Patent GB-904225-A (GB)
Title: Improvements in or relating to digital servo system
IPC: G05B19/29
Filed: 1959-07-07
Abstract: 904,225. Digital servo systems. SOC. NOUVELLE D'ELECTRONIQUE S.A.R.L. July 7, 1959 [July 30, 1958], No. 23372/59. Class 40 (1). In a digital servo system, a comparator delivers in series form as many error signals as there are mismatched digits in the " order " and " present position " numbers, and a reversible motor is energized by the most significant error signal in such a way that the shortest path to equalize the " order " and " present position " numbers is taken. In Fig. 7, a natural binary-coded order number is applied in parallel to bi-stable circuits BA1- BA4 which have a " 1 " output P and a " 0 " output Q. The present position number derived, for example, from a coded disc, is applied in series or parallel form to diode matrices MD1, MX2, MS3, MS4 on " 1 " and " 0 " terminals N, M respectively. Inductors 61, 62 may be included to convert the respective error signals from the diode matrices into serial form if a parallel input is employed, the most significant error signal appearing on X 1  or X 2  first. Matrices MS3, MS4 provide an output at A if a " 1 " is applied at MN and a " 0 " at PQ, and an output at B if a " 0 " is applied at MN and a " 1 " at PQ. No output at either A or B is obtained if the respective input digits D3, D<SP>1</SP>3 and D4, D<SP>1</SP>4 are matched. Matrix MX2 normally operates in a similar manner. To determine the sense of the shortest path to the ordered position, the first digits D1, D<SP>1</SP>1 are examined in matrix MD1 which provides an output at B if the digits are matched, or at A if they are unmatched. An output at B indicates that the two numbers are in the same half of the display circle shown in Fig. 8, and the sense of the most significant error signal correctly determines the sense of rotation of the motor required. If digits D1, D<SP>1</SP>1 are unmatched however, the sense of the most significant error signal no longer indicates the desired direction,. To deal with this situation the first digits are effectively ignored, thereby transferring one of the numbers into the same half of the display circle as the other, matrix MD1 merely provides an output at A if D1 and D<SP>1</SP>1 are different, irrespective of the sense of the difference, and inverter IS operates to transfer an error signal on X1 to X2 or vice versa. If the two numbers are exactly opposite each other on the display circle digits D2, D3, D4 will match digits D<SP>1</SP>2, D<SP>1</SP>3, D<SP>1</SP>4 and no error signal results, but in this case, a signal derived from bi-stable circuit BA1<SP>1</SP> on line T and applied to an RC network ML<SP>1</SP> switches matrix MX2 after a given time so that it operates as though one of its input digits were complemented, and an error signal results on X1 or X2 to select one of the two equal paths. Fig. 10 shows a similar comparison circuit for use when the numbers are coded on the reflected binary scale. Since the error signal produced by a comparison of two digits does not necessarily give the true sense of the shortest path, an inverter I1, I2, 13 is inserted between each pair of stages and is controlled to invert the signal on buses X1, X2 if the preceding digit of the order number is a " 1." In this way an error signal is complemented as many times as there are " 1's " in the preceding digits of the order number. Matrix MX2 operates as before when digits D1, D<SP>1</SP>1 are matched, but acts as if one of its inputs were complemented when D1, D<SP>1</SP>1 are unmatched so as to produce a correct error signal. When the two numbers are opposite each other on the display circle, no output is obtained from MX2 at first, but after a given time determined by circuit ML<SP>11</SP>, matrix MX2 is switched to operate as if neither input were complemented and an error signal results.