Diagnostic observers are a class of fault diagnosis technique that is based on quantitative models of the system. Relying on analytical redundancy, these check the actual system behavior against the system model for consistency. Any inconsistencies, also called residuals, are "artificial signals" reflecting the potential faults in the system and can be used for detection and isolation purposes. The residuals should be close to zero when no fault occurs but show "significant" values when the underlying system changes. The observer is designed such that faults leave distinct patterns in the residuals thereby making fault isolation possible. We are currently investigating the necessary and sufficient conditions for existence of such observers.
We are also working on diagonal forms of nonlinear observers with structured feedback. These observers are valid for a wider class of nonlinear systems and the synthesis procedure is markedly simpler than that discussed in conventional literature. These observers are demonstrated on numerical examples and simulations of typical chemical engineering systems like Continuous Stirred Tank Reactor and heat exchanger.