GB2057714A - A system permitting the control of the induction by chopping in a magnetic circuit - Google Patents

Abstract

A system permitting the control of the induction by chopping in a magnetic circuit particularly an electronic circuit for controlling the induction, starting from a direct-current source, of magnetically coupled windings and of the type comprising means for restoring the energy to the source when the current in the winding becomes higher than a reference value, characterised in that it comprises means for comparing the current with two variable limiting values and means for restoring the current to the source through an associated winding or placing the winding in a short-circuiting operation, according to the result of the comparison at a given instant and that of the preceding comparison. The invention is particularly applicable to electric motors.

Description

SPECIFICATION A system permitting the control of the induction by chopping in a magnetic circuit The present invention relates to electronic circuits, which are known by the name of "converters", which ensure the supply to, and the switching of, the windings of the stator of electric motors of the step-by-step, synchronous or asynchronous types, which are fed by a source of direct current.

In the known arrangements of this type, the current passing through the windings is kept either constant or zero, this having as its effect an irregular rotating field which is jerky and difficult to control and a likewise irregular motor torque. The disadvantages of such arrangements (especially oscillations of the rotor, resonance phenomena at certain frequencies and the impossibility of stopping the motor outside particular positions) are well known and are an obstacle to the use of such motors for the fine adjustment of the angular position.

In order to obtain a regular rotation, it is necessary to have a constant torque and consequently to modulate the current in the windings of the stator. Among the methods so far envisaged, it is possible to retain that which consists in the use of a chopping amplifier.

However, the known chopping amplifiers only comprise a single hysteresis comparator, permitting only two possible physical states, firstly a supply of energy from the source towards the windings, and, secondly, the short-circuiting of one of the windings (maintenance of the energy in this winding, except for the losses). Since the return of energy from the magnetic circuit towards the source is only carried out with inversions in sign of the currents and not before, it is impossible to control the regular decrease of the energy in the magnetic circuit.

The invention3 has specifically as an object an electronic circuit for controlling the induction in the windings of an electric motor so as to obviate these disadvantages and to permit the windings to be fed in accordance with a predetermined law of variation, without departing from this law when there is a decrease in the energy stored in the magnetic circuit.

In such a circuit, due to the data which is available in a logic system, the short-circuiting of a winding or the recovery of energy are triggered sufficiently early for correctly correcting the over-intensity.

The windings of the motor have an even number, are mounted in pairs with a middle point and the circuit comprises, for each pair of windings, an adder, a single comparator at the maximum value, a single comparator at the minimum value and a single logic system.

In possibly carrying out a combined operation with the foregoing, the comparators are hysteresis comparators delivering logic signals at two levels.

Possibly in combination with the foregoing, the logic system is formed by an ROM memory (or permanent store) re-looped on itself.

Furthermore, in possible combination with those previously mentioned, the said first means supplying a reference value variable as a function of time, are formed by a permanent store containing, in numerical form, a plurality of current waves which represent different optimised reference values.

Furthermore, a circuit of the aforementioned type may cause a variation in the law of evolution of the current as a function of the load on the motor by controlling the frequency of a clock which permits the scanning of the selected memory addresses.

More generally, the invention also has as an object, an electronic circuit permitting of providing a control by induction chopping in a magnetic circuit comprising two electromagnetically coupled windings or a single winding.

In accordance with the invention, an electronic circuit for feeding and controlling, from a direct-current source, the windings of the stators of electric rotating motors, of the type comprising means for restoring the energy to the source when the current in a winding becomes greater than a reference value, is characterised in that, so as to control the strength in the windings in operation to an essentially variable reference value and to maintain this control operational even outside the decrease of the reference value preceding the inversion of the direction of the current, it comprises:: means for comparing the current with two variable limit values, -and means for possibly restoring the energy to the source through an associated winding or bringing the winding into short-circuiting operation, following the result of the comparison at a given instant and that of the preceding comparison.

In order that the invention may be more readily understood it will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures la and ib are explanatory diagrams which permit the problem resolved by the invention to be better understood, Figure 2 is an explanatory diagram permitting a better understanding of the functioning of the logic system of the circuit according to the invention Figure 3 is a circuit diagram representing the principle of a preferred procedure for carrying the invention into effect, Figure 4 is an example showing development of the signals as a function of time, permitting a better understanding of the functioning of the circuit according to Fig. 1, Figure 5 is an explanatory diagram, Figure 6 shows a preferred example as regards the operation of the circuit in Fig. 3, Figures 7a and 7b represent recordings made during the operation of a stepping motor comprising a feeding circuit according to the invention.

Shown in Fig. 1 b in broken lines are the currents 11 and 12 which should be applied to the windings L, and L2 (Fig. 1 a), for example, of a stepping motor. In this figure, time is plotted as abscissae and the current strengths as ordinates.

It is in fact necessary that these currents are variable with the optimised forms in order to obtain a regular rotation and a regular torque. For simplification purposes, it is assumed in the figure that the motor comprises only two windings and that the windings shown in 1 a are, for example, the first and the third (or the second and the fourth) of a four-winding motor (number of phases n = 4).

In the known systems, comprising a chopping amplifier, the winding is brought into shortcircuiting operation when the current in the latter reaches the reference value. The voltages induced by the displacement of the rotor generally impose the curves which are represented as a continuous solid line, beneath the perfect theoretical curve, until the moment when there is produced the inversion of the direction of the reference value which causes a considerable disruption at the level of the torque. The short-circuiting, at this particular moment, as well as the different energy losses which become apparent, should permit the current to be restored to the reference value.However, because the reference value can quickly change (this depends on the slope of the curve of the reference value in the zone where the break is produced), because of the action of the mutual inductances of the windings and the voltages induced by the displacement of the rotor, the short-circuiting of the winding when the current in the latter becomes greater than the reference value is insufficient to restore the current level to a value close to the reference value. The amplifier then loses the control of the phenomenon, as can be seen from Fig. 1 b, in which the current 11 starts to increase (during the period A), when the energy stored in the magnetic circuit is too great for being-dissipated by Joule effect in the winding.At the moment of cutting off the supply to the winding L1, the energy contained in the magnetic circuit is manifested by the quasi-instantaneous appearance of a currents in the asociated winding. As long as the current is negative in the second winding, this latter returns the stored energy into the supply; consequently, the current 12 quickly reaches the reference value (period B). However, this operation occurs too late in order to compensate for the overintensity in the first winding, of which the current reaches the value lo before returning to zero.

To be able to better control the current at the cut-off in the latter, it is necessary to initiate more quickly the procedure as described, and for this to be possible, it is necessary to have available supplementary information as to the direction of evolution of an effective current relatively to the reference value and, better still, the direction of development of the current with respect to a level on either side of the reference value. It is precisely this which is achieved by the circuit according to the invention, which comprises a logic system which receives, at each instant, the positional data concerning the current between these levels and, when necessary, compares the instantantaneous position with the preceding position. This is illustrated very diagrammatically in Fig. 2, in which time has been plotted as abscissae and the current as ordinate.The curve e represents the current reference value. The curves e + a and e - a represent the limits of the admissible levels, the value a being the tolerance in absolute value. For example, the points B and D both represent currents within the limits of the levels and they correspond to different situations because the point B follows a value (point A) corresponding to an insufficient induction, while the point D follows a situation (point C) corresponding to an excess of induction. The situations A and C are situations which are only produced under normal conditions with the establishment of the current in one winding or with the chopping thereof and when the motor undergoes variations in load.

It is readily understood that the action to be applied is only the same on entering the levels after an increase or a decrease in induction. It is the logic circuit which then indicates in what situation one is situated, by comparison with the actual data and the preceding data. This is explained in detail later, with reference to the constructional example of the circuit according to the invention as shown in Fig. 3 for establishing the principles.

In this Figure, 11 and 1 2 represent two windings in phase opposition on the stator of an electric motor. These two windings are coupled magnetically in their mounting on the motor. In the present case, they are connected by a common end (central point mounting) to the positive terminal of a direct-current supply source 10 by way of a transistor 1, across the terminals of which a diode 4 is mounted in opposition. The second ends of the windings 11 and 1 2 are conected to the negative terminal of the supply system 10, respectively by way of a transistor 2 and a diode 5 mounted in opposition to the terminals of the latter, and a transistor 3 and a diode 6 mounted in opposition to the terminals of this latter.The currents 1" 12 circulating in these windings are measured by means of resistances 8 and 9 respectively mounted in series with said windings. The voltages at the terminals of these resistances, proportional to these currents, constitute the input signals c and d of an amplifier 13, of which the transfer function is: b = kc-kd k being considered as a constant in the effective working zone.

The amplifier 1 3 delivers a signal b which is directly proportional to the difference of the currents in the windings, i.e. at the level of the induction present in the magnetic circuit, if the material is not saturated.

This signal b is applied to two algebraic summators (an adder and a subtractor) 1 7 and 1 8 which likewise receive a reference signal a which represents the tolerance which is admissible on the strength of the current in the windings. The algebraic adders 1 7 and 1 8 provide the signals b + a and b - a, respectively applied to the comparators 1 5 and 1 6 to which is applied, on the other hand, the value e of the reference current, provided by a reference signal generator 14, and which constitutes the image of the induction level which it is desired to maintain in the magnetic circuit.

The comparators 1 5 and 16, in this example, are hysteresis comparators which have the double purpose of automatically assuring a certain degree of damping and of supplying binary output signals f, g, which can be applied, directly operated without any previous treatment, because of their nature and their damping, to the respective inputs 19.1 and 1 9.2 of a numerical logic system 1 9 which, in addition, directly receives at its input 1 9.3 from a generator 14, a binary signal indicating the sign of the reference value e. From these three input data and from the comparison of these data with the value of the data at the preceding instant, the binary control signals h, i, j, establish base signals for the transistors 1, 2 and 3.The logic system also establishes two supplementary signals land m, which will be hereinafter explained and which are used for possibly modifying the reference value e.

The reference 1 9.4 (signal k) which represents a reconnection of the logic system on itself, is intended to illustrate the exploitation by the latter of the data received at two successive instants. Depending on the nature of the logic system, of which the development is within the scope of the person skilled in the art, this "memory" permits a decoding of sequential type, which can be easily adapted to the extension of the principle.

The functioning of this circuit is hereinafter described: The assembly of the algebraic summators and of the comparators 1 5 and 1 6 forms the comparison means for the current, of which the induction, with the limits e + a and e - a, where e is the reference value and a is the tolerance, as well as the measured current, is within the limits as thus defined: The relationship as thus obtained is:: e - a < b < e + a which is represented by the signals S = b + a and D = b - a, at the output of the algebraic summators, such that S > e D < e and by the couple of logic signals f and g at the input of the circuit 1 9 fl g=0 When the induction becomes excessive (current such that the voltage at the terminals of the resistances 8 or 9 is higher than the sum of the absolute value of the reference value e and of the tolerance) i.e. b - al > lel, the case is accordingly that e is > O or < O, i.e. that n = 1 or O one of the group of signals indicated hereafter n = 1 b - a > e (and evidently also b + a > e) and f = 1 g=1 n = O b + a < e (and evidently also b - a < e) f=O g = 0.

Inversely, if the induction is insufficient Ib + al < lel there is obtained for n = 1 (e positive) f=O g=0 for n = O (e negative) f = 1 g=1 It is to be noted that what is understood here by "e positive" and by "e negative" is the fact that the reference number or record is lower or higher than a reference level representing the axis of symmetry of the variation curve of e. In a practical example, there is taken as maximum amplitude e = 2.5 volts with a tolerance a of a few tens of millivolts. For clarity in the figures, the tolerance is always represented relatively much more strongly in Fig. 4, where there is to ii seen a typical example of development of the signals b - a and b + a.This figure represents the functioning in time of the device of Fig. 3. The signals b + a and b - a established from the amplifier 1 3 and summators 1 7 and 1 8 are represented in broken lines, whereas the reference signal e is represented in a solid line. The discontinuities presented by the signal e are created by the switchings of the comparators 1 5 and 1 6 and serve a predominant part in the functioning of the device, because they limit the switching frequency of the transistors 1, 2 and 3 in those cases where the comparators 1 5 and 1 6 do not switch alternately.

In Fig. 4, this case is represented by the succession of the points 25 to 28.

Each change in state of one of the comparators 1 5 or 1 6 creates a new situation, which is interpreted by the logic system 19, which selects the best suited transfer of energy.

By way of example, the zone 22 is characterised by the saturated state of the single transistors 1 and 2 and consequently the supply of energy to the magnetic circuit. The zone 21 is characterised by the saturated state of the single transistor 2 and, consequently, the conservation of energy by the magnetic circuit, on account of a current circulating through the winding 11, the transistor 2 and the diode 7. The zone 20 is characterised by the blocking of the transistors 1, 2 and 3 and consequently the restoration of energy to the source by a current passing through the diodes 4 and 6 and the winding 1 2.

Summarising, the functioning is correct when the curve e is between the two curves in broken lines (b + a and b - a). There is an excess of induction when these two curves are above the curve e and an insufficiency of induction in the contrary case. The switchinqs of transistors as previously indicated are to be explained in greater detail below: When the induction is insufficient (logic state i), the signal H is at the logic level 1, causing the saturation of the transistor 1. If n = 1, the transistor 2 is saturated by the signal i = 1 and the transistor 3 is blocked by the signal i = O. The current then circulates normally in the winding II.

If nO, 0, it is the transistor 3 which is saturated by j = 1 and the transistor 2 is blocked by i=O. 0.

As the function is entirely symmetrical, the following description is limited to the case where n = 1.

If the induction is excessive (logic state 3 (b > e + a), then h=0 i=O and j=O.

The three transistors are blocked and energy is restored to the source by the circuit: diode 6, winding 12, diode 4, as indicated above.

In the two foregoing cases, the logic circuit operates from data (f, 9, n) at the moment, without taking into account earlier data, that is to say, the signal k is then without any effect on the output signals h to m. It is recalled that the signal k is not necessarily a signal available at the output, but symbolises the state of the combination of the signals f, G and f - 1, g - 1, values of f, g at the preceding moment.

For clarifying the matter, it is hereinafter assumed that: k = 0 when the induction was previously in excess and K = 1 if it was previously insufficient, When the induction is correct (f = 1, g = 0), the logic circuit compares the existing logic state with the preceding state.

If the induction was previously insufficient (logic state 2), the transistors 1 and 2 remain saturated and the transistor 3 remains blocked.

if the induction was previously excessive, the transistors 1 and 3 are blocked and the transistor 2 remains the sole conductor: the coil L1 is then in short-circuit via the transistor 2 and the diode 7, as previously indicated. This causes an inverse current, which quickly brings the current of the coil to the desired value. Fig. 5 recapitulates the possible successive logic situations, with their possible order of presentation and the Table describes the functioning of the circuit according to the invention in all the cases where e > o, n = 1, (coil L1 operating), e < o, n = 0 (coil L2 in operation).

In practice, because of the permanent actions of the logic system, if the magnetic circuit is not subjected to any strong disruptions, the logic states 2 and 2' are the most usual and have a considerable duration. The cases 1 and 3 are much briefer. If the cases 1 and 3 are decoded, the cyclic ratio of the signals as obtained will be almost zero, if the motor is not subjected to load variations or if the reference value does not vary suddenly.

However, when the magnetic circuit is subjected to disruptions or disturbances, one of the cases 1 or 3 becomes of longer duration and its mean value increases. The information or data which can be derived from this phenomenon is of great importance; actually, if the rotor of the motor is not at the "good" speed, it does not induce the "good" voltages in the windings and it is then a question of the length of the times 1 and 3 intended for restoring the current to its nominal value.

Inputs 19 outlet Outlets 19 # inlet # n f g k h i j l m

logic state induction observations a > o(coil L1 in use) b+a < e 1 0 0 1or0 1 1 0 0 1 1 insufficient Transistors 1 and 2 b-a < e saturated, 3 blocked, L1 fed, m can control 14 in order to reduce e 1 1 1 0 0 0 2 sufficient, Transistors 1 and 2 previously saturated, 3 blocked, insufficient L1 fed b-a < e < b+a 1 1 0 0 0 1 0 0 0 2 sufficient, Transistors 1 and 3 previously blocked, 2 saturated, insufficient L1 in short circuit All transistors blocked.

b+a > e 1 1 1 1or0 0 0 0 1 0 3 execessive Energy restored to the b-a > e source via L2. /can b-a > e control 14 to increase e n < o(coil L2 in use) All transistors blocked.

Energy restored to the b+a < e 0 0 0 1or0 0 0 0 1 0 3 excessive source via L1. /can b-a < e control 14 to reduce e 1 1 0 1 0 0 1 sufficient, Transistors 1 and 3 previously saturated. 2 blocked.

insufficient L2 fed b-a < e < b+a 0 1 0 0 0 0 1 0 0 2' sufficient, Transistors 1 and 2 blocked, previously 3 saturated. L2 in short excessive circuit.

b+a > e 0 1 1 1or0 1 0 1 0 1 1 insufficient Transistores 1 and 3 b-a > e saturated. 2 blocked. L2 fed, m can control 14 to reduce e It is possible in this way to know if the rotor is following the rotating field, or if it has fallen out of step, if it is oscillating about the rotating field, or if the resistant couple or torque is varying.

In this way, it is possible to respond to this information, for example: -by retarding the rotating field, if the torque demanded is increasing too much, or by increasing the amplitude of the reference value, and thus the current and the torque, -by advancing the rotating field, in the case, for example, of braking of an inertia without loss of angular position, or by increasing the amplitude of the reference value.

This second aspect of the procedure introduces a safety in operation to the controls with a step-by-step motor in open loop, reducing still further the major inconvenience of this type of control.

The treatment of the pulses 1 and 3 permits a control of the functional generator 14, for example, the increase in frequency of the reading or input clock, if the generator is formed of a memory and an input clock of the latter, as in the example of Fig. 6, in which the generator 14 is formed by a permanent store (ROM) 141 which can be programmed and which is synchronised by a clock 200 and having an output "e sign" (or output n) 141.a, coupled to the input 1 9.3 of the logic system 19, and an output 141 .b synchronised with the first and providing the signal e to a numerical-analog converter 300, which feeds the inputs of hysteresis comparators 1 5 and 16, these latter supplying logic signals which are able to act directly on the inputs 19.1 and 19.2 of the system 19.

Advantageously, this latter may also be formed simply of a permanent store comprising as many memory positions as there are possible logic combinations of the input signals f, g, n at a given instant and at the preceding instant. From that which has been previously seen, the memory must be capable of responding to four logic states of the combinations of input signals: the states 1, 2, 2' and 3.

In this example as regards operational procedure (logic system consisting of a permanent store), k effectively represents an output logic signal re-introduced into the memory.

The signal k corresponds to the logic combination f = 1, f~, = 0, if n is positive, and 9 = 1, g - 1 = 0, if n is negative, where f1 and 9~, are values of f and g at the preceding measurement.

The logic relationships resulting from Table A are taken into account at the time of the setting up of the memory. The knowledge of such relationships is thus not essential for the utilisation of the circuit, but may nevertheless be useful, if it is desired to employ a different technology with elementary logic circuit combinations instead of a pre-programmed permanent store.

In the case of a motor comparising p pairs of windings, the complete circuit will comprise p circuits, such as those described, which will preferably be synchronised by a single master clock 200.

To be seen in Figs. 7a and 7b are the recordings of the currents obtained with a step-by-step motor which is supplid in accordance with the invention and functioning respectively with a resistance couple or torque (Fig. 7a) and a no-load torque (Fig. 7b). On these recordings, the time is plotted as abscissae and the currents (at the output of the amplifier 13) as ordinates.

It is to be noted from the broken lines that the frequency of the returns of energy from the winding II towards the supply is relatively small, and that the switching time signals are shown below the recording.

The frequency of the pulses is much higher in the case where there is a no-load operation.

The establishment of the elements of the circuit as described has only been mentioned in respect of the memory or store, these elements being capable of numerous modifications, depending on the application which is visualised. Quite generally, the device which is the subject of the invention can be used in all those cases where a fine control of the magnetic induction has to be carried out, so as to permit permanent speed controls and very precise positioning operations (microsteppinggreater resolution in the case of step-by-step motors), while reducing to the maximum extent the losses of energy.

In the event of the magnetic circuit which is to be controlled forming part of a rotating or translating field motor, this device permits the actuation of synchronous, asynchronous or stepby-step motors, rotary or linear motors, at variable speed, without power, and starting from a direct-voltage source, this being of interest in the precision controls such as those encountered in connection with machine tools or peripheral computers, and in the forwarding operations where the possiblility of variation in the speed and the yield are criteria which can be chosen.

It is obvious that the invention is not limited to the embodiments which have just been described and illustrated and that it will be possible to incorporate numerous modifications as regards details without thereby departing from the scope of the invention.

Claims (8)

1. Supply and induction control electronic circuit, starting from a direct-current source, magnetically coupled windings and particularly windings of the stators of electric motors, of the type comprising means for restoring the energy to the source when the current in a winding becomes higher than a reference value, the said circuit being characterised in that, so as to control the strength, in the windings in operation at an essentially variable reference value, and to maintain this operational control even at the moment of cutting off the supply to the winding, it comprises:: means for comparing the current with two variable limit values, and means for possibly restoring the current to the source through an associated winding or placing the winding in operation in a short-circuiting condition, according to the result of the comparison at a given instant and that of the preceding comparison.

2. Circuit according to Claim 1, applied to a motor, characterised in that, so as to optimise the functioning of the motor as a function of the load, it comprises means for bringing the reference value under the control of the results of the comparison.

3. Circuit according to one of Claims 1 and 2, of the type comprising, in series with each winding, a control transistor and a diode mounted in opposition at the terminals of the latter, characterised in that it comprises, for each pair of windings of the motor: magnetic coupling means between the two coils of each pair of motor coils, a common control transistor for the supply of each pair of windings, coupled between the supply and the windings, and a diode mounted in opposition at the terminals of the transistor, a diode coupled between the windings and earth, first display means supplying a reference value variable as a function of time, second display means providing a reference value determined by the tolerance admissible on the current control and the admissible frequency of the switching operations due to the control, first comparison means coupled with the first and second display means and with the windings, comparing the current in each of the windings at the admissible maximum value defined by the first variable reference value, and by the second reference value of the second comparison means coupled with the preceding first and second display means and with the windings, comparing the current in each of the windings and at a minimum value defined by the first variable reference value and by the second reference value, a logic system establishing, at each instant, from output signals of the said first and second comparison means and the state of the system before the reception of the said signals, control signals from the said respective control transistors of the windings and from the common control transistor, as well as signals for modifying the law of variation of the reference values supplied as a function of time, so as to cause a variation in the speed or the torque of the motor as a function of the load.

4. Circuit according to any one of the preceding claims, characterised in that it comprises a first comparator and a second comparator receiving firstiy the variable reference value and secondly, respectively the output signals of the adding unit and of the subtracting unit, the said comparators preferably being hysteresis comparators.

5. Circuit according to one of claims 3 and 4, characterised in that the said logic system.

comprises a decoding memory or store looped on itself and having address inputs, respectively an input for the sign (n) of the variable reference value, and an input for the output signal of each of the comparators (f, g) and of the respective outputs (h, i, A connected to the bases of the transistors.

6. Circuit according to any one of the preceding claims, characterised in that the memory or store additionally comprises outputs possibly connected to the generator of the variable reference value.

7. Circuit according to any one of the preceding claims, characterised in that the store comprises outputs possibly connected to the amplifier, in order to control the reference value as regards frequency and/or amplitude or the two in combination.

8. A supply and induction control electronic circuit substantially as hereinbefore described with reference to the accompanying drawings.