Patent GB-730082-A (GB)
Title: Improvements in pneumatic control apparatus
Inventor: OLAH GEORGE
IPC: G05B11/50
Filed: 1949-05-03
Abstract: 730,082. Fluid-pressure servomotor-control systems; valves. PRECISION DEVELOPMENTS CO., Ltd. May 3, 1950 [May 3, 1949], No. 11831/49. Class 135. In a fluid-pressure servomotor control system of the kind yielding a pneumatic outgoing signal having components proportional to the magnitude, time derivative, and time integral of the incoming signal and having means to amplify or attenuate the incoming and/or outgoing signals in order to vary the proportional band, the adjustable fluid resistances incorporated in the means furnishing the derivative and integral components consist of valves giving, after any regulating adjustment, the same change in the ratio of the resistances before and after the adjustment irrespective of the initial position from which the adjustment was made, and means to indicate the differential displacement between the movable elements of the two valves is calibrated in values of the ratio between the constants of the derivative and integral components (i.e. Td/Ti) in the formula giving the output pressure in terms of the magnitude, time derivative, and time integral of the signal. The means to amplify or attenuate the incoming signal may be logarithmically arranged. The invention is applicable to a control system as shown in Fig. 1 wherein an electrical input signal from a line 1 is passed through an attenuating potentiometer 2 to a converter 4 similar to that described in Specification 730,081 to obtain a pneumatic pressure difference which is applied to two bellows 11, 13 acting in opposition on a lever 14 pivoted at 15. The air supply to the converter 4 is obtained from a line 8 which is provided with a gauge 9 and a filter 7. Downstream of the filter 7 is a branch line leading past a restriction 16 to an outlet 19 which is obturated by the lever 14 to produce a control pressure that actuates a further valve 20, and the latter regulates a further pressure supply through the lines 8, 21 to produce at the line 22 a varying pressure constituting the final output pressure of the apparatus. The output pressure from valve 20 is also applied to a bellows 26 and, through hydraulic links 29, 32 and 29, 40, to further bellows 34, 41, respectively, which act in opposed senses on the lever 14. The transmission through the hydraulic links is varied by means of valves 30, 39 constituting variable resistances and by spring- loaded bellows 35, 42 constituting loading capacitances. The final output pressure will be in the required form wherein the components corresponding to the magnitude, time derivative, and time integral of the input signal are produced by the impulses applied to lever 14 by bellows 11, 13, bellows 34, and bellows 41, respectively. In a practical embodiment, the valves 30, 39 are situated side-by-side and their controlling surfaces 97<SP>1</SP>, Fig. 7, are formed in accordance with a logarithmic curve. Their positions in the bores which they control are independently adjusted by means of concentric cams 131, 132 carried, respectively, by an outer disc 142 and an inner spindle 137, and the stems of the valves carry balls 112<SP>1</SP> to engage the cams. In each valve, the connection between the head carrying the cam follower and the valve spindle comprises an elastic ring 116, and the spaces 118 thus sealed off are filled with liquid similar to that used in the hydraulic links so as to compensate for any changes in viscosity of the controlled liquid following a change in temperature. Each valve is loaded by a spring 102 so that the cam follower 112<SP>1</SP> is kept in engagement with the appropriate cam, and the valve spindle is sealed off by a bellows 1041 the interior of which is in communication through passages 106<SP>1</SP>, 1071 with an appropriate part of the hydraulic link so that sudden variations in pressure in the hydraulic system are prevented. Situated above the valve assembly is the potentiometer 2, the casing of which can be rotated from a disc 141 through a gear quadrant 151 and a gear wheel 150 and the wiper of which is adjusted by a knob 155. The valves can be coupled together for simultaneous adjustment with a fixed ratio by coupling together spindle 137 and disc 142 by means of a screw 147. The operating knob on the inner spindle 137 carries an index mark to be read against a scale on disc 142 calibrated in values of Td/Ti, the disc 142 carries an index mark to be read against a scale on disc 141 calibrated in values of Ti, and disc 141 carries an index mark to be read against a fixed scale 164 of further values of Td/Ti. An alternative construction for the scales and indices is described.