Abstract
An ordinary Petri-net (PN) based approach is proposed to design a programmable logical controller (PLC) that prevents collisions among vehicles in an automated guided vehicle (AGV) system. First, a procedure is proposed to model an AGV system as a control-hardware PN. Second, a method is formulated to design a PN supervisor (the closed-loop PN) for collision prevention. This paper defines the line, merge, divide and intersection alignments, and then adds the transitions and arcs for these four alignments into the control-hardware PN, respectively. As a result, a closed-loop PN is obtained as the supervisor for preventing collisions among vehicles in an AGV system. Finally, a method is introduced to automatically translate a closed-loop PN into a ladder diagram by Boolean expressions. Consequently, the closed-loop PN can be implemented by a PLC. An example is used to illustrate this methodology.