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Tap capacitive sensor UI in next-gen CE devices

Posted: 02 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:capacitive sensor user interface? consumer electronic devices? capacitive sensor for CE interface?

Designers working on small systems such as mobile products, portable digital entertainment devices, remote controls and digicams continue to face major challenges as these products increase in complexity and functionality. For example, current generations of cellphones are facing user interface (UI) and ergonomic issues due to a crowded keypad and touchscreen combination.

As a result, designers are migrating toward the extra UI space that resistive touchscreens (RTS) offer. This particular sensor is a widely available commodity technology. However, even with the added space, embedded designers face new issues stemming from RTS limitations.

The two main challenges that embedded designers face with RTS are severely impaired optics of the underlying display and poor durability. Devices using RTS often fail in the field when they're dropped or the user presses the screen too hard.

Overcome RTS limitations
A more efficient and reliable alternative is a thin, transparent, capacitive sensor touchscreen that embedded users can place over any viewable surface for input and navigation. One implementation of this kind of capacitive sensor interface, called ClearPad, offers designers a solution to overcome RTS limitations.

The capacitive sensor module consists of a clear, thin transparent finger sensing region bonded to a flex circuit tail, which contains all of the sensing electronics. A finger on top of a grid of conductive traces changes the capacitance of the nearest traces (See figure below).

This change in trace capacitance is measured and the finger position computed. No pressure is needed to activate the capacitive sensor. A gentle stroke or glide along the surface of a capacitive pad is all that's required.

Specifically, the capacitive sensor's surface receives touch information from contact with the user and sends this information to the controller board. The controller board processes touch signals and conveys the information to the host. The host then uses finger position and contact information for various UI features, such as entering characters and data, and scrolling.

A capacitive sensor panel is solid-state. A finger on top of a grid of conductive traces changes the capacitance of the nearest traces. Trace capacitance change is measured and the finger position is computed.

In this UI technology, capacitive sensing is combined with a transparent trace matrix. The same materials used in RTS are used in the capacitive sensor interface approach, specifically indium tin oxide on polyethylene terephthalate (PET).

However, the capacitive sensor does not possess RTS's optical and durability issues. This is because it is a single laminate with no air gaps to degrade optics and is solid-state with no moving parts, thus it's highly reliable and durable. Resistive screens, on the other hand, are physical switches that must flex and require rub-in use, decreasing their useful lifetime.

Applications custom fit
Embedded designers working on applications like mobile, remote controls and digicams can specify size and shape customization of the capacitive sensor and supporting electronics module to meet specific application requirements. The assembly includes the sensor, controller board with proprietary IC and firmware.

Designs based on the capacitive sensor present improved optics and finger-sensing capabilities to end users. Text and graphics displayed on the underlying screen are crisp and clear, thanks to the sensor's matched optics that reduce internal reflections.

A capacitive sensor panel designed for a 4-inch diagonal TFT display, for example, has an active area of up to 60-by-80mm with 0.68mm sensor thickness (including a 0.075mm adhesive for laminating to the lens or casing). The active area is the sensor's transparent region reporting presence and location of a user's finger.

The viewing area, also in the sensor's transparent region, is outside the active area and does not detect the user's finger. The opaque PET inactive, non-sensing borders on three sides of the sensor's viewing area allow for low resistance trace routing. These borders are designed to be shielded both electrically and optically from the user. Lastly, the L-shaped or tail region connects to the sensor and houses the capacitive sensing electronics.

Since the sensor's finger-sensing region is transparent, it can ideally be used with contextual GUIs that change dynamically depending on the device's mode or application. Button arrays, sliders, soft-menus, cursor control and character recognition are possible. These interfaces give embedded designers a lot of design possibilities. The UI no longer needs to be fixed in hardware, but can now be entirely constructed in software to match the specific requirements for a given task or application.

Design considerations
Capacitive sensor technology ushers in more enriching design considerations than what embedded designers have become accustomed to with RTS. Thus, creating an intuitive UI involves more than optimizing the design of the capacitive sensor itself. In particular, designers must pay special attention to making accommodations in the device UI for input inaccuracies introduced by the user.

Since a capacitive sensor is optimized for finger usage, the UI designer must consider that typical users will not be able to reliably position and control their finger with great accuracy. Although designing for finger usage appears to be a limitation, such a constraint results in a more intuitive and simpler UI. Such a UI is more suitable for mass-market devices and users who may not be as technology-savvy or lack the time for substantial device training.

There are other ways to optimize the user's interaction with this sensor. They fall into two categories. One is static design, which includes control discoverability, layout and tactile definition. The other is dynamic control processing, which includes button activation methods, hysteresis in gestures and consistency in UI processing.

Control layout design
Another important interaction rule for control layout design is proper spacing and sizing of the UI elements. Although screen real estate is at a premium for most handheld devices, small control elements packed too closely will frustrate users.

It is important to determine the range of typical finger sizes of the device's target users. Buttons and UI elements smaller than the smallest anticipated finger contact area will cause usability problems and should be avoided. Furthermore, ensuring that enough space is provided between UI elements is important. Ideally, control elements should have a pitch (the spacing between the centers of UI elements) of at least a finger width.

One aspect of UI element design often overlooked is that the drawn element size does not need to match its activation size. For buttons, this implies that the graphic for a button does not need to correspond to its activation region.

For capacitive button layout, embedded designers should avoid arrangements where it is difficult for the user to touch one button at a time. Equally sized buttons are not equally accessible. To improve button accessibility while conserving space, some buttons can be made smaller and others larger, depending on their location and function in a product's overall design.

- Mariel Van Tatenhove
Senior Product Line Director

- Andrew Hsu
Strategic and Technical Marketing Manager
Synaptics Inc.

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