Field of the invention

This invention relates to the control of drive systems f transportation mechanisms typified by but not confined t industrial shuttle beam transfer devices, and more particularly to a method of and apparatus for controllin the speed of an electric motor so that a body driven by the motor is moved in accordance with a pre-deter ined l of motion.

Background to the invention

In the transportation of an object from one position to another as typified by but not confined to industrial shuttle type transfer systems, second order effects of acceleration and deceleration are applied to the object and the transportation mechanism, the amplitudes of whic are dependent on the law of motion employed. If the motion law involves abrupt changes in the rate of acceleration or deceleration, shock forces of considerab magnitude are liable to be applied to the object being transported and the transportation mechanism, with the consequent possibility of damage to the object and the mechanism.

It is known that these second order effects are minimal when the law of motion employed is that of a cycloid curve, with the velocity changing continuously over the full movement distance in accordance with the general equation y=c cosx + c between the limits x=180 and x=540

degrees. With this law of motion, the rate of change of acceleration follows thee equation y=sin x between the limits specified, with no abrupt changes to apply shock forces to the object being transported or the mechanism involved.

This known effect has hitherto been exploited by a method wherein a controlled change of velocity is imparted to a transportation mechanism by a mechanical arrangement of gears and levers driven by an electric or other force motor which may be started and stopped at the limits of travel, but which runs at substantially constant speed throughout the period of movement.

This means of producing cycloidal acceleration and deceleration has the desirable characteristic of providing accurate control of the initial and final position of the object being transported, but has the disadvantage of high equipment cost and requires the driving motor to overcome the friction and inertial effects of the gearing mechanism in addition to providing sufficient force for the acceleration of the transportation mechanism and the object being moved.

The known effect has also been exploited by means of second order closed loop servo mechanisms in which the transportation system is constrained to follow a pre¬ determined velocity profile by monitoring the position occupied by or volocity ot tne .uecliαnism c discrete intervals of time or distance, and applying a corrective signal or force when the actual position or velocity at any point diverges from the theoretical value determined for that point.

Such systems are complex and require integration of velocity and positional data with response lags determin by gain limits and stability constraints, and require th use of costly servo motors to provide the force necessar to drive the transportation system.

Further known means of exploiting the advantages of cycloidal velocity control employ electronic or other methods to directly control the speed of an electric or other force motor to produce cycloidal acceleration and deceleration by continuous or stepwise adjustment of the velocity control parameter over a pre-determined period time.

Such means may share with this invention the advantages simplicity and low equipment cost but they are unable to provide and maintain the postional accuracy obtainable with the invention disclosed herein without frequent adjustment to correct errors caused by variation in the operting performance or characteristics of the components contained in the control mechanism.

It is therefore an object of this invention to provide a simple reliable and low cost means of controlling the velocity imparted to a transportation mechanism by an electric or other force motor to produce velocity changes in accordance with a pre-determined law of motion and regulated during the deceleration phase by the motion imparted to the transportation mechanism in such a way as determine the position at which zero velocity occurs.

The invention

According to one aspect of the invention, there is

provided a method of producing a drive in accordance with a predetermined law of motion such as a cycloidal law for a body linearly driven by an electric motor, according to which, during deceleration of the motor to a stopped condition, a speed-determining parameter of the energising current supplied to the motor is incremented to reduce the motor speed in accordance with the predetermined law and in dependence on a sequence of digital valves read out from a memory device, the reading out of the digital values is sequenced by an encoding device itself driven responsively to rotation of the motor shaft, the encoding device being preset to match the reading out of the final digital value to the position of the driven body at which the motor is to be stopped, and when the digital value representing zero velocity is read out the motor comes to rest and a brake is applied.

According to another apsect of the invention, there is provided apparatus for producing a drive in accordance with a predetermined law of motion such as cycloidal law for a body linearly driven by an electric motor, comprising a memory device having stored therein a sequence of pre-seleσted digital values, an encoding device producing a sequence of output signals responsively to rotation of the motor shaft, means whereby the encoder output signals are utilised to read out the digital values stored in the memory device, a signal converting device whereby the read-out digital values are utilised to increment a speed-determining parameter cf the energising current to the motor so that the motor speed is reduced towards the stopped condition in accordance with the predetermined law, the encoding device being preset to match the reading-out of the digital value representing zero velocity to the position of the driven body at which

the motor is to be stopped.

A brake may be provided for instantaneously stopping the motor when the last digital value representing zero velocity is read out, thereby to prevent any possible uncontrolled movement.

It is known that a suitable electric motor whose speed i controllable by a parameter (amplitude or frequency) of the energising current supplied to the motor coils, can caused to produce a drive at its output shaft which follows a required law of acceleration and deceleration. However, the present invention provides, in particular, solution to the problem of stopping the motor of the driven body, eg the locating table, at an exact given position, which is a prime requirement in many situations.

Although reference is made above to use of the invention in driving a shuttle beam transfer device, the present invention is not limited to this application. The invention may be used, for example, to stop a moving, electrically driven vehicle in order to avoid a damaging impact. In this case, the encoding device will initiate the deceleration phase of motion responsively to the operation of a detector which senses when the vehicle passes a point which is a predetermined distance from the position at which an impact would occur, stopping the vehicle exactly at the end of the predetermined distance in accordance with a predetermined preferably cycloidal law of deceleration.

When the invention is employed to control a repetitive back and forth motion, such as a shuttle beam transfer

device, deceleration in both directions of movement will be controlled in the aforesaid manner. Conveniently, the acceleration phase will also be controlled by reading out digital values from the memory device, eg an EPROM, but the control will preferably be an open loop technique rather than the closed loop technique used during the deceleration phase. Thus, an encoding device is generally not required for the acceleration phase, as the exact position at which the acceleration phase is completed is generally not important. On the contrary, means may be provided for adjusting the interval at which the digital values are read out during the acceleration phase, so that the overall time span of this phase is likewise adjustable.

The acceleration and deceleration phases may or may not be immediately successive.

While microprocessor control is possible, this is not preferred. Thus, in a preferred arrangement, the memory device is an EPROM having say 8K bits, the first 4K of which are employed to store digital values for producing acceleration in accordance with a predetermined preferably cycloidal law (180 to 360 degrees) and the second 4K of which are employed to store digital values for producing deceleration in accordance with the same law (360 to 540 degrees) . A pair of counters, for example 12 bit counters, are employed to read the digital values respectively during the accelerai_lcr_ anύ det_.elc_rc.Lic.ri phases. The acceleration phase counter causes the first 4K bits to be read during a time span set by an adjustable voltage generator the output of which is fed to a voltage to frequency converter providing the trigger for the counter. The deceleration phase counter is triggered by

the encoding device driven by the output shaft of the electric motor. One or more devices such as limit switches reset the counters on completion of the reading out of each set of digital values.

The signals read out from the EPROM are preferably fed to a digital to analogue converter and thence via an amplifier to an inverter which provides the energising current controlling the motor speed. The deceleration phase is controlled over an exact number of revolutions o the motor shaft corresponding to the number of EPROM addresses employed to reduce the velocity parameter down to zero and the number of addresses incremented by the encoding device in each revolution of the shaft. The zer velocity value output from the EPROM can be accompanied b a flag at the zero velocity address to cause the motor to be disconnected and a mechanical or electrically actuated brake applied to prevent further uncontrolled movement of the driven mechanism. The resetting of the counters, coupled with reversal of two phases of the motor supply, enables motion of the driven body in the opposite direction (reverse rotation of the motor) to be controlle in an exactly analogous manner.

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

Figure 1 illustrates graphically a cycloidal velocity and acceleration characteristics; and

Figure 2 illustrates a control system in accordance with the invention for controlling an electric motor in accordance with a cycloidal law.

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Detailed description of the drawings

Figure 1 needs little comment. The graph shows the velocity curve (in full) and the acceleration curve (dotted) for a cycloidal motion.

In practice, the output shaft of the motor may drive a spur gear engaging a rack associated with a workpiece locating table which is to be repetitively driven back and forth to enable a manufacturing or assembly operation to be performed on each of a succession of workpieces.

Referring to the drawing, an a.c. geared brake motor 10, is controlled by the energising current supplied by an inverter control circuit 12 which is supplied via an operational amplifier 14 with the output signals of a digital to analogue converter 16 receiving the digital values read out from an 8K x 8 (8192 bit) EPROM 18.

Addresses 1 to 4096 in the EPROM hold digital values for producing acceleration in accordance with a predetermined law of motion such as a cycloidal law, and addresses 4097 to 8192 hold digital values for producing deceleration in accordance with the same predetermined law.

Acceleration of the body is produced by an open loop technique. A twelve bit counter 20 triggered by a voltage to frequency converter 22, fed from an adjustable voltage generator 24, causes the reading-out of the first set of digital values (1 to 4096) during a time span set at the voltage generator. The motor is thus accelerated from zero to maximum speed.

The count may be held at address 4096 so that the motor

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then maintains a constant speed until a signal received from an external source (a detector triggered by moveme of the locating table or other driven body) initiates th deceleration phase.

Deceleration is produced in accordance with a closed loo technique, in order to stop the motor after a predetermined number of revolutions of its output shaft, and thus stop the driven body at a predetermined require position, eg where a manufacturing or assembly operation is to be performed.

Thus, a twelve bit counter 26 causes the reading out of the second set of digital values (4097 to 8192) as it is stopped or by output signals obtained from an encoder 28 driven by the motor shaft. If, for example, a total travel of 2 metres is required during the deceleration phase to stop the driven body at a given position, and o revolution of the motor output shaft produces 1/2 mm of travel, then the encoder is pre-arranged to produce an output signal each time slightly less than one revolutio of the motor shaft has been completed (4000/4192 of a revolution) .

At address 8192 the byte value is zero, and a flag at th address disconnects the motor 10 and applies a brake 30.

A limit switch 32 resets the system ready for reverse rotation of the motor with two phases of the supply reversed, both counters receiving a reset signal.

It is to be noted that, although not apparent from the simplified block diagram shown in the drawing, acceleration is open loop and deceleration is closed loo

during reverse rotation of the motor, in exactly analogous manner to the method described for the motion in the first direction.

Various modifications of the above-described method and apparatus are possible within the scope of the invention hereinbefore defined.

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