Touch Display Shielding Electrode Layout for Screen Squeal
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Solution Overview
Problem
The screen squeal phenomenon in touch display devices, caused by electrostriction effects due to capacitance between touch electrodes and a metal back plate, results in audible vibrations and degrades user experience.
Innovation Solution
A touch display device with a shielding electrode grounded in series with the metal back plate forms a capacitor whose capacitance-resistance product is smaller than the pulse width of touch signals, converting rectangular pulses into spike pulses to neutralize induced charges and prevent electrostriction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a metal back plate is grounded to eliminate static electricity, then electrostatic discharge is prevented, but capacitance forms between touch electrodes and the metal back plate causing screen squeal
Solution Approach 1:
A shielding electrode is introduced as an intermediary component between the touch electrode and the metal back plate. This shielding electrode is connected to ground through a resistor, creating an RC circuit that neutralizes induced charges before they can cause electrostriction in the dielectric film. The shielding electrode acts as a mediator that prevents the direct capacitance coupling between the touch electrode and metal back plate, thereby eliminating screen squeal while maintaining electrostatic discharge protection.
Solution Approach 2:
The electrical parameters of the grounding system are modified by adding a resistor in series with the shielding electrode. This creates an RC time constant that is smaller than the pulse width of touch signals, enabling the circuit to respond quickly enough to neutralize induced charges during each pulse cycle. By changing the grounding configuration from direct connection to resistive connection with a specific time constant, the harmful electrostriction effect is eliminated while maintaining the protective grounding function.
2Ease of operation
If periodic high-frequency rectangular pulse signals are applied to touch electrodes, then touch detection function is enabled, but electrostriction effect causes mechanical vibration and audible noise
Solution Approach 1:
The shielding electrode serves as an intermediary that intercepts and neutralizes the induced charges generated by the high-frequency rectangular pulse signals on the touch electrode. By providing an alternative discharge path through the resistor to ground, the shielding electrode prevents these charges from accumulating and causing electrostriction in the dielectric film, thereby eliminating mechanical vibration while allowing the touch detection signals to pass through unaffected.
Solution Approach 2:
The induced charges that would normally cause harmful electrostriction and vibration are instead channeled through the RC circuit formed by the shielding electrode and resistor. The RC time constant is designed to be smaller than the pulse width, allowing the circuit to quickly neutralize these charges during each pulse cycle. This converts the potentially harmful induced charges into a beneficial rapid discharge mechanism that prevents vibration while maintaining touch signal integrity.
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 effectively suppresses mechanical vibrations, improving user experience by eliminating the screen squeal issue.
Implementation Method 1
the mathematical product of a capacitance value of the capacitor and a resistance value of the shielding electrode is smaller than a pulse width of touch signals applied to the touch electrodes
Implementation Method 2
an electric field formed between the touch electrodes and the metal back plate induce positive or negative charges, causing an electrostriction effect in a dielectric material film layer between the touch electrodes and the metal back plate, which results in elastic deformation and causes vibration
Implementation Method 3
the capacitance formed by the first touch control electrode layer and the conductive back plate can be avoided, and then an optical film group in the backlight module is prevented from resonating and sounding due to the piezoelectric effect
Data Source
Figure 1~2
Figure 3~4A
Figure 4B~5
AI summary
The application discloses a touch display device, including a liquid crystal display panel, a touch electrode, and a backlight module on a light incident side of the liquid crystal display panel. The backlight module includes a metal back plate. The touch display device further includes a shielding electrode on a side of the touch electrodes facing the backlight module, both the shielding electrode and the metal back plate are grounded. The touch electrode and the shielding electrode form a capacitor, and the product of a capacitance value of the capacitor and a resistance value of the shielding electrode is smaller than a pulse width of touch signals applied to the touch electrodes.