GB2128375A

GB2128375A - DC motor control system

Info

Publication number: GB2128375A
Application number: GB08228681A
Authority: GB
Inventors: Robert Frederick Gardner
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-10-07
Filing date: 1982-10-07
Publication date: 1984-04-26
Also published as: GB2128375B

Abstract

A speed control system for a DC electric motor M e.g. of machining equipment receives a demanded velocity signal in digital form from A/D converter 16, an actual speed feedback signal in digital form from A/D converter 14 and a motor current feedback signal in digital form from A/D converter 12. A processing system combines the digital demanded velocity and speed feedback signals, multiplies and delays the combined signal, then combines this with the current feedback signal and further multiplication and delay provides a digital speed-control signal for the motor. The processing system may be a microprocessor with a self-teach mode for setting-up optimal multiplication and delay factor which may then be stored in an EAROM. <IMAGE>

Description

SPECIFICATION DC Motor control system This invention relates to a speed control system for DC electric motors.

In a known control system, the DC motor is connected across a bridge the four arms of which include power switching transistors. These are controlled by a pulse width modulator so that alternately one opposite pair of transistors are in conduction and then the other opposite pair of transistors are in conduction. An analogue signal representing the actual speed is developed by a tachogenerator and fed back for combination with a "damanded velocity" analogue signal, the signal thus produced being supplied to a first amplifier.

The output of this amplifier is then combined with an analogue motor-current feedback signal and the signal thus produced is supplied to a second amplifier, the output of which is used to control the pulse width modulator.

In accordance with this invention, the demanded velocity signal is converted to (or provided as) a digital signal, the actual speed and motor current feedback signals are converted to digital signals, and the first and second amplifier (with their analogue gain and time delay functions) are replaced by a microprocessor control which effects the gain and time delay by digital multiplication and digital ramps respectively. With particular advantage, the control system may have a self-teach mode for setting up purposes, in which mode it monitors the response of the motor to a predetermined change in demand signal and incrementally changes the digital multiplication and ramp factors to achieve optimum values for the ideal response.

An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawing, the single figure of which is a schematic block diagram of a DC motor servo-control system in accordance with this invention.

The DC electric motor M is connected across a bridge the four arms of which include power switching transistors and these are controlled by a pulse width modulation power amplifier 10. A motor current feedback signal is passed to an analogue-to-digital converter 1 2 and a tachogenerator T is provided to pass a motor speed feedback signal to an analogue-to-digital converter 14. The demanded velocity signal is applied to an analogue-to-digital converter 1 6 (or otherwise is supplied in digital form).Within the microprocessor control (indicated schematically by the dotted box), the digitised (or digital) demanded velocity signal is combined with the digitised motor speed feedback signal the resultant undergoes gain and time delay at 20, 21 by the use of digital multiplication and digital ramp and then is combined with the motor current feedback signal. The resultant signal of this combination undergoes gain and time delay at 22, 23 (again by the use of digital multiplication and digital ramp). The signal now provided is current limited as indicated at 24, in dependence upon the digital current feedback signal as monitored at 26, and used at 28 for the software control of the pulse width modulation power amplifier 10.

As previously mentioned, the control system may incorporate a self-teach mode for setting up purposes. The values for the digital multiplying and ramp functions for optimum response of the motor are determined, firstly for the current loop and then for the speed loop, according to an appropriate algorithm and then stored, for example in EAROM (electrically alterable read only memory). In an initialisation phase each time the apparatus is used, the microprocessor retrives the stored values from this EAROM.

The microprocessor may also take account of preset speed limits, current limits and/or other limits which may either be pre-stored or otherwise keyed into the apparatus by the operator.

Instead of the tachogenerator providing an analogue feedback signal which is then digitised it may provide the feedback signal in digital form, for example as a train of pulses fed direct to and interpreted by the microprocessor.

Facility may be provided, for example at 30 or 31, for setting an offset value which is automatically combined with the signal at that point.

The described control may be effected with a single microprocessor or where desired with more than a single microprocessor. Moreover, the system may be arranged for the simultaneous control of more than one motor, for example in a multi-axis machining equipment.

Claims (filed on 7/10/83) 1. A speed control system for a DC electric motor, comprising signal processing means for receiving a demanded velocity signal in digital form and for receiving a feedback signal in digital form according to actual speed of the motor, said signal processing means being arranged to combine said digital demanded velocity and actual speed feedback signals and multiply and delay the combined signal to provide a digital control signal, and means responsive to said digital control signal to control application of power to the DC electric motor in accordance therewith.

2. A speed control system as claimed in claim 1, in which said signal processing means is arranged to receive a feedback signal in digital form according to actual current drawn by the motor and to combine this digital feedback signal with the multiplied and delayed said combined signal and effect further multiplication and delay to provide said digital control signal.

3. A speed control system as claimed in claim 2, further arranged to effect a current limit upon said control signal in accordance with the actual current feedback signal.

4. A speed control system as claimed in any

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION DC Motor control system This invention relates to a speed control system for DC electric motors. In a known control system, the DC motor is connected across a bridge the four arms of which include power switching transistors. These are controlled by a pulse width modulator so that alternately one opposite pair of transistors are in conduction and then the other opposite pair of transistors are in conduction. An analogue signal representing the actual speed is developed by a tachogenerator and fed back for combination with a "damanded velocity" analogue signal, the signal thus produced being supplied to a first amplifier. The output of this amplifier is then combined with an analogue motor-current feedback signal and the signal thus produced is supplied to a second amplifier, the output of which is used to control the pulse width modulator. In accordance with this invention, the demanded velocity signal is converted to (or provided as) a digital signal, the actual speed and motor current feedback signals are converted to digital signals, and the first and second amplifier (with their analogue gain and time delay functions) are replaced by a microprocessor control which effects the gain and time delay by digital multiplication and digital ramps respectively. With particular advantage, the control system may have a self-teach mode for setting up purposes, in which mode it monitors the response of the motor to a predetermined change in demand signal and incrementally changes the digital multiplication and ramp factors to achieve optimum values for the ideal response. An embodiment of this invention will now be described, by way of example only, with reference to the accompanying drawing, the single figure of which is a schematic block diagram of a DC motor servo-control system in accordance with this invention. The DC electric motor M is connected across a bridge the four arms of which include power switching transistors and these are controlled by a pulse width modulation power amplifier 10. A motor current feedback signal is passed to an analogue-to-digital converter 1 2 and a tachogenerator T is provided to pass a motor speed feedback signal to an analogue-to-digital converter 14. The demanded velocity signal is applied to an analogue-to-digital converter 1 6 (or otherwise is supplied in digital form).Within the microprocessor control (indicated schematically by the dotted box), the digitised (or digital) demanded velocity signal is combined with the digitised motor speed feedback signal the resultant undergoes gain and time delay at 20, 21 by the use of digital multiplication and digital ramp and then is combined with the motor current feedback signal. The resultant signal of this combination undergoes gain and time delay at 22, 23 (again by the use of digital multiplication and digital ramp). The signal now provided is current limited as indicated at 24, in dependence upon the digital current feedback signal as monitored at 26, and used at 28 for the software control of the pulse width modulation power amplifier 10. As previously mentioned, the control system may incorporate a self-teach mode for setting up purposes. The values for the digital multiplying and ramp functions for optimum response of the motor are determined, firstly for the current loop and then for the speed loop, according to an appropriate algorithm and then stored, for example in EAROM (electrically alterable read only memory). In an initialisation phase each time the apparatus is used, the microprocessor retrives the stored values from this EAROM. The microprocessor may also take account of preset speed limits, current limits and/or other limits which may either be pre-stored or otherwise keyed into the apparatus by the operator. Instead of the tachogenerator providing an analogue feedback signal which is then digitised it may provide the feedback signal in digital form, for example as a train of pulses fed direct to and interpreted by the microprocessor. Facility may be provided, for example at 30 or 31, for setting an offset value which is automatically combined with the signal at that point. The described control may be effected with a single microprocessor or where desired with more than a single microprocessor. Moreover, the system may be arranged for the simultaneous control of more than one motor, for example in a multi-axis machining equipment. Claims (filed on 7/10/83)

1. A speed control system for a DC electric motor, comprising signal processing means for receiving a demanded velocity signal in digital form and for receiving a feedback signal in digital form according to actual speed of the motor, said signal processing means being arranged to combine said digital demanded velocity and actual speed feedback signals and multiply and delay the combined signal to provide a digital control signal, and means responsive to said digital control signal to control application of power to the DC electric motor in accordance therewith.

4. A speed control system as claimed in any preceding claim, further in which said processing means includes means for setting an offset value for combining with the (or each) feedback signal in operation.

5. A speed control signal as claimed in any preceding claim, in which said processing means further comprises a store for preset limits (e.g.

speed limits, current limits) which in operation said processing means is responsive.

6. A speed control system as claimed in any preceding claim, arranged with a self-teaching mode for monitoring said feedback signals in response to predetermined changes in the demanded velocity signal and altering multiplication and delay factors for the (or each) combined signal to achieve a predetermined response for the motor.

7. A speed control system for a DC electric motor, substantially as herein described with reference to the accompanying drawing.

8. Machining equipment comprising at least one DC electric motor having associated therewith a speed control system as claimed in any preceding claim.