1969 Volume 2

Control of a complex organisation requires that its response to decision inputs be predictable . Traditionally, this has been made possible by almost breaking down the organisation into very simply coupled subsystems. This allows decision-makers to use a relatively simple and usually informal model.

Size and environmental rate of change are now tending to make the decoupling hard to make effective. Industrial dynamics provides both a theoretical structure and a computer simulation technique designed to provide decision-makers with a richer model of the dynamic feedback processes generally involved in industrial operations. The structure defines a system in terms of a closed boundary containing feedback loops, which in turn are formed by the relation between level and rate variables . The DYNAMO compiler allows models of systems conceived on these lines to be programmed in a straightforward way so as to allow experimental investigation of the system variables. The paper discusses case studies and concludes with notes on model validity and the problem of data.

Up to a few years ago, all industrial processes were either batch processes or were operated on a steady state basis, although some (like papermills, automatic looms and steel-rolling mills) operated in steady state only for a limited period until the specification or grade of the output had to be changed. Thus, industrial processes can be divided into three classes:

1. Batch processes.
2. Continuous processes.
3. Quasi-batch processes.

History and philosophy of control systems

The past decade has witnessed a major advance in the use of process control systems by the paper industry. Before that time, very few variables were controlled automatically, many instruments were either indicating or else, if controlling, were operated in the manual mode much of the time and the degree of instrument reliability and accuracy in the average mill did not promote operator confidence or use . Pneumatic controllers performing simple control loop strategies were the norm. Making paper was an art and frequently different papermakers would operate a machine in quite different ways to make the same given grade of paper. Fairly broad tolerances in paper specifications were accepted in the trade, since few machines could achieve or maintain very stable operations.

There is no difference, at least in principle, between control of administrative systems and control of other systems. The purpose of a control procedure is to specify the way in which the behaviour of a system can be affected. The controller of the system scrutinises information about the performance

of the system and every so often he has to make a decision, namely, to choose between several courses of action open to him. This decision is transmitted to the system with the expectation that it will react in a certain way. This cycle of events monitoring and evaluation of the system’s behaviour, followed by a decision for corrective action is the essence of the control process, irrespective of whether it is control of inanimate systems or managerial control of industrial enterprises.