Contactless flow detection with magnetostrictive bilayers

The paper concerns the contactless detection of an airflow through a tube by the use of a magnetostrictive bilayer strip as a sensor element. A bilayer mounted in the tube is magnetized by an exterior excitation coil. Due to its non-linear behavior, the bilayer generates a field exhibiting higher order harmonics which are detected by an exterior induction coil. The airflow yields bending of the bilayer and thus changed harmonic response, as a basis of signal establishment. For flow dynamics of 10 Hz, effective results are presented for different induction coil positions allowing flexible adjustments to individual geometric conditions. Furthermore, the shielding effect of conductive tube material (such as aluminum, copper and non-magnetic steel) was analyzed, the results indicating the need of low magnetization frequency. Finally, the significance of bilayer length was studied, increasing values yields high sensitivity, however, linked with decreasing mechanical resonance which is relevant in cases of high dynamic of airflow.