Patent GB-568634-A (GB)
Title: Improvements in or relating to control mechanisms for maintaining potential variables at desired values
IPC: G01G5/00
Filed: 1943-08-05
Abstract: 568,634. Fluid-pressure servomotor-control systems. FARRINGTON, G. H., and ILFORD, Ltd. Aug. 5, 1943, No. 12640. [Class 135] A control system for maintaining a variable at a desired value is fed with a pressure which varies in sympathy with the variable and delivers a correcting pressure either (Fig. 1) proportional to the departure of the variable from the set point, the time integral of said departure and the first time differential of the controlling pressure, or, Fig. 2 (not shown), proportional to the departure of the variable from the set point and to the first and second time derivatives of the variable, or, Fig. 2 (Provisional Specification not shown), proportional to the departure of the variable from the set point, the time integral of said departure and the first and second time derivatives of the variable. These effects are obtained by the aid of a series of " pressure balances," fixed and variable restrictions and capacity chambers. The pressure balances preferably comprise a hinged baffle controlling the flow of air from a jet and actuated by two or more bellows subject to balancing, loading and controlling pressures. According to the present invention, the contraction which produces a pressure difference proportional to the differential of the pressure impressed upon it is modified to produce only a fraction of said difference and the "pressure balance" to which the said fractional pressure difference is constructed to magnify the said fractional pressure difference in relation to the balancing. The latter is preferably effected by making some of the bellows of greater cross-section than the others and arranging them at a greater radial distance from the pivots of the buffers. As shown in Fig. 1, three "pressure balances" are employed, of which A produces a pressure proportional to the time integral of the departure, B a pressure proportional to the first time derivative of the variable and C combines these pressures with a pressure proportional to the departure. The balance A comprises a T-shaped lever 13 connected by a link 28, slidably connected to a hinged baffle 21 controlling the discharge from a jet 22 supplied with air under pressure through a filter 24 and restriction. The baffle is slightly over-balanced by air pressure in the nozzle acting on a bellows 26. Controlling pressure admitted through a pipe 11 and shown on a gauge 12 is admitted to a small bellows 15, while a loading pressure derived from the air supply conduit 23 through a restriction 35 and capacity chamber 36 acts on a small bellows 38 arranged at a radial distance from the pivot 4 of the bellows 15. The jet is connected through an adjustable needle valve 30 to a capacity chamber 62 and the pressures on the two sides of the needle valve are fed to an opposed pair of larger bellows 16, 34 arranged at a greater radial distance from the pivot 14 than the bellows 15. The balance B comprises a hinged baffle 51 loaded by a light spring 57 and adapted to control the discharge of air from a jet 54 supplied through a restriction 56 and filter 55. The baffle is connected by a link 59 with a lever acted on by a pair of opposed small bellows 45, 46. Controlling pressure is admitted to the bellows 45 through a series of capacity chambers separated by constrictions, while the jet is connected through an adjustable needle valve 49 with the bellows 46 and with a capacity chamber 71. The balance C comprises a hinged baffle 76 pressure balanced by a bellows 83 and adapted to control the discharge of air from a jet 79 supplied through a restriction 81 and filter 80. The baffle is coupled by a link 84 with a T-shaped lever 60 acted on by a small bellows. An opposed pair of large bellows 68, 69 are subject to the pressures on the two sides of the needle valve 49. The small bellows 65 arranged at a smaller radial distance from the pivot than the bellows 68, 69 is subject to the controlling pressure, while an opposed small bellows 73 is connected to the jet and to a pipe 75 leading to the regulating apparatus. A small bellows 67 arranged at half the radial distance of the bellows 65 is fed with a loading pressure from the capacity chamber 36. Similar arrangements of " pressure balances " are used in the other forms of the invention. In one form, described in the Provisional Specification, the baffles have an upstanding arm associated with a fine adjustment device to serve as a " zero " adjustment and/or to adjust the stiffness of the jet and baffle system. Fig. 3 shows one of the needle valves 30, 49. This valve comprises a base part 161 formed with inlet and outlet passages 162, 163 fitted with inlet and outlet filters 166, 167 held in place by unions 164, 165. Formed integrally with the base is an upstanding cylinder 174 formed with a valve seat 175 and an externally-threaded cylinder 169 adapted to receive an upper part 178. The valve needle 176 is secured to a plate 171 formed integrally with a ported tubular member 172 slidable on' the cylinder 174, said plate being connected to the base by a bellows 170. The valve spindle 177 has a screw-threaded engagement with the upper part and is fitted with a counter-mechanism visible through a window 186 and showing on one dial complete revolutions and on another fractional parts of a revolution. When the upper part is rotated, the two threads act differentially to adjust the needle. A ring 187 secured to the base serves to limit the movement of the upper part. Specifications 536,537 and 551,159 are referred to.