The speed of position control in many robotic applications is limited by the flexible modes of the structure. The flexible modes can sometimes be incorporated into the design of the overall control system if there is sufficient bandwidth in the main motor controller. An alternative strategy is to incorporate actuators and sensors into the flexible parts of the robotic system and use a high bandwidth local control system to suppress the flexible modes. The control problem then becomes similar to that of the control of unknown disturbances on smart structures.
In an initial feasibility study (Lee and Elliott, 1998) the tip motion of a long thin cantilever beam was controlled using a pair of piezoceramic actuators at the root. The dynamic response of the beam was dominated by a series of very lightly damped modes with the first natural frequency below 5Hz. The step response with feedback using conventional PID control and model-based Internal Model Control was compared and although the performance of the latter was much better than PID control, the stability was very sensitive to the assumed plant model.