Multiactivity paradigm for the design and coordination of FMSs

This paper introduces a methodology and a set of tools that allow a manufacturing engineer to integrate programable work cells such as flexible machining cells (FMCs) and flexible assembly cells (FACs), linked by an automated transport system, into an FMS that can be reprogrammed easily for the inclusion of additional products, thus making small- and medium-size production runs economical. The proposed method is based on the multiactivity paradigm that separates activity executions from their coordination by viewing systems as a set of assignable executors controlled by a coordinator. In this view, an FMS has at least three layers: a top layer in which the work cells are considered as assignable resources that can be programmed to execute the activities required to produce a given product; a middle layer in which the elements of the work cells (robot arms, NC machines, vision systems, etc.) are the assignable resources that execute in the required order the various machining, assembly and/or transfer operations; and a bottom layer that controls directly the various actuators and sensors. Because of the wide variety of work cell elements, only the top two layers are considered here. At the top level a control computer is used to coordinate the activities of the work cells and the transport system, and at the middle level each work cell includes a coordinator that oversees the operation of its elements. This paper introduces the process activity language, PAL, a high level specification language that matches the multiactivity view and allows the specification of the production steps and the operation of the work cells in a highly structured way. A number of PAL-based tools that aid the manufacturing engineer, to check for specification errors, to configure the FMS to meet the various production constraints, and that simplify the design and programming of the coordinators, have been developed and are outlined here.