The development of a control system for gravimetric feeding of a twin screw extruder

Abstract

The control of gravimetric feeding is generally done by control systems which utilize feedback control. The main problem with these control systems is that they generally revert to volumetric feeding during the refilling cycle. Furthermore the control system software is proprietary information and therefore not in the public sector and also very expensive. The main purpose of the study was therefore to review the basics of gravimetric feeding and to apply the knowledge in the design of a control system with applicable software for gravimetric feeding which is not affected by material bulk density variations, targeting food extrusion applications. It is also aimed at producing a test bench to test the control system and the developed software. The developed control system is a programmable logic controller (PLC) based control system. This allows for the control system to be easily integrated into any PLC controlled twin screw extruder. The control system was extensively tested to verify the developed gravimetric control algorithms. The results from the experimental tests illustrated that a fully functional gravimetric feeding system had been developed. Most importantly also that the volumetric characteristics of a material can be used, to assist a PI–controlled gravimetric feeding system in both the feeding cycle and the refilling cycle.