Modeling and process control of twin-screw cooking food extruders

Abstract

Twin-screw extruders exhibit highly non-linear and interactive dunamic behavior which makes their modeling and control difficult. Transfer function models determined from experimentally observed behavior are not only adequate in capturing the dynamics of the extrusion process, but are also invaluable tools in the design of multivariable control strategies for extruders. Suitable forms of transfer function models are taken from the literature to characterize a typical process used in cornmeal extrusion. The parameters for the transfer function models are also adapted from the literature and are used as an example in process control analysis and design. The degree of process loop interactions expected in this extrusion process for various controlled variable/manipulated variable pairings is studied using the relative gain array (RGA) technique. For minimal interaction, the MIMO system with screw speed and barrel heating as manipulated variables and product temperature and motor torque as controlled variables was found best. This system, using feed rate and moisture content as input disturbances and with realistic constraints on the manipulated variables, was simulated using standard software for controlled system performance. PID controllers without additional decoupling gave adequate performance results for various controlled system requirements.