Multiple Wheatstone Bridge Pressure Sensor Offset Cancellation
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Solution Overview
Problem
Conventional piezoresistive pressure sensors face challenges in increasing sensitivity while maintaining robustness to process variations and linearity, as larger diaphragms enhance sensitivity but are more fragile and costly, and misalignment during fabrication can lead to output offsets.
Innovation Solution
A multiple Wheatstone bridge configuration with piezoresistors placed equidistantly around the diaphragm edges, exhibiting mirror symmetry to cancel out misalignment-induced output offsets and enhance sensitivity through differential amplification.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the diaphragm size is increased to enhance sensitivity, then the pressure sensitivity and deflection are improved, but the device becomes more fragile, has larger die size, and incurs higher cost
Solution Approach 1:
The patent divides the single Wheatstone bridge into multiple Wheatstone bridges (e.g., four bridges) distributed around the diaphragm perimeter. Each bridge uses smaller piezoresistor elements, avoiding the need for a single large diaphragm while collectively achieving high sensitivity through additive output signals.
Solution Approach 2:
The patent combines the outputs of multiple Wheatstone bridges to achieve the sensitivity of a large diaphragm while using smaller individual diaphragm sections. The combined output voltage from multiple bridges equals or exceeds what a single large bridge would produce, without the associated fragility and cost penalties.
2Ease of manufacture
If the piezoresistors are placed symmetrically along the diaphragm edge to simplify fabrication, then the manufacturing process is easier, but misalignment with the pressure cavity causes output offset
Solution Approach 1:
The patent intentionally positions Wheatstone bridges at asymmetric locations around the diaphragm perimeter rather than symmetrically. This asymmetric distribution, with bridges placed at different radial distances from the center, makes the system robust against misalignment because no single symmetric alignment point is critical.
Solution Approach 2:
The patent transitions from a single-point measurement (one Wheatstone bridge at one location) to a distributed multi-point measurement system. By placing multiple bridges at different locations around the diaphragm edge, the system averages out misalignment effects and eliminates the need for precise single-point alignment.
3Device complexity
If a single Wheatstone bridge configuration is used to simplify the circuit, then the device complexity is reduced, but the sensitivity and output voltage are limited
Solution Approach 1:
The patent merges multiple Wheatstone bridge circuits to achieve fourfold or greater output voltage compared to a single bridge. The combined output of multiple bridges adds constructively, providing high sensitivity while maintaining relatively simple individual bridge configurations that are easy to fabricate.
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 achieves improved sensitivity and reduced sensitivity to process variations, resulting in enhanced device performance and cost savings by canceling output offsets and increasing the output voltage fourfold compared to single Wheatstone bridge configurations.
Implementation Method 1
Conventional piezoresistive pressure sensors are formed by a Wheatstone bridge that includes four piezoresistors. Under external pressure, the piezoresistors in parallel to the adjacent diaphragm edge are subjected to opposite stresses relative to the two piezoresistors that are normal to the adjacent diaphragm edge. This results in increased resistance for the piezoresistors that are normal and decreased resistance for the piezoresistors that are parallel.
Data Source
Figure 1~2
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AI summary
A pressure sensor includes a diaphragm suspended across a cavity in a substrate. A first group of piezoresistors is provided in the diaphragm proximate a first outer edge of the diaphragm, the piezoresistors of the first group being coupled to one another to form a first Wheatstone bridge. A second group of piezoresistors is provided in the diaphragm proximate a second outer edge of the diaphragm, the piezoresistors of the second group being coupled to one another to form a second Wheatstone bridge. The first and second Wheatstone bridges exhibit mirror symmetry relative to one another. Output signals from each of the first and second Wheatstone bridges are processed at respective first and second differential amplifiers. The output signals from each of the first and second differential amplifiers are processed at a third differential amplifier to produce a pressure output signal with enhanced sensitivity and reduced impact from process variation.