Hover sensing has the potential to revolutionize the handheld user interface by delivering a similar quality experience to that of a PC. Leading players in the portable media player/tablet PC market are starting to develop this technology into next-generation devices. Through a combination of self and mutual capacitive sensing, users can make selections by hovering a finger above the touch screen and through direct input. The following discussion will explore the capabilities and applications of hover technology in handheld devices.

Touch-screen technology has revolutionized how handheld device User Interfaces (UIs) are being designed. Eliminating buttons and switches can substantially reduce UI failure due to mechanical reasons. In addition, devices can have a flat profile, enabling them to be thinner and slicker. Touch screens also enable more intuitive UIs, and the support of gestures further increases device usability. However, touch screens introduce a variety of new problems for designers to solve.

Real touch-screen performance with gloves has long been a difficult challenge for handheld device designers. Gloves greatly hinder the ability to sense the capacitance of a fingertip, making navigation and selection all but impossible. However, users in cold climates need a more realistic solution than removing their gloves any time they need to operate their devices.

Many touch-screen controller manufacturers attempt to resolve this issue by dramatically slowing down screen refresh rates and significantly lowering the noise threshold. This allows the sensor to pick up very small touches, such as the kind produced when attempting to operate a touch-screen device with gloves.

Demonstrations of this type of functionality appear to work, but they are typically conducted on a demo kit that is a safe, noise-free environment. In an actual device, noise is an unavoidable occurrence. Once you take the technology contained in the demo kit and build it into a system where noise is present, the lowered thresholds and increased sensitivity that allowed glove functionality to work in the demo make the controller much more susceptible to phantom touches due to noise spikes (see Figure 1). In real systems, the ability to work with gloves falls apart quickly, leaving the glove-wearing user with an unresponsive touch screen that has poor accuracy and a higher incidence of false detection even when operated by bare hands.

An alternative to touch sensing

Hover technology provides a powerful means for supporting device operation with gloves – even very thick, heavy gloves. It allows the touch sensor to “see” finger movements without the finger touching the screen. Because hover operates in a different manner than touch sensing, false touches and performance disruption caused by noise under glove operation are eliminated (see Figure 2).

This technology also enables users with long fingernails to operate mobile devices. Touch sensors typically have difficulty picking up fingernail input for much the same reasons as with gloves; the capacitance of the body through a fingernail, especially false acrylic nails, is minimal. Long nails can make it almost physically impossible for users to navigate with their fingertips. These users need a more realistic navigation function. Hover functionality allows the widespread use of touch screens regardless of climate, gloves, or fingernails.

Furthermore, hover technology has the potential to significantly alter how users interact with mobile and portable devices. Hover capability is rising in importance among OEMs who are already aware of its potential.

Application areas

Hover has many possibilities for current handheld applications. For example, hover could be used to preview an e-mail inbox on a mobile device. Hovering a finger over new e-mails could reveal the first few sentences. If the e-mail demanded immediate attention, the user could use direct selection to open and respond to the entire message. Another potential application is personal media players, especially those with streaming capabilities. Hover could allow users to preview a song or play a movie trailer before downloading. Handheld gaming applications are also poised for increased functionality. In the lucrative and volatile gaming industry, developers are continually looking for new ways to innovate. No longer limited by simple taps or gestures, game designers could add new commands and options with the addition of hover.

The development of hover technology allows handheld devices to deliver the functionality of a PC in a very small form factor. Hover makes it possible to design a mouseover function for mobile devices. Users could hover over an area of the screen on a mobile device to magnify text – for example, to magnify a clickable link or keys on an on-screen keyboard for easier selection. Hover also enables the ability to view additional information than what is already being displayed on the screen. A user could use hover while scrolling through contacts in a mobile phone to see a person’s contact information and history, such as phone calls and text messages sent. App developers are already working to incorporate this technology.

It’s not unrealistic to think that a social networking mobile app could utilize hover so users could see a preview of another person’s profile, photos, or previous posts without direct selection. There are many plausible use-case scenarios: users could preview content on video-sharing sites like YouTube

Hover can be used in GPS devices or map apps on a mobile phone to browse nearby amenities like restaurants without having to navigate away from the current map. Hover also allows for a new and simplified method of text selection. Accurately placing a small cursor with a tap and then dragging a finger to highlight text can be difficult and increases the possibility of improper cut, copy, or place commands. Hovering could be used to select text, while direct input could be used to open the command menu and select specific commands.

Hover has even more possibilities beyond current applications. 3D displays have gained traction in the market, and manufacturers are working to incorporate them into mobile devices. With 3D interfaces, users need a 3D way to interact. Hover could allow users to navigate through multiple open applications on a 3D display, bringing different applications to the forefront of the screen, while other open applications are still visible behind it. For example, if a user was playing a game on a mobile phone and receive a text message, he or she could hover a finger over the incoming message to open and read it without having to close or navigate away from the game.

Touch-screen problems resolved

Designing hover into a mobile device does more than just create new commands and capabilities. In capacitive touch screens, the issue of Z-Force is a recurring problem. In this type of touch screen, Z-Force is a measure of cross-sectional area and signal, not a measurement of touch pressure, as it is in resistive touch screens. Applying more pressure with a finger flattens it against the screen, increasing the cross-sectional area of the finger/touch. A Z-Force measurement then evaluates the wider area touched by a fingertip in an effort to calculate force used. This is an issue because, by relying solely on area and signal, the touch controller has difficulty telling the difference between a light touch with a large finger and a hard press with a smaller finger.

Some UIs use a long press to open new command menus. Using a long press decreases the speed at which the user can interface with the phone and creates a slower and clumsier user experience. Adding hover functionality to a capacitive touch-screen device can remove the need for long presses or force measurement altogether. By enabling this kind of sensing, OS developers can preserve functionality without having to slow down the user experience or modify threshold and sensitivity levels to calculate Z-Force. With hover, a user no longer needs to apply varying force to perform certain functions or sacrifice efficiency when using the system.

The right combination of capacitance sensing

Hover sensing is enabled by a combination of self and mutual capacitance sensing. Both types have advantages and disadvantages.

Mutual capacitance enables multitouch functionality. Like self capacitance screens, mutual capacitance touch screens have horizontal and vertical rows of sensors, but measure capacitance through the intersections of these sensors (X*Y) instead of as individual sensors (X+Y). Best-in-class touch-screen controllers in the mobile space offer 32 sensor channels for sensing a 4.5" (16:9 aspect ratio) touch screen with an ideal sensor pitch of 5 mm. Because this type of measurement dramatically increases the possible number of sensors on a panel – a potential of 256 intersections, as opposed to 32 sensor lines for self capacitance – mutual capacitance scanning can deliver higher accuracy and true multitouch capability. Figure 3 shows five fingers on a mutual capacitance touch screen. All five input points are clearly identified, with no positional ambiguity.

The downside to mutual capacitance devices is that they require lower noise thresholds and increased sensitivity to support advanced features like hover technology, which makes them susceptible to performance disruptions from conducted noise.

Self capacitance, on the other hand, is a robust method of sensing in many ways. It generates a stronger signal and is capable of projecting larger fields than mutual capacitance. This increased strength and projection ability enables the touch-screen controller to accurately pick up the capacitance of objects like a finger hovering over the screen. Self capacitance can also provide more touch sensitivity without lowering the noise threshold, making it far less susceptible to false touches, poor accuracy, and delayed response times than mutual capacitance.

The problem with self capacitance is that it does not support true multitouch functionality. This is because of an issue known as ghosting, when there is ambiguity in the position of the two fingers on a screen. In self capacitance sensing, input is measured for change along the horizontal and vertical axes (X+Y). This results in positional ambiguity if the user touches two places on the same line. Resolving this problem becomes impossible with a third touch. Figure 4 shows this kind of ambiguity in a self capacitance touch screen. The green circles are actual touches on the X and Y sensor lines. Because each line reads a touch, the intersection of those lines registers touches (depicted with a red X) as well, even though none are present.

To deliver accurate hover functionality, a touch-screen controller must utilize both methods, which is not always easy to accomplish. Some touch-screen controller manufacturers use two different chips – one for self capacitance and one for mutual capacitance – to try and achieve this kind of functionality. The problem with this is that adding more silicon to a design significantly increases materials cost. It also imposes limitations on device size, as designers must make room for another chip on the board.

What device manufacturers truly need is a touch-screen controller capable of providing both self and mutual capacitance sensing on the same chip, with the ability to switch between both methods while in application. This keeps materials cost and device size down by eliminating the need for a second chip. A controller’s capability to actively switch between the two methods is important for maintaining responsiveness to user input. By using a touch-screen controller that combines self capacitance, mutual capacitance, and the ability to switch dynamically between the two, mobile device manufacturers can incorporate hover functionality into the UI.

The next big trend in mobile devices

Hover technology will dramatically change the way users interact with handheld devices. App, mobile, and OS developers are already designing with hover use cases in mind for their upcoming releases. Other emerging technologies like 3D mobile displays create new opportunities and applications for hover sensing.

Integrated hover and multitouch capability can only be enabled by touch-screen controllers that support both self and mutual capacitance sensing. To keep costs low, designers should look for touch-screen controllers with the ability to deliver both types of sensing on the same chip. By solving long-standing problems like force measurements and operation with gloves, plus adding new features, commands, and other UI options, hover is poised to make a major impact on the mobile device industry.

Christi Juchmes is a product marketing specialist at Cypress Semiconductor.

Cypress Semiconductor
[email protected]
www.cypress.com
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