Magnetostriction measurement by using dual heterodyne laser interferometers

Electrical machines and transformers have a core built out of laminations of ferromagnetic materials. A portion of the vibrations and noise of these devices is due to magnetic forces and magnetostriction arising from the magnetic core. Magnetic forces are well known, and analytical methods are extensively used to calculate them. Magnetostriction can be defined as the deformation of the ferromagnetic material in the presence of a magnetic field. Unlike magnetic forces, magnetostriction shows a rather complex behavior. It varies for every material, and it depends on the applied magnetic field and external pressure. Therefore, magnetostrictive behavior of every material needs to be determined experimentally by means of strain measurements. Strain gauge measurement techniques have been used before at the Electrical Energy Laboratory (EELAB), Ghent University, Ghent, Belgium. In this paper, a new measurement method using dual heterodyne laser interferometers is proposed to overcome the drawbacks of the old method. The proposed measurement setup and the working principles are explained. The possibility to apply both techniques on one and the same sample can also reveal some interesting results about the quality of both techniques

Magnetostriction and the advantages of using noncontact measurements

Magnetic noise in electrical machines and transformers are a large portion of the total noise of the device. Part of this magnetic noise is caused by the deformation of the ferromagnetic laminations due to the magnetic field. This effect is called magnetostriction, and it strongly depends on the applied magnetic field, the material properties and external pressure to the material. A strain gauge measurement setup has been applied before to measure the magnetostrictive behaviour of ferromagnetic materials. The results obtained by this setup suffered from some limitations such as the need to filter high-frequency harmonics. Also the measurement results for excitation below 0.8T were not easily distinguished from the present noise. Therefore, a new setup using heterodyne laser interferometers has been built. With this new setup, on the contrary to the strain gauge setup, the sample preparation is simple. This new setup and the gradual improvements toward the optimal performance of the setup are presented in this paper

Magnetostriction measurements on electrical steel and the effect of coating

Magnetostriction strain measurements on the samples of grain oriented and non-oriented electrical steels by using the heterodyne laser interferometer technique are presented. The measurements are performed on the samples with and without coating. The latter data show similar behaviour as of those obtained by using the strain gauge technique. Magnetostriction strains of the coated samples are smaller in amplitude compared to the non-coated samples. This difference is around 15 times lower for the grain oriented and 1.2 lower for the non-oriented samples

Quasi-static Transfer Function of the Rabbit Middle Ear‚ Measured with a Heterodyne Interferometer with High-Resolution Position Decoder

Due to changes in ambient pressure and to the gas-exchange processes in the middle ear (ME) cavity, the ear is subject to ultra-low-frequency pressure variations, which are many orders of magnitude larger than the loudest acoustic pressures. Little quantitative data exist on how ME mechanics deals with these large quasi-static pressure changes and because of this lack of data, only few efforts could be made to incorporate quasi-static behavior into computer models. When designing and modeling ossicle prostheses and implantable ME hearing aids, the effects of large ossicle movements caused by quasi-static pressures should be taken into account. We investigated the response of the ME to slowly varying pressures by measuring the displacement of the umbo and the stapes in rabbit with a heterodyne interferometer with position decoder. Displacement versus pressure curves were obtained at linear pressure change rates between 200 Pa/s and 1.5 kPa/s, with amplitude ±2.5 kPa. The change in stapes position associated with a pressure change is independent of pressure change rate (34 μm peak-to-peak at ±2.5 kPa). The stapes displacement versus pressure curves are highly nonlinear and level off for pressures beyond ±1 kPa. Stapes motion shows no measurable hysteresis at 1.5 kPa/s, which demonstrates that the annular ligament has little viscoelasticity. Hysteresis increases strongly at the lowest pressure change rates. The stapes moves in phase with the umbo and with pressure, but the sense of rotation of the hysteresis loop of stapes is phase inversed. Stapes motion is not a simple lever ratio mimic of umbo motion, but is the consequence of complex changes in ossicle joints and ossicle position. The change in umbo position produced by a ±2.5 kPa pressure change decreases with increasing rate from 165 μm at 200 Pa/s to 118 μm at 1.5 kPa/s. Umbo motion already shows significant hysteresis at 1.5 kPa/s, but hysteresis increases further as pressure change rate decreases. We conclude that in the quasi-static regime, ossicle movement is not only governed by viscoelasticity, but that other effects become dominant as pressure change rate decreases below 1 kPa/s. The increasing hysteresis can be caused by increasing friction as speed of movement decreases, and incorporating speed-dependent friction coefficients will be essential to generate realistic models of ossicle movements at slow pressure change rates