Diagnosis of an incipient sensor fault in a galvanising plant

In galvanising plant, the quality of the coating depends on the thermal cycle applied to a strip before entering the zinc bath. This temperature is measured by means of two pyrometers and one of them is used in the control-loop. It is required to know if a sensor is faulty and a triple redundancy is then necessary to locate the faulty sensor. Due to the closed-loop and an output which is always constant, it is not obvious to obtain a classical model sensitive to fault. In this paper, a new approach is developed in order to make the fault diagnosis possible.