Attenuation and Reconstruction of Signals in High Pulsed Fields Nd-Fe-B Magnets Characterization

Detailed design of machines incorporating permanent magnets requires precise knowledge of materials´ magnetization characteristics. Nd-Fe-B magnets are known for their “hard” properties and thorough characterization of these magnets in easy and hard directions requires very strong magnetic fields (in the range of MA/m). Application of such high fields results in high voltages induced in the pick-up coils-in presented case the induced voltage reaches up to 50-60 V. Signal is acquired using precise NI ADC accepting input voltage in range of ± 5 V max. This means that direct connection between pick-up and ADC is not possible. Obvious solution to this problem is to decrease number of turns in pick-up coils, however for high quality results the number of turns should be large and ideally an integer. The ADC could be exchanged but fast, precise and high-input-voltage ADC is not cost-effective. Additionally, especially in case of slow ADCs, transitions present in magnetization signal can exceed available bandwidths distorting the results. Perfect solution to highlighted problems is the attenuation of the signals and their further digital reconstruction. Attenuation might be performed using active integrators-reported error are in the level of 1% in ~3 MA/m field. It might be also based on passive devices-preferred approach eliminating input voltage limits and power considerations for the pick-up system. Reconstruction might be done on the basis of the model of an attenuator or reconstructive filtering (equalization). Two types of attenuators are presented in this paper. RC attenuator which provided reversible spectrum and amplitude adjustment and R attenuator affecting only the amplitude of the signal. Reconstruction error from RC attenuator was on the level of ±0.1% in the most important part of the signal. R attenuator provided comparable error with added merit of very easy de-attenuation procedure which makes it an attractive solution fo- - r high field measurements. Some results from Nd-Fe-B magnet characterization in ~11 MA/m (~14 T) field using similar R attenuator device are presented. Proposed passive attenuators made this high field characterization possible.