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FIELD OF THE INVENTION The present invention relates to ultrasonic sensors. [Prior Art] The
acoustic impedance of a ceramic or magnetostrictive transducer has an order of magnitude
greater impedance than that of the medium, and matching the acoustic impedance between the
transducer and the medium to increase the sound radiation efficiency. There is a need to do.
Conventionally, as a means for this purpose, a composite material obtained by kneading organic
or inorganic micro hollow spheres in a polymer (silicon, epoxy, etc.) resin is used to obtain an
acoustic impedance intermediate to that of the medium. And, this composite member is adhered
to the sound wave emitting surface of the transducer and used as a matching layer. However, it is
not possible to mix the hollow spheres uniformly in density in the polymer resin, and it is
necessary to carry out vacuum degassing in order to remove air bubbles generated by the air
introduced when kneading them. Since micro hollow spheres are powders with a very low ratio I,
special attention must be paid to prevention of airborne scattering in the wire mixing process,
etc., etc. with stable performance and also by economical means. There were many difficult sets
in manufacturing, and as a result, it was not possible to provide a stable and inexpensive
ultrasonic sensor as a product. [In the present invention, it is an object of the present invention to
stabilize the manufacturing process of the ultrasonic sensor and to achieve an economic effect,
and in particular, a transformer which largely causes the performance of the ultrasonic sensor. It
is intended to stabilize the effective performance by preventing the dispersion due to the
mismatch of the acoustic impedance of the medium and the air which is the medium or the water
by the economical material and the manufacturing method. It is well known that the cause of
acoustic impedance mismatch is that the difference in acoustic impedance between the
transducer and the medium is quite large, and to prevent this, it is necessary to use a layer
having an impedance matching function between them. It is also well known that it is a good idea
to place in However, as mentioned above, the method for stably and economically producing
these aligning members has not been established yet. Solution C In the present invention, readily
available wood is used as a production member. The echo impedance of wood shows a specific
value depending on the type of wood, for example Hebies concolor ([Trees]: 23.25 × 10 'kg / m
′ ′S eight cer to ampestris (Made in black): 20.46 × 10 ′ kg / m. 'sCalamus rotang:
12.65X10' kg / m'5 Cork IVood: 1.00X10 'kg / rn's (from the Nikkan Kogyo Shimbun, Ltd.,
published by Ultrasonic Technology Handbook) The acoustic impedance of the acoustic matching
member required now generally From the well-known matching condition, it is the geometric
mean value of the acoustic impedance of each of the transducer and the medium, its thickness is
preferably 1/4 wavelength, and when the medium is air, the acoustic impedance of air at room
temperature Is about 4 0 kg / m's, if transducers are ceramic, the acoustic impedance is 30X10
'kg / m's.
Therefore, the matching layer is 1. Acoustic impedance values of lX10 'kg / m's are preferred.
For example, Cork and Ba I sa materials can be mentioned as complete products which satisfy the
conditions and are easily available. The acoustic impedance of these products is 0.8-1.5 XIf) 'kg /
m's, which indicates a value suitable as a matching member between the transducer and air. The
acoustic matching layer made of these materials extremely efficiently radiates the energy
generated by the transducer into the medium, and the radiation level thereof shows a value equal
to or higher than that of the plastic matching layer according to the conventional method. In
order to improve the moisture resistance and waterproofness of these woods, it is also effective
to apply resin coating to the surface, side or whole of the wood matching layer to stabilize the
characteristics of the ultrasonic sensor. Furthermore, it is also possible to provide an acoustic
impedance gradient inside the wood piece by positively utilizing the porousness and water
absorption of the matching member, and impregnating the silicon or epoxy resin into the wicking
side of the wood piece of the wood alignment material. It belongs to the scope of the invention.
Also, if a layer made of silicon or epoxy resin is added as a second matching layer on the
transducer side and the above wood piece matching layer is superimposed on the layer, the
impedance matching between layers is extremely smooth, and a plurality of layers are
conventionally used. It is possible to reduce the reflection of the sound wave on the matching
layer overlapping surface, which has been considered as a drawback in the case of providing the
matching layer. An ultrasonic sensor having such a configuration has extremely stable
characteristics. [Example 1] An ultrasonic sensor formed by sticking a Ba1sa material on the
sound wave emitting surface of a piezoelectric element and fixing a wood piece outer peripheral
portion with a metal or plastic ring having an acoustic impedance larger than its acoustic
impedance is extremely stable. Performance and can be supplied to the market at an economical
price. [Example 2] A matching member in which a polymer resin (silicon, epoxy, etc.) is
impregnated into the transducer surface of the wood piece for alignment in the above-mentioned
example, and a gradient of acoustic impedance is provided in the sound wave radiation direction
Stable characteristics can be obtained. Of course, it is also possible to use the resin to be
impregnated into the wood piece to bond the transducer and the wood piece.
Brief description of the drawings
FIG. 1 is a cross-sectional view of a conventional ultrasonic sensor, and FIGS. 2 and 3 are crosssectional views showing the structure of an airborne ultrasonic sensor according to an
embodiment of the present invention.
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