Reliability of Fault Tolerant Control Systems: Part II 1

This paper reports Part I1 of a two part effort that is intended to delineate the relationship between reliability and fault tolerant control in a ,tuantitative mannet. Reliability properties peculiar t;o fault-tolerant control systems are emphasized, such a.s the presence of analytic redundancy in high proportion, the dependence of failures on control perforumnce, and high risks associated with decisions in redundancy management due to multiple sources of uncertainties and sometimes large processing requirements. As a consequence, coverage of failures through redundancy managernent can be severely limited. The paper proposes to formulate the fault tolerant control problem as an optimization problem that maximizes coverage of failures through redundancy management. Coverage modeling is att.empted in a way that captures its dependence on the control performance and on the diagnostic resolution. Under the proposed redudnacy management policy, it is shown that an enhanced overall system reliability can be achieved with a control law of a superior robustness, with an estimator of a higher resolution, and with a control performance requirement of a lesser stringency.