Techniques for real-time determination of the components of variance for control of continuous-sheet processes

The total variance of important output quality characteristics of continuous-sheet processes serves as a valuable qualitative guide for process control. Where significant correlation exists between a machine adjustment and a single component of the quality characteristic variance, quantitative decisions resulting in specific machine adjustments can be made. For improved control to be realized in such situations, however, the control system must be capable of performing real-time calculations of appropriate variance components. This paper presents three measurement-computing systems for the real-time, in-line calculation of the profile (cross-sheet), machine direction, and random components of variance of the output of a continuous-sheet process. A general analysis-of-variance model is developed in terms of actual sensor outputs and scanning patterns and then adapted for each of the three systems presented.