Principles and Classification A touchscreen system generally consists of two parts: a touch detection device and a touchscreen controller. The touch detection device is installed in front of the display screen to detect the user's touch position and sends the data to the touchscreen controller. The main function of the touchscreen controller is to receive touch information from the touch detection device, convert it into touch point coordinates, and then send it to the CPU. It can also receive and execute commands from the CPU. With technological advancements, touchscreen technology has undergone a gradual upgrade and development from low-end to high-end. Based on its working principle, it is currently generally classified into four categories: resistive touchscreens, capacitive touchscreens, infrared touchscreens, and surface acoustic wave (SAW) touchscreens. Resistive Touchscreens The screen of a resistive touchscreen is a multi-layered composite film. A layer of glass or acrylic glass serves as the base layer, coated with a transparent conductive layer (ITO film), and then covered with a hardened, smooth, scratch-resistant plastic layer. Its inner surface is also coated with ITO. Between the two conductive layers are numerous tiny (less than one-thousandth of an inch) transparent insulating dots that separate them. When a finger touches the screen, the two layers of ITO (Indium Tin Oxide) come into contact, causing a change in resistance. The controller calculates the coordinates of the contact point based on the detected resistance change and then performs the corresponding operation. Resistive touchscreens are classified into four-wire, five-wire, and other types based on the number of leads. The outer surface of a five-wire resistive touchscreen is conductive glass, not a conductive coating. This type of conductive glass has a longer lifespan and higher light transmittance. If the ITO coating of a resistive touchscreen is too thin, it is prone to breakage; if the coating is too thick, it reduces light transmittance and causes internal reflections, reducing clarity. Due to frequent touching, the surface ITO layer will develop small cracks and even deform after a certain period of use, thus its lifespan is not long. Resistive touchscreens are inexpensive and easy to manufacture, making them a popular choice. The emergence of four-wire, five-wire, seven-wire, and eight-wire touchscreens has made their performance more reliable and improved their optical characteristics. Capacitive Touchscreens Capacitive touchscreens have narrow electrodes plated on all four sides, creating a low-voltage alternating current electric field inside. A transparent thin film layer, a special metallic conductive material, is attached to the touchscreen. When a user touches a capacitive touchscreen, their finger and the surface form a coupling capacitor. Because a high-frequency signal is applied to the surface, the finger draws a small current, which flows out from the electrodes at the four corners of the screen. Theoretically, the current flowing through the four electrodes is proportional to the distance from the finger to the corner. The controller calculates the contact point position by precisely calculating the ratio of these four currents. The dual-glass design of capacitive touchscreens not only protects the conductors and sensors but also effectively prevents external environmental factors from affecting the touchscreen. Even if the screen is dirty, dusty, or oily, a capacitive touchscreen can still accurately calculate the touch position. However, because capacitance varies with temperature, humidity, and grounding conditions, its stability is relatively poor, often resulting in drift. Although not as widely used as resistive touchscreens, capacitive touchscreens are a popular choice. These devices are accurate, responsive, offer higher resolution even at slightly larger sizes, and are more durable (scratch-resistant), making them suitable for use as touchscreens in game consoles. Furthermore, emerging near-field imaging technology has improved the performance of capacitive touchscreens, reducing the drift that can occur in both capacitive and resistive touchscreens. Infrared Touchscreen An infrared touchscreen has infrared emitters and receivers arranged around its four sides, forming a cross-shaped infrared matrix. When a user touches the screen, their finger blocks the horizontal and vertical infrared rays passing through that location, and the controller calculates the touch point's position. Infrared touchscreens are also unaffected by current, voltage, and electrostatic interference, making them suitable for harsh environments. Their main advantages are low cost and easy installation, allowing them to be used on computers of all levels. Furthermore, due to the absence of a capacitor charging and discharging process, their response speed is faster than capacitive touchscreens, although their resolution is lower. Surface Acoustic Wave Touchscreen Surface acoustic waves (SAWs) are a type of ultrasonic wave, a mechanical energy wave that propagates shallowly on the surface of a medium (such as rigid materials like glass or metal). Using a wedge-shaped triangular base (strictly designed according to the wavelength of surface waves), directional, small-angle SAW energy emission can be achieved. SAWs are stable, easy to analyze, and possess very sharp frequency characteristics during transverse wave propagation, leading to rapid development in applications such as non-destructive testing, imaging, and waveguides in recent years. This type of touchscreen has ultrasonic transducers and receivers at each of its four corners, emitting ultrasonic waves that cover the screen surface. When a finger touches the screen, some of the sound wave energy is absorbed, causing a change in the received waveform—specifically, a momentary attenuation gap. The controller calculates the touch point position based on this attenuation signal. Surface acoustic wave (SAW) touchscreens are unaffected by environmental factors such as temperature and humidity, have extremely high resolution, excellent scratch resistance, a long lifespan (50 million failures), high light transmittance (92%), and maintain a clear and bright image. They exhibit no drift, requiring only one calibration during installation. They also have a third-axis (pressure axis) response, making them ideal for public places. However, SAW touchscreens are susceptible to water droplets and dust. Improvements can be made by adding dustproof strips or enhancing dirt monitoring to accurately distinguish between valid operations and contamination. Furthermore, because SAW screens can sense pressure, they implicitly add control mechanisms, greatly expanding screen functionality and thus broadening their application range. Basic Technologies of Touchscreens Absolute Coordinate System A touchscreen uses an absolute coordinate system, meaning the current positioning coordinates are independent of the previous ones. Each touch's data is directly converted into coordinates on the screen through calibration. Regardless of the situation, the output data for the same point using this coordinate system is stable. However, it cannot guarantee that every touch on the same point will be sampled identically; that is, absolute coordinate positioning cannot be guaranteed, which is the so-called drift problem. Positioning Various touchscreens rely on sensors to operate, and some touchscreens are even sensors themselves. Their respective positioning principles and the sensors they use determine the touchscreen's response speed, reliability, stability, and lifespan. The technical characteristics of various touchscreens are shown in Table 1. Table 1: Comparison of Basic Technologies of Various Touchscreens Touchscreen Performance Comparison Resistive touchscreens operate in a completely isolated environment. They are not afraid of dust, moisture, or oil, and can be touched with any object, making them suitable for industrial control applications. The disadvantage is that because the outer layer of the composite film is made of plastic, excessive force or the use of sharp objects may scratch the touchscreen. Capacitive touchscreens have high resolution and good light transmittance, well meeting various requirements, and are commonly found in public places. However, capacitive touchscreens use the human body as an electrode of a capacitor. When a conductor approaches and couples with a sufficiently large capacitance between the ITO working surface, the flowing current can cause malfunctions. Additionally, touching the screen while wearing gloves or holding an insulated object will result in no response due to the added insulating medium. Infrared touchscreens determine the touch position by measuring the on/off state of infrared light, regardless of the material of the transparent baffle used (some don't use any baffle at all). Therefore, using a baffle with good light transmittance and anti-reflective treatment can achieve excellent visual effects. However, due to the size limitations of the infrared emitter, high-density infrared light cannot be emitted, resulting in lower resolution. Furthermore, since infrared touchscreens rely on infrared sensing, changes in external light, such as sunlight or indoor lighting, can affect their accuracy. Surface acoustic wave (SAW) technology is very stable, and SAW touchscreen controllers calculate the touch position by measuring the position of the decay time on the time axis, resulting in very high accuracy. Surface acoustic wave (SAW) touchscreens also have a third axis (z-axis), which is the pressure axis—the force of a user's touch can be determined by calculating the attenuation at the point of signal attenuation, with up to 256 levels of pressure. The greater the force, the wider and deeper the attenuation gap on the received signal waveform. Among all touchscreens, only SAW touchscreens have the ability to sense touch pressure. Application Scenarios Based on the analysis of the structure, principle, and performance characteristics of touchscreens, the applicable scenarios for different touchscreens are as follows: Four-wire resistive touchscreen: Unaffected by dust, oil, and photoelectric interference, but susceptible to scratches. Suitable for public places with fixed users, such as industrial control sites, offices, and homes. Five-wire resistive touchscreen: Excellent sensitivity and light transmittance, long service life, unaffected by dust, oil, and photoelectric interference, suitable for various public places, especially for precision industrial control sites. Capacitive touchscreen: Due to the variation in capacitance with temperature, humidity, or grounding conditions, its stability is poor, often resulting in drift. Susceptible to electromagnetic interference and drift, not suitable for use in industrial control sites or places with interference. Infrared touchscreens: Suitable for public information inquiries requiring less precision; require frequent calibration and positioning. Infrared touchscreens: Lower resolution, but unaffected by current, voltage, and static electricity, suitable for harsh environments; suitable for various public places, offices, and industrial control sites where infrared and strong light interference are absent. Surface acoustic wave touchscreens: Made of pure glass, with the best light transmittance, long lifespan, and good scratch resistance, suitable for various public places with unknown users. However, they are susceptible to dust accumulation and oil contamination over time, so they are better suited for clean environments. Otherwise, regular cleaning services are required. Development Trends Touchscreen technology facilitates computer operation and is a highly promising interactive input technology. Countries worldwide are paying close attention to it and investing heavily in research and development, resulting in a continuous emergence of new touchscreen technologies. Stylus: Touchscreens operated with a stylus resemble whiteboards; in addition to displaying interfaces, windows, and icons, the stylus also has signature and marking functions. This type of stylus is a significant improvement over earlier light pens that only provided menu selection. Touchpad: Touchpads utilize pressure-sensitive capacitive touch technology and have the largest screen area. It consists of three parts: the bottom layer is the central sensor, used to monitor whether the touchpad is touched and then process the information; the middle layer provides interactive graphics and text; and the outermost layer is the touch surface, made of high-strength plastic. When a finger touches the outer surface, the information is sent to the sensor and processed within a millisecond. Besides being compatible with PCs, it also features high brightness, clear images, and easy interaction, making it ideal for point-and-click information query systems (such as electronic bulletin boards) with excellent results. In short, the development of touchscreens shows trends towards specialization, multimedia capabilities, three-dimensionality, and larger screen sizes. With the development of the information society, people need access to a wide variety of public information. Public information transmission systems using touchscreen technology as the interactive window employ advanced computer technology and utilize various forms such as text, images, music, narration, animation, and video to present information to people intuitively and vividly, bringing great convenience. It is foreseeable that with the rapid development of touchscreen technology, its application areas will become increasingly wider, and its performance will continue to improve.