Modular logic controllers for machining systems: formal representation and performance analysis using Petri nets

摘要

The machining systems considered are high volume transfer lines which are widely used in automotive manufacturing. In these machining systems, several machines linked together provide complete processing of a part. A logic controller is a discrete event supervisory system which controls parallel and synchronized sequences of elementary operations of each machine to achieve the goal of the machining system. Normal operation is governed by the auto-cycle function of a logic controller. A modular logic controller is introduced and formalized for high volume transfer lines. Its event-based functional properties are verified and its reconfigurability is considered. A live and safe marked graph can be directly transformed into a sequential function chart (which is one of the IEC 1131-3 languages) and, using this SFC representation, a modular logic controller can be implemented. The performance analysis of the modular logic controller is also introduced. The cycle time during normal operation is used as the performance metric of a transfer line. By adding time specifications to the model, a timed modular logic controller is generated. The time-based cyclic behavior of high volume transfer lines is thus characterized using timed Petri nets. Two efficient algorithms to compute the cycle time and critical operations are developed.