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Accueil > Recherche > Plasmas Spatiaux > Missions spatiales > Tirs fusées > Fusée CHARM2 (projet NASA)
PRINCIPLE OF THE COMBINED AC/DC MAGNETOMETER USING GIANT MAGNETO-IMPEDANCE SANDWICH onboarded on CHARM2 rocket
Sensor design : J. Moutoussamy & C. Coillot
Electronic design : D. Alison & C. Coillot
Scientific co-Investigator : P. Robert
Principle of classical sandwich giant magneto-impedance :
Giant magneto-impedance effect occurs in a ferromagnetic material supplied by a “high” frequency (few 100kHz up to GHz) alternating magnetic field biased with a static or low varying magnetic field (H). This one modifies the susceptibility of the ferromagnetic material and so on its impedance. GMI sandwich consists in a copper ribbon between 2 ferromagnetic ribbons (Figure 1 left). Value of the impedance modulus is related to the magnetic field H (Figure 1 right).
Bench test
A bench test presented on the figure below, using GPIB, has been used to characterize intrinsic sensitivities (SI= Z/H). All the relevant parameter can be swept to determine the optimal configuration (excitation current, bias field, excitation frequency).
A Helmholtz coil is used to generate the magnetic field to be measured, while a solenoid is used to generate the bias magnetic field. An example of characterization of Z(H) and SI(H) is presented below (Figures 3 - 4).
Intrinsic sensitivity curves exhibit a maximum at a characteristic field (effective anisotropic field). Many soft magnetic materials (Ni-Fe alloy but also Mn-Zn ferrite) display a strong GMI effect and demagnetizing coefficient of the magnetic material has be founded to be of great importance (Link to demagnetizing field coefficient modelling)
Wounded sandwich GMI for CHARM2 experiment
The combined AC/DC magnetometer uses a very high sensitivity wounded giant magneto-impedance sensor (figure 5) combined to search-coil sensor. A low noise amplifier using a double modulation to suppress offset and makes output voltage directly proportional to magnetic field is used.
Wounded GMI (Figure 5) has two advantages :
– low frequency excitation (few 100kHz instead of several MHz) and low current consumption (few mA) are used
– easy to manufacture (lamellas of magnetic material is manually cut off and wounded) => it is a potential low cost device.
Each sensor is characterized in terms of intrinsic sensitivity (Fig. 6), which represents the slope of the Z(H) curve of the sensor : with respect to magnetic field to find the optimum operating point. We make the measurement in 3 directions :
– orthogonal to Earth magnetic field ("Sens 1")
– parallel to earth magnetic field ("Sens 2")
– anti-parallel ("Sens 3")
That allows verifying the symmetrical behaviour of the sensor and its excursion inside Earth magnetic field. The optimal bias point is chosen at the maximum value of intrinsic sensitivity.
Low noise electronic is schematically represented on Figure 7. Excitation current and bias current are supplied using the same winding. The output signal is amplified and then demodulated twice (at excitation frequency and at bias frequency). The offset is suppressed thanks to the high pass filtering after the first demodulation at Fexc. The ouput signal is linearized as the bias is alternately on optimum bias point and opposite point. Thus, the slope of the intrinsic sensitivity curve is compensated.
In order to retrieve direction of magnetic field components of wave a tri-axis sensor associated to a 3 channels electronic amplifier has been realized and tested (cf. Figure 8 & 9).
Performances of the whole magnetometer are under investigation.
Preliminary measurements results demonstrate an ability to measure weak magnetic field lower than 1nT/sqrt(Hz) at 1Hz (cf. Figure 12). This ability is much better than the one of AMR sensor and Hall Effect sensor. Robustness of wounded giant magneto-impedance sensor should be mentioned.
The hybrid search-coil/GMI magnetometer has been on boarded on CHARM 2 NASA rocket in combination with an lowfrequency analyzer. The whole experiment is presented on Figure 13. The instrument installed on the CHARM2 rocket is presented on Figure 14.
Datas from CHARM2 rocket launch :
A quick look on rocket datas allows recognizing the different step of the launch. First of all, just after the launch the rocket starts to spin into the Earth magnetic field. This spin is related to the sinewave on GMI magnetometer datas (Figure 15 to Figure 18). Spin starts to increase rapidly to reach 3-4 rotations per second. Spin is noticeable on the three axes (Figure 16), even if it is smaller on Y axis (aligned with the spin axis).
In a second step, boom on which GMI magnetometer is mounted, is deployed. The spin is transferred from Y axis to Z axis (Figure 17). In this nominal configuration (Figure 18) Z axis is aligned with rocket while X et Y are in the spin place.
Finally the spectrogram on Figure 19 summarizes the measurement provided by the GMI magnetometer during the launch. A first line around 3Hz corresponds to the spin signature, while a second line at 1Hz corresponds to a small oscillation around spin axis.
Tir réussi pour la fusée CHARM2 le 16 février 2010
La fusée CHARM2 a été lancée avec succès par la NASA, le 16 février 2010, du Poker Flat Research Range, situé à Fairbanks en Alaska. A son bord, une chaine de mesure complète livrée par le LPP : un capteur de champs magnétiques AC/DC (du continu à 6kHz), trois axes, développé dans le cadre des activités de R&T de l’équipe instrumentale, et un analyseur de données embarqué, qui numérise les signaux issus du magnétomètre et effectue des traitements numériques (filtrage, décimation, FFT...). L’analyseur est un premier démonstrateur pour la contribution du LPP à la mission Solar Orbiter.
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contact : Christophe Coillot
(mise à jour 09/2010)
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