Available online at www.sciencedirect.com Available online at www.sciencedirect.com Procedia Engineering 00 (2011) 000–000 Procedia Engineering 25 (2011) 362 – 366 www.elsevier.com/locate/procedia Proc Eurosensors XXV, September 4-7, 2011, Athens, Greece AMR gradiometer for mine detection and sensing Jan Vyhnaneka, Michal Janoseka a*, Pavel Ripkaa a Czech Technical University in Prague, Faculty of Elec Engineering, Dept of Measurement, Technicka 2, Praha, Czech Republic Abstract We used anisotropic magnetoresistors (AMR’s) to build an advanced mine detector The sensing head involves gradiometric pair of AMR sensors and a continuous-wave driven excitation coil, so the gradiometer is capable of detection ferromagnetic materials as well as diamagnetic metals The sensors are specially arranged to suppress the large AC-excitation field, so the mine detector senses both DC and AC field gradient response of the object of interest Being limited by the sensor noise of 214 pTrms/√Hz at kHz, we were able to detect an 50x50x1.5 mm3 aluminum test object in a 20 cm-depth using a 300 T, 1kHz excitation field © 2011 Published by Elsevier Ltd Keywords: mine detection; magnetic sensors, AMR, gradiometer, eddy currents Introduction The mine detectors used for clearing landmines and other explosive remnants are nearly exclusively based on eddy currents and use induction coils for sensing They very reliably detect conducting objects, but they fail to discriminate dangerous objects from the scrap metal Work of deminers is often very slow, as they have to carefully excavate each concealed metal object [1] However, spatial resolution of induction coils is low – every suspicious target has to be “pinpointed” in a time-consuming manner Gradiometers with magnetic sensors are used to find deeply burried objects [2], however they can detect only ferrous targets We present a novel gradiometer based on AMR sensors, which offer much better spatial resolution than induction coils The gradiometer is able to work with a continuous-wave excitation field of kHz frequency, so it can sense the response of non-magnetic metals * Corresponding author Tel.: +420 22435 3964; fax: +420 233 339 929 E-mail address: michal.janosek@fel.cvut.cz 1877-7058 © 2011 Published by Elsevier Ltd doi:10.1016/j.proeng.2011.12.089 Jan Vyhnanek et al / Procedia Engineering 25 (2011) 362 – 366 Jan Vyhnanek/ Procedia Engineering 00 (2011) 000–000 in addition to DC response of ferrous objects Similar mine-detector with SDT sensors was presented in [3], however it worked with pulsed field and the DC noise of SDT sensors disqualifies them for combined DC and AC response sensing Gradiometer principle We used AMRs of the KMZ51 type (NXP, ex Philips) and arranged them in a distance of 40 mm, forming a vertical dBx/dx gradiometer (Fig 1a) The two gradiometric sensors in SO-8 package are soldered on the PCB together with signal preamplifiers (total gain of 1000x) As a proper mine-detector needs to sense diamagnetic metals too, we added a continuous-wave driven excitation coil and solved the most difficult part of suppressing the large excitation field The coil is symmetric to the gradiometric sensors and it is fed with a kHz harmonic signal resulting in an AC magnetic field of about 300 T amplitude (Fig 1b) The first (measuring) AMR sensor is located on the sensitive side of the detector head and experiences the same excitation magnetic field as does the second (compensation) sensor Without any deformation of the excitation or Earth’s field the gradiometer response is near zero, limited by the gain of the PI regulator in feedback loop Fig (a) The actual gradiometer with AMR sensors; b) Gradiometers combined with the excitation coil form a detector head, compensating and measuring sensor of the gradiometer are symmetrically placed on both sides of the excitation coil Fig Functional diagram of the gradiometer in the detector’s search head Gradiometric function is obtained by connecting compensation coils of measuring and compensated sensor in series 363 364 Jan Vyhnanek et al.Procedia / ProcediaEngineering Engineering00 25 (20111) (2011) 362 – 366 Jan Vyhnanek/ 000–000 Gradiometer circuitry In order to assure the magnetic state of the AMR sensors to improve stability of its parameters, the AMR sensors are periodically remagnetized – “flipped” at 30 kHz: the sensor field response becomes modulated (Fig 3b) High flipping pulses (1.5 A peak) are used in order to lower the sensor noise [4], while keeping maximum power dissipation by low duty-cycle Synchronous demodulators with the reference signal of 30 kHz provide reconstruction of the flipped output of AMR sensors After the 30 kHz demodulation stage there is a sampling circuit formed by switched integrator (Fig 3c), which suppresses the noisy time intervals until the sensor output recovers after a flipping pulse [4] The sensor connected to the feedback regulator is maintained at zero field reading by the compensation coil current from DC up to kHz The compensating current flows through the serially-connected compensating coils of both sensors, so the output of the second (measuring) sensor is proportional to the magnetic field gradient Suppressing the excitation field and also the Earth’s DC field by the compensator allows using higher excitation fields, therefore provides a reserve in DC gradient measurements and possibly improvement in S/N ratio There are four relevant outputs of the gradiometer processed according to Fig 3a: the DC field, DC field gradient, and the AC field gradient decomposed into the real and imaginary parts The DC field magnitude (homogeneous part as compensated by both sensors) is sensed on a shunt resistor in the feedback loop and it is of future use for i.e checking the “magnetically difficult” areas (a) (b) (c) Fig (a) Sensitive bands of the gradiometer defined by synchronous demodulators; (b) Gradiometer output before demodulation, effect of flipping; (c) Sensor output (top trace) and the reference (middle) of the switched integrator suppressing the most noisy intervals after flipping pulses (bottom trace) System noise and detection performance The noise levels achieved after the demodulation of the KMZ51 sensor output are 2.6 nT/√Hz @ Hz and 214 pT/√Hz @ kHz From the noise performance comparison of a few AMR sensor types in [6], which is consistent with our results, we can see that there is a chance for KMZ51 sensor being replaced in future by another type with lower noise levels As the sensitivity of AMR’s is generally low, the electronics should be very low-noise in the preamplifier stage, otherwise it could limit the overall system performance [7] In our case, the electronics noise at 1kHz was 150 pT/√Hz 365 Jan Vyhnanek et al / Procedia Engineering 25 (2011) 362 – 366 Jan Vyhnanek/ Procedia Engineering 00 (2011) 000–000 (a) (b) Fig (a) AMR sensor KMZ51 noise at Hz after demodulation – 2.6 nT/√Hz; (b) at kHz – 214 pT/√Hz Noise level decrease above kHz is caused by a low pass filter We tested the response of the gradiometer to aluminum and ferrous objects: the results are shown Figure In case of the aluminum object only phase response is usable as it is uniquely invertible This object of 5x5 size could be recognized from the noise level at the gradiometer output up to the distance of 20 cm The ferrous object was represented by a nail cm long and mm thick Phase response is changing with opposite sign to the aluminum response and the DC gradient signal allows to detect this object up to 10 cm distance The best approximation curve for both phase (AC) and DC gradient responses has turned out to be x-3 (Fig 5) 304 302 300 298 10 -200 -400 -600 -800 Distance [cm] (a) 10 10 Amplitude [mV] Amplitude [mV] Phase [rad] -0.1 DC gradient [mV] DC gradient [mV] Phase [rad] 0.1 10 12 14 16 18 20 10 12 14 16 18 20 14 16 18 20 600 400 200 -624 -625 -626 10 12 Distance [cm] (b) Fig (a) Static response of the gradiometer to a ferrous nail cm long, mm thick, (b) aluminium plate x cm, mm thick 366 Jan Vyhnanek et al.Procedia / ProcediaEngineering Engineering00 25 (20111) (2011) 362 – 366 Jan Vyhnanek/ 000–000 Conclusions The presented gradiometric mine-detector uses AMR sensors of KMZ51 type for metal detection together with kHz continuous-wave excitation As the gradient response of ferromagnetic and diamagnetic objects falls with distance approximately with 1/r3 rule, the most limiting factor of the maximum detection depth is the sensor noise – in our case we can detected a 50x50 mm aluminum plate up to 20-cm depth only being limited by the 214 pT/√Hz sensor noise at 1kHz Further improvement of the detection depth is possible when using AMR sensors with lower noise Prospectively, the high spatial resolution of AMR sensors should allow to recognize the objects by using signals from a sensor array Acknowledgements This project was funded by the internal grant of Czech Technical University No SGS10/205/OHK3/2T/13 “Compact sensors of magnetic field-gradient - development and application” and by the grant D102/09/H082 – “Sensors and intelligent sensor systems” of the Grant Agency of the Czech Republic References [1] Guelle D, Smith A, Lewis A, Bloodworth T Metal detector handbook for humanitarian demining European Communities, 2003 [2] Geneva Center for Humanitarian Demining: Mine Action Equipment E-Catalogue, www.gichd.org [3] Wold RJ, Nordman CA, Lavely EM, Tondra M, Lange E, Prouty M Development of a handheld mine detection system using a magnetoresistive sensor array Proc SPIE 1999; vol 3710 (1), pp 113-123 [4] Ripka P, Vopalensky M, Platil A, Doscher M, Lenssen KMH, Hauser H AMR magnetometer Journal of Magnetism and Magnetic Materials 2003; vol 254-255, pp 639-641 [5] Hauser H, Fulmek PL, Haumer P, Vopalensky M, Ripka P Flipping field and stability in anisotropic magnetoresistive sensors Sensors and Actuators 2003; vol 106, pp 121-125 [6] Zimmermann E, Verweed A, Glaas W, Tillmann A, Kemna A An AMR Sensor-Based Measurement System for Magnetoelectrical Resistivity Tomography IEEE Sensors Journal 2005; vol (2), pp 233-241 [7] Stutzke N, Russek SE, Pappas DP, Tondra M Low-frequency noise measurements on commercial magnetoresistive sensors Journal of Applied Physics 2005, vol 97, pp 10Q107-1 - 10Q107-3 ... Similar mine- detector with SDT sensors was presented in [3], however it worked with pulsed field and the DC noise of SDT sensors disqualifies them for combined DC and AC response sensing Gradiometer. .. loop Fig (a) The actual gradiometer with AMR sensors; b) Gradiometers combined with the excitation coil form a detector head, compensating and measuring sensor of the gradiometer are symmetrically... and detection performance The noise levels achieved after the demodulation of the KMZ51 sensor output are 2.6 nT/√Hz @ Hz and 214 pT/√Hz @ kHz From the noise performance comparison of a few AMR