Workload control concepts in job shops: A critical assessment

In the case of production environments with job shop characteristics, much research has been done on partial control such as priority dispatching. The development of comprehensive control concepts lags behind. However, the principles of workload control (WLC) have been elaborated to more comprehensive production control concepts. WLC concepts buffer the shop floor against external dynamics by creating a pool of unreleased jobs. The use of workload norms should turn the queueing or orders on the shop floor into a stationary process which can be characterised by an equilibrium. This paper compares and discusses the concepts of WLC. Assumptions of stationarity implied in the workload norms are exposed. A subdivision of workload definitions is chosen as a starting-point to trace assumptions of stationarity. The conflict between timing and balancing within the job release function leads to an examination of stationarity requirements on the job pool contents. The analysis of stationarity requirements within existing production control concepts provides guidelines for developing production control concepts for job shops working under dynamic circumstances.