Patent GB-816068-A (GB)
Title: Improvements in measuring or control apparatus dependent on the motion of a fluid relative to an aerodyne
IPC: G01P5/14
Filed: 1956-05-29
Abstract: 816,068. Electric measuring systems. LABORATOIRES DE PHYSIQUE APPLIQUEE L.E.G.P.A. May 29, 1956 [June 13, 1955], No. 16558/56. Class 40 (1). Apparatus for measuring the true and apparent air speeds of an aerodyne comprises means for measuring the static pressure and the dynamic pressure and combining them in a single indicator. Measuring static or dynamic pressure (Fig. 2).- A coil 3 is wound round the central limb 4 of a ferromagnetic iron body 15 with a gap in its base wherein is located an armature 5 carrying a diaphragm 16 subjected to a pressure p on its outer face, and a pressure P on its inner face. Associated with the armature 5 are contacts 6c, 6d which open or close depending on the relative value of the forces produced by the pressure difference and the electromagnetic force due to the Current in winding 3. Closing contacts 6c, 6d causes a current to flow in a winding 24 which is inductively coupled to a saturable coil 22 forming part of a bridge circuit, whereby the impedance of coil 22 is changed to cause the bridge unbalance current through coil 19 to change direction and cause reversal of a motor 8 of which it is the control winding. Motor 8 runs therefore in one direction or the other depending on whether contacts 6c, 6d are open or closed, and drives the slider 9 of potentiometer 10 to a position such that the average current in winding 3 produces an electromagnetic force on diaphragm 16 equal to that produced by the pressure difference, when the motor 8 is dynamically balanced, i.e. rotating equal amounts in both directions, and slider 9 is effectively at rest. To enable the motor 8 and slider 9 to remain actually at rest a further winding 30 (Fig. 4) is wound on the limb 4, being fed from a rectifier bridge 31 connected in parallel with winding 22, whereby opening contacts 6c, 6d causes winding 22 to have a high impedance, whereupon the measuring bridge current flows through the rectifier bridge 31 to coil 30 and causes contacts 6c, 6d to close, so that a vibratory motion of the contacts is produced, and the resultant rotation of motor 8 is negligible. In order to maintain proportionality between the current in winding 3 and the square root of the pressure difference on the diaphragm 16, switches 28a, 28b are operated by a circular cam driven by motor 8, so that the magnetic condition of the core 4 is constantly brought back to a zero position on the hysteresis loop. A further energized winding (not shown) may be located on core 4 and fed with a current adjusted to a value such that the current in coil 3 is zero when the pressure difference is zero. When the apparatus is used to measure the dynamic pressure the current in winding 3 is indicative of the apparent speed of the aerodyne, and may be determined by comparing the voltage across a resistor 40 (Fig. 9) with that across a tapped-off portion of a potentiometer 41, any difference being amplified at 44 to drive a motor 43, rotating the slider of potentiometer 41 until the difference is removed. Shunt resistors 50, 47 may be introduced as shown to open out the scale markings at low speeds. Measuring true air speed (Fig. 1).-Two pressure measuring devices 2a, 2b are respectively subjected to the dynamic pressure P and the static pressure p. The current in winding 3b is corrected by temperature-sensitive resistor 14 having the temperature To of the leading edge of the aerodyne, so that at balance between the pressure force and electromagnetic force on the diaphragm of device 2b the position of slider 9b is indicative of the quantities #p/To. This voltage is fed to the slider 9a associated with device 2a, so that the balance position of slider 9a, as indicated on a dial 12 driven by balancing motor 8a, is indicative of the ratio which is proportional to the true air speed. Specification 816,067 is referred to.