Patent GB-898160-A (GB)
Title: Fluid pressure responsive apparatus and servo apparatus therefor
IPC: G05B11/44
Filed: 1959-07-31
Abstract: 898,160. Toothed gearing; shaft couplings ; change-speed gear; cam mechanisms; linkwork. GARRETT CORPORATION. July 31, 1959 [Aug. 1, 1958], No. 26452/59. Classes 80 (1), 80 (2) and 80 (3). [Also in Groups XIX and XXXVIII] Apparatus for measuring fluid pressure comprises a beam 20 (Fig. 2) unbalanced by the difference in pressure between two bellows 22, 23 to produce an electrical output from a differential transformer 30, which is amplified and used via a motor 40 and spring 24 to rebalance the beam. As shown, bellows 23 is evacuated and attached to a fixed base 36: bellows 22 is subjected to atmospheric pressure. The output from transformer 30 is amplified at 31 and 53 to energize motor 40, which incorporates a tacho-generator feeding an antihunting signal back to amplifier 31. This signal, which is proportional to the rate of change of the log of atmospheric pressure, may also be fed to indicating or control apparatus. Motor 40 drives through gears 41, 45 two output shafts 54, 55, the latter producing an angular rotation proportional to pressure in bellows 22, and the former producing an angular rotation proportional to the logarithm of this pressure, by cam logarithmic gears 46, 47, which is also measured electrically by selsyn transmitter 52. Attached to shaft 55 is the spring 24 which transmits torque via lever arms 77, 104 to the beam 20 to eliminate the signal developed in transformer 30. To compensate for variations in the magnitude of the signal from differential transformer 30 at different atmospheric pressures, the gain of amplifier 53 is automatically controlled by potentiometer 133 whose spindle is driven by a rod 129 attached to a cylinder 115 rotated by shaft 55. The apparatus may be used on aircraft to determine altitude from the atmospheric pressure, the altitude being inversely proportional to the log of atmospheric pressure. The following mechanical details are included in the system to improve the accuracy and speed of operation: (a) To allow for variations in the areas of bellows 22, 23 each is coupled to the beam 20 by a separate knife edge. Bellows 22 is attached to a knife edge 61 engaging the beam 20, and also to a knife edge 63 engaging a yoke 60 attached to the base 36 by a spring blade 65. The evacuated bellows 23 is attached to a knife edge 27 resting in a notch in a slide portion 26 adjustable between the legs of yoke 60 to compensate for mismatching between the bellows. (b) The gears 41, 45 are coupled via electromagnetic clutches 43, 44 manually selected to provide fast or slow speed of shafts 54, 55. (c) The spring 24 is coupled to shaft 55 via a friction plate clutch 85, enabling the initial torque of the spring to be adjusted. (d) The reaction of the spring torque on the gears between shafts 54, 55 and the motor 40 is maintained constant by additional torque produced by a spring 98 acting on shaft 55 via an arm 99 pivoted at 100, and a roller 101 riding on a cam 83 attached to shaft 55. (e) Non-linearity between the spring torque and the angular rotation of shaft 55 is compensated for by a variable lever arrangement comprising a member 70 attached to the free end 71 of the spring, operating via knife edge 78 resting on a lever 104 attached to the beam 20. Lever 104 has a fulcrum 122 movable in accordance with a cam 115 located on the shaft 74 of spring 24. The cam surface 119 is a thin flexible band whose surface is adjusted as required by screws 121. Specifications 898,161 and 898,162 are referred to.