An Integrated Project for the Design and Appraisal of Mechanized Decision-Making Control Systems

IN the past, the main interest in the control of industrial processes has centred on process control and production control. Process control is the maintenance of certain physical quantities between given bounds or in given relationships to each other. This is achieved by regarding the plant as a mechanism with a given transfer function and adding feedback elements of a suitable kind to achieve the control of the physical quantities involved. It is, in fact, an industrial application of the theory of servo-mechanisms. Production control is concerned with the construction of a delivery schedule that can be met by using certain rules for passing different batch items through a complex network of batch processes. In those batch processes which involve a stochastic time-element, another form of control is necessary, the control of congestion. Congestion occurs, of course, in any multistage stochastic batch process, and several mathematical theories have been developed to study simple cases of these. Many industrial situations involve processes in which the process times will be dependent upon the waiting times of batches in the queues prior to these processes. No theory is available for this situation and the purpose of this paper is to describe a project for dealing with it. In the steel industry, attempts are made to complete the processing of steel in its initial hot condition. However, as the material passes through the various processes, it cools and ultimately, it is necessary to recover the lost heat by passing the steel through heating furnaces. The period in these furnaces will depend upon the length of time the material has taken on preceding stages. This lengthening of the total process time in periods of congestion means that, under certain conditions, it is possible to maintain a more rapid flow of material through the system by delaying the start of certain processes. In more complex plants, in which there is a multiplicity of possible routes for the steel through the plant, it is possible to minimize congestion and maximize the rate of flow by a scheduling procedure. This scheduling, however, is of quite a different nature from that concerned in normal production control problems. The main significance of this type of interaction between process and waiting times is the positive feedback effect, because as congestion increases, it leads to process times increasing, thereby reducing the effective capacity of the plant,