JP2009257978A - Ultrasonic sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultrasonic sensor for suppressing the reduction in detection accuracy due to humidity variation. <P>SOLUTION: In this ultrasonic sensor 10, a circuit device 20 applies voltage for oscillating ultrasonic wave to a laminated piezoelectric element 16, and detects that the receiving devices 12p-12r receive ultrasonic wave, when an output voltage output from a piezoelectric element 14p is equal to or greater than a threshold Vs. The circuit device 20 calculates a sound pressure P of the ultrasonic wave after propagating over a propagating distance (r) based on the humidity (h) calculated by the humidity arithmetic processing. When the lowest output voltage Vu determined from the sound pressure P is less than a value obtained by multiplying the threshold Vs by coefficient α, the circuit device 20 performs adjustment so as to decrease the threshold Vs. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an ultrasonic sensor that receives and detects an ultrasonic wave transmitted from a transmitting element and reflected by a detection target by a receiving element.

Conventionally, as an ultrasonic sensor that receives and detects an ultrasonic wave transmitted from a transmitting element and reflected by a detection target by a receiving element, there is an ultrasonic sensor device disclosed in Patent Document 1 below, for example. This ultrasonic sensor device includes one transmission element and four reception elements. When the AC voltage is applied to the electrode metal film of the piezoelectric vibrator, the transmitting element transmits the ultrasonic wave with the membrane resonating with the piezoelectric vibrator at a predetermined ultrasonic band frequency. This ultrasonic wave is reflected by the obstacle and received by each receiving element. In response to this reception, the distance and direction of the obstacle with respect to the ultrasonic sensor device are calculated from the time difference and phase difference of the reception signals output from the respective receiving elements.
JP 2006-242650 A

  By the way, when ambient humidity changes, the ultrasonic wave transmitted from the transmitting element attenuates and the sound pressure of the reaching sound wave changes. In the ultrasonic sensor device disclosed in Patent Document 1, ultrasonic waves having two different frequencies are transmitted from the transmitting element, the humidity is calculated from the difference in attenuation coefficient between the two ultrasonic waves, and the calculated humidity is set in advance. Used to correct the operating humidity.

  When the above-described humidity correction function is employed in an ultrasonic sensor having a transmitting element and a receiving element configured by joining an acoustic matching layer and a piezoelectric element, as a humidity compensating function, for example, reception of ultrasonic waves in a receiving element It is conceivable to eliminate the influence of humidity in ultrasonic detection by correcting a predetermined threshold used for determination according to humidity.

  However, since it is necessary to set the predetermined threshold value appropriately according to changes in the surrounding humidity, when the threshold value cannot be set appropriately in this way, for example, each receiving element may not be able to detect ultrasonic waves. As a result, errors are included in the time difference and the phase difference, so that there is a problem that the detection accuracy with respect to the obstacle is lowered.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide an ultrasonic sensor that can suppress a decrease in detection accuracy due to a change in humidity.

  In order to achieve the above object, in the ultrasonic sensor according to claim 1, the first piezoelectric element (16) capable of oscillating an ultrasonic wave and the ultrasonic wave oscillated by the first piezoelectric element are provided. A transmission element (11) having a first acoustic matching member (13) capable of transmitting the ultrasonic wave and transmitting the ultrasonic wave to the detected object; and the ultrasonic wave reflected by the detected object. A second piezoelectric element (14p) capable of detection and a second acoustic matching member (13p) capable of transmitting the ultrasonic wave reflected by the detected object to the second piezoelectric element; A receiving element (12p-12r) for receiving the ultrasonic wave reflected by the detector, a voltage for oscillating the ultrasonic wave to the first piezoelectric element, and the second piezoelectric element When the output voltage output from the first threshold is equal to or higher than the first threshold (Vs). A circuit element (20) for detecting reception of the ultrasonic wave by the element, humidity detecting means (20) for detecting humidity (h) around the transmitting element and the receiving element, and detection of the object to be detected A sound pressure (P) of the ultrasonic wave when propagating a propagation distance (r) corresponding to a round trip of a set distance is calculated based on the humidity detected by the humidity detecting means, and the sound pressure is calculated. Accordingly, when the amplitude value (Vu) of the output voltage output from the second piezoelectric element is smaller than the second threshold value set larger than the first threshold value, the first threshold value is adjusted to be lowered. And a threshold value adjusting means (20).

  In the first aspect of the invention, the circuit element applies a voltage for oscillating ultrasonic waves to the first piezoelectric element, and the output voltage output from the second piezoelectric element is equal to or higher than the first threshold value. The reception of the ultrasonic wave by the receiving element is detected. Then, the threshold adjustment means calculates the sound pressure of the ultrasonic wave when propagating the propagation distance corresponding to the reciprocation of the distance set so that the detection target can be detected based on the humidity detected by the humidity detection means. If the amplitude value of the output voltage output from the second piezoelectric element in accordance with the sound pressure is smaller than the second threshold value set larger than the first threshold value, the first threshold value is adjusted to be lowered.

  When the ultrasonic wave transmitted from the transmitting element is reflected by the object to be detected and propagates the propagation distance, the sound pressure is reduced. In particular, when the ultrasonic wave is further attenuated with a change in humidity and the sound pressure is reduced, even if an output voltage is output from the second piezoelectric element in accordance with the attenuated ultrasonic wave, the output voltage is the first threshold value. In some cases, it may not be possible to detect reception of ultrasonic waves that should be detected. On the other hand, when the first threshold value is simply lowered to eliminate such undetection, if the noise included in the output voltage output from the second piezoelectric element becomes equal to or higher than the first threshold value, the ultrasonic wave This means that the ultrasonic wave is detected even though it is not received, and the detection accuracy is lowered.

  Therefore, by calculating the sound pressure of the ultrasonic wave when propagating through the propagation distance based on the humidity detected by the humidity detecting means, the ultrasonic attenuation level considering the humidity state (the decrease level of the sound pressure). ) Can be estimated. Then, the first threshold is lowered based on the amplitude of the output voltage output from the second piezoelectric element in accordance with the ultrasonic attenuation, that is, the sound pressure of the ultrasonic, so that One threshold can be adjusted appropriately.

In particular, the threshold adjustment means is configured to output an amplitude value of the output voltage output from the second piezoelectric element in accordance with the sound pressure of the ultrasonic wave propagated through the propagation distance, than a second threshold value set to be larger than the first threshold value. When it is smaller, the first threshold value is adjusted to be lowered. If the amplitude value of the output voltage is directly compared with the first threshold value, an undetected state in which the ultrasonic wave to be detected before adjusting the first threshold value cannot be detected when the output voltage decreases can occur. Therefore, as described above, the undetected state can be eliminated by comparing the amplitude value of the output voltage output from the second piezoelectric element with the second threshold value set larger than the first threshold value.
Therefore, a decrease in detection accuracy due to humidity change can be suppressed.

According to a second aspect of the present invention, when the propagation distance is r, the ultrasonic frequency is f, the temperature is T, the saturated vapor pressure is Go, the atmospheric pressure is G, and the predetermined coefficients are A and M. Based on the humidity h detected by the humidity detecting means, a variable k is calculated by the following equation (1), an absorption coefficient m is calculated by the following equation (2) based on the variable k, and the absorption coefficient m When the sound pressure P of the ultrasonic wave is calculated by the following formula (3) based on the above, and the amplitude value of the output voltage output from the second piezoelectric element according to the sound pressure P is smaller than the second threshold value Is adjusted to lower the first threshold.
k = 1.92 × (Go / G × h) ^1.3 × 10 ⁵ (1)
m = (33 + 0.2T) f ² × 10 ⁻¹²
+ Mf / {k / (2πf) + (2πf) / k}} (2)
P = Ae− ^mr / r (3)

  Thus, based on the frequency f of the ultrasonic wave, the temperature T, the saturated vapor pressure Go, the atmospheric pressure G and the like in addition to the humidity h detected by the humidity detecting means, the above formulas (1) to (3) are used. The sound pressure P of the ultrasonic wave propagated through the propagation distance r may be calculated. In addition, said each formula (1)-(3) is based on description of reference documents: EJEvans and ENbazley Acustica 6 238-244 (1956), HOKnerser J.acoust.soc.Am.5 122 (1933). Is.

  In the invention of claim 3, the humidity detecting means applies a voltage having a first frequency slightly lower than the resonance frequency of the first acoustic matching member to the first piezoelectric element for a short time. When the ultrasonic wave transmitted from the transmitting element is transmitted to the second piezoelectric element, the first absorption is obtained by the above equation (3) based on the sound pressure obtained from the amplitude value of the output voltage of the second piezoelectric element. Calculate the coefficient. Further, the humidity detecting means applies a voltage having a second frequency slightly higher than the resonance frequency to the first piezoelectric element for a short time, and the ultrasonic wave transmitted from the transmitting element by the application is second. The second absorption coefficient is calculated by the above equation (3) based on the sound pressure obtained from the amplitude value of the output voltage of the second piezoelectric element. Then, the humidity detecting means substitutes the equation relating to the variable k obtained by substituting the first frequency and the first absorption coefficient into the equation (2), and the second frequency and the second absorption coefficient as the equation (2). ) And the humidity m is calculated and detected based on the equation relating to the variable k obtained by substituting for the above equation (1).

  Thereby, the humidity around the transmitting element and the receiving element can be detected without providing a special member such as a humidity sensor. Further, since the first frequency and the second frequency are in the vicinity of the resonance frequency of the first acoustic matching member, it is possible to improve the transmission sensitivity when detecting humidity. As a result, an increase in manufacturing cost due to the adoption of the humidity detecting means can be suppressed.

  In the invention of claim 4, the first acoustic matching member of the transmitting element is formed of a resin material. Both the first frequency and the second frequency in claim 3 need to be close to the resonance frequency of the first acoustic matching member in order to improve transmission sensitivity. Therefore, the first acoustic matching member of the transmission element is formed of a resin material having a low Q value, so that the Q value is higher than that of a transmission element having a membrane structure using a silicon substrate (see Patent Document 1 above). The resonance peak can be lowered. Thereby, it is possible to easily transmit ultrasonic waves of two different frequencies near the resonance frequency from one transmission element.

  In the invention of claim 5, a gel-like cushioning material that protects at least one of the transmitting element and the receiving element from a load of external force is provided. The humidity detection means detects the humidity around the transmitting element and the receiving element based on the dielectric constant of the buffer material.

  The dielectric constant of a gel-like buffer material, for example, a potting material, is about 3 to 6, and the dielectric constant of water is about 80. Then, when the humidity around the transmitting element and the receiving element is increased, the dielectric constant is increased in accordance with the increase in humidity. For this reason, humidity can be measured based on the dielectric constant of the buffer material. In particular, in order to measure the dielectric constant of the buffer material without providing a special member such as a humidity sensor, for example, the humidity is detected only by providing a pair of electrodes embedded in the buffer material in a state of being separated from each other. be able to. Thereby, the increase in manufacturing cost by having employ | adopted the humidity detection means can be suppressed.

  In the invention of claim 6, the surface acoustic wave element is provided on the surface of one of the first piezoelectric element and the second piezoelectric element. The humidity detecting means measures the humidity around the transmitting element and the receiving element based on the frequency change of the surface acoustic wave transmitted and received by the surface acoustic wave element.

  A surface acoustic wave element is usually provided with a film that changes according to humidity. As the humidity rises, the surface acoustic wave is absorbed by this film, and the amplitude value of the surface acoustic wave decreases. For this reason, the amplitude value of the surface acoustic wave received by the other electrode becomes smaller than the amplitude value of the surface acoustic wave transmitted from one electrode as the humidity increases. That is, the humidity can be measured from the difference between the amplitude value of the surface acoustic wave during transmission and the amplitude value of the surface acoustic wave during reception in the surface acoustic wave element. In this way, the humidity around the transmitting element and the receiving element may be measured.

  In the invention of claim 7, the first piezoelectric element is formed by laminating a plurality of piezoelectric elements. Thereby, the first piezoelectric element can oscillate ultrasonic waves with high sound pressure.

  In the invention of claim 8, the second piezoelectric element is formed of a lead zirconate titanate (PZT) material. Thereby, ultrasonic waves with low sound pressure can be received, and reception sensitivity can be improved.

  In the ninth aspect of the invention, the first piezoelectric element and the second piezoelectric element are made of a polyvinylidene fluoride (PVDF) material. Thereby, since the difference of the acoustic impedance with an acoustic matching member becomes small, attenuation | damping of an ultrasonic vibration can be made small. In addition, since the polyvinylidene fluoride (PVDF) -based material is a resin material, insert molding of the acoustic matching member is easy and can be suitably used.

  In the invention of claim 10, a plurality of receiving elements are provided, and the plurality of receiving elements are arranged in an array. Thereby, since the distance and azimuth angle to a to-be-detected body can be detected based on the ultrasonic wave received by each receiving element, the three-dimensional detection of the position of the to-be-detected body can be performed.

[First Embodiment]
A first embodiment of an ultrasonic sensor according to the present invention will be described with reference to the drawings. Here, an ultrasonic sensor mounted on a vehicle and used as an obstacle sensor will be described as an example. FIG. 1 is an explanatory diagram of the ultrasonic sensor according to the first embodiment. FIG. 1A is an explanatory plan view of the ultrasonic sensor as viewed from the acoustic matching member side, and FIG. 1B is a cross-sectional view taken along the line AA in FIG. Here, in FIG. 1, the front direction of FIG. 1 (A) and the upward direction of FIG. 1 (B) indicate the outside of the vehicle. Moreover, the ground exists in the downward direction of FIG. In each figure, for the sake of explanation, a part is enlarged and a part is omitted.

  As shown in FIGS. 1A and 1B, the ultrasonic sensor 10 includes a transmission element 11 that transmits ultrasonic waves, and a detection object (failure) that is transmitted from the transmission element 11 to the front of the vehicle and exists in front of the vehicle. The receiving elements 12p, 12q, and 12r that detect the ultrasonic waves reflected by the object), the vibration attenuating member 18 that prevents the transmission of the ultrasonic waves, and the transmitting element 11 and the receiving elements 12p, 12q, and 12r are loaded with external force and impact. A first buffer material 19 that protects from transmission, a transmission element 11 that is partitioned from reception elements 12p to 12r, a vibration separating member 90 that shields transmission of ultrasonic waves, and a circuit element that inputs and outputs voltage signals related to transmission and reception of ultrasonic waves 20 and a receiving element 12p, 12q, 12r, a transmitting element 11, a first cushioning material 19, and a box-shaped casing 31 having an open end for accommodating the vibration separating member 90.

Since the structures of the receiving elements 12p to 12r are the same, the receiving element 12p will be described here.
The receiving element 12p receives an ultrasonic wave oscillated from the transmitting element 11 and reflected by an obstacle, transmits an ultrasonic wave as vibration to the piezoelectric element 14p, and a piezoelectric element 14p that detects the ultrasonic wave. Are joined together.

  The piezoelectric element 14p is made of, for example, lead zirconate titanate (PZT), and a piezoelectric body formed in the shape of a rectangular column whose cross section is equal to the cross section of the acoustic matching member 13p is formed on the opposing surface with Pt. And a pair of electrodes 15p formed by sputtering of Cu or Ag, plating, baking of a conductive paste, or the like.

  The acoustic matching member 13p is formed using a resin material having excellent durability such as polycarbonate resin having an acoustic impedance larger than that of air and smaller than that of the piezoelectric element 14p.

  The acoustic matching member 13p is formed such that the thickness (length in the transmission direction of ultrasonic waves) L is about ¼ of the wavelength λ in the ultrasonic acoustic matching member 13p. A standing wave can be generated in the acoustic matching member 13p by forming the thickness L of the acoustic matching member 13p to be about 1/4 of the wavelength λ of the ultrasonic wave. As a result, it is possible to reduce the fact that the ultrasonic wave incident into the acoustic matching member 13p and the ultrasonic wave reflected at the interface between the acoustic matching member 13p and the piezoelectric element 14p interfere with each other and cancel each other. An ultrasonic wave can be efficiently transmitted to the element 14p. In addition, it is desirable that the width of the acoustic matching member 13p is set to be equal to or less than half the wavelength of ultrasonic waves in air.

  The transmitting element 11 is formed by bonding an acoustic matching member 13 that is similarly configured using the same material as the acoustic matching member 13p of the receiving element 12p and a laminated piezoelectric element 16 that oscillates an ultrasonic wave.

  The laminated piezoelectric element 16 is made of, for example, lead zirconate titanate (PZT), and is formed of a pair of electrodes 17 on a piezoelectric body that is formed in a rectangular column shape whose outer cross-sectional shape is equal to the outer cross-sectional shape of the acoustic matching member 13. Are alternately stacked in a comb-like shape. Thus, the laminated piezoelectric element 16 is equivalent to a shape in which a plurality of layers of piezoelectric elements are laminated, and in the present embodiment, it has a shape in which five layers of piezoelectric elements are laminated. Here, the number of stacked piezoelectric elements is variable according to the required sound pressure.

  The electrode 15p of each piezoelectric element 14p and the electrode 17 of the laminated piezoelectric element 16 are electrically connected to the circuit element 20 via wires 14a and 17a, respectively. The circuit element 20 is electrically connected to an ECU (Electronic Control Unit: not shown) provided in the vehicle.

  When transmitting the ultrasonic wave, the circuit element 20 has a predetermined frequency with respect to the laminated piezoelectric element 16 based on a control signal for controlling the sound pressure and phase of the transmitted ultrasonic wave output from the ECU. Output voltage signal (apply voltage). In addition, when the output voltage output from each piezoelectric element 14p is equal to or higher than a threshold value Vs adjusted by a threshold value adjustment process described later, the circuit element 20 detects reception of ultrasonic waves and performs a predetermined calculation process. The vibration signal is output to the ECU.

The detection of ultrasonic waves by the circuit element 20 will be described with reference to FIG. FIG. 2 is an explanatory diagram showing the relationship between the output voltage Vo from the piezoelectric element 14p and the threshold value Vs.
As shown in FIG. 2, when an ultrasonic wave is transmitted from the transmitting element 11 by applying an input voltage Vi having a predetermined frequency from the circuit element 20 to the laminated piezoelectric element 16, the ultrasonic wave is reflected by the detection object. And received by each piezoelectric element 14p. Each piezoelectric element 14p outputs an output voltage corresponding to the sound pressure of the received ultrasonic wave. When the output voltage Vo output from the piezoelectric element 14p is equal to or higher than the threshold value Vs (the position of the point X in FIG. 2), the circuit element 20 detects reception of ultrasonic waves with respect to the piezoelectric element 14p. The threshold value Vs is set to a value sufficiently larger than the voltage caused by noise or the like so as not to be affected by noise or the like.

  The acoustic matching member 13 of the transmitting element 11 and the acoustic matching member 13p of each of the receiving elements 12p to 12r are arranged between the acoustic matching members 13 and 13p adjacent to each other with a vibration damping member 18 that prevents transmission of ultrasonic waves interposed therebetween. It arrange | positions at an array form so that the space | interval d of a center part may become substantially equal to the half wavelength of an ultrasonic wave. However, the interval between the center portions depends on the angle of the detection area, and the angle can be detected even when the interval d is larger than a half wavelength.

  The vibration damping member 18 covers the reception surface 13j of each acoustic matching member 13p and the transmission surface 13s of the acoustic matching member 13, and is fixed to the opening of the housing 31. When this configuration is used, the interface between the acoustic matching members 13 and 13p and the vibration attenuating member 18 is not exposed to the outside, so that water or the like can be prevented from entering through the joint surface. 10 reliability can be improved. The casing 31 is attached to a predetermined position of the vehicle, for example, the bumper 100 so that the acoustic matching members 13 and 13p face outward.

  The vibration damping member 18 is made of a material having a smaller acoustic impedance and a higher damping constant than the acoustic matching members 13 and 13p, for example, silicon rubber. Furthermore, a material having a low elastic modulus and a material having a low density are preferably used for the vibration damping member 18. For example, a rubber material, a resin containing pores such as a foamed resin, a sponge, or the like can be used.

  Since the vibration damping member 18 formed of such a material is interposed between the acoustic matching members 13 and 13p, an ultrasonic wave is transmitted between the acoustic matching members 13 and 13p and causes noise. This can be prevented. Here, a portion of the vibration attenuating member 18 that covers the reception surface 13j and the transmission surface 13s is formed to have a thickness of 1 mm or less, for example, so as not to significantly inhibit transmission of ultrasonic waves.

  The first buffer material 19 is made of a material having a lower elastic modulus than that of the laminated piezoelectric element 16 and each piezoelectric element 14p, for example, a potting material. The first buffer material 19 surrounds a part of the laminated piezoelectric element 16 and the acoustic matching member 13 of the transmitting element 11 and a part of the piezoelectric element 14p and the acoustic matching member 13p of each of the receiving elements 12p to 12r. It is provided between the body 31 and the body 31. The first buffer material 19 may be made of a polymer material such as urethane, rubber, or silicone.

  By providing such a first buffer material 19, even when an impact is applied to each acoustic matching member 13, 13 p due to a collision of flying objects such as pebbles, the first buffer material 19 can be In order to absorb the impact transmitted to each of the receiving elements 12p to 12r and restrain the transmitting element 11 and each of the receiving elements 12p to 12r from being displaced toward the bottom surface 31a side of the casing 31, The elements 12p to 12r can be protected and destruction can be prevented. In addition, since environmental factors such as moisture that deteriorates the laminated piezoelectric element 16 and each piezoelectric element 14p can be blocked, the reliability can be improved.

  The vibration separating member 90 is formed in a plate shape from a material having a higher elastic modulus and acoustic impedance than the first buffer material 19. The vibration separating member 90 is provided between the transmitting element 11 and the adjacent receiving elements 12p and 12r, is erected from the bottom surface 31a of the housing 31, and one end is fixed by the vibration attenuating member 18, and the transmitting element 11, the interior of the housing 31 is partitioned. Here, the thickness of the vibration separating member 90 reduces the transmission of ultrasonic vibrations from the laminated piezoelectric element 16 to each acoustic matching member 13p, and inhibits the vibration of each acoustic matching member 13p in the vibration damping member 18. The thickness is not set.

With the thus constructed ultrasonic sensor 10 to the operating state, the circuit element 20, the adjustment based on the control signal inputted from the ECU, to be equal to the resonant frequency f _c of the acoustic matching member 13,13p The voltage having the frequency is applied to the laminated piezoelectric element 16 of the transmitting element 11. By the laminated piezoelectric element 16 vibrates in response to the applied voltage, the frequency of ultrasonic waves is equal to the resonant frequency f _c is sent from the transmitting surface 13s through the acoustic matching member 13 to the outside of the vehicle.

  Here, since the laminated piezoelectric element 16 of the transmitting element 11 is laminated in five layers, for example, when the same voltage is applied as compared with a piezoelectric element having only one layer, the displacement is five times, that is, five times. The sound pressure can be obtained. That is, the laminated piezoelectric element 16 can oscillate ultrasonic waves with high sound pressure.

  Further, since the vibration separating member 90 that partitions the transmission element 11 is formed of a material having a higher elastic modulus and acoustic impedance than the first buffer material 19, the vibration isolation member 90 is transmitted from the laminated piezoelectric element 16 through the first buffer material 19. Ultrasonic waves can be reflected at the interface between the vibration separating member 90 and the first buffer material 19. Thereby, even if an ultrasonic wave with a high sound pressure is oscillated from the transmitting element 11, vibration noise generated by transmitting the ultrasonic wave from the transmitting element 11 to each of the receiving elements 12p to 12r can be reduced.

  As described above, the ultrasonic wave transmitted from the transmission element 11 is reflected by the detection target, and then received by the reception surface 13j of the acoustic matching member 13p of each of the reception elements 12p to 12r. For example, the ultrasonic wave received on the receiving surface 13j of the acoustic matching member 13p of the receiving element 12p is transmitted to the piezoelectric element 14p via the acoustic matching member 13p.

  Each piezoelectric element 14 p outputs an output voltage corresponding to the transmitted sound pressure of the ultrasonic wave to the circuit element 20. When the amplitude value of the output voltage output from each piezoelectric element 14p is equal to or higher than a threshold value Vs adjusted by a threshold value adjustment process described later, the circuit element 20 detects reception of ultrasonic waves and performs a predetermined calculation process. And output as a vibration signal to the ECU.

  At this time, since the receiving elements 12p to 12r are arranged in an array, the time difference between the transmission and reception and the time difference or phase difference between the receiving elements 12p to 12r of the received ultrasonic waves are obtained. Based on the difference, three-dimensional detection of the position of the detected object such as an obstacle can be performed.

  In addition, since the vibration damping member 18 is interposed between the receiving elements 12p to 12r, ultrasonic waves can be separated and transmitted and detected for each of the receiving elements 13p to 13s. Characteristics can be obtained, and ultrasonic detection accuracy can be improved.

  Here, the flow of the threshold adjustment process executed by the circuit element 20 described above will be described in detail with reference to FIGS. 3 and 4 are flowcharts showing the flow of threshold adjustment processing in the circuit element 20.

First, when the temperature T, the atmospheric pressure G, and the saturated vapor pressure Go are acquired from a temperature sensor and an atmospheric pressure sensor (not shown) in step S101 in FIG. 3, a first detection voltage application process is performed in step S103. In this process, the common resonant frequency _{f c} slightly lower frequency than the acoustic matching member 13p of the acoustic matching member 13 and the respective receiving elements 12p~12r transmission element 11 (hereinafter, also referred to as a first frequency _{f 1)} One detection voltage is applied to the laminated piezoelectric element 16 of the transmission element 11 for a short time. When the laminated piezoelectric element 16 vibrates in response to this voltage application, the ultrasonic wave having the first frequency f ₁ is transmitted from the transmission surface 13 s to the outside of the vehicle via the acoustic matching member 13. The first frequency _{f 1,} for example, is set to 3kHz becomes lower than the resonant frequency _{f c.}

Next, in step S105, a first amplitude value acquisition process is performed. In this process, after the ultrasonic wave is transmitted in step S103, it is reflected by the object to be detected and transmitted to the piezoelectric element 14p via each acoustic matching member 13p, whereby the output voltage output from the piezoelectric element 14p is output. obtaining an amplitude value as a first amplitude V _1.

If S is the element sensitivity between the sound pressure P of the ultrasonic wave transmitted to the piezoelectric element 14p and the amplitude value V of the output voltage output from the piezoelectric element 14p, the relationship of the following equation (4) Is established.
V = S × P (4)
The ultrasonic wave attenuates when propagating a propagation distance (hereinafter also referred to as a propagation distance r) corresponding to a round trip of a distance set so that the detection object can be detected as the ultrasonic sensor 10. The relationship of the following formula | equation (3) is materialized between m and the sound pressure P.
P = Ae− ^mr / r (3)
Here, A is a predetermined coefficient, and is set based on, for example, a known initial oscillation sound pressure at a propagation distance r = 0.2 m. Further, the above formula (3) is the same relational formula as the formula (3) described in “Claims” and “Effects of the Invention”.

When the first amplitude _{V 1} is acquired by the step S105, in step S107, the first absorption coefficient calculation processing is performed. In this process, the first absorption coefficient m ₁ is calculated by the above equations (3) and (4) based on the first amplitude value V ₁ .

When the first absorption coefficient m ₁ is calculated as in step S109, the second detection voltage application processing is performed. In this process, slightly higher frequency than the resonance frequency f _c the second detection voltage (hereinafter, a second referred to as frequency f ₂₎ is applied only for a short time to the laminated piezoelectric element 16 of the transmission element 11. By the laminated piezoelectric element 16 vibrates in response to the voltage application, ultrasound of the second frequency f ₂ is transmitted from the transmitting surface 13s through the acoustic matching member 13 to the outside of the vehicle. Note that the second frequency _{f 2,} for example, are set to 3kHz higher relative resonant frequency _{f c.}

The first frequency f ₁ and the second frequency f ₂ described above, in order to improve the reception sensitivity, it is necessary to vicinity of the resonance frequency f _c. In the first embodiment, the transmitting element 11 is configured by bonding an acoustic matching layer 13 and a laminated piezoelectric element 16, and the acoustic matching layer 13 is a resin material and has a Q value of about 10. . Therefore, for example, compared with a transmission element having a membrane structure using a silicon substrate (see Patent Document 1 above), the Q value is lowered and the resonance peak can be lowered. In this manner, are easily transmit ultrasonic waves of two different frequencies near the resonance frequency f _c from one transmit element 11 (first frequency f ₁ and the second frequency f _2).

Next, in step S111, second amplitude value acquisition processing is performed. In this process, after the ultrasonic wave is transmitted in step S109, it is reflected by the object to be detected and transmitted to the piezoelectric element 14p through each acoustic matching member 13p, thereby outputting the output voltage output from the piezoelectric element 14p. obtaining an amplitude value as a second amplitude value V _2.

In step S113, a second absorption coefficient calculation process is performed. In this process, similarly to the first absorption coefficient calculation process described above, the second absorption coefficient m ₂ is calculated by the above equations (3) and (4) based on the second amplitude value V ₂ .

Next, humidity calculation processing is performed in step S115 of FIG. In this process, the following two formulas are obtained by substituting the first frequency f ₁ and the first absorption coefficient m ₁ , the second frequency f ₂ and the second absorption coefficient m _2, and the temperature T into the following formula (2). Then, the variable k is calculated from the following equation, and the humidity h is calculated based on the variable k, the atmospheric pressure G, and the saturated vapor pressure Go by the following equation (1).
m = (33 + 0.2T) f ² × 10 ⁻¹²
+ Mf / {k / (2πf) + (2πf) / k}} (2)
k = 1.92 × (Go / G × h) ^1.3 × 10 ⁵ (1)
Note that M is a predetermined coefficient. Moreover, the said Formula (1) and Formula (2) are the same relational expressions as Formula (1) and Formula (2) as described in "Claims" and "Effect of invention".

As described above, when the humidity h is calculated from the first frequency f ₁ and the first absorption coefficient m ₁ and from the second frequency f ₂ and the second absorption coefficient m ₂ , an absorption coefficient calculation process is performed in step S117. The In this process, for each frequency of the voltage applied from the circuit element 20 to the laminated piezoelectric element 16 based on the control signal from the ECU, the absorption coefficient m is calculated by the equations (1) and (2) based on the humidity h. Calculate.

  Next, in step S119, a minimum output voltage calculation process is performed. In this process, based on the absorption coefficient m calculated in step S117, the sound pressure P when the ultrasonic wave propagates through the propagation distance r and is transmitted to the piezoelectric element 14p is calculated by the above equation (3). To do. Based on this sound pressure P, the minimum output voltage Vu is calculated by the above equation (4). As described above, the minimum output voltage Vu is calculated so as to estimate the attenuation degree of the ultrasonic wave (the reduction degree of the sound pressure P) in consideration of the humidity state.

  When the minimum output voltage Vu is calculated as described above, it is determined in step S121 whether or not the minimum output voltage Vu is equal to or less than a value obtained by multiplying the threshold value Vs by the coefficient α. The reason for comparing the minimum output voltage Vu and the value obtained by multiplying the threshold value Vs by the coefficient α is that if the minimum output voltage Vu and the threshold value Vs are directly compared, the threshold value Vs is adjusted when the minimum output voltage Vu decreases. This is because a state in which the ultrasonic wave to be detected before the detection cannot be detected may occur. In the first embodiment, the coefficient α is set to 2.0. Further, the value obtained by multiplying the threshold value Vs by the coefficient α corresponds to a “second threshold value” recited in the claims.

  If the minimum output voltage Vu exceeds the value obtained by multiplying the threshold value Vs by α, it is not necessary to adjust the threshold value Vs. Therefore, it is determined No in step S121, and the vehicle ignition switch (IGSW: not shown). If is in the ON state and the vehicle is in the starting state (No in S125), the processes in and after step S101 are repeated.

  On the other hand, if the minimum output voltage Vu becomes equal to or less than the value obtained by multiplying the threshold value Vs by the coefficient α due to the change in the humidity h (Yes in step S121), the threshold value reduction process is performed in step S123. In this process, the threshold value Vs is set to be decreased so as to be equal to the value obtained by dividing the minimum output voltage Vu by the coefficient β. In the first embodiment, the coefficient β is set to be equal to the coefficient α. However, the present invention is not limited to this, and the coefficient β may be set to be different from the coefficient α.

  Thus, by detecting the ultrasonic wave based on the threshold value Vs adjusted in consideration of the humidity h, a decrease in detection accuracy of the ultrasonic sensor 10 due to a change in the humidity h is suppressed. Note that when the threshold value Vs is set to decrease in step S123 and a predetermined time has elapsed, the threshold value Vs may be returned to the initial value or gradually increased to the initial value. When the IGSW is turned off, it is determined Yes in step S125, and the threshold adjustment process is terminated.

  As described above, in the ultrasonic sensor 10 according to the first embodiment, the circuit element 20 applies a voltage for oscillating ultrasonic waves to the laminated piezoelectric element 16 and outputs from the piezoelectric element 14p. When the voltage is equal to or higher than the threshold value Vs, reception of ultrasonic waves by the receiving elements 12p to 12r is detected. The circuit element 20 calculates the sound pressure P of the ultrasonic wave when propagating the propagation distance r based on the humidity h calculated by the humidity calculation process, and the minimum output voltage Vu obtained from the sound pressure P. Is smaller than the value obtained by multiplying the threshold value Vs by the coefficient α, the threshold value Vs is adjusted to be lowered.

  As a result, it is possible to estimate the attenuation level of the ultrasonic waves (the decrease level of the sound pressure P) in consideration of the humidity state. Then, by adjusting the threshold value Vs according to the degree of attenuation of the ultrasonic wave, that is, the minimum output voltage Vu obtained from the sound pressure P of the ultrasonic wave, the threshold value Vs is appropriately adjusted according to the humidity change. can do.

In particular, in the threshold value adjustment process described above, the threshold value Vs is obtained when the minimum output voltage Vu obtained from the sound pressure P of the ultrasonic wave propagated through the propagation distance r is smaller than the value obtained by multiplying the threshold value Vs by the coefficient α. Adjust to lower. Thus, by directly comparing the minimum output voltage Vu and the threshold value Vs, it is possible to eliminate a state in which an ultrasonic wave to be detected before adjusting the threshold value Vs when the minimum output voltage Vu is reduced cannot be detected.
Therefore, a decrease in detection accuracy due to humidity change can be suppressed.

  In the ultrasonic sensor 10 according to the first embodiment, in the threshold adjustment process described above, the variable k is calculated by the above equation (1) based on the humidity h calculated by the humidity calculation process. Based on the above equation (2), the absorption coefficient m is calculated, and on the basis of the absorption coefficient m, the ultrasonic sound pressure P is calculated by the above equation (3). The minimum output voltage Vu obtained from this sound pressure P is When the threshold value Vs is smaller than a value obtained by multiplying the threshold value α by the coefficient α, the threshold value Vs is adjusted to be lowered.

  Thus, based on the frequency f of the ultrasonic wave, the temperature T, the saturated vapor pressure Go, the atmospheric pressure G and the like in addition to the humidity h calculated by the humidity calculation process, the above formulas (1) to (3) are used. By calculating the sound pressure P of the ultrasonic wave propagated through the propagation distance r, it is possible to estimate the attenuation level of the ultrasonic wave (decrease level of the sound pressure).

In the ultrasonic sensor 10 according to the first embodiment, the circuit element 20 applies a voltage of the first frequency f _{1 to} the multilayer piezoelectric element 16 for a short time, and the ultrasonic wave transmitted from the transmitting element 11 by this application. There calculates a first absorption coefficient m ₁ by the equation (3) from the first sound pressure P obtained from the amplitude value V ₁ of the time, which is transmitted to the piezoelectric element 14p. Further, the circuit element 20, a second amplitude when the second voltage of the frequency f ₂ is applied to the short time the laminated piezoelectric element 16, the ultrasonic wave transmitted from the transmitting device 11 by the application is transmitted to the piezoelectric element 14p The second absorption coefficient m ₂ is calculated from the sound pressure P obtained from the value V ₂ by the above formula (3). Then, the circuit element 20 calculates the equation regarding the variable k obtained by substituting the first frequency f ₁ and the first absorption coefficient m ₁ into the above equation (2), the second frequency f ₂ and the second absorption coefficient m ₂ . The humidity h is calculated on the basis of the equation relating to the variable k obtained by substituting the equation (2) and the equation (1).

Thereby, the humidity h can be detected without providing a special member such as a humidity sensor. The first frequency f ₁ and the second frequency f _2, since turned around the resonance frequency f _c of the acoustic matching member 13, it is possible to improve the transmission sensitivity in detecting the humidity. As a result, it is possible to suppress an increase in manufacturing cost for detecting humidity.

In the ultrasonic sensor 10 according to the first embodiment, the acoustic matching member 13 of the transmission element 11 is formed of a resin material. For this reason, for example, compared to a transmission element having a membrane structure using a silicon substrate (see Patent Document 1 above), the Q value is lowered and the resonance peak can be lowered. Thus, it is possible to easily transmit one ultrasonic resonance frequency f _c vicinity two different frequencies (first frequency f ₁ and the second frequency f ₂₎ from the transmission element 11 the acoustic matching member 13.

  In the ultrasonic sensor 10 according to the first embodiment, the laminated piezoelectric element 16 is formed by laminating a plurality of piezoelectric elements. Thereby, the laminated piezoelectric element 16 can oscillate ultrasonic waves with high sound pressure.

  In the ultrasonic sensor 10 according to the first embodiment, the piezoelectric element 14p is formed of a lead zirconate titanate (PZT) material. Thereby, ultrasonic waves with low sound pressure can be received, and reception sensitivity can be improved.

  The ultrasonic sensor 10 according to the first embodiment includes a plurality of receiving elements 12p to 12r, and the receiving elements 12p to 12r are arranged in an array. Thereby, since the distance and azimuth angle to a to-be-detected body can be detected based on the ultrasonic wave received by each receiving element 12p-12r, the three-dimensional detection of the position of a to-be-detected body can be performed.

[Second Embodiment]
Next, an ultrasonic sensor according to a second embodiment of the present invention will be described with reference to FIGS. FIG. 5 is a cross-sectional view of the ultrasonic sensor 10 in the second embodiment. FIG. 6 is a flowchart showing the flow of frequency adjustment processing by the circuit element 20 in the second embodiment.

  In the ultrasonic sensor 10 according to the second embodiment, a pair of electrodes 41 for detecting the dielectric constant of the first buffer material 19 is newly provided, and threshold adjustment processing in the circuit element 20 is performed with reference to FIGS. 3 and 4. 6 is different from the ultrasonic sensor according to the first embodiment in that it is performed based on the flowchart shown in FIG. Therefore, substantially the same components as those of the ultrasonic sensor according to the first embodiment are denoted by the same reference numerals and description thereof is omitted.

  As shown in FIG. 5, a part of the pair of electrodes 41 is embedded in the first buffer material 19 in a state of being separated from each other. Both electrodes 41 are electrically connected to the circuit element 20 via wires 41a. The circuit element 20 detects the dielectric constant of the first buffer material 19 from the capacitance between the electrodes 41. It has a function.

Hereinafter, threshold adjustment processing in the circuit element 20 in the second embodiment will be described with reference to the flowchart shown in FIG.
First, when the temperature T, the atmospheric pressure G, and the saturated vapor pressure Go are acquired in step S101 in FIG. 6, a humidity measurement process is performed in step S115a. In this process, the dielectric constant of the first buffer material 19 is detected from the capacitance between the electrodes 41 embedded in the first buffer material 19. The dielectric constant of the potting material constituting the first buffer material 19 is about 3 to 6, and the dielectric constant of water is about 80. Then, when the humidity h rises, the dielectric constant also rises according to this humidity rise. For this reason, the humidity h can be measured based on the dielectric constant of the first buffer material 19.

  When the humidity h is measured in this way, the processing after step S117 is performed based on the humidity h, and the threshold value Vs is appropriately adjusted according to the humidity h, as in the first embodiment. It becomes.

  As described above, in the ultrasonic sensor 10 according to the second embodiment, the humidity h is measured based on the dielectric constant between the electrodes 41 embedded in the first buffer material 19. In this way, it is only necessary to provide a pair of electrodes embedded in the first buffer material 19 in a state of being separated from each other without providing a special member such as a humidity sensor, thereby suppressing an increase in manufacturing cost related to humidity measurement. Can do.

[Third Embodiment]
Next, an ultrasonic sensor according to a third embodiment of the present invention will be described with reference to FIG. FIG. 7 is a partial cross-sectional view of the ultrasonic sensor 10 according to the third embodiment. For convenience of explanation, only the portion of the piezoelectric element 14p where the SAW element 50 is provided is not in a cross section.

  In the ultrasonic sensor 10 according to the third embodiment, instead of the pair of electrodes 41 described above, a surface acoustic wave element (hereinafter also referred to as a SAW element 50) is adopted in the second embodiment. Different from the ultrasonic sensor. Therefore, substantially the same components as those of the ultrasonic sensor according to the second embodiment are denoted by the same reference numerals, and description thereof is omitted.

  As shown in FIG. 7, the SAW element 50 includes an electrode 51 and an electrode 52, which are a pair of comb-like electrodes, and is provided on the surface of the piezoelectric element 14p so that the electrodes 51 and 52 are separated from each other by a predetermined distance. ing. Both electrodes 51 and 52 are electrically connected to the circuit element 20 via unillustrated wires, respectively.

  The SAW element 50 generates a surface acoustic wave having a predetermined frequency from the electrode 51 in response to a control signal from the circuit element 20, receives the surface acoustic wave transmitted through the surface of the piezoelectric element 14p by the electrode 52, and receives a predetermined value. It has a function of outputting an electrical signal. Further, the SAW element 50 is provided with a film that normally changes in accordance with the humidity. As the humidity increases, the surface acoustic wave is absorbed into the film, and the amplitude value of the surface acoustic wave becomes small.

Hereinafter, the threshold value adjustment process in the circuit element 20 in the third embodiment will be described with reference to the flowchart shown in FIG.
First, when the temperature T, the atmospheric pressure G, and the saturated vapor pressure Go are acquired in step S101 in FIG. 6, a humidity measurement process is performed in step S115a. In this process, unlike the second embodiment, the humidity h is set based on the frequency change of the surface acoustic wave transmitted and received between the electrodes 51 and 52 in the SAW element 50 in accordance with the control signal from the circuit element 20. taking measurement.

  Since the SAW element 50 is provided with a film that changes according to humidity, the surface acoustic wave received by the electrode 52 with respect to the amplitude value of the surface acoustic wave transmitted from the electrode 51 as the humidity increases. The amplitude value becomes smaller. That is, the humidity h can be measured from the difference between the amplitude value of the surface acoustic wave during transmission and the amplitude value of the surface acoustic wave during reception in the SAW element 50.

  When the humidity h is measured in this manner, the processing after step S117 is performed as in the second embodiment, and the threshold value Vs is appropriately adjusted according to the humidity h.

  As described above, in the ultrasonic sensor 10 according to the third embodiment, the SAW element 50 is provided on the surface of the piezoelectric element 14p. The humidity h is measured based on the frequency change of the surface acoustic wave transmitted and received between both electrodes 51 and 52 of the SAW element 50. Thus, the humidity h may be measured.

  Note that the SAW element 50 may be provided on the surface of an element other than the piezoelectric element 14p, and the humidity h may be measured based on the frequency change of the surface acoustic wave transmitted and received in the SAW element 50.

The present invention is not limited to the above embodiments, and may be embodied as follows. Even in this case, the same operations and effects as those of the above embodiments can be obtained.
(1) The laminated piezoelectric element 16 and each piezoelectric element 14p are not limited to being formed of lead zirconate titanate (PZT), and may be formed of, for example, a polyvinylidene fluoride (PVDF) material. Thereby, since the difference of acoustic impedance with each acoustic matching member 13 and 13p becomes small, attenuation | damping of ultrasonic vibration can be made small. In addition, since the polyvinylidene fluoride (PVDF) -based material is a resin material, insert molding of the acoustic matching members 13 and 13p is easy, and can be suitably used.

(2) In each of the above embodiments, the receiving surface 13j and the transmitting surface 13s are covered by the vibration attenuating member 18, but the present invention is not limited to this. For example, the vibration attenuating member 18 may employ a configuration in which the acoustic matching members 13 and 13p are fixed on the side surfaces near the reception surface 13j and the transmission surface 13s, and the reception surface 13j and the transmission surface 13s are exposed to the outside. . In addition, the exposed receiving surface 13j and transmitting surface 13s in this configuration may be covered with another member such as paint.

(3) The vibration separating member 90 can also be formed integrally with the housing 31. According to this, the number of parts can be reduced, and the positional accuracy of the vibration separating member 90 can be improved.

(4) The shape of each acoustic matching member 13, 13p is not limited to a quadrangular prism having a substantially square cross section, and may be, for example, a cylinder. According to this, the unnecessary vibration of each acoustic matching member 13, 13p can be suppressed.

(5) The number and arrangement of transmitting elements and receiving elements are arbitrary depending on the application. For example, if distance detection is performed, one transmitting element and one receiving element may be arranged. For angle detection, one transmitting element and two receiving elements may be arranged. Thereby, the angle detection of the direction which has arrange | positioned the receiving element can be performed.

It is explanatory drawing of the ultrasonic sensor of 1st Embodiment. FIG. 1A is an explanatory plan view of the ultrasonic sensor as viewed from the acoustic matching member side, and FIG. 1B is a cross-sectional view taken along the line AA in FIG. It is explanatory drawing which shows the relationship between the output voltage from a piezoelectric element, and a threshold value. It is a part of flowchart which shows the flow of the threshold value adjustment process by the circuit element in 1st Embodiment. It is a part of flowchart which shows the flow of the threshold value adjustment process by the circuit element in 1st Embodiment. It is sectional drawing of the ultrasonic sensor in 2nd Embodiment. It is a flowchart which shows the flow of the frequency adjustment process by the circuit element in 2nd Embodiment. It is a partial cross section figure of the ultrasonic sensor in a 3rd embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Ultrasonic sensor 11 ... Transmitting element 12p, 12q, 12r ... Receiving element 13 ... Acoustic matching member (1st acoustic matching member)
13p ... Acoustic matching member (second acoustic matching member)
14p: Piezoelectric element (second piezoelectric element)
16: Multilayer piezoelectric element (first piezoelectric element)
20 ... Circuit element (humidity detection means, threshold value adjustment means)
41 ... Electrode (humidity detection means)
50 ... SAW element (surface acoustic wave element)
h ... humidity P ... sound pressure r ... propagation distance Vs ... threshold value Vu ... minimum output voltage

Claims (10)

A first piezoelectric element capable of oscillating an ultrasonic wave and a first acoustic matching member capable of transmitting the ultrasonic wave oscillated by the first piezoelectric element, and transmitting the ultrasonic wave to an object to be detected. A transmitting element to perform,
A second piezoelectric element capable of detecting the ultrasonic wave reflected by the detected object and a second acoustic matching capable of transmitting the ultrasonic wave reflected by the detected object to the second piezoelectric element. A receiving element for receiving the ultrasonic wave reflected by the object to be detected.
When the voltage for oscillating the ultrasonic wave is applied to the first piezoelectric element, and the output voltage output from the second piezoelectric element is equal to or higher than a first threshold value, A circuit element for detecting reception;
Humidity detecting means for detecting the humidity around the transmitting element and the receiving element;
Based on the humidity detected by the humidity detecting means, the sound pressure of the ultrasonic wave when propagating a propagation distance corresponding to the reciprocation of the distance set so that the detected object can be detected is calculated. A threshold value that is adjusted to lower the first threshold value when the amplitude value of the output voltage output from the second piezoelectric element is smaller than the second threshold value that is set to be larger than the first threshold value according to the pressure. Adjusting means;
An ultrasonic sensor comprising: When the propagation distance is r, the ultrasonic frequency is f, the temperature is T, the saturated vapor pressure is Go, the atmospheric pressure is G, and the predetermined coefficients are A and M, the humidity detecting means The variable k is calculated by the following equation (1) based on the humidity h detected by the following equation (1), the absorption coefficient m is calculated by the following equation (2) based on the variable k, and based on the absorption coefficient m: When the sound pressure P is calculated by the following equation (3) and the amplitude value of the output voltage output from the second piezoelectric element according to the sound pressure P is smaller than the second threshold value, The ultrasonic sensor according to claim 1, wherein the ultrasonic sensor is adjusted to lower the first threshold value.
k = 1.92 × (Go / G × h) ^1.3 × 10 ⁵ (1)
m = (33 + 0.2T) f ² × 10 ⁻¹²
+ Mf / {k / (2πf) + (2πf) / k}} (2)
P = Ae− ^mr / r (3) The humidity detecting means includes
A voltage having a first frequency slightly lower than the resonance frequency of the first acoustic matching member is applied to the first piezoelectric element for a short time, and ultrasonic waves transmitted from the transmitting element by this application are applied. When transmitted to the second piezoelectric element, the first absorption coefficient is calculated by the equation (3) based on the sound pressure obtained from the amplitude value of the output voltage of the second piezoelectric element,
A voltage having a second frequency slightly higher than the resonance frequency is applied to the first piezoelectric element for a short time, and ultrasonic waves transmitted from the transmitting element by this application are applied to the second piezoelectric element. When transmitted, the second absorption coefficient is calculated by the equation (3) based on the sound pressure obtained from the amplitude value of the output voltage of the second piezoelectric element,
The equation relating to the variable k obtained by substituting the first frequency and the first absorption coefficient into the equation (2), and the second frequency and the second absorption coefficient into the equation (2). The ultrasonic sensor according to claim 2, wherein the humidity m is calculated and detected based on an equation relating to the variable k obtained by substitution and the equation (1).   The ultrasonic sensor according to claim 3, wherein the first acoustic matching member is formed of a resin material. A gel-like cushioning material that protects at least one of the transmitting element and the receiving element from an external force load;
The ultrasonic sensor according to claim 1, wherein the humidity detecting unit detects the humidity based on a dielectric constant of the buffer material. A surface acoustic wave element is provided on the surface of one of the first piezoelectric element and the second piezoelectric element,
The ultrasonic sensor according to claim 1, wherein the humidity detecting unit detects the humidity based on a frequency change of a surface acoustic wave transmitted and received by the surface acoustic wave element.   The ultrasonic sensor according to claim 1, wherein the first piezoelectric element is formed by stacking a plurality of piezoelectric elements.   The ultrasonic sensor according to claim 1, wherein the second piezoelectric element is formed of a lead zirconate titanate (PZT) -based material.   The ultrasonic sensor according to any one of claims 1 to 7, wherein the first piezoelectric element and the second piezoelectric element are made of a polyvinylidene fluoride (PVDF) material.   The ultrasonic sensor according to claim 1, wherein a plurality of the receiving elements are provided, and the plurality of receiving elements are arranged in an array.

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