Interface System for Servo Controller

1. Field of the Invention

The present invention relates to an interface system for a servo controller, between a servo system for driving a load such as a machine tool with a plurality of servomotors and a numerical control apparatus.

2. Description of the Related Art

In the above-mentioned field, servomotors are driven by command signals from a numerical control apparatus (hereinafter referred to as an "NC apparatus") or the like for accurately controlling a multi-axis machine tool or the like. Such servomotors have conventionally been controlled by an analog servo systems. With the recent advance of computerized control technology, various efforts have been made for efficiently controlling a plurality of servomotors with a single control apparatus.

FIG. 2 of the accompanying drawings is a block diagram of a servomotor control circuit and driver circuit. A control circuit 1 on the side of an NC apparatus is connected to a system bus line of a servo control microprocessor (servo CPU) so that servo system operation can be determined. The control circuit 1 comprises a memory 2 for reading in a position command, an arithmetic unit 3 for processing the position command and a present position signal that is fed back, and a positioning controller 4 for issuing a speed command VCMD based on the result of the arithmetic operation. A speed control circuit 5 on the side of a servomotor includes an arithmetic unit 6 for processing the speed command VCMD and a speed signal which is fed back, a speed controller 7, a current loop processor 8, a current controller 9, and an alarm processing circuit 10. Pulse width modulation (PWM) signals from the current controller 9 are supplied through an insulated amplifier 11 to an inverter 12 composed of a transistor bridge for driving a servomotor 13 with a determined drive current.

Designated at 14 is a detector such as a rotary encoder for detecting the speed and the present position of the servomotor.

The control circuit 1 receives data such as a position command from the servo CPU connected through a common RAM to a main CPU in the form of a microprocessor which effects sequential processing, and is also supplied with the speed and present position data fed back from the servomotor, through an interface circuit coupled to a data bus of the servo CPU.

The speed control circuit 5, which drives the inverter 12 connected to the servomotor 13, effects prescribed feedback control for a current and a speed on the servomotor 13. There are as many servomotors 13 as the number of the axes of the machine tool to be controlled. Where DC servomotors are employed, four transistors of the driver circuit shown in FIG. 3 are driven by the PWM signals A through D from the speed control circuit 5.

With the aforesaid conventional servo controller interface system, there are required as many speed control circuits 5 as the number of the axes of the machine tool which are to be servo-controlled. For highly accurate servo control, data should be transferred at a high speed between the NC apparatus and the speed control circuit 5 for the servo motor. Therefore, the interface circuit is complex out of necessity, and the maintenance of the hardware in general, and particularly that connected to the servomotor is difficult.

It is an object of the present invention to provide an interface system for a servo controller in the form of a microprocessor, which effectively utilizes time-sharing processing in digital control of servomotors, for achieving multi-axis time-sharing control with a low-cost servo controller of a simple circuit arrangement, without lowering the accuracy of the servo controller in the form of a microprocessor.

According to the present invention, there is provided an interface system for a servo controller which processes a position command signal from a numerical control apparatus and data fed back from a servomotor to control the servomotor, the interface system comprising a control circuit on the side of the numerical control apparatus for being supplied with a current value and speed data for the servomotor and for generating control signals for the current, speed, and position of the servomotor, and a driver circuit on the side of the servomotor, composed of a plurality of switching elements controllable by the control signals from the control circuit.

With the interface system for a servo controller according to the present invention, the servo controller can easily be software-implemented irrespective of the number of axes of a servo system to be servo-controlled, and multiple axes can be controlled on a time-sharing basis by a microprocessor, resulting in reduction of hardware.

FIG. 1 is a block diagram of an embodiment of the present invention;

FIG. 2 is a block diagram of a conventional interface system for a servo controller; and

FIG. 3 is a diagram of a DC servomotor driver circuit.

An embodiment of the present invention will hereinafter be described in specific detail with reference to the drawings.

FIG. 1 is a block diagram of an embodiment of the present invention. A control circuit 15 comprises a memory 2 for reading in a position command, an arithmetic unit 3 for processing the position command and a present position signal that is fed back and computing an error therebetween, a positioning controller 4 for issuing a speed command VCMD based on the result of the signal processing, an arithmetic unit 6 for processing the speed command VCMD and a speed signal that is fed back, a speed controller 7, a current loop processor 8, and a current controller 9.

A servo amplifier circuit 16 on the side of a servomotor has an insulated amplifier 11 and an alarm processing circuit 10, and is connected to the control circuit 15 through a prescribed number of data lines 17. PWM signals A through D are supplied via the data lines 17 and the insulated amplifier 11 to an inverter 12 which serves as a driver circuit for a servomotor 13.

The control circuit 15 for controlling the current, speed, and position of the servomotor 13 is disposed, together with a servo CPU, a main CPU, and a common RAM, on the side of an NC apparatus. Therefore, a local bus on the side of the servomotor can be coupled directly to a system bus of the main CPU. The inverter 12 may be composed of only a final-stage driver element such as a power transistor. Most of the complex control circuit elements may be disposed together on the NC apparatus side, whereas only a minimum number of circuit elements, such as the servo amplifier circuit 16 and the power transistor, are disposed on the side of the servomotor.

With the servo controller interface system thus arranged, a control mechanism, which has heretofore been disposed on the the servomotor side for current and speed control, is disposed on the NC apparatus side. Therefore, even if the number of axes of a servo system is increased, data to be fed back therefrom may be stored in the common RAM connected to the system bus line of the servo CPU. As a result, the interface between the servo system and the NC apparatus is not complicated. Mutiple axes can be controlled on a time-sharing basis without impairing the accuracy of a servo controller in the form of a microprocessor.

The present invention is not limited to the servo system employing a DC motor as described in the aforesaid embodiment, but is also applicable to a servo system employing an AC motor.

As described above, the interface system for a servo controller according to the present invention allows the servo controller to be easily software-implemented irrespective of the number of axes of a servo system to be servo-controlled, also allows multiple axes to be controlled on a time-sharing basis by a microprocessor, and can reduce hardware. Therefore, the interface system is preferably applied to robots and machine tools which are to be controlled highly accurately.