Robust control of a plasma wind tunnel

In this paper the problem of controlling a plasma wind tunnel, to simulate the re-entry conditions of space vehicles by reproducing desired trajectories in pressure and temperature on a test model, is considered. The proposed control strategy consists of two terms: a feedforward control action to guarantee the trajectory following in absence of external disturbances, and an output feedback control action to counteract possible misalignments between the desired trajectory and the actual one. The feedback controller is designed in the discrete-time setting. It is shown that the linearized model of the plant can be described by a LPV system depending on nine uncertain parameters; eight of such parameters exhibit small excursions and no bound on their rate of variation is available, while the last parameter turns out to be slowly-varying and a bound on the rate of variation is known. The controller is designed so as to robustly stabilize the overall closed loop system versus the above-mentioned uncertain parameters; an ad hoc technique, which takes into account the information on the rate of variation of the slowly-varying parameter, is used. Numerical simulations are carried out on the nonlinear system to show the effectiveness of the proposed control scheme.