Bøgh, S.A.

English

This thesis considered the development of fault tolerant control systems. The focus was on the category of automated processes that do not necessarily comprise a high number of identical sensors and actuators to maintain safe operation, but still have a potential for improving immunity to component failures. It is often feasible to increase availability for these control loops by designing the control system to perform on-line detection and reconfiguration in case of faults before the safety system makes a close-down of the process. A general development methodology is given in the thesis that carried the control system designer through the steps necessary to consider fault handling in an early design phase. It was shown how an existing control loop with interface to the plant wide control system could be extended with three additional modules to obtain fault tolerance: Fault detection and isolation, remedial action decision, and reconfiguration. The integration of these modules in software were considered. The general methodology covered the analysis, design, and implementation of fault tolerant control systems on an overall level. Two detailed studies were presented, one on fault detection and isolation design and one on design of the decision logic. Two application case studies were used to emphasize practical aspects of both the development methodology and the detailed studies. One was an electro-mechanical actuator in a position control loop for a diesel engine speed governor where the purpose was to avoid a total close-down in case of the most likely faults. The second was a fault tolerant attitude control system for a micro satellite where the operation of the system is mission critical. The purpose was to avoid hazardous effects from faults and maintain operation if possible. A method was introduced that, after a systematic examination of possible component failures, enables analysis of the relationship between failures and their consequences for the system's operation. This fault propagation analysis is based on coarse models of the subsystems describing the reaction to faults, as for example a variable being zero, low or high. Examples were given that illustrate how such models can be established by simple means, and yet provide important information when combined into a complete system. A special achievement was a method to determine how control loops behave in case of faults. This is not straight forward as the system behaviour depends on the character of the feedback. One of the detailed studies were the design of the decision logic in fault handling, realized as state-event machines. Guidelines for the design were provided, based on experience from the two case studies. Methods for verifying correct operation of the decision logic were described, where a completeness check against the fault propagation analysis is able to guarantee coverage of all considered faults. The usage of software tools to support the development process was illustrated with an off-the-shelf product for constraint logic solving and state-event machine analysis. The coarse system models and the decision logic were analyzed with the tool-box and it was shown how an easy analysis could be performed to verify correctness and completeness of the fault handling design. Experience from this study highlights requirements for a dedicated software environment for fault tolerant control systems design. The second detailed study addressed the detection of a fault event and determination of the failed component. A variety of algorithms were compared, based on two fault scenarios in the speed governor actuator setup. One was a position sensor fault and the second was an actuator current fault. The sensor fault detection was trivial, whereas the actuator fault was more challenging. The study demonstrated that many existing methods have a potential to detect and isolate the two faults, but also that the research field still misses a systematic approach to handle realistic problems such as low sampling rate and nonlinear characteristics of the system. The thesis contributed with methods to detect both faults and specifically with a novel algorithm for the actuator fault detection that is superior in terms of performance and complexity to the other algorithms in the comparative study

Fault Tolerant Control Systems:a Development Method and Real-Life Case Study

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Fault Tolerant Control Systems:a Development Method and Real-Life Case Study

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