R Fiber Optics Amplifier FIGURE 89.3(a)Light pen.( Courtesy of FTG Data Systems. )(b) Light pen schematic( Source: After S. Sherr, Electronic Displays, New York: John Wiley Sons, 1979, P. 388. With permission. somewhat difficult to use with raster systems. This device goes by a misleading name, as it does not emit light and is not a pen other than being somewhat similar to one in its physical appearance, as shown in Fig. 89.3(a) However, when we consider its functional characteristics, the validity of the term becomes apparent, as it is used to cause the electron beam to "write "patterns on the cathode ray tube( Crt) that are defined by the motion of the light pen on the CRT faceplate The light pen operates by sensing the existence or nonexistence of a pulse of light at the point on the screen f the CRT or surface of any other light-emitting device where the point of the pen is placed. This is accomplished by means of the circuit shown in Fig. 89.3(b), where the light pulse is collected and transmitted through the fiber optics to a light-sensitive device that converts the light pulse into an electrical pulse which is shaped by some form of electronics(of which a Schmitt trigger is one example). We need not concern ourselves with the exact form of the electronics except to note that this pulse is then sent to the computer, as shown in Fig. 89.4 and provides a complete, closed-loop system. As the electronic pulse occurs at the time when the light pulse passes under the light pen, the computer is informed of the location at which the designated operation is to be performed and may proceed accordingly. Thus, the light pen is a pointing device that designates a point on the display screen and can be used as an input device. Various light pen programs have been written to expand the capabilities of the original one, and it should be noted that the light pen is coming back into favor improvements in accuracy, ease of operation, and reliability occur There are two characteristics of light pen operation that affect the capabilities of this input device. The first is the sensitivity, given by e 2000 by CRC Press LLC
© 2000 by CRC Press LLC somewhat difficult to use with raster systems. This device goes by a misleading name, as it does not emit light and is not a pen other than being somewhat similar to one in its physical appearance, as shown in Fig. 89.3(a). However, when we consider its functional characteristics, the validity of the term becomes apparent, as it is used to cause the electron beam to “write” patterns on the cathode ray tube (CRT) that are defined by the motion of the light pen on the CRT faceplate. The light pen operates by sensing the existence or nonexistence of a pulse of light at the point on the screen of the CRT or surface of any other light-emitting device where the point of the pen is placed. This is accomplished by means of the circuit shown in Fig. 89.3(b), where the light pulse is collected and transmitted through the fiber optics to a light-sensitive device that converts the light pulse into an electrical pulse which is shaped by some form of electronics (of which a Schmitt trigger is one example). We need not concern ourselves with the exact form of the electronics except to note that this pulse is then sent to the computer, as shown in Fig. 89.4, and provides a complete, closed-loop system. As the electronic pulse occurs at the time when the light pulse passes under the light pen, the computer is informed of the location at which the designated operation is to be performed and may proceed accordingly. Thus, the light pen is a pointing device that designates a point on the display screen and can be used as an input device. Various light pen programs have been written to expand the capabilities of the original one, and it should be noted that the light pen is coming back into favor as improvements in accuracy, ease of operation, and reliability occur. There are two characteristics of light pen operation that affect the capabilities of this input device. The first is the sensitivity, given by FIGURE 89.3 (a) Light pen. (Courtesy of FTG Data Systems.) (b) Light pen schematic. (Source: After S. Sherr, Electronic Displays, New York: John Wiley & Sons, 1979, p. 388. With permission.)
Intensity decode (op=需_]一“mh FIGURE 89.4 Block diagram of light pen computer system. Source: S Sherr, Electronic Displays, New York: John wiley Sons, 1979, P. 389. with permission pAAm具 where E,= illuminance at photodetector, H,= photodetector sensitivity, A, preamplifier gain, A= main amplifier gain, u,- Schmitt trigger sensitivity, u,=flip-flop sensitivity, and ',=optical loss Equation(89.1)may be used to calculate the light output required from the display surface, which may be a CRt or other light-emitting device, but with the limitation that most of the flat panel units are matrix driven and must track the drive sequence in order to know the location of the light pen from the drive pulse timing When phosphors are involved as for the CRT, vacuum fluorescent displays(VFDs), thin-film electroluminescent (TFEL)units, and color liquid crystal displays(LCDs), the phosphor delays must be entered into the timing, and the total delay is given by E。=E( t/t) (892) where E.= voltage at triggering element, E: voltage equivalent of phosphor light output, t= time, and t of all delay These delays set limits to the positional accuracy, as the computer tracking the signal will be in error by this amount. TABLE 89.3 Light Pen Data Other inaccuracies are due to the dimensions of the optical Field of View Response Time sensitivity pickup surface, all of which somewhat negate the simplicity of operation. The result is the parameter values shown in Table 89.3 Data Tablet(Graphics, Digitizer) A very convenient means for data entry, retaining some of the ease of operation of the light pen but with much better accuracy, are the various forms of data tablets available. These tablets differ from the light pen in another ignificant way in that they do not require a moving spot of light to detect the location of the beam or direct it to a new location. This need for a moving light spot made the light pen difficult to use with the data tablets itially designed to overcome this limitation while still using a device with a pen-like input. The first successful example was the Rand tablet, a digital device that used an X-Y assembly from which a wand placed above some point on the X-y wire matrix could pick up pulse generator output that fed X and Y electrical pulses into the matrix By determining the number of pulses in a time period, the location of the wand is established. Another similar device used magnetostrictive rather than electrical signals to accomplish the same result, and this location is converted into display coordinates used to position a cursor on the CRT screen. The cursor may then be e 2000 by CRC Press LLC
© 2000 by CRC Press LLC S = ELmpApAmmsmf tL (89.1) where EL = illuminance at photodetector, mp = photodetector sensitivity, Ap = preamplifier gain, Am = main amplifier gain, ms = Schmitt trigger sensitivity, mf = flip-flop sensitivity, and tL = optical loss. Equation (89.1) may be used to calculate the light output required from the display surface, which may be a CRT or other light-emitting device, but with the limitation that most of the flat panel units are matrix driven and must track the drive sequence in order to know the location of the light pen from the drive pulse timing. When phosphors are involved as for the CRT, vacuum fluorescent displays (VFDs), thin-film electroluminescent (TFEL) units, and color liquid crystal displays (LCDs), the phosphor delays must be entered into the timing, and the total delay is given by Eo = Ei (1 – e – t/t) (89.2) where Eo = voltage at triggering element, Ei = voltage equivalent of phosphor light output, t = time, and t = sum of all delays. These delays set limits to the positional accuracy, as the computer tracking the signal will be in error by this amount. Other inaccuracies are due to the dimensions of the optical pickup surface, all of which somewhat negate the simplicity of operation. The result is the parameter values shown in Table 89.3. Data Tablet (Graphics, Digitizer) A very convenient means for data entry, retaining some of the ease of operation of the light pen but with much better accuracy, are the various forms of data tablets available. These tablets differ from the light pen in another significant way in that they do not require a moving spot of light to detect the location of the beam or direct it to a new location. This need for a moving light spot made the light pen difficult to use with the data tablets initially designed to overcome this limitation while still using a device with a pen-like input. The first successful example was the Rand tablet, a digital device that used an X–Y assembly from which a wand placed above some point on the X–Y wire matrix could pick up pulse generator output that fed X and Y electrical pulses into the matrix. By determining the number of pulses in a time period, the location of the wand is established. Another similar device used magnetostrictive rather than electrical signals to accomplish the same result, and this location is converted into display coordinates used to position a cursor on the CRT screen. The cursor may then be FIGURE 89.4 Block diagram of light pen computer system. (Source: S. Sherr, Electronic Displays, New York: John Wiley & Sons, 1979, p. 389. With permission.) TABLE 89.3 Light Pen Data Field of View Response Time Sensitivity 0.02–0.08 in. 120–150 ns 0.02–0.04 ft.L
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
Charge Flow Cursor or stylus plate fe Charge AMPR $V Voltage V Time for which switch is close FIGURE 89.5(a)Digitizer tablet.( Courtesy of Numonics )(b) Capacitive technology. Source: After T. E. Davies et al Digitizers and input tablets, in Input Devices, S Sherr, Ed, New York: Academic Press, 1988, P. 186. With permission. Mouse The mouse has gone a long way from its original invention by Engelbart in 1965, through its redesign and introduction by Apple as a main input device, and its general acceptance by computer users as an important addition to the group of input devices. It should be noted, in passing, that the mouse is essentially an upside down trackball, although the latter is now being referred to as an upside-down mouse. However, the trackball came first and is described further in the next se e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Mouse The mouse has gone a long way from its original invention by Engelbart in 1965, through its redesign at Xerox and introduction by Apple as a main input device, and its general acceptance by computer users as an important addition to the group of input devices. It should be noted, in passing, that the mouse is essentially an upsidedown trackball, although the latter is now being referred to as an upside-down mouse. However, the trackball came first and is described further in the next section. FIGURE 89.5 (a) Digitizer tablet. (Courtesy of Numonics.) (b) Capacitive technology. (Source: After T. E. Davies et al., “Digitizers and input tablets,” in Input Devices, S. Sherr, Ed., New York: Academic Press, 1988, p. 186. With permission.)
V mouse Motion FIGURE 89.6 Wheel showing velocities and slip angle. (Source: After C Goy, "Mice, " in Input Devices, S. Sherr, Ed, New fork: Academic Press, 1988, P. 225. With permission. Mice contain motion-sensing elements and are operated by moving mechanical or optical elements. One rm uses wheels and shafts to drive the sensing elements, as shown schematically in Fig. 89.6. The angular velocity(o)of the wheel and shaft is given by 0= V/R rad/s (894) where Vr= velocity of wheel and R= wheel radius The rotation angle(0)is given by (0)=X/R rad (895) where x= distance moved This type of mouse has two sets of wheels and shafts, one for horizontal and the other for vertical A more popular type of mechanical mouse is the one that uses a ball for the motion in Fig. 89.7. Again, the velocity of the ball circumference equals the velocity of the mouse, velocity is given by VIR rad/ where R,= shaft radius. The smaller the shaft the more rapid its rotation for a given mouse velocity. Another form of the ball-and-shaft mouse is the one that uses an optical interrupter, as shown in Fig. 89.8 In this form, the light from the light-emitting diodes(LEDs) V mouse is interrupted by the coded disks that are rotated by the shafts, and is then picked up by the phototransistors and converted into the digital signal that represents the disk rotation. An optical interrupter is also used for the optomechanical mouse, FIGURE 89.7 Ball and shaft.(Source: C. Goy and here the interrupter contains a set of slots; as the inter-"Mice,"in Input Devices, S. Sherr, Ed, New York: rupter rotates quadrature signals are created that correspond Academic Press, 1988. With permission.) to the shaft rotation In addition to the shaft and optomechanical mice, an early form of mouse used multiturn potentiometers connected to the wheels, and the output voltage that represented the motion varied in direct proportion to the mouse motion. The voltage was then converted by means of an analog-to-digital converter into digital form for input to the computer. Finally, there are the true optical mice that use a special surface that is printed with a set of geometric shapes, sually a grid of lines or dots, that are illuminated and focused on a light detector. The most common form uses a grid made up of orthogonal lines, with the vertical and horizontal lines printed in different colors. These colors absorb light at different frequencies so that the optical detectors can differentiate between horizontal e 2000 by CRC Press LLC
© 2000 by CRC Press LLC Mice contain motion-sensing elements and are operated by moving mechanical or optical elements. One form uses wheels and shafts to drive the sensing elements, as shown schematically in Fig. 89.6. The angular velocity (w) of the wheel and shaft is given by w = Vr/R rad/s (89.4) where Vr = velocity of wheel and R = wheel radius. The rotation angle (q) is given by (q) = X/R rad (89.5) where X = distance moved. This type of mouse has two sets of wheels and shafts, one for horizontal and the other for vertical motion. A more popular type of mechanical mouse is the one that uses a ball for the motion sensing device, as shown in Fig. 89.7. Again, the velocity of the ball circumference equals the velocity of the mouse, and the angular velocity is given by w = V/R1 rad/s (89.6) where R1 = shaft radius. The smaller the shaft the more rapid its rotation for a given mouse velocity. Another form of the ball-and-shaft mouse is the one that uses an optical interrupter, as shown in Fig. 89.8. In this form, the light from the light-emitting diodes (LEDs) is interrupted by the coded disks that are rotated by the shafts, and is then picked up by the phototransistors and converted into the digital signal that represents the disk rotation. An optical interrupter is also used for the optomechanical mouse, and here the interrupter contains a set of slots; as the interrupter rotates quadrature signals are created that correspond to the shaft rotation. In addition to the shaft and optomechanical mice, an early form of mouse used multiturn potentiometers connected to the wheels, and the output voltage that represented the motion varied in direct proportion to the mouse motion. The voltage was then converted by means of an analog-to-digital converter into digital form for input to the computer. Finally, there are the true optical mice that use a special surface that is printed with a set of geometric shapes, usually a grid of lines or dots, that are illuminated and focused on a light detector. The most common form uses a grid made up of orthogonal lines, with the vertical and horizontal lines printed in different colors. These colors absorb light at different frequencies so that the optical detectors can differentiate between horizontal FIGURE 89.6 Wheel showing velocities and slip angle. (Source: After C. Goy, “Mice,” in Input Devices, S. Sherr, Ed., New York: Academic Press, 1988, p. 225. With permission.) FIGURE 89.7 Ball and shaft. (Source: C. Goy, “Mice,” in Input Devices, S. Sherr, Ed., New York: Academic Press, 1988. With permission.)