Patent GB-603644-A (GB)
Title: Improvements relating to the measurement of the velocity of a moving body with respect to its surrounding medium
IPC: G01P5/24
Filed: 1945-08-08
Abstract: 603,644. Anemometers; logs. FERRANTI, Ltd., and TAYLOR, M. K. Aug. 8, 1945, No. 20305. [Classes 97 (ii) and 97 (iii)] [Also in Group XXXVIII] A method of measuring the velocity of, or the distance traversed in any desired time by a moving body with respect to its surrounding medium comprises means in which a factor dependent on the velocity of a pressure wave in a part of the medium which is stationary relative to the body is subtracted from a factor dependent on the velocity of a similar wave in a part of the medium which is moving relative to the body in the direction of travel of the wave, the difference between the factors being applied to means for indicating the required velocity and/or distance. The invention as applied to an air-speed indicator for aircraft comprises an acoustic radiator 1 mounted at a fixed distance in front of an acoustic receiver 2, both in the air stream surrounding the aircraft the receiver being coupled back to the radiator by way of an amplifier 3 and wave filter 4. The gain of the amplifier is such that sustained oscillations are set up, and, depending on the connections to either the radiator or receiver, will comprise frequencies corresponding to an odd or even number of half-wavelengths of the distance d in the radiator receiver A, Fig. 1. This frequency, the " medium controlled frequency " f is also dependent on the characteristics of the medium such as, temperature, density and its velocity relative to the moving body. In the present arrangement frequencies corresponding to more than one half-wavelength of the distance dare suppressed by the filter 4 which comprises an A.C. generator directly driven by an A.C. synchronous motor having a like number of poles so that the input and output frequencies are the same. Then f=(V+v)/2d where d=#/2, V= the velocity of sound in still air and v= the velocity of the aircraft relative to the surrounding air. A second acoustic system B (Fig. 1) in which the elements 5, 6, 7, 8 are similar to 1, 2, 3, 4 of system A mounted in a position in which the air is stationary with respect to the aircraft produces a " medium controlled frequency " f=V/2d. The speed of the aircraft equals f=f<1>=V/2d. This subtraction is effected in the circuit of Fig. 2, in which the signals f and f<1> are applied to transformers 9, 10 feeding the valves 15, 16 which in the absence of an input signal are biassed to cut-off by batteries 17, 18. During negative half-cycles of input to either valve the associated condenser 19, 20, is charged through the appropriate resistor 21, 22, and during positive half-cycles the valves conduct and discharge the condensers. The difference in potential between the anodes of each valve which is a measure of f-f<1> is read on a voltmeter 23 calibrated in the desired speed units. If the distance d is not the same in the two systems A, B, Fig. 1, the ratio of the time constants of the charging circuits of the condensers 19, 20 to the associated distance should have the same value. The potential applied to the voltmeter 23 may be employed to control servo-mechanism to maintain constant speed of the aircraft or, it may be integrated over a period of time, by charging a condenser by a current which is dependent on this potential, the accumulated condenser charge being a measure of the distance travelled. Alternatively the potential may be applied to a motor the speed of which is proportional to this potential and the distance travelled determined by a cyclometer. In a modification the outputs of the systems A, B are applied to two synchronous motors 24, 25 each of which drives a member of a differential gear 26, the third member 27, the speed of which is proportional to the difference in speed of the motors, being coupled to an instrument 28 of the tachometer type. Instead of synchronous motors ratchet wheels and pawls actuated electromagnetically by the oscillations may be used. In another arrangement the two frequencies f, f<1>, are mixed electronically and the resultant frequency applied to a frequency meter to indicate the velocity or to a ratchet and pawl device, as described above, to indicate the distance travelled. In a further arrangement the two oscillations are applied to two frequency meter elements rigidly connected together and acting in opposition so that the resultant movement is dependent on the velocity of the aircraft. To overcome objections which arise unless d is exceedingly small, i.e. large radiators, necessary because f is small, which would interfere with the air-flow and the large powers of radiation necessary, a carrier wave of supersonic frequency, about 50 Kc/s., is used ; the carrier being modulated by the " medium controlled frequency ". A magnetostrictive device, or a piezo-electric crystal of quartz, ammonium hydrogen phosphate, or Rochelle Salt, may form .both transmitter and receiver. In this arrangement the amplifier, which operates at carrier frequency and then demodulates, is fed by the supersonic receiver and supplies an audio signal at " mediumcontrolled frequency " to modulate the supersonic carrier oscillator. This modulated wave is fed to the supersonic transmitter. In another form of the invention only one supersonic oscillator and amplifier is provided and a rotating cam mechanism is arranged to switch those units alternately to each of the two acoustic receivers and radiators. At high speeds a correction should be made to allow for the fact that the velocity of sound in the stationary. airpath is greater than that in the moving airpath owing to the air being heated is bringing it to rest. The invention may also be applied to a body moving in a medium other than air, i.e. a submarine.