used as a visual feedback element so that the operator can correct the position of the wand until the cursor is properly placed. At this time the information from the tablet may also be transferred to either the host computer or the resident desktop or portable computer, as desired. Since the cursor is not used to signal its position to a pickup device, as is the case with the light pen, it may be used with any type of display system, including the non-light-emitting flat panel displays. Another advantage of the tablet is that it may be used to position cursors in the blank areas of the display, where no light pulses are available unless they are specially generated by the light pen There have been numerous improvements and new developments using a variety of technologies that include magnetostrictive, electromagnetic, electrostatic or capacitive, scanned X-Y grid, resistive, and sonic Of thes electromagnetic tablets dominate the digitizer market, and sonic is of interest because it does not requir tablet, but most of the other technologies are essentially restricted to touch input devices covered later. As ne previously, electromagnetic is the most popular technology for high-performance digitizer tablets. Operation based on transformer principles, whereby a conductor carrying ac creates a magnetic field around it that induces a current in a second conductor. The digitizer tablet uses the amplitude and phase of the induced current to determine digitizing data. The tablet contains an X-Y pattern of conductors beneath its surface, in manner similar to the Rand Tablet, but instead of counting pulses in a time period a circular conductor is sed as the pick-up element for the induced current. This coil is placed on the tablet surface, and its position determined by measuring the phase and amplitude of the current in the coil. Its center is interpolated by sweeping through the X-y grid lines and demodulating the signal in the coil to determine the phase reversal point, or by calculating this point using digitized data fed into a microprocessor. The X-Y coordinates may be solved to better than 0.025 mm using either of these two techniques. Figure 89.5(a) is a photograph of a representative digitizer tablet. Another digitizer technology is the one that uses the measurement of the time required for sound waves to travel from a source to movable microphone pickups. This sonic technology has the advantage that no special digitizing board is required, and either a stylus or a cursor can be used as the digitizer. Two sonic sources are ontained in an L frame so that both X and Y coordinates can be determined by calculating the time it takes for the sound wave to reach the microphones contained in the pickup device. This calculation is made on the basis of sound traveling at 345 m/s at 20C, and the accuracy is dependent on stable ambient conditions. This ends to limit the resolution to about 300 lpi, and the accuracy to +o. 1%. The device may also be implemented with a single sonic source as the digitizing means and a pair of microphones located outside the digitizing area. In this case the location of the transducer is calculated by triangulation and converted into Cartesian coordinates. Digitizers are used primarily for inputting accurate coordinate data from maps and engineering drawings. Their high accuracy requirements have led to relatively high prices. Alternative means for inputting data are the data and graphics tablets that meet most input requirements at a lower cost and accuracy. The main chnology is still electromagnetic, and the units are essentially the same as the digitizers, but with lower accuracies. However, several of the other technologies have also been used to achieve lower costs. Most successful among them are the capacitive and resistive versions, which may also be used as digitizers. The capacitive units, also termed electrostatic, use capacitive coupling where the coupling between the tablet and the cursor or stylus is determined by the capacitance made up of the tablet surface as one plate and the pickup element as the other. In this case, the capacitance is given by fc TM al d (893) where C= capacitance,= permittivity of dielectric, A= relative area of two plates, d= distance between plates, and f= proportionality factor. scanned grid approach is used to determine the location of the cursor. As in the electromagnetic tablet, an X-Y grid of conductors is embedded in the tablet, with semiconductor switches on each line providing contact on a scanned basis. The charge flowing from each capacitance is summed through a summing amplifier as shown in Fig. 89.5(b). The resultant voltage peaks twice, once for the X and once for the Y lines, as they are scanned. The peak positions are digitized by means of a counter that starts at the beginning of the scan, and runs at some multiple of the scan rate. The digital values represent the coordinates of the cursor location. e 2000 by CRC Press LLC© 2000 by CRC Press LLC used as a visual feedback element so that the operator can correct the position of the wand until the cursor is properly placed.At this time the information from the tablet may also be transferred to either the host computer or the resident desktop or portable computer, as desired. Since the cursor is not used to signal its position to a pickup device, as is the case with the light pen, it may be used with any type of display system, including the non-light-emitting flat panel displays. Another advantage of the tablet is that it may be used to position cursors in the blank areas of the display, where no light pulses are available unless they are specially generated by the light pen. There have been numerous improvements and new developments using a variety of technologies that include magnetostrictive, electromagnetic, electrostatic or capacitive, scanned X–Y grid, resistive, and sonic. Of these, electromagnetic tablets dominate the digitizer market, and sonic is of interest because it does not require a tablet, but most of the other technologies are essentially restricted to touch input devices covered later. As noted previously, electromagnetic is the most popular technology for high-performance digitizer tablets. Operation is based on transformer principles, whereby a conductor carrying ac creates a magnetic field around it that induces a current in a second conductor. The digitizer tablet uses the amplitude and phase of the induced current to determine digitizing data. The tablet contains an X–Y pattern of conductors beneath its surface, in a manner similar to the Rand Tablet, but instead of counting pulses in a time period a circular conductor is used as the pick-up element for the induced current. This coil is placed on the tablet surface, and its position is determined by measuring the phase and amplitude of the current in the coil. Its center is interpolated by sweeping through the X–Y grid lines and demodulating the signal in the coil to determine the phase reversal point, or by calculating this point using digitized data fed into a microprocessor. The X–Y coordinates may be resolved to better than 0.025 mm using either of these two techniques. Figure 89.5(a) is a photograph of a representative digitizer tablet. Another digitizer technology is the one that uses the measurement of the time required for sound waves to travel from a source to movable microphone pickups.This sonic technology has the advantage that no special digitizing board is required, and either a stylus or a cursor can be used as the digitizer. Two sonic sources are contained in an L frame so that both X and Y coordinates can be determined by calculating the time it takes for the sound wave to reach the microphones contained in the pickup device. This calculation is made on the basis of sound traveling at 345 m/s at 20°C, and the accuracy is dependent on stable ambient conditions. This tends to limit the resolution to about 300 lpi, and the accuracy to ±0.1%. The device may also be implemented with a single sonic source as the digitizing means and a pair of microphones located outside the digitizing area. In this case the location of the transducer is calculated by triangulation and converted into Cartesian coordinates. Digitizers are used primarily for inputting accurate coordinate data from maps and engineering drawings. Their high accuracy requirements have led to relatively high prices. Alternative means for inputting data are the data and graphics tablets that meet most input requirements at a lower cost and accuracy. The main technology is still electromagnetic, and the units are essentially the same as the digitizers, but with lower accuracies. However, several of the other technologies have also been used to achieve lower costs. Most successful among them are the capacitive and resistive versions, which may also be used as digitizers. The capacitive units, also termed electrostatic, use capacitive coupling where the coupling between the tablet and the cursor or stylus is determined by the capacitance made up of the tablet surface as one plate and the pickup element as the other. In this case, the capacitance is given by C = f (eA/d) (89.3) where C = capacitance, e = permittivity of dielectric, A = relative area of two plates, d = distance between plates, and f = proportionality factor. A scanned grid approach is used to determine the location of the cursor. As in the electromagnetic tablet, an X–Y grid of conductors is embedded in the tablet, with semiconductor switches on each line providing contact on a scanned basis. The charge flowing from each capacitance is summed through a summing amplifier as shown in Fig. 89.5(b). The resultant voltage peaks twice, once for the X and once for the Y lines, as they are scanned. The peak positions are digitized by means of a counter that starts at the beginning of the scan, and runs at some multiple of the scan rate. The digital values represent the coordinates of the cursor location