퍼지제어기를 이용한 가변속 냉동시스템의 과열도와 용량제어

Abstract

According to development of industrial technology and the growing demands for comfort in a residential environment, inverter-driven refrigeration system for energy saving is becoming more popular. Therefore, it is necessary to design high-performance and high-precision controller for obtaining precise temperature and energy saving. It is very well known that the control of capacity and superheat is basic control scheme in the variable speed refrigeration system. Superheat is maintained as a constant value to keep maximum COP, and capacity is controlled by changing compressor speed to cope with partial thermal load condition. Hence, precise indoor temperature and energy saving can be achieved at the same time by these two controls. To design high-performance and high-precision controller of the refrigeration system, an empirical model that represents dynamic characteristics of the system is necessary at first. The basic refrigeration system mainly is consisted of two exchangers(evaporator and condenser), an electronic expansion valve(EEV) and a compressor. It is very difficult to get a practical model of the system for the control due to its strong inherent non-linearity and interference loop between chamber temperature and superheat. A sophisticated mathematical model from the principle of the energy conservation is suitable for just numerical simulation, but it is a drawback for designing control system systematically due to its high-order differential terms in the model. This is the reason why a simple empirical model is more preferred for engineers in the industrial fields. Even though an empirical model obtained from a number of experiments also exists, systematic design of PI(Proportional, Integral) or PID controller for precise temperature control is not assured. Because the empirical model strongly depends on transfer functions which were linearized approximately and the model is still complicate due to interfere loops. Thus, the gains of PI or PID are determined by trial and errors manner based on tuning method such as Zigler-Nichols. To solve these problems, the applications of the fuzzy control based on the fuzzy inference are tried frequently. Most of the researches for temperature control in an air-conditioning system using fuzzy theory focus on treatment ambiguity of human thermal comfort. However, fuzzy controller proposed in this paper aimed at simultaneous control of the capacity and superheat without troublesome model of the system. Hence, the proposed controller enables maximum COP and energy saving at the same time even though the thermal load is varied. If we can get equivalent control performance to PI or PID controller by using the fuzzy control, it’ll be very a useful design method for engineers in the industrial fields.