Electricity: motive power systems – Positional servo systems – Antibacklash systems
Patent
1993-01-21
1994-11-22
Ro, Bentsu
Electricity: motive power systems
Positional servo systems
Antibacklash systems
G05B 19407
Patent
active
053672380
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a servo control method for controlling a servo-motor, which actuates a feed rod of a table and the like constituting a machine tool, and more particularly to a backlash acceleration correction applied when a shifting direction of the feed rod is reversed.
BACKGROUND ART
In machine tools, when an actuating direction of a servo-motor for actuating the table and the like is reversed, it is usual that a driven part of the machine cannot promptly response to or follow the reversing movement of the servo-motor because of backlash of feed screw or the effect of friction. For this reason, when a machine tool is performing profile machining, a protrusion may be formed on a cut surface of the workpiece when a shifting direction of a feed rod equipped in the machine tool is reversed.
For example, it is supposed that the machine tool operates to cut the workpiece in an arc shape on a plane defined by two axes of X-axis and Y-axis. And then, the table moves from one quadrant to another quadrant when the table is driven to move toward the plus in the direction of the X-axis and toward the minus in the direction of Y-axis. In this instance, if the table is actuated to move continuously in the same direction with respect to the Y-axis and, to the contrary, actuated to turn toward the minus in the direction of the X-axis, it is expected that no problem will occur with respect to the Y-axis because the cutting operation is continuously and smoothly carried out at the same speed in the direction of the Y-axis. However, the positional deviation in the direction of the X-axis becomes "0" and therefore, first of all, its torque command value becomes smaller, so that the servo-motor cannot reverse its turning direction immediately due to friction, and also, the shifting direction of the table cannot be immediately reversed due to backlash of a feed screw provided for feeding the table. Thus, the shifting movement of the workpiece in the X-direction cannot follow the shifting command and, therefore, there is caused a delay in the response of the workpiece. As a result of such a delay, a protrusion will be formed on the arc-shaped cut surface.
In order to eliminate or reduce this kind of protrusion, so-called backlash acceleration has been employed in such a manner that, when a shifting direction is reversed, a positional backlash correction is applied to a positional deviation and further, when the positional deviation is reversed, the servo-motor is accelerated in its reversing direction by adding an adequate amount of correction (i.e. an acceleration amount) to the speed command in order to reduce the protrusion in the transition phase from one quadrature to another quadrature, as disclosed, for example, in the Unexamined Japanese Patent Application JP, A, 4- 8451.
Furthermore, to reduce the amount of the positional deviation in a servo motor system for controlling a machine tool, a feedforward control is employed. Especially, in the case where a machine tool operates to cut a workpiece in a high-speed operating mode, a time lag in the servo system will cause an error in finished cut shape of the workpiece.
In order to reduce such a shape error, as disclosed in the Japanese Patent Application Serial No. 2-301154, filed by the same applicant of the present application, there has been developed a feedforward control wherein a feedforward amount is obtained by smoothing a shift command supplied from a numerical control apparatus, and thus obtained feedforward amount is added to a speed command that is calculated as an output of a position loop by multiplying a positional deviation by a position gain, thereby executing a speed loop processing on the basis of this corrected speed command.
This feedforward control will be explained with reference to FIG. 4. DDA (Digital Differential Analyzer) 10 splits a shift command Mcmd supplied from a CNC (Computer-equipped Numerical Controller) at a regular interval of a distribution period into shift commands corresponding to position and spe
REFERENCES:
patent: 5015935 (1991-05-01), Iwashita
patent: 5157597 (1992-10-01), Iwashita
patent: 5204602 (1993-04-01), Iwashita
Iwashita Yasusuke
Sakamoto Keiji
Fanuc Ltd.
Ro Bentsu
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