A computer-based method to fit Strejc's models for industrial processes responses

General purpose control systems still utilize PID-type algorithms, with low cost implementation. Today, to facilitate controllers setting procedures, many manufacturers offer auto-tuning options. In many cases these options rely upon simple process models which are derived by adapting simple identification procedures, still dominant in the practical use, to microcomputer implementation. To this aim, the present paper introduces a computer-oriented method to fit non parametric models, characterized by sigmoid-shaped step response, to the Srejc’s transfer function representation. The main idea is to force the first coefficients in the Laguerre spectrum of an assigned impulse response to coincide with the corresponding elements in the Laguerre spectrum of the Srejc’s model. These components, in their turn, are simply connected to the unknown parameters of the Srejc’s transfer function. This goal is achieved by appropriately choosing the scale factor on which the Laguerre functions depend. Some examples conclude the paper, showing the accuracy, the robustness of the method and the limited requirements of computer resources.v