Structured approach in PLC (programmable logic controller) programming for water/wastewater applications

This paper describes a methodology for efficient implementation of PLC programming for water/wastewater applications. The PLC was interfaced with a supervisory host computer which used touch screen equipped color monitors as operator interfaces. PLC ladder logic had to be designed to process real-world hardwired I/O as well as the I/O received from the host computer and/or touch screens, via a communications link. Standard "templates" of PLC networks were developed for (a) pump controls including provision for touch screen I/O; (b) PID control; (c) alarms; (d) motor run times; (e) square root extraction; (f) signal conversion, and (g) flow totalization. All logic was implemented using the standard templates. This structured approach led to efficient implementation, easy debugging/start-up, and easy to read uniform ladder logic.