Design and implementation of an advanced controller in plant distributed control system for improving control of non-linear belt weigh feeder

Abstract

A Belt Weigh Feeder (BWF) is a flat belt conveyor designed for feeding bulk material into a chemical process in a controlled fashion for better process control. The dynamic weight of bulk material is measured with a belt weigh bridge and the belt speed is controlled to compensate for any variation in the weight so that the mass feed rate is maintained as per the set feed rate. The problem of belt speed control is challenging as the dynamic response of system is non-linear and there are frequent changes in belt load due to variation in bulk material characteristics. The control accuracy of belt weigh feeder is fully dependent on the controller’s performance in providing precise control of speed of motor/belt. Any delay in achieving the set feed rate or frequent deviation between set and actual feed rate affects the quality and efficiency of downstream process. Conventional PI controller is unable to provide optimum control due to system non-linearity. To overcome this problem, in this paper first the operating data of the BWF system is analysed and the nature and cause for the nonlinearity is investigated. The system is then modelled using the design parameters of plant belt weigh feeder, which is then simulated to have a better insight into its non-linear response. Subsequently, based on simulation results, a PI Fuzzy Logic (PI-FL) controller is designed to improve the control accuracy of the system. Further, to ensure the stability of the system, an adaptive controller is introduced in cascade to fine tune the gains of PI-FL controller as per the operating speed of the BWF. Finally, an advanced PI-FL with cascade adaptive controller is implemented in the plant DCS (microprocessor based process control system). The actual test results indicate reduction in the Integral of Absolute Error (IAE) of the system by about 34% using this controller.