Under the Hood Teardown: Motion LS800 tablet PC senses where, senses who without touching

Motion Computing's LS800 tablet PC, Figure 1 , represents the company's second-generation product with an increased emphasis on portability vs. the previously launched LE1600.


Click to Enlarge Image
Figure 1: The Motion Computing LS800 Tablet PC uses a non-contact stylus based on inductive pickup principles.

Billed as an “ultramobile slate,” the LS800 is based on an Intel 1.2-GHz Pentium processor. The central interface is an 8.4-inch SVGA thin-film-transistor display.

The touchscreen is an important differentiating element of the design, as it provides for most of the user input to the LS800. Bypassing traditional resistive or capacitive overlays to the screen (either of which can compromise display quality), the LS800 uses a special pc board behind the LCD panel to sense in noncontact fashion where the companion touchscreen pen/stylus is located. Wacom is behind the technical solution and the stylus itself is passive, requiring no battery and emitting no internally generated signals.

The system works by scanning signals into an X-Y matrix board behind the display, where traces form the equivalent of rectangular, planar coils, Figure 2 .


Click to Enlarge Image
Figure 2: The design uses a special Wacom digitizer IC, supplemented with a Authentec fingerprint sensor.

Signals sent into the planar coil set are induced into a resonant coil circuit (pickup) internal to the pointing stylus. When the scan signals cease, the resonant excitation of the stylus coil emits a signal for detection in the planar coil loops. The coil that exhibits the greatest level of induced voltage corresponds to the physical location of the stylus. It all works quite well, and the pen can still be detected while hovering almost a full centimeter away from the glass surface.

Aside from pen and planar coil pcb, a controller board hosts a Wacom ASIC, which is responsible for the mixed-signal sending and sensing duties. To complete the input solution, routine housekeeping and communication tasks are covered by a Matsushita microcomputer.

Along with a pointing apparatus for sensing “where,” the design incorporates technology for sensing “who.” With a swipe-style fingerprint reader for biometric security, the LS800 can be trained to detect a user fingerprint as a means of controlling tablet access. The fingerprint reader is a sensor from AuthenTec, a device whose core technology is a CMOS-based 192 x 16-pixel array able to capture unique fingerprint patterns from the peaks and troughs of the finger's skin. The pattern sensing rests on variations in pixel-to-pixel capacitance read at scan time. The finger provides a “plate” of the capacitor, whose height (and therefore contribution to interpixel capacitance) varies with ridges in the skin.

With its screen-centric design, the sensor technology for input and pointing in the LS800 has stepped up to a more-expensive, but more satisfying, solution over resistive or capacitive touchscreens. Likewise, as the need for security has become more acute, sensor technologies have arrived to deliver biometrics for well under $10, a smart and affordable technical addition that serves a need far beyond the particular slate-style PC discussed here.

About the author
David Carey is president of Portelligent, www.teardown.com. The Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.