Two-Frequency Process Identification: An Extension of Ziegler-Nichols Type Methods

Controller tuning procedures that identify the process using an ultimate period method assume that the process can be sufficiently modeled in the frequency range important to control with two parameters. Ziegler-Nichols´ and Shinskey´s tuning methods are examples of these procedures. This paper discusses an extension of these methods, using two frequencies for identification, that allows the process to be modeled with a structure containing four parameters. In the frequency range important for control, this four-parameter model more accurately describes processes than the two-parameter model. Therefore, a controller designed from the four-parameter model will control a wider variety of processes more effectively.