Closed Loop End Piece Control Of A Servo-Controlled Manipulator

Abstract

This article discusses the possibility of attaining closed loop end piece control of a servo-controlled manipulator by externally modifying the independent control loop set point of each robot link. Tlus procedure leaves the existing servo control loops intact and thus has the benefit of acquiring the advantages of an advanced control strategy while retaining the inherent reliability and stability of the existing control loops. Algorithms are proposed for compensating for known disturbance torques, elinlinating end-piece position errors, and applying specified forces to the surroundinl9l with the end piece. These algorithms are demonstrated for the three-degree-of-freedom case using a simulation package that simulates the response of a servo-controlled manipulator to set point inputs. To compensate for known disturbance torques, the torques generated by the robot control system are assumed to be directly proportional to the changes in set points. These approximate relations are used to estimate the adjustment in set points necessary to generate the required additional joint torques. Simulation results show that this method is effective in compensating for gravity disturbance torques. Using the Jacobian combined with a weighting factor to prevent overshoot, an algorithm is developed that drives-end piece position errors to zero. The transpose of the Jacobian is used to relate forces exerted on the surroundinl9l by the end piece to generated joint torques. The required torques are created by adjusting the set points. Simulation results indicate that this method can be used to exert specified forces.