Capacitive Sensor Sigma-Delta Circuit for Linear Capacitance Sensing
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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 precision deteriorates due to exponential voltage relationship
Solution Approach 1:
The patent changes 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 oscillation rather than the voltage level, the system maintains noise immunity while achieving linear measurement that improves precision. The oscillator circuit converts the exponential voltage accumulation into a linear frequency output.
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 direct voltage measurement approach with an oscillator-based frequency measurement approach. Instead of measuring voltage and applying linearization algorithms, the system uses the charging capacitor to directly control an oscillator's frequency, eliminating the need for linearization processing while maintaining measurement capability.
3Adaptability or versatility
If conventional capacitance sensing is used, then mechanical controls can be replaced, but measurement accuracy deteriorates due to exponential relationship
Solution Approach 1:
The patent changes the measured parameter from voltage to frequency by introducing an oscillator circuit. This parameter transformation maintains the ability to replace mechanical controls with capacitive sensing while significantly improving measurement accuracy by establishing a linear relationship between the measured parameter and capacitance value.
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, reducing noise immunity issues and enabling reliable operation by converting capacitance into a measurable duty cycle, enhancing the accuracy and reliability of capacitance sensing.
Implementation Method 1
One class of capacitive sensor uses a charge transfer technique. Referring to FIG. 1, 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).
Implementation Method 2
Capacitance sensors are used to implement a variety of useful functions including touch sensors (e.g., touch pad, touch dial, touch wheel, etc.), determining the presence of an object, accelerometers, and other functions.
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.