Some hardware knowledge about the touch screen

I. Overview

Today, a lot of electronic equipment, whether industrial or consumer-level need man-machine interface, such as vehicle navigation, advertising machine, POS machine, tablet computer, handheld terminal equipment, intelligent wall switch, medical beauty equipment, etc., are equipped with touch monitor. Therefore, we need to know some basic knowledge about touch screen. second, Resistance screen and capacitive screen

The resistive touch screen is commonly known as "soft screen", which is used for mobile phones of Windows Mobile system.
Capacitive touch screen commonly known as "hard screen", such as the iphone and G1 and other machines using this screen quality. 1. Indoor visual effect

Both are usually good. 2, touch sensitive degree

1, Resistance touch screen, pressure to make the screen each layer of contact, you can use the fingers, even with gloves, nails, stylus and other operations. Supporting stylus is important in Asian markets, where gestures and word recognition are valued.
2. Capacitive touch screen, the slightest contact from the surface layer of the charged finger can also activate the capacitive sensing system at the bottom of the screen. Non-life objects, nails, gloves are not valid. Handwriting recognition is more difficult. 3. Precision

1, Resistance touch screen, precision at least to achieve a single display pixel, with the stylus can be seen. Easy handwriting recognition helps you operate under an interface that uses small control elements.
2, Capacitive touch screen, theoretical accuracy can reach several pixels, but in fact, the contact area of the finger limit. It is difficult for users to accurately click on a target smaller than 1cm2. 4. Cost

1, Resistance touch screen, very inexpensive.
2, capacitive touch screen, different manufacturers of capacitance screen price than the resistance screen more than 10% to 50%. This extra cost does not matter for the flagship product, but it may be prohibitive for medium-priced handsets. 5. Multi-point touch feasibility

1, Resistance touch screen, is not possible, unless the reorganization of the resistance screen and the machine circuit connection.
2, Capacitive touch screen, depending on the implementation of the way and software, has been in the G1 technology demo and iphone implementation. The G1 1.7T version already enables the browser's multi-touch features. 6, Damage resistance

1, Resistance touch screen, the fundamental characteristics of the resistance screen determines its top is soft, need to be able to press down. This makes the screen very easy to create scratches. Resistance screens require protective membranes and relatively more frequent calibrations. On the plus side, the use of plastic-resistive touch-screen devices is generally more vulnerable and less prone to breakage.
2, Capacitive touch screen, the outer layer can use glass. This is not indestructible and may shatter under severe shocks, but glass should be better for daily scratches and stains. 7, clean

1, Resistance touch screen, due to the use of stylus or nails to operate, it is not easy to leave fingerprints on the screen, grease and bacteria.
2, capacitive touch screen, to use the entire finger to touch, but the glass outer layer easier to clean. 8. Environmental adaptability

1, Resistance touch screen, the specific value is unknown. But there is evidence that Nokia 5800, which uses a resistive screen, can work at temperatures -15°c to +45°c, and that there is no demand for humidity.
2, capacitive touch screen, typical operating temperature between 0° to 35°, requires at least 5% of the humidity (operating principle limit) 9, the sun under the visual effects

1, resistive touch screen, usually very bad, the extra screen layer reflects a lot of sunlight. third, the type of touch screen 1, infra-red touch screen (infrared)

The infrared touch screen principle is very simple, only on the monitor plus light point frame frame, no need to add a coating on the screen surface or feeder controller. The light point is arranged in the four sides of the frame frame of the infrared emitter tube and receive tube, in the screen surface form an infrared net. When a user touches a screen with his or her finger, it blocks the two infrared rays that pass through the position, and the computer can instantly calculate the position of the touch point. Infrared touch screen is not subject to current, voltage and electrostatic interference, suitable for some harsh environmental conditions. Its main advantages are low price, convenient installation, no need card or any other controller, can be used in various grades of computers. However, because the frame is only added to the normal screen, the infrared emitter tube and the receiving tube around the frame frame are easy to be damaged and the resolution is lower in the course of use.

Figure 3.1 Infra-red touch screen

2, Capacitive touch screen (Surface capacitive)

Capacitive touch screen structure is mainly on the glass screen coated with a layer of transparent film body layer, and then a protective glass outside the conductor layer, double glass design can completely protect the conductor layer and sensors.

The capacitive touch screen is plated with a long narrow electrode on the four sides of the touch screen to form a low voltage AC electric field in the conductive body. When the user touches the screen, because of the human electric field, between the fingers and the conductor layer will form a coupling capacitor, the four edges of the electrodes emitted by the current will flow to the contact point, and the current strength and the distance between the finger to the electrode is directly proportional to the controller after the touch screen will calculate the proportion and strength of the current, accurate calculation of the location of the Capacitive touch screen of double glass not only can protect the conductor and sensors, more effective to prevent external environmental factors on the touch screen, even if the screen stained with dirt, dust or grease, capacitive touch screen can still accurately calculate the position of touch.

Resistive touch screen is controlled by pressure sensing. The main part is a resistor film screen which is very compatible with the display surface, this is a multi-layer composite film, with a layer of glass or hard plastic plate as the base, coated with a transparent metal oxide (ITO) conductive layer, which is covered with a layer of surface hardening, smooth scratch-resistant plastic layer ( There is also a layer of ITO coating on the inner surface, and there are many small (about 1/1000-inch) transparent intervals between them that insulate the two layers of Ito conductive layer. When the finger touches the screen, the usual insulation of the two layers of conductive layer in the touch point position has a contact, because one side of the conductive layer connected to the y-axis of the 5V uniform voltage field, so that the detection layer of voltage from 0 to Non-zero, the controller detected this connected, A/D conversion, and the resulting voltage value compared to 5V can get the point of the y-axis coordinates, the same as the x-axis coordinates, which is all resistance technology touch screen common principle.

Fig. 3.2 Capacitive touch-screen

3, Resistance technology touchscreen (resistive touch)

　　
The key of resistance type touch screen is material technology. The commonly used transparent conductive coating materials are:

①ito, indium oxide, weak conductive, characteristic is when the thickness drops to 1800 (aye = 10-10 meters) will suddenly become transparent, light transmittance is 80%, and then thin down the light transmittance, and then to 300 when the thickness rose to 80%. Ito is the main material used in touch screen and capacitive technology of all resistance technology, in fact, the resistance and capacitance technology touch screen face is ito coating.

② Nickel Gold coating, five-wire resistive touch screen of the outer conductive layer is a good ductile nickel-metal coating materials, the external conductive layer due to frequent touch, the use of ductile nickel gold materials to prolong the service life, but the process cost is more expensive. Nickel-Gold conductive layer, although the ductility is good, but only for transparent conductor, not as a resistance touch screen face, because of its high conductivity, and the metal is not easy to achieve a very uniform thickness, it is not suitable for the voltage distribution layer, only as a layer.

Fig. 3.3 Touch screen of resistance technology

1), four-wire resistive touch screen

Touch screen attached to the surface of the display, with the display, if you can measure the touch point on the screen coordinates position, you can according to the display on the display of the corresponding coordinates points or graphic characters to know the intention of the touch. The resistive touch screen is used more in the embedded system. Resistance Touch screen is a 4-layer transparent composite film screen, the bottom is the glass or plexiglass composition of the base, the top is a layer of external surface hardened to smooth scratch-resistant plastic layer, the middle is a two-layer metal conductive layer, respectively, on the base and plastic layer inside the surface, There are many small transparent isolation points separating them between the two conductive layers. When the finger touches the screen, the two conductive layers are contacted at the touch Point. The two metal conductive layer of the touch screen is the two faces of the touch screen, at each end of each working face coated with a silver glue, called the face of a pair of electrodes, if the electrode on a face on the application of voltage, then in the face will form a uniform continuous parallel voltage distribution. When a certain voltage is applied to an electrode in the x direction, when the Y-directional electrode is not added to the voltage, in the X parallel voltage field, the voltage value at the contact point can be reflected on the y+ (or y) electrode, and the X coordinate value of the contact point can be learned by measuring the voltage of the y+ electrode to the ground. Similarly, the y-coordinate of the contact point can be obtained by measuring the voltage of the x+ electrode when the voltage is added to the Y-electrode and the X-electrode is not added to the voltage.

Figure 3.3.1 14 inch (16:9) four wire resistive touch screen design drawing
　　

Four-wire resistance touch screen defect:
The B side of the resistive touch screen is often touched, and the B side of the four-wire resistive touch screen is ito, as we know, Ito is a very thin metal oxide, in the course of use, will soon produce a small crack, and once the crack is produced, the original flow through the field of the current forced to crack the line, the distribution of the voltage should be evenly distributed with the damage, Touch screen has a damage, the performance of the crack at the point is not allowed. As the cracks increase and increase, the touch screen will slowly become ineffective, so the service life is not long is the main problem of four-wire resistive touch screen.
　　

2), five-wire resistive touch screen

Five line Resistance technology touch screen of the base to the two-direction voltage field through the precision resistor network are added to the glass conductive face, we can simply understand that the two-direction voltage field time-sharing work added to the same working face, the outer layer of nickel-gold conductive layer is only used as a pure conductor, with a touch of time to detect the inner layer ITO Contact point x and y-Axis voltage value method to measure the position of the touch point. Five-wire resistive touch screen in the inner layer ITO need four leads, the outer layer only as a conductor only one, touch screen led a total of 5. Another proprietary technique of the five-wire resistive touch screen is to correct the linear problem of the inner ITO by a sophisticated resistor network: The distribution of the voltage is uneven due to the possible uneven thickness of the conductive coating.
　　

Figure 3.3.2 15 inch (4:3) five wire resistive touch screen design drawing

Resistance screen performance characteristics:
① they are a completely isolated work environment, not afraid of dust, water vapor and oil.
② can use any object to touch, can be used to write and draw, this is their relatively large advantage.
③ resistance touch screen accuracy depends only on a/D conversion accuracy, so can easily reach 2048*2048, compared to the five-line resistance than the four-wire resistance in order to ensure resolution accuracy is also superior, but the cost of large, so the price is very high.

Five-wire resistance touch screen Improvement:
First, the five-wire resistive touch screen of a surface is conductive glass rather than conductive coating, the process of conductive glass makes a surface life is greatly improved, and can improve the transmittance. Second, the five-wire resistive touch screen to the face of the task to the long life of a face, and B is used only as a conductor, and the use of ductility, low resistivity nickel-gold transparent conductive layer, therefore, B surface life is also greatly improved.
Another proprietary technique of the five-wire resistive touch screen is to correct the linear problem of a plane by using a sophisticated resistor network: Because of the uneven distribution of the voltage field due to the inevitable uneven thickness of the process engineering, the precision resistor network flows through most of the current while working, so it can compensate for the possible linearity distortion of the working face.
Five-wire resistive touch screen is currently the best resistance technology touch screen, the most suitable for military, medical, industrial control in the field of use. 4, Surface acoustic wave touch screen (surface acoustic wave)

The touchscreen part of the surface acoustic touch screen can be a flat, spherical or cylindrical glass plate, mounted in front of the CRT, LED, LCD or Plasma display screen. This glass slab is just a piece of pure tempered glass, unlike any other touch screen technology that has no foil and overlay. The top left corner of the glass screen and the lower right corner each fixed the vertical and horizontal direction of the ultrasonic emission transducer, the upper right corner fixed two corresponding ultrasonic receiver transducer. The four periphery of the glass screen is engraved with a very precise reflective stripe at a 45° angle from sparse to dense intervals.

The transmitter converts the electrical signals sent by the controller through the touchscreen cable into the acoustic energy and transmits to the left surface, the acoustic energy is then reflected by a set of fine reflective stripes on the bottom of the glass to pass through the upward homogeneous surface, and the acoustic energy passes through the surface of the screen, and then the right line is propagated to the receiving transducer of the x-axis by the reflection fringes above, The receiving transducer converts the returned surface acoustic energy into an electrical signal. The waveform of the receiving signal is exactly the same as the reference waveform when the signal is not touched. When the fingers or other objects capable of absorbing or blocking the acoustic energy touch the screen, the sound wave energy that goes up through the finger part of the x-axis is partially absorbed, and a attenuation gap is generated in the waveform at a given moment in the receiving waveform. Receive waveform corresponding to the finger block signal attenuation of a gap, the calculation of the gap position is the touch coordinates, the controller analysis to receive the signal attenuation and the location of the gap to determine the x coordinates. The y-coordinate of the touch point is then determined by the same process of the y-axis. In addition to the general touch screen can respond to the x, y coordinates, the surface acoustic wave touch screen also responds to the third axis z-axis coordinates, that is, can perceive the user touch pressure size value. Once the three axes are determined, the controller passes them on to the host.

Surface Acoustic wave touch screen is not affected by temperature, humidity and other environmental factors, high resolution, excellent scratch resistance, long life (50 million times without fault), high transmittance (92%), to maintain clear and translucent image quality; no drift, most suitable for use in public places. However, the induction converter of the surface sensing system will be damaged by the pressure of sound energy during long operation. General wool or leather gloves will receive part of the sound waves, the accuracy of the induction is also affected by certain. The fingers on the screen surface or touch screen, such as water stains, grease stains, dirt or dust, can also affect their performance and even stop the system from operating.

Fig. 3.4 Surface Acoustic Wave touch screen

Iv. Related Documents

Resistive touch-screen principle. pdf:http://download.csdn.net/detail/luckydarcy/9674972