Capacitive Sensor Sigma-Delta Circuit for Linear Noise-Immune Sensing
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Solution Overview
Problem
Conventional capacitance sensors face challenges in linearizing the exponential relationship between voltage on the summing capacitor and charge transfer cycles, making it difficult to accurately calculate capacitance over time, and they are susceptible to noise interference.
Innovation Solution
A capacitive sensor with a sigma-delta modulator that converts capacitance measurement into a substantially linear duty cycle of a feedback pulse signal, using a switching capacitor circuit, sigma-delta modulator, and measurement circuit to accurately determine capacitance changes while providing noise immunity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If charge transfer technique is used to measure capacitance, then noise immunity is improved, but the relationship between voltage and time becomes exponential requiring linearization
Solution Approach 1:
The patent transforms the measurement parameter from voltage (which has an exponential relationship with time) to frequency (which has a linear relationship with time). By measuring the frequency of oscillations rather than the voltage level, the system achieves accurate capacitance measurement without requiring linearization of exponential data, thus resolving the contradiction between noise immunity and measurement complexity
Solution Approach 2:
The patent replaces the conventional voltage-based measurement approach with a frequency-based measurement approach. Instead of measuring voltage levels that change exponentially, the system counts oscillation cycles, substituting a mechanical counting process for an electrical voltage measurement process, thereby eliminating the need for complex linearization algorithms
2Productivity
If voltage on summing capacitor is measured after predetermined time, then measurement speed is improved, but accuracy deteriorates due to exponential relationship
Solution Approach 1:
The patent changes the measurement parameter from voltage to frequency. By counting the number of oscillation cycles that occur within a fixed time period, the system achieves both fast measurement (high productivity) and high accuracy. The linear relationship between frequency and time eliminates the need for complex calculations, allowing rapid and precise capacitance determination simultaneously
3Device complexity
If conventional capacitance sensor is used, then circuit simplicity is maintained, but susceptibility to RF noise increases
Solution Approach 1:
The patent replaces the conventional voltage measurement circuit with a frequency measurement circuit that counts oscillation cycles. This substitution maintains relative circuit simplicity while dramatically improving RF noise immunity, as frequency counting is inherently more resistant to RF interference than voltage measurement, thus resolving the contradiction between circuit simplicity and noise susceptibility
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 enables accurate and noise-resistant capacitance measurement by converting the capacitance of the sensing capacitor into a linear duty cycle of a feedback pulse signal, effectively addressing the exponential relationship issue and improving noise immunity.
Implementation Method 1
Capacitive sensors are used to implement a variety of useful functions including touch sensors
Implementation Method 2
One class of capacitive sensor uses a charge transfer technique. Referring to FIG. 1A, the charge transfer technique charges a sensing capacitor Cx in one phase (switch SW1 closed, switch SW2 open) and discharges the sensing capacitor Cx into a summing capacitor Csum in a second phase (SW1 open, SW2 closed)
Data Source
AI summary
A capacitive sensor includes a switching capacitor circuit, a comparator, and a charge dissipation circuit. The switching capacitor circuit reciprocally couples a sensing capacitor in series with a modulation capacitor during a first switching phase and discharges the sensing capacitor during a second switching phase. The comparator is coupled to compare a voltage potential on the modulation capacitor to a reference and to generate a modulation signal in response. The charge dissipation circuit is coupled to the modulation capacitor to selectively discharge the modulation capacitor in response to the modulation signal.