In this paper, the performances of fuzzy proportional- integral-derivative (PID) and classic PID controllers are compared through simulation studies. For this purpose, the level control of a two interacting tanks system, temperature control of unstable continuous stirred tank reactor (CSTR), and pH control of pH neutralization process were selected. In the level control process, results indicated that both of classic and fuzzy PID controllers have approximately the same performance. However, adjusting the classic PID controller is simpler than fuzzy PID controller. Therefore, in simple processes like level control in two interacting tanks, classic PID controllers are preferred. In an unstable CSTR, classic PID controller is not suitable due to the instability of the system. Fuzzy PID controller is more useful than classic PID controller in this type of systems. In pH neutralization process, using classic PID controller is inappropriate because of nonlinearity of the system and the fuzzy PID controller is more efficient.

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