US-12616999-B2 - Ultrasonic sensor

US12616999B2US 12616999 B2US12616999 B2US 12616999B2US-12616999-B2

Abstract

An ultrasonic sensor that is less affected by humidity change is obtained. Ultrasonic sensor ( 1 ) is configured by sequentially laminating piezoelectric element ( 2 ), metal housing ( 3 ), first acoustic matching layer ( 4 ), and second acoustic matching layer ( 5 ). First acoustic matching layer ( 4 ) adjacent to piezoelectric element ( 2 ) with metal housing ( 3 ) interposed therebetween includes a thermoplastic resin and an inorganic filler. The weight fraction of the inorganic filler in first acoustic matching layer ( 4 ) is set to less than or equal to 30% and the weight fraction of the hollow structure filler in the inorganic filler is set to less than or equal to 50%.

Inventors

Tomoki Masuda
YUDAI ISHIZAKI
Hidetomo Nagahara

Assignees

PANASONIC INTELLECTUAL PROPERTY MANAGEMENT CO., LTD.

Dates

Publication Date: 20260505
Application Date: 20210208
Priority Date: 20200303

Claims (6)

1 . An ultrasonic sensor comprising: a piezoelectric element; and a plurality of acoustic matching layers laminated and bonded to each other, wherein the plurality of acoustic matching layers includes a first acoustic matching layer adjacent to the piezoelectric element, the first acoustic matching layer includes a thermoplastic resin and an inorganic filler, a weight percentage of the inorganic filler in the first acoustic matching layer is less than or equal to 30% with respect to a total weight of the first acoustic matching layer, the inorganic filler includes a needle-shaped filler and a hollow filler, and a weight percentage of the hollow filler in the inorganic filler is less than or equal to 50% with respect to a total weight of the inorganic filler.
2 . The ultrasonic sensor according to claim 1 , wherein the thermoplastic resin is a liquid crystal polymer.
3 . The ultrasonic sensor according to claim 1 , wherein the weight percentage of the inorganic filler in the first acoustic matching layer is less than or equal to 30% and greater than or equal to 10% with respect to the total weight of the first acoustic matching layer.
4 . The ultrasonic sensor according to claim 1 , wherein the weight percentage of the hollow filler in the inorganic filler is less than or equal to 50% and greater than or equal to 1% with respect to the total weight of the inorganic filler.
5 . The ultrasonic sensor according to claim 1 , further comprising a housing, wherein the first acoustic matching layer is arranged on an upper surface of a top panel of the housing, and the piezoelectric element is arranged on a lower surface of the top panel of the housing.
6 . The ultrasonic sensor according to claim 5 , wherein the plurality of acoustic matching layers includes a second acoustic matching layer arranged on an upper surface of the first acoustic matching layer.

Description

TECHNICAL FIELD The present invention relates to an ultrasonic sensor that transmits and receives ultrasonic waves. BACKGROUND ART When the difference in acoustic impedance between two different substances in contact with each other is small, an ultrasonic wave can pass through an interface between the two substances and propagates from one of the substances to the other. The acoustic impedance is a numerical value represented by the product of the density of a substance and the sound speed of the substance. When, however, the difference in acoustic impedance between two substances in contact with each other is very large, a larger portion of an ultrasonic wave reflects at an interface than a portion of the ultrasonic wave that propagates. Thus, the efficiency of ultrasonic energy propagation in two substances in contact with each other is higher for substances of which difference in acoustic impedance is smaller. However, a piezoelectric element used in an ultrasonic sensor is generally made of ceramics having a relatively high density and a relatively high sound speed. The density and sound speed of a gas such as air in which an ultrasonic wave propagates are significantly smaller than the density and sound speed of ceramics. Thus, the efficiency of ultrasonic energy propagation from a piezoelectric element to air is very low. To solve this problem, such a measure has been taken that an acoustic matching layer having an acoustic impedance smaller than the acoustic impedance of a piezoelectric element but larger than the acoustic impedance of air is interposed between the piezoelectric element and a gas. This raises the efficiency of ultrasonic energy propagation. From a viewpoint of acoustic impedance, the efficiency of ultrasonic energy propagation from a piezoelectric element to a gas through an acoustic matching layer takes the maximum value when the acoustic impedances of the substances satisfy the relationship represented by the following Formula (1). Z2{circumflex over ( )}2=Z1×Z3 (1) In Formula (1), Z1 is the acoustic impedance of the piezoelectric element, Z2 is the acoustic impedance of the acoustic matching layer, and Z3 is the acoustic impedance of the gas. Furthermore, to propagate an ultrasonic wave generated by a piezoelectric element in a gas with high efficiency, the energy loss of the ultrasonic wave propagating through the acoustic matching layer needs to be suppressed to a low level. A factor causing the energy loss of the ultrasonic wave propagating in the acoustic matching layer is dissipation of ultrasonic energy in the form of heat due to plastic deformation of the acoustic matching layer. Accordingly, to suppress the energy loss of the ultrasonic wave propagating in the acoustic matching layer to a low level, it is desirable that the substance used for the acoustic matching layer has high elasticity. However, as shown in Formula (1), the value of acoustic impedance Z2 of the acoustic matching layer needs to be reduced to bring acoustic impedance Z2 closer to acoustic impedance Z3 of the gas. Substances having low acoustic impedances are substances having a low sound speed and a low density, and in general, many of such substances deform easily. Such substances are not suitable for acoustic matching layers. Specifically, a piezoelectric element, which is a solid, and a gas have acoustic impedances of which values differ by about five orders of magnitude. Thus, to satisfy Formula (1), the acoustic impedance of the acoustic matching layer needs to be reduced to a value that differs from the acoustic impedance of the piezoelectric element by about three orders of magnitude. In this regard, studies have been made for an acoustic matching layer having two layers to cause an ultrasonic wave to propagate from a piezoelectric element to a gas with high efficiency. Here, an acoustic matching layer that is in contact with a gas and emits an ultrasonic wave into a gas is defined as a second acoustic matching layer, and an acoustic matching layer that is in contact with both the second acoustic matching layer and a piezoelectric element is defined as a first acoustic matching layer. The efficiency of ultrasonic energy propagation from the piezoelectric element to the gas through the first acoustic matching layer and the second acoustic matching layer takes the maximum value when the acoustic impedances of the substances satisfy the relationship represented by the following Formula (2) and Formula (3) derived from Formula (1). Z2A2=Z1×Z3 (2) Z3A2=Z2×Z4 (3) In Formula (2) and Formula (3), Z1 is the acoustic impedance of the piezoelectric element, Z2 is the acoustic impedance of the first acoustic matching layer, and Z3 is the acoustic impedance of the second acoustic matching layer, and Z4 is the acoustic impedance of the gas. Since an ultrasonic wave reflects at an interface where two different substances having acoustic impedances that greatly differ from each other are in contact with ea