Robust Averaging Level Control, Report no. LiTH-ISY-R-3023

Frequent inlet flow changes typically cause problems for averaging level controllers. For a frequently changing inlet flow the upsets do not occur when the system is in steady state and the tank level at its set-point. For this reason the tuning of the level controller gets quite complicated, since not only the size of the upsets but also the time in between them relative to the hold up of the tank have to be considered. One way to obtain optimal flow filtering while directly accounting for future inlet flow upsets is to use closed-loop robust MPC, as proposed here. The behavior of the robust MPC controller differs from earlier proposed level controllers as it does not return the tank level to a fixed set-point following an inlet flow upset. Guidelines on the tuning of the controller is presented and its performance is compared to that of a previously proposed MPC approach.