Steel Continuous Casting: Mold Oscillation Position

The hydraulic oscillation unit of a continuous-casting mould (non-sinusoidal oscillation) relies on a displacement closed loop to realise the set amplitude and frequency waveform. Unlike ordinary positioning control, the controlled quantity here is a continuously reciprocating displacement curve. The requirements on feedback are high refresh rate + high repeatability + no drift over the long term in a high-temperature, strong-magnetic environment. Distortion of the oscillation waveform is reflected directly in oscillation-mark depth on the strand surface and in mould-powder consumption — a typical scene in which “sensor figures directly determine product quality”.

Special requirements of mould oscillation on displacement feedback

Oscillation frequency is typically of the order of tens to hundreds of cycles per minute, with millimetre-scale amplitude. This means: first, the refresh rate must be high enough to collect enough points in one oscillation cycle to reconstruct the waveform (refresh rate about 0.5 ms to 5 ms depending on model and range; short-range models have the advantage); second, repeatability determines waveform consistency — repeatability of the order of ±0.002 mm is a necessary safeguard for millimetre-scale amplitude; third, phase is more critical than absolute accuracy — a phase deviation between the upper and lower oscillation cylinders causes mould tilt and uneven wear. The matching principle for refresh rate and control cycle is given in response time and the control cycle; the distinction among the three accuracy indicators is in the three accuracy terms.

High temperature and strong magnetic fields: two interference sources in the caster area

The mould sits next to high-temperature steel and the cooling-water system; ambient temperature is high, steam is plentiful and cooling-water spray is frequent. The sensor operating temperature must cover the actual environment (for the distinction see operating temperature vs. storage temperature). A protection rating of IP67 or better is recommended; take IP69K in washdown areas (see IP protection ratings). At the same time, electromagnetic stirrers and high-power equipment form a strong magnetic and electromagnetic environment, which is a direct challenge to a measuring method based on magnetic coupling: an external strong field may disturb coupling between the magnet and the waveguide. Mounting position should be kept as far as possible from the stirrer coils, with screening and earthing in place; for mechanisms and corrective thinking see EMC.

How oscillation displacement differs from ordinary positioning measuring points

Mounting form and model selection

Oscillation-cylinder stroke is short, so a short-range model should be specified — typical non-linearity <0.02% FS; converted from % FS, the absolute error of a short range is smaller and the refresh rate is faster. Prefer in-cylinder mounting so that the rod is not exposed to steam and spray, and the measuring chain is shortest. In-cylinder mounting requires a check of pressure rating (by series: Series 16/16R is 350 bar working / 530 bar peak, Series 17/17EX is 350/600 bar, Series 19 in-cylinder is 300/600 bar) and minimum piston-rod bore (generally ≥12.7 mm). Products to consider include Series 17 cylinder-integrated and Series 19H in-cylinder. The general framework for a hydraulic closed loop is given in why hydraulic cylinder position control uses magnetostrictive sensing. Schemes for other long-stroke measuring points on the mill line are in metallurgical rolling-mill roll-gap control.

Interfaces and synchronous sampling

A two-cylinder or multi-cylinder oscillation system requires each position channel to be sampled at the same instant, otherwise phase analysis is distorted. EtherCAT distributed clocks can meet the synchronisation need (Series 197 EtherCAT); configuration notes are in the practical fieldbus guide. Oscillation-waveform data should be retained over the long term to analyse mechanical degradation of the oscillation unit — under the same set parameters, increasing distortion of the actual waveform usually appears before a mechanical fault. Zero drift caused by temperature can be handled by reference to speed-of-sound temperature drift and compensation.