Sensing strain with Ni-Mn-Ga

• Published in: Sensors and actuators. A. Physical. 2018-01, Vol.269, p.137-144
• Main Authors: Hobza, Anthony, Patrick, Charles L., Ullakko, Kari, Rafla, Nader, Lindquist, Paul, Müllner, Peter
• Format: Article
• Language: English
• Subjects: Alloys; Anisotropy; Bias; Coils (windings); Compression tests; Crystal lattices; Deformation; Dynamic tests; Electric potential; Hall probes; Hysteresis; Magnetic circuits; Magnetic fields; Magnetic permeability; Magnetic properties; Magnetic shape memory; Magnetic susceptibility; Magneto-mechanics; Manganese; Mechanical tests; Nickel; Sensors; Shape memory alloys; Single crystals; Strain measurement; Strain rate; Strain sensor
• ISSN: 0924-4247; 1873-3069
• DOI: 10.1016/j.sna.2017.11.002

•Voltage amplitude in the sensing coil increases with decreasing tensile strain.•Strain sensing resolution decreases with increasing bias magnetic field.•Hysteresis in voltage occurred between loading and unloading during dynamic tests.•Static test matched the compressive portion of the dynamic test.•Hysteresis increased with increasing strain amplitude and frequency. Deformation rearranges the crystal lattice in magnetic shape memory alloys, which changes all anisotropic properties of the material. This study investigates leveraging the deformation-induced change of magnetic permeability for a strain measurement technique. A Ni-Mn-Ga single crystal placed inside a doubly wound coil with a primary and a secondary winding was used as a strain sensor. An AC voltage excited the primary coil and the secondary voltage was measured as the sample was strained from 0 to 5.2%. This method varies from other methods that utilize complex magnetic circuits, require high magnetic fields, or other sensing methods such as Hall probes. When the sensor element was tested statically by compressing the element manually against a bias magnetic field perpendicular to the load axis, the voltage output varied from 129.7mV to 164.2mV. The dynamic performance of the sensor was tested by cycling the element between 25 and 100Hz in compression against a bias magnetic field in a displacement controlled magneto-mechanical test system. The bias magnetic field was varied from 0.2 to 0.8T (0.16 to 0.64MA/m) while the cyclic displacement was varied from 0.5 to 4.5% strain. The voltage amplitude of the signal in the secondary coil increased with decreasing tensile strain. The full scale RMS voltage at a 200 mm stroke increased from 53.0mV to 78.4mV as the bias magnetic field decreased from 0.8T to 0.2T. As the element was compressed, there was no difference in the sensor output voltage between the static and dynamic tests. When the element expanded during unloading, the voltage output of the sensor from the static test matched the voltage output during compression. For the dynamic testing, the voltage output of the sensor exhibited a hysteresis from the loading voltage output, the hysteresis increased when the strain rate increased.