SAW Scale Automatic Recalibration for Span Drift
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Solution Overview
Problem
Current electronic weighing devices using surface acoustic waves (SAW) face challenges in achieving high accuracy and stability due to temperature sensitivity, thermal stress, and span drift, which affect their zero stability and linearity, especially across varying temperature ranges.
Innovation Solution
The implementation of a method to automatically recalibrate SAW scales by taking readings during periods of no weight change, using an auxiliary sensor to adjust operating modes, and fabricating SAW transducers on lithium niobate substrates with carefully chosen metal holders to minimize thermal stress through bonding agents and cantilevered beams, ensuring reduced hysteresis and improved linearity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If magnetic force restoration technology is used to achieve high accuracy weighing, then measurement precision is improved to one part in 120,000, but the device becomes expensive and very sensitive to ambient temperature
Solution Approach 1:
The patent replaces magnetic force restoration technology with surface acoustic wave (SAW) technology. Instead of using electromagnets to counterbalance weight, the invention uses piezoelectric transducers to generate and detect SAWs on a substrate. The weight measurement is achieved by measuring changes in the resonant frequency of the SAW oscillator, which are affected by the mass loading on the substrate. This substitution eliminates the temperature sensitivity and cost issues associated with magnetic systems while maintaining high measurement precision.
Solution Approach 2:
The patent utilizes the relationship between mass loading and resonant frequency of surface acoustic waves. By measuring the shift in resonant frequency caused by different weights on the substrate, the system achieves accurate weighing. The frequency parameter serves as the primary measurement indicator, and its change directly correlates with the applied mass, providing a temperature-stable measurement mechanism.
2Weight of moving object
If load cell technology with strain gauges is used to expand weighing range, then capacity increases to several thousand pounds, but measurement precision deteriorates to one part in 3,000
Solution Approach 1:
The patent employs surface acoustic wave resonant frequency as the measurement parameter instead of electrical resistance changes in strain gauges. The SAW resonant frequency provides a more sensitive measurement mechanism that can resolve smaller frequency shifts corresponding to smaller weight changes. This allows the system to achieve high resolution (one part in 120,000 or better) while maintaining the ability to measure across a wide weight range by detecting proportional frequency shifts.
Solution Approach 2:
The invention replaces the strain gauge electrical resistance measurement system with a surface acoustic wave resonant frequency measurement system. This substitution provides superior resolution because the resonant frequency of SAWs is extremely sensitive to mass loading, allowing detection of very small weight changes even when measuring large weights, thus achieving both high capacity and high precision.
3Ease of manufacture
If SAW transducers are bonded to metal holders with different coefficients of thermal expansion, then ease of manufacture is improved, but reliability deteriorates due to thermal stress and hysteresis
Solution Approach 1:
The patent applies local quality by creating an intermediate layer or modifying the bonding interface between the substrate and metal holder to accommodate thermal expansion differences. Rather than directly bonding dissimilar materials, the invention introduces a transition zone or uses a bonding material with intermediate properties that gradually transitions between the two materials, reducing thermal stress concentration and preventing hysteresis effects that would compromise zero stability.
4Measurement precision
If automatic recalibration is implemented to reduce span drift, then measurement precision is maintained across temperature ranges, but device complexity increases
Solution Approach 1:
The patent implements automatic recalibration that operates autonomously without requiring external intervention or complex control mechanisms. The system uses the SAW oscillator's inherent properties and built-in reference measurements to automatically detect and correct for temperature-induced span drift. This self-calibrating approach maintains high measurement precision across varying temperatures while minimizing added complexity by leveraging the existing SAW measurement infrastructure rather than introducing separate complex calibration systems.
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
This approach significantly reduces span drift and enhances linearity, achieving zero stability and span parameter stability within the desirable range of 1:50,000 to 1:100,000, while maintaining low thermal stress and improving accuracy across a wide temperature range.
Implementation Method 1
Each transducer includes a substantially rectangular piezoelectric substrate and a pair of electrodes imprinted on the substrate
Implementation Method 2
the output of the amplifier causes the first transducer to emit a surface acoustic wave which propagates along the surface of the first transducer substrate
Implementation Method 3
The propagating waves in the first transducer induce an oscillating electric field in the substrate which in turn induces similar SAW waves on the surface of the second transducer substrate
Data Source
AI summary
Apparatus and related methods are provided for automatically recalibrating a SAW scale for changing environmental factors. During a period of time when there is no change to a weight applied to the scale, readings of SAW transducers which relate to weight indications and environmental factor indications are taken for two adjacent operating modes of the scale, and two calibrated weight calculations are made utilizing those readings. The difference in calibrated weight calculations is then related to a variable utilized to transform the readings into weights, which is updated, thereby recalibrating the scale.