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^ ^'iirucDcs-R-7 2-491 coo 11+69-0198 
 
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 OBSERVER POSITION DETECTOR FOR THE 
 STEREOMATRIX 3-D DISPLAY SYSTEM 
 
 by 
 
 CHARLES RAYMOND PIRNAT 
 
 February, 1972 
 
 DEPARTMENT OF COMPUTER SCIENCE 
 
 UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN • URBANA, ILLINOIS 
 
 JRRK&ss 
 
UIUCDCS-R-72-491 
 
 OBSERVER POSITION DETECTOR FOR THE 
 STEREOMATRIX 3-D DISPLAY SYSTEM 
 
 by 
 
 CHARLES RAYMOND PIRNAT 
 
 February, 1972 
 
 Department of Computer Science 
 University of Illinois 
 Urbana, Illinois 6l801 
 
 This work was supported in part by Contract No. Atomic Energy Commission 
 AT (11-1) 1U69 and was submitted in partial fulfillment of the requirements 
 for the degree of Doctor of Philosophy in Electrical Engineering, February, 
 
 1972. 
 
OBSERVER POSITION DETECTOR FOR THE 
 STEREOMATRIX 3-D DISPLAY SYSTEM 
 
 BY 
 
 CHARLES RAYMOND PIRNAT 
 B.S., University of Illinois, 1966 
 M.S., University of Illinois, 1969 
 
 THESIS 
 
 Submitted in partial fulfillment of the requirements for the 
 for the degree of Doctor of Philosophy in Electrical Engineering 
 in the Graduate College of the 
 University of Illinois at Urbana-Champaign, 1972 
 
 Urbana, Illinois 
 
Digitized by the Internet Archive 
 in 2013 
 
 http://archive.org/details/observerposition491pirn 
 
OBSERVER POSITION DETECTOR 
 FOR 
 THE STEREOMATRIX 3-D DISPLAY SYSTEM 
 
 Charles Raymond Pirnat, Ph.D. 
 Department of Electrical Engineering 
 University of Illinois at Urb ana-Champaign 1972 
 
 STEREOMATRIX is a display system which presents computer-generated 
 three-dimensional line figures. This rear-projection laser display has two 
 interesting features. A lighted spot called the cursor can be moved around 
 
 in the viewing space by the observer with a joystick. This feature allows 
 the observer to select a point in the viewing space and relate it to the 
 computer space. It is also possible for the observer to draw in the viewing 
 space. The perspective of the line figure changes with observer movement in 
 such a way that the figure appears to be stationary in space. The position 
 of the cursor in the viewing space can be accurately determined by the 
 observer making small lateral movements to check the relative sensitiviby 
 of the cursor perspective change. This feature is provided by the observer 
 position detector system. 
 
 The position detectinn system is basically an infrared optical 
 scanning system with shaft encoders which yield digital sine and cosine 
 values of shaft angle. This digital information is held in registers and 
 converted to analog form for computation of the observer's coordinates for 
 each display frame. 
 
Ill 
 
 ACKNOWLEDGMENT 
 
 The author would like to thank Professor W. J. Poppelbaum for 
 suggesting the project and for his continued patience and advice. Thanks 
 also go to Professor W. J. Kubitz for many helpful discussions. 
 
 To the many people who performed supportive tasks toward the com- 
 pletion of this project, the author extends his thanks. 
 
IV 
 
 TABLE OF CONTENTS 
 
 1. INTRODUCTION 1 
 
 2. SYSTEM REQUIREMENTS FOR THE OBSERVER POSITION DETECTOR . . 7 
 
 3. SYSTEMS CONSIDERED 8 
 
 3-1 Mechanical Linkage 8 
 
 3.2 Ultrasonic Ranging 8 
 
 3.3 Radio Frequency Ranging 9 
 
 3 . 4 Optical Scanning 10 
 
 4. OPTICAL POSITION DETECTION SYSTEM 11 
 
 4.1 Optical Shaft Encoder 13 
 
 4.1.1 Disc Assembly 15 
 
 4.1.2 Reader Light Source 17 
 
 4.1.3 Reader Optic Fiber Mount . 24 
 
 4.2 Source - Detector Function 29 
 
 4.3 Reader Function 33 
 
 4.4 Inhibit Function 38 
 
 4.5 Memory - Converter Function 38 
 
 4.6 Clock Function 4l 
 
 4.7 Analog Computation Function 45 
 
 5. SUMMARY AND CONCLUSION 50 
 
 5 . 1 Summary 50 
 
 5.2 Conclusion 50 
 
 LIST OF REFERENCES 51 
 
 APPENDIX 52 
 
 VITA 59 
 
LIST OF FIGURES 
 
 Figure Page 
 
 1. Formation of a Stereoscopic Pair 2 
 
 2. STEREOMATRIX Block Diagram 3 
 
 3. Layout of STEREOMATRIX System 5 
 
 k. Observer Position Geometry 12 
 
 !?. Optical Position Detection System Block Diagram .... Ik 
 
 6. Picture of Encoder Disc 16 
 
 7. Detail of Disc Segments 18 
 
 8. Picture of Reader Light Source Assembly 19 
 
 9- Equivalent Tungsten Filament Color Temperature Curve for 
 
 the Reader Source Lamp 21 
 
 10. Spectral Compatibility of Glass Optic Fiber and FPT-100 
 Phototransistor 22 
 
 11. Spectral Compatibility of CR0F0N Plastic Optic Fiber and 
 FPT-100 Phototransistor 23 
 
 12. Picture of Shaft Encoder Assembly 25 
 
 13. Phototransistor Output vs. Disc Position for . 0^0" 
 Diameter Source and Pickup Fibers 26 
 
 Ik. Phototransistor Output vs. Disc Position for .020 Diameter 
 
 Source Fibers and . 0U0" Diameter Pickup Fibers 27 
 
 15. Alignment of Optic Fibers in the Reader Mount 28 
 
 16. Picture of Observer Glasses 30 
 
 17. Spectral Compatibility of FPT-100 Phototransistor and #87 
 Wratten Filter 31 
 
 18. Gold Mirror Spectral Reflectivity 32 
 
 19. Detector Function Block Diagram 3^- 
 
 20. Actual Phototransistor Output to Reader Circuit . . . . ' 36 
 
 21. Reader Function Block Diagram 37 
 
VI 
 
 Figure Page 
 
 22. Inhibit Function Block Diagram 39 
 
 23. Memory-Converter Function Block Diagram 1+2 
 
 2k. Clock Function Block Diagram J43 
 
 25. "U" Clock Truth Table 1+1+ 
 
 26. Worst Case Clock Timing I4.5 
 
 27. Reader and Clock Pulse Trains 1+7 
 
 28. Analog Computation Function Block Diagram kQ 
 
 A.l Detector Section Schematic Diagram 53 
 
 A. 2 Reader Board Schematic Diagram 54 
 
 A.3 Timing Board Schematic Diagram 55 
 
 A. k Digital-to-Analog Converter Board Pin Diagram 56 
 
 A. 5 Multiplier Board Schematic Diagram 57 
 
 A. 6 Divider Board Schematic Diagram 58 
 
1. INTRODUCTION 
 
 STEREOMATRIX is a display system which presents computer-generated 
 
 three dimensional line figures. There are three unique features in this 
 
 system. The figure displayed may be rotated, translated or scaled in the 
 
 virtual three dimensional space, the perspective of the figure changes as 
 
 the observer moves about, and state-of-the-art deflectors provide a random 
 
 scan. The display volume is a cube with 500 elements on a side. This 
 
 volume is simulated by presenting the stereoscopic pair for each point in the 
 
 volume on a two dimensional screen. Figure 1 illustrates the relationship of 
 
 the stereoscopic pair, P and P in the X-Y plane, to a point, P, in the 
 
 L R 
 
 display volume. The left and right eye positions are designated by points 
 L and R, respectively, where "a" is the fixed distance between the eyes. 
 The screen is in the X-Y plane and is the face through which the volume 
 is observed. 
 
 A block diagram of the STEREOMATRIX system is shown in Figure 2. 
 The six basic components of this system are the programmed storage, the 
 coefficient generator, the observer position detector, the cursor, the 
 transformer, and the display. The programmed storage is a small general 
 purpose computer (PDP-8) which converts the three dimensional graphic 
 information of a figure into a succession of points (X, Y, Z) in digital form 
 for the transformer. The coefficient generator provides a matrix of 
 coefficients to produce selected rotations and translations of the figure 
 in space. Determination of the observer's coordinates with respect to 
 the display is accomplished by the observer position detector. The cursor 
 provides its own X, Y, Z coordinates and compares them with each set in the 
 
Figure 1. Formation of a Stereoscopic Pair 
 

 
 
 
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 transformer to check for a coincidence. [1] The transformer operates on the 
 input X, Y, Z coordinates with the coefficients from the coefficient generator 
 scales and windows the coordinates, multiplexes the modified coordinates with 
 the cursor coordinates, generates stereoscopic pairs using position detector 
 information, and thus provides the intensity, blanking and deflection signals 
 for the display. In the display section, a one watt, CW argon laser is beam 
 split and rotated into two orthogonally polarized beams. Both beams have 
 equal vertical deflection, but different horizontal deflection for each 
 stereoscopic pair, P^y) and P R (x R , y). Deflection is accomplished with 
 acousto-optic deflectors by passing the beam to be deflected through a lead 
 molybdate crystal which is also passing a frequency modulated ultrasonic 
 signal. Bragg diffraction within the crystal is a function of the FM 
 ultrasonic signal. These deflectors provide greater than 500 spot resolution 
 for the random scan. The polarized and deflected beams are intensity 
 modulated and presented on a rear projection screen. Polarized glasses are 
 used to observe the stereoscopic pairs projected on the screen. The actual 
 layout of STEREOMATRIX is shown in Figure 3- 
 
 In the real world, a person encounters both stationary and moving 
 objects. When a stationary object is viewed, the portion seen is directly 
 related to the observer's position with respect to it. As an example, a 
 box on a table when viewed from the front and slightly above would have its 
 front face and a portion of its top showing. If the observer moved to his left 
 the perspective of the box would change with a portion of the left side com- 
 ing into his view. Observer movement to his right would allow him to see part 
 of the right side of the box. Another useful aspect of this perspective change 
 allows the observer to accurately determine the Z coordinate of the cursor in 
 the viewing space. Since the cursor is a point of light, it has no cue to 
 determine its depth in the viewing space. The observer can move his head 
 
6 
 
 slightly from side to side and note the sensitivity of the cursor's per- 
 spective change. If the cursor moves considerably, it is near the observer 
 and if it moves very little it is far from the observer. This unique char- 
 acteristic of relative position is incorporated into STEREOMATRIX to provide 
 a realistic 3-D display. 
 
 To present the observer with the view appropriate to his relative 
 position, it is necessary that his position be continuously monitored. It 
 is the purpose of the observer position detector to locate the observer and 
 
 to provide his current X and Z coordinates to the STEREOMATRIX transformer 
 
 o o 
 
 on a continuous basis. 
 
7 
 
 2. SYSTEM REQUIREMENTS FOR THE OBSERVER POSITION DETECTOR 
 
 The first set of conditions which must be considered are those 
 imposed by the operating rates of the transformer and the display sections. 
 Since points are processed in succession by the transformer, the observer 
 coordinates should be held constant until all stereoscopic pairs of the 
 figure have been displayed to prevent discontinuities in the image. Also, 
 new values of the observer coordinates should be available to the transformer 
 in less thne than is necessary to generate and display a single stereoscopic 
 pair because there is no pause between frames to allow for this change. The 
 position detector sample rate must be greater than or equal to the system 
 frame rate to provide a smooth transition of the image as the observer moves 
 about. This sample rate must also be sufficiently high to know the observer 
 coordinates to within a reasonable, chosen accuracy when he moves at his 
 maximum expected rate. 
 
 The position detection function should be accomplished while 
 allowing the observer maximum freedom of movement and minimum distraction 
 from the display. It should also operate correctly for observers of differ- 
 ent heights. 
 
 If several systems fulfill all of the above requirements, cost will 
 be the deciding factor. 
 
 Several systems will now be considered in the light of these 
 general requirements. 
 
3. SYSTEMS CONSIDERED 
 
 3.1 Mechanical Linkage 
 
 Probably the most obvious solution to the position detection pro- 
 blem is the use of a mechanical system which -would be attached to the 
 observer and would physically monitor his movements. This approach was 
 used in a 3-D display where two small cathode ray tubes were mounted in a 
 
 binocular arrangement to provide each eye with the correct stereoscopic 
 
 [2] 
 view of the figure . 
 
 A mechanical system restricts the observer's movement considerably 
 and would have to be complex to minimize friction in the linkages. Weight 
 of the assembly would be an important factor with respect to observer comfort, 
 
 3-2 Ultrasonic Ranging 
 
 The use of ultrasonic signals for the measurement of small dis- 
 tances is advantageous due to their low velocity of propagation in air. Upon 
 looking into the practical aspects of this system, two problems involving 
 the transducer arise. It is desirable to operate a tranducer in its 
 resonant mode to provide maximum energy transfer into the air. At relatively 
 low ultrasonic frequencies, the physical size of a resonant transducer is 
 too large to be carried by the observer. If a high ultrasonic frequency is 
 used, transducer size is acceptable, but the efficiency of the device in trans- 
 ferring energy to the air is very low. Given that an appropriate transducer 
 is available, it would be necessary to design a practical portable pulsed 
 driver. 
 
9 
 
 Another variation of this approach would be to measure the phase 
 difference between a reference signal and one propagated between the observer 
 and the display. This continuous wave system would be affected by reflec- 
 tions and possible resultant standing waves in the room. 
 
 3.3 Radio Frequency Ranging 
 
 It is interesting that the characteristic which makes radar so 
 useful for long range applications is an impediment for short range use. A 
 pulse reflected from a tar get ten feet from the transmitter-receiver would 
 return in about twenty nanoseconds. This time interval is only two to four 
 times the propagation delay of a single digital circuit which would make 
 accurate measurement by this means extremely difficult. 
 
 Some serious thought was given to a radio frequency phase measure- 
 ment scheme. The idea was to radiate a signal from the observer which 
 would be compared to a reference signal and the phase difference would pro- 
 vide a measure of the distance between the observer and the receiver. There 
 were two approaches to this measurement based on the choice of wavelength 
 of the transmitted signal. For a wavelength small with respect to the dis- 
 tance to be measured, the phase change between signals would be greater than 
 
 360 . It would be necessary to keep track of the number of full cycles in 
 
 o 
 addition to the differential phase measurement less than 360 . A simpler 
 
 method would be to make the signal wavelength greater than the maximum dis- 
 tance to be measured so that the phase measurement would always be less than 
 360 . Since phase could be measured accurately to within one-hundredth of 
 a degree, J it would be a feasible system. The problems encountered with 
 this system were related to the transmitting antenna. When the signal wavelength 
 was of the order of the distance to be measured, then the phase measuremert 
 
10 
 was being attempted in the near-field of the antenna where the phase was non- 
 Linear. Also, it was difficult to isolate the transmitting antenna as a radiat 
 ing source to prevent a phase locked condition between transmitter and 
 receiver. 
 
 3.U Optical Scanning 
 
 A common feature of the previously considered systems is that each 
 was designed to measure the observer's distance from a receiver. Utilizing 
 two receivers a fixed distance apart, the measured distances would complete 
 a triangle with the length of all sides known from which observer coordinates 
 could easily be calculated. It is equally simple to determine the observer's 
 coordinates if his angular position is known with respect to two points 
 separated by a fixed distance. To obtain the angular measurement, a shaft 
 encoder could be read when an optical scanner detected a light source located 
 on the observer. In this case, the speed of light in air insures that the 
 shaft encoder is read when the scanner "sees" the observer. 
 
 Some advantages of the optical scanner include accurate angular 
 measurement, use of a portable light source on the observer allowing freedom 
 of movement, choice of scan rates, simple noise filtering, and low cost compon- 
 ents. These characteristics of the optical scanner make it useful for the 
 position detection function. 
 
11 
 
 h. OPTICAL POSITION DETECTION SYSTEM 
 
 Having chosen the optical scanning system for this application, the 
 initial considerations are system geometry and related limitations. The 
 observer is able to move freely within a ten foot area of the rear pro- 
 jection screen with two limitations. A characteristic of this screen is that 
 the image intensity decreases as the observer moves away from the screen's 
 normal axis. This characteristic limits the observer's movement to less 
 than ^5 either side of the screen normal. It is also assumed that during 
 normal viewing the observer would never desire to be less than three feet 
 from the screen. A small lamp is mounted on the glasses which the observer 
 must wear to view the 3-D display. The two scanning detectors are positioned 
 with one at each top corner of the 3 high by h wide screen so that the 
 lamp will be approximately in their plane of rotation. A diagram of the 
 system geometry is shown in Figure h. 
 
 The equations for the observer coordinates in terms of the angles 
 
 6 and cp are easily derived and simply stated here. 
 
 _ R sin 6 cos cp - cos 6 sin cp 
 o " 2 sin 6 cos cd + cos fc sin cp 
 
 Z = 
 
 D sin 6 sin cp 
 
 o sin 6 cos cp + cos 6 sin cp 
 Where D is the distance between detectors, that is h feet. 
 These equations can be simplified to the form 
 
 D sin (6 - cp ) „ D sin 6 sin cp 
 
 "o " 2 sin (e + cp) " sin (8+ cp) 
 
 This simpler form of the equations requires nine analog operations 
 
 to generate the X and Z values from values of G and cp. If the sine and cosine 
 to o o 
 
 functions of the angles are available initially, then only seven analog opera- 
 tions are necessary to generate the X and Z values from the longer form of 
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 the equations. An added advantage to this second method is the elimination 
 
 of four sine function generators which are more costly. Commercial shaft 
 
 encoders are available which provide simultaneous sine and cosine function 
 
 values of the shaft angle for angular increments as small as 0.1 minute of 
 
 arc, but they are made on special order only and are very expensive. It was 
 
 decided to design a shaft encoder which would supply sine and cosine function 
 values at larger intervals for a considerably reduced cost. 
 
 A block diagram of the optical position detection system is shown in 
 Figure 5. In the following sections of this chapter, the purpose and 
 requirements of each portion of the detection apparatus are described along 
 with its hardware realization. 
 
 4.1 Optical Shaft Encoder 
 
 To establish the basic design of the shaft encoder, it is necessary 
 to determine the accuracy to which the observer must be located and the 
 sampling rate. The maximum frame rate determines the sample rate for the 
 detector as, at most, one new set of position coordinates is required for each 
 frame. To determine the accuracy, it is assumed that the observer's position 
 must be known to within 1 foot when he is moving on an arc with respect to 
 the detector. The radius of the arc is a minimum of 3 feet and a maximum of 
 10 feet with the worst case occurring at 10 feet. At 10 feet, the observer's 
 position must be known to within + 1/2 foot. Thus 
 
 - ^ = ll 2 it ' = 0-05 rad. = 2.9 deg. 
 
 Att - r 10 ft. 
 
 We choose to have the observer located to within 1 degree since this 
 is easily done with infrared accuracy. 
 
 The sample rate is variable in the range of 40Hz to 60Hz and, there- 
 fore, the position detector sample rate must be 60Hz. It is necessary to 
 
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 determine the accuracy provided by this sample rate to verify that accuracy 
 is within 1 degree. If the observer moves on a 3 foot radius arc at a maxi- 
 mum speed of 3 feet per second, his angular velocity would be 
 
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 which at 57 degrees per second represents 0.95 degrees of change. Consequently, 
 the observer, moving on a 3 foot arc at 3 feet per second, would move less 
 than 1 degree between detector samples preserving the desired accuracy. 
 This accuracy holds for asynchronous operation of the two detectors elimin- 
 ating the need for a more complex synchronous operation. 
 
 4.1.1 Disc Assembly 
 
 The disc assembly consists of a coded disc, a motor shaft collar, 
 and a mirror mount. The motor shaft collar secures both the disc and mirror 
 mount to the 60Hz motor shaft. 
 
 It is the disc which carries the coded sine and cosine values of 
 the shaft angle. Due to the system geometry, the observer will always be 
 somewhere within an angular sector from 23 to 135 as measured from the 
 
 plane of the screen in the direction of detector rotation. Therefore, the 
 
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 sine function occupies a 112 sector opposite the 112 sector for the cosine 
 
 function. Both functions are read simultaneously by reader mounts 180 apart. 
 A picture of the disc is shown in Figure 6. There are nine concen- 
 tric tracks which carry the binary representations of the two functions. 'The 
 track nearest to the disc center is the sign bit, the track nearest to the 
 disc edge is the inhibit bit, and the seven middle tracks provide the func- 
 tion magnitude. 
 
16 
 
 Figure 6. Picture of Encoder Disc 
 
17 
 Each radial segment is one degree wide and contains one coded por- 
 tion from each of the nine tracks. Half of the segment is a spacer the other 
 half is coded. A hole in the disc represents a binary zero, no hole is a 
 binary one. A detail of the disc is shown in Figure 7. Since .020" optic 
 fibers are used to read the disc, the tracks are made that width. The 
 disc diameter and thickness are kept to a minimum due to the forces imposed 
 on it by its high rotational speed. At 60Hz, the linear speed of the disc 
 periphery is about 88 ft. /sec. All tracks are spaced equally from the sign 
 track, so the disc diameter is determined by the radius of this track. The 
 minimum sign track radius is defined as that radius at which 0.5 of angle is 
 .020" of arc length. This is done to allow a maximum amount of light through 
 the disc hole from the .020" diameter optic fiber. 
 
 The disc is etched from a .005" thick sheet of stainless steel 
 flat stock to a diameter of 5 13/32". Its surfaces are blackened to prevent 
 reflections within the reader mount. Since the linear speed of the disc is 
 high, consideration was initially given to the distribution of holes for 
 proper balance. Upon calculating the areas of the holes and summing them, it 
 was found that this area removed from the solid disc was less than 0.7$ of 
 the total disc area. Removal of such a small distributed area has a negligible 
 effect on disc balance. 
 
 4.1.2 Reader Light Source 
 
 The reader light source consists of an incandescent lamp, two double 
 convex lenses, and two optic fiber bundle lamps in a light-tight, ventilated 
 box. A picture of the reader light source assembly is shown in Figure 8. 
 
18 
 
 INHIBIT BIT 
 LEAST SIGNIFICANT BIT 
 
 MOST SIGNIFICANT BIT 
 
 SIGN BIT 
 
 Figure 7. Detail of Disc Segments 
 
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 To illuminate the source optic fibers properly, the incandescent 
 lamp filament is focused on the end of the fiber bundle by a double convex 
 lens. One lens is focused on the sine bundle, the other on the cosine bundle. 
 The lens provides light within a cone angle of approximately 2h to the 
 fibers. This is advantageous even though the optic fiber acceptance cone 
 angle is 60 because the amount of light transmitted through the fiber drops 
 off rapidly as the off-axis angle increases. At a cone angle of 2k , at 
 least 70% of the incident light is transmitted into the fiber. The ends of 
 the fibers reflect about 8 to 10$ of the incident light. 
 
 An important consideration is the spectral compatibility of the lamp 
 radiation, the optic fibers, and the phototransistors in the reader system. 
 The equivalent tungsten filament color temperature curve of the reader lamp 
 was calculated for its operation at 90% of its rated voltage and is shown 
 in Figure 9- 
 
 Both glass and plastic fiber optics are available commercially. 
 Figure 10 shows the spectral compatibility of a glass optic fiber with an 
 FPT-100 phototransistor . Figure 11 shows the spectral compatibility of a 
 plastic optic fiber with a FPT-100 phototransistor. The resultant curves of 
 Figures 10 and 11 were graphically integrated and each composed to the area 
 under the phototransistor curve. The plastic fiber proved to be 12$ more 
 efficient than the glass fiber when used with this phototransistor. 
 
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 4.1.3 Reader Optic Fiber Mount 
 
 The reader mount aligns the source and pickup fibers for proper 
 reading of the nine digital tracks on the disc. It also aligns the pickup 
 fibers with their respective phototransistors for maximum illumination of 
 the photosensitive area. Figure 12 shows the two reader mounts in position 
 on the shaft encoder assembly. 
 
 Two experiments were performed to determine the fiber diameters 
 which would pass the maximum amount of light without crosstalk between adja- 
 cent tracks. Figure 13 shows the experimental set up and phototransistor 
 output for . 0U0" diameter fibers. The output voltage of the phototransistor 
 consists of an AC signal with a DC level. To distinguish between consecutive 
 holes in a particular track, the AC component of the composite signal should 
 be a maximum. In this case, the peak to peak value of the AC component is 
 only about 25% of the peak value of the composite signal. In an effort 
 to increase the peak to peak differential signal, with a minimum reduction 
 of source light, the source fiber diameter was reduced to a .020" diameter 
 as shown in Figure 1^-. With this combination of fiber diameters, the differ- 
 ential signal was increased to about 66% of the peak composite signal ampli- 
 tude facilitating detection of two adjacent disc holes. 
 
 The source fibers are clamped between two slotted aluminum bars 
 directly below the pickup fibers which are mounted the same way. Alignment 
 of the fibers is shown in Figure 15. The reader mount slides in a channel 
 with a screw adjustment so it can be aligned with the disc tracks. 
 
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 k. 2 Source - Detector Function 
 
 The source - detector function consists of a filtered incandescent 
 lamp mounted on the observer's glasses, a rotating mirror, a phototransistor, 
 and the necessary circuitry. 
 
 To keep the detection system "invisible" so the observer is not dis- 
 tracted from the 3-D display, the observer's lamp is fitted with a wratten 
 filter which blocks visible light and transmits only infra-red radiation of 
 wavelength longer than 0.7 microns. Power for the observer's lamp is 
 supplied by a 6 volt, 3 ampere-hour battery which is attached to his belt. 
 A picture of the observer's glasses is shown in Figure 16. 
 
 In order to eliminate noise due to fluorescent lights, a #87 wratten 
 filter is the window in the detector cover assembly. Figure 17 shows the 
 spectral compatibility of the #87 filter and an FPT-100 phototransistor. rhis 
 filter also prevents distraction of the observer by the rotating mirror. All 
 surfaces under the detector cover which might reflect light into the mirror 
 are painted flat black. 
 
 The detector mirror is mounted at h5 to the motor shaft. In this 
 position, it reflects infra-red radiation vertically into the detector photo- 
 transistor when facing the observer source. A gold mirror surface was 
 selected for its high reflectivity in the infra-red spectral region and its 
 reflective stability. The reflectivity of a gold mirror is shown as a func- 
 tion of wavelength in Figure 18. 
 
 When light is incident on the detector phototransistor, its output 
 is amplified 1000 times and level detected with a comparator. The comparator 
 output is compatible with transistor-transistor logic (TTL) and triggers 
 the first of two monostable multivibrators. These multivibrators (one-shots) 
 
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33 
 each have a potentiometer to adjust their pulse width. The first stage 
 provides a variable pulse delay for detector calibration and the second 
 stage provides a uniform output pulse width. 
 
 A phototransistor 's rise time is inversely proportional to its 
 collector current. Consequently, its rise time is slower for smaller drive 
 signals. The detector phototransistor rise time is typically 50 [4-sec when 
 the observer is ten feet from the detector. To maintain the angular measure- 
 ment accuracy to within 1 , the comparator threshold is set near its zero 
 input level. This threshold level is high enough to prevent false triggering 
 due to noise but is low enough to trigger within 5 H-sec of the initial rise 
 of the phototransistor output. As the observer moves closer to the detector, 
 the phototransistor rise time decreases and the pulse delay is less than 
 5 j-isec. At a rotational frequency of 60Hz, 1 is equivalent to a time inter- 
 val of kG (isec and the maximum detector delay is about 10% of the 1 
 interval. The output pulse width of the second monostable multivibrator is 
 adjusted to be greater than the phototransistor pulse width to prevent 
 false comparator triggering due to noise on the phototransistor pulse 
 trailing edge. A block diagram of the detector function is shown in Figure 19. 
 
 U.3 Reader Function 
 
 The reader function consists of the reader light source, optic 
 fiber mount assembly including eight phototransistors, eight adjustable 
 comparators, and eight inverters. When a disc hole is between the source 
 and pickup fibers, light is transmitted to a phototransistor by the pickup 
 fiber. A comparator converts the phototransistor output to a TTL compatible 
 pulse. The inverter at the comparator output serves as a buffer stage bet- 
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 A sample of the actual reader phototransistor output is shown 
 in Figure 20. The differences in the maximum and minimum peak values are 
 caused by slight differences in the size of both holes and spacers in 
 the disc. These non-uniformities occur because the disc is etched. As 
 long as the lowest peak is higher than the highest valley, the comparator 
 threshold can be set to detect all pulses. A block diagram of the reader 
 function is shown in Figure 21. 
 
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 1+.1+ inhibit Function 
 
 The inhibit function prevents the disc from being read when the 
 optic fibers are positioned over the metal spaces between coded segments. 
 There is a hole at every coded position of the inhibit track which is read 
 in the same way as the other holes (bits) on the disc. However, the inhibit 
 comparator output is fed into a monostable multivibrator to achieve an 
 adjustable pulse delay. The delayed output triggers a second monostable 
 multivibrator which determines the pulse width. Since edge triggered registers 
 are used in the memory section, the inhibit pulse is delayed until the lead- 
 ing edge is positioned at the center of the coded 0.5° segment. This delay 
 allows all bits to reach their correct logic level before being loaded into 
 the registers. The second monostable multivibrator provides a uniform 
 inhibit pulse width and enables the first multivibrator to adjust pulse 
 delay. The inhibit function block diagram is shown in Figure 22. 
 
 lj.,5 Memory - Converter Function 
 
 The memory- converter function consists of four sets of edge- 
 triggered registers and four digital-to-analog (D to A) converters. These 
 provide the interface between the asynchronous operation of the shaft encoders 
 and the synchronous STEREOMATRIX transformer. Pulse trains from the 
 reader function are continuously present at the first set of registers but 
 are loaded into the update registers only once during each detector cycle. 
 These registers contain the current value of the trigonometric function. 
 The second set of registers are the holding registers. They are loaded 
 with update register outputs at the end of each display frame and supply a 
 fixed value to the analog computation function for the duration of the frame. 
 
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 To make the correct choice of a D to A converter for this system, 
 it was necessary to consider the type of output, the output accuracy, and 
 the output settling time. The range of shaft angles which are measured 
 extends from the first into the second quadrant. Consequently, the cosine 
 function will change sign and a bipolar output is required. To preserve 
 the scale factor, a bipolar unit is also used for the sine function. If we 
 want to know the sine function value to within 1 accuracy, we must be able 
 to distinguish between values in the region where the function value changes 
 slowly. This worse case situation occurs for the sine function near 90 . 
 When an angle changes from 90 to 89 , the sine value changes by .0002. A 
 12 bit D to A converter would be required to obtain this accuracy. The 
 question of whether or not this much accuracy is warranted must be examined. 
 The important factor is the sensitivity of the computed coordinates to the 
 errors in the function values. To determine this sensitivity, a computer 
 program was written to calculate observer coordinates using function values 
 from 8, 10 and 12 bit D to A converters assuming 1% operational errors. 
 The results of this program indicated that the observer coordinates were calcu- 
 lated to within one degree for all three converter accuracies. The 8 bit 
 D to A converter was adequate for this calculation and another program was 
 
 written to tabulate the disc coding. This program lists the angles from 
 
 00 
 23 to 135 , each function value, and its 2's complement binary code. To 
 
 determine the settling time requirement, the maximum expected change in the 
 output voltage must be known. At the observer's maximum speed, he moves 
 about 1 in one display frame. A 1 change in the cosine function from 90 
 to 89 represents an increment of .017. For a converter with +5 volt output 
 swing, the output voltage equals 5 cos 0. Therefore, an increment of .017 is a 
 
in 
 
 voltage change of 5 (.017) = «°85 volts. If the slew rate of an inexpensive 
 converter is 0.4 volts/(isec, then the settling time for a voltage change of 
 .085 volts is 0.225 u-sec. Since the display takes 7 u-sec to present a point, 
 this converter settling time is acceptable. The D to A converter accepts an 
 8 bit digital input from the holding registers and provides a voltage of 
 5 times the function at its output. The memory-converter function block 
 diagram is shown in Figure 23. 
 
 4.6 Clock Function 
 
 The clock function controls the loading of the update and holding 
 registers. Figure 24 illustrates the clock function block diagram. To load 
 the update registers, with a "U" clock signal the detector and inhibit 
 signals to the clock function must be a logical one. Since the detector 
 pulse width is much greater than the inhibit pulse period, several inhibit 
 pulses will occur during one detector pulse. To prevent false "U" clock 
 signals after the initial coincidence, the gate output remains a logical 
 one for the duration of the detector pulse and then resets. The "U" clock 
 truth table is shown in Figure 25. Two "U" clock signals are provided, one 
 for the sine update registers, and one for the cosine update registers. 
 
 The "H" clock signal loads the holding registers when the frame 
 
 and "U" clock signals are a logical one. This arrangement insures that the 
 holding registers will not load while the update registers are loading. If 
 the frame pulse occurs just after "U" clock becomes a logical one, then the 
 holding registers would not load for the duration of the detector pulse 
 which is several hundred microseconds long. In this case, the old values 
 of the trigonometric functions would be used to display about forty points 
 
k2 
 
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1*5 
 
 in the new frame. To prevent this unnecessary delay, a release monostable 
 
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 The release multivibrator output remains a logical one until the "U" clock 
 input transitions to a logical zero. At this time, the release output goes 
 to a logical zero for 100 nanoseconds and returns to a logical one. This 
 100 nanoseconds allows adequate time for the update registers to load and 
 allows the holding registers to load the update values before the first point 
 of the new frame is displayed. This worse case timing is shown in Figure 26. 
 
 The timing relationships for the reader and clock pulses are 
 shown in Figure 27- 
 
 ^•7 Analog Computation Function 
 
 The analog computation function consists of three multipliers, two 
 
 dividers, two adders, and two inverters. This function calculates the 
 
 observer's X and Z coordinates from the converter section trigonometric 
 o o 
 
 outputs. A block diagram of the analog computation function is shown in 
 Figure 28. 
 
 The main considerations in the design of this function are accuracy 
 and settling time. Tests were performed on each unit to insure that the 
 operational accuracy was better than 1P] . Divider accuracy is a function 
 of the magnitude of the denominator so a computer program was written to 
 determine the denominator's range of values. The divider was then tested 
 over the range of expected denominator values. The divider output was 
 within 1% of the exact value for all expected denominator values. As was 
 the case for the D to A converter, the settling time of each unit is dependent 
 on the maximum expected voltage change at its output. The settling times 
 for each unit were measured for the maximum expected voltage change. These 
 
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 times were 0.5 usee for the multipliers or dividers and 0.3 |J.sec for the 
 adders and inverters. Total maximum settling time for the analog computa- 
 tion function is about 1.6 usee. 
 
 The multipliers and adders are scaled to maintain divider accuracy 
 and to yield the observer coordinates without additional stages for gain 
 adjustment. 
 
50 
 
 5. SUMMARY AND CONCLUSION 
 
 5.1 Summary 
 
 The realism of the STEREOMATRIX 3-D display is enhanced by the 
 operation of the position detector system. As the observer moves with respect 
 to the display, he is presented with a changing perspective of the figure 
 as if it were a real 3-D object. The sensitivity of movement of this 
 changing perspective is used to accurately determine the depth of the cursor 
 in the viewing space. 
 
 The observer coordinates are calculated from angular measurements 
 made by two optical shaft encoders. Digital circuits provide coding and 
 memory while the analog circuits provide simple multiplication and division. 
 The special purpose shaft encoder generates simultaneous sine and cosine 
 functions of shaft angle to eliminate the need for function generations in 
 the system. 
 
 5.2 Conclusion 
 
 The optical position detection system is automatic, compact, and 
 economical. It is a good example of a hybrid system which possesses the 
 advantages of optical, digital, and analog components. 
 
51 
 
 REFERENCES 
 
 1. R. T. Cheng, Coefficient generator and cursor for the STEREOMATRIX 3-D 
 display system, Report No. 484, Department of Computer Science, University 
 of Illinois, Urbana, Illinois. 
 
 2. I. E. Sutherland, "A head-mounted three dimensional display", Fall Joint 
 Computer Conference , I968. 
 
 3. D. E. Maxwell, "A 5 to 50 MHz direct-reading phase meter with hundredth- 
 degree precision", IEEE Transactions on Instrumentation and Measurement , 
 v. IM-15, n. k, December 1966, p. 304-310. ~~ 
 
 4. John Bliss, "Applications of Phototransistors in Electro-Optic Systems", 
 Motorola Semiconductor Products Inc., AN-508, December 1969. 
 
 5. R. F. Arnesen, "Boost discriminator performance with a differential 
 capacitor", Electronic Design 3, Feb. 1, 1970, p. 71-72. 
 
 6. R. L. Ehret, L. E. Wood and M. C. Thompson, Jr., "Linear integrated cir- 
 cuit phase meter", IEEE Transactions on Instrumentation and Measurement, 
 v. IM-18, n. 3, September 1969, p. 157-160. 
 
 7. R. H. Frater, "Accurate wideband multiplier-square-law detector", Review 
 of Scientific Instruments , v. 35, n. 7, July 1964, p. 8IO-813. 
 
 8. H. Hahn and R. J. Orgass, "Dynamic rf phase meter", Review of Scientific 
 Instruments , v. 34, n. 4, April 1963, p. 406-408. 
 
 9. E. W. Jones, "Space applications of IR instrumentation", Instruments and 
 Control Systems , v. kl y n. 4, April 1968, p. 105-110. 
 
 10. L. Mattera ed., "Fiber electron optics: new uses for an old technology", 
 Electronic Design 1, January 1971? P« 30-31. 
 
 11. B. B. O'Brien "Simple technique for high-resolution time-delay and group- 
 velocity measurements at radio frequencies", IEEE Transactions on 
 Instrumentation and Measurement , v. IM-18, n. 3, September 1969, p. 160- 
 162. 
 
 12. G. Schlisser and J. Insler, "Portable optical communicator rides laser 
 for secure voice transmissions", Electronics , March 16, 1970, p. 92-96. 
 
 13. R. Schmidhauser, "Measuring nanosecond time intervals by averaging", 
 Hewlett-Packard Journal , April 1970, p. 11-13. 
 
 14. R. Swirsky, "Variable phase-difference network has constant attenuation", 
 Canadian Electronics Engineering , November 1964, p. 26-28. 
 
 15. R. J. Thomas, "Fast light-pulse measurement schemes", IEEE Transactions on 
 Instrumentation and Measurement , v. IM-17, n. 1, March 1968, p. 12-18. 
 
 16. Y. P. Yu, "How to measure phase at high frequencies", Electronics , v. 34, 
 n. 11, March I96I, p. 54-56. 
 
52 
 
 APPENDIX 
 
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58 
 
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59 
 
 VITA 
 
 Charles Raymond Pirnat was born in Chicago, Illinois on April 19, 
 19^1. He graduated from St. Mel High School, Chicago, Illinois in 1958. In 
 1966, he received his B.S. in Electrical Engineering from the University of 
 Illinois at Urbana-Champaign. At that time, he joined the Aeronomy Labora- 
 tory of the Electrical Engineering Department as a Research Assistant under 
 Professor S. A. Bowhill. After completing his M.S. in Electrical Engineer- 
 ing in 1969? he joined the Circuit and Hardware Systems Research Group 
 of the Department of Computer Science under Professor W. J. Poppelbaum 
 to work toward a Ph.D. degree. 
 
 He was a Weapons Control Systems Technician with the United 
 States Air Force from 1959 to I963. In 196h, he was a technician with the 
 University of Illinois Antenna Research Laboratory. During the summer of 
 1965, he was an Assistant Engineer with the Special Services Engineering 
 Division of the Illinois Bell Telephone Company. The Maganavox Company, 
 Urbana, Illinois employed him as an Assistant Engineer in the Value 
 Engineering Department from June to September 1966. 
 
FormAEC-427 U.S. ATOMIC ENERGY COMMISSION 
 
 JcL^Ioa UNIVERSITY-TYPE CONTRACTOR'S RECOMMENDATION FOR 
 
 DISPOSITION OF SCIENTIF!C and technical document 
 
 ( See Instructions on Reverie Side ) 
 
 1. AEC REPORT NO. 
 
 coo 11+69-0198 
 
 2. TITLE 
 
 OBSERVER POSITION DETECTOR FOR THE STEREOMATRIX 
 3-D DISPLAY SYSTEM 
 
 3. TYPE OF DOCUMENT (Check one): 
 
 £J a. Scientific and technical report 
 
 |~[ b. Conference paper not to be published in a journal: 
 
 Title of conference 
 
 Date of conference 
 
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 □ c. Other (Specify) 
 
 4. RECOMMENDED ANNOUNCEMENT AND DISTRIBUTION (Check one): 
 
 Eel a. AEC's normal announcement and distribution procedures may be followed. 
 
 "2 b. Make available only within AEC and to AEC contractors and other U.S. Government agencies and their contractors. 
 J c. Make no announcement or distrubution. 
 
 5. REASON FOR RECOMMENDED RESTRICTIONS: 
 
 6. SUBMITTED BY: NAME AND POSITION (Please print or type) 
 
 Charles Raymond Pirnat 
 Research Assistant 
 
 Organization 
 
 Computer Science Laboratory 
 University of Illinois 
 TTrhana. THinnis &L&Q1 
 
 Signature 
 
 R v;~i 
 
 Date 
 
 February, 1972 
 
 FOR AEC USE ONLY 
 
 AEC CONTRACT ADMINISTRATOR'S COMMENTS, IF ANY, ON ABOVE ANNOUNCEMENT AND DISTRIBUTION 
 RECOMMENDATION: 
 
 PATENT CLEARANCE: 
 
 LJ a. AEC patent clearance has been granted by responsible AEC patent group. 
 LJ b. Report has been sent to responsible AEC patent group for clearance. 
 LJ c. Patent clearance not required. 
 
N<V 
 
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 <** 
 

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