Control system design issues for unstable linear systems with saturated inputs

The paper studies stable setpoint tracking for linear open-loop unstable systems with saturated control input. A linear reference filter is introduced that allows the independent design of the prestabilising and setpoint tracking controllers in an internal model control configuration. A nonlinear reference conditioning filter is also proposed that avoids saturation in the main control loop, which remains stable thereafter. Rules of thumb giving bounds on amplitude and frequency for sinusoidal references are given that ensure stable saturated operation of the nonlinear reference filter