Electricity: motive power systems – Positional servo systems – With compensating features
Reexamination Certificate
2002-08-07
2004-07-06
Leykin, Rita (Department: 2837)
Electricity: motive power systems
Positional servo systems
With compensating features
C318S630000, C318S633000, C318S638000, C318S652000
Reexamination Certificate
active
06759825
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control method and control apparatus for a feed system which comprises a drive mechanism for moving a slide along a straight line, a servo motor forgiving power to the driving mechanism, a rotational position detection means for detecting the rotational position of the servo motor, and a position detection means for detecting the position of the slide.
2. Description of the Prior Art
A feed system provided in an NC machine tool or the like comprises a drive mechanism for moving a slide such as a machine table along a straight line, a servo motor for giving power to the drive mechanism, a rotational position detection means for detecting the rotational position of the servo motor, etc. The drive mechanism comprises, for example, a ball screw (lead screw) driven by the servo motor, a nut screwed onto the ball screw, etc. and causes the table to move along the ball screw by means of the nut moving in the axial direction of the ball screw with the rotation of the ball screw.
The servo motor is feedback controlled by a control unit; in the case of a semi-closed system, the rotational position of the servo motor (equivalent to the position of the slide) and the rotational speed of the servo motor (equivalent to the feed speed of the slide) are feedback controlled based on the data detected by the rotational position detection means. On the other hand, in the case of a system employing full-closed feedback control, a position detection means such as a linear scale is included in order to detect the position of the slide, and the position control is performed based on the position data detected and fed back by the position detection means, while the speed control is performed based on the speed data detected and fed back by the rotational position detection means.
Here, a gap, or backlash, inherently exists between the thread grooves of the ball screw and nut and the balls constrained between them. There also occurs a lost motion due to the deflection of the ball screw. Accordingly, in the case of the semi-closed system that performs the position control by reference to the rotational position of the ball screw, when the direction of rotation of the ball screw, and hence the direction of movement (feed direction) of the slide, is reversed, a positioning error occurs by an amount equal to the amount of backlash and the degree of deflection of the ball screw. In the prior art semi-closed system, therefore, it has been practiced to detect the backlash amount in advance and to correct the position by an amount equal to the detected backlash amount when reversing the feed direction of the slide. In the full-closed feedback control, on the other hand, the positioning error due to the backlash or the deflection of the ball screw does not occur, since the control is performed by directly detecting the position of the slide.
Further, in both the semi-closed system and the full-closed system, when reversing the feed direction of the slide, a delay equal to the amount of backlash occurs in the reversing action of the slide (a delay in tracking the target position). In a machining center, when the tool is moved from one quadrant to another, for example, during a circular arc cutting, the above delay has resulted in the formation of a bump when cutting an outer circumferential arc, and a recess when cutting an inner circumferential arc. In the prior art, therefore, in order to obtain a smooth arc surface free from bumps or recesses, when reversing the feed direction of the slide it has been practiced to increase the feed speed during direction reversal (including the starting speed) in accordance with the backlash amount, thereby reducing the delay in tracking the target position.
The amount by which the feed speed is increased has been determined empirically by actually performing cutting operations, or based on correlation with the backlash amount. In the semi-closed system, the backlash amount is calculated from actual measurements, while in the full-closed system, the backlash amount is calculated from a difference between the position data of the slide detected by the position detection means and the position data of the slide calculated based on the rotational position data detected by the rotational position detection means.
However, the backlash amount is not constant, but increases with age due to the wear of the thread grooves of the ball screw and nut or the wear of the balls. As a result, if the amount by which the feed speed is to be increased during the reversal is left at its original setting, the backlash amount increasing with age cannot be accommodated and highly precise position control of the slide cannot be accomplished, thus leading to the problem that the bump or recess that occurs when switching from one quadrant to another gradually increases in size. Furthermore, if the pretension of the ball screw changes due to thermal expansion, the rigidity of the ball screw changes, and this change in rigidity causes the lost motion to vary. This also results in the problem that the bump or recess that occurs when switching from one quadrant to another changes in shape. In a machine tool designed to operate in a semi-closed control mode, this has lead to the problem that the accurate backlash amount and lost motion amount have to be re-measured periodically, that is, such time and labor consuming work has to be done periodically if highly precise machining is to be achieved.
On the other hand, in a machine tool designed to operate in a full-closed control mode, the above problem does not occur since the backlash amount and lost motion amount can be calculated from the position data detected by the position detection means and the rotational position data detected by the rotational position detection means, but in this case also, there arises the following problem.
That is, since the ball screw tends to thermally expand due to the heating of the screw supporting bearings and the frictional heat generated by the balls running in the thread grooves, and the balls tend to suffer deformation due to load, the backlash amount calculated from the position data detected by the position detection means and the rotational position data detected by the rotational position detection means, as described above, is inaccurate as it contains effects due to the thermal expansion of the ball screw and the deformation of the balls in addition to the actual backlash amount. As a result, if the amount by which the speed is to be increased during feed direction reversal is set based on such inaccurate backlash amount, highly precise position control of the slide cannot be accomplished.
In view of the above situation, it is an object of the present invention to provide a feed system control method and control apparatus that can calculate an accurate backlash amount in a periodic or non-periodic manner and can accomplish highly precise position control.
SUMMARY OF THE INVENTION
The present invention which solves the above problem concerns a feed system control apparatus and control method for controlling the operation of a feed system which comprises a drive mechanism for moving a slide along a straight line, a servo motor for giving power to the drive mechanism, a rotational position detection means for detecting the rotational position of the servo motor, and a position detection means for detecting the position of the slide, wherein the control apparatus and control method are provisioned to perform the processing of:
calculating position errors before and after a reversal in direction of movement of the slide, each from a difference between the position data of the slide detected by the position detection means and the position data of the slide calculated from the rotational position data of the servo motor detected by the rotational position detection means;
subsequently, calculating a backlash amount for the drive mechanism from a difference between the calculated position error before the reversal and the calcul
Leykin Rita
Mori Seiki Co. Ltd.
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