Note 8 Electric Actutor Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 1
1. Introduction In typicl cloed-loop, or feedbck, control of mchine or proce, the controller compre the ctul enor meurement with the deired vlue nd then djut the ignl to the ctutor ccordingly. The ctutor, or prime mover, convert ignl into phyicl quntity to initite motion, thereby regulting the controlled vrible. In generl, ctutor re clified into three ctegorie: electric, pneumtic, nd hydrulic. The following tble provide qulittive comprion mong thee type of ctutor. Tble 1 Comprion of Pneumtic, Hydrulic, nd Electric Actutor Electric ctutor convert electric power into mechnicl power. Electric ctutor re vilble in one of two type, direct current (DC) nd lternting current (AC). AC induction nd ynchronou motor re idel for contnt peed ppliction with little lod vrition. AC motor ue line current to directly provide more power compred to DC motor of imilr ize. For poition nd peed control ppliction involving vrible lod, DC motor re fvored. DC motor fll in one of three ctegorie: conventionl or bruhed DC motor, bruhle DC motor, nd tep motor. Servo re biclly DC motor fitted with ening nd control component. Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd
. DC Motor.1 Operting Principle of DC motor A DC motor i n electromechnicl device tht convert DC electricl energy into mechnicl energy. The principle of opertion of ny electric motor i bed on Ampere lw, which tte the conductor of length L will experience force F if n electric current I flow through the conductor t right ngle to mgnetic filed with flux denity B. Referring to Figure 1, the force i determined by the cro product F = ( B I) L = B I Linθ Figure 1: Force ct on current-crrying conductor in mgnetic field where θ i the ngle between the current flow nd the mgnetic flux denity. Bed on the foregoing, motor cn be contructed from two bic component: one to produce the mgnetic field, uully termed the ttor, nd one to ct the conductor, uully termed rmture or rotor. The ttor mgnetic my be creted either by field coil wound on the ttor pole or by permnent mgnet (PM). In bruhed DC motor, the rotor h the coil winding nd the ttor h the permnent mgnet. Beide, bruhed DC motor h mechnicl bruh pir on the motor frme nd mke contct with commuttor ring embly on the rotor in order to commutte current, or witch current from one winding to nother, function of rotor poition o tht the mgnetic field of the rotor nd ttor re lwy t 90 degree ngle reltive to ech other. Figure how the bruh nd commuttor rrngement nd torque function of rotor poition for different number of commuttor egment. Idelly, the lrger the number of commuttor, the mller torque ripple. However, there i prcticl limit on how mll the bruh-commuttor embly cn be ectioned. A bruhle DC motor i biclly n inide-out verion of bruhed DC motor, hown in Figure 3. The rotor h permnent mgnet, nd the ttor h the conductor winding, uully in three electriclly independent phe. The operting gol i the me, i.e., mintin the mgnetic field of the rotor nd ttor perpendiculr to ech other t ll time. The difference i in the commuttion. In the bruhed motor, the mgnetic flux generted by permnent mgnet of the ttor i fixed in pce; nd the mgnetic filed Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 3
generted by the rmture i lo mintined fixed in pce by the mechnicl bruhcommuttor embly nd perpendiculr to tht of the ttor. In the ce of bruhle DC motor, the field mgnetic i etblihed by the rotor nd it rotte in pce with the rotor. Therefore, the ttor winding current h to be controlled function of rotor poition o to keep the ttor generted mgnetic field lwy perpendiculr to the mgnetic field of the rotor. Figure : Commuttion nd torque vrition function of ngulr poition of the rotor. Figure 3: DC motor type: bruhed DC nd bruhle DC. Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 4
. Drive of DC Bruhed Motor Drive i conidered the power mplifiction tge of n electric motor. The mot common type of power tge mplifier ued for DC bruhed motor i n H-bridge mplifier (Figure 4). The H-bridge uue four power trnitor. When controlled in pir (Q 1 nd Q 4, or Q nd Q 3 ), it chnge the direction of the current, hence the direction of generted torque. Notice tht the pir of Q 1 nd Q 3, or Q nd Q 4 hould never be turned ON t the me time becue it would form hort-circuit pth between upply nd ground. The diode cro ech trnitor erve the purpoe of uppreing voltge pike nd provide freewheeling pth for the current to follow. Lrge voltge pike occur cro the trnitor in the revere direction due to the inductnce of the coil. The diode provide the ltertive current pth for inductive lod nd let current p through the coil. Figure 4: Block digrm of the bruhed DC motor drive: PWM mplifier with current feedbck control By controlling the current mgnitude through the power trnitor, the mgnitude of the torque i controlled. For thi purpoe, the pule width modultion (PWM) ignl i uully ued. The PWM circuit convert n nlog input ignl to fixed frequency but vrible pule width ignl. By modulting the ON-OFF time of the pule width, deired verge voltge cn be controlled..3 DC Motor: Electromechnicl Dynmic Model Motor-driven ytem with Ger. Motor-driven ytem re rrely een without ocited ger trin driving the lod. Figure 5 how ger driving rottionl inertil, pring, nd vicou dmper, in which N 1 nd N re the number of teeth of the input ger nd output ger, repectively. Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 5
Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 6 Figure 5: () Rottionl ytem driven by ger; (b) equivlent ytem t the output fter reflection of input torque; (3) equivlent ytem t the input fter reflection of impednce. The lod cn be reflected from the output to the input; ) ( ) ( 1 1 1 1 T N N N N D N N J = + + θ Dynmic of DC motor-driven ytem. Figure 6 () how the motor chemtic, nd the reulting toque drive typicl equivlent mechnicl loding on motor hown in Figure 6 (b). The trnfer function of the ytem i D J L R E G b t m m t m + + + = = ) )( ( ) ( ) ( ) ( θ Figure 6: () chemtic of DC motor nd (b) typicl equivlent mechnicl loding
Exmple 1 Given DC motor-driven ytem hown in the following figure. It i known from the motor pecifiction tht t = 40 N-m/A, b = V-/rd, nd the rmture reitnce R = 8 Ω, nd tht the rmture inductnce L, i mll nd it influence cn be ignored. Find the trnfer function G( ) = ( ) / E ( ). θ m Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 7
Torque-peed curve of DC motor. If DC voltge, e, i pplied, the motor will turn t contnt ngulr velocity t tedy-tte, ω m, with torque, T m. The reltionhip between ω m nd T m i chrcterized by = ω b t T m m + R R t e The bove eqution i tright line, ω m v. T m, nd i hown in the following figure. Thi plot i clled the torque-peed curve. The toque xi intercept occur when the ngulr velocity reche zero. Tht vlue of torque i clled the tll torque, T tll. Thu, T = tll R t e The ngulr velocity occurring when the torque i zero i clled the no-lod peed, ω no-lod. Thu, e ω no lod = b Figure 7: Torque-peed curve with n rmture voltge, e, prmeter. Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 8
Exmple Find the trnfer function G( ) = ( ) / E ( ), for the motor nd lod hown in the θ L following figure. The torque-peed curve i given by Tm = 8 ωm + 00 if the input voltge i 100 V. 3. Step Motor Step motor, lo clled tepper motor, electromechnicl contruction i uch tht it move in dicrete mechnicl tep. A chnge in phe current from one tte to nother crete ingle tep chnge in the rotor poition. If the phe current tte i not chnged, the rotor poition ty in tht tble poition. The operting principle of bic tepper motor i hown chemticlly in Figure 8, in which the rotor h one north nd one outh pole permnent mgnet; nd the ttor h four-pole, two-phe winding with four witche. At ny given time either witch 1 or, nd 3 or 4 cn be ON to ffect the polrity of electromgnet. For ech tte, there i correponding tble rotor poition. Conider the witching equence hown on the left four illutrtion t the bottom of Figure 8. At ny given time, ll of the ttor phe re energized; nd ech rotor pole i Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 9
ttrcted by two winding pole. Following the four witching equence, the rotor would tke the hown tble poition. The type of phe current witch, where both phe re energized, i referred to full-tep model of opertion. Conider the four equence of witch tt hown in the right-ide of Figure 8. In thi ce, only one of the ttor phe i energized while the other phed i OFF. The correponding tble rotor poition re hown in the figure. However, notice tht ince the mgnetic force pulling the rotor i provided by only one phe, the torque of the motor t thee witch tte i le thn (pproximtely ½) tht t the full tep mode. Thi mode of witching phe current i referred to the hlf-tep mode. Figure 8: Operting principle of tepper motor. Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 10
In relty, tepper motor i uully contructed to hve multiple "toothed" electromgnet rrnged round centrl ger-hped rotor, hown in Figure 9. The electromgnet re energized by n externl control circuit, uch microcontroller. To mke the motor hft turn, firt one electromgnet i given power, which mke the ger' teeth mgneticlly ttrcted to the electromgnet' teeth. When the ger' teeth re thu ligned to the firt electromgnet, they re lightly offet from the next electromgnet. When the next electromgnet i turned on nd the firt i turned off, the ger rotte lightly to lign with the next one, nd from there the proce i repeted. Ech of thoe light rottion i clled "tep." In tht wy, the motor cn be turned precie ngle. Figure 9: Stepper motor rotting mll ngel in ech tep. Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 11
Figure 10 how the ttor winding connection of two drive configurtion: unipolr drive, nd bipolr drive. The difference between thee configurtion i tht t witched on tte, only hlf of the winding i ued in the unipolr drive, nd the whole winding i ued in the bipolr drive. Figure 10: Sttor winding connection of two drive configurtion: () unipolr drive, nd (b) bipolr drive. 4. AC Induction Motor An AC motor i n electric motor tht i driven by n lternting current. An AC motor conit of two bic prt: (1) n outide ttionry ttor hving coil upplied with AC current to produce rotting mgnetic field, nd () n inide rotor ttched to the output hft tht i given torque by the rotting field. The number of phe of the motor i determined by the number of independent winding connected to eprte AC line phe. Number of motor pole refer to the number of electromgnetic pole generted by the winding. Typicl number of pole re P=, 4, or 6, hown in Figure 10. The coil wire for ech phe cn be ditributed over the periphery of the ttor to hpe the mgnetic flux ditribution. Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 1
Figure 10: Sttor winding of AC induction motor: () two pole, (b) 4 pole, nd (c) 6 pole. In n AC induction motor, the current in the ttor generte mgnetic field which induce current in the rotor conductor. Thi induction i reult of reltive motion between ttor mgnetic field (rotting electriclly due to AC current) nd the rotor conductor (which i initilly ttionry). Sttor AC current et up rotting flux field. The chnging mgnetic field induced emf voltge, hence current, in the rotor conductor. The induced current in the rotor in turn generte it own mgnetic field. The interction of the two mgnetic field (the mgnetic field of the rotor trying to keep up with the mgnetic field of the ttor) generte the torque on the rotor. When the rotor peed i identicl to the electricl rottion peed of ttor field, there i no induced voltge on the rotor, nd hence the generted torque i zero. Thi i the min operting principle of n AC induction motor. Deprtment of Mechnicl Engineering, Univerity Of Sktchewn, 57 Cmpu Drive, Sktoon, S S7N 5A9, Cnd 13