Patent GB-840004-A (GB)
Title: Course plotter
Inventor: JACKSON DONALD EDWIN
IPC: G01C21/00
Filed: 1957-06-27
Abstract: 840,004. Photo-electric control. SPERRY RAND CORPORATION. June 27, 1957, No. 20405/57. Class 40 (3). [Also in Groups XX and XL (c)] Relates to a system as described in Specification 840,003 for automatically positioning a carriage 10, Fig. 1, over a chart 18 having spaced position lines a, b, c, d, e marked thereon such that an index 17 on the carriage is located between the lines b and c and along a locus p, p<SP>1</SP> parallel to the direction of movement of the carriage at a distance x from the line b which is a predetermined fraction x/y of the distance y between the lines b and c, the said predetermined fraction x/y being given by the angular position of an input control shaft 45 ; alternatively the index 17 may record on a second adjacent chart representing the same area as the chart 18. When the position lines on the chart correspond to position lines in space defined by a Loran radio navigation system, the shaft 45 is controlled in accordance with the output of a Loran receiver to make a complete revolution as the craft carrying the receiver moves from one position line, e.g. b, to an adjacent position line c and according to the present invention when the craft moves to a point whose distance from a position line is less than a predetermined value the system interpolates between points spaced midway between the position lines instead of interpolating between the position lines themselves as described in Specification 840,003 so that the carriage moves continuously in accordance with the movement of the craft as it crosses a position line. As described a segment s, s<SP>1</SP> of the line p, p<SP>1</SP> at least equal to twice the greatest spacing between two adjacent position lines is scanned as described in Specification 840,003 by a phototube, Fig. 2 (not shown), mounted on the carriage 10 to produce (1) recurrent groups of scan pulses b, c, d, A, Fig. 3, corresponding in time to the lines b, c, d; and (2) reference pulses D, Fig. 3, corresponding to the position of the index 17. The scan pulses A are slightly delayed at 24 to give a corresponding series B of pulses b<SP>1</SP>, c<SP>1</SP> ,  d<SP>1</SP> which trigger a primary sawtooth generator 25 giving an output C which is sampled (1) in a gate circuit 32 by the reference pulses D to give a D.C. voltage I proportional to the distance of the index 17 from the chart line b, and (2) in a gate circuit 38 by pulses F corresponding to the c pulses to give a D.C. voltage G proportional to the distance y between the lines chart b and c ; the F pulses are derived from the trailing edges of pulses E which are initiated in a bi-stable multivibrator 33 by the reference pulses D and terminated by the next succeeding scan pulses, i.e. the c pulses as shown. The voltage G is applied to a circular potentiometer 42 whose continuously rotatable slider 43 is controlled by the input shaft 45 to give an output H proportional to the distance x of the craft from the space position line b. The voltages H and I are applied to a relay 49 and filter 53 to give an A.C. error signal K which controls a servomotor 55 to adjust the position of the carriage 10 until the voltages H and I are of equal amplitude. According to the present invention the signals C, A and H are only applied to their utilization circuits 32, 38 ; 33 and 49 when the angular position of the input shaft 45 is within limits corresponding to values of the fraction x/y between  and ¥, this being effected by relays 26, 35 and 48 controlled by a relay winding 74 whose energization is controlled by a rotary switch 71 ganged to the input shaft 45. An output from the sampling gate 38 is also applied to a potential divider 60, 61 giving a bias voltage B.L., C, Fig. 3, of amplitude equal to half that of the voltage G, the bias voltage being applied together with an output C from the primary sawtooth generator 25 to a comparator 27 and pulse generator 63 to give a pulse series M corresponding to the pulse series A but delayed relatively thereto by half the time interval between the b and c pulses. The pulse series M is slightly delayed at 64 and the resultant pulses N trigger an auxiliary sawtooth generator 65 to give an output O corresponding to waveform C but delayed relatively thereto by half the time interval between the b and c pulses. The potentiometer 42 is also provided with an additional slider 67 diametrically opposite to the slider 43 and producing an output H<SP>1</SP> proportional to the distance of the craft from a point midway between the space position lines b and c. When the craft approaches the position line c within a distance of a quarter of the separation between the lines b and c (reference pulses now shown as waveform D<SP>1</SP>), the relays 26, 33, 48 operate to select the signals O, M and H<SP>1</SP> in place of C, A and H. The resultant output G<SP>1</SP> from the sampling gate 38 is again proportional to the distance y between the lines b and c but the resultant output I<SP>1</SP> from the sampling gate 32 is now proportional to the distance of the index 17 from a point on the chart 18 midway between the lines b and c and the carriage is now controlled in accordance with any difference in amplitude between the voltages H<SP>1</SP> and I<SP>1</SP>. This mode of operation continues until the craft has crossed the position line c and reached a point whose distance from the line c is a quarter of the separation between the lines c and d when the switch 71 operates to restore the first mode of operation, the resultant voltages G, H and I being then related to the interval between the lines c and d and the new bias level being indicated in C Fig. 3 as B.L.<SP>11</SP>. Thus the carriage 10 continuously follows the movement of the craft as it crosses a position line. If the scanning is interrupted the bias level BL applied to the comparator 27 might become so high as to exceed the peak value of the primary sawtooth C and in order to prevent this a comparator 75 operates a relay 76 to discharge the auxiliary sawtooth generator 65 if the bias level exceeds half the maximum expected peak amplitude of the primary sawtooth C. Alternatively the magnitudes of the signals G, G<SP>1</SP>, H, H<SP>1</SP>, I, I<SP>1</SP> may be represented by electrical frequencies, digital code signals or mechanical displacements or rotations. Specification 840,002 also is referred to.