Measuring and testing – Specimen stress or strain – or testing by stress or strain... – Specified electrical sensor or system
Patent
1994-02-22
1995-08-22
Raevis, Robert
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
Specified electrical sensor or system
73705, G01L 110
Patent
active
054429633
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a temperature-compensated vibrating beam microsensor. It is particularly applicable to pressure measurements over a wide range of temperatures capable of going from several tens of degrees Celsius below 0.degree. C. to several hundreds of degrees Celsium above 0.degree. C.
Vibrating beam microsensors are known; descriptions of them can be found in the documents GB 2 185 106 and GB 2 194 049.
This type of sensor, applied to pressure measurement, is implemented in semiconductor material, for example silicon, and has a vibrating beam joined at its ends to a diaphragm subjected to pressure variations. The deformation of the diaphragm results in variations in the resonant frequency of the vibrating beam. These variations of the resonant frequency of the beam are therefore measured to deduce the pressure.
The static resonant frequency of the beam when not subjected to a force is determined during its implementation. The measurement of the variations of this frequency while the beam is subjected to a force can be effected by interferometry.
The interferometer, according for example to the document GB 2 185 106, is constituted by the end of the optical fibre directing a continuous monochromatic light beam for excitation of the beam, and the upper surface of the beam.
The transmission coefficient of the interferometer depends on the distance separating the end of the fibre and the facing surface.
When the transmission is maximum, the light beam is absorbed by the beam resulting in heating by photothermic effect of the upper part of the beam, which deforms by expansion. The fibre-beam distance varies, resulting in a reduction of the transmission coefficient and consequently a reduction of the heating, which tends to restore the beam to its initial state.
The beam thus begins vibrating at its resonant frequency, maintained by variations of the transmission coefficient. The phenomenon of autoexcitation is present.
The modulation of the light beam reflected by the beam is detected in order to measure the resonance frequency, which permits the deformation of the beam due to a pressure on the diaphragm to be deduced.
This detection presents some difficulties. In practice, to have maximum sensitivity, the distance between the end of the optical fibre and the upper surface of the beam must be precisely set at a value equal to .lambda./8(2k+1) where k is an integer and .lambda. is the wavelength of the light source. But the control of this distance with the required precision is very difficult to implement; on the other hand, the rest distance between the end of the fibre and the upper surface of the beam depends on external factors. For example, large variations in temperature result in the variation of this distance.
Another disadvantage of this type of sensor lies in its temperature dependence. In practice, the Young's modulus of silicon varies substantially as a function of temperature variations, which affects the resonant frequency of the vibrating beam and as a result distorts the measurements.
To alleviate this disadvantage, simultaneous temperature measurements are made, and the variation in Young's modulus is compensated in accordance with a previously determined calibration.
U.S. Pat. No. 4,972,076 describes a microsensor with two vibrating beams. The first, joined with the diaphragm at both of its ends, permits the measurement of pressure. The second, joined with the diaphragm at one of its ends only, the other end being free, is sensitive only to variations in temperature, so the measurement of its Young's modulus permits the necessary corrections to be applied to the pressure measurement.
The two beams are excited by the same modulated light beam, which results in the heating of the beams in accordance with their resonant frequencies. But it turns out that the simultaneous excitation of both beams is delicate to implement effectively.
The present invention has as an object to permit a temperature-compensated pressure measurement by means of a microsensor which is simpler to imp
REFERENCES:
patent: 5105665 (1992-04-01), Parsons et al.
Raevis Robert
Solartron Group Limited
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