Relay ladder logic and petri nets for discrete event control design : a comparative study

In the 1960\u27s and earlier discrete event systems (DES) were controlled by hardwired electromechanical relay systems. In 1969 an electronic programmable logic controller (PLC) was introduced. PLC\u27s have been programmed utilizing relay ladder logic (RLL). RLL is a graphical programming language with software devices used to emulate electromechanical devices. RLL programs, however, often become large and difficult to understand because its graphical representation of physical switching devices obscures the discrete event dynamics inherent in the process to be controlled. Petri nets are a methodology for modeling discrete event systems (DES). Using a Petri net based controller, a control strategy could be developed that captures the discrete event dynamics of the process. This should result in a control strategy that is much easier to understand, troubleshoot, modify and evaluate