Abstract: To resolve the impending deadlock problems in the path planning of Overhead Hoist Transporters (OHTs) in unified Automatic Material Handling Systems (AMHSs) of 300 mm wafer fabrication, an impending deadlock-free path planning strategy was proposed on the basis of the graphic theory. First, the impending deadlock problems were formally described. After introducing novel concepts of chains and share points, necessary and sufficient conditions were identified and an available approach was put forward using the graphic model. Then, a mathematic model was set up with an optimal objective function of minimizing the transporting time. Finally, by executing deadlock detection and resolution steps successively, an impending deadlock-free scheduling solution was assigned. Simulation experiments indicate that the proposed strategy can effectively detect and resolve the impending deadlocks occurring in OHTs' path planning during material handling processes. Thus, it can improve the security and stability of wafer fabrication systems.

Key words: impending deadlock, unified AMHSs, deadlock-free scheduling, path planning