A Magnetoelectric Laminate Based Passive Micro-Displacement Sensor

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Phys. Status Solidi RRL, 1–3 (2012) / DOI 10.1002/pssr.201206481

A magnetoelectric laminate based passive micro-displacement sensor
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pss www.pss-rapid.com Schematic diagram of the suggested micro-displacement sensor based on magnetostrictive/piezoelectric magnetoelectric laminate.
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Zheng Wu1, 2, Jun Zhang1, Ke Ma1, Yi Cao1, Yanmin Jia*, 1, Haosu Luo3, and Yihe Zhang4
Department of Physics, Zhejiang Normal University, Jinhua 321004, P.R. China College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, P.R. China 3 Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 201800, P.R. China 4 School of Materials Science and Technology, China University of Geosciences, Beijing 100083, P.R. China Received 8 November 2012, revised 27 November 2012, accepted 27 November 2012 Published online 29 November 2012 Keywords magnetoelectrics, laminates, displacement sensors, piezoelectrics
*

Corresponding author: e-mail yanmin_jia@yahoo.com.cn, Phone: +86 579 8229 8979, Fax: +86 579 8229 8188

A passive micro-displacement sensor (for ~ μm displacement) was fabricated based on a magnetoelectric laminate, in which the displacement change can result in a change of the magnetic flux around the magnetoelectric sensor. The displacement measurement was realized by measuring the magnetoelectric output voltage. The displacement detecting coefficient was ~ 2.5 mV/μm at a frequency of ~ 1 kHz. This passive displacement sensor possesses the advantages of low cost, high resolution, low energy consumption and good linearity and has potential for application in future displacement detectors.

1 Introduction The measurement of displacement or position is one of the most important and oldest tasks in sensor technology [1, 2]. Currently, the two main kinds of displacement sensors in practical application are based on optical and magnetic detecting technology [1, 2]. The optical displacement sensor is high resolution (~0.1 μm) and expensive, while the magnetic one is low resolution (~10 μm) and cheap. Developing magnetic displacement sensors with high detecting resolution is necessary for many applications in the automotive, hydraulic press, container security and food industries [3, 4]. In recent years, the magnetostrictive/piezoelectric magnetoelectric (ME) laminates have attracted considerable attention because of their ultrahigh magnetic-field sensitivity (~nT) [5, 6]. The ME conversion in the composites has been attributed to the joint effect of the magnetostrictive and the piezoelectric effect. The interestingly large ME coefficient of 384 mV/Oe has been reported in laminated composites composed of the magnetostrictive

TbxDy1–xFe2–y (Terfenol-D) alloy and the piezoelectric Pb(Mg1/3Nb2/3)1–xTixO3 (PMN-PT) crystal [7]. In this work, a passive ~μm ME displacement sensor was suggested and fabricated. 2 Experiment Figure 1 illustrates a schematic diagram of the suggested micro-displacement sensor, which consists of a permanent magnet to provide a dc bias magnetic field, some high-μ metal Fe yokes, a vibration Fe plate adhered to a shaker to generate the displacement, and a ME laminate to detect the magnetic field intensity. In our structure, the intrinsic magnetic flux in the permanent magnet keeps constant. On basis of Kirchhoff 's First Law on the magnetic circuit, the change of the air gap can induce a change in the magnetic flux around the ME laminate. Then the displacement measurement can be realized by monitoring the ME output voltage. The displacement of the Fe plate was generated by a shaker (Ling Dynamic Systems Ltd., model type V406) connected with a power
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Z. Wu et al.: A magnetoelectric laminate based passive micro-displacement sensor

Figure 2 (online colour at: www.pss-rapid.com) Waveforms of the measured output voltage due to an applied vibration displacement of ~ 10 μm peak at 1 kHz. Figure 1 (online colour at: www.pss-rapid.com) Schematic diagram of the suggested micro-displacement sensor based on the ME laminate. The arrows “P” and “M” are the polarization and magnetization directions, respectively.

amplifier (HEV-50, Nanjing Foneng Technology Co. Ltd.) and measured by a laser displacement meter (LK-GD500, Keyence Co. Ltd.). An oscilloscope was employed to monitor the output voltage from the ME laminate. The ME composite consists of two Terfenol-D alloy layers and one PMN-PT piezoelectric crystal layer. The Terfenol-D plates with dimensions of 12 × 6 × 1 mm3 were commercially supplied (Baotou Research Institute of Rare Earth). The PMN-PT crystal was grown in-house using a modified Bridgman technique [8]. The PMN-PT plate was cut to have same dimensions as the Terfenol-D plate and polarized along the thickness direction under an electric field of 1 kV/mm in a 120 °C silicone oil bath [9]. The ME laminate was fabricated by sandwiching the PMN-PT plate between two Terfenol-D plates using silver-loaded-epoxy (E-Solder No. 3021, ACME Division of Allied Products Co., USA) under a pressure of 10 MPa for 3 h to insure good mechanical coupling and strong bonding. 3 Results and discussion Figure 2 shows the waveforms of the measured output voltage (Vout ) due to an applied vibration displacement ( DLg ) of ~10 μm peak at the frequency of 1 kHz. Here Vout and DLg are of opposite phases, since the piezoelectric coefficient d 31 in the ME laminate carries a negative sign. The output voltage follows steadily the displacement variety and is up to ~25 mV, which demonstrates our device’s excellent performance to convert a micro-displacement to a voltage. Figure 3 plots the measured output voltage from the ME laminate as a function of the shaker’s vibration displacement amplitude at various vibration frequencies under an air gap of ~0.2 mm. It is seen that the output voltage has good linear responses to the displacement. The displacement detecting coefficient (dVout /dLg ), shown in the inset, is ~2.5 mV/μm and the minimum resolution of
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

the device is ~2 μm as shown in Fig. 3. The displacement detecting coefficient keeps constant under the measured frequency range of 0.2–1 kHz because of the low-frequency stability of ME laminates [5, 6]. On basis of Kirchhoff’s First Law, the magnetic flux in the magnetic circuit can be written as follows:

φ1 = φ2 + φ3 ,

(1)

where φ1 (φ1 = μ H1 S ), φ2 (φ2 = μ H 2 S ) and φ3 (φ3 = μ H 3 S = μ0 H g S ) are the magnetic flux of the bias magnet, the ME composite and the air gap, respectively. H1 , H 2 , H 3 and H g are the magnetic field intensity of the bias magnet, the ME composite, the edge of the yoke and the air gap, respectively. μ0 and μ are the magnetic permeability of vacuum and Fe yoke, respectively. S is the cross sectional area of the yoke. According to the Ampere’s circulation theorem on magnetic media, the following equation can be obtained:
2 H 3 L2 + H 3 L4 + 2 H g Lg - H 2 L3 = 0 ,

(2)

Figure 3 (online colour at: www.pss-rapid.com) Voltage output from the ME laminate as a function of the shaker’s vibration displacement amplitude under an air gap of ~ 0.2 mm. The inset figure shows the frequency dependence of the displacement detecting coefficient. www.pss-rapid.com Rapid Research Letter
Phys. Status Solidi RRL (2012) 3

decreases with the increase of the air gap as shown in Fig. 4. The experimental result agrees with the prediction from Eq. (5). It should be noted that decreasing the air gap will reduce the measurable displacement variety range, though it can effectively enhance the displacement detecting coefficient and detecting resolution. In order to meet the displacement measurement demand, a suitable value of the air gap should be selected in practical application. The advantages of low cost, high resolution, and good linearity in this passive ~μm displacement sensitive device make it promising for application in future displacement detectors.
Figure 4 Air gap dependence of the voltage output from the ME laminate under an applied peak vibration displacement of ~ 2 μm at the frequency of 1 kHz.

where L2 , L3 , L4 , Lg are the lengths of the yoke, the ME laminate, Fe displacement plate and air gap, respectively, as marked in Fig. 1. Combining Eqs. (1) and (2), the relationship between H 2 and Lg is expressed as

μ0 H 2 (2 L2 + L3 + L4 ) + 2μ H 2 Lg
= μ0 H1 (2 L2 + L4 ) + 2 μ H1 Lg .

(3)

4 Conclusion In summary, a passive ~μm magnetostrictive/piezoelectric ME displacement sensor was suggested and fabricated. The output voltage shows good linear response to the vibration displacement variety and the displacement detecting coefficient is up to ~2.5 mV/μm. The detecting coefficient can be further enhanced by reducing the air gap and increasing the ME coefficient. The passive displacement sensitive device possesses the advantages of low cost, high resolution, low energy consumption and good linearity and is suitable for application in future displacement detectors.
Acknowledgements This work was supported by the National Nature Science Foundation of China. (No. 51002141), Qianjiang Talents Project of the Technology Office of Zhejiang Province, China (No. 2011R10086), National Nature Science Foundation of Zhejiang Province (LY12A04001), Jinhua Science and Technology Bureau, Zhejiang Province, China (No. 2010-1051) and The Opening Project of the Key Laboratory of Inorganic Function Materials and Devices, Chinese Academy of Sciences (KLIFMD-2012).

By differentiating both sides of Eq. (3), we can obtain the following expression: dH 2 = H3 dLg . Lg + ( L2 + 0.5 L3 + 0.5L4 ) μ0 /μ

(4)

The displacement detecting coefficient can be obtained by substituting the definition of the ME coefficient (α ME = dVout /dH 2 ) into Eq. (4) as follows: dVout H 3α ME . = dLg Lg + ( L2 + 0.5L3 + 0.5L4 ) μ0 /μ

(5)
[1] [2] [3] [4] [5] [6] [7] [8] [9]

References
J. Gao et al., Sensors 10, 8424 (2010). F. Lina et al., Precis. Eng. 36, 620 (2012). O. Erb et al., Sens. Actuators A 26, 277 (1991). T. W. Ng et al., Sens. Actuators A 107, 21 (2003). Y. J. Wang et al., Phys. Status Solidi RRL 6, 265 (2012). Y. J. Wang et al., Phys. Status Solidi RRL 5, 232 (2011). Y. M. Jia et al., Chin. Sci. Bull. 53, 2129 (2008). H. S. Luo et al., Jpn. J. Appl. Phys. 39, 5581 (2000). Y. M. Jia et al., Appl. Phys. Lett. 94, 263504 (2009).

On basis of Eq. (5), the displacement detecting coefficient could be further enhanced by decreasing the air gap and increasing the ME coefficient. Figure 4 plots the air gap dependence of the measured voltage output from the ME laminate under an applied ~2 μm peak vibration displacement at the frequency of 1 kHz. The displacement detecting coefficient quickly

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© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

References: J. Gao et al., Sensors 10, 8424 (2010). F. Lina et al., Precis. Eng. 36, 620 (2012). O. Erb et al., Sens. Actuators A 26, 277 (1991). T. W. Ng et al., Sens. Actuators A 107, 21 (2003). Y. J. Wang et al., Phys. Status Solidi RRL 6, 265 (2012). Y. J. Wang et al., Phys. Status Solidi RRL 5, 232 (2011). Y. M. Jia et al., Chin. Sci. Bull. 53, 2129 (2008). H. S. Luo et al., Jpn. J. Appl. Phys. 39, 5581 (2000). Y. M. Jia et al., Appl. Phys. Lett. 94, 263504 (2009). On basis of Eq. (5), the displacement detecting coefficient could be further enhanced by decreasing the air gap and increasing the ME coefficient. Figure 4 plots the air gap dependence of the measured voltage output from the ME laminate under an applied ~2 μm peak vibration displacement at the frequency of 1 kHz. The displacement detecting coefficient quickly www.pss-rapid.com © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim