lecture notes set 5 -actuation systems - 2008-09 page 5.3 mfe3004 mechatronics i c. pace actuators...

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Page 5.1MFE3004 Mechatronics IC. Pace

MFE 3004Mechatronics I

Actuation SystemsDr Conrad Pace


Actuators• Actuators are the elements of a mechatronic system that are responsible for transforming the output of a control system into a physical controlling action on a machine or device

• Examples• An electrical output from a controller that has to be transformed to a linear or rotational motion to move a load• An electrical output that has to be used to control the flow rate through a pipe, etc..

Controller

Actuation

Sensing

ControlledEnvironment

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Actuators

• Actuation systems can be classified according to the source of the driving power

Pneumatic (Compressed Air power)Hydraulic (Pressurised Hydraulic Fluid Power)Electrical (Electrical power)

• Each of the above actuation systems have particular characteristics that make them suitable for certain applications.


Actuators

• Two components of an actuation system– Power Amplification and Modulation Stage

Power Amplification and Modulation Stage

Control Signal

(Low Power)

Power Required to Drive Actuator

(High Power)Un-modulated Power Source

• Provide high power switching in electrical drives

• Provide flow switching and fluid flow control in fluid power drives

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Actuators

• Two components of an actuation system– Energy Conversion Stage

Energy Converter

Modulated High Power Input

Output Work

(Physical output of Actuator)

• Examples :

• Electrical Drives

• Fluid Power Cylinder


Actuators• Two components of an actuation system

– Power Amplification and Modulation Stage– Energy Conversion Stage

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Electrical ActuationElectrical Actuators

Solenoids Linear Motors

Linear Motion Rotary Motion

DC Motors AC Motors Stepper Motors

Brush Type

Brushless

Asynchronous

Synchronous

Variable Reluctance

Permanent Magnet


Electrical ActuationDC Brush Type Motors

FieldPole

ArmatureFieldCoil

ArmatureConductors

Yoke

A four-pole DC motor

Current through armature coil

I = (V - kφω)R

V = voltage across coilR = coil resistancek = back e.m.f. constantφ = magnetic flux per poleω = rotational speed

• A magnetic field is generated by the stator winding / permanent magnet

• The flow of current through the rotor winding and the presence of a magnetic field causes a force to act on the winding which induces a rotating torque

• For continuous rotation the current must be reversed once for each pole pair, thus requiring a commutator and brushes.

Torque generated by Armature current flow

T = kTI kT = Torque constant

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.9MFE3004 Mechatronics IC. Pace

Electrical Actuation

DC Motors with Field Coils

ArmatureCoil

Field Coil Series Wound

• Highest starting torque

• Highest no-load speed

Shunt Wound

• Lowest starting torque

• Lowest no-load speed

• Stable speed over torque range

ArmatureCoil

FieldCoil

ArmatureCoil

FieldCoil

Field Coil Compound Wound

• Has characteristics of both series and shunt wound Armature

Coil

FieldCoil

Separately Excited

• Ideal set-up for speed control

• Speed control - Armature voltage control or Field voltage control



AC motors - Asynchronous Motors• Rotor consists of a set of conductors connected to each other by end rings forming a squirrel cage

• The stator winding is driven by an A.C. voltage which induces a rotating magnetic field

• The magnetic field induces an e.m.f. in the rotor which causes a current flow.

• The rotor current flow in the magnetic field causes a torque on the rotor, causing rotary motion

• The motor is not self-starting

Synchronous speed of motor = 2f/p

f = frequency of stator signal

p = no. of stator poles

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AC motors - Synchronous Motors• Stator similar to asynchronous motors, and carries an a.c. voltage

• Rotor consists of a permanent magnet

• Synchronous motors run at exactly the synchronous speed

• Speed control obtained through the variation of the signal frequency

~~

~~

Motor

Variablefrequency

A.C.Supply

VoltageInverter

RectifierVoltageregulator

AC motors - Speed Control


Electrical ActuationStepper Motors

• Motors that provide discrete angular displacements in steps (ex. 5° angular steps)

• Motors driven through a sequence of pulses where each pulse corresponds to an angular step.

• Continuous rotation can be provided by giving a continuous train of pulses, the speed being determined by the frequency of the pulses.

Variable Reluctance Stepper Motor• The ferromagnetic rotor consists of a number of poles (less than the stator poles)

N

S

Rotor

Stator

This pair of poles are energised by thecurrent being switched to them and therotor rotates to the position show adjacent.

N

S

This pair of poles are energisedby the current being switched tothem to give the next step.

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Permanent Magnet Stepper Motor• The rotor consists of a permanent magnet that aligns itself with the energised stator poles

• All stepper motors can have more than one stack so as to decrease the angular step

• The stacks are energised in sequence.



Hybrid Stepper Motor• The rotor consists of a permanent magnet

• The magnetic rotor is toothed so as to have a number of poles

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Typical Control Set-up for a motor• Motor Speed Control

• Motor Angular Displacement Control

+ _ + _

Position Error

Position Input Demand

Velocity Error Amplifier Motor

Motor Power Input Gear

Box

Rotational Output

Load

Position Output

Velocity Measurement

Position Measurement

Position Feedback

Velocity Feedback


Pneumatic Actuators - Cylinders

Bearing

Seal Piston rodExhaustInlet

Return SpringPiston

Bearing CapCylinder barrelPiston sealBase Cap

Single Acting Cylinders

Double Acting Cylinders

B A

Ao

AA

area on piston side area on piston rodside

Symbols

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Pneumatic Actuators

Rodless Cylinder

Limited Rotation Rotary Actuators

Symbol


Vane Type

Pneumatic Rotary Actuators

Continuous Rotation - Air Motors

Radial Piston Type

Axial Piston Type Symbol

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Control of Pneumatic Actuators

PressureControl

DrivingMeans

Regulator

Dire

ctio

nal

Con

trol

cylinder pistonpiston rod

Lubricator

AirCompressor

Filter

Air StorageTank

Pneumatic System

• The normal operating pressure in the system is generally 6 bar


Direction Control Elements

RP

A

P R

A

Operation of a Single Acting Cylinder via a 3 port / 2 position (3/2 way) valve

Operation of a Double Acting Cylinder via a 4/2 way valve

A

P R

B A

P R

B

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Pneumatic Actuation

Application Example : A pick-and-place Pneumatic System


Hydraulic Actuation

Agriculturalmachinery

Tractors, Combines

Constructionmachines andpublic servicevehicles

Excavators, graders, loaders,cranes, winches, elevatingplatforms, lifting and conveyingdevices, hydro-transmissions

Plasticmanufacturingmachines

blow-moulding machines,injection moulding machines

Heavy industries Steel mills,continuous-casting plant

Machine toolsnon-cutting

Materials-testing presses,ceramics and plastics presses,folding and bending presses,drawing presses.

Machine toolsmetal-cutting

Clamping devices,planers and slotting machines,lathes, drilling and turningm/c’s, grinding machines

100 200 300Pressure (Bar)

• Hydraulic actuation is used in a wide variety of applications with varying operating pressures

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Hydraulic Actuation

• The consideration of the flow characteristics is important due to the effect of viscosity

• High viscosity gives rise to high frictional forces in the pipes giving rise to substantial pressure drops

• Viscosity is greatly dependent on temperature, thus affecting Flow rate

Fundamental Principles of Hydraulics

• The hydraulic fluid has to fulfil following tasks:Pressure Transfer Lubrication CoolingDamping oscillations Corrosion protection Scuff removalSignal transmission Eliminate presence of air


Differentialcylinder(standard)

Piston return strokefaster than advancestroke

Synchronouscylinder

Pressurised area ofequal size. Advance andreturn speeds identical

Cylinder withend positioncushioning

To moderate the speedin the case of largemasses and prevent ahard impact

wiper seal

piston rodbearing

barrel

piston rod

piston withseals

end cap

rod sealventingscrew

Hydraulic Actuators - Cylinders

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Telescopiccylinder

Longer Strokes

PressureIntensifier

Increases pressure

TandemCylinder

When large forces arerequired and only smallcylinder diameters arepossible

Hydraulic Actuators - Cylinders


Hydraulic Actuators - Motors

Hydraulic Motors

Gear Motors Vane Motors Piston Type Motors

Axial Piston Motors Radial Piston Motors

Characteristics of Hydraulic MotorsApplication Motor Type

Low torque, low power Gear; Vane

Low torque, medium power Vane; Axial piston

Medium torque and/or power Axial piston; Radial piston (inward working)

High power, very high torque Axial piston; Radial piston (outward working)

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Hydraulic Actuators - Motors

Radial Piston Motors

Gear Motor Axial Piston Motor


Hydraulic Actuation

Hydraulic Power System

Driving Means

Pump

Non-return Valve

Pressure relief valve

To system control and working elements

Return

Oil reservoir (sump)

Return Filter

Suction filter

Pressure Filter

Hydraulic Circuits

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Hydraulic Actuation

Example of an Electro-Hydraulic Actuation System

SERVO VALVE

AMPLFIIER

Control Signal

HYDRAULIC FLUID SUPPLY

Hydraulic Fluid Drain

HYDRAULIC CYLINDER

Position Feedback

POSITION MEASUREMENT

Error Signal

Position Demand Signal


Hydraulic Pneumatics Electrical

Working Fluid Mineral Oil Air Voltage-Current

Working Pressure 500 Bar (maximum 6-7 Bar Up to 11kV

Available Force 100MN 10kN 100kN

Speed Low High Very High

Conversion Efficiency Over 70% Under 20% Over 80%

Capital Costs High Low Intermediate

Proportional Control Easy Difficult Easy

Hold Load Power-Off/Stability

Possible No (air is compressible) Possible

Precise Positioning Easy Difficult Easy

EnvironmentalInfluences

Sensitive in case oftemperature fluctuation,risk of fire in case ofleakage

Explosion proof,Insensitive totemperature

Risk of explosion incertain areas,insensitive totemperature

Energy storage Limited with the help ofcompressed gases

Easy Difficult, only in smallquantities usingbatteries

Linear Motion Simple using cylinders Simple using cylinders Difficult and expensive– with motion converter

Rotary Motion Simple Simple Simple

Actuator Power Systems

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Actuators

1 10 100 1,000 10,000

100,000

10,000

1,000

100

10

Pneumaticbellows

Pneumaticcylinders(rodless)

Pneumaticcylinders(rodded)

Solenoids

Pneumaticdiaphragms

HydraulicCylinders

LinearElectric

Motor andscrew

Piezoelectricdevices

Distance(mm)

Force (N)


1 10 100 1,000 10,000

100,000

10,000

1,000

100

10

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NOTEServo & steppermotors may operateat low speeds,including incrementalmotion

Hydraulic DrivesElectrical DrivesPneumatic Drives

Pneumatic(limited rotation)

Hydraulic(limited rotation)

Hydraulicradial piston

Hydraulicaxial piston

Hydraulicvane

Hydraulic gear

PneumaticDrives

PermanentMagnet stepper

High FrequencyInverter Drives

Induction motorhigh power

inverter drives

Output Speed (rev/min)

Output Torque (Nm)

Actuators

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Electricalslipcouplings

Conepulley

Vee beltand chains

Torques to250,000Nm parallelshaft (marine gears)

Unit hydraulicdrives

Harmonicdrives

1 10 100 1,000 10,000

100,000

10,000

1,000

100

10

1Friction wire

wrap

ToothedBelt

Ball ordiscfrictiondrive

Parallel shaftWormFluid couplingsand torqueconverters

Epicyclicgears

Output Speed (rev/min)

Output Torque (Nm)

Actuators


Actuator Performance Characteristics

• Motion Requirements• Type of motion required (linear or rotary)• Travel required (e.g. stroke lengths or angles or rotation)

• Velocity• Defining Acceleration and Deceleration Characteristics

• Operational Power• Power sufficient for application (effort x flow)• Power due to varying loads

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• Resolution, Accuracy and Repeatability

• Resolution : the smallest controllable change in output possible by the actuator

• Accuracy : how close to the intended target an actuator is expected to locate (error between demanded and actual output)

• Dependent on internal actuator factors (looseness of mechanical couplings, stiction, etc..)

• Dependent on external actuator factors (sliding friction of someexternal slide)

• Repeatability : the ability of the actuator to return repeatedly to the same point.

• Influenced by component rigidity (under changing loads)• Effected by actuator hysteresis and backlash.



• Responsiveness/ Dynamic Behaviour• Dynamic response to a demand signal• Dependent on

• Actuator’s velocity and acceleration characteristics• Effectiveness of feedback loop and control

• Generally analysed based on the • Step response (Time Domain analysis)• Frequency response (Frequency Domain analysis)

• Compliance• The movement of a component in reaction to a force exerted on it. • Stiffness of the system (low compliance) or sponginess/softness (high

compliance)• Compliance requirements depend on the type of application

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• Non-Linearities• General Assumption – Actuation System behaves linearly (follows the

principle of superposition)• Most actuation systems exhibit deviations from the linear behaviour (non-

linearities)• Static/ Coulomb Friction• Eccentricity• Backlash• Saturation• Deadband



• Non-Linearities

Static/ Coulomb Friction Eccentricity

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• Non-Linearities

Backlash Saturation



• Non-Linearities

Deadband

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• Other Actuation System Selection Parameters• Control Parameters

• parameters that influence actuator control• Power Source Requirements

• Type of power source available • Method of Actuator performance measurement• System Integration• Costs• Safety (as part of the system)

lecture notes set 5 -actuation systems - 2008-09 page 5.3 mfe3004 mechatronics i c. pace actuators...

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