Patent GB-1207198-A (GB)
Title: Mark sensing apparatus
IPC: G06K7/10
Filed: 1968-01-04
Abstract: 1,207,198. Sensing records. INTERNATIONAL BUSINESS MACHINES CORP. 4 Jan., 1968 [9 Jan., 1967], No. 632/68. Heading G4M. Apparatus for sensing multiple-choice examination answers includes a function generator producing an output which is a function of mark density and width. Scanning system Fig. 3a includes a continuously rotating mask drum 30 having staggered apertures (31), Fig. 1 (not shown) for successively sensing the response positions in a row of answer sheet 20, timing apertures (33) for sensing the timing marks associated with the rows on sheet 20, and sync. apertures (35) for sensing light reflected from a fixed surface on base-plate 40. A stationary drum 50 is coaxial with drum 30 and houses bundles of illuminating optical fibres 54, 54a-54g receiving light from a common lamp 52, and further bundles of sensing optical fibres 58a-58e, 59a, 60a for sensing the row positions on sheet 20, the timing marks and the sync. apertures respectively, these bundles being led to photodetectors 62, 64 and 66. Each of the staggered apertures (31) in drum 30 is preceded by a reflective area on the inside surface of drum 30, these areas providing a reference level as described below. Circuit, Fig. 8. Just before the sensing of a mark position on sheet 20, the reflective area inside the drum passes the ends of a pair of optical fibre bundles 54, 58, causing the input to amplifier 100 to go negative. Peak storage circuit 101 includes a capacitor which is charged a capacitor which is charged to the maximum negative voltage attained by the input, and its output is used to control the gain of amplifier 100 to normalize its response. A read gate signal which is produced by a timing circuit for a number of complete drum revolutions, starting at a datum point in a drum cycle following first detection of a timing mark on a sheet 20, is gated with the sync. signals to sample the output of amplifier 100 as successive positions in a row are being sensed. The maximum sampled amplitude is stored at 104 to provide an input W to amplifier 108. Simultaneously, the sampled signal is applied to turn on a ramp generator 106, provided the signal exceeds threshold 105 which is set to correspond to the minimum mark density to be detected. The output of ramp generator 106 rises to a value proportional to the width of the detected mark, and this value X is stored at 107. The gain of amplifier 108 is normally set by threshold 109, so that its output Y initially follows input W. If the mark width exceeds that corresponding to threshold 109, the gain of amplifier 108 is increased by signal X, so that for marks of the same density (W), increased weight is given to wide marks. Signal Y is compared at A time with a reference Z which initially is a minimum mark threshold, and if Y>Z comparator 113 produces an an output DM. Provided Y>Z, Y replaces Z in store 112 at B time; and detection of subsequent marks proceeds, store 112 retaining the value corresponding to the most valid mark sensed previously, and comparator 113 only producing an output DM if this stored value Z is exceeded by a newly sensed mark. The circuit 112 is reset at the end of several drum cycles sensing the same row of sheet 20. The output of comparator 113 is fed to a register which comprises stages settable in turn under control of the sync.-derived timing pulses A-C and comparator output DM, only that stage corresponding to the most valid mark remaining set at the end of a row scanning operation. The timing circuit may feed one pulse per row of sheet 20 to a row counter.