JP3455585B2 - Aerial ultrasonic transducer - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an aerial ultrasonic vibrator, and more particularly to a structure of an aerial ultrasonic vibrator including a piezoelectric vibrator and a matching plate.

[0002]

2. Description of the Related Art It is well known that piezoelectric vibrators are used as ultrasonic vibrators used in ultrasonic sensors and the like.
Some ultrasonic transducers that emit ultrasonic waves in the air or receive ultrasonic waves in the air have a structure in which a matching plate is attached in order to match the acoustic impedance between the air and the piezoelectric vibrator.

FIG. 6 is a diagram showing an example of a conventional ultrasonic vibrator having a structure in which a matching plate is attached. In the drawing, 1 is a piezoelectric vibrator and 2 is a matching plate. Thickness Tm of this matching plate
Is a quarter wavelength of the resonance frequency f ₀ of the piezoelectric vibrator 1. That is, when λm is the propagation wavelength of the ultrasonic wave in the matching plate and Vm is the propagation velocity in the matching plate, the thickness Tm of the matching plate is Tm = λm / 4 = Vm / 4f ₀ .

[0004]

Since the conventional ultrasonic transducer for aerial use as described above is constructed as described above,
As shown in FIG. 7, it has a high sensitivity only in the vicinity of the resonance frequency f ₀ and has only a so-called single use band characteristic. For example, when an ultrasonic sensor requires a high frequency band and a low frequency band, etc. Had to be equipped with two different ultrasonic transducers for aerial use.

The present invention has been made to solve the above problems, and it is possible to make two or more frequency bands usable band characteristics by combining one piezoelectric vibrator and one matching plate. It is intended to provide a sound wave oscillator.

[0006]

An ultrasonic transducer according to the present invention is an ultrasonic transducer for air , comprising a piezoelectric transducer having a plurality of different vibration modes and a matching plate.
The thickness of the matching plate is set to Tm, and in each vibration mode of the piezoelectric vibrator
The ultrasonic waves generated by the vibration of the plate propagate through the matching plate
, Λ ₁ , λ ₂ , λ ₃ , ... (λ ₁ , λ
_{, 2} , λ ₃ , ... Are different values , Tm = λ ₁ (n ₁ +1/4) = λ ₂ (n ₂ +1/4) =
_{λ 3 (n 3 +1/4) =} ·· · (n 1, n 2, n 3, ·
... is a different number, and satisfies 0 or a positive integer).

Further, a disk-shaped or ring-shaped piezoelectric vibrator is used, and the ultrasonic wave generated by the vibration in the vibration mode in the thickness direction and the vibration in the vibration mode in the radial direction is upward.
Wavelength when propagating in serial alignment plates, each of the above lambda ₁
And λ ₂ .

Further, by using a rectangular piezoelectric vibrator , the vibration in the vibration mode in the thickness direction and the vibration mode in the lateral direction are used.
Due to vibrations in the
It is characterized in that the wavelengths of the generated ultrasonic waves when propagating through the matching plate are λ ₁ , λ ₂ and λ ₃ , respectively.

[0009]

With the above-described structure, the aerial ultrasonic vibrator according to the present invention can have two or more frequency body bands as used band characteristics even though it is a single vibrator. Further, by using the high frequency band for the primary wave and the low frequency band for the secondary wave, it is possible to configure a parametric transducer for transmission and reception.

[0010]

【Example】

Example 1. An embodiment of the present invention will be described below with reference to the drawings. According to the present invention, the plurality of vibration modes originally possessed by the piezoelectric vibrator are used as they are, and the shape (size) of the matching plate is determined so that the acoustic impedance matching with respect to the plurality of vibration modes can be performed with one matching plate. To do. That is,
For example, when the piezoelectric vibrator has a disc shape, the piezoelectric vibrator originally has a vibration mode in the thickness direction and a vibration mode in the radial direction.

In the conventional ultrasonic vibrator as shown in FIG. 6, a matching plate that matches one of the vibration modes is attached and only one vibration mode is used. However, the matching condition of the matching plate is a λ / 4 wavelength plate, which is equivalent to λ (n ± 1/4). Therefore, the thickness Tm of the matching plate is set to the matching condition λ _{1 of one} vibration mode.
/ 4 and other vibration mode matching condition λ ₂ (n ± 1/4)
If both and are set to satisfy the thickness at the same time, both vibration modes can be used.

FIG. 1 is a diagram showing a first embodiment of an ultrasonic vibrator according to the present invention, in which 10 is a piezoelectric vibrator and 20 is a matching plate. The piezoelectric vibrator 10 has a disk shape having a diameter D and a thickness T. Therefore, the vibration mode in the radial direction determined by the diameter D and the vibration mode in the thickness direction determined by the thickness T as described above. The resonance frequencies f _P and f _t of these two vibration modes can be expressed by f _P = N ₁ /D...(1), f _t = N ₄ /T...(2) it can. Where f _{P is the} radial vibration resonance frequency N ₁ is the radial vibration frequency constant that is determined by the material constant of the piezoelectric vibrator f _{t is} the vibration resonance frequency in the thickness direction N ₄ is the thickness direction that is determined by the piezoelectric vibrator material constant Is the vibration frequency constant of.

Therefore, the thickness Tm of the matching plate 20 is determined as follows. The vibration / acoustic conversion characteristics of the matching plate 20 are
Since it is considered to be equivalent to a lossless distributed constant line, it can be represented by a two-terminal pair Fundamental matrix as shown in FIG. Therefore, the thickness Tm of the matching plate is λ / 4 and λ
It can be seen that (n + 1/4) is equivalent (n is 0
Or a positive integer ).

Here, the thickness Tm of the matching plate 20 is Tm = t ₁ + t ₂ =, as shown in FIG. 1, where the ultrasonic wave propagation wavelength of the matching plate at the resonance frequency of the vibration mode in the thickness direction is λt. It can be considered that λt (1/4 + n) = λt / 4 + nλt (4), and from the above equation (2), t ₁ = λt / 4 = T · Vm / 4N ₄ (5) t ₂ = n λt = nT · Vm / N ₄ (6) Here, n is 0 or a positive integer , and Vm and N ₄ are constants. Therefore, matching of acoustic impedance in the vibration mode in the thickness direction is performed according to the thickness T of the piezoelectric vibrator 10.
The thickness of the matching plate 20 may be determined by the equations (5) and (6).

Next, assuming that the ultrasonic wave propagation wavelength of the matching plate at the resonance frequency of the vibration mode in the radial direction is λ _P , Tm = t ₃ + t ₄ = t ₁ + t ₂ as in the above equation (4). (Not shown) Tm = t ₃ + t ₄ = λ _P (1/4 + n ) = λ _P / 4 + nλ _P (7) Then, from the above equation (1), t ₃ = λ _P / 4 = D · Vm / 4N ₁ (8) t ₄ = λ _P = nD · Vm / N ₁ (9)

Therefore, in order to match the acoustic impedance of the vibration mode in the thickness direction, the diameter D of the piezoelectric vibrator 10 and the thickness Tm of the matching plate 20 are determined so as to satisfy the equations (8) and (9). It will be good. Here, since the equations (6) and (9) can be determined in the same manner as the equations (5) and (8), respectively, in the following description, the n-th order is defined as the variables (α, β, γ, δ) will be omitted,
The thickness T of the matching plate is set so as to satisfy the equations (5) and (8).
m and the dimensions D and T of the disk-shaped piezoelectric vibrator are determined, the resonance frequency of the vibration mode in the radial direction is determined by one piezoelectric vibrator 10 and one matching plate 20 as shown in FIG. f
_{It is possible} to realize an airborne ultrasonic transducer having two use band characteristics in the vicinity of _{P and in} the vicinity of the resonance frequency f _t of the vibration mode in the thickness direction.

Example 2. FIG. 4 is a view showing another embodiment of the present invention, in which 11 is a ring-shaped piezoelectric vibrator and 21 is a matching plate. As for the vibration mode of the ring-shaped piezoelectric vibrator 11, the vibration mode in the thickness direction is determined by the thickness T thereof. Therefore, from the relationship with the above equation (5), the vibration frequency constant in the length direction is N ₂ , When the vibration frequency constant of is set to N ₄ , the relationship of Tm = α + T · Vm / 4N ₄ (10) is established. On the other hand, in the radial (low order) vibration mode, assuming that the outer diameter is A and the inner diameter is B, Tm = β + {[(A + B) /
2] π · Vm} / 4N ₂ (11),
Therefore, in order to satisfy the equations (10) and (11), the thickness Tm of the matching plate, the outer diameter A of the ring-shaped piezoelectric vibrator 11,
By determining the inner diameter B and the thickness T, one ultrasonic transducer 11 and one matching plate 21 can realize an ultrasonic transducer for air having two band characteristics.

Example 3. Although the above-described first and second embodiments have described the airborne ultrasonic vibrator having two use bands, for example, by using a rectangular piezoelectric vibrator, a thickness direction, a vertical direction, and a horizontal direction can be obtained. It is also possible to realize an airborne ultrasonic transducer having three use bands by utilizing three vibration modes. FIG. 5 is a diagram for explaining the third embodiment, in which 12 is a rectangular piezoelectric vibrator,
22 is a matching plate.

When the horizontal dimension of the rectangular piezoelectric vibrator 12 is X, the vertical dimension is Y, and the thickness is T, the vibration mode in the thickness direction is the same as in the first and second embodiments. Can be determined by the thickness T. Further, when the oscillation frequency constant in the longitudinal direction and N _2, lateral vibration mode and the longitudinal vibration mode which is a low-order vibration mode, when the oscillation frequency constant in the longitudinal direction and N _2, Tm = γ + X · Vm / 4N ₂・ ・ ・ (12) Tm = δ + Y · Vm / 4N ₂・ ・ ・ (13) Therefore, similarly, the thickness Tm of the matching plate and the width of the rectangular piezoelectric vibrator 12 are the same. By determining the dimension X in the direction, the dimension Y in the vertical direction, and the thickness T, one piezoelectric vibrator 12 and one matching plate 22 are used to form an airborne ultrasonic vibrator having three use band characteristics. realizable.

Barium titanate and lead zirconate titanate (PZT) are typical materials for the piezoelectric vibrator, and the matching plate material is hollow glass with a size of several tens of μm. A ball made of epoxy resin is often used.

[0021]

As described above, the aerial ultrasonic vibrator of the present invention has the matching plate thickness satisfying the matching condition of ¼ wavelength at the same time for different vibration modes of the piezoelectric vibrator. Therefore, one piezoelectric vibrator and one matching plate
An aerial ultrasonic transducer having the above frequency band as a use band can be obtained. If the high frequency band of this aerial ultrasonic transducer is used as the primary wave and the low frequency band is used as the secondary wave, it can be used as a parametric transducer.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a first embodiment of the present invention.

FIG. 2 is a diagram showing a Fundamental matrix of two terminal pairs.

FIG. 3 is a diagram showing sensitivity characteristics of the ultrasonic transducer of the first embodiment.

FIG. 4 is a diagram for explaining a second embodiment of the present invention.

FIG. 5 is a diagram for explaining a third embodiment of the present invention.

FIG. 6 is a diagram for explaining a conventional ultrasonic transducer for aerial use of this type.

FIG. 7 is a diagram showing sensitivity characteristics of a conventional ultrasonic transducer.

[Explanation of symbols]

10, 11, 12 Piezoelectric vibrator 20,21,22 Matching plate T Thickness of piezoelectric vibrator D Piezoelectric vibrator diameter A Piezoelectric vibrator outer diameter B Piezoelectric vibrator inner diameter Tm Matching plate thickness

Claims (4)

(57) [Claims] And 1. A piezoelectric vibrator having a plurality of different vibration modes, the air ultrasonic transducers made of the alignment plate, the thickness of the alignment plate and Tm, the vibration in each vibration mode of the piezoelectric vibrator Super sound produced by
The wavelengths of the waves propagating through the matching plate are λ ₁ and λ, respectively.
₂ , λ ₃ , ... (λ ₁ , λ ₂ , λ ₃ , ... Are different
Value) , Tm = λ ₁ (n ₁ +1/4) = λ ₂ (n ₂ +1/4) =
_{λ 3 (n 3 +1/4) =} ·· · (n 1, n 2, n 3, ·
... is a different number, 0 or a positive integer) aerial ultrasonic vibrator and satisfies the. 2. A disk-shaped or ring-shaped piezoelectric vibrator is used, and ultrasonic waves generated by vibration in a vibration mode in the thickness direction and vibration in a vibration mode in the radial direction propagate in the matching plate. _2. The airborne ultrasonic transducer according to claim ₁ , wherein the wavelengths are respectively set to λ ₁ and λ ₂ . 3. Using a rectangular piezoelectric vibrator, vibration in the vibration mode in the thickness direction, in the vibration mode in the transverse direction
Raw the vibration in the vibration mode of vibration and its longitudinal direction
_2. The airborne ultrasonic transducer according to claim ₁ , wherein the wavelengths of the ultrasonic waves propagated through the matching plate are λ ₁ , λ ₂ and λ ₃ , respectively. 4. The piezoelectric vibrator according to any one of claims 1 to 3, wherein the piezoelectric vibrator and the matching plate are bonded and fixed to each other.
The ultrasonic transducer for aerial use according to claim 1.