Patent GB-535519-A (GB)
Title: Improvements in or relating to control apparatus
IPC: G05D23/12
Filed: 1939-08-10
Abstract: 535,519. Fluid-pressure servomotor-control systems. NEGRETTI, H. N., NEGRETTI, P. E., and IBBOTT, H. W. Aug. 10, 1939, No. 23148. [Class 135] An automatic control apparatus for regulating pressure, temperature &c. includes a displaceable member subjected to forces including a force whose effect upon the member, for a given strength of the force, is variable responsive to changes in the physical condition, so that.when a change occurs the said member is displaced to an extent dependent upon the amount of said change and thereby causes actuation to a corresponding extent of control means for varying the physical condition in a sense to counter the change. The figure shows the invention applied to the control of the temperature in a vessel 11 heated by a coil'12 under the control of a power-operated valve 25. The supply and exhaust of compressed air to and from the actuating chamber of the valve 25 is controlled by a relay device of the kind described in Specification 524,077. This device conprises an inlet valve 31 co-operating with a fixed seat and formed integrally with an exhaust valve 30 co-operating with a seat carried by the movable wall of a capsule 29. This wall is coupled by a lever 32 having alternative fulcra 34, 35 to a capsule 27 subject to a variable fluid pressure produced by a control valve. The control valve, which cooperates with a nozzle 6 fed with compressed air through a conduit with a constriction 24, is carried by a member 1 mounted on pivots 2, 3 on an axis YY mounted in the member 1 on an axis XX inclined, preferably at a rightangle, is a bell-crank lever 8 coupled by a link 9 to a Bourdon tube 10 connected to a thermal feeler in the vessel 11. A loading spring 15 adjusted by a knob 16 acts on a pivoted member 14 coupled by a strut 13 with the free end of thebell-crank lever. The pivotal connection between the member 14 and the strut 13 is arranged to coincide with the axis XX when the member 1 is in its normal equilibrium position so that when the connection between the strut and the bell crank lies on the axis YY no turning moment is applied to the member. As the bell-crank is turned consequent on a variation in the controlling impulse an increasing turning moment is applied to the member 1. Where plant lag is large a delay action device may be fitted to increase the control movement when the variation of the physical characteristic is sustained. In the form shown this device comprises a capsule 17 subject to the pressure in the control conduit and coupled through two further capsule stacks containing oil and connected by a throttled passage to a spring 22 acting on the member 1. The throttle may comprise a length of wire inserted in the conduit. The delay action may be obtained by throttling the conduit 18 or a dashpot may be fitted. In a modification the displaceable member is in the form of a rocking member and the force is exerted through a roller which is moved along the arm to vary the leverage in accordance with the changes in the physical condition. Specification 324,635, [Class 135], also is referred to.