Speed Control System With Comparison Of A Sawtooth Wave With A Reference Level

Abstract

A speed control system comprising means for producing a sawtooth wave signal of a frequency corresponding to a frequency-modulated signal relating to the revolution of a rotary member, means for controlling the revolving speed of the rotary member with the sawtooth wave signal and a circuit for changing the inclination of the sawtooth wave.

Description

BACKGROUND OF THE INVENTION

This invention relates to a speed control system for controlling the speed of rotary members such as, for example, a motor, capstan and so in on in magnetic recording and reproducing devices, and more particularly to a speed control system which ensures stable and highly sensitive speed control of the rotary members and enables selection of the reference speed of the rotary members over a wide range.

DESCRIPTION OF THE PRIOR ART

Hitherto, various systems have been proposed for controlling the speed of motors and other rotary members. With these prior art systems, however, no satisfactory followup property of control is obtained and, in addition, the rotational speed of the rotary members cannot be freely established at a desired value. Further, the revolving speed control of DC motors encounters a "self-start" problem, as set forth in copending application, U.S. Ser. No. 586,771 assigned to the same assignee as the present application. In addition, the provision of a speed control circuit in the form of an integrated circuit is difficult, since heat generation often leads to breakage of the integrated circuit.

SUMMARY OF THE INVENTION

This invention is directed to a speed control system for rotary members in which a sawtooth wave signal, frequency-modulated in accordance with the speed of the rotary members, is produced; the speed of the rotary members is controlled in response to the sawtooth wave signal; and the inclination of the sawtooth wave is varied to select the speed to be maintained. Consequently, the speed control circuit operates with a pulse signal and hence is low in power consumption and is easy to construct in the form of an integrated circuit. Further, the use of this system in the speed control of DC motors enables self-start of the motors.

The primary object of this invention is to provide a speed control system for rotary members which permits free selection of the speed of the rotary members over a wide range and is excellent in the followup property of speed control.

Other objects, features and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit connection diagram of a rotary member speed control system in accordance with one embodiment of this invention;

FIG. 2 is a series of waveform diagrams for explaining the speed control system exemplified in FIG. 1;

FIG. 3 is a circuit connection diagram showing another embodiment of the present invention system; and

FIG. 4 is a series of waveform diagrams for explaining the speed control system depicted in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1 reference numeral 1 designates a signal generator SG which is mounted on a rotary shaft of a motor M and rotates with the rotation of the motor M and from which a signal S.sub.1 is derived such as depicted in FIG. 2A which is frequency-modulated in accordance with the rotational angular velocity of the motor M. The frequency-modulated signal S.sub.1 is fed to a waveform shaping circuit 2 constituted by, for example, an NPN-type transistor Ir.sub.1. Thus, there is derived from the waveform shaping circuit 2 a rectangular wave signal S.sub.2 such as depicted in FIG. 2B which corresponds to the frequency-modulated signal S.sub.1. The resulting rectangular wave signal S.sub.2 is applied to a differentiation circuit 3 consisting of a capacitor C.sub.1 and a resistor R.sub.1 to produce differentiated pulses S.sub.3 and S.sub.4 respectively at the rise and fall of the rectangular wave signal S.sub.2 as shown in FIG. 2C.

The differentiated pulses S.sub.3 and S.sub.4 are fed to a sawtooth wave generator 4 consisting of transistors Tr.sub.2 and Tr.sub.3 to drive the generator 4 with either one of the differentiated pulses S.sub.3 and S.sub.4, that is, S.sub.3 in the illustrated example, thus providing a sawtooth wave signal S.sub.5 such as shown in FIG. 2D which has a period equal to that of the differentiated pulse S.sub.3 and consequently varies its peak value in accordance with the period.

In this case the inclination of the sawtooth wave signal S.sub.5 is rendered variable. Namely, the time constant for charge of the signal S.sub.5 is made variable. In the illustrated example, such variation of the time constant is made possible by connecting the collector of the transistor Tr.sub.2 of the sawtooth wave generator 4 to a movable contact S.sub.F of a changeover switch S which is selectively engageable with one of the switch contacts S.sub.V1, S.sub.V2 and S.sub.V3 respectively connected to a power source terminal 6a through series circuit R.sub.10, R.sub.20 or R.sub.30 respectively consisting of resistor R.sub.11, R.sub.21 or R.sub.31, and variable resistor R.sub.12, R.sub.22 or R.sub.32, and a resistor R.sub.8.

With such an arrangement, changeover of the switch S or adjustment of the variable resistor R.sub.12, R.sub.22 or R.sub.32 causes a change in the charge time constant thereby to change the inclination of the sawtooth wave signal S.sub.5. It is also possible, of course, to change the time constant by making variable a capacitor C.sub.4 of the sawtooth wave generator 4, or by selectively changing over a plurality of capacitors which are provided instead of the capacitor C.sub.4 or by rendering the resistance and capacitance values variable.

It is preferred, in this case, to improve the linearity of the sawtooth wave signal S.sub.5 by a positive feedback circuit in which the collector output of the transistor Tr.sub.2 is applied to the base of the transistor Tr.sub.3 and the emitter output of the transistor Tr.sub.3 is fed through a capacitor C.sub.3, the series circuit R.sub.10, R.sub.20 or R.sub.30, and the switch S to a collector load circuit of the transistor Tr.sub.2.

The sawtooth wave signal S.sub.5 is applied to the base of a transistor Tr.sub.4 of a differential amplifier circuit consisting of the transistors Tr.sub.4 and Tr.sub.5 of, for example, a comparator circuit 5.

The differential amplifier circuit can be constructed by applying to the base of the transistor Tr.sub.5 a reference DC voltage E, produced by dividing the voltage of, for example, the power source terminal 6a with the variable resistor R.sub.2 and the resistors R.sub.3 and R.sub.4, and by connecting the connection point of the emitters of the transistors Tr.sub.5 and Tr.sub.4 to the other power source terminal 6b through an emitter resistor R.sub.7.

Thus, the sawtooth wave signal S.sub.5 fed to the base of the transistor Tt.sub.4 is compared with the sum E.sub.1 of a voltage, yielded in the emitters of the transistors Tr.sub.4 and Tr.sub.5 by the aforementioned reference DC voltage E fed to the base of the transistor Tr.sub.4, and a forward drop voltage between the base and emitter of the transistor Tr.sub.4, thus deriving from, for example, the collector of the transistor Tr.sub.4 a signal S.sub.5 ' shown in FIG. 2D'.

In the differential amplifier circuit it is possible, if necessary, to adjust the amplitude of the signal S.sub.5 ' by making the aforementioned resistor R.sub.2 variable to thereby render the reference DC voltage E variable. This leads to the variations in the pulse width of a signal S.sub.6 described later and hence enables adjustment of the rotational speed of the motor M. Further, the differential amplifier connection of the transistors Tr.sub.4 and Tr.sub.5 leads to reduction of drift of the rotational speed of the motor which is caused by the influence of the ambient temperature variations on the signal S.sub.5 '. The signal S.sub.5 ' is a triangular wave having a varying peak value and, in order to convert the triangular wave signal S.sub.5 ' into a width-modulated rectangular wave, the signal S.sub.5 ' is applied to, for example, the base of an emitter-follower type transistor amplifier consisting of a transistor Tr.sub.6 and is thereby amplified, the amplified output being fed to a pulse width modulator circuit 7.

The pulse width modulator circuit 7 is provided with, for example, an NPN-type transistor Tr.sub.7 having its emitter directly connected to the power source terminal 6b , its collector connected to the power source terminal 6a through a collector load resistor and its base connected to the emitter of, for example, the aforementioned transistor Tr.sub.6 (or the collector of the transistor Tr.sub.4 ) at least through a Zener diode Z.sub.D.

Accordingly, the transistor Tr.sub.7 is adapted to be in the on state when the level of the signal S.sub.5 ' exceeds the sum E.sub.2 (FIG. 2D' ) of the Zener voltage of the Zener diode Z.sub.D and the forward drop voltage between the base and emitter of the transistor Tr.sub.7 and to be in the off state when the level of the signal S.sub.5 ' becomes lower than the sum of the voltages. As a result of this, the pulse width modulator circuit 7 produces a rectangular wave comparison signal S.sub.6 such as depicted in FIG. 2E which has been modulated in pulse width in accordance with the peak value of the triangular wave signal S.sub.5 '.

With the above arrangement, the frequency, which is capable of feeding a sawtooth pulse S.sub.5 of a peak amplitude permitting the start of the pulse width modulating action of the pulse width modulator circuit, that is, the frequency of the output signal S.sub.1 of the signal generator 1 can be determined dependent upon the reference DC voltage E fed to the base of the transistor Tr.sub.5 of the differential amplifier circuit. In addition, the reference value, that is, the revolving number of the motor is established according to the Zener voltage of the Zener diode Z.sub.D of the pulse width modulator circuit 7 and the forward drop voltage between the base and emitter of the transistor Tr.sub.7 and an emitter-follower type transistor circuit is interposed as a buffer between the differential amplifier circuit and the pulse width modulator circuit so as to insure sufficient switching action of the pulse width modulator circuit.

The reason why the reference value is thus established in two stages is to improve the servo control sensitivity by sharpening the rise of the signal fed to an integration circuit 8 described later when the revolving number of the motor varies in excess of the set value.

Namely, the signal S.sub.5 or S.sub.5 ' derived from the comparator circuit 5 is a triangular wave whose height and base vary, so that the rise of the signal S.sub.5 or S.sub.5 ' not so sharp and this presents a problem especially when accurate servo is required. To settle the problem, in the present example the signal S.sub.5 or S.sub.5 ' is converted by the pulse width modulator circuit into a pulse-width-modulated wave S.sub.6 the peak value of which is constant and only the pulse width of which varies with changes in that of the signal S.sub.5 or S.sub.5 ' and accordingly the base thereof.

Consequently, since the peak value of the signal S.sub.6 at the rise thereof becomes the same as those of the subsequent pulses, the rise of the signal can be rendered sharp and the ripple factor of the value integrated by the integration circuit 8 described later can be reduced by selecting the Zener voltage of the Zener diode so as to approximate the pulse width of the pulse-width-modulated wave signal to the base length of the triangular wave. If the same ripple factor is permissible, the integration time constant can be decreased to provide for enhanced followup property in servo control.

The rise and fall of the waveform of the signal S.sub.6 can be improved by connecting the collector of the transistor Tr.sub.7 through the resistor R.sub.6 to the base of the transistor Tr.sub.4 as shown and by positively feeding the rectangular wave comparison signal S.sub.6 from the transistor Tr.sub.6 back to the stage preceding it.

The rectangular wave comparison signal S.sub.6 is applied to a low-pass filter, that is, the integration circuit 8 consisting of a resistor R.sub.5 and a capacitor C.sub.2 to provide a DC voltage signal such as shown in FIG. 2F which corresponds to the pulse width of the rectangular wave comparison signal S.sub.6, that is, a demodulated signal S.sub.7 of the frequency-modulated signal S.sub.1 derived from the signal generator 1, shown in FIG. 2A. The demodulated signal S.sub.7 is fed to a DC amplifier 9 consisting of transistors Tr.sub.8 and Tr.sub.9. The motor M is connected, for example, between the collector of an output transistor Tr.sub.9 of the DC amplifier 9 and the power source terminal 6a and is supplied with a current depending upon the level of the DC voltage signal S.sub.7 fed to the DC amplifier 9.

With the above arrangement, an increase in the revolving speed of the motor M causes an increase in the frequency of the frequency-modulated signal S.sub.1 derived from the signal generator 1 thereby to shorten the period of the differentiated pulse S.sub.3, that is, the repeating cycle of the sawtooth wave signal S.sub.5. This leads to a decrease in the peak value of the sawtooth wave signal S.sub.5 , by which the pulse width of the rectangular wave comparison signal S.sub.6 is decreased to lower the level of the DC voltage S.sub.7 applied to the DC amplifier 9. As a result of this, the collector current of the output transistor Tr.sub.9, that is, the current flowing to the motor M decreases to reduce its speed. While, a decrease in the revolving speed of the motor M causes an increase in the current fed to the motor M to increase its speed. Accordingly, self-start of the motor M is possible and its revolving speed is always controlled to be constant.

With the above-described speed control system of this invention, stable and highly sensitive control can be achieved and since the inclination of the sawtooth wave signal S.sub.5 is rendered variable, the reference speed of the rotary member, that is, the motor M in the illustrated example can be selected over a wide range.

Even if the revolving number of the motor is invariable, the frequency-modulated signal S.sub.1 is often slightly amplitude-modulated, as indicated by a broken line S.sub.1 ' in FIG. 2A, by a displacement of the signal generator unit between the magnetic center of the rotor and that of the stator which is caused by a change in thrust in the axial direction of the motor shaft. Accordingly, where the rectangular wave signal S.sub.2 has been produced by shaping the waveform of the signal S.sub.1 as above described, there is the possibility that the pulse width of the signal varies even if the frequency of the signal S.sub.1 is invariable. Provided that the period of the signal S.sub.2, for example, at its rising portion agrees with that of the frequency-modulated signal S.sub.1, the period of the signal S.sub.2 at the falling portion does not agree with that of the signal S.sub.1. Consequently, when a servo signal (the signal S.sub.5 in FIG. 2) is produced by using both the rise and fall of the signal S.sub.2, an error is introduced to make it impossible to obtain accurate comparison output, and in the case of controlling the revolution of a motor the established revolving number of the motor is caused to vary.

With the rotary member speed control system of the present invention, however, the frequency-modulated signal S.sub.1, which is generated in relation to the revolution of the rotary member, that is, the motor in the illustrated example, is shaped in waveform to produce the rectangular wave signals S.sub.2, and the sawtooth wave generator 4 is driven by either one of the differentiated pulses S.sub.3 and S.sub.4 of the signal S.sub.2 at its rise and fall, that is, by the differentiated pulse S.sub.3 only in the illustrated example, thereby producing one sawtooth wave signal S.sub.5, that is, one rectangular wave comparison signal S.sub.6 every one cycle of the frequency-modulated signal S.sub.1. Consequently, even if the frequency modulated signal S.sub.1 is amplitude-modulated to vary the differentiated pulse, for example, S.sub.4, there is no possibility that the variation in the pulse S.sub.4 appears as an error component. Thus, the speed control system of this invention is extremely high in precision and accordingly it ensures stable control of the motor which is not affected by the influence of the variations in the thrust of the motor shaft and so on.

As previously described, the reference speed of the motor M depends upon the inclination of the sawtooth wave signal S.sub.5, the sum E.sub.1 of the emitter voltages of the transistors Tr.sub.4 and Tr.sub.5 and the forward drop voltage between the base and emitter of the transistor Tr.sub.4 and the sum E.sub.2 of the Zener voltage of the Zener diode Z.sub.D and the forward drop voltage between the base and emitter of the transistor Tr.sub.7. In the present invention, however, means for changing the inclination of the sawtooth wave signal S.sub.5, that is, the means for changing the time constant for charge of the signal S.sub.5 is provided, so that the reference speed of the Motor M can be selected at will over a wide range by changing over the switch S as illustrated to select the resistors R.sub.10, R.sub.20 and R.sub.30.

Further, since the currents and voltages in the overall control system vary in an intermittent manner, heat generation is low and hence the device for the speed control system can be readily produced in the form of an integrated circuit.

In FIG. 3 there is illustrated another embodiment of this invention which is identical in fundamental circuit construction with the example of FIG. 1, and accordingly elements similar to those in FIG. 1 are identified by the same reference numerals and no further detailed description will be repeated. The circuit of this example is simpler in construction than that of FIG. 1 in constituting the sawtooth wave generator 4 with one transistor Tr.sub.2 and in elimination of the emitter-follower type transistor amplifier consisting of the transistor Tr.sub.6 and the Zener diode Z.sub.D in FIG. 1. The Zener diode Z.sub.D is incorporated to stabilize the power source voltage.

The present example is similar in the fundamental operation to that of FIG. 1. In FIG. 4 waveforms corresponding to those in FIG. 2 are designated with the same reference numerals. A sawtooth wave signal S.sub.5 such as shown in FIG. 4D, which is derived from a sawtooth base generator 4, is applied to the base of a transistor Ir.sub.4 of a differential Zener consisting of transistors Tr.sub.4 D and T r.sub.5 of a comparator circuit 5 and the signal S.sub.5 is compared with the sum E.sub.1 of the emitter voltages of the transistors Tr.sub.4 and Tr.sub.5 and a forward drop voltage between the base and emitter of the transistor Tr.sub.4 as in the case of FIG. 1 to generate in the collector of the transistor Tr.sub.4 a signal S.sub.5 ' depicted in FIG. 4D', thereby producing in the base of a transistor Tr.sub.7 a signal S.sub.5 " shown in FIG. 4D". The resulting signal S.sub.5 " is compared with a forward drop voltage E.sub.2 ' between the base and emitter of the transistor Tr.sub.7 to produce in the collector of the transistor Tr.sub.7 a rectangular wave comparison signal S.sub.6 shown in FIG. 4E. The subsequent operations are the same as those in the example of FIG. 1.

Also in the example of FIG. 3 stable and highly sensitive control can be achieved and the reference speed of a motor M can be freely selected over a wide range by changing the inclination of the sawtooth wave signal S.sub.5 by changing over a switch S as in the case of FIG. 1.

Although, in the foregoing, the sawtooth wave signal S.sub.5 derived from the sawtooth wave generator 4 is fed to the comparator circuit 5 and the rectangular wave comparison signal S.sub.6 resulting from comparing the signal S.sub.5 with the reference DC voltage is applied to the integration circuit 8, it is also possible to apply the sawtooth wave signal from the sawtooth wave generator 4 to the integration circuit and to apply the output of the integration circuit to the comparator circuit to be compared with reference DC voltage.

While the present invention has been described in connection with the case where a signal, frequency-modulated in accordance with the rotational angular velocity of the motor M, is derived from the signal generator 1 mounted on the rotary shaft of the motor M and is demodulated to control the revolving speed of the motor M, the frequency-modulated signal may be produced by other methods, Further, the present invention is suitable for use in speed control of rotary members such as a motor, a capstan and the like of magnetic recording and reproducing devices and so on which is achieved by controlling a brake mounted on a rotary shaft of the rotary member.

It will be apparent that many modifications and variations may be effected without departing from the scope of the novel concepts of this invention.

Claims

I claim as my invention:

1. A speed system for a rotary member comprising means for generating a sine wave signal that is frequency-modulated in accordance with the rotational speed of said rotary member, means for producing a sawtooth wave signal for each complete cycle of said frequency-modulated sine wave signal with the pitch of each said sawtooth wave signal, and hence the peak voltage thereof, corresponding to the period of the respective cycle of said frequency-modulated signal, means for comparing said peak voltage of each sawtooth wave signal with a reference voltage and for producing a comparison signal in response to said peak voltage being in excess of said reference voltage with said comparison signal corresponding to the extent of said excess, means responsive to said comparison signal for providing a corresponding control signal, means for exerting a driving force on the rotary member in accordance with said control signal, and means for varying the inclination of said sawtooth wave signal and thereby determining the speed of the rotary member which is to be maintained in correspondence to said reference voltage.

2. A speed control system according to claim 10, wherein said rotary member is a DC motor.

3. A speed control system according to claim 1, wherein said rotary member is a capstan.

4. A speed control system according to claim 1, wherein said means for producing a sawtooth wave signal includes shaping circuit means shaping said frequency-modulated signal to provide a rectangular wave having a pitch corresponding to the period of said frequency-modulated sine wave, differentiating circuit means differentiating said rectangular wave to provide sets of differentiated pulses at the rise and fall, respectively, of said rectangular wave, and sawtooth wave generating means triggered by one of said sets of differentiated pulses.

5. A speed control system according to claim 1, wherein said means for providing the control signal further includes pulse width modulating circuit means providing a wave which is pulse width modulated in accordance with said comparison signal.

6. A speed control system according to claim 5, wherein said pulse width modulating circuit means provides said wave only when said comparison signal exceeds a predetermined level.

7. A speed control system according to claim 6, wherein said means for providing the control signal further includes integration circuit means integrating the pulse width modulated wave to provide said control signal in the form of a varying DC voltage.