Displacement transducer using impedance variations due to core torsion

The wide obtainable range of mechanical compliance encourages the application of helical magnetostrictive springs in force and pressure transducers as well as for direct displacement sensing. In the usual mode of operation, an alternating current is conducted through the magnetostrictive wire, and an output signal (related to the axial compression or extension of the spring via torsional stress anisotropy) is obtained from as insulated copper wire solenoid wound on the helical spring wire. Sensing the strain by the variation in impedance of this solenoidal winding provides an alternative mode of operation that allows a substantial decrease in the excitation current; an attractive prospect for applications concerned with intrinsic safety and energy usage. Suitable spring materials are found among the Ni-Fe Permalloys. Impedance is found to decrease monotonically with torsional strain (displacement). Variations of more than 2 to 1 are found to be markedly insensitive to excitation current and frequency over several orders of magnitude. This mode of operation is easily implemented with simple electronic multivibrator or other oscillator circuits, physically small enough to be incorporated within the case of practical transducers.