HIERARCHICAL PERFORMANCE-ORIENTED CONTROL OF FLEXIBLE MANUFACTURING CELLS

In a job shop, each product may have a different processing route through the system. Automated Flexible Manufacturing Cells (FMC) that adopt this flow pattern are highly prone to deadlocks. A supervisor is a controller that uses available data via feedback loops to characterize the current behavior of the cell, and modify the equipment controllers to achieve the desired operational specifications in a deadlock-free manner. This paper proposes a hierarchical control system divided into an upper level scheduler and a lower level supervisor to control FMCs. The scheduler is responsible for determining a deadlock-free allocation of the resources that optimizes some performance measure, based on the current production requirements, and the supervisor guarantees that the flow plan (behavior) determined by the scheduler is realized on the shop floor. For that purpose, a formal method that can transform a production schedule into a supervisor, in real time, is also proposed. The supervisor is an augmented Marked Graph (MG) that captures all the events that can take place in the cell. The proposed approach is validated by generating and simulating the supervisors for two benchmark problems.