Non-linear and adaptive control of a refrigeration system

Abstract

In a refrigeration process heat is absorbed in an evaporator by evaporating a flow of liquid refrigerant at low pressure and temperature. Controlling the evaporator inlet valve and the compressor in such a way that a high degree of liquid filling in the evaporator is obtained at all compressor capacities ensures a high energy efficiency. The level of liquid filling is indirectly measured by the superheat. Introduction of variable speed compressors and electronic expansion valves enables the use of more sophisticated control algorithms, giving a higher degree of performance and just as important are capable of adapting to variety of systems. This paper proposes a novel method for superheat and capacity control of refrigeration systems; namely by controlling the superheat by the compressor speed and capacity by the refrigerant flow. A new low order nonlinear model of the evaporator is developed and used in a backstepping design of a nonlinear adaptive controller. The stability of the proposed method is validated theoretically by Lyapunov analysis and experimental results show the performance of the system for a wide range of operating points. The method is compared to a conventional method based on a thermostatic superheat controller.

Control robusto QFT de un sistema de refrigeración

“…La diferencia en el desempeño temporal de estos controladores sería inapreciable. (14), respectivamente, para un mayor número de frecuencias que (16). Para cada lazo, se representan las magnitudes de las respuestas frecuenciales para todas las plantas del dominio de incertidumbre y la cota de especificación.…”

“…Por otro lado, el enfoque del problema no ha de ser necesariamente multivariable, sino que las características particulares de la planta de refrigeración pueden simplificar mucho la estrategia de diseño de los controladores. Se partirá de un modelo de simulación [8][9] de un sistema de refrigeración por compresión de vapor de una etapa, de validez contrastada [4][5][14] [16], y que se corresponde con un sistema real. Se mostrará cómo para este caso el empleo de un control descentralizado más un desacoplador en una sola dirección es suficiente para cumplir los objetivos.…”

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

“…Some authors (Perez et a l, 2004) suggest that the evaporators are complex systems that are required to be linearized in the equilibrium point to control the process. Another work in the control of heat exchangers is presented in Rasmussen and Larsen (2011), in this work a low order non linear model of the evaporator is developed and used in a backstepping design of a nonlinear adaptive controller. The proposed m ethod allows work in a wide range of operating points.…”

Modeling and output feedback control of automotive air conditioning system

“…Similarly, a low-order nonlinear evaporator model was developed for backstepping design of a nonlinear adaptive controller [15]. A model predictive control algorithm was applied to compute an optimal command profile of each actuator by formulating a multiple criterion including cycle efficiency and technological constraints [16], based on a low order nonlinear model of a vapor compression cycle proposed in [17].…”