Electric field sensor

Abstract

The present invention comprises a cantilever having a first end that is fixed and a second end that is an end section on the opposite side from the first end and serves as a free end, and having a piezoelectric film that is formed from a piezoelectric material. A light source is further fixed to the second end side, which is the free end of the cantilever. The present invention also comprises a light receiving element that is provided in a location where the same can receive light emitted from the light source. The present invention provides an electric field sensor that is suitable for continuing electric field sensing even in a severe environment in which large currents and high voltages occur.

Description

electric field sensor

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

[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. Since an angular difference occurs in the detection structure when it is in a detection environment and when it is in a non-detection environment, the electric field detection information for that detection environment is determined based on that angular difference.

[0003] Chinese Patent Publication No. 115575730

[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.

[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.

[0008] It can continue field sensing even in harsh environments where high currents and high voltages occur.

[0009] This figure shows an example configuration of an electric field sensor. This figure shows another example configuration of an electric field sensor. This figure shows the cantilever bending due to the electric field. This figure shows a bimorph electric field sensor. (MgHf) ｘ Al １－ｘ This figure shows the characteristics of N. This figure shows a modified example of the cantilever shape.

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

[0011] The cantilever 12 has a first end 12a and a second end 12b opposite to the first end 12a. The first end 12a is a fixed end. In the example in Figure 1, the first end 12a is fixed to a fixing part 18. In one example, the fixing part 18 is a part that fixes the first end 12a with insulating material. The second end 12b is not fixed and is a free end.

[0012] A light source 20 is fixed to the second end 12b of the cantilever 12. The light source 20 is, for example, a light-emitting diode. Various well-known methods can be used to supply power to the light source 20. For example, the light source 20 can be powered using well-known contactless power supply technology.

[0013] The electric field sensor 10 of this embodiment includes a light-receiving element 30 that receives light from a light source 20. The light-receiving element 30 can be a segmented photodiode, such as a two-segment photodiode or a four-segment photodiode. Any segmented photodiode can be used depending on the required position resolution. Figure 2 shows an example of an electric field sensor with a load impedance. The load impedance 32 is connected to the cantilever 12. In operating environments where excessive charge accumulation may occur on the cantilever 12, charge accumulation can be avoided by connecting the load impedance 32 to the cantilever 12.

[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 portion in Figure 3 represents the cantilever that has been displaced by the inverse piezoelectric effect when an electric field is applied. In accordance with this displacement of the cantilever, the intensity distribution of the light emitted from the light source 20 to the photodetector 30 changes. 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 light-receiving element 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 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 a 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 when the cantilever and light source are separated, and alignment is greatly simplified. That is, when the light source 20 is fixed to the cantilever 12, only the position of two points, the light source 20 (i.e., the tip of the cantilever) and the photodetector 30, is required, whereas when the light source is fixed at a position different from the cantilever, optical adjustment of three points, the light source, the tip of the cantilever, and the measurement system photodetector, is required. Therefore, fixing the light source 20 to the cantilever 12 simplifies the optical design and thereby improves the alignment accuracy.

[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 was 10 mm, the length (width) in the depth direction of Figure 4 was 5 mm, the thickness of the substrate 14 was 10 μm, the thickness of the piezoelectric film 16a was 5 μm, and the thickness of the piezoelectric film 16b was 5 μm. In this case, the material of the substrate 14 was SUS, and the materials of the piezoelectric films 16a and 16b were 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 cantilever 12 is perfectly straight with no flex, the displacement of the second end 12b is 0 nm, and the downward displacement is negative. The simulation results showed that the displacement of the second end 12b is: - -6096.1 nm when gravity is applied and the electric field strength is -20 kV / m - -6138.7 nm when gravity is applied and the electric field strength is 0 kV / m - -6181.3 nm when gravity is applied and the electric field strength is +20 kV / m Therefore, it was found that the strain of the cantilever 12 increases by 2.13 nm when the electric field strength increases by 1 kV / m. The strain z of the cantilever can be calculated from the following formula: z [nm] = -2.13 × electric field strength [kV / m] - 6138.7 Also, the natural frequencies of this bimorph cantilever were as follows. First natural frequency: 250.1Hz Second 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 in which the cantilever 12 in Figure 1 had a lateral length of 10 mm, a 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 the second end 12b of the cantilever 12 bends downward by 15 μm due to the influence of gravity. When the cantilever 12 is perfectly straight with no flex, and the displacement of the second end 12b is set to 0 nm, and the downward displacement is set to negative, the simulation results showed that the displacement of the second end 12b is: -15101 nm when gravity acceleration is applied and the electric field strength is -20 kV / m -15135.5 nm when gravity acceleration is applied and the electric field strength is 0 kV / m -15170 nm when gravity acceleration is applied and the electric field strength is +20 kV / m Therefore, it was found that the strain of the cantilever 12 increases by 1.73 nm when the electric field strength increases by 1 kV / m. The strain z of the cantilever can be calculated from the following formula: z [nm] = -1.73 × electric field strength [kV / m] - 15135.5 Also, the natural frequencies of this unimorph cantilever were as follows. First natural frequency: 159.3Hz Second natural frequency: 1001.5Hz Third natural frequency: 2803.8Hz

[0020] These simulations showed that the bimorph configuration had a primary natural frequency of 250.1 Hz, which is higher than the unimorph configuration's primary natural frequency of 159.3 Hz. Therefore, the bimorph configuration can be considered a superior cantilever with a higher primary natural frequency.

[0021] Next, we will consider the material of the piezoelectric film. The piezoelectric film in this disclosure can be made from various materials that, when an electric field is applied, undergo polarization and cause strain (displacement) in the crystal, i.e., exhibit an inverse piezoelectric effect. In that sense, the piezoelectric film can be formed from any material, and the type of piezoelectric film is not particularly limited in this disclosure. The capability of the sensor output is given by the figure of merit (FOM) of the piezoelectric thin film = (d ３３ ) ２ / ε, where d ３３ is the piezoelectric constant and ε is the permittivity. By using a material with a high performance index as the piezoelectric film of the electric field sensor of the present disclosure, the electric field sensitivity can be enhanced. As materials with a high performance index, 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 performance index 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 the MgHf component ratio in (MgHf) ３３ Al ｘ N. Stainless steel was used as the substrate supporting the piezoelectric film. From FIG. 5, the MgHf concentration dependence of d １－ｘ and the FOM can be grasped. From this figure, when the component ratio of MgHf is about 40%, the performance index becomes as high as about 60 GPa, and the sensitivity of the electric field sensor can be enhanced. From FIG. 5, the performance index increases as the value of the MgHf component ratio of (MgHf) ３３ Al ｘ N increases from 0, and increases up to about 40%, but approaches a saturation state from around 40%. From FIG. 5, it can be seen that when the value of the MgHf component ratio is from 40% to about 50%, it has a high performance index, and the sensor output can be particularly enhanced within 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 instead distributed nearly uniformly throughout. Therefore, the effect of the electric field, i.e., the amount of deflection and polarization, becomes more accurate and more likely to match the calculated value.

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

Claims

1. An electric field sensor comprising: 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; and a light-receiving element that receives light from the 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.

Images