Multifrequency Capacitive Touch Detection Eliminates Phantom Touches
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Solution Overview
Problem
Projected capacitive touchscreens face issues with phantom touches and electromagnetic interference, making them unsuitable for critical environments like aircraft cockpits, and they struggle with sensitivity when users wear gloves or in noisy electromagnetic conditions.
Innovation Solution
A dual-frequency capacitive touch-sensitive device that uses emission voltages at two different frequencies to determine touch presence and position, eliminating phantom touches and interference, and allowing use with gloved hands, by analyzing impedance variations at specific frequencies using synchronous demodulators.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If self capacitive detection is used, then the acquisition speed is improved (only N+M acquisitions needed), but phantom touches cannot be resolved
Solution Approach 1:
The patent introduces a temporal dimension by performing multiple acquisitions at different time instants. By comparing the temporal evolution of capacitance values across multiple readings, the system can distinguish between real touches and phantom touches, resolving the ambiguity that plagues single-reading self-capacitive systems.
Solution Approach 2:
The system performs preliminary acquisitions to establish a baseline state before making the final touch detection decision. By acquiring capacitance values at multiple prior time points and analyzing their evolution, the system prepares the necessary information to accurately distinguish real touches from phantom touches in the current reading.
2Measurement precision
If mutual capacitive detection is used, then phantom touches are resolved through coupling capacitance measurement, but the system requires a large number of analog switches which increases coupling capacitance to ground
Solution Approach 1:
The patent extracts and eliminates the problematic analog switches from the measurement circuitry. By using a different measurement approach that doesn't require switching between row/column pairs, the system removes the source of excessive coupling capacitance to ground while still maintaining the ability to resolve phantom touches through temporal analysis.
Solution Approach 2:
The patent replaces the mechanical switching system with an electronic signal processing approach. Instead of physically switching between different measurement configurations, the system uses temporal sampling and computational analysis to achieve the same discrimination function, thereby eliminating the capacitance penalty of analog switches.
3Measurement precision
If analog switches are added to resolve phantom touches, then touch position accuracy is improved, but sensitivity is reduced due to increased coupling capacitance
Solution Approach 1:
The patent removes the analog switches that were causing the sensitivity degradation. By eliminating these switching components, the system restores its original sensitivity while maintaining touch position accuracy through the alternative temporal analysis method that doesn't require additional switching hardware.
4Measurement precision
If charge transfer measurement method is used, then signal-to-noise ratio is improved, but electromagnetic interference from burst signals is generated
Solution Approach 1:
The patent employs periodic sampling at carefully selected time intervals rather than continuous burst signaling. By performing measurements at discrete, periodic moments and analyzing the temporal evolution, the system achieves good signal-to-noise ratio through integration over time while avoiding the broadband electromagnetic interference generated by high-frequency burst signals.
Solution Approach 2:
The system dynamically adjusts the measurement timing and frequency based on the operational context. By flexibly selecting when to perform acquisitions and how frequently to sample, the system optimizes the signal-to-noise ratio for each measurement while minimizing electromagnetic interference generation, adapting to the specific requirements of each measurement scenario.
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 device is free from phantom touches and electromagnetic interference, maintains sensitivity with gloved hands, and complies with aeronautical electromagnetic standards, providing accurate and reliable multitouch detection.
Implementation Method 1
The so-called projected capacitive detection consists in producing a detection matrix arranged so as to detect the local variations of capacitance introduced by the proximity of the fingers of the user or of any other conductive designating object
Implementation Method 2
a first periodic emission voltage emitted at a first frequency denoted working frequency and a second periodic emission voltage emitted at a second frequency denoted discrimination frequency... determine, for each row and for each column: the value of a first reception voltage at the working frequency and the value of a second reception voltage at the discrimination frequency
Data Source
AI summary
The general field of the invention is that of touchscreen devices with projected capacitive detection comprising a matrix-form touch pad comprising conductive rows and columns, said pad being linked to control means and electronic reception and analysis means. The control means generate, for each conductive row and column, a first periodic emission voltage emitted at a first working frequency and a second periodic emission voltage emitted at a second discrimination frequency, different from the first frequency. The electronic reception and analysis means are arranged so as to determine, for each row and for each column, the impedance of a first reception voltage at the working frequency and the impedance of a second reception voltage at the discrimination frequency. According to predetermined values, the values of the two impedances are representative of a touch on the touch pad and its location on the row or on the column concerned.