Design of inductive capacitive touch control system from the perspective of controller

Introduction

Since the introduction of the iPhone® in 2007, the use of capacitive touchscreens has continued to expand. Despite this, integrating capacitive touchscreens into devices remains challenging, especially in environments with liquid crystal displays (LCDs), interference from peripheral devices, and noisy environments. One effective solution is to use a touchscreen controller with a high signal-to-noise ratio (SNR) to combat noise. A high SNR controller has other advantages, which are described in detail below.

SNR is defined as the power ratio of the signal (useful information) to the noise (unwanted signal). If the signal and noise are measured under the same load, the SNR can be obtained by calculating the square of the root mean square (RMS) amplitude. The value of the power ratio (PS/PN) is usually large and is usually described in logarithms (dB). SNR can be expressed as:

SNRdB = 10log10(PS/PN) = 10log10(RMSS/RMSN)² = 20log10(RMSS/RMSN)

A high SNR means that the measured signal strength is higher than the background noise.

The overall

touch performance is mainly determined by two components: the touchscreen sensor and the touchscreen controller. There are many types of touch screen sensors, and their names vividly describe their shapes and structures, such as triangle, diamond, snowflake, stripe, etc. For example, "diamond" is a grid structure of diamonds, while "strip" is a grid of rows and columns that cross, like the streets of a city. Some sensor types use a single layer of ITO, while others require two or three layers, depending on the required system performance and touch screen controller chip. The

touch screen sensor pattern and layer structure ("stack") are usually determined by the touch screen controller structure to maximize SNR. For example, in a single-layer mutual capacitance diamond pattern with crossovers (bridges), the distance from the touch screen surface to the X and Y layers of ITO is the same, which reduces gain error and makes the SNR of rows and columns very similar. However, a shielding layer is still needed to protect the sensor from LCD noise. Using a high SNR touch screen controller can reduce the cost of the touch screen sensor, relax design constraints, and use more patterns and layer structures. As will be discussed below, a high SNR touch screen controller can also provide additional benefits, such as easier to find the center of the touch, reduced sensitivity of the touch screen to environmental noise, and allow the use of gloves or fine conductive pens.

Controller Architectures

Self-capacitive and mutual-capacitive are the two main capacitive touchscreen sensing technologies. The characteristics of self-capacitive and mutual-capacitive are briefly summarized as follows:

Self-capacitive

• An early technology still in use today.
• Limited by "ghosts" (false touch locations relative to the true touch location), usually one or two touches.
• Diamond pattern is most common.
• Poor LCD noise rejection.
• Simple, low-cost controller.

Mutual-capacitive

• A new generation of designs that are taking over the market.
• True two or more touch points.
• Higher accuracy.
• More flexibility in sensor pattern design, which helps maximize SNR.
• Better noise rejection.
• More complex, high-cost controller.

Many applications require only one or two touch points, so self-capacitive solutions are more attractive, especially when the touch location of the user interface can be controlled to eliminate "ghosts." Self-capacitive solutions have a typical SNR of more than 30dB, and usually require a shielding layer between the LCD and the bottom of the touch layer of the sensor, which increases cost and reduces display brightness.

Other technologies can be used with self-capacitive solutions to further improve SNR. These include (a) increasing the number of samples per channel; (b) increasing the sensor drive voltage, which increases the signal amplitude under fixed noise (such as noise from the LCD); and (c) sampling at different frequencies to avoid fixed frequency interference, such as avoiding 60Hz (this is called "frequency jitter"). However, this technique usually reduces the frame rate and increases power consumption, both of which are undesirable.

From the above discussion, it is clear that in order to maximize SNR and support two or more touch points, mutual capacitance is the most promising sensing detection technology. The system block diagram of Figure 1 summarizes the implementation of mutual capacitance, that is, an excitation signal is applied to one pole of the touch screen sensor capacitor, and the other pole is connected to the analog front end (AFE) of the touch screen controller. The output of the AFE is converted into digital format and further processed in the digital signal processor (DSP).
 

Figure 1. Block diagram of a mutual capacitance system

Design Challenges

There are many technical challenges when integrating capacitive touch screen sensors into touch-enabled devices. The following situations can all benefit from a high SNR touch screen controller.

Sensor layer design: There are a variety of touch screen sensor layer structures today, each with different requirements for materials, thickness, performance, and cost. This is shown in Figure 2. Single-layer or multi-layer substrates, "up" or "down" architecture, changes in X and Y layer thickness, changes in optical clear adhesive (OCA) thickness, and other factors all affect the signal amplitude generated by the sensor. Since a high SNR touch screen controller can handle a wider dynamic range of touch screen sensor signals, the effects caused by structural differences will be reduced. This gives designers more freedom to choose the sensor layer structure.
 

Figure 2. Layer structure of a mutual-capacitive touchscreen sensor (not to scale).

Thick protective cover: Some applications, such as bank ATM machines, may require a thick glass cover to prevent the display from being damaged. However, the thick glass cover will reduce the touch signal strength and reduce the accuracy of touch position detection. This is because the distance between the finger and the touch screen sensor becomes larger, resulting in a larger capacitance range and lower signal amplitude, making it difficult to determine the exact touch position. Wearing gloves will also produce the same effect. [page]
 

LCD VCOM type: LCD VCOM is the "common mode voltage" and is the reference voltage for the LCD screen. Depending on the system requirements, either AC VCOM or DC VCOM may be used. AC VCOM is alternating, while DC VCOM is a constant voltage. The former generates more noise.

Air gap between touch sensor and protective lens: One of the most common problems reported by users of touch devices is a broken protective lens. To make the product thinner, capacitive touch sensors can be pressed onto the back of the protective lens, but when a broken protective lens is replaced, the touch sensor must also be replaced, which increases repair costs. To avoid this cost, as well as the cost of low yield in the lamination process, a spacer is usually used to separate the touch sensor from the protective lens.

However, when an air gap is formed between the touch sensor and the protective lens, it is difficult for the touch sensor to detect a finger touch because the dielectric constant of air is low and the signal strength generated by a finger touch is also low. One way to solve this problem is to increase the sensitivity threshold of the touch system, but this can be dangerous because the sensor will pick up some stray signals, such as LCD or other environmental noise, making it difficult for the touch sensor to distinguish the touch from the noise.

Industrial design requirements: Some device manufacturers make the touch screen sensor directly on the display to make the overall design thinner. But this is also risky because the touch screen sensor is placed directly on the noise source. One solution is to add a shielding layer between the touch screen sensor and the display. But adding an extra layer of ITO increases the overall material cost and affects the light transmittance.

Integrated touch screen sensor: To reduce production costs, LCD manufacturers began to make the touch screen sensor directly on the color filter under the polarizer. This method does not require an external sensor and lamination, but the touch screen sensor is closer to the display, further increasing the noise received by the sensor.

Touch screen controller location: Capacitive touch screen controllers are usually located on the touch screen cable (chip on the wire or PCB), and sometimes directly on the touch screen sensor (chip on glass). However, for ease of testing, some designs require the touch screen controller to be placed on the system board. This may require a long flexible circuit board (FPC) to connect the touch screen sensor and the controller. The long FPC will act as an antenna and easily absorb noise, making it difficult for the touch screen controller to process the weak signal from the touch screen sensor.

Other noise sources: The main noise sources of mobile devices are LCD screens, LCD inverters, WiFi antennas, GSM antennas, and various high-speed circuits in the device. Environmental noise also has a great impact on the touch system. For example, some AC power supplies can generate strong noise, which can be transmitted through the AC adapter. Similarly, when the device is placed near a strong noise source such as a desktop fluorescent lamp, the touch system will mistake the noise for a valid touch behavior.

Under normal conditions, for normal-sized fingers (>7mm), high-SNR controllers do not have a great advantage over low-SNR controllers. The advantage will only be reflected in a strong noise environment, such as using a writing pen or using a gloved finger input, when the signal is very weak. Low-SNR controllers cannot distinguish signals from noise. If the sensor threshold is lowered to increase the detection sensitivity, the touch system will be easily triggered by mistake, causing misoperation, which is absolutely not allowed in practical applications.

Application Challenges

Touch accuracy: Touch accuracy is an important indicator in the design of touch screen sensors. For example, in virtual keyboard applications, characters are compactly arranged in a small area, and it is critical to accurately respond to touch actions and avoid mis-entering characters. One way to improve accuracy is to add more sensor channels to the controller to support a higher touchscreen sensor grid density. However, this will come at a cost because both the touchscreen sensor and the touchscreen controller require more pins. In addition, more sensor channels require more traces to be added to the touchscreen border, which increases the border width. A

high SNR touchscreen controller can enhance detection accuracy because it has a stronger ability to detect weak signals and collects sampled data from a larger peripheral range. The larger detection range provides more reference points so that the touch position can be accurately calculated. Figure 3 shows the impact of the touchscreen controller SNR on the line drawing accuracy. This is a straight line drawn by a robotic arm holding a 4mm metal sheet. The line drawn by the high SNR controller is obviously smoother than the line drawn by the low SNR controller. Note that these measurements were recorded from the same touchscreen sensor and the same post-processing software to ensure a fair comparison.
 

Figure 3. A robot arm holding a 4mm metal sheet and drawing a straight line. The left side uses a high SNR touch screen controller; the right side uses a low SNR touch screen controller

Stylus: Users of resistive touch screens have long been accustomed to using a stylus with a pointed tip. The tip of a typical resistive touch screen stylus is less than 1mm in diameter and is usually made of non-conductive plastic. For capacitive touch systems, detecting such a small, non-conductive device is difficult because the signal it can provide to the touch screen controller is very weak. Many touch systems on the market use stylus tips with large diameters (3-9mm), which makes writing and drawing difficult because the thick tip makes the writing blurry.

As long as the stylus is covered with conductive material (a relatively small sacrifice), a high SNR touch screen controller can detect a stylus with a 1mm diameter tip. Figure 4 shows the impact of touch screen controller SNR on the detection of a stylus with a 2mm conductive tip. A low SNR controller has difficulty distinguishing a small tip stylus from the background noise, especially on the noisiest parts of the screen. Using a 1mm tip stylus in a low SNR situation will cause the useful signal to be buried in the background noise, rendering the stylus unusable.
 

Figure 4. Capacitance profiles of a 2 mm conductive stylus on a 4-inch screen. The left side of the profile uses a high SNR touchscreen controller; the right side uses a low SNR touchscreen controller. The stylus is at the top of the green cone; the height of the white plane represents the background noise. The increase in signal-to-noise ratio effectively reduces the amplitude of the background noise, as shown on the left. If the stylus in the right figure moves to the left side of the screen, the signal will be overwhelmed by the noise and the stylus will not work.

Non-contact detection: Proximity detection is increasingly being adopted in touchscreen applications. For example, by increasing the sensitivity of the touch system, when using an e-book, the user can turn the page with a gesture without actually touching the screen. However, the increased sensitivity of the touch system also makes it easier for the environment to trigger it. Designers have been working hard to find the best balance between maximizing the proximity distance and not causing false triggers. Mitsubishi has done some interesting research in this area. They built a touch system that automatically adjusts the sensitivity based on whether the touch finger is suspended or actually touching. [2]

Glove operation: In medical applications, touch screens need to be able to work while wearing surgical gloves. Similarly, a car touchscreen GPS needs to work with gloves in winter. Most gloves are made of dielectric materials, which makes it difficult for the touchscreen sensor to detect touches. Increasing the sensitivity of the touchscreen controller may cause false triggers when the user is not wearing gloves. The only solution is to require the application (or the user) to choose a different sensitivity according to the situation.

Conclusion

High SNR capacitive touchscreen controllers have many advantages. They can meet a wide range of design and application requirements such as stylus, small fingers and gloves. It can help improve touch accuracy without the need for specialized ITO sensor patterns or additional sensor channels. It can meet the requirements of various displays and different backlights while maintaining good touch performance. It provides more flexible options for sensor design and production. It enables the touch system to work in a strong noisy environment and reduces the noise of the device itself from the LCD, WiFi antenna, GPS antenna and AC adapter. It gives device OEMs more freedom to choose components. Finally, from a performance point of view, it provides precise touch accuracy. In summary, high SNR touchscreen controllers can help end users achieve more reliable applications.