stepper_motor_drive (Скамко)

APPLICATION NOTESTEPPER MOTOR DRIVINGBy H. SAXDedicated integrated circuits have dramatically simplified stepper motor driving. To apply these ICs designers need little specific knowledge of motor driving techniques, but an under-standing of the basics will helpin finding the best solution. This note explains the basics of stepper motor driving and describes the drivetechniques used today.From a circuit designer’s point of view stepper motors can be divided into two basic types : unipolarand bipolar.A 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 2 : ICs for Unipolar and Bipolar Driving.UNIPOLARFigure 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.a)BIPOLARb)AN235/07881/17APPLICATION NOTEThe advantage of the bipolar circuit is that there isonly one winding, with a good bulk factor (low winding resistance).

The main disapuantages are thetwo changeover switches because in this case moresemiconductors are needed.The 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 involved.Unipolar motors are still popular today because thedrive circuit appears to be simpler when implemented with discrete devices. However with the integrated circuits available today bipolar motors can bedriver with no more components than the unipolarmotors. Figure 2 compares integrated unipolar andbipolar devices.CONSTANT CURRENT DRIVINGIn order to keep the motor’s power loss within a reasonable limit, the current in the windings must becontrolled.A 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 complicated but also more efficient and precise solutionis the inclusion of a current generator (fig. 4b), toachieve independence from the winding resistance.The supply voltage in Fig. 4b has to be higher thanthe one in Fig. 4a. A comparison between both circuits in the dynamic load/working order shows visible differences.Figure 4 : Resistance Current Limiter (a) and Current Generator Limiting.BIPOLAR 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 increasing the current.A 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 resistance because of the double cross section of thewire. The winding current may be increased by thefactor √2 and this produces a direct proportional affect on the torque. At their power loss limit bipolarmotors 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 reduce the motor size or the power loss.Figure 3 : Bipolar Motors Driver Deliver MoreTorque than Unipolars.2/17Figure 5 : At High Step Frequencies the WindingCurrent cannot Reach its Setting Valuebecause of the Continuous DirectionChange.APPLICATION NOTEIt has already been mentioned that this power of themotor is, among others, proportional to the windingcurrent.In the dynamic working order a stepper motor changes poles of the winding current in the same statorwinding after two steps.

The speed with which thecurrent changes its direction in the form of an exponential function depends on the specified inductance, 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 windings are changed more often, which correspondsto a high step frequency, the current no longer reaches its saturating value because of the limitedchange time ; the power and also the torque diminish clearly at increasing number of revolutions (fig.5).MORE TORQUE AT A HIGHER NUMBEROF REVOLUTIONSHigher torque at faster speeds are possible if a current generator as shown in Fig. 4b is used.

In thisapplication the supply voltage is chosen as high possible to increase the current’s rate of change. 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 winding.Fig. 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 maintained in the winding for a longer time.

The torquedecrease starts only at much higher speeds.Fig. 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 range.voltage until the current, detected across RS, reaches the desired nominal value.

At that moment theswitch, formerly connected to + VS, changes position and shorts out the winding. In this way the current 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 changing switches changes back to + VS, starting an induction recharge and the clock-regulation-cyclestarts again.Figure 6 : With a Step Current Slew, it is not aProblem to Obtain, even at High Stepfrequencies Sufficient Current in Windings.Figure 7 : Constant Current Control of the Step-per Motor Means more Torque at HighFrequency.Since the only losses in this technique are the satu-EFFICIENCY - THE DECISIVE FACTORThe current generator combined with the high supply voltage guarantees that the rate of change of thecurrent in the coil is sufficiently high.At the static condition or at low numbers of revolutions, 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 bad.Help comes from a switched current regulationusing the switch-transformer principle, as shown infig.

8. The phase winding is switched to the supply3/17APPLICATION NOTEration loss of the switch and that of the coil resistance, the total efficiency is very high.The average current that flows from the power supply line is less than the winding current due to theconcept of circuit inversion. In this way also the power unit is discharged. This king of phase currentcontrol that has to be done separately for each motor phase leads to the best ratio between the supplied electrical and delivered mechanical energy.POSSIBLE IMPROVEMENTS OF THE UNIPOLAR 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 disadvantage.From the economic point of view a reasonable andjustifiable improvement is the "Bi-Level-Drive"(fig.

9). This circuit concept works with two supply voltages ; with every new step of the motor both windings are connected for a short time to a high supplyvoltage. This considerably increases the current rateof change and its behaviour corresponds more orless to the stabilized power principle. After a pre-determined the switch opens, a no a lower supply voltage is connected to the winding thru a diode.This kind of circuit by no means reaches the performance of the clocked stabilized power control as perfig.

8, as the factors : distribution voltage oscillation,B.e.m.f., 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 application.Figure 8 : With Switch Mode Current Regulation Efficiency is Increased.4/17APPLICATION NOTEFigure 9 : At Every New Step of the Motor, it is Possible to Increase the Current Rate with a Bilevel Circuit.ADVANTAGES AND DISADVANTAGES OFTHE HALF-STEPAn essential advantage of a stepper motor operating at half-step conditions is its position resolutionincreased by the factor 2. From a 3.6 degree motoryou achieve 1.8 degrees, which means 200 stepsper revolution.This is not always the only reason.

Often you are forced to operate at half-step conditions in order toavoid that operations are disturbed by the motor resonance. 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 changing magnetic field of the stator forms a springmass-system that may be stimulated to vibrate. Inpractice, the load might deaden this system, but onlyif there is sufficient frictional force.In most cases half-step operation helps, as the course covered by the rotor is only half as long and thesystem is less stimulated.The fact that the half-step operation is not the dominating or general solution, depends on certain disadvantages :-the half-step system needs twice as manyclock-pulses as the full-step system ; theclock-frequency is twice as high as with thefull-step.-in the half-step position the motor has onlyabout half of the torque of the full-step.Figure 10 :The Motor has no More Torque in Certain Step Frequency Ranges with FullStep Driving.For this reason many systems use the half-step operation only if the clock-frequency of the motor is within the resonance risk area.The dynamic loss is higher the nearer the load moment comes to the limit torque of the motor.

This effect decreases at higher numbers of revolutions.5/17APPLICATION NOTETORQUE LOSS COMPENSATION IN THEHALF-STEP OPERATIONIt’s clear that, especially in limit situations, the torqueloss in half-step is a disadvantage. If one has tochoose the next larger motor or one with a doubleresolution operating in full-step because of some insufficient torque percentages, it will greatly influencethe costs of the whole system.In this case, there is an alternative solution that doesnot increase the coats for the bipolar chopping stabilized current drive circuit.The torque loss in the half-step position may becompensated for by increasing the winding currentby the factor √2 in the phase winding that remainsactive. This is also permissible if, according to themotor data sheet, the current limit has been reached, because this limit refers always to the contemporary supply with current in both windings in thefull-step position.

The factor √2 increase in currentdoubles the stray power of the active phase. Thetoal dissipated power is like that of the full-step because the non-active phase does not dissipate power.The resulting torque in the half-step positionamounts to about 90 % of that of the full-step, thatmeans dynamically more than 95 % torque compared to the pure full-step ; a neglectable factor.The only thing to avoid is stopping the motor at limitcurrent conditions in a half-step position because itwould be like a winding thermal phase overload concentrated in one.The best switch-technique for the half-step phasecurrent increase will be explained in detail later onFig.