Schlumberger Industries

Philip J Parsons

Jolyon P Willson

Eric Metcalf

In pressure sensor systems based upon a micromachined silicon pressure sensor comprising a resonantly vibratable beam supported on a diaphragm, the beam is excited into resonant vibration by directing an optical excitation signal at the beam resonant frequency, via an optical fibre 26, onto a part of the sensor other than the beam, preferably the diaphragm. To detect the vibrations, the underside of the beam 16 and the adjacent upper surface of the diaphragm 18 are together arranged to define a Fabry-Perot cavity, and a continuous optical detection signal is directed at this cavity, also via the optical fibre 26. The optical detection signal is thus modulated at the vibration frequency of the beam 16 by the cavity, and the modulated signal is reflected back into the optical fibre 26.

Sensor systems

In a micromachined silicon pressure sensor comprising a resonantly vibratable beam supported on a diaphragm, the beam is indirectly excited into resonant vibration by directing an optical excitation signal at the beam resonant frequency onto a part of the sensor other than the beam, preferably the diaphragm. Preferably, the optical excitation signal is of a wavelength to which the sensor is fairly transparent, and is directed through the beam and diaphragm to be absorbed by a suitable coating on the underside of the diaphragm. The optical excitation signal produces local heating, and the resulting expansions and contractions at the beam resonant frequency propagate through the sensor structure to excite the beam into resonant vibration. Another optical signal is used to detect the frequency of vibration of the beam, and a positive feedback loop maintains the frequency of the excitation signal equal to the detected beam vibration frequency. In a modification, the indirect excitation is achieved by forming resistors or piezoelectric devices in a part of the sensor other than the beam, so that the expansions and contractions mentioned above can be excited electrically rather than optically.

Sensors

A solid state sensor, micromachined in silicon, comprised first and second beams which can be optically excited into resonant vibration. The first beam is arranged to be acted upon by a diaphragm, so that the tension in it, and therefore its resonant frequency, is affected by the pressure applied to the diaphragm. The second beam is provided for temperature correction purposes, and to this end is positioned close to the first beam but free at one end, so that it is insensitive to the pressure applied to the diaphragm, while being subject to the same temperature variations as the first beam. Additionally, the second beam is positioned sufficiently close to the first beam that it can be optically excited simultaneously with the first beam via a single optical fibre.

Solid state sensor with dual resonant vibratable members

An optical transducer system for producing an optical output signal representative of the rotational speed of a rotary member, particularly a shaft of a gas turbine engine, comprises a conventional inductive speed probe positioned adjacent the shaft to sense the passing of the teeth of a toothed wheel secured to and rotatable with the shaft. The inductive sensor thus produces an AC output signal whose frequency is proportional to the rotational speed of the shaft, and this signal is used to excite electrostatically vibration of a cantilever beam micromachined in silicon. The vibrating beam is arranged in an optical path defined by one or more optical fibres, such that its vibrations modulate light directed along the optical path at a frequency which is again proportional to the rotational speed of the shaft.

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Enfield, United Kingdom

Philip J Parsons