Time-Delay Compensation and Disturbance Rejection in the Control of Robots with Visco-elastic Joints Using Loop-Shaping Approach

In many medical applications, it is necessary to precisely control a robot to achieve exact positioning. In such cases, some components of robot which had been considered rigid should be modeled with visco-elastic elements to present more exact model of robot, and hence controllers designed under the assumption of rigidity may not accurately control them, especially when time-delay and disturbances have been appeared in the closed-loop system. In this study, we present a new control approach to force such robots to have rigid and exact motions, while they have visco-elastic components. Time-delay in the feedback path defects transient state response, while disturbances change the steady state response. Therefore different controllers are required to correct the robot behaviors. In this regard, robot dynamics is extracted and converted to the characteristic equation. Then, by an appropriate system identification method, an order-reduced model is obtained. Loop-shaping technique is exploited on the magnitude frequency response to design different model-based controllers for principal zones of the frequency response. The simulation results illustrate appropriate time-delay compensation and disturbance rejection in the time response.