Patent GB-710348-A (GB)
Title: Improvements in automatic control of physical characteristics
IPC: G05B11/01
Filed: 1951-03-15
Abstract: 710,348. Automatic temperature control systems. LEEDS & NORTHRUP CO. March 15, 1951 [March 15, 1950], No. 6286/51. Class 38(4) In Fig. 1, to control automatically the temperature of a furnace 10, the temperature within the furnace is detected by a thermocouple 11 arranged in a selfbalancing potentiometer circuit which includes a galvanometer 12 associated with a mechanical relay 13 operating a temperature indicator and recorder 17, 18 and a potentiometer 31. Deviation of temperature from a value predetermined by the setting of a potentiometer 42 produces, by adjustment of potentiometer 31, a direct voltage E, dependent in magnitude and polarity on the extent and sense of temperature deviation. This voltage is arranged in opposition to another direct voltage E 3  and the resultant applied to the input of a high impedance amplifier 27 which energizes a relay 25 or 26 to run a reversible electric motor 22 in the appropriate direction to correct the temperature deviation by adjusting a valve 20 in the supply of heating or cooling medium through a pipe 21. Motor 22 also adjusts a potentiometer 35 so that a direct voltage dependent in magnitude and polarity on the extent and sense of valve adjustment is produced as a component of a voltage E 3 . The amount of movement of potentiometer 35 and hence range of adjustment of valve 20 necessary to correct a given movement of potentiometer 31 is adjustable by rheostat 36 or 45 altering the voltage applied to potentiometers 31 and 35 respectively. Rheostats 36 and 45 may be ganged for simultaneous adjustment in opposite directions and calibrated as a percentage of the range of indicator 17. A differentiating circuit including resistances 52, 53 and capacitor 54 introduces such a component into voltage E 3  that motor 22 is also controlled in accordance with the integral of the temperature deviation. An integrating circuit including resistance 58 and capacitor 62 introduces a component dependent on the rate of temperature deviation. Resistance 58 is adjustable to vary the magnitude of the rate component, an anti-hunting resistance 60 being simultaneously adjusted in the same directon by knob 65. Potentiometer 42 may be adjusted by a programme controller to vary the temperature datum. The temperature datum may alternatively be adjusted by moving potentiometer 31 relative to its slider. Typical values are given for the components of the integrating and differentiating circuits, a low-leakage capacitor of polystyrene film in place of paper being recommended for capacitor 54. If an inverse rate component is desired resistance 58 is short-circuited by closing a switch 68, the desired inverse rate component being developed by capactor 62 and resistances 60. 61. Alternatively switch 68 and a switch 69 are opened and switches 70, 125 closed. A resistance 72 and a capacitor 71 then produce the inverse rate component without an immediate proportional component. Cam-operated contacts 82a 86a determine the limits of operation of motor 22, signal lamp 87 or 88 being extinguished at the limit. The signal lamps also indicate the direction of operation of motor 22. Further integrating and/or differentiating circuits may be added to the system. In Fig. 6 a single two-way relay 148 replaces relays 25, 26 of Fig. 1. Instability due to the fact that, following a persistent temperature deviation, capacitor 54 may become charged to the maximum voltage. is avoided in Fig. 6 by a relay 135 which is energized when the temperature reaches a lower limit such that contacts 141 are closed by a travelling nut 136 operated by mechanical relay 13. Energization of relay 135: (a) operates motor 22, through contact 135d, to fully open valve 20, (b) substitutes a potentiometer 144 for potentiometer 35, through contacts 135b, 135c, (c) short-circuits the input to amplifier 27 through contacts 135f. Potentiometer 144 applies to capacitor 54 any voltage selected by the setting of a knob 145 thereby preventing undue rise of voltage across the capacitor.. When furnace temperature rises, relay 135 is maintained energized until contacts 139 open. In a modification, Fig. 7 (not shown), a persistent temperature deviation which does not reach the lower limit but which results in movement of the valve to its fully open position causes operation of the limit switch which also energizes a delayed relay to introduce, after a predetermined time, a pre-set potentiometer limiting the voltage across the integrating condenser. In this arrangement, which includes several relays, the input to the amplifier is not short-circuited. In Fig. 8 (not shown), however the limitation of the voltage across the integrating condenser is effected without relays by a preset potentiometer which is introduced by the conductivity of a rectifier at the voltage limit. In order to ascertain the position of valve 20, Fig. 1, switches 81, 116, 110 are operated to the "RV" position thereby opening the circuit of motor 22, shortcircuiting the voltage source E 1 , and introducing a voltage from a potentiometer 113 to oppose the voltage from potentiometer 35 in the input to amplifier 27. The position of valve 20 is then indicated on the dial of potentiometer 113 when the latter has been adjusted to reduce the input of amplifier 27 to zero as indicated by lamps 87, 88 To permit manual control of motor 22, switches 117, 68, 125, 126 are operated to the " M " position thereby short-circuiting the voltage source E 1 , short-circuiting resistances 58, 60, and applying to the input of amplifier 27 the difference between the voltages from potentiometers 35, 113. Motor 22 can then be run to operate valve 20 in either direction by adjustment of potentiometer 113. Recommended sequences of operation for smooth changeover from manual to automatic control are discussed.