Patent GB-797451-A (GB)
Title: Improvements in and relating to programming control systems
IPC: G05B19/35
Filed: 1955-12-30
Abstract: 797,451. Programming for machine tools. GENERAL ELECTRIC CO. Dec. 30, 1955 [Dec. 30, 1954], No. 37369/55. Class 83 (3). [Also in Group XXXVIII] A follow-up device for controlling the movement of an object from a record comprises fine and coarse director devices 8-11, the accuracy of the first being a multiple of that of the latter, and means. to move the fine-director device direct to a position as indicated by the record. The table 1 of a turret punch-press is moved to a desired position through a lead-screw 21 by a D.C. shunt-type motor 2, the armature of which is supplied with current for forward and reverse rotation by thyratrons 4, 5. The supply of control voltages to the input circuits of the thyratrons is provided by a director 6 and a position-indicator 7, each of which has four rotary induction devices 8-11 and 12-15. Windings 12a-15a are supplied with A.C. through lines 16 and induce voltages in the windings 8a-11a according to their angularpositions. The devices are geared together by 10 : 1 gearing so that one revolution of the device 12 respectively produces 10, 100 and 1000 revolutions of the devices 13-15. The latter device is connected to the lead-screw 21 through gearing 20 such that one revolution of the device 15 causes 1 inch of movement of the table. Since the polarity of the voltage induced in the secondary winding of each of the rotary induction devices in the director becomes reversed when the angular disagreement between it and its counter-part in the position indicator exceeds 180 degrees, the zone of working of each must be limited to less than 180 degrees. A take-over circuit 22, consisting of fixed resistors such as 23 and voltagesensitive non-linear resistors such as 24, is therefore provided to effect transfer of control of the motor 2 selectively and successively between the four pairs of devices. The resulting alternating error-voltage is amplified by a twin triode cathode-coupled valve 49, and is then passed through a discriminator 51, having an output-resistor 54, across which a direct voltage is produced which has a polarity corresponding to the direction of the error and is approximately proportional to the error for small values thereof. This voltage output is supplied to the input-circuit of an amplifier of the cathode-follower type, and thence to the motor-control circuit 3. When the rotors of the position indicator and director are in balance, the voltage across the resistor 54 is zero and the voltage of a terminal 63a in the cathode-follower circuit will have a value such that both halves of a cathode-coupled twintriode 67 are conducting equal amounts of current in approximately mid-range, so that the thyratrons 4, 5 supply equal small currents in opposite directions to the motor 2, thus holding it stationary. When the rotors are not in correspondence, but are displaced, say, forwardly, an error-voltage appears across the output resistor 54 of the discriminator, and the current conducted by the cathode-follower 58 increases, thereby making the voltage at point 63a more positive. This results in increasing the current in the right-hand path of the valve 67 and decreasing it in the left-hand path. The more-positive voltage at the anode 67a advances the firing-point of the forwardthyratron 4 to increase the current to the motor 2. Conversely, the reverse-thyratron 5 ceases to conduct. The motor 2 thus rotates until correspondence has been re-established and the table moved to its new position. Each position to which the table must be moved is recorded on a punched tabulating-card 75, Fig. 2. The card is read by a device 76 and stored in a device 77. Such stored information is then utilized to move the rotors in the director 6 into angular positions corresponding to those which the rotors of the position-indicator must occupy when the machine table is in the required position. The card-reader 76 comprises a contact-roll 79 which completes a circuit to a stepping switch 82 whenever a brush 83 finds a hole in the card. Columns are provided on each card for tens, down to thousandths, but only the storage device 77 for the hundredths information is shown. In this device, the arm 82c of the stepping-switch 82 operates as soon as the hundredths brush 83 makes contact with the roll 79. An operating-coil 82a is then energized to advance the stepper according to pulses received from a switch 81 operated by a wheel on the roll 79. Corresponding to each stepping-switch 82 there is a ratchet-wheel in a rotor-positioning device 78, of which ratchet wheels only that (96) for the hundredths column will be described. The wheel is freely mounted on a shaft 87 which is oscillated through 300 degrees, once for each card, by a constantspeed motor 88 driving through a one-revolution clutch 90, a crank 92 and gears 91, 93. On the return swing of an oscillation, initiated by the completion of the previous card-reading, a wiper-contact 99a on the shaft 87 completes a circuit with the previously-set stepping switch 82c to operate an electromagnet 98a. The electromagnet advances a pawl 98c to hold the returning ratchet-wheel 96 in its appropriate position. Thus, each ratchet-wheel is set in a position determined by the card. The ratchetwheels are connected to the rotary induction devices 8-11 through differential gearing 105, 109, 112-115, Fig. 4. There are no rotary induction devices corresponding to the hundredths and thousandths columns of the card as the devices 8-11 provide the necessary refinement of control. Consequently, a tenth of the rotation of the thousandths ratchetdevice 104 is added directly or through the differential 115 to the differential 114, and a tenth of that rotation together with a tenth of that of the hundredths ratchet-device 103 is added to the device 11 through the differential 113. Each lower-order device thus feeds <SP>1</SP>/10<SP>th</SP> of its rotation to the upper-order device. An electronic circuit 128, Fig. 1, prevents movement of the table until exact correspondence has been reached, the circuit having coupledresistors and capacitors such as 135, 137 to make the circuit fast to indicate error and slow to indicate correspondence. As the table is moved by the motor 2, the output voltage of the take-over network is dominated successively by the voltages in the windings 8a-11a, so that the motor 2 gradually slows to a stop with the table at the required position. In a modification, the stepping-switches 82 are eliminated by reading the card while it is stationary, such as by placing it between pin-boxes 120, 121, Fig. 9. In a further modification, the stepping switches are eliminated by synchronizing the movement of the ratchet-wheels with the movement of the card. Linear, instead of rotary, induction devices may be used.