SENSOR DEVICE

Abstract: A pressure sensor includes a resonator element (14) mounted on an optical fibre ferrule (11) whereby light signals for maintaining the resonator in a state of oscillation and for returning a modulated signal to a remite station are coupled to the resonator. The resonator is located in an evacuated cavity (16) and is mounted on a flexible diaphragm (13) whereby pressure changes may be detected as changes in the resonant frequency.

Description: This invention relates to pressure sensor devices e.g. for well logging applications. Background of the Invention Pressure sensor devices currently in use require some form of electrical connection between a sensing station and a sensor head. Whilst such sensors are satisfactory in many applications they have not found favour in enviroments subject to fire hazard such as are encountered in gas or oil well logging. In these applications there is great reluctance to install any system that involves electrical connections One approach to this problem is discussed in earlier European patent application No. 87 302 706.4, published as EP-A2-0244086. This document describes a pressure transducer comprising a pair of beam resonators supported on a pressure responsive diaphragm. The beam resonators are maintained in an oscillatory condition by optical signals coupled to the transducer via an optical fibre. Flexure of the diaphragm in response to an applied pressure determines the resonator frequency. Summary of the Invention According to the invention there is provided a remote pressure sensor, including a single crystal silicon transducer structure incorporating a pressure responsive diaphragm on which a resonator element is supported, said resonator element being responsive by variation of its resonant frequency to flexure of the diaphragm resulting from a pressure applied thereto, and an optical fibre disposed in alignment with the resonator element whereby to provide means for maintaining the element in a resonant condition at a frewquency corresponding to the applied pressure by an optical signal modulated at that resonant frequency, characterised in that the optical fibre is mounted in a bore (111) in a glass ferrule (11), that the ferrule has a flat end surface on which the transducer structure is mounted in register with the optical fibre, that the diaphragm has a peripheral flange whereby the transducer structure is sealed to the end face of the ferrule so as to define an air-tight cavity containing the resonator element, and that said cavity is evacuated so as to define an absolute pressure reference. Brief description of the Drawings Embodiments of the invention will now be described with reference to the accompanying drawings in which:- Fig. 1 is a sectional view of a pressure sensor together with its associated control system, Fig. 2 shows an alternative sensor construction. and Fig. 3 shows an alternative control system. Description of preferred Embodiment. Referring to Fig. 1, the sensor includes an optical fibre ferrule 11 on one end of which a pressure transducer 12 is mounted. The transducer 12 has a flexible diaphragm 13 which, in use, is exposed to an external pressure. Mounted on the diaphragm 13 is a resonator element 14 the vibrational frequency of which is a function of strain applied thereto by distortion of the diaphragm. The diaphragm 13 and resonator 14 are formed, preferably as an integral structure, by selective etching from a body of single crystal silicon. The diaphragm 13 is secured to the ferrule 11 via a peripheral flange 15, e.g. by electrostatic bonding, to define a cavity 16 in which the resonator element 14 is disposed. The cavity 16 is evacuated to minimise the viscous damping of the resonator and to provide an absolute, tenperature independent, reference pressure. An optical fibre 17 e.g. a silica fibre, is mounted in the ferrule such that the fibre end is adjacent the resonator element 14 whereby optical signals may be applied to the resonator to maintain it in a state of oscillation. Techniques for driving a resonator from an optical signal are described in the afore-mentioned specification No. EP-A-0244086 and in No. GB-B-2121953. In use, optical power is provided by a source 18 coupled to the fibre, and return signals modulated with the resonator frequency are fed via a detector 19 to a frequency meter 20. A pressure applied to the diaphragm 13 generates a corresponding strain in the resonator 14 thus determining its vibrational frequency. Typically the ferrule 11 is formed from a borosilicate glass whose thermal coefficient of expansion matches that of silicon. The fibre 17 may be sealed in the central bore 111 of the ferrule by metallising the surfaces of both the fibre and the bore followed by soldering or welding of the two surfaces to provide an hermetic seal therebetween. Alternatively the ferrule may be heated to its softening point and fused to the fibre. There will be localised strain arising from the difference between the expansion coefficients of glass and silica, but this strain is compressive and does not lead to fracture of the structure. After the fibre has been secured the end face of the ferrule is polished to provide a smooth flat surface to facilitate bonding of the transducer 12. A modified sensor is shown in Fig. 2 of the accompanying drawings. In this arrangement the ferrule 20 is made in two parts one of which (20 a ) has a tapered opening 22 coaxial with the bore in the other ferrule part 20 b and in which opening a ball lens 23 is located in register with the end of optical fibre 17 supported in the ferrule part 20 b . The ferrule part 20 a may be made of single crystal silicon. The ball lens 23 provides optical coupling between the fibre end and a resonator element 24 supported on a flexible single crystal silicon diaphragm 25. As before, the diaphragm 25 has a peripheral flange 26 sealed to the ferrule part 20 a so as to define a cavity in which the resonator element 24 is contained. This cavity is evacuated to provide an absolute reference pressure and to minimise damping of the resonator vibrations. An alternative control circuit for the system is shown in Fig. 3. The circuit incrporates an optical portion 31 a and an electronic portion 31 b . Optical power to the resonator 32 is provided e.g. by laser 33 driven by a power amplifier 34. The modulated output from the resonator is converted into an electrical signal by photodetector 35 is then fed via a preamplifier 36 to a gain control (AGC) circuit 37. Typically the preamplifier 36 comprises a junction field effect transistor. The AGC output is then fed to the power amplifier 34 thus completing the feedback loop. To ensure oscillation of the loop resonant frequency the input and output signals to and from the resonator are in phase. This phase relationship is provided either by the amplifier or by an additional phase control circuit (not shown) in the loop. Other techniques of driving the resonator and detaining the vibrational frequency will be apparent to those skilled in the art. The sensors of Figs, 1 and 2 are of particular application to hostile enviroments such as are encountered in well logging. As the sensors are purely optical in operation, i.e. there is no electrical connection thereto, there is no fire hazard involved in their use.

Claims: A remote pressure sensor, including a single crystal silicon transducer structure (12) incorporating a pressure responsive diaphragm (13) on which a resonator element (14) is supported, said resonator element being responsive by variation of its resonant frequency to flexure of the diaphragm resulting from a pressure applied thereto, and an optical fibre (17) disposed in alignment with the resonator element whereby to provide means for maintaining the element in a resonant condition at a frequency corresponding to the applied pressure by an optical signal modulated at that resonant frequency, characterised in that the optical fibre is mounted in a bore (111) in a glass ferrule (11), that the ferrule has a flat end surface on which the transducer structure is mounted in register with the optical fibre, that the diaphragm has a peripheral flange (15) whereby the transducer structure is sealed to the end face of the ferrule so as to define an air-tight cavity (16) containing the resonator element, and that said cavity is evacuated so as to define an absolute pressure reference. A pressure sensor as claimed in claim 1, characterised in that the diaphragm and resonator are formed as a unitary body of single crystal silicon. A pressure sensor as claimed in claim 1 or 2, characterised in that the diaphragm is bonded electrostatically to the ferrule end face. A pressure sensor as claimed in claim 1, 2 or 3, characterised in that a ball lens (23) is provided between the optical fibre and the resonator. A sensor system incorporating one or more pressure sensors as claimed in any one of the preceding claims.

Inventors: GRAVES, PETER WILLIAM, GREENWOOD, JOHN CHRISTOPHER, NEAT, ROSAMUND CAROL
Applicants: NORTHERN TELECOM LIMITED
Application year: 1987
Main classification: G01L11
IPC codes: G01D5, G01L11, G01L1, G01L9