APPLICATION NOTE

AN235/0788

STEPPER MOTOR DRIVINGBy H. SAX

From a circuit designer’s point of view stepper mo-tors can be divided into two basic types : unipolarand bipolar.

A stepper motor moves onestep when the directionof current flow in the field coil(s) changes, reversingthe magnetic field of the statorpoles. The differencebetweenunipolar 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 Driving.

UNIPOLAR

a)

b)

BIPOLAR

Dedicated integrated circuits have dramatically simplified stepper motor driving. To apply these ICs desi-gners need little specific knowledgeof motor driving techniques,but an under-standingof the basics will helpin finding the best solution. This note explains the basics of stepper motor driving and describes the drivetechniquesused today.

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The advantage of the bipolar circuit is that there isonly one winding, with a good bulk factor (low win-ding resistance). The main disapuantages are thetwochangeoverswitches because in this casemoresemiconductorsare needed.

The unipolar circuit needs only one changeoverswitch. Its enormousdisadvantageis, 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 implemen-ted with discrete devices. However with the integra-ted circuits available today bipolar motors can bedriver with no more components than the unipolarmotors. Figure 2 compares integratedunipolar andbipolar devices.

BIPOLAR PRODUCES MORE TORQUE

Thetorqueof thesteppermotor isproportionaltothemagneticfield intensity of thestator windings. It maybe increased only by addingmore windings or by in-creasing the current.

A natural limit against any current increase is thedangerof 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 oneadvantageof 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. 3)than unipolar motors built on the same frame.

If a higher torque is not required, one may either re-duce the motor size or the power loss.

Figure 3 : Bipolar Motors Driver Deliver MoreTorque than Unipolars.

CONSTANT CURRENT DRIVING

In order to keep the motor’s power loss within a rea-sonable limit, the current in the windings must becontrolled.

A simple andpopularsolutionis to give onlyas 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 independencefrom the winding resistance.The 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 differences.

Figure 4 : ResistanceCurrent Limiter (a) andCur-rent GeneratorLimiting.

Figure 5 : At High Step Frequencies the WindingCurrent cannot Reach its SettingValuebecause of the Continuous DirectionChange.

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It has already beenmentioned that thispower of themotor is, among others, proportional to the windingcurrent.

In the dynamicworking order a steppermotor chan-ges 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 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 valueVL/RL beforethe 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 increasingnumber of revolutions (fig.5).

MORE TORQUE AT A HIGHER NUMBEROF REVOLUTIONS

Higher torque at faster speeds are possible if a cur-rent generator as shown in Fig. 4b is used. In thisapplicationthesupplyvoltage is chosen ashighpos-sible 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 main-tained in the winding for a longer time. The torquedecrease starts only at much higher speeds.

Fig. 7 shows the relation between torqueand speedin the normalgraphicscheme, 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.

EFFICIENCY - THE DECISIVE FACTOR

The current generator combined with the high sup-ply voltageguaranteesthat the rate of changeof thecurrent in the coil is sufficiently high.

At the static condition or at low numbers of revolu-tions, however, this meansthat 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 supply

voltage 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 again.

Figure 6 : With a Step Current Slew, it is not aProblem to Obtain, even at High Stepfrequencies Sufficient Current in Wind-ings.

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-

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ration loss of the switch and that of the coil resistan-ce, the total efficiency is very high.

The average current that flows from the power sup-ply line is less than the winding current due to theconcept of circuit inversion. 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 energy.

POSSIBLE IMPROVEMENTS OF THE UNI-POLAR CIRCUIT

It would make no sense to apply the same principleto a stabilized current controlled unipolar circuit, astwo more switches per phase would be necessaryfor theshorteningout of the windings during the freephase and thus the number of components wouldbe thesame asfor thebipolarcircuit ; and moreover,there would be the well known torquedisadvantage.

From the economic point of view a reasonable andjustifiable improvement is the ”Bi-Level-Drive”(fig. 9). This circuit conceptworks with two supplyvol-tages ; with every new step of the motor both win-dings 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-de-termined the switch opens, a no a lower supply volt-age is connected to the winding thru a diode.

This kind of circuit by no means reaches the perfor-manceof the clocked stabilizedpowercontrolas 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 RegulationEfficiency is Increased.

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Figure 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 opera-ting 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 for-ced to operate at half-step conditions in order toavoid that operationsare disturbed by the motor re-sonance.Thesemay beso strongthat themotorhasno more torque in certainstep frequencyranges 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 deadenthissystem,butonlyif there is sufficient frictional force.

Inmost caseshalf-stepoperationhelps,as thecour-se covered by the rotor is only half as long and thesystem is less stimulated.

The 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 ; 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 hasno More Torque in Cer-tain Step Frequency Ranges with FullStep Driving.

For thisreasonmanysystemsusethehalf-stepope-ration only if the clock-frequency of the motor is wi-thin the resonance risk area.

The dynamic loss is higher the nearer the load mo-ment comes to the limit torque of the motor. This ef-fect decreases at higher numbers of revolutions.

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TORQUE LOSS COMPENSATION IN THEHALF-STEP OPERATION

It’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 becauseof some in-sufficient torquepercentages,it will greatly influencethe costs of the whole system.

In this case, there is analternativesolution that doesnot increase the coats for the bipolar chopping sta-bilized 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 rea-ched,becausethis limit refers always to the contem-porary 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 be-cause the non-active phase does not dissipate po-wer.

The resulting torque in the half-step positionamounts to about 90 % of that of the full-step, thatmeans dynamically more than 95 % torque compa-red 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 itwouldbelike a winding thermalphaseoverloadcon-centrated in one.

The best switch-technique for the half-step phasecurrent increase will be explained in detail later onFig. 11 shows the phase current of a steppingmotorin half-stepcontrolwith an without phase current in-crease and the pertinent curves of stap frequencyand torque.

Figure 11 : Half Step Driving with Shaping Allowsto Increase the Motor’s Torque toabout 95 % of that of the Full Step.

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Figure 12 : Only Two Signals for Full Step Driving are Necessary while Four (six if three-state is neededon the output stages) for half Step.

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DRIVE SIGNALS FOR THE MICRO ELEC-TRONIC

A direct current motor runs by itself if you supply ifwith voltage, whereas the stepping motor needsthecommutation signal in for of several separated butlinkable commands. In 95 % of the applications to-day, the origin of these digital commands is a micro-processor system.

In its simplest form,a full-stepcontrolneedsonly tworectangular signals in quadrature. According towhich phase is leading, the motor axis rotatesclock-wise or counter-clockwise, whereby the rotationspeed is proportional to the clock frequency.

In the half-step system the situation becomes morecomplicated. The minimal two control signals beco-me four control signals. In some conditions as manyas six signals are needed. If the Tri-state-commandfor the phase ranges without current, necessary forhigh motor speeds, may not be obtained from the 4control signals. Fig. 12 shows the relationship be-tween the phase current diagram and the control si-gnal for full and half-step.

Since all signals in each mode are in defined rela-tions with each other, it is possible to generate themusing standard logic. However, if the possibility to

choose full and half-step is desired, a good logic im-plementationbecomes quite expensive and an ap-plication specific integrated circuit would be better.Such an application specific integratedcircuit couldreducethe number ofoutputsrequired from a micro-processor from the 6 required to 3 static and dyna-mic control line.

A typical control circuit that meets all these require-ments is the L297 unit (fig. 13).

Four signals control the motor in all operations :

1. CLOCK : The clock signal, giving the step-ping command

2. RESET : Puts the final level signals in a de-fined start position

3. DIRECTION :Determinesthe sense of rotationofthe motor axis.

4. HALF/FULL : Desides whether to operate in fullor in half-step.

Another inhibit input allows the device to switch themotor output into the Tri-state-mode in order to pre-vent undesiredmovementsduringundefinedopera-ting conditions, such as those that could occurduring.

Figure 13 : The L297 avoids the Use of Complicated Standard Logic to Generate Both Full and Half-stepDriving Signals Together with Chopper Current Control.

RS1 RS2 = 0.5 ΩVF ≤ 1.2 V @ i = 2A

D1 to D8 = 2 A fast diodes trr ≤ 200 ns

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SWITCH-MODE CURRENT REGULATION

The primary functionof the current regulationcircuitis to supply enough current to the phase windingsof the motor, even at high step rates.

The functionalblocks required fora switchmodecur-rent control are the same blocks required in swit-ching power supplies ; flip-flops, comparators ; andan oscillator are required. These blocks can easilybe includedin the same IC that generatesthephasecontrol signals. Let us consider the implementationof chopper current control in the L297.

The oscillator on pin 16 of the L297 resets the twoflip-flops at the start of each oscillator period. Theflip-flop outputs are then combined with the outputsof the translator circuit to form the 6 control signalssupplied to the power bridge (L298).

When activated, by the oscillator, the current in thewinding will raise, following the L/R time constantcurve,untilthe voltage acrossthe senseresistor (pin1, 15 of L298) is equal to the reference voltage input(pin 15, L297) the comparator thensets the flip-flop,causing the outputof theL297to changeto an equi-phase condition, thus effectively putting a short cir-cuit across the phase winding. The bridge isactivated into a diagonally conductive state whenthe oscillator resets the flip-flop at the start of thenext cycle.

Using a common oscillator to controlbothcurrent re-gulators maintainsthe samechoppingfrequencyforboth, thus avoiding interferencebetween the two.

The functionalblock diagramof theL297and thepo-wer stage (L298) are shown in Figure 14 alone withthe operating wave forms.

An importantcharacteristicsof thiscircuit implemen-tation is that, during the reset time, the flip-flops arekept reset. The reset time can be selectedby selec-ting the impedance of the R/C network or pin 16. Inthis way, thecurrent spike andnoiseacross the sen-

se resistors that may occur during switching will notcause a premature setting of the flip-flop. Thus therecovery current spike of the protection diodes canbe ignored and a filter in the sense line is avoided.

THE RIGHT PHASE CURRENT FOR EVE-RY OPERATING CONDITIONThe Chopper principle of the controller unit revealsthat the phase current in the motor windings is con-trolled by two data : the reference voltage at pin 15of the controllerand the valueof thesenseresis-tan-ce at pins 1 and 15 of the L298, that is IL = VREF/RS.By changing VREF it is very easy to vary the currentwithin large limits. Theonlyquestionis for whichpur-pose and at which conditions.

More phase current means more motor torque, butalso higher energy consumption.

An analysis of the torque consumption for differentperiods and load position changes shows that thereis no need for differentenergies.

There is a high energyneed during the accelerationor breakphases,whereasduring continuous opera-tion or neutral or stop position less energy has to besupplied. A motor with its phase current continuou-sly oriented at the load moment limit, even with theloadmoment lacking,consumesneedlesslyenergy,that is completely transformed into heat.

Therefore it is useful to resolve the phase current inat least two levels controllable from the processor.Fig. 18 shows a minimal configuration with two re-sistance and one small signal transistor as change-over switch for the reference input. With anotherresistance and transistor it is possible to resolve 2Bits and consequently4 levels. That is sufficient forall imaginable causes.Fig. 16 shows a optimal phase current diagram du-ring a positioning operation.

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Figure 14 : Two ICs andvery Few ExternalComponentsProvide Complete Microprocessor to Bipolar Step-per Motor Interface.

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Figure 15 : Because of the Set-dominant Latch Inside the L297 it is Possible to Hide Current Spikes andNoise Across the Sense Resistors thus Avoiding External Filters.

Figure 16 : More Energy is needed During The Acceleration and break Phases Compared the ContinuousOperation,Neutral or Stop Position.

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HIGH MOTOR CLOCK RESETS IN THEHALF-STEP SYSTEM

In the half-steppositiononeof the motorphaseshasto be without current. If the motor moves from a full-step position into a half-step position, this meansthat one motor winding has to be completely di-scharged.From the logic diagramthis means for thehigh level bridgean equivalent statusof the input si-gnals A/B, for example in the HIGH-status. For thecoil this means short circuit (fig. 17 up) and conse-quentlya low reductionof the current. Incaseofhighhalf-step speeds the short circuit discharge timeconstant of the phase winding is not sufficient to di-scharge the current during the short half-step pha-

ses. The current diagram is not neat, the half stepis not carried out correctly (fig. 17 center).

For this reason the L297 controller-unit generatesan inhibit-command for each phase bridge, thatswitches the specific bridge output in the half-stepposition into Tri-state. In this way the coil can startswinging freely over the external recovery diodesand discharge quickly. The current decrease rate ofchange corresponds more or less to the increaserate of change (fig. 17 below).

In case of full-step operation both inhibit-outputs ofthe controller (pin 5 and 8) remain in the HIGH-sta-tus.

Figure 17 : The Inhibit Signal Turns Off Immediately the Output StagesAllowing thus a Faster Current De-cay (mandatory with half-step operation).

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Figure 18 : With This Configuration it is Possible to Obtain Half-step with ShapingOperationand ThereforeMore Torque.

MORE TORQUE IN THE HALF-STEP POSI-TION

A topic that has already been discussed in detail.Sowe will limit our considerations on how it is carriedout, in fact quite simply because of the referencevoltage controlled phase current regulation.

With the help of the inhibit-signals at outputs5 and8 of the controller, whichare alternativelyactiveonlywhen the half-step control is programmed, the ref-erence voltage is increased by the factor 1.41 witha very simple additional wiring (fig. 18), as soon asone of the two inhibit-signals switches LOW. This in-creases the current in the active motorphase pro-portionally to the reference voltage andcompensatesthe torque loss in thisposition.Fig. 19shows clearly that the diagram of the phasecurrentis almost sinusoidal, in principle the ideal form of thecurrent graph.

To sum up we may say that this half-step version of-fers most advantages. The motor works with poorresonance and a double position resolution at atorque, that is almost the same as that of the full-step.

BETTER GLIDING THAN STEPPING

If astepper motor is supposedto workalmost glidingand not step by step, the form of the phase currentdiagram has to be sinusoidal.

The advantages are very important :

- no more phenomena of resonance

- drastic noise reduction

- connected gearings and loads are treatedwith care

- the position resolution may be increased fur-ther.

However, the use of the L297 controller-unit descri-bed until now is no longer possible of themore sem-plicated form of the phase current diagram theController may become simpler in its functions.

Fig. 20 shows us an example with the L6505 unit.This IC containsnothingmore than theclockedpha-se current regulation which works according to thesame principle as L297. The four control signalsemitting continuously a full-step program are nowgenerateddirectlyby themicroprocessor. Inorder toobtain a sinusoidal phase current course the refer-ence voltage inputs of the Controller are modulatedwith sinusoidal half-waves.

The microprocessor that controls thedirection of thecurrent phase with the control signals also genera-tes the two analog signals.

For many applications a microprocessor with dedi-cated digital to analog converters can be chosen.Eliminating the need for separate D/A circuits.

About5 bit have proved to be the most suitablesud-

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Figure 19 : The Half-step with Shaping Positioning is Achieved by Simply Changing Reference Voltages.

division of the current within one full-step. A higherresolution brings no measurable advantages. Onthe contrary, the converter clock frequency is alrea-dy very high in case of low motor revolutions andvery difficult to process by the processor-software.It is recommended to reduce the D/A resolution athigh step frequencies.

In case of higher motor revolutions it ismore conve-nient to operate only in full-step, since harmoniccontrol is no longer an advantageas thecurrent hasonly a triangular waveform in the motor winding.

PRECISION OF THE MICRO STEP

Any desired increase of the position resolution be-tween the full step position has its physical limits.Those who think it is possible to resolve a 7.2° -

stepper motor to 1.8°with the same precision as a1.8° - motor in full-step will be received,as there areseveral limits :The rise rate of the torque diagram correspondingto the twisting angle of the rotor for the 7.2° - motoris flatter by a factor of 4 then for the original 1.8° -motor. Consequently with friction or load moment,the position error is larger (fig. 21).For most of the commercial motors there isn’t a suf-ficiently precise, linear relationship betweena sinu-soidal-current-diagram and an exact micro stepangle. The reason is a dishomogeneousmagneticfield between the rotor and the two stator fields.Above all, problems have to be expected with mo-tors with high pole feeling. However, there are spe-cial steppermotors in which anoptimizedmicro stepoperation has already been considered during theconstructionphase.

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Figure 20 : L6506Unit Gives The Possibility to Modulate Separately the Two Reference Voltage Inputs inOrder to obtain a Sinusoidal Phase Current.

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Figure 21 : Better Resolution is achieved with Low Degree Motor but More torque is delivered with highDegree Motor.

CONCLUSIONS

The above described application examples of mo-dern integrated circuits show that output and effi-ciency of stepper motors may be remarkablyincreased without any excessive expense increaselike before.

Working in limit areas, where improved electronicswith optimized drive sequencesallow theuse of lessexpensivemotors, it is evenpossible to obtaina costreduction.

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Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility forthe consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from itsuse. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifica-tions mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information pre-viously supplied. SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices orsystems without express written approval of SGS-THOMSON Microelectronics.

1995 SGS-THOMSON Microelectronics - All Rights Reserved

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