A touch screen consists of a touch detection component and a touch screen controller. The touch detection component is installed in front of the display screen to detect the user's touch position and send the information to the touch screen controller. The main function of the touch screen controller is to receive touch information from the touch point detection device, convert it into touch point coordinates, and then send it to the CPU. It can also receive commands from the CPU and execute them.
Main types of touch screens
Based on their working principles and information transmission media, touchscreens are classified into four types: resistive, capacitive, infrared, and surface acoustic wave (SAW) touchscreens. Each type has its own advantages and disadvantages. To understand which type of touchscreen is suitable for which application, it is crucial to understand the working principles and characteristics of each type of touchscreen technology.
1. Resistive Touchscreen
This type of touchscreen uses pressure sensing for control. The main component of a resistive touchscreen is a resistive film screen that fits perfectly against the display surface. This is a multi-layered composite film, consisting of a glass or hard plastic plate as the base layer, coated with a transparent metal oxide (transparent conductive resistor) conductive layer, and then covered with a hardened, smooth, scratch-resistant plastic layer. Its inner surface is also coated. Between these layers are numerous tiny (less than 1/1000 of an inch) transparent insulating dots that separate and insulate the two conductive layers. When a finger touches the screen, the two conductive layers make contact at the touch point, causing a change in resistance and generating signals in both the X and Y directions, which are then sent to the touchscreen controller. The controller detects this contact, calculates the (X, Y) position, and operates in a manner similar to simulating a mouse. This is the basic principle of resistive touchscreen technology. The key to resistive touchscreens lies in materials science; commonly used transparent conductive coating materials include:
1. ITO, indium oxide, is a weak conductor. Its characteristic is that when the thickness is reduced to below 1800 angstroms (angstrom = 10⁻¹⁰ meters), it suddenly becomes transparent with a light transmittance of 80%. The light transmittance decreases when the thickness is further reduced, but rises back to 80% at a thickness of 300 angstroms. ITO is the main material used in all resistive and capacitive touch screens. In fact, the working surface of resistive and capacitive touch screens is the ITO coating.
2. Nickel-gold coating: The outer conductive layer of a five-wire resistive touchscreen uses a highly ductile nickel-gold coating material. Due to frequent touches, the use of this ductile material extends the lifespan of the outer conductive layer, but the manufacturing cost is relatively high. Although the nickel-gold conductive layer has good ductility, it can only be used as a transparent conductor and is not suitable as the working surface of a resistive touchscreen because of its high conductivity. Furthermore, it is difficult to achieve a very uniform thickness with metal, making it unsuitable as a voltage distribution layer; it can only be used as a probe layer.
Four-wire resistive screen
The four-wire resistance analog signal technology uses two transparent metal layers, each with a constant 5V voltage applied: one vertically and one horizontally. A total of four cables are required. Features: High resolution, high-speed transmission response. Surface hardening treatment reduces scratches and provides chemical resistance. Available in glossy and matte finishes. One-time calibration ensures high stability and prevents drift.
Five-wire resistive screen
Five-wire resistive touchscreens apply voltage fields in two directions to the conductive working surface of the glass through a precision resistor network. This can be simply understood as the voltage fields in two directions being applied to the same working surface in a time-division manner, while the outer nickel-gold conductive layer acts only as a pure conductor. The touch point's position is determined by time-division detection of the X and Y axis voltage values of the inner ITO contact point after a touch. A five-wire resistive touchscreen requires four leads for the inner ITO layer and only one lead for the outer layer (which acts only as a conductor), resulting in a total of five leads. Features: High resolution, high-speed transmission response. High surface hardness, reducing scratches and requiring chemical resistance. Can withstand 30 million touches at the same point and still be used. Conductive glass is the substrate medium. One-time calibration, high stability, and no drift. The disadvantages of five-wire resistive touchscreens are high price and high environmental requirements.
1.3 Limitations of Resistive Screens
Both four-wire and five-wire resistive touchscreens operate in a completely isolated environment, unaffected by dust and moisture. They can be touched with any object and used for writing and drawing, making them suitable for industrial control and limited-person office use. A common drawback of resistive touchscreens is that the outer layer of the composite film is made of plastic; improper handling or the use of sharp objects can scratch the entire touchscreen, rendering it unusable. However, within limits, scratches will only damage the outer conductive layer. Scratches on the outer conductive layer are not a problem for five-wire resistive touchscreens, but are fatal to four-wire resistive touchscreens.
2. Capacitive Touch Screen
2.1 Capacitive Touchscreen Technology
It works by utilizing the electrical current sensing of the human body. A capacitive touchscreen is a four-layer composite glass screen. The inner surface and interlayer of the glass screen are each coated with a layer of ITO (Indium Tin Oxide). The outermost layer is a thin silica glass protective layer. The ITO coating in the interlayer serves as the working surface, with four electrodes extending from the four corners. The inner ITO layer acts as a shielding layer to ensure a good working environment. When a finger touches the metal layer, due to the human body's electric field, a coupling capacitor is formed between the user and the touchscreen surface. For high-frequency currents, the capacitor is a direct conductor, so the finger draws a small current from the contact point. This current flows out from the electrodes at the four corners of the touchscreen, and the current flowing through these four electrodes is proportional to the distance from the finger to the corner. The controller determines the position of the touch point by accurately calculating the ratio of these four currents.
Advantages of capacitive touchscreens:
1. Novel operation. The capacitive touchscreen supports multi-touch, making operation more intuitive and fun.
2. Less prone to accidental touches. Because capacitive touchscreens need to sense the electrical current from the human body, only the human body can operate them. Touching them with other objects will not elicit a response, thus largely avoiding the possibility of accidental touches.
3. High durability. Compared to resistive touchscreens, capacitive touchscreens perform better in terms of dust resistance, water resistance, and wear resistance.
Disadvantages of capacitive touchscreens:
1. Lower accuracy. Due to technical limitations, capacitive touchscreens are less accurate than resistive touchscreens. Furthermore, they can only be input using fingers, making it difficult to recognize complex handwriting input on small screens.
2. Susceptible to environmental influences. Changes in environmental factors such as temperature and humidity can cause instability or even drift in capacitive touchscreens. For example, users moving their bodies close to the screen while using the device can cause drift, and even operating the device in a crowded place can lead to drift. This is mainly due to the working principle of capacitive touchscreen technology. Although the user's finger is closer to the screen, there are many other electric fields with a volume much larger than the finger acting simultaneously near the screen, which affects the judgment of the touch position.
3. High cost. In addition, there are still some technical difficulties in the process of attaching the touchpad to the LCD panel for capacitive touch screens, resulting in a low yield rate, which in turn increases the cost of capacitive touch screens.
3. Infrared touchscreen
Infrared touchscreens use a dense matrix of infrared rays along the X and Y directions to detect and locate user touches. An infrared touchscreen has a circuit board frame mounted in front of the display. Infrared emitters and receivers are arranged around the four sides of the screen, forming a crisscrossing infrared matrix. When a user touches the screen, their finger blocks the horizontal and vertical infrared rays passing through that location, thus allowing the system to determine the touch point's position on the screen.
An external touchscreen is a highly integrated electronic circuit product. An infrared touchscreen contains a complete integrated control circuit, a set of high-precision, anti-interference infrared emitters, and a set of infrared receivers, cross-mounted in two opposite directions on a highly integrated circuit board to form an invisible infrared grating. The intelligent control system embedded in the control circuit continuously emits pulses to the diodes, forming an infrared polarized beam grating. When a touching object, such as a finger, enters the grating, it blocks the beam. The intelligent control system detects the change in light loss and transmits a signal to the control system to confirm the X-axis and Y-axis coordinate values.
Advantages of infrared touchscreens:
1. High stability, it will not drift due to changes in time or environment.
2. Highly adaptable, unaffected by current, voltage, and static electricity, suitable for certain harsh environmental conditions (explosion-proof, dustproof).
3. High light transmittance with no intermediate medium, up to 100% compliance.
4. Long service life, highly durable, scratch-resistant, and long touch screen life.
5. Excellent usability; no force is required for touch, and there are no special requirements for the touch surface.
Disadvantages of infrared touchscreens:
1. It is susceptible to strong infrared interference, such as from remote controls, high-temperature objects, sunlight, or incandescent lamps illuminating the infrared receiver tube.
2. It is susceptible to strong electromagnetic interference, such as from transformers.
4. Surface acoustic wave touch screen
Surface acoustic wave touch screen working principle
Surface acoustic waves (SAWs), a type of ultrasonic wave, are mechanical energy waves that propagate 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. Transducers for emitting and receiving sound waves in the X and Y directions are attached to the three corners of the acoustic screen (the transducers are made of special ceramic materials and are divided into emitting and receiving transducers; they convert the electrical signals sent from the controller via the touchscreen cable into acoustic energy and the surface acoustic wave energy converged by reflective stripes into electrical signals). Reflective stripes reflecting the ultrasonic waves are engraved on the four sides. When a finger or soft object touches the screen, some of the acoustic energy is absorbed, thus changing the received signal. This signal is then processed by the controller to obtain the X and Y coordinates of the touch.
Advantages of surface acoustic wave touch screens:
1. High clarity and good light transmittance.
2. Highly durable and scratch-resistant (compared to resistors, capacitors, etc., which have surface coatings).
3. Quick response.
4. Unaffected by environmental factors such as temperature and humidity.
5. High resolution,
6. Long lifespan (50 million cycles under proper maintenance);
7. High light transmittance (92%), maintaining clear and bright image quality;
8. No drift, only one calibration is required during installation;
9. There is a third axis (i.e., pressure axis) response.
Disadvantages of surface acoustic wave touchscreens:
Surface acoustic wave (SAW) touchscreens require frequent maintenance. Dust, oil, and even spilled beverages can clog the waveguide grooves on the screen surface, preventing proper wave emission or altering the waveform so the controller cannot recognize it, thus affecting the normal operation of the touchscreen. Users must pay strict attention to environmental hygiene. The screen surface must be wiped frequently to keep it clean, and a thorough cleaning should be performed periodically.
