Error Reference Control of nonlinear two-mass flexible servo systems

This paper recapitulates the adaptive (time- varying) control strategy funnel-control (FC) and introduces its direct derivative error reference control (ERC) with specially designed Funnel boundaries and auxiliary reference. Both controller designs are comparatively applied to a nonlinear two-mass flexible servo system for speed control. ERC (as derivative of FC) is based on the high-gain controllability of minimum-phase systems with relative degree one and known high-frequency gain. Both implementations allow prescribed transient behavior without identification and/or parameter estimation, although the plant is only structurally known (huge parameter deviations are tolerated). As most industrial applications - also the considered two-mass system - exhibit higher relative degrees, the control strategy is not directly applicable. Therefore a state-feedback like extension is introduced, which assures the reduction of the relative degree and the minimum- phase property. By an additional implementation of a Pi-like structure, disturbance rejection and asymptotic tracking of the load speed for a given time-varying velocity trajectory is achieved. Measurement results underpin the achievable performance.