Capacitive Touch Timing Detection Using FIR Signal Filtering
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Solution Overview
Problem
Capacitive touch sensors face challenges in accurately detecting touch points due to small and variable capacitance changes, influenced by finger shape, size, positioning, and the use of gloves, leading to unreliable detection and the need for additional complex force sensors for active haptic feedback.
Innovation Solution
A method using digital filtering with FIR filters for pulse compression and offset elimination, combined with downsampling and dynamic threshold evaluation to enhance signal-to-noise ratio and compensate for signal drift, allowing for precise detection of touch points independently of finger characteristics and enabling fast actuation responses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a predefined signal threshold is used for touch detection, then the detection method is simple, but the detection reliability deteriorates due to variability in capacitance changes caused by different finger characteristics and glove usage
Solution Approach 1:
The patent implements dynamic threshold adjustment by continuously adapting the reference threshold based on the measured capacitance baseline. Instead of using a fixed predefined threshold, the system dynamically calculates thresholds that adapt to changing environmental conditions, finger characteristics, and glove materials, thereby maintaining high detection reliability across varying conditions without requiring complex additional hardware
Solution Approach 2:
The patent changes the evaluation parameter from absolute capacitance values to capacitance change rates and differential signals. By evaluating the rate of change rather than absolute values, and by using differential measurements that cancel out common-mode variations, the system achieves reliable detection independent of finger size, shape, and glove thickness while keeping the detection method relatively simple
2Adaptability or versatility
If the signal threshold is adjusted to detect touches with gloves, then glove operation is enabled, but false touch detection occurs several millimeters above the sensor surface when operating without gloves
Solution Approach 1:
The patent employs periodic sampling and evaluation of capacitance changes over time. By analyzing the temporal pattern and rate of capacitance change rather than single instantaneous values, the system can distinguish between genuine touch events (which show characteristic rapid change patterns) and false detections (which show different temporal patterns), enabling glove operation while minimizing false positives
Solution Approach 2:
The system uses feedback mechanisms where the detected baseline capacitance and its variations are continuously fed back to adjust the detection thresholds and evaluation criteria. This adaptive feedback loop allows the system to learn the specific characteristics of the current operating condition (with or without gloves) and adjust accordingly, preventing false detections while maintaining glove compatibility
3Device complexity
If capacitive sensors are used for touch detection, then the sensor structure remains simple, but the measurement precision deteriorates due to very small and variable capacitance changes
Solution Approach 1:
The patent replaces direct measurement of small capacitance values with measurement of derived electrical quantities such as voltage changes across known capacitors, current measurements, or charge transfer quantities. By converting the measurement task to these more easily measurable electrical parameters with higher signal-to-noise ratios, the system achieves high measurement precision while maintaining simple sensor structure
Solution Approach 2:
The patent uses differential measurement techniques where two measurements are taken simultaneously or in rapid succession and their difference is evaluated. By subtracting the baseline capacitance from the current capacitance measurement, or by using differential capacitor configurations, the system amplifies the small capacitance change signal while canceling out common-mode noise and interference, achieving high precision without increasing sensor complexity
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 method achieves high accuracy and reliability in detecting touch points with minimal delay, effectively handling varying finger sizes and glove usage without requiring additional force sensors, ensuring precise and timely actuation responses.
Implementation Method 1
When an object touches or comes near the surface of a capacitive touch sensor, the capacitance of the sensor element changes
Implementation Method 2
A method using digital filtering with FIR filters for pulse compression and offset elimination
Data Source
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AI summary
The invention relates to a method for determining a time of contact on a capacitive sensor element, wherein the capacitive value of the sensor element is continuously measured and is processed into a digital sensor signal. A filtering of said sensor signal by means of a digital filter, preferably a FIR filter, which carries out a pulse compression at the same time as offsetting elimination, allows a filter signal, which reproduces in its temporal course the dynamic behaviour of the sensor signal, is be produced. The observation of said filter signal not only makes it possible to generally make contact with the capacitive sensor element but to also determine, with a high degree of reliability, the precise time of said contact.