A Petri-net-based structure for AS/RS operation modelling

This paper presents a Petri-net (PN)-based hierarchical structure for AS/RS operation modelling. The complete AS/RS operation is modelled by four diff erent layer modules, including the command typing layer (CTL), the command profile layer (CPL), the command compilation layer (CCL) and the command ex ecution layer (CEL). To simplify the structure, macro-places are created to represent complicated activities associated with some specific groups of f unctions. The complete modelling system is implemented in a compact 'Visual C'-based object-oriented computer program. The 'click' function is introdu ced to allow macro-places to be zoomed in for detailed internal activity vi sualization. In a typical AS/RS, operation commands are generated and issue d by the management system. Each command is first typed and queued in the C TL module and then delivered one by one to the CPL module for work-domain ( i.e. the starting and ending locations of the crane) evaluation. After the work-domain is decided, the information is delivered to the CCL module for command sequence compilation. The compiled command sequence is then execute d in the CEL module by the crane device. By employing the proposed four-lay er modelling procedure, basic AS/RS operation modules of any size can be de veloped and then assembled to construct a complete AS/RS-PN model. By caref ully analysing the properties of the complete AS/RS-PN model, the robustnes s of the AS/RS structure is verified.