Patent GB-1162364-A (GB)
Title: Pressure to Altitude Converter
Inventor: OLAH GEORGE
IPC: G01C5/06
Filed: 1965-11-24
Abstract: 1,162,364. Measuring altitude electrically. ELLIOTT BROS. (LONDON) Ltd. Nov.23, 1966 [Nov. 24, 1965], No.49980/65. Heading G1N. Altitude is measured by comparing in a differential amplifier 15, Fig. 1, the output of a pressure transducer 11, and a continuously varying voltage derived by discharging a capacitor C through a resistor r, the time taken for the varying voltage to change from an initial value to a value equal to the transducer output being measured as an indication of altitude. The transducer may be a strain gauge type supplying a D. C. signal directly to the differential amplifier, or a capacitive or inductive type, in which case it is connected in an A. C. bridge 12 feeding a rectifier 16. The capacitor C is charged by a lead 13 and a measuring sequence is initiated by the sampler unit 17 which simultaneously changes over the switch 14 to connect the resistor r to the capacitor C, and opens a gate to allow pulses from a generator 20 to start passing to a counter 19. When the discharging capacitor voltage drops to the level of the pressure signal, the output of amplifier 15 is zero, and the counting operation is stopped. Since the temperature of the atmosphere falls with increasing altitude until a certain altitude is reached, and thereafter is assumed to be constant, the measured pressure varies non-linearly with altitude up to this point and thereafter varies linearly, and this is accommodated by arranging that the value of the capacitor C is itself varied linearly with time until the decaying voltage thereon reaches a value corresponding to the altitude at which the temperature ceases to vary. A sawtooth generator 18 varies the bias to a variable capacitance semi-conductor element constituting the capacitor C, and a Zener diode is arranged to determine the changeover from varying to constant capacitance operation. As an alternative to the pulse arrangement, the time delay may be measured by using the initial changeover of switch 14 and the attainment of zero output from the differential amplifier to respectively start and stop the charging of a capacitor or Miller integrator (neither shown) from a constant current source. The stored charge is subsequently conveyed to a further capacitor or Miller integrator which provides a D. C. output voltage indicative of altitude. The D. C. voltage may adjust the angular position of a shaft. In an alternative embodiment Fig. 2 (not shown), the pulse arrangement of Fig. 1 for measuring the time delay is replaced by a servo-balancing circuit in which the position of a servomotor shaft (25) indicates the time delay. In this case a constant speed motor (23) provides the sawtooth waveform to the capacitor C, Fig. 1, and the start of a measuring sequence is made to correspond to the shaft of the motor (23) passing through a zero reference position. The motor (23) may alternatively vary the capacitance C directly or vary a resistance in the lead 13. The shaft positions of the motor (23) and of the temporarily stationary servomotor (24) are compared (at 26) and the error signal supplied via a switch (27) to charge a capacitor (C m ). When the output of the amplifier 15, Fig. 1, reaches zero, the switch (27) is changed over and the signal stored on capacitor (C m ) is used to drive the servomotor (24) so that its shaft (25) is turned to a position corresponding to the shaft position of motor (23) at which the zero was detected. The apparatus is then ready to carry out a further sequence. Instead of varying the capacitor C, Fig.1, to compensate for the temperature variation, the voltage used to charge it may be made to vary in value (V ox , Fig. 3, not shown), the time delay being measured by a pulse system as in Fig. 1.