One of the main objectives of our research is to build several ARMA linear models and nonlinear neuro models for DAT systems such as developed in [3], [4]. Based on these models we have been interested to build some feedback control strategies for these systems[3]-[5]. Due to the highly complexity of the HVAC systems the requirements for reliability, availability, and security grow significantly and consequently investigations in this field become necessary. The objective of our paper is to develop some strategies for the fault detection and diagnosis (FDD) of the HVAC systems based on frequency and spectral analysis of the system response. The main interest is focused on dealing with the unanticipated valves actuators failures in the most general formulation based on the frequency analysis of the system response. In our study we consider that the increasing of the backlash opening in the valve actuators could create serious problems by degrading drastically the HVAC system performance.Based on the spectrum analysis we are capable to gather the most significant patterns that characterize well each fault mode detected, and finally a systematic procedure and technique for proper fault accommodation under the unanticipated failures will be developed. As tools we use MATLAB with SIMULINK toolboxes from control systems, system identification and signal processing.
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