Dynamic analysis of an inventory and order based production control system

Abstract

In an inventory and order based production control system (IOBPCS), there are three fundamental system parameters. One of these, the production delay time, is beyond the control of the IOBPCS designer. Indeed, the production delay may be highly variable with time due to a variety of problems commonly found in manufacturing industry. This uncertainty must be countered by the proper use of a feedback loop which detects and operates on inventory deviations. The IOBPCS designer is thus left with the forward path demand averaging time and feedback path gain to adjust, as best he can. This paper studies the ability of the IOBPCS to recover from shock demands, and to protect the manufacturing process from random variations in consumption. It is shown that the feedback path is of fundamental importance in good control, and that adequate performance cannot be obtained by feedforward alone. Dynamic recovery is examined graphically by separately identifying the feedforward and feedback components. Random disturbance rejection is studied via the noise bandwidth concept. Finally, performance optimization is undertaken using coefficient plane models related to ‘standard forms’ and hardware systems. One good compromise design achieved has the time-to-adjust inventory, demand averaging time, and production delay time all of comparable magnitude.