Apparatus for and method of compensating for positioning error

A servo control apparatus comprising:

• a motor rotated by a driving current under the control of a current command signal;
• a ballscrew rotating with a rotation of said motor;
• a workpiece position detection means for detecting a position of said workpiece;
• a table, coupled to said ballscrew and supporting a workpiece, moving in a table travel direction in response to the rotation of said ballscrew;
• travel direction reversing timing signal generating means for generating a travel direction reversing timing signal indicating a reversal of travel direction of said table;
• ballscrew deformation error detecting means for detecting a ballscrew deformation error indicating the magnitude of a change in deformation amount of said ballscrew at the reversal of said table travel direction;
• ballscrew deformation error storage means for storing said ballscrew deformation error detected by said ballscrew deformation error detecting means;
• current command compensating means for combining said ballscrew deformation error stored in said ballscrew deformation error storage means with said current command signal from a point of time based on said travel direction reversing timing signal to compensate said current command signal;
• change slowing means for receiving the ballscrew deformation error stored in said ballscrew deformation error storage means and outputting to said position compensating means, a signal based on said received ballscrew deformation error that rises slowly with time;
• wherein the current command compensating means adds or subtracts the output signal of said change slowing means to or from the current command signal from a point of time based on the travel direction reversing timing signal to compensate said current command signal;
• second parameter detecting means for detecting a second parameter indicating the timewise change of state of the deformation amount of the ballscrew at the reversal of the table travel direction;
• and
• second parameter storage means for storing the second parameter detected by said second parameter detecting means;
• wherein the change slowing means outputs said signal that rises slowly with time based at least in part on said second parameter stored in said second parameter storage means.

A servo control apparatus comprising:

• a motor rotated by a driving current under the control of a current command signal;
• a workpiece detection means for detecting a position of said workpiece;
• a ballscrew rotating with the rotation of said motor;
• a table, coupled to said ballscrew and supporting a workpiece, moving in response to the rotation of said ballscrew;
• table position detecting means for detecting a position of said table;
• position command means for outputting a position command signal for specifying the position of said table;
• travel direction reversing timing signal generating means for generating a travel direction reversing timing signal indicating a reversal of travel direction of said table;
• ballscrew deformation error detecting means for detecting a ballscrew deformation error indicating the magnitude of a change in deformation amount of said ballscrew at the reversal of said table travel direction;
• ballscrew deformation error storage means for storing said ballscrew deformation error detected by said ballscrew deformation error detecting means;
• current command compensating means for combining said ballscrew deformation error stored in said ballscrew deformation error storage means with said current command signal from a point of time based on said travel direction reversing timing signal to compensate said current command signal;
• machine deformation detecting means for detecting a machine deformation error;
• machine deformation error storage means for storing the machine deformation error indicated by the magnitude of a change in difference between the position of said workpiece and the position of said table as detected by said table position detecting means at the reversal of said table travel direction;
• position command compensating means for combining said machine deformation error stored in said machine deformation error storage means with said position command signal from a point of time based on said travel direction reversing timing signal to compensate said position command signal;
• and
• motor driving means for rotating said motor so that the detection output of said table position detecting means matches the output of said position command compensating means.

In a servo control apparatus having a table coupled to a ballscrew and supporting a workpiece, said ballscrew being rotated by a motor in response to a driving current, said table having a position along a travel direction corresponding to rotation of said ballscrew, a method of controlling a servo comprising the steps of:

• generating a driving current for rotating said motor;
• detecting the position of said table;
• detecting a position of said workpiece;
• outputting a position command signal for specifying the position of said table;
• generating a travel direction reversing timing signal upon reversal of travel direction of said table;
• detecting a ballscrew deformation error indicating the magnitude of a change in deformation amount of said ballscrew at the reversal of said table travel direction;
• compensating said driving current by combining said driving current with said ballscrew deformation error thus detected;
• detecting a machine deformation error indicating the magnitude of a change in difference between the position of the workpiece and the position of said table at the reversal of said table travel direction;
• compensating said position command by combining said machine deformation error with said position command from a point of time based on said reversal of said table travel direction;
• and
• generating a current command signal for driving said motor;
• and
• driving said motor so that the position of said table matches said compensated position command.