Sensor foreign object removal device and piezoelectric vibrator, and method for driving the piezoelectric vibrator

Abstract

The present invention provides a piezoelectric vibrator with feedback that can suppress a rapid rise in temperature due to self-heating and can also control that self-heating. [Solution] A piezoelectric vibrator 12 for a sensor foreign object removal device that is attached to a cover member (3) of an optical sensor or an electromagnetic wave sensor, comprising a piezoelectric body 4 polarized in the thickness direction, electrodes 13a and 13b for applying an AC voltage to the piezoelectric body 4, and a feedback electrode 14 that outputs a detection signal indicating a voltage change corresponding to the expansion and contraction of the piezoelectric body 4 to which the AC voltage is applied.

Description

TECHNICAL FIELD

[0001] The present invention relates to a piezoelectric vibrator attached to a cover member that protects optical sensors such as cameras and electromagnetic wave sensors such as millimeter-wave radars, a driving method therefor, and a foreign matter removing device using the piezoelectric vibrator. BACKGROUND ART

[0002] In recent years, for the purpose of driving assistance and autonomous driving, optical sensors such as cameras and LiDAR (Light Detection And Ranging) have been installed not only on the front part of vehicles but also on the rear and side parts of vehicles. In addition, optical sensors such as surveillance cameras are installed outdoors for the purposes of crime prevention and fixed-point monitoring. These optical sensors are provided with a cover member made of glass or transparent resin on the front surface so as not to be directly exposed to rain, wind and snow. However, when foreign matters (water droplets, snow-covered and frozen ice, dust, insects, etc.) adhere to the cover member, light is blocked by the adhering foreign matters, and accurate sensor information such as images may not be obtained. Accordingly, foreign matter removing devices for removing foreign matters adhering to the cover member have been provided. In addition, electromagnetic wave sensors such as millimeter-wave radars are sometimes used for autonomous driving sensors and surveillance cameras. Electromagnetic waves have good transmittance in fog and snowfall and are resistant to disturbances, and the cover member does not necessarily need to be transparent. However, when foreign matters such as dust, ice and snow adhere to the cover member, the electromagnetic waves may be blocked or absorbed, so that accurate sensor information may not be obtained.

[0003] Some recent foreign matter removing devices utilize vibration generated by a piezoelectric vibrator. Generally, a piezoelectric vibrator includes a polarized piezoelectric body, and when an alternating voltage is applied to the piezoelectric body, the piezoelectric body expands and contracts to generate vibration. By vibrating the cover member using the piezoelectric vibrator, foreign matters (water droplets, snow-covered and frozen ice, dust, insects, etc.) adhering to the cover member are removed from the surface of the cover member.

[0004] Patent Document 1 describes an optical device equipped with a piezoelectric vibrator that vibrates a protective cover of an optical sensor. This optical device vibrates the piezoelectric vibrator in a first vibration mode with an AC voltage frequency of 50 kHz or higher, or in a second vibration mode with an AC voltage frequency of 500 kHz or higher. In the first vibration mode, water droplets adhering to the protective cover can be atomized. In the second vibration mode, the fluidity of the water droplets can be increased by controlling the temperature to create a temperature gradient in which the temperature increases from the periphery to the center of the protective cover.

[0005] Patent Document 2 describes an optical sensor cover equipped with a vibration mechanism (piezoelectric element) that vibrates the cover glass. This optical sensor cover removes cleaning solution adhering to the cover glass by vibrating the cover glass with the vibration mechanism during or after cleaning the cover glass. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] International Publication No. 2020 / 230420 [Patent Document 2] Japanese Patent Publication No. 2011-244417 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, the foreign matter removal device that uses the piezoelectric vibrator described above to vibrate the cover glass, i.e., the cover member, to remove foreign matter has the following problems. When a piezoelectric vibrator is driven, it generates heat due to internal friction. Therefore, if the piezoelectric vibrator is vibrated continuously without considering this self-heating, its temperature will rise rapidly, which may result in damage to the piezoelectric vibrator.

[0008] In the optical device described in Patent Document 1, the rapid temperature rise due to self-heating of the piezoelectric vibrator is not considered. Therefore, when the piezoelectric vibrator is continuously vibrated in the first vibration mode, the temperature of the piezoelectric vibrator may rise rapidly, potentially damaging it. In the second vibration mode, the vibration of the piezoelectric vibrator is used to regulate the temperature of the protective cover, but the rapid temperature rise due to self-heating is not considered. Therefore, the piezoelectric vibrator may also be damaged in the second vibration mode.

[0009] In the optical sensor cover described in Patent Document 2, rapid temperature rise due to self-heating is not taken into consideration. Therefore, if the piezoelectric element, which is a vibration mechanism, is continuously vibrated, the temperature of the piezoelectric element may rise rapidly and it may be damaged.

[0010] Until now, no foreign matter removal device has been provided that takes into account the temperature rise due to self-heating of the piezoelectric vibrator. In addition, in order to suppress damage due to the temperature rise of the piezoelectric vibrator, it is necessary to measure the temperature of the piezoelectric vibrator, but until now, no technology has been provided to directly measure the temperature of the piezoelectric vibrator itself.

[0011] In view of the above problems, the object of the present invention is to provide a piezoelectric vibrator with feedback that can suppress a rapid rise in temperature due to self-heating and control such self-heating, as well as a foreign matter removal device using the piezoelectric vibrator with feedback. [Means for solving the problem]

[0012] This invention encompasses the following: [1] A piezoelectric vibrator that is attached to the cover member of an optical sensor or an electromagnetic wave sensor, A piezoelectric material polarized in the thickness direction, An electrode for applying an AC voltage to the piezoelectric element, A piezoelectric vibrator for a sensor foreign object removal device, characterized by having a feedback electrode that outputs a detection signal indicating a voltage change corresponding to the expansion and contraction of the piezoelectric body to which the AC voltage is applied.

[0013] [2] The piezoelectric vibrator is A first drive region and a second drive region, each independently drivable, The piezoelectric element has a non-driving region provided between the first driving region and the second driving region in a direction intersecting the thickness direction, The piezoelectric vibrator for a foreign object removal device for a sensor according to [1], characterized in that the feedback electrode is provided in the non-driving region.

[0014] [3] A piezoelectric vibrator for a foreign object removal device for a sensor according to [2], characterized in that the direction of polarization of the first drive region and the direction of polarization of the second drive region are in the same direction.

[0015] [4] A piezoelectric vibrator for a foreign object removal device for a sensor according to [2], characterized in that the direction of polarization of the first drive region and the direction of polarization of the second drive region are opposite to each other.

[0016] [5] The piezoelectric vibrator has a first surface located on the cover member side and a second surface located on the opposite side of the piezoelectric vibrator, The electrode provided on the first surface is used as the ground electrode. The electrodes provided on the second surface are configured as an application electrode for applying an AC voltage and an output feedback electrode. Furthermore, the piezoelectric vibrator for a foreign object removal device for a sensor according to [1] is characterized in that a portion of the ground electrode extends from the first surface side to the second surface side.

[0017] [6] At least one piezoelectric vibrator as described in any one of [1] to [5], The system includes a control device for controlling the drive of the piezoelectric vibrator, The foreign matter removing apparatus for a sensor is characterized in that the control device supplies a control signal of a predetermined frequency, which is an AC voltage, to an electrode for applying the AC voltage to vibrate the piezoelectric vibrator, and stops supplying the control signal when the frequency and / or voltage of the detection signal output from the feedback electrode exceeds a preset threshold value.

[0018] [7] The foreign matter removing apparatus for a sensor according to [6], characterized in that the piezoelectric vibrator is provided so as to surround a light-transmitting region or an electromagnetic wave-transmitting region of the cover member.

[0019] [8] The foreign matter removing apparatus for a sensor according to [6], characterized in that in addition to the piezoelectric vibrator described, it comprises a piezoelectric vibrator that is not provided with the feedback electrode.

[0020] [9] It comprises at least one piezoelectric vibrator according to any one of [1] to [5], and a control device that performs drive control of the piezoelectric vibrator, The foreign matter removing apparatus for a sensor is characterized in that the control device supplies a control signal of a predetermined frequency, which is an AC voltage, to the electrode for applying the AC voltage to vibrate the piezoelectric vibrator, and changes the frequency of the control signal to a frequency different from the predetermined frequency when the frequency and / or voltage of the detection signal output from the feedback electrode exceeds a preset threshold value.

[0021]

[10] The foreign matter removing apparatus for a sensor according to [9], characterized in that the piezoelectric vibrator is provided so as to surround a light-transmitting region or an electromagnetic wave-transmitting region of the cover member.

[0022]

[11] The foreign matter removing apparatus for a sensor according to [9], characterized in that in addition to the piezoelectric vibrator described, it comprises a piezoelectric vibrator that is not provided with the feedback electrode.

[0023]

[12] A method for driving a piezoelectric vibrator attached to a cover member of an optical sensor or an electromagnetic wave sensor, comprising a piezoelectric body polarized in the thickness direction, an electrode for applying an AC voltage to the piezoelectric body, and a feedback electrode that outputs a detection signal indicating a voltage change corresponding to the expansion and contraction of the piezoelectric body to which the AC voltage is applied, A control signal of a predetermined frequency, which is the aforementioned AC voltage, is supplied to the electrode for applying the AC voltage to vibrate the piezoelectric vibrator. A method for driving a piezoelectric vibrator for a sensor foreign object removal device, characterized in that the supply of the control signal is stopped when the frequency and / or voltage of the detection signal output from the feedback electrode exceeds a preset threshold.

[0024]

[13] A method for driving a piezoelectric vibrator attached to a cover member of an optical sensor or an electromagnetic wave sensor, comprising a piezoelectric body polarized in the thickness direction, an electrode for applying an AC voltage to the piezoelectric body, and a feedback electrode that outputs a detection signal indicating a voltage change corresponding to the expansion and contraction of the piezoelectric body to which the AC voltage is applied, A control signal of a predetermined frequency, which is the aforementioned AC voltage, is supplied to the electrode for applying the AC voltage to vibrate the piezoelectric vibrator. A method for driving a piezoelectric vibrator for a sensor foreign object removal device, characterized in that when the frequency and / or voltage of the detection signal output from the feedback electrode exceeds a preset threshold, the frequency of the control signal is changed to a frequency different from the predetermined frequency. [Effects of the Invention]

[0025] According to the present invention, it is possible to predict the overheating state of the piezoelectric vibrator due to self-heating, thereby preventing damage to the piezoelectric vibrator due to a rapid rise in temperature, and controlling the temperature of the piezoelectric vibrator, and consequently the cover member to which the piezoelectric vibrator is attached. [Brief explanation of the drawing]

[0026] [Figure 1] This is a schematic diagram illustrating a piezoelectric material polarized in the thickness direction. [Figure 2] This is a block diagram showing the configuration of a foreign matter removal device including a feedback piezoelectric vibrator according to a first embodiment of the present invention. [Figure 3] This is a schematic cross-sectional view showing the cross-sectional structure of a feedback-equipped piezoelectric vibrator according to the first embodiment of the present invention. [Figure 4] Figure 2 is a schematic diagram illustrating an example of the application of the foreign object removal device shown. [Figure 5] This diagram illustrates an example of mode switching for the drive control of a piezoelectric vibrator and the temperature rise state of the piezoelectric vibrator. [Figure 6] This diagram illustrates another example of mode switching for piezoelectric oscillator drive control and the temperature rise state of the piezoelectric oscillator. [Figure 7] This is a block diagram showing the configuration of a foreign matter removal device including a feedback piezoelectric vibrator according to a second embodiment of the present invention. [Figure 8] This is a schematic cross-sectional view showing the cross-sectional structure of a feedback-equipped piezoelectric vibrator according to a second embodiment of the present invention. [Figure 9] This is a schematic diagram showing a first modified example of a piezoelectric oscillator with feedback. [Figure 10] This is a schematic diagram showing a second modified example of a piezoelectric oscillator with feedback. [Figure 11] This is a schematic diagram showing a third modified example of a piezoelectric oscillator with feedback. [Figure 12] This is a schematic diagram showing a fourth modified example of a piezoelectric oscillator with feedback. [Modes for carrying out the invention]

[0027] Embodiments of the present invention will be described in detail below with reference to the drawings. However, the components described in the embodiments are merely examples and are not intended to limit the scope of the present invention to them.

[0028] First, let's briefly explain the basic structure of a piezoelectric vibrator. Generally, a piezoelectric vibrator is equipped with a polarized piezoelectric body, and when an AC voltage is applied to this piezoelectric body, it expands and contracts, generating vibrations. In this invention, a piezoelectric body polarized in the thickness direction is used.

[0029] Figure 1 is a schematic diagram illustrating a piezoelectric material polarized in the thickness direction. In Figure 1, (a) shows the state of the piezoelectric material before polarization, and (b) shows the state of the piezoelectric material after polarization. As shown in Figure 1(a), the direction of spontaneous polarization in a fired piezoelectric material (ceramics) is random. As shown in Figure 1(b), by applying a high voltage to the piezoelectric material (ceramics), the direction of spontaneous polarization can be aligned in the thickness direction (direction of arrow A in the figure). This polarized piezoelectric material (ceramics) maintains its state of having the direction of spontaneous polarization aligned in the thickness direction even after the voltage application is stopped.

[0030] When an AC voltage is applied to a polarized piezoelectric material, the piezoelectric material expands and contracts in the thickness direction. Conversely, when external pressure is applied to a polarized piezoelectric material, a positive charge is generated on one side of the piezoelectric material and a negative charge is generated on the other side. Conversely, when a polarized piezoelectric material is stretched, a negative charge is generated on one side of the piezoelectric material and a positive charge is generated on the other side. In this invention, the overheating state of the piezoelectric material is estimated by detecting the voltage change corresponding to the expansion and contraction of the piezoelectric material, i.e., the change in the magnitude of the voltage and the change in the period (frequency) of the voltage displacement, while vibrating a piezoelectric vibrator due to the expansion and contraction of the piezoelectric material. Furthermore, by controlling the vibration (expansion and contraction) of the piezoelectric vibrator based on this estimation, it becomes possible to control the temperature of the piezoelectric vibrator and, by extension, the cover member to which the piezoelectric vibrator is attached.

[0031] (First Embodiment) Figure 2 is a block diagram showing the configuration of a foreign matter removal device including a feedback-equipped piezoelectric vibrator according to a first embodiment of the present invention. Figure 3 is a schematic cross-sectional view showing the cross-sectional structure of the feedback-equipped piezoelectric vibrator shown in Figure 2. In Figure 3, (a) is a cross-sectional view along line AA, (b) is a cross-sectional view along line BB, (c) is a cross-sectional view along line CC, and (d) is a cross-sectional view along line DD. Figure 4 is a schematic diagram showing an application example of the foreign matter removal device shown in Figure 2. In Figures 2 to 4, the illustrated components are depicted schematically and do not differ from their actual size (thickness, width, etc.).

[0032] Referring to Figures 2 and 3, the foreign object removal device 10 includes a feedback piezoelectric vibrator 12 of this embodiment and a control device 11 that controls the driving of the piezoelectric vibrator 12. As shown in Figure 4, the optical sensor 1 is housed in a housing 2. The optical sensor 1 is, for example, an in-vehicle camera, LiDAR, or surveillance camera. The housing 2 has an opening in the imaging direction of the camera of the optical sensor 1, and a cover glass 3 is attached to this opening as a cover member to protect the optical sensor 1. The piezoelectric vibrator 12 is attached to the surface of the cover glass 3 on the optical sensor 1 side. The cover glass 3 is made of a material (transparent material) that allows the optical sensor 1 to detect objects.

[0033] The piezoelectric vibrator 12 comprises a piezoelectric body 4 polarized in the thickness direction, electrodes 13a and 13b for applying an AC voltage to the piezoelectric body 4, a feedback electrode 14 that outputs a detection signal indicating a voltage change corresponding to the expansion and contraction of the piezoelectric body 4 to which the AC voltage is applied, and a ground (GND) electrode 15. The piezoelectric body 4 extends so as to surround a predetermined region 3a of the cover glass 3. Here, the predetermined region 3a means the light transmission region, that is, the region necessary for camera imaging by the optical sensor 1 (camera field of view).

[0034] In the example shown in Figure 2, the predetermined region 3a is rectangular in shape, and the piezoelectric element 4 is provided to surround the predetermined region 3a. Electrode 13a is provided to surround the upper half of the predetermined region 3a, and electrode 13b is provided to surround the lower half of the predetermined region 3a. Electrodes 13a and 13b are, for example, U-shaped. A feedback electrode 14 is provided between one end of electrode 13a and one end of electrode 13b. A ground electrode 15 is provided between the other end of electrode 13a and the other end of electrode 13b. Electrodes 13a, 13b, the feedback electrode 14, and the ground electrode 15 are electrically insulated from each other.

[0035] As shown in Figure 3(a), near the feedback electrode 14, the piezoelectric element 4 has driving regions 16a, 16b, and a non-driving region 17a. Driving region 16a can be called the first driving region, and driving region 16b can be called the second driving region. Driving regions 16a and 16b can each be driven independently. Driving regions 16a, 16b, and non-driving region 17a are all polarized in the thickness direction (Z direction) (the direction of spontaneous polarization is aligned in the thickness direction). In Figure 3, the direction of polarization is represented by "-" and "+" for convenience. The piezoelectric element 4 has a first surface 4a located on the cover glass 3 side and a second surface 4b located on the opposite side of the piezoelectric element 4 from the first surface 4a, with the side of the first surface 4a being "+" and the side of the second surface 4b being "-". Here, the direction of polarization in the driving region 16a and the direction of polarization in the driving region 16b are the same, and the direction of polarization in the non-driving region 17a is the same as the direction of polarization in the driving regions 16a and 16b.

[0036] In the extending direction (X direction) of the piezoelectric element 4, the non-driving region 17a is located between the driving region 16a and the driving region 16b. In the driving regions 16a and 16b, the electrode 13a is provided on the second surface 4b, and the ground electrode 15 is provided on the first surface 4a. In the non-driving region 17a, the feedback electrode 14 is provided on the second surface 4b, and the ground electrode 15 is provided on the first surface 4a. The ground electrode 15 is located on the opposite side of the piezoelectric element 4 from the electrodes 13a, 13b and the feedback electrode 14. The piezoelectric resonator 12 is fixed to the cover glass 3 via adhesive 18.

[0037] As shown in Figures 3(b) and 3(c), the piezoelectric element 4 has a non-driven region 17b that is not polarized. In the direction of extension of the piezoelectric element 4 (X direction), the non-driven region 17b is located between the driven region 16a and the driven region 16b. In the non-driven region 17b, a portion of the ground electrode 15 extends from the side of the first surface 4a to the side of the second surface 4b, wrapping around the end of the piezoelectric element 4. The cross-sectional shape of the ground electrode 15 in the non-driven region 17b is, for example, U-shaped. The extension of the ground electrode 15 to the side of the second surface 4b allows connection to the control device 11 on the side of the second surface 4b.

[0038] In the portion surrounding the upper half of a predetermined area 3a of the cover glass 3, the piezoelectric element 4 extends in the X direction, and in this portion, as shown in Figure 3(d), the piezoelectric element 4 has a drive region 16a. The drive region 16a is the same as that shown in Figure 3(a). In the drive region 16a, the ground electrode 15 is formed on the first surface 4a, and the electrode 13a is formed on the second surface 4b. Although not shown, in the portion surrounding the lower half of the predetermined area 3a of the cover glass 3, the piezoelectric element 4 has the same structure as that shown in Figure 3(d).

[0039] Next, the operation of the foreign object removal device 10 shown in Figures 2 and 3 will be described. In the foreign matter removal device 10, the control device 11 is connected to electrodes 13a and 13b of the piezoelectric vibrator 12 via control signal lines, and to the feedback electrode 14 of the piezoelectric vibrator 12 via a detection signal line. The control device 11 is also connected to the ground electrode 15 of the piezoelectric vibrator 12 via a ground line. The ground line is branched and connected to the ground.

[0040] The control device 11 controls the drive of the piezoelectric vibrator 12. The drive control has two modes: vibration mode and temperature rise suppression mode. When the control device 11 detects a predetermined overheating state of the piezoelectric vibrator 12 while driving it in vibration mode, it switches from vibration mode to temperature rise suppression mode. Figure 5 is a diagram illustrating the mode switching of the drive control of the piezoelectric vibrator 12 and the temperature rise state of the piezoelectric vibrator 12. In Figure 5, (a) shows the control signal (drive voltage) 11a, the temperature of the piezoelectric vibrator 12, and the output (detection signal) of the feedback (FB) electrode 14, respectively, and (b) is an enlarged view of the output of the feedback (FB) electrode 14 immediately before mode switching. Note that the waveforms shown in Figure 5 are schematic representations and do not represent actual waveforms.

[0041] As shown in Figure 5(a), the control device 11 first drives the piezoelectric vibrator 12 in vibration mode. In vibration mode, the control device 11 supplies a control signal 11a, which is an AC voltage (driving voltage), to the electrodes 13a and 13b of the piezoelectric vibrator 12. Preferably, the frequency of the control signal 11a is either the resonant frequency of the piezoelectric vibrator 12 (e.g., 1.6 MHz) which is set in advance or determined by learning, or the combined resonant frequency of the piezoelectric vibrator 12 and the cover glass 3 (e.g., 950 kHz). The resonant frequency can be appropriately selected depending on the foreign matter to be removed and the removal method. By vibrating the piezoelectric vibrator 12 at the resonant frequency or the combined resonant frequency, foreign matter (water droplets, ice, dust, insects, etc.) adhering to a predetermined area 3a of the cover glass 3 can be removed. Here, the resonant frequency or the combined resonant frequency can be called a predetermined frequency.

[0042] In the piezoelectric vibrator 12, the feedback electrode 14 outputs a detection signal 11b indicating a voltage change corresponding to the expansion and contraction (vibration) of the piezoelectric body 4, i.e., a change in the magnitude of the voltage, and / or the period (frequency) of the voltage displacement. For example, in the structure shown in Figure 3(a), the drive regions 16a and 16b adjacent to both sides of the non-drive region 17a expand and contract in the thickness direction (Z direction) in response to a control signal 11a that generates a combined resonance frequency between the piezoelectric vibrator 12 and the cover glass 3. The drive regions 16a and 16b alternately repeat a contracted state and an extended state, and the cover glass 3 vibrates accordingly. Pressure is applied to the non-drive region 17a in response to the expansion and contraction of these drive regions 16a and 16b and the vibration of the cover glass 3. As a result, the non-drive region 17a also alternately repeats a contracted state and an extended state. In the non-driven region 17a, when it is contracted, a positive charge is generated on one side of the non-driven region 17a and a negative charge is generated on the other side. When it is extended, charges with the opposite positive and negative charges are generated on both sides of the non-driven region 17a compared to the contracted state. These positive and negative charge generation correspond to the expansion and contraction of the driven regions 16a and 16b and the vibration of the cover glass 3. Therefore, the feedback electrode 14 provided in the non-driven region 17a outputs a detection signal 11b that indicates a voltage change corresponding to the expansion and contraction of the driven regions 16a and 16b.

[0043] During period T1, when the temperature of the piezoelectric vibrator 12 is not rising rapidly, the vibration waveform of the piezoelectric vibrator 12 (driving regions 16a, 16b) approximately matches the resonant frequency of the control signal 11a, and the frequency of the detection signal 11b approximately matches the frequency of the control signal 11a.

[0044] When the piezoelectric vibrator 12 is vibrated continuously, self-heating occurs due to internal friction in the piezoelectric vibrator 12. If the continuous vibration period becomes long enough, the temperature of the piezoelectric vibrator 12 rises rapidly (period T2). During period T2, due to the effect of self-heating, the resonant frequency (or combined resonant frequency) of the piezoelectric vibrator 12 shifts upward from the initial resonant frequency, causing a discrepancy with the frequency of the control signal 11a and reducing the vibration amplitude of the piezoelectric vibrator 12. Consequently, the magnitude of the voltage of the detection signal 11b decreases. For example, during period T2, as shown in Figure 5(b), the amplitude of the waveform of the detection signal 11b gradually decreases.

[0045] The control device 11 estimates the overheating state of the piezoelectric vibrator 12 based on the detection signal 11b output from the feedback electrode 14, and when it detects that a predetermined overheating state has been reached, it switches the drive control mode from vibration mode to temperature rise suppression mode. Here, the predetermined overheating state means the changes shown in Figure 5(b). Specifically, when the amplitude of the vibration (amplitude of the voltage) of the detection signal 11b output from the feedback electrode 14 falls below a predetermined threshold, the control device 11 switches the drive control mode from vibration mode to temperature rise suppression mode. In the temperature rise suppression mode in this example, the control device 11 changes the frequency of the control signal 11a to a frequency different from a predetermined frequency (resonance frequency or combined resonance frequency) (a frequency in which resonance does not occur). As a result, the effect of self-heating is reduced, and the temperature of the piezoelectric vibrator 12 decreases. Note that in temperature rise suppression mode, the control device 11 may also stop supplying the control signal 11a. In this case as well, the effect of self-heating can be reduced, and the temperature of the piezoelectric vibrator 12 can be lowered.

[0046] The temperature rise suppression mode is terminated after the temperature of the piezoelectric vibrator 12 has sufficiently decreased. For example, the period during which the temperature of the piezoelectric vibrator 12 has sufficiently decreased may be predetermined, and the temperature rise suppression mode may be terminated based on this period. Alternatively, after a predetermined time has elapsed, a test drive may be performed, and if the detection signal 11b of the feedback electrode 14 reaches a predetermined value at a predetermined resonant frequency, the temperature of the piezoelectric vibrator 12 may be considered to have decreased, and the temperature rise suppression mode may be terminated. After the temperature rise suppression mode is terminated, the control device 11 drives the piezoelectric vibrator 12 again in vibration mode.

[0047] Figure 6 is a diagram illustrating the mode switching of the drive control of the piezoelectric vibrator 12 and the temperature rise state of the piezoelectric vibrator 12 when using a different control device 111 from the control device 11 described in Figure 5. In this explanation, the same reference numerals are used for components similar to those in Figure 5. In Figure 6, the control device 111 has the function of monitoring the output (detection signal) of the feedback (FB) electrode 14 and supplying a control signal 11a with a frequency that matches the resonant frequency of the piezoelectric vibrator 12 or the combined resonant frequency of the piezoelectric vibrator 12 and the cover glass 3 to the electrodes 13a and 13b of the piezoelectric vibrator 12, i.e., a resonant frequency tracking function.

[0048] As shown in Figure 6(a), the control device 111 first drives the piezoelectric vibrator 12 in vibration mode. In vibration mode, the control device 111 supplies a control signal 11a of a predetermined frequency to the electrodes 13a and 13b of the piezoelectric vibrator 12, similar to the control device 11 in Figure 5.

[0049] In the piezoelectric vibrator 12, the feedback electrode 14 outputs a detection signal 11b that indicates a voltage change corresponding to the expansion and contraction (vibration) of the piezoelectric body 4, i.e., a change in the magnitude of the voltage, and / or the period (frequency) of the voltage displacement.

[0050] During period T1, when the temperature of the piezoelectric vibrator 12 is not rising rapidly, the vibration waveform of the piezoelectric vibrator 12 (driving regions 16a, 16b) approximately matches the resonant frequency of the control signal 11a, and the frequency of the detection signal 11b approximately matches the frequency of the control signal 11a.

[0051] When the piezoelectric vibrator 12 is vibrated continuously, it generates heat due to internal friction. After a certain period of continuous vibration, the temperature of the piezoelectric vibrator 12 rises rapidly (period T2). During period T2, due to the effect of self-heating, the resonant frequency (or combined resonant frequency) of the piezoelectric vibrator 12 shifts upward from the initial resonant frequency, causing a discrepancy with the frequency of the control signal 11a and reducing the vibration amplitude of the piezoelectric vibrator 12. Consequently, the magnitude of the voltage of the detection signal 11b decreases. When the control device 111 senses the decrease in the detection signal 11b, it shifts the frequency of the output control signal 11a upward to match the resonant frequency of the piezoelectric vibrator 12 that has shifted upward, thereby maintaining the vibration amplitude of the piezoelectric vibrator 12. If the resonant frequency of the piezoelectric vibrator 12 shifts further upward due to overheating of the piezoelectric vibrator 12, the control device 111 repeats the operation of shifting the frequency of the control signal 11a further upward to match this. For example, during period T2, as shown in Figure 6(b), the frequency of the waveform of the detection signal 11b increases rapidly.

[0052] The control device 111 estimates the overheating state of the piezoelectric vibrator 12 based on the detection signal 11b output from the feedback electrode 14, and when it detects that a predetermined overheating state has been reached, it switches the drive control mode from vibration mode to temperature rise suppression mode. Here, the predetermined overheating state means the changes shown in Figure 6(b). Specifically, when the vibration frequency (voltage vibration frequency) of the detection signal 11b output from the feedback electrode 14 exceeds a predetermined threshold S, the control device 111 switches the drive control mode from vibration mode to temperature rise suppression mode. In the temperature rise suppression mode in this example, the control device 111 stops outputting the control signal 11a. As a result, the vibration of the piezoelectric vibrator 12 is stopped, self-heating is eliminated, and the temperature of the piezoelectric vibrator 12 decreases. In the temperature rise suppression mode, the control device 111 may stop or reduce the vibration of the piezoelectric vibrator 12 by setting the frequency of the control signal 11a to a frequency different from the predetermined frequency (a frequency in which resonance does not occur). In this case as well, the effect of self-heating can be reduced, and the temperature of the piezoelectric vibrator 12 can be lowered.

[0053] The temperature rise suppression mode is terminated after the temperature of the piezoelectric vibrator 12 has sufficiently decreased. For example, the period during which the temperature of the piezoelectric vibrator 12 has sufficiently decreased may be predetermined, and the temperature rise suppression mode may be terminated based on this period. Alternatively, after a predetermined time has elapsed, a test drive may be performed, and if the detection signal 11b of the feedback electrode 14 reaches a predetermined value at a predetermined resonant frequency, the temperature of the piezoelectric vibrator 12 may be considered to have decreased, and the temperature rise suppression mode may be terminated. After the temperature rise suppression mode is terminated, the control device 11 drives the piezoelectric vibrator 12 again in vibration mode.

[0054] The feedback-equipped piezoelectric vibrator 12 of this embodiment, as described above, provides the following effects.

[0055] The feedback electrode 14 outputs a detection signal 11b that indicates a voltage change corresponding to the expansion and contraction of the piezoelectric element 4. The frequency of the detection signal 11b depends on the period of the vibration waveform of the piezoelectric vibrator 12. Therefore, when the piezoelectric vibrator 12 is vibrated at its resonant frequency, the frequency of the detection signal 11b approximately matches the resonant frequency of the piezoelectric vibrator 12. In general, the resonant frequency of the piezoelectric vibrator 12 depends on its temperature. As the temperature of the piezoelectric vibrator 12 rises, the resonant frequency of the piezoelectric vibrator 12 rises accordingly, and if a discrepancy occurs between the frequency of the control signal 11a of the control device 11 and the resonant frequency, the magnitude (voltage) of the detection signal 11b decreases. Therefore, when the temperature of the piezoelectric vibrator 12 rises while it is vibrating at its resonant frequency, the resonant frequency of the piezoelectric vibrator 12 rises, and the magnitude of the detection signal 11b also decreases. Based on such changes in the detection signal 11b, it is possible to infer that the piezoelectric vibrator 12 is in an overheated state.

[0056] Furthermore, when the piezoelectric vibrator 12 is vibrated at its resonant frequency by the control signal 11a of the control device 111, which has a resonant frequency tracking function, as the temperature of the piezoelectric vibrator 12 rises, the resonant frequency of the piezoelectric vibrator 12 rises accordingly. As a result, a discrepancy occurs between the frequency of the control signal 11a of the control device 111 and the resonant frequency, causing the magnitude (voltage) of the detection signal 11b to decrease. Based on the output of the detection signal 11b, if the frequency of the control signal 11a of the control device 111 is increased in accordance with the rise in the resonant frequency of the piezoelectric vibrator 12, the frequency of the detection signal 11b also increases. Based on such changes in the detection signal 11b, it is possible to infer that the piezoelectric vibrator 12 is in an overheated state.

[0057] Furthermore, with the foreign matter removal device 10 equipped with the feedback piezoelectric vibrator 12 of this embodiment, damage to the piezoelectric vibrator 12 due to a rapid rise in temperature can be prevented by switching between the vibration mode and the temperature rise suppression mode. In this embodiment, the non-driven region 17b is not polarized, but it is not necessarily required to use a non-polarized piezoelectric body 4.

[0058] Furthermore, while the explanation for Figures 5 and 6 described an example where the resonant frequency shifts upward due to overheating of the piezoelectric vibrator 12, depending on the material of the piezoelectric vibrator 12, overheating may also cause the resonant frequency to shift downward. In such cases, it is natural to detect the overheating state of the piezoelectric vibrator 12 in accordance with the downward shift in its resonant frequency.

[0059] Furthermore, according to the present invention, by adjusting the duration of the vibration mode and the temperature rise suppression mode and repeating them in a pulsed manner, it is possible to maintain the piezoelectric vibrator 12 in a constant heated state while preventing damage due to internal heat generation of the piezoelectric vibrator 12. In other words, it is possible to control the temperature of the cover glass (cover member) to which the piezoelectric vibrator 12 is attached. As a result, the foreign matter removal device 10 equipped with the feedback piezoelectric vibrator 12 of the present invention can efficiently remove ice and snow adhering to the cover member by melting it.

[0060] (Second embodiment) Figure 7 is a block diagram showing the configuration of a foreign matter removal device including a feedback-equipped piezoelectric vibrator according to a second embodiment of the present invention. Figure 8 is a schematic cross-sectional view showing the cross-sectional structure of the feedback-equipped piezoelectric vibrator shown in Figure 7. In Figure 7, (a) is a cross-sectional view along line AA, and (b) is a cross-sectional view along line BB. In Figures 7 and 8, the illustrated components are depicted schematically and do not differ from their actual size (thickness, width, etc.).

[0061] Referring to Figures 7 and 8, the foreign matter removal device 10 includes the feedback piezoelectric vibrator 12A of this embodiment and a control device 11 that controls the driving of the piezoelectric vibrator 12A. The piezoelectric vibrator 12A is mounted on a cover glass 3 that protects the optical sensor 1 shown in Figure 4.

[0062] The piezoelectric vibrator 12A has the same structure as the feedback-equipped piezoelectric vibrator 12 of the first embodiment, except that the polarity directions of the drive region 16a and the drive region 16b are different from each other, and the drive region 16a and the drive region 16b are alternately arranged along the outer circumference of a predetermined region 3a. Below, the structure that differs from the first embodiment will be described, and the description of the same structure will be omitted.

[0063] As shown in Figure 8(a), near the feedback electrode 14, the driving regions 16a, 16b, and non-driving region 17a are all polarized in the thickness direction (Z direction). The polarization direction of the driving region 16a is "+" on the side of the first surface 4a and "-" on the side of the second surface 4b. The polarization direction of the driving region 16b is "-" on the side of the first surface 4a and "+" on the side of the second surface 4b. The polarization direction of the non-driving region 17a is "-" on the side of the first surface 4a and "+" on the side of the second surface 4b. In the extending direction (X direction) of the piezoelectric element 4, the non-driving region 17a is located between the driving region 16a and the driving region 16b.

[0064] As shown in Figure 8(b), a non-driven region 17b that is not polarized is provided between the driven regions 16a and 16b near the ground electrode 15. The polarization direction of the driven region 16a is "+" on the side of the first surface 4a and "-" on the side of the second surface 4b. The polarization direction of the driven region 16b is "-" on the side of the first surface 4a and "+" on the side of the second surface 4b. In other words, the polarization directions of the driven region 16a and the driving region 16b are opposite to each other.

[0065] In the portion surrounding the upper half of a predetermined area 3a of the cover glass 3, the piezoelectric element 4 extends in the Y direction, and in this portion, drive regions 16a and drive regions 16b are arranged alternately. Similarly, in the portion surrounding the lower half of a predetermined area 3a of the cover glass 3, the piezoelectric element 4 extends in the Y direction, and in this portion, drive regions 16a and drive regions 16b are arranged alternately. Electrodes 13a are provided for each drive region 16a, and electrodes 13b are provided for each drive region 16b.

[0066] The control device 11 is connected to the electrodes 13a of each drive region 16a and the electrodes 13b of each drive region 16b via control signal lines, and is connected to the feedback electrode 14 via a detection signal line. The control device 11 is connected to the ground electrode 15 via a ground line. The ground line is branched and connected to ground.

[0067] In the piezoelectric vibrator 12A, the feedback electrode 14 outputs a detection signal 11b indicating a voltage change (change in voltage magnitude and / or period (frequency) of voltage displacement) corresponding to the expansion and contraction of the piezoelectric body 4. For example, in the structure shown in Figure 8(a), the drive regions 16a and 16b adjacent to both sides of the non-drive region 17a expand and contract in the thickness direction (Z direction) in response to the control signal 11a. The drive regions 16a and 16b alternately repeat a contracted state and an extended state, but when one is in a contracted state, the other is in an extended state. Pressure is applied to the non-drive region 17a in response to the expansion and contraction of these drive regions 16a and 16b and the vibration of the cover glass 3. As a result, the non-drive region 17a also alternately repeats a contracted state and an extended state. The feedback electrode 14 provided in the non-drive region 17a outputs a detection signal 11b indicating a voltage change corresponding to the expansion and contraction of the drive regions 16a and 16b.

[0068] The feedback-equipped piezoelectric vibrator 12A of this embodiment also provides the same effects and advantages as the feedback-equipped piezoelectric vibrator 12 of the first embodiment. Furthermore, in the foreign matter removal device 10 equipped with the feedback piezoelectric vibrator 12A of this embodiment, drive control is performed in vibration mode and temperature rise suppression mode as shown in Figure 5. By switching between vibration mode and temperature rise suppression mode, damage to the piezoelectric vibrator 12A due to rapid temperature rise can be prevented.

[0069] In each of the embodiments described above, the shape of the feedback piezoelectric vibrators 12 and 12A attached to the cover glass 3 is not limited to the illustrated shape. Hereinafter, variations of the feedback piezoelectric vibrators 12 and 12A will be described.

[0070] (Variation 1) Figure 9 is a schematic diagram showing a first modified example of a piezoelectric vibrator with feedback. As shown in Figure 9, when a predetermined region 3a of the cover glass 3 is circular in shape, the piezoelectric vibrator with feedback 12A is formed in a circular shape along the circumferential direction. The electrodes 13a of the driving region 16a and the electrodes 13b of the driving region 16b are arranged alternately in the circumferential direction, and in a part of the circumferential direction, the feedback electrode 14 of the non-driving region 17a is positioned between electrodes 13a and 13b, and in another part of the circumferential direction, the ground electrode 15 of the non-driving region 17b is positioned between electrodes 13a and 13b. In this modified example, the feedback electrode 14 and the ground electrode 15 are positioned on opposite sides of the predetermined region 3a (here, the upper and lower sides). Similar to the piezoelectric vibrator 12A, the feedback-equipped piezoelectric vibrator 12 can also be formed in a circular shape so as to surround a predetermined region 3a. This arrangement of piezoelectric vibrators 12 is particularly suitable for applications such as surveillance cameras, when it is desired to form a convex shape at the center of the region 3a of the cover glass 3. In this modified example, the feedback electrode 14 and ground electrode 15 do not contribute to driving the piezoelectric vibrator 12A, and therefore it is preferable to arrange them on opposite sides of the circumference so as not to hinder the driving of the piezoelectric vibrator 12, but this is not the only option.

[0071] (Modification 2) Figure 10 is a schematic diagram showing a second modified example of a feedback piezoelectric vibrator. As shown in Figure 10, a predetermined region 3a of the cover glass 3 is rectangular in shape, and two feedback piezoelectric vibrators 12 are arranged above and below the predetermined region 3a. Each feedback piezoelectric vibrator 12 is formed in a straight line. In each feedback piezoelectric vibrator 12, the feedback electrode 14 in the non-driven region 17a is positioned between the electrode 13a in the driven region 16a and the electrode 13b in the driven region 16b, and the ground electrode 15 in the non-driven region 17b is positioned at one end. Note that the feedback piezoelectric vibrator 12 may be provided on only one side, either the upper or lower side. Alternatively, a feedback piezoelectric vibrator 12A may be provided in a straight line instead of the feedback piezoelectric vibrator 12.

[0072] (Variation 3) Figure 11 is a schematic diagram showing a third modified example of a feedback piezoelectric vibrator. As shown in Figure 11, a predetermined region 3a of the cover glass 3 is rectangular in shape, and two feedback piezoelectric vibrators 12 are arranged on the left and right sides of the predetermined region 3a. Each feedback piezoelectric vibrator 12 is formed in a straight line. In each feedback piezoelectric vibrator 12, the feedback electrode 14 in the non-driven region 17a is positioned between the electrode 13a in the driven region 16a and the electrode 13b in the driven region 16b, and the ground electrode 15 in the non-driven region 17b is positioned at one end. Note that the feedback piezoelectric vibrator 12 may be provided on only one side, either the left or the right. Alternatively, a feedback piezoelectric vibrator 12A may be provided in a straight line instead of the feedback piezoelectric vibrator 12.

[0073] (Modification 4) Figure 12 is a schematic diagram showing a fourth modified example of a piezoelectric vibrator with feedback. As shown in Figure 12, a predetermined region 3a of the cover glass 3 is circular in shape, and two piezoelectric vibrators with feedback 12 are arranged above and below the predetermined region 3a. Each piezoelectric vibrator with feedback 12 is formed in an arc shape. In each piezoelectric vibrator with feedback 12, the feedback electrode 14 in the non-driven region 17a is positioned between the electrode 13a in the driven region 16a and the electrode 13b in the driven region 16b, and the ground electrode 15 in the non-driven region 17b is positioned at one end. Note that the piezoelectric vibrator with feedback 12 may be provided on only one side, either the upper or lower side. Alternatively, a piezoelectric vibrator with feedback 12A may be provided in a linear shape instead of the piezoelectric vibrator with feedback 12.

[0074] The above-described modifications 1 to 4 are examples, and the present invention is not limited to these modifications. For example, three or more feedback piezoelectric vibrators 12 (12A) may be provided so as to surround a predetermined area 3a of the cover glass 3. For example, modifications 2 and 3 may be combined. Alternatively, multiple piezoelectric vibrators may be arranged to surround a predetermined area 3a of the cover glass 3, and at least one of these piezoelectric vibrators may be configured as a feedback-equipped piezoelectric vibrator 12(12A). In this case, if a predetermined overheating state is detected in at least one piezoelectric vibrator configured as a feedback-equipped piezoelectric vibrator 12(12A), both that at least one piezoelectric vibrator and the other piezoelectric vibrators may be driven in a temperature rise suppression mode.

[0075] In the first embodiment, the second embodiment, and modifications 1 to 4, the piezoelectric vibrator 12(12A) is attached to the cover glass 3 of the optical sensor 1, but is not limited to this. For example, in a foreign matter removal device for sensors, the piezoelectric vibrator 12(12A) may be attached to the cover member of an electromagnetic wave sensor such as a millimeter-wave radar. In this case, the piezoelectric vibrator 12(12A) may be provided so as to surround a predetermined area (electromagnetic wave transmission area) of the cover member of the electromagnetic wave sensor. With this configuration, by vibrating the piezoelectric vibrator 12(12A) at the resonant frequency or combined resonant frequency, foreign matter such as dust and ice and snow attached to the cover member can be removed, and the effect of self-heating can be reduced by the temperature rise suppression mode. The cover member does not necessarily have to be transparent. Furthermore, in the sensor foreign matter removal device, in addition to the piezoelectric vibrator 12 (12A), a piezoelectric vibrator without a feedback electrode may also be provided. [Explanation of symbols]

[0076] 1 Optical sensor 2 cabinets 3. Cover glass 4 Piezoelectric element 10 Foreign matter removal device 11, 111 Control device 11a Control signals 11b Detection signal 12. Piezoelectric oscillator with feedback 13a, 13b electrode 14 Feedback electrodes 15 Ground electrode 16a, 16b drive area 17a, 17b Non-driven region

Claims

1. A piezoelectric vibrator that is attached to the cover member of an optical sensor or an electromagnetic wave sensor, A piezoelectric material polarized in the thickness direction, An electrode for applying an AC voltage to the piezoelectric element, A piezoelectric vibrator for a sensor foreign object removal device, characterized by having a feedback electrode that outputs a detection signal indicating a voltage change corresponding to the expansion and contraction of the piezoelectric body to which the AC voltage is applied.

2. The piezoelectric vibrator is, A first drive region and a second drive region, each independently drivable, The piezoelectric element has a non-driving region provided between the first driving region and the second driving region in a direction intersecting the thickness direction, The piezoelectric vibrator for a foreign object removal device for a sensor according to claim 1, characterized in that the feedback electrode is provided in the non-driving region.

3. A piezoelectric vibrator for a foreign object removal device for a sensor according to claim 2, characterized in that the direction of polarization of the first drive region and the direction of polarization of the second drive region are in the same direction.

4. A piezoelectric vibrator for a foreign object removal device for a sensor according to claim 2, characterized in that the polarization direction of the first drive region and the polarization direction of the second drive region are opposite to each other.

5. The piezoelectric vibrator has a first surface located on the cover member side and a second surface located on the opposite side of the piezoelectric vibrator. The electrode provided on the first surface is used as the ground electrode. The electrodes provided on the second surface are configured as an application electrode for applying an AC voltage and an output feedback electrode. Furthermore, the piezoelectric vibrator for a foreign object removal device for a sensor according to claim 1, characterized in that a portion of the ground electrode extends from the first surface side to the second surface side.

6. A piezoelectric vibrator according to any one of claims 1 to 5, The system includes a control device for controlling the drive of the piezoelectric vibrator, The control device is characterized in that it supplies a control signal of a predetermined frequency, which is an AC voltage, to an electrode for applying the AC voltage to vibrate the piezoelectric vibrator, and stops supplying the control signal when the frequency and / or voltage of the detection signal output from the feedback electrode exceeds a preset threshold.

7. The foreign matter removal device for a sensor according to claim 6, characterized in that the piezoelectric vibrator is provided so as to surround the light-transmitting region or electromagnetic wave-transmitting region of the cover member.

8. The foreign matter removal device for a sensor according to claim 6, characterized in that it includes a piezoelectric vibrator without the feedback electrode, in addition to the piezoelectric vibrator.

9. A piezoelectric vibrator according to any one of claims 1 to 5, The system includes a control device for controlling the drive of the piezoelectric vibrator, The control device is characterized in that it supplies a control signal of a predetermined frequency, which is an AC voltage, to an electrode for applying the AC voltage to vibrate the piezoelectric vibrator, and when the frequency and / or voltage of the detection signal output from the feedback electrode exceeds a preset threshold, it changes the frequency of the control signal to a frequency different from the predetermined frequency.

10. The foreign matter removal device for a sensor according to claim 9, characterized in that the piezoelectric vibrator is provided so as to surround the light-transmitting region or electromagnetic wave-transmitting region of the cover member.

11. The foreign matter removal device for a sensor according to claim 9, characterized in that it includes a piezoelectric vibrator without the feedback electrode, in addition to the piezoelectric vibrator.

12. A method for driving a piezoelectric vibrator, which is attached to a cover member of an optical sensor or an electromagnetic wave sensor, comprising a piezoelectric body polarized in the thickness direction, an electrode for applying an AC voltage to the piezoelectric body, and a feedback electrode that outputs a detection signal indicating a voltage change corresponding to the expansion and contraction of the piezoelectric body to which the AC voltage is applied, A control signal of a predetermined frequency, which is the aforementioned AC voltage, is supplied to the electrode for applying the AC voltage to vibrate the piezoelectric vibrator. A method for driving a piezoelectric vibrator for a sensor foreign object removal device, characterized in that the supply of the control signal is stopped when the frequency and / or voltage of the detection signal output from the feedback electrode exceeds a preset threshold.

13. A method for driving a piezoelectric vibrator, which is attached to a cover member of an optical sensor or an electromagnetic wave sensor, comprising a piezoelectric body polarized in the thickness direction, an electrode for applying an AC voltage to the piezoelectric body, and a feedback electrode that outputs a detection signal indicating a voltage change corresponding to the expansion and contraction of the piezoelectric body to which the AC voltage is applied, A control signal of a predetermined frequency, which is the aforementioned AC voltage, is supplied to the electrode for applying the AC voltage to vibrate the piezoelectric vibrator. A method for driving a piezoelectric vibrator for a sensor foreign object removal device, characterized in that when the frequency and / or voltage of the detection signal output from the feedback electrode exceeds a preset threshold, the frequency of the control signal is changed to a frequency different from the predetermined frequency.

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