Patent GB-1044246-A (GB)
Title: Improvements in or relating to electronic servo apparatus for following the contour of line patterns
IPC: G09G1/08
Filed: 1965-05-10
Abstract: 1,044,246. Automatic character reading. INTERNATIONAL BUSINESS MACHINES CORPORATION. May 10, 1965 [June 24, 1964], No. 19553/65. Heading G4R. In electronic servo apparatus for following the contour of a line pattern there are means for following one boundary of a line and for producing signals indicative of a discontinuity in the opposite boundary of the line. The symbol 213a, Fig. 1 (not shown), is scanned by a moving spot of light from C.R.T. 210 and reflected light is received by photo-cell 214. Attenuators 203 have two levels of gain according to the control signals on leads 203a, 205a. The inputs are sinusoidal oscillations shifted by 10 and 100 degrees so as to represent sine and cosine waveforms. In the absence of signals on leads 203a and 205a the sine and cosine waveforms pass unchanged to integrators 204, 206 giving cosine and sine. The former is inverted at 207 and the cosine and sine waveforms are applied to summing amplifiers 208, 209 causing the C.R.T. beam to trace a circular pattern on the screen. When the line 51, Fig. 3 (not shown), is cut at B by the scan, the gain of attenuators 203, 205 is reduced to half over arc D-E and forms an integrated value in the integrators 204, 206 so that the circle moves along the line in a series of connected substantially circular paths, the centres of which lie within the line being traced. The high frequency components of the deflection waveforms are removed by filters 247, 246 to provide X and Y signals representing the path of the centre of the scanning spot. The circuits of block 100 give extra data. For example, if three black areas are cut in a single circular scan a T-join is indicated. In the block 100, Fig. 2 (not shown), the video signal is clipped at 11 and applied through gate 12 to gates 13 and 14. Only gate 13 is open and single shot 15 fires and passes a gating pulse to enable gate 16. This passes the unclipped video signals to a circuit 17 which tests for black, ignoring smudges and specks. The time of single shot 15 is 90 degrees after the interception of black at B (Fig. 3), i.e. the gating pulse G ceases at D. When the single-shot resets it gives a pulse P which is combined in gate 18 with the black test signal 17a. The output of gate 18 passes through Or gates 19, 20 to fire single-shot 21. The gate output 9 resets the black test circuit 17 and provides the gain control signal on leads 203a, 205a for the attenuators 203, 205 (Fig. 1). The radius of the search circle is then reduced for 60 degrees, the period of single shot 21. At point E (Fig. 3) the circle resumes its normal size. When the single shot 21 resets the P pulse operates single shot 22 which opens gate 12 to blind the circuit to the white/black transition at F. As it resets single shot 22 re-opens gate 12 at H and fires single shot 24 which remains fired until point J. During the period H-J gate 14 is enabled and any black signal obtained will fire single shot 27, the signal 9 opening gate 28 to pass subsequent video signals to the early video evaluator. This is the same as the other test circuit 17 but has a higher black threshold. Its output is combined in gate 30 with the reset output of single shot 27 to pass a signal through Or gates 19 and 20 to single shot 21 to repeat the operations described so that the scan will follow round a corner to the right. If no line is out on a circle it may be an end or a gap. Single shot 26 causes an artificial attenuation so that the trace moves ahead in an attempt to bridge the gap. If a T-join or other feature is present on the lower side of the line the scan will encounter black during the period D to E, i.e. while the attenuation signal is present. Gate 35, enabled by this signal, passes the video signal relating to the lower side of the line to terminal 100b from which it may be passed to analyzing circuits.