Patent GB-591270-A (GB)
Title: Improvements in or relating to apparatus for automatically controlling a variable ofa process
IPC: G05B11/44
Filed: 1944-03-31
Abstract: 591,270. Fluid pressure servomotor-control systems. FOXBORO CO. March 31; 1944, No. 6022. Convention date, April 7, 1943. [Class 135] In apparatus for controlling a variable according to three effects, one proportional to the deviation of the variable from its control point, the second proportional to the rate of change of such deviation, and the third consisting of giving to the regulator a rate of change proportional to the deviation of the variable, the three effects can be adjusted simultaneously, directly, and proportionately, and the second and third effects can also be further adjusted simultaneously but in opposite directions to each other. The invention is described with reference to the maintenance of a desired level of water in a tank a, Fig. 2, into which water can flow through pipes b, c and out of which it can flow through a pipe d containing a restriction e. Pipe b is provided with a valve f and when the setting of this valve is altered, the flow through pipe c must'be altered by means of a diaphragm-operated valve g to bring the level in a back to its former value. The pressure due to the head of water in the tank acts on a servo device h which controls the valve g in the manner set out above. In a further form, a number of tanks are connected in series with restrictions in the interconnecting pipes, and pipe c leads into the first tank in the series and pipe b into the last tank, in which the level is to be controlled. Variations in the level or head of water in tank a cause corresponding variations in a helical tube 11, the movements of this tube being transmitted to a spring-pressed pivoted baffle plate 53 through a lever 23, link 24, cranked lever 25, link 32; cranked lever 33, link 38, pivoted arm 39, link 49, and lever 50 which carries a pin 52 pressing on baffle plate 53. The latter plate obturates a jet 55 and its position determines the pressure in a chamber 56 fed with compressed air past a reducing valve 16 and restriction 57. The variations in pressure in chamber 56 act on a bellows 59 which actuates a combined inlet and exhaust shuttle valve 61 to cause corresponding variations in the pressure in a pipe 63 leading to the valve motor h. The operative range of movement of plate 53 is not large and the control apparatus will always tend to bring the plate 53 into a position corresponding to its operative range. To give a regulating effect proportional to the rate of deviation of the variable, the varying pressure in pipe 63 is allowed to act on a spring-loaded bellows 46 and through a restriction 64 on to a bellows 42, both bellows acting in the same sense on a floating 41 to which the lever 39 is pivoted. The third control effect is obtained by allowing the pressure variations in bellows 42, after passing through further restrictions, such as 65, to act on a further spring-loaded bellows 43 which is also connected to the floating frame 41, but acts in the opposite sense to bellows 42, 46. It thus constitutes a " resetting " mechanism. Adjustment of the position of link 49 along arm 39 will vary the response in output pipe 63 for a given movement of link 38, and thus all three controlling effects will be adjusted simultaneously, directly, and proportionately.. In the adjustment of the second and third control effects above, the times taken by the two sets of bellows to develop appropriate pressures are altered by increasing the volume of bellows 42 and by adding resistances to the passages transmitting pressures from bellows 42 to bellows 43. So that the adjustment effects will be in opposite senses, valves 66 ... 69, are provided. Each valve casing is open to the pressure in bellows 42 and each valve can be operated by a screw 82 to close up a pipe 66b leading from the end of the casing. Thus, if valve 68 is opened and all the other valves are closed, the volume of a chamber 73 will be added to that of bellows 42 and, at the same time restrictions (resistances) 65, 72, 74 and a volume 71 will be inserted into the pipe between bellows 42 and bellows 43. The closure mem'- bers of the valves are mounted onbellows to prevent leakage. To adjust the,point at which the level in tank a is controlled, the cranked lever 33 is pivoted on a pivoted plate 110, adjustment of the portion of which will vary the position of that end of lever 39 connected to link 49. Plate 110.is also connected through a link 115 and lever 116 to a pointer (not shown) which gives an indication of the point at which the level is being controlled.