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2. Introduction to the Principles of Feedback

2.3.1 Performance Specifications

There are two fundamental specifications for the continuous caster: safety and profitability.

These ultimate requirements can be further broken down to derive a control-system specification that quantifies the target to be met by controller-design.

  • Safety: In this example, the safety requirement translates rather straightforwardly into a constraint on the level of molten steel in the mould (called the mould level). Clearly, the mould level must never be in danger of overflowing or emptying, because either case would result in molten metal spilling, with disastrous consequences.

  • Profitability: Obviously, the system needs to be operated in a cost-effective fashion. Aspects which contribute to this requirement include the following:

    • Product quality: It turns out that high-frequency changes in the mould level reduce bloom quality, because molten metal has a viscosity similar to that of water, and turbulence mixes impurities into the steel. Furthermore, because of the cooling applied to the mould, an oscillating mould level results in uneven cooling of the steel, which also reduces its quality. Thus, product quality demands steady control of the mould level.

    • Maintenance: Through an oxidizing reaction, the nozzle through which the metal flows into the mould is subject to intense wear at the mould level. The nozzle's lifetime is therefore maximized if the mould level is controlled so that it slowly ramps across the wear band of the nozzle. Maintenance costs are further reduced if the lifetime of the control valve is maximized by having relatively nonaggressive control actions. Thus, keeping the cost of maintenance low requires steady regulation around a set-point that slowly ramps across the nozzle wear band while avoiding aggressive valve movements.

    • Throughput: Throughput is a direct function of casting speed. The casting speed, however, is also a function of numerous up-stream process factors (availability of the necessary grades of raw material, furnace capacity, etc.) as well as down-stream factors (product demand, shipping capacity, etc.). Thus, the mould-level control system should maintain performance at all casting speeds to avoid being a limiting factor in the overall operation of the process.