Forcefree Control for Flexible Motion of Industrial Articulated Robot Arms

Abstract

Operations such as cutting and welding can be easily achieved by using industrial robot arms. These operations are carried out through contour control in that the tip of the robot arm moves along a given path, and point-to-point (PTP) control in that the tip moves between previously assigned points. These operations are tractable as the rigidity of the robot arm advances. Therefore, the industrial robot arms are designed with high rigidity. Recently, control of a contact force is required in order to carry out assembling, handling, inlaying, pull-out work and grinder operations. On the contrary, it is difficult to control the contact force if the robot arm possesses high rigidity. In view of this, the property of low rigidity for the industrial robot arms is required to control the contact force. On the other hand, flexible motion is also required for the safety operation such as contact between the robot and human operator. Generally, an emergency shutdown switch is built in to the servo controller. When the operator is sandwiched between the tip of the robot arm and the environment, the emergency halt becomes more dangerous. If the robot arm can be actuated with flexibility, the operator is released from the sandwiched situation. Some of the problems have been solved by particularly designed robots or by remodeling of robots. However, a number of industrial robot arms are manufactured for general purpose, and such robot arms have a lower cost compared with that of special purpose robots. In this view, flexible motion by general purpose industrial robot arms is required in industry. A servo controller of industrial robot arm includes a position loop and a velocity loop. Input to the industrial robot arm is usually the joint position of each link. Hence, the industrial robot arms should be considered as the combination of the mechanism of the robot arm and the servo controller. Recently, a study of force control of industrial robots has developed rapidly and the achievement of such force control has been the major concern. A number of force control methods for the change of rigidity of robot arms such as impedance control These methods are apparently good enough to achieve the requirement of force control. However, to apply these methods in industry, there are difficult problems to be solved. For general purpose robots including the servo controller, these methods require changes in the control strategy in the servo controller. Modification of the servo controller is almost impossible from the user side, and modification by the maker is very expensive even upon request from the user side. Presently, available methods for the achievement of flexible contact