Abstract:
Earlier hardwired electromechanical relays were used to realize and control discrete event systems, then a steep rise of the PLC was observed and today sequencing is normally implemented in software instead of relays. PLC's are industrially hardened microcomputer that performs discrete and continuous control functions in a variety of processing plants and control environments.
Originally PLC's were introduced as a relay replacement tool but in present days they fmd usage in all sorts of industrial control applications. From their onset in market one of the languages that gained prominence for programming PLC's is ladder logic diagrams of IEC 1131-3, recently flexible industrial automation are challenging the use of LLD's and other high level programming language methods. LLD's grow too large and difficult to understand and troubleshoot once complexity in sequence control increases.
Petri Nets originated from the field of computer science and is used as a tool for modeling asynchronous and concurrent activity, lately it is gaining popularity as an effective technique for sequence control of industrial automation systems. The popularity of Petri Nets is due to the fact that unlike LLD's not only they successfully control the system but they also help in system analysis, evaluation and simulation thereby making control strategies, more easier to understand, troubleshoot, modify and evaluate.
In this dissertation complexity of sequence controllers designed•using both LLD and PN for process control applications are compared based on number of basic elements used to model strategies and number of rules and logic transformations required to implement them. It is shown that with increasing complexity in applications Petri Nets (PN) supercedes its LLD, counterpart -in both measures thereby establishing PN as a better solution for designing. controllers for discrete event systems.
Real Time Petri Net Controllers for different process control applications are synthesized on FPGA using VHDL, the syntactic and semantic compatibility of Petri Net descriptions with VHDL is observed for considered applications. Further it is observed that Field Programmable Logic for controller design results in creation of near-optimal controller implementations in terms of performance
