Evaluation of Sequential Function Charts execution techniques. The Active Steps Algorithm.

Programmable logic controllers (PLCs) play a significant role in the control of production systems and sequential function chart (SFC) is one of the main programming languages. The reaction time of a PLC is a fundamental matter in discrete event control systems. We show that the reaction time of PLC depends greatly on the SFC structure, on the events sequence and also on the algorithm that executes the SFC. Five algorithms have been analyzed: brute force, enabled transitions, representing places, deferred transit and the immediate transit evolution models. The analysis has been carried out over a SFC library composed by well known models which can be scaled using a parameter. Finally we propose a new SFC execution technique adapted to efficiently execute a subclass of SFCs. We call this technique the active steps algorithm.