Baumjohann, W

Brown, P

Carr, CM

Delva, M

Hagen, C

Leitner, S

Magnes, W

Valavanoglou, A

Whiteside, BJ

IEEE Transactions on Magnetics

Spiral - Imperial College Digital Repository

MP20  1 Design of the magnetoresistive magnetometer for ESA's SOSMAG project  S. Leitner1, A. Valavanoglou1, P. Brown2, C. Hagen1, W. Magnes1, B. J. Whiteside2, C. M. Carr2, M. Delva1 and W. Baumjohann1  1Space Research Institute, Austrian Academy of Sciences, Graz, Austria 2The Blackett Laboratory, Imperial College London, London, UK  A prototype of a Service Oriented Spacecraft Magnetometer (SOSMAG) is being developed for ESA's Space Situational Awareness program, which shall serve as a ready-to-use space weather monitoring system to be mounted on a variety of different spacecraft built without a magnetic cleanliness program. The separation between the natural magnetic field in space and the artificial magnetic field generated by the spacecraft is one of the key issues for its successful performance. The SOSMAG design is based on two types of magnetic sensors. One or two low noise fluxgate sensors for the required high measurement accuracy will be mounted along a boom of one meter in length, and two anisotropic magneto-resistance (AMR) based sensors will be used within the spacecraft for detection and characterization of its magnetic disturbances. This paper presents the design and performance of the AMR magnetometer which contains an application-specified integrated circuit (ASIC) and a hybrid sensor as core elements. The magnetometer front-end ASIC (MFA-3), originally developed for NASA's Magnetospheric Multiscale (MMS) mission, is based on a fourth-order sigma-delta A/D converter design. All the active electronics needed for the readout of the AMR sensor and its digitization, as well as for digitizing the magnetometer's housekeeping data are part of the ASIC. Each of the three-axis hybrid sensors consists of a ceramics circuit board with three HMC1021 sensor elements, one bypass capacitor, a temperature sensor and a MOSFET driver mounted in chip-on-board technology. A novel approach is used to set and reset the AMR elements. It is done in a narrow-pulsed way at 32 kHz with a pulse width of only 238 ns, thus minimizing the required excitation power to 30 mW in total.  Index Terms—AMR, magnetometer, space instrumentation, miniaturization  I. INTRODUCTION pace weather monitoring and forecasting grows more important to scientific and commercial interests as technology advances. It is key to predict risks posed to actual spacecraft, systems on Earth and in Earth's ionosphere, which are caused by space weather effects. To cover all these grounds entire arrays of reliable and radiation robust sensor instruments are needed operating in various orbits. It is intended one of these instruments shall be the Service Oriented Spacecraft Magnetometer (SOSMAG). This magnetometer is to be deployed in utterly distinct orbits such as low and middle Earth orbits (LEO/MEO), highly elliptical ones (HEO) and orbits in Earth's first Lagrangian point (L1). The orders of magnitude for magnetic fields in these orbits range from 256 nT (L1) to 64000 nT (LEO). Another goal of the SOSMAG project is to detect magnetic disturbances on the spacecraft itself, to characterize them and correct their effects on the magnetometer data on-the-fly. This could minimize the need of a magnetic cleanliness program for the spacecraft, hence saving money and time. One or two high resolution boom-mounted fluxgate-magnetometers are provided by Magson GmbH and the Technical University of Braunschweig. For detection and characterization of magnetic disturbers on the spacecraft two magnetometers based on the anisotropic magnetoresistive (AMR) effect are being developed in a joint effort by Imperial College London and the Space Research Institute in Graz. AMR sensors in general become increasingly attractive in space applications (e.g. [1], [2]), are low-priced, small-sized lightweight, and their basic elements are commercially available, thus making them an interesting option in the afore-mentioned scenario. II. OVERALL DESIGN OF THE AMR MAGNETOMETER The two major components of one AMR magnetometer are a hybrid sensor designed in cooperation with Imperial College London and near sensor electronics built around a mixed signal application-specified integrated circuit (ASIC) - the Magnetometer Front-end ASIC (MFA-3) - developed by the Space Research Institute in Graz [3]. As the SOSMAG instrument houses two AMR magnetometers, both near sensor electronics, an Actel ProASIC FPGA, the generation of supply voltages and a dual redundant power and RS-422 digital data interface are combined onto a single AMR electronics board.  Fig. 1. Photo of the 3-axis AMR hybrid sensor  One 3-axis hybrid sensor (see Fig. 1) is composed of three Honeywell HMC1021 AMR sensor elements, a PT1000 S Manuscript received June 13, 2014. Corresponding author: Stefan Leitner (e-mail: stefan.leitner@oeaw.ac.at). Digital Object Identifier inserted by IEEE MP20  2 temperature sensor and a die of a Micrel MIC4424 MOSFET driver IC with one corresponding non-magnetic bypass capacitor all mounted onto a ceramics circuit board of 1.6 cm in diameter. The HMC1021 was favored over the HMC1001 sensor element, although it has an inferior sensitivity that consequently results in a higher noise floor, since the offset constants of the internal offset straps differ by about a factor of ten (HMC1001: 0.51 mA/µT, HMC1021: 0.05 mA/µT). Especially when operating in Earth's field ranges (ca. 60 µT) this leads to significantly lower currents and power dissipation in the feedback circuits. On the AMR electronics board, one MFA-3 is designated to every magnetometer. In the fully differential analog part of this ASIC for each of the three axes of the hybrid sensor, a low noise amplifier (LNA), an adjustable synchronous demodulator (ASD) and a 2–2 cascaded sigma-delta modulator is embedded. An additional fourth modulator (single ended) is destined for digitizing the housekeeping data of the entire magnetometer (e.g. supply voltages, supply currents, sensor and board temperatures). On the digital side primary and secondary decimation filter stages (128 Hz and 2-64 Hz datarate at 24 bit resolution respectively), a serial synchronous digital interface as well as a control and command logic are included. The chip area amounts to about 20 mm2 and the total power consumption to 60 mW. More on the specifics regarding the MFA-3 are to be found in [3]. Further functional blocks of the AMR magnetometer are depicted in Fig. 2 and their purpose in the setup is described in the remainder of this section.  Fig. 2. Block diagram of a single AMR magnetometer (1-axis principle)  The preset-magnetization of the AMR sensor element's thin-film structure along its 'easy axis' is periodically flipped by alternating current pulses via the Set/Reset strap. This also switches the polarity of the AMR Wheatstone bridge's output attributed to the external magnetic field component along the 'hard axis', and therefore this information on the external magnetic field component is modulated onto a rectangular carrier with a frequency equivalent to the pulsing of the Set/Reset straps i.e. the Set/Reset frequency (detailed explanations about the AMR effect in general, the definitions of easy/hard axis and the effects of preset-magnetizing and other stabilizing techniques for thin-film structures are given in [4]). The modulated rectangular bridge output of the AMR sensor element is fed into a fully differential amplifier cascade (Intersil ISL70227SRH preamp + MFA-3 LNA) each in inverting bandpass configuration, boosting it by 45 dB in total. Next the signal experiences a demodulation (Set/Reset frequency: 32.8 kHz) back into the baseband and is processed in the 2-2 cascaded sigma-delta modulator forming a 4th-order modulator. After filtering and decimation of the noise shaped digital data from 8192 Hz to a maximum of 128 Hz, the FPGA handles the communication with the data interface. The 1-bit output of the first 2nd-order modulator is converted back to the analog domain and fed into the current feedback circuit, providing a full field feedback of about +/- 15000 nT via the offset strap. All the functional blocks of Fig. 2 mentioned so far but the FPGA form an extended 2-2 cascaded sigma-delta converter, where the summation point of the sigma-delta modulator is not in the electrical domain, but in the magnetic domain, and is in fact in the AMR sensor element itself. For additional magnetic field compensation beyond the full field feedback of +/-15000 nT, a current offset circuit using a Maxim MAX542 D/A converter is implemented. It provides an additional DC current through the offset strap which is controlled by an algorithm in the FPGA, to ensure that the extended 2-2 cascaded sigma-delta converter remains in its functional input range. This additional compensation amounting up to +/-60000 nT leads to an alteration to the sigma-delta converter's digital data, which is then corrected by the FPGA. Two more blocks in Fig. 2, namely the DC offset and the MIC4424, form the new approach of a Set/Reset circuit, which is explained more thoroughly in the next section. III. ALTERNATE SET/RESET CIRCUIT The MFA-3 is based on a fully synchronous clock design, meaning its CLK frequency must be in a fixed ratio to the Set/Reset frequency of the AMR sensor elements and to the maximum digital datarate. For the SOSMAG project the latter was defined to be 128 Hz resulting in a Set/Reset frequency of 32.8 kHz and a MFA-3 CLK frequency of 4194.3 kHz.  The proposed circuit by Honeywell [5] shows at such a high Set/Reset frequency its drawbacks when combining a totem-pole stage with a capacitive load i.e. the ESR capacitor. These include the linear increase in capacitive load power consumption with frequency as well as the problem to adjust the maximum current peak of the Set/Reset pulses and the timing constant of their exponential decay, because both factors are tied to the capacitance of the ESR capacitor. Given that a repeatable Set/Reset operation requires two parameters to be fulfilled, namely the amplitude of the Set/Reset current peak and the duration where this peak is above a certain threshold, one extreme is to minimize the timing constant for a fast exponential decay which subsequently results in an excessive overshoot of the maximum current peak. The other extreme is to minimize the current peak overshoot, and to accept the exceeding MP20  3 exponential decay in time, which does not contribute to a more directed preset-magnetizing of the AMR thin-film structure along its 'easy-axis', but only to additional power dissipation and is further limiting the maximum Set/Reset frequency.  Fig. 3. Schematics of the alternate Set/Reset circuit  Fig. 3 shows the topology of the alternate Set/Reset circuit used in the SOSMAG AMR magnetometer. The main idea behind this new Set/Reset circuit consists in using the Micrel MIC4424 Dual 3A-Peak Low-Side MOSFET Driver, which – depending on its logical input signals – switches its outputs to the driver's supply voltages, thus delivering the pulses into the connected Set/Reset straps of the AMR elements. To adapt the logical signals originally generated by the FPGA to MIC4424 supply voltage levels they get passed through a combination of capacitive coupling and a voltage divider (DC offset circuit) to the MOSFET driver on the AMR hybrid sensor. To support the IC and supply enough charge during these steep transitions of the Set/Reset operations a proper non-magnetic bypass capacitor near the MIC4424 supply pins is essential. In this setup the only increase in capacitive load power consumption comes from the parasitic capacitance at the output stages of the MIC4424. Further the problem regarding the exponential decay does not occur at all, as there is no ESR capacitor in series to the Set/Reset straps and the amplitude of the Set/Reset current peak is set directly by the supply voltage of the MIC4424. The duration where this peak is above a certain threshold corresponds to the pulse width of the logical input signals, and the maximum Set/Reset frequency is determined by the MIC4424 parameters rise time, fall time and minimum pulse width.  The following performance tests with the alternate Set/Reset circuit were conducted with HMC1001 sensor elements. Because of their higher sensitivity and the resulting lower noise floor, deviations from the expected behavior of the Set/Reset circuit are more easily detected.  Table 1 shows the power consumption of the MIC4424 based circuit, when varying the Set/Reset frequency and the MFA-3 CLK frequency in a constant ratio to each other (see synchronous ASIC design earlier this section). The Set/Reset pulse width is set to one MFA-3 CLK period. By doubling the MFA-3 CLK frequency the Set/Reset frequency also doubles, and the Set/Reset pulse width is halved. So ideally the supply current and the power dissipation in Table 1 should stay constant, but finite rise and fall times of the MIC4424 output stages limit the effective Set/Reset pulse width. This causes the slight decrease in power dissipation especially at higher frequencies. TABLE I SET/RESET CIRCUIT'S POWER DISSIPATION AS A FUNCTION OF FREQUENCY CLK Frequency [kHz] Set/Reset Frequency [kHz] MIC4424 supply current measured [mArms] Power dissipation of Set/Reset circuit a [mW] 4194.3 32.8 3.7 30.7 2097.2 16.4 4.1 34 1048.6 8.2 4.4 36.5 524.3 4.1 4.6 38.2 ... ... ... ... 65.5 0.5 4.65 38.6 aMIC4424 supply voltage 8.3 V, Set/Reset pulse width 1 CLK cycle, 3x HMC1001 Set/Reset straps connected, measurements performed at room temperature  The length of the Set/Reset pulses can be adjusted by multiples of the MFA-3 CLK cycles. Its effect on power dissipation is shown in Table 2 and should reveal a linear dependency. TABLE 2 SET/RESET CIRCUIT'S POWER DISSIPATION AS A FUNCTION OF PULSE WIDTH Pulse width [CLK cycles] Power dissipation of Set/Reset circuit a [mW]  CLK Freq.: 4194.3 kHz Set/Reset Freq.: 32.8 kHz CLK Freq.: 131 kHz Set/Reset Freq.: 1 kHz 1 30.7 38.2 2 67.2 76.4 3 103.8 112.9 4 141.9 150.2 aMIC4424 supply voltage 8.3 V, 3x HMC1001 Set/Reset straps connected, measurements performed at room temperature  Deviations from the ideal behavior are again attributed to the already above-mentioned finite transition time of the MIC4424 output stages. Another essential benchmark of the MIC4424 based Set/Reset circuit is the performance over a wide range of supply voltages. TABLE 3 SET/RESET CIRCUIT'S PERFORMANCE AS A FUNCTION OF SUPPLY VOLTAGE MIC4424 supply voltage [V] standard deviation a,c [pTrms] noise density b,c [pTrms/sqrt(Hz)] 15-8 220-240 30-40 7 230 30-40 6 280 40 5 1400 200 4 52000 8000 astandard deviation of 128 Hz digital magnetic field data of MFA-3 primary filter output bnoise density in the power spectral density (parameters see section IV) of 8192 Hz magnetic field data of the MFA-3 4th-order sigma-delta modulator's output, measured in the AMR magnetometer's white noise region (>5 Hz) cAMR bridge voltage 10 V, Set/Reset frequency 32.8 kHz, CLK frequency 4194.3 kHz, Set/Reset pulse width 1 CLK cycle, 3x HMC1001 Set/Reset straps connected, measurements performed at room temperature with AMR sensor in 3-layer µ-metal shielding (residual fields: <1 nT planar, <5 nT axial)  The indication of proper functionality is represented by the repeatability of the magnetic domain alignment in the AMR sensor elements and can be derived from spectral plots of the digitized magnetometer data. A malfunction would manifest in spectral anomalies or in an increase of noise. Table 3 lists the MP20  4 changes in these indicator figures when varying the supply voltage of the MIC4424. It is seen, that proper operation is assured for voltages greater than 6 V at given parameters. IV. PERFORMANCE OF THE AMR MAGNETOMETER To give representative data and key performance indicators of the AMR magnetometer, several long-term tests as well as spectral analyses were made and are depicted in the following figures. In Fig. 4 the 2 Hz digital data of an overnight measurement with the AMR hybrid sensor deposited in a 3-layer µ-metal shielding (inner diameter 276 mm, height 764 mm, residual fields: <1 nT planar, <5 nT axial) is shown, which was performed at room temperature. The DC offset in this data roughly expresses the offset value of the uncalibrated magnetometer.  Fig. 4. 15 h long-term measurement of the AMR magnetometer  Key figures like noise performance, signal to noise ratio (SNR), correctness of the gain settings of the sigma-delta modulator as well as spectral anomalies of the AMR magnetometer are revealed in the power spectral density (PSD) plot in Fig. 5.  Fig. 5. PSD of the fourth-order sigma-delta modulator's 8192 Hz digital output  It corresponds to an on-the-fly PSD (Welch's method, 65536 points, 50% overlap, Hanning window, 50 averages) of the fourth order 6-bit output of the 2-2 cascaded sigma-delta modulator with its typical noise shaping at higher frequencies (80 dB/decade), which is an indicator of proper gain settings in the modulator's control loop. The noise floor is free of any artefacts in the region of the usage bandwidth and dominated by the AMR sensor element's and the preamplifier input resistors' Johnson noise, and the preamplifier's voltage noise in equal proportions. It has a white noise characteristic and lies at about 100 pT/sqrt(Hz). This measurement was performed at room temperature with the AMR hybrid sensor in the same 3-layer µ-metal shielding characterized prior in this section and an additional DC offset field to counteract the offset value of the uncalibrated magnetometer and to maximize the dynamic range for the SNR estimation. At the time acquiring the data in Fig. 5 also a 13 Hz sine signal with an amplitude of 9500 nTp was active and gives a signal to noise ratio (SNR) of about 86 dB. Additional specs of the SOSMAG AMR instrument are listed in Table 4.  TABLE 4 TABLE OF SOSMAG AMR INSTRUMENT SPECS Overall instrument a  Field range +/-70000 nT Noise density 100 pT/sqrt(Hz) @1Hz SNR >85 dB Measurement bandwidth 0-30 Hz (-3 dB corner frequency) Power dissipation 1460 mW b Nominal input voltage 28 V c Input voltage range 18-36 V Temperature range -30° C to +55° C (sensor and electronics) AMR hybrid sensor  AMR sensor elements HMC1021 Bridge voltage 10 V Set/Reset supply voltage 10 V Set/Reset frequency 32.8 kHz Set/Reset pulse width 1 CLK cycle Bypass capacitor Electrolytic 10 µF/15 V Mass 1.1 g (w/o harnessing) Volume 1.67 cm³ (16 mm diameter, 8.3 mm height) AMR electronics board  Board dimensions 183 mm × 123 mm × 15 mm CLK frequency 4192 kHz Digital Interface RS-422 (physical), UART @115.2 kBaud c Mass 206 g (incl. 3x SUBHD26 connectors) aoffset stability, long term drift, temperature coefficients, radiation hardness are yet to be determined b2x AMR magnetometer, incl. DC/DC converter losses, w/o RS-422 interface connected, both AMR sensors exposed to 60000 nT field credundant power and data interfaces REFERENCES [1] P. Brown et al.. (2012, April 04). Magnetoresistive magnetometer for space science applications. Measurement Science and Technology [Online]. 23(2). Available: http://stacks.iop.org/MST/23/025902 [2] M. Díaz-Michelena. (2009, March 30). Small Magnetic Sensors for Space Applications. Sensors [Online]. 9(4), pp. 2271-2288. Available: http://http://www.mdpi.com/1424-8220/9/4/2271 [3] W. Magnes et al.. (2008, Sept. 30). Highly integrated front-end electro-nics for spaceborne fluxgate sensors. Measurement Science and Techno-logy [Online]. 19(11). Available: http://stacks.iop.org/MST/19/115801 [4] S. Tumanski, Thin film magnetoresistive sensors. Bristol, United Kingdom: Institute of Physics Publishing, 2001. [5] Honeywell International Inc., “1- and 2-Axis Magnetic Sensors“, HMC1001/1002/1021/1022 datasheet, Aug. 2008 

Design of the Magnetoresistive Magnetometer for ESA's SOSMAG Project

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Design of the Magnetoresistive Magnetometer for ESA's SOSMAG Project

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