Capacitive Touch Measurement Using Square Wave Excitation
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Solution Overview
Problem
Conventional capacitive measurement techniques suffer from quadrature error due to the resistive nature of touch surfaces, leading to inaccurate detection of capacitance between electrodes and objects, especially when using thick dielectrics or styluses.
Innovation Solution
A method involving a processor unit that applies a non-sinusoidal excitation signal with discontinuities to the resistive electrode plane, allowing for partial demodulation to separate capacitive effects from resistive effects, thereby attenuating quadrature errors and improving capacitance measurement accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional sinusoidal excitation signal is used for capacitive measurement, then the measurement system can operate with simple excitation, but quadrature error occurs due to resistive leakage that degrades measurement accuracy
Solution Approach 1:
The patent changes the excitation signal from sinusoidal to square wave, fundamentally altering the signal parameters. This parameter change causes the capacitive current to be in phase with the excitation signal during the flat portions of the square wave, eliminating the quadrature error that plagues sinusoidal excitation systems. The resistive leakage current becomes distinguishable from the capacitive current, allowing for accurate capacitance measurement despite the presence of resistive electrodes.
2Illumination intensity
If resistive transparent conductive material like ITO is used for electrodes, then the touch surface can be made optically transparent, but resistive leakage creates phase offset that degrades capacitive measurement
Solution Approach 1:
The patent applies square wave excitation specifically to overcome the limitations of resistive transparent conductive materials like ITO. By using this non-sinusoidal excitation, the system can maintain optically transparent electrodes while eliminating the phase offset problem caused by their resistive nature. The square wave excitation creates distinct temporal characteristics that allow separation of capacitive and resistive current components.
3Measurement precision
If uniform conductive plane is placed behind measuring electrodes to eliminate parasitic coupling, then parasitic capacitive coupling is reduced, but the resistive nature of the plane still creates measurement errors
Solution Approach 1:
The patent extends the square wave excitation to the guard plane, synchronizing its excitation with the measuring electrodes. This coordinated parameter change ensures that both the measurement signal and the guard plane operate in the same temporal reference frame, allowing the benefits of parasitic coupling elimination while avoiding the introduction of additional quadrature errors from the guard plane's resistive leakage.
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 effectively reduces quadrature errors, enabling precise capacitance measurement even with resistive electrodes, allowing for reliable touch commands through thick dielectrics or styluses, enhancing the sensitivity of capacitive touch interfaces.
Implementation Method 1
measuring the value of the capacitances created between electrodes and the object to be detected
Implementation Method 2
measure the value of the capacitances created between electrodes and the object to be detected in order to perform a command
Data Source
AI summary
A method of capacitive measurement between an object and an electrode plane includes applying an excitation signal to the resistive electrode plane, and determining a value of capacitance on the basis of a measurement of a measurement signal originating from the electrode plane. The excitation signal is a nonsinusoidal signal having at least one discontinuity. To determine the value of capacitance, the measurement signal is attenuated over a time window, this time window being the duration for which the resistivity of the electrode plane is manifested on the measurement signal in response to the at least one discontinuity.