Ultrasonic Sensor Acoustic Matching Layers for Humidity Stability
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Solution Overview
Problem
Ultrasonic sensors face low efficiency in propagating ultrasonic energy from piezoelectric elements to gases due to large differences in acoustic impedance, and existing materials for acoustic matching layers are prone to density variations and moisture absorption, affecting performance under varying environments.
Innovation Solution
The use of a thermoplastic resin with a specified blending percentage of inorganic fillers, including needle-shaped and hollow fillers, to create acoustic matching layers that can be injection molded, ensuring consistent density and reduced moisture absorption, thereby enhancing humidity resistance and acoustic impedance matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an acoustic matching layer with low acoustic impedance is used to improve ultrasonic energy propagation efficiency from piezoelectric element to gas, then the acoustic impedance matching is improved, but the material becomes prone to deformation and density variation
Solution Approach 1:
The acoustic matching layer uses a composite material consisting of a rubber base material (providing elasticity and low acoustic impedance) and a hollow filler (providing density control and structural stability). This composite structure achieves both good acoustic impedance matching with gas and resistance to deformation under pressure differential, resolving the contradiction between propagation efficiency and compositional stability.
Solution Approach 2:
The hollow filler creates a porous structure within the acoustic matching layer that reduces the overall density of the material, thereby lowering its acoustic impedance to match better with gas. Simultaneously, the porous structure maintains structural integrity and resistance to deformation, addressing both acoustic matching and stability requirements.
2Reliability
If a material with very low acoustic impedance is used to match gas impedance, then acoustic impedance matching is improved, but the material deforms easily under pressure differential
Solution Approach 1:
The composite of rubber base material and hollow filler creates a material that combines the low acoustic impedance needed for gas matching with the structural strength provided by the rubber matrix. The hollow filler reduces density for impedance matching while the rubber base material provides deformation resistance under pressure differential.
Solution Approach 2:
The acoustic matching layer exhibits local quality differentiation where the hollow filler distribution and rubber matrix structure are optimized to provide different properties in different aspects: low density for acoustic matching and high elasticity for deformation resistance, allowing the material to satisfy both requirements simultaneously.
3Reliability
If traditional acoustic matching layer materials are used to achieve low acoustic impedance, then ultrasonic energy propagation is improved, but moisture absorption increases under high humidity conditions
Solution Approach 1:
The rubber base material is selected and formulated to have specific parameters including low moisture absorption characteristics while maintaining the required acoustic properties. By changing the material parameters (selecting rubber with appropriate cross-linking density, filler content, and composition), the material achieves both good ultrasonic energy propagation and resistance to moisture absorption in high humidity environments.
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 solution results in a highly reliable ultrasonic sensor with improved acoustic energy propagation efficiency and stability across varying environmental conditions, including high humidity and temperature changes.
Implementation Method 1
an ultrasonic sensor that transmits and receives ultrasonic waves... a piezoelectric element used in an ultrasonic sensor
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
an ultrasonic wave can pass through an interface between the two substances and propagates from one of the substances to the other
Data Source
Figure 1
Figure 2
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%.