Patent GB-790368-A (GB)
Title: Improvements in automatic electrical process-control systems
IPC: G05D27/02
Filed: 1954-10-18
Abstract: 790,368. Automatic control systems. MANNING, MAXWELL & MOORE, Inc. Oct. 18, 1954 [Nov. 2, 1953], No. 29936/54. Class 38 (4). [Also in Group XXXVIII] In an automatic process control system a torque proportional to a variable to be controlled is applied to a pivoted beam, movement of which controls an electrical oscillator from which is derived a direct current output signal, proportional to the variable, which is fed back to a moving coil applying a balancing torque to the beam. Applications. As applied to the control of petrol refining a liquid level gauge of the displacement type detects changes of level in, and controls an outlet valve of a stripping column to which superheated steam and crude oil are supplied. As applied to the control of a heat exchanger, a gauge responsive to steam flow to the exchanger controls a steam supply valve, and a thermocouple responsive to the temperature of fluid heated by the exchanger controls the datum of steam flow. The electrical transmitter. In Fig. 3 the moving member of the liquid level gauge (not shown) applies torque at 209 to a pivoted metallic beam 210 which moves relatively to a spirally wound inductance 212, with changes in level, to alter the frequency of a high-frequency oscillator including a thermionic valve 275. Balance of the beam is maintained by feedback to a moving coil 214 so that the direct voltage output from terminals 290, 294, which is dependent on oscillator anode current, is representative of liquid level. Potentiometers 297, 293 are adjusted according to the sensitivity required and the specific gravity of the liquid respectively. In the other application, similar transmitters are associated with the flow gauge and the thermocouple, although in the latter case torque is applied to the beam by a second moving coil connected to the thermocouple. Construction of the balanced beam is described in detail. The recorder and datum selector. The direct voltage from the transmitter is applied to terminals 420, 423, Fig. 20. A portion of this voltage is applied to a moving coil 429 acting on a pivoted metallic beam 425 which moves relative to an inductance 426 to alter the frequency of an oscillator including valves 427, 428. Oscillator anode current energizes a solenoid 435 of an electromagnetically operated pen-carrying arm 360 recording values of the variable. Balance of the beam is maintained by mechanical feedback from arm 360 applying torque to the beam. Hunting is opposed by rate feedback from a coil 445 inductively coupled to solenoid 435 energizing another moving coil 430 associated with the beam. Slight mains ripple, permitted by filter 437,438 reduces the effect of static friction. Another portion of the transmitter voltage is applied to a moving coil 451 acting on another beam 450 which moves relative to an inductance 452 to alter the frequency of an oscillator 453. A direct voltage output derived through a bridge circuit from the oscillator anode current and applied to terminals 454,455 is representative of deviation of the variable from a value determined by the setting of a manually adjustable lever 373 applying torque to beam 450. Balance of the beam is maintained by feedback to a moving coil 459. Temperature compensating networks are associated with moving coils 429, 451 respectively. Capacity coupling to inductance 452 reduces electrostatic effects. Construction of the recorder and datum selector is described in detail. The servo-amplifiers. The error voltage from the datum selector is applied to input leads 55, 56, Fig. 23, of a direct-coupled thermionic valve amplifier in which a feedback network 520, 521 introduces an adjustable differential component and a feedback network 552, 553 introduces an adjustable integral component. Either component may be made zero. Mains ripple is eliminated by a filter 504, 505. Condensers connected in Miller fashion prevent oscillation by attenuating high-frequency components. Potentiometers 512, 549 are initially adjusted to produce zero output in the absence of an input to the amplifiers. Sensitivity is adjustable by a potentiometer 550. With switch 65 in the " Automatic control position shown, the amplified error voltage appears at terminals 563,564. The pneumatically operated valve. The amplified error voltage is applied to terminals 611, 612, Fig. 25, to energize a moving coil 613 acting on a pivoted beam 610 so that the obstruction of air issuing from a nozzle 615 produces such a pressure drop across a sapphire orifice 624 that a spring-loaded relay valve 628 operates to control the pressure on a diaphragm 608 and hence the position of a steam or other valve 46 to correct the variable. Mechanical feedback is applied to beam 610 from valve 46. A temperature-compensating network is associated with moving coil 613. Detailed construction of the valve control gear, including Flameproof housing, cleaning devices, adjustments and devices for preventing overstrain is described. Manual control. Change-over to manual control is effected by first setting a manual control potentiometer 574, Fig. 23, to a calibrated position corresponding to the reading of a meter 70 measuring the automatic control current. Switch 65 may then be operated to its other manual control position so that a relay 560 substitutes an equal control current from a D.C. source 570. The pneumatically operated valve may then be controlled by potentiometer 574 through a series valve regulator 572. Under these conditions a relay 580 feeds to the servo amplifiers a signal dependent on the manual control current so that change back to automatic control may be effected without shock.