Analysis Techniques For Man-Machine Systems Design

NATO UNCLASSIFIED EAC/243(Panel 8 TR/7Volume 2- 36 -orderconfirmationmmunication order is processing operator 1 communication order is sent toorder arrives waiting starts is active is terminated complete sendsystem is idle,waiting for ordersr communication order order operator 2 communication order is con tiocommnicaionordeissenttoorder arrives waiting processing is active is terminated complete cstartscommandstartsFigure 2.10:A Petri net model of a communication system in a conflict situationTable 2.4: Some typical interpretations of Transitions and Places (Murata, 1989)Input places Transition Output placesPreconditions Event Post-conditionsInput data Computation step Output dataInput signals Signal processor Output signalsResources needed Task or job Resources releasedConditions Clause in logic ConclusionsBuffers Processor BuffersInputs to the techniqueThe events and the conditions of a system must bedescribed and modelled. The sequencing between theevents must be available, as well as their prioritiesand duration times.Outputs of the techniqueThe execution of the Pctri net results in a sequence ofdiscrete events that can be analysed. Further, severaltechniques have been developed for analyzing Pctri netswithout execution of the neL Some typical questions thatcan be addressed by this type of model are:1. Reachability of certain markings, i.e., unsafe systemstates.2. Resource administration and allocation, i.e., overflowand conflict situations, and deadlock problems and"starvation" of the system.When to usePetri nets can be used in the latter stages of design definition, or when other systems engineering specialities havemade them available.NATO UNCLASSIFIED- 36-