Capacitance Bridge Measurements of Magnetostriction for Engineering Applications of Magnetoelastic Materials

Recently developed magneto-elastic torquesensing technology usually employs circumferentially magnetized shafts or rings of specially formulated alloy steels that have undergone unique processing treatments to enhance their efficiency. Identifying these suitable alloys and assuring their appropriate sensor performance has proven to be an expensive and tedious process. In order to promote the widespread applicability of such devices, it is now essential to economically and reliably predict the suitability of additional specific alloys and processing treatments. Although the specification of mechanical properties of commercially available alloys is readily accessible, in the vast number of cases magnetic properties appear to be either unobtainable or unpublished. In particular, the magnetostriction level of the alloy is functionally one of the most important characteristics in determining its efficacy in torquesensing technology. Magnetostriction effects were investigated for five different materials by using a simple method recently developed in our laboratory. The magnetostriction effects were generated by a large oscillating magnetic field produced by high current 60Hz AC, capable of reaching saturation levels for the material, and then detected by a change in capacitance between a hollow cylindrical sample and a concentric brass ring. This capacitance change was continuously monitored at a high frequency rate by a standard laboratory capacitance bridge meter. The output voltage of the bridge was fed into a storage oscilloscope and its voltage versus time signals were then analyzed by a computer program. Four ferromagnetic rings, constructed of pure elemental nickel and iron, and of high-speed steels 4620 and 4340 (which have proven applicability for use in magneto-elastic torque sensing), were used as the experimental samples, while a paramagnetic aluminum ring was used for the control sample. We have found this experimental method to be both reproducible and sufficient to rank different ferromagnetic materials by their magnetostriction level, which is a significant consideration in producing effective magneto-elastic torque sensors, and would significantly aid in the identification of additional suitable alloys. According to classical magnetostriction theory, a sinusoidally oscillating circumferential magnetic field produced by the application of a large alternating current to our coaxial insulated ferromagnetic sample results in a corresponding oscillatory variation in the radius of our sample ring, thus affecting its capacitance with a concentric nonferromagnetic holder according to the well-known formula for a cylindrical capacitor (where C is the capacitance of the device, l is the length and r is the outer radius of the sample ring, and R is the inner radius of the holder): C = 2πε0l ln R r ( )