Patent GB-1353919-A (GB)
Title: Optical curve tracers
IPC: G06K11/02
Filed: 1971-09-23
Abstract: 1353919 Optical line tracing OPTOGRAMS Inc 23 Sept 1971 44488/71 Heading G3N An edge of lines on a master print of plastics, e.g. "Mylar" (Registered Trade Mark), or a drawing or a workpiece is optically scanned in sequential steps in ŒX, ŒY direction under control of an optical head, comprising a "zoom" lens system, and a single, or group of five, Fig. 21 (not shown) circular section photo-sensors, each associated with an optical fibre, cylindrical or tapered with smallest aperture at the light receiving end, see Fig.1 (not shown). A single sensor 60 passes its output to digital comparators 65-68, whose thresholds, automatically compensated via sensor 112 and potential divider 116 for variations in the ambient light level, and which change state in the sequence C  LL , C L  C H , C HH  as the unobscured portion of the sensor sensitive section increases. The corresponding threshold currents (I LL , I L  I H  , I HH ) are set for a normal tracking state in which half the section is obscured, Figs. 5, 6 (not shown), or somewhat less than half, Figs.7, 8 (not shown). The signals from comparators C H , C L  pass via a decoder 74, see Fig. 10 (not shown), to binary updown ring counter 96, comprising a pair of J-K flip-flops, which counts up, or down respectively under control of master clock 90 whose frequency is set to a desired scanning rate, only provided that both or neither of comparators C L , C H  are triggered, corresponding to a misalignment between the line edge and the sensor, whence decoder driver controls rotary or linear stepping motors 108, 109 to change the stepping direction of the reading head, in one or other of a pre-set clockwise or anti-clockwise sense between the possible ŒX, ŒY directions in steps of length less than the projected photo-sensor diameter. When C L  and C H  are in different states, a succeeding step has the direction of the preceding one. A logic circuit 77, see Fig. 15 (not shown) controls gate 86 to ensure the counter 96 is not merely counted up and down in an oscillatory manner, to avoid the effects of backlash in the system, unless either of C LL , or C HH  is triggered, whence, via logic circuits 72, 80 see Fig. 17 (not shown) counter 96 is enabled to control a change in step direction every two or more at choice, clock pulses in the pre-set clockwise or anti-clockwise sense, thus enabling the tracing of an acute angled line, Fig. 16 (not shown). In a simplified embodiment, Fig.14 (not shown) the comparators C LL , C HH  logic circuits L 1 , L 2 , L 3  and gate 86 are omitted. Scanning by the group of five sensors. To give accurate tracking with less sensitivity to irregularities in the line to be scanned, see Fig. 19 (not shown), four circular sensors in a square array with a central circular sensor may be used,  normal tracking taking place with two of the four outer sensors being blanked out, Fig. 21 (not  shown). The control system, Figs. 20, 20A (not shown) is such that, under normal conditions, the control sensor (190) has no effect on the operation, controlled by the four outer sensors. When the illumination state on the said four attains either of logic levels 0000, or 1111, the unit "BLIND" is triggered to transfer control to the central sensor (190) controlling digital comparators (196, 198), decoder (200) and counter (207). For greater tracking accuracy, there may be a pulse multiplier between the reading and actuating systems. A further system moves simultaneously with the reading head to operate a machine tool for a workpiece, or a writing mechanism to produce a drawing. Instead of uniform illumination, a light source coaxial with the sensor(s) may be directed at the line edge. The master print may be developed on a table using a precision straight edge, and circular and other segments which can be manipulated. In scanning the master print may be moved instead of the head, or a combination of both types of scanning may be used.