Electricity: motive power systems – Positional servo systems
Patent
1993-09-30
1995-04-11
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
318571, 318600, 318569, 31856818, G05B 1101
Patent
active
054061822
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to a method of controlling a servomotor for actuating feed shafts of a machine tool or robot arms, and more particularly, to a feedforward control for a servomotor.
DESCRIPTION OF THE RELATED ART
When controlling feed shafts of the machine tool or robot arms by the servomotor, especially when carrying out high-speed cutting operation by the machine tool, shape errors usually occur due to time lag in following-up of the servo system. Thus, to reduce the positional deviation and correct the servo delay, a feedforward control used to be incorporated into the position loop. For such feedforward control, in each of position-speed processing cycles, a shift command is differentiated. Then, this differentiated value is multiplied by a feedforward coefficient and added to a speed command obtained by the ordinary position loop processing for correcting the speed command. Then, the speed loop processing is executed on the basis of this corrected speed command.
A typical distribution cycle (ITP cycle), which is generally defined by a time required for transmitting a shift command from a numerical control apparatus to the servo system, i.e., to a position loop, is about 8 msec. Typical cycles of the position loop and the speed loop cycles are 2 msec to 1 msec respectively. In the position loop, the ITP cycle is equally divided, so that the shift command supplied from the numerical control device is evenly given to each position loop cycle.
Consequently, even if an acceleration/deceleration time constant is occasionally added to the shift command outputted from the numerical control device, the shift command is equally modified in each position-speed loop cycle Ts within the same ITP cycle. In other words, the shift command remains constant within the same ITP cycle at a value determined by the given acceleration/deceleration time constant. Thus, the value of the shift command varies largely immediately before and after the ITP cycle goes to the next cycle.
It is feared that this sudden change may become a significant large value. If this value is differentiated in a feedforward term, it will become larger. Thus, the speed command will contain a high frequency component. It will, therefore, become unable to be followed up in the speed loop. Hence, the positional deviation will cause hunting phenomenon, thereby further causing a motor or a machine to be accompanied by a significantly large shock.
To overcome this kind of drawback, the inventor of the present invention has proposed a method of controlling a servomotor with a smoothing operation (i.e. smoothing processing) wherein the hunting phenomenon is eliminated by inserting an acceleration/deceleration processing into the feedforward term of the position control and speed control, as disclosed in the International Application (PCT/JP90/00380) which was later published as W090/11562.
The present inventor has further improved the servomotor control method and developed a method by employing an advance smoothing processing, in which, unlike the case of the aforementioned smoothing method characterized by a mere averaging of the past data, the shift commands of the position/speed loop cycle coming before and after the relevant ITP cycle are averaged by using the shift command to be outputted after the relevant ITP cycle. The value obtained through this advance smoothing processing, as a feedforward quantity, is added to the speed command obtained through the ordinary position loop processing (Refer to the international Application PCT/JP92/00666 and PCT/JP92/01150).
According to the advanced-smoothing processing an average value A of the shift commands is obtained by the following equation (1). Thus obtained average value is multiplied by a feedforward coefficient .alpha., and the product is used as a position feedforward amount. (1)
Where, N represents a value obtained by dividing the ITP cycle by the position-speed loop cycle; Z.sup.-1, a delay corresponding to one position-speed loop processing cycle; and
REFERENCES:
patent: 3958109 (1976-05-01), Doherty et al.
patent: 5248921 (1993-09-01), Kato et al.
English translation of International Search Report, Apr., 1993.
Fanuc Ltd.
Masih Karen
Shoop Jr. William M.
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