Micrometric non-contact position magnetoimpedance sensor

Highlights: • Non-contact position technology based on Giant Magnetoimpedance effect. • Sensitivity analysis in terms of the magnetic field gradient. • Optimum sensitivity is achieved in shorter soft magnetic wires. • Closure domain structure at wire's ends plays a dominant role. • Micrometric sensitivity is achieved, suitable for agronomic sensing purposes. In this work a sensitive micrometric non-contact position sensor based on the Giant MagnetoImpedance effect (GMI) is analyzed. A nearly zero magnetostrictive CoFeSiBCr wire was employed as sensor nucleus. The sensing principle is based on the changes in the high frequency electric impedance, Z, of the soft magnetic element as a function of the relative position of a permanent magnet generating a non-uniform magnetic field along the wirés axis. The sensor sensitivity is analyzed in terms of the magnetic field gradient and wire's length. The comparison between the sensing response of a single wire element and a long wire (12 cm in length) with different voltage contacts along its axis is performed. Higher micrometric sensitivities are achieved in wires with a certain critical length. A slight enhancement of the sensor sensitivity is found under the single wire configuration below the critical wire length. These results are interpreted as the contribution of the characteristic closure domain structure at the sample ends in these soft magnetic wires. Finally, the application of the sensor for the detection of the daily micrometric trunk shrinkage variations in a lemon tree is presented. The results indicate that this type of magnetic sensors can be easily implemented in the agricultural sector, providing a low cost and sensitive detection technique regarding water monitoring purposes.