CH626759A5 - Device for controlling the speed of a DC motor - Google Patents

Description

The present invention relates to the control of the speed of DC motors, by action on their armature current. It therefore applies equally to motors with permanent magnet excitation and to motors with electric excitation.

It is already known to supply the armature of a DC motor with voltage pulses. The operation of the motor then depends on the average energy applied to it by these pulses. By setting a tachometer generator on the motor shaft, we know how to detect its speed of rotation, and act accordingly on the average energy supplied to it. This way of regulating speed suffers from an obvious lack of simplicity and economy. In addition, it is not applicable in installations already carried out where the addition of a silent generator is impossible.

The present invention aims to provide a control device for speed regulation which is free of these drawbacks, and which applies very well in the two aforementioned cases.

2c The licensee observed that, when the armature current is substantially canceled out, between two supply pulses, the voltage across the armature becomes, after a transient state, equal to the electromotive force of the motor (sometimes called counter-electromotive force). And this electromotive force is itself representative of the speed of rotation of the motor.

Thus, it is possible to determine, between the supply pulses of the armature of the motor, the voltage across the armature when the induced current is substantially canceled, that is to say the electromotive force of the motor. , and then to act on the average energy of the control pulses as a function of the relationship between the voltage thus determined and a set value (desired speed), which allows precise regulation of the speed of rotation of the motor.

The control device which is the subject of the invention, which implements this principle, is defined by claim 1.

According to a first embodiment, the detector means comprises, in addition to a low-pass filter, an auxiliary comparator connected to the terminals of the armature of the motor to detect there 40 reverse overvoltages consecutive to each supply pulse, and a switch auxiliary coupled to this auxiliary comparator and connected to the low-pass filter to maintain the output of the latter at a high potential during said reverse overvoltages, the output voltage of the filter then decreases gradually 45 to a value equal to the electromotive force of the motor.

According to a second embodiment, the detector means comprises, in addition to the low-pass filter, an auxiliary comparator connected to the terminals of the armature of the motor to detect there 50 reverse overvoltages consecutive to each supply pulse, logic means to produce an activation signal outside the motor supply pulses and after the reverse overvoltages which follow them, and a sampler-blocker connected to the output of the low-pass filter and controlled by these logic means, the voltage stored in the neuro-blocker sample thus being representative of the electromotive force of the motor.

According to another embodiment, the low-pass filter com-60 takes a simple capacitor chosen to achieve critical damping of the reverse overvoltage across the terminals of the armature, (the equivalent diagram of the armature includes the electromotive force, inducatance and resistance). The output voltage of the filter then progressively decreases, after each supply pulse, to a value equal to the electromotive force of the motor.

The regulating means and the control pulse source can be of two types.

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In the first type, the source of control pulses comprises a triggered monostable multivibrator and the regulating means acts on the instant of triggering of this multivibrator.

In the second type, the control pulse source comprises a monostable multivibrator with controlled pulse duration, and the regulating means acts on the pulse duration of the monostable multivibrator.

In the first case, the duration of these pulses can also be constant, we then simply act on the repetition rate of the pulses, and this with each pulse. In the second case, the repetition rate of the pulses can itself be constant, and one acts simply on the duration of each supply pulse applied to the armature of the motor. Finally, we can also combine the two types of intervention.

Each of the embodiments of the detector means can be associated with one and / or the other of the types of embodiment of the regulation.

It is also possible to regulate the speed of a DC motor by supplying it with pulses of two polarities. The regulation then operates whatever the direction of rotation (bidirectional control).

A first way of achieving this bidirectional control consists in using two control devices of the previously defined type operating with opposite polarities, but controlled by the same setpoint voltage. Means are provided to avoid simultaneously triggering pulses of the two polarities, in particular due to small variations of zero in the comparators.

Another way is to alternate the polarity of the power pulses applied by the power switch to the motor armature. Equal and alternating time intervals are assigned to each supply polarity. For a zero difference at the level of the comparator, it will then be possible to apply to the motor brief pulses of opposite direction which follow one another and neutralize themselves taking account of the inertia of the motor.

Other details and advantages of the invention will appear on reading the detailed description which follows, given with reference to the appended drawings, given by way of example and in which:

- Figure 1 illustrates the block diagram of the first embodiment of the device object of the invention, with the regulation of the first type acting on the interval between supply pulses;

- Figure 2 illustrates signal shapes at different points in Figure 1;

- Figure 3 illustrates an example of a detailed electrical diagram applying the block diagram of Figure 1;

- Figure 4 illustrates the block diagram of the second embodiment of the device object of the invention, with the regulation of the second type acting on the duration of the supply pulses;

- Figure 5 illustrates signal shapes at different points in Figure 4; and

- Figure 6 illustrates the electrical circuit diagram of a bidirectional control based on the embodiment of Figure 1.

FIG. 1 illustrates the electrical circuit diagram of a first embodiment of the device which is the subject of the invention, associated with a direct current motor M, represented symbolically. A monostable multivibrator 10 acts as a source of control pulses. This monostable 10 has an input for triggering each pulse, and thereby defining the spacing between these control pulses. The output of the monostable 10 controls a circuit 11, shown diagrammatically in the form of a switch, which is a power amplifier-switch. The power switch 11 is mounted in series between a positive voltage source + 24 volts and a terminal of the armature of the motor M, the other terminal of the armature being connected to ground.

In parallel on the armature of the motor M is mounted a reverse overvoltage limiter device, consisting of a Zener diode 12 s and a conventional diode 13. The diode 13 allows the current to flow only when the armature voltage is reverse of the power supplied by the switch means 11, which occurs when the switch means are open, in the form of a reverse overvoltage due to the inductance of the armature of the motor. The Zener diode 12 then limits this reverse overvoltage.

The hot spot C of the armature of the motor M (the terminal which is connected to the power switch 11) is connected to a low-pass filter constituted here by a resistor 17 and a capacitor 16. The common point of resistor 17 and capacitor 16, is the output of the low-pass filter.

The hot spot C of the armature of the motor M is also connected to the inverting input of an auxiliary comparator 14 located to the left of the figure. Here the inverting inputs are represented by a small circle. The non-inverting input of comparator 14 re-20 receives the ground voltage, like the other terminal of the motor armature. Thus, the output of comparator 14 will only be excited when the hot point C of the armature of the motor M is negative relative to the mass. In this case, the comparator 14 acts on a switching transistor 15 illustrated diagrammatically, which applies a voltage of + 24 volts to the capacitor 16 to maintain the latter at the voltage it received during the pulse d feeding that has just ended.

The regulator here comprises a main comparator amplifier 18, whose non-inverting input receives the output of the low-pass filter, that is to say here the voltage across the capacitor 16. The inverting input of the comparator 18 receives a reference voltage representative of the desired speed. And, at the time of equality, the output of comparator 18 activates the monostable circuit 10 to trigger the application of a new supply pulse to the armature of the motor M.

We will now describe with reference to FIGS. 2a to 2c the operation of the device in FIG. 1.

FIG. 2a represents the output of the monostable circuit 10, the output pulses of which have a preset duration. FIG. 40 2b illustrates the voltage present at the terminals of the armature of the motor M. During the supply pulses supplied by the power switch 11, which correspond to the control pulses of FIG. 2a, the armature voltage of the motor is at + 24 volts. At the end of the command pulse, it decreases abruptly 45 until it takes negative values, due to the inductance of the armature. As previously indicated, the negative overvoltage 21 is clipped by the joint action of the Zener diode 12 and the diode 13. After this negative overvoltage phase 21, the voltage rises to a level 22. Pen- 50 during this period, the switch 11 does not drive. And the overvoltage limiter device formed by elements 12 and 13 stopped driving after the overvoltage. Consequently, the bearing 22 simply corresponds to the electromotive force of the motor, when the latter is driven by its mechanical load between two control pulses. And this electromotive force is, in a manner known per se, representative of the speed of rotation of the motor.

As already indicated, the voltage across the armature 6o of the motor M is applied across the resistor 17 to a capacitor 16. The resistor 17 and the capacitor 16 define a time constant, of low-pass filtering . In addition, the switching transistor 15 controlled by the comparator 14 also maintains the capacitor 16 at a positive voltage of + 24 65 volts during the negative overvoltage across the armature of the motor. Thus, as can be seen in FIG. 2c, the voltage at the capacitor 16 is 24 volts while the power switch 11 applies the voltage of 24 volts to the motor. At

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at the time of the negative overvoltage at the motor terminals, it remains 24 volts. Then, the voltage across the capacitor 16 decreases progressively until asymptotically reaches the electromotive force of the motor.

When the voltage at the capacitor 16 reaches the set value (point 25 of FIG. 2c), the comparator 18 applies a control pulse to the monostable 10, which has the effect of supplying the motor with a new pulse of 24 volts.

We can now examine in detail with the help of Figures 2b and 2c the operation of the speed control device of the invention. In Figure 2c before the first 24-volt pulse, the voltage across the motor was the voltage at point 26, assumed to be equal to the setpoint. It is assumed that during the application of the first pulse 23 (of 24 volts), the load on the motor has increased, for example due to a transient increase in dry friction. So, after this first pulse 23, the electromotive force of the motor at point 22 (Figure 2b) is less than the electromotive force at point 26. If we now consider Figure 2c, we see that the decrease in the charge of the capacitor 16 after the pulse 23 is fairly rapid, since it must descend to the value 22 of the electromotive force which is lower than previously (26). The next motor control pulse is then commanded relatively early, at point 25.

On the contrary, after the second supply pulse 27, it can be seen that the motor returns to an electromotive force 28 which was equal to the initial value 26. In this case, in FIG. 2c, the discharge of the capacitor 16 is slower, and the triggering of the next pulse takes place at point 29, which is further from the previous control pulse than was at point 25. Thus the speed regulation acts on the average energy of the pulses supplied to the engine.

We will now describe with reference to FIG. 3 a particular embodiment of the device, the principle diagram of which is given in FIG. 1.

FIG. 3 shows the motor M, and, in parallel on the armature thereof, the Zener diode 12 and the diode 13.

The power switch means apply, when they are controlled, a voltage of 24 volts to the armature of the motor M via a power diode 110 (advantageously used to protect the power transistor).

These power switch means consist of a power transistor 111, its control transistor 113 and a protection transistor 112. The emitter of the power transistor 111 is connected to the anode of the diode 110, while that its collector is connected to the + 24 volt line via a resistor 1110. This collector is also connected by a resistor 1112 to the base of transistor 112. The emitter of transistor 112 is connected directly to line + 24 volts; its collector is connected to the base of the control transistor 113, to limit the control current (in particular in the event of a short circuit). The collector of transistor 113 is connected via a resistor 1130 to the emitter of transistor 111 as well as to its base for controlling it. Finally, the emitter of transistor 113 is connected to the line + 24 volts by a resistor 1131. The base of transistor 113 is connected to the common point of two resistors, the first resistor 1132 still going towards the line + 24 volts, while the resistor 1133 connects these power switch means to the output of the monostable circuit.

The monostable circuit consists of two comparator amplifiers 101 and 102 (the output of which is a switch transistor which normally gives a zero level). The output of comparator 101 which constitutes the output of the monostable circuit is connected to the inverting input of comparator 102. The non-inverting input of the latter is connected to the inverting input of comparator 101, as well as at the midpoint a divider bridge with two resistors 103 and 104, connected between the line + 24 volts and the earth. Finally, the output of comparator 102 is also connected by a last resistor 105 to the common point of this resistive divider bridge. The non-inverting input of comparator 101 is connected by a resistor 107 to the + 24 volt line, and by a capacitor 108 to ground. It constitutes the entrance to the monostable circuit.

In the power switch means, it will be noted that the power transistor 111 is blocked when the transistor 113 is itself blocked, that is to say when the base of io thereof, and consequently the output of the amplifier 101, is in the vicinity of + 24 volts. It is precisely the state of rest that exists when the capacitor 108, fully charged by the voltage of + 24 volts, exceeds the voltage of the midpoint of the resistive bridge 103,104,105 (the resistor 105 being set to zero by the comparator 102 whose l the pouring input is more positive than the non-inverting input and saturates the comparator 101 in the open state; the output of the comparator 101 then rises to + 24 volts due to the resistors 1133 and 1132).

Furthermore, the voltage across the armature of the motor M 2 is applied across the resistor 17 to the capacitor 16. And the point common to the resistor 17 and to the capacitor 16 constitutes the input of a comparator amplifier 18. The inverting input of comparator 18 receives a reference voltage which is supplied to it by a potentiometer 180 from a voltage of 24 25 volts advantageously stabilized. Of course, in place of the potentiometer, one can have any other source of setpoint voltage, possibly variable if necessary. As in FIG. 1, a switch transistor 15 acts as an inhibitor for the capacitor 16 during the negative overvoltages which occur at the terminals of the armature of the motor M. For this purpose, the emitter of the transistor 15 is connected to the + 24 volt line and its collector at the point common to the resistor 17 and to the capacitor 16. The base of the transistor 15 is also connected to the + 24 volt line by a resistor 150, as well as by a resistor 35151 at the output of the differential operational amplifier 14. The inverting input thereof is connected to ground. Its non-inverting input is connected by a resistor 140 to receive the voltage across the motor armature. It will be noted here that, unlike FIG. 1, the armature voltage of the motor is not taken directly from the motor, but from the anode of the diode 110. Thus, the amplifier 14 will be saturated for all the voltages which are higher than the threshold voltage of the diode 110 (a fraction of a volt). The amplifier 14 will therefore be blocked for negative overvoltages and for slightly positive voltages, which makes operation safer. At this time, it brings a low voltage to the base of the transistor 15, which makes it conductive and maintains the charge of the capacitor 16 at the potential of + 24 volts.

We now find the operation previously so described. The capacitor 16 is therefore at a voltage of + 24 volts as long as the motor is supplied by the power switch and in particular by the power transistor 111. It remains at the voltage of 24 volts during the negative overvoltage due to the conduction of transistor 15. And then it discharges through tra-55 towards resistor 17 up to the residual voltage which represents the electromotive force of the motor. The discharge voltage is compared by the comparator amplifier 18 to the reference value defined by the potentiometer 180. As long as it remains higher, the output of the comparator 18 is at a high level, and 60 we have seen that this prevents the operation of the monostable and of the power switch means. On the contrary, as soon as the voltage across the capacitor 16 becomes lower than the set value, the output of the amplifier 18 drops to ground level which very quickly discharges the condenser-65 108, thereby controlling the circuit monostable, therefore the recharging of this same capacitor up to the level defined by the divider 103,104,105 being out of service during this interval because the inverting input of comparator 102 is

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lower (output 101 = zero) than the non-inverting input, and consequently the comparator output transistor 102 is blocked. It follows a control pulse which is used by the power switch means (transistors 113, 112 and 111) to supply the motor M again with a 24 volt pulse, the preset duration of which is essentially defined by the capacitor 108 and the resistor. 107, and also by the level of the voltage divider constituted by the resistors 103, 104 and 105.

The low-pass filter used here simply consists of a resistor followed by a capacitor. It follows that the charge of the capacitor will decrease up to the electromotive force of the motor according to the well known exponential law. We can obviously use low pass filters (or integrator filters)

more complex providing laws of different variations, provided that these are well known and, preferably, monotonous.

Thus, for example, the resistor 17 can be replaced by a current source controlled by the armature voltage of the motor. This gives a ramp voltage decreasing in a straight line up to the electromotive force of the motor. The expression low-pass filter should be understood here as applying to such an integrating assembly.

The filter makes it possible to eliminate the transients which are due to the delay in the circuits, and in particular to the delay in the rise of the overvoltage, because the armature of the motor comprises not only an inductance, but also losses intervening like a resistance in parallel on this inductance. (In the absence of this loss resistance, the voltage of the motor would instantly go up to the electromotive force at the time of the extinction of the negative overvoltage, while this rise is progressive as seen in Figures 2b and 5c) .

Filtering has another advantage. Indeed, the voltage compared to the set value which defines the desired speed is not the electromotive force of the motor itself for a zero current, but a voltage which asymptotically approaches it by excess. This asymptotic approximation by excess establishes a gradual relationship between the difference (the difference between the electromotive force and its set value) and the power supplied to the motor. This defines the gain of the speed control device, and its stability.

The use of the filter also offers a variant of the first embodiment of the invention. In this, a comparator 14 was used to identify the period during which the negative overvoltage occurs at the terminals of the armature. By adjusting the time constant defined by the resistor 17 and the capacitor 16 so that it produces an elimination by filtering of the negative armature overvoltage, in a manner practically free from oscillation, the voltage can be used directly across the capacitor 16, without forcibly maintaining the potential thereof at the value + 24 volts during the overvoltage. In FIG. 1, this corresponds to the suppression of the comparator 14 and of the switching transistor 15. However, the time constant thus obtained for the resistor 17 and the capacitor 16 is quite high, and supposes that the difference between control pulses is important. This can therefore only be used for low power applications, the wait between the pulses being in principle relatively long, and for this same reason, applications where one can tolerate a relatively long response time of the servo ; this process also results in a gain and therefore lower precision.

In addition, in FIG. 3, a threshold can be provided for the action of comparator 18 by connecting its non-inverting input to the + 24 volt line by a resistor (not shown). We will now describe with reference to Figure 4 the second form of execution of the device object of the invention. There is the DC motor M, and the power switch means 11 which supply it from the voltage at + 24 volts. We find

also in parallel on the armature of the motor M, the overvoltage limiter device consisting of the Zener diode 12 and the diode 13.

FIG. 4 also includes, like FIG. 1, a differential amplifier 14 operating as a negative overvoltage detector at the terminals of the armature of the motor M. The armature terminals of the motor remain connected to a resistor 17 connected in series on a capacitor 16.

Unlike the first embodiment, that of FIG. 4 comprises at the outlet of the capacitor 16 and in parallel thereon a sampler-blocker device 40. In known manner, the sampler-blocker 40 consists of a switch 401 (advantageously produced in the form of a field effect transistor), followed by a capacitor forming an analog memory 402, and an amplifier 403 which is capable of supplying the voltage stored on the capacitor 402 without substantially discharging it. It is clear that the voltage stored in the capacitor 402 depends on the instant of closure of the switch 401, hence the name sampler-blocker-20.

The output of the sampler-blocker, that is to say that of the amplifier 403, is connected to the non-inverting input of an amplifier 41. As in the other embodiment, the input this amplifier receives a setpoint voltage which is representative of the desired speed. The output of amplifier 41 provides a signal representing the difference between the voltage of the sampler-blocker and the reference value.

According to this difference, the output of the amplifier 41 controls a monostable circuit 42, which is of the type with controlled pulse duration 30. An oscillator 43, such as an astable multivibrator, is connected to the monostable 52 to control the start of each pulse. And the duration of the pulses of the monostable 42 comprises a fixed part and a part substantially proportional to the difference provided by the output of the amplifier 41. It is these 35 control pulses which are applied to the power switch means 11 to define the moments of supply of the motor M.

Furthermore, the same control pulses are applied with the output of the comparator 14 to an NI gate 45, which 40 controls the contact 401 of the sampler-blocker 40 through a delay element 46. Under the action of this gate NI 45, contact 401 is only closed when firstly the DC motor is not supplied, and secondly there is no negative overvoltage at these terminals (this being defined 45 by comparator 14), after a slight delay defined by the delay element 46).

Figures 5a to 5d illustrate the waveforms at different points in the circuit of Figure 4. Figure 5a shows the periodic output of the astable multivibrator 43. Figure 5b shows the 50 output of the monostable 42 which defines the instants of motor supply. It can be seen that the duration of the pulses supplied at the output of the monostable 42 is variable, but that the start of these pulses takes place at fixed time intervals defined by the astable multivibrator 43.

Figure 5c illustrates the waveform across the armature of the motor M. During each control pulse, the armature voltage is + 24 volts. Immediately after each control pulse, there is a negative armature overvoltage, then a gradual rise to the electro-60 driving force of the motor.

As before, it is assumed that after the first supply pulse 50, there has been a temporary increase in dry friction of the load and from which has resulted a decrease in the speed of the motor. Under these conditions, during the 65 course of stage 51 which follows the first control pulse 50, the armature voltage rises to a value which is slightly lower than the value 52 which was present before the pulse 50.

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Shortly after the negative overvoltage, the sampler-blocker 40 is controlled by the NI gate 45 to take the voltage proportional to the electromotive force of the motor. The voltage stored in the sampler 40 is shown in Figure 5d. Until the first negative overvoltage, it had the value 52. From that moment on, it had the value 51 which is slightly lower.

The output of the comparator amplifier 41 provides a voltage equal to the difference between the stored voltage and the setpoint voltage. When these two values are equal, the monostable 42 provides a pulse of duration equal to the fixed part already mentioned (pulse 53 of FIG. 5b). When the voltage of the sampler-blocker takes the value 51, lower than the set value, the monostable 42 then supplies a pulse 54 of slightly extended duration. It follows that the average energy supplied to the motor is greater and that the latter will rise to the desired speed of rotation. This is what occurs during the following cycles, where it can be seen in FIG. 5c that the electromotive force covers the value 52. Under these conditions, in FIG. 5b, the control pulses recover the duration of the initial control pulse 53.

If we now compare the embodiments of FIGS. 1 and 4, it appears that in the first case, it is the moment of command of the pulses which is advanced or delayed as a function of the difference between the electromotive force of the motor and the reference voltage. This results in variations in the repetition rate of the supply pulses applied to the motor. In the second case, the repetition rate is fixed and defined by the astable multivibrator 43. And it is the duration of the pulses which is modified as a function of the difference between the electromotive force of the motor and the reference voltage.

In a very simple way, the two effects can be combined by acting both on the duration of the pulses and on their repetition rate. It would suffice for this purpose to replace the astable multivibrator 43 of FIG. 4 by a triggered monostable, and to constitute a control channel of the type of that of FIG. 1 between the capacitors 16 and this triggered monostable multivibrator. In this variant, the output of the comparator 14 would still command a switch of the type of the switch 15 of FIG. 1, mounted between the common point of the capacitor 16 and of the resistor 17 and the line at + 24 volts.

The use of pulses of constant duration with variable repetition rate (first type of embodiment) allows the motor to overcome a relatively high dry friction torque even at low speed. Indeed, each supply pulse then corresponds to a shock given to the motor. The inertia of the motor allows it to maintain its speed of rotation between two supply pulses. And, when this speed tends to decrease, the supply pulses are brought together. The embodiment of FIG. 3, which uses this principle, has given satisfaction for small-sized motors (power approximately 30 Watts) operating up to the very low speed of 4 revolutions per minute by driving a reducer comprising important hard points.

To have a known response time, it may prove more advantageous to use, according to the second type of embodiment, variations in the duration of the pulses, either as has been described with reference to FIG. 4 by giving the pulses supply a constant repetition rate, or even by giving these pulses a repetition rate which is also adjustable, which combines the advantages of the two embodiments.

Referring to Figure 6, we will now describe a bidirectional control achievable with two control devices, or to control the motor speed, use supply pulses at + 24 volts and supply pulses at - 24 volts ( bidirectional control).

In FIG. 6, we find the motor M, a first anti-overvoltage device constituted by the diode 13 towards the + 24 volts, and a second anti-overvoltage device constituted by the diode 113 towards the - 24 volts. In the upper part of the figure, there are also most of the components of the block diagram of Figure 1, which will therefore not be described again. As in Figure 1, these elements are connected to the supply line at + 24 volts.

In the lower part of the figure, there is a circuit of identical constitution, but connected from a supply line at - 24 volts. The bottom elements of Figure 6 have the same reference as the homologous top elements, plus the number of hundreds 1.

It will be noted that the power switch 111 receives the power supply of - 24 volts. The comparator 118 operates in reverse of its counterpart 18: the latter will act when the voltage of the capacitor 16 becomes lower than the set value; the first 118 acts when the voltage of the capacitor 116 becomes greater than the set value.

The same is true for the auxiliary comparators 14 and 114, which detect the non-conduction of the diodes 113 and 13, respectively.

The first forces the capacitor 16 to + 24 volts as long as the armature voltage is negative, and less than - 24 volts. For its part, the auxiliary comparator 114 forces the capacitor 116 to - 24 volts as long as the armature voltage is positive, and greater than + 24 volts.

Therefore, whatever happens, the capacitor 16 will discharge from + 24 volts to the value of the electromotive force of the motor, and the capacitor 116 will charge from - 24 volts to the electromotive force of the motor. As the comparators 18 and 118 operate in opposite directions around the set value, only one of these comparators will act according to the direction of the difference between the electromotive force of the motor and the set value. 35 And the acting comparator can excite the monostable 10 or the monostable 110, depending on whether the actual speed is lower or higher than the desired speed (it is assumed here that the motor turns in the direction corresponding to a supply voltage of generally positive).

40 An astable multivibrator 20 produces regularly spaced pulses 60 alternating with pulses 160. The AND gate 19 does not take into account the comparison signal 18 until the instant of a pulse 60; similarly, the AND gate 119 reacts to the output of the comparator 118 only during the pulses 45 160. This alternation makes it impossible to simultaneously activate the power switches 10 and 110; it remains to guard against the engagement of a channel while the other is already engaged; this is the purpose of inhibitors 61 and 161. As soon as one channel is engaged, these put the capacitor 16 or 116 n / a of the other channel at its resting potential (24 volts positive or negative, respectively).

The circuit of FIG. 6 does indeed allow bidirectional operation, since the speed regulation is effective whatever the direction of rotation of the motor.

55 The bidirectional control device which has just been described was produced according to the diagram in FIG. 1. Of course, it is also possible to transpose the diagram in FIG. 4 for a bidirectional control.

The reciprocal interdiction device 60 connected between the monostables 10 and 110, including the AND gates 19 and 119, can be replaced by opposite direction thresholds placed between the reference voltage and the comparators 18 and 118. This nevertheless gives a slightly worse regulation around the desired speed.

Another variant of bidirectional control consists in reserving a priori for each polarity of the motor supply pulses equal and alternating time intervals. During a first time interval, only the switching means

11 power would be allowed to power the engine; during a second time interval, only the power switch means 111 would be authorized to supply the motor in the other direction, and so on.

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In this case, for a zero difference between the electromotive force of the motor which represents its speed and the reference voltage, it will be possible to have supply pulses of opposite directions which follow one another at the terminals of the motor, and are neutralized to maintain 5 speed.

VS

4 sheets of drawings

Claims (8)

626,759? CLAIMS 1. A device for controlling the speed of a DC motor, comprising a source of adjustable control pulses, power switch means for applying, according to these control pulses, supply pulses to the armature of the DC motor, a means for detecting the voltage across the armature, and a regulating means for controlling the pulse source as a function of the relationship between the detected armature voltage and a set value, characterized by fact that the detector means is arranged to react to an electric armature parameter to receive the armature voltage when the armature current is substantially canceled, apart from the supply pulses. 2. Device according to claim 1, characterized in that the detector means comprises a low-pass filter connected directly to the terminals of the armature of the motor, even during the supply pulses, and that the regulating means comprises a comparator of the output voltage of the filter at the set value, the output of the low-pass filter thus approaching by higher values the electromotive force of the motor with zero armature current. 3. Device according to claim 2, characterized in that the detector means further comprises an auxiliary comparator coupled to the armature of the motor to detect therein reverse overvoltages consecutive to each supply pulse, and an auxiliary switch coupled to it auxiliary comparator and connected to the low-pass filter to maintain the output of the latter at a high potential during said reverse overvoltages, the output voltage of the filter then decreasing gradually to a value equal to the electromotive force of the motor. 4. Device according to claim 2, characterized in that the detector means further comprises an auxiliary comparator coupled to the armature of the motor to detect therein reverse overvoltages consecutive to each supply pulse, a logic means for producing a signal activation outside the motor supply pulses and after the reverse overvoltages which follow them, and a sampler-blocker connected to the output of the low-pass filter and controlled by this logic means, the voltage stored in the 'sampler-blocker being thus representative of the electromotive force of the engine. 5. Device according to one of claims 3 and 4, characterized in that the auxiliary comparator is arranged to also detect the slightly positive voltages at the terminals of the armature, which keeps the output of the low-pass filter at its potential high for these slightly positive tensions. 6. Device according to one of claims 1 to 5, characterized in that the source of control pulses comprises a triggered monostable multivibrator and that the regulating means acts on the instant of triggering of this multivibrator. 7. Device according to one of claims 1 to 5, characterized in that the source of control pulses comprises a monostable multivibrator with controlled pulse duration, and that the regulating means acts on the pulse duration of the multivibrator monostable. 8. Device according to claim 6, characterized in that the triggered monostable multivibrator consists of two comparator amplifiers, one of which provides the monostable output and receives as inputs on the one hand the output of the comparator constituting the regulating means and on the other hand the voltage of a voltage divider, and the other receives as inputs on the one hand the output of the first and on the other hand the voltage of the same voltage divider, while its output is connected by a resistance to this same voltage divider.