Patent GB-1206768-A (GB)
Title: Improvements in or relating to graphical data digitizers
IPC: G06K11/00
Filed: 1967-12-11
Abstract: 1,206,768. Digitizing graphical data. CALMA CO. 11 Dec., 1967 [15 Dec., 1966], No. 66157/67. Heading G4H. In apparatus for digitizing graphical data comprises manually operated means for following the data and means for recording digital values corresponding to the movements of the follower over the graphical data, means are provided for varying the sampling interval so that it is proportional to the speed of movement of the follower. The apparatus is generally similar to that disclosed in Specification 1,166,265. It comprises a tracing bed on which a sheet may be placed carrying a graph which is traced by an operator using a stylus or crosshairs. The stylus is mounted on a carriage slidable along a vertical track rod which, in turn is horizontally movable on top and bottom rails. Cables connected to the end of the track rod are wound around an X-digitizing shaft and a cable connected to the carriage is wound round a Z- digitizing shaft. Z is the sum of X and Y. Both shafts carry toothed discs each having three reading heads T1, T2, T3 and T4, T5, T6, Fig. 9, energized by clock synchronized driver M3. The reading head signals are gated at 201 to storage flip-flops N1-N6 and the previous signals are stored on flip-flops S1-S4. The present signals in N1-N6 are compared with the previous signals in 208 and 209. The X increment signals are subtracted from the Z increments by a logic circuit 225, the output of which represents Y increments. Circuits 63, 64 apply the X and Y increments on forward and backward leads to counters 60, 61 which accumulate algebraically the X and Y increments over a preset interval of time, e.g. 4 milliseconds, which includes 32 digitizing periods i.e. the counters could accumulate a maximum of 31 increments. The count and sign digit are transferred by circuits 250 to flip-flops B1-B8 and BA in the output register 70 and the counters are reset. The stored values are gated to the recorder 37. Clock pulses from oscillator C1 are applied to a counter comprising five stages, outputs of which are gated together. If there has been insufficient movement in a sampling period to warrant making a recording, recording may be prevented by X and Y increment selectors 75, 76. These are controlled by switches 31, 32 which are preset so that no recording takes place until a given X or Y movement has taken place. In circuit 75 the contacts of switch 31, except that corresponding to the minimum increment 0À01 inch, are connected to diode gates there being a corresponding input to each gate from the counter stages X1-X4. Thus the next smallest increment 0À02 inch is gated with the X2 counter stage output and if the switch is set to this position no output will appear until there have been at least two steps in the X-direction. Recording is prevented until this minimum movement has been made. The Y increment selector is similar. The operator can override these selectors by pressing a data point switch 33 which causes a recording to be made irrespective of the amount of movement. Switches 35 control the variable increment selector 77 to set the maximum distance that the stylus can travel in each recording period. The X and Y maximum distances on switches 35 are compared with the contents of counters 60 and 61 and if the maxima are exceeded, an alarm is given by light 29 and speaker 78. If the distance is set to the maximum 0À15 inch (all switches closed) the sampling interval is 0À01 inch but this increases automatically to 0À02 inch if the stylus speed exceeds 150 inches per minute, to 0À03 inch if it exceeds 300 inches per minute and so on up to a maximum tracing speed of 2250 inches per minute. Data hold switch 34 disables the circuits 63, 64 so that no signals can pass to the counters. This avoids recording data relating to movements of the stylus which are not part of the graphical data.