Resonator measuring device and method involving same

0001The invention relates to a device and a method for detecting a fault in a measuring device comprising a resonator and means for measuring a resonant frequency of the resonator. The measuring device comprises for example a fluid pressure sensor. The invention finds particular utility in aeronautics where pressure measurements are essential for the flight control of an aircraft. In fact, the altitude of a flight level required for an aircraft is determined by the static pressure of the air surrounding the aircraft. In addition, as air traffic increases, traffic control authorities seek to reduce the gap between two neighboring flight levels. The detection of a fault with a pressure sensor is essential to guarantee the safety of air traffic.

0002The invention can also be implemented for other devices using a resonator such as for example in an accelerometer as described in the French patent application <patcit id="pcit0001" dnum="FR2848298"><text>FR 2 848 298</text></patcit>, a gyrometer as described in the French patent application <patcit id="pcit0002" dnum="FR2834055"><text>FR 2 834 055</text></patcit> or even in a time base. The following description will only be made with reference to an air pressure sensor without of course limiting the invention.

0003To measure the pressure of the ambient air, pressure sensors are commonly used comprising a chamber maintained at a reference pressure generally close to vacuum. An example of this type of sensor is described in the French patent application.<patcit id="pcit0003" dnum="FR2687783"><text>FR 2 687 783</text></patcit>. The pressure sensor measures a pressure difference between the chamber and the air. The guarantee of the precision in the pressure measurement depends essentially on the maintenance of the vacuum prevailing inside the chamber during the whole lifetime of a sensor, or at least between two calibrations of the pressure sensor. Several phenomena can degrade the vacuum prevailing in the chamber, such as in particular leaks which can occur at the junctions of different components of the walls of the chamber or even the degassing of the walls or components located in the chamber.

0004The pressure sensor described in the French patent application <patcit id="pcit0004" dnum="FR2687783"><text>FR 2 687 783</text></patcit> comprises a resonator, one end of which is subjected to a force as a function of the pressure difference between the interior of the chamber and the ambient air. The principle of pressure measurement consists in measuring the resonant frequency of the resonator.

0005It has also been found that at constant air pressure, the ambient temperature influences the value of the resonance frequency. It is possible to add a temperature sensor to the pressure sensor. During a calibration phase, a function is established combining the measured temperature and the resonant frequency to determine the pressure. This function can be established empirically. This correction does not take into account any change in the pressure in the chamber. To date, only a recalibration of the pressure measurement device allows such a modification to be known. The document<patcit id="pcit0005" dnum="US6085594A"><text>US 6085594</text></patcit> describes a pressure sensor in which it is the resonant frequency which gives the pressure. This same document describes the fact that the quality factor at the resonant frequency is used to determine the pressure.

0006The invention aims to improve knowledge of the level of precision of the pressure sensor during its use and to avoid the obligation to periodically recalibrate the sensor in a preventive manner. Another object of the invention is to maintain a level of precision of the order of 0.1 hPa.

0007To this end, the subject of the invention is a measuring device comprising a resonator and means for measuring a resonant frequency of the resonator, characterized in that it comprises means delivering information representative of the quality coefficient of the resonator at the resonant frequency, the information making it possible to detect a fault in the device.

0008The invention also relates to a method of using an air pressure measuring device comprising a pressure sensor and a chamber maintained at a reference pressure, the pressure sensor measuring a pressure difference between the chamber and the air, the sensor comprising a resonator excited by an oscillation controlled by automatic amplitude control means characterized in that the device comprises means for detecting a variation in pressure in the chamber and in that the method consists in comparing a first gain of the automatic control of the amplitude of the excitation measured during the pressure measurement with a second gain of the automatic control of the amplitude of the excitation calculated at the frequency of the excitation measured from parameters defined during a calibration of the device so as to detect a fault in the device when the difference between the two gains is greater than a given value.

0009The invention will be better understood and other advantages will appear on reading the detailed description of an embodiment given by way of example, description illustrated by the attached drawing in which:<ul id="ul0001" list-style="dash" compact="compact"><li>the <figref idref="f0001">figure 1</figref> shows in the form of a block diagram an example of a device according to the invention;</li><li>the <figref idref="f0002">figure 2</figref> represents an exemplary embodiment of part of the diagram of the <figref idref="f0001">figure 1</figref>.</li></ul>

0010The <figref idref="f0001">figure 1</figref> shows an air pressure measuring device comprising a pressure sensor 1 and a chamber 2 maintained at a reference pressure, generally close to vacuum. The pressure sensor 1 measures a pressure difference between the chamber 2 and the air surrounding the pressure sensor 1.

0011Advantageously, the device comprises a resonator 3 and means for measuring a resonant frequency of the resonator 3. The resonator 3 is for example produced by means of a silicon wafer 4 which can enter into resonance under the effect of a electrical excitation signal E. The silicon blade 4 is located in the chamber 2. The silicon blade 4 is embedded at one of its ends 5 in a body 6 of the resonator 3 and at the other of its ends 7 on a thinned wall 8 of the chamber 2. The wall 8 is subjected on one of its faces to the pressure of the air, pressure to be measured, and on the other of its faces to the pressure of the chamber 2. The wall 8 deforms as a function of the pressure difference between room 2 and the air. This deformation of the wall 8 causes a stress in the silicon wafer 4. The stress changes as a function of the pressure difference between the air and the chamber 2. The resonant frequency of the silicon wafer 4 is therefore also a function of the pressure difference between the air and the chamber 2. A more detailed explanation of the making of this example of a resonator can be obtained by reading the French patent application. <patcit id="pcit0006" dnum="FR2687783"><text>FR 2 687 783</text></patcit>. It is of course possible to use another type of pressure sensor using a resonator and in which the resonator is located outside a chamber maintained at a reference pressure.

0012Resonance is detected by capacitive effect between the silicon wafer 4 and the body 6 of the resonator 3 by means of an electrical signal D taken from the body 6 of the resonator 3. The electrical signal D is amplified by an amplifier 9 then filtered by means of a bandpass filter 10 so as to keep only the resonance frequency and to be delivered to means for automatic control of the amplitude of the excitation signal E, means commonly called automatic gain control and marked CAG on the <figref idref="f0001">figure 1</figref>. The automatic gain control is controlled by a setpoint C. The automatic gain control delivers the excitation signal E. the excitation signal E forms the signal Fp used by a computer 14 to determine the air pressure.

0013The device further comprises means 15 for measuring the air temperature. The means 15 comprise for example a resistance with a negative temperature coefficient. The means 15 deliver a signal St to the computer 14 to correct the air pressure measurement. This correction is for example calculated as a function of the signal St and of the signal Fp by means of a polynomial function defined during a calibration of the device. This calibration is carried out using a pressure measurement campaign carried out at different temperatures. The polynomial function is for example of the form:<maths id="math0001" num=""><math display="block"><mi mathvariant="normal">P</mi><mo mathvariant="normal">=</mo><mi mathvariant="normal">AT</mi><mo>⁢</mo><mn mathvariant="normal">0</mn><mo mathvariant="normal">+</mo><mi mathvariant="normal">AT</mi><mo>⁢</mo><mn mathvariant="normal">1.</mn><mo>⁢</mo><mi>Fp</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">AT</mi><mo>⁢</mo><mn mathvariant="normal">2.</mn><mo>⁢</mo><mi>St</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">AT</mi><mo>⁢</mo><mn mathvariant="normal">3.</mn><mo>⁢</mo><mi>Fp</mi><mn mathvariant="normal">.</mn><mi>St</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">AT</mi><mo>⁢</mo><mn mathvariant="normal">4.</mn><mo>⁢</mo><msup><mi>Fp</mi><mn mathvariant="normal">2</mn></msup><mn mathvariant="normal">.</mn><mi>St</mi><mo mathvariant="normal">+</mo><mi mathvariant="normal">AT</mi><mo>⁢</mo><mn mathvariant="normal">5</mn><mspace width="1em" /><mi>Fp</mi><mn mathvariant="normal">.</mn><msup><mi>St</mi><mn mathvariant="normal">2</mn></msup></math><img file="EP1831663B1_D0001.tif" /></maths>where P represents the air pressure and where Ai represent constants. It has been found that a fifth order polynomial function makes it possible to obtain sufficient precision for the value of the pressure P.

0014According to the invention, the device comprises means for detecting a pressure variation in the chamber 2. These means advantageously deliver information representative of the quality coefficient of the resonator at the resonant frequency, for example in the form of a gain S3 of the automatic gain control delivered to the computer 14 to detect a fault in the device. Furthermore, during the calibration, the signal S3 was measured for each pressure measurement carried out. The measurements made during calibration make it possible to calculate, for any subsequent pressure measurement, a value that signal S3 should take if the chamber pressure remained unchanged. As before, it was found that the signal S3 is a function of the signals Fp and St and that this function can be approximated by means of a polynomial function.

0015To detect a fault with the device, a method consists in comparing a first signal S3 measured during the pressure measurement with a second signal S3 calculated from parameters defined during the calibration of the device and as a function of the signals Fp and St measured. The device is then declared in default if the difference between the measured signal S3 and the calculated signal S3 is greater than a given value. The comparison and the various calculations are carried out by the computer 14.

0016Advantageously, it is possible to correct the pressure difference measurement between the chamber 2 and the air as a function of the difference between the two signals S3. The pressure P is calculated for example as a function of the signals Fp, St and S3 measured. This calculation can be done using a function whose parameters are defined during the calibration phase. The function is here again for example polynomial. Thus, even if the pressure sensor 1 were to drift, due to a pressure drift of the chamber 2, it is possible to compensate for this drift by using the gain S3 of the automatic control of the amplitude of the excitation signal E of the resonator 3.

0017The <figref idref="f0002">figure 2</figref> represents an exemplary embodiment of part of the diagram of the <figref idref="f0001">figure 1</figref>. In order not to overload the<figref idref="f0002">figure 2</figref>, the computer 14 and the means 15 for measuring the air temperature have not been shown.

0018The amplifier 9 and the bandpass filter 10 are formed around an operational amplifier 20 driven on its inverting input by the signal D. The non-inverting input of the operational amplifier 20 is connected to ground. A feedback from the operational amplifier is formed by a resistor 21 and a capacitor 22 connected in parallel between the inverting input and the output of the operational amplifier 20. A capacitor 23 is connected to the output of the operational amplifier 20 to deliver a signal to the automatic gain control AGC which can weaken this signal by means of a resistor 24 and a field effect transistor 25. The signal thus weakened is polarized and shaped by crossing a circuit 26. the bias voltage is a voltage V₀ supplied to circuit 26. The output of circuit 26 provides the excitation signal E. The signal Fp is formed from the signal E through a circuit 27 whose function is to depolarize the signal E by means of a capacitor 28 and to amplify the signal E by means of an operational amplifier 29. the signal Fp is then rectified by means of a circuit 30 to be delivered to the automatic gain control AGC. The automatic AGC gain control is controlled by a setpoint C. The automatic gain control AGC has a first integration stage produced around an operational amplifier 31 whose output forms the signal S3 which drives a gate G of the field effect transistor 25.