This paper proposes a robust force control which adapts to wide parameter variations. A robust force control by disturbance ob server has been proposed successfully. The control system, how ever, becomes unstable if the parameter var iations of manipula tor and environment related to the force control is too large. We propose an environmental stiffness identification to adjust the pa rameters of the force controller and the disturbance observer. Ow ing to the parameter's adjustment, the robust strategy using the disturbance observer becomes fairly stable to the wide stiffness variation.
IntroductionForce control has been developed for robots performing contact tasks [1]. During the contact, force response is influ enced by various disturbances to actuator such as gravity force, friction force, and reactive force from outside, and by variations of actuator parameters such as mass and viscosity. Moreover, it is also influenced by parameter variations of environment on which the force is imposed such as mass, viscosity, and stiffness [2]. To obtain accurate force response, the disturbance and the parameter variations of both robot and environment should be compensated.The disturbance and the parameter variations of actua tor have been successfully compensated by using the dist urbance observer. It was applied in control of a direct drive motor and a robot manipulator which is a highly nonlinear control object; and was proved to be valid and useful [3][4][5] [6]. The distur bance observer is of simple design and its application results in the realization of robust control system.The robust force control, however, can not be realized by the conventional disturbance observer, because condition of en vironment on which the force is imposed influences the force re sponse. In order to realize the robust force control, the change of the environment should also be considered. To compensate the parameter variations of environment, we have developed a new observer which estimates the disturbance due to the pa rameter variations of environment, and is termed environment observer[7][8J. The proposed strategy is combination of the dis turbance observer which estimates the disturbance and the pa rameter variations of the actuator, and the environment observer which estimates the parameter variations of the environment. We have also proposed another robust strategy which is based on force-based disturbance observer which has the similar function to the former two observers [7][8].0-7803-0582-5 ,'12$3.00©1992 IEEE Since the control system is fixed to a nominal system by the observers, second derivative of force can be controlled, and quick force response can be obtained. We have also proposed a force control strategy using controller of the second derivative of force[7][8][1O]. It is different from conventional force controllers [1][4][6], because it is based on the second derivative of force.In this paper, we improve the dynamic performance of the robust force control using the disturbance observer. Since wide stiffness change cau...