Inductive level measurement
The surface level of liquid metals in containers – as e.g. moulds in steel casting – is an important parameter in industrial manufacturing processes. Its exact surveillance and control can give a significant contribution to the quality of the final product. Level measurement system have to withstand the often hot and aggressive surrounding of liquid metals. Here, inductive methods excel by not needing direct contact with the melt. Thus, they are easier to protect from the challenging environment.
The measurement method is based on the induction of eddy currents in the melt: An excitation coil generates an alternating magnetic field, which penetrates the container. Through the temporal variation of the magnetic field, electric currents are induced into the melt, which generate a secondary magnetic field themselves. The strength of this secondary field is dependent on the shape of the electrically conducting volume in the container, i.e. of the position and shape of the surface of the melt. The secondary magnetic field is measured using induction coils. From the resulting voltage signal the surface position can be deduced by means of calibration.
Figure 1: Inductive surface level sensor. (a) Schematic of excitation and induction coils. (b) Picture of the sensor.
The magnetic field of interest is in general much weaker than the excitation field (typically by a factor of 100). To negate the influence of the imposed magnetic field a gradiometric coil is used to measure the secondary field. For this, two identical coils are placed in symmetrical positions to the excitation coil. This way the excitation field produces the same voltage in both detector coils. Now the two coils are connected in a way, so that these voltages cancel out. The only remaining contribution to the measurement signal is then the asymmetric part of the secondary magnetic field. This leads to a significant improvement of the measurement accuracy (in the example in Figure 1 from a relative error of 35 % to 1 %.
Figure 2: Experimental setup (left) and measured voltage amplitude for several different surface levels (right).
Additional advantages of this method are, that the induction principle only acts on electrically conducting materials. Thus, electrically insulating slag, that often is present on top of the melt, does not influence the measurement. Furthermore, the coils can be easily scaled and are thus adjustable for many different kinds of applications.
Measurement if the surface topology
The inductive surface detection is an integral measurement method, i.e. the measured signal is generated by all eddy currents in the melt. For example, if the coils span the whole container, a mean position of the whole surface is measured. On the other hand, if the excitation coil has much smaller dimensions than the container, only a small area of the melt is penetrated by the magnetic field. The measured level is then representative of the mean surface position in only a small area close to the sensor. Using multiple sensors surrounding the container, the surface topology can be reconstructed from the individual local measurements.
Figure 3: Reconstruction of an imposed surface shape (black line) in the experiment in Figure 2. The sensor was placed at different positions along the container and had been calibrated with a plane surface beforehand.
Zürner, T.; Ratajczak, M.; Wondrak, T.; Eckert, S.
Inductive measurement of the free surface of liquid metals
Measurement Science and Technology 28(2017)11, 115301