Touch interfaces 101

Here are some basics for undertaking an interactive interface design while emphasizing the hardware selection process

BY BRUCE DeVISSER
Fujitsu Components America
Sunnyvale, CA
http://www.fcai.fujitsu.com

Increasingly, a broad range of entertainment electronics products use, or will use, an interactive touch interface as the main human machine interface (HMI). The touch interface provides a very user-friendly way to operate sometimes-complex consumer entertainment devices with a very small learning curve.

For designers, implementing an interactive user interface (HMI, MMI, GUI pick your favorite acronym) is a combination of using common sense and best technology. This approach is necessary to create a visual and physical interactive experience that meets all requisite goals with a technology that suits the device’s intended use.

Interactive touch display simplified subsystem block diagram: Video data and position must be displayed by the operating system and underlying software; a touch event is detected by the touch controller and reported to the operating system, which uses a software filter to test if the touch xy matches any video xy of interest.

With the rapid rise of HMI interface popularity, creating a design outline will help to quickly bring together the right parameters for the many new touch-based consumer devices.

Create a design outline

First, it is helpful to establish a base on which to build knowledge. Although not all-encompassing, there are a few key points to consider when starting a touch-input-based project:

The product’s external envelopeDisplay type, size, and specificationsOperating environmentIndustrial design objectivesDesign extensibility

The product’s external envelope will determine the size of the available display placement area and the location of the display. This can vary from a small portion of the product to the majority, or even the entirety, of the external envelope surface area.

This information can be used to sketch basic display requirements, since the space available dictates the range of display sizes that can be accommodated and the associated physical parameters. For example, if a 5.7-in. LCD is too big and full-VGA (FVGA) capability is needed, a smaller FVGA display may require some compromises, such as using a landscape-mode display or changing the user interface design due to the smaller active-display area available.

Sometimes compromises lead to other choices that actually enhance the design. For example, a landscape-mode (16:9 or 16:10 aspect ratio) display could present movie clips without any reformatting.

Display parameters should be set with the operating environment in mind, so designers must identify early where the product will be used, such as home, office, vehicle, personal (handheld or worn), indoors, outdoors, etc. If the product is intended for only one environment or if the range of ambient light is limited, the display choice is easier than if several environments must be considered.

Adding a touch panel will also somewhat affect viewing quality. It is best to factor this in at the beginning of the design cycle, but realize that adding touch usually needs some sort of mock-up to check the acceptability of any design through testing in a simulated or actual operating environment.

User interaction must also be considered. But when a product is completely new, it may be difficult to accurately define how it will be used. Alternatively, a user-model could be developed based on data from how similar devices are used, or at least from what is imagined by the product-defining team.

The next step is to choose a touch interface. The two most popular for entertainment electronics are resistive and projected capacitive. After determining which LCD type will be used, the next data point needed in choosing the touch interface is the product industrial design (ID).

The current ID trend for portable consumer electronics is a glossy-top surface finish, edge-to-edge flush panel, similar to some of the newer cell phones and personal entertainment devices. However, a home entertainment device design may be very different. It may include a panel-mounted display or one incorporated into the faceplate of the product.

Once a panel technology is chosen, manufacturers can provide appropriate engineering design support to achieve a successful product. Whichever school of design meets the product goal, ID is mentioned because it influences the touch technology selection and implementation.

Another key consideration affecting touch technology choice is whether a stylus will be part of the interface operational design. This feature is a common reason for selecting resistive, as it responds very accurately to contact by any solid implement.

Some engineers prefer a continuous touch surface. But even if the touch device could offer this, the display must still have some minimum border (frame) dimension, to which space for wiring and enclosure thickness must be added.

Beyond all this, each touch technology has its own specific physical and electrical design requirements. The most common, resistive, requires the outer border area (frame) be protected somehow from point pressure. Typically, this isn’t a problem for designs that use some sort of bezel around the display or can mount the display assembly behind a panel, which insulates the touch panel frame area from pressure points.

If the design calls for a flush top, and the preferred or required choice is resistive touch, specialized design methods can create a flush surface. Resistive panel sizes range from 1.8 to over 17 in. and touch panel thickness is typically 1.0 to 2.0 mm.

If the product calls for finger-only input and a glossy, edge-to-edge surface finish is desired, the current, rising star of touch technology, projected capacitive, can provide a continuous, edge-to-edge, hard-front surface. This panel type needs a masked frame (border) area of a few millimeters to hide the internal wiring of the touch panel, which is typically applied to the required plastic or glass top cover sheet.

This technology’s main feature is the ability to provide simultaneous, multiple-touch detection. Current sizes are targeted to devices in the 2 to 5-in. range, with the touch-panel-plus-cover-sheet thickness varying from 1.25 to 2.2 mm.

With both resistive and projected capacitive technologies, there are specific assembly integration considerations, such as the z-axis spacing between the touch panel and display, which typically ranges from 0.5 to 1.0 mm. Maintaining this spacing can be critical for EMI purposes, especially with projected capacitive.

Expanding functionality

Perhaps the most exciting area of design consideration is extensibility. One of the most attractive aspects of an interactive interface is the ability to completely alter its appearance and function solely by changing the application software.

What was once a difficult choice of retooling mechanical components when a new function was needed can now be avoided. Also, when designing a family of products, it may be possible to reuse the touch display assembly in multiple devices to significantly save design time and cost.

A final consideration is whether the device must emit sound or music. New technologies enable a product to generate high-quality stereo sound from the flat surface of the touch panel by incorporating solid-state generators on the underside of the touch panel. This requires a minor concession to stack-up height, but providing sound without an aperture on the front surface is very attractive for some designs. ■