GB1579121A - Stepper motors and starting circuits therefor - Google Patents

Description

(54) STEPPER MOTORS AND STARTING CIRCUITS THEREFOR (71) We, NATIONAL RESEARCH DEVELOPMENT CORPORATION, a British Corporation established by Statute, of Kingsgate House, 66 - 74 Victoria Street, London, S.W.1, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to stepper motors their drive circuits and starting circuits for such motors. It is particularly concerned with increasing the starting rate of stepper motors.

The starting rate of a stepper motor is normally considered as the maximum pulse rate that can be applied to a stepper motor which is at rest and to which the motor will respond without loss of synhcronism. In order to enable the motor to operate at a higher speed than tbis there are techniques by which the applied pulse rate is increased from a standing start. However such arrangements involve complex circuitry and it is an object of the invention to provide simple circuits by which the starting rate of a motor can be increased.

Accordingly the present invention comprises the combination of a stepper motor a drive circuit for the stepper motor and a starting circuit wherein the starting circuit comprises a pair of input terminals for connection to a source of supply voltage and a pair of output terminals connected to the drive circuit, a diode connected in series between one of said input terminals and one of said output terminals, a capacitor and a charging resistor connected in series across the output terminals and a switch connected across the series combination of diode and capacitor, the arrangement being such that when the switch is open the voltage across the output terminals is the supply voltage across the input terminals and when the switch is closed the voltage across the output terminals comprises the supply voltage plus the voltage across the capacitor whereby to augment the starting voltage applied to the drive circuit.

In the starting circuit set forth above the capacitor is charged from the input terminals prior to starting the motor to a voltage equal to the input voltage so that when the switch is closed on starting the voltage across the output terminals is initially double the input voltage and decays to the value of the input voltage as the capacitor discharges by supply of load current to the motor. By this means there is an initial augmentation of the operating voltage of the stepper motor on starting which is available for the first few steps. This enables the motor to maintain step synchronism at a higher stepping rate than would be possible if the rated input voltage was applied to the motor.

To enable the motor to stop from a stepping rate that would cause the motor to overshoot the above-mentioned starting circuit can equally well be used as a stopping circuit by closure of the switch therein for the last few stepping pulses.

One preferred embodiment of the invention includes pulse generating means providing an initial sequence of trigger pulses for the drive circuit which pulses have graded changes of time intervals between successive ones of said pulses to enable the motor to reach a higher stepping rate from rest than would be possible with a corresponding sequence of constant rate trigger pulses.

The pulse generating means need only provide a few additional pulses, typically three such pulses and these may be obtained from suitable electronic circuits. Optimisation of the time intervals between successive pulses will depend on the particular operating conditions of a motor and the magnitude of the frictional or inertial load which it is required to drive.

In order that the invention may be more fully understood reference will now be made to the drawings accompanying this specification in which: Figure I illustrates a circuit in accordance with one embodiment of the invention, Figure 2 illustrates in block diagrammatic form another embodiment of the invention, Figure 3 is a graph showing timed trigger pulses provided in the Figure 2 circuit, Figure 4 shows a modification of part of the circuit of Figure 2, Figure 5 shows yet another modification of the Figure 2 circuit, and Figure 6 is a position/time curve of a stepper motor on starting.

Referring now to Figure 1 there is shown therein part of a drive circuit for a stepper motor. A stepper motor has a plurality of drive coils such as coil 1 wound on its stator and these coils are energised from motor input terminals 2 and 3 in accordance with the action of switches such as switch 4 connected in series with drive coil 1. A forcing resistor 5 is connected in series with coil 1 and a reverse biassed freewheel diode 6 is connected across coil 1 and forcing resistor 5. Each stator pole is provided with a coil similar to coil 1, for example coil 1' and associated switch 4', forcing resistor 5' and freewheel diode 6'. An input voltage Vm is applied between terminals 2 and 3 and this will normally be the supply voltage available from an external source.Transistor switches 4, 4' and so on are controlled by a suitable control circuit (not shown) so that a stepped pattern of energisation of the stator and corresponding stepping action of the motor is obtained.

In order to enable the motor to follow a sequence of energisation which is switched at a high stepping rate a starting circuit is interposed between input terminals 7 and 8, to which a supply voltage Vs is applied, and terminals 2 and 3 of the motor. This starting circuit comprises a capacitor C and charging resistor R connected between terminals 2 and 3 and a diode D connected to be forward biassed for the voltage Vs and through which capacitor C is charged. A transistor switch S is connected across diode D and capacitor C in series.

When the motor is initially at rest, switch S is open and series capacitor C will have been charged from the power supply at terminals 7 and 8 through diode D so that there is a potential Vs across it. It is arranged that the first step command pulse also closes switch S causing diode D to be reverse biassed. The voltage Vm available between terminals 2 and 3 is now Vs plus the voltage across capacitor C and thus has a magnitude of 2Vs. This doubling of the supply voltage provides not only a higher value of current during the first few steps of the motor but also a faster rate of rise of current through those drive coils that are switched on. Since the motor torque increases with higher current in the coils the torque therefore rises to a higher value in a shorter time permitting higher acceleration to be achieved.As capacitor C discharges the value of Vm falls towards Vs at a rate depending on the size of capacitor C and the current taken by the motor. After the first few steps switch S can be opened so that Vm then becomes equal to Vs and capacitor C now recharges through charging resistor R.

The value of capacitor C is chosen so that its discharge time through the motor windings is sufficiently long for increased torque to be available throughout the time of acceleration to the desired starting speed but in general the higher the value of the capacitor C the higher is the maximum achievable starting rate.

It is possible to increase the starting rate still further by cascading additional starting circuits each with their individual capacitors but in practice the increment in starting rate that can be achieved for each additional stage falls rapidly with the number of stages since saturation causes the motor torque to approach a limiting value no matter how high the value to which the initial drive current is raised.

With a single starting circuit as shown in Figure 1 it is possible to increase the starting rate by 150% to 200% depending on the load conditions of the motor.

The above arrangement is readily applicable to any stepping motor system and only one additional power transistor S and diode D is needed so that the additional cost is small. Any normal power supply will be capable of dealing with the high shortduration loads caused by the need to charge capacitor C provided that the start/stop commitments for the motor are not too concentrated in the operation cycle. The drive circuit described and illustrated in Figure 1 is by way of example only and alternative arrangements of drive circuit can equally well be used.

Figure 2 shows a stepper motor drive circuit which includes provision for an initial sequence of trigger pulses for starting purposes. Motor 21 has a drive circuit 22 which provides operating pulses to energise the stator windings of motor 21 in an appropriate pattern. This pattern is switched in a set sequence in drive circuit 22 by trigger pulses applied to the input to drive circuit 22.

The trigger pulses are supplied to drive circuit 22 from two alternative sources. One source is an oscillator 23 which is used in the normal running of the motor while the other source is a starting pulse circuit 24 which is used to provide an initial sequence of timed trigger pulses. The outputs from oscillator 23 and starting pulse circuit 24 are fed through an OR gate 25 to the input to drive circuit 22. The resultant pulse train fed to drive circuit 22 is as shown in Figure 3 where the starting pulse circuit 24 is illustrated as providing an initial sequence of four trigger pulses defining three starting pulse periods after which the step frequency oscillator 23 provides a continuous train of pulses.

In the operation of the arrangement shown in Figure 2 starting pulse circuit 24 is set to provide trigger pulses at those instants which ensure that the resulting operational pulses from drive circuit 22 will produce positive torque in the rotor of motor 21.

The first trigger pulse from starting pulse circuit 24 will cause drive circuit 22 to energise the motor to travel through an initial angle. Starting pulse circuit 24 is set to produce its second trigger pulse at the instant when the position of the rotor is such that maximum torque will be produced.

After the rotor has travelled through a further appropriate angle a third trigger pulse will be generated. The time interval between the second trigger pulse and the third trigger pulse will generally but not necessarily be less than the time between the first and second trigger pulses since the rotor is now travelling at a higher velocity.

In like manner there is generally but not necessarily a progressively shorter time interval between successive trigger pulses and between the final trigger pulse and the initiation of the pulse train from oscillator 23.

A suitable starting pulse circuit is shown in Figure 4. The circuit comprises monostables 31, 32, 33 and 34. The time delays in the monostables are tl, t2, t3 and t4 respectively. The outputs from each of the monostables 31, 32, 33 are taken to an OR gate 35 the output of which is applied to monostable 34. A starting signal is applied both as an input to OR gate 35 and as an input to monostable 31. Monostable 32 and 33 are triggered from the outputs of monostable 31 and 32 respectively. Monostable 34 provides the output from the circuit and is applied to the OR gate 25 of Figure 2. The output from monostable 33 is applied through a RS flip flop 36 to switch on oscillator 23.

Monostable 34 is set to provide a very narrow trigger pulse on receipt of an input thereto. On the application of a starting signal monostable 34 immediately provides the first trigger signal shown in Figure 3 and in addition monostable 31 is triggered by the start signal and after a time interval tl provides an output to monostable 34 to give the second trigger signal. Thus the time interval between the first and second trigger signals is given by tl. The output from monostable 31 also triggers monostable 32 which similarly provides for the third trigger signal after a time interval t2. Thus t2 is the time interval between the second and third trigger pulses. On initiation of the third trigger pulse monostable 33 is set and after a time interval t3 provides a fourth trigger signal. In addition it sets the flip flop 36 to switch oscillator 23.The time interval t3 can be made equal to the period of the oscillator 23. The time interval t3 can be made equal to the period of the oscillator step frequency or can be given a slightly longer value if desired.

In Figure 5 there is shown an alternative arrangement in which oscillator 23 and starting pulse circuit 24 are combined into a single circuit. In the Figure 5 arrangement a high frequency clock pulse generator 41 feeds four down-counters 42, 43, 44 and 45 in parallel and a divider 47 through an AND gate 48. A second input to AND gate 48 is obtained from an RS flip flop 46 triggered from the output of counter 45. The outputs from all of the down counters and divider 47 are fed to an OR gate 49 which replaces the OR gate 25 of Figure 2 and the output from which is applied to the drive circuit 22.

Each of the down-counters is set to a predetermined count. Thus counter 42 is set to a number N1, counter 43 is set to a number N2 greater than N1, counter 44 is set to a number N3 greater than N2 and counter 45 is set to a number N4 greater than N3. All of the counters provide an output when their count reaches zero. Thus after providing a starting signal to clock pulse generator 41 each of the counters will count down and provide trigger pulses in sequence. The time intervals between successive pulses from the respective counters will depend upon the values of the numbers set in them and it can be readily arranged that these time intervals correspond to times tl, t2 and t3. When all of the counters have given their outputs the final pulse from counter 45 also sets RS flip flop 46 thereby opening AND gate 48 and admitting clock pulses to divider 47.Divider 47 now provides a constant frequency pulse train for the motor through OR gate 49.

The setting up of the monostables 31, 32 and 33 of Figure 4 to give appropriate times tl, t2 and t3 for given load conditions is described with reference to Figure 6. This figure shows a curve of rotor position against time on starting for an assumed constant torque equal to the average torque of the rotor. Parallel to the ordinate there is shown a torque/position curve for successive patterns of energisation. In that curve it is assumed that there is a total of three different patterns labelled A, B and C but in practice there may be a greater number of such patterns. The rotor angles 01, 02, 03 etc. at which the next pattern in a sequence gives a greater torquc are marked on the position/time curve.It is desirable to pro vidc trigger pulses when the rotor is at or close to these angular positions to achieve optimum acceleration. With a given position/time curve it is a simpie matter to derive the times ti, t2 and t3 and in addition the optimum value for the time interval t between successive pulses from oscillator 23. The trigger pulses are marked along the abscissa and correspond to the trigger pulses shown in Figure 3.

The position/time curve that is obtained by assuming a constant torque will be a reasonable approximation to the true position/timc curve and if necessary the time intcrvals between the initial trigger pulses can be adjusted by trial and error.

Where the alternative Figure 5 arrangemcnt is used the times t1, t2 and t3 are obtained by the preliminary setting of the down counters. In this arrangement the time intervals are proportional to the diffcrences between settings on adjacent counter.

WHAT WE CLAIM 15:- In In combination a stcppcr motor a drive circuit for the stcppcr motor and a starting circuit wherein the starting circuit comprises a pair of input terminals for connection to a source of supple voltage and a pair of output terminals connected to the drive circuit. a diode connected in series between one of said input terminals and one of said output terminals. a capacitor and a charging resistor connected in series across the output terminals and a switch connected across the series combination of diode and capacitor. the arrangement being such that when the switch is open the voltage across the output terminals is the supply voltage across the input terminals and when the switch is closed the voltage across the output terminals comprises the supple voltage plus the voltage across the capacitor whereby to augment tlie starting voltage applied to the drive circuit 2. The combination as claimed in Claim 1 and including pulse generating means providing an initial sequence of trigger pulses for tlre drive circuit which pulses have graded changes of time intervils between successive oncs of s;iid pulses to enable the motor to reach a higher stepping rate from rest th:in would be possible with a corres- ponding sequcnce of constant rite trigger pulses 3.The combinition as claimed in Cl'-im 2 in which the pulse generating means comprises a starting pulse circuit antl an oscillator. said st.lrtillg pulse circuit provide ing the limited initial sequence of trigger pulses for starting the oscillator providing a continuous train of trigger pulses for subse- quent running.

4. The combination as claimed in Claim 3 in which the starting pulse circuit comprises a cascaded series of monostables each providing an output trigger pulse after a preset time delay from an input trigger pulse, the initial monostable being triggered by a start signal and each subsequent monostable being triggered by a trigger pulse from an immediately preceding monostable, and in which the last monostable switches on the oscillator.

5. The combination as claimed in Claim 2 in which the pulse generating means comprises a clock pulse generator, a plurality of counters each triggered by the output of the clock pulse generator and each providing one of the trigger pulses of the starting sequence after an interval determined by the initial state of that counter, and a divider also supplied with the output of the clock pulse generator through a gate and which provides a continuous train of trigger pulses for running, which gate is opcned only when the last of the start trigger pulses is generated.

6. In combination a stepper motor a drive circuit and a starting circuit substantially as described with reference to Figure 1 of the accompanying drawings.

7. In combination a stepper motor a drive circuit and a starting circuit substantiallv as described with reference to all of the figures of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   such patterns. The rotor angles 01, 02, 03 etc. at which the next pattern in a sequence gives a greater torquc are marked on the position/time curve. It is desirable to pro vidc trigger pulses when the rotor is at or close to these angular positions to achieve optimum acceleration. With a given position/time curve it is a simpie matter to derive the times ti, t2 and t3 and in addition the optimum value for the time interval t between successive pulses from oscillator 23. The trigger pulses are marked along the abscissa and correspond to the trigger pulses shown in Figure 3.

   The position/time curve that is obtained by assuming a constant torque will be a reasonable approximation to the true position/timc curve and if necessary the time intcrvals between the initial trigger pulses can be adjusted by trial and error.

   Where the alternative Figure 5 arrangemcnt is used the times t1, t2 and t3 are obtained by the preliminary setting of the down counters. In this arrangement the time intervals are proportional to the diffcrences between settings on adjacent counter.

   WHAT WE CLAIM 15:- In In combination a stcppcr motor a drive circuit for the stcppcr motor and a starting circuit wherein the starting circuit comprises a pair of input terminals for connection to a source of supple voltage and a pair of output terminals connected to the drive circuit. a diode connected in series between one of said input terminals and one of said output terminals. a capacitor and a charging resistor connected in series across the output terminals and a switch connected across the series combination of diode and capacitor. the arrangement being such that when the switch is open the voltage across the output terminals is the supply voltage across the input terminals and when the switch is closed the voltage across the output terminals comprises the supple voltage plus the voltage across the capacitor whereby to augment tlie starting voltage applied to the drive circuit
2. 2. The combination as claimed in Claim 1 and including pulse generating means providing an initial sequence of trigger pulses for tlre drive circuit which pulses have graded changes of time intervils between successive oncs of s;iid pulses to enable the motor to reach a higher stepping rate from rest th:in would be possible with a corres- ponding sequcnce of constant rite trigger pulses
3. 3.The combinition as claimed in Cl'-im 2 in which the pulse generating means comprises a starting pulse circuit antl an oscillator. said st.lrtillg pulse circuit provide ing the limited initial sequence of trigger pulses for starting the oscillator providing a continuous train of trigger pulses for subse- quent running.
4. 4. The combination as claimed in Claim 3 in which the starting pulse circuit comprises a cascaded series of monostables each providing an output trigger pulse after a preset time delay from an input trigger pulse, the initial monostable being triggered by a start signal and each subsequent monostable being triggered by a trigger pulse from an immediately preceding monostable, and in which the last monostable switches on the oscillator.
5. 5. The combination as claimed in Claim 2 in which the pulse generating means comprises a clock pulse generator, a plurality of counters each triggered by the output of the clock pulse generator and each providing one of the trigger pulses of the starting sequence after an interval determined by the initial state of that counter, and a divider also supplied with the output of the clock pulse generator through a gate and which provides a continuous train of trigger pulses for running, which gate is opcned only when the last of the start trigger pulses is generated.
6. 6. In combination a stepper motor a drive circuit and a starting circuit substantially as described with reference to Figure 1 of the accompanying drawings.
7. 7. In combination a stepper motor a drive circuit and a starting circuit substantiallv as described with reference to all of the figures of the accompanying drawings.