electric field sensor

Abstract

This disclosure aims to provide an electric field sensor that is particularly suitable for continuing electric field sensing even in harsh environments where large currents and high voltages occur. [Solution] The invention comprises a cantilever having a fixed first end and a second end that is a free end on the opposite side of the first end, and having a piezoelectric film made of a piezoelectric material; a light source fixed to the second end side which is the free end of the cantilever; and a light receiving element provided to receive light from the light source.

Description

[Technical Field]

[0001] This disclosure relates to an electric field sensor. [Background technology]

[0002] Patent Document 1 discloses an electric field measuring sensor including a piezoelectric layer. This electric field measuring sensor includes a positive electret and a negative electret for forming an electrostatic field. Furthermore, this electric field measuring sensor has a detection structure in which a cantilever and a piezoelectric layer are bonded together. An angular difference occurs in the detection structure when it is in a detection environment and when it is in a non-detection environment, and the electric field detection information for that detection environment is determined based on that angular difference. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Chinese Patent Publication No. 115575730 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] Electric field sensors are implemented in various systems, such as wind power generation, railways, and electric vehicles, to monitor their operating status. Electric field sensors used for electric field monitoring must be able to continue normal operation without failure even in harsh environments where sudden high voltages may occur. However, connecting an electric field sensor to the object being measured applies high current and high voltage to the sensor. For example, connecting an electric field sensor to a generator via a voltage divider circuit can cause sudden high voltages to be applied to the sensor, potentially damaging it. This can prevent the sensor from being used continuously. Therefore, there has been a need to develop an electric field sensor suitable for continuous electric field sensing even in harsh environments where high current and high voltage occur.

[0005] This disclosure was made to solve the problems described above, and aims to provide an electric field sensor that can continue electric field sensing even in harsh environments. [Means for solving the problem]

[0006] The electric field sensor according to this disclosure comprises a cantilever having a fixed first end and a second end which is a free end opposite to the first end, and having a piezoelectric film formed of a piezoelectric material; a light source fixed to the second end side of the cantilever; and a light-receiving element that receives light from the light source.

[0007] Other features of this disclosure are outlined below. [Effects of the Invention]

[0008] It can continue field sensing even in harsh environments where high currents and high voltages are generated. [Brief explanation of the drawing]

[0009] [Figure 1] This figure shows an example of the configuration of an electric field sensor. [Figure 2] This figure shows another example configuration of an electric field sensor. [Figure 3] This diagram shows how a cantilever bends due to an electric field. [Figure 4] This figure shows a bimorph electric field sensor. [Figure 5] This figure shows the properties of (MgHf)xAl1-xN. [Figure 6] This diagram shows variations in the shape of a cantilever. [Modes for carrying out the invention]

[0010] Embodiment. FIG. 1 is a diagram showing a configuration example of an electric field sensor according to an embodiment. This electric field sensor 10 includes a cantilever 12. According to an example, the cantilever 12 includes a substrate 14 and a piezoelectric film 16. The substrate 14 can be any material that does not prevent the deformation of the cantilever 12. The substrate 14 is, for example, stainless steel (SUS). The piezoelectric film 16 is not particularly limited as long as it is an arbitrary piezoelectric material, but can be, for example, the following piezoelectric materials. ·(MgHf) x Al 1-x N (x is a value greater than 0 and less than 1.) ·ScAl-N According to another example, the substrate 14 can be omitted, and the cantilever can be only a piezoelectric film formed of a piezoelectric material.

[0011] The cantilever 12 has a first end 12a and a second end 12b on the side opposite to the first end 12a. The first end 12a is a fixed end. In the example of FIG. 1, the first end 12a is fixed to a fixing portion 18. According to an example, the fixing portion 18 is a portion that fixes the first end 12a with an insulating material. The second end 12b is not fixed and is a free end.

[0012] A light source 20 is fixed on the second end 12b side of the cantilever 12. The light source ~ 20 is, for example, a light-emitting diode. Various well-known methods can be adopted for supplying power to the light source 20. For example, power can be supplied to the light source 20 using well-known non-contact power supply technology.

[0013] The electric field sensor 10 of the present embodiment includes a light-receiving element 30 that receives the light of the light source 20. The light-receiving element 30 can be a split-type photodiode such as a two-split photodiode or a four-split photodiode. Any split-type photodiode can be used according to the required position resolution. FIG. 2 is a diagram showing an example of an electric field sensor provided with a load impedance. A load impedance 32 is connected to the cantilever 12. In an operating environment where excessive charge can accumulate on the cantilever 12, connecting the load impedance 32 to the cantilever 12 can avoid charge accumulation.

[0014] Figure 3 shows an example of the operation of the electric field sensor 10 when an electric field is generated in the measurement environment. When an electric field is generated in the measurement environment, the electric field is applied to the piezoelectric film 16, and the electric field strength is converted into the displacement of the cantilever 12 by the inverse piezoelectric effect. In other words, the cantilever 12 is deflected. If a DC electric field is generated, the cantilever 12 deflects in one direction, and if an AC electric field is generated, the cantilever 12 vibrates up and down in accordance with the time change of the electric field. To prevent the cantilever from being unable to follow the electric field when the AC frequency is large, the cantilever can be made smaller. The dashed line in Figure 3 represents the cantilever that has been displaced by the inverse piezoelectric effect when an electric field is applied. The intensity distribution of the light emitted from the light source 20 to the photodetector 30 changes according to this displacement of the cantilever. The photodetector 30 can detect this change in light intensity distribution and thereby detect the amount of displacement of the cantilever 12. For example, a correspondence table between the amount of displacement of the cantilever 12 and the electric field strength may be stored in a computer in advance. In this case, the computer that receives the electrical signal from the photodetector 30 calculates the electric field strength using a correspondence table based on the content of the electrical signal. By performing this operation, for example, continuously, periodically, or at the user's request, the electric field strength of the measurement environment can be determined.

[0015] As shown in Figures 1, 2, and 3, the electric field sensor 10 is not connected to the object being measured, so it is not susceptible to damage from large currents or high voltages from the object being measured. Such unconnected electric field sensors have a low risk of damage and enable continuous electric field monitoring. By enabling electric field sensing even in environments where large currents or high voltages may occur, maintenance frequency can be reduced, and stable operation of the equipment can be achieved.

[0016] As an example different from the electric field sensor shown in Figure 1-3, the light source can be separated from the cantilever, light can be shone from the light source onto the cantilever, and the reflected light can be detected by the photodetector. Monitoring of the electric field is also possible with such a configuration. However, by fixing the light source 20 to the cantilever 12, the optical design becomes easier compared to the case where the cantilever and light source are separated, and alignment is greatly simplified. In other words, when the light source 20 is fixed to the cantilever 12, only two points of position adjustment are required: the light source 20 (i.e., the tip of the cantilever) and the photodetector 30. In contrast, when the light source is fixed at a position different from the cantilever, three points of optical adjustment are required: the light source, the tip of the cantilever, and the measurement system photodetector. Therefore, fixing the light source 20 to the cantilever 12 simplifies the optical design and thereby improves the accuracy of alignment.

[0017] Figure 4 shows an example of a bimorph cantilever configuration. When the piezoelectric film 16 is provided on only one side of the substrate 14, as in the cantilever in Figure 1, it is called a unimorph specification, and when the piezoelectric films 16a and 16b are provided on both sides of the substrate 14, as in the cantilever in Figure 4, it is called a bimorph specification.

[0018] As an example of the configuration of a bimorph cantilever 12, a simulation was performed for a configuration in which the horizontal length in Figure 4 is 10 mm, the length (width) in the depth direction of Figure 4 is 5 mm, the thickness of the substrate 14 is 10 μm, the thickness of the piezoelectric film 16a is 5 μm, and the thickness of the piezoelectric film 16b is 5 μm. Here, the material of the substrate 14 is SUS, and the materials of the piezoelectric films 16a and 16b are AlN. The simulation revealed that the second end 12b of the cantilever 12 bends downward by 6 μm due to the influence of gravity. When the displacement of the second end 12b is 0 nm and the downward displacement is negative when the cantilever 12 is not bending and is a perfect straight line, the displacement of the second end 12b is: • When gravitational acceleration is applied and the electric field strength is -20kV / m, the wavelength is -6096.1nm. • When gravitational acceleration is applied and the electric field strength is 0 kV / m, the value is -6138.7 nm. • When gravitational acceleration is applied and the electric field strength is +20kV / m, the wavelength is -6181.3nm. The following simulation results were obtained. Therefore, it was found that when the electric field strength increases by 1 kV / m, the strain of cantilever 12 increases by 2.13 nm. The strain of the cantilever z can be calculated from the following equation. z[nm] = -2.13 × electric field strength[kV / m] - 6138.7 Furthermore, the natural frequencies of this bimorph cantilever were as follows: Primary natural frequency: 250.1Hz Secondary natural frequency: 1570.4Hz Third natural frequency: 4397.6Hz

[0019] Next, we also performed a simulation for a unimorph cantilever. The simulation was conducted for a configuration where the lateral length of the cantilever 12 in Figure 1 was 10 mm, the length (width) in the depth direction of the paper in Figure 1 was 5 mm, the thickness of the substrate 14 was 10 μm, and the thickness of the piezoelectric film 16 was 5 μm. The material of the substrate 14 was SUS, and the material of the piezoelectric film 16 was AlN. The simulation revealed that, due to the influence of gravity, the second end 12b of the cantilever 12 bends downward by 15 μm. When the displacement of the second end 12b is 0 nm and the downward displacement is negative, the displacement of the second end 12b is: • When gravity is accelerating and the electric field strength is -20kV / m, the value is -15101nm. • When gravitational acceleration is applied and the electric field strength is 0 kV / m, the value is -15135.5 nm. • When gravity is accelerating and the electric field strength is +20kV / m, the wavelength is -15170nm. The following simulation results were obtained. Therefore, it was found that when the electric field strength increases by 1 kV / m, the strain of cantilever 12 increases by 1.73 nm. The strain of the cantilever z can be calculated from the following equation. z[nm] = -1.73 × electric field strength[kV / m] - 15135.5 Furthermore, the natural frequencies of this unimorph cantilever were as follows: First natural frequency: 159.3 Hz Second natural frequency: 1001.5 Hz Third natural frequency: 2803.8 Hz

[0020] As a result of these simulations, in the bimorph specification, the first natural frequency was 250.1 Hz, which was higher than the first natural frequency of 159.3 Hz in the unimorph specification. Therefore, it can be said that the bimorph specification is a better cantilever with a higher first natural frequency.

[0021] Next, the material of the piezoelectric film is considered. As the piezoelectric film of the present disclosure, various materials in which polarization occurs and strain (displacement) occurs in the crystal when an electric field is applied, that is, materials in which the inverse piezoelectric effect occurs can be adopted. In that sense, the piezoelectric film may be formed of any material, and the type of the piezoelectric film is not particularly limited in the present disclosure. The ability of the sensor output is represented by the figure of merit (FOM) of the piezoelectric thin film = (d 33 ) 2 / ε. Here, d 33 is the piezoelectric constant, and ε is the dielectric constant. By using a material with a high figure of merit as the piezoelectric film of the electric field sensor of the present disclosure, the electric field sensitivity can be increased. As materials with a high figure of merit, as described above, the following two materials can be mentioned. ·(MgHf) x Al 1-x N (x is a value greater than 0 and less than 1.) ·ScAl-N (MgHf) x Al 1-x N is a new material disclosed in Japanese Patent No. 6994247. (MgHf) x Al 1-x N has a very high figure of merit value compared to other piezoelectric materials. FIG. 5 is a diagram showing the piezoelectric coefficient d x Al 1-x N and the FOM. The horizontal axis is (MgHf) 33 Al x Al 1-xThis is the MgHf component ratio in N. Stainless steel was used as the substrate to support the piezoelectric film. From Figure 5, d 33 Furthermore, the MgHf concentration dependence of FOM can be understood. From this figure, it can be seen that if the MgHf component ratio is set to about 40%, the figure of merit becomes very high at about 60 GPa, and the sensitivity of the electric field sensor can be increased. From Figure 5, the figure of merit is (MgHf) x Al 1-x As the MgHf component ratio of N is increased from 0, it increases up to about 40%, but from around 40%, it approaches a saturation state. Figure 5 shows that a high performance index is obtained when the MgHf component ratio is set to around 40% to 50%, and the sensor output can be particularly increased in this component ratio range.

[0022] ScAl-N is (MgHf) x Al 1-x Although inferior to N, it is a material with a high figure of merit. Therefore, using ScAl-N as a piezoelectric film can increase the sensitivity of electric field sensors. Note that while piezoelectric materials with a low characteristic index can be used as piezoelectric films, in that case the sensitivity of the cantilever to the electric field will be low, and the displacement and polarization of the piezoelectric film will be small, so it is necessary to detect small displacements and small polarizations.

[0023] Figure 6 is a plan view showing a modified cantilever shape. Since Figure 6 is a top view of the cantilever, only the piezoelectric film 16 is shown, but the cantilever may include a substrate and may be bimorph or unimorph. This cantilever has a tapered shape from the first end 12a to the second end 12b. The cantilever can have various tapered shapes. For example, the cantilever can be trapezoidal, triangular, or have a rounded tip in plan view.

[0024] According to simulations conducted by the inventor, tapering the cantilever's planar shape, specifically making it trapezoidal, reduces static deflection due to gravity and increases the natural frequency compared to a rectangular planar shape. The trapezoidal shape suppresses static deflection of the cantilever due to gravity because the width of the cantilever's tip is smaller compared to the rectangular shape. A tapered cantilever can improve the signal-to-noise ratio. In a rectangular cantilever, the deflection is large, so stress tends to concentrate at the base. Therefore, the effect of the electric field is concentrated at the base, and the cantilever as a whole is not significantly affected by the electric field. On the other hand, in a tapered cantilever, the deflection of the cantilever is suppressed, so the stress becomes nearly uniform, and the effect of the electric field does not concentrate at the base, but is affected nearly uniformly throughout. Therefore, the effect of the electric field, i.e., the amount of deflection and polarization, becomes more accurate and is more likely to match calculated values. [Explanation of Symbols]

[0025] 10,40 Electric field sensor, 12 Cantilever, 14 Substrate, 16 Piezoelectric film, 20 Light source, 30 Photodetector

Claims

1. A cantilever having a fixed first end and a second end that is a free end opposite to the first end, and having a piezoelectric film formed of a piezoelectric material, A light source fixed to the second end side of the cantilever, An electric field sensor comprising a light-receiving element that receives light from the aforementioned light source.

2. The electric field sensor according to claim 1, wherein the cantilever has a tapered shape from the first end to the second end.

3. The piezoelectric film is such that x is greater than 0 and less than 1 (MgHf). ｘ Al １－ｘ The electric field sensor according to claim 1 or 2, wherein the material is N or ScAl-N.

4. The electric field sensor according to claim 1 or 2, wherein the cantilever is provided with a substrate that supports the piezoelectric film, with the piezoelectric film provided on one or both sides.

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