A Petri-net-based structure for automated storage and retrieval system operation modelling

This paper presents a Petri-net (PN)-based hierarchical structure for AS/RS operation modelling. The complete automated storage and retrieval system (AS/RS) operation is modelled by four different layer modules, including the command typing layer (CTL), the command profile layer (CPL), the command compilation layer (CCL) and the command execution layer (CEL). To simplify the structure, macro-places are created to represent complicated activities associated with some specific groups of functions. The complete modelling system is implemented in a compact ‘Visual C’-based object-oriented computer program. The ‘click’ function is introduced to allow macro-places to be zoomed in for detailed internal activity visualization. In a typical AS/RS, operation commands are generated and issued by the management system. Each command is first typed and queued in the CTL 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 executed in the CEL module by the crane device. By employing the proposed four-layer modelling procedure, basic AS/RS operation modules of any size can be developed and then assembled to construct a complete AS/RS-PN model. By carefully analysing the properties of the complete AS/RS-PN model, the robustness of the AS/RS structure is verified.