Acceleration Signal Corrected Motor Speed Control System

1. Field of the Invention

This invention relates to an improvement in a method of controlling the speed of a motor. This invention particularly relates to an improvement in a method of controlling the speed of a low-speed motor mounted on a supporting base the position of which is changed by rotation, swinging or the like when the motor is operating.

2. Description of the Prior Art

In portable audio apparatuses such as cassette decks and radiation image recording and read-out systems using a stimulable phosphor as described, for example, in Japanese Patent Application No. 57(1982)-193767, which are placed and used in a vehicle, the motors used therefor are mounted on supporting bases the positions of which are changed during operation of the motor installations. In the motors of this type, the stator is subject to the same movement as that of the supporting base, and the rotor is required to undergo a desired movement with respect to the stator.

In the aforesaid motors, when a change in the position of the supporting base causes a change in angular velocity in a plane normal to the shaft of the motor, namely when angular acceleration arises with respect to the shaft of the motor, the change in the position of the supporting base causes the rotation speed of the motor to change. This occurs because the rotor has inertia and tends to maintain the previous movement including that of the supporting base carrying the motor thereon.

In order to control the speed of the motor the rotor of which is subject to a movement relative to the stator, it is necessary to regulate speed control so as to compensate for the movement of the rotor relative to the stator. The compensation for the movement of the rotor relative to the stator is of course not necessary when the movement is at a constant angular velocity, but is necessary only when such movement involves angular acceleration of the motor shaft. Actually, the aforesaid compensation is found to be significant only when the degree of angular acceleration generated with respect to the shaft of the motor by the aforesaid relative movement is significantly large with respect to the rotation speed of the rotor of the motor.

Although there have been such motors wherein the stator undergoes movement which is likely to disturb the rotation speed of the rotor, in general, the degree of angular acceleration of the stator relative to the rotor was very small with respect to the rotation speed of the rotor so that there were few cases where compensation for the movement of the supporting base relative to the rotor was required.

However, in the aforesaid radiation image recording and read-out systems using a stimulable phosphor, or the like, the rotation speed of the rotor of the motor used in the systems is very low and, in general, ranges from about 0.4 rpm to 2 rpm. On the other hand, the degree of angular acceleration produce by the movement of the supporting base on the shaft of the motor is nearly 200 mrad/sec.sup.2. Therefore, the effect of the movement of the supporting base on the rotation speed of the rotor is not negligible. In the speed control of the motor used in the aforesaid new applications, it is desirable to eliminate the adverse effect of a change in the position of the stator supporting base on the rotation speed of the rotor.

The primary object of the present invention is to provide a method of controlling the speed of a motor supported on a supporting base the position of which is changed during operation of the motor, the method eliminating the adverse effect of the movement of the supporting base on the rotation speed of the rotor.

Another object of the present invention is to provide a method of controlling the speed of a motor supported on a supporting base the position of which is changed during operation of the motor, the method being very suitable for the speed control of a low-speed motor supported on such a supporting base.

The aforesaid objects are accomplished by a method of controlling the speed of a motor supported on a supporting base the position of which is changed by rotation, swinging or the like during operation of the motor, wherein the improvement comprises correctively modifying one of the speed control command signals by means of a signal detected by an angular acceleration detecting means already well known in the art, per se, such as a rotary encoder, a resolver, an Inductosyn or a tachometer generator, which is arranged on said supporting base for limiting detection of angular acceleration of angular displacement of the shaft of said motor, in directions perpendicular thereto i.e. in a plane normal to the shaft of the motor found to be significant as aforementioned.

In the method of the present invention, the degree of angular acceleration of the supporting base is calculated by differentiating the output signal of the angular acceleration detecting means such as a rotary encoder, a resolver, an Inductosyn or a tachometer generator, positioned on the motor supporting base so that the axis of the angular acceleration detecting means is in the same direction as that of the shaft of the motor. A negative sign is given to the angular acceleration signal thus calculated, and the signal is fed, as one of the speed control command signals, to the speed control system of the motor. For example, when a motor exhibiting a rated torque of 40 kg-cm, a rated rotation speed of 1 rpm and a rotor inertia of 20 kg-cm-sec.sup.2 was supported on a supporting base which was swung so as to exert angular acceleration of 200 mrad/sec.sup.2 on the shaft of the motor, a speed disturbance within the range of .+-.0.1 rpm inevitably arose in the conventional technique. However, when the method in accordance with the present invention was used in this case, no disturbance in the rotation speed was observed and it was found that the method in accordance with the present invention very accurately eliminated the adverse effect of the movement of the supporting base on the rotation speed of the rotor. The method of the present invention is very suitable for controlling the speed of a low-speed motor mounted on a supporting base the position of which is changed by rotation, swinging or the like during operation of the motor.

The single FIGURE of the drawing is a block diagram showing an embodiment of the method of controlling the motor speed in accordance with the present invention.

The present invention will hereinbelow be described in further detail with reference to the accompanying drawing.

By way of example, the drawing shows the case where a DC motor 1 fed with a direct current of a desired level obtained by a gate turn-off thyristor 3 and speed-controlled through the negative feedback of a speed signal detected by a tachometer generator 4 is mounted on a supporting base 2 the position of which is changed during operation of the motor 1. The gate turn-off thyristor 3 receives an alternating current, rectifies it and feeds a direct current to the motor 1. A speed command signal A is compared with a speed signal B detected by the tachometer generator 4 driven by the motor 1. A speed error signal C indicating the amount of deviation between the speed command signal A and the speed signal B is then amplified by an amplifying circuit 5 to generate a current command signal D. The current command signal D is then compared with a current signal E detected by a shunt circuit 6. A current error signal F indicating the amount of deviation between the current command signal D and the current signal E is amplified by an amplifying circuit 7 to generate a speed control signal G, which is then fed to the gate turn-off thyristor 3. An angular acceleration detecting means 8 (as disclosed for example in U.S. Pat. No. 3,295,377 comprising a rotary encoder, a resolver, or the like), and a differentiating circuit is supported on the supporting base 2 for detecting the degree of angular acceleration of the supporting base 2 in a plane normal to directionally limited to displacement of the motor shaft the rotational axis of the shaft of the motor 1. Thus directionally limited angular velocity values when detected by the rotary encoder, the resolver or the like are differentiated to obtain an angular acceleration signal H. The sign of the angular acceleration signal H is reversed (e.g. a positive angular acceleration signal is converted to a negative acceleration signal) and correctively added to the speed command signal A for comparison of the corrected speed command signal with the speed signal B to obtain the speed error signal C. The speed error signal C is processed as described above to control the speed of the motor 1.

In this embodiment, since the speed command signal A for controlling the speed of the motor 1 is corrected on the basis of the angular acceleration signal H indicating the degree of angular acceleration of the supporting base 2, it is possible to eliminate the adverse effect of a change in the position of the supporting base 2 on the rotation speed of the motor 1.

When a motor exhibiting a rated torque of 40 kg-cm, a rated rotation speed of 1 rpm and a rotor inertia of 20 kg-cm-sec.sup.2 was secured to the supporting base 2 and the speed thereof was controlled by the method shown in the drawing, no disturbance in the rotation speed of the motor was observed, and the technical effect of the present invention was verified. When the speed control was conducted according to the conventional technique, a speed disturbance within the range of about .+-.0.1 rpm was detected.