Development of Online Multi-Loop PID Controller in PLC For A Cascaded Level - Flow Process

The cascaded control schemes are one of the complex multi-loop control systems preferred in process industries to have better control over the process variable. The PID controllers are the most preferred controller in industries even today for its effective control performance. The Programmable Logic Controller (PLC) is widely used in process industries for implementing many of the control schemes. The PLCs provide fast, reliable and easy implementation of control schemes in process industries. This paper aims at developing and implementing complex control scheme, cascade control with simple ladder logic program developed in a low-cost PLC rather than implementing with the built-in tools in relatively high cost Distributed Control Systems (DCS). The PID controllers are tuned for both the primary and secondary process, namely, level and flow. The multi-loop controller is developed with simple ladder logics in PLC. The PLC is interfaced with level-flow process and real-time results are obtained. The time-domain parameters of the process are obtained. Thus a complex control scheme is developed using ladder logic program in PLC and results are obtained for the level-flow process with a similar cascade controller developed using a digital PID. This control program can directly be implemented in process industries for online control of cascade systems with a low-cost PLC rather than adapting a high-cost DCS systems.