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application note AN235/0788 stepper motor driving by h. sax from a circuit designer's point of view stepper mo- tors can be divided into two basic types : unipolar and bipolar. a stepper motor moves one step when the direction of current flow in the field coil(s) changes, reversing the magnetic field of the stator poles. the difference between unipolar and bipolar motors lies in the may that this reversal is achieved (figure 1) : figure 1a : bipolar - with one field coil and two chargeover switches that are switched in the opposite direction. figure 1b : unipolar - with two separate field coils 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-standingof the basics will help in finding the best solution. this note explains the basics of stepper motor driving and describes the drive techniques used today. 1/17
the advantage of the bipolar circuit is that there is only one winding, with a good bulk factor (low win- ding resistance). the main disapuantages are the two changeover switches because in this casemore semiconductors are needed. the unipolar circuit needs only one changeover switch. its enormous disadvantageis, however, that a double bifilar winding is required. this means that at a specific bulk factor the wire is thinner and the resistance is much higher. we will discuss later the problems involved. unipolar motors are still popular today because the drive circuit appears to be simpler when implemen- ted with discrete devices. however with the integra- ted circuits available today bipolar motors can be driver with no more components than the unipolar motors. figure 2 compares integrated unipolar and bipolar devices. bipolar produces more torque thetorque of the stepper motor is proportionaltothe magneticfield intensity of the stator windings. it may be increased only by adding more windings or by in- creasing the current. a natural limit against any current increase is the danger of saturating the iron core. though this is of minimal importance. much more important is the maximum temperature rise of the motor, due to the power loss in the stator windings. this shows one advantageof the bipolar circuit, which, compared to unipolar systems, has only half of the copper resi- stance because of the double cross section of the wire. the winding current may be increased by the factor 2 and this produces a direct proportional af- fect on the torque. at their power loss limit bipolar 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- duce the motor size or the power loss. figure 3 : bipolar motors driver deliver more torque 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 be controlled. a simple and popularsolutionis to give onlyas much voltage as needed, utilizing the resistance (r l )of the winding to limit the current (fig. 4a). a more com- plicated but also more efficient and precise solution is the inclusion of a current generator (fig. 4b), to achieve independencefrom the winding resistance. the supply voltage in fig. 4b has to be higher than the 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) and cur- rent generator limiting. figure 5 : at high step frequencies the winding current cannot reach its setting value because of the continuous direction change. application note 2/17
it has already been mentioned that this power of the motor is, among others, proportional to the winding current. in the dynamic working order a stepper motor chan- ges poles of the winding current in the same stator winding after two steps. the speed with which the current 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. 5a shows that at a low step rate the winding current i l reaches its nominal value v l /r l before the direction is changed. however, if the poles of the stator win- dings are changed more often, which corresponds to a high step frequency, the current no longer rea- ches its saturating value because of the limited change time ; the power and also the torque dimi- nish clearly at increasing number of revolutions (fig. 5). more torque at a higher number of revolutions higher torque at faster speeds are possible if a cur- rent generator as shown in fig. 4b is used. in this application the supply voltage is chosen as high pos- sible to increase the current's rate of change. the current generator itself limits only the phase current and becomes active only the moment in which the coil current has reached its set nominal value. up to this value the current generator is in saturation and the supply voltage is applied directly to the winding. fig. 6, shows that the rate of the current increase is now much higher than in figure 5. consequently at higher step rates the desired current can be main- tained in the winding for a longer time. the torque decrease starts only at much higher speeds. fig. 7 shows the relation between torque and speed in the normal graphic scheme, typical for the stepper motor. it is obvious that the power increases in the upper torque range where it is normally needed, as the load to be driven draws most energy from the motor in this range. efficiency - the decisive factor the current generator combined with the high sup- ply voltage guaranteesthat the rate of change of the current 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 the current generator dramatically increases, although the motor does not deliver any more energy in this range ; the efficiency factor is extremely bad. help comes from a switched current regulation using the switch-transformer principle, as shown in fig. 8. the phase winding is switched to the supply voltage until the current, detected across r s , rea- ches the desired nominal value. at that moment the switch, formerly connected to + v s , changes posi- tion and shorts out the winding. in this way the cur- rent is stored, but it decays slowly because of inner winding losses. the discharge time of the current is determined during this phase by a monostable or pulse oscillator. after this time one of the pole chan- ging switches changes back to + v s , starting an in- duction recharge and the clock-regulation-cycle starts again. figure 6 : with a step current slew, it is not a problem to obtain, even at high step frequencies sufficient current in wind- ings. figure 7 : constant current control of the step- per motor means more torque at high frequency. since the only losses in this technique are the satu- application note 3/17
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 the concept of circuit inversion. in this way also the po- wer unit is discharged. this king of phase current control 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 principle to a stabilized current controlled unipolar circuit, as two more switches per phase would be necessary for the shorteningout of the windings during the free phase and thus the number of components would be 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 and justifiable improvement is the obi-level-driveo (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 supply voltage. this considerably increases the current rate of change and its behaviour corresponds more or less 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 per fig. 8, as the factors : distribution voltage oscillation, b.e.m.f., thermal winding resistance, as well as the separate coil current regulation are not considered, but it is this circuit that makes the simple unipolar r/l-control suitable for many fields of application. figure 8 : with switch mode current regulation efficiency is increased. application note 4/17
figure9: at every new step of the motor, it is possible to increase the current rate with a bilevel circuit. advantages and disadvantages of the half-step an essential advantage of a stepper motor opera- ting at half-step conditions is its position resolution increased by the factor 2. from a 3.6 degree motor you achieve 1.8 degrees, which means 200 steps per revolution. this is not always the only reason.often you are for- ced to operate at half-step conditions in order to avoid that operations are disturbed by the motor re- sonance. thesemay be so strong that the motor has no more torque in certain step frequencyranges and looses completely its position (fig. 10). this is due to 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. in practice, the load might deaden this system, but only if there is sufficient frictional force. in most cases half-step operationhelps, as the cour- se covered by the rotor is only half as long and the system 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 many clock-pulses as the full-step system ; the clock-frequency is twice as high as with the full-step. - in the half-step position the motor has only about half of the torque of the full-step. figure 10 : the motor has no more torque in cer- tain step frequency ranges with full step driving. for thisreason 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. application note 5/17
torque loss compensation in the half-step operation it's clear that,especially in limit situations,the torque loss in half-step is a disadvantage. if one has to choose the next larger motor or one with a double resolution operating in full-step because of some in- sufficient torquepercentages,it will greatly influence the costs of the whole system. in this case, there is an alternative solution that does not increase the coats for the bipolar chopping sta- bilized current drive circuit. the torque loss in the half-step position may be compensated for by increasing the winding current by the factor 2 in the phase winding that remains active. this is also permissible if, according to the motor 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 the full-step position. the factor 2 increase in current doubles the stray power of the active phase. the toal 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 position amounts to about 90 % of that of the full-step, that means 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 limit current conditions in a half-step position because it would be like a winding thermal phase overloadcon- centrated in one. the best switch-technique for the half-step phase current increase will be explained in detail later on fig. 11 shows the phase current of a stepping motor in half-step control with an without phase current in- crease and the pertinent curves of stap frequency and torque. figure 11 : half step driving with shaping allows to increase the motor's torque to about 95 % of that of the full step. application note 6/17
figure 12 : only two signals for full step driving are necessary while four (six if three-state is needed on the output stages) for half step. application note 7/17
drive signals for the micro elec- tronic a direct current motor runs by itself if you supply if with voltage, whereas the stepping motor needs the commutation signal in for of several separated but linkable 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 controlneeds only two rectangular signals in quadrature. according to which phase is leading, the motor axis rotates clock- wise or counter-clockwise, whereby the rotation speed is proportional to the clock frequency. in the half-step system the situation becomes more complicated. the minimal two control signals beco- me four control signals. in some conditions as many as six signals are needed. if the tri-state-command for the phase ranges without current, necessary for high motor speeds, may not be obtained from the 4 control 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 them using 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 could reducethe 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 :determines the sense of rotation of the motor axis. 4. half/full : desides whether to operate in full or in half-step. another inhibit input allows the device to switch the motor output into the tri-state-mode in order to pre- vent undesiredmovements during undefinedopera- ting conditions, such as those that could occur during. figure 13 : the l297 avoids the use of complicated standard logic to generate both full and half-step driving signals together with chopper current control. r s1 r s2 =0.5 w v f 1.2 v @ i = 2a d1 to d8 = 2 a fast diodes { trr 200 ns application note 8/17
switch-mode current regulation the primary function of the current regulation circuit is to supply enough current to the phase windings of the motor, even at high step rates. the functionalblocks required for a switchmode cur- rent control are the same blocks required in swit- ching power supplies ; flip-flops, comparators ; and an oscillator are required. these blocks can easily be included in the same ic that generatesthe phase control signals. let us consider the implementation of chopper current control in the l297. the oscillator on pin 16 of the l297 resets the two flip-flops at the start of each oscillator period. the flip-flop outputs are then combined with the outputs of the translator circuit to form the 6 control signals supplied to the power bridge (l298). when activated, by the oscillator, the current in the winding will raise, following the l/r time constant curve,untilthe voltage across the senseresistor (pin 1, 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 is activated into a diagonally conductive state when the oscillator resets the flip-flop at the start of the next cycle. using a common oscillator to control both current re- gulators maintains the same choppingfrequencyfor both, thus avoiding interference between the two. the functionalblock diagramof the l297 and the po- wer stage (l298) are shown in figure 14 alone with the operating wave forms. an importantcharacteristicsof this circuit implemen- tation is that, during the reset time, the flip-flops are kept reset. the reset time can be selected by selec- ting the impedance of the r/c network or pin 16. in this way, the current spike and noise across the sen- se resistors that may occur during switching will not cause a premature setting of the flip-flop. thus the recovery current spike of the protection diodes can be ignored and a filter in the sense line is avoided. the right phase current for eve- ry operating condition the chopper principle of the controller unit reveals that the phase current in the motor windings is con- trolled by two data : the reference voltage at pin 15 of the controllerand the value of the sense resis-tan- ce at pins 1 and 15 of the l298, that is i l =v ref /r s . by changing v ref it is very easy to vary the current within large limits. the only question is for which pur- pose and at which conditions. more phase current means more motor torque, but also higher energy consumption. an analysis of the torque consumption for different periods and load position changes shows that there is no need for different energies. there is a high energy need during the acceleration or break phases, whereas during continuous opera- tion or neutral or stop position less energy has to be supplied. a motor with its phase current continuou- sly oriented at the load moment limit, even with the load moment lacking,consumes needlessly energy, that is completely transformed into heat. therefore it is useful to resolve the phase current in at 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 another resistance and transistor it is possible to resolve 2 bits and consequently4 levels. that is sufficient for all imaginable causes. fig. 16 shows a optimal phase current diagram du- ring a positioning operation. application note 9/17
figure 14 : two ics and very few external components provide complete microprocessor to bipolar step- per motor interface. application note 10/17
figure 15 : because of the set-dominant latch inside the l297 it is possible to hide current spikes and noise across the sense resistors thus avoiding external filters. figure 16 : more energy is needed during the acceleration and break phases compared the continuous operation, neutral or stop position. application note 11/17
high motor clock resets in the half-step system in the half-step position one of the motor phases has to be without current. if the motor moves from a full- step position into a half-step position, this means that one motor winding has to be completely di- scharged. from the logic diagram this means for the high level bridge an equivalent status of the input si- gnals a/b, for example in the high-status. for the coil this means short circuit (fig. 17 up) and conse- quentlya low reductionof the current. incase of high half-step speeds the short circuit discharge time constant 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 step is not carried out correctly (fig. 17 center). for this reason the l297 controller-unit generates an inhibit-command for each phase bridge, that switches the specific bridge output in the half-step position into tri-state. in this way the coil can start swinging freely over the external recovery diodes and discharge quickly. the current decrease rate of change corresponds more or less to the increase rate of change (fig. 17 below). in case of full-step operation both inhibit-outputs of the 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). application note 12/17
figure 18 : with this configuration it is possible to obtain half-step with shaping operation and therefore more torque. more torque in the half-step posi- tion a topic that has already been discussed in detail.so we will limit our considerations on how it is carried out, in fact quite simply because of the reference voltage controlled phase current regulation. with the help of the inhibit-signals at outputs 5 and 8 of the controller, which are alternativelyactive only when the half-step control is programmed, the ref- erence voltage is increased by the factor 1.41 with a very simple additional wiring (fig. 18), as soon as one of the two inhibit-signals switches low. this in- creases the current in the active motorphase pro- portionally to the reference voltage and compensatesthe torque loss in this position. fig. 19 shows clearly that the diagram of the phase current is almost sinusoidal, in principle the ideal form of the current graph. to sum up we may say that this half-step version of- fers most advantages. the motor works with poor resonance and a double position resolution at a torque, that is almost the same as that of the full- step. better gliding than stepping if a stepper motor is supposedto work almost gliding and not step by step, the form of the phase current diagram has to be sinusoidal. the advantages are very important : - no more phenomena of resonance - drastic noise reduction - connected gearings and loads are treated with 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 the controller may become simpler in its functions. fig. 20 shows us an example with the l6505 unit. this ic containsnothing more than the clocked pha- se current regulation which works according to the same principle as l297. the four control signals emitting continuously a full-step program are now generateddirectly by the microprocessor. in order to obtain a sinusoidal phase current course the refer- ence voltage inputs of the controller are modulated with sinusoidal half-waves. the microprocessor that controls the direction of the current 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 suitable sud- application note 13/17
figure 19 : the half-step with shaping positioning is achieved by simply changing reference voltages. division of the current within one full-step. a higher resolution brings no measurable advantages. on the contrary, the converter clock frequency is alrea- dy very high in case of low motor revolutions and very difficult to process by the processor-software. it is recommended to reduce the d/a resolution at high step frequencies. in case of higher motor revolutions it is more conve- nient to operate only in full-step, since harmonic control is no longer an advantageas the current has only 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 a 1.8 - motor in full-step will be received, as there are several limits : the rise rate of the torque diagram corresponding to the twisting angle of the rotor for the 7.2 -motor is 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 step angle. the reason is a dishomogeneousmagnetic field 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 an optimized micro step operation has already been considered during the construction phase. application note 14/17
figure 20 : l6506 unit gives the possibility to modulate separately the two reference voltage inputs in order to obtain a sinusoidal phase current. application note 15/17
figure 21 : better resolution is achieved with low degree motor but more torque is delivered with high degree motor. conclusions the above described application examples of mo- dern integrated circuits show that output and effi- ciency of stepper motors may be remarkably increased without any excessive expense increase like before. working in limit areas, where improved electronics with optimized drive sequencesallow the use of less expensivemotors, it is even possible to obtain a cost reduction. application note 16/17
information furnished is believed to be accurate and reliable. however, sgs-thomson microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. 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 or systems without express written approval of sgs-thomson microelectronics. ? 1995 sgs-thomson microelectronics - all rights reserved sgs-thomson microelectronics group of companies australia - brazil - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thaliand - united kingdom - u.s.a. application note 17/17

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