Detection and Isolation of Switch Faults

Identification of System Mode Changes in the Presence of Parametric Faults 

In the previous section, each discrete switch state has been taken into account by a row in the FSM. System mode changes can be viewed as faults in discrete switch states. This means that there are far more fault candidates than sensors. Accordingly, discrete switch states will share the same component fault signature so that a switch fault cannot be isolated. If multiple simultaneous faults have happened only in parameters that change continuously with time then parameter estimation can be used to isolate them. However, if there are discrete switch state faults among the multiple simultaneous faults then parameter estimation may result in meaningless real values for the discrete switch states. Therefore, in [6], Alavi and her co-authors propose to chose a combination of switch states, to insert them into the functional to 

ARRs derived from hybrid system models are mode-dependent. For ARR-based FDI it is therefore necessary to know the current system mode so that ARRs with the correct set of discrete switch state values are evaluated. When ARR residuals are outside admissible parameter uncertainty bounds it is not clear whether a parametric fault has occurred or a mode change has happened. If a mode change has happened then an evaluation of ARRs with the discrete switch states of the last known mode yields wrong and misleading results. This chapter considers system mode identification for healthy systems only. For the more complex case of system mode changes in the presence of faults, it is suggested to see latest publications, e.g. [4], 

The current system mode of a healthy system can be identified by evaluating all ARRs for all feasible switch state combinations. This may require considerable computational effort. The computational time, however, can be reduced by distributing the task on multiple parallel processors. Moreover, this task is only necessary if an initial system mode is not known or if the last known system mode is no longer valid because rapid system modes have taken place while system mode identification is still in progress. Once the current system mode is known, the all-mode FSM can be consulted to identify a subset of ARRs that is to be evaluated to identify the new current system. This has been illustrated by application to a simple circuit with three semiconductor switches. 

Finally, the knowledge of the current values of discrete switch states can be used to detect and to isolate switch faults by rule-based reasoning. If switches toggle then-state at a high frequency little time is left for mode identification. However, again, parallel processing can help to cope with the time constraints. 

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