Capacitive Pressure Sensor with Vented Reference Cavity
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Solution Overview
Problem
Capacitive pressure sensors exhibit non-linear capacitance-pressure behavior, requiring numerous calibration points and complex polynomial fits, leading to increased error and cost due to the need for extensive calibration, especially at higher pressures, and the Runge interpolation phenomenon limits accurate extrapolation outside the calibration range.
Innovation Solution
A capacitive pressure sensor design incorporating a sealed cavity sensor and a vented cavity reference sensor, with an integrated CMOS circuit, allows for reduced calibration points by using ambient pressure measurements and resonance frequency differences to derive cavity pressure, enabling a more accurate non-linear capacitance-pressure function fit with fewer measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If more calibration data points are used to improve accuracy, then measurement precision improves, but calibration time and cost increase significantly
Solution Approach 1:
The patent introduces a reference capacitor as an intermediary element that experiences the same environmental conditions (temperature, pressure, humidity) as the sensing capacitor but does not respond to the target measurand. By measuring the reference capacitor's characteristics, the system can compensate for environmental drifts and extract accurate calibration parameters with fewer calibration points, thus reducing calibration time while maintaining measurement precision.
Solution Approach 2:
The patent replaces the traditional mechanical approach of applying multiple controlled pressure levels during calibration with an electrical field-based method. By using the reference capacitor and analyzing capacitance ratios, the system can determine calibration parameters through electrical measurements alone, eliminating the need for complex mechanical pressure control equipment and reducing calibration time.
2Measurement precision
If higher order polynomials are employed to improve accuracy, then measurement precision improves, but error at outer calibration points increases due to Runge interpolation phenomenon
Solution Approach 1:
The reference capacitor serves as a mediator that provides information about environmental conditions without being affected by the target measurand. By using the reference capacitor's capacitance changes, the system can separate environmental drift from actual measurement signals, enabling the use of simpler polynomial fits that are more reliable at outer calibration points while maintaining overall accuracy.
Solution Approach 2:
The patent changes the calibration approach from using absolute capacitance values to using capacitance ratios or differential measurements between sensing and reference capacitors. This parameter transformation reduces the impact of environmental variations and allows for more stable polynomial fitting across the entire measurement range, including outer calibration points.
3Measurement precision
If more calibration points are taken at higher pressure regions, then accuracy in non-linear regions improves, but calibration complexity and cost increase
Solution Approach 1:
The reference capacitor acts as a mediator that provides real-time information about environmental conditions affecting the sensing capacitor. By continuously monitoring the reference capacitor during operation, the system can compensate for drifts in the high-pressure region without requiring additional calibration points, thus improving accuracy while reducing calibration complexity.
Solution Approach 2:
The patent implements a feedback mechanism where the reference capacitor's measurements are continuously used to adjust and correct the sensing capacitor's readings. This real-time compensation allows the system to maintain high accuracy in non-linear pressure regions without requiring complex pre-calibration procedures or multiple calibration points at high pressures.
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 calibration time and cost while improving measurement accuracy, allowing for accurate pressure sensing with minimal calibration points and eliminating the need for controlled pressure measurements.
Implementation Method 1
capacitive pressure sensor, measuring a capacitance between the electrodes of the first sensor device
Implementation Method 2
the cavity shape is changed in dependence on a pressure exerted on the cavity roof
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
The sensor has at least two membranes of which one membrane is sealed and one other membrane has an opening to ambient pressure, and functions as a reference device. The sealed membrane deflects as function of pressure whereas the deflection of the "open" membrane remains unaltered. The reference device enables determination of relevant calibration parameters such as capacitance offset, dC/dP, parasitic capacitance, membrane stiffness, back side pressure. This then enables an accurate capacitance pressure function to be obtained with only two calibration points and without needing to control pressure levels.