March 2000

Portable electronic devices offer mobility. But with mobility comes risk. Cellular phones, notebook computers, personal digital assistants and pagers drop to the floor or are bumped against unyielding objects every day. Yet users expect them to continue operating.

That's where the work of such researchers as Suresh Goyal comes in. Goyal, a member of the technical staff in the Wireless Components and Packaging Research department at Bell Labs in Murray Hill, NJ, analyzes products and studies how impacts affect them. He then works with designers to improve the impact tolerance of products. Bell Labs, whose scientists are responsible for many of the greatest inventions of the past century, from the transistor to the laser and cellular telephony, is the research and development arm of Lucent Technologies.

Impact-Induced Loads

During impact testing, the force on a portable device such as a cellular phone may range from hundreds to thousands of g's (1 g is the acceleration of an object because of gravity) over the course of several milliseconds. As a result, the product's housing may become deformed or come apart, and such fragile components as ceramic substrates or liquid crystal displays may fracture.

"A dropped portable electronic device is most likely to strike the floor at an angle," Goyal said. "First one corner touches down, then the object rotates, and other corners bounce and clatter. The result is that the ends of the product strike at much higher velocities and receive much more powerful impacts than the initial drop would lead you to expect."

The most common approach to protecting portable electronic equipment against such impact forces comes in the form of plastic housings with extra-thick walls, numerous screws, snaps, and hooks to hold the casing together, and extra space between components. But in communications, the market demands small, lightweight products. So designers must find other ways to protect the delicate internal electronics.

The Role of High-Speed Imaging

To observe and record the complex sequence of events in impact testing, Goyal employs a high-speed camera -- preferably one that can record from 1000 to 10,000 frames per second. "The higher the frame rate, the better. The higher the time resolution, the greater the information we have to work with," he explains.

At present Goyal is using the high-speed motion analyzer Model 4540 from Roper Scientific MASD (formerly Eastman Kodak Company's Motion Analysis Systems Division). The imager captures up to 4500 full-frame grayscale images per second at 256 x 256 pixels and up to 40,500 partial frames at lesser resolutions. Typically, he uses the analyzer at 4500 to 18,000 frames per second.

The high-speed motion analyzer captures an event digitally in dynamic RAM. The sequence of images can then be saved to a hard disk or written to CD, like any computer data, or recorded on analog videotape. A direct digital interface from the analyzer's CPU to a desktop computer makes it simple to store the images for later review and quantitative analysis using a computer.

Using the high-speed digital motion analyzer to document and study impact testing lets Goyal frame the shot, record, and view the image series in minutes. If he wants to see different views of the impact event, he can stage the experiment again until he succeeds in getting the scenes he wants.

The high-speed motion analyzer he uses, configured with 1536 megabytes of DRAM, can capture and store 24,576 frames (5.46 seconds of recording at 4500 frames per second). Out of that total, only a few hundred capture the impact event. The lab's previous digital-motion analyzer could capture only 0.66 seconds of data. The extended period of capture now makes it much more likely that the event will be sufficiently recorded in a single attempt, since it isn't necessary to synchronize the event and trigger the motion analyzer so precisely.

Once the images are captured, Goyal can review them on a video monitor at a variety of playback speeds, from real-time replay to freeze frame. A few milliseconds can be stretched out to minutes.

Selected frames can be analyzed in detail. Goyal can choose a subset of the event -- a half dozen frames that capture the most important moments -- and share them over e-mail with designers in the next building or thousands of miles away.

Making Sense of the Images

What do the images show? The body of a device with a clamshell case flexes when it strikes a surface. It deforms and buckles, and the top and bottom halves of the case separate momentarily. These are effects that can be seen and understood only by viewing the high-speed images. A second after a device is dropped, it may appear completely normal and unaffected. Only the digital images reveal the violence it has endured.

"The high-speed images are used to validate analytical results and to design modifications," Goyal said. The final product design is tested before commercial release.

The value of this work can be measured in the increased reliability of products. Greater impact tolerance means fewer warranty claims, which translates directly to the bottom line, and a reputation for reliability in the marketplace.

The value of testing makes even more sense as the value of the product increases. Devices with special capabilities and proprietary interfaces may sell for hundreds or thousands of dollars, but they are likely to come with an iron-clad warranty. Whatever happens to them, the customer can take them back for replacement. For products like that, impact tolerance is critical.

The value of impact testing is clearly more important than ever before. Impact test data can be used to optimize designs almost from the start of the design process.

"Equally important, it confirms the public perception of products as sturdy, reliable, and of the highest quality," Goyal said. "It's difficult to put a price on that."

For more information, contact Wendy Telford at Roper Scientific MASD, 11633 Sorrento Valley Rd., San Diego, CA 92121; (858) 535-2909; e-mail: wendytel@aol.com.

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