Capacitive Resonant Sensor Detection Through Moisture Coverings
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Solution Overview
Problem
Capacitive proximity sensors fail to detect the approach of objects effectively when moisture, such as water or wet cloths, is present between the sensor electrode and the electrical ground potential, as moisture acts as a large capacitance, preventing the detection of changes in the electrical field.
Innovation Solution
A capacitive proximity sensor with a resonant circuit that includes an inductance and capacitance relative to a reference potential, where the sensor electrode is integrated into the resonant circuit, and the circuit is periodically excited with a limited number of excitation oscillations, allowing the evaluation circuit to distinguish changes in amplitude even with resistive components like damp cloths, ensuring detection of object approaches regardless of moisture presence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a capacitive proximity sensor uses conventional operating principles (fixed frequency excitation or simple RC charging), then the sensor structure is simple, but moisture between the sensor electrode and ground potential acts as a large capacitance that prevents detection of object approach
Solution Approach 1:
The patent applies dynamic excitation by periodically varying the excitation frequency around the resonant frequency of the LC circuit. This dynamic approach allows the sensor to distinguish between moisture-induced detuning (which occurs at all frequencies) and object-induced detuning (which shows characteristic frequency-dependent patterns), thereby maintaining detection reliability in the presence of moisture.
Solution Approach 2:
The patent changes the operating parameter from fixed frequency to variable frequency excitation. By sweeping through a frequency range and analyzing the resonant response, the system can differentiate between the capacitive effect of moisture (which shifts resonance frequency) and the capacitive effect of approaching objects (which causes additional detuning), thus overcoming moisture interference.
2Measurement precision
If the resonant circuit is continuously excited to maintain steady-state operation, then the output signal is stable, but the sensor cannot distinguish between transient detuning caused by moisture and detuning caused by approaching objects
Solution Approach 1:
The patent uses periodic excitation pulses instead of continuous excitation. Each pulse excites the resonant circuit temporarily, allowing the system to observe the transient response and decay characteristics. This periodic action provides distinct temporal signatures for moisture-induced detuning versus object-induced detuning, enabling precise object detection while filtering out moisture interference.
Solution Approach 2:
The patent performs preliminary excitation pulses before object detection to establish a baseline resonant frequency. By comparing subsequent responses against this baseline, the system can identify deviations caused by approaching objects while compensating for static moisture effects, thereby improving measurement precision.
3Adaptability or versatility
If a wet cloth completely covers the sensor surface and connects to ground potential, then the capacitance between sensor electrode and ground is greatly increased, but the sensor can still detect object approach through the wet cloth
Solution Approach 1:
The patent employs dynamic frequency sweeping to adapt to the changed capacitive conditions. By continuously adjusting the excitation frequency and monitoring the resonant response, the system maintains sensitivity to object approach even when the baseline capacitance is significantly increased by moisture, thus achieving versatility under adverse conditions.
Solution Approach 2:
The patent changes the detection parameter from absolute capacitance measurement to frequency shift measurement. This allows the sensor to operate reliably despite large changes in baseline capacitance caused by moisture, as the frequency-based detection method adapts to the new capacitive environment while remaining sensitive to object approach.
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
The solution enables clear detection of object approaches, such as hands, even through damp cloths or water, by maintaining a constant or changed amplitude signal, effectively differentiating between capacitive changes with resistive components and purely capacitive changes, thus overcoming the limitations of existing sensors.
Implementation Method 1
a resonant circuit with at least one inductance L and at least one capacitance C relative to a ground potential 1.2... In a state where a high-frequency signal with a first detection frequency is input into an LCR resonant circuit
Implementation Method 2
The capacitance between the sensor electrode and the electrical ground potential is measured... another capacitance of an object approaches the sensor electrode
Implementation Method 3
a high-frequency signal with a first detection frequency is input into an LCR resonant circuit... a high-frequency signal with a second detection frequency is introduced into the resonant circuit
Implementation Method 4
A medium with a resistive component, such as moisture, can be placed between the sensor electrode and a ground potential, causing the resonant circuit to detune
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
A capacitive proximity sensor comprises a sensor assembly having a resonant circuit which has an inductor (1.3) and a capacitor (1.6), the capacitor (1.6) being formed relative to a ground potential (1.2). The resonant circuit is excited via an exciter circuit (1.1). An output signal (1.8) that can be tapped on the resonant circuit is analysed by means of an analysis circuit (1.9) in order to determine the proximity of an object. A sensor electrode (1.4) is incorporated as resonance capacitor in the resonant circuit. The resonant circuit is excited in a starting state by the exciter circuit (1.1) with a frequency below a resonance frequency of the resonant circuit. A medium (1.7) with a resistive component arrangeable between the sensor electrode (1.4) and the basic potential (1.2) leads to a detuning of the resonant circuit with an output signal (1.8) having an amplitude that is the same or practically the same as in the starting state, whereas the possibility of a further capacitor approaching the sensor electrode (1.4) leads to a detuning of the resonant circuit with an output signal (1.8) having an amplitude that is altered in relation to the starting state, even in the presence of the medium (1.7) having a resistive component. The analysis circuit (1.9) identifies the approach of the further capacitor on the basis of the change to the amplitude of the output signal (1.8). Since the exciter circuit (1.1; 11.6) can be operated such that the resonant circuit is excited periodically with N exciter vibrations, wherein N is a number less than 10, wherein the analysis circuit (1.9) is intended and suitable for analysing, on the resonant circuit, the amplitude of the output signal (1.8; 11.4) during and/or shortly after the N exciter vibrations, a capacitive proximity sensor and a method for determining the proximity of an object to the sensor assembly are provided which do not respond to moderately conductive coverings, such as a closed water film, moist contaminations, a covering of snow or ice, even if these coverings are connected to the electric ground potential and cover the sensor electrode fully or partially. The approach of a hand through these grounded coverings can thus be recognised perfectly.