E.M.F. Sensing Controlled Variable Speed Drive System of a Linear Stepper Motor Loránd SZABÓ oan-adrian VOREL Zoltán KOVÁCS Technical University of Cluj-Napoca Department of Electrical Engineering P.O. Box 358, 3400 CLUJ NAPOCA, Romania Abstract: The application of variable speed linear drive systems using hybrid linear stepper motors seems to be one of the best solutions for industrial linear movements. An improved drive control strategy is proposed, based on the E.M.F. sensing of the un-energized motor coil. A typical positioning task is simulated by means of a coupled circuit field mathematical model. Cite as: Szabó, L. Viorel,.A. Kovács, Z.: E.M.F. Sensing Controlled Variable Speed Drive System of a Linear Stepper Motor, Proceedings of the Power Electronics, Motion Control Conference (PEMC '94), Warsaw, (Poland), pp. 366-371, 1994. See attached the scan of the paper REFERENCES 1. SZABO, L. - VOREL,.A. - KOVACS, Z.: Computer Simulation of a Closed-Loop Linear Positioning System, Proceedings of the Power Conversion & ntelligent Motion Conference (PCM '93), Nürnberg (Germany), vol. ntelligent Motion, pp. 142-151, 1993. 2. SZABO, L. - VOREL,.A. - KOVACS, Z.: Variable Speed Conveyer System Using E.M.F. Sensing Controlled Linear Stepper Motor, Proceedings of the nternational Conference on Power Conversion & ntelligent Motion (PCM '94), Nürnberg (Germany), vol. ntelligent Motion, pp. 183-190, 1994. 3. VOREL,.A. KOVACS, Z. SZABO, L.: Sawyer Type Linear Motor Modelling, Proceedings of the nternational Conference on Electrical Machines (CEM '92), Manchester (UK), pp. 697-701, 1992. 4. VOREL,.A. - KOVACS, Z. - SZABO, L.: Dynamic Modelling of a Closed-Loop Drive System of a Sawyer Type Linear Motor, Proceedings of the Power Conversion & ntelligent Motion Conference (PCM '92), Nürnberg (Germany), vol. ntelligent Motion, pp. 251-257, 1992. 5. VOREL,.A. - SZABO, L. - KOVACS, Z.: Quadrature Field-Oriented Control of a Linear Stepper Motor, Proceedings of the Power Conversion & ntelligent Motion Conference (PCM '93), Nürnberg (Germany), vol. ntelligent Motion, pp. 64-73, 1993.
PEMC'94 CONFERENCE PUBLCATON WE-ST - EAST TECHNOLOGY BRDGE NTERNATONAL CONFERENCE ON POWER ELECTRONCS, MOTON CONTROL AND ASSOCATED APPLCATONS 20-22 September 1994, Warsaw, Poland Volume EDTORS: R.BARLK K.DUSZCZYK M.KAZMERKOWSK W.KOCZARA Published by NSTTUTE OF CONTROL AND NDUSTRAL ELECTRONCS WARSAW UNVERSTY OF TECHNOLOGY ul. Koszykowa 75, 00-662 WARSZAWA, POLAND FAX: +48 2 6256633, TEL: +48 22 294991, E-MAil: PEMC@NOV.lSEP.PW.EDU.Pl
PEMC'94 o 20-22 SEP'TEMBE" 1104 WARSAW.,"OLANO <- E.M.F. SENSNG CONTROLLED VARABLE SPEED DRVE SYSTEM A LNEAR STEPPER MOTOR L. SZABO,.A. VoREL, Z. KOVACS Technical University of Cluj Department of Electrical Engineering P.O. Box. 358., 3400-CLUJ, Romania Abstract. The appl ica tion of variable speed lin ear drivl systems using hybrid linear stepper motors seems to one of the best solutions for industrial linear movements. An improved drive control strategy is proposed, based on the E.M.F. sensing of t un-energized motor coi 1. A typical posi tioning task 11 simulated by means of a coupled circuit-fielf, mathematical model. Keywords. hybrid linear stepper motor, E.M.F. detection dynamic simulation m p r P NTRODUCTON n automatized factories many movements have to be performed linearly. The application of variable speed linear drive systems using outer magnet type hybrid linear stepper motors seems to be one of the best solutions for these purposes. They have the capability of carrying products at variable speed with high positioning accuracy. n open-loop drive mode the operating frequency of the coi 1 current pulses is given by an external source. f the load is varying the motor may demand acceleration rates which exceed the step capability 01 the motor, resulting in dynamic instabilities and loss of synchronism between the motor position and excitation changes, and in amplifying the vibrations of the mover. The total positioning capabilities and dynamic performances of the motor can be improved by operating under closed-loop control in conjunction with position sensing feedback. All these sensors are expensive, unreliable and need suppleme nter electronic circuits. All thesl disadvantages may be eliminated' utilizing a control method based on the E.M.F. (electrical moti for c e ) sen sin g 0 f t 'll un-energized coil of the which will allows the speed to vary wi th the load. n this case the operati"9 frequency will depend only the capabi 1 i ty of the motor rea 1 i ze a step under conditions as load and power. THE HYBRD LNEAR STEPPER MOTOR The hy br id linear 5 motor, shown in figure 1., variable reluctance, perman magnet e)(ci ted synchron motor. t is operated under combined pr inci pies of vari reluctance motors (tending C( W' or 0:> NSTTUTE OF CONTROL AND NDUSTRAL ElECTRONCS 366
5 N Figure 1 :ed i are!c se ted,sed live,,,,t he 'or, tor n!ing on j to ven,put per s a ent nus the ble to to wards the aligned teeth in which magnetic is minimum) and of permanent magnet motors (having life-long excitation) [3]. The moveable armature consists of two electromagnets.. ith coi s and a perman en t magnet their tqp. Each electromagnet, aade of insulated low-loss lolmination, has two poles, and all poles have the same number of teeth. A command coil is placed on each pole. The back iron closes the motor magneti c. circui t. The suspended over a fixed stator (the platen), a toothed ferromagneti c structure having the same fine teeth pi tch wi th the.oveable ar.mature. The hybrid linear stepper eotor presen ts high track ing force to volume performance, mainly because of the small air gap and good magnetic circuit utilization. t has the ability to hold fix position under applied load. The motor is characterized by high servo stiffness which is essential for quick move-and-settle appl ica tions. THE CONTROL SYSTEM The direct-tim,,", controller of the varioible speed linear drive system (shown in fig ure 2.) is microprocessor based, having two hysteresis current controlled voltage source, full-h bri dge PWM inverters. The proposed control method is based on the quadrature field-or ien ted con trol method described in detail in [5]. As the flux linkage through the un-energi zed coi of the motor is on y function of the mover position and speed it is independent of the phase currents or of any other circuit parameters. The commutation of the command current from one to another coil is determined by the value of the detected back EMF divided by the actual mover speed. The command currents must be commuted before the mover is reaching position minimize ripple. its equilibrium [2J in order to the tangential force The currents of the command coils are controlled instantaneously. The imposed phase currents are prescribed by the controller in such a way as the resu ting, load f uctua tion independent speed keeps on the imposed velocity profile best suited for the movement of specific load. The velocity f"",,,",dback loop is c l osed by 367
A(... e1 '1 f- D PWM speed M ref". nverter ret". f- 0 MOTON T >09 1 t.1 on p CONTROL 0 12 ret". D f- ref". l- inverter PWM R Ftcce. met.ar e ;) )( S v 1 S eo Figure 2 sensing ad integrating the acceleration of the mover obtained by a piezoelectrical accelerometer disposed on the moveable armature. DYNAMC SMULATON OF THE CONTROL SYSTEM The main reason for simu a ting any sys tem through software is to verify that the design concept is optimal and provides the best balance of price and per forman ce. The compu ter simulation of the above mentioned variable speed drive system, performed by a combined circuit-field mathematical model, offers the possibility to calculate the motor parameters, and is an accurate tool for designers. The dynamic behavior of the inear hybrid stepper motor can no t be covered accura te 1 y by an usual mathematical model bec::ause of the complex toothed configuration of the motor armatures, the magnetic saturation of the iron parts and the permanen t magnet operating point changes due to air-gap variable reluctance and control ampert urns. Th e coupled circuit-field model was described in detai 1 in several papers, [1 J and [4 J. t consists of the following three main parts (see figure 3.): 1. circuit submodel where the coils currents are computed. The imposed curren t waveform is given by a velocity controller in function of the actua l and imposed speed of the mover. At a certain time value, with the constant input voltages and imposed curren t wave forms given, the control currents are calculated by sol ving the usual differential equation for each phase, tak ing in to accoun t the modific::ations of the coils inductances wi th the variations of the magnetic fluxes through the c::oi 1 s. The back EMF generated in the un-energized 368
r---------------------------,---------------------------------t---------------------------------, l CRCUT! FELD! MECHANCAL SlJE.«JDEL SUEoVOEL SUEoVDEL, r.' uc'tanc:e )( modl1'c:at:.lon Rm '" V orc. v.foelt.y dl&ilpla..oenwnt. c-'..otrplrtn1' on ccrtpllte:t 'on C:Cl'tlon COl"pllt.ot Jon )(, l JL J Figure 3 l- coi 1 is, simu 1 taneous 1 y, compu ted. 2. field submodel where magnetic field is computed. The t ield prob 1 em has been reduced to a computationally simple, analytical model based on the solving of the non 1 inear equivalent magnetic circuit. A numerical method, such as finite elements or finite differences method, shou 1 d give more accuracy, but, because it needs longer computer time, it is useless in a dyna mic simulation problem. The magnetic reluctances are computed tak ing fu 11 y in to accoun t the modifications of the operating points on the magnetization, r e spe c t i v e l y o n t h e demagnetization curves. 3. mechanical submodel where the position of the moveable armature is computed. The total normal and tangential forces developed by the motor are computed at every ti me ite ration Vld the alr-gap magnetic energy. n add i tion to the weight of the mover and the friction force they form a simply force structure, resulting the acceleration of the mover. The simultaneous solution of the mechanical equation def ines the speed and the resultant displacement of the motor at each time interval. The velocity controller compares the imposed speed in accord with the preselected velocity profile. and the measured actual speed of the mover. The detected error signal is the determined and transferred to circuit submodel. The combined field-circuit model is conceived to be solved by means of compu ter, and the computational process consists of an iterative' calculation. The computer program based on the proposed model offers the possibility to calculate all the motor charac ter i s ti cs (such as forces, acceleration, velocity, displacement, back E.M.F. etc.). The model is well-suited for the simulation of both static and dynamic behavior of the 369
'4(. described variable speed conveyer system.. The digital simulation is an accurate tool for designers because they can try.out many different control a i gorithm without prototyping hardware. RESULTS AND CONCLUSONS The hybrid linear stepper motor of the variable speed drive syst under study has four poles, 5 teeth on each pole. The tooth pitch is of 0.002 m and the tooth length is of 0.001 - m. The permanent magnet of the mover is of VACOMAX-145 type with residual flux density 0.9 T and coercive force 650 ka/m. n figure 4. are presented some results of dynamic simulation of the variable speed drive system controlled by the above presented E.M.F. detetion based method: the velocity, the command current in coil 1., the tangential force, the acceleration and the mover displacement versus time. The simulated task for the system was the following: the motor moves to the left with no-load at a speed of O.Bm/s. t stays stopped 20ms. After this the motor is moved at a lower (0.5m/s) speed to ttie right, back to the in i tia 1 posi tion, having a O. 5kg load. Trapezoidal velocity profiles were adopted in both cases. The maximum slew speed is imposed by the needs of the posi tioning system. The maximum motor thrust force and the load mass def ines the maximum acceleration of the mover. The acceleration and deceleration times are taken equals. The command current has grea t va 1 ues during the two accelerations. The resulting thrust is high and the acceleration is relatively cons tan t during the accel era t ions. When the motor is moving at the slew speed the tangential force is constant and the acceleration is near ni. n order to decelerate the motor opposite tangen tia 1 forces are appl ied. Du ing the movements in both directions the displacement is near linear. By evaluating the results obtained on computer model it can be seen that the imposed task was success fu 11 y fu 1 fill. t is quite clear that the current control loop based on the back E.M.F. detection of the un-energ i zed command coi and speed sensing assures high control accuracy and very good performances. REFERENCES 1. SZABO L. VOREL.A. KOVACS Z. : Computer Simulation of a Closed-Loop. Linear Posi tioning System, Proceedings of PCM - ntelligent Motion Conference 1993, pp. 142-151. 2. SZABO L. VOREL.A. KOVACS Z. : Variable Speed Conveyer System Using E.N.F. Sensing Controlled Linear Stepper Motor, to be presented at the PCM - ntelligent Motion Conference 1994. 3. VOREL.A. KOVACS SZABO L. Sawyer Type Linear Motor Modelling, Proceedings of the nternational Conference on Electrical Machines 1992, pp. 697-701. 4.. V OREL. A. KOVACS. SZABO L. : Dynamic Modell ing of C l osed -Loop Drive System of Sawyer Ty pe Linear Motor, Proceedings of PCM-ntelligent Motion Conference 1992, 257. 5. VOREL KOVACS 1. A. Z : Field-Oriented SZABO Control Linear S t e ppe r Poceedings of the ntell igent 1993, pp. 64-73. Motion pp. 251'". 370
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