Hierarchical Scheduling of Robotic AssemblyOperations in a Flexible Manufacturing

An approach to hierarchical scheduling for assembly in a fully automated exible manufacturing system is presented. The three-level production system consists of a master production plan, which incorporates new customer requests and makes long-term production guarantees; a production ow level, which plans and monitors the ow of subassemblies among workstations; and a scheduler level, which sequences and dispatches assembly operations. An independent scheduler at each workstation coordinates robot motions and conveyor transferals to interleave the operations related to diierent subassembly products. The higher priorities of tardy subassemblies are weighed against immediate opportunities to exploit the current connguration of the workstation and maximize overall through-put. We focus on the scheduler level, wherein heuristics and combinatorial search are used to exploit each workstation's capacity optimally. Two scheduling algorithms are described | a ne-granularity scheduler, which analyzes robot-conveyor interactions with precise timing, and an anytime scheduler, which stochastically generates and evaluates alternative schedules until the tradeoo between expected plan improvements and dispatching delays is no longer favorable.