Patent GB-783654-A (GB)
Title: Digital potentiometer for reflected binary code
IPC: H03M1/00
Filed: 1955-08-30
Abstract: 783,654. Electric selective signalling and control systems. BENDIX AVIATION CORPORATION. Aug. 30, 1955 [Sept. 7, 1954], No. 24882/55. Class 40 (1). In an electric signalling or control system employing reflected binary code, digital relays responsive to the code signals connect a number of fixed impedances into a circuit in such combination as to produce in the circuit a total impedance corresponding to the value of the code. The circuit may be in the form of a continuous closed potentiometer which replaces the conventional transmitter in a synchro system. Fig. 1 shows a four-digit potentiometer comprising sections I, II, III, IV, each comprising a line 10, and a line 11 including a resistor the value of which doubles in successively higher digital sections. Digital switches 12 are controlled by relays D1 to D4 and for binary O complete the connection shown in solid lines and for binary 1 the connection shown in dotted lines. Actuation of any switch 12 transposes all the resistors between the switch and a main terminal B into the opposite line, thereby deleting impedance between the terminals C and B and inserting it between terminals A and B. A centre-tap T may be added at the 7¢R point on the resistor 8R and regardless of the position of the switches. the resistance between the tap T and each of the terminals A and C will always be one half the total resistance of 15R, the arrangement being the equivalent of a conventional potentiometer having a centre-tap T, fifteen equal steps of resistance R connected between terminals A and C, and a movable tap B. Fig. 4 shows a servo system in which a closed potentiometer 50, operated by a knob 52, is substituted for the conventional synchro transmitter. Two digital potentiometers of the type described in Fig. 1 may be arranged as in Fig. 5 to act as an exact replacement of the potentiometer 50. For each position of the rotor of the synchro 51 there is an angular position of the potentiometer 50 in which the potential between the movable contacts is zero. If the potentiometer contacts are rotated out of the null position a potential is developed which is transmitted over leads P1, P2, amplified at 54, and compared at 55 with the potential across the rotor of synchro 51, to cause a motor 56 to drive the rotor to the position corresponding to the new position of the potentiometer and thereby restore the null across its contacts. The arrangement shown in Fig. 5 responds to sixdigit reflected binary signals according to a table given in the Specification and provides a resistance of 16R between each adjacent pair of taps T1, T2, T3, and a resistance of 24R in each circuit between P1 and P2, as in Fig. 4. Each successive signal shifts the points of connection of lines P1 and P2 an amount corresponding to one increment clockwise corresponding to an angular movement of 7¢ degrees. At position 16 relay D5 operates to transpose the connections of L1 and L3, and at position 32 relay D6 operates to transpose the connections of L1 and L2. To reduce the inherent error between the position of the potentiometer and that of the synchro, which takes the form of a successive lead or lag between each successive 30 degree point, the resistance 4R is reduced to 3 11/12R and R/12 is introduced into the other side of that section, Fig. 7 (not shown), and further correction may be introduced by causing relay D1 when energized to change the resistance 3 11/12R to 3 5/6R and the resistance R/12 to R/6, Fig. 8 (not shown). When only one rotating contact of a closed potentiometer is to be simulated one of the two digital potentiometers 30, 31, is dispensed with and a simple impedance element of the same value is substituted.