Detailed description of the classification, principles and features of the touch panel

Touch technology as a key interaction method for smart phones brings us a lot of convenience. So do you know the concept, characteristics, classification and principles of touch panels? This article will give you a comprehensive introduction to the basics of touch panels and hope to help you.

What is a touch panel?

The touch panel is also called Touch Panel, or Touch Screen, or Touch Pad, etc.. All electronic devices use the screen. If you don't want your screen to be occupied by a boring keyboard, you must use the touch screen. As the medium of human-machine dialogue, touch screen is the latest computer input device. It is the simplest, convenient and natural human-computer interaction method. It gives multimedia a new look and is an attractive new multimedia interactive device.

The touch panel was first introduced in 1965 by EA Johnson, a short article describing the capacitive touch screen, and then in 1967, there was an article with a truth. In 1970, two transparent engineers, invented by two engineers at the CERN (European Council for Nuclear Research) in the early 1970s, were put into use in 1973. Then in 1975, an American George Samuel Hurst invented the resistive touch panel and obtained the US patent (#3, 911, 215), and commercialized in 1982.

1. Technical points of the touch panel:

From the technical point of view, the touch screen is a transparent absolute positioning system. First, it must be transparent. Secondly, it is absolute coordinates. Which finger is the touch, unlike the mouse, a cursor is required for relative positioning, so it is very It's easy to distract, because you have to pay attention to where the cursor is.

The structure is usually covered on a semi-reflective liquid crystal panel (ITO transparent conductive electrode), which is highly sensitive to pressure. When an object is pressed on it, a current signal is generated and a pressure source is determined. Location and dynamic tracking. This is the on-cell technology that we report in the media. Now, the In cell Touch touch component is integrated into the display panel, so that the panel itself has a touch function, and the touch effect and application can be achieved without the need for additional bonding and assembly with the touch panel, mainly Apple is researching.

Next, we mainly introduce the differences and principles of different touch technologies from transparency and positioning methods.

2. Classification and principle of touch panel:

From the technical principle to distinguish the touch screen, it can be divided into five basic categories: vector pressure sensing technology touch screen, resistance technology touch screen, capacitive technology touch screen, infrared technology touch screen, surface acoustic wave technology touch screen. Among them, the vector pressure sensing technology touch screen has withdrawn from the historical stage; the infrared technology touch screen is inexpensive, but its outer frame is fragile, easy to produce light interference, and the surface is distorted; the surface acoustic wave touch screen almost solves all kinds of defects of the touch screen, clear and not It is easy to be damaged and suitable for various occasions. The fatal disadvantage is that if there are water droplets and dust on the surface of the screen, the touch screen will become dull or even not working, so it is difficult to popularize. The following mainly talk about resistive and capacitive screens.

Resistive: It's good to use!

The resistive touch panel is mainly composed of two film substrates coated with ITO (indium tin oxide) on one side, and the upper and lower plates need to be filled with transparent elastic spacers to separate them. It is shown that the lower plate must be rigid thick glass to prevent deformation, while the upper plate needs to induce external force to deform, so it is necessary to explode glass or plastic.

During normal operation, the upper and lower plates are connected to the voltage and are in the off state. When the external force is pressed, the upper plate is deformed and the lower plate is in contact with the lower plate. At this time, a voltage change is generated, and the touch point coordinates can be accurately measured by the voltage change ( Because the upper and lower plates are touched by the upper and lower plates, the upper and lower plates are changed into a resistor divided into two, and the value of the resistance value is proportional to its distance from the edge to calculate the X and Y coordinates. Therefore, the accuracy of the resistive touch screen mainly depends on the conversion accuracy of this coordinate voltage, so it is very dependent on the accuracy of the A/D converter (voltage sensitivity of the magnitude).

Because the resistive screen is controlled by pressure, it does not have to be controlled by hand. Pens, credit cards, etc. can be operated. Even if it is worn, it does not matter, and it is isolated from the outside world, so it has the advantage of dust and dirt; If the "touch" is too light, the resistive screen will not respond, so you should use a light stamp. The resistive screen has low cost and low technical threshold. Moreover, when the resistive touch screen is operated, it needs to be tapped, so it is easy to be bad, and the sensitivity is not so good, and the drawing and writing are not smooth.

Capacitive: Enjoy light and smooth!

However, what really brings the trend of smartphones is the capacitive touch screen, which consists of a glass substrate coated with a conductive film on both sides, and is covered with a thin layer of SiO2 dielectric on the upper plate. As shown in the figure, the upper electrode is used to form a plate capacitance sensing capacitance change with the human body (ground), and the lower plate is used to shield external signal interference.

During operation, the upper transparent electrode needs to be connected to the voltage and draw four electrodes at the four corners. Therefore, when the finger touches the upper SiO2 layer, since the human body is electrically conductive, sufficient coupling capacitance is generated between the human body and the upper transparent electrode. And calculate the touch position coordinates according to the change of the capacitance value measured with the four corners (or the periphery) (the closer the touch position is, the larger the capacitance). However, this surface capacitive touch (Surface CapaciTIve) is still unable to meet the popular multi-touch. If you want to achieve multi-touch, you must use a new technology called Projected-CapaciTIve Touch, which mainly changes the surface of the sensing electrode. Laying in one or two layers and patterning (mainly diamonds), one layer is responsible for the X direction and one layer is responsible for the Y direction. It is then positioned by a change in the electrode capacitance in the X and Y directions.

Since the mainstream is now multi-touch (MulTI-Touch), I will talk a little bit about his evolution. There are two main types of multi-touch Projected CapaciTIve: Self-Capacitive and Mutual. Capacitive). Self-capacitance It scans the electrode capacitance of each X and Y directly, so when two touch points are used, two additional ghost points (Ghost Points) will be generated. As shown in the figure, the left side is a graphical representation of the two-layer electrode (multiple For the diamond shape, it requires only one ITO layer to form the X and Y electrodes by photolithography. On the right side is the schematic diagram. From the schematic diagram, when touching (X2, Y0) and (X1, Y3) at the same time, since the capacitance of the four electrodes is measured, two extra points (X1, Y0) and two extra points are added. (X2, Y3), this is Ghost Points, which can only be solved by software. Although the self-capacitance has the problem of Ghost-Points, the self-capacitance position has high precision and sensitivity. The biggest advantage is that it can be used as a Single layer ITO film. However, when the size is large ("15"), the number of pins increases, resulting in an increase in the number of pins. Very high, and after the number of points, the middle line will not go out, the ITO must be thinned, so the resistance will increase, and the number of scans will increase. It seems that there is no advantage, but now the Apple phone seems to be walking. Self-capacitance touch technology, these technologies should have broken through.

Mutual-Cpacitive or Trans-Capacitive requires two layers of ITO film. The X and Y electrodes are separated at each node by a special structure, so that it scans the Intersection capacitor. Not the electrode capacitance. Only the contact of the two layers of ITO at the intersection must be separated, and it is necessary to use MEMS technology to bridge it like an interchange.

Both self-capacitance and mutual capacitance rely on the introduction of capacitance from the body capacitance to the electrode, so these two technologies are called charge-transfer capacitive touch (Charge-Transfer).

The advantage of capacitive touch is that it is fast and can be used without slipping. However, it can only be manipulated with conductive objects. It has the disadvantage that if the touch area is large (palm), you may have an action before you touch it. Because the large coupling capacitance is large, the screen is triggered, so it It is sensitive to electric field changes caused by external electric field or temperature and humidity. However, it is a layer of glass plate structure, so the transmittance is up to 90% higher than the resistance type.

However, it is difficult to achieve a uniform electric field for both resistive and capacitive touch screens, so it can only be used for panel sizes up to 20 inches. If you want to make large-screen touch, you must use wave touch technology (mainly surface acoustic wave or infrared wave). It mainly installs infrared or acoustic transmitter/receiver at the four corners or edges. When the touch blocks the sound wave or In the case of infrared rays, the corresponding receiver can not determine the coordinates when it receives no signal. This touch screen is afraid of dirty, afraid of oil, too delicate, and is easily affected by environmental fluctuations.

Each type of touch screen has its own advantages and disadvantages. To understand which touch screen is suitable for which occasion, the key is to understand the working principle and characteristics of each type of touch screen technology.

3. The circuit part of the touch panel:

I spent most of the time on the principle and structure of the sensor module of the touch panel, but like the Sensor (touch is also a kind of Sensor), it has a circuit part in the sensing part, and the circuit part of the touch is mainly responsible. Things are: signal detection, coordinate positioning, and gesture recognition (sliding/zooming/clicking).

For the MCU circuit, which circuit units are mainly needed, the most important one is the ADC (this is necessary for all Sensors), followed by Scan Control and DSP (signal processing), and the scanning circuit must require the clock signal, so the Timer is required. . Gesture recognition is achieved by a unit called Finger Tracking. Finally, the code of User configure needs to use EEPROM or Flash.

There are two main difficulties in design: 1) high resistance and increased capacitance, and 2) noise coupling (Noise Coupling). The former is mainly due to the increasing size of the screen, so that the ITO electrode is getting longer and longer, so the resistance is getting larger and larger, and the electrode is getting longer and longer, so that the capacitance area is getting larger and larger, so the capacitance is also larger. The final problem is RC-delay. Extend, and the solution is either to speed up the scan by increasing the voltage, or to change the metal wiring (Ag). The latter is mainly due to the larger and larger panels. It is easy to receive the interference of environmental noise mainly by shielding, and the other interference comes from the interference of the switching power supply. This can only be achieved by adding Noise Cancellation before the ADC. achieve.

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