Accurate measurement of electrode length in electric arc furnaces will result in decreased maintenance time and improved plant productivity. This thesis describes the development of a microwave-based Soderberg electrode length-measurement system.
Various methods of electrode-length measurement were investigated and it was found that a microwave measurement system based on a conventional frequency modulated continuous wave (FMCW) radar presented most feasible technique. In this system, microwaves are propagated down a waveguide placed in the electrode. As the waveguide melts, they continue propagating in the resulting cavity until they are reflected by the discontinuity at the bottom of the electrode. The time taken for the return journey to the bottom of the electrode and back is measured, and the electrode length calculated.
FMCW radar measures return time by mixing a transmitted linear frequency sweep with the reflection to produce the difference frequency. The resulting beat frequency is then prorportional to the distance from the reflecting object. An investigation into the required linearity of the microwave source showed that the linearity is crucial to obtaining high signal-to-noise ratios in the beat frequency and therefore also to achieving good measurement accuracy. Although the designed radar had built-in linearisation capabilities, it was found to be more cost-effective to use a temperature-stabilised linear voltage controlled oscillator.
Ultimately, the accuracy with which electrode length can be determined depends on how accurately the peaks in the beat frequency spectrum can be determined. Various spectral estimation techniques were investigated, and it was found that the fast Fourier transform in conjunction with zero padding and weighted averages provided a good combination of fast, robust and accurate results.
A steel pipe with the same dimensions as the one to be placed in the electrode was used to test the measurement system. Electrode tip erosion was modelled by moving a reflective plunger up the waveguide. Length measurements were performed between 4.5m and 6.9m at one-centimetre intervals. The RMS error associated with the measurement was found to be 1.37cm, and the linearity of the system was excellent. Further experiments up to 9m long confirmed the accuracy and linearity. Finally, a 40cm long section of Soderberg electrode was placed at the end of the electrode to confirm that the waves would continue to propagate after the waveguide had melted. The length-measurement system performed as well as in previous experiments with the electrode section having no adverse effects on the measurements. Various other experiments to determine the effects of waveguide joints, temperature changes and electrode terminations also had favourable results.
A radar-based system was thus designed to measure the length of the Soderberg electrodes in electric arc furnaces. The system was then built and tested, and it was found that it provided excellent measurement accuracy under a range of conditions. The system must now be implemented in a real furnace in order to evaluate the effect of the environment further.