The hottest surface capacitive touch technology pr

Surface capacitive touch technology promotes the new revolution of man-machine interface

compared with the traditional resistive touch technology with the highest market share at present, capacitive touch technology has brought many advantages to users, including: as high as 97% penetration and more realistic color presentation bring us better visual enjoyment; The realization of touch function requires only a touch, or even no actual contact with the screen, which brings users more relaxed and flexible control; With a longer service life, the touch life of the capacitive screen is about 200 million times, 200 times that of the four wire resistive screen (onemillion times) and five times that of the five wire resistive screen (40 million times)

the signal detected by capacitive touch technology comes from small changes caused by touch. According to different working principles, it can be roughly divided into surface capacitive touch (SCT) and projected capacitive touch (PCT). The former is commonly used in large-scale outdoor applications, such as public information platform (POI) and public service (Sales) platform (POS) and other products, while the latter is hyped by Apple's multi touch iPhone

from the perspective of the development process of touch technology, the first market to introduce touch technology is the industrial control field. Its purpose is to integrate the complex and huge mechanical equipment control panel into a single window, multi paged screen. At that time, medium and large size resistive screens were used. However, the defects such as the insufficient service life and tolerance of the resistance screen can not meet the needs of the industrial control field. Therefore, as soon as the medium and large-size SCT came out, the high-end equipment immediately switched to the SCT scheme. Until around 2003, due to the reduction of the manufacturing cost of resistive screens, small sizes began to be used in PDA, GPS and other portable products, and touch technology officially entered the consumer market. In 2006, the iPhone adopted small-size PCT. Its excellent optical properties and multi-point touch function set off a wave and became the most eye-catching touch technology in recent years

from the above, it is not difficult to find that at present, the consumer market with small size as the mainstream has only two choices of touch technology: resistive and projection capacitive. Although the former is low-cost, its poor optical performance and tolerance have been criticized by the market for a long time; Although the latter has many advantages, there are only a few suppliers of real energy production, and the price is naturally quite expensive, so that it is only seen in a few high unit price products

at present, the reason why SCT is rarely used in small-scale markets is mainly the cost. SCT panel manufacturers lack key optical coating technology for a long time, so they must subcontract gb/t 90.2 (2) 002 fastener acceptance inspection, marking and packaging processing, while SCT touch IC is controlled by a few technology manufacturers, and the price remains high. In addition, unlike resistive screens that can be matched with resistive ICs at will, SCT screens and ICs must have excellent compatibility to work stably. The foregoing factors make the price of SCT in small-scale consumer applications close to that of PCT, which is naturally difficult to be adopted by the customer base

however, compared with resistance technology, SCT can significantly improve its defects; Compared with PCT, SCT technology is more mature and stable, and can be introduced in mass production. Therefore, we can reasonably infer that when the overall cost of SCT decreases significantly due to the strategic alliance and technical resource integration of industry members, SCT will have the opportunity to become the best solution for small-scale consumer applications

the following will briefly introduce the basic principles of PCT and SCT, and compare the advantages and disadvantages of the two technologies

projected capacitive touch technology

pct is built on the concept of matrix. In the touch screen production part, the ITO of the PCT panel is etched to produce a specific pattern, in order to improve the SNR value of each touch point and enhance the accuracy of recognition. By copying the above pattern several times in the x-axis and Y-axis directions (the number of times depends on the screen size), a PCT matrix similar to a keyboard is formed. Figure 1 is the most common diamond pattern at present

Figure 1: diamond pattern layout of projection capacitive screen

the orange diamond pattern in Figure 1 forms ITO wires in the x-axis direction (a total of M), while the green diamond pattern forms ITO wires in the y-axis direction (a total of n); The PCT controller will drive these wires in turn to detect whether there is any increase in capacitance due to touch

Figure 2: the detection waveform on the X2 wire before and after the PCT equivalent RC circuit and finger touch

take the RC oscillation scheme with the simplest structure as an example We simplify the equivalent circuit of the X2 conductor in the x-axis to figure 2 to form an RC circuit composed of N RP and N CP, where RP and CP respectively represent the equivalent internal resistance of the ITO sub report and the inherent capacitance of each node of the PCT (at the intersection of the XY axis). When fingers approach or touch the screen, a capacitance (CF) will be added on the screen; For this RC oscillation circuit, the appearance of CF means that the oscillation period becomes longer and the frequency decreases. By calculating the change of oscillation period and frequency on x2 wire before and after finger touch, PCT controller can distinguish the position of touch, and even the distance between finger and screen (that is, provide z-axis information)

surface capacitive touch technology

sct panel is a uniformly coated ITO layer, and there is an outgoing line (ur, UL, LR, ll) at each corner of the panel to connect with the SCT controller. In order to detect the exact position of the touch point, the SCT controller must first establish a uniform electric field on the SCT panel, which is achieved by charging the panel by the driving circuit inside the IC. When the finger touches the screen, it will cause a small amount of current flow; At this time, the sensing circuit in the IC will analyze the current on the four lines respectively, and calculate the XY coordinates of the touch point according to the formula in Figure 3. In order to overcome the influence of interference, hardware filters or software filters can be used to process the calculated coordinate values

Figure 3: block schematic diagram of 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; From this point of view, PCT seems to be better than SCT, but in fact, it is not entirely so

the current market situation of practical touch technology with major strategies such as capacitance and "one belt and one way"

the working principle of PCT is not complex, so it is not difficult to provide an exemplary prototype display; 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 come one after another. The main challenges include the following three aspects

the new material design methods such as big data and artificial intelligence (AI) technology used by Toyota Central Research Institute in Japan to be detected, the "material Informatics" number is weak (made into coarse song), which is easy to be affected by the environment and become unstable, resulting in inconsistent sensitivity of touch function, and even misoperation. The capacitance change caused by touch is actually very easy to be affected by temperature and humidity. At present, the commonly used countermeasure is to use timed automatic calibration to overcome this problem. In addition, other electrical appliances being used or other functions of the product itself (such as RF) will interfere with signal measurement. This part must be achieved by improving SNR in the form of software or hardware; Up to now, IC designers still have room for efforts in this regard

the yield of mass production needs to be improved (made into coarse song). PCT technology inherently has the advantage of multi touch. Therefore, since the advent of iPhone, almost all the focus has been on PCT. The touch industry, including touch screen manufacturers, IC design companies and solution developers, has invested great resources in development; But until today, there are still not many PCT products in the market. The problem is that the overall production yield is still not high, resulting in high costs. The reasons for the low yield are: the existing PCT controller is still not mature, and its self adjustment ability in different application environments is still insufficient, so the touch menu is now unstable. In this case, the characteristics of the PCT panel (such as the internal resistance and capacitance of the panel) have to be strictly limited to reduce variables; Due to the capacity limitation of PCT controller, the strict requirements on the characteristic consistency of PCT panel are often the main reason for the low yield

patent problem (made into coarse song): because many basic gestures and multi finger touch functions have been patented and recognized by some manufacturers, the multi-point advantages of PCT products cannot be brought into play in many application fields (especially in the main market)! On the whole, PCT scheme will definitely occupy a place in the market in the future because of its advantages of multi touch. But at present, it is still not a mature plan

in contrast, although SCT technology also faces the problem that capacitive signals are vulnerable to interference in controller design, through the continuous efforts of IC designers, SCT scheme has been widely used in various environments. For example, the SCT touch IC (wt5750f) jointly developed by Weiquan electronics and Wanda optoelectronics can easily overcome the differences between a variety of external environments and the size of the touch screen and stably provide a high-quality touch experience through various adjustment mechanisms contained in the IC. In addition, in order to meet the needs of handheld devices for mechanism miniaturization, Weiquan Electronics' SCT touch IC has made great efforts to improve product integration, so it can meet the two important demands of miniaturization and cost reduction at the same time

in addition to IC, the touch screen structure of SCT is also simpler than that of PCT, and it is easier to overcome the problem of noise interference in hardware. In addition, through process improvement and breakthroughs in key technologies, SCT's overall solution has a cost competitive advantage comparable to resistance. Taking the optical coating technology developed by Wanda optoelectronics as an example, the light penetration rate of the touch screen can be increased from 87% to 97%, and the reflectivity can be reduced from 12% to 2%. More importantly, the yield of optical processing can be controlled at more than 90%, the steps of outsourcing processing can be omitted, the technical autonomy and production capacity can be improved, and the manufacturing cost can be greatly reduced

sct technology is one of capacitive touch technologies, so it also retains many advantages of capacitive touch: "better visual enjoyment", "easier and flexible handling" and "longer service life"; In addition, compared with PCT, SCT has relatively loose requirements for touch screen characteristics and more mature mass production technology, which makes it have the advantages of high yield and low cost at the same time. Moreover, in practice, users' demand for single point selection, single point gesture and handwriting functions is more urgent than multi touch, and these functional requirements happen to be stable provided by SCT at present. Therefore, before the PCT scheme is mature (stable function, patent problems, high production yield and low cost), the SCT scheme is a touch scheme that meets the expectations of the market. (end)