Turning point

The tablet PC class of pen-based mobile computers brings an old fable to mind. With the birds and animals poised for a battle, both sides ask the bat to join them. The bat refuses, on the grounds that, as a flying animal, he doesn't really fit in with either side. Both the birds and animals promptly turn against him. The moral of the story: Sometimes you have to take sides.

Most mobile-computer makers seem to have taken this tale to heart. Their tablet PC offerings clearly take sides. Some go the pure "slate" route and have no built-in keyboard at all. Other "convertible" models resemble gussied-up notebook computers with swiveling screens that fold back to cover the keys during pen use.

Unlike the other tablet PCs on the market, the new HP TPC 1000 doesn't take sides. It functions both as a pure slate and as a convertible machine, thanks to a keyboard that can either detach or fold under the slate, depending on the user's whim. And for good measure, this machine also fits into a docking station that provides connections for a full range of desktop peripherals. Ted Clark, the HP vice president in charge of these new computers, says that the Houston-based engineers responsible for this computer "skipped over the first and second generation and went straight to a third generation design."

Coming up with a machine that can do triple duty—in a meeting as a slate, on the road as a lightweight notebook computer, or in the office as a desktop—created a slew of mechanical design challenges that simply don't apply to traditional notebook computers. Steve Homer, the mechanical program manager for the tablet PC, experienced these challenges firsthand as soon as the project kicked off three years ago. "I had done some work with PDAs and thought the tablet PC would be similar," he recalls. And in some ways they are.

Like a handheld PDA, the HP tablet houses all its main components within a keyboardless box. "But the tablet PC design was surprisingly difficult," Homer continues, explaining that the design team faced more stack-up difficulties than they would have with a conventional notebook computer. For example, all of the usual computer innards—like the motherboard and hard drive—had to share the same slim enclosure with the LCD display, the digitizer that accepts the pen inputs, and the piece of coated tempered glass that provides a rugged writing surface. Making matters worse, the design team had to accommodate all the space-consuming extras that define a tablet PC.

Component selection also turned out to be more difficult than for previous mobile computers given the tablet PC's intended use. With their wireless capabilities and small size, tablet PCs are likely to spend much of their working lives cradled in their users' arms, away from a desktop. So controlling heat and extending battery life became more important than ever. The system also needed decent computing power—in part, so that latency from handwritten input wouldn't frustrate users. "Finding low-power components that didn't sacrifice performance was a crucial design task," says Peter Hunt, the tablet PC's director of engineering.

But for all the effort devoted to finding the right components and shoehorning them into a tight space, HP's take on the tablet PC literally hinges on what might usually be thought of as the least interesting part of a mobile computer—its hinge. The TPC 1000 features a distinctive rotating hinge that does far more than those found on clamshell mobile-computer designs. It not only allows the tablet to fully detach from its keyboard, but also joins the tablet to its keyboard in two different orientations that correspond to the slate and notebook modes. Until the design team committed to developing this complex hinging concept, the idea of a tablet PC that works in three modes would have been just a pipe dream, in Hunt's view. "The whole design follows from the hinge," he says.

The business end of HP's hinge assembly consists of a pair of rigid hooks that extend from the hinge's magnesium housing and engage a spring-release mechanism inside the tablet. These hooks, along with the rest of the hinge assembly, mount eccentrically on a rotating disk built into the top of the keyboard base. To use the TPC as a notebook, users rotate this disk until the screen comes into view. With the hinge in this position, the slate hovers over the center of the keyboard base. "In this design, once you rotate the tablet, it sits above the center of the keyboard, rather than having its hinge at the back like a clamshell notebook," Hunt explains. Yet like a clamshell notebook, the hinge assembly contains a standard friction clutch that allows the panel to tilt forward or backwards. "It was important that we not restrict the user's viewing angle," Hunt notes.

To fold up the computer, users rotate the disk 180°. This motion brings the hinge assembly just beyond the edge of the keyboard base and faces the screen away from the keys. From this position, the hinge works in the opposite direction as a viewing angle adjustment, and it allows the keyboard to fold up underneath the slate. It then locks in place with its keys sandwiched between the base and the bottom of the tablet. According to Hunt, this kind of hinge design has no parallel in the mobile-PC industry. It required the engineering team to work through three equally distinctive design challenges.

For one, this hinge concept only works with the screen pointing in the right direction—and that direction may not always be the obvious one. Whenever the disk rotates to its outward position, the tablet must point away from the keys to be in position for folding. "Some users will find that counter-intuitive," says Homer. So HP engineers added a locating feature—a post—to the magnesium hinge housing to ensure that the tablet mounts properly.

HP engineers also had to deal with the related issue of how the computer balances in its notebook mode. Homer points out that ordinary notebooks carry about three times more weight below their hinge than above it. "This computer is the inverse," he says, stressing that the tablet portion contains processor, drive, screen, and all the electronics needed for a stand-alone computer. Homer notes that the standard clutch mechanism HP used for the computer easily handles up to 15-in. panels, so it didn't pose a problem. "Our challenge was more a lack of weight on the base than the weight of the tablet," he says. And that challenge required the engineering team to fine-tune the tablet's balance. Much of the machine's ability to balance the 3-lb tablet on a thin featherweight keyboard comes from the design and location of the hinge assembly in the center of the keyboard. But the design team also looked beyond the hinge itself to other areas of the computer. They specified and located some internal components with an eye toward fine-tuning the center of gravity location—even when it might complicate the stack-up of the components. "For example, we used the largest disk drive we could to shift the center of gravity forward," Homer says.

Finally, the hinge assembly, particularly the arms, needed to be even more robust than a fixed hinge—partly because of the extra forces from the "weight inversion" and partly because users look for solid feel in detachable components. The additional forces required them to beef up the hooks, which are an integrated feature on the hinge's magnesium housing. The solid feel also comes from the fact that HP engineers extend the hooks "well into the body of the tablet," say Homer. They also tightened the dimensional tolerances for the hinge assembly—to about three times as snug as those for an ordinary notebook computer.