Patent GB-945786-A (GB)
Title: Improvements in or relating to position control systems
Inventor: TURNER BASIL AMORY
IPC: G05B19/23
Filed: 1959-04-14
Abstract: 945,786. Numerical control systems. GENERAL ELECTRIC CO. Ltd. April 8, 1960 [April 14, 1959], No. 12627/59. Heading G3N. [Also in Divisions G1, G4 and H2] In a numerical control system for positioning the work-table of a vertical boring machine, the table follows the movements of a pilot member driven by a pilot leadscrew which is rotated under the control of a comparator. The comparator compares a decimal coded command signal (from a punched card or manual selector switches) with the decimal binary coded output of a digitiser coupled to the pilot leadscrew. Only one axis of movement is described, the second axis being identical. Switches between the digitiser and the comparator enable any table position to be used as the datum zero by modifying the digitiser output so that it equals the command signal. A reversing switch between the card reader and the comparator enables settings either side of the datum to be selected. Card reader and digit selector. The command signal is read at 18 from a decimal coded punched card in which a hole in a selected one of ten columns represents a number from 0 to 9, and six lines of holes represent the six digits of a decimal number (tens, units and four decimal places) which defines the desired position of the work-table. The card is read line by line under the control of a digit selector 20. This latter, (Fig. 12 (not shown), comprises a chain of six bistable transistor circuits which are switched in sequence after each decimal digit has been compared with the corresponding decimal digit from the digitiser 14. Alternatively the digit selector may be reset to the first (i.e. most significant) decimal digit. Thus a signal is provided to the comparator 17 on one of ten lines, the process being repeated six times (18 or more, to include repetitive checking, see "Pilot leadscrew controls" below) to define one command. Digitiser. The position of the pilot member 3 is measured by a digitiser 14 driven by the pilot leadscrew 4. The digitiser 14 consists of 6 slotted discs, one for each decimal digit, each providing a 5 bit binary coded output as disclosed in Specification 890, 729. This output may be modified by the zero adjusting circuits, see below. In any case, the coded output in respect of each of the six decimal digits is passed in turn to the comparator 17, by gates 21 controlled by the digit selector 20. Comparator. Use is made Figs. 11a, 11b (not shown) of the characteristics of the particular binary code chosen to provide eighteen lines representing outputs from the digitiser of >0; <1; >1; &c. up to <9. The value >4 corresponds to the change from 1 to 0 of the most significant bit, and <5 is the reverse of this. The other values are obtained, by coincidence gating of selected pairs of the ten input lines from the digitiser 17 (i.e. lines representing '1' to '0' for each of the 5 code bits). These 18 lines are connected to 'AND' gates together with the ten input lines from the card reader so that, for example, the '2' line from the card reader is gated with the <2 line from the digitiser so that an output is obtained if the digitiser is less than the command, and this is passed to one output line. The '2' line is also gated with both the >1 and the <3 lines so that an output is obtained if the digitiser equals the command, and this is passed to a second output line. The '2' line is also gated with the >2 line so that an output is obtained if the digitiser. exceeds the command, and this is passed to a third output line. Similar trios of gates are provided for all ten lines from the card reader (except 0 and 9 which each need only a pair of gates) and their outputs are parallelled on to the 3 output lines. Pilot lead screw controls. The three outputs from the comparator 17 (digitiser equal to, greater than, or less than the command) control a twospeed reversible D.C. motor 15 which drives the pilot lead screw 4 through a two-speed gear box 16. The "greater than" and "less than" signals are connected in the normal mode of operation, via manually controlled switches, to motor reversing relays. Circuits are provided, Figs. 13a...e (not shown) whereby, following a "start" signal, the motor 15 is energized at full voltage and the gear box 16 is in high gear to drive the pilot leadscrew 4 at full speed until the "equals" signal is obtained in respect of the most significant digit. The digit selector 20 is then stepped and the same procedure repeated in respect of the 2nd and 3rd digits. The motor 15 is then energized at reduced voltage, the digit selector is reset and the 1st, 2nd 3rd and 4th digits are successively checked at the second speed. The gear box is then changed down, the motor re-energized at effectively full voltage, the nut of the pilot member 3 is tightened on to the leadscrew 4 as disclosed in Specification 891,036 so as to elimate backlash, the digit selector 20 is reset, and the 1st to 5th digits are successively checked at the third speed. The motor 15 is then energized at reduced voltage, the digit selector 20 is reset and all 6 digits are successively checked at the slowest speed. If the motor 15 is still running when the "equals" signal appears, the 6 digits are repeatedly checked at the slowest speed until exact coincidence is achieved. Sensing device. Positional coincidence between the pilot member 3 and the table 1 is detected by an 'E' type pick-off 475, Fig. 25 mounted on the pilot member 3 and two mild steel bars 482, 483 mounted beneath the work-table 1 so that bar 482 is between the centre and one side limb of the pick-off when the pilot member and table are misaligned in one sense, and the bar 483 is between the centre and the other side limb when the pilot member and table are misaligned in the opposite sense. The output of the pick-off is passed by a phase correcting network to a phase sensitive detector, Fig. 15 (not shown) to provide 'high' and 'low' signals. Table control circuits. The two outputs of the sensing device 10 control a reversible 3 phase motor 5 and a two-speed reversible motor 8 which operate in succession to drive the main lead screw 2. The 'high' and 'low' signals control reversing relays common to both motors. Circuits are provided, Figs. 17a...g (not shown) whereby, following a "start" signal passed on from the pilot lead screw control circuit, an electro- . mechanical clamp 12 is released and the A.C. motor 5 is energized to drive the table until the sensing device passes through zero. On receipt of a signal from the pilot lead screw control circuit that the gear box 16 is in low gear, the motor 5 is again energized to drive to zero. The D.C. motor 8 is then energized at full voltage to drive the table beyond true zero to a false zero obtained by biasing the phase sensitive detector. Then motor 8 is energized at reduced voltage to drive the table back to true zero at low speed. If a signal is now received to indicate that the positioning of the pilot member is complete, the table is clamped. Zero setting. Instead of the table 1 following the pilot member 3 the table may be moved under manual control to a desired datum position and the pilot member 3 is then automatically moved into coincidence, the output of the digitiser 14 being changed until it agrees with whatever command is being obtained from the card reader 18. The manually controlled switches mentioned under "pilot lead screw controls" above are changed over so that the motor 15 is controlled by the sensing device 10, then the circuits of Figs. 13a... e (not shown) operate to energize the motor 15 at high speed with box in high gear to drive the pilot member to the false zero. It returns to true zero (and overshoots) at low speed high gear, then return toward zero at high speed low gear and is finally positioned at low speed low gear. A signal then initiates the stepping in turn of banks of switches, Figs. 19 and 20 (not shown) one bank for each of the six decimal digits, so as to effectively rearrange the binary coding and so advance each decimal digit one digit at'a time until it is in agreement with the command signal. Conditions calling for a "carry" signal are sensed by gates from the characteristics of the code in use, and the process is repeated from the most significant downwards until all the necessary carrying has taken place. Specification 945, 787 also is referred to.