MICROMACHINED SENSOR DEVICE

Abstract: In a method for monitoring strain variations of a strain responsive element of a micromachined sensor device while subjected to outside parameter conditions, at least two oscillation resonance modes of said element are activated and interrogated optically. Thereby correspondingly at least two resonance frequencies are obtained. From parameter/frequency characteristics of said device correspondingly at least two parameter values are derived.

Description: MICROMACHINED SENSOR DEVICE The present invention relates to a micromachined sensor device for determination of parameters affecting said device. In particular the present invention relates to a method for monitoring strain variations of a strain responsive element of a micromachined sensor device, the method comprising the steps of: irradiating said element by a beam of light with at least a frequency swept modulated intensity for activating said element in order to oscillate in a corresponding oscillation resonance mode at its resonance frequency, - determining frequency characteristics of said device when subjected to predefined parameter conditions, subjecting said element to outside parameter conditions, and detecting the beam of light after being modified correspondingly by said activated element. Such a method is known from GB 2 223 582 which clearly addresses a pressure responsive device, for example to be used as a "down well" sensor in oil drilling and exploration. Only pressure response is known for this device. In particular said sensors are made from single crystal silicon. In the article by Andres et al. , "Sensitivity and mode spectrum of a frequenc -output silicon pressure sensor", Sensors and Actuators, 15 (1988), pages 417-426, different vibrational modes of a so-called butterfly silicon sensor integral with a thin diaphragm are shown. The modes are investigated by activating electrically and interrogating optically. In particular for said modes relationships between resonance frequency and pressure are determined. The modes are classified in accordance with nm position and direction of node-axes. In the article by Uttamchandani et al., "Optically excited resonant beam pressure sensor" , Electronic Letters, 3rd December 1987, Vol. 23, No. 25, pages 1333-1334, resonance frequency/pressure and resonance frequency/temperature relationships for the fundamental mode of vibration of a beam- ype micromachined silicon resonator are determined. No further interrelationships or dependencies are shown. Moreover from the above documents the usual way of driving such resonators and detecting resonance frequencies by optical activation/interrogation techniques is known. In particular two light sources, i.e. a pulsed mode source for activation and a continuous-wave source for interrogation are employed. Light transmitted by said sources is joined and guided in one fibre to the sensor, thus avoiding complex instrumental arrangements at the measuring position. hereas from the above advantageous use of one single fibre for activation and interrogation of the vibrational modes of such sensors is known only determination of corresponding one parameter responsible for operation conditions as mentioned is realised. However, in most cases it is necessary to determine a set of parameters to qualify accurately working conditions in hostile environmental situations. For example in the above mentioned downhole circumstances exploration and production activities should be carried out at operation conditions as safe as possible. Especially pressure/ emper ture combinations should be monitored closely and reliably. Moreover, with respect to production activities, permanently installed sensor devices have to be used for long time periods. To remedy the shortcomings as addressed above several solutions are proposed. For example in the article by Vincent et al. , "An All-optical Single-fibre Micromachined Silicon Resonant Sensor: Towards a Commercial Device", Sensors and Actuators A, 25-27 (1991), pages 209-212, both the use of a set of sensors, called multiplexing sensors, with different resonance frequencies, and mechanical temperature compensation are disclosed. A further compensation method is known from EP 371 592. Also one fibre communication linkage is applied. The sensor device shown comprises a couple of two strain responsive silicon beams, the one being affected by both pressure and temperature variations, the other having a free end only being subjected to temperature variations. Thus temperature corrections can be made. However, the solutions and options as presented in the documents discussed, either seem to represent only a device development stage, or employ multicomponent sensor devices and related complex or incomplete parameter monitoring programmes. Thus it is a main object of the present invention to present a method capable of determining a set of outside parameters by means of a single micromachined sensor device in a reliable and accurate way. It is a further object of the invention to determine said outside parameters in only one frequency sweep. Therefore the present invention provides a method as shown above, further characterized by: activating subsequently at least two oscillation resonance modes, measuring the corresponding two resonance frequencies, and determining correspondingly two parameter values by fitting the resonance' frequencies upon the corresponding frequency characteristics. Furthermore in accordance with the invention a system is presented for monitoring strain variations of a strain responsive element of a micromachined sensor device arranged in varying outside parameter conditions, the system further comprising: a light beam supply means for irradiating said element, a frequency oscillator means for generating a beam with frequency swept modulated intensity in order to activate said element in at least two oscillation resonance modes at their resonance frequencies, detecting means for detecting the beam of light modified by said activated element, measuring means for measuring the resonance frequencies from said modified beam, and processing means for processing resonance frequency values and deriving therefrom at least correspondingly two outside parameter values. The present invention will now be described by way of example in more detail with reference to the accompanying drawing, wherein Figure 1 shows an enlarged top view of a cross-section of a measuring tube including a micromachined sensor device for carrying out the method of the present invention, and Figure 2 shows an enlarged top view of a cross-section of a butterfly sensor. In both igures the same sensor components are referred to b, the same reference members. Referring to Figure 1 a micromachined sensor means 1 is mounted within a tubular housing 2 shown in a cross-sectional view. Said housing is subjected to outside parameter variations and conditions to be monitored and sensed by said sensor device. Sensor device 1, of which a top view is shown, has the form of a thin plate and is mounted within and connected to the housing 2 by means of frits 9 of which by way of example two ones, black coloured, are shown. More in detail sensor means 1 comprises a so-called butterfly sensor 3. Said sensor 3 generally comprises two rectangular paddles 4 arranged in line and at their mutually facing short sides connected to each other by means of a hinge 5. Each paddle 4 is at its two long sides coupled to a support means 7 by means of at least corresponding two string means 6. Paddles 4, hinge 5 and string means 6 extend in the same plane and form the strain responsive element. As can be seen in said figure said string means exist of a V-formed pair of strings, having V-bottoms arranged on the above said long sides, and the V-ends arranged on said support means 7. Typically said sensor or sensor devices have sizes up to 1000 μ . They are made from amorphous silicon, polycrystalline silicon, single crystal silicon, or even from suitable metals. Recesses, holes, and further relief features are obtained for example by etching or deposition techniques which are well known to those skilled in the art. Tubes and sensor means are made from the same material. Usually said sensors are addressed as micromachined silicon sensors. While only a so-called butterfly sensor device is shown, a plurality of further sensor embodiments is known from the literature as discussed above. Especially beam type sensor devices are investigated widely. As can be seen in said figure the butterfly sensor 3 is coupled to support means 7 which in turn is connected to a leverage construction 8 connected with the above said frits 9 to the housing and enabling advantageous activation of the butterfly. In this arrangement strain affecting variations are transferred through the tubular housing to the leverage which in turn influences strain geometry in the string means. Besides the above leverage construction, support means 7 can also form a well known console-means which are arranged upon and connected to, i.e. mostly integrally formed with, an end wall of said tubular housing. In that case said end wall functions as a diaphragm for transferring said outside parameter conditions. As will be clear to those skilled in the art the sensor devices as mentioned above borrow their action on creating strain variations in their strain responsive element which is effected by means of the string means shown. Thus, in principle any strain related parameter can be sensed by such a sensor device. Evident examples of such parameters are pressure and temperature. Further examples may be flow, vibration, displacement which is kine- matically related to flow and vibration, and pressure fluctuations caused by passing chemical interfaces or fluctuating chemical conditions. In a further embodiment of the present invention it is possible to evacuate the tubular housing to optimise vibration of the strain responsive elements. In order to operate such a sensor device it is well known to use both electrical and optical activation and interrogation. Especially the optical method is preferred because the use of one light beam covers both activation and interrogation and reduces circuitry complexity of such detector and sensor systems substantially. Preferably optical fibres are used for guiding light signals and creating a relatively narrow beam irradiating the sensor device. For the sensor device in accordance with the present invention in principal all kinds of fibres can be used i.e. single mode, multimode, graded index, step index, related to corresponding fibre diameters, for example from 4 μ to 1000 μm. As known from the literature such sensor devices have a set of vibrational modes. Since the geometry and mechanics of said modes have to be distinguished clearly as to number and position of for example their node-axes, they can be referred to as M , as shown in the above cited article by Andres et al. However different mode characterisations can be used and are per se strictly related to the above M . In the present invention in particular M_„ and M-., , nm r r 02 04 also referred to as the third and sixth mode, are activated. As is well known to those skilled in the art optical activation and interrogation can be carried out in di ferent ways. Principally irradiating the sensor by means of a beam of light induces temperature gradients causing strain variations. In the case of irradiating by means of a non-continuous wave beam having a non-continuous intensity, said strain variations cause oscilla¬ tions- Dependent on the frequency of said light intensity variations, resonance frequencies can be excited, and dependent on the resulting oscillation of the sensor, light is reflected correspondingly. Thus from the reflected beam as modified by said oscillations, resonance frequency modes are detected, and consequently said resonance frequencies are measured. For said activation well known intensity variations like block pulse, sinusoidal, or saw-tooth variations are used. For the M_„ and M_, resonance modes, resonance frequencies respectively around 60 and 170 kHz are employed. In order to cover a set of modes in one activation run the frequency for the variations or modulations of the intensity of the irradiating beam of light is swept for example from 10 to 500 kHz, preferably from 50 to 200 kHz. Further to the above activation light signal, it is well known to employ a continuous-wave signal component, thereby improving detection of reflected beams. Generally intensity variations, in particular interference variations, are detected. Especially the latter case is accomplished as a fibre, which guides the above signals, is used, and the space between fibre end and sensor surface is employed as a well known resonance cavity. Generally the irradiating beam forms a spot upon the sensor device. By matching fibre characteristics, like diameter and direction, with sensor characteristics, like mass and width, modified intensity variations as induced by the sensor device are detected optimally. Prior to operating it is necessary to set, to calibrate, and to characterize some sensor features in order to use the sensor as a reliable measuring and detecting device. For example location of the spot on the sensor device, direction of the beam of light irradiating the sensor, angle between beam or fibre axis and normal of the sensor plane, and distance between fibre end and sensor plane have to be set accurately. Especially as to outside parameter conditions it has appeared possible to characterise relationships between said parameters and said resonance frequencies. Dependent on particular strain properties, sets of parameters are matched on said properties. In the case of detecting both pressure and temperature dependencies, at least two twofold frequency/pressure- temperature relationships have to be determined. In accordance with the present invention it has appeared advantageous to determine pressure and temperature characteristics for the above-mentioned M„„ and M n . resonance modes. Said 02 04 characteristics include for each resonance frequency an equation comprising specific temperature and pressure dependencies. Combining the two corresponding characteristics enables deriving pressure and temperature values. Thus by applying the above discussed method in accordance with the present invention pressure and temperature values, respectively between 0 and 2000 bar, and 0 and 350 °C, preferably between 35 and 1000 bar, and 20 and 200 "C, are determined. A system for carrying out the procedures in accordance with the method of the present invention includes a light beam supply means for irradiating the strain responsive element of the micromachined sensor, a frequency oscillator means for generating a beam with frequency swept modulated intensity in order to activate said element in at least two oscillation resonance modes at their resonance frequencies, detecting means for detecting the beam of light modified by said activated element, meaning means for measuring the resonance frequencies from said modified beam, and processing means for processing resonance frequency values and deriving therefrom at least correspondingly two outside parameter values. Advantageously the system comprises a continuous wave generating means for generating an additional continuous-wave signal component. EXAMPLE In an example as given below further details as to the method and system of the invention when using a butterfly sensor are shown. Referring to figure 2 a butterfly sensor 3 comprises the above shown paddles 4, hinge 5, string means 6, and support means 7. In more detail the butterfly sensor comprises holes 10 alongside hinge 5 in order to make the paddles 4 more flexible. Usually said holes 10 are rectangular. In the experiments carried out with the sensor as shown in figure 2 the following operation conditions were used: pulsed mode laser diode, wave length 1550 nm, - continuous-wave mode laser diode, wave length 1300 nm, spot location alongside one of the holes, fibre axis/normal - angle, 1° . fibre end/paddle surface - distance, 100 μm. With respect to the above it is noted that at least 60% of the beam light energy should arrive on the paddle, more advantageously at least 80%. Furthermore, it has appeared that said fibre axis/- normal - angle should be less than 5° , preferably less than 2° , and distances between fibre end and paddle surface should lie in the range from 20 to 200 μm, preferably from 50 to 150 μm, to give advantageous parameter values. In the above conditions, resulting in resonance frequencies of 62 kHz and 173 kHz for respectively the M.,-- and the n -mode, temperature and pressure values, respectively between 20 and 100 °C and up to 200 bar are determined. Errors in said values did not • exceed 0.1 °C and 0.1 bar. By applying the above method and system it has appeared possible to monitor conditions in hostile environments reliably and accurately which is of great interest, especially with regard to safety requirements. Various modifications of the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawing. Some modifications are intended to fall within the scope of the appended claims.

Claims: C L A I S 1. A method for monitoring strain variations of a strain responsive element of a micromachined sensor device, the method comprising the steps of: irradiating said element by a beam of light with at least frequency swept modulated intensity for activating said element in order to oscillate in a corresponding oscillation resonance mode at its resonance frequency, determining frequency characteristics of said device when subjected to predefined parameter conditions, - subjecting said element to outside parameter conditions, and detecting the beam of light after being modified correspondingly by said activated element, characterized in that the method further comprises: activating subsequently at least two oscillation resonance modes, measuring the- corresponding two resonance frequencies, and determining correspondingly two parameter values by fitting the resonance frequencies upon the corresponding frequency characteristics. 2. The method as claimed in claim 1, wherein the device is a silicon sensor. 3. The method as claimed in claim 1 or 2, wherein the device is a butterfly sensor, which comprises at least two rectangular paddles arranged in line and at their mutually facing short sides connected to each other by means of a hinge, each paddle at its two long sides coupled to a support means by means of at least corresponding two string means, paddles, hinge and string means extending in the same plane and forming the strain responsive element. 4. The method as claimed in claim 3, wherein said string means exist of a V-formed pair of strings, the V-bottoms arranged on said sides, and the V-ends arranged on said support means. 5. The method as claimed in any one of the foregoing claims, wherein the device is arranged within a substantially evacuated tubular housing which is subjected to said outside parameter conditions transferred to said device through said support means. 6. The method as claimed in claim 5, wherein said support means form a console-means, arranged upon and connected to an end wall of said housing, the end wall operating as a diaphragm for said outside parameter conditions. 7. The method as claimed in claim 5, wherein said support means are a part of a leverage construction, said construction being connected to the cylindrical wall of said housing for transferring said outside parameter conditions. 8. The method as claimed in any one of the foregoing claims, wherein the beam of light is supplied through an optical fibre. 9. The method as claimed in any one of the claims 3 to 8, wherein the M_„ and M n , oscillation resonance modes are activated. 10. The method as claimed in claim 1, wherein the frequency is swept between 10 and 500 kHz, preferably between 50 and 200 kHz. 11. The method as claimed in any one of the claims 3 to 10, wherein the beam of light irradiating the butterfly sensor is focussed to a spot upon one of said paddles, the major part of the spot being localised alongside the hinge. 12. The method as claimed in any one of the foregoing claims, wherein the beam of light irradiating said element further comprises a continuous-wave signal component. 13. The method as claimed in any one of the foregoing claims, wherein both pressure and temperature values are determined. 14. The method as claimed in any one of the foregoing claims . wherein pressure and temperature values are measured, respectively between 0 and 2000 bar and 0 and 350 °C, preferably between 35 and 1000 bar and 20 and 200 °C. 15. A system for monitoring strain variations of a strain responsive element of a micromachined sensor device arranged in varying outside parameter conditions, the system further comprising: a light beam supply means for irradiating said element, a frequency oscillator means for generating a beam with frequency swept modulated intensity in order to activate said element in at least two oscillation resonance modes at their 5 resonance frequencies, detecting means for detecting the beam of light modified by said activated element, measuring means for measuring the resonance frequencies from said modified beam, and *- ~ - processing means for processing resonance frequency values and deriving therefrom at least correspondingly two outside parameter values.

Inventors: DOUMA BINDERT SJOERD, EIGENRAAM PETER, DOUMA, BINDERT, SJOERD, EIGENRAAM, PETER
Applicants: SHELL CANADA LTD, SHELL INT RESEARCH, SHELL CANADA LIMITED, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1993
Main classification: G01L9
IPC codes: G01L1, G01L9