Research On Control Method For The Marine Vav Air-Conditioning System

Variable-air-volume (VAV) air-conditioning (A/C) system has great application advantages in ships due to its significant energy-saving and its ability to realize independent temperature control for cabins. In order to carry out experimental research on the marine VAV A/C system, a test rig has been built. The test rig was composed of a refrigerating unit, an air-handling chamber, an air-mixing chamber, a variable-frequency fan, a supply air static-pressure control unit, three pressure-independent VAV box and diffusers etc. The test rig was of direct-expansion refrigeration type. Since system's air side interacted directly with the refrigeration side, the control loops of marine VAV A/C system affected each other significantly. As a result the stability of control system became very important for the marine VAV A/C system and should be highlighted.The control loops of marine VAV A/C system were complex and had a lot of uncertainties. The traditional PID controller with pre-set control parameters can usually have the good control effect only in the corresponding specific condition. However, the system's working condition changes greatly during navigation; it is very difficult for the traditional PID controller to achieve ideal control effect. Hence the new intelligent controller needs to be developed for the marine VAV A/C system. This paper carried out some research work from the following aspects regarding the control study of marine VAV A/C system.Firstly, requirements for hardware and software for a measuring and control system of the marine VAV A/C system were analyzed and the suitable hardware was selected out. A measuring and control system was then developed based on LabVIEW to provide software platform for the realization of system's control method. The measuring and control system had the functions of data acquisition, processing, storage, query and report generation.Secondly, according to characteristics of the control loops of marine VAV A/C system the mathematical models respectively for the system's supply air static pressure loop and supply air temperature loop were established. The time and frequency domain analysis were made for the two loops to provide theoretic basis for the selection of control method.Thirdly, two fuzzy adaptive PID controllers were then designed respectively for the system's supply air static pressure loop and supply air temperature loop. The deviation and deviation change rate were taken as control input of the fuzzy adaptive PID controller, and the controller output was used to real-time adjust PID control parameters according to the given fuzzy rules.Lastly, the fuzzy adaptive PID controllers developed were applied separately to control system's supply static pressure and supply air temperature. The experimental studies were carried out to investigate the control effect for the controllers developed. The experimental results showed that good control effect can be achieved for the marine VAV A/C system by using fuzzy adaptive PID controllers.