This invention relates to apparatus for controlling the position of the table of a machine tool, for example, and more particularly to apparatus for correcting a mechanical error of a driving system for driving a table or the like, and a pitch error of a position detector or the like.
To control the position of the table of a machine tool, apparatus shown in FIG. 3 has been used. In the apparatus shown in FIG. 3, an increment type position detector 24 is connected to an electric motor 23 which moves a table 21 in the horizontal direction through a well known ball screw 22, and the displacement data detected by position detector 24 is supplied to a detected position register 25 storing the absolute position reached by a previous movement (or an initial position set after the table has returned to an original position at the time of closing a source switch). The content of the register 25 is subtracted from the content of an instructed position register 26 by using a subtractor 27, and based on a difference thus obtained, the motor 23 is controlled through speed control apparatus 28. Position detection for feedback is made by indirectly detecting the position of table 21 by a position detector 24 provided for motor 23 so that the actual position of the table 21 and the detected position do not always coincide with each other due to errors caused by the accuracies of the ball screw 22 and the position detector 24, thus disenabling to accurately move table 21 to an instructed position.
For eliminating this problem, an error correction device 29 has been provided wherein the error between the actual position and the detected position of table 21 corresponding to the instructed position or the detected position is measured by a high precision measuring device so as to store the measured error in an error table or a circuit is constructed so that an approximate correction value or signal can be obtained. The errors or differences between the actual position and the detected position of the table can be made to cancel with ecch other by correcting the instructed position or detected position previously used with a corresponding error data so that it is possible to correctly move the table 21 to the instructed position.
However, with the prior art driving apparatus utilizing an increment type position detector, it can detect only relative movement data so that in order to obtain absolute position data, it is necessary to use an absolute position utilized as a reference. Accordingly, for setting the reference point of the increment type position detector it is necessary to return the driving device after closing the source switch, thus requiring an initializing operation for storing an initial value acting as an origin in the detected position register and an instructed position register 26, thus decreasing the operating efficiency.