The steel casting industry is always improving its manufacturing plants and techniques in terms of both product quality and safety. In this framework, a lot of open challenging problems need to be solved.
As far as continuous casting is concerned, where several very long steel products (e.g. billets) are manufactured in a single machine run, the steel level in the mold should be controlled with great accuracy in order to obtain a high-quality output and to avoid breakouts. This kind of measurement is very complex owing to both the high-temperature environment (more than 1000° C) and the presence of the molding powders on the steel surface. The latter, which completely hide the steel level, should be characterized and controlled as well, because it also affects the final steel quality.
Nowadays, the steel level measurement is commonly carried out with a radioactive sensor, which is fairly accurate, but it also require a lot of care in terms of safety. On the contrary, the available measurement systems are still not reliable for the powder thickness.
In the framework of a fruitful collaboration with Centro Sviluppo Materiali S.p.A. and ORI-Martin S.p.A., a new radio-frequency sensor which can simultaneously measure both the steel level and the powder thickness, without being dangerous for the employee, has been developed and patented.
The sensor is mainly based on an electromagnetic cavity, whose resonance condition has been obtained in spite of the various fairly-big apertures that need to be present on the top wall (see picture). The measurement system exploits the basic principle according to which a variation of the air volume, due to the variation of the steel level in the cavity, causes a corresponding variation of the cavity resonance frequency. An analogous behavior is also clearly observable in acoustics: consider a trombone with its slide mechanism that varies the length of the instrument to change the pitch. The transmission bandwidth and amplitude are instead related to the powder amount which basically operates like a lossy/damping material. The measure sensitivity is related to the quality of the confinement of the electromagnetic field distribution inside the open cavity. This crucial point has been solved by studying a particular geometrical configuration which exploits a higher order mode resonance.
In the operative conditions, the resonance frequency and bandwidth of the cavity can be measured with a proper electronic equipment. The relationship between the electrical parameters and the geometrical ones has already been characterized with both simulations and measurements on a low-temperature prototype (see picture). Consequently, the required geometrical parameters i.e. steel level and powder thickness can be directly computed from the measured electrical data.
The prototypes designed for ORI-Martin S.p.A. demonstrated very good results on long-term measurement campaigns with a particular emphasis on the powder thickness measurement. As a matter of fact, the obtained powder thickness signal has recently been used to control the powder injection automatically. Finally, it should be remarked that the obtained results are driving a further industrial development of this new cavity-based electromagnetic sensor focused to both the performance improvement and the extension of its field of application.
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