The Principle And Classification Of Touch LCD Technology

Touch technology has been widely used in consumer electronic products such as smart phones and tablets.

Touch LCD screen technology

Touch LCD screen technology is a new type of human-computer interaction input method. Compared with traditional keyboard and mouse input methods, touch LCD screen input is more intuitive. With recognition software, touch LCD screen can also realize handwriting input.

Principle of touch LCD screen

Resistive touch liquid crystal screen technology Resistive touch screen is a touch liquid crystal screen technology that uses the change in resistance caused by the surface of the touch LCD screen to change with the pressure on the screen, and the resistance changes caused by the uneven deformation of the screen to achieve precise positioning. The performance of resistive screens has the following characteristics: ①They are a kind of working environment that is completely isolated from the outside world, not afraid of dust, water vapor and oil. The accuracy of the resistive touch screen only depends on the accuracy of the A/D conversion, so it can easily reach 4096*4096. According to different implementation principles, the resistive touch LCD screen is divided into four-wire and five-wire types. The surface acoustic wave (SAW) type SAW touch LCD screen is composed of a glass coating with piezoelectric sensors for sending and receiving for the X and Y axes. The controller sends electrical signals to the transmitter sensor and converts the signals into ultrasonic waves in the surface of the glass. Through the reflector array, these waves cover the entire touch LCD screen. The opposite reflector collects and controls these waves to the receiving sensor and converts them into electrical signals. Repeat this process for each axis. When the user touches, a part of the propagating wave is absorbed. The received signals corresponding to the X and Y coordinates are compared with the stored digital distribution map to identify changes and calculate the coordinates.

The touch LCD screen is attached to the surface of the display and is used in conjunction with the display. The analog electrical signal is generated by touch, and the coordinate of the touch point is calculated by the microprocessor after being converted into a digital signal, so that the operator's intention is obtained and executed. Touch LCD screens can be divided into five categories according to their technical principles: vector pressure sensing type, resistive type, capacitive type, infrared type and surface acoustic wave type. Among them, resistive touch LCD screens are more used in practical applications. The resistive touch LCD screen is composed of 4 layers of transparent and thin, the bottom is a base layer made of glass or plexiglass, and the top is a plastic layer whose outer surface is hardened to make it smooth and scratch-resistant. It is attached to the upper and lower inner surfaces. The two layers are metal conductive layers (OTI, indium oxide), which are insulated by small transparent isolation points. When a finger touches the screen, the two conductive layers touch at the touch point.

The two metal conductive layers of the touch LCD screen are used to measure the coordinates in the X-axis and Y-axis directions, respectively. The conductive layer used for X coordinate measurement leads to two electrodes from the left and right ends, denoted as X+ and X-. The conductive layer used for Y coordinate measurement leads out two electrodes from the upper and lower ends, which are denoted as Y+ and Y-. This is the lead composition of the four-wire resistive touch LCD screen. When a voltage is applied to a pair of electrodes, a uniform and continuous voltage distribution is formed on the conductive layer. If a certain voltage is applied to the electrode pair in the X direction and no voltage is applied to the electrode pair in the Y direction, in the X parallel voltage field, the voltage value at the contact can be reflected on the Y+ (or Y-) electrode , By measuring the voltage of the Y+ electrode to the ground, the X coordinate value of the contact can be known. Similarly, when a voltage is applied to the Y electrode pair and no voltage is applied to the X electrode pair, the Y coordinate of the contact can be obtained by measuring the voltage of the X+ electrode. The measurement principle is shown in Figure 1.

The five-wire touch LCD screen is different from the four-wire type. The main difference is that the five-wire touch LCD screen leads out the four ends of one of the conductive layers as four electrodes, and the other conductive layer is only used as the measured conductor to output the voltage in the X and Y directions. The measurement must alternate between the X and Y directions. Y applies voltage upwards.

Classification of touch LCD technology

According to the different positioning principles of the screen surface, touch LCD technology can be divided into Acoustic Pulse Recognition (APR) technology, Surface Acoustic Wave (SAW) technology, capacitive touch LCD technology, and resistive touch LCD technology, infrared/optical technology.

Acoustic Pulse Recognition (APR) technology APR consists of a glass display coating or other hard substrate, with 4 piezoelectric sensors installed on the back. The sensor is installed on two opposite corners of the visible area and connected to the control card through a bent cable. When the user touches the LCD screen, the finger or the drag between the stylus and the glass collides or rubs, so sound waves are generated. The wave radiation leaves the contact point and travels to the sensor, generating an electrical signal in proportion to the sound wave. These signals are amplified in the control card and then converted into digital data streams. Compare the data with the pre-stored sound list to determine where to touch the LCD. APR is designed to eliminate environmental influences and external sounds because these factors do not match the stored sound list.

Surface Acoustic Wave (SAW) technology SAW touch LCD screen is composed of a glass coating with piezoelectric sensors for sending and receiving for the X and Y axis. The controller sends electrical signals to the transmitter sensor and converts the signals into ultrasonic waves in the surface of the glass. Through the reflector array, these waves cover the entire touch LCD screen. The opposite reflector collects and controls these waves to the receiving sensor and converts them into electrical signals. Repeat this process for each axis. When the user touches, a part of the propagating wave is absorbed. The received signals corresponding to the X and Y coordinates are compared with the stored digital distribution map to identify changes and calculate the coordinates.