Modelling and Control of Servomechanisms

Two major sources of load oscillations in servo systems are dealt with in this contribution, namely pulsating torque disturbances and torsional flexibility. After reviewing the state of the art in torque ripple compensation, a compact model of torque disturbances is presented and algorithms are given for the identification of model parameters. Thanks to its compactness, the model allows the implementation of a straightforward and effective disturbance compensation technique. Torsional flexibility is modelled by a classical two-mass model with spring and damper. A short review on the control of elastic joints is given. Then a careful analysis of the properties of the two-mass model is performed, leading to the design of a P/PI controller where suppression of load oscillations rather than fast setpoint tracking of the motor position is pursued. Further improvements of the load behaviour can be achieved by a notch filter placed outside the velocity loop. Finally, the most complete solution available with only motor position measurements, an LQG controller with feedforward actions for load setpoint tracking, is investigated. Experimental and simulation results are given to assess the effectiveness of the proposed approach.