Electricity: motive power systems – Positional servo systems – Antibacklash systems
Patent
1991-08-12
1993-04-20
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
Antibacklash systems
31856822, 318561, 318632, 364566, G05B 1101
Patent
active
052046028
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a control method for a servomotor for driving a feed screw, and more particularly, to a method for controlling the drive of the servomotor so that a feed delay caused at the time of the reversal of the feed screw due to backlash of the feed screw can be eliminated.
BACKGROUND ART
In a machine having a movable part arranged to be reciprocated along each of one or more feed screws, typically, each feed screw is rotated in a desired direction by means of a corresponding servomotor, which is drivingly controlled by a servo system, so as to move the machine movable part in desired directions along the feed screws, in order to perform a desired operation. When the moving direction of the movable part is reversed, however, the movable part sometimes cannot accurately follow up a command for movement, due to backlash of the feed screws or the like, thus failing to perform the desired operation with accuracy.
In machining a workpiece into a hollow cylinder, for example, servomotors for X and Y axes typically are rotated so that a cutting tool mounted on a tool table moves relatively to the workpiece along a tool path within an XY plane. In doing this, the cutting tool on the Y axis is first moved in the negative direction along the X axis, as well as in the negative direction along the Y axis, in order that it is moved along the tool path within the second quadrant of the XY plane, for example. Then, the tool is moved in the positive direction along the X axis, as well as in the negative direction along the Y axis, in order that it is moved within the third quadrant of the XY plane. Further, the tool is moved in the fourth and first quadrants of the XY plane. When the tool transfers between the adjacent quadrants, a positional deviation in the servo system corresponding to the axis (feed screw) associated with the reverse movement of the tool is generally reduced to zero, and a torque command from the servo system is reduced, while a frictional force produced in the mechanical system acts in different directions before and after the transfer. Thus, when the tool transfers between the adjacent quadrants, the servomotor cannot immediately produce an output torque great enough to resist the frictional force, so that the servomotor sometimes cannot be reversed at once. When the tool transfers from the second quadrant to the third, for example, the reversal of the X-axis motor may suffer a delay. Since the feed screws of the tool table are subject to backlash, moreover, the tool table sometimes cannot follow up the movement command during the transfer between the quadrants, so that the reverse movement of the table is delayed. If the follow-up performance is lowered in this manner, the cutting work is insufficient, so that projections are formed on the machined surface of the workpiece.
Conventionally, in order to eliminate these problems, backlash correction and backlash acceleration correction are effected in the servo system when the servomotor is reversed from a first rotating direction to a second rotating direction. For example, a conventional digital servo circuit (FIG. 8), which is provided with a processor for executing position, speed and current loop processes on software-basis in response to a position command from a numerical control device having a computer built-in, is arranged to receive backlash correction data which is delivered from the control device when the sign of the position command is inverted. When the correction data is inputted, the processor which functions as backlash correction means 13 adds this correction data to a value (positional deviation) stored in an error counter 10. Then, the processor which functions as position loop means 11 multiplies the positional deviation after the correction by a position gain Kp, thereby generating a speed command VCMD. Further, the processor which functions as speed loop means 12 and current loop means (not shown) successively executes the speed and current loop processes in accordance with a speed c
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Fanuc Ltd
Martin David
Shoop Jr. William M.
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