Design of industrial automated systems via relay ladder logicprogramming and Petri nets

摘要

For the past few decades, programmable logic controllers (PLCs)nusing relay ladder logic (RLL) programming have been the workhorse forncontrolling event-driven industrial automated systems. RLL proved to benflexible compared to the hardwired RLL control implementation, due tonits feature of software implementation. As automated systems become morencomplex, they also become more difficult to understand and maintain. Itntakes tremendous effort to accommodate specification changes (which arenbecoming frequent) to meet today's flexible automation needs. Severalnmethods are emerging to overcome the shortcomings of RLL. Petri netsn(PNs), initially proposed as a modeling tool, have been developed asnsuch a method. This paper adopts an industrial-scale system to comparenRLL and PN design methods so that the advantages of PN-like approachesnare fully recognized. The criteria are: (i) the understandability thatnrelates to the ability to evaluate the programmed logic, to verify itsncorrectness and to maintain the control system, and (ii) the flexibilitynthat relates to the easy modification of logic when the specificationnchanges. This network takes an existing industrial system, conductsndiscrete event control designs by using both RLL programming and PNnmethods, and performs a comparative study on them. Together withnprevious comparison results using small-scale systems, the results ofnthis study support that PN-like advanced discrete event control designnmethods are better than RLL in terms of the understandability andnflexibility of the resulting control design