Electricity: motive power systems – Positional servo systems – Adaptive or optimizing systems including 'bang-bang' servos
Patent
1998-03-24
2000-02-01
Nappi, Robert E.
Electricity: motive power systems
Positional servo systems
Adaptive or optimizing systems including 'bang-bang' servos
318560, G05B 1300
Patent
active
06020706&
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of controlling a servomotor for driving a feed shaft of a machine tool, etc., and more specifically, to a servomotor control method for preventing an overshoot during a positional feedback control in which the position of a movable part driven by the feed shaft is detected by a position detector such as a scale detector.
BACKGROUND ART
In positioning control of a feed shaft of a machine tool and the like using a servomotor, according to a conventional method for preventing an overshoot during positioning, the output torque of a servomotor is reduced by decreasing a value of an integrator in a velocity loop when a position deviation becomes close to "0" (i.e., a movable part of the machine comes close to a commanded position), to thereby reduce the overshoot.
However, in a full-closed loop control, the position of the movable part driven by the feed shaft is detected by a position detector such as a scale detector for performing a positional feedback control based on the detected position. Further, in a case of such control a mechanical system between the servomotor and the position detector such as a scale detector may have an insufficient rigidity. If so, then the servomotor will, due to the torsion of the mechanical system, advance too much by an amount corresponding to the torsion at the time when the position deviation has become close to "0" and the movable part has reached the commanded position. That is, when the rigidity of the mechanical system between the servomotor and the position detector is insufficient, the movable part is driven while the mechanical system is distorted. The result is that, when the position is detected by the position detector as being the commanded position, the servomotor actually takes a position which makes the movable part to advance beyond the commanded target position by the amount corresponding to the torsion of the mechanical system. When the position deviation becomes "0" and the positioning is completed to stop the rotation of the servomotor, the torsion of the mechanical system is released gradually, causing the mechanical movable part to move further forward by the amount corresponding to the torsion of the mechanical system (in the same direction as it has been moving), thereby to overshoot the commanded target position. As the movable part of the machine moves forward, the position deviation increases. Therefore, in order to cancel the increase in the position deviation, the servomotor moves in the opposite direction, so that the movable part is positioned at the commanded target position. Thus, in the full-closed loop positional control, the movable part tends to overshoot the commanded position due to the torsion of the movable part.
SUMMARY OF THE INVENTION
An object of the present invention is to prevent an overshoot of a movable part of a machine or to reduce an overshooting amount when positional control is performed in a full-closed loop.
A servomotor control method of the present invention comprises the steps of: detecting a position of a movable part of a machine, which is driven by a servomotor through a mechanical system; controlling a position of the movable part using a motion command distributed from a numerical control device and the detected position of the movable part by a feedback control system including a position loop; and positioning the movable part so that torsion of the mechanical system between the servomotor and the movable part is released by reducing a torque command for the servomotor for a period immediately before termination of the motion command inputted to the position loop.
In the case where the motion command distributed from the numerical control device is subjected to the acceleration/deceleration process by an acceleration/deceleration processing section and then inputted to the position loop, the torque command for the servomotor is reduced when an input to the acceleration/deceleration processing section becomes zero. In the case where the acceleration/
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Iwashita Yasusuke
Kawamura Hiroyuki
Fanuc Ltd.
Leykin Rita
Nappi Robert E.
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