GB2122773A

GB2122773A - Stepper motor control

Info

Publication number: GB2122773A
Application number: GB08217110A
Authority: GB
Inventors: Mervyn Brian Oakes
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1982-06-12
Filing date: 1982-06-12
Publication date: 1984-01-18

Abstract

A control system for a stepper motor 2 has the analogue signals from a transducer 4 for actual and desired motor positions compared by a differential amplifier 6 and the analogue difference signal used to drive, via a voltage controlled oscillator 10, a counter 20. If the difference signal voltage is outside the range +/-V1, the counter 20 counts up or down depending upon the sense of the difference signal. The digital output from the counter 20 is decoded by decoder 22 into a suitable digital form for driving the stepper motor 2 towards the desired position. <IMAGE>

Description

SPECIFICATION Stepper motor control This invention relates to arrangements for controlling stepper motors.

A stepper motor is a motor which can be moved, by the application of one or more discrete control signals, between a plurality of discrete positions. Thus, stepper motors are inherently suitable for control by digital electronics.

In a first known arrangement for controlling a stepper motor, a computer directly controls the stepper motor. In this arrangement the computer has to produce a new control signal in the form of a new bit pattern to drive the stepper motor (via power translation devices such as VMOS transistors) to each new position; the computer has also to compute the direction and total movement required and also has to provide some form of failure detection, preferably with a general monitoring and supervisory function. These duties are very time-consuming and can frequently require the computer to spend a large proportion of its time just in controlling one or more stepper motors, possibly slowing the computer's response to more fundamental needs.

In a second known arrangement for controlling a stepper motor, the position of the motor is sensed by a position encoder and a digital signal produced thereby is passed to a digital processor (usually in the form of a microprocessor or logic array) which compares the sensed motor position with a desired position and produces appropriate control signals in the form of bit patterns to drive the stepper motor (via power translation devices such as VMOS transistors) accordingly. In this arrangement the position encoder produces a parallel digital output corresponding to the position of the stepper motor. Such position encoders are reliable, accurate and produce a ready-formatted digital output.Unfortunately, such position encoders are costly, complex, of large size and require a large number of connections (one connection for each bit of the output and one or more connections for power lines).

It is also known to control a D.C. motor for movement between desired positions by sensing the position of the motor using an analogue position transducer and comparing the analogue position signal produced thereby with a desired position signal to produce an error signal which drives (via a suitable power amplifier) the motor.

Such an analogue control arrangement is simple, but suffers from a number of disadvantages.

Normally, power dissipation in the power amplifier is high and may well exceed the useful power that is delivered to the motor; control theory must be applied to enable the feedback loop criteria to be optimised and open loop characteristic analysis is often required; also various lead or lag components may have to be inserted into the loop to achieve an acceptable degree of stability.

It is an object of the present invention to provide a circuit for controlling a stepper motor wherein the above disadvantages may be overcome or at least alleviated.

In accordance with the invention an arrangement for controlling a stepper motor comprises analogue position transducer means for providing at its output an analogue actual position signal representative of the actual position of the stepper motor; analogue comparator means connected to the output of the analogue position transducer means actual position signal and connected to receive an analogue desired position signal representative of a desired position of the stepper motor for providing at its output an analogue position difference signal representative of the difference between the actual position of the stepper motor and the desired position of the stepper motor; digital counter means connected to the output of the analogue comparator means for counting in response to said analogue difference signal and for providing at its output a digital count signal representative of its count; and decoding and power driving means connected to the output of the digital counter means for decoding the digital count signal into a suitable form and for amplifying the signal to a suitable power for application to the stepper motor to drive the stepper motor from the actual position to the desired position.

One arrangement in accordance with the invention for controlling a stepper motor will now be described, by way of example only, with reference to the accompanying drawing, which shows a block-schematic diagram of the arrangement.

Referring to the drawing, a rotary stepper motor 2 has attached thereto a position transducer in the form of a potentiometer 4. The output of the potentiometer 4 is connected to the inverting input of a differential amplifier 6.

The output of the differential amplifier 6 is connected via an amplitude modulus generator 8 to the control input of a voltage controlled oscillator (VCO) 1 0. The output of the VCO 10 is connected to one input (ii) of a three-input logical AND gate 12.

The output of the differential amplifier 6 is also applied to the inputs of two differential amplifiers 14 and 1 6, to the other inputs of which are applied reference voltages +V1 and -V1 respectively. The outputs of the amplifiers 14 and 16 are applied to the inputs of a logical OR gate 18, whose output is applied to an input (iii) of the AND gate 12.

The outputs of the amplifiers 14, 16 are also applied to direction sensing inputs of a counter 20. The output of the AND gate 12 is connected to the counting input of the counter 20. The output of the counter 20 is connected to the input of a logic decoder and power driver 22 whose output is connected to drive the stepper motor 2.

In controlling the stepper motor 2, the arrangement operates in the following manner.

The potentiometer 4 produces an analogue signal A representative of the actual position of the stepper motor. This signal is compared in amplifier 6 with an analogue signal D representative of the desired position of the stepper motor applied to the non-inverting input of the amplifier. Thus the differential amplifier 6 acts as an ' error: amplifier, its output E being representative of the error (if any) between its input signals.

The error signal E output from amplifier 6 is applied to the differential amplifiers 14, 1 6 which produce negative outputs unless the signal E is outside the range +V1. If the signal E is outside the range +V1 then the output of a respective one of the amplifiers 14 and 1 6 becomes positive, thus causing a positive voltage to be applied through the OR gate 18 to the input (iii) of the AND gate 12 and causing a positive voltage to be applied to a respective one of the direction inputs of the counter 20.

The amplitude modulus generator 8 produces a signal /E/ equal to the modulus of the error signal E from the amplifier 6, and this modulus signal /E/ is applied to the VCO 10. The VCO 10 produces an output F which oscillates at a frequency proportional to the amplitude of the error signal E.

The signal F is applied to the input (ii) of the AND gate 1 2. An enabling signal EN is normally applied tp the input (i) of the AND gate 12, but this may be withdrawn if it is desired to "freeze" the position of the stepper motor 2.

Assuming that the enabling signal is applied to the input (i) of the AND gate 12, then if the error signal E is outside the range +V1 (causing a positive voltage to be applied to the input (iii) of the AND gate 12) positive pulses will be applied from the AND gate 12 to the counting input of the counter 20 at the frequency of the signal F. In response to each pulse from the AND gate 12 the counter 20 counts up or down (depending on which of its direction inputs is positive) by one unit.

The count of the counter 20 is applied to the logic decoder and power driver 22 which decodes the digital count number of the counter 20 into a corresponding motor control bit pattern and applies this digital bit pattern to the motor 2 via VMOS power transistors (not shown).

It will be appreciated that the use of the VCO 10 enables the stepping rate to vary in accordance with the distance which the stepping motor has to travel, the rate of stepping being initially high but the rate reducing as the stepper motor approaches its desired position so as to prevent overshoot.

It will also be appreciated that in the above described arrangement for controlling a stepper motor the components 4 and 6 are analogue components, allowing a medium quality, singleturn potentiometer to be used as the position transducer, and requiring a minimum of interconnecting leads, whereas the components 20 and 22 are digital components which are readily suitable for connection to the stepper motor 2.

It will also be appreciated that the arrangement uses throughout uncomplex, commonly available and inexpensive components.

It will also be appreciated that the overall performance of the above described control arrangement is simple to calculate and simple to achieve in practice.

Claims

1. An arrangement for controlling a stepper motor, comprising: analogue position transducer means for providing at its output an analogue actual position signal representative of the actual position of the stepper motor; analogue comparator means connected to the output of the analogue position transducer means and connected to receive an analogue desired position of the stepper motor for providing at its output an analogue position difference signal representative of the difference between the actual position of the stepper motor and the desired position of the stepper motor; digital counter means connected to the output of the analogue comparator means for counting in response to said analogue difference signal and for providing at its output a digital count; and decoding and power driving means connected to the output of the digital counter means for decoding the digital count signal into a suitable form and for amplifying the signal to a suitable power for application to the stepper motor to drive the stepper motor from the actual position to the desired position.

2. An arrangement according to claim 1 further comprising range sensing means connected to the output of the analogue comparator means for preventing said digital counter means for counting when the value of said analogue difference signal lies within a predetermined range.

3. An arrangement according to claim 1 or 2 further comprising counting rate varying means connected to the output of said analogue comparator means for varying the rate of counting of said digital counter means in response to the amplitude of said analogue difference signal.

4. An arrangement according to claim 3 wherein said counting rate varying means comprises modulus generating means connected to the output of said analogue comparator means for generating at its output a signal representative of the modulus of said analogue difference signal and voltage controlled oscillator means connected to the output of said modulus generating means for producing at its output a series of pulses whose frequency is controlled by the voltage of said analogue difference signal, the output of said voltage controlled oscillator means being connected to said digital counter means.

5. An arrangement according to any preceding claim wherein said analogue position transducer means comprises a potentiometer.

6. An arrangement for controlling a stepper motor substantially as hereinbefore described with reference to the accompanying drawings.