Cascade unknown input observers applied to a quadruple tank process

A technique of nonlinear unknown input observer (UIO) is developed for a large class of MIMO nonlinear systems in order to simultaneously estimate the state variables as well as the unknown inputs. In fact, for every unknown input a high gain observer (HGO) is designed before putting the whole of observers on cascade to simultaneously estimate the non-measured states and the unknown inputs. The process investigated herein is the quadruple tank process (QTP) that has been introduced in the laboratories of many schools as a didactically multivariable system. It is a benchmark for testing advanced algorithm of control and observation. In this work, the flow rates of moto-pump conjointly with the upper liquid level are estimated using an UIO.