Passive stylus design for large capacitive touchscreensManufacturers of mobile devices want to differentiate themselves through advanced features that meet consumer demand for a natural user interface with the ease of pen and paper, combined with the flexibility of a PC.
A small tip, passive stylus with palm rejection would give manufacturers the ability to deliver a low cost solution that enables writing, editing, signature capture, precise navigation, and other new applications. However, implementing such capabilities introduces several challenges for passive stylus developers to meet the necessary performance requirements using capacitive sensing technology on larger touchscreens.
Specifically, advanced algorithms and sensing methods are required that are able to detect a very small signal of interest from the stylus while rejecting the large, unwanted signal from the user’s hand. Devices also must be able to dynamically switch back and forth between stylus and multitouch input while maintaining the speed, precision and responsiveness required for a desirable user experience.
As capacitive touch screen sizes grow, it becomes more intuitive to use a writing device similar to paper and pen. The most common ways for manufacturers to enable pen capabilities are through an active or passive stylus.
An active stylus includes electric components, requires a power source, and transmits a signal to the host device. This allows for advanced features such as hovering above the display, pressure sensing, button support, and eraser functionality.
A passive stylus is a piece of conductive material that is essentially an extension of the user. The capacitive coupling from the individual’s hand enables a signal to be sent when the stylus touches the screen. There is no active communication between the pen and the host platform, so it can be difficult to differentiate between a finger and a passive stylus.
In many cases, the features that can be implemented using either type of stylus are not worth the added cost to the system. The additional components and power requirements for an active stylus make it a tough sell, and the poor performance and/or large tip of a passive stylus make for an unnatural writing experience.
This said, the user experience for a passive stylus can be improved if the stylus can have a small tip on the order of 1-2 mm, support the user resting his or her palm on the screen while writing, and maintaining sufficient speed and accuracy so that “ink” deposits directly under the contact point.
To create a viable implementation that supports a finger plus a small tip passive stylus, many use cases must be considered. For example, developers need to consider how fast the system needs to be able to switch between detecting fingers and writing with stylus.
Similarly, they must define how the system should react if the stylus touches before the finger or palm, after the finger or palm, or simultaneously. Other important details include configuring how close the stylus can be to a hand before the stylus signal is no longer detected. Figure 1 shows example state machine progressions of stylus use cases.
Figure 1 - The Stylus Paradox
Passive stylus detection is a complex problem for touch engineers, with the root of the problem being the stylus paradox. The stylus paradox is that the signal profile for a passive stylus is several times smaller than that of a normal touch inut, but the fine point of the stylus makes the user believe that it will be more accurate.
Accuracy and linearity are proportionally related to the signal to noise ratio of the system. Since the noise floor generally will not change relative to the input, a reduction in signal greatly affects SNR. A rough approximation for the signal level of a capacitive touch screen is the area of coverage for the touch input. This means that a 2 mm passive stylus will have 25 times lower signal strength than a typical 10 mm touch.
It is this signal gap that causes so many problems for the touch engineer. The firmware must be able to detect the small stylus signal, even in the presence of the larger touch signal. This will often require separate sensor scanning modes at the expense of noise immunity and refresh rate. Additionally, the most natural fit for a passive stylus is a large touch pad, which already has lower refresh rates or larger pitch sensors, each of which lower system performance metrics.
Fundamentally, there are two issues related to managing the signal gap. The stylus must first be detected despite its very low signal. Once it is detected, the stylus must be accurately reported. Both of these issues present their own difficulties.
Conceptually, the most common sense approach to detecting the stylus is maximizing the sensor signal. This is often accomplished by minimizing the dynamic range of the sensor to signal levels that are very near the expected levels, or even through software multiplication and filtering.
However, high gain systems are easily saturated by larger inputs such as normal finger touches. So care must be made to accommodate both normal touches and small stylus signals. One common method is to perform two separate scans at each expected signal level to decipher normal touches from stylus inputs.
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