Designing a Fuzzy Inference System Another way to design the FLC is by using the Fuzzy Logic Toolbox in MATLAB. Keeping in mind that this FLC will be used to control the flow of liquid in the plant; the Fuzzy Inference System (FIS) is designed to have two inputs i.e. the error, Eand the rate of change of flow which is,delPV. The reason why the derivative of the process variable is taken instead of the derivative of error is because the derivative mode amplifies the sudden changes in controller input signal
and cause large variation in the controller output. For example, derivative of a step change goes to infinity and practically this may completely open/close the control valve. Hence, by taking the derivative of the process variable, this could reduce the extreme variation in the manipulated variable [8]. The output is the control signals to the control valve, hence the manipulated variablemv. The relationship between the inputs and output are shown in Table 1. The input-output relationships are used to develop the IF-THEN rules for the fuzzy inference system. A total of 81 rules can be developed. There are 81 rules intotal; the reason why such a large number of rules are applied is to ensure better control performance, with better accuracy especially at smaller errors