Analysis of 6 types of touch screens and 4 technology development trends

    After Apple launched a series of smart terminals, the touch screen industry has developed rapidly in recent years, and related products have become popular in the market. In the future, the application fields of touch screens will enter daily life more widely. Let's take a deep look at the types and development trends of touch screens.

　　Analysis of touch screen types and various technology development trends

　　1. Types of touch screen technology

　　A touch screen is a positioning device that allows users to directly input coordinate information into a computer with their fingers. Like a mouse and keyboard, it is also an input device. The touch screen has many advantages, such as durability, fast response, space saving, and easy communication. With this technology, you can operate the host by simply touching the icons or text on the computer display with your fingers, making human-computer interaction more direct. This technology greatly facilitates users who do not know how to operate a computer. It has been widely used in control, information query and other aspects in the fields of industry, medicine, and communications.

　　1. Resistive touch screen

　　Analog resistive screen

　　Analog resistive touch screen is what we usually call "resistive screen", which is a touch screen controlled by pressure sensing. It uses two layers of ITO (indium tin oxide) plastic film with conductive function. The two ITO sheets are equipped with microparticle fulcrums, so that there is a certain gap between the two layers of ITO when the screen is not pressed, and it is in a non-conductive state. When the operator presses the screen with a fingertip or a pen tip, the pressure will cause the film to concave, and the ITO layer will be in contact and conductive due to deformation. Then, the corresponding pressure point is calculated by detecting the voltage changes on the X-axis and Y-axis to complete the touch processing mechanism of the entire screen. At present, analog resistive touch screens have 4-wire, 5-wire, 6-wire and 8-wire types, as shown in Figure 1. The more lines, the higher the detectable precision, but the cost will also be relatively higher. Resistive screens do not support multi-touch, have high power consumption, short life, and long-term use will cause detection point drift, which requires calibration. However, resistive screens have simple structures and low costs. Before the maturity of capacitive touch screens, they once occupied most of the touch screen market.

　　Digital resistive screen

　　The basic principle of digital resistive screen is similar to that of analog screen. Unlike analog resistive screen, which evenly coats ITO layer on glass substrate, digital resistive screen only uses substrate with ITO stripes. The ITO stripes of upper and lower substrates are perpendicular to each other. Digital resistive screen is more like a simple switch, so it is usually used as a membrane switch. Digital resistive screen can realize multi-touch.

　　2. Capacitive touch screen

　　Surface capacitive

　　The surface capacitive touch screen senses the touch behavior on the screen surface by electric field induction. Its panel is a piece of evenly coated ITO layer, and there is a wire at each of the four corners of the panel connected to the controller. When working, a uniform electric field is generated on the surface of the touch screen.

　　When a grounded object touches the screen surface, the electrodes can sense the change in the charge on the screen surface and determine the coordinates of the touch point. Surface capacitive touch screens have a long service life and high light transmittance, but low resolution and do not support multi-touch. They are currently mainly used in large-size outdoor touch screens, such as public information platforms (POI) and public service (sales) platforms (POS) and other products.

    Projected capacitive screen

　　Projected capacitive touch screens use the electrostatic field lines emitted by the touch screen electrodes for sensing. Projected capacitive sensing technology can be divided into two types: self-capacitance and interactive capacitance. Self-capacitance, also known as absolute capacitance, uses the sensed object as the other plate of the capacitor. The object induces a charge between the sensing electrode and the sensed electrode, and the position is determined by detecting the change in the coupling capacitance. However, if it is a single-point touch, there is only one set of coordinates determined in the X-axis and Y-axis directions through the capacitance change, and the combined coordinates are also unique; if there are two touch points on the touch screen and the two points are not in the same X direction or the same Y direction, there are two coordinate projections in the X and Y directions respectively, and 4 coordinates are combined. Obviously, only two coordinates are real, and the other two are commonly known as "ghost points." Therefore, the self-capacitive screen cannot achieve true multi-point touch; interactive capacitance is also called transcapacitance, which is the capacitance generated by the coupling of adjacent electrodes. When the sensed object approaches the electric field line from one electrode to another, the change of interactive capacitance will be felt. When the horizontal electrodes send out excitation signals in turn, all the vertical electrodes receive the signals at the same time, so that the capacitance value of all the intersection points of the horizontal and vertical electrodes can be obtained, that is, the capacitance value of the two-dimensional plane of the entire touch screen. When the human finger approaches, it will cause the local capacitance to decrease. According to the data of the two-dimensional capacitance change of the touch screen, the coordinates of each touch point can be calculated. Therefore, even if there are multiple touch points on the screen, the true coordinates of each touch point can be calculated. In the above two types of projected capacitive sensors, the sensing capacitor can be designed according to a certain method so that the touch of the finger can be detected at any given time. The touch is not limited to one finger, but can also be multiple fingers. Since 2007, the great success of Apple's iPhone and iPad series products has begun the development of projected capacitive screens, quickly replacing resistive touch screens and becoming the mainstream touch technology in the current market.

　　3. Infrared touch screen

　　Infrared touch screens use a dense infrared matrix in the X and Y directions to detect and locate the user's touch. The infrared touch screen installs a circuit board frame in front of the display. The circuit board arranges infrared emitting tubes and infrared receiving tubes on the four sides of the screen, corresponding to a horizontal and vertical cross infrared matrix. When the user touches the screen, the finger will block the two horizontal and vertical infrared rays passing through the position, based on which the position of the touch point on the screen can be determined. Infrared touch screens have the advantages of high light transmittance, no interference from current, voltage and static electricity, and high touch stability. However, infrared touch screens will be affected by changes in ambient light, infrared sources such as remote controls, high-temperature objects, and incandescent lamps, which will reduce its accuracy. Early infrared touch screens appeared in 1992 with a resolution of only 32×32. They are easily affected by environmental interference and malfunction, and require use in a certain light-shielding environment.

　　After 20 years of development, the current advanced infrared touch screen has a service life of more than 7 years under normal working environment. When tracking the movement trajectory of fingers, the accuracy, smoothness and tracking speed can meet the requirements. The user's writing can be converted into image trajectory very smoothly, and handwriting recognition input is fully supported. Infrared touch screens are mainly used in various public places, offices and industrial control places without infrared and strong light interference, as well as industrial control places that do not require very precise control.

　　4. Sonic wave touch screen

　　Surface Acoustic Wave Touch Screen

　　Surface acoustic wave touch screen is a touch technology that uses sound waves to locate. Sensors for transmitting and receiving sound waves in the X and Y directions are attached to the four corners of the touch screen, and 45° reflective stripes are engraved around it. When a finger touches the screen, the finger absorbs part of the sound wave energy, and the controller detects the attenuation of the received signal at a certain moment, thereby calculating the position of the touch point.

　　Surface acoustic wave technology is very stable and has very high precision. In addition to the X and Y coordinates that general touch screens can respond to, it also responds to its unique third axis Z axis coordinate, that is, the pressure axis response. With this function, each touch point is not just two digital switch states of touch and no touch, but becomes an analog switch that can sense force: the greater the pressure, the wider and deeper the attenuation gap on the received signal waveform.

    Among all types of touch screens, only surface acoustic wave touch screens have the ability to sense touch pressure. Surface acoustic wave touch screens are not affected by environmental factors such as temperature and humidity, have high clarity (extremely high resolution), good light transmittance, high durability, good scratch resistance, sensitive response, long life, can maintain clear and bright image quality, no drift, only need one calibration during installation, good anti-violence performance, and are most suitable for public information inquiries and use in offices, government agencies and public places with relatively clean environments.

　　Curved Sonic Wave Touch Screen

　　The curved acoustic wave touch screen is a technology based on sound pulse recognition. When an object touches the surface of the touch screen, the sensor will detect the frequency of the sound wave, and determine the location of the touch point by comparing the frequency with the standard frequency pre-stored in the chip. In this way, it can eliminate false recognition caused by environmental factors such as clothing, luggage, dust and insects. The sound waves of the surface touch screen propagate along the surface of the substrate, while the sound waves of the curved touch screen propagate inside the substrate, so the curved touch screen has better anti-environmental interference performance than the surface touch screen. At present, curved touch screens are generally used in information kiosks, financial equipment and vending machines with an area of ​​5 inches or more.

　　5. Optical imaging touch screen

　　Optical imaging touch screen is a touch technology that uses light for positioning. Light sources and light capture sensors are set at the four corners of the screen. When an object touches the touch screen surface, the light changes, and the touch IC module analyzes the changes in the light sensor to determine the touch position. Optical imaging touch screens are highly durable and suitable for use in complex environments. They also support multi-touch, but are easily affected by ambient light, dust, insects, etc. and may cause misidentification. Currently, this technology is only used in desktop monitors larger than 10 inches, education/training, etc.

　　6. Electromagnetic induction touch screen

　　The sensor of the electromagnetic induction touch screen is set behind the display screen. The sensor generates an electromagnetic area on the display surface. When the electronic pen touches the display surface, the sensor can determine the location of the touch point by calculating the change in the electromagnetic field. Compared with other touch screen technologies, the electromagnetic induction touch screen has the highest accuracy and resolution, low power consumption, and is thinner and lighter. It is particularly suitable for use in war environments and construction environments. Currently, this technology is mainly used by the US military.

　　Other touch screen technologies In addition to the above-mentioned touch technologies, there are also various other touch technologies on the market, such as pressure sensing, digital acoustic wave guided, and oscillating pointer, which are generally used for special purposes.

　　2. Development trend of touch screen

　　1. Embedded touch screen structure

　　At present, touch screens basically adopt an external structure. The display module and touch module of this structure are two relatively independent devices. Then the two devices are integrated through the back-end bonding process. However, this relatively independent external structure will affect the thickness of the product, which is not in line with the development trend of increasingly thin and light touch display products. As a result, the concept of embedded touch screens was born. The embedded structure embeds the touch module into the display module, so that the two modules are integrated into one, rather than two relatively independent devices. Compared with the traditional external structure, the advantages of the embedded structure are: only 2 layers of ITO glass are required, the material cost is reduced, the light transmittance is improved, and it is lighter and thinner; there is no need for back-end bonding of the touch screen module and the TFT module, which improves the yield rate; the touch screen group and the TFT module are produced at the same time, which reduces the transportation cost of the module. Embedded touch screens can be divided into two types: In-cell technology and On-cell technology.

    The definitions of the two technologies are slightly different, but the principles are similar, both of which embed the touch screen into the liquid crystal module. In-cell technology integrates the touch screen under the color filter. Since the touch sensor is placed inside the liquid crystal panel, it occupies a part of the display area, so some display effects are sacrificed, and the process becomes complicated, and high yield is difficult to achieve. On-cell technology integrates the touch screen on the color filter, instead of embedding the touch sensor inside the liquid crystal panel. It only needs to form a simple transparent electrode between the color filter backplane and the polarizer, which reduces the technical difficulty. The main challenge of On-cell is the amount of noise that the display couples to the sensing layer. The touch screen element must use sophisticated algorithms to process this noise. On-cell technology provides all the benefits of integrating the touch screen into the display, such as making the touch panel thinner and significantly reducing costs, but the overall system cost reduction is still far less than that of Incell technology. The concept of embedded was first proposed by TMD in 2003, and then Sharp, Samsung, AUO, LG and other companies successively proposed this concept and announced some research results, but due to technical problems, none of them have been able to achieve commercialization. Embedded touch screens have been under development for nearly 10 years, and are still some distance away from commercialization. However, embedded touch screens represent the future development direction of touch screens, and manufacturers who actively reserve embedded technology will be in a relatively advantageous position in future market competition.

　　2. Multi-touch technology

　　In 2007, Apple implemented the multi-touch function through projected capacitive technology, which provided an unprecedented user experience and was different from other touch technologies at the time, making multi-touch technology a market trend. Currently, multi-touch technology has evolved from only being able to achieve two-finger zooming, three-finger scrolling, and four-finger panning to being able to support more than 5-point touch recognition and multiple input methods. In the future, multi-touch technology will develop in the direction of achieving more detailed screen object control and greater freedom.

　　3. Hybrid touch technology

　　目前虽然触控技术类型众多，但每种技术都各有利弊，没有一种技术是完美的。近年来有人开始提出混合式触控技术的概念，即在一块触控面上采用两种或者两种以上的触控识别技术，达到多种触控技术之间实现优劣互补的目的。目前已经研发出基于电容式和电阻式的混合式触摸屏，该触摸屏可以通过手写笔和手指操作、支持多点触控等，显著提高触摸屏的识别效率。

　　As users' requirements for touch technology continue to increase, a single touch technology will certainly not be able to meet people's needs, so hybrid touch technology will definitely become one of the development directions of touch technology in the future.

　　4. Haptic feedback technology

　　The continuous development of touch display technology brings people convenient operation methods and good visual effects, but ignores giving users a tactile feedback during touch operation. At present, there is not much research on tactile feedback technology. Immersion, an American company, has launched a tactile feedback technology called "Forcefeedback". This technology uses mechanical motors to generate vibration or movement. It can simulate tactile effects such as jumping, falling objects and damping movement. It is also a tactile feedback technology that is currently used more. Senseg's "E-sense" technology uses the principle of bioelectric field to generate a tactile feedback. The development of more realistic tactile feedback technology can bring users a new touch experience, so tactile feedback technology is also a direction for the development of touch technology in the future.