Electricity: motive power systems – Positional servo systems – 'reset' systems
Patent
1990-12-03
1992-07-28
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
'reset' systems
318610, 318611, 318561, 318616, 364161, 364162, G05B 1136
Patent
active
051343541
DESCRIPTION:
BRIEF SUMMARY
Field of Art
The present invention relates to an apparatus which controls a servo motor that drives a machine tool to machine an object according to instructions from an command function generator and, more particularly, to a technique for compensating for the delay of the response of a servo motor incorporated in a machine whose internal characteristics differ according to direction because of gravity or other reasons, the delay being introduced when the motor is reversed or started.
BACKGROUND OF THE INVENTION
As shown in FIG. 1, a servo motor control circuit having a position feedback loop and a digitally controlled velocity loop is generally equipped with both a velocity feedback loop for controlling the rotational speed of a motor 10 and a current feedback loop for controlling the current flowing through the motor 10. In addition there is the position feedback loop for controlling the position, angular or otherwise of the movable mechanical part (not shown) of the motor 10. In the current feedback loop, the motor current I.sub.d is controlled so that instructed analog current I.sub.s ' equals motor current I.sub.d, as detected with a resistor 11. The motor velocity is controlled so that the difference .DELTA.V between motor velocity V.sub.d, found by a velocity-calculating portion 21, and velocity V.sub.s, instructed from a position control portion 19, is reduced down to zero. The motor position is controlled by the position feedback loop in such a way that the differenoe .DELTA.l between position l.sub.d and instructed value l.sub.s produced from an command function generator 16 is reduced down to zero. The position l.sub.d is detected by a position detector 15 such as a pulse encoder or a linear scale and counted by a position counter 20. Also shown in FIG. 1 are a velocity control portion 12, a D/A converter 14, a power amplifier 17, and subtracters 18a, 18b, 18c.
In the area surrounded by the dot-and-dash line, signals are processed digitally. Digital signals are indicated by , while analog signals are indicated by .fwdarw.. Usually, the position control section 19 performs an arithmetic function given by of PI (proportional plus integration) action, the velocity control portion 12 performs arithmetic functions given by ##EQU1## where K.sub.v and K.sub.I are constants, and .SIGMA. means summation. Where the velocity is controlled by making use of I-F action, the velocity control portion 12 which determines the relation among the instructed velocity V.sub.s, the velocity V.sub.d, and the instructed current 1, performs arithmetic functions given by ##EQU2##
In the prior art servo motor control apparatus construction as shown in FIG. 1, if the rigidity of the motor shaft is ideal, then the instructed motor velocity V.sub.s, and the motor current I.sub.d, which are taken when the rotation of the motor is reversed, show characteristics as indicated by 3OV.sub.s, 31V.sub.d, 32i.sub.d, respectively, in FIG. 2. In these graphs, the instant at which an instruction for reversing the direction is given at the origin, these three characteristics V.sub.s, V.sub.d, and I.sub.d are plotted against time t. I.sub.o is a value obtained by transforming the friction torque produced in the machine into a motor current. In the illustrated sample, the instructed velocity is reduced at a constant rate, i.e., the acceleration is a constant negative value. As can be seen from these graphs, when an instruction for reversing the direction is entered at instant t=0, the motor current I.sub.d indicated by the curve 32i.sub.d decreases gradually, but the velocity V.sub.d indicated by the curve 31Vd is kept null until the motor current I.sub.d reaches-I.sub.o, whereupon the motor begins to reverse. That is, a time delay of T.sub.1 occurs between the instant at which an instruction for reversing the direction is entered and the instant at which the motor begins to reverse.
Of course, this time delay produces an error in controlling the machining process. More specifically, as shown in FIG. 3, when a command pulse train is dis
REFERENCES:
patent: 4743822 (1988-05-01), Futquri et al.
patent: 4851754 (1989-07-01), Sakamoto et al.
patent: 4882526 (1989-11-01), Iino et al.
patent: 4999557 (1991-03-01), Inoue
Hiraga Yoshiji
Tohya Takayuki
Yamamoto Yo-ichi
Bergmann Saul M.
Kabushiki Kaisha Yaskawa Denki Seisakusho
Shoop Jr. William M.
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