Actuator failures have too often plagued chemical processes, often leading to deteriorating product quality and potentially dangerous process operation, such as runaway conditions. Motivated by the importance of the aforementioned problem, the issue of fault tolerant and fault accommodating controller design has been an active research topic in the chemical engineering community. However, while there has been extensive research from the control community on fault detection and diagnosis of finite dimensional systems using model-based robust and adaptive control techniques, the issue of fault tolerant control for distributed processes has been investigated only recently [1,2].

To address the issue of fault tolerance we embark on a completely new direction; we explicitly consider the spatial variability that transport-reaction processes naturally enjoy, and explicitly employ a new concept of identifying actuator locations that while being physically apart they have the same authority on the process states [3]. This artifact is only applicable to distributed parameter systems wherein two or more different locations within the spatial domain of definition can provide the same level of controllability and at the same time the same feedback gain can be applicable to all such locations. We capitalize on this property of spatially distributed parameter systems to find locations for different groups of actuators with similar controllability levels, and when a given actuator group fails, then simply deactivating the faulty actuator group and activating another actuator group constitutes the fault accommodation policy. The control signal in this case is not changed, a situation that cannot be implemented in lumped parameter systems.

Therefore the major conceptual contribution of this work is to employ ``optimal'' actuator locations that have the same level of controllability and design a single feedback controller for the archetypal actuator group. An on-line supervisor is subsequently built that includes a fault detection scheme that monitors for possible faults by comparing the plants performance with that of the nominal performance. When a deviation is observed, then a fault is declared. The fault accommodation scheme in this special case does not require a control reconfiguration, but simply the deactivation of the current actuator group and the activation of another healthy actuator group. The profound simplicity of the fault accommodation significantly reduces the costs associated with process supervision with obvious economic and performance savings.

[1] M. A. Demetriou, “Utilization of LMI methods for fault tolerant control of a flexible cable with faulty actuators,” Proceedings of the 2001 40th IEEE Conference on Decision and Control, pp. 1885-1890, vol. 2, Orlando, Florida, December 4-7, 2001.

[2] A. Armaou and M. A. Demetriou, “Robust Detection and Accommodation of Incipient Component and Actuator Faults in Nonlinear Distributed Processes,” AIChE J., in press, 2008.

[3] A. Armaou and M. A. Demetriou, “Optimal Actuator/Sensor Placement for Linear Parabolic PDEs Using Spatial H2 Norm,” Chem. Eng. Sci., 61, 7351-7367, 2006.