Capacitance Sensor Accuracy via Microprocessor Calibration
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Solution Overview
Problem
Capacitance-type sensors, particularly those measuring cryogenic liquid levels in mobile storage vessels, face challenges in accuracy due to variable capacitance and difficulty in recalibration, leading to errors of up to 20-25% and increased wear on fuel pump components from frequent refueling.
Innovation Solution
A measurement circuit with a microprocessor and calibration capacitors of known capacitance is used to calculate correction values for measured data, improving accuracy by compensating for errors introduced by the measurement circuit and environmental factors, with calibration capacitors positioned outside the storage vessel for accessibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If capacitance-type sensors are used to measure cryogenic liquid levels in mobile storage vessels, then liquid level detection is enabled, but measurement accuracy deteriorates with errors of up to 20-25%
Solution Approach 1:
The system performs preliminary calibration by measuring the capacitance of known reference capacitors to establish correction values before actual liquid level measurement. This preliminary characterization of the measurement circuit's error behavior enables subsequent correction of liquid level measurements, improving accuracy from 20-25% error down to approximately 1% error.
Solution Approach 2:
The system uses feedback by comparing measured capacitance values against correction values derived from reference capacitor measurements. The microprocessor continuously applies correction based on the difference between expected and actual readings, creating a closed-loop system that compensates for measurement circuit drift and environmental variations.
2Measurement precision
If recalibration of capacitance-type sensors is performed to improve accuracy, then measurement precision improves, but device complexity and operational difficulty increase
Solution Approach 1:
The system uses reference capacitors as simplified copies or models of the sensor capacitance. Instead of performing complex recalibration procedures on the actual sensor, the system measures known reference capacitors with identical or similar capacitance values to the expected sensor readings, thereby characterizing measurement circuit errors without direct sensor manipulation.
Solution Approach 2:
The system changes the parameter being measured from unknown sensor capacitance to known reference capacitor capacitance during calibration mode. By switching between measuring unknown sensor values and known reference values, the system can determine correction factors that account for measurement circuit variations without requiring physical recalibration of the sensor itself.
3Measurement precision
If calibration capacitors are positioned inside the storage vessel with the sensor, then measurement consistency is improved, but accessibility for maintenance and replacement deteriorates
Solution Approach 1:
The system segments the measurement system into two distinct parts: the sensor assembly positioned inside the storage vessel for continuous measurement, and the reference capacitors positioned outside for accessibility. This segmentation allows the reference capacitors to be easily accessed, replaced, or recalibrated without requiring access to the interior of the storage vessel, while still providing the necessary calibration function.
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 reduces measurement errors to about 1% for cryogenic liquid levels, enhancing the reliability of liquid level detection and reducing operational inefficiencies and component wear.
Implementation Method 1
A measurement circuit is provided that includes a calibration capacitor with a known capacitance
Implementation Method 2
For a given capacitor there is a known relationship between charge, capacitance and voltage. The voltage is proportional to the amount of charge and the circuit detects an increase of capacitance when there is an increase in voltage.
Data Source
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AI summary
An apparatus and method is disclosed for improving the accuracy of measurements taken with a capacitance-type sensor. In addition to the sensor the apparatus comprises a measurement circuit and a microprocessor. One of the sensor or a calibration capacitor with a known and fixed capacitance is connectable to the measurement circuit by a switch that has its position controlled by the microprocessor. The microprocessor sends measurement signals to the measurement circuit and receives data therefrom. According to the disclosed method, the measurement circuit is programmed to determine an error between measured calibration data and the known capacitance and to use this error to calculate a correction value, which it applies to the measured sensor data to calculate corrected sensor measurements.