I'm working on a pantograph device, which is not a linear plant, and implemented 2 different control schemes. First one is computed torque; For this control technique, I modeled nonlinear terms in the equation of motion and canceled them by injecting their model within the control input, which is torque for two motors. This way I achieved a very fast response but since the controller was PD, there is always a steady state error, which gets smaller as the P gain increases though. The second method was the classic PID method. Again I used PD controller for each motor and what I observed is a very slow response, like 3-5 seconds to catch the reference. Furthermore, the gains I used was extremely high even to be able to get this response, like Kp = 15000, Kd = 15000 My question is; Is the reason why PD controller performs too slow, due to non-linear plant ? And one more question for the Computed torque technique; I used the following control input; Torque = M(q)*[qd'' + Kv*e' + Kp*e] - H(q,u) - Bf(q)*Fext Where H and Bf terms are the non-linear parts I cancel. qd = desired joint angle; qactual = actual joint angle; e = qd-qactual e' = derivative of error qd'' = desired acceleration M(q) = mass matrix So, this model is the PD controller. Is it possible to build a PID controller with Computed torque technique ?