Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Patent
1995-04-18
1997-01-28
Sircus, Brian
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
318609, 318630, 364161, G05B 1101
Patent
active
055980775
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a control apparatus and a control method for a servomotor which drives feed axes of a table or the like of a machine tool, etc., and more particularly, to backlash acceleration correction for the time of reversal of the moving direction of the feed axes.
BACKGROUND ART
In reversing the direction of drive of servomotors for driving tables, etc. in a machine tool or the like, the machine normally cannot be reversed at once, due to influences of backlash and friction of feed screws. When the quadrant changes while arcuate cutting or the like is being carried out by the machine tool, projections are formed on an arcuate cut surface. In subjecting a workpiece to arcuate cutting on X- and Y-axis planes, for example, the quadrant changes as the machine is driven in the positive direction with respect to the X axis and in the negative direction with respect to the Y axis. When the movement crosses the X axis, for example, the machine is driven in the negative direction with respect to the Y axis without change in direction and in the negative direction with respect to the X axis, switched from the positive direction. In this case, cutting is carried out at the same speed with respect to the Y axis as before the changeover. With respect to the X axis, however, the position deviation becomes "0", so that the torque command value is small, and friction prevents the servomotors from being reversed at once. Moreover, the movement of the tables cannot follow up movement commands and is subject to delay, due to the backlash of the feed screws for feeding the tables. The reduction of the torque command value and the generation of the backlash result in formation of projections on arcuate cut surfaces.
Conventionally, in order to prevent the formation of the projections on the cut surfaces or reduce the height of the projections, a motor control method based on the so-called backlash acceleration correction has been carried out so that the position deviation is subjected to positional backlash correction when the moving direction is reversed. Further a suitable value (acceleration value) is added to a speed command to effect acceleration in the reverse rotating direction of the servomotors, thereby reducing quadrant projections.
FIG. 11 is a block diagram illustrating a motor control method based on backlash acceleration correction (see Jpn. Pat. Appln. KOKAI Publication No. 3-228106, for example) as one method of backlash acceleration. According to this conventional motor control method based on backlash acceleration correction, backlash acceleration correction is effected in a manner such that the value in a speed control loop integrator (term of K1/S in FIG. 11) just before the reversal of direction is obtained, and a value obtained by inverting the sign of this value is used as a target value after the reversal. Further, the backlash acceleration correction is effected in such a manner that and in each speed control loop process within a set time after the reversal of direction, the product of a suitable constant value and a value obtained by subtracting the value in the integrator for each speed control loop process from the target value is used as a backlash acceleration value for each speed control loop process.
However, the conventional motor control method described above has a problem that satisfactory backlash acceleration correction cannot be achieved when the speed of arcuate motion increases.
Referring to FIGS. 12A-12H, the reason for this circumstance will be described. Ideally, the value in the speed control loop integrator should be equal to the sum of frictional torque and acceleration torque components. As the motor rotation is reversed, the sign of the frictional torque is inverted, as indicated by ft1 in FIG. 12C and ft2 in FIG. 12G. On the other hand, the acceleration torque component forms a cosine wave which is obtained by differentiating a speed sine wave with time, and its absolute value has a maximum at the point of time of the reversal of moto
REFERENCES:
patent: 4271385 (1981-06-01), Azusawa
patent: 5101146 (1992-03-01), Teshima
patent: 5204602 (1993-04-01), Iwashita
patent: 5440218 (1995-08-01), Oldenkamp
patent: 5467004 (1995-11-01), Matsuo et al.
Iwashita Yasusuke
Matsubara Shunsuke
Okita Tadashi
Fanuc Ltd.
Sircus Brian
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