Robust encoder for German blast furnace

Stockport, UK – Krupp Mannesmann has deployed a new encoder system to overcome production issues on the blast furnaces at its steelworks in Duisburg, Germany. The device is being used on the plugging machines --the most critical parts of the plant, which produces 5.2 million tonnes of pig iron a year.

The plugging machines open tapholes in the furnace several times a day to discharge the hot metal ready for the next stage of its processing. To seal the hole, a taphole gun rotates about its attachment point to drive its stopper head to seal the previously drilled taphole with rapidly hardening plug material.

The plugging machine must never apply pressure to the furnace breast and so must travel precisely to its end position at the taphole in order to position the plug. The new encoder, a Sensorline GEL 235 from Lenord + Bauer, measures the angle of rotation of a plugging machine at the bottom of the blast furnace where the hot pig iron emerges at around 1,600 degrees C. This enables the movement to come to a halt at precisely the right moment.

Previously, encoders performing this function have suffered frequent failures because of the extreme, fluctuating temperatures in the vicinity of the plugging machines. Optical sensors in particular regularly refused to function as a result of temperature-related material tensions between the steel encoder shaft and the optically transparent plastic or glass code discs.

Absolute rotary encoders in elaborate protective housings also failed to deliver the required reliability and the temperature cycles of the environment accelerated the aging of thealways susceptible LED sources.

The L+B device features a new type of contour disk made of ferromagnetic steel. Etched into the steel disk using optical lithography and chemical etching processes are three incremental tracks with stepped numbers of ridges.

The tracks are scanned magnetically using giant magneto-resistive sensor elements and the absolute position is determined by ingeniously applying a Vernier scale to the readings. In such a way the new encoder is accurate to 0.1 of a degree and has an overall resolution of 28 bits (16 bit single turn and 12 bit multi-turn).

Its use of a magnetic measuring principle means that the new GEL 235 absolute encoder is immune to ageing effects and is unaffected by wide fluctuations in temperature. The ferromagnetic steel contour disk is form fixed to the encoder shaft resulting in a highly robust unit. Unlike optical systems that use transparent glass or plastic code discs it is practically impervious to dirt or vibration and is impervious to condensation.