Temperature control in a plastic extruder control system

Abstract

The chapter presents the development of a new control system for plastic extruders. The extrusion machine consists of a hopper (where the raw plastic material is stored), a large screw (within a barrel) that transports the plastic material up to the die, and heating and cooling zones distributed along the barrel. In order to satisfy all the requirements four control blocks had to be developed: temperature control, parison control, hydraulic control, and sequence control. The chapter concentrates on temperature-control problems, since a precise temperature profile along the barrel is very important for maintaining high quality plastic. The problem is addressed using auto-tuning and adaptation algorithms for each of the 17 PID controllers activating the heating/cooling zones along the barrel. The auto-tuner uses an original PID tuning method, referred to as the “Magnitude Optimum Multiple Integration (MOMI)” method, which is based on a process step response and multiple integrations of the time response. The adaptation algorithm is based on assessing the behaviour of the process. It relies on four detectors: an overshoot detector, an oscillation/decay detector, a long-tail detector, and a noise detector. The control blocks have been realised on standard modular programmable logic controllers (PLC) and tested on an industrial plastic extruder. The chapter concludes with a discussion of problems and limitations in applying theory to solve this control problem.