Processing of the fluxgate output signal

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1 Processing of the fluxgate output signal P. Ripka, S. Kawahito* Dept. of Measurement, Faculty of Electrical Engineering CTU, Technicka 2, Praha 6, Czech Republic, Dept. of Information and Computer Sciences, Tyohashi University of Technology, Tempaku-cho, 441, Japan Abstract Fluxgate sensors measure magnetic field with a resolution up to 10 pt. New methods of the output signal processing allow to decrease the sensor size, lower the energy consumption and increase the working frequency. Tuning the voltage output may substantially increase the sensitivity, but in certain cases it may cause unstability. Fluxgate in current-output mode requires lower number of turns of the pick-up coil; its output may be processed by gated integrator rather than by the classical synchronous detector. The output of the short-circuited fluxgate can be tuned by serial capacitor. The use of synchronous filters (revolving and SC) brought no practical advantages. The digital magnetometers performing the detection in numerical form in the DSP have appeared. The methods of3 increasing the frequency range of the fluxgate magnetometer, the crossfield effect and the problems associated with the use of fluxgates in multi-sensor systems are also discussed. Keywords: magnetic sensor, fluxgate, magnetometer, gradiometer 1. Introduction Fluxgates are the most suitable sensors for precise measurement of the DC and low-frequency AC magnetic field vector in the range of 1 nt..1 mt [1]. They are solid-state sensors resistant to rough environment and they are stable over the wide temperature range. In many applications such as magnetic ink reading, security and navigation it is necessary to lower the sensor size, increase the frequency range and lower the power consumption. Other applications such as ultrastable gradiometers require lowering of the sensor noise and increasing of the offset stability. Such demands call for new methods of the processing of the fluxgate output signal. We review the presently used techniques and propose the new solutions suitable for integrated fluxgate sensors. 2. Tuned voltage output Tuning the voltage-output pick-up coil by parallel capacitor increases the sensitivity at second harmonic frequency [2]. This effect of parametric amplification was theoretically analyzed in [3]. Increase of the sensitivity by the factor of 10 to 100 is easily achievable. For high quality factor (low pick-up coil resistance) the circuit may become unstable [4]. This situation is usual for very sensitive fluxgates, for example race-track core sensors. Fig. 1 shows the unloaded output voltage of the race-track fluxgate sensor [17] for the measured field of 5µT. Fig. 2 shows the same sensor tuned by parallel capacitor C 2 = 11 nf. The sensor is in the unstable mode: due to the high quality factor of the nonlinear resonant circuit the output oscillates even for zero measured field. The sensor may be stabilized by damping resistor either in series or parallel to the tuning capacitor. Fig. 3 shows the same

2 sensor stabilized by serial resistor of 10 Ω. It should be noted that the value of the damping resistor necessary to stabilize the sensor is small comparing to the DC resistance of the pick-up coil which was 45 Ω. There are indications that the output tuning may decrease the sensor noise. For the case of using conventional synchronous detector, which is sensitive only to one frequency component, we observed mild reduction of the voltage-output sensor noise associated with the concentration of the output information from all even harmonics to second harmonic frequency [5]. 3. Current output Fig. 1 excitation current and voltage output of the untuned sensor Fig. 2 Unstable sensor Short circuited mode of the sensing coil [6] suppresses the influence of the parasitic capacitances. The output is broadband and the sensitivity is inversely proportional to the number of coil turns [7]. This mode may be advantageous over the traditional voltage output for the integrated fluxgate sensors as they have low number of turns. Fig. 4 shows the output current of the shortcircuited ring-core fluxgate sensor for the measured field of 5 µt. The sensor core is made of 18/22 mm rings etched from 50 µm thick 79Ni4Mn permalloy. The sensor was excited by 4 khz /5 V sinewave and the excitation winding was tuned by parallel capacitor C 1 = 2.2 nf. The pick-up coil of 100 turns (0.3-mm diameter wire) was connected to the current-to voltage converter with 5 kω feedback resistor. Current-output fluxgate may be tuned by serial capacitor [8]. Fig. 5 shows the same ring-core sensor tuned by C 2 = 1.7 µf. The second harmonic sensitivity was increased only by the factor of 5, which shows that the damping is much higher than in case of tuning the voltage output. Fig. 3 Sensor stabilised by damping resistor Ring-core sensors are less sensitive than the race-tracks due to the higher demagnetization factor. If their pick-up coil is wound of the thin wire, they are often stable for each value of the tuning capacitor and measured field. Fig. 4 Untuned current output

3 Komachi and Tanaka [10], uses a second switch which is delayed by d periods of the clock signal. The modified cell suppresses combs on kdf 0. The output of the Fig. 5 Tuned current output 2. Analog feedback vs. ranging Most of the fluxgate magnetometers work in the feedback mode, i.e. they compensate the measured field by the current into the compensation coil. The main objective is to improve the linearity. Fully analog feedback has the best dynamic performance for large steps of the measured field and is the simplest solution. The field range may be switched by feedback resistor to increase the resolution for low-field measurements. However, precise applications may require mixed architecture using stable ADCs for partial digital compensation and analog feedback with limited range, which has fast response. 3. Synchronous filters and gated integrators. Synchronous revolving and SC filters are attractive for fluxgate application, but their performance is limited by noise and harmonic distortion [9]. N-path revolving synchronous filters have a comb characteristics with bandpass at each multiple of the basic center frequency kf 0, except for inf 0 multiples. This feature may be advantageous for the fluxgate, as its output signal bears the information on each even harmonic frequency (except of the case of efficiently tuned sensors, which have the information concentrated in a single frequency). The modified topology proposed by revolving filter is a step-wise approximation of the extracted sinewave. A substantial disadvantage of this class of filters is a regular presence of large spurious signals at the N-1 and N+1 harmonic; thus practical applications require large N. Switching capacitor (SC) filters The resonance frequency of the SC filter depends on the ratio of two capacitor values, which is more stable than RC or LC product in the traditional analog filters. The SC structure can be easily integrated into monolithic structures. SC filters can be easily synchronized with the excitation frequency, which further increases the stability of the parameters. Thus even the structures with very high selectivity may have good phase stability; however, the SC cells with very high Q factor have increased distortion. Gated integrators are similar to switching phase-sensitive detectors, but both the length and phase of the switching pulse are adjustable. They are ideal for processing of the currentoutput fluxgate as their switching time may be fitted to the shape of the output signal to get maximum sensitivity [6]. For the practical construction of the magnetometer the requirements for the detector settings are rather different. As almost all magnetometers work in the feedback mode, sensor sensitivity is not the primary criterion. The practical experience had shown that the optimum switching pulse length is rather shorter than the length found for the maximum sensitivity; it was shown that the shorter reference pulses improve the sensor offset stability vs. temperature.

4 5. DSP magnetometers Fully digital fluxgate magnetometer performs the analogto-digital conversion of the sensor output signal right after the pre-amplification and eventual analog pre-filtering to suppress the unwanted signals: i) high-frequency components which may cause aliasing ii) feedthrough voltage at the excitation frequency. The harmonic distortion in the ADC would cause false signal output. As the feedthrough changes with temperature, this effect could degrade the offset stability. The phase-sensitive detection and further filtration is performed numerically in DSP. The digital magnetometer based on ADSP21020 CPU was described in [11]. The instrument has rms noise of 71 pt in the band of 0.25 Hz to 10 Hz (400 pt p-p) while the noise of the fluxgate sensor of this type used in analog magnetometer was significantly lower (15 pt rms in the 0.03 Hz to 12 Hz frequency band, 80 pt p-p). It should be noted that the increased noise level of the mentioned instrument is due to the design compromises to lower the power consumption: DSP based laboratory lock-in amplifiers such as SR 830 have very low noise. 6. Multi-axis magnetometers and gradiometers Transverse field effect Although fluxgate sensors are vectorial, they also react on fields perpendicular to their sensing axis. The transverse or cross-field effect can be observed as change of the sensing axis direction with the amplitude of the field or as a degradation of the sensor linearity in the presence of large perpendicular field. The resulting error may be more than 20 nt in the nt Earth s field and the effect is temperature dependent [12]. Brauer et al. [13] had shown that in case of the ring-core sensor this effect may be explained by the variation of the core susceptibility along the core diameter. They also proved that decreasing of the sensor diameter lowers the effect. Another possibility is to use the core shape, which has large demagnetization for perpendicular fields such as race-track [14] or rod. The problem with the crossfield sensitivity may be fully eliminated by positioning the sensor into the complete magnetic vacuum: the three-axial sensor head developed for the Oersted satellite [15] has all the sensors inside the compact spherical 3-axial compensation coil system. The compact single axis compensated three-axial magnetometer developed for the Swedish satellite Astra [16] has 3 closely mounted 17 mm ring-core fluxgates. All the mechanical parts are made of machinable ceramics MACOR. The dimension of the sensor head is 32*47*55 mm. Close vicinity of the sensors increases the nonlinearities caused by the crosstalk and crossfield effect up to 8 nt p-p in the nt range. Thanks to the low temperature expansion of MACOR (9.4 ppm/ 0 C) the temperature drift of the sensitivity is for individual sensors is between 10 to 13 ppm/ 0 C. The offset drift was 0.01 and 0.02 nt/ 0 C for x and y axes respectively, but degraded to 0.45 nt/ 0 C for z-axis, which is more magnetically coupled to the other sensors due to the holder geometry. This effect is again caused by the crossfield sensitivity and may be suppressed by increasing the sensor distance or decreasing of the core diameter. The temperature drift of the sensor angles was 0.17, 0.33 and 0.5 /0C. Fluxgate gradiometers Single-core fluxgate gradiometer was described in [17]. The sensor has two sections of the pick-up coil connected antiserially and works in the open-loop. The main problem of such sensor was long-term stability of the astatisation: the sensor should be periodically adjusted to have low response to the homogeneous (zero gradient) field. Another possibility is to use two or more individual sensors; in this case the gradiometric base may be longer. The compensation coils should have same low tempco and should be rigidly mounted. Arrays of low-noise

5 fluxgate sensors are used for detection systems. The sensor signals are processed numerically to compensate for the crosstalks, individual sensitivities and temperature coefficients. 7. Increasing the bandwidth There are three basic approaches how to increase the frequency range of the fluxgate: increasing the excitation frequency, exploitation of the direct induction effect in the pick-up coil and using of the AC error signal of the DC loop [18], [19]. The limiting factor of the excitation frequency is the velocity of the domain wall movement and eddy currents, which decrease the field inside the core. The amorphous materials are advantageous over crystalline permalloys as they have higher resistivity and lower thickness: the maximum excitation frequency for 20 µm amorphous tape core is below 100 khz. Cores made of ferrites or thin layers made by sputtering or electrodeposition may work at MHz frequencies, but these materials usually have higher noise level. AC magnetic fields induce voltage (or short-circuited current) of the same frequency; while the induced voltage should be integrated, short-circuited current ideally follows the waveform of the magnetic flux. The mentioned AC fields simultaneously create sidebands of the excitation frequency (and also its higher even harmonics) by the fluxgate effect; it is always easy to keep these signals well separated. The AC error signal of the slow DC feedback loop was used to measure AC fields in the Thunderstorm rocket experiment. The frequency band of the AC output was 3 khz, while the upper frequency limit of the DC feedback loop. 8. Integration Miniature fluxgate sensors are creating new applications of fluxgate sensors, because of their features of small size, light weight, mass-production, and the integration of onchip electronics [20]. Simple PCB construction of the 15 mm long fluxgates is described in [21]. The annealed core made of amorphous foil is sandwiched between two layers of PCB, which have outer metal layers forming the halves of the winding. The layers are connected by electroplating. Orthogonal fluxgate with flat excitation and pick-up coil was described in [22]. The sensor 10 mm diameter ring core is also etched from Vitrovac 6025 amorphous ribbon. The sensor resolution is 40 nt and the linearity error in the 400 µt range is 0.5%. Planar fluxgate sensor with flat coils was described in [23]. The sensor core is in the form of two serially configured strips of sputtered permalloy 2 µm film. The flat excitation coil saturates the strips in oposite directions, the differencial flux is sensed by two antiserially connected flat pick-up coils. The maximum sensitivity of 73V/T was reached for 1 MHz/150 ma p-p excitation current. The excitation current and output voltage of such sensor for B = 20 µt is shown in Fig. 6. Iex Vout Fig. 6 Miniature fluxgate Currently, though the sensitivity of the micro fluxgates is not so high compared with traditional fluxgates, the sensitivity and the stability are sufficiently high compared with small size magnetic sensors such as Hall elements and MR elements. Another merit of the miniature structure is the use of high excitation frequency, which leads to a wide frequency response.

6 Acknowledgment This work was partly supported by the Grant Agency of the Ministry of Education under No. ME 275. References [1] P. Ripka: Magnetic sensors for industrial and field applications, Sensors and Actuators A, 42 (1994), No.1-3, pp [2] F.Primdahl, and P.A.Jensen, Noise in the tuned fluxgate, J. Phys. E, Vol. 20, pp , [3] Z. Gao, R. D. Russel: Fluxgate sensor theory: Sensitivity and phase plane analysis, IEEE Trans. Geosci. 25 (1987), [4] Z. Gao, R. D. Russel: Fluxgate sensor theory: Stability study, IEEE Trans. Geosci. 25 (1987), [5] P.Ripka, W. Billingsley: Fluxgate: Tuned vs. untuned output, IEEE Trans. Magn. 34 (1998), [6] F. Primdahl, J. R. Petersen, C. Olin, K. Harbo Andersen, The short-circuited fluxgate output current, J. Phys. E: Sci. Instrum., Vol. 22, pp , 1989 [7] F. Primdahl, P. Ripka, J.R. Petersen, and O. V. Nielsen, The Short-Circuited Fluxgate Sensitivity Parameters, Meas. Sci. Technol., Vol. 2, pp , [8] P. Ripka, F. Primdahl: Tuning the current-output fluxgate, Transducers 1999, submitted [9] P. Ripka : Coherent signal processing in sensor technology, Proc. Eurosensors VII conf., Budapest [10] Y. Komachi and S. Tanaka: Lock-in amplifier using a sampled data synchronous filter, J. Phys. E8 (1975), [11] J. P. Henriksen, J.M.G. Merayo, O.V. Nielsen, H. Petersen, J.R.Petersen, F. Primdahl: Digital detection and feedback fluxgate magnetometer, Meas. Sci. Technol. 7 (1996), [12] P.Ripka, W. Billingsley: Crossfield effect at fluxgate, Proc. EMSA conference, Sheffield 1998, p. 62, to appear in Senors and Actuators. [13] P. Brauer, J.M.G. Merayo, O.V. Nielsen, F. Primdahl, J.R. Petersen, Transverse field effect in fluxgate sensors, Sens. and Actuators A 59 (1997) [14] Ripka, P.: Race-track fluxgate sensors, Sensors and Actuators A, (1993), pp [15] O.V.Nielsen, J.R.Petersen, F.Primdahl, P.Brauer, B.Hernando, A.Fernandez, J.M.G.Merayo, P.Ripka: Development, construction and analysis of the 'Orsted' fluxgate magnetometer, Meas. Sci. Technol. 6 (1995), [16] P. Brauer, O.V. Nielsen: Fluxgate sensor for the vector magnetometer onboard the Astrid 2 satellite, Proc. EMSA conference, Sheffield 1998, p. 65, to appear in Sensors and Actuators. [17] P. Ripka, P. Navratil: Fluxgate sensor for magnetopneumometry, Sensors and Actuators A60 (1997), [18] Primdahl F., Nielsen O.V., Petersen J.R. and Ripka P.: High frequency fluxgate sensor noise, Electronic Letters 30 (1994), No. 6, pp [19] P. Ripka, F. Primdahl, O.V. Nielsen, J.R. Petersen, A.Ranta: AC magnetic field measurement using the fluxgate, Sensors and Actuators A, (1995), pp [20] S. Kawahito, H. Satoh, M. Sutoh, and Y. Tadokoro: High-resolution micro-fluxgate sensing elements using closely coupled coil structures," Sensors and Actuators A, Vol. 54, pp , 1996 [21] O. Dezuari, E. Belloy, S.E. Gilbert, M.A.M. Gijs: Printed circuit board integrated fluxgate sensor, Proc. EMSA conference, Sheffield 1998, p. 99 [22] P. Kejik, L. Chiesi, B. Janossy, R.S. Popovic: A new compact 2D planar fluxgate sensor with amorphous metal core, Proc. EMSA conference, Sheffield 1998, p , to appear in Sensors and Actuators. [23] S. O. Choi, S. Kawahito, Y. Matsumoto, M. Ishida, Y. Tadokoro, "An integrated micro fluxgate magnetic sensor," Sensors and Actuators, 55, pp (1996).

INDUCTION COILS: VOLTAGE VERSUS CURRENT OUTPUT

INDUCTION COILS: VOLTAGE VERSUS CURRENT OUTPUT P. Kašpar and P. Ripka Czech Technical University, Faculty of Electrical Engineering Department of Measurements, Technicka, 166 7 Praha 6, Czech Republic