10- Electro-Mechanical Actuators & Their Drives


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Electro-Mechanical Actuators & Their Drives
Contents
Electromechanical Actuators
Low Power Drivers
Medium power Drivers
Insulated Gate Bipolar Transistor (IGBT)
 IGBT is a voltage-controlled transistor that has the terminals identified in the
same way as BJTs.
 IGBT is a four-layer device that has the similar construction of a MOSFET
with an additional p layer.
 Figure 1 shows the schematic symbol and equivalent circuit for an IGBT.
 IGBT has the combined characteristics of the BJT and MOSFET.
 Similar to MOSFET, it has high input impedance and high switching
frequency. It also has high power handling capacity like the BJT
Figure 1 schematic symbole
Typical Power Amplifiers for Electromechanical Actuators
Poweramplificationandmodulationforelectromechanicalactuatorsareclassifiedintotwobasiccategories,basedonthemethodstherespectivepowerelectronicsaredriven.
1.LinearamplifiersdrivetheBJTsintheiractivelinearregion
2.Switchingamplifiersdrivethetransistorsinon-offswitchmode
Dependingonthecontrolobjective,thecommandsignaltotheamplifiercanbeeitheravoltageorcurrentcommandthatintendstomodulatetheelectricenergydeliveredtotheenergyconversiondevice.
SincemostoftheelectromechanicalactuatorinvolvesdrivinganinductanceloadsuchasacoilwindingofanelectromagnetortherotorofaDCmotor,inthefollowingdiscussion,wewillusetheDCmotorasanexampleofaninductanceloadforthepoweramplifier.
Linear Amplifiers
Figure2showsthebasicdrivecircuitforlinearvoltageandcurrentcontrolamplifiers.
Bothschemeshavethefollowingcommonalities:
1.TheinputcommandvoltageVi(t)isappliedtothebaseofthetransistor.
2.TheelectricpowerneededtodrivetheloadisprovidedbyaDCsupply.
3.Thetransistorsaredrivenintheactivelinearregion.
Figure 2 Linear Amplifier
Voltage Control (Mode) Amplifier
InFig.2(a),themotorisdrivenasaloadofanemittercircuit.
Ifthebase-emittervoltageisignored,thevoltageacrossthemotorVMisdirectlycontrolledbytheinputvoltageVINandthecurrentissuppliedbythepowersupply.
Theamountofcurrent,ontheotherhand,dependsontheappliedvoltage,speed,andthemotorparameter.
Figure 2a Voltage Control Mode
Linear Amplifiers Cont.
Toprovideadjustablevoltagegain,Fig.3showsavariablegainvoltage-modeamplifier,wheretheapparentamplifiergainfromthecommandinputVINtotheappliedmotor(winding)voltageVMcanbeapproximatedby(RA+RB)/RA,whichcanbeadjustedbyproperselectionoftheresistorsRAandRB.
Ifalargemotorcurrentisrequired,transistorQ2canbereplacedbyaDarlingtontransistorpair.
TheamplifiersshowninFig.2(a)
andFig.3canonlydrivethecurrent
throughthemotor(load)inone
direction.Hence,theyarealsocalled
unipolaramplifiers.
Figure3variablegainvoltage-modeamplifier
Linear Amplifiers Cont.
Toprovidebidirectionalcurrentflow,twotransistorscanbeconnectedwiththemotorinapush-pulltypeconfiguration,asshowninFig.4.
Thetwodiodesinthecircuitareusedtoeliminatethedead-zonecreatedbythebase-emittervoltagedropforthetransistors.Noticethatabipolarvoltagesourceisneededforthisconfiguration.
Figure4bidirectionalDrivers
Voltage-Mode Driver
Figure5showsabipolarvoltage-modedriverwithvariablegain(RA+RB)/RA.
Similarly,iflargermotorcurrentisrequired,transistorsQ3andQ4canbereplacedbyDarlingtontransistorpairs.
Figure5bipolarvoltage-modedriverwithvariablegain
Current Control (Mode) Amplifier
 As previously discussed, in many electromagnetic actuators, the output force or
torque of the device has strong correlation with the winding current, e.g., for a
permanent magnet DC motor and a voice coil actuator, the output torque and
force are proportional to the input current.
 Therefore, in many motion control applications, it is more desirable to have a
voltage-to-current conversion (current-mode amplifier) at the power stage, where
the input voltage command is proportional to the current flowing into/out of the
motor (winding).
 The relationship between the emitter (motor)
 current iM and the input voltage command
 VIN is:
Figure 2(b) shows a basic circuit for a current-mode amplifier
Current Control (Mode) Amplifier Cont.
If the base-emitter voltage is ignored, the voltage across the motor current iMis proportional to the input voltage VIN, i.e., iM≈ (1/RS)VIN.
Figure 6 shows a basic bipolar current-mode amplifier. An Op-Amp is used to close the current loop.
The resistor RS, often called the sensing resistor, is used to sense the motor current for feedback to the Op-Amp.
Depending on the desired current magnitude, the sensing resistor needs to have adequate power rating to dissipate the heat (i2MRs) generated by flowing current through the resistor.
For a zeroth order approximation, at steady state, the Op-Amp will try to equalize the potential at the positive and the negative terminals, i.e., it will try to make
Which implies, the motor current:
Current Control (Mode) Amplifier Cont.
 Figure 6 shows a basic bipolar current-mode amplifier.
 An Op-Amp is used to close the current loop.
 Although a current amplifier tends to have a linear relationship between the
command input and the winding current, there is practical limitation due to the limited
source voltage
Rs
Figure 6 bipolar current-mode amplifier
Switching Amplifiers
Linearamplifiersaresimpleanddonotgenerateelectricalnoises.However,sincethefinalstagetransistorsareoperatingintheactivelinearregion,significantpowerisdissipatedintoheat;thisreducestheefficiencyofthedeviceaswellasrequireslargeheatsinkstoprotectthecomponents.
However,asshowninFig.7,whenoperatinginthesaturationregion,ifthecollector-emittervoltagedropisintheorderof1Vorless,thepowerlossacrossthetransistorissignificantlyless,giventhesameamountofcurrentflow.Thetradeoffisthatadditionalcircuitsareneededtoprovidethemodulationforcurrentorvoltagecontrol.
Figure7bipolarcurrent-modeamplifier
Switching Amplifiers
 Figure 8 shows a simple switching amplifier that is simply a transistor
connecting a load. It is essentially the same as the basic linear amplifier
shown in Fig. 2(a). The difference is in the way the transistor is controlled.
For a switching amplifier, the input (base) voltage only takes on two values
(states), high and low. When the base (input) voltage is high, the transistor is
turned on in the saturation mode and current will flow through the load.
Figure 8 Simple switching amplifier with switching input
Switching Amplifiers
Ifweneglectthecollector-emittervoltagedrop,thevoltageacrosstheloadisapproximatelythesupplyvoltage.Whenthebasevoltageislow,thetransistoristurnedoffinthecutoffstateandnovoltageisappliedtotheload.Iftheloadhasalowpasscharacteristic,theaveragecurrent/voltageacrosstheloadwillbeproportionaltotheturn-ontime.
Therefore,iftheswitchingfrequencyissufficientlyhigh(relativetotheloadimpedance),theeffectivevoltage/currentacrosstheloadcanbemodulatedbythepercenthighinputvoltage,e.g.,iftheVINishigh80%ofthetime,theaveragevoltageacrosstheloadwillbecloseto80%ofthesuppliedvoltageV.
Thisistheso-calledPulseWidthModulation(PWM).AnotherbenefitofusingswitchingamplifiersisthatVINcanbedirectlyinterfacedwithadigitaldevicewithouttheneedforaDAC.
Push-Pull (Class B) Power Amplifier
 The switching amplifier shown in Fig. 8 is unipolar, i.e., it can only drive
current through the load in one direction.
 The circuit in Fig. 9 is very similar to the bipolar voltage-mode amplifier
shown Fig. 10
Figure 9 Switching push-pull amplifier
Push-Pull (Class B) Power Amplifier
 in Fig. 10. The difference is also in the way the transistors are controlled. When the
base voltage VIN is sufficiently positive (+V), the push transistor Qpush will be turned
on and the pull transistor Qpull will be turned off. This results in a load current flowing
from positive supply to ground.
 If the base voltage is sufficiently negative (−V), Qpush will be turned off and Qpull will
be turned on, which results in a current flow from ground to the negative supply.
Figure 10 Using diodes to reduce switching voltage when driving inductive loads
H-Bridge Driver
Tomodulatetheloadvoltage/current,PWMcanalsobeused.
ThisconfigurationisalsocalledahalfH-bridgedriverorhalf-bridgedriverforshort.
Fromanimplementationperspective,thisdevicerequiresbothapositivesupplyandanegativesupply,whichtendstoincreasethecomplexityandcostofthecircuit.
H-bridgeconfigurationisaneatsolutiontoachievebipolaroperationwithunipolarsupply.Figure11showsasimpleH-bridgecircuitdrivingaload.
H-Bridge Driver Configration
 An H-bridge consists of four transistors that are connected in a Wheatstone bridge
configuration. By turning on/off different pairs of transistors (Q1-Q3) or (Q2-Q4),
bipolar voltage across the load can be achieved using a unipolar supply, see Fig. 11.
 In many applications, the transistors pairs in the H-bridge can be directly driven by
the output of a digital device (TTL or CMOS).
 The n-p-n or n-channel transistors can be turned on to saturation by a high output
from the digital port and turned off by a low output.
Figure 11 H-Bridge driver
Pulse-Width Modulation (PWM)
 PWM is one of the more common ways of encoding analog information using digital
signal. A PWM signal is a wave of fixed frequency and varying duty cycle (pulse
width).
 The duty cycle in PWM context refers to the percentage of time that the signal is in
the active state—usually this means a state of logic 1, see Fig. 12.
 In essence, PWM encodes (modulates) the information in the time domain rather
than the voltage domain as with analog signals.
Figure 12 PWM duty cycle
Pulse-Width Modulation (PWM)
PWMactuationhasseveraladvantagesovertheuseofD/Aconvertersandlinearcomponents.Oneistheefficiencywheretheswitchingamplifiersaremoreefficientthantheirlinearcounterparts.
Advantagesare
1.thereisnoneedforD/Aconversion.
2.digitalsignalismaintainedfromthemicroprocessor/microcontrollertothepoweramplifier.
3.inadditionaltohavingbetternoiserejectioncapability,thisalsoreducestheneedforaDACandtendstomakethecircuitsimplerandmorecosteffective.
OnedrawbackforusingPWMandswitchingamplifiersasawholeisthat:
1.thehighfrequencyswitchinginducesradiofrequencyinterference(RFI)andelectromagneticinterference(EMI).

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