Working and Types of Touch Screen

Catalog

Ⅰ Background of touch screen

Dr. Samuel Hurst invented a touch sensor in 1971. This sensor is the prototype of a touch screen. Three years later, he designed the first transparent touch screen. In 1977, touch screen technology was greatly improved, and it is still widely used and developed rapidly to this day. In the earliest days, touch screens often appeared in a high-end form. The earliest touch screen that we can trace back to be used on mobile devices should be Motorola's A6188 in 1999. The emergence of this model has completely changed everyone's concept of mobile phone operation. Of course, the more important significance should be the introduction of handwriting technology into the mobile phone field, and the intelligent function of mobile phone integrated PDA has also begun to appear.

Ⅱ Working principle of touch screen

1.Traditional four-wire resistive touch screen technology

Early mobile phone touch screen technology, Motorola A6188 mobile phone adopts the traditional "analog four-wire resistive touch screen" technology. This touch screen is composed of two layers of glass and plastic coated with transparent conductive material. The surface touched by the finger is a hard coating. To protect the PET (polyester film) layer underneath, there are two transparent conductive layers ITO (indium oxide, weak conductor) between the surface protection hard coating and the glass bottom layer, corresponding to the X and Y axes, and between them, the fine and transparent insulating particles are insulated, and the pressure generated by the touch will make the two conductive layers connect. When pressing different points, the resistance value from the point to the output terminal is also different, and output the voltage signal corresponding to the position of the point (analog ). The coordinate values of X and Y can be obtained after A/D conversion. This is the most basic principle of resistive technology touch screens. This type of technology is currently mature because of its low price and ease of production. It is now used in low-end mobile phones.

2.TOUCH LENS technology

The traditional mobile phone resistive touch screen is assembled with the mobile phone case, which has a concave and convex surface and the structure is not sealed. Nowadays, the more front-end application in the market is a touch screen using TOUCH LENS technology. It has now been widely recognized and applied, with Apple iPhone as the main driving force. It is divided into the resistive type and capacitive type. The iPhone uses capacitive technology. Previously, the resistive type is more widely used in the market, and its working principle is the same as that of a traditional resistive touch screen.

The main features of TOUCH LENS: (1) The surface of the touch panel and the phone case is completely flat, sealed in structure, and dustproof; (2) Irregular shapes can be processed to make the appearance of the phone more beautiful; (3) Handwriting is smooth, It feels comfortable, the screen surface is clean, the appearance is beautiful, the material is excellent, and it is not easy to break; (4) Because the upper and lower electrode layers are all membrane structures, the thickness is thinner than traditional touch screens, which has advantages in structural design.

Ⅲ Technological development of touch screen

Regardless of the traditional four-wire resistive touch screen or TOUCHLENS structure, the above mobile phones can only be touched by a single point, which cannot satisfy the rich touch action experience. The hot multi-touch technology promotes the further development of resistive touch screens. Today, when capacitive screens are popular, resistive touch screen solutions still occupy a place in the market due to their inherent simplicity, low cost, and support for multiple input media (conductor and non-conductor). Compared with capacitive touch screen solutions, the durable and multi-touch are the two major weaknesses of resistive screens. However, one of the technical problems-multi-touch has been a breakthrough. The current multi-touch applications of resistive screens are explained below. The current resistive multi-touch technology can be roughly divided into three types: digital matrix resistance DMR, analog matrix resistance AMR, and five-wire multi-point resistance MF.

1.Analog matrix resistance AMR technology

AMR is to etch blocks arranged in parallel on the ITO layer along with the X and Y directions, which is equivalent to dividing the entire touch screen into many small matrix blocks. Each small matrix is equivalent to a small analog four-wire resistive touch screen. Each block is independent of the other. When the finger presses on the corresponding block, the block will send out the corresponding proportional voltage, and the controller will translate it into coordinate information after receiving the voltage. The method of using the four-wire resistive touch screen to realize the multi-touch technology: at the first moment, apply a voltage to the X1 electrode, and the Y1, Y2, and Y3 electrodes read the X coordinates detected by the A, B, and C touch units; In the same way, read the X coordinates of the remaining touch units in sequence at each subsequent time. After obtaining the X coordinates of all touch units, voltage is applied to the Y electrodes in turn to obtain the Y coordinates of each touch unit.

The analog matrix resistance AMR is different from the pure digital DMR technology multi-touch screen system. AMR is a digital-analog hybrid system. Therefore, on the basis of the scanning circuit, AD conversion circuit, and control circuit, various auxiliary components need to be added to reduce the interference of external noise on the analog circuit. Especially for AD conversion, in order to improve the accuracy of the conversion, it is necessary to add a pull-down resistor to the hardware circuit to avoid the phenomenon that the AD input terminal is floating when no touch occurs. The control circuit will control the scan circuit to generate the appropriate scan signal, and make the AD conversion circuit perform data sampling and conversion at the appropriate time. For the AD conversion circuit, you can make a trade-off between serial conversion and parallel conversion. The serial conversion structure is simple, the number of AD modules required is small, but the total conversion frequency is low; the number of AD modules required for parallel conversion is slightly more, but the total conversion frequency can be improved. So the basic circuit architecture can be divided into serial and parallel.

2.Capacitive touch screen

The capacitive 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 protective layer of silica glass with a thickness of only 0.0015mm. The interlayer ITO coating is used as the working surface, and the four corners lead out four electrodes. The inner layer of ITO is the screen layer to ensure the working environment. When the user touches the capacitive screen, due to the electric field of the human body, the user's finger and the working surface form a coupling capacitor. Because the working surface is connected to a high-frequency signal, the finger absorbs a small current, which flows from the four corners of the screen. The current flowing through the four electrodes is theoretically proportional to the distance from the fingertip to the four corners. The controller calculates the position of the four current ratios accurately.

3.Infrared touch screen

The infrared touch screen uses the densely distributed infrared matrix in the XY direction to detect and locate the user's touch. The infrared touch screen installs a circuit board frame in front of the display. The circuit board is arranged with infrared emitting tubes and infrared receiving tubes on the four sides of the screen, forming a one-to-one correspondence. When the user touches the screen, the user's finger will block the two infrared rays passing through the position. Therefore, it can be judged that the touchpoint is on the screen. Any touch object can change the infrared rays on the contact point to realize the early operation of the touch screen.

In concept, infrared touch screens have technical limitations such as low resolution, limited touch methods, and susceptibility to environmental interference and misoperation, so they were once out of the market. After that, the second-generation infrared screens partially solved the problem of anti-light interference. The fourth generation and the fourth generation have also improved in terms of resolution and stability, but they have not made a qualitative leap in key indicators or overall performance. However, everyone who knows touch screen technology knows that infrared touch screens are not interfered with by current, voltage, and static electricity, which is suitable for harsh environmental conditions. Infrared technology is the final development trend of touch screen products.

Infrared touch screen

Touch screens using acoustics and other material science technologies have their insurmountable barriers, such as the damage and aging of a single sensor, and the touch interface is afraid of pollution and destructive use. As long as the infrared touch screen achieves high stability and high resolution, it will replace other technical products and become the mainstream of the touch screen market. The resolution of infrared touch screens in the past is determined by the number of infrared pairs in the frame, so the resolution rate is low. In addition, infrared screens are more sensitive to light environment factors and will misjudge or even crash when the light changes greatly.

The weakness of infrared screens and the latest technology, the resolution of the fifth generation infrared screen depends on the scanning frequency of the infrared pair of tubes and the difference algorithm. The resolution has reached 1000X720. As for the infrared screen is unstable under light conditions, since the second generation of infrared touch screens, it has overcome the weakness of anti-light interference. The fifth generation of infrared touch screens is a new generation of intelligent technology products, which realizes 1000*720 high-resolution multi-level self-adjustment and self-recovery hardware adaptability and highly intelligent identification. It can be used in various harsh environments for a long time and can be customized for users with extended functions, such as network control, sound sense, human proximity sensing, user software encryption, protection of infrared data transmission, and other original media propaganda infrared touch screens. One of the main disadvantages is poor riot resistance. In fact, the infrared screen can use any riot glass that the customer thinks is satisfactory without increasing the cost and affecting the performance. This is something other touch screens cannot imitate.

4.Surface acoustic wave touch screen

The transmitting transducer converts the electrical signal sent by the controller through the touch screen cable into sound wave energy and transmits it to the left surface, and then a set of precision reflection stripes under the glass plate. The sound wave energy is reflected into an upward uniform surface for transmission. The sound wave energy passes through the surface of the screen, and then is gathered into a rightward line by the upper reflection fringe to propagate to the X-axis receiving transducer, and the transducer will turn the return energy of the surface acoustic wave into an electrical signal. After the transmitting transducer emits a narrow pulse, the sound wave energy reaches the receiving transducer through different paths. The one on the far right arrives first, the one on the far left arrives at the latest, the early arrival and the late arrival. The sound wave energy is superimposed into a wider waveform signal.

It is not difficult to see that the received signal is a collection of all the sound wave energy-returning through different long and short paths in the X-axis direction. The distance they travel on the Y-axis is the same, but on the X-axis, the farthest one travels twice the X-axis maximum distance than the nearest one, so the time axis of this waveform signal reflects the position before each original waveform is superimposed, that is, the X-axis coordinate transmit signal and receive signal waveforms are received when they are not touched. The waveform of the signal is exactly the same as the reference waveform.

When a finger or other object that can absorb or block the sound wave energy touches the screen, the sound wave energy going up on the X axis through the finger part is partially absorbed, which is reflected in the received waveform, that is, the waveform at a certain moment. There is an attenuation notch. The received waveform corresponds to the signal at the part blocked by the finger. A notch is attenuated. The touch coordinate is calculated by the notch position. The controller analyzes the attenuation of the received signal and determines the X coordinate by the notch position. The Y-axis determines the touchpoint in the same process In addition to the XY coordinates that the general touch screen can respond to, the surface acoustic wave touch screen also responds to the third axis Z coordinate, that is, it can sense the magnitude of the user's touch pressure. The principle is to calculate the attenuation at the received signal attenuation.

5.MTK touch screen

MTK resistive four-wire touch screen principle

The touch screen is attached to the surface of the display and used in conjunction with the display. If the coordinate position of the touchpoint on the screen can be measured. The toucher's intention can be obtained according to the display content or icons of the corresponding coordinate point on the display screen. Among them, resistive touch screens are used more in embedded systems. The resistive touch screen is a 4-layer transparent composite film screen. The bottom layer is a base layer made of glass or plexiglass. The top layer is a plastic layer whose outer surface is hardened to make it smooth and scratch-resistant. In the middle are two metal conductive layers. On the base layer and the inner surface of the plastic layer, there are many small transparent isolation points between the two conductive layers to separate them.

When a finger touches the screen, the two conductive layers touch at the touchpoint. The two metal conductive layers of the touch screen are the two working surfaces of the touch screen. A silver glue is coated on both ends of each working surface, which is called a pair of electrodes on the working surface. If a voltage is applied to a pair of electrodes on a working surface, a uniform and continuous parallel voltage distribution will be formed on the working surface. As shown in Figure, when a certain voltage is applied to the electrode pair in the X direction and no voltage is applied to the electrode pair in the Y direction, in the X parallel voltage field, the voltage value at the contact can be reflected on the electrode at Y+(or Y -), the X coordinate value of the contact can be obtained by measuring the voltage of the Y+ electrode to the ground. Similarly, when a voltage is applied to the Y electrode pair and no voltage is applied to the X electrode pair, the Y coordinate of the contact can be obtained by measuring the voltage of the X+ electrode.

6.Resistive touch screen

The resistive touch screen structure

A resistive touch screen is a sensor that converts the physical position of the touchpoint (X, Y) in a rectangular area into voltages representing X and Y coordinates. Many LCD modules use resistive touch screens. This screen can use four, five, seven, or eight wires to generate screen bias voltage and read back the voltage at the touchpoint. The resistive touch screen is basically a structure of thin film and glass. The adjacent surfaces of the thin film and the glass are coated with ITO (Nano Indium Tin Metal Oxide) coating. ITO has good conductivity and transparency. When touching operation, the ITO of the lower layer of the film will contact the ITO of the upper layer of the glass, and the corresponding electrical 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 the calculation to complete the point. The selected action is displayed on the screen.