Patent GB-988911-A (GB)
Title: Mark selection circuit
IPC: G06K17/00
Filed: 1962-06-07
Abstract: 988,911. Statistical apparatus. UNITED AIRCRAFT CORPORATION. June 7, 1962 [June 12, 1961], No. 22060/62. Heading G4M. In a system for sensing marks on an examination answer card, means are provided for detecting when two marks have relative densities within a predetermined range have been made in the alternative answer positions relating to a single question. Answer card. The card, Fig. 2, is divided into rows, each of which has an associated row marker at the left-hand edge. Rows 1, 2, 3 and 7 are filled in by the candidate to indicate his identity and the tests being answered. The remaining rows are for entering the answers to the tests,there being nine questions in each test. Each row is divided into three items each comprising three positions of which one only should be marked to indicate the selected answer. Circuit details. The answer card is carried on a rotating drum 21, Fig. 1. A photo-cell 25 detects the presence of the card and enables gate 29 during the sensing operation to pass row marker pulses from photo-cell 15 to the row counter 49. A sweep generator 13 is triggered by each row marker pulse to sweep a beam of light from a galvanometer mirror across the respective rows of the answer card, and also to provide position marking pulses from a photo-cell 52 associated with a notched transparent rod. The position marker pulses pass through gate 33 to a counter 91,a gate 53 and a delay network 95, 97, 99. The output of a mark sensing photo-cell 36 is clipped to a preselected background noise level and passes through gate 53 to charge a capacitor 57. Further capacitors 65,71 are charged to levels corresponding to the maximum mark density in an item being sensed, capacitor 65 producing an output pulse from capacitor 87 whenever the density of a mark being sensed exceeds that of any preceding mark in the item. Capacitor 57 is discharged through gate 55 by each delayed positionmarking pulse, whereas capacitors 65, 71 are discharged at the end of each item through respective gates 63, 69. The density of a mark being sensed, represented by the voltage across capacitor 57, is compared with the preceding maximum mark density, represented by the voltage across capacitor 71,by means of differential amplifiers 143, 145 and potentiometers 133, 139. These potentiometers may be set so that if the density of a mark being sensed is more than half but less than twice the maximum density of the preceding marks, bi-stable circuit 153 is triggered to its YES state to indicate a multiple marking of an item, this providing an output at terminal W to indicate that the answer must be regarded as being wrong. The answers to the various questions are obtained as signals at terminals A, B, C as follows. Each time a mark is sensed which is denser than any preceding marks, the pulse from capacitor 87 opens gates 121 and the corresponding mark position registered in counter 91 is stored as charge on the appropriate capacitor 43, 45 or 47, any previously stored information being erased. Provided no multiple marking condition is detected, gates 123 are opened at the end of the item. A differential amplifier 81 ensures that only marks having more than a predetermined minimum density are registered in the above manner. If no mark in an item has the required density, bi-stable circuit 159 is set to its YES state through gate 157 to indicate an answer which is so faintly marked as to require further inspection. The output of bi-stable circuit 159 is therefore used to operate a paper shunt mechanism 27 through gate 181 and divert the answer card to a separate stack. The answer card is also diverted to this stack if, in any item of rows 1, 2, 3 or 7 reserved for the identity of the candidate and the tests, there is no mark (gate 171) or a multiple mark is detected (gate 173).