Ultrasonic sensor

Abstract

To provide an ultrasonic sensor capable of correcting a sound velocity based on a temperature at a low cost.SOLUTION: An ultrasonic sensor includes an ultrasonic element that transmits an ultrasonic wave to an object, a temperature calculation unit that measures an ambient temperature of an environment in which the ultrasonic element is installed, and a sound velocity correction unit that corrects a sound velocity of the ultrasonic wave based on the ambient temperature, wherein the ultrasonic element includes a substrate having an opening, a diaphragm that closes the opening, and a piezoelectric element provided on a surface of the diaphragm opposite to the opening, the temperature calculation unit specifies a resonance frequency of the ultrasonic element by changing a frequency of a drive voltage applied to the ultrasonic element, and calculates the environmental temperature based on the resonance frequency.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an ultrasonic sensor. [Background technology]

[0002] Conventionally, ultrasonic sensors are known that transmit ultrasonic waves, receive the ultrasonic waves reflected by an object, and measure the distance to the object based on the time from transmission to reception of the ultrasonic waves and the speed of sound. Since the speed of sound varies with the ambient temperature, such ultrasonic sensors must measure the temperature and correct the speed of sound or distance. For example, the ultrasonic sensor described in Patent Document 1 performs a correction process based on the temperature measured by a temperature sensor. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-61120 Summary of the Invention [Problem to be solved by the invention]

[0004] However, the ultrasonic sensor of Patent Document 1 requires a separate temperature sensor, which poses problems such as the need for installation space and costs for the temperature sensor, and the cost of a control circuit that controls the temperature sensor. [Means for solving the problem]

[0005] An ultrasonic sensor according to one aspect of the present disclosure includes an ultrasonic element that transmits ultrasonic waves to an object and receives the ultrasonic waves reflected by the object, a temperature calculation unit that measures the ambient temperature around where the ultrasonic element is installed, and a sound speed correction unit that corrects the sound speed of the ultrasonic waves based on the ambient temperature, wherein the ultrasonic element includes a substrate having an opening, a vibration plate that closes the opening, and a piezoelectric element provided on the surface of the vibration plate opposite to the opening, and the temperature calculation unit determines the resonant frequency of the ultrasonic element by changing the frequency of a drive voltage applied to the ultrasonic element, and calculates the ambient temperature based on the resonant frequency. [Brief explanation of the drawings]

[0006] [Figure 1] FIG. 1 is a schematic diagram showing a schematic configuration of an ultrasonic sensor according to a first embodiment. [Figure 2] FIG. 2 is a plan view showing a schematic configuration of the ultrasonic element according to the first embodiment. [Figure 3] FIG. 3 is an enlarged plan view of one of the ultrasonic transducers shown in FIG. 2. [Figure 4] A cross-sectional view of the ultrasonic transducer taken along line AA in Figure 3. [Figure 5] 5A and 5B are diagrams illustrating the deflection of a vibrating part due to a change in environmental temperature in the ultrasonic transducer according to the first embodiment. [Figure 6] FIG. 10 is a diagram showing the relationship between the environmental temperature and the impedance of an ultrasonic transducer. [Figure 7] FIG. 10 is a diagram showing the relationship between the environmental temperature and the resonance frequency. [Figure 8] 4 is a flowchart showing the operation of the ultrasonic sensor according to the first embodiment. [Figure 9] FIG. 10 is a plan view showing a schematic configuration of an ultrasonic element according to a second embodiment. [Figure 10] 10 is a schematic cross-sectional view of the ultrasonic element cut along line BB in FIG. 9. DETAILED DESCRIPTION OF THE INVENTION

[0007] [First embodiment] A first embodiment of the present disclosure will be described below. FIG. 1 is a schematic diagram showing a schematic configuration of an ultrasonic sensor according to a first embodiment. As shown in Fig. 1, the ultrasonic sensor 1 is configured to include an ultrasonic element 10 and a control unit 20. This ultrasonic sensor 1 is a distance measuring sensor that uses ultrasonic waves to measure the distance to an object W. That is, the ultrasonic sensor 1 transmits ultrasonic waves from the ultrasonic element 10 toward the object W, and receives the ultrasonic waves reflected by the object W at the ultrasonic element 10. The control unit 20 of the ultrasonic sensor 1 then calculates the distance from the ultrasonic element 10 to the object W using the time from the transmission timing to the reception timing of the ultrasonic waves and the sound speed of the ultrasonic waves.

[0008] [Configuration of ultrasonic element 10] Fig. 2 is a plan view showing a schematic configuration of the ultrasonic element 10. Fig. 3 is an enlarged plan view showing one of the ultrasonic transducers Tr shown in Fig. 2. Fig. 4 is a cross-sectional view of the ultrasonic transducer Tr taken along line AA in Fig. 3. 4, the ultrasonic element 10 is configured to include an element substrate 11, which is a base material, a vibration plate 12, and a piezoelectric element 13. Although not shown in the figure, a configuration may be adopted in which a protective film made of, for example, Al2O3 or the like is separately provided to cover the piezoelectric element 13.

[0009] The element substrate 11 is a semiconductor substrate such as Si, and has a predetermined thickness to support the diaphragm 12. The element substrate 11 has a first surface 11A and a second surface 11B opposite to the first surface 11A. In the following description, the direction from the first surface 11A toward the second surface 11B is referred to as the Z direction, the direction perpendicular to the Z direction is referred to as the X direction, and the direction perpendicular to the X and Z directions is referred to as the Y direction. The first surface 11A and the second surface 11B are parallel to the XY plane. In this embodiment, an example is shown in which the Y direction is perpendicular to the X direction, but the Y direction may be inclined at an angle other than 90° with respect to the X direction. In the following description, the X direction, Y direction, and Z direction may be referred to as directions even when they do not include orientations.

[0010] The element substrate 11 is provided with a plurality of openings 111 arranged in a two-dimensional array along the X and Y directions. These openings 111 are through-holes that penetrate the element substrate 11 in the Z direction from the first surface 11A to the second surface 11B. Furthermore, a diaphragm 12 is provided on the first surface 11A of the element substrate 11, and the opening end on the -Z side of the opening 111 is closed by the diaphragm 12. In other words, the portion of the element substrate 11 where the opening 111 is not provided forms a wall portion 112, and the diaphragm 12 is laminated on this wall portion 112.

[0011] The diaphragm 12 is made of, for example, a laminate of SiO2 and ZrO2, and as described above, is provided on the first surface 11A of the element substrate 11. The thickness of the diaphragm 12 is sufficiently smaller than that of the element substrate 11. Here, the portion of the diaphragm 12 that is laminated on the wall portion 112 constitutes a support portion 121, and the portion of the diaphragm 12 that closes the opening 111 constitutes a vibrating portion 122. The vibrating portion 122 and the piezoelectric element 13 constitute an ultrasonic transducer Tr.

[0012] The piezoelectric element 13 is provided on the surface of the vibrating portion 122 of the diaphragm 12 opposite to the element substrate 11. This piezoelectric element 13 is configured by laminating a first electrode 131, a piezoelectric film 132, and a second electrode 133 in this order on the diaphragm 12. Note that although a configuration in which the piezoelectric element 13 is laminated on the diaphragm 12 is shown as an example, a configuration in which another layer is provided between the piezoelectric element 13 and the diaphragm 12 may also be used. The first electrode 131 is made of a conductive material such as Pt, and is formed elongated in the X direction, for example, as shown in Fig. 2. The first electrode 131 is common to multiple ultrasonic transducers Tr arranged in the X direction, and first input terminals 131P connected to the control unit 20 are provided at the ±X side ends. The piezoelectric film 132 is made of, for example, PZT or the like, and is driven to expand and contract when a drive voltage is applied between the first electrode 131 and the second electrode 133 . The second electrode 133 has a two-layer structure formed by, for example, using Ti as a base layer and laminating Ir on top. The second electrode 133 is, for example, arranged along the Y direction and connected to lead electrodes 134, which will be described later, at the ±Y side ends. The lead electrodes 134 arranged at the ±Y side ends of the ultrasonic element 10 are each provided with a second input terminal 133P, which is connected to the control unit 20.

[0013] The piezoelectric element 13 expands and contracts when a voltage is applied between the first electrode 131 and the second electrode 133. When the piezoelectric element 13 expands and contracts, the vibration part 122 of the vibration plate 12 on which the piezoelectric element 13 is provided vibrates, and an ultrasonic wave is transmitted from the ultrasonic transducer Tr. Furthermore, when ultrasonic waves are input to the vibration part 122 from the opening 111, the vibration part 122 vibrates, and a potential difference occurs above and below the piezoelectric film 132 of the piezoelectric element 13. Therefore, by detecting the potential difference occurring between the first electrode 131 and the second electrode 133, it is possible to detect the reception of ultrasonic waves.

[0014] In this embodiment, as shown in Fig. 2, the ultrasonic element 10 has a plurality of ultrasonic transducers Tr arranged in an array along the X and Y directions. In this embodiment, ultrasonic transducers Tr adjacent to each other in the X direction share a first electrode 131, constituting one channel CH. In addition, a plurality of channels CH are arranged along the Y direction. Therefore, independent drive signals can be input to the first input terminals 131P corresponding to each channel CH, and each channel CH can be driven individually. On the other hand, the second electrode 133 is connected to an ultrasonic transducer arranged in the XY direction via a lead electrode 134, and the same common potential is applied from a second input terminal 133P provided on the lead electrode 134 at the ±Y side end.

[0015] 3, in this embodiment, the width of the vibrating part 122 in the Y direction is shorter than the width in the X direction. In this configuration, the vibration frequency when the vibrating part 122 vibrates is dominated by the minor axis direction of the vibrating part 122 (the Y direction in this embodiment). In this embodiment, a lead electrode 134 that also functions as a stress adjusting member of the present disclosure is provided on the -Z side of the diaphragm 12. More specifically, the lead electrode 134 extending along the X direction is arranged so as to bridge between adjacent vibrating parts 122 in the Y direction. In other words, in each vibrating part 122, the lead electrode 134 is arranged on the ±Y side in the Y direction, which affects the vibration frequency. Here, if the portion of the vibrating part 122 where the piezoelectric film 132 is laminated is defined as the central portion 122A, and the portion from the edge of the piezoelectric film 132 to the outer peripheral edge of the vibrating part 122 is defined as the arm portion 122B, the lead electrode 134 is provided from the support portion 121 of the vibration plate 12 to the arm portion 122B. More specifically, the lead electrode 134 covers the arm 122B in a range from the ±Y side edge 122C (see FIG. 4) of the vibrating part 122 to a first distance u. Here, when the width of the arm (the distance in the Y direction from the ±Y side edge 122C of the vibrating part 122 to the piezoelectric film 132) is H, the first distance u satisfies u≧H / 2.

[0016] Furthermore, the lead electrodes 134 are made of a material with a linear expansion coefficient greater than that of the material making up the diaphragm 12. Table 1 below shows the linear expansion coefficients of the materials making up the ultrasonic element 10 in this embodiment.

[0017] [Table 1]

[0018] In this embodiment, the diaphragm 12 includes a ZrO2 layer, and in this case, the lead electrode 134 is preferably made of at least one of Au and NiCr, or a laminate of Au and NiCr, which have a linear expansion coefficient greater than that of ZrO2. Such an ultrasonic element 10 is significantly affected by the linear expansion coefficient of the lead electrode 134, which is a stress adjusting member, and the resonant frequency of each ultrasonic transducer Tr changes with changes in the ambient temperature.

[0019] 5 is a diagram showing a schematic diagram of the deflection of the vibration part 122 due to a change in the environmental temperature in the ultrasonic transducer Tr of this embodiment. In FIG. 5, the deflection amount of the vibration part 122 at a reference temperature (for example, 23° C.) when no drive voltage is applied to the piezoelectric element 13 (when the vibration part 122 is stationary) is set to an initial deflection amount δ0. When the ambient temperature becomes higher than the reference temperature, the tensile stress of the lead electrode 134 decreases, causing the vibrating part 122 to bend convexly toward the opening 111, with the amount of bending δ1 becoming larger than the initial amount of bending δ0. As the vibrating part 122 bends in this way, the resonant frequency f1 of the ultrasonic transducer Tr becomes lower than the resonant frequency f0 at the reference temperature. On the other hand, when the environmental temperature becomes lower than the reference temperature, the tensile stress of the lead electrode 134 increases, causing the deflection δ2 of the vibrating part 122 to become smaller than the initial deflection δ0. As the deflection of the vibrating part 122 decreases in this manner, the resonant frequency f2 of the ultrasonic transducer Tr becomes higher than the resonant frequency f0 at the reference temperature. Furthermore, the initial deflection δ0 is preferably set to 0.1% or less of the minimum opening width (the length in the minor axis direction of the vibrating part 122) of the opening 111. In this embodiment, as described above, the Y direction is the minor axis direction of the opening 111, so the initial deflection δ0 is set to 0.1% or less of the width in the Y direction of the opening 111 (that is, the width in the Y direction of the vibrating part 122).

[0020] [Configuration of control unit 20] The control unit 20 is configured to include various circuits that control the ultrasonic element 10. Specifically, the control unit 20 includes a voltage control circuit 21, a receiving circuit 22, an impedance detection circuit 23, a memory unit 24, and one or more processors 25. The voltage control circuit 21, the receiving circuit 22, and the impedance detection circuit 23 are connected to the first input terminal 131P and the second input terminal 133P of the ultrasonic element 10, respectively.

[0021] The voltage control circuit 21 controls the drive voltage input to the ultrasonic element 10 based on a command from the processor 25. In addition, the voltage control circuit 21 is configured to be able to change the frequency of the drive voltage input to each ultrasonic transducer Tr of the ultrasonic element 10 within a predetermined frequency range.

[0022] The receiving circuit 22 receives the reception signals of each ultrasonic transducer Tr input from the ultrasonic element 10. In this embodiment, a plurality of ultrasonic transducers Tr constitute one channel CH, and therefore reception signals are output from each of these channels CH. In this embodiment, the reception timing of the received signal is acquired in order to calculate the distance from the ultrasonic element 10 to the target W based on the time from the transmission timing to the reception timing of the ultrasonic waves. The reception timing is acquired by measuring a change in the signal value of the received signal and identifying the reception timing by detecting the timing when the signal value becomes equal to or exceeds a predetermined threshold value or the zero-crossing point of the signal.

[0023] The impedance detection circuit 23 measures the impedance of each channel CH of the ultrasonic element 10. Although details will be described later, in this embodiment, the environmental temperature is estimated by measuring the impedance when the drive voltage applied to each channel CH is swept.

[0024] The storage unit 24 is configured by a recording device such as a semiconductor memory, and stores various data and programs related to driving the ultrasonic elements 10 and temperature estimation. In addition, the storage unit 24 stores relational data indicating the relationship between the environmental temperature and the resonant frequency of the ultrasonic elements 10, which will be described in detail later.

[0025] The processor 25 is configured with an arithmetic circuit such as a CPU (Central Processing Unit), and realizes various functions by reading and executing programs stored in the storage unit 24. Specifically, by executing the various programs, the processor 25 functions as a temperature calculation unit 251, a sound speed correction unit 252, and a distance calculation unit 253, as shown in FIG.

[0026] The temperature calculation unit 251 calculates the environmental temperature around the ultrasonic element 10 prior to the distance calculation process using the ultrasonic element 10.

[0027] Fig. 6 is a diagram showing the relationship between the ambient temperature and the impedance of the ultrasonic transducer Tr. Fig. 7 is a diagram showing the relationship between the ambient temperature and the resonant frequency. In Fig. 7, the solid line indicates the change in resonant frequency when the initial deflection δ0 is 0.1% of the length of the vibrating section 122 in the minor axis direction (Y direction). The dashed line indicates the change in resonant frequency when the initial deflection δ0 is 0.5% of the length of the vibrating section 122 in the minor axis direction (Y direction). The dashed line indicates the change in resonant frequency when the initial deflection δ0 is 1.0% of the length of the vibrating section 122 in the minor axis direction (Y direction).

[0028] As shown in Figure 6, when the frequency of the drive voltage applied to the ultrasonic transducer Tr is swept, the impedance of the ultrasonic transducer Tr changes. A maximum value in this change in impedance is observed when the frequency of the drive voltage applied to the ultrasonic transducer Tr matches the resonant frequency. Therefore, by sweeping the drive voltage applied to the ultrasonic transducer Tr and detecting its impedance, the resonant frequency of the ultrasonic transducer Tr can be determined. Furthermore, as shown in Figures 6 and 7, the resonant frequency of the ultrasonic transducer Tr changes depending on the environmental temperature. In the example shown in Figure 7, the reference temperature is 23°C, and the change in the resonant frequency with respect to the change in environmental temperature is shown, relative to the resonant frequency at the reference temperature. As shown in Figure 7, as the environmental temperature decreases, the resonant frequency increases, and as the environmental temperature increases, the resonant frequency decreases.

[0029] Furthermore, the amount of change in the resonant frequency with respect to a change in the environmental temperature differs depending on the initial deflection δ0 of the vibrating part 122. In this embodiment, as described above, the opening size of the opening 111, the position of the lead terminal 134 relative to the vibrating part 122 (opening 111), and the material of the lead terminal 134 are selected so that the initial deflection δ0 is 0.1% or less of the length of the vibrating part 122 in the minor axis direction (Y direction). In this case, as shown in Fig. 7, the resonant frequency changes significantly in the range of -15% to +10% with respect to a change in the environmental temperature, so that the change in the resonant frequency can be detected with high resolution, and the environmental temperature can be calculated with high accuracy. On the other hand, as shown by the dashed line and the dashed line in FIG. 7, when the initial deflection δ0 is large, the amount of change in the resonant frequency due to a change in the environmental temperature becomes small, making it difficult to detect the change in the resonant frequency and increasing the possibility of an error being included.

[0030] The temperature calculation unit 251 of this embodiment calculates the temperature by utilizing the above-described characteristics of the ultrasonic transducer Tr. Specifically, the temperature calculation unit 251 outputs a command signal to the voltage control circuit 21 to sweep the frequency of the drive voltage input to the ultrasonic element 10 within a predetermined frequency range, and acquires from the impedance detection circuit 23 the impedance of the channel CH when the drive voltage of each frequency is input. Then, the temperature calculation unit 251 identifies the resonance frequency of the ultrasonic element 10 (ultrasonic transducer Tr or channel CH) from the obtained impedance observation results, and calculates the environmental temperature for that resonance frequency. For example, relational data (solid line data in FIG. 7) showing the relationship between the resonance frequency of the ultrasonic transducer Tr and the environmental temperature is stored in advance in the storage unit 24, and the environmental temperature corresponding to the resonance frequency is calculated based on the relational data.

[0031] The sound speed correction unit 252 corrects the sound speed based on the environmental temperature calculated by the temperature calculation unit 251. The sound speed corresponding to the temperature can be calculated using a predetermined calculation formula according to the medium through which the ultrasonic waves propagate. For example, when transmitting ultrasonic waves from the ultrasonic element 10 into the air, the sound speed correction unit 252 uses the calculated environmental temperature t to calculate the sound speed c according to the following formula (1). [Number 1] c=331.5+0.61t …(1)

[0032] The distance calculation unit 253 calculates the distance from the ultrasonic element 10 to the object W using the time from the transmission timing when the ultrasonic element 10 transmits the ultrasonic wave to the reception timing when the ultrasonic wave reflected by the object W is received, and the corrected sound speed c.

[0033] [Ultrasonic sensor operation] FIG. 8 is a flowchart showing the operation of the ultrasonic sensor 1 of this embodiment. In the ultrasonic sensor 1 of this embodiment, when distance measurement is performed, first, the temperature calculation unit 251 sweeps the frequency of the drive voltage applied to the ultrasonic element 10 within a predetermined frequency range (step S1). Then, the temperature measurement unit 241, when sweeping the frequency of the drive voltage, identifies the drive frequency at the timing when the impedance takes a maximum value as the resonant frequency based on the change in impedance detected by the impedance detection circuit 23 (step S2).

[0034] Here, the predetermined frequency range is a range in which a resonant frequency at which the impedance reaches its maximum value can be obtained, and the resonant frequencies that can be obtained within the range of environmental temperature change can be measured in advance. For example, for an ultrasonic sensor 1 used in an environmental temperature range from T0°C to T1°C, the minimum resonant frequency of the ultrasonic transducer Tr corresponding to T0°C and the maximum resonant frequency of the ultrasonic transducer Tr corresponding to T1°C can be measured in advance, and the frequency of the drive voltage can be swept between the minimum and maximum resonant frequencies. Alternatively, the frequency can be swept within a range from the minimum to the maximum resonant frequency, plus a predetermined margin.

[0035] Furthermore, the impedance detection circuit 23 measures the impedance of each of the multiple channels CH. At this time, the resonant frequency corresponding to the maximum value may differ from one another due to individual differences between the individual channels CH. Therefore, it is preferable that the temperature calculation unit 251 sets a channel CH for temperature measurement in advance and identifies the resonant frequency from the impedance of the channel CH for temperature measurement. Alternatively, a representative value of the resonant frequencies measured in each channel CH may be used as the resonant frequency of the ultrasonic element 10. Examples of the representative value include the average value, mode, median, etc. of the impedance of each channel CH. In this embodiment, a single channel CH is formed by connecting multiple ultrasonic transducers Tr, but when each ultrasonic transducer Tr is controlled independently, the impedance of each ultrasonic transducer Tr is measured, and the resonant frequency of the ultrasonic element 10 is calculated based on the representative value.

[0036] Next, the temperature calculation unit 251 calculates the environmental temperature based on the resonance frequency measured in step S2 (step S3). Specifically, the temperature calculation unit 251 calculates the environmental temperature corresponding to the resonance frequency corresponding to step S2 using relationship data that is recorded in the storage unit 24 and that records the relationship between the resonance frequency and the environmental temperature.

[0037] Next, the sound speed correction unit 252 corrects the sound speed using the environmental temperature calculated in step S3 (step S4). For example, when transmitting ultrasonic waves from the ultrasonic element 10 into the air, the above-mentioned formula (1) is used.

[0038] Then, the distance calculation unit 253 drives the ultrasonic element 10 to perform ultrasonic transmission and reception processing, and measures the time (TOF: Time Of Flight) from the transmission timing of the ultrasonic waves to the reception timing of the ultrasonic waves reflected by the object W (step S5). Then, the distance calculation unit 253 calculates the distance from the ultrasonic element 10 to the object W based on the sound speed corrected in step S4 and the TOF measured in step S5 (step S6).

[0039] [Effects of this embodiment] The ultrasonic sensor 1 of this embodiment includes an ultrasonic element 10 and a control unit 20. The control unit 20 functions as a temperature calculation unit 251 that measures the environmental temperature and a sound speed correction unit 252 that corrects the sound speed of ultrasonic waves based on the environmental temperature. The ultrasonic element 10 also includes an element substrate 11 having an opening 111, a diaphragm 12 that closes the opening 111, and a piezoelectric element 13 that is provided on the surface of the diaphragm 12 opposite to the opening 111. The temperature calculation unit 251 then sweeps and changes the frequency of the drive voltage applied to the ultrasonic element 10 (more specifically, each channel CH of the ultrasonic element 10), identifies the resonant frequency of the ultrasonic element 10, and calculates the environmental temperature based on the identified resonant frequency. This makes it possible to calculate the environmental temperature around the ultrasonic element 10 without using a separate component such as a temperature sensor. Because there is no need to provide a temperature sensor, a control circuit for controlling the temperature sensor is also not required, which enables space saving and cost reduction, and promotes miniaturization of the ultrasonic sensor 1.

[0040] In the ultrasonic sensor 1 of this embodiment, the diaphragm 12 is provided with a lead electrode 134 as a stress adjusting member having a linear expansion coefficient greater than that of the diaphragm 12 . Therefore, when the environmental temperature changes, the amount of deflection of the vibrating part 122 changes significantly due to the difference in the linear expansion coefficient. As a result, the resonance frequency also changes significantly, so the environmental temperature can be accurately calculated. That is, when the variation in the amount of deflection of the vibrating part 122 is small with respect to the change in the environmental temperature, the variation in the resonance frequency also becomes small. In such a case, the resolution deteriorates, and when measuring the impedance, if an error is included, it becomes difficult to identify the resonance frequency. In contrast, in the present embodiment, by providing the lead electrode 134, which is a stress adjustment member, on the vibrating part 122, the variation in the amount of deflection of the vibrating part 122 when the environmental temperature changes becomes large. Therefore, even if an error is included when measuring the impedance, the resonance frequency can be accurately identified, and the environmental temperature corresponding to the identified resonance frequency can be accurately calculated.

[0041] In the ultrasonic sensor 1 of the present embodiment, the lead electrode 134, which is a stress adjustment part, is preferably disposed on the arm part 122B. Thereby, the amount of deflection of the arm part 122B when the environmental temperature changes can be changed, and the amount of change in the resonance frequency also becomes large. Therefore, the resonance frequency can be easily and accurately identified, and the environmental temperature can be accurately calculated based on the resonance frequency.

[0042] In the ultrasonic sensor 1 of the present embodiment, the lead electrode 134, which is a stress adjustment member, is provided on the arm part 122B from the ±Y side edges 122C of the arm part 122B to a first distance u, and satisfies u≧H / 2. When u < H / 2, the influence of the thermal contraction of the lead electrode 134 on the deflection of the arm part 122B is small, and the variation in the amount of deflection of the vibrating part 122 when the environmental temperature changes becomes small. In contrast, in the present embodiment, a large difference occurs in the amount of deflection of the arm part 122B when the environmental temperature changes, and the resonance frequency also changes significantly. Therefore, the ultrasonic sensor 1 can accurately detect the resonance frequency and can accurately calculate the environmental temperature based on the resonance frequency.

[0043] In this embodiment, the deflection of the vibrating part 122 at a predetermined reference temperature and when no voltage is applied to the piezoelectric element 13 is defined as an initial deflection amount δ0, and the initial deflection amount δ0 is 0.1% or less of the length of the vibrating part 122 in the minor axis direction (Y direction) (the minimum opening width of the opening 111). If the initial deflection exceeds 0.1%, the change in the deflection of the diaphragm when the environmental temperature changes becomes small, making it difficult to identify the resonant frequency. In contrast, by setting the initial deflection to 0.1% or less, the change in the deflection of the diaphragm when the environmental temperature changes can be made large, making it easier to identify the resonant frequency and allowing the resonant frequency to be identified with high accuracy.

[0044] In this embodiment, the lead electrode 134, which is a stress adjusting member, is made of a material containing at least one of Au and NiCr. This allows a stress adjusting material with a large linear expansion coefficient to be provided on the diaphragm 12 formed of SiO2 or ZrO2, thereby increasing the amount of deflection of the diaphragm 12 due to changes in ambient temperature and changing the resonant frequency.

[0045] In the ultrasonic sensor 1 of this embodiment, the control unit 20 is provided with a storage unit 24, which stores relational data indicating the correspondence between the environmental temperature and the resonant frequency. This allows the temperature calculation unit 251 to easily calculate the environmental temperature corresponding to the identified resonant frequency based on the relational data.

[0046] In the ultrasonic sensor 1 of this embodiment, the processor 25 of the control unit 20 functions as a distance calculation unit 253, and calculates the distance from the ultrasonic element 10 to the object W based on the sound speed corrected by the sound speed correction unit 252 and the TOF acquired by the ultrasonic element 10. As a result, even if a temperature sensor is not provided, the distance from the ultrasonic element 10 to the object W can be calculated using the sound speed corrected in accordance with the environmental temperature.

[0047] [Second embodiment] Next, a second embodiment will be described. In the first embodiment, the lead electrode 134 connected to the second electrode 133 in the ultrasonic element 10 is shown as an example of the stress adjusting member of the present disclosure. In the second embodiment, another example of the stress adjusting member will be described.

[0048] FIG. 9 is a plan view showing a schematic configuration of an ultrasonic element 10A of the second embodiment, and FIG. 10 is a schematic cross-sectional view of the ultrasonic element 10A taken along line BB in FIG. In this embodiment, as shown in FIG. 10, a sealing plate 14 is disposed on the −Z side of diaphragm 12, and a plurality of legs 141 (walls according to the present disclosure) extend from sealing plate 14 toward diaphragm 12. These leg portions 141 are each provided longitudinally along the X direction, and a plurality of leg portions 141 are arranged in the Y direction. Each leg portion 141 is joined to the diaphragm 12 via a resin material 142 serving as a stress adjusting member.

[0049] In this embodiment, the leg portion 141 and the lead electrode 134 on the diaphragm 12 are joined by a resin material 142. In this embodiment, the lead electrode 134 is disposed on the support portion 121 of the diaphragm 12 and does not protrude onto the vibrating portion 122. Instead, in this embodiment, the resin material 142 functions as a stress adjusting member of the present disclosure, and covers the region of the vibrating portion 122 (arm portion 122B) from the ±Y side edge 122C of the vibrating portion 122 to the first distance u. The first distance u satisfies u≧H / 2, as in the first embodiment. The resin material 142 is made of a material having a linear expansion coefficient greater than that of the diaphragm 12. For example, in this embodiment, the linear expansion coefficient is 6.2×10 -5 Use an epoxy resin with a temperature of 1000KJ / ℃. Also in this embodiment, the initial deflection δ0 of the vibrating part 122 is configured to be 0.1% or less of the width in the minor axis direction (Y direction) of the vibrating part 122 (the minimum opening width of the opening 111) at the reference temperature.

[0050] [Effects of this embodiment] In this embodiment, the same effects as those of the first embodiment can be achieved. In this embodiment, legs 141 are provided as walls on the −Z side of diaphragm 12, and resin material 142 that joins legs 141 to diaphragm 12 is made of epoxy resin. This allows a stress adjusting material with a large linear expansion coefficient to be provided on the diaphragm 12 compared to SiO2 or ZrO2, which are commonly used for the diaphragm 12, thereby increasing the amount of deflection of the diaphragm due to changes in ambient temperature and changing the resonant frequency.

[0051] [Variations] The present invention is not limited to the above-described embodiments and variations, and includes modifications, improvements, and configurations obtained by appropriately combining the embodiments within the scope of achieving the object of the present invention.

[0052] [Variation 1] For example, in the first embodiment, after the processes of steps S1 to S4 are performed, the ultrasonic measurement process is performed in step S5 to measure the TOF. Alternatively, when the drive voltage is swept in step S1 to drive the ultrasonic transducer Tr, the received signal may be received to measure the TOF.

[0053] However, when one ultrasonic element 10 is used to both transmit and receive ultrasonic waves, the ultrasonic waves are transmitted at a constant cycle so that the vibration of the vibration part 122 when transmitting ultrasonic waves does not overlap with the vibration of the vibration part 122 when receiving ultrasonic waves. Therefore, since the time required to measure the impedance for all frequencies becomes long, it is preferable to perform the distance measurement process after performing the sound speed correction process, which does not require reception of a received signal, as in the above embodiment.

[0054] The ultrasonic element 10 may be configured to include a transmitting unit that transmits ultrasonic waves and a receiving unit that receives ultrasonic waves. In this case, the temperature calculation unit 251 sweeps the frequency of the drive voltage applied to the transmitting unit to identify the resonance frequency, and can calculate the environmental temperature based on the identified resonance frequency, and the impedance measurement time can be reduced to the same time as in the first embodiment.

[0055] [Variation 2] In the above embodiment, as an example of a case where the minor axis direction of the opening 111 is the Y direction, an example has been shown in which the lead electrodes 134 or the resin material 142 constituting the stress adjusting member are disposed from the ±Y side edges 122C of the vibrating part 122 to a position that is the first distance u toward the piezoelectric element 13. However, a configuration may also be adopted in which the lead electrodes 134 or the resin material 142 constituting the stress adjusting member are similarly provided in the X direction, which is the major axis direction.

[0056] [Variation 3] In the above embodiment, a configuration in which the lead electrode 134 and the resin material 142 are provided as the stress adjustment member has been exemplified, but the present disclosure is not limited to this. The stress adjustment member of the present disclosure is provided to accurately detect the resonant frequency by increasing the amount of deflection of the vibrating part 122 due to changes in the environmental temperature, and the stress adjustment member does not necessarily have to be provided. Even in this case, the resonant frequency changes with changes in the environmental temperature, so the environmental temperature can be measured and calculated using the same method as in the above embodiment.

[0057] [Variation 4] In the above embodiment, a distance sensor that measures the distance to the object W is given as an example of an ultrasonic sensor, but the present invention is not limited to this. For example, the ultrasonic sensor of the present disclosure may be applied to a fluid device in which an ultrasonic element is disposed in a flow path through which a fluid flows and generates ultrasonic waves within the flow path. In this case, it is possible to calculate the temperature of the fluid from the resonant frequency of the ultrasonic element and correct the speed of sound in the fluid based on the calculated fluid temperature.

[0058] Summary of this disclosure An ultrasonic sensor according to a first aspect of the present disclosure comprises an ultrasonic element that transmits ultrasonic waves to an object, a temperature calculation unit that measures the ambient temperature around where the ultrasonic element is installed, and a sound speed correction unit that corrects the sound speed of the ultrasonic waves based on the ambient temperature, wherein the ultrasonic element comprises a substrate having an opening, a diaphragm that closes the opening, and a piezoelectric element provided on the surface of the diaphragm opposite to the opening, and the temperature calculation unit determines the resonant frequency of the ultrasonic element by changing the frequency of a drive voltage applied to the ultrasonic element, and calculates the ambient temperature based on the resonant frequency. This makes it possible to calculate the environmental temperature around the ultrasonic element without using a separate configuration such as a temperature sensor, etc. Since there is no need to provide a temperature sensor, there is also no need for a control circuit to control the temperature sensor, which makes it possible to save space and reduce costs.

[0059] In the ultrasonic sensor of the present disclosure, it is preferable that a stress adjusting member having a linear expansion coefficient larger than that of the diaphragm is provided on the diaphragm. As a result, when the ambient temperature changes, the deflection of the diaphragm changes significantly compared to when the stress adjusting member is not provided due to the difference in the linear expansion coefficient, and accordingly the resonant frequency also changes significantly. Therefore, the ambient temperature can be calculated accurately based on the resonant frequency.

[0060] In the ultrasonic sensor of the present disclosure, it is preferable that in the ultrasonic element, the portion of the vibration plate that blocks the opening is a vibration portion, and the portion of the vibration portion that does not overlap with the piezoelectric element is an arm portion, and the stress adjustment portion is arranged on the arm portion. This allows the amount of deflection of the arm to be changed when the environmental temperature changes, and the amount of change in the resonant frequency also increases. Therefore, the resonant frequency can be easily and accurately determined, and the environmental temperature can also be calculated accurately based on the resonant frequency.

[0061] In the ultrasonic sensor of the present disclosure, when the distance from the outer peripheral edge of the vibrating part to the edge of the piezoelectric element is H, it is preferable that the stress adjustment member is provided at a position where the distance from the outer peripheral edge is H / 2 or more. This causes a large difference in the amount of bending of the arm when the environmental temperature changes, and the resonant frequency also changes significantly. Therefore, the ultrasonic sensor can accurately detect the resonant frequency and accurately calculate the environmental temperature based on the resonant frequency.

[0062] In the ultrasonic sensor of the present disclosure, when the portion of the vibration plate in the ultrasonic element that blocks the opening is defined as the vibration portion, and the amount of deflection of the vibration portion at a predetermined reference temperature and in a state where no voltage is applied to the piezoelectric element is defined as the initial deflection, it is preferable that the initial deflection be 0.1% or less of the length of the vibration portion in the minor axis direction. If the initial deflection exceeds 0.1%, the change in the deflection of the diaphragm when the environmental temperature changes becomes small, making it difficult to identify the resonant frequency. In contrast, by setting the initial deflection to 0.1% or less, the change in the deflection of the diaphragm when the environmental temperature changes can be made large, making it easier to identify the resonant frequency and allowing the resonant frequency to be identified with high accuracy.

[0063] In the ultrasonic sensor of the present disclosure, the stress adjusting member preferably contains at least one of Au and NiCr. This allows the diaphragm to be provided with a stress adjusting material with a larger linear expansion coefficient than SiO2 or ZrO2, which are commonly used for diaphragms, thereby increasing the amount of deflection of the diaphragm due to changes in ambient temperature and changing the resonant frequency.

[0064] In the ultrasonic sensor of the present disclosure, the ultrasonic element may include a wall portion arranged on the opposite side of the vibration plate from the substrate, and the stress adjustment member may be made of epoxy resin that bonds the wall portion and the vibration plate. This allows the diaphragm to be provided with a stress adjusting material with a larger linear expansion coefficient than SiO2 or ZrO2, which are commonly used for diaphragms, thereby increasing the amount of deflection of the diaphragm due to changes in ambient temperature and changing the resonant frequency.

[0065] The ultrasonic sensor of the present disclosure preferably includes a storage unit that records the correspondence between the environmental temperature and the resonant frequency. This allows the temperature calculation unit to easily calculate the environmental temperature corresponding to the identified resonant frequency.

[0066] In the ultrasonic sensor of the present disclosure, it is preferable to include a distance calculation unit that calculates the distance from the ultrasonic element to the object based on the sound speed corrected by the sound speed correction unit and the time it takes for the ultrasonic element to transmit the ultrasonic wave and receive the ultrasonic wave reflected by the object. This makes it possible to calculate the distance from the ultrasonic element to the object using the sound speed corrected in accordance with the environmental temperature, even if a temperature sensor is not provided. [Explanation of symbols]

[0067] 1...ultrasonic sensor, 10, 10A...ultrasonic element, 11...element substrate (substrate), 12...vibration plate, 13...piezoelectric element, 14...sealing plate, 20...control unit, 21...voltage control circuit, 22...receiving circuit, 23...impedance detection circuit, 24...memory unit, 25...processor, 111...opening, 122...vibration unit, 122A...central portion, 122B...arm portion, 122C...±Y side edge (outer peripheral edge) of vibration unit, 134...lead electrode (stress adjustment member), 141...leg portion (wall portion), 142...resin material (stress adjustment member), 241...temperature measurement unit, 251...temperature calculation unit, 252...sound speed correction unit, 253...distance calculation unit.

Claims

1. an ultrasonic element that transmits ultrasonic waves to an object; a temperature calculation unit that measures the ambient temperature around the ultrasonic element; a sound velocity correction unit that corrects the sound velocity of the ultrasonic wave based on the environmental temperature, The ultrasonic element includes a substrate having an opening, a vibration plate that closes the opening, and a piezoelectric element provided on a surface of the vibration plate opposite to the opening, The temperature calculation unit determines a resonant frequency of the ultrasonic element by changing the frequency of a drive voltage applied to the ultrasonic element, and calculates the environmental temperature based on the resonant frequency.

2. a stress adjusting member having a linear expansion coefficient larger than that of the diaphragm is provided on the diaphragm; The ultrasonic sensor of claim 1 .

3. In the ultrasonic element, a portion of the vibration plate that closes the opening is a vibration portion, and a portion of the vibration portion that does not overlap with the piezoelectric element is an arm portion, The stress adjustment member is disposed on the arm portion. The ultrasonic sensor according to claim 2 .

4. In the arm portion, when a distance from an outer peripheral edge of the vibrating portion to an edge of the piezoelectric element is H, the stress adjusting member is provided up to a position where the distance from the outer peripheral edge is H / 2 or more. The ultrasonic sensor according to claim 3 .

5. In the ultrasonic element, when a portion of the vibration plate that closes the opening is a vibration portion, and the amount of deflection of the vibration portion at a predetermined reference temperature and in a state where no voltage is applied to the piezoelectric element is an initial deflection amount, the initial deflection amount is 0.1% or less of the length of the vibration portion in the minor axis direction. The ultrasonic sensor of claim 1 .

6. the stress adjusting member contains at least one of Au and NiCr; The ultrasonic sensor according to claim 2 .

7. The ultrasonic element includes a wall portion disposed on the opposite side of the vibration plate from the substrate, the stress adjusting member is made of an epoxy resin that bonds the wall portion and the diaphragm; The ultrasonic sensor according to claim 2 .

8. a storage unit that records the correspondence relationship between the environmental temperature and the resonance frequency; The ultrasonic sensor of claim 1 .

9. a distance calculation unit that calculates a distance from the ultrasonic element to the object based on the sound speed corrected by the sound speed correction unit and the time from when the ultrasonic element transmits the ultrasonic wave to when the ultrasonic element receives the ultrasonic wave reflected by the object, The ultrasonic sensor of claim 1 .

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