Designing touchscreens with true glove functionality
Most capacitive touchscreens simply do not work with gloves. Glove material is generally non-conductive and the material dielectric properties are closer to that of air. One method for getting gloves to work with capacitive touchscreens is to increase the sensitivity of the touchscreen. However, this poses its own challenge: The sensitivity may now be too high to process normal finger touches and may also be more prone to various noise sources. To address this, most existing glove-based systems have to go into a special high sensitivity mode that must be enabled by the user. The complexity rises further when you need to support multi-touch, as well as a wide range of glove material and thicknesses.
The signal from a gloved finger as measured on a capacitive touchscreen can be more than 10 times smaller than the signal from a normal finger. Figure 1 shows the signal measurement (2126 peak counts) from a normal finger on a capacitive touchscreen. Figure 2 shows the signal measurement (189 peak counts) with the same human finger inserted into a thick ski glove (5mm thick glove material). The relatively small signal from a gloved finger can be difficult to detect, especially in the presence of display and charger noise. The low signal-to-noise ratio (SNR) impacts the complex algorithms used to detect and report touch locations to the host, and this results in unresponsive and inaccurate touch performance.
The natural solution is to boost the signal sensitivity to be able to reliably detect the small signal from a gloved finger. However, this poses several issues. First of all, if sensitivity for gloved finger input is increased, this could lead to the system being saturated on touches from larger signal inputs like a normal finger. Another issue is that different types of glove material and thicknesses lead to different signal levels. Hence, simply boosting the sensitivity will not address the issue of multitude of glove types. In addition, an increase in touchscreen sensitivity also increases susceptibility to noise sources.
A robust glove-based system needs to be able to detect each of these objects reliably without saturating the system while at the same time detecting enough signal for the touch algorithms to work reliably and accurately. To achieve this, different sensing modes can be used, depending on the input type. The touch system enters a high sensitivity mode if a glove is used and normal mode if finger touch is the input. A touchscreen controller that has enough dynamic range to be able to support these two different modes is required. Detecting a thick ski glove with glove material as thick as 5 mm will be much more challenging than a thin cotton glove which has glove material thickness of less than 1 mm. Additional modes, and hence a wider dynamic range, may be required if there is a requirement to be able to further distinguish between thin and thick gloves for the best user experience.
A system that supports different input modes depending on object signal level needs some sort of mode switching mechanism. One mechanism relies on a host setting the sensitivity mode based on user settings or the application in use. An example of this is a mobile phone user going into the phone “settings” menu and selecting “enable glove”. Users may actually have to take their gloves off to make the settings change. Moreover, as discussed earlier, the drawbacks of this approach of setting the phone always in high sensitivity mode are increased susceptibility to noise and concerns about saturation when a normal finger is used. A more versatile approach is to use an automatic mode switching scheme where the touchscreen controller goes into the appropriate mode based on detection of the signal intensity and signature.
When the touchscreen switches to the high sensitivity mode to reliably detect glove touches, it leaves the system more vulnerable to noise. Hence, any solution that enables glove use in this manner needs to have a high SNR and be tolerant to noise. Traditionally, touch systems have used schemes such as adding an air gap or a shield layer to mitigate the effects of display noise from coupling to the touchscreen sensor. While these methods are effective and may be necessary for highly noisy displays, a growing number of device and touch module makers have been moving away from these approaches to save cost and make thinner modules. This leaves the battle against noise up to the touchscreen controller. A high-end touchscreen controller that supports gloves can achieve a high SNR through schemes like using a high transmit (Tx) voltage and transmitting on multiple Tx lines to average out noise. A sophisticated analog front end, hardware and software filtering techniques, and intelligent algorithms can further help the touchscreen controller to defeat noise from both displays and chargers.
Touchscreen users expect gloves to work like normal fingers and are frustrated in having to use special gloves with conductive tips or even take their gloves off to operate their devices. By designing in innovative and sophisticated touchscreen controllers, such as the TMA4XX and TMA5XX devices from Cypress, systems can support the dynamic range necessary to address sensitivity issues. With different sensitivity modes, automatic mode switching capability, and high SNR and noise immunity, systems can seamless support gloves with the enhanced user experience that the masses have been demanding.
1. Product Overview for TrueTouch advanced features: http://www.cypress.com/?docID=44683
2. Glove feature for Cypress TrueTouch: http://www.cypress.com/touch/#Features
3. Cypress TMA4XX and TMA5XX controllers: http://www.cypress.com/touch/#Solutions