Patent GB-1025769-A (GB)
Title: Automatic stereomapping system
IPC: G08C21/00
Filed: 1963-06-04
Abstract: 1,025,769. Automatic stereo mapping system. THOMPSON RAMO WOOLDRIDGE Inc. June 4, 1963 [June 4, 1962], No. 22229/63. Heading H4D. [Also in Division G4] In an automatic stereomapping system for deriving contour information from a projected stereo image, of the type wherein an elementary portion of the stereo image is scanned to derive video information, indicative of the height of said portion, and the scanning means is caused to traverse the stereo image so that an indication of the height of each portion of the image is produced, the scanning means is moved bodily in the direction of height measurement so as to follow the contour of the stereo image, such that the position of the scanning means in the said direction indicates the height of the portion of the image being instantaneously scanned. The embodiment described is a modified Kelsh photoplotter, basically comprising, Fig. 1, two diapositive photographs 115 and 116, associated light sources 112, a scanner 118 comprising a Nipkow scanning disc 140 and two photomultipliers 150 and 151, respectively detecting the scanned images of photographs 116 and 115, and two image directing assemblies 129, the light sources and image directing assemblies being coupled to the scanner and photo-multipliers such that the images of the portions of the photographs are focused and directed on to the scanning part of the disc 140 and the respective photo-cells, irrespective of how the scanner 118 is moved horizontally .The scanner 118 is moved in a television type scan, Fig. 2 (not shown), by a Y-direction worm drive and motor, 133 and 134, and an X-direction worm drive and motor, 135 and 136. The scanner 118, Fig. 6 and 8 comprises a scanning disc housing 160 having two windows therein 172 and 170 whereby the frame and line sync. pulses may be generated and the chosen portion of the stereo image may be scanned. The Nipkow disc 140, Fig. 9 (not shown), which is rotated within the housing 160 by a motor 162, has three groups of fifteen line sync. apertures 214 and three frame sync. apertures 216, these sync. apertures being aligned with window 172, together with three spiral groups of fifteen line scanning apertures 212, aligned with window 170. The disc rotates at 3600 r.p.m. so that the portion of the stereo image is scanned at a rate of 180 frames/sec. with a resolution of 15 lines per frame. The sync. pulses are generated by two photo-cells 174 and 175, Fig. 7 (not shown) mounted on the housing 160 above aperture 172, under the illumination of light source 176. The housing 160 is movable in the vertical or Z-direction by a worm drive 165 and Z-motor 166. A counter 168 shows the vertical position of the housing, indicative of the height of the portion of the stereo image. The housing 160 may be tilted about the Y- direction by a motor such that better correspondence may be obtained with the actual terrain intilted areas of the image. Signals from the two photo-multipliers are fed to correlating circuitry to provide a signal to energize Z-motor 166 and adjust the height of the scanning disc 140 until maximum correlation is achieved between the two photomultiplier signals, the height of the scanning disc then representing the height of the image position. The photo-multiplier signals are also fed to a tilt control stage to control the tilt of the scanning disc and to a printing arrangement whereby an orthographic photograph is printed from the most suitable of the two signals, together with a contour representation. The system is shown diagrammatically in Fig. 10 (not shown), wherein the signals from the two photo-multipliers 150 and 151 are fed via preamplifiers 180 and 181 to a dead time compensation 183 which is provided so that the drop in the output of the photo-multipliers which occurs between one scanning aperture 212 leaving the field of aperture 170 and the next entering, is not passed on to the rest of the Z-motor control circuitry. The dead time compensator comprises, Fig. 16 (not shown), for one channel, a capacitor 256 which, during a line scan period, is charged up by the photo-multiplier signal, applied via amplifier 250 and switch 253. Just before the end of the line scan a line sync. pulse 259 causes, via circuit 186A, switches 252, 253 and 254 to switch over, such that the voltage across capacitor 256, representing the average signal received during the line scan, is applied by emitter follower 258 to the rest of the circuit. The thus adapted outputs P1 and P2 of the dead time compensator are passed via AGC amplifiers 188 and 189 to a Z- axis control stage 192 and a tilt control stage 194. The Z-axis control stage 192 is shown in Figs. 13 and 18 (both not shown) and comprises two correlators 234 and 235 which (see Division G4) respectively compare for correlation, signal P2 with signal P1 delayed in 231 by 20 Ás, and signal P1 with signal P2 delayed in 232 by 20 Ás. If signals P1 and P2 are equal and in phase, the outputs of the two correlators will be equal and the output of a switch 260, which samples the two outputs alternatively at 400 c/s. will be zero. If, however, signal P1 is out of phase with P2, i.e. the scanner is in the wrong Z-position, the outputs of the correlators will be different and a 400 c/s. A.C. signal will be developed at the output of switch 260, representing in amplitude and phase, the amplitude and sense of the Z-position error. The signals P1 and P2 are also fed, undelayed, to a third correlator 236, which develops a maximum signal when P1 and P2 are equal and in phase. The output of correlator is sampled by switch 262, alternately with the average of the outputs of correlators 234 and 235, at a rate of 400 c/s. to provide a signal to control the Y-speed of the scanner carriage, such that the speed may be decreased when good correlation is lost. A Z-threshold circuit 264 and a no-track control 265 are fed with the outputs of switches 260 and 262, such that the Y-motion of the scanner may be stopped when bad or no correlation occurs. The Z-position error signal from switch 260 is fed via a gating amplifier 336, Fig. 18 (not shown), to the Z-drive motor 331 (166 in Fig. 8) to position the scanning disc 140 in the Z- direction. A Z-limit switch 333, when actuated blocks gating amplifier 336. The tilt control stage, shown in Fig. 14 (not shown), is based on the fact that if the scanning disc 140 is not aligned with the terrain in the stereo image, the Z-position error signal will change as the scanning aperture 212, Fig. 9, sweeps across the image viewed through aperture 170. A comparison is therefore made between the Z- position error signals of each half of each time scan. Delay circuits 241 and 242 and correlators 244 and 245 function in the same way as those in Fig. 13 (not shown). The correlator outputs are fed into a reversing switch which, under the control of the output of a monostable multivibrator, triggered by line sync. pulses, reverses the outputs at the end of each line scan and in the middle thereof. The switched outputs are sampled at 400 c/s. in switch 267. If the disc is aligned with terrain the output of switch 267 will be a continuous train of constant amplitude pulses, each pulse and interval being 1/2700 sec. in duration, whilst non-alignment is shown by a pulsed output wherein the amplitudes of the pulses change every 1/800 sec. The tilt control stage is blocked by a tilt limit circuit 339, Fig. 18, when further tilting of the disc 140 would result in mechanical interference with the photo-multipliers &c. on cutting off the light passing through the apertures 212. The tilt limit stage is fully described in Specification 1, 025, 770. The X and Y drive system for the carriage is shown in Fig. 17 (not shown), wherein the Y-direction drive motor 310 (134 in Fig. 2) drives the scanner carriage 119 in a Y-direction until Y limit switch 318 is actuated. This passes a signal to X drive motor control 323 and to Y direction reversing control 321. An X-direction incremental step is set as a number of pulses in a setter 326, and said number is conveyed to counter/comparator 325. On receiving the signal from switch 318 control 323 starts to activate X-direction drive motor 311 by a series of pulses. These pulses are compared in counter 325, and the X-drive is stopped when the count of these pulses equals that set in setter 326. On receiving the signal from switch 318 control 321 causes the carriage to slow down by reducing the voltage applied to motor 310, and then reverses the said voltage to stop the carriage and accelerate it into the reverse Y-direction. A-Y speed control 320 receives the signals from the no-track control 265, Fig. 13, and the switch 262 and controls the speed of the carriage accordingly. X and Y resolvers 314 and 315 feed signals to corresponding drive means on the printing carriage 206, Fig. 20 (not shown), whereby the carriage, which carries two CRT's 202 and 203, is moved such that the CRT's describe paths relative to respective film plates, slaved to the motion of the scanner carriage. The grid of CRT 202 is controlled via amplifier 365 by the output of photo-multiplier 150 or 151, Fig. 10, depending on which is chosen by a signal selector 190, Fig. 10. The signal selector is shown in Fig. 19 (not shown), wherein a summing amplifier 358 sums a signal representing by sign and amplitude the tilt of the disc 140, and a signal representing by sign and amplitude the position of the carriage in the X-direction. The ummation signal is fed directly to one input of AND gate 350, having the signal P1 at its other input, and, via an invertor 359, to an input of AND gate 351. The outputs of the gates are fed, via an inhibitor gate 352, whereby character marks may be introduced to the grid of the CRT. The operation of the circuit of Fig. 19 is such that the photo-multiplier signal chosen is that resulting from a scanning in a direction most perpendicularly to the projecting light.