sensors Article

A Miniature Magnetic-Force-Based Three-Axis AC Magnetic Sensor with Piezoelectric/Vibrational Energy-Harvesting Functions Chiao-Fang Hung 1 , Po-Chen Yeh 1 and Tien-Kan Chung 1,2, * 1 2

*

Department of Mechanical Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan; [email protected] (C.-F.H.); [email protected] (P.-C.Y.) International College of Semiconductor Technology, National Chiao Tung University, Hsinchu 30010, Taiwan Correspondence: [email protected]; Tel.: +886-3-571-2121 (ext. 55116); Fax: +886-3-572-0634

Academic Editors: Manuel Gasulla and Ferran Reverter Received: 29 August 2016; Accepted: 4 February 2017; Published: 8 February 2017

Abstract: In this paper, we demonstrate a miniature magnetic-force-based, three-axis, AC magnetic sensor with piezoelectric/vibrational energy-harvesting functions. For magnetic sensing, the sensor employs a magnetic–mechanical–piezoelectric configuration (which uses magnetic force and torque, a compact, single, mechanical mechanism, and the piezoelectric effect) to convert x-axis and y-axis in-plane and z-axis magnetic fields into piezoelectric voltage outputs. Under the x-axis magnetic field (sine-wave, 100 Hz, 0.2–3.2 gauss) and the z-axis magnetic field (sine-wave, 142 Hz, 0.2–3.2 gauss), the voltage output with the sensitivity of the sensor are 1.13–26.15 mV with 8.79 mV/gauss and 1.31–8.92 mV with 2.63 mV/gauss, respectively. In addition, through this configuration, the sensor can harness ambient vibrational energy, i.e., possessing piezoelectric/vibrational energy-harvesting functions. Under x-axis vibration (sine-wave, 100 Hz, 3.5 g) and z-axis vibration (sine-wave, 142 Hz, 3.8 g), the root-mean-square voltage output with power output of the sensor is 439 mV with 0.333 µW and 138 mV with 0.051 µW, respectively. These results show that the sensor, using this configuration, successfully achieves three-axis magnetic field sensing and three-axis vibration energy-harvesting. Due to these features, the three-axis AC magnetic sensor could be an important design reference in order to develop future three-axis AC magnetic sensors, which possess energy-harvesting functions, for practical industrial applications, such as intelligent vehicle/traffic monitoring, processes monitoring, security systems, and so on. Keywords: AC magnetic sensor; magnetic force; piezoelectric; mechanical; 3-axis; energy harvesting

1. Introduction To date, magnetic sensing technology is still one of the important research fields with respect to sensors, and has created many energy, commercial–electronic, medical, and industrial applications. Novel magnetic sensors, using smart materials, are used for a number of applications, including for condition monitoring of current-carrying wires in smart grid applications [1–3], magnetic-field detection of magnetic read–write heads in computer hard disks and other data-storage devices [4], magnetic-flux detection of interlocking nails in bone-fracture surgeries [5,6], and magnetic-flux detection/conversion of magnetic energy-harvest-powered wireless sensing systems [7], amongst others. Recently, magnetic sensors have developed rapidly in practical industrial applications, which require a sensing range of a few gauss (such as for application in vehicle/traffic monitoring, processes monitoring, security systems, and so on) [8]. Based on the magnetic-sensing principles for these few-gauss-range applications, magnetic sensors are divided into several representative categories. In these categories, currently, the most developed and comprehensively-used are Hall Effect magnetic

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sensors [9] and anisotropic magnetoresistive (AMR) magnetic sensors [10,11]. However, compared to Hall Effect and AMR magnetic sensors, microelectromechanical systems (MEMS) resonant magnetic sensors with movable structures have some advantages, such as low-costs, larger output signals, and high sensitivities [12]. Regarding MEMS magnetic sensors, owing to their movable structures, recently, researchers have demonstrated Lorentz-force-based MEMS magnetic sensors [12]. However, these sensors have to be powered/enabled by applying currents. Furthermore, in order to enhance the effect of the Lorentz force, the current-carrying wires of the sensors must be long and thin, which increase resistance, consequently produce more heat dissipation, and eventually increase power consumption. On the other hand, some researchers have demonstrated magnetoelectric-based MEMS magnetic sensors [13–17], which also use moveable structures [17]. However, an additional DC magnetic field is typically needed to enable the magnetic sensing functions of the sensor. Providing this field requires additional energy supply and, thereby, increase the overall power consumption with respect to the system. Therefore, magnetoelectric-based MEMS magnetic sensors, as well as the Lorentz-force-based MEMS magnetic sensors, have power consumption issues. In addition, according to the developing trend of smart sensing technology, the above-mentioned applications can further integrate with a wireless transmission module in order to become a wireless sensor module. However, sometimes the wireless sensor module is located in hard-to-reach areas [18] and, thus, suffer power supply problems [19]. Thus, we believe that if these sensors can have energy-harvesting functions, the sensors not only can be self-powered, but can also power other devices, such as wireless transmission modules (i.e., achieving a self-powered wireless sensor module). In order to add energy-harvesting functions to sensors, combining magnetic sensors with piezoelectric materials (rather than using magnetoelectric-based approaches) would be one of best solutions. To date, several representative piezoelectric-based magnetic sensors and magnetic energy harvesters have been demonstrated in the literature [3,20–24]. Both, the sensing and energy harvesting approaches, use magnet-induced magnetic force interactions to deflect/deform a single piezoelectric cantilever beam in order to produce piezoelectric voltage responses. However, the single-beam design can only be used to sense a single-axis magnetic field. On the other hand, researchers have recently demonstrated piezoelectric-based, single-mechanical-mechanism three-axis accelerometers and energy harvesters [25–27]. That is, by using compact single-mechanical mechanisms (for example, a cross-linked beam), the accelerometers and the energy harvesters can achieve three-axis mechanical motion-sensing and three-axis vibrational energy-harvesting, respectively. According to this, by combining a magnetic material and a piezoelectric-based, single-mechanical mechanism, we can achieve a novel single-mechanical-mechanism-configured, magnetic-force-based, three-axis AC magnetic sensor with piezoelectric/vibrational energy-harvesting functions. Our proposed sensor has the same advantages (i.e., being cost-effective and space-efficient) as commercial three-axis magnetic sensors [28]. Moreover, regarding our proposed sensor with respect to specific applications, some researchers have recently reported a three-axis current sensor for evaluating the integrity of concentric neutrals in underground power distribution cables [29], and an AC magnetic field sensing system for a motion tracking applications [30]. We think that these two applications can more clearly show the practical applications of our sensor (for more details of these applications, please see the Supplemental Materials). Thus, in this paper, we report a three-axis AC magnetic sensor with piezoelectric/vibrational energy-harvesting functions. The most important features of the sensor are the compact, single-mechanical-mechanism-configured, three-axis magnetic field sensing and the three-axis vibrational-energy harvesting. The design, fabrication, testing, and results of the sensor are described in the following sections. 2. Design The design of the sensor is shown in Figure 1a. The sensor is comprised of four piezoelectric lead-zirconate-titanate/copper-beryllium (PZT/CuBe)-layered cantilever beams, a neodymium iron

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boron (NdFeB) magnet, a connector, and top and bottom mechanical frames. The free end of the CuBe sheet of each PZT/CuBe beam is connected by the connector. The fixed end of each PZT/CuBe beam boron (NdFeB) a connector, and top and The bottom mechanical frames. asThe free end of is clamped by themagnet, top and bottom mechanical frames. four beams are connected a cross-linkedthe CuBe sheet of each PZT/CuBe beam is connected by the connector. The fixed end of each beam. Finally, the NdFeB magnet is fixed on the surface, at the center of the cross-linked-beam. The PZT/CuBe beam is clamped by the top and bottom mechanical frames. The four beams are connected magnetic sensing principle of the sensor is shown in Figure 1b,d. In Figure 1b,d, the sensor is under as a cross-linked-beam. the NdFeB is fixed on the surface, an at the center of the an initial/static state (i.e., Finally, zero magnetic fieldmagnet and zero ambient vibrations), x-axis (in-plane) cross-linked-beam. The magnetic sensing principle of the sensor is shown in Figure 1b,d. In Figure 1b,d, magnetic field, and a z-axis (out-of-plane) magnetic field, respectively. When the x-axial (or y-axial) the sensorfield is under an initial/static stateas(i.e., zeroinmagnetic field zero magnet ambient(magnetization vibrations), an magnetic is applied to the sensor, shown Figure 1c, theand NdFeB x-axis (in-plane) magnetic field, and a z-axis (out-of-plane) magnetic field, respectively. the direction is along the z-axis) experiences a magnetic force (Fx (or Fy)) and torque (T). The When magnetic x-axial (ortorque y-axial) magnetic field is applied to the sensor,deform as shown Figure the NdFeB magnet force and rotate the NdFeB magnet and, therefore, thein two x-axis1c, (or y-axis) cantilever (magnetization direction is along the z-axis) experiences a magnetic force (Fx (or Fy)) and torque (T). beams. Because of the piezoelectric effect, the deformed beams produce voltage outputs. Thus, the The magnetic force and torque rotate the NdFeB magnet and, therefore, deform the two x-axis (or sensor achieves x-axis (or y-axis) magnetic-field sensing. When the z-axial magnetic field is applied y-axis) cantilever beams.inBecause of the effect, the deformed beams produce voltage to the sensor, as shown Figure 1d, the piezoelectric NdFeB magnet is subjected to a magnetic force (Fz) and, outputs. Thus, the sensor achieves x-axis (or y-axis) magnetic-field sensing. When the z-axial magnetic thus, is projected along the z-axis; the z-axial projection deflects the beams. Due to the piezoelectric field isthe applied to the sensor, as shown in Figure 1d,Therefore, the NdFeBthe magnet subjected to amagneticmagnetic effect, deformed beams produce voltage outputs. sensorisachieves z-axis forcesensing. (Fz) and, thus, is projected along the z-axis; the z-axial projection deflects the beams. Due to the field piezoelectric effect, the deformed beams voltage outputs. Therefore, sensor achieves z-axis In addition, when the sensor doesproduce not need to sense magnetic fieldsthe (i.e., magnetic sensing magnetic-field sensing. function is disable or in standby mode), the sensor can be used as a three-axis piezoelectric/vibrational In addition, when sensor three-axis does not need to sense magnetic (i.e., magnetic function energy harvester, whichthe converts ambient vibration intofields voltage outputs (i.e.,sensing the vibrational is disable or in standby mode), the sensor can be used as a three-axis piezoelectric/vibrational energy energy harvesting function is enabled or in active mode). The piezoelectric/vibrational harvester, which converts three-axis ambient vibration into voltage outputs (i.e., the vibrational energy energy-harvesting principle of the sensor is fundamentally the same as that of the three-axis energy harvesting function is enabled or in active mode). The piezoelectric/vibrational energy-harvesting harvester, previously report by our group [27]. Thus, the details of the principles are not described in principle this work.of the sensor is fundamentally the same as that of the three-axis energy harvester, previously report by our group [27]. Thus, the details of the principles are not described in this work.

NdFeB Magnet

(a)

NdFeB Magnet

(b)

PZT-3

PZT-4 Connector PZT-1

Connector PZT Z

Z

X

X

Y

Top Mechanical Frame

PZT-2

Bottom Mechanical Frame

CuBe

CuBe

(d)

T PZT Z X CuBe Bending

Top Mechanical Connector Frame Bending

Bottom Mechanical Frame

NdFeB Magnet Connector PZT

Z X CuBe Bending

Magnetic Field

Magnetic Field

(c)

Top Mechanical Frame

Bottom Bending Mechanical Frame

Figure 1. Magnetic-sensing principle of the 3-axis magnetic-force-based AC magnetic sensor: Figure 1. Magnetic-sensing principle the 3-axis magnetic-force-based AC magnetic sensor:field (a) (a) isometric view of the sensor. Theofsensor is under (b) initial/static state; zero magnetic isometric view of the sensor. The sensor is under (b) initial/static state; zero magnetic field and zero and zero ambient vibration, (c) x-axis magnetic field, and (d) z-axis magnetic field. ambient vibration, (c) x-axis magnetic field, and (d) z-axis magnetic field.

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3.3.Fabrication Fabrication The Thefabrication fabricationof ofthe thesensor sensorcomprises comprisesthree threeparts: parts:(I) (I)mechanical mechanicalframes; frames; (II) (II) PZT/CuBe PZT/CuBelayered layered cantilever cantileverbeams; beams;and and(III) (III)aaconnector connectorwith withaaNdFeB NdFeBmagnet. magnet.First, First,acrylic acrylicplates plateswere weremachined machinedto to serve as the top and bottom mechanical frames. Second, a PZT-5H sheet (length × width × thickness: serve as the top and bottom mechanical frames. Second, a PZT-5H sheet (length × width × thickness: 7.5 1 mm) was fixed onon a CuBe sheet (length × width × thickness: 9 mm × 4 mm 0.25× 7.5mm mm××4 4mm mm× × 1 mm) was fixed a CuBe sheet (length × width × thickness: 9 mm × 4 ×mm mm) as a as PZT/CuBe-layered cantilever beam. Third, the the screw, shim, and 0.25 mm) a PZT/CuBe-layered cantilever beam. Third, screw, shim, andNdFeB NdFeBmagnet magnetwere were assembled to serve as a connector with a NdFeB magnet. Finally, the fixed end of each PZT/CuBe assembled to serve as a connector with a NdFeB magnet. Finally, the fixed end of each PZT/CuBe beam beamwas wasclamped clampedby bythe thetop topand andbottom bottommechanism mechanismframes, frames,while whilethe thefree freeend endof ofeach eachbeam beamwas was connected connectedby bythe theconnector connectorwith withthe theNdFeB NdFeBmagnet. magnet.The Thefabricated fabricatedsensor sensorisisshown shownin inFigure Figure2.2.The The dimensions of the sensor and its components are summarized in Table 1. dimensions of the sensor and its components are summarized in Table 1.

(a)

(b) Magnet with Connector

Magnet with Connector Cantilever Beams

Cantilever Beams

Mechanical Frame

Mechanical Frame

Figure2.2.The Thephotographs photographsofofthe thefabricated fabricatedAC ACmagnetic magneticsensor: sensor:(a) (a)top topview viewand and(b) (b)isotropic isotropicview. view. Figure Table1.1.The Thedimensions dimensionsof ofthe theAC ACmagnetic magneticsensor sensorand andits itscomponents. components. Table

TheSensor Sensorand and Components The Components Sensor

Sensor

CuBe Sheet

Size Size (length × width × thickness)

(length × width × thickness) 3 3 2 cm 2 ×22××2 2×cm

(length × width × thickness)

CuBe Sheet (length × width × thickness) (sandwiched bybymechanical 3.5 × 3.5 × 0.25 mm3 (sandwiched mechanical clamp) 3.5 × 3.5 × 0.25 mm3 clamp) PZT Sheet (length × width × thickness)

(sandwiched bySheet mechanical clamp) (length × 3.5width × 3.5×× thickness) 1 mm3 PZT (sandwiched by Sheet mechanical CuBe (length width × thickness) 3 3.5 ××3.5 × 1 mm clamp) (outside of mechanical clamp) 5.5 × 3.5 × 0.25 mm3 CuBe (length × width × thickness) PZTSheet Sheet (length × width × thickness) 3 (outside clamp) 5.5 ×43.5 × 0.25 (outsideofofmechanical mechanical clamp) × 3.5 × 1 mm mm3 2 ××πthickness) PZT Sheet (length(radius × width × high) (outside of mechanical clamp) 4 × 3.52 × 1 mm3 3 Magnet 4.35 × π × 6 mm 2 (radius π× With a 12 × ×π × 6high) mm3 hole 2 Magnet 4.35 × π 2× 6 mm3 (radius × π × high) Connector With a 12 2× π × 6 mm3 hole 4.0 × π × 0.49 mm3 Connector

4. Testing

(radius2 × π × high) 4.02 × π × 0.49 mm3

Weight Weight 7.1 7.1 g g 0.01 0.01 g g 0.06 g 0.06 g

0.02 g 0.02 g g 0.07 0.07 g

2.27 g 2.27 g

0.01 g 0.01 g

4. Testing 4.1. Magnetic Sensing Functions

testing the performance of the AC magnetic sensor, we have to 4.1. Before Magneticquantitatively Sensing Functions qualitatively confirm that the deflection, bending, and torsion of the cross-linked-beams of the sensor, Before quantitatively testing the performance of the AC magnetic sensor, we have to qualitatively under different magnetic fields, are in line with our design (i.e., design validation). To do this, we confirm that the deflection, bending, and torsion of the cross-linked-beams of the sensor, under used a high-speed camera with a controlling computer [27] in order to record the motion of the sensor different magnetic fields, are in line with our design (i.e., design validation). To do this, we used under the x-axis magnetic field, which is produced using a standard commercial electromagnet with a high-speed camera with a controlling computer [27] in order to record the motion of the sensor a function generator. From the motion recording, we were able to observe and analyze the beams’ corresponding deflections, bending, and torsion.

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under the x-axis magnetic field, which is produced using a standard commercial electromagnet with a function generator. From the motion recording, we were able to observe and analyze the beams’ After qualitative test, a quantitative test is conducted. An illustration and photograph of the corresponding deflections, bending, and torsion. quantitative test of the sensor are shown in Figure 3. The testing process is described as follows: First, After qualitative test, a quantitative test is conducted. An illustration and photograph of the according to the specification of the Helmholtz coils, we apply an AC voltage to the Helmholtz coils quantitative test of the sensor are shown in Figure 3. The testing process is described as follows: First, in order to produce an AC magnetic field. Consequently, we placed the Hall probe of a gauss meter according to the specification of the Helmholtz coils, we apply an AC voltage to the Helmholtz coils in to the central location of the coils in order to measure the magnitude of the AC magnetic field, as order to produce an AC magnetic field. Consequently, we placed the Hall probe of a gauss meter to shown in Figure 3a,b. After this, we tuned the frequency and magnitude of the applied AC voltage the central location of the coils in order to measure the magnitude of the AC magnetic field, as shown to ensure that the magnitude of the produced AC magnetic field remained the same at different in Figure 3a,b. After this, we tuned the frequency and magnitude of the applied AC voltage to ensure frequencies; that is, providing a constant AC magnetic field at different frequencies to the central part that the magnitude of the produced AC magnetic field remained the same at different frequencies; that of the coils. After using the Hall probe of the gauss meter to measure the magnitude of the magnetic is, providing a constant AC magnetic field at different frequencies to the central part of the coils. After field produced by the coils, we removed the Hall probe and the gauss meter, and, consequently, used using the Hall probe of the gauss meter to measure the magnitude of the magnetic field produced by a position aligner in order to place the magnetic sensor in the same central location of the coils for the coils, we removed the Hall probe and the gauss meter, and, consequently, used a position aligner further testing, as shown in Figure 3c,d. Under the AC magnetic field, the sensor produced a in order to place the magnetic sensor in the same central location of the coils for further testing, as corresponding piezoelectric voltage output. Therefore, by setting the reference signal as the AC shown in Figure 3c,d. Under the AC magnetic field, the sensor produced a corresponding piezoelectric voltage produced by the AC power supply, we used a lock-in amplifier in order to measure voltage output. Therefore, by setting the reference signal as the AC voltage produced by the AC piezoelectric voltage output. Based on the testing setup, the resonant frequency of the sensor was power supply, we used a lock-in amplifier in order to measure piezoelectric voltage output. Based measured using a frequency sweeping approach. During frequency sweeping, the gauss meter was on the testing setup, the resonant frequency of the sensor was measured using a frequency sweeping used to ensure that the magnitude of the magnetic field remained the same when the frequency of approach. During frequency sweeping, the gauss meter was used to ensure that the magnitude of the the magnetic field was varied. By comparing the frequencies of the AC magnetic field and the voltage magnetic field remained the same when the frequency of the magnetic field was varied. By comparing output of the sensor, the resonant frequency of the sensor could be determined. After obtaining the the frequencies of the AC magnetic field and the voltage output of the sensor, the resonant frequency resonant frequency of the sensor, sensitivity measurements of the sensor were conducted. To measure of the sensor could be determined. After obtaining the resonant frequency of the sensor, sensitivity the sensitivity of the sensor, we recorded the voltage outputs of the sensor when it was operated in measurements of the sensor were conducted. To measure the sensitivity of the sensor, we recorded the resonant mode, but with different magnetic field magnitudes. According to the correlation the voltage outputs of the sensor when it was operated in the resonant mode, but with different between the recorded voltage response and the magnetic field magnitudes, the sensitivity of the magnetic field magnitudes. According to the correlation between the recorded voltage response and sensor could be estimated. the magnetic field magnitudes, the sensitivity of the sensor could be estimated.

(a)

(b) Helmholtz coils Gauss Meter

Hall Probe

Helmholtz coils

Gauss Meter Probe

AC Power Supply

Gauss Meter

(c)

(d) Helmholtz coils

Helmholtz coils

Magnetic Sensor

Helmholtz coils

Lock-in Amplifier

AC Power Supply

Magnetic Sensor Helmholtz coils

Figure3.3.Testing Testingsetup setupof ofthe thequantitative quantitativetest. test.(a) (a)Illustration Illustrationand and(b) (b)photograph photographofofthe theHall Hallprobe probe Figure placedininthe thecentral centralarea areaof ofthe thecoils. coils.(c) (c)Illustration Illustrationand and(d) (d)photograph photographofofthe theAC ACmagnetic magneticsensor sensor placed placed in the central area of the coils. placed in the central area of the coils.

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Piezoelectric/Vibrational-Energy-Harvesting Functions Functions 4.2. Piezoelectric/Vibrational-Energy-Harvesting 4.2. A photograph photograph of of the the piezoelectric/vibrational piezoelectric/vibrational energy-harvesting energy-harvesting test [27] isisshown shown in Figure 4. piezoelectric/vibrational energy-harvestingtest test[27] [27]is shownin inFigure Figure4. 4. A A function functiongenerator generatorand anda power power amplifier were used to operate operate Ling Dynamic Dynamic Systems (LDS) amplifier were used to operate a Ling Systems (LDS) shaker A function generator and aa power amplifier were used to aa Dynamic Ling Systems (LDS) shaker with different different orientations in generate order to to x-axis generate x-axis and z-axis z-axis in vibrations inrespectively, the sensor, sensor, with different orientations in order to andx-axis z-axis and vibrations the sensor, shaker with orientations in order generate vibrations in the respectively, as shown shown inAn Figure 4a,b. An Anwas oscilloscope was used used to record record the voltage voltage outputs of the the as shown in Figure 4a,b.in oscilloscope used to record the voltage outputs of theoutputs sensor. of respectively, as Figure 4a,b. oscilloscope was to the sensor. sensor. (b) (a) (b) (a) (a) (b)

Magnetic Magnetic Sensor Sensor

Power Function LDS Power Function LDS Amplifier Oscilloscope Generator Generator Amplifier Shaker Oscilloscope Shaker

Magnetic Magnetic Sensor Sensor

Power Function LDS Power Function LDS Amplifier Oscilloscope Generator Generator Amplifier Shaker Oscilloscope Shaker

Figure 4. 4. Testing Testingsetup setup of the piezoelectric/vibrational energy-harvesting functions of the sensor sensor Figure Testing setup piezoelectric/vibrational energy-harvesting functions the Figure ofof thethe piezoelectric/vibrational energy-harvesting functions of theof sensor under under (a) vibration x-axis vibration vibration and (b)vibration. z-axis vibration. vibration. under (a) x-axis (b) z-axis (a) x-axis and (b)and z-axis

5. Results Results and Discussion 5. 5. Results and and Discussion Discussion 5.1. Magnetic-Sensing Magnetic-Sensing Functions 5.1. Magnetic-Sensing Functions Functions 5.1. For design designvalidation, validation,the the images captured and the movie movie recorded by, ausing using high-speed For images captured andand the movie recorded by, using high-speed camera, For design validation, the images captured the recorded by, aa high-speed camera, areinshown shown in Figure 5. In In Figure 5, by by comparing comparing the of axial line of the the NdFeB magnet (i.e., are shown Figurein 5.Figure In Figure 5, Figure by comparing the axial line theline NdFeB magnet (i.e., green (i.e., line) camera, are 5. 5, the axial of NdFeB magnet green line) and the reference line fixed on the background (i.e., blue line), we observe that the NdFeB and the reference fixed on thefixed background (i.e., blue line), that the NdFeB magnet of green line) and theline reference line on the background (i.e., we blueobserve line), we observe that the NdFeB magnet of experiences the sensor sensor experiences experiences small angle angle rotation with aa small small horizontal displacement. displacement. This the sensor a small angleaa rotation withrotation a small horizontal displacement. This shows thatThis the magnet of the small with horizontal showsand thaty-axis the x-axis x-axis and y-axis y-axis piezoelectric beams of the the slight sensorbending experience slight bending bending and x-axis piezoelectric beams of the sensor experience and torsion, respectively shows that the and piezoelectric beams of sensor experience slight and torsion, respectively (i.e., the sensor design is proven). (i.e., the respectively sensor design is proven). torsion, (i.e., the sensor design is proven).

(a) (a)

(b) (b)

Figure 5. 5. The captured images (front view) show the beam beam motion of the theof AC magnetic sensor under: under: Figure captured images (front view) show the motion of AC magnetic sensor Figure 5.The The captured images (front view) show the beam motion the AC magnetic sensor (a) zero magnetic field (initial state) and (b) x-axis magnetic field. Note: The shooting speed and (a) zero(a) magnetic field (initial state) and x-axis magnetic field. field. Note:Note: The shooting speedspeed and under: zero magnetic field (initial state)(b) and (b) x-axis magnetic The shooting sensitivity for image capture of the high-speed camera is 500 fps and ISO-1600, respectively. sensitivity for image capture of the high-speed camera is is 500 and sensitivity for image capture of the high-speed camera 500fps fpsand andISO-1600, ISO-1600, respectively. respectively. Additionally, for for videos, videos, please please see see the the Supplementary Supplementary Materials Materials for for a clearer clearer motion motion of of the the sensor sensor Additionally, please Supplementary Materials for aa clearer motion of the sensor under an an in-plane in-plane magnetic magnetic field. field. under under an in-plane magnetic

Regarding to to the the resonant resonant frequency frequency measurement, measurement, we we first first measure measure the the magnetic-field-induced magnetic-field-induced Regarding resonant frequency. That is, we apply the magnetic field with same field magnitude but varying varying the the resonant frequency. That is, we apply the magnetic field with same field magnitude but magnetic field’s field’s frequency frequency (with (with an an incremental incremental step step of of 11 Hz) Hz) from from 20 20 Hz Hz to to 200 200 Hz Hz for for the the x-axis x-axis magnetic

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Regarding to the resonant frequency measurement, we first measure the magnetic-field-induced resonant frequency. That is, we apply the magnetic field with same field magnitude but varying the magnetic field’s frequency (with an incremental step of 1 Hz) from 20 Hz to 200 Hz for the x-axis magnetic Sensors field2017, test17,and The 308 from 20 Hz to 250 Hz for the z-axis magnetic field test, respectively. 7 of 14 measured voltage outputs of the sensor under the magnetic field with different frequencies are plotted field test and from 20 Hz to 250 Hz for the z-axis magnetic field test, respectively. The in Figure 6.magnetic Through the plots, the magnetic-field-induced resonant frequency of the sensor is obtained measured voltage outputs of the sensor under the magnetic field with different frequencies are when the voltage output isThrough maximized. Thetheresults show the resonant frequency plotted in Figure 6. the plots, magnetic-field-induced resonant frequencyofofthe the sensor sensor is 100 Hz and 142 Hz for the x-axis and z-axis magnetic field test, respectively. is obtained when the voltage output is maximized. The results show the resonant frequency of the sensor is 100 Hz and 142 Hz for the x-axis and z-axis magnetic field test, respectively.

(a)

Input: x-axis magnetic field (sine-wave, 3.2 gauss) Output:PZT-1 (x-axis)

100 Hz

158 Hz

Voltage, Vpp(mV)

Voltage, Vpp(mV)

30 25 20 15 10 5 0 0

50

100 150 Frequency (Hz)

200

9 8 7 6 5 4 3 2 1 0

100 Hz

158 Hz

0

50

100 150 Frequency (Hz)

200

Input: z-axis magnetic field (sine-wave, 3.2 gauss) Output:PZT-1

(c) Voltage, Vpp(mV)

Input: x-axis magnetic field (sine-wave, 3.2 gauss) Output:PZT-2 (y-axis)

(b)

11 10 9 8 7 6 5 4 3 2 1 0

142 Hz

0

50

217 Hz

100 150 200 Frequency (Hz)

250

Figure 6. Experimental results of the measurement of the magnetic-field-induced resonant frequency: Figure 6. Experimental results of the measurement of the magnetic-field-induced resonant frequency: The voltage output vs. magnetic-field-frequency plots of the AC magnetic sensor under: (a,b) x-axis The voltage output vs.(sine-wave, magnetic-field-frequency plots of the AC magnetic magnetic field 3.2 gauss), and (c) z-axis magnetic field (sine-wave, 3.2 sensor gauss). under: (a,b) x-axis magnetic field (sine-wave, 3.2 gauss), and (c) z-axis magnetic field (sine-wave, 3.2 gauss).

After the resonant frequencies of the sensor were obtained, and we applied the x-axis and z-axis magnetic fields with different magnitudes at the sensor’s resonant frequency to the sensor, Afterrespectively. the resonant of versus the sensor were obtained, and applied the x-axis Thefrequencies voltage outputs magnetic-field magnitudes arewe plotted and correlated by and z-axis magnetic fields with different magnitudes at the 7,sensor’s resonant the sensor, respectively. curve fitting, as shown in Figure 7. In Figure the voltage outputs frequency have a goodto linearity with the magnetic-field magnitudes. The voltage outputs with the are sensitivities the PZT-1, PZT-2,by PZT-3, The voltage outputs versus magnetic-field magnitudes plottedofand correlated curve fitting, PZT-4 of the sensor under the x-axis magnetic field (sine-wave, 100 Hz, 0.2–3.2 gauss) are 1.13– as shown and in Figure 7. In Figure 7, the voltage outputs have a good linearity with the magnetic-field 26.15 mV with 8.79 mV/gauss, 0.43–8.21 mV with 2.66 mV/gauss, 1.33–27.43 mV with 8.48 mV/gauss, magnitudes. voltage outputs with the sensitivities of the outputs PZT-1,ofPZT-2, and the PZT-4 of the and The 0.45–8.08 mV with 2.58 mV/gauss, respectively. The voltage the PZTPZT-3, sheets along sensor under the x-axis magnetic field (sine-wave, 100 Hz,than 0.2–3.2 aresheets 1.13–26.15 mV x-axis (PZT-1, PZT-3) are approximately three-times larger thosegauss) of the PZT along the y- with 8.79 axis0.43–8.21 (PZT-2, PZT-4). This is 2.66 reasonable, as researchers have reported that the piezoelectric output of 0.45–8.08 mV/gauss, mV with mV/gauss, 1.33–27.43 mV with 8.48 mV/gauss, and PZT sheets in the bending mode is larger than the torsion mode [31], and our sensor’s PZT sheets do mV with 2.58 mV/gauss, respectively. The voltage outputs of the PZT sheets along the x-axis (PZT-1, experience this. Thus, the voltage outputs of the PZT sheets along the x-axis (PZT-1, PZT-3) and the PZT-3) arey-axis approximately larger than those of the PZT they-axis y-axis (PZT-2, (PZT-2, PZT-4)three-times can be correlated to indicate the magnitude of the sheets applied along x-axis and PZT-4). This is reasonable, as researchers reported that thethepiezoelectric output of PZT sheets magnetic field, respectively. In Figure 7c,have the voltage outputs with sensitivities of PZT-1, PZT-2, PZT-3,mode and PZT-4 of the than sensorthe under the z-axis magnetic field our (sine-wave, 142PZT Hz, 0.2–3.2 in the bending is larger torsion mode [31], and sensor’s sheetsgauss) do experience

this. Thus, the voltage outputs of the PZT sheets along the x-axis (PZT-1, PZT-3) and the y-axis (PZT-2, PZT-4) can be correlated to indicate the magnitude of the applied x-axis and y-axis magnetic field, respectively. In Figure 7c, the voltage outputs with the sensitivities of PZT-1, PZT-2, PZT-3, and PZT-4 of the sensor under the z-axis magnetic field (sine-wave, 142 Hz, 0.2–3.2 gauss) were 1.31–8.92 mV with

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were 1.31–8.92 mV withmV 2.63with mV/gauss, 1.33–9.31 mV with 2.73 mV/gauss, mV with 2.79 2.63 mV/gauss, 1.33–9.31 2.73 mV/gauss, 1.22–9.55 mV with 2.791.22–9.55 mV/gauss, and 1.44–9.63 mV/gauss, and 1.44–9.63 mV with 2.80 mV/gauss, respectively. Because of the geometric symmetry mV with 2.80 mV/gauss, respectively. Because of the geometric symmetry of the cross-linked beams, of the cross-linked beams, the voltage output of each PZT sheet is similar. Therefore, the voltage the voltage output of each PZT sheet is similar. Therefore, the voltage outputs of any of the four outputs of any of the four PZT sheets can be correlated in order to indicate the magnitude of the PZT applied sheets can be magnetic correlated in order to according indicate the magnitude the applied z-axis magnetic z-axis field. Finally, to these results, of our sensor successfully achieves field. Finally, according to these results, our sensor successfully achieves three-axis magnetic sensing. three-axis magnetic field sensing. Furthermore, because the voltage outputs are large and arefield directly Furthermore, thesheets voltage outputs are magnetic large andfields, are directly produced the PZT produced because by the PZT under different the sensor is able toby perform thesheets energyunder different magnetic fields, the sensor is able to perform the energy harvesting function. harvesting function.

Input: x-axis magnetic field Output:PZT-1, PZT-3 (x-axis direction)

(a) 25

20 15 10 5

PZT-1 y = 8.79 x - 0.87 PZT-3 y = 8.48 x - 1.37

0 0

1

2

3

Input: x-axis magnetic field Output:PZT-2, PZT-4 (y-axis direction)

9

Voltage, Vpp(mV)

Voltage, Vpp(mV)

30

(b) 8 7 6 5 4 3 2

PZT-2 y = 2.66 x - 0.36 PZT-4 y = 2.58 x - 0.34

1 0 4

0

1

Magnetic-field (gauss)

Voltage, Vpp(mV)

(c)

2

3

4

Magnetic-field (gauss)

Input: z-axis magnetic field Output:PZT-1 to PZT-4

11 10 9 8 7 6 5 4 3 2 1 0

PZT-1 PZT-2 PZT-3 PZT-4 0

1

2

y = 2.63 x + 0.48 y = 2.73 x + 0.48 y = 2.79 x + 0.29 y = 2.80 x + 0.40 3

4

Magnetic-field (gauss) Figure 7. Experimental results theperformance performance test magnetic sensor. TheThe voltage outputs Figure 7. Experimental results ofofthe testofofthe theAC AC magnetic sensor. voltage outputs of the AC magnetic sensor under: (a,b) the x-axis magnetic fields (sine-wave, 100 Hz, 0.2–3.2 gauss), of the AC magnetic sensor under: (a,b) the x-axis magnetic fields (sine-wave, 100 Hz, 0.2–3.2 gauss), andthe (c) z-axis the z-axis magnetic fields (sine-wave, 142 142 Hz, Hz, 0.2–3.2 and (c) magnetic fields (sine-wave, 0.2–3.2gauss). gauss).

The sensitivity of our magnetic-force-interaction-based AC magnetic sensor, and other The sensitivity of our magnetic-force-interaction-based magnetic sensor, representative magnetic-force-interaction-based magnetic sensors,AC are summarized in Tableand 2 forother representative sensors, are output summarized in Table comparison.magnetic-force-interaction-based Because the volume of each sensor is magnetic different and the voltage is proportional to 2 for the volume of the sensor/sensing-element (due to definition power density), the is sensitivity of comparison. Because the volume of each sensor is the different andofthe voltage output proportional to each sensor has to be normalized for a reasonable comparison. For normalization, the sensitivity of the volume of the sensor/sensing-element (due to the definition of power density), the sensitivity of sensor is to normalized using the of the movable structure and the entire device. shown each the sensor has be normalized forvolume a reasonable comparison. For normalization, theAs sensitivity of in Table 2, the normalized sensitivity of our sensor, and those of the representative sensors, are in a the sensor is normalized using the volume of the movable structure and the entire device. As shown comparable level (note: for the noise and temperature issues, please see Supplemental Materials). in Table 2, the normalized sensitivity of our sensor, and those of the representative sensors, are in Moreover, our sensor is the only one that uses a compact single mechanism in order to achieve threea comparable levelsensing, (note: while for thethe noise temperature issues, see Supplemental Materials). axis magnetic otherand sensors have to use three,please individual, single-axis magnetic Moreover, sensor is thethe only one that uses compact single mechanism in order to achieve sensorsour in order to achieve same purpose. This ademonstrates that our sensor has more advantages three-axis magnetic sensing, whilemechanism, the other sensors have to less use volume, three, individual, single-axis magnetic (i.e., compact single-mechanical dual functions, easier assembly works, etc.) sensors order tosensors. achieve the same purpose. This demonstrates that our sensor has more advantages thaninthe other

(i.e., compact single-mechanical mechanism, dual functions, less volume, easier assembly works, etc.) than the other sensors.

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Table 2. Comparison of the sensitivity of our, and other representative, magnetic-force-interactionbased magnetic sensors. Magnetic Sensors

Our Sensor

Han et al. [21]

Yu et al. [22]

Features

Three-Axis

Single-Axis

Single-Axis

VolMS

(mm3 )

103.27

2460

a

1733.33 a

VolSensor (mm3 )

8000

25,800 a

36,000 a

Sin-plane (mV/gauss)

PZT-1: 8.79 PZT-3: 8.48

Sample S1 b : 790 a Sample S2 b : 920 a Sample S3 b : 960 a

N/A

PZT-1: 1.02 e PZT-3: 0.98 e

Sample S1 b : 3.84 a,e Sample S2 b : 4.48 a,e Sample S3 b : 4.66 a,e

N/A

PZT-1: 1.02 e PZT-3: 0.98 e

Sample S1 b : 9.46 a,e Sample S2 b : 11.01 a,e Sample S3 b : 11.49 a,e

N/A

Normalized Sin-plane by VolMS

c

Normalized Sin-plane by VolSensor

d

PZT-1: PZT-2: PZT-3: PZT-4:

2.63 2.73 2.79 2.80

N/A

7a

c

PZT-1: PZT-2: PZT-3: PZT-4:

0.96 f 0.99 f 1.01 f 1.02 f

N/A

0.15 a,f

Normalized Sout-of-plane by VolSensor

PZT-1: PZT-2: PZT-3: PZT-4:

0.96 f 0.99 f 1.01 f 1.02 f

N/A

0.19 a,f

Sout-of-plane (mV/gauss)

Normalized Sout-of-plane by VolMS

d

a

Value estimated or extrapolated from data in reference; b Sample S1, S2, and S3 uses same pair of PZT cantilever beams but with different gap/spacing distance of 4 mm, 8 mm, and 12 mm, respectively; c Divided by the volume of the moveable structure; d Divided by the volume of the sensor. The single-axis sensor has to combine with two single-axis sensors to achieve 3-axis magnetic field sensing (i.e., thus the volume is increased 3-times); e Normalized by setting the average sensitivity of two PZTs of our sensor (i.e., average: 8.635 mV/gauss) as 1 for the normalization base; f Normalized by setting the average sensitivity of four PZTs of our sensor (i.e., average: 2.738 mV/gauss) as 1 for the normalization base. VolMS : volume of the movable structure of the sensor; VolSensor : volume of the sensor; Sin-plane : In-plane sensitivity of the sensor tested in x-axis magnetic field; Sout-of-plane : out-of-plane sensitivity of the sensor tested in z-axis magnetic field.

5.2. Piezoelectric/Vibrational Energy-Harvesting Functions Through vibration-induced resonant frequency testing (i.e., varying the applied vibration’s frequency, from 20 Hz to 190 Hz, for x-axis vibrations, and from 20 Hz to 240 Hz for z-axis vibrations, and then recording the voltage responses), the vibration-induced resonant frequency of the sensor is 100 Hz for the x-axis vibration and 142 Hz for the z-axis vibration, as shown in Figure 8a. When comparing Figures 8a and 6, a slight asymmetry is shown to exist between the curves; this is caused by using different excitations (i.e., magnetic-fields or vibrations) [32]. Furthermore, through optimum load testing [27], the optimum load resistance of PZT-1 and PZT-2 of the sensor, under the x-axis vibration, is 578 kΩ and 758 kΩ, respectively, while the optimum load resistance of each PZT sheet of the sensor under the z-axis vibration is 375 kΩ, as shown in Figure 8b. Furthermore, in Figure 8c, the root-mean-square (RMS) voltage outputs of PZT-1, PZT-2, PZT-3, and PZT-4 of the sensor under x-axis vibration (sine-wave, 100 Hz, 3.5 g, tested at resonant and optimum loads), are approximately 439 mV, 59.7 mV, 424 mV, and 61.9 mV, respectively. In Figure 8d, the RMS voltage outputs of PZT-1, PZT-2, PZT-3, and PZT-4 of the sensor under z-axis vibration (sine-wave, 142 Hz, 3.8 g, tested at resonant and optimum loads) are approximately 138 mV, 132 mV, 128 mV, and 127 mV, respectively. By using the

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equation P = V2 /R [27,33] (P: Average power output, V: RMS voltage output, R: Load resistance), the average power outputs of PZT-1, PZT-2, PZT-3, and PZT-4 of the sensor under the x-axis vibration (sine-wave, 100 Hz, 3.5 g, tested at resonant and optimum loads) are approximately 0.333 µW, 0.005 µW, 0.311 µW, and 0.005 µW, respectively. The average power outputs of PZT-1, PZT-2, PZT-3, and PZT-4 of the sensor under the z-axis vibration (sine-wave, 142 Hz, 3.8 g, tested at resonant and optimum loads) are approximately 0.051 µW, 0.046 µW, 0.044 µW, and 0.043 µW, respectively. According to our previous work [27], there are many sensors that consume ultra-low amounts of power nowadays, in which only few nW (or even a few pW) are sufficient to drive them. The power harnessed by our AC magnetic sensor is able to drive these nW–pW level sensors. These results confirm that our AC magnetic sensor possesses a decent three-axis piezoelectric/vibrational energy-harvesting function.

Figure 8. Test results of the piezoelectric/vibrational energy-harvesting function of the AC magnetic sensor: (a) voltage vs. frequency plots and (b) power vs. load resistance plots. The voltage outputs of each piezoelectric lead-zirconate-titanate (PZT) sheet of the sensor under (c) x-axis vibration and (d) z-axis vibration.

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Finally, after the magnetic sensing function and the piezoelectric/vibrational energy-harvesting function of our AC magnetic sensor were performed, we provided guidelines of our sensor for applications. In general, our sensor’s magnetic-sensing function is enabled when using applications that typically require magnetic sensors (for example, vehicle/traffic monitoring, processes monitoring, and security system applications need to use three-axis magnetic sensors). However, the magnetic sensing function can be disabled when the application is turned off. Thus, in this condition, the vibrational energy-harvesting function of the senor can be enabled in order to harness ambient vibrational energy. To switch (i.e., enable or disable) between the magnetic sensing function and the vibration energy-harvesting function, a set of program-controlled switches are used (shown in Figure 9). When turning on the application, program-controlled switch 1 is set to a closed state, and switch 2 is set to an open state, as shown in Figure 9a. Therefore, the sensor enables the magnetic-sensing function and, thus, the voltage output is transmitted to Vout1 . When the magnetic-sensing function is disabled, or is in standby mode, program-controlled switch 1 is set to an open state and switch 2 is set to a closed state, as shown in Figure 9b. Therefore, the sensor enables the piezoelectric/vibrational energy-harvesting function, and, thus, the voltage output is transmitted to Vout2 . By using the setting protocols, the sensor achieves switching between the magnetic-sensing function and the piezoelectric/vibrational energy-harvesting function. The switching significantly improves the energy utilization of the sensor (note: some researchers may have a concern on a mechanical-coupling/decoupling issue when our sensor utilizes a single mechanical-mechanism to demonstrate two functions. For this issue, please see Supplemental Materials). To illustrate the importance of the two functions (sensing and harvesting) of our sensor, we found some applications that require magnetic field sensing, and that also produce vibrations for energy harvesting; this is to say that these applications match our sensor’s features on both magnetic-field sensing and vibrational energy-harvesting. The first application was intelligent vehicle monitoring [34]. Magnets are fixed to the movable structures of the vehicle’s components. When the vehicle components are used, AC magnetic fields are produced. A magnetic sensor is needed to sense the AC magnetic fields in order to monitor the operating conditions of vehicle components (such as steering-torque sensing/monitoring and motor-position sensing/monitoring). In addition, while operating vehicle components, the moveable structures of the vehicle components generate a periodic rotating movement, which causes rotational vibration. In addition, while driving the vehicle, three axial vibrations are generated in the vehicle. Due to these vibrations, our sensor can detect magnetic fields and also harvest/convert the vibrations into electrical energy. The second application used magnetic sensors for vibration sensing in milling-machining monitoring [35]. Wear/damage to ball bearings from machining spindles can produce a local magnetic flux change. A magnetic sensor is needed in order to sense the magnetic flux change to monitor the health of the ball bearings and the spindle. In addition, while milling, rotational vibration occurred in the spindle and three axial vibrations occurred in the milling-machine. According to this, our sensor can detect magnetic fields, and also harvest/convert vibrations to electrical energy. The third application is a speed measurement of a differential gear system [36]. Rotation of the rotary gear produces periodic and alternative magnetic fluxes. A magnetic sensor is needed in order to sense the periodic/alternative magnetic fluxes, so as to measure the rotational speed of the rotary gear. In addition, while operating the rotary gear, rotational motion is produced by the gears and three axial vibrations are produced by the entire gears system. Accordingly, our sensor can detect the magnetic fields and also harvest/convert the motion and vibrations into electrical energy. In summary, our sensor’s features can match the above-mentioned applications; which is to say that our sensor can detect AC magnetic fields produced by the critical components of the vehicle/machine/system, and also harvest the produced vibrations. In addition, when the magnetic-sensing and energy-harvesting functions are performed by the sensor, some researchers may have a concern on the sensors’ power-budget analysis (i.e., the comparison between energy required by the sensor read-out electronics and the energy that can be potentially harvested by the sensor). For the power-budget analysis of our sensor, please see Supplemental Materials.

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(a)

Program Controlled Switch 1

＋ Vout1

－

Magnetic Sensing Function

Program Controlled Switch 2

＋ Vout2

－

Magnetic Sensor

Program Controlled Switch 1

(b)

Disconnect

＋ Vout1

－

Disconnect

Program Controlled Switch 2

＋ Magnetic Sensor

Vout2

－

Piezoelectric/Vibrational Energy-Harvesting Function

Figure 9. Illustration of the electrical switching approach, which uses a set of program-controlled Figure 9. Illustration of the electrical switching approach, which uses a set of program-controlled switches and setting protocols to switch between the functions of the sensor: (a) enabling the sensing switches and setting protocols to switch between the functions of the sensor: (a) enabling the sensing function while disabling the energy-harvesting function, and (b) disabling the sensing function while function while disabling the energy-harvesting function, and (b) disabling the sensing function while enabling the energy-harvesting function. enabling the energy-harvesting function.

6. Conclusions Conclusions 6. In this this paper, three-axis AC AC magnetic magnetic sensor sensor with with In paper, aa miniature miniature magnetic-force-based, magnetic-force-based, three-axis piezoelectric/vibrationalenergy-harvesting energy-harvestingfunctions functions was was successfully successfully demonstrated. demonstrated. By By using using aa piezoelectric/vibrational high-speed camera, the motion of the movable structure of the magnetic sensor was qualitatively high-speed camera, the motion of the movable structure of the magnetic sensor was qualitatively confirmed, and, and, thus, thus, our our design Furthermore, the the results results of of the the quantitative quantitative tests tests confirmed, design was was verified. verified. Furthermore, showed that that the the sensor achieved three-axis three-axis magnetic-field magnetic-field sensing sensing and and three-axis showed sensor successfully successfully achieved three-axis piezoelectric/vibrational energy harvesting. According to these features, the sensor could be be an an piezoelectric/vibrational energy harvesting. According to these features, the sensor could important and and alternative alternative approach approach for for three-axis three-axis AC AC magnetic magnetic sensors sensors possessing possessing energy energy harvesting harvesting important functions for intelligent vehicle/traffic monitoring, processes monitoring, security systems, and so soon. on. functions for intelligent vehicle/traffic monitoring, processes monitoring, security systems, and Supplementary Materials: The Supplementary Materials are available online at http://www.mdpi.com/1424Supplementary Materials: The Supplementary Materials are available online at http://www.mdpi.com/14248220/17/2/308/s1. 8220/17/2/308/s1. Acknowledgments: This work is supported by the Ministry of Science and Technology, Taiwan (Grant Number: 105-2628-E-009-001-MY2). Chiao-Fang Hungby would like to thank Chin-Chung Chen for Taiwan helping(Grant the quantitative Acknowledgments: This work is supported the Ministry of Science and Technology, Number: tests of magnetic-sensing functions. 105-2628-E-009-001-MY2). Chiao-Fang Hung would like to thank Chin-Chung Chen for helping the quantitative tests of magnetic-sensing functions. Hung designed the device, designed and conducted all experiments/ Author Contributions: Chiao-Fang fabrication/testing/analysis, analyzed data and results, wrote manuscript, conducted literature review, conducted Author Contributions: Chiao-Fang Hung designed the device, designed and conducted all most details of the work. Po-Chen Yeh measured/calculated power consumption, conducted power budget experiments/fabrication/testing/analysis, analyzed data and results, wrote manuscript, conducted literature analysis, partially helped on data-analysis/manuscript-writing/literature-review. Tien-Kan Chung conceived and designed the device, designed andwork. guided all experiments/fabrication/testing/analysis, analyzed data and review, conducted most details of the Po-Chen Yeh measured/calculated power consumption, conducted results, wrote conductedhelped literature guided direction and all details of the work. Tien-Kan power budgetmanuscript, analysis, partially on review, data-analysis/manuscript-writing/literature-review. Chung conceived andThe designed device, guided all experiments/fabrication/testing/analysis, Conflicts of Interest: authorsthe declare no designed conflict ofand interest. analyzed data and results, wrote manuscript, conducted literature review, guided direction and all details of the work.

Conflicts of Interest: The authors declare no conflict of interest.

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References 1.

2. 3.

4.

5.

6.

7.

8. 9. 10. 11.

12. 13.

14.

15. 16.

17.

18.

Reig, C.; Cubells-Beltran, M.D.; Munoz, D.R. Magnetic Field Sensors Based on Giant Magnetoresistance (GMR) Technology: Applications in Electrical Current Sensing. Sensors 2009, 9, 7919–7942. [CrossRef] [PubMed] Ouyang, Y.; He, J.L.; Hu, J.; Wang, S.X. A Current Sensor Based on the Giant Magnetoresistance Effect: Design and Potential Smart Grid Applications. Sensors 2012, 12, 15520–15541. [CrossRef] [PubMed] Yeh, P.C.; Chung, T.K.; Lai, C.H.; Wang, C.M. A magnetic-piezoelectric smart material-structure utilizing magnetic force interaction to optimize the sensitivity of current sensing. Appl. Phys. A-Mater. Sci. 2016, 122, 29. [CrossRef] Jogschies, L.; Klaas, D.; Kruppe, R.; Rittinger, J.; Taptimthong, P.; Wienecke, A.; Rissing, L.; Wurz, M.C. Recent Developments of Magnetoresistive Sensors for Industrial Applications. Sensors 2015, 15, 28665–28689. [CrossRef] [PubMed] Wong, T.H.; Chung, T.K.; Liu, T.W.; Chu, H.J.; Hsu, W.Y.; Yeh, P.C.; Chen, C.C.; Lee, M.S.; Yang, Y.S. Electromagnetic/Magnetic-Coupled Targeting System for Screw-Hole Locating in Intramedullary Interlocking-Nail Surgery. IEEE Sens. J. 2014, 14, 4402–4410. [CrossRef] Lee, M.S.; Hsu, P.J.; Sun, A.; Wong, T.H.; Hsu, W.; Chung, T.K. A Removable Mechanism for Positioning Magnet for Distal Locking in Intramedullary Nailing Surgery. In Proceedings of the 14th International Federation for the Promotion of Mechanism and Machine Science World Congress (IFToMM 2015), Taipei, Taiwan, 25–30 October 2015. Chung, T.K.; Yeh, P.C.; Lee, H.; Lin, C.M.; Tseng, C.Y.; Lo, W.T.; Wang, C.M.; Wang, W.C.; Tu, C.J.; Tasi, P.Y.; et al. An Attachable Electromagnetic Energy Harvester Driven Wireless Sensing System Demonstrating Milling-Processes and Cutter-Wear/Breakage-Condition Monitoring. Sensors 2016, 16, 269. [CrossRef] [PubMed] SMART DIGITAL MAGNETOMETER HMR2300—Farnell. Available online: http://www.farnell.com/ datasheets/46088.pdf (accessed on 13 January 2017). Fontana, M.; Salsedo, F.; Bergamasco, M. Novel Magnetic Sensing Approach with Improved Linearity. Sensors 2013, 13, 7618–7632. [CrossRef] [PubMed] Sanz, R.; Fernandez, A.B.; Dominguez, J.A.; Martin, B.; Michelena, M.D. Gamma Irradiation of Magnetoresistive Sensors for Planetary Exploration. Sensors 2012, 12, 4447–4465. [CrossRef] [PubMed] Chen, Y.J.; Yeh, P.C.; Chung, T.K. A Novel AMR Magnetic Sensor Utilizing Nanoscale Magnetic-Domain Transformation. In Proceedings of the IEEE International Magnetics Conference (INTERMAG), Beijing, China, 11–15 May 2015. Herrera-May, A.L.; Aguilera-Cortes, L.A.; Garcia-Ramirez, P.J.; Manjarrez, E. Resonant Magnetic Field Sensors Based On MEMS Technology. Sensors 2009, 9, 7785–7813. [CrossRef] [PubMed] Chung, T.K.; Wang, H.M.; Chen, Y.J.; Lin, S.H.; Chu, H.J.; Lin, P.J.; Hung, C.F. Magnetic-Field-Assisted Electric-Field-Controlled Rotation of Magnetic Stripe Domains in a Magnetoelectric Ni Microbar/ [Pb(Mg1/3Nb2/3 )O3 ]0.68 -[PbTiO3 ]0.32 heterostructure. Appl. Phys. Express 2016, 9, 043003. [CrossRef] Garcia-Arribas, A.; Gutierrez, J.; Kurlyandskaya, G.V.; Barandiaran, J.M.; Svalov, A.; Fernandez, E.; Lasheras, A.; de Cos, D.; Bravo-Imaz, I. Sensor Applications of Soft Magnetic Materials Based on Magneto-Impedance, Magneto-Elastic Resonance and Magneto-Electricity. Sensors 2014, 14, 7602–7624. [CrossRef] [PubMed] Priya, S.; Ryu, J.; Park, C.S.; Oliver, J.; Choi, J.J.; Park, D.S. Piezoelectric and Magnetoelectric Thick Films for Fabricating Power Sources in Wireless Sensor Nodes. Sensors 2009, 9, 6362–6384. [CrossRef] [PubMed] Kreitmeier, F.; Chashin, D.V.; Fetisov, Y.K.; Fetisov, L.Y.; Schulz, I.; Monkman, G.J.; Shamonin, M. Nonlinear Magnetoelectric Response of Planar Ferromagnetic-Piezoelectric Structures to Sub-Millisecond Magnetic Pulses. Sensors 2012, 12, 14821–14837. [CrossRef] [PubMed] Marauska, S.; Jahns, R.; Kirchhof, C.; Claus, M.; Quandt, E.; Knochel, R.; Wagner, B. Highly sensitive wafer-level packaged MEMS magnetic field sensor based on magnetoelectric composites. Sens. Actuators A-Phys. 2013, 189, 321–327. [CrossRef] Regini, E.; Rosing, T.S. An Energy Efficient Wireless Communication Mechanism for Sensor Node Cluster Heads. In Proceedings of the 2009 International Conference on Intelligent Sensors, Sensor Networks and Information Processing (ISSNIP 2009), Melbourne, Australia, 7–10 December 2009.

Sensors 2017, 17, 308

19. 20. 21. 22. 23.

24. 25. 26.

27. 28.

29.

30.

31. 32. 33. 34.

35.

36.

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Chen, W.; Cao, Y.L.; Xie, J. Piezoelectric and electromagnetic hybrid energy harvester for powering wireless sensor nodes in smart grid. J. Mech. Sci. Technol. 2015, 29, 4313–4318. [CrossRef] Cui, N.Y.; Wu, W.W.; Zhao, Y.; Bai, S.; Meng, L.X.; Qin, Y.; Wang, Z.L. Magnetic Force Driven Nanogenerators as a Noncontact Energy Harvester and Sensor. Nano Lett. 2012, 12, 3701–3705. [CrossRef] [PubMed] Han, J.C.; Hu, J.; Wang, Z.X.; Wang, S.X.; He, J.L. Magnetoelectric effect in shear-mode Pb(Zr, Ti)O3 /NdFeB composite cantilever. Appl. Phys. Lett. 2015, 106, 182901. [CrossRef] Yu, A.F.; Song, M.; Zhang, Y.; Kou, J.Z.; Zhai, J.Y.; Wang, Z.L. A self-powered AC magnetic sensor based on piezoelectric nanogenerator. Nanotechnology 2014, 25, 455503. [CrossRef] [PubMed] Chung, T.K.; Wang, C.M.; Yeh, P.C.; Liu, T.W.; Tseng, C.Y.; Chen, C.C. A Three-Axial Frequency-Tunable Piezoelectric Energy Harvester Using a Magnetic-Force Configuration. IEEE Sens. J. 2014, 14, 3152–3163. [CrossRef] Chen, C.C.; Chung, T.K.; Tseng, C.Y.; Hung, C.F.; Yeh, P.C.; Cheng, C.C. A Miniature Magnetic-piezoelectric Thermal Energy Harvester. IEEE Trans. Magn. 2015, 51, 9100309. Kunz, K.; Enoksson, P.; Stemme, G. Highly sensitive triaxial silicon accelerometer with integrated PZT thin film detectors. Sens. Actuators A-Phys. 2001, 92, 156–160. [CrossRef] Kanda, K.; Iga, Y.; Matsuoka, J.; Jiang, Y.G.; Fujita, T.; Higuchi, K.; Maenaka, K. A Tri-Axial Accelerometer with Structure-Based Voltage Operation by Using Series-Connected Piezoelectric Elements. In Proceedings of the 24th Eurosensor Conference, Linz, Austria, 5–8 September 2010. Hung, C.F.; Chung, T.K.; Yeh, P.C.; Chen, C.C.; Wang, C.M.; Lin, S.H. A Miniature Mechanical-PiezoelectricConfigured Three-Axis Vibrational Energy Harvester. IEEE Sens. J. 2015, 15, 5601–5615. [CrossRef] Three-Axis Magnetic Sensor HMC1043L. Available online: https://aerospace.honeywell.com/~/media/ aerospace/files/datasheet/hmc1043l_3-axis_magnetic_sensor_preliminary.pdf (accessed on 13 January 2017). Seidel, M.; Paprotny, I.; Krishnan, K.; White, R.; Evans, J. AMR Current Sensors for Evaluating the Integrity of Concentric Neutrals in In-Service Underground Power Distribution Cables. In Proceedings of the Conference Record of the 2010 IEEE International Symposium on Electrical Insulation (ISEI), San Diega, CA, USA, 6–9 June 2010. Sosnicki, O.; Porchez, T.; Michaud, G.; Bencheikh, N.; Claeyssen, F. AC magnetic field detection system applied to motion tracking. In Proceedings of the Sensoren und Messsysteme 2010-15, ITG/GMAFachtagung, Nürnberg, Germany, 18–19 May 2010. Yasuhide, S.; Fumio, N. Dynamic bending/torsion and output power of S-shaped piezoelectric energy harvesters. Int. J. Mech. Mater. Des. 2014, 10, 305–311. Li, M.; Rouf, V.T.; Thompson, M.J.; Horsley, D.A. Three-Axis Lorentz-Force Magnetic Sensor for Electronic Compass Applications. J. Microelectromech. Syst. 2012, 21, 1002–1010. [CrossRef] Roundy, S.; Wright, P.K. A piezoelectric vibration based generator for wireless electronics. Smart Mater. Struct. 2004, 13, 1131–1142. [CrossRef] Sensor Solutions for Automotive, Industrial and Consumer Applications. Available online: http://www. infineon.com/dgdl/Infineon-Sensor_Solutions_for_Automotive_Industrial_and+Customer_ Appl_BR-2015. pdf?fileId=5546d4614937379a01495212845c039f (accessed on 24 January 2017). Goh, K.M.; Chan, H.L.; Ong, S.H.; Moh, W.P.; Tows, D.; Ling, K.V. Wireless GMR Sensor Node for Vibration Monitoring. In Proceedings of the 5th IEEE Conference on Industrial Electronics and Applications, Taichung, Taiwan, 15–17 June 2010. Hall Effect Differential Gear Tooth Sensors CYGTS101DC-S. Available online: http://www.hallsensors.de/ CYGTS101DC-S.pdf (accessed on 24 January 2017). © 2017 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).