Capacitive Field Sensor Using Sigma-Delta Duty Cycle Measurement
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Solution Overview
Problem
Conventional capacitance sensors face challenges in accurately calculating capacitance due to the exponential rise of voltage on the summing capacitor over time/cycles, requiring linearization, which complicates the measurement process.
Innovation Solution
A capacitive sensor with a sigma-delta modulator is introduced, which 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.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional charge transfer technique is used to measure capacitance, then the sensing capacitor can be charged and discharged reliably, but the voltage on the summing capacitor rises exponentially versus time/cycles requiring linearization
Solution Approach 1:
The patent applies feedback by using the output of the sigma-delta modulator to control a digital-to-analog converter (DAC) that generates a feedback voltage. This feedback voltage is applied to the summing capacitor to counteract the exponential voltage rise, thereby linearizing the relationship between the feedback pulse duty cycle and the sensed capacitance value.
Solution Approach 2:
The patent changes the measurement parameter from exponential voltage accumulation to linear duty cycle modulation. By using a sigma-delta modulator, the system converts the capacitance measurement into a pulse train where the duty cycle is linearly proportional to the capacitance value, eliminating the need for post-measurement linearization.
2Productivity
If exponential voltage accumulation is used in summing capacitor, then charge transfer can be implemented, but linearization is required to calculate capacitance accurately
Solution Approach 1:
The feedback mechanism uses a DAC controlled by the sigma-delta modulator output to generate a feedback voltage that is applied to the summing capacitor. This feedback voltage compensates for the exponential voltage rise, creating a linear relationship between the feedback pulse duty cycle and the capacitance being measured, thereby improving measurement precision.
Solution Approach 2:
The patent replaces the conventional voltage accumulation method with a sigma-delta modulation approach. Instead of directly measuring exponential voltage rise, the system uses a modulator to convert the capacitance information into a pulse train with linear duty cycle characteristics, substituting a more sophisticated signal processing mechanism for the simple voltage integration method.
3Measurement precision
If conventional capacitance sensing is used, then basic measurement can be performed, but sensitivity to RF fields and RF noise remains a challenge
Solution Approach 1:
The sigma-delta modulator acts as an intermediary between the capacitance sensing circuit and the measurement system. It converts the analog capacitance signal into a modulated pulse train, providing noise immunity through its inherent filtering properties and making the measurement more resistant to RF interference and noise.
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 provides a linear relationship between the duty cycle of the feedback pulse and capacitance, simplifying the measurement process and improving the accuracy of capacitance determination, while also offering noise immunity and efficient proximity detection of objects.
Implementation Method 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
A capacitive sensor with a sigma-delta modulator is introduced, which converts capacitance measurement into a substantially linear 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.