Capacitive Sensor Sigma-Delta Circuit for Linear Duty-Cycle Readout
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Solution Overview
Problem
Conventional capacitance sensors face challenges in accurately measuring capacitance due to the exponential relationship between the voltage on the summing capacitor and the charge transfer time/cycles, requiring linearization for reliable capacitance calculation.
Innovation Solution
A capacitive sensor with a sigma-delta modulator is introduced, which converts the measurement of the sensing capacitor's capacitance into the duty cycle of a feedback pulse signal, providing a substantially linear relationship and using a switching capacitor circuit, sigma-delta modulator, and charge dissipation circuit to achieve this conversion.
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 measurement linearity deteriorates due to exponential voltage relationship
Solution Approach 1:
The patent transforms the measurement parameter from voltage (which has an exponential relationship with time) to frequency/duty cycle (which has a linear relationship with capacitance). The sigma-delta modulator converts the capacitance measurement into a frequency-domain signal where the duty cycle is directly proportional to the capacitance value, eliminating the exponential nonlinearity while preserving noise immunity through the modulator's inherent filtering capability.
Solution Approach 2:
The patent replaces the conventional voltage-based measurement system with a frequency-based system using a sigma-delta modulator. This substitution transforms the measurement from the time domain (exponential voltage rise) to the frequency domain (linear duty cycle), where the output frequency or duty cycle is directly proportional to the capacitance, achieving linearity without sacrificing noise immunity.
2Measurement precision
If voltage measurement on summing capacitor is used, then capacitance can be determined, but linearization is required due to exponential voltage-time relationship
Solution Approach 1:
The patent replaces the voltage measurement approach with a frequency measurement approach. Instead of measuring the exponential voltage rise on the summing capacitor and applying linearization algorithms, the sigma-delta modulator directly converts the capacitance into a linear frequency or duty cycle signal, eliminating the need for post-processing linearization and simplifying the overall system.
Solution Approach 2:
The patent changes the measurement parameter from voltage to frequency/duty cycle. The sigma-delta modulator produces an output signal whose duty cycle is linearly proportional to the capacitance value, directly providing a linear measurement without requiring additional linearization circuitry or computational processing.
3Adaptability or versatility
If conventional capacitance sensing is used, then touch detection is achieved, but user interface functionality is limited compared to mechanical controls
Solution Approach 1:
The patent enhances the versatility of capacitive sensing by enabling multiple user interface functions (touch pad, dial, wheel, etc.) through a single sensing mechanism. The improved linearity and accuracy of the sigma-delta modulator enable precise detection of various touch interactions, allowing one system to replace multiple mechanical controls while maintaining reliability under harsh conditions.
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 allows for accurate and efficient capacitance measurement by converting the capacitance of the sensing capacitor into a duty cycle, enhancing the reliability and linearity of capacitance calculations, thereby improving the sensing accuracy and noise immunity.
Implementation Method 1
Capacitance sensors are used to implement a variety of useful functions including touch sensors
Implementation Method 2
A capacitive sensor with a sigma-delta modulator is introduced, which converts the measurement of the sensing capacitor's capacitance into the duty cycle of a feedback pulse signal
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.