Patent GB-1286493-A (GB)
Title: Improvements in and relating to numerically controlled servo mechanisms
Inventor: TINKLER KENNETH
IPC: G05B19/39
Filed: 1969-09-17
Abstract: 1286493 Numerically controlled servo positioning apparatus SERVOTEL CONTROLS Ltd 5 Nov 1970 [17 Sept 1969] 45703/69 Headings G4G and G4H] Numerically controlled apparatus for positioning a member 2 by means of a screw 1 driven by a motor 3, 4 comprises, for each pair of digits of equal significance in respectively a first number representing the actual position of the member 2, and a second number representing the desired position, a comparator for comparing two analogue signals respectively representing those digits so as to provide a respective error signal, means to provide in respect of each digit in each of the two numbers a digital signal representing that digit, and for converting separately each digital signal into a corresponding analogue signal. As shown, a desired position may be represented either from an array of switches 16 or a tape reader 18 to provide components of a 6 decimal digit number, each digit of each component of which is transmitted as four binary bits to corresponding arrays, of 6 switching channels 12b, 12c, each channel comprising a digital-to-analogue converter, Fig. 4 (not shown), for providing corresponding analogue current inputs to an array of 6 analogue adders 13 to provide outputs corresponding to each digit of the combined number, which outputs are taken to a subtractor unit 14 for comparison with corresponding analogue signals representing each digit of a decimal number representing the actual position of the member 2. This latter information is derived photoelectrically by a binary coded plate system 8, 9, as in Fig. 3 (not shown), binary coded outputs corresponding to the least significant digits being derived directly, higher digits being derived from a pulse delivered to an array of 4 counters 11a- 11d at each complete revolution of the disc 8, analogue signals corresponding to each digit being taken to the unit 14 where respective error signals are derived and fed to a logic unit 21 to control the drive 3, 4. The drive is adapted to run at a maximum speed until the error signal corresponding to a particular digit is zero together with all higher digits, and it then runs at a speed proportional to the residual error. In the event of the least significant digit only being non-zero, a potentiometer controlled drive may be adopted and the event of no error being indicated a sine wave signal derived from a track of equispaced openings on the disc 8 in co-operation with a photocell 22 is employed and the member 2 brought to rest at a position indicated by a part of zero amplitude. Analogue Adding and subtracting circuits.- Each adding circuit as shown in Fig. 7 comprises a differential amplifier 101 having outputs to further amplifiers 102-104. The two digits to be added are presented in the form of analogue voltages and the addition is firstly performed in the DIA converter, of Fig. 4 (not shown), using Kirchaff laws at 101. In the event of the analogue voltage input representing a digit less than or equal to 9, an analogous output is derived directly via amplifier 104. In the event of the input being e.g. 5+5, the analogue voltage is such that the polarity of the output of amplifier 103 is reversed to cause triggering of transistor 74 via 73 and an output analogous to a number 10 derived by resistors 75, 76 and is subtracted from the output of 101, a number analogous to a 1 being carried to the next decade via resistor 77. Each subtracting circuit is as shown in Fig. 5. The input of a magnitude and pularity according to the subtracted quantities is provided as a differential amplifier 61 and if e.g. 5 is being subtracted from 7, a voltage analogous to a number plus 2 is derived directly at 69 via resistor 70, and amplifier 63. In the event of 7 being subtracted from 5 an anlogue of minus 2 will be the input and the output at 61 will be of inverse polarity and cause, via differential amplifier 62, the triggering of a transistor to feed an input equal to a number 10 to amplifier 63 whose output will now be 10-2=8, and to feed an input 1 to the input of the next higher decade subtractor.