Structure and working principle of resistance screen

       First of all, the resistance screen is divided into four-wire type and five-wire type, but we often see four-wire type and five-wire type, and their working principles are almost the same. The biggest difference is that its accuracy will be different after being affected by external forces.
       In fact, simply speaking, the resistance screen is divided into two layers, separated by a spacer in the middle. When the two layers collide with each other,    the current will have an impact, and the chip will respond by calculating the data between the force and the current to assess which position on the screen is pressed. Because the resistive screen needs the upper and lower layers to collide before it can react. Therefore, when two points are pressed at the same time, the pressure on the screen becomes unbalanced, resulting in touch error. Therefore, this principle makes it difficult for the resistance screen to realize multi-touch, and even if the multi-touch sensitivity is realized by technical means, it is not easy to adjust, and the phenomenon of A-point sensitivity and B-point dullness often occurs. In addition, because the resistive touch screen needs a certain pressure, it is easy to wear the surface material for a long time, or the upper and lower layers lose elasticity, which leads to poor contact, thus affecting the normal service life of the product.


Detailed introduction of resistance touch screen
 

This touch screen is controlled by pressure sensing. The main part of the resistive touch screen is a resistive film screen that fits well with the display surface. It is a multi-layer composite film. It takes a glass or hard plastic flat plate as the base layer, and its surface is coated with a transparent metal oxide (transparent conductive resistance) conductive layer, which is covered with a plastic layer with hardened outer surface and smooth scratch resistance. Its inner surface is also coated with a coating, and there are many tiny (less than 1/1000 inch) layers between them. When the finger touches the screen, the two conductive layers contact at the touch point, and the resistance changes, which generates signals in the X and Y directions and then sends them to the touch screen controller. The controller detects this contact and calculates the position of (x, y), and then operates according to the way of simulating the mouse. This is the most basic principle of resistive touch screen. The resistive touch screen can be operated with hard objects. The key of resistive touch screen lies in material science and technology. Commonly used transparent conductive coating materials are:

ITO, indium oxide, a weak conductor, is characterized in that when the thickness drops below 1800 Angstroms (Angstroms = 10-10 meters), it will suddenly become transparent, with a light transmittance of 80%. When it is thinner, the light transmittance will decrease, and it will rise to 80% when it reaches 300 Angstroms. ITO is the main material used in all resistive touch screens and capacitive touch screens. In fact, the working surface of resistive and capacitive touch screens is ITO coating.

IT nickel-gold coating, the outer conductive layer of the five-wire resistance touch screen is made of nickel-gold coating material with good ductility, Because the outer layer is frequently touched, the purpose of using nickel-gold material with good ductility is to prolong the service life, but the process cost is relatively high. Although the nickel conductive layer has good ductility, it can only be used as a transparent conductor, which is not suitable for working face of resistive touch screen. Because of its high conductivity, and the metal thickness is not easy to be very uniform, it is not suitable for voltage distribution layer, but only for probe layer.

1. Four-wire resistance screen

When the two transparent metal layers of the four-wire resistance analog technology work, a constant voltage of 5V is added to each layer: one vertical direction and one horizontal direction. A total of four cables are required. Features: High resolution, high speed transmission response. Surface hardness treatment to reduce scratches, scratches and chemical treatment. With smooth and matte finish. Once correction, high stability, never drift.

 2-wire and five-wire resistance screen

The basic layer of the five-wire resistance touch screen applies the voltage fields in two directions to the conductive working surface of the glass through the precision resistance network. We can simply understand that the voltage fields in two directions are applied to the same working surface in time sharing, while the outer nickel-gold conductive layer is only used as a pure conductor. The position of the touch point is measured by detecting the voltage values of the X axis and Y axis of the inner ITO contact point in time sharing after touching. The inner ITO of the five-wire resistance touch screen needs four leads, and the outer one is only a conductor. There are five leads of the touch screen.

Features: High resolution, high-speed transmission response. High surface hardness, reducing scratches, scratches and chemical treatment. It can still be used after 30 million point contacts. Conductive glass is the substrate medium. Once correction, high stability, never drift. Five-wire resistance touch screen has the disadvantages of high price and high environmental requirements.

3. Limitations of resistance screen

Whether it is a four-wire resistive touch screen or a five-wire resistive touch screen, they are a completely isolated working environment from the outside world. Not afraid of dust and water vapor, it can be touched by any object, can be used for writing and drawing, and is more suitable for limited people in industrial control field and office. The common disadvantage of resistive touch screen is that the outer layer of the composite film is made of plastic material, and people who don't know it may scratch the whole touch screen if they touch it too hard or touch it with sharp instruments, resulting in scrapping. However, within the limit, the scratch will only hurt the outer conductive layer. The scratch of the outer conductive layer is not related to the five-wire resistance touch screen, but fatal to the four-wire resistance touch screen.

Structure and working principle of capacitive screen
      After understanding the working principle of resistance screen, let's learn about the structure and working principle of capacitive screen. Actually, capacitive screen and resistance screen have two layers, but the difference is that capacitive screen does not react through the collision between the two layers. Basically, the capacitive screen uses the lower layer to transmit signals to the upper layer, and when the upper layer is contacted by a conductor, the lower layer can receive messages and make calculations. Therefore, there is no need for direct contact between the two screens, and only the information received by the lower layer can be calculated to determine the position touched by the finger. Because of this, capacitive screen can not only support multiple points at the same time, but also greatly improve the sensitivity of touch. Because the human body itself is a conductor, the capacitive screen can react when the finger touches the screen. Compared with resistance screen, capacitive screen has the advantage that capacitive screen is driven by static electricity and resistance screen is driven by force, and capacitive screen can be used under harsh conditions (high temperature, high humidity and low temperature), but the use of resistance screen will be affected to some extent.

Detailed introduction of capacitive touch screen

1. capacitive touch screen

It works by the current induction of the human body. The capacitive touch screen is a four-layer composite glass screen. The inner surface of the glass screen and the interlayer are coated with a layer of ITO, and the outermost layer is a thin layer of silica glass protective layer. The interlayer ITO coating is used as the working surface, and four electrodes are led out at four corners. The inner layer ITO is the shielding layer to ensure a good working environment. When the finger touches the metal layer, due to the electric field of human body, a coupling capacitor is formed between the user and the touch screen surface. For high-frequency current, the capacitor is a direct conductor, so the finger sucks 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 these four electrodes is proportional to the distance between the finger and the four corners. The controller can get the position of the touch point by accurately calculating the proportion of these four currents.

2. The defect of capacitive touch screen.

The transmittance and clarity of the capacitive touch screen are better than those of the four-wire resistance screen,

Of course, it can't be compared with surface acoustic screen and five-wire resistance screen. The reflection of capacitive screen is serious. Moreover, the four-layer composite touch screen with capacitive technology has uneven light transmittance of each wavelength, which leads to color distortion. Because of the reflection of light between layers, the image characters are blurred. In principle, the capacitive screen uses the human body as an electrode of a capacitor element. When a conductor is close to a capacitor with sufficient capacitance coupled with the interlayer ITO working face, the current flowing away is enough to cause the misoperation of the capacitive screen. As we know, although the capacitance is inversely proportional to the distance between electrodes, it is directly proportional to the relative area, and it is also related to the dielectric coefficient. Therefore, when the palm of a large area or a hand-held conductor is close to the capacitive screen instead of touching it, the malfunction of the capacitive screen can be caused, especially in wet weather, and the malfunction of the capacitive screen can be caused by holding the display with one's hand, keeping the palm close to the display within 7cm or keeping the body close to the display within 15cm. Another disadvantage of capacitive screens is that they don't respond when touched by gloved hands or non-conductive objects, because of the addition of more insulating media. The main disadvantage of capacitive screen is drift: when the ambient temperature and humidity change, and the ambient electric field changes, the capacitive screen will drift, resulting in inaccuracy. For example, the temperature rise of the display will cause drift after the boot: the user touches the screen while the other hand or one side of the body approaches the display will drift; Large objects near the capacitive touch screen move back and forth, and when touching, if someone comes around to watch, it will also cause drift; The drift of the capacitive screen is due to the inherent technical deficiency. Although the environmental potential surface (including the user's body) is far away from the capacitive touch screen, it is much larger than the finger area, which directly affects the determination of the touch position. In addition, many relationships that should be linear in theory are actually nonlinear. For example, the total amount of current sucked by people with different weights or different degrees of finger wetting is different, while the change of total current and the change of four partial currents are nonlinear. The self-defined polar coordinate system of the four corners adopted by the capacitive touch screen has no coordinate origin, so the controller can't detect and recover it after it drifts. Moreover, after the four A/Ds are completed, the values of the four partial currents are at right angles to the touch point. Because there is no origin, the drift of capacitive screen is cumulative, and it often needs to be calibrated at the job site. The outermost silica protective glass of the capacitive touch screen is very scratch-resistant, but for fear of nails or hard objects knocking, knocking out a small hole will hurt the interlayer ITO. Whether it hurts the interlayer ITO or the inner surface ITO layer during installation and transportation, the capacitive screen will not work normally.