Capacitive Sensor Resonant Frequency Ratio Measurement
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Solution Overview
Problem
Existing capacitive sensors face challenges due to the cost and complexity of analog switching circuitry and ADCs, as well as the need for precise frequency references and accurate inductive coil measurements, which affect their accuracy and expense.
Innovation Solution
A capacitive sensor design that includes a variable capacitor, a fixed capacitor, an inductor, and a switch, where the controller alternates between connecting the variable capacitor and the fixed capacitor to the inductor to generate oscillations, allowing the identification of capacitance based on the ratio of their resonant frequencies, reducing the need for precise inductance measurements and using a low-cost RC oscillator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a switched capacitor circuit and ADC are used to generate a digital value from capacitance, then the capacitance can be identified, but the cost and complexity of the analog switching circuitry and ADC increase
Solution Approach 1:
The patent replaces the mechanical/electrical analog switching circuitry and ADC with a resonant frequency-based measurement system. The variable capacitor is connected to form a resonant circuit, and the resonant frequency is measured directly by a frequency counter or microcontroller timer, eliminating the need for complex analog-to-digital conversion circuitry.
Solution Approach 2:
The patent changes the measurement parameter from direct capacitance measurement to resonant frequency measurement. By measuring the resonant frequency of the circuit containing the variable capacitor, the capacitance value can be determined indirectly through the relationship between frequency and capacitance in a resonant circuit, simplifying the required electronics.
2Measurement precision
If a resonance sensor with crystal oscillator is used to measure frequency, then the frequency can be identified, but the cost increases due to the need for precise frequency reference
Solution Approach 1:
The patent uses inexpensive RC oscillators or microcontroller internal oscillators instead of expensive crystal oscillators. While these cheaper oscillators have less precision, the patent compensates through differential measurement techniques and calibration, achieving sufficient accuracy without the high cost of crystal frequency references.
3Power
If an inductive coil is used in the variable oscillator, then the oscillation can be generated, but the accuracy reduces due to changes in inductance from relative permeability variations
Solution Approach 1:
The patent extracts or removes the inductive element from the resonant circuit, using only capacitive elements (the variable capacitor and fixed capacitors) to generate the resonant frequency. This eliminates the source of error related to inductance variations caused by changes in relative permeability of surrounding materials.
Solution Approach 2:
The patent creates a capacitive resonant circuit with localized electric fields rather than using an inductive coil with magnetic fields that are sensitive to surrounding materials. The resonant frequency is determined solely by the capacitive elements, making the measurement locally determined and insensitive to external magnetic environment variations.
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 simplifies the sensor design, reduces costs, and maintains accuracy by compensating for frequency drift and noise in the oscillator, enabling precise capacitance measurement without requiring expensive crystal oscillators or precise inductance values.
Implementation Method 1
identify a frequency of a first oscillation of the variable capacitor in the parallel electrical circuit with the inductor based on the periodic signal generated by the oscillator when the switch is in the first position, identify a frequency of a second oscillation of the fixed capacitor in the parallel electrical circuit with the inductor based on the periodic signal generated by the oscillator when the switch is in the second position
Data Source
AI summary
A sensor includes a variable capacitor, a fixed capacitor, an inductor, a switch that electrically connects the variable capacitor with the inductor or the fixed capacitor with the inductor, an oscillator that generates a periodic signal, and a controller connected to the switch, the oscillator, and the inductor. The controller operates the switch, identifies a frequency of a first oscillation of the variable capacitor and the inductor based on the periodic signal from the oscillator, identifies a frequency of a second oscillation of the fixed capacitor and the inductor based on the periodic signal from the oscillator, and identifies a capacitance of the variable capacitor based on a ratio of the frequency of the first oscillation to the frequency of the second oscillation.