Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Patent
1996-10-21
1998-04-21
Ip, Paul
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
318570, 318571, 31856817, 901 9, 36447406, 36447403, G05B 1900
Patent
active
057421382
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention related to a control technology of an industrial robot, and more particularly to a control technology capable of modifying or variably setting, at need, softness of a servo system for controlling a servomotor for driving axes of a robot.
BACKGROUND ART
Respective axes of servomotors for driving the arm of a robot are usually controlled by a servo system having a position control loop and a velocity control loop. FIG. 1 of the accompanying drawings is a block diagram showing the servo system, in which reference numeral 1 designates a term of a position loop gain Kp and 2 designates a term of a velocity loop gain Kv. Further, 3 and 4 designate terms of a transfer function of a servomotor; Kt, a torque constant; and J, inertia. In addition, 5 designates a transfer function for obtaining a motor position q by integrating a motor velocity v. s represents Laplace operator.
A position error e is calculated from a motion command r produced in a robot controller, and the motor position q, and the position error e is multiplied by the position loop gain Kp to output a velocity command vc. Further, a velocity error ev is calculated from the velocity command vc and the motor velocity v, and the velocity error ev is multiplied by the velocity loop gain Kv to output a torque command tc. Then, a driving current in accordance with the torque command tc is supplied to the servomotor. In the velocity loop control, PI (proportional and integral) control may be adopted in place of (proportional) control.
When a tool center point of the robot, whose motors for respective axes are controlled by the above-mentioned servo system (PI control), is encountered and comes into contact with any obstruction in moving toward a target position, it will attempt to continue moving toward the target position against the obstruction.
This phenomenon is explained as follows; Although the servo motor attempts to move toward the target position irrespective of presence of the obstruction, actually it is blocked from reaching the target position so that the position error e increases. As a result, the velocity command vc, which is obtained by multiplying the position error e by the position gain Kp, also increases. The deference between the increased velocity command vc and the actual motor velocity v (the velocity v is assumed to be near "0" when it is in contact with the obstruction) increases as integrated by an integrator in the velocity loop, so that the torque command tc becomes a large value. As a result, the servomotor outputs a maximum torque as it attempts to realize the movement toward the target position, to make a cause of a stop of the robot or an accident (interference accident) of breaking a workpiece, an end effector, etc.
For avoiding the foregoing inconvenience, there is adopted a servo system in which increases of the velocity command vc and the torque command tc are restrained by lowering the position loop gain Kp and the velocity loop gain Kv when necessary. In this servo system, generally, gain values Kp' and Kv' for softness control are previously set and are substituted for Kp and Kv, respectively, when a softness control command is input.
In this conventional servo system, "a soft floating function on the space of each robot axis (hereinafter referred to each axis)" is realized by modifying or variably setting the softness of the servo system for each axis.
For example, if Kp' and Kv' are set to be very small with respect to three axes of the wrist of the robot, a servo system particularly soft in three axes of the wrist is realized. If Kp' and Kv' are set to be very small for all axes, the servo system is very soft in all the axes.
However, in the above conventional method, since the softness of the servo system can be adjusted only for each axis, it is difficult to adjust the softness independently in different directions in the working space and to give the robot a softness suitable for actual operations, such as fitting and deburring.
For example, in the fitting operation, it
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Kato Tetsuaki
Tsuchida Yukinobu
Fanuc Ltd.
Ip Paul
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