Eric Fuhs is not your average mechanical engineer. His team of Engineers at Palm designs not just PDAs, but Palm's premium, "high-fashion" PDAs-the company's smallest, sexiest, top-of-the-line hand-held devices that are not only packed with features, but also have the look and feel of luxury. Fuhs and his guys are good. They know how to make small. They know how to make solid. They know their stuff.

So imagine yourself in this meeting at Palm. Palm's industrial designers-the people who specify how a future PDA will look and feel and operate-come to Fuhs with a nonfunctional block model of what would be one of the smallest PDAs ever. They want this PDA to fit in a shirt pocket, even though it requires a bigger than usual display, a bigger than usual battery, and has a raft of gee-whiz features. And that's not all they want. They want this PDA to physically stretch. They want you to be able to pull on one end and extend the PDA to reveal even more features. They want a lot. A whole lot. And, finally, the industrial designers turn to Fuhs and his guys and ask if it can be done. And Fuhs and his guys say no. But that "no" was only tentative, fortunately, or this story would end right here. When feasibility studies showed that the proposed PDA was theoretically possible, but very challenging, the answer changed to yes. Barely a year later, Palm's Tungsten T was launched.

A Stretch

Both literally and figuratively, the Tungsten T is a stretch. When closed, it's only 4 inches long-small enough to fit in a shirt pocket. When open, it stretches to 4.8 inches. And even when it's closed, you can use it to retrieve information from its application programs-calendar, address book, and to-do list-and to write digital-ink notes with its stylus. When it's open, revealing Palm's traditional Grafitti character-recognition area, you can use the stylus to input information to all those applications and many more.

But designing the Tungsten T's sliding portion was a huge challenge, and the main reason for the engineers' initial reluctance. The slider isn't just an extension of the T's housing, it also contains some of the PDA's working components-a keypad, a peripheral connector, and a switch to detect when the slider is open or closed. In addition, a flex circuit in the slider provides electrical connections with the main assembly. The slider also needed to include a durable, long-lasting sliding mechanism that wouldn't increase the PDA's overall thickness. "The biggest problem was trying to get the sliding mechanism as thin and small as we could get it," says Fuhs, "and on top of that, there were our ruggedness requirements, our drop tests." Palm's tests required survival after a 4-ft drop onto a carpeted surface and a 3-ft drop onto a harder surface-a tough test for any PDA, let alone one with a mechanism that slides open and shut. So tentative were the engineers initially, that they began simultaneously developing a conventional, non-sliding PDA as a backup.

The slider concept gained impetus, however, with a jump-start idea from Lawrence Lam, Palm's manager of industrial design. Lam suggested using rails on the sides of the PDA's sliding component that would mesh with rail guides in the main body. By being on the side, these rails could provide stability to the slider mechanism without adding to the T's thickness. Fuhs and his MEs liked the idea. "The rails gave us a nice, long glide surface that worked out really well," Fuhs says.

Fuhs and his team, and especially engineer Mark Luzbetak, took it from there. The stainless steel rails that they designed are riveted to the sides of the slider component and slide into rail guides molded into the Tungsten T's plastic midframe, the PDA's main structural component. The rails keep the slider from wobbling up and down and from rocking back and forth. To do that, though, the rails needed to be strong. "One of our concerns early on was that we couldn't make the rails stiff enough, so that when you try to open the slider, it would rock to one side, and then bind, and then rock to the other side, like opening a dresser drawer if you pull on one side," Fuhs adds.

Stability

Taking no chances, Luzbetak designed 1.3 mm thick stainless steel rails that are more than stiff enough for the needed stability. "We did a series of calculations to determine thickness," Luzbetak says. "But at the end of the day, we just had to go with our gut feel. We didn't have time to fully task the job before we decided which direction to go. I think we ended up with rails that are probably much stronger than they need to be." For even more stability, the engineers added a pair of rails to the back of the PDA. Recessed tracks for these rails include detents for the open and closed positions. "The main rails on the side are what really attach the slider," Luzbetak says. "The rails on the back stabilize the slider and serve as a stop."

Not everything went smoothly in the slider's design, however. The sliding part of the PDA includes a keypad, which includes at its center a five-button navigation ring. The engineers discovered that pushing on the buttons caused the keypad platform to bow, thereby touching and activating the character-recognition digitizer that's hidden underneath it when the PDA is closed.

Solving the bowing problem required finite element analysis. The engineers knew the force needed to activate the navigation buttons, so they multiplied that force by ten and used FEA to estimate how much the keypad platform would deflect as a result. With that knowledge, Luzbetak stiffened the platform by reinforcing it with a plate of stainless steel, again implementing a conservative "overkill" design. In subsequent prototype testing, Fuhs says, his team measured the amount of force needed to force the keypad platform down to the digitizer and found that it was significantly higher than the FEA showed.

Surprisingly, the Tungsten T's stylus also proved to be a design challenge. Any stylus short enough to stow in the T's closed configuration, the engineers soon discovered, was so short that it wouldn't nestle properly between a user's thumb and forefinger during use. As a remedy, Palm's industrial designers suggested a telescoping stylus, but there were problems with that, too. A simple "pick-and-pull" stylus requires some feature that lets you grab onto it; otherwise, it has to protrude far enough from the PDA's body to let you grab it, thus negating the benefit of shrinking the Tungsten T's dimensions. An alternative was to minimize protrusion by incorporating a flat disc on the end of the stylus that could be pulled out using only a fingernail. The problem with that idea was that the disc would dig into a user's finger when the stylus was used for writing.

Push-Push

Finally, someone-no one seems to remember who-came up with the idea of a "push-push" telescoping mechanism: Push once on the end of the stylus to collapse it into the Tungsten T, push again to extend it far enough to grab and remove it. In essence, it's a lot like a retractable ballpoint pen, which served as inspiration. "We went out and bought several Bic pens to see how they work," says engineer Joel Friedrick. "The problem with Bic pens, though, is that they're too big in diameter, so we couldn't use their mechanism." The Tungsten T's stylus, Friedrick explains, has an outside diameter of only 5.2 mm; the inside diameter, where a push-push mechanism needs to be, is even smaller.

Ultimately, Fuhs's team came up with two viable push-push concepts, both similar to, but still significantly different from, ballpoint mechanisms. "We took them both through prototyping," Fuhs says, "and both performed very well." One version, though, couldn't make it through the required 200,000 cycles of life testing. The one designed by Friedrick and others could, however, and thus became the winning design. Surprisingly, considering its simple but elegant look, it contains over a dozen components.

There remained, however, the problem of packing all of the Tungsten T's features into its small package, and especially onto its tiny circuit board. The T's slider, by allowing the PDA to close into a compact form, significantly reduces the PCB's length compared to circuit boards in other, noncompacting PDAs. In addition, Fuhs's engineers were trying to make the T as narrow as possible to better fit in a user's hand-a feature being pushed by the industrial designers and the marketers as a potentially big selling point. At one point, the engineers made an important step toward that goal by providing storage space for the PDA's stylus behind the display, rather than beside it as in earlier models. That reduced the PCB's width by 5 mm, and switching from a custom battery to a smaller, standard battery took away another couple of millimeters-not much for many circuit boards, but a lot in this case. When it was all over, Friedrick notes, the Tungsten T's PCB provided only about half the area of boards in other PDAs.

All the while, though, Fuhs's team had been trying to shrink only the PDA and not the circuit board, making mechanical design compromises in order to give the electronics engineers as much board space as possible for their circuitry. Realizing the difficulty that the MEs were facing, Electrical Design Manager Mike Cortopassi took up the challenge of fitting circuitry into whatever space, however small, Fuhs's team could provide. As Fuhs relates it, Cortopassi said, "Don't mess around anymore. Give us the board you guys want, and we'll fit our stuff in there." And, in an iterative process with the mechanical engineers, they did, arranging and re-arranging components until everything fit like pieces of a puzzle.

3D World

But fitting those pieces together wasn't easy. "We had to nest components," Fuhs says. "We nested a taller capacitor in one area where we had the height, and we put lower resistors and capacitors over in another area where we didn't have as much height." Fuhs's MEs assisted in that process by applying Pro/ENGINEER, a 3D CAD program from Needham, MA-based PTC, to 2D layouts created by the electrical team. "The PCB layout guys pretty much live in a 2D world," Fuhs says. "They look at a screen; they put the components down and route the traces. Mechanicals live in a 3D world." And there was also the problem of making the Tungsten T aesthetically pleasing with a look that says "high fashion." That look comes largely from a housing made of what the Palm team calls cosmetic metal, in this case etched aluminum. The engineering team had used this metal on the earlier Palm 5 PDA, and Luzbetak had become an expert in dealing with its eccentricities. Most of the housing consists of metal pieces 0.6-mm thick, but the piece that covers the keypad is thinner at 0.4 mm. Luzbetak explains that thicker metal wasn't needed on the keypad for strength-the underlying stainless steel sheet provides that-and saving even 0.2 mm was important in keeping the Tungsten T appropriately thin.

And thin the Tungsten T is, although not quite as thin as the earlier Palm 5, which doesn't have a sliding mechanism. The T's slider, however, makes possible a PDA that is very short and narrow and that fits in either a hand or a shirt pocket exceptionally well. That small size, the people at Palm are convinced, is the main reason the Tungsten T has sold so well in its short time on the market.

How the T came to be so small, though, is a story of good engineering that vey few consumers are aware of or will appreciate, even as they enjoy its benefits. The T's slider is plainly visible, yet most users will likely take it for granted. They will, however, appreciate how the slider makes the Tungsten T so small. That, at least, is an open-and-shut case.

Contact Contributing Editor Gary Legg at[email protected].