control Criteria .............................................................................................. 19
3.2 Shifting Strategy ............................................................................................ 23
3.2.1 Shifting Based on the Driver Pedal ......................................................... 23
3.2.2 Shifting Based on Hydraulic Motor Conditions .......................................... 25
3.3 Control Algorithm ......................................................................................... 25
Chapter 4 Simulation Design ............................................................................ 27
4.1 Transmission Model ...................................................................................... 27
4.2 Controller System Model .............................................................................. 29
4.2.1 Driver Shifting Controller ....................................................................... 30
4.2.2 Speed Computation ................................................................................. 31
4.2.3 Braking Controller .................................................................................. 33
4.2.4 Priority Selection ..................................................................................... 34
4.2.5 Overall Speed Violation of the Hydraulic Motor .................................... 35
4.2.6 Dwell Time Controller ............................................................................ 36
4.2.7 Gear Memory .......................................................................................... 36
4.2.8 Gear Ratio Matching ............................................................................... 37
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Chapter 5 Simulation Results............................................................................ 39
5.1 Fuel Consumption ......................................................................................... 40
5.2 Gear Shifting Schedule .................................................................................. 42
5.3 Tracking Performance ................................................................................... 44
5.4 Hydraulic Motor Speed ................................................................................. 47
5.5 Summary ....................................................................................................... 48
Chapter 6 Summary and Conclusion .............................................................. 50
6.1 Summary ....................................................................................................... 50
6.2 Conclusion ..................................................................................................... 51
6.3 Future Work ................................................................................................... 51
References................................................................................................................. 53
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List of Tables
Table 5.1 Simulation results comparison between controller and original model ....... 49
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x
List of Figures
Figure 1-1 Configuration block diagram of the hydraulic hybrid vehicle ..................... 2
Figure 2-1 Parallel configuration [5] ............................................................................. 7
Figure 2-2 Series configuration [5] ................................................................................ 8
Figure 3-1 Plots of efficiency vs. motor displacement of each gear ............................ 21
Figure 3-2 Plots of motor displacement vs. acceleration of each gear ........................ 21
Figure 4-1 Original model with default shifting schedule ........................................... 28
Figure 4-2 Block diagrams with the optimal shifting controller in orange .................. 29
Figure 4-3 All block diagrams of the optimal controller top layer .............................. 30
Figure 4-4 Driver shifting controller block diagrams in orange .................................. 31
Figure 4-5 Speed computation block diagrams in grey ............................................... 33
Figure 4-6 Braking controller block diagrams in orange ............................................. 34
Figure 4-7 Hydraulic motor speed limit controller block diagrams in red .................. 35
Figure 4-8 Dwell time controller block diagrams in magenta ..................................... 36
Figure 4-9 Gear memory by using unit delay .............................................................. 37
Figure 4-10 Gear ratio matching controller ................................................................. 38
Figure 5-1 EPA Federal Urban Driving Schedule (FUDS) .......................................... 40
Figure 5-2 Fuel consumption simulation results comparison: fuel consumptionKg
vs. time (seconds)......................................................................................................... 41
Figure 5-3 Controller model simulation results: gear ratio vs. time (seconds) ............ 42
Figure 5-4 Original model simulation results: gear ratio vs. time (seconds) ............... 43
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Figure 5-5 Controller model simulation results: actual and desired vehicle speed (m/s)
vs. time (second) .......................................................................................................... 44
Figure 5-6 Original model simulation results: actual and desired vehicle speed (m/s)
vs. time (second) .......................................................................................................... 45
Figure 5-7 Simulation results comparison between original and controller model:
vehicle speed error (m/s) vs. time (second) ................................................................. 45
Figure 5-8 Controller model simulation results: hydraulic motor speed
(radian/second) vs. time (second) ................................................................................ 47
Figure 5-9 Original model simulation results: hydraulic motor speed (radian/second)
vs. time (second) .......................................................................................................... 48
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Nomenclature
CVT
EPA
FUDS
HHV
IC
SwRI
Continuously variable transmission
Environmental Protection Agency
Federal Urban Driving Schedule
Hydraulic hybrid vehicle
Internal combustion
Final drive ratio
Transmission gear ratio
Minimum transmission gear ratio
Radius of the vehicle tire
Southwest Research Institute
Output torque corresponding to
Output torque corresponding to
Vehicle speed
Motor speed
Torque convertor efficiency
Minimum torque convertor efficiency
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1
Chapter 1
Introduction
1.1 Background of Research
In many applications especially those where high power densities are
required, hydraulic hybrid systems can offer a more efficient alternative to those
driven by electric motors. Hydraulic technology can be used to capture and transfer
high levels of energy extremely quickly compared with similarly sized electric
systems, which generally require long periods over which batteries have to be
charged. Hydraulic systems are also likely to have a longer operating life than
battery-powered devices.
Similar to an electric hybrid vehicle, which includes a gas or diesel engine
and battery, the hydraulic hybrid vehicle (HHV) includes a diesel engine and a
hydraulic power system that, in laboratory testing, has achieved significant fuel
economy over traditional UPS vehicles. Hydraulic hybrid technology includes two
power sources that propel the vehicle: a fuel-efficient diesel combustion engine and
hydraulic components. This technology replaces a conventional drive train with a
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hydraulic one, which eliminates the need for a mechanical transmission and
driveline.
These vehicles can store energy from the hydraulic system, even after the
vehicle is turned off. This storage allows the vehicle to start with this energy, instead
of relying on the engine to propel the vehicle [1].
To improve fuel economy of a military vehicle or family of vehicles, the
hydraulic hybrid technology is applied and the ways it can benefit ancillary vehicle
functions and enhance the mission usefulness of the vehicle are investigated as the
purpose of this project. Figure 1-1 below is the configuration of the hydraulic hybrid
vehicle developed in this project.
Figure 1-1 Configuration block diagram of the hydraulic hybrid vehicle
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1.2 Problem StatementThe objective of this thesis is to explore an optimal gear shifting strategy for
the best fuel economy with a seven-speed automatic transmission used on a
hydraulic hybrid vehicle. A simulation model of this automatic transmission was
developed in SIMULINK by engineers at Southwest Research Institution that takes
the seven-speed gear ratios as a shifting schedule input to the transmission model
and exports the vehicle speed, vehicle distance, driver pedal position, current gear
ratio, fuel consumption, simulation time, motor pressure, motor displacement, motor
speed and efficiency as outputs to a MATLAB workspace. Though shown as a
seven- speed model, the strategy discussed is for the range from 2nd to 7th gear as a
shift directly from 1st to 2nd gear is not allowed while the vehicle is moving.
In addition, since the transmission is no longer driven by the IC engine, the
instantaneous fuel consumption cannot be computed from outputs of the
transmission model. Part of the work of this thesis is to research and identify better
operational measures by which fuel economy can be optimized.
1.3 Work Outline
Chapter 2 is the literature review which represents the development of
hydraulic hybrid vehicles along with illustrations of two main configurations:
parallel and series. It also introduces some different approaches to transmission
optimization.
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Chapter 3 represents the shifting criteria and analyzes the shifting strategy
for the transmission.
Chapter 4 shows the simulation block diagrams built in
MATLAB/SIMULINK according to the shifting strategy.
Chapter 5 discusses the simulation results and compares the results to the
original shifting model simulation.
Chapter 6 is the conclusion of the optimal shifting strategy simulation of the
transmission and represents the future work.
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Chapter 2
Literature Review
2.1 Hydraulic Hybrid Vehicles
A hybrid vehicle combines two or more sources of power. Currently, hybrid
technology is widely recognized as the most effective measure to solve the energy
problem. Heavy vehicles such as city buses have the characteristics of high
stop-and-go duty cycles and high power flow braking energy, which needs to find an
efficient way to store and reuse the braking energy [2]. The additional power source
can be electrical, chemical, hydraulic, flywheel operated or any other form of power
storage and production [3]. Within the many hybridization options, battery and fuel
cell have the characteristics of high energy density and are well suited for light
vehicles. However, the high internal resistances and handling the wasted battery are
major obstacles for commercialization. Both fuel cell and battery hybrid vehicles can
only marginally recover the braking energy. Moreover, high frequency charging and
discharging will lead to overheating and battery destruction. An ultra- capacitor (UC)
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has higher power density than a battery because their operation does not employ a
chemical reaction, but the high cost and relatively lower reliability constrain their
applications [2,5,4].
As an important method of hybrid technology, hydraulic hybrid vehicles
(HHV) now attract the attention of worldwide research institutions and commercial
industrial companies. The first-ever hydraulic-hybrid diesel urban-delivery vehicle
reportedly improves fuel economy by 60 to 70% and reduces carbon-dioxide
emissions more than 40% in initial laboratory testing. EPA estimates that the
technology has the potential to save more than 1,000 gallons/yr for each
urban-delivery vehicle [11].
In a hydraulic hybrid vehicle, the hydraulic power assists the conventional
internal combustion engine by providing additional torque to the driveshaft [6]. The
hydraulic hybrid system with accumulators and hydraulic pump/motors have the
potential for improving fuel economy by operating the engine in the optimum
efficiency range and making use of the regenerative braking during deceleration.
When braking, a hydraulic hybrid can recover and reuse braking energy that is
normally wasted. As the vehicle stops, energy from the wheels pumps fluid from a
low-pressure reservoir into the high-pressure accumulator. When the vehicle
subsequently accelerates, the stored energy propels the vehicle. According to EPA
officials, this process recovers and reuses more than 70% of the energy normally
wasted during braking, and it also reduces brake wear by about 75%, substantially
increasing the savings [11].
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There are two main configurations for hydraulic hybrid vehicles, parallel and
series. Both of them have the main components which consist of a high-pressure
accumulator storing energy by using hydraulic fluid to compress nitrogen gas stored
inside each accumulator, a low-pressure reservoir storing hydraulic fluid after it has
been used by the pump/motor, a pump/motor converting high-pressure hydraulic
fluid into rotating power for the wheels and transmitting braking energy back to the
high-pressure accumulator, and an engine pump transmitting pressurized hydraulic
fluid to the pump/motor, the high-pressure accumulator, or both [1].
2.1.1. Parallel
Shan [7] reports parallel hydraulic hybrid vehicles are easier to implement,
but efficiency gains are limited by the solid link between the wheels and engine.
Figure 2-1 Parallel configuration [5]
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The rear pump/motor acts as the drivetrain, such that in the motor mode, it
drives the axle using high pressure fluid and, as the vehicle brakes, the pump/motor
directs the fluid to the high pressure accumulator by switching into the pump mode.
The high pressure accumulator is usually designed to satisfy internal pressure loads
up to 35MPa (5000 psi), whereas the low pressure accumulator holds the internal
pressure of up to 1.4MPa (200 psi) [8, 9].
2.1.2 Series
Series hydraulic hybrid vehicles allow engine speed to be decoupled from
vehicle speed. This permits a control strategy where the engine and other hydraulic
components operate only near maximum efficiency.
Figure 2-2 Series configuration [5]
In this configuration, the conventional driveline is completely removed; it is
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connected through hydraulic pipes and the drive pump/motors are used to transfer
power and propel the vehicle. The drive pump/motor converts high pressure
hydraulic fluid into rotating power in the motor mode and transmits regenerative
braking energy back to the high pressure accumulator in the pump mode. Since the
direct link between the engine and the driveline components is removed, the engine
is separated from the road and higher efficiencies are anticipated. [7].
2.2 Optimal Transmission Strategies
As it is important part of the vehicle powertrain to convert torque and
rotation, the optimization of the transmission can increase the performance of the
vehicle efficiency and fuel economy significantly. This depends on different working
conditions and rules, and various optimal control methods are applied to different
kinds of transmissions.
A. Haj-Fraj [10] introduces an optimal control approach for gear shift
operations in automatic transmissions as a multistage decision process by making
use of dynamic programming method. Starting from a verified model of a typical
powertrain, Haj-Fraj considers three optimization parameters as a performance
measurement for evaluating the gear shift process: the control data for the clutch
pressure in the gear box, the engine load-reduction and the evaluation of the gear
shift duration. As the passengers comfort is a subjective issue and varies from driver
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to driver, it can generally be stated that the smoother the acceleration change is and
the smaller the peaks of the jerk are, the more comfortable the gear shifting is. To
engage the clutch of the target gear with a very smooth and slow rising pressure in
order to get a very slow transition in the dynamical behavior of the powertrain in
order to get a smooth acceleration but a very long gear shift process. A control law is
derived analytically in an explicit form by minimizing the performance measure over
each process stage and the optimization finally is solved with a sequential quadratic
programming algorithm.
The results have shown that the passengers comfort and the duration of the
gear shift operation represent contrary issues. This means that an improvement in
one criterion leads to a deterioration in the other one. Therefore the optimization
problem represents be leads to find a reasonable solution that can define a
compromise between the two issues.
The shift schedule proposed by Gong Jie [13] for the ground vehicle
automatic transmission by studying the function of the torque converter and
transmission shift schedule can keep the torque converter working in the high
efficiency range under all the working conditions except in the low efficiency range
on the left when the transmission worked at the lowest shift, and in the low
efficiency range on the right when the transmission worked at the highest shift. In
order to evaluate the economic performance of the torque converter, the efficiency of
the torque converter is required no less than an ideal value in use ( =
75% in engineering machinery, = 80% in the normal automobile). The shift
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schedule aims to control and set the common working point of engine and torque
converter so that the torque converter efficiency is no less than the ideal value
so that . In other words, the points satisfying the condition
are used as the shift points.
The gear ratio and corresponding torque are analyzed as the key
factors to determine the shift points. After the experimental test-bed results, the
torque converter efficiency was controlled to be over 75% in the high efficiency
range by gear shift according to the actual load acting on the drivetrain.
This method focuses most on the relationship between the fixed gear ratio
and the continually changing actual gear ratio along with its torque value, which is
much easier to control compared to the dynamic controller stated in Haj-Frajs.
Toshinichi Minowa [15] investigates a powertrain control model for an
automatic transmission providing efficient control for both the engine and the
transmission which leads to better fuel economy and acceleration feeling by
optimizing the shift timing and throttle valve opening. The gear shift timing consists
of the vehicle speed and the throttle valve opening simulating the engine load. To
achieve the optimal fuel consumption, gear shift timing is calculated by comparing
the efficiency of the torque transmitted to the wheels at each gear shift ratio, using
the fuel flow rate characteristic of the engine and torque convertor. The controller is
developed based on the concept that the optimal gear shift timing is selected with the
driven horse-power required for running, and the engine torque is controlled by the
throttle valve opening so as to maintain the driven shaft torque demanded by the
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driver which can be calculated from accelerator pedal angle and vehicle speed. To
avoid the busy shift and fluctuations of the driven shaft torque, hysteresis are
applied to the fuel flow rate calculated every gear shift to prevent gear command
signal changing several times.
The test pattern model used to verify the optimization shift timing results is
very simple and straight forward, so the simulation results are relatively better with
known driving circle.
M. Pachter [17] presents and analyzes differential game model of an
automatic transmission for a road vehicle control which leads to a synthesis of the
optimal feedback strategy for the selection of the transmission ratio of the automatic
gearbox. The vehicle is considered as object, which is governed by two controlling
agents: the setting by the driver of the throttle and the gear-change controller. He
remarks that most road vehicles are equipped with manual or automatic gearboxes
which have a finite number of discrete transmission ratios. But for CVT where the
transmission ratio is a continuous variable, his analysis hinges on the fact that the
dynamic function assumes a discrete set of values. Therefore, in order to analyze the
case of a continuous secondary control variable, one should discretize the
transmission ratio or alternatively, use an adaptation of our theory to a discrete-time
model for the dynamical system.
Quan Zheng [33] introduces a new coordinated engine-transmission control
approach for the neutral idle input clutch application phase aiming at improving fuel
economy during urban driving and reducing engine vibrations transmitted to the
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passenger compartment. It involves ramping up engine speed smoothly and
simultaneously engaging the forward clutch smoothly. Effective coordination can be
realized by controlling the engine and turbine speeds closed loop so that they follow
specified trajectories. Therefore, the controller design objective is to realize optimal
trajectory tracking by synthesizing the control inputs so that the engine and turbine
speeds track the specified desired trajectories closely. Due to the Multi-Input
Multi-Output nature of the control problem, an optimal Linear Quadratic Regulator
with Explicit Model Following is used to allow the system dynamic response to
track two desired trajectories for engine and turbine speeds. The results show that the
proposed control strategy can achieve satisfactory performance.
There are several issues that remain to be explored in this paper. The first
issue is state estimation. In the current formulation, the states are assumed to be
available for feedback implementation. However, state estimation is needed to obtain
clutch pressure and the derivative of clutch pressure, and dynamic estimation of
engine indicated torque is also necessary. The second issue is better inclusion of the
vehicle dynamics in the design. One limitation of the current implementation is that
the state related to vehicle acceleration is ignored in the process of model
simplification. This limits accurate studies of vehicle vibrations under different
control strategies.
In Alarico Macors [34] work, the design of a hydro -mechanical
transmission is defined as an optimization problem in which the objective function is
the average efficiency of transmission, that is to minimize the total loss of the
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transmission, while the design variables are the displacements of the two hydraulic
machines and gear ratios of ordinary and planetary gears. Since the object function
doesn't have an analytical formulation, the optimization problem is solved by a
direct-search algorithm based on the swarm method, which showed a good speed
convergence and the ability to overcome local minima. And the use of evolutionary
algorithms is also able to reduce the importance of the initial research point and the
trapping in local minima far from absolute minimum.
However the advantage of the continuous speed variation of the power-split
drives is counterbalanced by a reduced efficiency, caused by the double energy
conversion taking place in the hydrostatic transmission. Therefore, the design of the
power split drive still need further study. The proposed procedure does not depend
on experience and previous knowledge because no assumption had to be made on the
component's sizing; the optimality of the output is based on the implemented search
algorithm while the quality of the classical designs depends strongly on the
designer's experience.
B.Mashadi [18] designs a gear-shifting strategy of an automated manual
transmission by taking into consideration the effects of these parameters, with the
application of a fuzzy control method. The controller structure is formed in two
layers. In the first layer, two fuzzy inference modules are used to determine the
necessary outputs. In the second layer a fuzzy inference module makes the decision
of shifting by upshift, downshift, or maintain commands. The behavior of the fuzzy
controller is examined by making use of ADVISOR software. It is shown that at
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different driving conditions the controllers make correct decisions for gear shifting
accounting for the dynamic requirements of the vehicle. Both the engine state and
the drivers intention also eliminate unnecessary shifts that are present when the
intention is overlooked. Both strategies for the vehicle to reach the maximum speed
starting from rest allow the gear shift to be made consecutively.
Considerable differences are observed between the two strategies in the
deceleration phase. The engine-state strategy is less sensitive to downshift, taking
even unnecessary upshift decisions. The state intention strategy, however, interprets
the drivers intention correctly for decreasing speed and utilizes engine brake torque
to reduce the vehicle speed in a shorter time.
Magnus Pettersson and Lars Nielsen [21] uses engine control during the
gear shift for a manual transmission without using the clutch during the shift event.
To minimize the total time needed for a gear shift will excite the driveline
resonances which may lead to problems with disengaging the old gear and
synchronizing speeds for engaging the new gear. Internal driveline torque control is a
novel idea introduced in their work for handling resonances and increasing shift
quality. By estimating the transmitted torque and controlling it to zero by engine
control, the gear can systematically be disengaged with minimized driver
disturbances and faster speed synchronization.
Two main advantages of the control system are: fast shifts to neutral gear,
despite disturbances and driveline oscillations at the start of the gear shift; the
control scheme is simple and robust against variations among different gears and
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damping of driveline resonances can be obtained with an observer in combination
with a PID feedback structure.
Different from the manual or automatic transmission with fixed gear ratios,
continuously variable transmission (CVT) with its continuous ratio offers the
potential to substantially improve the part-load fuel efficiency of spark-ignited
engines. The control of CVT has traditionally been designed using static arguments,
like by identifying the best efficiency points in the engine map for each constant
power requirement and by following that curve using some heuristics as much as
possible also in transients.
R. Pfiffner [36] presents the solution of the fuel-optimal control problem for
transient conditions using the numerical optimization package DIRCOL. Based on
this optimal solution a simplified but causal control strategy is proposed which offers
almost the same benefits. The resulting engine operation trajectory is pictured in a
engine torque vs. engine speed figure and the optimal solution therefore consists of
bringing the system to the corresponding best efficiency curve as fast as possible,
and to keep the system on this curve as long as possible. During the periods when
the system moves towards or away from this curve the gear ratio has to be changed
with maximal possible speed while the optimal engine power trajectory is more or
less constant over time.
Michiel Pesgens [35] develops a transmission ratio controller for a
hydraulically actuated metal push-belt continuously variable transmission (CVT),
which consists of an anti-windup PID feedback part with linearizing weighting and a
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set point feedforward which is generated by the hierarchical (coordinated) controller.
Physical constraints on the system, especially with respect to the hydraulic pressures,
are accounted for using a feedforward part to eliminate their undesired effects on the
ratio. The total ratio controller guarantees that at least one of the pressure setpoints is
always minimal with respect to its constraints, while the other is raised above the
minimum level to enable shifting.
This approach has potential for improving the efficiency of the CVT,
compared to non-model based ratio controllers with experimental results showing
that adequate tracking is obtained together with good robustness against actuator
saturation. The largest deviations from the ratio setpoint are caused by actuator
pressure saturation. It is further revealed that all feedforward and compensator terms
in the controller have a beneficial effect on minimizing the tracking error.
Most optimal approaches stated above depend on the working condition of
engine and torque converter. For this case, in this thesis, as the transmission is no
longer driven by the IC engine, the instantaneous fuel consumption cannot be
computed from outputs of the transmission model. Therefore the optimal gear
shifting decision cannot be made based on the best fuel economy of a specific gear
under a certain driving condition. A new method based on the current transmission
model is designed and introduced in the following chapters.
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Chapter 3
Control Criteria and Shifting Strategy
The function of a vehicle transmission is to adapt the traction available from
the drive unit to suit the vehicle, the surface, the driver and the environment. It has a
decisive effect on the reliability, fuel consumption, ease of use, road safety and
transportation performance [37]. The optimization of the transmission can
significantly increase the performance of the vehicle efficiency and fuel economy.
As introduced in chapter 1, a simulation model of a truck was developed in
SIMULINK by engineers at Southwest Research Institution based on the
configuration of the hydraulic hybrid vehicle shown in Figure 1-1. It is a
seven-speed transmission which can provide more efficient fuel performance than a
normal four-speed one. The strategy discussed is for the range between 2nd and 7th
gear as a shift directly from 1st to 2nd gear is not allowed while the vehicle is moving.
In the transmission model, a shifting command that consists of one of the
seven-speed gear ratios is taken as an input, and it outputs the vehicle speed, vehicle
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distance, driver pedal position, current gear ratio, fuel consumption, simulation time,
motor pressure, motor displacement, motor speed and efficiency as outputs to a
MATLAB workspace.
In this chapter, an optimal control strategy for the gear shifting schedule of
the automatic transmission is developed depending on the proper application of the
decision-making algorithm for best fuel economy. Since the transmission is no
longer driven by the IC engine, the instantaneous fuel consumption cannot be
computed from outputs of the transmission model. The control strategy developed in
this chapter is to research and identify better operational measures by which fuel
economy can be optimized.
3.1 Control Criteria
The design of a controller requires certain performance criteria to be
established as objectives. The overall objective of this controller was to provide a
gear shifting schedule that would minimize fuel consumption for a hydraulic hybrid
truck. Unlike conventional vehicles, this trucks drive wheels are not directly
attached to the internal combustion engine. Therefore the torque and wheel speeds
are controlled independently from motor speed. While the internal combustion
engine must provide power to a storage system, the rate at which the power is
consumed does not need to match the rate of power production. Therefore, the
internal combustion engine can continuously operate at the point of its best
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efficiency. When sufficient power has been stored, the internal combustion engine
can be turned off until the stored power has dropped to some replenishment level. At
that time, it would again recharge the storage system. Thus the objective of
minimizing fuel consumption will not depend on matching engine torque and speed
to immediate driving conditions. As it will be explained later, meeting this objective
will depend on promoting higher displacements in a variable displacement hydraulic
motor.
First of all, the analysis of the truck model shows that higher efficiency of
the motor is obtained in a higher gear of the transmission. Therefore, the
transmission should be in the highest possible gear as long as the torque demand can
be met while driving.
The analysis of data produced by the SwRI s-function black box model of
the hydraulic motor in Figure 3-1 shows that the highest efficiency for the motor is
near full displacement. Therefore, the controller should strive to keep the hydraulic
motor working near full displacement. Figure 3-2 shows that high acceleration
requires higher torque, thus requiring a lower gear. However, taking this plot into
account, the transmission should upshift to get higher motor displacement as long as
the torque requirement can be satisfied. So, the highest possible gear criterion is
made for this controller.
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Figure 3-1 Plots of efficiency vs. motor displacement of each gear
Figure 3-2 Plots of motor displacement vs. acceleration of each gear
Another criterion that the controller must meet is to provide the wheel
speed and torque necessary to meet the EPA Federal Urban Driving Cycle. This
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cycle, meant for comparing automobile fuel efficiency consists of 1369 seconds of
velocities simulating large city stop-and-go traffic. For a large truck, this is a very
demanding schedule to meet as the accelerations required are at the torque limits of
the propulsion system. However, the driving cycle cannot be used directly. Initially,
the demands for torque and the conditions for braking must be interpreted from the
actions of the acceleration pedal which will be discussed in the strategy section.
There are also certain safety standards imposed. Based on the limitations
of the truck components, shifting from a lower gear to a higher gear, or shifting from
a higher gear to a lower gear requires that the transmission be shifted into each gear
in between. For example, a shift from 4th gear to 2nd gear, as might be required
driving a braking event, requires that shift sequence is from 4th gear to 3rd gear and
then to 2nd gear. This is required in order to sequence the transmission internal
clutches so the transmission is not destroyed or so that the gear shift can be
successfully made. The vehicle must start from a stop in 2 nd gear. 1st gear is not used
for normal driving conditions.
Another transmission requirement is that each gear during an upshift
requires a dwell time in that gear for conditions to stabilize as the transmission is
under driving force. This requirement is relaxed during downshift so that maximum
use can be made of regenerating braking. Besides that, the gear shifting time is also
taken into consideration as the gear shifting needs certain time to complete.
The hydraulic motor has a rotational speed limitation of 3000 RPM. At
speeds that exceed this, the motors rotating parts produce enough inertia such that
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the motor could explode or otherwise be damaged.
3.2 Shifting Strategy
The optimal shifting strategy is mainly developed from two different
aspects of the truckmodel. One approach is based on the drivers intention which is
given as pedal position output of the transmission model. Based on the concept that
the driving cycle is unknown to the controller, drivers intention is the only input can
be taken into account as a prediction of the driving cycle. The intention of the driver
pressing the pedal indicates the torque or the acceleration is required from driver and
releasing the pedal indicates braking.
In addition to interpreting drivers behavior, another aspect that is taken into
consideration is the hydraulic motor displacement, which can guarantee the
hydraulic motor working within the range where higher efficiency can be achieved.
3.2.1 Shifting Based on the Driver Pedal
The intention of the driver is interpreted from the position of the driver
pedal. In the most basic form, the depression of the pedal is interpreted as the driver
is demanding torque to accelerate while releasing of the pedal is interpreted as the
driver intends to decelerate.
There are two parameters that can be considered here under the drivers
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control, one is the pedal position, and the other is the pedal rate. The pedal position
is represented with a numerical value from 0 to 1 with regard to the full pedal
position as 1. The pedal rate is defined as the rate of increase of pedal position
during one sample time T (T=5 milliseconds). When the driver uses a high rate of
increase in pedal position, it is interpreted as an urgent torque demand. By observing
the relationship between pedal rate and acceleration requirement, the average value
of the pedal rate was found to be 0.01 per sample time from averaging all of the rates
of increase over the FUDS driving cycle. So when driver presses the pedal down
with rate double of the average value per sampling time, a downshift command is
applied to increase torque. In addition, an upshift decision is made when pedal rate is
negative which is interpreted as the driver wants to decelerate by retarding the pedal
based on the highest possible gear criterion.
The release of the pedal indicates that the driver wants to decelerate the
vehicle. The vehicle can decelerate in two ways: coast down and active braking.
However, braking is neither an input nor an output of the truck model. Therefore, a
surrogate had to be developed. If the pedal position is less than 0.05, it is considered
as a potential sign for braking, so a downshift decision is made for braking mode.
Otherwise, in the case of pedal position greater than 5% along with pedal
rate within the range from 0 to 0.005 will keep on the current gear.
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3.2.2 Shifting Based on Hydraulic Motor Conditions
Another aspect that can be considered for minimizing the fuel consumption
is to maximize the hydraulic motor efficiency. According to the relationship between
efficiency and motor displacement, and efficiency and motor speed, the higher motor
displacement and speed, the higher efficiency. So the hydraulic motor needs to work
in the high range of displacement and speed to get high efficiency. As shown in
Figure 3-1, the highest efficiency, 95%, of the motor occurs at a high motor
displacement range from 0.7 to 0.95. A downshift will be applied when motor
displacement value is more than 0.95 to prevent over driving the displacement.
3.3 Control Algorithm
Based on the analysis of the control criteria and strategy, the main algorithm
of the controller can be determined by taking all the input and output parameters of
the transmission model into account.
First of all, the controller will always upshift the transmission to the highest
possible gear provided the driving power requirements can be met based on the
highest possible gear criterion. There are some exceptions for downshift, pedal
position is less than 5%, which indicates braking mode, and pedal rate more than
0.005 per sampling time which indicates more torque requirement from the driver.
Otherwise, when the pedal rate is less than 0.005 per sampling time the controller
will stay in the current gear. Secondly, the maximum motor displacement limitation
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is set to 95% and downshift will be applied if it exceeds that value. When the current
motor speed reaches 3000 RPM (314 radian/second), an upshift is always applied to
reduce the motor speed. In addition, any command derived from the drivers
intention will also be checked with the current vehicle speed to protect the motor
speed from exceeding the maximum value. All the shifting commands must be
checked with the 2nd to 7th gear range controller and then combined with the
2-second dwell time other than when braking. Finally, as the gear shifting needs
certain time to accomplish, the shifting in process (SIP) signal is presented with
either 1 which shows the shifting is still in process or 0 which means the shifting is
already completed. The shifting command only can be applied when the shifting in
process (SIP) signal is 0. In other words, every proposed shifting command must be
checked with SIP signal before it can be delivered to the final output.
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Chapter 4
Simulation Design
SIMULINK provides a platform to simulate the performance of the
optimal shifting controller by running the transmission model in a certain driving
cycle.
4.1 Transmission Model
The transmission model shown in Figure 4-1 is a discrete-time based
SIMULINK model which runs a 1369-second simulation driving cycle with a
0.005-second sampling time. The model simulates the shifting of a 6-speed
transmission under conditions imposed by the EPA Federal Urban Driving Cycle for
a hydraulic hybrid truck. The truck dynamics are contained within a black box
s-function provided by Southwest Research Institute. This s-function uses the gear
ratio as the input, and then provides the vehicle speed, the driver pedal position, the
hydraulic motor speed, the motor displacement and the hydraulic pressure among
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other factors.
Figure 4-1 Original model with default shifting schedule
A simulation model of the controller was developed in SIMULINK based
on the original truck model provided by Southwest Research Institute, presented in
Figure 4-2. The highest level of this controller-truck model consists of three
components: the controller system, the truck s-function and the outputs. The
s-function black box is the same as the original truck model which uses the gear
ratio from the controller system block and outputs 12 parameters including vehicle
speed, vehicle distance, driver pedal position, shifting in process signal (SIP),
simulation time, current gear ratio, fuel consumption, simulation time, motor
In1
Out1
Out2
Out3
Out4
Out5
Out6
Out7
Out8
Pressure (N/m2)
Motor Disp (0-1)
Motor Speed (rad/s)
P*eff
VehicleWithSeriesHydraulicPowerTrainAndFTPDriveCycle+Driver1
VehDistance
VehDistance (m)
tt
Simulation Time (tt) seconds
SIP
Shift In Progress (SIP)
Pressure
Pressure (N/m2 )
PtimesEff
Press*Eff
MotorSpeed
MotorSpeed (rad/sec)
MotorDisp
MotorDisp (0-1)
FuelConsumed
FuelConsumed (Kg)
DriverPedal
DriverPedal (0-1)
DesVehSpd
DesVehSpd (m/s)
CurrentGearRatio
CurrentGearRatio-1
Autom atic S hifti ng
or Des Gear Ratio
ActVeh Spd
ActVehS pd (m/ s)
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pressure, motor displacement, motor speed and efficiency as matrices into MATLAB
workspace.
Figure 4-2 Block diagrams with the optimal shifting controller in orange
4.2 Controller System Model
The controller is a five-input and one-output system which uses vehicle
speed, driver pedal position, shifting in process signal (SIP), motor displacement and
motor speed. The output is the gear ratio for the transmission. The controller
contains the following components which interpret the control algorithm discussed
in chapter 3.
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Figure 4-3 All block diagrams of the optimal controller top layer
4.2.1 Driver Shifting Controller
The shifting decisions made by the controller will be presented in signals of
1, -1 and 0, which represent upshift, downshift, and on action respectively.
The torque requirements are interpreted from the driver pedal position and
the pedal position rate. The pedal position is represented with a numerical value
from 0 to 1 and the pedal rate is defined as the rate of increase of pedal position
during one sample time T (T=0.005 second).
The controller always implements an upshift command unless the torque
requirement cannot be satisfied by the current gear. As mentioned before, a high rate
of increase in pedal position with a value of 0.005 per sampling time is interpreted as
Algorithm ofthe Controller:
1.HighestGear Optimization.
2.Max Motor Speed(314rad/s) firstpriority,upshiftwhenexceed.
3.Max Displacementlimit0.95,downshiftwhenexceed.
4.Always upshiftunless:
1).Pedalposition=0.005/sampletime,thendownshift.
5.Dwelltime5sec, may causeproblems whenbrakingtimetoa stopis soshort
thatthegear cannotshifttothe2ndgear intime,
sonodwelltimeis appliedwhenvehiclespeed< =0.3m/s.
Value is1 if shift is in progress, zero other
used to inhibit shifting
-1,0,1 values
Note:
gear number 1corresponds to2nd Gear
gear number 6corresponsedto7th Gear
Prop GearNumber(1 to 6)
1
GearRat ioz
1
used to
breakalg loop
>= 314
Speed Check
If Prop gearviolatesmotor speed
use current gear(no shift)
Shift Cmd
afterspd ck
> 0
SIP Check
No shifting if SIP
Proposed Gear
MotorSpeed Limit Shift
G e ar R a n ge L i m it e d G ea r
Limit Gearsto Range 2nd -7th
P r op G e a r R a ng e L i mi t ed G e a r
Limit Gearsto Range 1 to 6
(gears2nd to 7th)
Mo to r S p ee d G e ar S h if t C o mma nd : 1 = u p s h if t 0 = n o a c t i o n
If 1 A MotorSpeed Violoation isOccurring!
Mandatesupshift if motorspeed exceedsset value
Highest Priority: Not subject to Dwell Time
VehSpeed
Propgear command without dwelltime
Propgearcommandafterdwel l time
PedalPosition
Propgear
Give priority to Braking VehSpeed 0
Give priority
to an upshift
GearSent
z
1
GearMemory
GearDelay
PropGearafter l imi t
GearCommand Givento Transmission
PropGearafterDwellTime
Dwell Time Controller
DriverPedal
PedalPosition
Displacement
DriverShift Command
DriverCommand Shifting
DriverCommand
Create gearnumber
to go to
1
2
3
4
5
6
*Convert to
GearRation
Current VehVelocity
ProposedGear
ComputedGearMotorSpeed
Computed MotorSpeed
Add
0.78
7thGearRatio1
0.90
6thGearRatio1
1.20
5thGearRatio1
1.69
4thGearRatio1
2.24
3rd GearRatio1
4.18
2nd GearRatio1
5
Displacement
4
MotorSpeed
3
Actual Velocity
2
DriverPedal
1
SIP
Current GearNumber (1to 6)
Current GearNumber (1to 6)
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an urgent torque demand from the driver, a downshift command is applied to
increase torque in this case. In addition, pedal position greater than 0.05 with the
pedal rate staying in the range from 0 to 0.005 indicating the driver is demanding
torque gently and will keep the shifting command in the current gear. This is shown
in Figure 4-4.
Figure 4-4 Driver shifting controller block diagrams in orange
4.2.2 Speed Computation
The maximum hydraulic motor rotational speed is 3000 RPM which can be
l i l :
. l i i . i i i , i .
. l i i i i i i i , .
. l i i i i . l i , i .
. i l li i . , i .
-1 or 0 values
-1 or 0 values1
Driver Shift Command
1
Up
Shift
z
1
Unit Delay
0
No Shift
> 0.95
If HysMotor Disp isgreater
than 0.95: downshift
> 0
If Driver isdepressing pedal:
Torque Demanded : Do not up shift
otherwise allow upshift
2nd priority
>= 0
Give priority of
choice to pedal vel
cmd
>= 0
Give priority of
choice to disp
cmd
>= 0
Final Gear Command
Prioity given to braking
(down shift)
>= 0.05
Driver Pedal Retard:
If pedel depress< 5%
assume braking m odes
i
-1
Down shift
Assume l owest gear
slowing
Difference of
Driver Pedal
position
> 0.005
If pedal depression velocity
isgreated than .005/Ts,
the driver isurgently
demaning torque
1st priority
2
Displacement
1
Pedal Position
0 or 1 values
-1,0,1 values
-1,0,1 values
-1, 0, 1 values
-1 or 0 values-1 or 0 values
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converted into radians per second as:
The proposed gear command made by the driver shifting controller also
had to be considered in the proposed corresponding hydraulic motor rotation speed
based on the proposed gear ratio, , and checked with the maximum hydraulic
motor speed limitation based on the criteria:
The ratio for the final drive is and the radius of the vehicle tire is
This is implemented as shown in Figure 4-5.
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Figure 4-5 Speed computation block diagrams in grey
4.2.3 Braking Controller
In order to get to the lowest gear when the vehicle stops, a downshift
command will be made during the braking process. As there is no braking pedal
signal in the transmission model, the driver pedal position of less than 5% is
predicted braking. A downshift decision is made in the pedal controller to provide
sufficient time to get to the 2nd gear before the vehicle stops. A figure for the
braking controller is shown below.
1
Computed
Gear Motor
Speed
13.4483
Velocity factor (Nf/re)
1
2
3
4
5
6
*Proposed
Gear Ratio
ComputeMotor Speed
from gear ratio
0.78
7thGear Ratio
0.90
6thGear Ratio
1.20
5thGear Ratio
1.69
4thGear Ratio
2.24
3rd Gear Ratio
4.18
2nd Gear Ratio
2
Proposed Gear
1
Current Veh Velocity
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Figure 4-6 Braking controller block diagrams in orange
4.2.4 Priority Selection
In the driver pedal shifting controller, the first priority is given to the
urgent torque demand from the driver with a pedal rate greater than 0.005/T.
Otherwise the algorithm maintains the current gear for torque demand for the
increasing rate of the pedal less than 0.005/T, which indicates moderate torque
demand from the driver. The upshift command with decreasing pedal rate is
secondary to increasing pedal rate. Within the pedal controller, which consists of the
driver pedal shifting controller, braking controller, and hydraulic motor efficiency
controller, the first priority is given to the braking process. The second priority is the
hydraulic efficiency controller, with 95% limitation, which can keep the hydraulic
motor working within the high efficiency range. For the overall controller of the
transmission, first priority is given to the shifting in process (SIP) signal controller as
no new action can be initiated while a previous shift command is being executed.
-1 or 0 values1
Driver Shift Command
0
No Shift
>= 0
Final Gear Command
Prioity given to braking
(down shift)
>= 0.05
Driver Pedal Retard:
If pedel depress < 5%
assume braking modes
downshift
-1
Down shift
Assume l owest gear
slowing
1
Pedal Position
-1,0,1 values
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The hydraulic motor speed controller is the second priority as long as the violation of
the maximum hydraulic motor speed limitation is not violated.
4.2.5 Overall Speed Violation of the Hydraulic Motor
The maximum hydraulic motor rotational speed limitation of 3000 RPM
(134 radians/second), must be strictly obeyed regardless of current hydraulic motor
speed condition or the proposed hydraulic motor speed after calculation. An upshift
is applied when the current hydraulic motor speed exceeds the maximum limitation
to reduce the motor speed. Furthermore, no command can be made by the shift
controller if the corresponding proposed hydraulic motor speed exceeds the
maximum limitation. Figure 4-7 shows the hydraulic motor speed limit controller.
Figure 4-7 Hydraulic motor speed limit controller block diagrams in red
Purpose: to enforce an upshift on motor speed violation
1
Gear Shift Command:
1 = upshift
0 = no action
1
Next Gear Up
>= 314
Motor Speed
Check
0
Hold Gear Constant
1
Motor Speed
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4.2.6 Dwell Time Controller
A two-second dwell time used to stabilize the gear during upshift is applied
to the transmission by using the dwell time controller, except for braking processes.
The controller shown in Figure 4-8 uses a delay loop which lasts for two seconds to
be the timer of the dwell time. It is initiated by a gear change signal to reset the dwell
loop.
Figure 4-8 Dwell time controller block diagrams in magenta
4.2.7 Gear Memory
Every new gear command is made on the basis of the previous gear
Gear delay Loop
-1,0,1 values
1
Prop Gear after
Dwell Tim e
> 0
Switch2
~= 0
Reset for Shift time
When Gear Shift
z
1
Last Shift Time Memory
>= 2
Dwell Tim e
Setting in Seconds
12:34
Digital Clock1
12:34
Digital Clock
Del T
Check Gear Shift
3
Gear Command
Given to Transmission2
Prop Gear after limit
1
Gear Delay
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command. The last gear command of the controller is stored by using a unit delay
block. As shown in Figure 4-9 every proposed shifting command has to be combined
with the last gear command and stored to the next step sampling time. For example,
a downshift command with a value of -1 will be combined with the last gear
command of 4 (5th gear ) and output a gear command as 3.
Figure 4-9 Gear memory by using unit delay
4.2.8 Gear Ratio Matching
Every gear command must be within the 2nd to 7th gear range according to
the criteria in Chapter 3. The gear range controller checks every gear command
before delivering it to the transmission by setting the lowest gear to be 1 (2nd gear)
proposed gear command
1
Gear Ratioz
1
used to
break alg loop
> 0
SIP Check
No shifting if SIP
Proposed Gear1
Gear Sent
z
1
Gear Memory
Create gear number
to go to1
1
2
3
4
5
6
*Convert to
Gear Ration
0.78
7thGear Ratio1
0.90
6thGear Ratio1
1.20
5thGear Ratio1
1.69
4thGear Ratio1
2.24
3rd Gear Ratio1
4.18
2nd Gear Ratio1
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and highest gear to be 6 (7th gear). The the gear ratios of 0.78, 0.9, 1.2, 1.69, 2.24
and 4.18 are matched with the gear numbers from the highest gear 6(7 th gear) to the
lowest gear 1(2nd gear). In doing this, the gear range controller can match each gear
command with its corresponding gear ratio and then output the gear ratio to the truck
model. For instance, the gear range controller takes in a gear command number 4
(5th gear), finds its gearratio 1.2 and then outputs it to the truck model. Figure 4-10
shows the gear ratio matching controller.
Figure 4-10 Gear ratio matching controller
Propsed ge ar
1
Gear Ratioz
1
used to
break alg loop
Gear Sent
1
2
3
4
5
6
*Convert to
Gear Ration
0.78
7thGear Ratio1
0.90
6thGear Ratio1
1.20
5thGear Ratio1
1.69
4thGear Ratio1
2.24
3rd Gear Ratio1
4.18
2nd Gear Ratio1
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Chapter 5
Simulation Results
In this chapter, the simulation results are presented and discussed by
comparing them with the default gear shift schedule. Results include fuel
consumption, tracking performance, and motor speed limit. The simulation runs in a
1369-second velocity-base EPA Federal Urban Driving Schedule (FUDS) with a
discrete 0.005-second sampling time.
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Figure 5-1 EPA Federal Urban Driving Schedule (FUDS)
5.1 Fuel Consumption
Figure 5-2 shows that the total fuel consumption of the controller simulation
is 3.7367 Kg, which is 5.22% less than the fuel consumption of the original default
shifting simulation, 3.9427 Kg. The number of gear changes in the controller
simulation is 315, compared to 197 in the default simulation.
0 200 400 600 800 1000 1200 14000
5
10
15
20
25
30Federal Urban Driving Schedule (FUDS)
Time(sec)
VehicleSpeed(m/s
)
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Figure 5-2 Fuel consumption simulation results comparison: fuel consumption
Kg vs. time (seconds)
As the default shifting schedule of the original model does not include the
2-second dwell time criterion, the fuel consumption of the original model with
2-second dwell time will be a little higher than 3.9427 Kg without dwell time.
0 200 400 600 800 1000 1200 14000
0.5
1
1.5
2
2.5
3
3.5
4Fuel Consumption
Time(sec)
FuelConsumed(Kg
)
Controller Model FuelConsumption
Original Model Fuelconsumption
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5.2 Gear Shifting Schedule
Figure 5-3 Controller model simulation results: gear ratio vs. time (seconds)
The gear shifting schedule of the controller simulation shows no unexpected
oscillations of the gear selection when a 2-second dwell time applied, as illustrated
in Figure 5-3. All the downshifts are either due to the urge demand of torque from
the driver or violations of the hydraulic motor speed and displacement limitations as
expected in the strategy. Shifting with less than 2 seconds dwell time only occurs
when the hydraulic motor speed reaches the maximum limit 314 radian/second
(3000 RPM), or during braking downshift process, as introduced in the previous
chapters.
0 200 400 600 800 1000 1200 14000.5
1
1.5
2
2.5
3
3.5
4
4.5GearRatio o f Controller
Time(sec)
GearRatio
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Figure 5-4 Original model simulation results: gear ratio vs. time (seconds)
The default gear shifting schedule of the original model, shown in Figure 5-4,
presents relatively simple gear changes and unexpected oscillation at approximately
700 seconds, which may be due to the lack of dwell time.
0 200 400 600 800 1000 1200 14000.5
1
1.5
2
2.5
3
3.5
4
4.5GearRatio of Original Shifting
Time(sec)
GearRatio
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5.3 Tracking Performance
Figure 5-5 Controller model simulation results: actual and desired vehicle speed
(m/s) vs. time (second)
0 200 400 600 800 1000 1200 14000
5
10
15
20
25
30VehicleSpeed of Controller
Time(sec)
VehicleSpeed(m/s)
Actual Vehicle Speed
Desired Vehicle Speed
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Figure 5-6 Original model simulation results: actual and desired vehicle speed
(m/s) vs. time (second)
Figure 5-7 Simulation results comparison between original and controller model:
vehicle speed error (m/s) vs. time (second)
0 200 400 600 800 1000 1200 1400
0
5
10
15
20
25
30VehicleSpeed of Original Shifting
Time(sec)
VehicleSpeed(m/s)
Actual V ehicle Speed
Desired Vehicle Speed
0 200 400 600 800 1000 1200 1400-4
-2
0
2
4
6
8
10
VehicleSpeed Error
Time(sec)
VehicleSpeedError(m/s)
Original Model Vehicle Speed Error
Controller Model Vehicle Speed Error
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The combined time-based plots of actual vehicle speed (m/s) and desired
vehicle speed (m/s) are depicted in Figure 5-5 and Figure 5-6. The results of
figure-20 indicate that the truck controlled by the optimal gear shifting controller can
track the desired driving cycle well. One exception occurs between 200 seconds and
300 seconds with a 8m/s peak value difference during the high-acceleration and
high-speed period around 17m/s to 25m/s. This difference can be seen clearly from
Figure 5-7 which shows the error of the actual vehicle speed from both original and
controller model when compared to the desired vehicle speed. The vehicle speed
error stays no more than 3m/s except for the period between 200 seconds and 300
seconds as mentioned before. In fact, the FUDS is used for light-duty vehicle test.
Therefore, the most aggressive acceleration rates might not be realistic for
heavy-duty truck [7], which can account for the peak error of vehicle speed
displayed between 200 seconds and 300 seconds.
The total actual simulation driving distance of the truck is 11428m which is
4.68% less than the desired total distance of FUDS 11989m. The tracking
performance of the original model, as shown in figure-21, has a total driving distance
of 11592m, a 3.31% difference between actual driving distance and desired driving
distance. Though the controller model drives 1.37% less distance than the original
one, the difference is less than the designated maximum difference of 5%. Therefore,
the simulation results of the controller still show good performance on tracking the
desired driving cycle.
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5.4 Hydraulic Motor Speed
Figure 5-8 Controller model simulation results: hydraulic motor speed
(radian/second) vs. time (second)
0 200 400 600 800 1000 1200 14000
50
100
150
200
250
300
350MotorSpeed of Controller Model
Time(sec)
MotorSpeed(rad/s)
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Figure 5-9 Original model simulation results: hydraulic motor speed
(radian/second) vs. time (second)
The hydraulic motor speed is well below the maximum limit of 14
radian/second (3000RPM) , as presented in Figure 5-8. When compared to the
results of original model in Figure 5-9, it is obvious that the hydraulic motor speed
of the controller model is higher, or in other words, keeps in the high-speed range
more often, which eventually results in higher motor efficiency and better fuel
economy.
5.5 Summary
This chapter presents and analyzes the results of the controller model
0 200 400 600 800 1000 1200 14000
50
100
150
200
250
300MotorSpeed of Original Shifting
Time(sec)
MotorSpeed(rad/s)
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simulation and compares them with the results of the original model. Table-1 shows
the comparison between the original model with default shifting schedule and the
model with shifting controller in term of fuel consumption, number of gear changes,
total driving distance, and the difference between actual and desired distance.
Table 5.1 Simulation results comparison between controller and original model
Fuel
consumption
(Kg)
Total
driving
distance (m)
Driving distance
difference between
actual and desired (%)
Shifting
change
times
controller 3.7367 11428 4.68 315
original 3.9427 11592 3.31 197
The controller simulation showed improved fuel consumption and
acceptable tracking performance. However, the original model has slightly better
tracking performance.
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Chapter 6
Summary and Conclusion
6.1 Summary
This paper proposes a fuel-economy optimization gear shifting strategy for
a seven-speed automatic transmission used on a hydraulic hybrid vehicle in order to
maximize fuel economy.
This strategy is designed with a highest possible gear criterion as long as the
torque requirement can be satisfied, except for braking process and torque
demanding situations. The optimization strategy takes several other criteria into
consideration, such as high motor displacement criterion, to improve efficiency and
fuel economy as well. Then the optimization strategy is developed on the basis of
these criteria from two main aspects of the existing SIMULINK truck model. One
approach is based on the hydraulic motor working conditions, such as motor
displacement, and the other is based on the drivers intention, which is interpreted as
the driver pedal position. This controller is able to recognize the drivers intention to
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change the speed and incorporate it into gear shifting decision making.
This paper then develops a SIMULINK controller model based on the
optimal gear shifting strategy and criteria and validates the model both in fuel
economy and power performance by analyzing the simulation results in the Federal
Urban Driving Cycle.
6.2 Conclusion
The simulation results show that the SIMULINK optimal gear shifting
controller model is able to increase the fuel economy by 5.22% with a 3.7367 Kg
fuel consumption compared to the original default shifting schedule.
The controller model is also able to keep the hydraulic motor speed below
the 3000RPM maximum speed limitation when driving. Moreover, by keeping the
hydraulic motor speed higher, the hydraulic motor efficiency can stay in high range
more often to get better fuel economy.
The controller model performs well in the tracking with 4.68% distance
difference between the actual and desired total driving distance which is less than the
5% designed standard.
6.3 Future Work
As there is no braking pedal in the truck model, the braking intention can
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only be predicted from the driver behavior trend on the accelerating pedal. And due
to many influence factors in the truck model, the positive correlation between the
hydraulic motor speed and the displacement can only be concluded approximately.
Therefore, a more efficient schedule for high speed driving period can be developed
based on a braking pedal input and accurate relationship between the hydraulic
motor speed and the displacement, which can track the driving cycle or road
conditions better. Furthermore, with instantaneous fuel consumption output from the
truck model, the optimal shifting strategy can be improved with more efficient
choices. The FUDS only simulates the driving test on flat roads and roads with
slopes are left to be developed in the future.
The optimal gear shifting strategy for a seven-speed automatic transmission
developed in this thesis is part of the research work for a future hydraulic hybrid
truck. The work done by this thesis is presented in simulation, which still needs to be
applied on the hydraulic hybrid truck transmission and adjusted according to the real
conditions of the truck in the future.
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