9-Actuators


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9-Actuators
Contents
Electromechanical Actuators
Electrical Machines
Hydraulic and Pneumatic Actuation Systems
Piezoelectric Actuators
Introduction
Mechatronics Systems
Figure 1 Mechatronics System
Metal strain gauges
ActuatorFunctionalDiagram
Figure 2 Actuator Functional Diagram
Electro-Hydraulic & Electromechanical Actuators
Figure 3 Electro-Hydraulic & Electromechanical Actuators
Electric Motors
 Electric motors are the most widely used electromechanical actuators.
They can either be classified based on functionality or electromagnetic
characteristics. The differences in electric motors are mainly in the rotor
design and the method of generating the magnetic field. Figure 4 shows
the composition of a permanent magnet DC motor. Some common
terminologies for electric motors are:
Figure 4 the composition of DC motor
Electric Motors
 Stator is the stationary outer or inner housing of the motor that supports
the material that generates the appropriate stator magnetic field. It can be
made of permanent magnet or coil windings.
 Field coil (system) is the portion of the stator that is responsible for
generating the stator (field) magnetic flux.
Figure 4 the composition of DC motor
Electric Motors
 Rotor is the rotating part of the motor. Depending on the construction, it
can be a permanent magnet or a ferromagnetic core with coil windings
(armature) to provide the appropriate armature field to interact with the
stator field to create the torque.
 Armature is the rotor winding that carries current and induces a rotor
magnetic field.
 Air gap is the small gap between the rotor and the stator, where the two
magnetic fields interact and generate the output torque.
 Brush is the part of a DC motor through which the current is supplied to
the armature (rotor).
Figure 5 rotor structure
Electric Motors
CommutatoristhepartoftheDCmotorrotorthatisincontactwiththebrushesandisusedforcontrollingthearmaturecurrentdirection.
Commutationcanbeinterpretedasthemethodtocontrolthecurrentdirectionsinthestatorand/orthearmaturecoilssothatadesiredrelativestatorandrotormagneticfluxdirectionismaintained.
ForACmotors,commutationisdonebytheACappliedcurrentaswellasthedesignofthewindinggeometry.
ForsteppingmotorsandbrushlessDC(BLDC)motors,commutationsaredoneinthedriveelectronicsand/ormotorcommands.
Torquegenerationinanelectricmotoriseitherthroughtheinteractionofthearmaturecurrentandthestatormagneticfield(LorentzLaw)orthroughtheinteractionofthestatorfieldandthearmaturefield.
Method of Connections
 The Shunt-Wound Motor
 The shunt-wound motor is wired such that the armature and field coils are
connected in parallel with the supply. Under normal operating conditions, the
field current will be constant. As the armature current increases, the armature
reaction effect will weaken the field and the speed will tend to increase.
Figure 6 Shunt-Wound Motor
Method of Connections
However,theinducedvoltagewilldecreaseduetotheincreasingarmaturevoltagedropandthiswilltendtodecreasethespeed.Thetwoeffectsarenotselfcancellingandoverallthemotorspeedwillfallslightlyasthearmaturecurrentincreases.
Themotortorqueincreasesapproximatelylinearlywiththearmaturecurrentuntilthearmaturereactionstartstoweakenthefield.Itcanalsobeseenthatnotorqueisdevelopeduntilthearmaturecurrentislargeenoughtoovercometheconstantlossesinthemachine.
Sincethetorqueincreasesdramaticallyforaslightdecreaseinspeed,theshunt-woundmotorisparticularlysuitablefordrivingequipmentlikepumps,compressors,andmachinetoolelementswherethespeedmustremain“constant”overawiderangeofloadconditions
The Series-Wound Motor
Theseries-woundmotorisshowninFigure7.Astheloadcurrentincreases,theinducedvoltage,E,willdecreaseduetothearmatureandfieldresistancedrops.
Becausethefieldwindingisconnectedinserieswiththearmature,thefluxisdirectlyproportionaltothearmaturecurrent.
Thespeed–armaturecurrentcharacteristicwilltaketheformofahyperbola
Similarly,thetorque–armaturecurrentcharacteristicwillbeapproximatelyparabolic
Torque–Speed Characteristic
Figure 7 Series-Wound Motor
The Series-Wound Motor
Thesegeneralcharacteristics,alongwiththederivedtorque–speedcharacteristicinFigure7.
Thegeneralcharacteristicsindicatethatiftheloadfallstoaparticularlylowvaluethenthespeedmaybecomedangerouslyhigh.
Aseries-woundmotorshouldneverbeused,therefore,insituationswheretheloadislikelytobesuddenlyrelaxed.
Themainadvantageoftheseries-woundmotoristhatitprovidesalargetorqueatlowspeeds.
Serieswoundmotorsareextremelysuitable,therefore,forapplicationswherealargestartingtorqueisrequired.Thisincludes,forexample,lifts,hoists,cranes,andelectrictrains.

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