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 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 Driving.

UNIPOLAR

a)

b)

BIPOLAR

Dedicated 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. This note explains the basics of stepper motor driving and describes the drivetechniques used 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 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 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 integrated unipolar andbipolar devices.

BIPOLAR PRODUCES MORE TORQUE

The 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 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 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 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 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 : Resistance Current Limiter (a) and Cur-rent Generator Limiting.

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

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It has already been mentioned that this power of themotor is, among others, proportional to the windingcurrent.

In the dynamic working order a stepper motor 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 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 (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 thisapplication the supply voltage is chosen as high pos-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 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.

EFFICIENCY - THE DECISIVE FACTOR

The current generator combined with the high sup-ply voltage guarantees that the rate of change of thecurrent in the coil is sufficiently high.

At 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 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 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 vol-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-mance 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.

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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 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 force.

In most cases half-step operation helps, as the cour-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 has no More Torque in Cer-tain Step Frequency Ranges with FullStep Driving.

For this reason many systems use the half-step ope-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 because of some in-sufficient 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 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, because this 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 itwould be like a winding thermal phase overload con-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 stepping motorin half-step control with 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 needs thecommutation 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-step control needs only tworectangular signals in quadrature. According towhich phase is leading, the motor axis rotates clock-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-plementation becomes quite expensive and an ap-plication specific integrated circuit would be better.Such an application specific integrated circuit couldreduce the number of outputs required 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 : Determines the sense of rotation ofthe 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 undesired movements during undefined opera-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 function of the current regulation circuitis to supply enough current to the phase windingsof the motor, even at high step rates.

The functional blocks required for a switchmode cur-rent control are the same blocks required in swit-ching power supplies ; flip-flops, comparators ; andan oscillator are required. These blocks can easilybe included in the same IC that generates the phasecontrol 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, until the voltage across the sense resistor (pin1, 15 of L298) is equal to the reference voltage input(pin 15, L297) the comparator then sets the flip-flop,causing the output of the L297 to change to 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 control both current re-gulators maintains the same chopping frequency forboth, thus avoiding interference between the two.

The functional block diagram of the L297 and the po-wer stage (L298) are shown in Figure 14 alone withthe operating wave forms.

An important characteristics of this circuit implemen-tation is that, during the reset time, the flip-flops arekept reset. The reset time can be selected by selec-ting the impedance of the R/C network or pin 16. Inthis way, the current spike and noise across 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 controller and the value of the sense resis-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. The only question is for which pur-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 different energies.

There is a high energy need during the accelerationor break phases, whereas during 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 theload moment lacking, consumes needlessly energy,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 consequently 4 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 and very Few External Components Provide 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-step position one of the motor phases hasto 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 diagram this means for thehigh level bridge an equivalent status of the input si-gnals A/B, for example in the HIGH-status. For thecoil this means short circuit (fig. 17 up) and conse-quently a low reduction of the current. In case of highhalf-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 Stages Allowing 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 Shaping Operation and 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 outputs 5 and8 of the controller, which are alternatively active onlywhen 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 andcompensates the torque loss in this position. Fig. 19shows clearly that the diagram of the phase currentis 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 a stepper motor is supposed to work almost 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 the more 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 contains nothing more than the clocked pha-se current regulation which works according to thesame principle as L297. The four control signalsemitting continuously a full-step program are nowgenerated directly by the microprocessor. In order toobtain a sinusoidal phase current course the refer-ence voltage inputs of the Controller are modulatedwith sinusoidal half-waves.

The microprocessor that controls the direction 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.

About 5 bit have proved to be the most suitable sud-

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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 is more conve-nient to operate only in full-step, since harmoniccontrol is no longer an advantage as the current 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 between a sinu-soidal-current-diagram and an exact micro stepangle. The reason is a dishomogeneous magneticfield 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 stepper motors in which an optimized micro stepoperation has already been considered during theconstruction phase.

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Figure 20 : L6506 Unit 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.

CONCLUSIONSThe 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 sequences allow the use of lessexpensive motors, it is even possible to obtain a 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.

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