Speaker
Description
Our group has experimented in the past with development of contactless position sensors suitable for retrofitting to pneumatic actuators - i.e. piston position sensing [1,2]. The proposed advantage was AC-operation enabling use of synchronous detection and suppression of noise (including DC). Excitation at low frequency (<100 Hz) made possible measurement through aluminum sheath (piston barrel) without internal permanent magnet fixed to movable piston. An array of external fluxgate magnetometers (Texas Instruments DRV425) monitors the response to excitation from internal metallic target (aluminum piston and ferrous piston rod).
While results with axial-excitation model were promising [2], the radial-excitation design achieved only moderate level of accuracy [1]. The approach published in [1] utilized simple position estimation options: i) finding the sensor with maximum signal or ii) calculation of weighted average from two sensors with strongest signal or iii) least-square fitting to known response function of single magnetometer. The reported position errors were $\pm$15 mm, $\pm$5 mm and $\pm$2 mm, respectively [1].
However, with modified data processing approach, improved positioning results are achievable. For illustration, data from only three sensors in the larger array are shown here. With variable piston position z, the raw magnetic field data are shown in Fig.1a and the corresponding gradients (differences from magnetometer pairs 30 mm apart) in Fig.1b.
Fig. 1 Three channels of raw data (a) and corresponding gradients (b) with “linear” parts.
The gradient curves are piecewise monotonic and approximately linear near zero-crossing. Within each segment of array, the appropriate pair of sensors is selected as source of signal based on the relative signal strength (amplitude). The gradient can then be recalculated into position estimation. Achieved position error level was $\pm$1 mm using polynomial approximation of 3rd order. Early results indicate potential for further improvement. More improvement could possibly be achieved with additional processing of another (perpendicular) component of measured response field.
Acknowledgements
This work was supported by GACR project 24-12705S Novel Magnetic Position Sensor.
References
[1] J. Vyhnanek, P. Ripka, and A. Chirtsov, “Linear Position Sensing through Conductive Wall without Permanent Magnet,” Proceedings of Eurosensors 2017, Paris, France, 3–6 September 2017. MDPI, Aug. 08, 2017. doi: 10.3390/proceedings1040390.
[2] P. Ripka, A. Chirtsov, and V. Grim, “Contactless Piston Position Transducer With Axial Excitation,” IEEE Transactions on Magnetics, vol. 53, no. 11. Institute of Electrical and Electronics Engineers (IEEE), pp. 1–4, Nov. 2017. doi: 10.1109/tmag.2017.2715073.
