Threshold Magnetic Field Sensor Based on the ... - IEEE Xplore

IEEE SENSORS JOURNAL, VOL. 15, NO. 11, NOVEMBER 2015

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Threshold Magnetic Field Sensor Based on the Oscillator With a Magnetosensitive Piezoelectric Transducer in the Feedback Vladimir N. Serov, Leonid Y. Fetisov, Aleksandr A. Morozov, and Yuri K. Fetisov, Senior Member, IEEE Abstract— A threshold direct current magnetic field sensor with turn ON tunable level based on the oscillator containing a wideband amplifier, resistive voltage divider, as well as a resonance magnetosensitive transducer in the feedback loop has been described. The transducer is a bimorph plate of piezoelectric lead zirconate titanate with an electromagnetic coil wound on it. It uses a combination of the Ampere force, piezoelectric effect, and the acoustic resonance. The voltage transmission coefficient of the transducer depends on the dc magnetic field applied to the bimorph plate. Switching magnetic field of the oscillator can be tuned within the 5–600 Oe range by adjusting the parameters of the resistive voltage divider. The sensor is sensitive to the dc magnetic field direction. Index Terms— Magnetic field sensor, threshold sensor, oscillator, magneto-sensitive transducer, piezoelectric resonator.

I. I NTRODUCTION

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AGNETIC field sensors of various types are widely used at present in energy and transportation industries, in safety and control systems, for navigation, in geophysics and medicine [1]–[3]. In order to measure quasi-static magnetic fields in the range from few mOe to several kOe the use of the Hall effect sensors [4], giant magnetoresistance sensors [5], microelectromechanical sensors [6], [7], and magnetoelectric sensors [8] has received wide acceptance. The output signal of such analog sensors is usually a dc or ac electrical voltage whose amplitude is proportional to the field measured. Furthermore, there are many applications where the threshold sensors, which produce the output signal only if the field is above a certain predefined level, are required. Among these are the contactless threshold current sensors for accidents prevention in electrical grids, magnetic proximity sensors, and magnetic sensors for vehicle identification systems [9]. In present paper the threshold dc magnetic field sensor of a new type with the low-frequency output has been suggested and investigated. The sensor is based on the oscillator containing a wide-band amplifier, resistive voltage divider, and a Manuscript received May 2, 2015; revised June 30, 2015; accepted July 6, 2015. Date of publication July 22, 2015; date of current version September 4, 2015. This work was supported by the Ministry of Education and Science of Russian Federation under Project RFMEF158314X0009. The associate editor coordinating the review of this paper and approving it for publication was Prof. Zeljko Ignjatovic. The authors are with the Moscow State University of Information Technologies, Radio Engineering, and Electronics, Moscow 119454, Russia (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JSEN.2015.2459725

Fig. 1.

Block-diagram of the dc magnetic field sensor.

resonance magneto-sensitive transducer in the feedback loop. The voltage transmission coefficient of the transducer depends on the dc magnetic field that allows controlling of the threshold level by adjusting the parameters of the voltage divider. II. B LOCK -D IAGRAM AND P RINCIPLE OF THE S ENSOR O PERATION The block-diagram of the threshold magnetic field sensor is shown in Fig. 1. The main elements of the sensor are the wide-band amplifier with the gain coefficient K 1 , phase-shifter with the transmission coefficient K 2 , output amplifier with the gain coefficient K 3 , magneto-sensitive resonance piezoelectric transducer with the transmission coefficient K 0 , tuned voltage divider on the resistors R and R0 , and a switch S. All elements are connected in series in a closed loop. The precision instrumentation amplifier INA326 with single-polar supply was used as a wide-band amplifier. Within the frequency band of 0-1.5 kHz it had the voltage gain coefficient K 1 = 100, input impedance of 1 M, output impedance of several , and the output saturation level of ∼2 V. The phase-shifter made on the RC chain had the voltage transmission coefficient K 2 = 0.33 and at the frequency of 790 Hz yielded the outrunning phase-shift of 90°. The output operation amplifier OPA2314 and a pair of complementary transistors ZXTC2045E6 provided the due current for the transducer’s coil and had the voltage transmission coefficient K 3 = 3. The magneto-sensitive resonance piezoelectric transducer used the combination of the Ampere force, piezoelectric effect, and the acoustic resonance [10]. The transducer contained a bimorph piezoelectric element with an excitation electromagnetic coil wound on it. The coil was rigidly fixed on the bimorph element with glue. The bimorph element consisted of two piezoelectric lead zirconate titanate (PZT) slabs with

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dimensions of 14 mm × 3 mm × 0.2 mm each, mechanically connected to each other. During the fabrication process, 2 μm thick Ag electrodes were fabricated on the slabs’ surfaces and between them. After sintering, the PZT slabs were poled on opposite directions by applying 2 kV dc voltage to the electrodes. The bimorph element was rigidly fixed at one end allowing bending oscillations with the resonance frequency of f 0 = 790 Hz. The coil contained N = 100 turns of 0.2 mm wire and had the resistance of R0 = 4 . The dc magnetic field H = 0 −1 kOe was applied perpendicularly to the bimorph plane. The variable resistor R = 1 k was used in the voltage divider to change the transmission coefficient K 4 = R0 /(R0 + R) from 4·10−3 up to 1. The principle of the threshold sensor operation is as follows. The voltage u 1 cos(2πft) with the amplitude u 1 and frequency f is applied to the transducer coil and creates the ac current with the amplitude I ≈ u 1 /R0 . This current produces ac magnetic moment M( f ) = NS(u 1 /R0 )cos(2πft) directed along the long axis of the bimorph, where S is the cross-section of the coil. The field H acts on the magnetic moment and generates a variable torque which deflects the PZT-bimorph with the coil from its equilibrium position. This results in an excitation of bending oscillations of the PZT-bimorph. Periodic deformation of the bimorph generates ac voltage between its outer electrodes with the amplitude u 2 and the frequency f . The amplitude of the voltage is proportional to the field u 2 ∼ H and increases by a quality factor Q when the excitation frequency coincides with the frequency of the structure bending oscillations. Thus, the transmission coefficient of the transducer K 0 (H ) = u 2 (H )/u 1 depends on the magnetic field and it may be tuned over a wide range by varying the field H . The resistive voltage divider allows changing of the voltage u 1 applied to the input of the piezoelectric transducer. It is well known that under the fulfillment of the amplitude balance and phase matching conditions K i ≥ 1 and φi = 2πn, (1) i

i

where K i and ϕi are the transmission coefficient and the phase shift in the i –th element of the block-diagram, respectively, and n is the integer, the oscillations appear in the loop. In this case the oscillations frequency will be equal to the frequency of the PZT-bimorph bending oscillations, while the voltage amplitude u 4 is limited by the amplifier saturation level. Equation (1) will be fulfilled for various values of the transducer transmission coefficient K 0 , when the coefficient K 4 is changed. As each value of K 0 corresponds to the certain magnetic field, it is possible to tune the field H1 when oscillations in the loop start. It follows that the oscillator described may be used as a threshold dc magnetic field sensor with tuned switching level. III. E XPERIMENTAL R ESULTS The measured amplitude-frequency and phase-frequency responses of the resonance piezoelectric transducer, which determine basic parameters of the threshold magnetic field sensor, are illustrated in Fig. 2a. The data were taken for the

Fig. 2. (a) The amplitude-frequency u 2 ( f ) and phase-frequency ϕ( f ) responses of the PZT-transducer for H = 550 Oe. (b) The dependence of the transducer output signal u 2 on the magnetic field H for f 0 = 790 Hz and input signal u 1 = 80 mV.

input voltage u 1 = 80 mV and dc magnetic field H = 600 Oe. One can see a peak on the amplitude-frequency response with the central frequency of f 0 = 790 Hz and the quality factor of Q ≈ 19. The peak corresponds to excitation of the PZT-element bending oscillations. The resonance frequency estimation using a formula for the main resonance frequency of the one end fixed beam [11] and parameters of the bimorph element corresponding to the experiment, gives the value of ∼995 Hz. The estimated frequency is in reasonable agreement with the measured one. Presence of the coil causes a decrease in the resonance frequency due to influence of additional mass. It follows from the phase-frequency response, that the piezoelectric transducer induced phase shift is equal to φ ≈ 900 at the resonance. Note, that the phase shift induced by the electromagnetic coil itself is negligible. In order to compensate this phase shift and fulfill the phase-matching conditions, the phase-shifter shown in Fig.1 was used. Figure 2b shows the voltage at the transducer output u 2 (H ) as a function of magnetic field H for fixed voltage applied to the coil u 1 = 80 mV. It is seen, that the voltage transmission coefficient of the transducer K 0 is gradually increases from zero at H = 0 up to 4.7 at the field H = 1000 Oe. Figure 3 shows the signal waveforms at different points of the oscillator, elucidating specific features of its operation. Let us first consider the oscillograms in Fig.3a and Fig. 3b taken for the open oscillator loop and magnetic field H ≈ 300 Oe. The harmonic voltage with the amplitude u 1 ≈ 5 mV applied to the coil (Fig. 3a) produces harmonic signal with the amplitude of u 3 ≈ 0.85 V and the phase delay of 900 at the amplifier output. The voltage gain in the transduceramplifier section of the loop is equal to K 0 K 1 ≈ 170. The phase-shifter provides the phase shift of φ = +90° to fulfill the phase-matching condition. After closing the loop at R ≤ 500 , when the voltage divider transmission coefficient K 4 ≥ 7.9 · 10−3 , the oscillator starts to generate a signal with the frequency f 0 = 790 Hz, equal to the piezoelectric transducer resonance frequency (Fig. 3c). The distortion of the output signal is due to the saturation of the amplifier at the level of u 4 = 1.9 V.

SEROV et al.: THRESHOLD MAGNETIC FIELD SENSOR

Fig. 3. The signal waveforms at the: (a) transducer input, (b) amplifier output, and (c) at the sensor output.

Fig. 4. The dependence of the sensor output signal u 4 on the field H for increasing and decreasing fields. The arrows show directions of the field variation.

The dependence of the signal amplitude u 4 on the magnetic field H , measured for the fixed R = 500 is shown in Fig. 4. The curves were taken for the velocity of the magnetic field sweeping ∂ H /∂t ≈ 0.8 Oe/s. The arrows indicate the directions of the field increase or decrease. One can see that a hysteresis takes place at the oscillator turn on and turn off. The generation starts at H1 ≈ 308 Oe as the field is increased and it vanishes at H2 ≈ 276 Oe as the field is decreased. The threshold magnetic fields H1 and H2 were determined in Fig. 4 as the points of intersection of the tangent lines to the vertical curves with horizontal lines at u 4 = 0 and u 4 = 1.9 V. The error in the field definition was H ≈ 2 Oe. The width of the hysteresis loop is H1 − H2 ≈ 32 Oe. The appearance of the hysteresis is due to the nonlinearity of the transconductance coefficient of complimentary transistors. For small input signals (≤200 mV) the complementary transistors have low transconductance coefficient, while for high signals (≥200 mV) the transconductance coefficient increases. This leads to the nonlinearity in the amplification and, thus, to the “hard” regime of the oscillator excitation followed by a formation of the hysteresis loop. To eliminate the hysteresis, one should use an output amplifier with linear characteristics. One can see from Fig. 4 that the oscillator

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Fig. 5. The threshold “turn on” H1 and “turn off” H2 fields as function of the resistance R in the feedback loop of the oscillator.

“turn on” and “turn off” times were about ∼5 s and were practically equal to each other. These characteristics times can be decreased by 1-2 orders or more by using the bimorph element of smaller sizes with higher resonance frequency. Figure 5 shows measured dependences of the characteristic “turn on” H1 and “turn off” H2 magnetic fields on the resistance R for the threshold sensor. Both fields linearly grow as the resistance is increased from zero up to 1 k. As this takes place, the hysteresis loop width remains nearly constant and equal to ∼30 Oe. In our oscillator the smallest reliably reproducible “turn on” field was H1min ≈ 5 Oe and the highest field was H1max = 600 Oe. For the resistor value R > 1 k, the condition (1) was not satisfied and the oscillations in the loop did not appear. It has been shown that the sensor is sensitive to the direction of dc magnetic field. Upon reversal the H direction, the oscillations did not appear for any value of the field. It is conditioned by the change in the direction of the torque acting on the piezoelectric beam followed by a corresponding 180 degree change in the phase of the piezoelectric bimorph generated signal. As a result, the phase-matching condition (1) is violated. To restore generation at the H direction reversal, it was required to insert additional phase shift of 1800 in the loop by using the phase shifter. The “turn on” field H1 of the sensor increased for fixed R when the field H was deflected from the normal to the PZT-bimorph. The deflection of the field resulted in a decrease in the normal component of dc field which determines interaction of the coil’s magnetic moment with the field and, hence, a decrease in the amplitude of the piezoelectric transducer output signal. Power consumption of the sensor described is determined mainly by the amplifiers power consumption and was ∼20 mW for magnetic fields lower than the threshold and was not higher than 50 mW in the oscillation regime. IV. C ONCLUSION Thus, the dc magnetic field sensor with the controlled threshold level have been fabricated and investigated. The sensor is based on the oscillator containing a wideband amplifier, resistive voltage divider, and a resonance magneto-sensitive transducer in the feedback loop. Due to the dependence of

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the transducer transmission coefficient on the magnetic field, the sensor “turn on” field can be tuned within the limits of 5-600 Oe using a variable resistor. The frequency of the sensor output signal is determined by the resonance frequency of the piezoelectric transducer and in our case was 790 Hz. It has been shown that the sensor is sensitive to the orientation of the dc field: the oscillation disappeared at the field direction reversal; the threshold level increased as the field was deflected from the normal to the transducer plane. The range of operational magnetic fields for the described threshold sensors can be widen up to several kOe and the frequency of the output signal can be increased up to some MHz by choosing the characteristics of the piezoelectric transducer and electrical parameters of the oscillator electronic circuit. The threshold magnetic field sensors may be used, for example, to prevent accidents in electrical grids, in proximity sensors, in the car identification systems and so on.

Vladimir N. Serov received the Ph.D. degree in electronics from the Moscow Engineering Physics Institute, in 1976. He jointed the Moscow State Technical University of Radio Engineering, Electronics, and Automation as a Researcher in 1978. Since 1980, he has been an Associate Professor with the Faculty of Electronics. He has co-authored four books, over 30 papers in peer-reviewed journals, 20 patents, and numerous presentations at conferences.

Leonid Y. Fetisov received the degree and the Ph.D. degree in physics of magnetism from M. Lomonosov Moscow State University, Russia, in 2010 and 2013, respectively. Since 2013 up to 2015, he has been a Leading Specialist with JSC Ruselectronics, Moscow. In 2015, he joined the Moscow State Technical University of Radio Engineering, Electronics and Automation. He has co-authored 20 papers in peer-reviewed journals. His current research interests are in the field of multiferroics, magnetoelectric effects, and magnetic field sensors.

R EFERENCES [1] P. Ripka, Magnetic Sensors and Magnetometers. Norwood, MA, USA: Artech House, 2000. [2] J. Lenz and A. S. Edelstein, “Magnetic sensors and their applications,” IEEE Sensors J., vol. 6, no. 3, pp. 631–648, Jun. 2006. [3] M. Díaz-Michelena, “Small magnetic sensors for space applications,” Sensors, vol. 9, no. 4, pp. 2271–2288, 2009. [4] Sensing and Control, Honeywell Inc. (2014). Hall Effect Sensing and Application. [Online]. Available: http://sensing.honeywell.com [5] C. Reig, S. Cardoso, and S. C. Mukhopadhyay, Giant Magnetoresistance (GMR) Sensors. Berlin, Germany: Springer-Verlag, 2013. [6] A. L. Herrera-May, L. A. Aguilera-Cortés, P. J. García-Ramírez, and E. Manjarrez, “Resonant magnetic field sensors based on MEMS technology,” Sensors, vol. 9, no. 10, pp. 7785–7813, 2009. [7] S. Tadigadapa and K. Mateti, “Piezoelectric MEMS sensors: State-ofthe-art and perspectives,” Meas. Sci. Technol., vol. 29, no. 9, 092001 (30 pp), Sep. 2009. [8] Y. Wang, J. Li, and D. Viehland, “Magnetoelectrics for magnetic sensor applications: Status, challenges and perspectives,” Mater. Today, vol. 17, no. 6, pp. 269–275, Jul./Aug. 2014. [9] A. Daubaras and M. Zilys, “Vehicle detection based on magneto-resistive magnetic field sensor,” Electron. Elect. Eng., vol. 118, no. 2, pp. 27–32, 2012. [10] Y. K. Fetisov, “Piezoelectric resonance sensors of DC magnetic field,” IEEE Sensors J., vol. 14, no. 6, pp. 1817–1821, Jun. 2014. [11] S. Timoshenko, Vibration Problems in Engineering, 3rd ed. Toronto, ON, Canada: Van Nostrand, 1955.

Aleksandr A. Morozov received the B.S. degree in robotics from the Moscow State Technical University of Radio Engineering, Electronics and Automation, Russia, in 2014, where he is currently pursuing the master’s degree. His research interests are in the field of sensors and control systems.

Yuri K. Fetisov (SM’96) received the Ph.D. degree in solid-state physics from the Moscow Engineering Physics Institute, in 1981. He joined the Moscow State Technical University of Radio Engineering, Electronics and Automation (MIREA) as a Researcher in 1983, and was advanced to Professor of the Physics Department in 1994 and the Dean of the Faculty of Electronics in 2007. He is currently the Head of the Research-Educational Center Magnetoelectric Materials and Devices with MIREA, where he is involved in research on spin wave processes in thin magnetic films, magnetoelectric effects in multilayer composite structures, and design of solid state signal processing devices and magnetic field sensors. He has co-authored over 120 papers in peer-reviewed journals.