Valuable materials-development history of TOUCH Technology

Compared with the traditional resistive touch technology with the highest market share, capacitive touch technology brings many advantages to users, including: with a penetration rate of up to 97% and a more authentic color presentation, we can enjoy a better visual experience. To achieve the touch function, we only need to touch or even not actually touch the screen, it provides users with more ease and flexibility. With a longer service life, the touch life of the capacitive screen is about 0.2 billion times, which is 1 million times that of the four-line resistance screen (two hundred times) and the five-line resistance screen (40 million times) five times.

Capacitive touch technology detects a small amount of changes caused by a touch. Depending on the working principle, the surface capacitive touch technology (SCT, surface capacitive touch) and projection capacitive touch technology (PCT, projected capacitive touch) can be roughly divided ). The former is common in outdoor applications of large sizes, such as public information platforms (POI) and Public Service (sales) platforms (POS, while the latter is excited by Apple's multi-touch iPhone.

From the perspective of the development of the touch technology, the market that first introduced the touch technology was the industrial control field, and its purpose was to control the complex and large mechanical equipment, integrated into a single window, multi-page screen, the use of large and large sizes of resistance screen. However, the defects such as insufficient life and tolerance of the Resistance screen cannot meet the needs of the industrial control field. Therefore, when the large-scale SCT was launched, the high-end equipment was immediately switched to the SCT solution. Until around 2003, due to the reduction in the manufacturing cost of the Resistance screen, small sizes were applied to portable products such as PDA and GPS, and the touch technology entered the consumer market. In 2006, the iPhone adopted the small-size PCT, and its excellent optical characteristics and multi-point touch functions set off a storm, making it the most eye-catching touch technology in recent years.

 

From the above, it is not difficult to find that the consumer market with small size as the mainstream only has two options: Resistance Type and projection Capacitive Type. Although the former is cost-effective, however, poor optical performance and tolerance have long been criticized by the market. Although the latter has many advantages, there are only a few suppliers of real energy production, and the price is naturally quite expensive, so it is only seen in a few high-unit-price products.

 

At present, the small-size market seldom uses SCT, mainly due to cost issues. SCT panel manufacturers have long lacked key optical coating technologies and must be outsourced for external processing, while SCT touch IC is controlled by a few technical vendors and the prices remain high. In addition, unlike the resistive screen, which can be used with a resistive IC at will, the SCT screen and IC must have excellent compatibility to work stably. These factors make the price of the Small-size consumption application of SCT not far from that of the PCT, And it is naturally difficult for the customer group to adopt it.

However, compared with the Resistance Technology, SCT can greatly improve its defects. Compared with PCT, The SCT technology is more mature and stable, and can be imported in mass production. Therefore, we can reasonably infer that when the overall cost of SCT decreases significantly due to the integration of strategic alliances and technical resources of industrial members, SCT will have the opportunity to become the best solution for small-size consumption applications.

The following describes the basic principles of PCT and SCT, and compares the advantages and disadvantages of these two technologies.

Projection capacitive touch technology

PCT is built on the concept of matrix. In Touch Screen production, the ITO of the PCT panel is etching to produce a specific pattern, in order to improve the SNR value of each touch point and enhance the recognition accuracy. By copying the preceding pattern several times in the X and Y axes respectively (the number of times varies depending on the screen size), a PCT matrix similar to the keyboard is formed, figure 1 shows the most common diamond pattern.

Figure 1: diamond pattern layout of the projected capacitive screen.

 

The orange diamond pattern in Figure 1 forms the ITO wire (m in total) in the X axis direction, while the green diamond pattern forms the ITO wire in the Y axis direction (N in total ); the PCT controller will drive these wires in sequence to detect changes in electrical capacity due to touch.

Figure 2: PCT equivalent RC Circuit and the waveform detected on the X2 wire before and after the finger touch.

 

Take the simplest RC oscillator scheme as an example. the equivalent circuit of the X2 wire in the X axis is simplified to Figure 2 to form an RC circuit consisting of n RP and n cp, the RP and CP represent the inherent capacitance values of the equivalent Ito segment internal resistance and each PCT node (xy Axis Intersection. When the finger approaches or comes into contact with the screen, an electrical capacity (CF) will be added to the screen. For this RC oscillating circuit, the appearance of CF means that the oscillation cycle gets longer and the frequency decreases. By calculating the oscillation cycle and frequency change on the X2 wire before and after the finger touch, the PCT controller can identify the touch position, it can even tell the distance between the finger and the screen (that is, provide the Z axis information ).

Surface capacitive touch technology

The SCT panel is an even coated ITO layer. Each of the four corners of the panel has an egress (UR, UL, LR, LL) connected to the SCT controller. In order to detect the exact position of the touch point, the SCT controller must first create an even Electric Field on the SCT panel, which is achieved by charging the panel with the driving circuit inside the IC. When the finger touches the screen, it will lead to a trace of current flow. At this time, the sensing circuit in the IC will separately parse the four online electrical flows, calculate the XY coordinate of the touch point according to the formula in figure 3. To overcome the interference, hardware filters or software filters can be used to process the calculated coordinate values.

Figure 3: Schematic diagram of the cell bar of the SCT panel and controller.

The biggest difference between PCT and SCT is that PCT has the opportunity to achieve multi-touch, while SCT can only achieve single-touch, PCT seems to be better than SCT, but in fact it is not exactly the case.

Market Status of capacitive touch technology

The working principle of PCT is not complex, so it is not difficult to provide a prototype demonstration. However, when engineers try to convert the PCT Demo set into a mass production plan and prepare a large number of copies, various technical challenges are coming soon. The main challenges include the following three aspects.

The weak signal to be detected is vulnerable to environment instability, resulting in inconsistent sensitivity of the touch function and even incorrect operations. The capacitive changes caused by the touch are very likely to be affected by the temperature and humidity. Currently, the common countermeasure is to use timed automatic calibration to overcome this problem. In addition, other electrical appliances or other functions of the product itself (such as mobile phone RF) that are being used will interfere with signal measurement. This part has to be achieved through improving SNR in the form of software or hardware; so far, the icdesign industry still has room for effort in this regard.

The quality of mass production needs to be improved (to make it rough). The PCT technology has the advantage of multi-touch. Therefore, since the advent of the iPhone, almost all focus has been on the PCT, the Touch industry, including touch screen manufacturers, icdesign companies, and solution developers, have invested a lot of resources for development. However, up to today, there are still not many PCT products in the market, and the problem is that the overall production yield is still low, this causes high costs. The reasons for the low yield are: the current PCT controller is still not mature enough, and the self-adjustment capability in different application environments is still insufficient. Therefore, the touch menu is unstable, in this case, we have to strictly limit the characteristics of the PCT Panel (such as the resistance value and capacitance value in the Panel) to reduce variables. Due to the limitation of the PCT controller, strict requirements on PCT panel feature consistency are often the main cause of low yield.

Patent problems (made in rough song) because many basic gestures and multi-finger touch functions have been applied for by some manufacturers for patents and are recognized, the multi-point advantage of PCT products is not available in many application fields (especially the main mobile phone market! Overall, the PCT solution will certainly have a place in the market in the future because of its multi-touch advantage. However, in the current situation, it is still not a mature solution.

In contrast, although the controller design of the SCT technology is also faced with the problem that capacitive signals are susceptible to interference, the SCT solution has been widely used in various environments through the continuous efforts of the icdesigner. For example, the SCT touch IC (wt5750f) jointly developed by weiyun electronics and Wanda Opto Electronics can easily overcome the differences between external environments and touch screen sizes by various adjustment mechanisms embedded in the IC, stable and high-quality Touch Experience. In addition, in order to meet the requirements of handheld devices for the miniaturization of the Mechanism, weiyun electronics's SCT touch IC has made great efforts to improve product integration, therefore, it can meet the needs of both miniaturization and cost reduction.

 

In addition to the IC, The SCT touch screen structure is simpler than that of the PCT, and it is easier to overcome the noise interference problem on the hardware. In addition, by improving the process and making breakthroughs in key technologies, the overall SCT solution has a competitive cost advantage comparable to the resistance type. Taking the optical coating technology independently developed by Wanda optoelectronics as an example, the light transmittance of the touch screen can be increased from 87% to 97%, and the reflectivity is reduced from 12% to 2%. More importantly, the optical processing yield can be controlled at a rate of more than 9, and outsourced processing steps are omitted to improve technical autonomy and production capacity, and the manufacturing cost can also be greatly reduced.

 

The SCT technology is one of the capacitive touch technologies, so it also has many advantages of Capacitive: "Better visual enjoyment", "easier and flexible handling", and "longer service life; coupled with the fact that SCT has relatively loose requirements for touch screen features than PCT, and its mass production technology is more mature, it also has the advantage of high yield and low cost. In addition, in actual operations, the user's need for single point selection, single point gesture and handwriting functions is more urgent than multi-touch ), these functional requirements happen to be provided stably by SCT at present. Therefore, before the PCT solution is mature (with stable functions, patent problems, high production yield and low cost), The SCT solution is a touch solution that is very suitable for the market.

Davide Giaco mini

Valuable materials-development history of TOUCH Technology)