Switching Policies for Mold Level Control in Continuous Casting Plants

The problem of controlling the liquid level in the mold of a continuous casting process is considered. Essentially, the system operates in two different modes: (a) one in which the system is affected only by small-amplitude high-frequency disturbances, and (b) one in which the system is affected by a comparatively large disturbance related to an abrupt unclogging phenomenon. To cope with the first situation, the control system must exhibit a narrow bandwidth, whereas the second situation requires a much larger bandwidth. The two conflicting requirements lead naturally to the design of two distinct controllers and a switching rule that activates either controller according to the operating condition. Due to the time-varying nature of the resulting control system, the controllers should be properly realized to guarantee stability; to this purpose, a recent result on switching systems is invoked. Since the sudden switching between controllers can cause controller and actuator output bumps, a soft switching policy that still ensures stability is also proposed. Simulations and laboratory experiments carried out on an electronic analogue of the process show the effectiveness of the suggested control schemes.