Systematic design of membership functions for fuzzy logic control of variable speed refrigeration system

The systematic design of membership functions (MFs) for the fuzzy logic control (FLC) of a variable speed refrigeration system (VSRS) is presented in this paper to reduce trial and error as well as dependence on expert knowledge. In particular, the influence of MFs on the error and error change rate of controlled variables is investigated in detail to verify the validity of the designed MFs. To this end, the range of MFs obtained based on the control accuracy and experimental data for the static and dynamic states of the VSRS were used as the criterion values. Two different values from the criterion were then used to compare the control performance through simulations and experiments. Some simulations and real experiments for the VSRS were conducted to verify the validity of the designed MFs. The experimental results showed good agreement with the simulation results. The error change rate and sampling time strongly affected the control performance of the VSRS in the transient state. Moreover, the desired control accuracy at steady-state, an essential element in the design specifications, was successfully achieved using the proposed approach. The suggested method can be applied using the control accuracy and simple experimental results from the static and dynamic states of the target system without depending on iterative work when designing MFs, which will be helpful for beginners.