A flatness based Active Disturbance Rejection Controller for the four tank benchmark problem

A flatness based decentralized Active Disturbance Rejection Control (ADRC) approach is here proposed as a means to deal with the fully nonlinear, exogenously perturbed, version, of the four-tank benchmark process problem under non-minimum phase behavior for the traditionally chosen system outputs. The liquid level control task, related to the two lower tanks, is transformed into a corresponding, indirect, multi-input multi-flat-output decoupling control problem for the flat outputs. Decentralized robust active disturbance rejection controllers are proposed for the flat outputs, thus circumventing the well known non-minimum phase problems. The effectiveness of the proposed approach is validated via extensive realistic simulation results, wherein unknown chaotic inputs are proposed as exogenous disturbances.