Technical procedures to select basic parameters of a fluxgate magnetometer

Abstract

ABSTRACT. In fluxgate magnetometers, the external magnetic field is generally measured from the second harmonic induced voltage in the pick-up coil. Thus, to improve the performance of a fluxgate magnetometer design, careful considerations should focus on some operational parameters and how to calibrate them to attain an optimal performance with a minimum intrinsic noise at the output. Basically, two main factors are considered: i) the output dependence on variations of the even harmonics amplitudes with the excitation current; and ii) adjustments for a perfect timely match between the core saturation and the time-width of the sampling. The best sensitivity of the magnetometer is obtained in a portion of the hysteresis curve where the 2 nd harmonic component predominates. Since this region depends on the amplitude of the excitation, it becomes necessary to calibrate the excitation amplitude in order to get the highest 2 nd harmonic amplitude in the pick-up coil. When the amplitude of the excitation current is modified, the amplitude of the even harmonics also changes, and consequently the magnetometer will operate below its optimal sensitivity, depending on how much the current has changed. Similar result is observed when the core is built with a different inductance (different number of wraps) which requires operational tests to be carried out to verify the level of the core saturation by adjusting the amplitude and the frequency to get the best performance from the magnetometer. Changes in the amplitudes of the excitation current generally occur with changes in the excitation current waveform, affecting the timing of the core saturation. To avoid bad synchronization between the excitation current and the sampling, the phase shift circuit is locked with the excitation current circuit in order to work together and variations on the excitation current phase is automatically corrected in the sampling circuit. In summary, finding the moment when the 2 nd harmonic has its greatest value, adjusting the time-width of the synchronous detector and avoiding phase's shifts between excitation and detection circuits can produce a magnetometer performing at its lowest noise output.Keywords: magnetometer, fluxgate, magnetic noise, vitrovac, computer simulations. RESUMO.Nos magnetômetros tipo fluxgate, o campo magnético externoé geralmente medido utilizando-se da tensão do segundo harmônico induzido na bobina de detecção. Assim, para melhorar o desempenho de um magnetômetro fluxgate, deve-se observar com cuidado alguns parâmetros operacionais e o modo de calibrá-los, com o objetivo de colocá-los num pontoótimo de operação. Basicamente dois fatores são considerados: i) a dependência das variações do segundo harmônico com a corrente de excitação; e ii) o ajuste perfeito entre a saturação do núcleo e a janela de amostragem. A melhor sensibilidadeé obtida em uma porção da curva de histerese onde o segundo harmônicoé predominante. Visto que essa região depende da amplitude da corrente de excitação, torna-se necessário calibrar esta...