How to solve the problem of large capacitive touch screen input? Passive stylus brings natural control experience
Author: Cypress Semiconductor Corporation Todd Severson
Mobile device manufacturers want to provide consumers with a natural and easy-to-use user interface, as easy as using paper and pens, and also have a high degree of flexibility in computers. Such advanced features can help manufacturers differentiate their products. A small, pointed passive stylus with palm error elimination function allows manufacturers to provide low-cost solutions that support various new application functions such as handwriting, editing, signature capture, and precise navigation. However, the implementation of the above-mentioned functions also faces some challenges. Developers of passive styluses must meet a series of performance requirements of capacitive sensing technology on larger touch screens. Specifically, advanced algorithms and sensing methods need to be used to detect the very small signals sent by the stylus, while also excluding the large useless signals caused by the user's palm. In addition, the device must be able to dynamically switch back and forth between the stylus and multi-touch input, while maintaining a certain speed, accuracy, and responsiveness to ensure an ideal user experience.
As the size of capacitive touch screens grows larger, it becomes more intuitive and convenient to use writing devices like paper and pens. The most common method for manufacturers to support the stylus function is to use an active stylus or a passive stylus. The active stylus uses electronic components, requires a power supply, and emits signals to the host device. Using an active stylus can support advanced features such as hovering, pressure sensing, key support, and erasing on the display. The passive stylus uses conductive materials, which is equivalent to the extension of the user's body. The capacitive coupling of the user's hand allows the passive stylus to send a signal when it touches the screen. There is no active communication between the stylus and the host platform, so how to distinguish between the finger and the passive stylus is a problem.
In many cases, if both the active stylus and the passive stylus can achieve the same characteristics, there is no need to add additional cost to the system. The additional components and power requirements of the active stylus make it difficult to open the market, and the poor performance of the passive stylus and / or the bulky head will bring an unnatural handwriting experience. Therefore, if the tip of the passive stylus is 1 to 2 mm, the palm of the user can rest on the screen while writing, while maintaining sufficient speed and accuracy, and ensuring that the contact point is just the "inking place" of "ink". It can improve the user experience of passive stylus.
In order to create a practical implementation scheme that can support both finger and passive stylus operations, many different use cases must be considered. For example, developers should consider how quickly the system should switch between finger and stylus input. Similarly, they also need to define how the system responds when the stylus touches the screen before, after, or simultaneously with the finger / palm. Other important factors include how close the stylus is to the hand and the stylus signal is no longer detected. Figure 1 shows an example of a state machine process under the stylus use case.
Figure 1 Example of using state machine under passive stylus use case
The paradox of stylus
Passive stylus detection is a complex issue for touch control engineers. The root of the problem lies in the "stylus paradox". The so-called "stylus paradox" refers to that the signal of the passive stylus is much smaller than the normal finger touch input, and the user thinks that the tip of the stylus is so thin and should be more accurate than the finger.
The accuracy and linearity are proportional to the signal-to-noise ratio of the system. Since the noise floor does not change with input, signal attenuation will have a greater impact on the signal-to-noise ratio. The signal level of the capacitive touch screen basically depends on the coverage area of ​​the touch input. This means that the signal strength of a 2 mm passive stylus is 25 times smaller than the signal strength of a typical 10 mm finger touch. This gap in signal strength has caused many problems for touch engineers. Even in the case of a large touch signal, the firmware must be able to detect a small stylus signal, which often requires different sensor scanning modes, and noise resistance and refresh rate are affected. In addition, passive stylus pens are best suited for use with larger touchpads, but large touchpads have inherently low refresh rates or require larger distance sensors, both of which can affect system performance.
Fundamentally speaking, there are two problems to be solved when dealing with the gap in signal strength. First, despite the extremely low signal strength, the stylus must be detected first. Second, once the stylus is detected, an accurate report must be made. These two major problems have their own difficulties. Conceptually, the most reasonable stylus detection method is to maximize the sensor signal. Generally, the problem is solved by minimizing the dynamic range of the sensor to the signal level (very close to the expected signal level), or even by using software multiplication and filtering. However, high-gain systems can easily saturate through larger inputs such as normal finger touch, so normal touch and smaller stylus signals must be handled carefully. A common method is to perform two independent scans at each expected signal level to distinguish normal touch from stylus input.
Figure 2 Introduction to touch and stylus
This mode switching is easily affected by false detection, so the false detection must be filtered out. A typical example is when a finger approaches or leaves the touch screen. When the finger is close, the signal level is very low (in the passive stylus area), and the signal level is also very low when leaving, so other judgers must be used to confirm any detected stylus input.
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