Design of Built-In Tests for Active Fault Detection and Isolation of Discrete Faults

A method is proposed to improve system reliability in terms of self-detection and isolation of discrete faults. The application of this method on the design of built-in tests for maintenance is presented using the case of aircraft environmental control system maintenance testing. Built-in testing during aircraft on-ground maintenance allows for wider system input variability due to the absence of operational constraints and requirements. This provides an opportunity to optimally design tests that improve the fault detection and isolation capabilities for operators and maintenance crews. A general mathematical framework for built-in test design is shown using a model-based active fault detection and isolation technique. The motivation for this paper is first illustrated through a case study showing the inability to detect and isolate faults at different conditions, demonstrating how system states contribute to these issues. The success of the proposed framework is demonstrated by designing built-in tests that are capable of fully detecting and isolating a multiplicity of faults common to aircraft environmental control systems. We conclude by presenting the value of the proposed method of detecting and isolating faults, as a solution to a constrained optimization problem with the admissible system inputs as manipulated variables.