Servo control device for performing correction based on amount of stretch or contraction of ball screw

• US 9,448,545 B2
• Filed: 10/29/2013
• Issued: 09/20/2016
• Est. Priority Date: 10/30/2012
• Status: Active Grant

• Current Assignee: Fanuc Corporation
• Original Assignee: Fanuc Corporation
• Inventors: Iwashita, Yasusuke, Takayama, Kenichi, Ikai, Satoshi
• Primary Examiner(s): Luo, David S
• Application Number:
  US14/065,925
• Publication Number:
  US 20140121818A1
• Time in Patent Office: 1,057 Days
• Field of Search: 318/685, 318/671, 318/560
• US Class Current: 1/1
• CPC Class Codes: G05B 13/024 in which a parameter or coefficient is automatically adjusted to optimise the performance
• CPC Class Codes: G05B 19/404 characterised by control arrangements for compensation, e.g. for backlash, overshoot, tool offset, tool wear, temperature, machine construction errors, load, inertia G05B19/19, G05B19/41 take precedence

Abstract

A servo control device in the present invention corrects a position command to a feed axis, so as to compensate for an amount of stretch or contraction of a ball screw during operation, by taking into account an influence of tension applied to the ball screw. The amount of stretch or contraction of the ball screw is calculated, based on the tension acting on the ball screw on the side farther from the servo motor, a distance between two fixed units supporting the ball screw at its opposite ends, a distance from the fixed unit situated closer to the servo motor to a moving body, and a torque command to the servo motor. An amount of position correction of the feed axis is calculated, based on the calculated amount of stretch or contraction of the ball screw.

Claims

1. A servo control device for controlling a feed axis which converts rotational motion of a servo motor into linear motion of a moving body by way of a ball screw,

wherein the ball screw is supported at opposite ends thereof by a first fixed unit arranged at one end of the ball screw closer to the servo motor, and by a second fixed unit arranged at the other end of the ball screw, respectively,

the servo control device comprising:

a position command generating unit for generating a position command associated with a position of the feed axis;

a torque command generating unit for generating a torque command for the servo motor;

a thermal displacement amount calculating unit for calculating an amount of thermal displacement of the ball screw;

a tension calculating unit for calculating tension acting between the ball screw and the second fixed unit, based on a distance from the first fixed unit to the second fixed unit, a distance from the first fixed unit to the moving body, the amount of thermal displacement calculated by the thermal displacement amount calculating unit, and the torque command generated by the torque command generating unit;

a stretch/contraction amount calculating unit for calculating an amount of locally-generated stretch or contraction of the ball screw during operation of the servo motor, based on the tension calculated by the tension calculating unit, the distance from the first fixed unit to the second fixed unit, the distance from the first fixed unit to the moving body, and the torque command generated by the torque command generating unit; and

a position correction amount calculating unit for calculating an amount of position correction adapted to correct the position command associated with a position of the feed axis, based on the amount of locally-generated stretch or contraction calculated by the stretch/contraction amount calculating unit.

2. The servo control device according to claim 1, further comprising a judging unit for determining whether the tension calculated by the tension calculating unit acts in a stretching direction that stretches the ball screw, or in a contracting direction that contracts the ball screw,

wherein the stretch/contraction amount calculating unit further comprises:

a first stretch/contraction amount calculating unit for calculating an amount of locally-generated stretch of the ball screw, based on the distance from the first fixed unit to the moving body, the distance from the first fixed unit to the second fixed unit, and the torque command generated by the torque command generating unit, when the judging unit determines that the tension acts in the stretching direction; and

a second stretch/contraction amount calculating unit for calculating an amount of locally-generated contraction of the ball screw, based on the distance from the first fixed unit to the moving body, and the torque command generated by the torque command generating unit, when the judging unit determines that the tension acts in the contracting direction.

3. The servo control device according to claim 2, wherein the first stretch/contraction amount calculating unit is configured to calculate the amount of locally-generated stretch of the ball screw by multiplying, with a predetermined constant, a product of:

the distance from the first fixed unit to the moving body;

a distance from the first fixed unit to the second fixed unit;

an added distance obtained by adding a value of the amount of thermal displacement calculated by the thermal displacement amount calculating unit to an entire length of the ball screw at an ordinary temperature; and

the torque command generated by the torque command generating unit.

4. The servo control device according to claim 2, wherein the second stretch/contraction amount calculating unit is configured to calculate the amount of locally-generated contraction of the ball screw by multiplying, with a predetermined constant, a product of:

the distance from the first fixed unit to the moving body; and

the torque command generated by the torque command generating unit.

5. The servo control device according to claim 1, wherein the distance from the first fixed unit to the second fixed unit is greater than an entire length of the ball screw at an ordinary temperature, and

wherein the ball screw is fixed by the first fixed unit and the second fixed unit such that the ball screw is deformable in a direction that stretches the ball screw, while the ball screw is not deformable in a direction that contracts the ball screw.

6. The servo control device according to claim 1, wherein the thermal displacement amount calculating unit is configured to calculate the amount of thermal displacement of the ball screw, based on an average speed during a unit time.