The w Plane—Applications of a New Design Aid for Thermal Process Control

Many common thermal processes (boilers, vats, furnaces, ovens, plasma torches, fluidized beds, etc.) used in industry can be represented by a model comprising an open loop gain μ, a salient time constant T, and a transit delay L. When such processes are to be closed-loop controlled, the question of stability arises. This paper introduces a time-domain normalization and a compound parameter v which incorporates μ, L, and T. By use of displays in another plane, the w plane, stability criteria and transient response can be readily determined for almost all combinations of command and disturbance likely to be encountered in industrial operations. The basic principle of this approach is explained, typical graphical displays are presented and the approach demonstrated by examples. The presentation is intended for the professional engineer concerned with the design and control of processes in industry.