GB2089069A

GB2089069A - Automatic control of speed

Info

Publication number: GB2089069A
Application number: GB8130184A
Authority: GB
Original assignee: CHALOV EVGENY I; FOMIN ALEXANDR F; SUMACHEV YURI N
Current assignee: CHALOV EVGENY I; FOMIN ALEXANDR F; SUMACHEV YURI N
Priority date: 1980-11-05
Filing date: 1981-10-06
Publication date: 1982-06-16
Also published as: SU1053247A1; GB2089069B; FR2493630A1; DD230395A3; FI813019L; DE3141238C2; DE3141238A1; FR2493630B1

Abstract

The signal from a first tacho pick-up (2) representing the rotation of a shaft is fed via limiting amplifier (4), differentiator (5) and sawtooth generator (6) to a comparator (7), whence a sawtooth signal offset from zero by an amount set by a d.c. reference (8) is supplied to an access-storage circuit (9). The value of the sawtooth voltage supplied to the circuit (9) is stored on the arrival of a pulse from a second tacho pick-up displaced in the direction of rotation of the shaft from the first, and is held constant until the arrival of a second pulse. The output voltage of the access-storage circuit (9) is supplied to the input of an amplifier (10) supplying the motor (1) driving the shaft. <IMAGE>

Description

SPECIFICATION Apparatus for stabilizing the rotation frequency of an electric motor The present invention relates to electrical engineering and, more particularly, it relates to an apparatus for stabilizing the rotation frequency of an electric motor.

The herein disclosed apparatus can be used to advantage for stablilizing the speed of rotation of an electric motor.

High requirements are placed nowadays upon electric motors used in automatic control systems, in data recording, reproducing and transmitting devices, in optomechanical systems and electromechanical devices of radioelectronic equipment in what regards the stability of the rotor rotation frequency upon variation of load moment, supply voltage, temperature and other destabilizing factors.

Widely used in such equipment along with synchronous motors are contactless d.c. electric motors and asynchronous electric motors characterized by a high reliability in operation, low levels of natural vibrations and acoustic noise and fast response.

Very rigid requirements with respect to the stability of rotation frequency (the frequency shall not vary by more than 0.1 %) are placed upon some electric motors, especially those which have a low rotation frequency (up to 100 r.p.m.), for example, electric motors designed for directly driving theturnt- able of electric record player in equipment for hi-fi reproduction of sound from phonograph records.

The present invention resides essentially in that an apparatus for stabilizing the rotation frequency of an electric motor, comprising, connected in series, a tachometric frequency pickup, a limiting amplifier, a differentiator and a sawtooth voltage generator, as well as a comparison circuit to whose first input there are supplied data on the rotation frequency of an electric motor shaft, a direct voltage source with an output connected to a second input of the comparison circuit, and a power amplifier connected to the electric motor and coupled with the output of the comparison circuit, according to the invention, further comprises, connected in series, an additional tachometric frequency pickup whose sensing element is shifted relative to the sensing element of the former tachometric pickup in the direction of rotation of the electric motor shaft, the modulator of the formertachometric pickup serving as the modulator of said additional tachometric pickup, an additional limiting amplifier, an additional differentiator and an access-storage circuit whose second input is connected to the output ofthe comparison circuit while its output is connected to the input of the power amplifier.

For increasing the accuracy of stabilizing by way of reducing the static error, it is expedient that the connection of the output of the access-storage circuit to the input of the power amplifier be effected via series-connected integrator and adder, a second input of the latter being combined with the integrator input.

The apparatus for stabilizing the rotation frequency of an electric motor, embodying the present invention, provides for the stabilization of the instantaneous rotation frequency, a substantial reduction of the effect of tachometric pickup manufacturing errors upon the uniformity of electric motor rotation and for an increased speed of response of the elctronic part ofthe apparatus.

The present invention will be better understood upon considering the following detailed description of examplary embodiments thereof, with due reference to the accompanying drawings in which: Figure 1 shows a functional diagram of the herein disclosed apparatus for stabilizing the rotation frequency of an electric motor; Figure 2 is a structural diagram which shows the positioning of tachometric frequency pickups in the herein disclosed apparatus utilizing pickups ofthe photoelectric type; Figure 3-ditto, side view; Figure 4 illustrates the diagram of Fig. 1, with additionally connected integrator and adder; Figure 5-ditto, with the comparison circuit connected to the output of the access-storage circuit; Figure 6 - ditto, with the comparison circuit connected to the integrator input;; Figure 7a, b, c, d, e, f, g, h shows time diagrams illustrating the operation of the apparatus as shown in Fig. 1, where time is plotted on the axis of abscissae and voltages at the outputs of the functional elements of the apparatus - on the axis of ordinates; and Figure 8a, b, c, d, e, f, g, h, k, I shows time diagrams illustrating the operation of the apparatus as shown in Fig. 4, where time is plotted on the axis of abscissae and voltages at the outputs of the functional elements of the apparatus - on the axis of ordinates.

Referring now to Fig. 1 of the accompanying drawings, the herein disclosed apparatus for stabilizing the rotation frequency of an electric motor 1 comprises two tachometric frequency pickups 2 and 3, i.e., main and additional ones, respectively, with a common modulator (described below in more detail). Connected in series with the first (in the direction of rotation) tachometric pickup 2 are a limiting amplifier 4, a differentiator 5, a sawtooth voltage generator 6, a comparison circuit 7 to whose other input there is connected a direct voltage source 8, a circuit 9 designed for the access and storage of analog signal, and a power amplifier 10 connected to the electric motor 1. Connected in series with the additional tachometric pickup 3 are an additional limiting amplifier 11 and an additional differentiator 12 coupled to the control input ofthe access-storage circuit 9.The access-storage circuit 9 includes, connected in series, an electronic key, a storage capacitor and a source follower. When stablilizing the rotation frequency of a d.c. electric motor, the power amplifier is connected to the electric motor directly, as shown in Fig. 1, and, when stabilizing the rotation frequency of an a.c. electric motor, said power amplifier is connected via controlled bridge diode circuit or some other shaper of control signal (not shown in the drawings).

In the apparatus according to the invention use can be made of the various tachometric frequency pickups such as those of the induction, inductance, capacitance or photoelectric type, which generate at the output an alternating or pulse voltage at the modulation frequency of the magnetic, electromagnetic, electrostatic or light flux, respectively.

The modulator common to both tachometric pickups is rigidly coupled to the rotating shaft (rotor) of the electric motor, with the angle pitch of modulation being somewhat greater than the pickup position angle.

Shown by way of example in Figs. 2 and 3 is the positioning of photoelectric tachometric pickups.

The tachometric frequency pickups 2 and 3 include a radiant energy source 13, an optical system (lens) 14, a modulator 15 rigidly attached to the shaft of the motor 1, and two sensing elements, i.e., photo detectors 16 and 17, respectively, positioned adjacent each other with a shaft relative to each other in the direction of the electric motor rotation The optical system 14 and modulator 15 are positioned in series between the source 13 and photodetectors 16 and 17. The modulator iSis fashioned as a lighttight disk with slotted openings the pitch between which is somewhat greater than the spacing between the photodetectors 16 and 17.The output ofthe photodetector 16, the first sensing element in the direction of the motor rotation, corresponds to the output of the tachometric frequency pickup 2, and the output of the photodetector 17 -to the output of the tachometric frequency pickup 3.

As distinct from the embodiment of Fig. 1, in the embodiment of the apparatus for stabilizing the rotation frequency of an electric motor, shown in Fig. 4, the connection of the output ofthe access-storage circuit 9 to the input of the power amplifier 10 is effected via series-connected integrator 18 and adder 19, a second input of the latter being combined at a point 20 with the intput of the integrator 18.

As distinct from the embodiment of Fig. 4, in the apparatus of Fig. 5 the comparison circuit 7' connected in series with the direct voltage source 8 is connected between the output of the access-storage circuit 9 and the point 20 at which the inputs of the integrator 18 and adder 19 are combined.

As distinct from the embodiment of Fig. 4, in the apparatus for stabilizing the rotation frequency of an electric motor 1, shown in Fig. 6, the comparison circuit 7" series-connected with the direct voltage source 8 is connected between the point 20 at which the inputs of the integrator 18 and adder 19 are combined and the input of the integrator 18.

The apparatus according to the invention, illustrated in Fig. 1, operates in the following manner.

Upon rotation of the shaft of the motor 1,the photodetectors 16 and 17 are lit alternately through one and the same opening of the modulator 15 shown in Figs. 2 and 3 while at the outputs of said photodetectors there emerge e.m.f.'s at a pulse repetition rate equal to the frequency of modulation of the light flux from the source 13. The output voltage pulses of the tachometric pickups 2 and 3 (Fig.

7a, b) are shifted relative to each other by a delay timer equal to the time of passing the modulation opening between the tachometric pickups 2 and 3 the distance between which is constant. The delay time r between pulses (Fig. 7a, b) art a constant rotation frequency of the motor 1 together with the modulatJr 15 is constant and varies inversely with the variation of the rotation frequency ofthe motor 1.

In the structural embodiment shown in Figs. 2 and 3, the optical system 14 converts the light flux of the point source 13 located nearthemodulator 15to a right flux with a virtual radiation center on the rotation axis of the modulator 15 while ensuring identical conditions of light flux modulation for each one of the photodetectors 16 and 17. In this case, the errors in the manufacture of the modulator 15 such as eccentricity, instability of the angle pitch between the openings and beating do not affect the time of passing one and the same modulation opening from one photodetector to the other, i.e., the time r is not affected by synchronous frequency errors of the tachometric pickups 2 and 3.Signals from the outputs of the tachometric pickups 2 and 3 are supplied to the respective limiting amplifiers 4 and 11 and further to the differentiators 5 and 12 which convert the trains of square pulses shifted in time to trains of narrow positive pulses (Fig. 7c, d) corresponding to the leading (trailing) edges of square pulses at the inputs of the differentiators 5 and 12. From the output of the differentiator 5 the narrow pulses (Fig. 7c) are supplied to the input of the sawtooth voltage generator 6 while the pulses from the output of the differentiator 12 (Fig. 7d) are supplied to one of the inputs ofthe access-storage circuit 9. A narrow positive pulse from the output of the differentiator 5 (Fig.

7c) sets the output voltage of the generator 6 equal to zero and, upon termination of said pulse, the output voltage of the generator 6 increases linearily until the moment of arrival of the subsequent pulse.

Thus, a sawtooth voltage emerges at the outlet of the generator 6 (Fig. 7e) at a frequency equal to the lig.htflux modulation frequency. Said voltage is applied to one of the inputs of the comparison circuit 7 to whose other input there is applied reference voltage from the direct voltage source 8. From the output of the circuit 7, the sawtooth voltage (Fig. 7f) shifted relative to zero by the value of reference voltage Iso is applied to the second input ofthe accessstorage circuit 9 which stores the value of sawtooth voltage (Fig. 7f) at the moment a narrow positive pulse arrives from the differentiator 12 (Fig. 7d)to the second input of the access-storage circuit 9 and keeps the value of said voltage constant until the moment of arrival of the subsequent pulse. The voltage from the output of the access-storage circuit 9 (Fig. 79) is supplied to the input of the power amplifier 10 and further to the electric motor 1 to close the control loop.

The mean value ofthe constant component of voltage at the output of the circuit 9 upon rotation of the electric motor 1 in the steady-state mode depends upon the level of reference voltage UO by whose value the comparison circuit 7 shifts, relative to zero, the sawtooth voltage at the input of the circuit 9 (Fig.

7f). The mean value of the contstant component of voltage at the output of the power amplifier 10 (Fig.

7h) defines the rated rotation frequency of the electric motor 1.

Upon setting a lower value of reference voltage from the direct voltage source than that defining the rated rotation frequency of electric motor, the value of the constant component of voltage at the output of the circuit 9 (Fig. 7f) will increase, this leading to an increase of the rated rotation frequency of the electric motor 1 which will be maintained constant within the steady-state error. When a higher value of reference voltage is set, a reverse process occurs.

The stabilization of the rotation frequency ofthe electric motor is effected in the following manner.

Upon the effect of external factors, for example, upon a decrease of the load moment M of the electric motor 1 or upon an increase of its supply voltage, the rotation frequency of the electric motor 1 will increase. The light flux modulation frequency, pulse repetition rate at the output of the tachometric pickups 2 and 3 (Fig. 7a, b) and the triggering frequency of the generator 6 will all increase accordingly, while the delay timer between trains of narrow pulses at the outputs ofthe differentiators 12 and 5 (Fig. 7d, c) will decrease.Inasmuch as the rate of linear increase of sawtooth voltage (Fig. 7e) at the output of the generator 6 does not vary upon an increase of the frequency of triggering said generator by the differentiator 5 while the time interval between the triggering of the generator 6 and the moment of storing the current value of sawtooth voltage (Fig. 7f) by the access-storage circuit 9, defined by the moment of arrival of pulse from the differentiator 12 (Fig. 7d), is reduced, the level of voltage at the output of the circuit 9 (Fig. 79) will decrease.

There will accordingly decrease the voltage at the output of the power amplifer 10 (Fig. 7h) and the rotation frequency of the electric motor 1, tending to the rated value.

Upon an increase of the load moment of the electric motor 1 causing a decrease of the rotation frequency of the latter, a reverse process takes place because the voltage at the output of the apparatus for stabilizing the rotation frequency of an electric motor is directly dependent upon the delay time between trains of pulses at the output of the tachometric pickups.

In this manner, by varying the voltage supplied to the electric motor 1, the closed control loop com pensates for the variation of the rotation frequency of said motor under the effect of external factors, maintaining said frequency stable within permiss ible static error.

The apparatus embodiment of Fig. 4 operates analogously with that illustrated in Fig. 1, shaping square pulses at the outputs of the limiting amplifiers 4 and 11 (Fig. 8a, b), narrow pulses at the outputs of the differentiators 5 and 12 (Fig. 8c, d), sawtooth voltage at the output of the sawtooth voltage generator 6 (Fig. 8e) shifted relative to zero by the value of reference voltage U,, sawtooth voltage at the output of the comparison circuit 7 (Fig. 8f) and staircase voltage at the output of the access-storage circuit 9 (Fig. 8g).

The voltage from the output of the access-storage circuit 9 (Fig. 89) is supplied to one of the two inputs of the adder 19 and, simultaneously, to the input of the integrator 18 whose output is connected with the other input of the adder 19 at the point 20. Propor tionallyto the integration constant, a rising, voltage emerges at the output of the integrator 18 upon the occurrence of positive voltage at the input thereof, a continuously decreasing voltage - upon the occurence of negative voltage at said input, and an unvarying voltage -upon the occurrence of zero voltage at the input of the integrator 18. The adder 19 adds up the output voltages of the access-storage circuit 9 (Fig. 89) and of the integrator 18 (Fig. 8h).

The voltage from the output of the adder 19 (Fig. 8k) is supplied to the input of the power amplifier 10 and further to the electric motor 1 to close the control loop.

The value of reference voltage from the soirce 8 governs the tuning of the closed control loop to the rated rotation frequency of the electric motor 1.

Upon settling a lower value of reference voltage from the direct voltage source 8 than that defining the rated rotation frequency of the electric motor, the value of the constant component of voltage at the output of the access-storage circuit 9 (Fig. 89) will increase, with the voltage at the output of the integrator 18 (Fig. 8h) increasing continuously, which results in a continuous increase of voltage at the output of the adder 19 and power amplifier 10 (Fig.

8k, I). Upon an increase of voltage supplied to the electric motor 1, the rotation frequency of the latter increases and, accordingly, there decrease the delay time 7 between the trains of pulses at the outputs of the tachometric pickups 2 and 3 (Fig. 8a, b) and the voltage at the output of the access-storage circuit 9 (Fig. 8g). On reaching the zero level of output voltage of the circuit 9, the integrator 18 ceases to integrate and its voltage is maintained constant and, accordingly, the voltage supplied from the amplifier 10 to the electric motor 1 and the rotation frequency of the latter are maintained constant.

When a higher value of reference voltage is set up, a reverse process takes place.

The stabilization of the rotation frequency of the electric motor is effected in the following manner.

Under the effect of external factors, for example, upon an increase of the load moment M of the electric motor 1 or upon a decrease of its supply voltage, the rotation frequency of the electric motor 1 decreases, as well as the pulse repetition rate at the output of the tachometric pick-ups 2 and 3 (Fig. 8a, b) and the triggering frequency of the generator 6 (Fig.

8e), while the delay timer between trains of narrow pulses at the outputs of the differentiators 12 and 5 (Fig. 8c, d) increases. Inasmuch as the rate of linear increase of sawtooth voltage at the output of the generator 6 (Fig. 8e) does not vary upon the variation of its triggering frequency by the differentiator 5 while the time interval between the triggering of the generator 6 and the moment of storing the current value of sawtooth voltage (Fig. 8f) by the access storage circuit 9, defined by the moment of arrival of pulse from the differentiator 12 (Fig. 8d) to the control input of the latter circuit, increases, the level o-i voltage at the output of the circuit 9 (Fig. 89), inversely proportional to the deviation ofthe actual rotation frequency from the rated one, will increase discretely. The voltage at the output of the integrator 18 (Fig. 8h) starts increasing continuously, which leads to an increase of voltage at the output of the adder 19 and power amplifier 10 (Fig. 8k, I). Upon an increase of voltage supplied to the electric motor 1, its rotation frequency increases while overcoming the resistance of the load moment and, accordingly, there decrease the delay time T between the trains of pulses at the outputs of the tachometric pickups 2 and 3 (Fig. 8a, b) and the voltage at the output of the access-storage circuit 9 (Fig. 89).

On reaching the zero level ofthe output voltage of the circuit 9, i.e., with the actual and rated rotation frequencies coinciding, the output voltages ofthe integrator 18 and adder 19, the voltage supplied to the electric motor 1 from the power amplifier 10 and the rotation frequency of the electric motor 1 are maintained constant.

Upon a decrease of the load moment of the electric motor 1 causing an increase of the rotation frequency of the latter, a reverse process will take place during which the voltage supplied to the electric motor 1 will decrease until the actual rotation frequency of the electric motor 1 coincides with the rated one.

In this manner, by varying the voltage supplied to the electric motor, the closed control loop compensates for the variation ofthe the rotation frequency of said motorundertheeffect of external factors, maintaining said frequency stable upon mismatch of the actual and rated rotation frequency within an error tending to zero. I.e., the embodiment of the apparatus for stabilizing the rotation frequency of an electric motor, as shown in Fig. 4, provides for a more accurate stabilization as compared with the apparatus shown in Fig. 1.

Upon operation ofthe apparatus of Fig. 5, the comparison of voltage directly proportional to the delay time between trains of pulses at the outputs of the tachometric pickups 2 and 3 with reference voltage is done at the output of the access-storage circuit 9. In this case, the output voltage ofthe accessstorage circuit 9 is inversely proportional to the actual rotation frequency of the electric motor 1 while the difference from zero of the output voltage of the comparison circuit 7' is proportional to the opposite-sign deviation ofthe actual rotation frequency from the rated one.

Upon operation of the apparatus illustrated in Fig.

6, the voltage at the output of the access-storage circuit 9, directly proportional to the delay time bet weentrains of pulses at the outputs of the tachometric pickups 2 and 3 is summed up with the output voltage of the integrator 18 to whose output the voltage is supplied after the circuit7" compares the output voltage of the access-storage circuit 9 with the reference voltage of the source 8.

In this latter case, the output voltage of the access-storage circuit 9 is inversely proportional to the actual rotation frequency of the electric motor 1 while the output voltage of the comparison circuit 7" depends upon the deviation of the actual rotation frequency from the rated one.

When induction-type tachometric pickups are used in the apparatus of the invention, a magnetic flux source is provided by a d.c.-supplied coil or a permanent magnet, a modulator- by a multipole member of varying magnetic conductivity or a multipole magnet, and a sensing element ofthe tachometric pickup includes a coil at whose output the e.m.f. has a frequency multiple of the electric motor rotation frequency.

The herein disclosed apparatus features a considerably faster response than analogous prior art apparatus; the manufacturing error of tachometric pickups does not affect the accuracy of stabilization.

The apparatus ofthe invention stabilizes the instantaneous rotation frequency of a slow-running electric motor while ensuring a practically uniform rotation with a relatively quick-response turntable.

The technical effect yielded by the apparatus shown in Figs. 4, 5 and 6 is due to a considerably higher accuracy of stabilizing the rotation frequency of an electric motor. For example, on a variation of the load moment which causes, in the apparatus shown in Fig. 1, a deviation of the actual rotation frequency from the rated one equal to 1%, such deviation in the apparatus illustrated in Figs. 4, 5 and 6 does not exceed 0.05%.

From the viewpoint of the accuracy of stabilizing the rotation frequency of an electric motor, the apparatus embodiments shown in Figs. 4through 6 are practically similar in efficiency, however, the embodiment of Fig. 4 is somewhat simpler to realize while the embodiments of Figs. 5 and 6 are more convenient in operation inasmuch as they provide for a possibility of connecting instruments monitoring the actual rotation frequency and its deviation from the rated value.

Claims

1. An apparatus for stabilizing the rotation frequency of an electric motor, comprising, connected in series, a tachometric frequency pickup, a limiting amplifier, a differentiator and a sawtooth voltage generator, as well as a comparison circuit to whose first input data are supplied on the rotation frequency of an electric motor shaft, a direct voltage source with an output connected to a second input of the comparison circuit, and a power amplifier connected to the electric motor and coupled with the output of the comparison circuit, said apparatus further comprising, connected in series, an additional tachometric frequency pickup whose sensing element is shifted relative to the sensing element of theformertachometric pickup in the direction of rotation of the electric motor shaft, a modulator of the formertachometric pickup serving as a modulator of said additional tachometric pickup, an additional limiting amplifier, an additional differentiator and an access-storage circuit whose second input is connected to the output of the comparison circuit while its output is connected to the input of the power amplifier.

2. An apparatus for stabilizing the rotation fre quency of an electric motor, according to claim 1, wherein the connection of the output of the accessstorage circuit to the input of the power amplifier is effected via series-connected integrator and adder, a second input of the latter being combined with the integrator input.

3. An apparatus according to claim 2, wherein the comparison circuit is connected between the output of the access-storage circuit and a point at which the inputs of the integrator and adder are combined.

4. An apparatus according to claim 2, wherein the comparison circuit is connected between a point at which the inputs of the integrator and adder are combined and the integrator input.

5. An apparatus for stabilizing the rotation frequency of an electric motor, substantially as hereinbefore described in the preceding claims and specification and illustrated by the accompanying drawings.