Electricity: motive power systems – Positional servo systems – Program- or pattern-controlled systems
Patent
1998-09-10
2000-10-31
Ip, Paul
Electricity: motive power systems
Positional servo systems
Program- or pattern-controlled systems
3185681, 31856818, 901 23, G05B 19408, G05B 1918
Patent
active
061407887
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a control method for an industrial robot, in which the robot is enabled to perform proper operation when it touches an external object, and a control system for carrying out the control method.
DESCRIPTION OF THE RELATED ART
In the most popular cases, control of an industrial robot (hereinafter referred to simply as "robot") is based on a system in which the respective positions of motors for individual axes of the robot are controlled by means of a servo control system. Referring now to the block diagram of FIG. 1, this servo control system will be described.
The servo control system shown in FIG. 1 is a conventional one that has a position control loop and a speed control loop, including a term 1 for a position loop gain Kp, term 2 for a speed loop gain Kv, term 3 for a torque constant Kt, term 4 for an inertia J, and transfer function 5 with which an actual motor position q is obtained by integrating an actual motor speed v. Among these terms, the term 3 for the torque constant Kt and the term 4 for the inertia J constitute a transfer function term for the motor M. Symbol s represents a Laplace operator.
A position deviation e is computed from a move command r generated in a robot controller (not shown in FIG. 1) and the motor position q, and a speed command vc is outputted by multiplying the position deviation e by the position loop gain Kp. Further, a speed deviation ev is computed from the speed command vc and the actual motor speed, and a torque command tc is outputted by multiplying the speed deviation ev by the speed loop gain Kv. A driving current corresponding to the torque command tc is supplied to the motor. For the speed loop control, proportional-plus-integral control or integral-plus-proportional control may be used in place of proportional control,in some cases.
Conventionally, in this robot control, high rigidity (loop gain) is given to the servo control system to effect accurate positioning. In order to prevent breakage when the robot touches a peripheral object, however, the robot operation is made flexible by lowering the loop gain of the servo control system below a normal value for a section in which contact or collision is possible. This method is referred to as "floating based on software" or "soft-floating." Changeover to soft-floating control in response to a command from a CPU of the robot controller is referred to as "effectuation of a soft-floating function."
According to another known method to cope with the contact between the robot and an external object, moreover, danger is avoided by detecting contact by means of the outputs of various sensors (force sensor, proximity sensor, visual sensor, etc.) or a disturbance observer based on software and effecting an emergency stop.
In the case where the former technique (soft-floating) is applied for coping with the contact between the robot and the external object, however, it is necessary previously to teach in a program a part (section) in which the contact occurs. Accordingly, the soft-floating control is not carried out outside the taught section, so that contact may possibly be made in an unexpected section, thus resulting in breakage.
In the case where the latter technique (emergency stop after detection of contact) is adopted, on the other hand, no such control is effected as to reduce the rigidity of the servo control system immediately. Although the emergency stop is effected after detecting the contact by utilizing the various sensors or the disturbance observer, therefore, the robot is urged to move for a short distance along a taught path, with the loop gain of the servo control system kept high, thus inevitably interfering with the external object. Possibly, therefore, the peripheral object, gripped workpiece, hand, etc. may be damaged.
Whichever method we choose to cope with the contact, it is difficult to avoid danger unless the speed is sharply lowered for sections in which contact is supposed to occur. If the robot operation is stopped with every contact
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Kato Tetsuaki
Tsuchida Yukinobu
Watanabe Atsushi
Fanuc Ltd.
Ip Paul
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