Patent GB-1446318-A (GB)
Title: Pressure measuring apparatus
IPC: G01L9/00
Filed: 1973-07-30
Abstract: 1446318 Altimeter APPARATEBAU GAUTING GmbH 30 July 1973 [31 July 1972] 36252/73 Heading H4D  To measure a parameter which effects the resonant frequency of a mechanical oscillator in a non-linear manner, a signal indicative of the oscillator frequency is passed through a circuit having a non-linear transfer function, at least approximating to said non-linear manner, to give a signal varying in a linear manner with the parameter. The invention is described as applied to an altimeter wherein the air pressure P, Fig. 7, varies non-linearly with the altitude H but causes a linear variation of the oscillatory frequency of a tuning fork 1, Fig. 1. The tuning fork includes damping members 3 on the end of its tines 2 comprising semicylindrical members Fig. 1 or parallel discs 16, (17), Fig. 5, having partly or wholly overlapping upwardly turned edges 18, 19 and rotatable shutters (20), 21 for varying the size of fine frequency tuning holes 23, 24. The fork is excited by a solenoid 5 and the resulting oscillatory output picked up by an output transducer 4 to appear on line 9. A temperature sensing element 7 provides a signal on line 10. A bimetallic temp. compensating strip may directly rotate the shutters, Fig. 6 (not shown). The relationship between the altitude H and the normalized pressure P/Po (where Po is a standard pressure) may be approximated by a single hyperbolic function where a, b, n are constants. To produce a transfer function equivalent to the above, the digital circuit of Fig. 8 may be used, the signal on line 9 being converted to be in the form of a pulse train of frequency f 1  proportional to P/Po. The signal frequency f 1  is fed to the input of a digital frequency changer 32 which passes or blocks pulses in the signal f 1  in accordance with a digital control signal on k control lines 34. The resultant signal at output 33 has pulses missing to give an average frequency of fa, where: where M = control signal. Or converting to base 10 Pulse signal is fed to the reverse input of a digital up-down counter 35, having a constant frequency f 2  pulse train fed to its forward input, where f 2  = constant <f 1 . The counter output becomes the control signal M and by retroactive action fa is brought to f 2 . The control signal M therefore is the required hyperbolic function of the signal f 1 . In practice the actual variation of altitude with air pressure (or frequency) cannot be approximated with a single hyperbolic function. The upper half of a first function y = (n1/x), Fig. 9 must be combined with the lower half of a second function y=(n2/x), the former function being shifted through x and y distances a 1  and b 1  to become: and the latter function being shifted through - a 2  and - b 2  to become: To perform this combination, the arrangement of Fig. 10 must be used, incorporating the circuit of Fig. 8 together with circuitry for the a 1 , a 2 , b 1 , b 2  shifts, the n 1 , n 2  coefficients and the automatic change over from the set of curve constants to the other. Thus the output of the tuning fork oscillator 11 is mixed with the output of oscillator 39 which receives a control signal from the temperature sensing element 7. The resultant signal, independent of temperature, is filtered (41), frequency multiplied (42), frequency shifted (44, 43), filtered (45) and formed into the pulsed f 1  signal (47). The f 1  signal is fed to a frequency discriminator 85 whereby a control signal is fed to a switch 86 which switches over the control inputs of frequency changers 64, 73, 73 respectively setting the values of n, b, a, at the required function (1) to function (2) transition point. An OR gate 81 combines the a output of changer 77 (fed with the divided down output of oscillator 44) with the f 1  signal, so that (f 1 +a) is fed to frequency changer 31. The signal (f 1 +a) is also fed to frequency changer 73 to become b (f 1 +a), which is then combined at 69 with the output of frequency changer 31 for application to the backwards input of counter 35. Frequency changer 64 receives the divided down (46, 68) output of oscillator 44 as signal frequency f 2  and produces an output of frequency f 2 n which is fed to the forward input counter 35. The signal M produced by the counter 35 is the desired general function: which signal may be indicated at 57. A further frequency changer 48 and up-down counter 52 is feedback connection, utilizes the signals fed to the two inputs of counter 35 to provide an indication of the rate of change of altitude. It is stated that by having a ram tube 38 connected to the tuning fork chamber it is possible to produce a linear output indicating air speed.