2. Introduction to the Principles of Feedback
To make the above general discussion more concrete, we next present a simplified, yet essentially authentic, example of an industrial control problem. The example, taken from the steel industry, is of a particular nature; however, the principal elements of specifying a desired behavior, modeling and the necessity for trade-off decisions are generic. Some details of the example might not be quite clear at this early stage, but they will set the scene for future work.
One of the products of the steel industry is a so-called bloom, which is a rectangular slab of steel. Blooms are produced in a process called a continuous caster. A diagram of an industrial bloom caster is given in Figure 2.1. The principal components of such a system relevant to our discussion here are shown in Figure 2.2. A photograph of a bloom caster can be found on the web page.
The tundish can be thought of as a large container that acts as a reservoir for molten steel. A control valve regulates the rate of flow of steel that enters the mould mounted under the tundish. The mould, whose cross-sectional area equals the cross-sectional area of the desired bloom, is open from above and below. By intense cooling, steel in the mould is cooled to a semi-solid state. In this state, it is sufficiently firm so that the strand can be withdrawn continuously from the mould by rolls. The resulting continuous strand is then subjected to further cooling and finally cut into blooms.