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STEPPER MOTOR DRIVING - University of Texas at Austin

APPLICATION NOTEAN235/0788 STEPPER MOTOR DRIVINGBy H. SAXFrom a circuit designer s point of view STEPPER mo-tors can be divided into two basic types : unipolarand STEPPER MOTOR moves one step when the directionof current flow in the field coil(s) changes, reversingthe magnetic field of the stator poles. The differencebetween unipolar and bipolar motors lies in the maythat this reversal is achieved (figure 1) :Figure 1a : BIPOLAR - with One Field Coil andTwo Chargeover Switches That areSwitched in the Opposite Direction. Figure 1b : UNIPOLAR - with Two Separate FieldCoils and are Chargeover Switch. Figure 2 : ICs for Unipolar and Bipolar )b)BIPOLARD edicated integrated circuits have dramatically simplified STEPPER MOTOR DRIVING . To apply these ICs desi-gners need little specific knowledge of MOTOR DRIVING techniques, but an under-standing of the basics will helpin finding the best solution.

STEPPER MOTOR DRIVING By H. SAX ... motors thus deliver about 40 % more torque (fig. 3) than unipolar motors built on the same frame. If a higher torque is not required, one may either re- ... In the dynamic working order a stepper motor chan-ges poles of the winding current in the same stator

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Transcription of STEPPER MOTOR DRIVING - University of Texas at Austin

1 APPLICATION NOTEAN235/0788 STEPPER MOTOR DRIVINGBy H. SAXFrom a circuit designer s point of view STEPPER mo-tors can be divided into two basic types : unipolarand STEPPER MOTOR moves one step when the directionof current flow in the field coil(s) changes, reversingthe magnetic field of the stator poles. The differencebetween unipolar and bipolar motors lies in the maythat this reversal is achieved (figure 1) :Figure 1a : BIPOLAR - with One Field Coil andTwo Chargeover Switches That areSwitched in the Opposite Direction. Figure 1b : UNIPOLAR - with Two Separate FieldCoils and are Chargeover Switch. Figure 2 : ICs for Unipolar and Bipolar )b)BIPOLARD edicated integrated circuits have dramatically simplified STEPPER MOTOR DRIVING . To apply these ICs desi-gners need little specific knowledge of MOTOR DRIVING techniques, but an under-standing of the basics will helpin finding the best solution.

2 This note explains the basics of STEPPER MOTOR DRIVING and describes the drivetechniques used advantage of the bipolar circuit is that there isonly one winding, with a good bulk factor (low win-ding resistance). The main disapuantages are thetwo changeover switches because in this case moresemiconductors are unipolar circuit needs only one changeoverswitch. Its enormous disadvantage is, however, thata double bifilar winding is required. This means thatat a specific bulk factor the wire is thinner and theresistance is much higher. We will discuss later theproblems motors are still popular today because thedrive circuit appears to be simpler when implemen-ted with discrete devices. However with the integra-ted circuits available today bipolar motors can bedriver with no more components than the unipolarmotors.

3 Figure 2 compares integrated unipolar andbipolar PRODUCES MORE TORQUEThe torque of the STEPPER MOTOR is proportional to themagnetic field intensity of the stator windings. It maybe increased only by adding more windings or by in-creasing the natural limit against any current increase is thedanger of saturating the iron core. Though this is ofminimal importance. Much more important is themaximum temperature rise of the MOTOR , due to thepower loss in the stator windings. This shows oneadvantage of the bipolar circuit, which, compared tounipolar systems, has only half of the copper resi-stance because of the double cross section of thewire. The winding current may be increased by thefactor 2 and this produces a direct proportional af-fect on the torque. At their power loss limit bipolarmotors thus deliver about 40 % more torque (fig.)

4 3)than unipolar motors built on the same a higher torque is not required, one may either re-duce the MOTOR size or the power 3 :Bipolar Motors Driver Deliver MoreTorque than CURRENT DRIVINGIn order to keep the MOTOR s power loss within a rea-sonable limit, the current in the windings must simple and popular solution is to give only as muchvoltage as needed, utilizing the resistance (RL) ofthe winding to limit the current (fig. 4a). A more com-plicated but also more efficient and precise solutionis the inclusion of a current generator (fig. 4b), toachieve independence from the winding supply voltage in Fig. 4b has to be higher thanthe one in Fig. 4a. A comparison between both cir-cuits in the dynamic load/ working order shows visi-ble 4 :Resistance Current Limiter (a) and Cur-rent Generator 5 :At High Step Frequencies the WindingCurrent cannot Reach its Setting Valuebecause of the Continuous NOTE2/17It has already been mentioned that this power of themotor is, among others, proportional to the the dynamic working order a STEPPER MOTOR chan-ges poles of the winding current in the same statorwinding after two steps.

5 The speed with which thecurrent changes its direction in the form of an expo-nential function depends on the specified inductan-ce, the coil resistance and on the voltage. Fig. 5ashows that at a low step rate the winding current ILreaches its nominal value VL/RL before the directionis changed. However, if the poles of the stator win-dings are changed more often, which correspondsto a high step frequency, the current no longer rea-ches its saturating value because of the limitedchange time ; the power and also the torque dimi-nish clearly at increasing number of revolutions ( ).MORE TORQUE AT A HIGHER NUMBEROF REVOLUTIONSH igher torque at faster speeds are possible if a cur-rent generator as shown in Fig. 4b is used. In thisapplication the supply voltage is chosen as high pos-sible to increase the current s rate of change.

6 Thecurrent generator itself limits only the phase currentand becomes active only the moment in which thecoil current has reached its set nominal value. Up tothis value the current generator is in saturation andthe supply voltage is applied directly to the 6, shows that the rate of the current increase isnow much higher than in Figure 5. Consequently athigher step rates the desired current can be main-tained in the winding for a longer time. The torquedecrease starts only at much higher 7 shows the relation between torque and speedin the normal graphic scheme, typical for the steppermotor. It is obvious that the power increases in theupper torque range where it is normally needed, asthe load to be driven draws most energy from themotor in this - THE DECISIVE FACTORThe current generator combined with the high sup-ply voltage guarantees that the rate of change of thecurrent in the coil is sufficiently the static condition or at low numbers of revolu-tions, however, this means that the power loss in thecurrent generator dramatically increases, althoughthe MOTOR does not deliver any more energy in thisrange ; the efficiency factor is extremely comes from a switched current regulationusing the switch-transformer principle, as shown infig.

7 8. The phase winding is switched to the supplyvoltage until the current, detected across RS, rea-ches the desired nominal value. At that moment theswitch, formerly connected to + VS, changes posi-tion and shorts out the winding. In this way the cur-rent is stored, but it decays slowly because of innerwinding losses. The discharge time of the current isdetermined during this phase by a monostable orpulse oscillator. After this time one of the pole chan-ging switches changes back to + VS, starting an in-duction recharge and the clock-regulation-cyclestarts 6 :With a Step Current Slew, it is not aProblem to Obtain, even at High Stepfrequencies Sufficient Current in 7 : Constant Current Control of the Step-per MOTOR Means more Torque at the only losses in this technique are the satu-APPLICATION NOTE3/17ration loss of the switch and that of the coil resistan-ce, the total efficiency is very average current that flows from the power sup-ply line is less than the winding current due to theconcept of circuit inversion.

8 In this way also the po-wer unit is discharged. This king of phase currentcontrol that has to be done separately for each mo-tor phase leads to the best ratio between the sup-plied electrical and delivered mechanical IMPROVEMENTS OF THE UNI-POLAR CIRCUITIt would make no sense to apply the same principleto a stabilized current controlled unipolar circuit, astwo more switches per phase would be necessaryfor the shortening out of the windings during the freephase and thus the number of components wouldbe the same as for the bipolar circuit ; and moreover,there would be the well known torque the economic point of view a reasonable andjustifiable improvement is the "Bi-Level-Drive"(fig. 9). This circuit concept works with two supply vol-tages ; with every new step of the MOTOR both win-dings are connected for a short time to a high supplyvoltage.

9 This considerably increases the current rateof change and its behaviour corresponds more orless to the stabilized power principle. After a pre-de-termined the switch opens, a no a lower supply volt-age is connected to the winding thru a kind of circuit by no means reaches the perfor-mance of the clocked stabilized power control as perfig. 8, as the factors : distribution voltage oscillation, , thermal winding resistance, as well as theseparate coil current regulation are not considered,but it is this circuit that makes the simple unipolarR/L-control suitable for many fields of 8 : With Switch Mode Current Regulation Efficiency is NOTE4/17 Figure 9 :At Every New Step of the MOTOR , it is Possible to Increase the Current Rate with a Bilevel AND DISADVANTAGES OFTHE HALF-STEPAn essential advantage of a STEPPER MOTOR opera-ting at half-step conditions is its position resolutionincreased by the factor 2.

10 From a degree motoryou achieve degrees, which means 200 stepsper is not always the only reason. Often you are for-ced to operate at half-step conditions in order toavoid that operations are disturbed by the MOTOR re-sonance. These may be so strong that the MOTOR hasno more torque in certain step frequency ranges andlooses completely its position (fig. 10). This is dueto the fact that the rotor of the MOTOR , and the chan-ging magnetic field of the stator forms a spring-mass-system that may be stimulated to vibrate. Inpractice, the load might deaden this system, but onlyif there is sufficient frictional most cases half-step operation helps, as the cour-se covered by the rotor is only half as long and thesystem is less fact that the half-step operation is not the domi-nating or general solution, depends on certain di-sadvantages :-the half-step system needs twice as manyclock-pulses as the full-step system.


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