Capacitive touch screen

1. The working principle of capacitive screen is different from resistive touch screen, capacitive touch screen does not rely on finger pressure to create and change the voltage value to detect coordinates. Capacitive screens work through any object that holds electrical charge, including human skin. (Charge carried by the human body) Capacitive touch screens are made of materials such as alloys or indium tin oxide (ITO), and the charges are stored in tiny electrostatic nets that are thinner than hair. When a finger taps on the screen, a small amount of current will be absorbed from the contact point, causing a voltage drop in the corner electrode. The touch is achieved by sensing the weak current of the human body. There is an inductive capacitor between any two conductive objects. A button, a pad and the ground can also form an inductive capacitor. When the surrounding environment is unchanged, the inductive capacitor value is a fixed and unchanging small value. When a human finger approaches the touch button, the inductive capacitor formed by the human finger and the ground in parallel with the pad and the ground will increase the total inductive capacitance. After the capacitive touch button IC detects that the sensing capacitance value of a button has changed, it will output a confirmation signal that a button is pressed. Because capacitive touch buttons have no mechanical structure, all detections are small changes in power, so they are more sensitive to various interferences. Therefore, touch button design, touch panel design, and touch IC selection are critical.

2. Capacitive screen module composition

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

Touch IC: When the capacitive screen is touched, the position coordinates of the touch points are analyzed, which is calculated and processed by this chip.

There are two main types of capacitive touch screens:

(1) Surface capacitive type: The surface capacitive type uses sensors located in the four corners and a film evenly distributed across the surface. It has an ordinary ITO layer and a metal frame. When a finger touches the screen, it is released from the board. The charge is sensed at the four corners of the touch screen, no complicated ITO patterns are required;

(2) Projected Capacitors: One or more carefully designed, etched ITO layers are used to form multiple horizontal and vertical electrodes through corrosion, and independent chips with interlaced rows/columns with sensing functions are used. Now tablets, mobile phones, vehicles, etc. mostly use projected capacitors, so our later analysis shows the composition of projected capacitors.

Axial-coordinate sensing unit matrix of projected capacitance: The separate rows and columns of the axis-coordinated sensing unit are realized by two crossed sliders. X-axis slider and Y-axis slider are used to detect the capacitance change of each grid sensing unit.