Excitation circuit, vibration device and vehicle

Abstract

This excitation circuit (31A-31E) comprises: an output circuit (37A, 37B) which contains a series circuit of a first switch (35) and a second switch (36), connected to a DC power source (33, 33A), and in which a piezoelectric element (15) is connected to a connection point (C1) of the first switch (35) and the second switch (36); a current-detecting circuit (38A, 38E) that detects a current flowing in the first switch (35) and / or a current flowing in the second switch (36), and outputs a detection signal that indicates a value based on the detected current; and a control circuit (32) that, in order to apply a voltage of a prescribed frequency from the output circuit (37A, 37B) to the piezoelectric element (15), executes a switching process in which the first switch (35) and the second switch (36) are complementarily switched between on and off at a switching frequency that corresponds to the prescribed frequency, and that has a search mode in which, on the basis of the value indicated by the detection signal output from the current-detecting circuit (38A, 38E), the resonance frequency of a resonator (17) vibrated by the piezoelectric element (15) is determined.

Description

[Technical Field]

[0001] The present disclosure relates to an excitation circuit, a vibration device, and a vehicle. [Background technology]

[0002] Conventionally, a technology has been studied in which a piezoelectric element is vibrated by applying a drive signal having a frequency component, and the vibration vibrates the lens to clean the lens. For example, Patent Document 1 discloses an ultrasonic cleaning system for cleaning lenses, in which a vibration drive signal is provided to an ultrasonic vibrator, and a driver integrated circuit controls the frequency of the drive signal based on a current detection signal that indicates the drive current flowing through the vibrator. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent No. 10,401,618 Summary of the Invention [Problem to be solved by the invention]

[0004] By using the technology disclosed in Patent Document 1, a piezoelectric element installed in a specific device can be driven at a resonant frequency. 、 The device can be vibrated in a predetermined vibration mode. However, if the control circuit drives the piezoelectric element with one polarity in order to detect the magnitude of the drive current, which is a signal with a predetermined frequency, this can promote migration of the piezoelectric element and lead to failure.

[0005] The present disclosure aims to provide an excitation circuit, a vibration device, and a vehicle that can detect the magnitude of a current flowing through a piezoelectric element while reducing the possibility of migration occurring in the piezoelectric element. [Means for solving the problem]

[0006] The excitation circuit according to the present disclosure comprises: an output circuit including a series circuit of a first switch and a second switch connected to a DC power source, with a piezoelectric element connected to the connection point between the first switch and the second switch; a current detection circuit that detects at least one of the current flowing through the first switch and the current flowing through the second switch and outputs a detection signal indicating a value based on the detected current; and a control circuit having a search mode that performs a switching process that complementarily switches the first switch and the second switch on and off at a switching frequency corresponding to a predetermined frequency in order to apply a voltage of a predetermined frequency from the output circuit to the piezoelectric element, and determines the resonant frequency of a vibrator including an object vibrated by the piezoelectric element and the piezoelectric element based on the value indicated by the detection signal output from the current detection circuit.

[0007] A vibration device according to the present disclosure includes an excitation circuit, a piezoelectric element, and a light-transmitting protective cover that is vibrated by the piezoelectric element.

[0008] A vehicle according to the present disclosure includes a vibration device and an imaging device that detects light transmitted through a protective cover. [Effects of the Invention]

[0009] According to the present disclosure, it is possible to provide an excitation circuit, a vibration device, and a vehicle that can detect the magnitude of the current flowing through a piezoelectric element while reducing the possibility of migration occurring in the piezoelectric element. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a perspective view of a vibration device according to a first embodiment; [Figure 2] 1 is a schematic cross-sectional view of a configuration of an imaging unit according to a first embodiment; [Figure 3] 1 is a schematic circuit diagram of an oscillator circuit according to a first embodiment; [Figure 4] A graph showing the relationship between the frequency of the drive signal applied to the piezoelectric element and the impedance [Figure 5] Timing chart showing input and output signals of each element of the excitation circuit [Figure 6A] 1 is a graph showing the change over time in a drive signal having a predetermined resonance frequency applied to a piezoelectric element and the change over time in the amount of displacement of a protective cover when the piezoelectric element is driven at that frequency. [Figure 6B] A graph showing the change over time in a drive signal having a predetermined resonant frequency applied to a piezoelectric element and the change over time in the amount of displacement of a protective cover when the piezoelectric element is driven at a frequency 1 / 3 of that frequency. [Figure 7A] An example of control by the first sweep method of the control circuit for determining the resonant frequency [Figure 7B] An example of control by the second sweep method of the control circuit for determining the resonant frequency [Figure 7C] An example of control by the third sweep method of the control circuit for determining the resonant frequency [Figure 8] 1 is a graph showing the impedance of a piezoelectric element relative to a switching frequency near a resonance frequency, and the phase difference between the voltage applied to the piezoelectric element and the current flowing through the piezoelectric element. [Figure 9] 1 is a schematic circuit diagram showing a modified example of an excitation circuit according to the first embodiment; [Figure 10] 1 is a flowchart illustrating vibration processing of a vibration device by a control circuit of an excitation circuit according to a first embodiment. [Figure 11] FIG. 1 is a schematic circuit diagram showing an example of a low-pass filter of an excitation circuit according to a first embodiment; [Figure 12] 1 is a schematic circuit diagram showing a modified example of an excitation circuit according to the first embodiment; [Figure 13] Schematic circuit diagram of an oscillator circuit according to a second embodiment. [Figure 14] Schematic circuit diagram of an oscillator circuit according to a third embodiment DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, a first embodiment, a second embodiment, and a third embodiment according to the present disclosure will be described with reference to the drawings. However, the configurations described below are merely examples of the present disclosure, and the present disclosure is not limited to the following embodiments. Even if the configurations are different from these embodiments, various modifications can be made depending on the design, etc., as long as they do not deviate from the technical concept of the present disclosure.

[0012] (First embodiment) 1-1.Configuration example An excitation circuit according to a first embodiment of the present disclosure includes an output circuit including a series circuit of a first switch and a second switch connected to a DC power supply, with a piezoelectric element connected to the junction between the first switch and the second switch; a current detection circuit that detects at least one of the currents flowing through the first switch and the second switch and outputs a detection signal indicating the detected current; and a control circuit that controls the switching frequency of the first switch and the second switch, performs a switching process that complementarily switches the first switch and the second switch on and off to apply a voltage having a switching frequency to the piezoelectric element, and has a search mode that determines the resonant frequency of a vibrator including the piezoelectric element and an object vibrated by the piezoelectric element based on the current indicated by the detection signal output from the current detection circuit. With this configuration, the control circuit of the excitation circuit controls the switching frequency at which the switching process is performed to control the frequency of the voltage applied to the piezoelectric element. The control circuit can detect the magnitude of the current flowing through the piezoelectric element even if the average current flowing through the piezoelectric element or the average voltage applied to the piezoelectric element at that frequency is zero, thereby determining the resonant frequency of the vibrator. Therefore, the excitation circuit can detect the current flowing through the piezoelectric element while reducing the possibility of migration occurring in the piezoelectric element to which a voltage is applied. Also, the excitation circuit can perform switching based on the magnitude of the detected current. process The switching frequency at which the

[0013] 1-1-1. Vibration device FIG. 1 is a perspective view of a vibration device 10 according to a first embodiment of the present disclosure. The vibration device 10 according to the first embodiment includes a protective cover 11, a vibrating body 13, a piezoelectric element 15, and an excitation circuit 31A (described later). The vibrating body 13 includes a first cylindrical body 13a, a spring portion 13b, a second cylindrical body 13c, and a diaphragm 13d. The vibration device 10 and an imaging unit 100 (described later in detail) including the vibration device 10 are examples of devices vibrated by the excitation circuit 31A according to the present embodiment (described later), but are not limited thereto. The piezoelectric element 15 vibrates a predetermined object. The object includes the protective cover 11 and the vibrating body 13. The structure including the protective cover 11, the vibrating body 13, and the piezoelectric element 15 has a predetermined resonant frequency (described later) relative to the vibration of the piezoelectric element 15. Hereinafter, this structure will be referred to as a vibrator 17.

[0014] The protective cover 11 transmits light of a predetermined wavelength. The predetermined wavelength is, for example, a wavelength detected by the imaging device 20 (see FIG. 2) of the imaging unit 100. The predetermined wavelength is not limited to a wavelength in the visible light range, and may be a wavelength in the invisible light range.

[0015] The protective cover 11 is supported by an end of a cylindrical first cylindrical body 13a. Specifically, the rear surface of the protective cover 11 is supported by the first cylindrical body 13a.

[0016] The protective cover 11 has a hemispherical dome shape. When viewed from the height direction of the vibration device 10, the protective cover 11 has a circular shape. The shape of the protective cover 11 is not limited to a circle. When viewed from the height direction of the vibration device 10, the shape of the protective cover 11 may be a polygon, an ellipse, or the like. The protective cover 11 is not limited to a hemispherical dome shape. For example, the protective cover 11 may have a shape in which a cylinder is connected to a hemisphere, or a curved shape that is smaller than a hemisphere. The protective cover 11 may be a flat plate. The protective cover 11 may function as an optical element such as a lens.

[0017] The first cylindrical body 13a is formed in a cylindrical shape having one end and the other end. The first cylindrical body 13a supports the protective cover 11 at one end. For example, the protective cover 11 and the first cylindrical body 13a are joined together. There is no particular restriction on the method for joining the protective cover 11 and the first cylindrical body 13a. Examples of joining methods include adhesion with an adhesive, welding, fitting, and press-fitting.

[0018] In the first embodiment, the first cylindrical body 13a has a flange 13aa at one end. The flange 13aa is a plate-like member that extends outward from one end of the first cylindrical body 13a. The flange 13aa is formed in the shape of an annular plate. The flange 13aa increases the contact area between the first cylindrical body 13a and the protective cover 11, thereby stably supporting the protective cover 11.

[0019] The other end of first cylindrical body 13a is supported by elastically deformable spring portion 13b. In other words, first cylindrical body 13a is supported by spring portion 13b on the side opposite to protective cover 11.

[0020] The first cylindrical body 13a is a hollow member with a through-hole formed therein. The through-hole is formed in the height direction of the vibration device 10, and openings of the through-hole are formed at one end and the other end of the first cylindrical body 13a. The first cylindrical body 13a has, for example, a cylindrical shape. When viewed from the height direction of the vibration device 10, the outer shape of the first cylindrical body 13a and the opening of the through-hole are formed in a circular shape.

[0021] The shape of the first cylindrical body 13a is not limited to a cylindrical shape, and may be, for example, a polygonal cylindrical shape or an elliptical cylindrical shape.

[0022] The material of first cylindrical body 13a may be, for example, metal or synthetic resin. Also, the material of first cylindrical body 13a may be moldable and / or cuttable ceramic or glass. This also applies to spring portion 13b, second cylindrical body 13c, and diaphragm 13d.

[0023] The spring portion 13b supports the first cylindrical body 13a so that it can be displaced relative to the second cylindrical body 13c. The spring portion 13b is an annular leaf spring. The inner peripheral portion of the spring portion 13b supports the other end of the first cylindrical body 13a. The outer peripheral portion of the spring portion 13b is supported by the second cylindrical body 13c. When viewed from the height direction of the vibration device 10, the outer peripheral shape and inner peripheral shape of the spring portion 13b are circular.

[0024] The outer and inner peripheral shapes of spring portion 13b are not limited to a circular shape, and may be polygonal or elliptical when viewed from the height direction of vibration device 10.

[0025] The second cylindrical body 13c has a cylindrical shape with one end and the other end, and the one end of the second cylindrical body 13c supports the outer periphery of the spring portion 13b.

[0026] A diaphragm 13d is disposed at the other end of second cylindrical body 13c.

[0027] The second cylindrical body 13c is not limited to a cylindrical shape, and may be, for example, a polygonal cylindrical shape or an elliptical cylindrical shape.

[0028] Diaphragm 13d is disposed at the other end of second cylindrical body 13c, and vibrates in the height direction of vibration device 10. Specifically, diaphragm 13d is disposed at the other end of second cylindrical body 13c, i.e., on the bottom surface.

[0029] Piezoelectric element 15 is provided on the bottom surface (lower surface) of vibration plate 13d. Vibration of piezoelectric element 15 causes vibration plate 13d to vibrate, causing second cylindrical body 13c to vibrate in the height direction of vibration device 10. For example, piezoelectric element 15 vibrates when a voltage is applied thereto.

[0030] The piezoelectric element 15 is in the shape of an annular plate. When viewed from the height direction of the vibration device 10, the outer and inner peripheral shapes of the piezoelectric element 15 are circular. However, the outer and inner peripheral shapes of the piezoelectric element 15 are not limited to circular. When viewed from the height direction of the vibration device 10, the outer and inner peripheral shapes of the piezoelectric element 15 may be, for example, polygonal or elliptical.

[0031] The piezoelectric element 15 has a piezoelectric body and electrodes. Examples of the material of the piezoelectric body include barium titanate (BaTiO), lead zirconate titanate (PZT: PbTiO.PbZrO), lead titanate (PbTiO), lead metaniobate (PbNbO), and bismuth titanate (BiTiO). 12 Examples of suitable piezoelectric ceramics include (K,Na)NbO3, and suitable piezoelectric single crystals such as LiTaO3 and LiNbO3. The electrodes may be, for example, Ni electrodes. The electrodes may be formed by sputtering and made of a thin metal film such as Ag or Au. The electrodes can be formed by sputtering, plating, or vapor deposition.

[0032] The diaphragm 13d is in the form of a circular ring plate and supports the bottom surface of the second cylindrical body 13c.

[0033] Protective cover 11, first cylindrical body 13a, spring portion 13b, and second cylindrical body 13c are configured so that the resonant frequency of protective cover 11 is greater than the resonant frequency of spring portion 13b. Specifically, by determining the materials and dimensions of protective cover 11, first cylindrical body 13a, spring portion 13b, and second cylindrical body 13c described above, the resonant frequency of protective cover 11 is made greater than the resonant frequency of spring portion 13b.

[0034] First cylindrical body 13a, spring portion 13b, second cylindrical body 13c, and diaphragm 13d are integrally formed, but first cylindrical body 13a, spring portion 13b, second cylindrical body 13c, and diaphragm 13d may be formed separately or as separate members.

[0035] As described above, the vibration device 10 includes an excitation circuit 31A that applies a drive signal to the piezoelectric element 15 to generate vibration. The excitation circuit 31A is connected to the piezoelectric element 15, for example, via a power supply conductor. The piezoelectric element 15 vibrates in the height direction of the vibration device 10 based on the drive signal from the excitation circuit 31A. Vibration of the piezoelectric element 15 causes the vibration plate 13d to vibrate in the height direction of the vibration device 10, and the vibration plate 13d vibrates the second cylindrical body 13c in the height direction of the vibration device 10. Vibration of the second cylindrical body 13c transmits the vibration of the piezoelectric element 15 to the first cylindrical body 13a via the spring portion 13b. In the vibration device 10, vibration of the first cylindrical body 13a causes the protective cover 11 to vibrate, thereby removing foreign matter such as raindrops adhering to the protective cover 11.

[0036] The excitation circuit 31A applies a drive signal to the piezoelectric element 15 so that the first cylindrical body 13a and the second cylindrical body 13c vibrate in opposite phases in the height direction of the vibration device 10. The excitation circuit 31A can vibrate the vibration device 10 in a vibration mode other than the mode in which the first cylindrical body 13a and the second cylindrical body 13c vibrate in opposite phases in the height direction of the vibration device 10 by the drive signal applied to the piezoelectric element 15.

[0037] Fig. 2 is a schematic cross-sectional view of the configuration of the imaging unit 100 according to this embodiment. Fig. 2 is a cross-sectional view of the vibration device 10 of Fig. 1 cut along a plane passing through the center of the vibration device 10 as viewed from the height direction of the vibration device 10. The imaging unit 100 is a unit that is attached to the front or rear of a vehicle, for example, and captures an image of an object to be imaged. Note that the imaging unit 100 is not limited to being attached to a vehicle, and may be attached to other devices such as a ship or an aircraft.

[0038] The imaging unit 100 includes a vibration device 10 and an imaging device 20. The imaging device 20 is housed within the vibration device 10. The imaging device 20 includes an imaging element such as a CMOS or CCD. The imaging device 20 can form an image based on light transmitted through the protective cover 11. The imaging unit 100 further includes a base member 21, a main body member 22, and a support member 23. The main body member 22 is circular and plate-shaped. The base member 21 is located at the center of the upper surface of the main body member 22. The imaging device 20 is fixed onto the base member 21. The support member 23 extends upward from the outer periphery of the main body member 22. The vibration device 10 is supported by the support member 23. The imaging unit 100 may include one or more optical members, such as lenses, between the protective cover 11 and the imaging device 20.

[0039] When the imaging unit 100 is attached to a vehicle or the like and used outdoors, foreign matter such as raindrops, mud, dust, etc. may adhere to the protective cover 11 that covers the imaging device 20, and the protective cover 11 may also freeze. The vibration device 10 can generate vibrations that remove foreign matter such as raindrops that have adhered to the protective cover 11, or vibrations that defuse the freezing.

[0040] 1-1-2. Oscillatory circuit 3 is a schematic circuit diagram of an oscillation circuit 30A according to this embodiment, including an excitation circuit 31A and a piezoelectric element 15. The excitation circuit 31A includes a control circuit 32, a DC power supply 33, an output circuit 37A including a series circuit of a first switch 35 and a second switch 36, a current detection circuit 38A, a capacitor 39, and a resistor 40.

[0041] The control circuit 32 controls the switching frequencies of the first switch 35 and the second switch 36. The control circuit 32 includes a general-purpose processor, such as a CPU or MPU, that executes programs to implement predetermined functions. The control circuit 32 is configured to be able to communicate with a storage device and executes programs stored in the storage device to implement various processes in the control circuit 32, such as switching of the first switch 35 and the second switch 36. The control circuit 32 is not limited to a configuration in which hardware resources and software work together to implement predetermined functions, but may also be a hardware circuit designed specifically to implement the predetermined functions. In other words, the control circuit 32 can be implemented by various processors, such as a CPU, an MPU, a GPU, an FPGA, a DSP, or an ASIC. Such a control circuit 32 can be implemented, for example, by a signal processing circuit, which is a semiconductor integrated circuit.

[0042] The DC power supply 33 has an output terminal that generates a predetermined voltage between itself and a reference potential 34. The DC power supply 33 may be, for example, a battery, and the output terminal may be the positive terminal of the battery. Note that the DC power supply 33 may be a known device that can apply a predetermined voltage to the piezoelectric element 15 in combination with the reference potential 34.

[0043] The reference potential 34 may be, for example, ground or a body earth connected to the negative terminal of the battery.

[0044] The output circuit 37A is connected to the DC power supply 33. As shown in FIG. 3 , in this embodiment, the output circuit 37A is connected to the reference potential 34 via a current-voltage conversion circuit 42A, which will be described later. As described above, the output circuit 37A includes a series circuit of a first switch 35 and a second switch 36 connected to the DC power supply 33. The series circuit of the first switch 35 and the second switch 36 is also referred to as a “first leg 41A” in this specification. In the first leg 41A of the output circuit 37A, a connection point C1 between the first switch 35 and the second switch 36 is connected to the piezoelectric element 15 via a capacitor 39.

[0045] The first switch 35 is, for example, a metal-oxide semiconductor field-effect transistor (MOSFET), but is not limited to this. The first switch 35 has one end (for example, a source) and the other end (for example, a drain). One end of the first switch 35 is connected to the DC power supply 33. The other end of the first switch 35 is connected to the second switch 36. The other end of the first switch 35 is connected to the piezoelectric element 15 via a capacitor 39. The control circuit 32 is connected to a control end (for example, a gate) of the first switch 35 and can switch the first switch 35 on / off as described above. That is, by switching the first switch 35 on / off, the control circuit 32 can control the first switch 35 to electrically connect / disconnect an electrical path between the DC power supply 33 connected to the first switch 35 and the piezoelectric element 15.

[0046] The second switch 36 is, like the first switch 35, for example, a MOSFET, but is not limited to this. The second switch 36 has one end (e.g., a source) and the other end (e.g., a drain). One end of the second switch 36 is connected to the other end of the first switch 35. That is, like the other end of the first switch 35, one end of the second switch 36 is connected to the piezoelectric element 15 via a capacitor 39. The other end of the second switch 36 is connected to the reference potential 34 via the current-voltage conversion element 45 of the current-voltage conversion circuit 42A. The control circuit 32 is connected to a control end (e.g., a gate) of the second switch 36 and can switch the second switch 36 on and off as described above. That is, by switching the second switch 36 on and off, the control circuit 32 can control the second switch 36 to electrically connect and disconnect the electrical path between the piezoelectric element 15 connected to the second switch 36 and the reference potential 34.

[0047] The current detection circuit 38A can detect at least one of the current flowing through the first switch 35 and the current flowing through the second switch 36, and output a detection signal indicating the magnitude of the detected current to the control circuit 32. The current detection circuit 38A according to this embodiment includes a current-voltage conversion circuit 42A, a low-pass filter 43, and an analog-to-digital conversion circuit (AD conversion circuit) 44.

[0048] The current-voltage conversion circuit 42A includes a current-voltage conversion element 45. The current-voltage conversion element 45 can convert the current flowing through the current-voltage conversion element 45 into a voltage corresponding to the magnitude of the current flowing through the current-voltage conversion element 45. The current-voltage conversion element 45 can be configured to detect, for example, the current flowing through the first switch 35 or the current flowing through the second switch 36 as a voltage. In this embodiment, the current-voltage conversion element 45 is connected between the second switch 36 and the reference potential 34. The current-voltage conversion element 45 can detect the current flowing from the piezoelectric element 15 to the reference potential 34 via the second switch 36. The current-voltage conversion circuit 42A may include two current-voltage conversion elements, one of which detects the current flowing through the first switch 35 and the other of which detects the current flowing through the second switch 36. In this embodiment, the current-voltage conversion element 45 is a resistor (shunt resistor) having a predetermined resistance value. The current-voltage conversion element 45 is not limited to a shunt resistor and may be a Hall element. In this case, the current-voltage conversion element 45 may be disposed near the second switch 36 so as to detect a magnetic field caused by a current flowing through the second switch 36. In this manner, the current-voltage conversion element 45 may be a known element capable of converting a current into a voltage.

[0049] The low-pass filter 43 is a filter circuit that removes signals having frequency components higher than the cutoff frequency. In this embodiment, the low-pass filter 43 is connected to the connection point between the current-voltage conversion element 45 and the second switch 36. The low-pass filter 43 teeth, The voltage input from the current-voltage conversion circuit 42A is smoothed and output to the AD conversion circuit 44.

[0050] The AD conversion circuit 44 is a circuit that converts the voltage (analog signal) smoothed by the low-pass filter 43 into a digital signal that can be input to the control circuit 32. The AD conversion circuit 44 outputs the digital signal to the control circuit 32 as a detection signal. The current detection circuit 38A does not include the AD conversion circuit 44, and does not include the low-pass filter 43. inThe smoothed voltage may be output to the control circuit 32 as a detection signal.

[0051] Current detection circuit 38A according to the present embodiment outputs a detection signal, which is a digital signal generated based on the magnitude of the current flowing through second switch 36, to control circuit 32. However, this is not limiting. For example, current detection circuit 38A may be configured to include only current-voltage conversion circuit 42A and low-pass filter 43, and to output a detection signal, which is an analog signal rather than a digital signal, to control circuit 32.

[0052] As described above, the piezoelectric element 15 has a piezoelectric body and an electrode. The piezoelectric element 15 has one end and the other end, and one end is connected to the capacitor 39 and the other end is connected to the reference potential 34. Specifically, the electrode on one end of the piezoelectric element 15 is connected to the capacitor 39, and the electrode on the other end of the piezoelectric element 15 is connected to the reference potential 34.

[0053] In a first state described below, the capacitor 39 can store charge based on the voltage applied by the DC power supply 33. In a second state described below, the capacitor 39 can release the stored charge to the reference potential 34 via the second switch 36. As a result, the excitation circuit 31A can pass the currents I1 and I2 through the oscillation circuit 30A, as described below, by the control circuit 32 controlling the switching processes of the first switch 35 and the second switch 36. In this way, the capacitor 39 functions as a polarity reversing circuit that reverses the polarity of the voltage applied to the piezoelectric element 15 between the first state and the second state.

[0054] Resistor 40 is connected between the connection point of piezoelectric element 15 and capacitor 39 and reference potential 34. When the switching process by control circuit 32 is completed, one end of piezoelectric element 15 is connected to reference potential 34 via resistor 40, so that one end and the other end are at the same potential.

[0055] 1-2. Example of operation An example of the operation of the excitation circuit 31A according to the first embodiment will be described with reference to Fig. 3. As described above, Fig. 3 shows the oscillation circuit 30A including the excitation circuit 31A and the piezoelectric element 15.

[0056] The control circuit 32 of the excitation circuit 31A according to the first embodiment performs switching processing to complementarily switch the first switch 35 and the second switch 36 at a switching frequency. That is, the control circuit 32 controls the first switch 35 and the second switch 36 so that when the first switch 35 is on, the second switch 36 is off (referred to as the "first state" as appropriate). The control circuit 32 also controls the first switch 35 and the second switch 36 so that when the first switch 35 is off, the second switch 36 is on (referred to as the "second state" as appropriate). The control circuit 32 complementarily switches the first switch 35 and the second switch 36, thereby applying a voltage (e.g., a rectangular wave voltage) having a frequency corresponding to the switching frequency as a drive signal to the piezoelectric element 15 based on a predetermined voltage from the DC power supply 33.

[0057] In the first state, a current I1 flows in the oscillation circuit 30A via the first switch 35. The current I1 is indicated by a dashed arrow in Fig. 3. As shown in Fig. 3, the current I1 flows from the DC power supply 33 to the piezoelectric element 15 via the first switch 35. Therefore, a voltage that makes the excitation circuit 31A side have a higher potential is applied to the piezoelectric element 15.

[0058] In the resonator circuit 30A, when a voltage is applied to the piezoelectric element 15 in the first state, a positive charge accumulates on the output circuit 37A side and a negative charge accumulates on the reference potential 34 side of the capacitor 39 located between the output circuit 37A and the piezoelectric element 15. When the control circuit 32 changes the output circuit 37A from the first state to the second state, the capacitor 39 and the piezoelectric element 15 release the charge. In the second state, the released charge flows as a current I2 through the resonator circuit 30A via the second switch 36. The current I2 is indicated by a dashed arrow in FIG. 3. As shown in FIG. 3, the current I2 flows from the piezoelectric element 15 to the reference potential 34 via the second switch. Furthermore, a negative charge accumulates on the output circuit 37A side of the capacitor 39 and a positive charge accumulates on the piezoelectric element 15 side. Therefore, a voltage is applied to the piezoelectric element 15, causing the excitation circuit 31A side to have a low potential.

[0059] In this way, the control circuit 32 can apply a voltage with reversed polarity at a predetermined frequency to the piezoelectric element 15 by switching the first switch 35 and the second switch 36. Therefore, the vibration circuit 30A according to the present embodiment can reduce the possibility of ion migration occurring in the piezoelectric element 15.

[0060] When a drive signal (e.g., a rectangular wave voltage having a predetermined frequency) is applied to the piezoelectric element 15, the impedance of the piezoelectric element 15 varies depending on the frequency of the drive signal. For example, FIG. 4 is a graph showing the relationship between the frequency of the drive signal applied to the piezoelectric element 15 and the impedance. As shown in FIG. 4, the piezoelectric element 15 has a plurality of frequencies at which the impedance locally decreases. These frequencies correspond to the resonant frequencies of the vibrator 17. In the vibration device 10 according to this embodiment, the resonant frequencies are, for example, approximately 31 kHz (arrow A), approximately 110 kHz (arrow B), and approximately 550 kHz (arrow C). When a voltage (drive signal) having a frequency corresponding to one of these resonant frequencies is applied to the piezoelectric element 15, the piezoelectric element 15 vibrates the protective cover 11 in a different vibration mode for each frequency. For example, when a voltage having a frequency of approximately 31 kHz is applied, the piezoelectric element 15 vibrates the protective cover 11 via the vibrating body 13 in a first removal mode, which is a vibration mode in which the protective cover 11 is vibrated as a whole. The first removal mode is a vibration mode that can atomize and remove foreign matter, such as droplets, adhering to the protective cover 11. When a voltage having a frequency of approximately 110 kHz is applied, the piezoelectric element 15 vibrates the protective cover 11 via the vibrating body 13 in the second removal mode, which is a vibration mode in which the center of the protective cover 11 vibrates more strongly than the peripheral portion. The vibration in the second removal mode corresponds to the resonant frequency of the protective cover 11. When a voltage having a frequency of approximately 550 kHz is applied, the piezoelectric element 15 vibrates the protective cover 11 via the vibrating body 13 in the thawing mode, which is a vibration mode in which the protective cover 11 easily heats up. The vibration around approximately 550 kHz vibrates the protective cover 11 in a higher-order vibration mode with more nodes than the vibration at approximately 110 kHz. In the thawing mode, the impedance of the piezoelectric element 15 is low, so a large amount of power is applied to the piezoelectric element 15, allowing the protective cover 11 to heat up quickly. The above resonant frequency is an example and can be changed depending on the shape and material of the vibration device 10. The piezoelectric element 15 may be configured to apply vibrations to the protective cover 11 in modes other than those described above.

[0061] 4, when a voltage having a frequency corresponding to the resonant frequency is applied, the impedance of piezoelectric element 15 becomes a local minimum. Therefore, by detecting the value of the current flowing through piezoelectric element 15, control circuit 32 can determine whether the frequency of the voltage applied to piezoelectric element 15 is the resonant frequency.

[0062] 5 is a timing chart showing signals (for example, current values ​​and voltage values) input to or output from each element of the excitation circuit 31A. The horizontal axis of FIG. 5 represents time. FIG. 5 shows the timing chart of signals DT1, DT2, and current I R , input voltage V AD . The signal DT1 is an example of a signal used by the control circuit 32 to control the on / off of the first switch 35. The signal DT2 is an example of a signal used by the control circuit 32 to control the on / off of the second switch 36. The first switch 35 and the second switch 36 are turned on when the signals DT1 and DT2 are at a high level (i.e., the first switch 35 electrically connects the DC power supply 33 and the piezoelectric element 15, and the second switch 36 electrically connects the piezoelectric element 15 and the reference potential 34). The first switch 35 and the second switch 36 are turned off when the signals DT1 and DT2 are at a low level (i.e., the first switch 35 electrically disconnects the DC power supply 33 and the piezoelectric element 15, and the second switch 36 electrically disconnects the piezoelectric element 15 and the reference potential 34). The current I R indicates the current flowing through the current-voltage conversion element 45. Current I R corresponds to the voltage input to the low-pass filter 43 based on the current-voltage conversion circuit 42A. AD denotes the smoothed voltage input from the low-pass filter 43 to the AD conversion circuit 44. As shown in FIG. 5, in this embodiment, V AD is a signal having a DC component.

[0063] In Figure 5, the current I R The current I is shown as a solid line. Ris an example of a waveform of a current flowing through the current-voltage conversion element 45 when the switching frequency of the first switch 35 and the second switch 36 corresponds to the resonance frequency of the vibrator 17 (i.e., during resonance). The current I R is an example of a waveform of a current flowing through current-voltage conversion element 45 when the switching frequency of first switch 35 and second switch 36 does not correspond to the resonant frequency of vibrator 17 (i.e., when not resonant). As is clear from Fig. 5, the current when resonant is larger than the current when not resonant.

[0064] Similarly, in Figure 5, the input voltage V AD The input voltage V is shown as a solid line. AD is an example of a waveform of a voltage output from the low-pass filter 43 and input to the AD conversion circuit 44 during resonance. The input voltage V AD is an example of a waveform of a voltage output from the low-pass filter 43 during non-resonance and input to the AD conversion circuit 44. As is clear from Fig. 5, the input voltage during resonance is larger than the input voltage during non-resonance.

[0065] Thus, the signal (voltage) input to the AD conversion circuit 44 has a larger value during resonance than during non-resonance. Therefore, the detection signal input from the AD conversion circuit 44 to the control circuit 32 also has a larger value during resonance than during non-resonance. Therefore, based on the detection signal input from the AD conversion circuit 44, the control circuit 32 can determine whether the switching frequency of the first switch 35 and the second switch 36, i.e., the frequency of the drive signal input to the piezoelectric element 15, is the resonant frequency. For example, the control circuit 32 acquires the values ​​of the detection signals input from the AD conversion circuit 44 at two or more switching frequencies when the switches 35 and 36 are operated at a specific switching frequency. The control circuit 32 then compares the values ​​of the detection signals at different switching frequencies and determines that the switching frequency corresponding to the larger detection signal is closer to the resonant frequency. Therefore, the control circuit 32 can switch the switches 35 and 36 at multiple switching frequencies within a predetermined frequency range and compare the values ​​of the multiple detection signals corresponding to the multiple switching frequencies to determine the switching frequency closest to the resonant frequency within the predetermined frequency range.

[0066] The periods of the signals DT1 and DT2 differ between when they are resonant and when they are not. However, for simplicity, FIG. 5 shows the signal waveforms with the same width even when they have different periods. Therefore, the current I R The period during which the current flows actually differs between when the current is resonant and when it is not resonant.

[0067] In this way, the control circuit 32 can acquire the current flowing through the current-voltage conversion element 45 of the current-voltage conversion circuit 42A as a DC component based on the switching process. Therefore, unlike when detecting the current flowing through the piezoelectric element 15, the control circuit 32 does not need to set the sampling frequency for detecting the current sufficiently higher than the resonance frequency of the vibrator 17, thereby reducing the cost of the current-voltage conversion circuit 42A. Furthermore, by detecting the current, the control circuit 32 can calculate the impedance of the piezoelectric element 15 and determine the resonance frequency of the vibrator 17.

[0068] As described above, the control circuit 32 controls the switching frequency to change the frequency of the voltage applied to the piezoelectric element 15, thereby determining the resonant frequency of the vibrator 17 based on the value of the detection signal input from the current detection circuit 38A. For example, the control circuit 32 can determine the resonant frequency of the vibrator 17 using multiple methods. The excitation circuit 31A according to this embodiment has three sweep methods: a first sweep method, a second sweep method, and a third sweep method (each of which will be described in detail later). The first sweep method, the second sweep method, and the third sweep method differ in the method of changing the switching frequency for determining the resonant frequency of the vibrator 17. The control circuit 32 has multiple sequences to be executed in each of the first sweep method to the third sweep method. In this embodiment, the multiple sequences include a search mode and a drive mode.

[0069] In the search mode, the control circuit 32 changes the switching frequency within a predetermined frequency range (hereinafter referred to as the “first frequency range”) to determine the resonant frequency. Hereinafter, the act of the control circuit 32 changing the switching frequency by a predetermined increment (or decrement) within a given frequency range to determine the resonant frequency is also referred to as “sweeping.” As described above, the control circuit 32 can determine the switching frequency at which the detection signal output from the AD conversion circuit 44 has the largest value as the resonant frequency. Therefore, if the resonant frequency is included within the first frequency range, the control circuit 32 can determine the resonant frequency. If the detection signal output from the AD conversion circuit 44 has the largest value at the upper limit frequency within the first frequency range, the switching frequency may not be the resonant frequency. Therefore, in such a case, the control circuit 32 may change the first frequency range to include a higher frequency, change the switching frequency within the range, and then determine the resonant frequency again. Similarly, if the detection signal output from the AD conversion circuit 44 has the largest value at the lower limit frequency within the first frequency range, the control circuit 32 may change the first frequency range to include a lower frequency and then determine the resonant frequency again. If the control circuit 32 determines that there are multiple switching frequencies at which the value of the output detection signal is locally greatest, the control circuit 32 may execute the sweep again.

[0070] When the control circuit 32 determines the resonant frequency in the search mode, it can vibrate the protective cover 11 in a predetermined vibration mode corresponding to that frequency (e.g., first removal mode, second removal mode, or melting mode) by switching at that frequency. However, the resonant frequency can vary due to various factors. For example, the resonant frequency can vary in response to temperature changes in the protective cover 11. The resonant frequency can also vary when foreign matter adheres to the protective cover 11. Therefore, the excitation circuit 31A according to this embodiment is configured to respond to changes in the frequency in the drive mode.

[0071] In the drive mode, the control circuit 32 changes the switching frequency within a predetermined frequency range (hereinafter referred to as the "second frequency range") narrower than the first frequency range and determines the resonant frequency. When transitioning from the search mode to the drive mode, the control circuit 32 sets the second frequency range so that the resonant frequency determined in the search mode is at its center, and changes the switching frequency within the second frequency range. The control circuit 32 sweeps the switching frequency within the second frequency range, determines the switching frequency at which the detection signal output from the AD conversion circuit 44 has the largest value, and determines the determined switching frequency as the current resonant frequency of the vibrator 17. After determining the current resonant frequency of the vibrator 17, the control circuit 32 changes the frequency set at the center of the second frequency range to the current resonant frequency and updates the second frequency range. The control circuit 32 again sweeps the switching frequency within the updated second frequency range and repeats the above-described update of the second frequency range. By operating in this drive mode, the control circuit 32 can cause the switching frequency to track the resonant frequency of the vibrator 17 even if the resonant frequency changes.

[0072] When vibrating the piezoelectric element 15, the resonant frequency of the vibrator 17 may not be the same when the frequency is changed from a low frequency to a high frequency and when the frequency is changed from a high frequency to a low frequency. Therefore, the control circuit 32 of the excitation circuit 31A according to this embodiment is configured to sweep the switching frequency using multiple methods when determining the resonant frequency using the search mode or the drive mode. In this embodiment, as described above, the control circuit 32 has a first sweep method, a second sweep method, and a third sweep method. In the first sweep method, the control circuit 32 changes the switching frequency from a low frequency to a high frequency (hereinafter also referred to as an "upward sweep"). In the second sweep method, the control circuit 32 changes the switching frequency from a low frequency to a high frequency and then from the high frequency to a low frequency (hereinafter also referred to as an "upward and downward sweep"). In the third sweep method, the control circuit 32 changes the switching frequency from a higher frequency side to a lower frequency side (hereinafter also referred to as "sweeping down").

[0073] The excitation circuit 31A according to this embodiment is configured to operate the protective cover 11 in a predetermined vibration mode by matching the switching frequency of the first switch 35 and the second switch 36 with the resonance frequency of the vibrator 17. In this regard, even when the first switch 35 and the second switch 36 are operated at a switching frequency that has a predetermined ratio to the resonance frequency, the impedance is locally minimized. Here, the frequency that has the predetermined ratio is a frequency that is 1 / (2n+1) times the resonance frequency (n is a positive integer).

[0074] 6A is a graph showing the time change in a drive signal (voltage) having a frequency of 31.5 kHz, which is a frequency near one of the resonant frequencies, applied to piezoelectric element 15, and the time change in the amount of displacement of protective cover 11 when piezoelectric element 15 is driven at that frequency. In FIG. 6A, waveform S1 shows the time change in the drive signal, and waveform D1 shows the time change in the amount of displacement. The amount of displacement of protective cover 11 is obtained by measuring the displacement of protective cover 11 using, for example, a laser Doppler meter, and waveform D1 in FIG. 6A shows the time change in the voltage value obtained by converting the measured amount of displacement into voltage. The horizontal axis of the graph shown in FIG. 6A represents time, and the vertical axis represents voltage.

[0075] Fig. 6B is a graph showing the change over time in a drive signal having a frequency of 10.5 kHz, which is one-third the frequency of 31.5 kHz, applied to piezoelectric element 15, and the change over time in the amount of displacement of protective cover 11 when piezoelectric element 15 is driven at that frequency. In Fig. 6B, waveform S2 shows the change over time in the drive signal, and waveform D2 shows the change over time in the amount of displacement. The horizontal axis of the graph shown in Fig. 6B represents time, and the vertical axis represents voltage.

[0076] As can be seen from FIGS. 6A and 6B, even if the frequency of the drive signal is 1 / 3 of the resonant frequency, the displacement frequency of the protective cover 11 (i.e., the vibration frequency of the protective cover 11) is equal to the resonant frequency. Furthermore, as can be seen from FIGS. 6A and 6B, the maximum displacement amount when the piezoelectric element 15 is driven at a frequency 1 / 3 of the resonant frequency is approximately 1 / 3 of the maximum displacement amount when the piezoelectric element 15 is driven at the resonant frequency. This relationship holds when the frequency of the drive signal is 1 / (2n+1) times the resonant frequency (n is a positive integer). That is, when the frequency of the drive signal is 1 / (2n+1) times the resonant frequency, the maximum displacement amount of the protective cover 11 is approximately 1 / (2n+1) times the maximum displacement amount when the piezoelectric element 15 is driven at the resonant frequency. By utilizing this change in displacement amount based on the difference in the frequency of the drive signal, the vibration device 10 according to this embodiment can achieve various effects.

[0077] For example, in the search mode, the control circuit 32 can sweep the switching frequency through a first frequency range that includes a frequency equivalent to one-third of the resonant frequency to determine the frequency corresponding to the resonant frequency. The control circuit 32 then determines the frequency that is three times the switching frequency determined to correspond to the resonant frequency as the resonant frequency, and sets a second frequency range centered around the three-times frequency to execute the drive mode. This allows the control circuit 32 to reduce the power consumption required for this determination while suppressing the temperature rise of the piezoelectric element 15. Furthermore, by reducing the current value, the control circuit 32 can suppress vibrations that occur when executing the search mode and suppress fluctuations in the resonant frequency due to changes in the state of foreign matter, etc., that are caused by the vibrations.

[0078] The above relationship also holds true between the resonant frequency and its 2n+1 times higher frequency (n is a positive integer). For example, when the control circuit 32 applies a drive signal having a frequency three times the resonant frequency to the piezoelectric element 15, the time change in the displacement of the protective cover 11 has a frequency corresponding to the resonant frequency, as in the case of FIG. 6A. Furthermore, the maximum displacement of the protective cover 11 is approximately one-third of the maximum displacement when a drive signal having the resonant frequency is applied. Therefore, in order to suppress a temperature rise in the piezoelectric element 15, the control circuit 32 may set the switching frequency for switching the first switch 35 and the second switch 36 on and off to (2n+1) times the resonant frequency and operate the switches.

[0079] The control circuit 32 determines whether or not a foreign object has adhered to the protective cover 11 by combining changes in the resonant frequency and changes in the impedance. The resonant frequency of the vibrator 17 decreases as the temperature increases. Similarly, the minimum impedance (local minimum value of impedance) of the piezoelectric element 15 decreases as the temperature increases. In contrast, if a foreign object (e.g., water) adheres to the protective cover 11, the resonant frequency of the vibrator 17 decreases as the amount of adhering water increases. Furthermore, the rate of change in the minimum impedance of the piezoelectric element 15 increases as the amount of adhering water increases. In this way, the control circuit 32 can determine whether or not a foreign object has adhered to the protective cover 11 by referring to changes in the temperature and the minimum impedance. The temperature change can be acquired, for example, by a temperature sensor that can be provided in the vibration device 10. The control circuit 32 may drive the piezoelectric element 15 in the search mode at a frequency that is 1 / (2n+1) times the resonance frequency (n is a positive integer) as described above until a foreign object adheres, and then switch to the drive mode and drive the piezoelectric element 15 at the resonance frequency when it determines that a foreign object has adhered. By driving the piezoelectric element 15 in this manner, the control circuit 32 can reduce the power consumption of the vibration device 10.

[0080] Figure 7A shows an example of control by the control circuit 32 using a first sweep method to determine the resonant frequency. Figure 7B shows an example of control by the control circuit 32 using a second sweep method to determine the resonant frequency. Figure 7C shows an example of control by the control circuit 32 using a third sweep method to determine the resonant frequency.

[0081] FIG. 7A shows an example of the search mode and drive mode processing by the control circuit 32 using the first sweep method. In this embodiment, the control circuit 32 executes the search mode by setting a first frequency range to include a frequency that is approximately 1 / 3 of the resonant frequency. In FIG. 7A, the first frequency range is indicated by fsearch1. The control circuit 32 sweeps the switching frequency in the upward direction to find a frequency fr within the first frequency range where the current is maximum. u When the frequency fr u Multiply the value of fdrive by 3 uAs shown in Fig. 7A, the control circuit 32 executes a sweep for a period tsearch1.

[0082] The control circuit 32 calculates the fdrive u The drive mode is executed by setting the second frequency range so that the second frequency range is centered on fdrive1. In FIG. 7A, the second frequency range is indicated by fdrive1. The control circuit 32 sweeps the switching frequency in the upward direction within the second frequency range, determines the frequency at which the current value is maximized, and sets fdrive2. u to that frequency. As shown in FIG. 7A, the control circuit 32 executes a sweep of the second frequency range in the period tsweep1. The control circuit 32 updates the second frequency range each time a sweep is executed, and again executes a sweep in the updated second frequency range in the period tsweep1. The period tdrive1 indicates the period during which the piezoelectric element 15 is driven in the drive mode. By operating in this manner, the control circuit 32 can vibrate the protective cover 11 at a more accurate frequency while tracking the fluctuating resonance frequency. After executing the drive of the piezoelectric element 15 in the drive mode for a predetermined period, such as the period tdrive1, the control circuit 32 again executes the drive of the piezoelectric element 15 in the search mode. 15 Alternatively, when the control circuit 32 determines that the adhesion of foreign matter has been eliminated based on, for example, a change in temperature and a change in impedance, the control circuit 32 may switch from driving in the drive mode to driving in the search mode. The control circuit 32 may not switch from the drive mode to the search mode, but may instead 15 The same applies to the second sweep method and the third sweep method described later.

[0083] FIG. 7B shows an example of the search mode and drive mode processing by the control circuit 32 using the second sweep method. In this embodiment, the control circuit 32 sets a first frequency range to include the frequency corresponding to the resonant frequency, and executes the search mode. In FIG. 7B, the first frequency range is indicated by fsearch2. The control circuit 32 sweeps the switching frequency in the upward direction to find the frequency fr at which the current is maximized within the first frequency range. u When the frequency fr u Based on fdrive u The control circuit 32 determines the determined fdrive u The control circuit 32 sets the second frequency range in the upward direction so that the frequency fr at which the current becomes maximum within the first frequency range is set to be the center. d When the frequency fr d Based on fdrive d The control circuit 32 determines the determined fdrive d The second frequency range in the down direction is set so that the frequency is centered at . As shown in Fig. 7B, the control circuit 32 executes the up sweep and the down sweep in the period tsearch2. Note that the up sweep period tsearch2 and the down sweep period tsearch2 may be the same length or different lengths.

[0084] When the control circuit 32 sets the second frequency ranges in the up and down directions, it executes the drive mode. The control circuit 32 sweeps the switching frequency within each second frequency range in the up and down directions, determines the frequency at which the current value is maximized, and sets the fdrive u and fdrive dto each frequency. As shown in FIG. 7B, the control circuit 32 performs an up sweep and a down sweep of the second frequency range in each period tsweep2. The control circuit 32 updates the second frequency range each time an up sweep or a down sweep is performed, and then performs a sweep in the updated second frequency range again in period tsweep2. Period tdrive2 indicates the period during which the piezoelectric element 15 is driven in drive mode. By operating in this manner, the control circuit 32 can vibrate the protective cover 11 at a more accurate frequency while tracking the fluctuating resonance frequency for each of the up sweep and the down sweep.

[0085] FIG. 7C shows an example of the search mode and drive mode processing by the control circuit 32 using the third sweep method. In this embodiment, the control circuit 32 executes the search mode by setting a first frequency range to include the frequency corresponding to the resonant frequency. In FIG. 7C, the first frequency range is indicated by fsearch3. The control circuit 32 sweeps the switching frequency downward to find the frequency fr at which the current is maximized within the first frequency range. d When the frequency fr d Based on fdrive d As shown in FIG. 7C, the control circuit 32 executes a sweep with a period tsearch3.

[0086] The control circuit 32 determines the fdrive d The drive mode is executed by setting the second frequency range so that the second frequency range is centered on fdrive3. In FIG. 7C, the second frequency range is indicated by fdrive3. The control circuit 32 sweeps the switching frequency downward within the second frequency range, determines the frequency at which the current value is maximum, and sets fdrive4. dto that frequency. As shown in FIG. 7C, the control circuit 32 executes a sweep of the second frequency range in the period tsweep3. The control circuit 32 then updates the second frequency range each time a sweep is executed, and executes a sweep in the updated second frequency range again in the period tsweep3. The period tdrive3 indicates the period during which the piezoelectric element 15 is driven in drive mode. By operating in this manner, the control circuit 32 can vibrate the protective cover 11 at a more accurate frequency while tracking the fluctuating resonance frequency.

[0087] The control circuit 32 may use, for example, the first sweep method described above in the first removal mode. The control circuit 32 may use the second sweep method described above in the second removal mode. The control circuit 32 may use the third sweep method described above in the ice-melting mode. The sweep methods used in each vibration mode are not limited to those described above, and the control circuit 32 may vibrate the piezoelectric element 15 in any combination. In the first sweep method described above, the control circuit 32 drives the piezoelectric element 15 using a frequency that is one-third the resonant frequency in the search mode and drives the piezoelectric element 15 using the resonant frequency in the drive mode, but this is not a limitation. In the second and third sweep methods described above, the control circuit 32 drives the piezoelectric element 15 using the resonant frequency in the search mode and the drive mode, but this is not a limitation. For example, the control circuit 32 may drive the piezoelectric element 15 using the resonant frequency in the search mode and the drive mode using at least one of the first to third sweep methods. Furthermore, in at least one of the first to third sweep methods, the control circuit 32 may drive the piezoelectric element 15 using a frequency that is 1 / (2n+1) times the resonance frequency in the search mode, and may drive the piezoelectric element 15 using the resonance frequency in the drive mode. Furthermore, in at least one of the first to third sweep methods, the control circuit 32 may drive the piezoelectric element 15 using the resonance frequency in the search mode, and may drive the piezoelectric element 15 using a frequency that is 1 / (2n+1) times the resonance frequency in the drive mode. Furthermore, in at least one of the first to third sweep methods, the control circuit 32 may drive the piezoelectric element 15 using a frequency that is 1 / (2n+1) times the resonance frequency in the search mode and the drive mode.

[0088] FIG. 8 is a graph showing the impedance of piezoelectric element 15 versus switching frequency near a certain resonant frequency, and the phase difference between the voltage applied to piezoelectric element 15 and the current flowing through piezoelectric element 15. As shown in FIG. 8, when the switching frequency changes near the resonant frequency, the impedance changes. As described above, the frequency at which the impedance is locally minimized corresponds to the resonant frequency. Also, as shown in FIG. 8, when the switching frequency changes near the resonant frequency, the phase difference between the voltage applied to piezoelectric element 15 and the current flowing through the piezoelectric element changes. When control circuit 32 switches first switch 35 and second switch 36 at the resonant frequency, the phase difference becomes zero. Therefore, by configuring excitation circuit 31A to detect the phase difference, the switching frequency corresponding to the resonant frequency can be more accurately determined.

[0089] Fig. 9 is a modified example of the excitation circuit 31A according to the first embodiment. Fig. 9 shows an oscillation circuit 30B. The oscillation circuit 30B includes an excitation circuit 31B and a piezoelectric element 15. The excitation circuit 31B further includes a phase comparator 46 in addition to the excitation circuit 31A. The excitation circuit 31B is configured so that the phase comparator 46 can compare the phase difference between the voltage applied to the piezoelectric element 15 and the current flowing through the piezoelectric element, as described above.

[0090] The phase comparator 46 is, for example, a multiplier. The phase comparator 46 can detect a voltage based on the current flowing through the current-voltage conversion element 45. The phase of the current used by the phase comparator 46 is the current flowing through the current-voltage conversion element 45 when the second switch 36 is on. The control circuit 32 can also output a control signal to the phase comparator 46 when switching the first switch 35 and the second switch 36. Therefore, the phase comparator 46 can compare the phase of the voltage applied to the piezoelectric element 15 with the phase of the current flowing through the piezoelectric element based on the phase of the control signal. The phase comparator 46 can be configured to, for example, compare the phase of the control signal for driving the second switch 36 with the phase of the voltage based on the current flowing through the current-voltage conversion element 45, and output a predetermined signal (for example, a voltage) to the control circuit 32 if there is a phase difference. The phase comparator 46 may output a positive voltage to the control circuit 32 if the phase of the control signal leads the phase of the voltage based on the current flowing through the current-voltage conversion element 45, and a negative voltage if the phase of the control signal lags. With this configuration, the control circuit 32 can detect the presence or absence of a phase difference between the current and voltage in the piezoelectric element 15 based on the signal output from the phase comparator 46. The control circuit 32 can also detect whether the phase of the current leads or lags the voltage. As can be seen from FIG. 9 , when the switching frequency is near the resonant frequency, the phase lead or lag between the voltage applied to the piezoelectric element 15 and the current flowing through the piezoelectric element 15 depends on whether the switching frequency is higher or lower than the resonant frequency. Therefore, the control circuit 32 can determine, based on the phase difference, whether the switching frequency needs to be changed to a higher or lower frequency to match the resonant frequency. The control circuit 32 can control the switching frequency based on the phase difference detected by the phase comparator 46, thereby more appropriately matching the switching frequency to the resonant frequency of the vibrator 17. Conversely, the phase comparator 46 may output a voltage having a negative value to the control circuit 32 if the phase of the control signal leads the phase of the voltage based on the current flowing through the current-voltage conversion element 45, and a voltage having a positive value if the phase of the control signal lags.

[0091] Next, the vibration processing of the vibration device 10 by the control circuit 32 will be described with reference to a flowchart. Fig. 10 is a flowchart for explaining the vibration processing of the vibration device 10 by the control circuit 32 of the excitation circuit 31A according to this embodiment. In this vibration processing, the control circuit 32 executes a search mode within a first frequency range that includes a frequency that is one-third of the resonance frequency to drive the piezoelectric element 15. Then, when the control circuit 32 determines that a foreign object has adhered to the protective cover 11, it determines the current resonance frequency and executes a drive mode within a second frequency range that includes the current resonance frequency to drive the piezoelectric element 15.

[0092] First, the control circuit 32 calculates a frequency that is 1 / 3 times the resonant frequency of the vibration mode in which the piezoelectric element 15 is driven (S10). After calculating the 1 / 3 frequency, the control circuit 32 sets a first frequency range that includes the calculated frequency (S11). After setting the first frequency range, the control circuit 32 drives the piezoelectric element 15 within the first frequency range in a search mode and determines the current resonant frequency (S12). That is, the control circuit 32 sweeps the switching frequencies of the first switch 35 and the second switch 36 within the first frequency range and determines the current resonant frequency based on the magnitude of the current detected by the current detection circuit 38A.

[0093] The control circuit 32 determines whether a foreign object is attached to the protective cover 11 based on, for example, the impedance calculated from the magnitude of the detected current as described above (S13). If the control circuit 32 determines that a foreign object is not attached (S13: No), it executes step S12 again and determines the current resonant frequency again. If the control circuit 32 determines that a foreign object is attached (S13: Yes), it sets a second frequency range centered on the current resonant frequency at that time (S14). After setting the second frequency range, the control circuit 32 drives the piezoelectric element 15 within the second frequency range in drive mode and determines the current resonant frequency (S15). That is, the control circuit 32 sweeps the switching frequencies of the first switch 35 and the second switch 36 within the second frequency range and determines the current resonant frequency based on the magnitude of the current detected by the current detection circuit 38A.

[0094] As in step S13, the control circuit 32 checks whether any foreign matter remains on the protective cover 11, for example, based on the impedance calculated from the magnitude of the detected current (S16). If it is determined that any foreign matter remains on the protective cover 11 (S16: No), the control circuit 32 executes step S14 again to set a second frequency range centered on the current resonance frequency determined in step S15. That is, the control circuit 32 updates the second frequency range to a range centered on the current resonance frequency. Then, the control circuit 32 drives the piezoelectric element 15 in drive mode until the foreign matter is removed. If it is determined that the foreign matter has been removed (i.e., there is no foreign matter remaining on the protective cover 11) (S16: Yes), the control circuit 32 stops driving the piezoelectric element 15 (S17). In this way, the control circuit 32 can remove any foreign matter from the protective cover 11. Furthermore, the control circuit 32 can reduce the power required to remove the foreign matter.

[0095] 8, the impedance value of piezoelectric element 15 at each resonance frequency corresponding to each vibration mode differs depending on the frequency. Therefore, the value of the current flowing through piezoelectric element 15 when first switch 35 and second switch 36 are switched at each resonance frequency differs for each frequency. For this reason, current detection circuit 38A needs to be configured to correspond to the vibration mode in which the current flowing through current-voltage conversion element 45 is the largest (i.e., the impedance value of piezoelectric element 15 is the lowest).

[0096] 11 is a schematic circuit diagram showing an example of low-pass filter 43 configured to be able to change the amplification factor. Low-pass filter 43 can change the amplification factor for the input voltage, so it can determine the resonant frequency even if the magnitude of the current flowing through current-voltage conversion element 45 is different.

[0097] The low-pass filter 43 includes an operational amplifier 50, a variable resistor 51, a resistor 52, and a capacitor 53. The inverting input terminal of the operational amplifier 50 is connected to an input terminal Vin (i.e., a terminal different from the reference potential side of the current-voltage conversion element 45) via the variable resistor 51, the non-inverting input terminal is connected to a reference potential, and the output terminal is connected to an output terminal Vout (i.e., a terminal that outputs to the AD conversion circuit 44). The variable resistor 51 is disposed between the input terminal Vin and the inverting input terminal of the operational amplifier 50. The resistor 52 is disposed so as to connect the inverting input terminal and output terminal of the operational amplifier via the resistor 52. The capacitor 53 is disposed in parallel with the resistor 52 so as to connect the inverting input terminal and output terminal of the operational amplifier 50 via the capacitor 53. By changing the resistance value of the variable resistor 51, the low-pass filter 43 can change the amplification factor (i.e., gain) of the voltage input from the input terminal Vin. Therefore, the resonant frequency can be determined even if the magnitude of the current flowing through the current-voltage conversion element 45 is different. The frequency control circuit 32 can change the amplification factor based on, for example, the vibration mode in which the piezoelectric element 15 is vibrated. The control circuit 32 may also change the amplification factor based on a frequency included in a first frequency range in which the piezoelectric element 15 is driven in a search mode. By including the low-pass filter 43 having such a configuration, the excitation circuit 31B can accurately detect the peak current in a plurality of vibration modes in which the peak current differs.

[0098] Fig. 12 is a modified example of the excitation circuit 31A according to the first embodiment. Fig. 12 shows the configuration of an oscillation circuit 30C. The oscillation circuit 30C includes an excitation circuit 31C and a piezoelectric element 15. The excitation circuit 31C includes a DC power supply 33A and a negative power supply circuit 33B instead of the DC power supply 33 in the excitation circuit 31A. Furthermore, the excitation circuit 31C does not include the capacitor 39 in the excitation circuit 31A. The negative power supply circuit 33B functions as a polarity inversion circuit in place of the capacitor 39 in the excitation circuit 31A.

[0099] In this embodiment, the DC power supply 33A is connected to the first switch 35 instead of the DC power supply 33 in the excitation circuit 31A. The DC power supply 33A outputs a positive voltage. The negative power supply circuit 33B is connected to the opposite side of the DC power supply 33A with respect to the series circuit of the output circuit 37A. Specifically, the negative power supply circuit 33B is connected to the second switch 36 instead of the reference potential 34 in the oscillation circuit 30A via the current-voltage conversion circuit 42A. The negative power supply circuit 33B outputs a negative voltage. That is, the negative power supply circuit 33B has a potential whose polarity is inverted relative to the DC power supply 33A, with respect to the potential of the reference potential 34. For example, if the potential of the reference potential 34 is zero and the potential of the DC power supply 33A is +Vp, the potential of the negative power supply circuit 33B may be -Vp. The DC power supply 33A and the negative power supply circuit 33B may each be a known device capable of applying a predetermined voltage to the piezoelectric element 15 in combination with a reference potential.

[0100] With this configuration, when the control circuit 32 executes switching processing of the first switch 35 and the second switch 36, it is possible to apply voltages of opposite polarity to the piezoelectric element 15 between the first state and the second state. For example, when the potential of the reference potential 34 is zero, the potential of the DC power supply 33A is +Vp, and the potential of the negative power supply circuit 33B is −Vp, the control circuit 32 can apply a positive voltage of +Vp to the piezoelectric element 15 in the first state and a negative voltage of −Vp to the piezoelectric element 15 in the second state. In this case, the control circuit 32 can apply a voltage that averages to zero through switching processing to the piezoelectric element 15. By applying a voltage of opposite polarity to the piezoelectric element 15, the resonating circuit 30C can reduce the possibility of ion migration occurring in the piezoelectric element 15, similar to the resonating circuit 30A.

[0101] (Second embodiment) 2-1.Configuration example A vibration device according to a second embodiment of the present disclosure will be described. Note that in the second embodiment, differences from the first embodiment will be mainly described. In the second embodiment, components that are the same as or equivalent to those in the first embodiment will be denoted by the same reference numerals. Also, in the second embodiment, descriptions that overlap with those in the first embodiment will be omitted.

[0102] FIG. 13 is a schematic circuit diagram of an oscillation circuit 30D including an excitation circuit 31D and a piezoelectric element 15 according to a second embodiment of the present disclosure. The excitation circuit 31D of the oscillation circuit 30D has an output circuit 37B, instead of the output circuit 37A of the excitation circuit 31A, which further includes a series circuit of a third switch 60 and a fourth switch 61 connected to a DC power supply 33. The series circuit of the third switch 60 and the fourth switch 61 is also referred to herein as a "second leg 41B." The second leg 41B is connected in parallel with the first leg 41A between the DC power supply 33 and a reference potential 34. As shown in FIG. 13, in this embodiment, the second leg 41B is connected to the reference potential 34 via a current-voltage conversion element 45 of a current-voltage conversion circuit 42A. Alternatively or additionally, the second leg 41B may be connected to the DC power supply 33 via the current-voltage conversion element 45 of the current-voltage conversion circuit 42A. 13, the piezoelectric element 15 of the resonant circuit 30D is not connected to the reference potential 34, unlike the resonant circuit 30A according to the first embodiment, but is instead connected to the connection point C2 between the third switch 60 and the fourth switch 61 of the second leg 41B. Therefore, the piezoelectric element 15 is connected between the connection point C1 between the first switch 35 and the second switch 36 and the connection point C2 between the third switch 60 and the fourth switch 61. Note that the resonant circuit 30D according to the second embodiment does not necessarily have to include the capacitor 39 included in the resonant circuit 30A according to the first embodiment.

[0103] The third switch 60 is, like the first switch 35, for example, a MOSFET, but is not limited to this. The third switch 60 has one end (source) and the other end (drain). One end of the third switch 60 is connected to the DC power supply 33. Also, one end of the third switch 60 is connected to one end of the first switch 35. The other end of the third switch 60 is connected to one end of the fourth switch 61. Also, the other end of the third switch 60 is connected to the end opposite to the end connected to the first leg 41A of the piezoelectric element 15. The control circuit 32 is connected to the control end of the third switch 60 and can switch the third switch 60 on / off. By switching the third switch 60 on / off, the control circuit 32 controls the DC power supply 33 and the piezoelectric element 15 connected to the third switch 60. 15 The third switch 60 can be controlled to electrically connect / disconnect the circuit between the first and second terminals.

[0104] The fourth switch 61 is, like the first switch 35, for example, a MOSFET, but is not limited to this. The fourth switch 61 has one end (source) and the other end (drain). One end of the fourth switch 61 is connected to the other end of the third switch 60. That is, like the other end of the third switch, one end of the fourth switch 61 is connected to the piezoelectric element 15. The other end of the fourth switch 61 is connected to the reference potential 34 via the current-voltage conversion element 45 of the current-voltage conversion circuit 42A. The control circuit 32 is connected to the control end of the fourth switch 61 and can switch the fourth switch 61 on and off. By switching the fourth switch 61 on and off, the control circuit 32 can control the fourth switch 61 to electrically connect and disconnect the circuit between the piezoelectric element 15 connected to the fourth switch 61 and the reference potential 34.

[0105] 2-2. Example of operation An example of the operation of the excitation circuit 31D according to the second embodiment will be described with reference to Fig. 13. As described above, Fig. 13 shows the oscillation circuit 30D including the excitation circuit 31D and the piezoelectric element 15.

[0106] The control circuit 32 of the excitation circuit 31D according to the second embodiment controls the third switch 60 and the fourth switch 61 so as to complementarily switch in addition to the first switch 35 and the second switch 36. That is, the control circuit 32 controls the on / off switching of each of the switches 35, 36, 60, 61 so that the third switch 60 and the second switch 36 are synchronized and the fourth switch 61 and the first switch 35 are synchronized. 60 The control circuit 32 controls the first switch 35 to the fourth switch 61 so that when the first switch 35 and the third switch 60 are on, the second switch 36 and the fourth switch 61 are off (referred to as the "third state" as appropriate). Furthermore, when the first switch 35 and the third switch 60 are off, the control circuit 32 controls the first switch 35 to the fourth switch 61 so that the second switch 36 and the fourth switch 61 are on (referred to as the "fourth state" as appropriate). The control circuit 32 can reverse the polarity of the voltage applied to the piezoelectric element 15 by switching each of the switches 35, 36, 60, and 61 between the third state and the fourth state.

[0107] By operating the control circuit 32 in this manner, the current detection circuit 38A can detect the current flowing from the DC power supply 33 to the reference potential 34 through the first switch 35, the piezoelectric element 15, and the fourth switch 61 in the third state. Furthermore, the current detection circuit 38A can detect the current flowing from the DC power supply 33 to the reference potential 34 through the third switch 60, the piezoelectric element 15, and the second switch 36 in the fourth state. In the resonant circuit 30A according to the first embodiment, the current detection circuit 38A detects the current only in the second state. However, in the resonant circuit 30D according to the second embodiment, the current detection circuit 38A detects the current in both the third state and the fourth state. That is, unlike the current detection circuit 38A according to the first embodiment, which detects the current flowing in the current-voltage conversion element 45 in the second state, the current detection circuit 38A according to the second embodiment detects the current flowing in the current-voltage conversion element 45 in both the third state and the fourth state. Therefore, the value output from the AD conversion circuit 44 to the control circuit 32 via the low-pass filter 43 is substantially based on the sum of the currents flowing through the second switch 36 and the fourth switch 61. Therefore, the excitation circuit 31D can improve the S / N ratio of the signal input from the AD conversion circuit 44 to the control circuit 32.

[0108] (Third embodiment) 3-1.Configuration example A vibration device according to a third embodiment of the present disclosure will be described. Note that in the third embodiment, differences from the first embodiment will be mainly described. In the third embodiment, components that are the same as or equivalent to those in the first embodiment will be denoted by the same reference numerals. Also, in the third embodiment, descriptions that overlap with those in the first embodiment will be omitted.

[0109] 14 is a schematic circuit diagram of an oscillation circuit 30E including an excitation circuit 31E and a piezoelectric element 15 according to a third embodiment of the present disclosure. The excitation circuit 31E according to the third embodiment includes a current-voltage conversion circuit 42E instead of the current-voltage conversion circuit 42A. The current-voltage conversion circuit 42E of the excitation circuit 31E includes a current-voltage conversion element 45A between the second switch 36 and the reference potential 34, and a current-voltage conversion element 45B between the DC power supply 33 and the first switch 35. The current-voltage conversion element 45A corresponds to the current-voltage conversion element 45 in the first embodiment. In this embodiment, the current-voltage conversion elements 45A and 45B are resistors (shunt resistors) having a predetermined resistance value, similar to the current-voltage conversion element 45, but are not limited to this and may be known elements capable of converting current into voltage, such as Hall elements.

[0110] The current-voltage conversion circuit 42E has a differential circuit 70 between the connection point between the second switch 36 and the current-voltage conversion element 45A and the low-pass filter 43. The connection point between the current-voltage conversion element 45B and the first switch 35 is connected to the differential circuit 70.

[0111] The difference circuit 70 is, for example, a differential amplifier circuit configured to have an amplification factor of 1, but is not limited to this and any known circuit may be used. The current-voltage conversion element 45A converts the current flowing to the current-voltage conversion element 45A via the second switch 36 into a voltage corresponding to the magnitude of the current flowing to the current-voltage conversion element 45A. The current-voltage conversion element 45B converts the current flowing to the current-voltage conversion element 45B via the first switch 35 into a voltage corresponding to the magnitude of the current flowing to the current-voltage conversion element 45B. The difference circuit 70 outputs a voltage indicating the difference between the voltage input from the current-voltage conversion element 45A and the voltage input from the current-voltage conversion element 45B as a detection voltage to the low-pass filter 43.

[0112] The current-voltage conversion element 45A is arranged on the low potential side (low side) of the piezoelectric element 15, and the current-voltage conversion element 45B is arranged on the high potential side (high side) of the piezoelectric element 15. Considering FIG. 14 and the direction of the current, the polarities of the voltages converted by the current-voltage conversion elements 45A and 45B are opposite.

[0113] Therefore, when the difference circuit 70 obtains the difference between these voltages, the current flowing through the current-voltage conversion element 45B in the first state can be detected.

[0114] In the resonant circuit 30E according to the third embodiment, the current detection circuit 38E detects the current in each of the first and second states. Unlike the current detection circuit 38A, which detects the current flowing through the current-voltage conversion element 45 in the second state, the current detection circuit 38E according to the third embodiment detects the current flowing through the current-voltage conversion elements 45A and 45B in each of the first and second states. Therefore, when the differential circuit 70 obtains the difference in voltage values ​​based on the currents flowing through the current-voltage conversion elements 45A and 45B, the value output from the AD conversion circuit 44 to the control circuit 32 is substantially based on the sum of the currents flowing through the first switch 35 and the second switch 36. In this way, the excitation circuit 31E obtains the difference in voltage values, thereby canceling out common-mode noise flowing through the elements 45A and 45B and improving the S / N ratio of the signal input from the AD conversion circuit 44 to the control circuit 32.

[0115] The current detection circuit 38E of the excitation circuit 31E according to this embodiment acquires the difference between the voltages converted by the current-voltage conversion element 45A and the current-voltage conversion element 45B using a differential circuit 70, but is not limited to this. For example, if Hall elements are used for the current-voltage conversion elements 45A and 45B, the current detection circuit 38E may include an arithmetic circuit that adds up the voltages obtained by the Hall elements instead of the differential circuit 70.

[0116] (Summary of the embodiment) The excitation circuit, vibration device, and vehicle according to the present embodiment described above may be configured as follows.

[0117] (Mode 1) The excitation circuit includes an output circuit including a series circuit of a first switch and a second switch connected to a DC power source, with a piezoelectric element connected to the connection point between the first switch and the second switch; a current detection circuit that detects at least one of the current flowing through the first switch and the current flowing through the second switch and outputs a detection signal indicating a value based on the detected current; and a control circuit that performs a switching process that complementarily switches the first switch and the second switch on and off at a switching frequency corresponding to a predetermined frequency in order to apply a voltage of a predetermined frequency from the output circuit to the piezoelectric element, and has a search mode that determines the resonant frequency of a vibrator including the piezoelectric element and an object vibrated by the piezoelectric element based on the value indicated by the detection signal output from the current detection circuit.

[0118] (Embodiment 2) The excitation circuit of embodiment 1 may include a current detection circuit that detects at least one of the current flowing through the first switch and the current flowing through the second switch and outputs a detected voltage based on the detected current, and a low-pass filter that smooths the detected voltage from the current-voltage conversion circuit and outputs the smoothed detected voltage.

[0119] (Embodiment 3) The excitation circuit of embodiment 2 may further include an analog-to-digital conversion circuit in which the current detection circuit receives the smoothed detection voltage from the low-pass filter and outputs a digital signal indicating the smoothed detection voltage from the low-pass filter as the detection signal to the control circuit.

[0120] (Aspect 4) In the excitation circuit of aspect 2 or aspect 3, the current-voltage conversion circuit may have a first current-voltage conversion element that converts the current flowing through the first switch into a voltage and outputs it, a second current-voltage conversion element that converts the current flowing through the second switch into a voltage and outputs it, and an arithmetic circuit that outputs a voltage indicating the difference or sum of the current flowing through the first switch and the current flowing through the second switch based on the voltage output from the first current-voltage conversion element and the voltage output from the second current-voltage conversion element to the low-pass filter as a detection voltage.

[0121] (Embodiment 5) The excitation circuit of any one of embodiments 1 to 4 may further include a polarity reversal circuit that reverses the polarity of the voltage applied to the piezoelectric element when the first switch is on and the second switch is off and when the first switch is off and the second switch is on.

[0122] (Embodiment 6) In the excitation circuit of embodiment 5, the polarity reversal circuit may include a capacitor connected between the connection point of the first switch and the second switch and the piezoelectric element.

[0123] (Embodiment 7) The excitation circuit of embodiment 5 may include a negative power supply circuit in which the DC power supply outputs a positive voltage and the polarity inversion circuit is connected on the opposite side of the DC power supply to the series circuit of the output circuit, and outputs a negative voltage.

[0124] (Embodiment 8) In the excitation circuit of any one of embodiments 1 to 7, the current detection circuit may further include a phase difference detection circuit that detects the phase difference between the current flowing through the second switch and the voltage applied to the piezoelectric element, and the control circuit may adjust the switching frequency based on the detected phase difference.

[0125] (Aspect 9) The excitation circuit of any one of aspects 1 to 8 may further have a drive mode in which the search mode changes the switching frequency within a first frequency range, obtains changes in the value of the detection signal in response to changes in the switching frequency within the first frequency range, and determines the resonant frequency of the vibrator based on the frequency within the first frequency range at which the value of the detection signal is maximized, and the control circuit changes the switching frequency within a second frequency range that includes the resonant frequency of the vibrator and is narrower than the first frequency range, obtains changes in the value of the detection signal in response to changes in the switching frequency within the second frequency range, and repeats the operation of updating the resonant frequency of the vibrator based on the frequency within the second frequency range at which the value of the detection signal is maximized.

[0126] (Embodiment 10) In the excitation circuit of embodiment 9, the control circuit may change the gain of the current detection circuit based on a frequency included in the first frequency range.

[0127] (Embodiment 11) In the excitation circuit of embodiment 9 or embodiment 10, the first frequency range may include frequencies that are 1 / (2n+1) or (2n+1) times the resonant frequency of the vibrator, and the second frequency range may include the resonant frequency of the vibrator, which is the frequency within the second frequency range at which the value of the detection signal is maximum, where n is a positive integer.

[0128] (Embodiment 12) In the excitation circuit of embodiment 9 or embodiment 10, the first frequency range may include a resonant frequency of the vibrator, and the second frequency range may include a frequency that is 1 / (2n+1) or (2n+1) times the resonant frequency of the vibrator, where n is a positive integer, and is the frequency within the second frequency range at which the value of the detection signal is maximized.

[0129] (Embodiment 13) In the excitation circuit of any one of embodiments 1 to 12, the piezoelectric element may have a first end and a second end, the first end of the piezoelectric element may be connected to a connection point of the first switch and the second switch, and the second end of the piezoelectric element may be connected to a reference potential having a lower potential than the output terminal of the DC power supply.

[0130] (Embodiment 14) In the excitation circuit of any one of embodiments 1 to 12, the output circuit may further include a series circuit of a third switch and a fourth switch connected to a DC power supply in parallel with the series circuit of the first switch and the second switch, a piezoelectric element may be connected between the connection point of the third switch and the fourth switch and the connection point of the first switch and the second switch, an end of the first switch opposite to the second switch and an end of the third switch opposite to the fourth switch are connected to each other, an end of the second switch opposite to the first switch and an end of the fourth switch opposite to the third switch are connected to each other, and a switching process may complementarily switch on and off the pair of the first switch and the fourth switch and the pair of the second switch and the third switch at a switching frequency.

[0131] (Embodiment 15) A vibration device includes the excitation circuit of any one of embodiments 1 to 14, a piezoelectric element, and a light-transmitting protective cover that is vibrated by the piezoelectric element.

[0132] (Embodiment 16) A vehicle includes the vibration device of embodiment 15 and an imaging device that captures an image and detects light that passes through the protective cover.

[0133] The excitation circuit, vibration device, and vehicle described in the present disclosure are realized by the cooperation of hardware resources, such as a processor and memory, and software resources (computer programs). [Industrial Applicability]

[0134] According to the present disclosure, it is possible to provide an excitation circuit, a vibration device, and a vehicle that can detect the magnitude of the current flowing through a piezoelectric element while reducing the possibility of migration occurring in the piezoelectric element, and therefore the present disclosure can be suitably used in this type of industrial field. [Explanation of symbols]

[0135] 10 Vibration device 11 Protective cover 13 Vibration body 15 Piezoelectric element 17 Oscillators 20 Imaging device 30A, 30B, 30C, 30D, 30E vibration circuit 31A, 31B, 31C, 31D, 31E Excitation circuit 32 Control circuit 33, 33A DC power supply 33B Negative power supply circuit 34 Reference potential 35 First Switch 36 Second Switch 37A, 37B output circuit 38A, 38E Current detection circuit 39 Capacitor 40 Resistance 42A, 42E Current-to-voltage conversion circuit 43 Low-pass filter 44 Analog / Digital Conversion Circuit 45, 45A, 45B Current-to-voltage conversion element 60 Third Switch 61 4th Switch 70 Differential circuit C1, C2 connection points

Claims

1. an output circuit including a series circuit of a first switch and a second switch connected to a DC power supply, and a piezoelectric element connected to a connection point between the first switch and the second switch; a current detection circuit that detects at least one of a current flowing through the first switch and a current flowing through the second switch and outputs a detection signal indicating a value based on the detected current; a control circuit that executes a switching process of complementarily switching on and off the first switch and the second switch at a switching frequency corresponding to a predetermined frequency in order to apply a voltage of the predetermined frequency from the output circuit to the piezoelectric element, and has a search mode that determines a resonance frequency of a vibrator including the object vibrated by the piezoelectric element and the piezoelectric element based on a value indicated by the detection signal output from the current detection circuit; Equipped with Excitation circuit.

2. The current detection circuit a current-voltage conversion circuit that detects at least one of a current flowing through the first switch and a current flowing through the second switch, and outputs a detection voltage based on the detected current; a low-pass filter that smoothes the detected voltage from the current-voltage conversion circuit and outputs the smoothed detected voltage; Equipped with 2. The excitation circuit of claim 1.

3. The current detection circuit an analog-to-digital conversion circuit that receives the smoothed detection voltage from the low-pass filter and outputs a digital signal indicating the smoothed detection voltage from the low-pass filter as the detection signal to the control circuit; 3. The excitation circuit of claim 2.

4. The current-voltage conversion circuit a first current-voltage conversion element that converts a current flowing through the first switch into a voltage and outputs the voltage; a second current-voltage conversion element that converts the current flowing through the second switch into a voltage and outputs the voltage; an arithmetic circuit that outputs a voltage indicating a difference or a sum of a current flowing through the first switch and a current flowing through the second switch as the detection voltage to the low-pass filter based on a voltage output from the first current-voltage conversion element and a voltage output from the second current-voltage conversion element; having 3. The excitation circuit of claim 2.

5. a polarity reversal circuit that reverses the polarity of the voltage applied to the piezoelectric element when the first switch is on and the second switch is off and when the first switch is off and the second switch is on; 2. The excitation circuit of claim 1.

6. the polarity reversing circuit includes a capacitor connected between the piezoelectric element and a connection point between the first switch and the second switch.

6. The excitation circuit of claim 5.

7. the DC power supply outputs a positive voltage; the polarity inversion circuit includes a negative power supply circuit connected to the series circuit of the output circuit on the opposite side to the DC power supply and outputting a negative voltage.

6. The excitation circuit of claim 5.

8. the current detection circuit further includes a phase difference detection circuit that detects a phase difference between a current flowing through the second switch and a voltage applied to the piezoelectric element; The control circuit adjusts the switching frequency based on the detected phase difference.

2. The excitation circuit of claim 1.

9. The search mode is changing the switching frequency within a first frequency range and acquiring a change in the value of the detection signal in response to the change in the switching frequency within the first frequency range; and determining a resonant frequency of the vibrator based on a frequency within the first frequency range at which the value of the detection signal is maximized; the control circuit changes the switching frequency within a second frequency range that includes a resonant frequency of the vibrator and is narrower than the first frequency range, acquires a change in the value of the detection signal in response to the change in the switching frequency within the second frequency range, and repeats an operation of updating the resonant frequency of the vibrator based on the frequency at which the value of the detection signal becomes maximum within the second frequency range, Further having 2. The excitation circuit of claim 1.

10. the control circuit changes a gain of the current detection circuit based on a frequency included in the first frequency range.

10. The excitation circuit of claim 9.

11. the first frequency range includes frequencies that are 1 / (2n+1) times or (2n+1) times the resonant frequency of the vibrator; the second frequency range includes a resonant frequency of the vibrator, which is a frequency within the second frequency range at which the value of the detection signal is maximum, n is a positive integer, 10. The excitation circuit of claim 9.

12. the first frequency range includes a resonant frequency of the vibrator; the second frequency range includes a frequency that is 1 / (2n+1) times or (2n+1) times the resonant frequency of the vibrator, which is a frequency within the second frequency range at which the value of the detection signal is maximum, n is a positive integer, 10. The excitation circuit of claim 9.

13. the piezoelectric element has a first end and a second end; a first end of the piezoelectric element is connected to a connection point between the first switch and the second switch; a second end of the piezoelectric element is connected to a reference potential having a lower potential than the output end of the DC power supply; 2. The excitation circuit of claim 1.

14. The output circuit further including a series circuit of a third switch and a fourth switch connected to the DC power supply in parallel with the series circuit of the first switch and the second switch; the piezoelectric element is connected between a connection point between the third switch and the fourth switch and a connection point between the first switch and the second switch; an end of the first switch opposite to the second switch and an end of the third switch opposite to the fourth switch are connected to each other, an end of the second switch opposite to the first switch and an end of the fourth switch opposite to the third switch are connected to each other; the switching process complementarily switches on and off a pair of the first switch and the fourth switch and a pair of the second switch and the third switch at the switching frequency; 2. The excitation circuit of claim 1.

15. an excitation circuit according to claim 1; The piezoelectric element; a light-transmitting protective cover that is vibrated by the piezoelectric element; Equipped with Vibration device.

16. The vibration device according to claim 15; an imaging device that detects light passing through the protective cover; Equipped with vehicle.

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