Patent GB-815633-A (GB)
Title: Improvements in and relating to fluid pressure servomotor control systems
IPC: G05B6/05
Filed: 1956-10-26
Abstract: 815,633. Fluid-pressure servomotor-control systems. MILTON ROY CO. Oct. 26, 1956 [Nov. 3, 1955], No. 32770/56. Class 135. In a control system for positioning an element in accordance with the magnitude of a variable condition a rotating type reversible fluid-operated motor mechanically connected to the element is rotated in a direction and at a speed in dependence upon the magnitude of the variable condition. A measuring instrument 15, Fig. 1, of known type supplies a pneumatic signal of from 3-15 p.s.i. via line 18 to the chamber 20a of a pneumatic positioner 19. The magnitude of the signal is proportional to the magnitude of the controlled condition. The positioner 19 comprises a casing divided into pressure chambers 20a, 21a, 22a by diaphragms 20, 21, 22 interconnected by a member 23 and loaded by a spring 24 adjustable, as described later, by a feedback connection. A valve member 25 is biased by a spring 28 to close the port 27 between the chambers 21a, 22a and to close the exhaust passage 26 leading from the chamber 22a. The chamber 21a is supplied via line 73 with air at 25 p.s.i. The positioner 19 is connected to a reversing relay 34 and to two similar booster relays 37, 38. The reversing relay 34 comprises a pair of diaphragms 42, 43 loaded by an adjustable spring 50 and interconnected by a member 44 having a central passage 44a. A valve member 52 is biased by a spring to close the passage 44 and the port 51a between the chambers 53 and 45 of the relay 34. The chamber 53 is supplied via line 55 with air at 25 p.s.i. The passage 44 leads from chamber 45 to an exhaust line 47. Each booster relay 37, 38 comprises two diaphragms 60, 61 interconnected by a rigid member 62 the passage 62a of which leads to an exhaust line 63. Each relay is supplied with air at say 60 p.s.i. via a line 67 and is provided with a line connection 39 or 40 to a reversible rotary motor 11. A valve member 64 is biased by a spring 66 to close the passage 62a and a port interconnecting the line 67 with line 39 or 40. In operation, if the pressure of the signal from the measuring instrument rises the diaphragms 20, 21, 22 are raised so that the exhaust passage 26 is closed and the port 27 is opened to allow air from the supply line 73 to pass into the chamber 22a and the lines 30, 35 to the booster relay 37. The diaphragms 60, 61 of relay 37 move downwardly to close the exhaust passage 62a and open the outlet of chamber 65 so that air from the supply line 67 passes to the line 39. The increased pressure in the line 30 is also transmitted via line 31 to the chamber 46 of the reversing relay 34. The diaphragms 42, 43 and the interconnecting member 44 move upwardly so that the line 36 is connected via the passage 44a to the exhaust line 47. The resultant fall in pressure in the line 36 causes upward movement of the diaphragms 60, 61 of the booster relay 38 so that the line 40 is connected to the exhaust line 63 via passage 62a. In the same way a decrease in the pressure signal from the instrument 15 results in a decrease of the pressure in line 39 and an increase of the pressure in line 40. The arrangement is such that with the pressure in chamber 22a at 12¢ p.s.i. the pressure in lines 39, 40 is equal at 25 p.s.i. and the motor 11 is in a stalled or neutral position. Is the pressure in the chamber 22a falls below for rises above 12¢ p.s.i. the difference in the presure in lines 39, 40 rises so that the motor 11 is rotated at a speed proportional to the difference and in a direction determined by which pressure is the greater. A shaft extension of the motor 11 is connected by a worm and worm wheel 71, 72 and a mechanical linkage 70 to the spring 24 which is adjusted accordingly so that the rotation of the output shaft 11 is always proportional to the instantaneous air signal from the instrument 15. The opposite end of the motor shaft is connected via a sprocket and chain to the adjusting screw 78 of a stroke control device 79 of the plunger 12 of a pump 13 which regulates the correcting condition. The lines 35, 36 are interconnected by a line 90 containing a bypass valve 91 adjustment of which controls the rate of response of the booster relays 37, 38 to eliminate hunting or oscillation of the motor 11.