The relative contributions of input and output controls on the performance of a workload control system in Make-To-Order companies

Workload control (WLC) systems can be seen as a natural extension of input-output control systems which allow both the control of jobs into the shop and the control of capacity which affects the flow of finished goods out of the shop. In previous research, it has been shown that WLC can help to reduce manufacturing lead times, even if the total workload processed by the shop in a given time period is maintained at a constant rate. However, the relative contributions of the input control mechanisms versus the output control mechanisms have not been clearly determined. This paper seeks to explore this issue using a simulation of a real job shop, in which the WLC system that has been developed at Lancaster University is used. It explores the use of input control alone and the use of input control plus output control. In addition, the relation between workload and various performance measures, such as lead time, queueing time and capacity utilisation, is explored. The results of the experiments show that one should consider input control and output control as two levers of control that are both to be pulled rather than relying on the single lever of stopping the input when the WIP reaches some preset level.