Forcefree Control for Flexible Motion of Industrial Articulated Robot Arms

Many industrial robot arms are operated in industry, and some robotic applications in the industry, such as a pulling-out of products made by die casting, require the flexible motion excited by an external force. Here, the flexible motion means that the robot arm moves passively according to the external force. Industrial robot arms, however, are difficult to be moved by the external force because the servo controller of the industrial robot arm controls the motion of the robot arm excited by an input signal responsible for the motion. The torque generated by the external force is a kind of disturbance for the robot control system and it can be compensated by the servo controller. Impedance control (Hogan 1985; Scivicco & Siciliano, 2000) and compliance control (Mason, 1981; Micheal et al., 1982) were proposed in order to achieve the flexible motion, and these methods have been applied to industrial robots (Ciro et al., 2000). Most of these control methods impose desired dynamic characteristics between an end-effector and an environment by setting inertia, friction and stiffness. Usually an elastic spring behavior is introduced in order to achieve the flexible motion of the robot arm. The potential force of the elastic spring behavior is a conservative force, and it is impossible to achieve the passive motion away from the environment caused by the external force is impossible to be achieved by using these control methods. In this research, the forcefree control, which achieves the passive motion of the robot arm according to the external force, is proposed. Moreover, the forcefree control is extended to the forcefree control with independent compensation, the forcefree control with assigned locus and the position information based forcefree control. The effectiveness of the proposed forcefree control is assured by comparing the experimental results with simulation results. Comparison between the forcefree control and other force control methods such as impedance control are also described. Finally, applications of the forcefree control of pull-out work, direct teaching and rehabilitation robot are demonstrated.