Limitations in control of elastic servos with co-located sensors

Performance limitations of elastic servomechanisms where only the motor position is measured emerge considering also the load position behavior. Limitations inherent to PID control, the most common solution in commercial products are dealt with first. The authors explicitly relate a bound of the H_∞ norm of the motor position setpoint to load position transfer function to the motor position loop bandwidth and to the other servo design parameters. Then they show that introducing the torque disturbance observer the load oscillates almost freely at the locked frequency, when excited either from the setpoint or from a torque disturbance at the load side. Finally, they point out that even a very general two-way control structure, combining a feedforward action and an observer based algebraic feedback, cannot improve sensibly the performance achieved with the PID controller.