PLC Program Design For Automated Guided Vehicle Systems Based On Petri Nets

With the high speed development of modern logistics technology, automatedguided vehicle (AGV) systems are being increasingly used in automatic logisticssystems and automobile industries since they have the advantages of high degree ofautomation, flexible application and low cost. However, the reliability, reuse andrapidly development for programmable logic controller (PLC) programs in an AGVsystem need much research to develop the corresponding methods.An ordinary Petri net (PN) based approach is proposed to design the PLCprograms, which are for the collision prevention and sequence control in an AGVsystem，and the main points of this paper are summarized as follows:1．A method is proposed to design the control-element PN after analyzing thehardware structure of an AGV system. Based on this method, two methods areproposed to design the control-hardware PN of an AGV system for the collisionprevention and sequence control, respectively.2．A method is proposed to design a PN supervisor that prevents collisionsamong vehicles in an AGV system. Firstly, according to the characteristics of zonedistribution, four alignments are defined, which are the line, divide, merge andintersection ones. Secondly, according to the control specifications of these fouralignments, four methods are proposed to design the collision prevention controllerby adding the transitions and arcs of actuators into its control-hardware PN. As aresult, the collision prevention controller is obtained.3．A method is proposed to design the PN supervisor, which is for the sequencecontrol in an AGV system. Firstly, according to the control specifications of thesequence control, a method is proposed to design the sequence controller byaugmenting its control-hardware PN. As a result, the sequence controller is obtained.Further, the important properties, including deadlock free, liveness and reversibility,can be analyzed by the reachable graph of the sequence controller.4． A systematic and comprehensive algorithm is proposed to translate aclosed-loop PN into an LD program in order to realize the safety and reliabilitydesign of PLC programs for an AGV system. Firstly, describe the firing conditions ofthe transitions for actuators in an AGV system by Boolean expressions. Further, build the connections between the variables of Boolean expressions and the contacts，andcoils of LD programs. As a result, safty and reliability LD programs of an AGVsystem are obtained.A laboratory AGV system is used to illustrate the methods which are proposedin this paper.