Wireless Passive SAW Temperature Sensor Design
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Solution Overview
Problem
Existing wireless surface acoustic wave (SAW) sensors for temperature measurement face challenges such as increased fabrication costs due to unique photo-lithographically patterned bit sequences for identification, reduced energy for sensing due to energy used for identification, and impractical monitoring of phase and individual waves, especially in portable and inexpensive systems.
Innovation Solution
A wireless passive temperature sensor using a surface acoustic wave delay line on a piezoelectric substrate with a thermal coefficient of frequency, featuring a launch interdigital transducer and a reflection interdigital transducer, and an antenna for RF pulse transmission and reception, allowing remote measurement of thermal profiles and heat flux without the need for batteries or wires, using a pulsed radar-type measurement and operating in the 915 MHz ISM band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If unique photo-lithographically patterned bit sequences are used for identification in SAW devices, then self-identification capability is achieved, but fabrication cost increases due to unique photo-masks and tracking requirements
Solution Approach 1:
The patent applies universality by using a common photo-mask pattern for all SAW sensors in the array, eliminating the need for unique photo-masks for each sensor. The identification is achieved through the spatial arrangement and resonance characteristics rather than unique lithographic patterns, reducing fabrication complexity and cost while maintaining self-identification capability.
Solution Approach 2:
The patent changes the identification approach from unique lithographic patterns to variations in resonance frequency and delay time parameters. By tuning these parameters across the sensor array, each sensor can be uniquely identified without requiring unique photo-masks, thus reducing fabrication costs while preserving adaptability.
2Adaptability or versatility
If energy is used for identification in SAW devices, then self-identification is enabled, but energy available for sensing measurement is reduced
Solution Approach 1:
The patent makes the RF pulse transmitted by the external reader serve multiple functions: it simultaneously provides energy for sensing measurement and enables identification through the sensor's resonance response characteristics. This eliminates the need for separate identification energy consumption, as the same RF field used for excitation also carries identification information through the sensor's frequency and delay response.
Solution Approach 2:
The patent merges the identification function with the measurement function by using the same RF pulse and response mechanism for both purposes. The identification is derived from the characteristics of the sensing response itself (resonance frequency and delay time), combining what were previously separate functions into a unified process that maximizes energy efficiency.
3Measurement precision
If phase and individual waves are monitored at high frequencies (100 MHz to several GHz), then pressure, strain, and torque can be monitored, but the system becomes impractical for portable and inexpensive applications
Solution Approach 1:
The patent replaces the complex high-frequency phase monitoring system with a time-domain pulse echo measurement system. Instead of monitoring phase at frequencies of 100 MHz to several GHz, the system uses lower frequency RF pulses and measures the time delay of the acoustic wave round trip, which is much easier to implement with portable and inexpensive hardware while maintaining measurement precision.
Solution Approach 2:
The patent uses periodic RF pulse excitation instead of continuous high-frequency signals. By transmitting periodic pulses and measuring the time delay of reflected pulses, the system achieves accurate measurements without requiring complex high-frequency phase monitoring hardware, making the system practical for portable applications.
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
Enables precise temperature measurement over a wide range (−60° C. to 190° C.) with uncertainties of 0.9° C., independent of physical distance, and allows for the deployment of arrays of sensors with different center frequency bands and transit delays, providing a cost-effective and portable solution for thermal profiling and heat flux measurement.
Implementation Method 1
a launch interdigital transducer, disposed on the surface of the piezoelectric substrate and adapted to launch a surface acoustic wave at a resonance frequency in the piezoelectric substrate when a RF electric field is applied across the launch interdigital transducer
Implementation Method 2
a reflection interdigital transducer, disposed in the acoustic path of and spaced a delay length from the launch interdigital transducer on the piezoelectric substrate and adapted to at least partially reflect the launched surface acoustic wave back to the launch interdigital transducer
Implementation Method 3
an antenna adapted to receive a transmitted RF pulse and apply the RF electric field across the launch interdigital transducer to launch the surface acoustic wave, and to retransmit an RF pulse from the reflected surface acoustic wave received by the launch interdigital transducer
Implementation Method 4
a piezoelectric substrate having a thermal coefficient of frequency
Data Source
AI summary
A wireless passive temperature sensor comprising a surface acoustic wave (SAW) delay line is constructed on a piezoelectric substrate having a thermal coefficient of frequency. An array of addressable, wireless passive temperature sensors can be used to monitor an array of temperature points on a structure using a wireless reader. Each sensor can be monitored by measuring the frequency of maximum reflection in their respective bands. A wireless passive heat flux gauge uses two temperature sensors with different frequency and/or time delay responses to measure that temperature differential across a thermal conductor that has a well characterized thermal conductivity and thickness.