In the field of electronic application equipment, touch screen products have been seen everywhere, and the most common ones are our daily necessities: smart phones and tablet computers. These electronic application settings have made the touchable LCD display interface an indispensable part of our lives. People can easily issue their own instructions to these electronic application devices with their own fingers, and get the results and goals they want.
Technology brings not only technological progress but also a convenient lifestyle. You must know that 10 years ago, there were not so many intelligent electronic application devices on the market. In the age when the iphone was not yet invented, most people could only rely on buttons or electronic screen stylus to implement information interaction related actions with electronic application devices.
1. What is a capacitive touch screen
Capacitive touch screen technology uses the current induction of the human body to work. The capacitive touch screen is a four-layer composite glass screen. The inner surface and the interlayer of the glass screen are each coated with a layer of ITO. The outermost layer is a thin layer of silica glass protective layer. The interlayer ITO coating is used as the working surface, and the four corners are led out Four electrodes, the inner ITO is a shielding layer to ensure a good working environment. When a finger touches the metal layer, due to the electric field of the human body, a coupling capacitor is formed between the user and the surface of the touch screen. For high-frequency current, the capacitor is a direct conductor, so the finger draws a small current from the contact point. This current flows from the electrodes on the four corners of the touch screen, and the current flowing through the four electrodes is proportional to the distance from the finger to the four corners. The controller calculates the position of the touchpoint by accurately calculating the ratio of these four currents.<
2.what is a resistive touch screen
The resistive touch screen is a kind of sensor, which is basically a structure of thin film and glass. The adjacent sides of the thin film and glass are coated with ITO (Nano Indium Tin Metal Oxide) coating. ITO has good conductivity and transparency Sex. When touch operation, the ITO of the lower layer of the film will contact the ITO of the upper layer of the glass, and the corresponding electric signal will be transmitted through the sensor, and then sent to the processor through the conversion circuit, which is converted into the X and Y values on the screen through calculation, and the point is completed. The selected action is displayed on the screen.
The composition and technical principle of touch screen
The main difference between resistive capacitive and projected capacitive touch technology lies in the composition of the touch screen, which will seriously affect technical functions and costs. In fact, the high transparency and high resistivity characteristics of indium tin oxide (ITO) were first realized and utilized through resistive touch technology.
1. When a finger is pressed on the resistive touch screen sensor, it will make actual electrical contact. The screen consists of a glass substrate covered with two layers of polyethylene terephthalate (PET), each layer coated with indium tin oxide (ITO). They are separated by air gaps and separated by isolation points. Below it is an insulating substrate usually made of glass. Touching the screen will cause physical contact between the top and bottom ITO layers.
The resistive touch screen consists of two layers of polyethylene terephthalate (PET), each layer is coated with ITO (Figure 1). The two layers of PET are separated by air gaps and isolation points. The bottom PET layer is placed on an insulating substrate usually made of glass. The protective layer of the hard coat is placed on top of another PET layer. When a finger presses on the touch screen, this action causes physical contact between the top and bottom ITO layers, which indicates that the finger has touched the ground. Resistive touch screens are available in 4-wire, 5-wire, 6-wire and 8-wire, which provide different degrees of durability and noise suppression.
2. In a capacitive touch screen with an integrated display, the entire assembly is laminated with the display below the display.
The standard capacitive touch screen system (Figure 2) includes a projected capacitive touch screen sensor laminated to a protective protective mirror, an adhesive flexible printed circuit (FPC) with a touch screen controller installed, and a display. The FPC connects the touch screen controller to the host processor. The display is located below the touch screen sensor, usually separated by an air gap or directly laminated.
Projected capacitance does not use pressure for touch detection, it can even detect the lightest touch. This technology reads finger touches based on the difference in capacitance when the finger is placed on the touch screen.
Without pressure-based detection, projected capacitors will eliminate the bendable protective cover. Instead, a thicker plastic or glass protective cover that is stronger and scratch-resistant can be deployed. In contrast to resistive touch screens, projected capacitive touch screens can utilize glass or PET substrates, and can be single-layer or double-layer. OEMs also have multiple stacking options for projected capacitive touch screens. Please note that a single-layer sensor with ITO deposited on a glass substrate will greatly improve the optical clarity of the touch screen.
In addition, major advancements made by touch screen controller suppliers such as Cypress have led to passive pens and gloves supported by projected capacitance. A few years ago, such non-finger input support could only be provided by resistive touch screens. Looking to the future, further innovations in projected capacitance for new functions such as hovering (detecting a finger hovering over a certain distance above the touch screen) open up possibilities for even more revolutionary and rich user experiences.
Defects of resistive touch screen
Although resistive touch screens have been widely used, there are still many drawbacks, which provide other touch technologies with opportunities to establish business and expand market share. Users are always frustrated with resistive touch screens because they inaccurately report button activation on different parts of the touch screen, or the press is not activated as expected. Since resistive touch screens rely on pressure-activated touches, they need to move and bend different layers in their stack. Moreover, the top hard coat layer needs to be thin enough to maintain the flexibility of the touch screen panel. The combination of the moving layer and the thin protective layer results in reduced durability and susceptibility to scratches.
The light transmittance of resistive touch screen is also low (<80%). The construction of a resistive touch screen with two layers of PET, air gaps and spacing points may cause loss of refracted and reflected light, resulting in a more shadowy display. Another negative feature is that resistive touch screens suffer from aging, which starts to occur when PET is damaged at high temperatures. Signs of aging include discoloration of the touch screen, which starts to turn white. Resistive touch screens also have the disadvantage of requiring calibration. Calibration is required to compensate for ITO sheet resistance drift on PET.
Projected capacitive technology does not have the disadvantages of resistive touch screens, and they are not used in many high-end and high-volume applications. Due to the attractive characteristics of projected capacitive touch screens, more and more consumers require devices using this technology. However, in order to support high-end devices as well as mid- and low-end devices, touch screen and touch controller suppliers must reduce costs while maintaining full feature set innovation.
How to improve the future development space of resistive touch screen
In order to achieve a more cost-effective implementation, system designers using capacitive technology must understand how component integration affects system cost and performance, both of which can be optimized through smart design choices.
Cover plate: The cover plate and the touch screen sensor are complex structures that constitute a stack structure of the touch screen. The protective mirror is the top layer and can be made of a variety of materials. Choosing a lens made of polymethyl methacrylate (PMMA) instead of glass can reduce the cost of the cover lens by up to 50%. PMMA is shatterproof, but may reduce signal sensitivity.
Touch screen sensors: Figure 3 shows several stacking options for touch screen sensors. Each layer has a customized pattern and structure that can be etched on ITO (better optical clarity) or PET substrate (better noise resistance) on glass. Cost can be reduced through integration layer. For example, the cost of a single-layer sensor can be reduced by up to 50%, making it attractive for applications that traditionally use resistive touch screens or have not yet moved to touch screen-based interfaces.
Flexible printed circuit board (FPC): FPC interconnects touch screen panel, touch screen controller and host processor. More efficient FPC routing helps to integrate it with the rest of the system. While increasing signal integrity, single-layer routing can also minimize costs.
Display: The noise is coupled to the touch screen sensor, thereby reducing sensitivity and increasing the possibility of false touches. In order to reduce noise, an additional ITO shielding layer can be placed between the display and the touch screen sensor. However, this increases the cost and thickness of the module. Alternatively, an air gap of 0.2 to 0.5 mm may be used for separation. This helps reduce costs, but still requires additional thickness.
Touch screen controller: The ability of the touch screen controller to process noise-sensitive signals affects performance, functionality, and user experience. The controller needs at least a high-quality analog front end, built-in noise processing functions and complex processing algorithms. By providing a high signal-to-noise ratio (SNR) and effective noise processing, the controller can compensate for the drop in signal strength caused by noise sources such as cheap protective cover lenses or noisy displays. The controller also needs an algorithm compatible with the sensor used. To benefit from a single-layer FPC, the controller pins must support flexible routing. The controller also determines which advanced functions the system can support, such as waterproofing or hovering.
Capacitive touch screens and resistive touch screens are like two twins, mutually promoting the continuous development of each other’s technology. Although in recent years, with the continuous development of the intelligent electronic application equipment industry, more and more capacitive touch screen styles and types have been developed, resulting in the illusion that a resistive touch screen is about to be eliminated by the market. But the real situation is not at all a sign of this error. In the latest survey report on the touch screen LCD monitor industry in 2019, data released by relevant authoritative research institutions show that in special environments and certain equipment, the use of resistive touch screens has not only not declined, but has increased on a small scale. These special use environments and special equipment require resistive touch screens that cannot be replaced by capacitive touch screens. These authoritative survey data use facts to show that the resistive touch screen industry has not ushered in the sunset at all, but has exploded with new vitality. In the future development direction of touch screens, authoritative research institutions predict that capacitive touch screens will occupy a larger market share in more intelligent devices, and it is likely to be smart home life equipment and smart wearable devices. And resistive touch screen products are more likely to explode new vitality in complex use environments and specific equipment industries, such as industrial and mining machinery, vehicle and ship manufacturing industries, and large-scale aerospace equipment. It is worth noting that the future of the touch screens of these two electronic devices will not and should not be against each other, but more about the mutual integration and common progress of technologies. Let us look forward to the coming of this beautiful era!
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