Capacitive Sensor Sigma-Delta Circuit for Linear Capacitance Sensing
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Solution Overview
Problem
Conventional capacitance sensors face challenges in accurately calculating capacitance due to the exponential relationship between the voltage on the summing capacitor and charge transfer cycles, requiring linearization for reliable capacitance measurement.
Innovation Solution
A capacitive sensor with a sigma-delta modulator is introduced, which converts the measurement of sensing capacitor 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 relationship requiring linearization
Solution Approach 1:
The patent changes the measurement parameter from voltage (which has exponential relationship with time) to duty cycle (which has linear relationship with capacitance). The sigma-delta modulator converts the capacitance measurement into a duty cycle measurement, where the duty cycle is directly proportional to the capacitance value, eliminating the need for linearization and improving measurement precision while maintaining noise immunity.
2Ease of operation
If voltage on summing capacitor is used for capacitance calculation, then calculation is simplified, but measurement precision deteriorates due to exponential relationship
Solution Approach 1:
The patent measures the duty cycle of the feedback pulse signal instead of the voltage on the summing capacitor. The duty cycle has a substantially linear relationship with capacitance, making it equally simple to calculate while providing superior measurement precision. The sigma-delta modulator generates this duty cycle signal directly from the capacitance measurement.
3Device complexity
If conventional capacitance sensor is used, then device complexity is reduced, but measurement precision deteriorates due to exponential relationship requiring linearization
Solution Approach 1:
The patent employs a sigma-delta modulator with feedback mechanism where a feedback pulse signal is generated and fed back to the summing capacitor. The duty cycle of this feedback pulse is measured to determine capacitance. This feedback approach linearizes the measurement process, providing high precision capacitance measurement while maintaining reasonable device complexity through integrated circuit implementation.
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 solution allows for accurate and reliable capacitance measurement by converting the capacitance of the sensing capacitor into a duty cycle of a feedback pulse signal, enhancing noise immunity and simplifying the calculation of capacitance changes, particularly in the presence of objects or noise.
Implementation Method 1
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
Implementation Method 2
converts the measurement of sensing capacitor 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.