Magnetic Field Measurement Using Ultrasonics

Abstract

The detection and measurement of magnetic field intensity for purposes of field plotting and uniformity studies is a most important consideration in the field of magnetics. Field plotting and uniformity studies have been conducted on certain sizes of magnetic geometries with rotating coil instruments and Hall generators and quartz crystal magnetometer. The obvious limit of such devices is their minimum resolving power which can be defined in terms of sampling area and mimimum air gap. For the rotating coil instruments, the minimum air gap is 14 in. and the sampling area is roughly 1.2 × 10−2 sq in. Commercially available Hall generators which can be fitted into air gaps of approximately 0.011 in. have sampling areas in the order of 3.2 × 10−3 sq in. Hall generators with sampling areas in the order of 10−7 sq in. have been reported in the literature. Minimum thickness of this type device is presumably less than 0.01 in. The more recent work with the vibrating quartz crystal magnetometer has been used to measure flux density normal to the surface of permanent magnets. The sampling area of this type of equipment is 0.56 × 10−6 sq in. The minimum thickness of this equipment has not yet been reported in the literature. The flux-measuring equipment being described in this paper has a sampling area of approximately 6.25 × 10−6 sq in. and a senstivity of approximately 0.01 × 10−3 v/gauss. The probe consists of a single turn inductor of length 116 in. which vibrates ±0.00005 in. The inductor is fastened to a probe of 0.01 in. thickness and the 0.0001 in. motion is perpendicular to this 0.01 in. probe thickness. The physical geometry and size of this probe permits measurements in a variety of positions relative to the magnetic circuit. This type of probe is applicable for measuring flux density in air gaps as small as 0.017 in. The driving device for this equipment is a 60 Kcps magnetostrictive transducer. The complete probe consists of the transducer reactor, a coupling bar of nonmagnetic stainless steel and the thin strip of the nonmagnetic stainless to which the inductoris fastened. The geometry and size of this thin strip can be changed to accommodate variations in both inductor length and displacement.