A stable model predictive control with zone control

GONZÁLEZ, ALEJANDRO HERNÁN; ODLOAK, DARCI

JOURNAL OF PROCESS CONTROL

Elsevier

Lugar: doi:10.1016/j.jprocont.2008.01.003; Año: 2008

0959-1524

Some MPC applications implement a control strategy in which the system outputs are controlled within specified ranges or zones, rather than to fixed set points (Maciejowski, 2002). This means that the outputs will be treated as controlled variables only when their predicted future values lie outside the boundaries of the corresponding zones. The zone control is usually implemented by selecting an appropriate weighting matrix for the output error in the control cost function. When an output prediction is inside its zone, the corresponding weight is zeroed, so that the controller ignores this output. When the output prediction lies outside the zone, the error weight is made equal to a specified value and the distance between the output prediction and the boundary of the zone is minimized. The main problem of this approach, as long as stability of the closed loop is concerned, is that each time an output is switched from the status of non-controlled to the status of controlled, or vice versa, a different linear controller is activated. Thus, throughout the continuous operation of the process, the control system keeps switching from one controller to another. Even if a stabilizing control law is developed for each of the control configurations, switching among stable controllers not necessarily produces a stable closed loop system. Here, a stable MPC is developed for the case of zone control of a nominal stable system. Focusing on the practical application of the proposed controller, it is assumed that in the control structure of the process system there is an upper optimization layer that defines optimal targets to the system inputs. The performance of the proposed strategy is illustrated by simulation of a subsystem of an industrial FCC system