Patent GB-689217-A (GB)
Title: Improvements in electronic control systems
IPC: G05D5/03
Filed: 1951-01-30
Abstract: 689,217. Automatic thickness control systems. AMERICAN BRASS CO. Jan. 30, 1951 [Feb. 8, 1950], No. 2322/51. Class 38 (iv). In an automatic control system an alternating signal voltage dependent on the output of an electrical gauge is compared with an alternating reference voltage in anti-phase with the signal voltage, the difference voltage being coupled to two valve control grids, the two corresponding anode circuits of which are supplied with voltages in anti-phase and include electromagnetic relays, one or other of which is energized according to whether the voltage is greater or less than the reference voltage. In a metal rolling mill a direct voltage derived from a gauge (not shown), e.g. an ionization gauge responsive to beta rays measuring the thickness of metal emerging from the rolls, is developed across a resistor R5, Fig. 1, and from it an alternating signal voltage of corresponding magnitude is produced by an electromagnetic vibrator 12s, across the secondary winding 20 of a transformer T3. From the direct voltage output of a full-wave rectifier VI an alternating reference voltage in antiphase to the signal voltage is produced by a second electromagnetic vibrator 12r across the secondary winding 17 of a transformer T2. The magnitude of this reference voltage is adjustable by a potentiometer R3. Since the electromagnets 15r, 15s of the vibrators are energized from a mains transformer T1, both the signal and reference voltages are of mains frequency. The two secondary windings 17 and 20 are so connected that a resultant error signal, dependent in magnitude and phasesense on the magnitude and sense of the difference between the signal and reference voltages, is produced and applied to a twostage thermionic valve amplifier V3. The A.C. output of amplifier V3 is applied in phase to the grids of a double-triode valve V4 the anodes of which are energized in anti-phase from a secondary winding S3 of mains transformer T1 through electromagnetic relays RL-1, RL-2, respectively. One or other of these relays is thus energized according to the sense of the difference between the signal and reference voltages and hence according to whether the metal is thicker or thinner than desired. The change of thickness necessary to cause these relays to operate may be pre-set by a potentiometer R12 which determines the cathode bias of valve V4. These relays control through continuously interrupted contacts 70 further electromagnetic relays RL-11, RL-12 which control motors running in opposite directions or a reversible motor (not shown) correcting thickness by adjusting the spacing of the rolls. Contacts 70 are continuously interrupted to have a short " on " time and a relatively longer " off " time as the result of the alternate firing of thyratron valves V6, V7 by relays RL-5, RL-6, rheostats R41, R42 permitting adjustment of the " on " and "off" times respectively. If the thickness error is large, one or other of further relays RL-3, RL-4 associated with a double-triode valve V5 operates according to the sense of the error in addition to one of the relays RL-1, RL-2. The magnitude of the error necessary for this to occur may be pre-set by a potentiometer R15 controlling the cathode bias of valve V5 and, when it occurs, energization of relay RL-11 or RL-12 is then completed through continuously interrupted contacts 71 instead of contacts 70. Contacts 71 operate as the result of the alternate firing of thyratron valves V8, V9 by relays RL-7, RL-8, rheostats R43, R44 being set to give a relatively longer " on " time and shorter " off " time than contacts 70 so that more rapid correction of the larger error is effected. To prevent false operation with no metal in the rolls a switch 69 is closed by the response of a photo-cell (not shown) detecting the presence of the metal, whereupon a relay RL-10 causes a thyratron valve V10 to conduct after a delay to prepare the circuit of relay RL-11 or RL-12. The following modifications are referred to: (a) capacity-coupled inductances replacing transformers T2, T3; (b) a double vibrator having a single electromagnet replacing vibrators 12r, 12s and capacity-coupled resistances replacing transformers T2, T3; (c) a variable reluctance gauge in series with an A.C. mains supply to the primary of transformer T3 and a potentiometer in series with an A.C. mains supply to the primary of transformer T2, thereby dispensing with vibrators 12r, 12s; (d) an arrangement similar to (c) above with resistances replacing transformers T2, T3. Specification 414,785, [Group XXXVII], is referred to.