Digitizing Technology for Pen-Based Systems (90439)

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Resistive Digitizers

Resistive technologies basically consists of a single or a double layer of transparent conductive sheets spread over a protective glass surface that lies above the top polarizing layer of the LCD screen. The double-layered version relies on membrane touch-panel technology. The pen causes the two sheets to make contact, and the currents are measured for an x and y coordinate of the pen position. The single layer version requires a tethered pen to conduct current through the single transparent sheet.

The two-layer resistive digitizing technology is relatively inexpensive and it is easy to implement on an LCD system. However, it has several drawbacks. The two conductive sheets on top of the LCD screen reduce brightness more than any of the other digitizing methods. Also, the two sheets, which are separated by spacers, are easily influenced by temperature and humidity in such a way as to adversely affect the uniform resistivity of the conductive sheets. As a result, the positional measurements for the pen are less accurate. Similar, though less pronounced problems, occur with the single-conductive-layer resistive digitizer design. A final flaw with both versions of the resistive digitizing technology is that it does not allow for proximity sensing of the pen.

Normally, pen development standards would expect the pen to register when the pen is close but not touching the pen writing surface, but this feature is not available with resistive digitizers.

Electrostatic/Capacitive Digitizers

Electrostatic or capacitive digitizers require a tethered pen, and a transparent conductive layer must be deposited on the underside of the protective glass shield. In this way, when the pen comes close to the surface of the glass, the electronic signal in the pen creates a capacitive effect with the conductive sheet on the underside of the glass. Relative current measurements determine the x and y coordinate position of the pen.

The electrostatic/capacitive digitizer has an advantage over resistive digitizers because it implements proximity sensing for the pen. Not only can the electrostatic method provide information about pen position above the writing surface, but it also provides information about pen height as well as pen tilt. The disadvantages of the electrostatic/capacitive digitizer is that it requires a tethered pen, and it reduces optical transmissions to about the same degree as the single-layer resistive technique (that is, the transparent conductive layer reduces LCD transmission by about 15 percent).

Electromagnetic Digitizers

The electromagnetic digitizer relies on a series of looped coils on a sensor board beneath the LCD screen. A magnetic field is created through the coils. The pen may either reflect these electromagnetic waves back to the sensor board by its own coil inside of the pen, or the pen is tethered and will send back its own signal. Pen position is determined by measuring the magnetic signal from the pen relative to the position of the sensor board coils under the LCD.

The main advantage that the electromagnetic digitizer has over the other two technologies is that it does not interfere with the optical transmission of the LCD. Because the sensor board is behind the LCD screen, only a protective clear glass layer needs to be placed above the top of the LCD display panel. A disadvantage of the electromagnetic system is that it requires the "light-pipe" design for backlighting. This method of backlighting requires that the light source be placed at the side of the LCD screen rather than directly behind the screen. Additionally the electromagnetic digitizer requires that a magnetic shield be placed between the sensor board and the system motherboard. Examples of currently released products that use the electromagnetic digitizing technique are the NCR 3125 clipboard, the WACOM 648A tablet, and the WACOM 510C tablets.