Sensor fault compensation via software sensors: Application in a heat pump's helical evaporator

Abstract

This work presents a sensor fault compensation system, applied to a heat pump's helical evaporator. The mathematical model of the evaporator is given by algebraic and differential equations. These equations were selected according to the phases and regime of the fluid to be evaporated. The sensor fault compensation is based on fault detection and isolation system and a MPC (model predictive control) strategy. The fault detection isolation system is based on a bank of two high-gain observers which have two main purposes. The first one is to generate adequate residuals when a sensor fault occurs. The second purpose is to act as a software sensor, meaning, the measure estimated by the observer replaces the measure given by physical sensor when a sensor fault occurs. The high-gain observers were selected because they are easy to implement and tune. Furthermore, they provide an adequate estimation of the process outputs (when a sensor fault occurs) to a model predictive control (MPC) strategy. The MPC has been implemented to regulate the steam outlet temperature. Several experiments were carried out to show that the MPC regulates the process output even if a fault occurs. The experiments in the evaporator of the absorption heat pump have shown reliability of the method presented in this work to detect a sensor failure, isolate the sensor failure and regulate the steam temperature when a total or partial failure occurs.