It is very difficult to obtain a good control effect for HVAC systems that possess multivariables, long delay times, and nonlinear characteristics by traditional control methods. Fuzzy control is already an efficient way to implement the control process in HVAC. Fuzzy logic reasoning is a basis of fuzzy control, and the method of the compositional rule of inference (CRI) is used widely. This paper puts forward a new concise method of fuzzy logic reasoning¾the functioning-fuzzy-subset inference (FFSI)—and analyzes and compares the performance of the FFSI with CRI for the nonsingleton fuzzy logic control system with discrete domain, triangular membership function, approximate fuzzifier, Mamdani style fuzzy control rules, and maximum-minimum inference. Then the paper discusses the control effects of the FFSI, CRI, and PID by simulation for a controlled system with one order inertia and delay times, which is very general in HVAC. In order to utilize the FFSI in HVAC, a single-chip microcomputer, which will be an important device to carry out the fuzzy control experiment in HVAC, has been developed and introduced in brief.

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