Electricity: motive power systems – Positional servo systems – Antibacklash systems
Patent
1993-02-17
1994-08-30
Shoop, Jr., William M.
Electricity: motive power systems
Positional servo systems
Antibacklash systems
G05B 1101
Patent
active
053431324
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a method of controlling a servomotor for actuating a table of a machine tool, a robot arm or the like, and more particularly, to a backlash acceleration correction to be executed when the moving direction of the table or robot arm is reversed.
Description of the Related Art
In reversing the rotating direction of a servomotor for actuating a table feed shaft of a machine tool, a robot arm or the like, driven parts of the machine cannot be reversed at once due to the effects of the friction and the backlash of a feed screw or reduction gear. Thus, if the rotating direction of the servomotor is reversed while contouring operation is carried out with the machine tool or the robot arm is making circular movement, a swell is formed in the cut surface of a workpiece or in the locus of the robot arm.
For example, in the case of a circular cutting performed on a workpiece in a plane defined by two axes, i.e., X and Y axes, when driven to move from a state of the movement of which the X component and Y component are negative and positive respectively into another state of the movement of which both the X and Y components are negative, causing a transition from one quadrant to another, the cutting speed with respect to the Y axis remains the same, but, for the movement in the direction of the X axis, the position deviation becomes "0", and thus the torque command value becomes smaller. Since the rotation of the servomotor cannot be reversed at once due to friction and the movement of the table cannot be reversed at once due to the backlash of the feed screw for feeding the table, the movement of the workpiece in the X-axis direction will not be able to respond to the move command at once, thereby causing the delay of action. As a result, a swell will be formed on the circular surface to be cut.
To eliminate or lessen such a swell, the so-called backlash acceleration is applied, as disclosed in, e.g., Unexamined Japanese Patent Publication JP-A-4-8451, in which, when the moving direction is reversed, position backlash correction is carried out on a position deviation, and when the position deviation is reversed, an appropriate value (acceleration value) is added to a velocity command to accelerate the servomotor in the reverse direction, thereby reducing a swell caused by quadrant change.
Further, in servomotor systems for machine tools, feedforward control is employed to reduce the amount of position deviation. Especially, in carrying out high-speed cutting with a machine tool, a shaping error occurs due to a follow-up delay of the servo system. To lessen the shaping error, a feedforward control method has been developed, as proposed by the applicant of the present application in International Patent Application No. PCT/JP91/01537, in which a smoothing processing is executed on a move command output from a numerical control device to obtain a feedforward quantity; the feedforward quantity is then added to a velocity command from a position loop, which is obtained by multiplying the position deviation by a position gain, to obtain a corrected velocity command, and the processing of a velocity loop is executed by using the corrected velocity command.
This type of feedforward control will be described with reference to FIG. 4 (PRIOR ART). A DDA (digital differential analyzer) 10 divides a move command Mcmd, which is outputted from a CNC (computer numerical control) device in every distribution period, into move commands for respective position/velocity loop processing periods. An error counter 11 adds up the values obtained by subtracting a position feedback quantity Pfb from the move command outputs delivered by the DDA 10 to obtain a position deviation. An element 12 calculates a velocity command by multiplying a position gain Kp by the position deviation stored in the error counter 11. Reference numeral 13 denotes a velocity loop term, in which a velocity loop processing is executed based on the velocity command to obtain a torque comman
REFERENCES:
patent: 5015935 (1991-05-01), Iwashita
patent: 5204602 (1993-04-01), Iwashita
Fanuc Ltd.
Masih Karen
Shoop Jr. William M.
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