Capacitance Sensing Circuit with Modulated Potentials and Non-Linearity Compensation
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Solution Overview
Problem
Capacitive sensors face challenges with reduced accuracy due to small rest capacitance and increased stray capacitance, leading to degraded signal-to-noise ratios and non-linearity, which limits their error specifications.
Innovation Solution
A capacitive sensing circuit with a voltage divider comprising first and second capacitors, where at least one capacitor's capacitance varies with the process variable, and a control circuit that adjusts modulated potentials to compensate for non-linearity and noise, using a detector to provide a process variable output in the baseband frequency range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If capacitive sensor size is reduced to decrease product size and cost, then manufacturing cost and product size are improved, but signal-to-noise ratio deteriorates due to smaller rest capacitance and larger relative stray capacitance
Solution Approach 1:
The patent applies periodic modulation of the capacitive sensor at a carrier frequency, converting the DC capacitance measurement into an AC signal measurement. This periodic action allows the use of lock-in detection techniques that reject noise outside the modulation frequency band, thereby improving signal-to-noise ratio while maintaining small sensor size
Solution Approach 2:
The patent introduces an intermediary modulation signal at carrier frequency that mediates between the small capacitive signal and the measurement system. By modulating the sensor capacitance with an AC signal and detecting the modulated response, the system can distinguish the sensor signal from stray capacitance and noise, improving measurement precision without increasing sensor size
2Volume of moving object
If capacitive sensor size is reduced to decrease product size and cost, then product size is improved, but measurement accuracy deteriorates due to degraded signal-to-noise ratio
Solution Approach 1:
The patent uses periodic modulation at carrier frequency to convert small capacitive changes into detectable AC signals. This allows accurate measurement of small capacitance values in miniaturized sensors by using frequency-domain separation to reject noise and stray capacitance effects
Solution Approach 2:
The patent implements feedback through a control circuit that uses the detected carrier frequency components to adjust and control the baseband envelope of modulated potentials. This feedback mechanism compensates for non-linearity and improves measurement accuracy in miniaturized capacitive sensors
3Measurement precision
If demand for improved accuracy increases, then measurement precision requirement is improved, but sensor non-linearity becomes more significant and limits error specifications
Solution Approach 1:
The patent implements feedback through a control circuit that uses the detected carrier frequency components to adjust and control the baseband envelope of modulated potentials. This feedback mechanism compensates for non-linearity and improves measurement accuracy
Solution Approach 2:
The patent changes the operating parameters by using modulation at carrier frequency and controlling the baseband envelope as a function of the detector output. This parameter transformation allows linearization of the capacitive sensor response through electronic control, reducing non-linearity errors
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 enhances the accuracy of capacitive sensors by reducing noise and compensating for non-linearity, resulting in improved signal-to-noise ratios and more precise measurements of process variables.
Implementation Method 1
a voltage divider that includes first and second capacitors... The divider has divider ends that receive modulated potentials... The divider has a center tap connection that provides a detector input
Implementation Method 2
The circuit comprises a detector with a detector output. The detector output represents carrier frequency range components of the detector input
Implementation Method 3
The control circuit controls the baseband envelope of the modulated potentials as a function of the detector output... The non-linear control function compensates the process variable output for non-linearity with respect to the process variable
Data Source
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AI summary
A circuit (100) that senses a process variable (PROC. VAR.), comprises a voltage divider (106) that includes first (102) and second (104) capacitances. At least one of the capacitances is varied by the process variable. Divider ends (128,130) receive modulated potentials (El, E2), and a divider center tap (108) coupled to a detector. The detector has a detector output (118) representing carrier frequency range components. A control circuit (120) controls a baseband envelope of the modulated potentials as a function of the detector output.