1. Field of the Invention
The present invention relates to pressure measuring devices using the properties of a resonating cavity sensor whose resonance frequency is modified by mechanical deformation of a wall under the effect of the pressure to be measured.
2. Description of the Prior Art
This kind of sensor whose fundamental principle is well known has proved advantageous for the instantaneous measurement of static and dynamic pressures more especially for applications related to the equipment of airborne vehicles. In fact, considering on the one hand, the severity of the stresses, not only dimensional but also environmental to which modern airborne vehicles are subjected, and on the other hand considering the organic redundance which is generally imposed on their equipment, it has become indispensable to have pressure measuring means which are not only small, light and robust but also capable of supplying practically instantaneous information to the navigating and piloting systems of the aircraft while remaining accurate and reliable in an extensive range of variation of the factors of influence.
Now, traditional devices with which airborne vehicles are equipped for measuring static and total pressures are essentially formed by a tubular probe disposed in the airstream at a distance from the outer wall of the vehicle. This probe (PITOT tubes) is generally provided with front and side orifices for aerodynamically picking up pressures which be conveyed, for use, to sensors disposed inside the structure of the aircraft through pneumatic ducts of suitable length and section.
This arrangement presents well known disadvantages which are the following:
First of all, the great length of the pneumatic connecting ducts between the external probe and the internal sensors associated with the apparatus for using the pressure measurements causes a lack in response of the device all the longer and more troublesome since high speed acquisition and processing of the data relative to the pressure is required for piloting the aircraft and vehicles flying at speeds which may exceed mach 2.
Then the very existence of these pneumatic ducts affect the real reliability of the measurements, because of the environmental temperature variations related more especially to the electrothermic de-icing of the probe as well as the aerodynamic heating of the surface walls of the aircraft.
In addition, for maintenance purposes, the need to divide ducts of relatively large section (6 to 12 mm in diameter) into segments all along their path from the outer probe to the inside of the structure of the aircraft where the associated sensors and measuring apparatus are situated (generally in multiple copies so as to increase operating reliability), entails the provision of a certain number of connecting elements which increase the volume, the weight and the cost of the installation, besides increasing the usual risks of leaks and stoppages more especially by icing.
This is why, in order to overcome these disadvantages, it has already been proposed, more especially by French Pat. No. 1 598 497 to incorporate the pressure sensor or sensors inside the tubular probe outside the aircraft and to convey the information collected by electrical means to the internal user unit of the aircraft, and more precisely to provide a fluid pressure measuring device comprising an electronic circuit using the properties of a resonating cavity sensor whose resonance frequency is modified by the variations of electric capacity due to the mechanical deformation of a wall under the effect of the pressure to be measured, the device using a single coaxial cable for electrically connecting the sensor to the user circuit.
However, with the above mentioned device, when it is desired to make static pressure measurements, the defects due to the connecting cable disturb the measurements.