GB2029136A - Speed control circuit for a direct current motor having armature control - Google Patents

Abstract

A direct current motor 12 is controlled by a switching transistor bridge 20, 22, 28, 30 responsive to comparators 50, 52, 56, 58 controlled by a signal 44 and a sawtooth 54, operation being such that comparators 50, 52 cycle transistors 20, 30 on and off in phase opposition, whilst comparators 56, 58 permanently hold one of transistors 22, 28 on and the other off, or vice versa, according to whether the signal 44 represents one or other direction of rotation of the motor, the signal level determining the cyclic ratio and hence the motor speed. Diodes 32, 34, 38, 40 maintain motor current during transistor off periods and diodes 38, 40 complete a rheostatic braking circuit through the motor, resistor 42 and transistor 22 or 30, when signal 44 is reduced below the range producing speed reduction only by reduction of the cyclic ratio. Excess braking current detected by members 36, 70 turns off transistor 22 or 30 to produce regenerative braking through diode 32 or 34. When signal 44 is changed to reverse the motor, braking occurs through circuit 12, 30, 40 or 12, 22, 38 in addition to the cyclical reverse current braking through circuit 28, 12, 30 or 20, 12, 22. Excess driving current detected by members 42, 80 causes transistor 20 or 28 to turn off. Bridge short circuits are prevented by comparator 90 receiving the sawtooth 54 and a reference and controlling comparators 52, 58 to block and unblock transistors 22, 30 when the sawtooth is respectively greater than the reference and less than signal 44. <IMAGE>

Description

SPECIFICATION Method of controlling the speed of a d.c.

motor having armature control and a supply device for such a d.c. motor The invention relates to a method of controlling the speed of rotation of a direct current motor having armature control, and likewise to a supply device for such a direct current, which puts the said method into effect.

Supply devices are already known for a direct current motor having armature control, connected into a bridge which includes on the one hand a first pair of arms connected respectively to each end of the said direct current motor and to the terminals of a source or supply so as to be traversed by a current which controls the rotation of the motor in a first direction of rotation and on the other hand a second pair of arms connected respectively to each end of the said motor and to the terminals of the source of supply so as to be traversed by a current which controls the rotation of the motor in a second direction of rotation, with an element for ail-or-nothing control of the flow of current connected into each of the arms of first and second pairs.

Such systems of control of motors are usually employed in particular in precision servomechanisms, for ensuring speed control of the motor in both directions of rotation. The conduction of the control elements is controlled by signal sequences proceeding from choppers the cyclic ration of which is determined by the level of the operating order which is transmitted to the motor. Amongst the different systems of operation of the bridge formed by the four arms previously mentioned, maybe cited that in which the control elements of the first pair of arms are operated into conduction by a first signal proceeding from a chopper, whereas the control elements of the second pair of arms are operated into conduction in phase opposition with the operation into conduction of the control elements of the first arm.

In this type of circuit the two control elements of a first one of the pairs of arms are respectively simultanously conductive or blocked. Consequently when these two elements are conductive the current starts to be established from the positive pole towards the negative pole of the source through one of the control elements, the motor armature, and the other of the control elements. When these two elements are rendered non-conductive the current will be able to continue to flow only be recovery of energy towards the source, the voltage of the motor becoming necessarily higher than that of the source of supply.

If the source does not offer a sufficiently weak resistive and self-inductive impedance, considerable over-voltages may appear at the terminals of the motor, which tend to generate interference and are dangerous for the semi-conductors constituting the control elements. In addition, this disadvantage may likewise appear during the course of a phase called the "brakings" phase, during the course of which the current passing through the motor armature has the tendency to operate the latter in a direction of rotation which is the reverse of the effective direction of rotation of the motor.

The type of circuit previously described has likewise as a disadvantage that of subjecting the motor to reversals of polarity in time with the chopping frequency of the chopper, which is not desirable.

In addition, all the control elements of the bridge being constantly operated by chopping, considerable losses of energy result during the transitions from the flow state to the blocked state.

The aim of the present invention is to correct the various disadvantages which have just been mentioned.

The invention proposes a method controlling the speed of rotation of a direct current motor having armature control, connected into a bridge which includes on the one hand a first pair of arms connected respectively to each end of the direct current motor and to the terminals of a source of supply so as to be traversed by a current which controls the rotation of the motor in a first direction of rotation and on the other hand a second pair of arms connected respectively to each end of the motor and to the terminals of the said source of supply so as to be traversed by a current which controls the rotation of the motor in a second direction of rotation, an element for all-or-nothing control of the flow of current being connected into each of the arms of the first and second pairs, characterized in that the control elements connected into the two arms of the bridge which are connected to one of the ends of the motor are operated cyclically into conduction in phase opposition, and in that simultaneously the control elements connected into the two arms of the bridge which are connected to the other end of the motor are normally operated respectively into conduction and into blocking.

The invention likewise proposes a supply device for a direct current motor having armature control, connected into a bridge which includes on the one hand a first pair of arms connected respectively to each end of the said direct current motor and to the terminals of a source of supply so as to be traversed by a current which controls the rotation of the motor in a first direction of rotation and so on the other hand a second pair of arms connected respectively to each end of the said motor and to the terminals of the source of supply so as to be traversed by a current which controls the rotation of the motor in a second direction of rotation, an element for all-or-nothing control of the flow of current being connected into each of the arms of the first and second pairs, characterized in that it includes first generator means which deliver to the respective control elements in the two arms of the bridge which are connected to a first end of the motor at least one first operating signal which controls the conduction of the said elements in phase opposition, and second generator means which deliver likewise to the respective control elements in the two arms of the bridge which are connected to the other of the ends of the motor at least one second operating signal which controls the conduction of the said elements in phase opposition, the said first and second generator means being sensitive to a control signal representative of the operating voltage of the said motor in order to deliver respectively the said first signal in a cyclic manner and the said second operating signal in a substantially permanent manner or vice versa, according to whether the said control signal is representative of a control of the rotation of the motor in the first direction of in the second direction.

The invention is now described by way of example by referring to the single Figure of the drawings, which represents one embodiment of the supply device in accordance with the invention.

If reference is made to Fig. 1 there is represented by the general reference 10 a supply device for a direct current motor 1 2 having armature control, connected into a bridge intended to supply the motor with a current for controlling it in rotation in a first direction of operation or in a second direction of operation according to the desires of the user. The motor is fed from a source of supply consisting of a source of voltage which delivers a positive voltage over a line 14. The bridge includes a first pair of arms 1 6 and 1 8 respectively connected to the ends of the motor 1 2 and to the source of supply in order to enable flow of the current in the motor in a first direction.A PNP transistor 20 is connected into the arm 16, which forms an element for all-or-nothing control of the flow of current in it. In the same way a NPN transistor 22 which likewise forms an element for all-or-nothing control of the flow of current is connected into the arm 1 8. The bridge includes otherwise a second pair of arms 24 and 26 respectively connected to the ends of the motor 1 2 and to the source of supply in order to enable the flow of current in the motor in a second direction opposite to the first direction. A PNP transistor 28 is arranged in the arm 24, which forms an element for allor-nothing control of the flow of current.Simi larly a NPN transistor 30 the switching operation of which is identical with that of the transistors 20, 22 and 28, is provided in the arm 1 8. As has been said previously, this type of bridge is well known to specialists in the technique of the control of direct current motors, in particular for precision servomechanisms. In order to conclude the description of the bridge it will be observed that each end of the motor 1 2 is connected to the supply line 14 by way of diodes 32 and 34 connected in reverse, respectively in parallel with the emitter-collector circuits ofthe transistors 20 and 28. As will be explaned in greater detail later these diodes are intended for draining towards the source the energy supplied by motor in the braking phase.Further, each end of the motor 1 2 is likewise connected to a resistor 36 which in turn is connected to earth, by way of diodes 38 and 40 connected in reverse. In addition the emitter of each of the transistors 22 and 30 is connected to earth by way of a resistor 42.

The diodes 38 and 40 are the diodes called "freewheel" diodes, well known in operational servomechanisms.

The bridge which has just been described is operated by an electronic operating device, as a function of the level of a control signal delivered by an operating voltage generator 44. The level of the signal delivered by the generator 44 is capable of being developed on opposite sides of a means value (zero in the embodiment described) in order to control the direction and the speed of rotation of the motor as a function of this level. When the value of the signal delivered by the generator 44 is equal to the said mean value the motor 1 2 is not operated in rotation.In a general way, when the motor is operated in rotation in a first direction corresponding with the flow of current in the arms 1 6 and 18, the operating device is designed so that the transistors 20 and 30 are operated into conduction in a complementary way, that is to say, in phase opposition by trains of cyclic pulses and that simultaneously the transistors 22 and 28 are operated so that the transistor 22 is conductive and the transistor 28 is blocked. If on the contrary the motor is operated in rotation in a second direction opposite to the first direction, the transistors 28 and 22 are operated into conduction in a complementary way, that is to say, in phase opposition, by trains of cyclic pulses and the transistors 20 and 30 are respectively blocked and conductive. The op erating device includes first generator means comprising a first comparator 50 and a sec ond comparator 52 which compare the signal delivered by the generator 44 and a periodic signal of sawtooth shape delivered by a gener ator 54. The comparator 50 delivers an oper ating signal which controls the conduction of the transistor 20 when the signal delivered by the generator 54 is less than the signal deliv ered by the generator 44. On the contrary the comparator 52 delivers an operating signal which controls the conduction of the transistor 30 when the signal of sawtooth shape deliv ered by the generator 54 is greater than the signal delivered by the generator 44.The operating device likewise includes second generator means which comprises a third comparator 56 and a fourth comparator 58 which compare the signal of sawtooth shape delivered by the generator 54 with a reference signal delivered by an amplifier 60 of gain-l the input to which receives the operating signal delivered at the output from the generator 44. The comparator 56 delivers a signal which operates the conduction of the transistor 28 when the signal delivered by the generator 54 is less than the signal delivered by the amplifier 60. Similarly the comparator 58 delivers a signal for operating the conduction of the transistor 22 when the signal delivered by the generator 54 is greater than the signal delivered by the amplifier 60.It will be observed that the first and second generator means could comprise only one comparator each, the output signal from which would control the conduction in phase opposition of the pairs of transistors associated with each of the said generator means. Nevertheless it is desirable that each of the generator means include two comparators for reasons which will become apparent during the course of the description.

In operation, if the signal delivered by the operating voltage generator 44 has a positive level corresponding with the mean current passing through the motor 1 2 which is flowing through the arms 1 6 and 18, it may be understood that the transistors 20 and 30 are operated by chopping in phase opposition, whilst the transistors 28 and 22 are respectively blocked and conductive. On the contrary, if the signal delivered by the generator 44 has a negative level corresponding with a mean current in the motor 1 2 which is flowing in the arms 24 and 26, it may be understood that the transistors 28 and 22 are being operated by chopping in phase opposition, whilst the transistor 20 and 30 are respectively blocked and conductive.

In short, the first generator means 50, 52 deliver to the control elements 20, 30 connected into the arms 1 6 and 26 connected to a first end of the motor at least one first operating signal which controls the conduction of the elements 20 and 30 in a complementary manner, that is to say, in phase opposition. Similarly the second generator means 56, 58 deliver to the control elements 28 and 22 connected into the arms 24 and 18 connected to the second end of the motor 12, at least one second operating signal which controls the conduction of the elements 28 and 22 in a complementary manner.The first and second generator means are sensitive to the level of the control signal delivered by the generator 44 in order normally to deliver respectively the said first operating signal in cyclic manner and the said second operating signal permanently, or vice versa, according to whether the level of the control signal is representative of operation of the rotation of the motor in the first direction or in the second direction.

The operating circuit likewise includes a comparator 70, which compares the voltage at the terminals of the resistor 36 (which is representative of the whole of the current flowing through the freewheel diodes) with a reference voltage delivered at its negative in put terminal 72 in order to generate an output signal which blocks the comparators 52 and 58 in a state for which the transistors 22 and 30 are rendered non-conductive. Inasmuch as the the voltage at the terminals of the resistor 36 is representative of the braking current a de vice has thus been realised which limits the intensity of braking. Of course the braking current might be measured by means other than than a resistor, such as an opto-electronic device, for example.

The operating circuit further includes a comparator 80 which compares the voltage at the terminal at the resistor 42 with another reference voltage delivered at its inverter ter minal 82 in order to generate an output signal which blocks the comparators 50 and 56 in a state for which the transistors 20 and 28 are non-conductive. Inasmuch as the voltage at the terminals of the resistor 42 is representa tive of the driving intensity, it may be under stood that the blocking of the transistors 20 and 28 which is produced at a time when the driving intensity is greator than a given value, has as a result a flow of the current passing through the motor through the diode 38 and the transistor 22 or else through the diode 40 and the transistor 30.When the driving cur rent has fallen back below the threshold value for which the comparator 80 has been re leased, the trasistors 20 and 28 are no longer operated to blocking by the comparator 80.

Inasmuch as in the course of operation the transistors 20 and 30 are operated in phase opposition, the same as the transistors 22 and 28, it is necessary to take precautions against the risks of short-circuit which are capable of being produced if, for example, the transistor 30 becomes conductive before the transistor 20 has ceased to conduct, or vice versa. In order to do this, the comparators 52 and 58 are connected so as to switch from the blocked state to the state of flow with a slight hysteresis with respect to the moment when the signal of sawtooth shape becomes greater than the reference signal, that is to say, at a moment when it is certain that the transistor 20 or the transistor 28 is blocked.Similarly the operating device includes inhibitor means intended to prevent the transmission of the whole of the pulsed operating signals deliv ered by the comparators 52 and 54 and resulting from the comparison of the signal of sawtooth shape with the signals delivered by the generator 44 and by the inverter 60.

More precisely, the inhibitor means block the transmission of an operating pulse from the transistors 22 and 30 just before the end of this operating pulse. In order to do this the inhibitor means include means of detection of the moment when the value of the signal of sawtooth shape is equal or close to its crest value, that is to say, slightly before an order for operation into conduction is transmitted to the transistor 20 or to the transistor 28, in order to act upon the amplifiers 52 and 58 so as to operate blocking of the transistors 22 and 30.The means of detection previously mentioned are formed by a comparator 90 which compares the signal of sawtooth shape with a reference signal in order to deliver momentarily a signal for operating blocking of the transistors 22 and 30 by way of the comparators 58 and 52 when the signal of sawtooth shape has a value greater than the reference signal, until the moment when the signal of sawtooth shape becomes less than the control signal delivered by the generator 44.

The supply device which has just been described operates in the following manner.

It will be assumed first of all that the motor is operating in an established state, that is to say, that a control signal of constant value (positive, for example) is being delivered by the generator 44 so that the speed of the motor is stabilised. Inasmuch as it is being assumed that the level of the control signal is positive, the result is that the comparators 50 and 52 deliver to the transistors 20 and 30 chopped operating signals intended to render them conductive in phase opposition. In view of the polarities of the transistors 20 and 30, it is to be understood that the operating signals applied to their bases are identical in order to render then conductive in phase opposition.Nevertheless an operating bridge may equally well be conceived in which the signals for operation by chopping are strictly complementary without thereby departing from the scope of the present invention.

On the other hand, in view of the level of the voltages applied to the input terminals to the comparators 56 and 58, the result is that the transistor 28 is normally operated permanently into blocking, and that the transistor 22 is on the contrary operated permanently into conduction. Thus the motor is operated in rotation at a speed which is a function of the cyclic ratio of the chopped signal transmitted to the transistors 20 and 30, that is to say, which is a function of the level of the operating signal delivered by the generator 44.

The permanent state is split up into two alternate cyclic phases: when the transistor 20 is operated into conduction and the transistor 30 is operated into blocking, the current flows from the line 14 as far as earth by way of the transistor 20, the motor 12, the transistor 22 and the resistor 42. The current which passes through the motor increases with a rate of increase which is essentially limited by the inductance of the motor. When the transistor 20 is operated into blocking and the transistor 30 is operated into conduction, the self-inductive effect which results is greater than the counter-electromotive force of the motor and the current then continues to pass through the motor whilst decreasing. The current then flows by way of the freewheel diode 38, the motor 1 2 and the transistor 22.It will be observed that the NPN transistor 30, although operated into conduction, cannot pass current from earth towards the motor, with the result that the whole of the current flows through the diode 38.

If the operating signal which is delivered by the generator 44 is reduced whilst preserving the same positive polarity, two modes of operation may be produced: in a first mode in which the level of the signal has been little modified, the modification of the cyclic ratio of the chopping signal applied to the transistors 20 and 30 is insufficient for reversing the direction of the current passing through the motor, and there is then produced a succession of increases and reductions in the driving current in a way substantially identical with operation in the established state, but with a mean value of the current which diminishes progressively until the new established state has been reached; in a second mode of operation, the modification of the cyclic ratio is sufficiently large to reverse the direction of the current during the course of the periods of time when the transistor 30 is operated into conduction. The braking current then flows through the diode 40, the motor 12, the transistor 30 and the resistor 42. This phase of operation then corresponds with a period of rheostatic braking. If during the course of this braking period the braking current detected in the resistor 36 reaches a value higher than the authorized threshold, the comparator 70 then blocks the operation of the transistor 30 into conduction and the current then flows by recovery of energy towards the source, through the diode 32.

If the polarity of the operating signal delivered by the generator 44 is changed (from positive to negative) the transistor 20 is then operated permanently into blocking and the transistor 30 is operated permanently into conduction. The transistors 28 and 22 are on the contrary operated into conduction in phase opposition.

As long as the voltage of counter-electromotive force of the motor is of opposite sign to that of the mean value of the new operating voltage of the motor, imposed by the cyclic ratio of the chopped signal applied to the bases of the transistors 28 and 22, the motor is traversed by a braking current which flows in the diode 40, the motor 1 2 and the transistor 30. This braking phase continues until the moment when the voltage of counter-electromotive force changes sign. Then the motor is operated in rotation in the reverse direction by the energy proceeding from the source.

It is quite obvious that the operation of the supply device which has just been described is totally symmetrical. In other words, the circuit operates in the same way when the motor is in an established state with a negative value of the operating signal, and when this signal is modified in the direction of positive values. In this case the transistors 28 and 22 are operated at the start into conduction by chopping whilst the transistors 20 and 30 are respectively blocked and conductive.

On the other hand although the elements for control of the flow of current which have been employed in the embodiment described above are transistors, it is quite obvious than any other equivalent device such as a thyristor or triac, might be aqually suitable, without thereby departing from the scope of the present invention.

Claims (18)

1. A method of controlling the speed of rotation of a direct current motor having armature control, connected into a bridge which includes on the one hand a first pair of arms connected respectively to each end of the direct current motor and to the terminals of a source of supply so as to be traversed by a current which controls the rotation of the motor in a first direction of rotation and on the other hand a second pair of arms connected respectively to each end of the motor and to the terminals of the said source of supply so as to be traversed by a current which controls the rotation of the motor in a second direction of rotation, an element for all-or-nothing control of the flow of current being connected into each of the arms of the first and second pairs, chracterized in that the control elements connected into the two arms of the bridge which are connected to one of the ends of the motor are operated cyclically into conduction in phase opposition, and in that simultaneously the control elements connected into the two arms of the bridge which are connected to the other and of the motor are normally operated respectively into conduction and into blocking.

2. A method as claimed in Claim 1, characterized in that the control elements operated cyclically into conduction are operated by periodic pulses the cyclic ratio of which is representative of the operating voltage of the direct current motor.

3. A method as in Claim 1 or Claim 2, characterized in that one of the control elements operated cyclically into conduction is rendered conductive after the other of the said elements has become non-conductive.

4. A supply device for a direct current motor having armature control, connected into a bridge which includes on the one hand a first pair of arms connected respectively to each end of the said direct current motor and to the terminals of a source of supply so as to be traversed by a current which controls the rotation of the motor in a first direction of rotation and on the other hand a second pair of arms connected respectively to each end of the said motor and to the terminals of the source of supply so as to be traversed by a current which controls the rotation of the motor in a second direction of rotation, an element for all-or-nothing control of the flow of current being connected into each of the arms of the first and second pairs, characterized in that it includes first generator means which deliver to the respective control elements in the two arms of the bridge which are connected to a first end of the motor at least one first operating signal which controls the conduction of the said elements in phase opposition, and second generator means which deliver likewise to the respective control elements in the two arms of the bridge which are connected to the other of the ends of the motor at least one second operating signal which controls the conduction of the said elements in phase opposition, the said first and second generator means being sensitive to a control signal representative of the operating voltage of the said motor in order to deliver respectively the said first operating signal in a cyclic manner and the said second operating signal in a substantially permanent manner or vice versa, according to whether the said control signal is representative of a control of the rotation of the motor in the first direction or in the second direciton.

5. A supply device as in Claim 4, characterized in that the first generator means include at least one comparator for comparing a periodic signal of sawtooth shape having a given polarity, with the said control signal, the latter being capable of having a first or a second polarity according to whether the motor is being controlled in rotation in the first or the second direction, the said comparator delivering the said first operating signal.

6. A supply device as in Claim 5, characterized in that the second generator means include at least one comparator for comparing the said periodic signal of sawtooth shape with a reference signal the value of which is opposite to that of the said control signal, the said comparator delivering the said second operating signal.

7. A supply device as in one of the Claims 5 or 6, characterized in that the first generator means include a first comparator which delivers an operating signal which controls the conduction of a first one of the control elements connected into the arm which connects the first end of the motor and a first terminal of the source of supply, and a second compar ator which delivers an operating signal which controls the conduction of a second one of the control elements connected into the arm which connects the said first end and the other terminal of the source of supply, the first and second comparators delivering signals which operate in phase opposition the conduction of the first and second control elements.

8. A supply device as in Claim 7, characterized in that the second generator means include a third comparator which delivers an operating signal which controls the conduction of a third one of the control elements connected into the arm which connects the second end of the motor and the first terminal of the source of supply, and a fourth comparator which delivers an operating signal which controls the conduction of a fourth one of the control elements connected into the arm which connects the second end of the motor and the other terminal of the source of supply, the third and fourth comparators delivering signals which operate in a complementary manner the conduction of the third and fourth control elements.

9. A supply device as in Claim 7, characterized in that it includes inhibitor means which act upon one of the first and second comparators in order to inhibit the operating signal delivered by one of the said comparators at the start and at the finish of the operating signal.

10. A supply device as in Claim 8, characterized in that it includes inhibitor means which act upon one of the third and fourth comparators in order to inhibit the operating signal delivered by one of the said comparators at the start and at the finish of this operating signal.

11. A supply device as in Claim 9 or Claim 10, characterized in that the said comparator from the first generator means, which is sensitive to the inhibitor means is connected up in order to generate a signal for operating conduction of its associated control element when the signal of sawtooth shape is greater than the control signal, the inhibitor means including in addition means of detecting the transition of the signal of sawtooth shape to the vicinity of its maximum value and of preventing the transmission of the said operating signal in response to the said detection, so as to hasten the end of the said operating signal.

1 2. A supply device as in Claim 11 in combination with Claim 10, characterized in that the said comparator from the second generator means, which is sensitive to the inhibitor means is connected up in order to generate a signal for operating conduction of its associated control element when the signal of sawtooth shape is greater than the signal of value opposite to that of the control signal, the said means of detection preventing the transmission of the said operating signal in response to the said detection, so as to hasten the end of the latter.

1 3. A supply device as in Claim 9 and Claim 10 in combination, characterized in that one of the terminals of the source of supply is connected in a manner in itself known to the ends of the motor by way of a first and a second freewheel diode arranged in parallel with with the control elements associated respec- tively with the said one of the first and second comparators and with the said one of the third and fourth comparators, the said device in cluding likewise means of measuring the cur rent passing through the freewheel diodes, and means which act upon the said one of the first and second comparators in order to oper ate interruption of the conduction of its associ ated control element when the said current is greater than a second predetermined value.

14. A supply device as in Claim 13, char acterized in that the means of operating inter ruption act likewise upon the said one of the third and fourth comparators in order to oper I ate interruption of the conduction of its associ ated control element when the said current is greater than the said second predetermined value.

1 5. A supply device as in Claim 13, char acterized in that it includes means of measur ing the current passing through the control elements connected in parallel with the freewheel diodes, and means which act upon the other of the first and second comparators in order to operate interruption of the conduc tion of its associated control element when the said current is greater than a third predet ermined value.

1 6. A supply device as in Claim 15, char acterized in that the said means of operating interruption of the conduction act likewise upon the other of the third and fourth compar ators in order to interrupt the conduction of its associated control element when the said cur rent is greater than the said third predeter mined value.

1 7. A method of controlling the speed of rotation of a direct current motor having arma ture control substantially as described with reference to the accompanying drawings.

18. A supply device for a direct current motor having armature control substantially as described and as shown in the accompanying drawings.