Since the advent of computers, people have been thinking about how to realize the dialogue between people and computers in a more effective way, which is the so-called human-computer interaction technology. Capacitive touch technology, especially mutual capacitance technology, because of its direct, efficient, accurate, smooth and stylish characteristics, has greatly improved the efficiency and convenience of dialogue between people and computers. In the future, it will definitely replace the mouse and keyboard and become the future consumer. The mainstream.
Projected capacitive screen touch detection principle
Projected capacitive screens can be divided into two types: self-capacitance screens and mutual capacitance screens. The horizontal and vertical electrode arrays are made of ITO (a transparent conductive material) on the glass surface. These horizontal and vertical electrodes form a capacitance with the ground. This capacitance is commonly known as self-capacitance, which is the capacitance of the electrode to ground. . When the finger touches the capacitive screen, the capacitance of the finger will be superimposed on the capacitance of the screen body, so that the capacitance of the screen body increases.
During touch detection, the self-capacitance screen sequentially detects the horizontal and vertical electrode arrays respectively, and determines the horizontal and vertical coordinates according to the change in capacitance before and after the touch, and then combines them into planar touch coordinates. The self-capacitance scanning method is equivalent to projecting the touch points on the touch screen into the X-axis and Y-axis directions, and then calculating the coordinates in the X-axis and Y-axis directions, respectively, and finally combining the coordinates of the touch points.
If it is a single touch, the projections in the X and Y directions are unique, and the combined coordinates are also unique; if there are two touches on the touch screen and the two points are not in the same X direction or the same Y direction, Then there are two projections in the X and Y directions, respectively, and 4 coordinates are combined. Obviously, only two coordinates are real, and the other two are commonly known as "ghost points". Therefore, the self-capacitance screen cannot achieve true multi-touch.
The mutual capacitance screen is also made of ITO on the glass surface to make horizontal electrodes and vertical electrodes. The difference between it and the self-capacitance screen is that the two sets of electrodes will form a capacitor at the intersection, that is, the two sets of electrodes form the two poles of the capacitor. When a finger touches the capacitive screen, it affects the coupling between the two electrodes near the touch point, thereby changing the capacitance between the two electrodes. When detecting the mutual capacitance, the horizontal electrodes sequentially emit excitation signals, and all the vertical electrodes simultaneously receive the signals, so that the capacitance value at the intersection of all horizontal and vertical electrodes can be obtained, that is, the capacitance of the two-dimensional plane of the entire touch screen. According to the two-dimensional capacitance change data of the touch screen, the coordinates of each touch point can be calculated. Therefore, even if there are multiple touch points on the screen, the true coordinates of each touch point can be calculated.
Figure 1. The mechanism of self-capacitance ghosting.
FocalTech mutual capacitance solution
FocalTech is one of the companies that started the mutual capacitance touch screen technology research and development earlier. It has dozens of domestic and international patents in the mutual capacitance field, including the design of mutual capacitance touch screen body, mutual capacitance touch detection circuit , Touch detection algorithm, environment adaptive algorithm and other technologies. Using FocalTech's own patented technology, the following performance of mutual capacitance touch screens can be greatly improved:
1) Anti-electromagnetic interference capability
Anti-electromagnetic interference is the most critical factor in the performance of capacitive touch screen systems. Since 2007, some companies have started to provide capacitive touch screen technology with self-capacitance scheme. However, due to the poor anti-electromagnetic interference design, it is often impossible to answer the call when the call comes in, or cannot hang up the phone when the call ends. In addition, the frequent failure of the touch screen during environmental changes has caused the failure of multiple capacitive touch screen mobile phone projects, and even indirectly caused the closure of some program companies. Focaltech draws on the frequency hopping technology in the field of modern wireless communication, and at the same time increases the TX transmission power, which effectively suppresses electromagnetic interference while improving the system's signal-to-noise ratio.
2) Signal-to-noise ratio (SNR)
SNR is defined as the ratio of received signal power to noise power. SNR is another key factor in the performance of touch screen systems. Its height directly determines the accuracy, linearity and resolution of touch. FocalTech mainly improves SNR in three ways. The first is to increase the signal transmission power. Increasing the signal transmission signal power correspondingly increases the received signal power, thereby increasing the SNR. Secondly, noise reduction is also an effective method. FocalTech provides design solutions for touch screens. These solutions have very good shielding design, such as adding a ground plane at the bottom of the touch screen to the LCD side and increasing ground isolation around the screen body. These measures can effectively reduce the power of noise. Another way is to increase the amount of capacitance change caused by touch. The amount of touch capacitance change is proportional to the signal power. That is, the greater the touch change amount, the greater the detected signal power. FTS's unique touch screen patented technology can greatly increase the rate of capacitance change caused by touch, which can usually reach more than 30%, which is much higher than the iPhone's touch screen with only 18% change.
3) Environmental adaptability
Automatically adapting to environmental changes is also very important for touch screen systems. The touch screen is directly exposed to the air, and the temperature and humidity of the air will affect the capacitance of the touch screen body. The water droplets on the surface of the touch screen may directly cause accidental touch. A good design must be able to adapt to changes in ambient temperature and humidity in a very large range, and can be touched normally in the presence of a small amount of water. FocalTech has specially developed an environment adaptive algorithm and cooperated with the corresponding touch screen body design, which has completely solved the problem of the impact of environmental changes on the touch screen.
4) Power consumption
For portable devices, power consumption is also critical. The mutual capacitance technology uses two-dimensional detection instead of self-capacitance one-dimensional detection, which greatly increases the power consumption of the detection circuit and the power consumption of post-processing data. Generally speaking, for touch screens of the same specification, mutual capacitance technology consumes 2 to 3 times the power consumption of self-capacitance technology. Therefore, reducing power consumption becomes critical. FocalTech also uses multiple technologies to reduce power consumption. In the IC design, the power consumption is listed as the first constraint, such as the use of low-power structures, low-power processes, and the addition of hardware accelerators. In the coordinate calculation, FocalTech has developed a fast coordinate calculation scheme, which can simplify the calculation amount and greatly reduce the time and power consumption of data post-processing. In addition, multiple power consumption modes are designed. The system can use these modes flexibly to reduce the overall power consumption. According to actual measurements, the power consumption of FocalTech's solution is only about half of that of similar solutions.
Figure 2. Basic architecture of FocalTech touch chip.
Technology and market trends
With the deepening of research, the mutual capacitance technology continues to mature, and its basic functions and basic architecture have been determined. Future developments will mainly focus on detailed optimization, such as further improving the signal-to-noise ratio, improving touch accuracy (especially edge accuracy), and reducing Power consumption, increase the ability to adapt to changes in the environment, increase the scanning speed, multi-point ID recognition, multi-point application development, reduce the impact of floating touch, etc.
At present, mutual capacitive touch screens have become mainstream in the European and American markets, and shipments are continuously rising, which also greatly promotes the improvement of the capacitive touch screen industry chain. With the gradual improvement of the industrial chain, the addition of many suppliers, and the application of new materials, new technologies, and new processes have also brought down various costs, thereby reducing the overall module price. Some large manufacturers of capacitive touch screen modules predict that in 2011, the price of ITO glass with electrode etching will be reduced to 0.3 ~ 0.5 US dollars / inch, less than half of the current price; at present, the shipment prices of some small factories in Shenzhen have been Very close to this interval. We expect that there will be a significant price reduction for capacitive touch screens in the future. In 2011, the price of 3.5 â€modules can be reduced to 5-6 US dollars; while some low-end products are more likely to reach about 4 US dollars, or even lower . At that time, capacitive screens will significantly squeeze the current market of resistive screens. Regardless of the brand or cottage market, capacitive screens will become mainstream.
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