2DOF PID for Current Driven Servo

A design method for 2DOF PID continuous or discrete controller for a current driven servo described by a double integrator is presented. The closed-loop time constant is the single design data. The method results in the rules for three PID settings and two PID weights b, c that reduce gains for P and D modes for the set-point input. The PID settings provide a triple real pole of the closed-loop system to assure smooth transients. The weights b, c eliminate overshoot of the step response. In the case of continuous control, the weights are fixed, independent of the design data. In the discrete case, the weights decrease when the discretization step increases. Laboratory experiments confirm fairly good correspondence of system response to simulations, but also demonstrate the effects of friction when the output is close to a set-point. Role of additional parameters of an industrial PID function block is also examined.