GB2063595A

GB2063595A - Monitoring electric motors

Info

Publication number: GB2063595A
Application number: GB7939548A
Authority: GB
Original assignee: Fujitsu Fanuc Ltd
Current assignee: Fanuc Corp
Priority date: 1979-11-15
Filing date: 1979-11-15
Publication date: 1981-06-03
Also published as: GB2063595B

Abstract

Failure of a tachogenerator TG driven by an electric motor DCM and providing an output used to control operation of the motor may lead to runaway of the motor. Circuitry for detecting this fault operates in dependence upon the tachogenerator output and a driving voltage applied to the motor, and may be arranged to provide an abnormality indicating response if the tachogenerator output has a value outside a predetermined operating range (e.g. less than Vr2) at a time when the driving voltage is within a preselected working range (e.g. greater than Vr1). In another embodiment the absolute value of the difference between signals representing the tachogenerator output and the driving voltage is compared with a threshold. The response provided may be used to initiate measures for checking such runaway of the motor. <IMAGE>

Description

SPECIFICATION Circuitry for detecting an abnormality in the operation of a controlled motor arrangement This invention relates to circuitry for detecting an abnormality in the operation of a controlled motor arrangement, for example an abnormality detecting system for DC motor control apparatus adapted to drive a DC motor at a rotational speed which follows a command speed signal.

In DC motor control apparatus in which a tachogenerator (tachometer) or like speed signalling device is coupled with a DC motor to perform drive control of the DC motor in accordance with a signal that is the difference between an actual-speed output signal from the speed signalling device and a command speed signal applied to drive the DC motor at a rotational speed represented by the command speed signal, absence of the actualspeed output signal owing to failure of the speed signalling device can produce such an effect on the said difference signal that the rotational speed of the DC motor rises abnormally high. The DC motor can thus enter a runaway state, possibly leading also to damage in a load driven by the DC motor.

According to a first aspect of the present invention there is provided, for use in monitoring operation of a controlled motor arrangement comprising an electric motor coupled to drive a speed signalling device for providing an electrical actual-speed output signal dependent upon the running speed of the motor, abnormality-detection circuitry operable, in dependence upon the said actual-speed output signal and a driving voltage applied to the motor, to provide an abnormality-indicating response to an operating condition characterised by the actual-speed output signal having a value outside a predetermined operating range at a time when the said driving voltage is within a preselected working range.

According to a second aspect of the present invention there is provided an abnormality detecting system for DC motor control apparatus in which a DC motor is driven in accordance with a signal of the difference between a revolving speed signal from a speed detector coupled with the DC motor and a command speed signal to drive the DC motor at a revolving speed indicated by the command speed signal, characterized in that when a voltage to be applied to the DC motor and the revolving speed signal from the speed detector are both detected, the operation of DC motor control apparatus is decided as normal; and when the voltage to the DC motor is detected but the revolving speed signal from the speed detector is not detected, the operation of the DC motor control apparatus is decided as abnormal and an alarm signal is provided.

Thus, in an embodiment of the present invention, when a voltage to be applied to a DC motor and a rotational speed signal from a speed detector coupled with the DC motor are both detected, the DC motor arrangement is regarded as running normally. When the voltage to the DC motor is detected but the rotational speed signal is not detected, the arrangement is regarded as running abnormally and an alarm signal is produced.

Reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a block diagram of a first embodiment of the present invention; and Figure 2 is a block diagram of circuitry contained in another embodiment of this invention.

In the embodiment shown in Fig. 1, a tachogenerator TG is coupled with a DC motor DCM. A speed command signal SPC and an actual-speed output signal B from the tachogenerator TG are applied to an adder AD to obtain a signal representing the difference between the aforementioned two signals. The output from the adder AD is amplified by an amplifier AMP and applied to the DC motor DCM to drive it at a rotational speed that varies with the speed command signal SPC.

Circuitry is included to detect abnormality of the output of the tachogenerator TG, this circuitry operating in dependence upon drive voltage, applied to the DC motor DCM, and the actual-speed output signal from the tachogenerator TG. Thus, a voltage A applied to the DC motor DCM is compared with a constant refere-.se voltage Vr1 in a comparator CMP1, and the actual-speed signal B is compared with a constant reference voltage Vr2 in a comparator CMP2. When A > Vr1 and B > Vr2, the outputs from the comparators CMP1 and CMP2 are both logic "1".Accordingly, the output from a gate circuit G1 is "O", so that a bistable FF1 is retained in an initial state, in which it is reset by a reset signal rst such that the signal at a Q output terminal of the bistable is "0", indicating the normal state.

When a fault is developed in the tachogenerator TG such that an appropriate rotational speed signal B is not provided thereby, A > Vr1 and B < Vr2, causing the comparators CMP1 and CMP 2 to provide outputs "1" and "0", respectively. As a consequence, the output from the gate circuit G1 becomes "1" to set the bistable FF1 and thereby make the signal at the Q output terminal become ''1", this response serving to provide an alarm signal "arm" constituting an abnormality detection.

It is desirable to select the voltage Vr1 and Vr2 so that, in the normal state, the output from the comparator CMP1 becomes "1" at least (very shortly) after the output from the comparator CMP2 becomes "1".

The above embodiment has been described in connection with the case where the DC motor DCM rotates in one direction, but in the case of a DC motor rotating in both forward and reverse directions there may be provided respective abnormality detecting arrangements for forward and reverse rotations.

Fig. 2 shows in block form the principal part of another embodiment of this invention, in which a level converter LVC is provided for deriving from the voltage A, to the DC motor, a voltage A' equal to the normal voltage value of the rotational speed signal B from the tachogenerator and in which the converter output A' is provided to an adder AD1 for an addition of A' + (- B) and then the adder output is applied to an absolute value circuit ABS via a filter composed of a resistor R and a capacitor C. The output from the adder AD1 becomes positive or negative in dependence on the rotational direction of the DC motor, but this output is converted by the absolute value circuit ABS to a signal of one preselected polarity.The output from the absolute value circuit ABS and a constant reference voltage Vr are compared by a comparator CMP3, and when the difference between the output A' from the level converter LVC and the rotational speed signal B is larger, that is, when the revolving speed signal B is not available due to a fault in the tachogenerator, the output from the comparator CMP3 becomes "1" to set a flip-flop FF2, causing output Q to be set to "1" '' so as to provide an alarm signal "arm".

This embodiment employs only one comparator although the DC motor rotates in both forward and reverse directions.

A tachometer of a kind that generates an AC voltage or pulses of a frequency proportional to the rate of rotation of the DC motor may be employed in place of the instrument TG, which directly provides a DC voltage proportional to its rate of rotation, if such a tachometer is provided with a converter for deriving the rotational speed signal B by converting the AC voltage, or the pulses, into a DC voltage.

Accordingly, in an embodiment of the present invention concerned with DC motor control aparatus in which a speed detector is coupled with a DC motor, the operation of a tachogenerator is regarded as normal when a voltage for driving the DC motor and a rotation speed signal from the speed detector are both detected, and upon occurrence of an abnormality such as, for example, disconnection of the tachogenerator, no rotational speed signal is provided therefrom, so that an alarm signal is produced as a response to indicate the occurrence of abnormality. This response may be utilized to bring about control measures for checking (terminating or preventing) runaway of the DC motor. Such control can be effected automatically or manually.

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

Claims

1. For use in monitoring operation of a controlled motor arrangement comprising an electric motor coupled to drive a second signalling device for providing an electrical actual-speed output signal dependent upon the running speed of the motor, abnormality-detection circuitry operable, in dependence upon the said actual-speed output signal and a driving voltage applied to the motor, to provide an abnormality-indicating response to an operating condition characterised by the actual-speed output signal having a value outside a predetermined operating range at a time when the said driving voltage is within a preselected working range.

2. Abnormality-detection circuitry as claimed in claim 1, operatively connected in such a controlled motor arrangement in which the electric motor is a D.C. motor and the speed signalling device is a tachogenerator, the arrangement including electrical supply means connected to drive the motor in accordance with a signal that is a measure of difference between the said actual speed output signal and a command speed signal applied to the arrangement, when it is in use, to bring about operation of the motor at a rotational speed which is dependent upon magnitude of the command speed signal.

3. Abnormality-detection circuitry as claimed in claim 1 or 2, including a first comparator connected for comparing the said driving voltage with a first reference voltage, and a second comparator connected for comparing the said actual-speed output signal with a second reference voltage, the circuitry having a normal operating condition in which the respective electrical outputs from the first and second comparators are such as to indicate that the driving voltage is greater than the first reference voltage and that the actualspeed output signal is greater in voltage value than the second reference voltage, and there being response means connected to receive the outputs of the comparators such as to provide the said abnormality-indicating response if the output of the first comparator indicates that the driving voltage is greater than the first reference voltage at an instant when the output of the second comparator indicates that the actual-speed output signal is smaller in voltage value than the second reference voltage.

4. An abnormality detecting system for DC motor control apparatus in which a DC motor is driven in accordance with a signal of the difference between a revolving speed signal from a speed detector coupled with the DC motor and a command speed signal to drive the DC motor at a revolving speed indicated by the command speed signal, characterized in that when a voltage to be applied to the DC motor and the revolving speed signal from the speed detector are both detected, the operation of DC motor control apparatus is decided as normal; and when the voltage to the DC motor is detected but the revolving speed signal from the speed detector is not detected, the operation of the DC motor control apparatus is decided as abnormal and an alarm signal is provided.

5. An abnormality detecting system for DC motor control apparatus according to claim 4, which comprises a first comparator for comparing the voltage to the DC motor with a reference voltage, and a second comparator for comparing the revolving speed signal from the speed detector with a reference voltage, wherein when the outputs from the first and second comparators are equal to each other, the operation of the DC motor control apparatus is decided as normal, and wherein when the outputs from the first and second comparators are different from each other, the operation of the DC motor control apparatus is decided as abnormal and the alarm signal is provided.

6. An abnormality detecting system for DC motor control apparatus according to claim 4, which comprises a level converter for converting the level of the voltage to the DC motor, a circuit for obtaining the absolute value of the difference between the output signal from the level converter and the revolving speed signal from the speed detector, and a comparator for comparing the output signal from the circuit with a reference voltage, wherein when the output signal from the circuit is larger than the reference voltage, the operation of the DC motor control apparatus is decided as normal, and wherein when the output signal from the circuit is smaller than the reference voltage, the operation of the DC motor control apparatus is decided as abnormal and the alarm signal is provided.

7. Abnormality-detection circuitry, as claimed in claim 1 and substantially as hereinbefore described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

8. An electrical motor in combination with supply circuitry therefore and a tachogenerator and abnormality-detection circuitry, the combination being substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

8. Abnormality-detection circuitry, operatively connected in a controlled motor arrangement, as claimed in claim 2 and substantially as hereinbefore described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

9. An electrical motor in combination with supply circuitry therefore and a tachogenerator and abnormality-detection circuitry, the combination being substantially as hereinbefore described with reference to Fig. 1 or Fig.

2 of the accompanying drawings.

CLAIMS (4 Mar 1981)

6. Abnormality-detection circuitry, as claimed in claim 1 and substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

7. Abnormality-detection circuitry, operatively connected in a controlled motor arrangement, as claimed in claim 2 and substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.