Android Capacitive-Touchscreen (i): The basic principle of capacitive screen

Keywords: android capacitive screen TP ITO
Platform Information:
Kernel: linux2.6/linux3.0
System: android/android4.0
Platform: s5pv310 (Samsung Exynos 4210)

xubin341719 (Welcome reprint, please specify the author)

Android Capacitive-Touchscreen (i): The basic principle of capacitive screen

Android Capacitive-touchscreen (ii): Basic concept of drive commissioning

Android Capacitive-Touchscreen (iii): Driver Analysis of driver debugging

First, capacitive screen working principle

The working principle of the touchscreen is to report the values of coordinate values, x-axis and y-axis. Before we analyzed the resistive touch screen, it is through the ADC to detect the calculation of x, Y coordinate values, below we analyze the capacitive touch screen working principle, see how it is to detect the x, y coordinates of the value.

Unlike resistive touchscreen, capacitive touch screens do not rely on the finger to create and change the voltage value to detect coordinates. The capacitive screen works by any object that holds an electric charge, including human skin. (The charge of the body) a capacitive touchscreen is made up of materials such as alloys or indium Siena oxides (ITO), which are stored in a miniature static grid with a thinner root than the hair. When the finger hits the screen, it absorbs a small amount of current from the point of contact, causing the pressure drop of the corner electrode, using the method of sensing the weak current of the human body to achieve the purpose of touch. (This is why when you wear gloves to touch the screen, there is no reason for the reaction), you can clearly explain how the capacitive screen works.

Second, the Capacitive screen module composition

Touch screen: The transparent part of our hand touch operation;

Touch IC: when the capacitive screen touch, to resolve to the position of the contact coordinates, is through the chip to calculate processing.

1. There are two main types of capacitive touch screen:

(1), surface capacitance: surface capacitive use of sensors located in four corners and evenly distributed across the surface of the film, there is a common ito layer and a metal frame, when a finger touch the screen, from the plate surface discharge charge, sensing in the corner of the touch screen complete, do not need a complex ito pattern ；

(2), projected capacitance: the use of one or more carefully designed, the corrosion of Ito, these ito layers through the decay to form a plurality of horizontal and vertical electrodes, the row/column staggered with the sensor function of the independent chip. Now the tablet, mobile phone, car and other multi-use projected capacitance, so we analyzed the structure of the projected capacitance.

Axial-coordinate sensing unit matrix for projected capacitance: axis-coordinate sensing unit discrete rows and columns, two crossed sliders for x-axis slider y-axis slider to detect capacitance changes for each sensor unit. (medium capacitance, actually transparent)

2, capacitive touch screen resolution, the number of channels;

As shown, the accuracy and resolution of the X, Y axis of the transparent electrode capacitive screen is related to the number of channels on the × and y axes, the more channels, the higher the resolution.

3, Capacitive touch screen structure classification:

(1), single-layer Ito

Advantages: Low cost, high transmission rate,

Cons: Poor anti-jamming capability

(2), single layer Ito

Advantages: Good performance, high yield

Cons: Higher cost

(3), double-sided single-layer Ito

Advantages: Good performance, strong antistatic ability

Cons: Poor anti-jamming capability

3. The classification and working principle of Capacitive touch screen

(1), self-born capacitive touch screen

cp-parasitic capacitance

Increased parasitic capacitance when finger touch: Cp ' =cp/cfinger

Detects the change in parasitic capacitance and determines the position of finger touch

(2), mutual capacitive touch-screen

cm-coupling capacitance

Reduced coupling capacitance during finger touch, detects coupling capacitance change, and determines position of finger touch

Four, why will appear ghost Point, Ghost Point how to eliminate

1. Why is there a ghost point?

When a finger is pressed, the X and y axes have only one intersection, and two are pressed at the same time, there will be 4 intersections, as shown, the point we don't expect to get is the ghost point.

2, eliminate the ghost point method

(1), time-sharing method : Based on the multi-touch, the assumption of multi-point touch, the operation interval sequel milliseconds;

(2), Partition method : The entire touch screen physically divided into several areas by judging the touch into the introduction of the corresponding area, so that the point of the ghost points out the true point.