GB536537A - Improvements in or relating to control mechanisms for maintaining potential variables at desired values - Google Patents

Abstract

536,537. Fluid-pressure servomotor-control systems. FARRINGTON, G. H. Nov. 16, 1939, Nos. 30230/39, 5646/40 and 9100/40. [Class 135] An automatic control system for maintaining a variable such as temperature, humidity, liquid level, fluid flow or pressure, pH concentration, voltage or amperage of an electric current, speed, the feeding of solid material or the intensity of light or sound at a desired value includes a series of fluid pressure balances or equivalent devices whereby a controlling effect is obtained having three components which at any given time are proportional respectively to the departure of the variable from the desired value, to the time integral of the said departure and to the time differential of the said departure. Second and subsequent derivative components may also be included. In the form shown in Fig. 1 a balance beam subject to a controlling impulse f and to the action of two bellows 7 , 8 actuates a leak-off valve on a supply conduit 5 fitted with an adjustable throttle 4 to produce a pressure difference p<SP>1</SP> across an adjustable restriction 9 proportional to the controlling impulse. The flow past the restriction 9 which is such that the flow is proportional to the pressure difference passes into the bellows 8 and associated chambers to produce a pressure p<SP>2</SP> proportional to the time integral of p<SP>1</SP>. A second balance beam is actuated by three bellows 11, 12, 13 to actuate a leak-off valve 14 on a supply conduit fitted with a throttle. The bellows 11, 12 are subject to the pressures in either side of the restriction 9 and the bellows 13 receives pressure through an adjustable restriction 15 under the control of the valve producing a pressure p across the restriction 15 proportional to the time differential of the controlling impulse. A third balance beam is subject to the action of a bellows 16 receiving the pressure p<SP>2</SP> and bellows 17 receiving a pressure p<SP>5</SP> equal to the sum of the pressures p<SP>1</SP> and p<SP>3</SP> and actuates a leak-off valve 20 on a throttled fluid supply conduit to produce a pressure in a bellows p<SP>6</SP> and the connecting devices proportional to the sum of these three pressures. The balance beams are loaded by weights 10, 10a 10b preferably having a value approximating half the pressure range of the device. These weights may, however, be replaced by bellows subject to the supply pressure or to a further regulating impulse. In a modification only two balance beams are employed: In this arrangement the bellows 12 and the third balance beam are omitted. In another form using three balance beams the first beam is actuated by a single bellows while the others are each acted on by four bellows.