Ultrasonic Sensor Matching Layer for Humidity-Resistant Transmission
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Solution Overview
Problem
The efficiency of ultrasonic energy propagation from a piezoelectric element to air is low due to the significant difference in acoustic impedance, and existing materials for acoustic matching layers are prone to moisture absorption and density variation, affecting sensor reliability.
Innovation Solution
An ultrasonic sensor with a first acoustic matching layer composed of a thermoplastic resin and inorganic filler, where the inorganic filler is limited to less than 30% by weight and includes a mixture of needle-shaped and hollow fillers, allowing for injection molding and improved humidity resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a piezoelectric element made of ceramics is used to transmit ultrasonic waves to air, then the ultrasonic sensor can generate ultrasonic waves, but the efficiency of ultrasonic energy propagation is very low due to large difference in acoustic impedance between ceramics and air
Solution Approach 1:
An acoustic matching layer is introduced as an intermediary substance between the piezoelectric element and air. This matching layer has acoustic impedance values between those of ceramics and air, enabling gradual impedance transition and improving ultrasonic energy propagation efficiency while avoiding the need to directly interface high-impedance ceramics with low-impedance air
Solution Approach 2:
The acoustic matching layer is constructed as a composite material combining resin and inorganic filler. The resin provides elasticity to reduce energy loss, while the inorganic filler adjusts the acoustic impedance to an optimal value between ceramics and air. This composite structure simultaneously achieves both mechanical performance and acoustic impedance matching requirements
2Loss of energy
If substances with low acoustic impedance are used for the acoustic matching layer to satisfy the impedance matching condition, then the ultrasonic energy propagation efficiency is improved, but these substances have low density and low sound speed and deform easily, making them unsuitable for acoustic matching layers
Solution Approach 1:
The acoustic matching layer uses a composite structure where resin provides the elastic matrix for energy propagation while inorganic filler particles provide structural stability. The filler particles are dispersed within the resin matrix, creating a material that combines the low acoustic impedance needed for efficient ultrasonic transmission with the mechanical strength required to prevent deformation
Solution Approach 2:
The acoustic impedance, density, and sound speed of the matching layer are precisely controlled by adjusting the ratio of inorganic filler to resin. By optimizing these material parameters, the matching layer achieves the desired acoustic impedance for efficient energy propagation while maintaining sufficient structural stability through appropriate filler loading and distribution
3Manufacturing precision
If a high-temperature curing reaction process is used to produce the acoustic matching layer, then the material properties can be controlled, but the production process becomes complex and time-consuming with high variability in density
Solution Approach 1:
The chemical curing process is replaced with a mechanical injection molding process. The injection molding method uses mechanical injection to form the acoustic matching layer, eliminating the need for high-temperature curing reactions. This substitution provides better control over density and dimensional accuracy while significantly reducing production time and process complexity
Solution Approach 2:
The production process transitions from chemical parameter control (curing temperature, reaction time) to mechanical parameter control (injection pressure, injection temperature, mold design). This parameter change enables more precise and consistent density control of the acoustic matching layer while improving manufacturing efficiency and reducing process variability
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The sensor achieves high reliability and humidity resistance by controlling density variation and reducing moisture absorption, ensuring efficient ultrasonic wave propagation.
Implementation Method 1
a piezoelectric element used in an ultrasonic sensor is generally made of ceramics
Implementation Method 2
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
Implementation Method 3
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
Data Source
AI summary
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%.