Model Predictive Control with Constraints on Accumulated Degradation of Actuators

Process industry has experienced important changes over the last two decades. The development of new technologies has made automated systems more complex and, eventually, more vulnerable to faults. Fault-tolerant operation in the presence of actuator faults requires some form of redundancy, especially if it is necessary to keep the system running until the next scheduled maintenance. This period of time is defined in the present work as the maintenance horizon. In some cases the control effort can be redistributed among the available actuators to alleviate the work load and the stress factors on the equipments with worst conditions to avoid their break down. For this purpose, an appropriate policy should be developed to redistribute this effort until maintenance actions can be taken. In this paper, a solution to this problem is proposed by using a Model Predictive Control (MPC) approach. An important feature of MPC is the possibility of explicitly including input and state constraints in the control law. In the proposed approach, the plant model is augmented with additional states associated to the accumulated degradation of the actuators. It is assumed that the degradation rate is proportional to the control effort demanded from each actuator. Constraints are imposed on the additional states to ensure that the accumulated degradation will be acceptable at the end of the maintenance horizon. The onset of an actuator fault is modelled as either an increase in the degradation rate or a step increment in the accumulated degradation. A simulated tank level control system is used as a case study to illustrate the efficiency of the proposed approach. This system is composed by a set of redundant pumps that supply fluid to the tank. The results show that the predictive controller distributes the control effort in a suitable manner to relieve the pumps with larger accumulated damage. Moreover, this distribution is promptly changed upon the occurrence of a fault.