Patent GB-666413-A (GB)
Title: Improvements relating to apparatus for the measurement of the velocity of a moving body with respect to its surrounding fluid medium
IPC: G01P5/24
Filed: 1949-01-18
Abstract: 666,413. Measuring velocity utilizing sounds and like vibrations; electrical measuring systems. FERRANTI, Ltd., McCALLUM, D. M., and GENT, H. March 27,1950. [Jan. 18, 1949; May 21, 1949 ; Oct. 6, 1949] Nos. 1365/49, 13608/49 and 25710/49. Classes 40 (i) and 118 (ii). [Also in Groups XXXVI and XL (c)] Relates to a means for measuring the velocity V of a moving body, e.g. an aircraft, relative to a surrounding fluid medium utilising the Doppler effect on the velocity of propagation of pressure waves in the relatively moving fluid. In one embodiment, Fig. 1 (1365/49), two electro-acoustic receiving transducers 10 and 11 are spaced rearwardly from an electroacoustic transmitting transducer 13 along directions 17 and 18 making angles # and # with the direction of motion 16 of the body and the Specification shows that the required velocity V is given by the equation where V1 and V2 are the velocities of propagation of pressure waves measured along directions 17 and 18. When the axis 16 of the transmitter/receiver combination does not coincide with the crafts direction of motion the computed value of V will be erroneous and such error may be reduced by locating two further receivers 10<SP>1</SP> and 11<SP>1</SP> on opposite sides of the axis with respect to 10 and 11, the velocity V1 being taken as the mean of the velocities V1<SP>1</SP> and V1<SP>11</SP> between 13 and 10 and between 13 and 10<SP>1</SP> and similarly for V2. Alternatively, the receivers may be located in front of the transmitter and/or the two receivers may co-operate with separate transmitters or there may be one receiver co-operating with two transmitters. When the velocity V of the body is greater than the speed of sound, a radiated compressional wave will give rise to two time-displaced components at a receiver spaced behind the transmitter along a direction making an angle # with the direction of motion, the two components having different propagation velocities V1 and V2 in which case V=V1+V2/2 cos # and a simplified embodiment is described, Fig. 1 and Figs. 2-4, (not shown), (Prov. 13608/49), for determining the velocity in such circumstances. In both embodiments the velocities V1 and V2 are determined by measuring by electronic means the time intervals t1 and t2 between the transmission of a pressure wave pulse and its reception at the receivers or receiver and deriving reciprocal quantities proportional to V1 and V2. The transmitter or transmitters may be triggered spark gaps of the type disclosed in Specification 666,414, [Group XL (c)], and the receivers or receiver may be piezo-electric crystal transducers. First embodiment. As shown in Fig. 1 (1365/49), the timing is controlled by a multivibrator unit 14 producing two out-of-phase square waves (a) and (b) Fig. 3 (1365/49). Short positive pulses 44, Fig. 3 (c), are derived from the trailing edges 43 of waveform (a) and higher amplitude positive pulses (not shown) are derived from the trailing edges of pulses 44 and applied through a trigger circuit 12 to trigger the spark gap transmitter 13 and produce synchronous electrical initiating pulses (d) which are applied over lead 25 to unit 24 of the reciprocal computer 24, 27. Pressure wave pulses (e) are received at 10 and 11 after delay times t1 =d/V1 and t2 = d/V2 where d is the distance of each receiver from the transmitter and the corresponding electrical wave pulses are applied alternately to computer unit 24 through switch 22 which is actuated via a link 26 by a relav in unit 14 operated by waveform (a). Unit 24 includes an amplifier and rectifier 60, Fig. 5 (Prov. 1365/49), (not shown), which is blocked during transmission of the pressure wave pulse by a negative pulse 64, Fig. 3(b), derived from the trailing edge of waveform (a) applied over lead 29 and the output pulses (f) are applied to terminate negative square wave pulses 62 and 63, Fig. 3 (g) which are initiated in a pulse generator 61, Fig. 5 (Prov. 1365/49) (not shown), by the initiating pulses (d) applied from the multivibrator unit 14 over lead 25. The pulse generator 61 comprises a negatively biassed pentode 301 Fig. 1 (Comp.) which is rendered conductive by a positive initiating pulse (d) applied over lead 25 through diode 307 to charge capacitor 317 connected to the signal grid, the pentode being cut off by the leading edge of a subsequent positive terminating pulse (f) applied over lead 323 to the grid of triode 320 whose anode is also connected to the pentode signal grid. The output pulses 62 or 63, Fig. 3 (g), having a duration t1 or t2 depending on the position of switch 22 are applied over lead 28 to unit 27 of the reciprocal computer together with waveforms (a) and (b) from multivibrator 14 to produce shaft rotations or slider displacements proportional to V1 or V2. Unit 27 comprises a Miller integrator 70, Fig. 7 (1365/ 49), with feedback capacitor 72 the grid being connected through diode 90, resistor 74 and switch 87 to a variable tapping 75 or 75<SP>1</SP> on potentiometer 76 and the grid being also connected through triode 92 and diode 97 to tappings on a negative potentiometer 94. A triode 100 is connected between the junction of diode 90 and resistor 74 to a tapping on a negative potentiometer 101 and waveforms (a) and (g) are applied to the grids of triodes 92 and 100 respectively. Assuming switches 22 and 87 to be such that receiver 10 is connected to the computer and resistor 74 is connected to tapping 75, prior to time T2 triodes 92 and 100 and diode 97 will be conductive, diode 90 will be cut off and consequently the Miller valve 70 will be cut off. At time T2 waveform (a) cuts off triode 92 removing the negative bias from the grid of valve 70 and at time T3 pulse 62, waveform (g), cuts off triode 100 rendering diode 90 conductive and thereby connecting the grid of valve 70 to tapping 75 so that the anode voltage of valve 70 decreases linearly (line 104 Fig. 3 (k) ) the slope being proportional to E3/RC where E3 is the voltage at tapping 75 and R, C are the values of resistor 74 and capacitor 72. When pulse 62 terminates at time T4, triode 100 becomes conductive and diode 90 is cut off thereby terminating the anode voltage run down 104 of valve 70 and the anode voltage subsequently increases slowly along line 107, Fig. 3 (k), due to the high resistance 91 shunted across diode 90. The anode voltage of valve 70 is applied via lead 106 and diode 120, Fig. 8 (1365/49), to the signal grid of one pentode 124 of a balanced modulator 124, 125 the grid of the other pentode 125 being maintained at a reference voltage E2 provided by a tapping on a potentiometer 78. The diode 120 in association with capacitor 121 functions as a peak rectifier so that after the termination of the Miller run down at time T4 the grid of pentode 124 is held at a voltage level 105, Fig. 3 (1), equal to the minimum anode voltage 105, Fig. 3 (k), of valve 70 and if this level differs from the reference voltage E2, the 400 c/s voltage applied at 138 in the common cathode circuit of the modulator 124, 125 causes a resultant 400 c/s voltage to appear at the anode of valve 124 which is applied through amplifier 142, lead 79 and switch 88, Fig. 7 (1365/49), to one field winding 80 of a two-phase induction motor 81 which shifts tapping 75 to vary the voltage E3 and thence the slope of the subsequent Miller run down so that the run down terminates at voltage level E2, the rotation of motor 81 being thus inversely proportional to the time duration t1 and therefore directly proportional to VI. The modulator 124, 125 is alternately blocked and gated by waveform (b) from multivibrator unit 14 applied over lead 30 to the suppressor grids and before each run down occurs the capacitor 121 is charged to a voltage E5 by momentarily closing switch 141. When receiver 11 is connected to the computer the tapping 751 is controlled in a similar manner so that the rotation of motor 81<SP>1</SP> is proportional to V2, the two rotations being applied by shafts 31 and 32 to the output computer 33. When the distances from the transmitter 13 to the receivers 10 and 11 are different, the contacts of switch 87 are connected to tappings 75 and 75<SP>1</SP> through separate resistors 74 and 74<SP>1</SP>, Fig. 9 (1365/49), (not shown), which replace resistor 74. If two separate reciprocal computers are employed the switching is eliminated. The output computer 33 comprises two valves 342, 352, Fig. 2 (Comp.), having feedback resistors 347 and 353 of value RE and RG between anode and grid, the anodes being connected to a reference point 350 by resistors 351 and 354 of value RF and RH. A potentiometer 330 has two equal windings 331 and 332 connected in series, two ganged sliders 334 and 335 being controlled by input shaft 31 (i.e. settings proportional to VI) and a third slider 337 being controlled by input shaft 32 (i.e. setting proportional to V2) so that the resistance Rs between sliders 335 and 337 is proportional to (V1 + V2) and the resistance R D  between sliders 334 and 337 is proportional to (V1 - V2) the resistances Rs and R D  being connected in series between the grids of valves 342 and 352 and an A.C. source 355 which is also connected to the reference point 350 via a variable resistor 360 of value Rx. The Specification shows that when resistor 360 is adjusted so that' the A.C. potential e between the reference point 350 and earth is zero then and this balanced condition is produced automatically by applying potential e through an amplifier 361 to one winding of a two-phase induction motor 362 which is coupled to resistor 360. When the values of resistors 347 and 353 are manually adjusted to make the terms RE/RF and RG/RH proportional to (cos # + cos #) and (cos #-cos #) respectively, Rx is proportional to the required velocity V and this value may be displayed by a pointer 34 driven by motor 362. In a modified form of reciprocal computer a strobe .pulse is initiated