Time Series Analysis: Forecasting and Control, Fourth Edition

CHAPTER FIFTEEN

Aspects of Process Control

The term process control is used in different ways. Shewhart charts and other quality control charts are frequently employed in industries concerned with the manufacture of discrete “parts” in what is called statistical process control (SPC). By contrast, various forms of feedback and feedforward adjustment are used, particularly in the process and chemical industries, in what we shall call engineering process control (EPC). Because the adjustments made by engineering process control are usually computed and applied automatically, this type of control is sometimes called automatic process control(APC). However, the manner in which adjustments are applied is a matter of convenience, so we will not use that terminology here. The object of this chapter is to draw on the earlier discussions in this book to provide insight into the statistical aspects of these control methods and to appreciate better their relationships and objectives.

We first discuss process monitoring using, for example, Shewhartcontrol charts and contrast this with techniques for process adjustment. In particular, a common adjustment problem is to maintain an output variable close to a target value in a dynamic system subject to disturbances by manipulation of an input variable, to obtain feedback control. Feedback control schemes use only the observed deviation of the output from target as a basis for adjustment of the input variable. We consider this problem first in ...