Electronic Design
Designing Larger Touchscreens Is No Small Gesture

Designing Larger Touchscreens Is No Small Gesture

Riding along with the tidal wave of personal computing, device manufacturers are now moving faster than ever to port touchscreen technologies to large-format hardware. However, the transition from small screens and simple touch-enabled applications to a new paradigm, where hands and fingers are the primary tools for interacting with full-scale computers, is likely anything but straightforward.

Manufacturers need to rethink the way that consumers will use touchscreens and address a new and more demanding set of requirements. To cite just one example, “multi-touch” capabilities chiefly consist of a few finger strokes on today’s 5-in. screens. What will they mean on a 12- or 40-in. device—or when multiple users interact simultaneously using both hands? What wildly popular new applications will emerge for large-format touchscreens, and how can manufacturers ensure that their devices will support them?

For device manufacturers, these are not academic questions. But while you can’t predict the future, you can certainly prepare for it. Thus, it’s a good idea to take a closer look at some of the key requirements for building successful touchscreens and how those requirements change for large-format devices and applications.

Basics of Touchscreen Technologies
Large or small, the success of any touchscreen device is a function of the technology choices made in designing it, the most important being projected capacitance technology, sensor design, and driver chip.

Today’s devices overwhelmingly use capacitive touchscreens, which operate by measuring small changes in capacitance—the ability to hold an electrical charge—when an object (such as a finger) approaches or touches the surface of the screen. However, not all capacitive touchscreens are created equal. Choices in the capacitive-to-digital conversion (CDC) technique and the spatial arrangement of the electrodes that collect the charge determine the device’s potential performance and functionality.

Device manufacturers can arrange and measure a touchscreen’s capacitance changes in one of two ways: self-capacitance and mutual-capacitance. Most early capacitive touchscreens relied on self-capacitance, which measures an entire row or column of electrodes for capacitive change.

Though this approach is fine for one-touch or simple two-touch interactions, it presents serious limitations for more advanced applications because it introduces positional ambiguity when the user touches down in two places. Effectively, the system detects touches at two (x) coordinates and two (y) coordinates, but has no way to know which (x) goes with which (y). This leads to “ghost” positions when interpreting the touch points, reducing accuracy and performance.

Alternatively, mutual-capacitance touchscreens use transmit and receive electrodes arranged as an orthogonal matrix, allowing them to measure the intersection point of a row and column of electrodes. In this way, they detect each touch as a specific pair of (x,y) coordinates. For example, a mutual-capacitance system will detect two touches as (x1,y3) and (x2,y0), whereas a self-capacitance system will detect simply (x1,x2,y0,y3) (see the figure).

The underlying CDC technique also affects performance. The receive lines are held at zero potential during the charge acquisition process, and only the charge between the specific transmitter X and receiver Y electrodes touched by the user is transferred.

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Other techniques are available, but CDC holds an edge because it’s immune to noise and parasitic effects. Such immunity allows for additional system design flexibility. For example, the sensor IC can be placed either on the flexible printed circuit (FPC) immediately adjacent to the sensor or farther away on the main circuit board.

Electrode pitch, a key parameter in sensor design, refers to the density of electrodes—or more specifically, (x,y) “nodes”—on the touchscreen. Also, it largely determines the touchscreen resolution, accuracy, and finger separation.

Naturally, different applications have different resolution requirements. But today’s multi-touch applications, which need to interpret fine-scale touch movements such as stretching and pinching fingertips, require high resolutions to uniquely identify several adjacent touches.

Typically, touchscreens require a row and column electrode pitch of approximately 5 mm or less (derived from measuring the tip-to-tip distance between the thumb and forefinger when pinched together). This allows the device to properly track fingertip movements, support stylus input, and with proper firmware algorithms, reject unintended touches.

At the core of any successful touch-sensor system is the underlying chip and software technology. As with any other chip design, the touchscreen driver chip should feature high integration, minimal footprint, and close to zero power consumption. Moreover, it must have the flexibility to support a broad range of sensor designs and implementation scenarios. Any driver chip will be measured by its overall balance of speed, power, and flexibility.

Supersizing the Touchscreen
These considerations apply to a touchscreen device of any size. But what are the specific considerations for moving to large-format devices? Manufacturers will find that the key requirements for modern touchscreen technologies—multi-touch support, performance, flexibility, and efficiency—become even more critical when users adopt larger screens that feature more complex touch applications.

Users of the Apple iPhone and other contemporary devices will be familiar with today’s multi-touch gestures, typically pinching or stretching two fingers. With a larger screen, however, it becomes possible to envision much more complex multi-touch gestures.

Imagine painting and music applications for young students, for instance, that involve gesturing with all 10 fingers and thumbs. Or consider new tablet-based games that pit two or more users against each other on the same screen.

As large-format touch computing evolves, application developers will demand the flexibility to take full advantage of new kinds of touchscreen interactions. Device manufacturers don’t want to stand in their way, and they certainly don’t want to build a device that’s unable to support the next hugely popular touch application.

As large-format touch applications begin using four, five, and 10 touches, it’s important to consider how new applications might exploit these capabilities and how the controller chip will use this richer information to create a better user experience. For example, the ability to track incidental touches around the edge of a screen and classify them as “suppressed” becomes even more important on a large-format device than on a smaller product.

Much like a mobile phone’s touchscreen ability to recognize when a user is holding the phone or resting the screen against one’s cheek, a larger-format system must account for the different ways that users will hold and use the device. Examples of this include resting the edge of the hand on the screen when using a stylus or resting both palms when using a virtual keyboard.

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It’s not enough to identify and suppress incidental touches. The device must track them so they remain suppressed even if they stray into the active region. The more touches that a controller can unambiguously resolve, classify, and track at once, the more intuitive and accurate the experience for the user.

Performance is a function of six basic factors. First, accuracy means the fidelity with which the touchscreen reports the user’s finger or stylus location on the touchscreen. An accurate touchscreen should report touch position better than ±1 mm.

Linearity measures the “straightness” of a line drawn across the screen. It depends on sound screen pattern design and should be accurate within ±1 mm. Next, finger separation describes how closely the user can bring two fingers together before the device recognizes them as a single touch.

Response time measures how long it takes the device to register a touch and respond. For basic touch gestures such as tapping, the device should register the input and provide feedback to the user in less than 100 ms.

Factoring in various system latencies, that typically means that touchscreens need to report a first qualified touch position in less than 15 ms. Applications such as handwriting recognition require even faster response.

Fifth, resolution is the smallest detectable amount of finger or stylus motion. It’s important to reduce the resolution to a fraction of a millimeter for a number of reasons, particularly when it comes to enabling stylus-based handwriting and drawing applications.

Finally, signal-to-noise ratio (SNR) refers to the touchscreen’s ability to discriminate between the capacitive signal arising from real touches and the capacitive signal arising from accidental noise. Capacitive touchscreen controllers measure very small changes in the row-to-column coupling capacitance.

The way those measurements are performed strongly influences the controller’s susceptibility to external noise. Large-format touchscreens are especially challenging in this regard, because the LCD itself is one of the most significant noise generators.

As touchscreens grow larger and support more simultaneous touches—and more complex interactive content—top performance in these categories becomes that much more critical.

Flexibility and Efficiency
Most of today’s small touchscreens are designed to support a specific device, and often, specific software and applications. Emerging large-format touchscreens, however, will need much greater versatility.

For example, a paper-sized tablet device is a natural fit for handwritten input using a stylus. To support that, though, the touchscreen requires a higher resolution than one intended for fingertip gestures on a 5-in. screen.

In a mobile device, Windows Hardware Quality Labs (WHQL) certification may be incidental. For a large-format device that will function as a PC—and support common computing applications—WHQL certification is essential. Large-format touchscreen devices will also double as e-book readers, as well as video devices. To fully enjoy any of these applications, a 10-in. or larger screen requires superior visual quality.

While they won’t be used like a mobile phone, tablet devices will still need to be light enough to be comfortably used in a variety of positions and locations. This requires small, lightweight batteries, which means highly efficient microcontrollers with excellent power consumption.

Balancing this requirement against the need for greater accuracy and responsiveness (after all, a larger screen inherently means higher resolutions, more nodes to process, and exponentially higher processing power) remains one of the sturdiest challenges in designing an effective large-format touchscreen.

Moving from a smaller to larger screen for touch isn’t trivial, but vendors have done so based on their expertise in microcontrollers and touch algorithms. Designers should look for the features and requirements outlined here in looking to migrate from a smaller to larger screen using touch technologies.

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