Patent GB-1115881-A (GB)
Title: Position controlling system
Inventor: MACDONALD ALEXANDER TORRANCE
IPC: G05B19/39
Filed: 1964-08-13
Abstract: 1,115,881. Digital control systems. SPERRY RAND Ltd. 13 Aug., 1965 [13 Aug., 1964], No. 32992/64. Heading G4H. [Also in Division G3] A control system for positioning a movable member includes drive means, control means for the drive means, a digital number store in which the positional error from a commanded position is set up, means for producing pulses in number proportional to distance moved which are applied to the number store to cause the stored number to remain proportional to the position error, means sensing the speed of the movable member, means for algebraically adding a signal from the speed sensing means to a signal from the number store to produce a coarse control signal which is applied to the control means and switch means operable at a predetermined number to disconnect the coarse error signal from the control means and apply instead a fine error signal. The embodiment relates to a carriage, having a vacuum-operated pick-up head for the lifting of green bricks from a conveyer and placing them in a stack to suit a firing kiln, which is positioned by movements in horizontal directions which are mutually at right angles. As shown, position information for one direction is derived from a punched tape by a reader (not shown) and fed on a line 18 to a coarse register 20, containing the whole 11 bit number, and a fine register 19 containing the last 7 bits. A digital to analogue converter 22 produces a D.C. error signal, the amplitude of which is proportional to the number in the register 20, which is modulated to an alternating form in a device 23, controlled through a phase switch 2 to determine the direction of travel, is fed to a non- linear device 37 deriving the square-root of the error. The square-root error signal is applied to a summer 36 together with a velocity feedback signal from an A.C. tachogenerator 34 which tends to slow down the positioning as the error falls. The difference signal from the summer is applied through a preamplifier 25 to a servo amplifier 26 positioning servo-valves 14, 15 controlling an hydraulic motor 13 which drives a leadscrew 11 for positioning the piok-up head, not shown. Pulses derived from a photoelectrical cell 30, responsive to a castellated disc 29 driven by the screw, are fed through a gate 32 to reduce the count in the coarse register 20. At a predetermined count in the register 20 a detector 39 changes-over a two-way switch 27 to apply a fine error signal to the servo-amplifier from a summer 41 deriving the difference between a fine position signal from a resolver 33 and the velocity signal from the A.C. tachogenerator 34. The resolver 33 is energized from the register 19 through a digital to analogue converter 35. As the resolver motor approaches the null plane the fine error signal falls to reduce the speed of motor 13. When the required carriage motion ceases an electromagnetic clutch 44 is engaged. Continued movement of the lead screw is sensed by a contact device 46 which through a safety unit 47 shuts off, through valves 51, the supply of pressurized fluid to the motor 13 and operates an alarm bell. The safety unit is also operated by the output of an overspeed D.C. tachogenerator 48 and by manually initiated emergency signals on lines a-d.