Patent GB-807218-A (GB)
Title: Improvements in digital-to-analog converters for automatic shaft control
IPC: G05D3/14
Filed: 1956-05-14
Abstract: 807,218. Digital-to-analogue converter; electric correspondence control FARRAND OPTICAL CO. Inc. May 14, 1956 [Oct. 17, 1955], No. 14931/56. Class 40 (1). In a digital-to-analogue converter the input is from a punched card which has digital representations of angular values (e.g. 60‹ and 4‹) wherein the sum of the values represent a total angular value (64‹) and a converter responsive to these inputs to produce output signals proportional respectively to the sine and cosine of the sums of the input values (i.e. of 64‹). This total value represents the angular position of a shaft, the sine and cosine outputs being applied to the quadrature windings of a selsyn motor. The conversion is effected by stepping switches responsive to different fields of the input record card. As shown in Fig. 1, the 100‹ field 25 controls stepping switch 101 by applying a voltage to the appropriate one of contacts 5. A stepping magnet then steps the wiper 3 over the contacts until it touches the live contact, the resulting current thereupon energizing coil 6 to open contact 7 and disconnect the stepping magnet 2. A wiper 4 moves with wiper 3 and applies a voltage to a corresponding contact 23 and to a corresponding wiper 13, 14, 15 or 16. These form part of a ten-position stepping switch 102 responsive in the same way to the tens of degrees applied from field 26 of the input record. Connections from contact banks 18, 19, 20 and 21 to lines Q1, Q2, Q3, Q4 are such that these lines are made alive according to whether the angle represented falls in the first, second, third or fourth quadrant. Line Q1 is selected if the angle is from 0-80‹; line Q2 if the angle is 90-170‹ ; line Q3 if the angle is 180-260‹ and Q4 if the angle is 270-350‹. The signal on one of the quadrant lines governs a four-position stepping switch 103, Fig. 6, the contact banks 194-197 of which are inter-connected so that the sign of the output signal is corrected according to the sign of the sine or cosine of the angle as determined by which quadrant it is in. This stepping switch also, through wiper 91 and lines 27 selects one of the wipers C11-C14, Fig. 2, of stepping switch 104 the contact banks of which are connected by lines 108 to contacts 22, Fig. 1, one of which is made alive by the stepping switch 102. The contact banks 111-114 are selected so that, according to the quadrant, an angle will be selected by wipers C15 and C16 the sine and cosine of which is the same as the values for the angle represented by the digital input but without reference to the sign. The sign is corrected by the cross-connections of the contact banks of the quadrant stepping switch. The contacts 56 are connected to a stepped potentiometer 52 the steps of which are graded so as to be proportional to the sine of the angle represented and the contacts of wiper C16 are connected to a stepped potentiometer 54 representing the cosine. These values are applied to the primaries 57, 58, Fig. 5, of transformers, the secondaries of which apply a proportional voltage to potentiometers 59, 75 which are stepped according to the tangent of the units of degrees, the appropriate contact being selected by stepping switch 105 controlled by the units of degrees field 70 of the input record card. Connections are made (see Fig. 6) so that the sine a value on line 77 is added to the cosine a X tangent b value on line 78 and similarly the cosine a value on line 83 is added to the sine a X tangent b on line 85, the resultant values being proportional to sin (a and b) and cos (a and b) respectively. These values are applied through the sign correction network referred to above to the quadrature windings of the control device 37, Fig. 7, for a coarse angular positioning of shaft 8 by servomotor 124. The digital representations of the 0.1‹ and the 0.01‹ values are used to control stepping switches as before which select tappings of transformer windings so as to provide an output proportional to the values, the output from the 0.01‹ transformer supplying the inputs to two tapped transformers the tappings of which are selected by a stepping switch controlled by the 0.1‹ field of the input record card. The output from these transformers represents the sine and cosine of the sum of these small angles multiplied by 72 for the reasons set out below and are applied to the primary transformer windings 138, 141, Fig. 5. The secondary windings have tappings in steps of 72‹. The reason for this is that the fine control device 68, Fig. 7, has 144 poles, i.e. it has 72 cycles for each revolution of shaft 8. One degree of angle B is therefore represented by 72 ‹ on the tappings of the secondary and 0.1‹ of angle A is represented by 7.2‹. The sin A and cos A input signals are therefore made proportional to the sine of the small angles read off the record card multiplied by 7.2. Stepping switch 105 responsive to the 1‹ digital input selects appropriate values of angle B in units so that the secondary transformer tapping represents the sine or cosine this angle B multiplied by 72. The products cos B sin A, sinB cos A are therefore applied on lines 180, 181 to the fine control winding 131 and the products sin B sin A and cos B cos A are applied on lines 182, 183 to winding 130. The shaft 8 may be used to control a machining operation. Specifications 801,516, 801,517, [both in Group XXXV], and 807,219 are referred to.