The present invention relates .to a control system for a motor and is particularly concerned with the system for controlling motor to move a member driven by the motor, such as a robot arm, rapidly to an accurately predeter¬ mined position.

It is already known to mount on a motor an angular position transducer or shaft encoder to provide informa¬ tion regarding the position of the rotor. When the rotor is in a predetermined position, the position can be maintained by means of a negative feedback loop in which the motor current is controlled by the output of the angular position transducer.

If, for example, the, encoder produces a triangular waveform as the motor shaft rotates, then at every second zero crossing there is a point of stable equilibrium at which movement of the shaft away from the equilibrium point results in a motor current which applies a torque to return the rotor to the equilibrium position.

In such a system where a motor is controlled by feed - back from a position transducer, every other zero crossing is a point of unstable equilibrium. In this case, movement away from the point of equilibrium results in a torque which rapidly increases to move the rotor away from the point of unstable equilibrium.

Though the servo system described above can be used to maintain the position of a motor once it has reached a predetermined position, it offers no solution to the problems of reaching that position.

The invention seeks to - enable a rotor to be moved rapidly to an accurately predetermined position.

According to the one aspect of present invention, there is provided a method of controlling the position of the rotor of a motor connected to a position transducer operative to produce a cyclic signal in which at consecutive zero crossings the time derivative of the output signal is of opposite sign, the method comprising controlling the motor current in a servo loop in dependence upon the output of the position transducer whereby to define a plurality of positions of stable equilibrium spaced from one another by points of unstable equilibrium and, in order to move the rotor by one increment, applying a curreαt pulse to the motor to provide an initial bias to deflect the rotor in the direction of desired motion and reversing the polarity of the feedback signal in the servo loop to change the point of stable equilibrium into one of unstable equilibrium and the adjacent point of unstable equilibrium into one of stable equilibrium, whereby the rotor moves rapidly to the next zero crossing at which the rotor is prevented from further movement by the servo loop.

In accordance with a second aspect of the invention, there is provided a control circuit for an electric motor which comprises an angular position transducer for producing a cyclic signal having consecutive zero crossings of opposite slope, means for applying a current to the motor in dependence upon the output signal of the transducer to stabilise the position of the motor at a zero crossing, means for reversing the polarity of the feedback signal to provide incremental movement of the rotor to a point corresponding to an adjacent zero crossing and means for applying an initial acceleration pulse to the motor to ensure movement in the desired direction when the polarity is reversed.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which:

Figures la and lb show output signals in phase quadrature derived from an angular position transducer, and

Figure 2 shows a block circuit diagram of a control system in accordance with the invention.

In Figure 1, there is shown the output of a transducer. The waveform illustrated is triangular but alternatively it could be in the form of a sine wave. The zero crossings of the triangular wave form designated A and B correspond to points of equilibrium in that at these points the motor current determined by the servo loop will be zero and there will be no torque on the motor to move. Some of these points will however be stable and others unstable.

Assuming that the points designated A are those of stable equilibrium. In these cases, movement of the rotor to one side of the point will result in a current of such polarity as to tend to return the rotor to the point of stable equilibrium. At the other points, any movement away from the point of equilibrium will result in a current causing the motor to move still further away from that point. If the polarity of the feedback signal is reversed, then the points A will be unstable and thepoints B will be stable.

It is known to position the motor by other means until its position corresponds to one of the points of stable equilibrium and having reached that position to switch on the feedback loop for the purpose of maintaining that position.

In the present invention, each of the zero crossings is a possible point at which the motor can be brought to rest. If the feedback signal is reversed when the motor is at position Al then that point will be unstable but both of the adjacent points Bl and B2 will be stable. The motor shaft will move in an arbitrary direction and on reaching one of the points Bl or B2 will come to rest again.

In the system of the invention, to move the motor from the position Al to the position Bl a pulse is first applied to displace the rotor in the right direction and subsequently the polarity of the feedback loop is reversed. In other words, the rotor is given an accele¬ ration in the right direction before the polarity is reversed. Using this method of control, a conventional motor can be used to provide accuracy of positioning comparable with that of a stepper motor with the rotor moving by increments from one zero crossing to the next. Furthermore, on reaching the desired position the servo loop will ensure that no further movement should occur.

A circuit for implementing this- method of control is shown in Figure 2 in block diagram form. The output of the shaft encoder is applied to a circuit 10 which produces position and velocity information. The output signal of the circuit 10 is applied by a way of a switch 16, a summation amplifier 14 and a gained controlled amplifier 20 to produce a control signal for the motor on the output line, thereby completing the servo loop. The movement of the motor is effected by means of a control logic circuit 18 receiving three inputs from suitable position setting means. The first is a signal as shown in Figure IB in quadature with the output signal of the transducer applied to the circuit 10, the second signal is a pulse indicating that movement is required and the third is ^~ a signal on a line to indicate direction of movement required.

When it is desired to move the rotor, the control logic circuit 18 applies an input to the acceleration unit 12 which superimposes a pulse on the error signal. The duration of this pulse is determined by the quadrature signal (see Fig. IB), the acceleration pulse being

^' removed when or before the quadrature signal crosses through zero corresponding to a peak of the error signal. The control logic 18 also applies a signal to the gain controlled amplifier 20 to alter the polarity of the control signal thereby converting the stable point of equilibrium to one of unstable equilibrium and causing an incremental movement of the motor.

By switching the polarity of the feedback loop every 180 electrical degrees, it is possible to increment the motor fairly rapidly to the desired position and when the desired position is reached the polarity of the feed-back loop is allowed to remain fixed so that the motor remains in the desired set position.

The purpose of the switch 16 is to enable the feedback loop to be open circuited so as to permit open loop control of the motor. When the motor is to be moved by a considerable distance, the position control servo is open circuited and the motor is controlled by a separate control loop to achieve coarse positioning in as rapid a time as possible, once the arm is within close distance of its desired position the incremental servo described above is switched in by closing the switch 16 to take over control and move the arm in small steps until the desired position is achieved with precision.

An advantage of the control system as described above is that while it does not use stepper motors, it enables control of a motor of a robot or a machine tool to be effected by means of a digital computer with which the logic circuit may interface directly.