WO1993009422A1 - Sensor - Google Patents

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Publication number
WO1993009422A1
WO1993009422A1 PCT/FI1992/000294 FI9200294W WO9309422A1 WO 1993009422 A1 WO1993009422 A1 WO 1993009422A1 FI 9200294 W FI9200294 W FI 9200294W WO 9309422 A1 WO9309422 A1 WO 9309422A1
Authority
WO
WIPO (PCT)
Prior art keywords
filter
spacer
sensor
window panes
piezoelectric
Prior art date
Application number
PCT/FI1992/000294
Other languages
French (fr)
Inventor
Pentti NIEMELÄ
Original Assignee
Valtion Teknillinen Tutkimuskeskus
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Valtion Teknillinen Tutkimuskeskus filed Critical Valtion Teknillinen Tutkimuskeskus
Publication of WO1993009422A1 publication Critical patent/WO1993009422A1/en

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/12—Generating the spectrum; Monochromators
    • G01J3/26—Generating the spectrum; Monochromators using multiple reflection, e.g. Fabry-Perot interferometer, variable interference filters
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/25—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands
    • G01N21/31—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry
    • G01N21/35—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light
    • G01N21/3504—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry using infrared light for analysing gases, e.g. multi-gas analysis
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N2201/00—Features of devices classified in G01N21/00
    • G01N2201/02—Mechanical
    • G01N2201/022—Casings
    • G01N2201/0221—Portable; cableless; compact; hand-held

Definitions

  • the invention relates to a sensor, especially a gas sensor, which is used for infrared spectroscopic measurements of concentration, for analyzing a radia ⁇ tion coming from an external source of radiation through a sample space containing one gas or several gases, the sensor comprising, besides a filter defin ⁇ ing a wave number range, a collimator lens and an in- frared detector, also an adjustable filter of Fabry- Perot type, which filter is formed of two parallel window panes at least partially coated with a parti ⁇ ally reflective coating and of a piezoelectric actua ⁇ tor, by means of which the distance between the win- dow panes can be adjusted by a voltage switched over the actuator, and passbands of which filter are ar ⁇ ranged in pairs to correspond to absorption peaks of a gas to be measured within a wave number range de ⁇ termined by the filter defining the wave number range.
  • the present invention is connected with mea ⁇ surements of gas component concentrations in flue gases, for instance, and the invention can be util ⁇ ized for the measurement of emissions from the pro- duction of energy and for the measurement of pollut ⁇ ant concentrations in the air, etc.
  • the infrared method is impeded by the high percentage of water vapour in the flue gas, and to eliminate that, the gas sample to be measured must in several cases be dried before the measurement.
  • a mea ⁇ surement taking place directly in a gas flue has sig ⁇ nificant advantages, but it makes certain demands on the properties of the infrared analyzer, the most important of these properties being a good spectral resolution.
  • FTIR spectro- meters and Fabry-Perot spectrometers have a suffi ⁇ cient resolution.
  • the Fabry-Perot has a simpler structure and a more advantageous price and it is more suitable for the measurement of an individual gas component.
  • the present solution is of Fabry-Perot type.
  • the most central part of a Fabry-Perot spectrometer is formed of two window panes separated by a narrow air slot.
  • the window panes are transparent and the inner surfaces thereof are coated with a partially reflective coating.
  • the transmission spectrum of such a structure consists of several passbands located at equal distances within a wave number range, which passbands can be displaced by moving one of the win- dows.
  • only one passband is utilized at a time and an adjustment range corresponding to the distance between two se ⁇ quential passbands is defined by a separate filter.
  • a generally external piezoelectric actuator is ordina- rily used for moving the window, which actuator elon ⁇ gates under the influence of the voltage directed thereto and displaces the windows closer to each other.
  • the solutions of the patents DE-39 25 692, DE-38 12 334 and DE-39 23 831 show a structure in which a spacer is made of an electro- optically or thermooptically adjustable material, in which the index of refraction of the spacer changes thanks to a change in the electric field directed to the spacer or a temperature change. Consequently, the spacer in question is not a piezoelectric spacer changing the distance between the reflective coatings physically. In the solution of the invention on the contrary, the piezoelectric spacer changes the physi- cal distance between the window panes having reflec- tively coated inner surfaces.
  • DE-37 06 833 discloses a method and a device for a Fabry-Perot spectroscopic measurement.
  • the de ⁇ vice described in this publication comprises a piezo- electrically operated actuator between window panes, by means of which actuator the distance between the panes is changed.
  • the actuator is, however, considerably longer than the distance between the reflective inner surfaces of the window panes in order that the filter can be provided with a sufficiently long distance of movement, for the distance of movement of an actuator is proportional to its length.
  • the actuator is posi- tioned within the edge zone of the window panes in a space thus being considerably longer than the dis ⁇ tance between the very reflective surfaces, and it is therefore clear that the significant advantages due to the shortness of the required distance of movement which are provided by the present invention cannot be achieved by means of this structure. It is also clear that it will be substantially difficult to meet the demands on dimensional accuracy of grinding. Conse ⁇ quently, this structure does not represent a pure etalon structure.
  • DE-39 39 359 discloses a spectrometer of Fabry- Perot type, the filter part of which comprises piezo ⁇ electric actuators for changing the distance between the window panes, but these actuators are located outside the panes, not between the panes within the area between the reflective coatings.
  • the known Fabry-Perot spectrometer construc ⁇ tions show several problems. For a good spectral re ⁇ solution in combination with a wide adjustment range is required that the window surfaces are very even and the inner surfaces of the windows very parallel. Because the windows of some known constructions are not supported by each other, the demand on paral ⁇ lelism presupposes a high dimensional accuracy of the body, the windows and the piezo actuator of the spec ⁇ trometer and a low heat expansion of the materials used. In addition, a wide adjustment range requires a big piezo actuator, which makes it difficult to in ⁇ tegrate the construction to a small-sized sensor.
  • the object of the present invention is to pro ⁇ vide a novel sensor construction mainly suitable for gas measurements, by means of which construction the problems associated with the known constructions can be remarkably reduced.
  • This object is achieved by means of the sensor solution of the invention, which is characterized in that the piezoelectric actuator is located between the re lective coatings of the window panes to form one spacer or several spacers with a thickness essen- tially corresponding to the distance between the re ⁇ flective coatings, the piezoelectric spacer forming together with the window panes the adjustable etalon filter.
  • the solution of the invention is based on a novel way of positioning the piezo actuator of a Fabry-Perot spectrometer and on a way of going through a certain spectral range by means of the ac ⁇ tuator by utilizing the periodicity of the spectral peak system of a gas component.
  • the piezoelectric spacer or spacers positioned directly within the area between the coat ⁇ ings change the physical distance between the window panes having reflectively coated inner surfaces.
  • the alleviated demands on dimensional accuracy concern only the piezo actuator now, because the coatings of the windows support each other through that actuator.
  • Another embodiment of the invention which is possible if the piezoelectric material is transparent, has an even simpler structure.
  • the ac ⁇ tual separate windows have been omitted and the par ⁇ tially reflective coatings, which are in this case also electrically conductive, are located on the sur ⁇ faces of the piezo actuator, while in the other em ⁇ bodiments of the i vention the inner surfaces of each window pane have a partially reflective coating.
  • the alleviated demands on tolerance facilitate the man- ufacture of the constructions of the invention and reduce the manufacturing costs.
  • the con ⁇ structions can be made in a small size, which makes it possible to integrate them together with closely associated components to a compact sensor. The invention will be described in greater de- tail in the following referring to the enclosed draw ⁇ ings, in which
  • Figure 1 shows a structure of a sensor and how it is located in a measuring environment
  • Figure 2 shows a first embodiment of an adjust ⁇ able etalon filter.
  • Figure 3 shows a second embodiment of the ad ⁇ justable etalon filter
  • Figure 4 shows a third embodiment of the ad- justable etalon filter.
  • Figure 5 shows an infrared spectrum of carbon monoxide and a transmission spectrum of the etalon filter arranged for the measurement of carbon monox ⁇ ide
  • Figure 6 shows a further integrated sensor so ⁇ lution.
  • Figure 1 shows the structure of an sensor of the invention schematically and how it is located in a measuring environment.
  • the sensor 1 is especially a gas sensor, which is used for infrared spectroscopic measurements of concentration, for analyzing a radia ⁇ tion coming from an external source 2 of radiation through a sample space 3.
  • the sample space 3 can be for instance a gas flue containing one gas or several gases.
  • the sensor 1 comprises, besides a filter 4 de ⁇ fining a wave number range, a collimator lens 5 and an infrared detector 6, an adjustable etalon filter 7 of Fabry-Perot type formed of two parallel window panes 10 and 11, the inner surfaces of which are coated with a partially reflective coating 8 and 9, and of a piezoelectric actuator 12, by means of which a distance d between the window panes can be adjusted by a voltage U switched over the actuator 12.
  • the voltage can be switched to the actuator e.g. by coat- ing its upper and lower surfaces with a metal film.
  • the piezoelectric actuator 12 is a spacer located between the window panes 10 and 11, which spacer unequivocal ⁇ ly determines the distance d between the window panes 10 and 11 and forms together with the window panes 10 and 11 the adjustable etalon filter 7.
  • Figure 5 shows an infrared spectrum of a gas, in this case carbon monoxide (CO), and a transmission spectrum of the etalon filter arranged for the measurement of carbon monoxide, whereby, referring to the Figures 1 and 5 and still in the manner described by the basic idea of the invention, passbands 13a to 13j of the etalon filter are arranged in pairs to correspond to absorp ⁇ tion peaks 14a to 14j of the gas to be measured with- in a wave number range S determined by the filter 4 defining the wave number range.
  • the structure shown in Figure 1 further comprises a second collimator lens 15 positioned between the source 2 of radiation and the filter 4 defining the wave number range, by ⁇ which lens the radiation to be obtained from the source 2 of radiation can be collimated.
  • the most important part of the sensor 1 is the adjustable etalon filter 7, which is formed of the two parallel window panes 10 and 11 and the spacer to separate them, which spacer accordingly is the piezoelectric actuator 12.
  • the sensor 1 itself comprises the broad-band infrared filter 4, by means of which a desired amount of the absorption peaks 14a to 14j in the transmission spec- trum shown in Figure 5 of the gas to be examined can be defined from the wave number range.
  • the sensor thus comprises further the adjustable etalon filter 7, the collimator lens 5 and the detector 6 sensitive to infrared radiation, all of them located in the same casing in a preferred embodiment of the inven- tion, which casing is indicated by a broken line 1. This way to realize the structure forms a very com ⁇ pact and durable whole.
  • the structure of the sensor 1 can still be simplified by providing the free outer surface of the window pane 10 facing the source 2 of radiation with a broad-band infrared filter 4 and by combining the function of the collimator lens 5 with the window 11 facing the detector 6, by giving the outer surface of the window a curved form.
  • the filter 4 forms a thin film on the surface of the window pane 10.
  • an external source 2 of radiation is also needed in the * measuring arrangement, which source emits broad-band radiation in the infrared region.
  • the sample space 3, such as a gas flue, is located between the source 2 of radiation and the sensor 1.
  • the sensor 1 is used for spectroscopic measure- ments, for measuring and analyzing radiation coming from the external source 2 of radiation through the sample space 3 containing gases to the detector 6.
  • the measurement is based on the radiation being ab ⁇ sorbed by the gas components of the sample space 3, which is seen as a decreasing intensity of the radia ⁇ tion coming to the detector 6.
  • Strong absorption belts of different gas components are usually located within different wave number ranges, and even if they overlap, the distances between periodic series of peaks differ from each other. This makes the sensor a very selective concentration meter.
  • the upper part of Figure 5 shows the transmis ⁇ sion spectrum of carbon monoxide (CO) set forth as an example within the strongest absorption belt area and the lower part of Figure 5 shows the transmission spectrum of the etalon filter arranged for the measu ⁇ rement of the left branch.
  • the peaks 14a to 14j di ⁇ rected downwards in the gas spectrum represent the areas of strong absorption. The distances between the peaks increase slowly when moving to the right, but within the narrower area S defined by the filter the distances are very equal, the passbands 13a to 13j of the etalon filter corresponding in pairs to the ab ⁇ sorption peaks 14a to 14j of the gas.
  • the piezo actuator In the starting situation of a measuring period the piezo actuator is without voltage and its thickness is dimensioned in such a manner that the passbands 13a to 13j come be ⁇ tween the absorption peaks 14a to 14j and the radia ⁇ tion coming to the detector is at its strongest.
  • the control voltage When the control voltage is gradually raised, the distance between the windows increases and the passbands move to the right.
  • the radiation obtained by the detector decreases simultaneously and reaches the minimum when the passbands 13a to 13j and the absorption peaks 14a to 14j are opposite to each other.
  • the passbands 13a to 13 come to the area of weak absorption again.
  • the adjustment range dd of the distance between the win ⁇ dow panes for going through the defined wave number range S corresponds substantially to the distance between two adjacent passbands of the etalon filter, the passbands 13a and 13b, for instance, which makes the use of the sensor structure of the invention as appropriate and effective as possible.
  • a change in a signal (I) depends on the concen- tration (p) of a gas component according to the equa ⁇ tion
  • k is a coefficient representing the intensity of absorption. Since the concentration depends only on the ratio of the signals, no special demands are made on the stability of the source of radiation, especially as the duration of a measuring period is relatively short.
  • the spectral transmission of etalon depends primarily on the distance d between the windows and on a reflection coefficient R of the coatings.
  • the average wavelengths lambda-,- of the passbands are determined by the equation
  • the finesse value can be decreased by non-idealities of the structure, such as an uneven- ness of the reflective surfaces and a deflection of the surfaces from the parallel direction. These are described by another finesse value
  • 1/F 2 1/F R 2 + 1/F F 2 .
  • the CO sensor chosen as an example can be allowed to have 10 times looser manufacturing tolerances than the prior art structures have.
  • Table 1 The applicability of the structure of the in- vention to the measurement of various gas components can be examined by means of Table 1.
  • the parameters of the table are the order m of the midmost transmission band of the etalon filter, the thickness d of the piezo actuator and the change dd in the thickness of the piezo actuator, the change corresponding to a displacement measuring the distance l/2nd) between the passbands in the transmission spectrum of the ilte .
  • PLZT and PMN changes in thickness of 0.2 % have been achieved, which makes it possible to measure carbon monoxide (CO), nitrogen monoxide '(NO) and nitrogen dioxide (N0 2 ), in addition to the previous gases.
  • the rest of the gases studied require actuators even more effec- tive than this.
  • PLZT is transparent at wave number values higher than 1500 1/cm, which makes it possible to measure the same gas components, except for S0 2 , also with a sim- pier etalon of plate type.
  • the actuator thickness (d) and its change (dd) shall then be divided by the index of refraction (about 2.4) of PLZT.
  • the spacer functioning as a piezoelectric actuator is formed of a disc 12, the middle part of which comprises an opening 16, through which radiation can propagate.
  • the structure of this solution is simple and does not place any restric- tions on the selection of the material of the window panes 10 or 11.
  • the embodiment shown in Figure 2 is similar to that of Figure 1, i.e. an embodiment in which the middle part of the piezoelectric actuator 12 comprises an opening.
  • the spacer is formed of at least three actuators 12a to 12c positioned symmetri ⁇ cally between the windows 10 and 11.
  • detectors 17a to 17c are positioned between the ac ⁇ tuators.
  • detectors 17a to 17c 3 detec- tors measuring the distance between the window panes.
  • This solution is more complicated than the previous one, but it facilitates the manufacture of the construction as far as the demand on the windows being parallel is concerned, for the piezo actuators 12a to 12c can be controlled separately in such a manner that small errors will be amended.
  • the dis ⁇ tance measuring detectors 17a to 17c are for instance capacitive detectors.
  • the whole etalon filter 7 con- sists of a transparent piezoelectric disc 12, the surfaces of which are provided with partially re ⁇ flective coatings 8 and 9, which are in this case also electrically conductive, since the control vol- tage is connected to the actuator through them. Then the spacer, i.e. the actuator 12, forms a plate-like etalon filter.
  • This solution has the simpliest struc ⁇ ture of all, but makes various demands on the materi ⁇ als used.
  • the partially reflective coatings 8 and 9 are positioned on the inner surfaces of the window panes 10 and 11.
  • the embodiment of Figure 4 does not show any actual separate window panes, but the partially reflective coating is positioned on the surface of the piezoelectric actuator 12. Consequently, as to the embodiment of Figure 4, the definition of the existence of the window panes in the wording of the main claim shall be understood in such a way that the partially reflective coating includes both the coat- ing and the window pane, even though this structure does not actually show any separate window panes.
  • the solution of Figure 4 has a good producibility, for the demands on dimensional accuracy can be met more easily, since there are no separate window panes, but the surfaces of the piezoelectric spacer are provided with the coatings 8 and 9.

Abstract

The invention relates to a sensor, especially a gas sensor, which is used for infrared spectroscopic measurements of concentration, for analyzing a radiation coming from an external source (2) of radiation through a sample space (3) containing one gas or several gases. The sensor comprises, besides a filter (4) defining a wave number range, a collimator lens (5) and an infrared detector (6), also an adjustable filter (7) of Fabry-Perot type, which is formed of two parallel window panes (10 and 11) coated with a partially reflective coating (8 and 9) and of a piezoelectric actuator (12), by means of which the distance (d) between the window panes can be adjusted by a voltage switched over the actuator (12). The piezoelectric actuator (12) is a spacer positioned between the reflective coatings (8, 9) of the window panes, the thickness of which spacer essentially corresponds to the distance (d) of the coatings and forms together with the window panes the adjustable etalon filter (7).

Description

SENSOR .
The invention relates to a sensor, especially a gas sensor, which is used for infrared spectroscopic measurements of concentration, for analyzing a radia¬ tion coming from an external source of radiation through a sample space containing one gas or several gases, the sensor comprising, besides a filter defin¬ ing a wave number range, a collimator lens and an in- frared detector, also an adjustable filter of Fabry- Perot type, which filter is formed of two parallel window panes at least partially coated with a parti¬ ally reflective coating and of a piezoelectric actua¬ tor, by means of which the distance between the win- dow panes can be adjusted by a voltage switched over the actuator, and passbands of which filter are ar¬ ranged in pairs to correspond to absorption peaks of a gas to be measured within a wave number range de¬ termined by the filter defining the wave number range.
The present invention is connected with mea¬ surements of gas component concentrations in flue gases, for instance, and the invention can be util¬ ized for the measurement of emissions from the pro- duction of energy and for the measurement of pollut¬ ant concentrations in the air, etc. At flue gas mea¬ surements, the infrared method is impeded by the high percentage of water vapour in the flue gas, and to eliminate that, the gas sample to be measured must in several cases be dried before the measurement. A mea¬ surement taking place directly in a gas flue has sig¬ nificant advantages, but it makes certain demands on the properties of the infrared analyzer, the most important of these properties being a good spectral resolution. As to known analyzer types, FTIR spectro- meters and Fabry-Perot spectrometers have a suffi¬ cient resolution. The Fabry-Perot has a simpler structure and a more advantageous price and it is more suitable for the measurement of an individual gas component.
The present solution is of Fabry-Perot type. The most central part of a Fabry-Perot spectrometer is formed of two window panes separated by a narrow air slot. The window panes are transparent and the inner surfaces thereof are coated with a partially reflective coating. The transmission spectrum of such a structure consists of several passbands located at equal distances within a wave number range, which passbands can be displaced by moving one of the win- dows. In some known spectrometer solutions only one passband is utilized at a time and an adjustment range corresponding to the distance between two se¬ quential passbands is defined by a separate filter. A generally external piezoelectric actuator is ordina- rily used for moving the window, which actuator elon¬ gates under the influence of the voltage directed thereto and displaces the windows closer to each other.
As to prior art, the solutions of the patents DE-39 25 692, DE-38 12 334 and DE-39 23 831 show a structure in which a spacer is made of an electro- optically or thermooptically adjustable material, in which the index of refraction of the spacer changes thanks to a change in the electric field directed to the spacer or a temperature change. Consequently, the spacer in question is not a piezoelectric spacer changing the distance between the reflective coatings physically. In the solution of the invention on the contrary, the piezoelectric spacer changes the physi- cal distance between the window panes having reflec- tively coated inner surfaces.
DE-37 06 833 discloses a method and a device for a Fabry-Perot spectroscopic measurement. The de¬ vice described in this publication comprises a piezo- electrically operated actuator between window panes, by means of which actuator the distance between the panes is changed. In the solution set forth in this publication, the actuator is, however, considerably longer than the distance between the reflective inner surfaces of the window panes in order that the filter can be provided with a sufficiently long distance of movement, for the distance of movement of an actuator is proportional to its length. In said structure de¬ scribed in said publication, the actuator is posi- tioned within the edge zone of the window panes in a space thus being considerably longer than the dis¬ tance between the very reflective surfaces, and it is therefore clear that the significant advantages due to the shortness of the required distance of movement which are provided by the present invention cannot be achieved by means of this structure. It is also clear that it will be substantially difficult to meet the demands on dimensional accuracy of grinding. Conse¬ quently, this structure does not represent a pure etalon structure.
DE-39 39 359 discloses a spectrometer of Fabry- Perot type, the filter part of which comprises piezo¬ electric actuators for changing the distance between the window panes, but these actuators are located outside the panes, not between the panes within the area between the reflective coatings.
The known Fabry-Perot spectrometer construc¬ tions show several problems. For a good spectral re¬ solution in combination with a wide adjustment range is required that the window surfaces are very even and the inner surfaces of the windows very parallel. Because the windows of some known constructions are not supported by each other, the demand on paral¬ lelism presupposes a high dimensional accuracy of the body, the windows and the piezo actuator of the spec¬ trometer and a low heat expansion of the materials used. In addition, a wide adjustment range requires a big piezo actuator, which makes it difficult to in¬ tegrate the construction to a small-sized sensor. The object of the present invention is to pro¬ vide a novel sensor construction mainly suitable for gas measurements, by means of which construction the problems associated with the known constructions can be remarkably reduced. This object is achieved by means of the sensor solution of the invention, which is characterized in that the piezoelectric actuator is located between the re lective coatings of the window panes to form one spacer or several spacers with a thickness essen- tially corresponding to the distance between the re¬ flective coatings, the piezoelectric spacer forming together with the window panes the adjustable etalon filter.
The solution of the invention is based on a novel way of positioning the piezo actuator of a Fabry-Perot spectrometer and on a way of going through a certain spectral range by means of the ac¬ tuator by utilizing the periodicity of the spectral peak system of a gas component. In the solution of the invention the piezoelectric spacer or spacers positioned directly within the area between the coat¬ ings change the physical distance between the window panes having reflectively coated inner surfaces. Several significant advantages are achieved by means of the sensor solution of the invention. The structure of the sensor becomes very compact and, as far as the dimensional accuracy required is concern¬ ed, looser to manufacture. The use of several (n) passbands for going through a certain spectral range increases the distance between the windows to n-fold. When the necessary distance of movement remains un¬ changed, the relative change in thickness required of the piezo actuator positioned between the windows de¬ creases to a 1/nth part, which makes it possible to measure several gas components even by means of the piezoelectric materials known at present. When the same band width is used for the passbands in both cases, the finesse value required decreases to a 1/nth part, which increases the amount of permissible errors in the evenness of the window surfaces and the parallelism of the inner surfaces to n-fold. In addi¬ tion, the alleviated demands on dimensional accuracy concern only the piezo actuator now, because the coatings of the windows support each other through that actuator. Another embodiment of the invention, which is possible if the piezoelectric material is transparent, has an even simpler structure. The ac¬ tual separate windows have been omitted and the par¬ tially reflective coatings, which are in this case also electrically conductive, are located on the sur¬ faces of the piezo actuator, while in the other em¬ bodiments of the i vention the inner surfaces of each window pane have a partially reflective coating. The alleviated demands on tolerance facilitate the man- ufacture of the constructions of the invention and reduce the manufacturing costs. Moreover, the con¬ structions can be made in a small size, which makes it possible to integrate them together with closely associated components to a compact sensor. The invention will be described in greater de- tail in the following referring to the enclosed draw¬ ings, in which
Figure 1 shows a structure of a sensor and how it is located in a measuring environment, Figure 2 shows a first embodiment of an adjust¬ able etalon filter.
Figure 3 shows a second embodiment of the ad¬ justable etalon filter,
Figure 4 shows a third embodiment of the ad- justable etalon filter.
Figure 5 shows an infrared spectrum of carbon monoxide and a transmission spectrum of the etalon filter arranged for the measurement of carbon monox¬ ide, Figure 6 shows a further integrated sensor so¬ lution.
Figure 1 shows the structure of an sensor of the invention schematically and how it is located in a measuring environment. The sensor 1 is especially a gas sensor, which is used for infrared spectroscopic measurements of concentration, for analyzing a radia¬ tion coming from an external source 2 of radiation through a sample space 3. The sample space 3 can be for instance a gas flue containing one gas or several gases. The sensor 1 comprises, besides a filter 4 de¬ fining a wave number range, a collimator lens 5 and an infrared detector 6, an adjustable etalon filter 7 of Fabry-Perot type formed of two parallel window panes 10 and 11, the inner surfaces of which are coated with a partially reflective coating 8 and 9, and of a piezoelectric actuator 12, by means of which a distance d between the window panes can be adjusted by a voltage U switched over the actuator 12. The voltage can be switched to the actuator e.g. by coat- ing its upper and lower surfaces with a metal film. According to the basic idea of the invention, the piezoelectric actuator 12 is a spacer located between the window panes 10 and 11, which spacer unequivocal¬ ly determines the distance d between the window panes 10 and 11 and forms together with the window panes 10 and 11 the adjustable etalon filter 7. Figure 5 shows an infrared spectrum of a gas, in this case carbon monoxide (CO), and a transmission spectrum of the etalon filter arranged for the measurement of carbon monoxide, whereby, referring to the Figures 1 and 5 and still in the manner described by the basic idea of the invention, passbands 13a to 13j of the etalon filter are arranged in pairs to correspond to absorp¬ tion peaks 14a to 14j of the gas to be measured with- in a wave number range S determined by the filter 4 defining the wave number range. The structure shown in Figure 1 further comprises a second collimator lens 15 positioned between the source 2 of radiation and the filter 4 defining the wave number range, by< which lens the radiation to be obtained from the source 2 of radiation can be collimated.
According to Figure 1, the most important part of the sensor 1 is the adjustable etalon filter 7, which is formed of the two parallel window panes 10 and 11 and the spacer to separate them, which spacer accordingly is the piezoelectric actuator 12. The sensor 1 itself comprises the broad-band infrared filter 4, by means of which a desired amount of the absorption peaks 14a to 14j in the transmission spec- trum shown in Figure 5 of the gas to be examined can be defined from the wave number range. The sensor thus comprises further the adjustable etalon filter 7, the collimator lens 5 and the detector 6 sensitive to infrared radiation, all of them located in the same casing in a preferred embodiment of the inven- tion, which casing is indicated by a broken line 1. This way to realize the structure forms a very com¬ pact and durable whole.
By means of a further integrated sensor solu- tion shown in Figure 6, the structure of the sensor 1 can still be simplified by providing the free outer surface of the window pane 10 facing the source 2 of radiation with a broad-band infrared filter 4 and by combining the function of the collimator lens 5 with the window 11 facing the detector 6, by giving the outer surface of the window a curved form. The filter 4 forms a thin film on the surface of the window pane 10. As before, an external source 2 of radiation is also needed in the * measuring arrangement, which source emits broad-band radiation in the infrared region. The sample space 3, such as a gas flue, is located between the source 2 of radiation and the sensor 1.
The sensor 1 is used for spectroscopic measure- ments, for measuring and analyzing radiation coming from the external source 2 of radiation through the sample space 3 containing gases to the detector 6. The measurement is based on the radiation being ab¬ sorbed by the gas components of the sample space 3, which is seen as a decreasing intensity of the radia¬ tion coming to the detector 6. Strong absorption belts of different gas components are usually located within different wave number ranges, and even if they overlap, the distances between periodic series of peaks differ from each other. This makes the sensor a very selective concentration meter.
The upper part of Figure 5 shows the transmis¬ sion spectrum of carbon monoxide (CO) set forth as an example within the strongest absorption belt area and the lower part of Figure 5 shows the transmission spectrum of the etalon filter arranged for the measu¬ rement of the left branch. The peaks 14a to 14j di¬ rected downwards in the gas spectrum represent the areas of strong absorption. The distances between the peaks increase slowly when moving to the right, but within the narrower area S defined by the filter the distances are very equal, the passbands 13a to 13j of the etalon filter corresponding in pairs to the ab¬ sorption peaks 14a to 14j of the gas. In the starting situation of a measuring period the piezo actuator is without voltage and its thickness is dimensioned in such a manner that the passbands 13a to 13j come be¬ tween the absorption peaks 14a to 14j and the radia¬ tion coming to the detector is at its strongest. When the control voltage is gradually raised, the distance between the windows increases and the passbands move to the right. The radiation obtained by the detector decreases simultaneously and reaches the minimum when the passbands 13a to 13j and the absorption peaks 14a to 14j are opposite to each other. At the maximum control voltage value corresponding to a displacement measuring the distance between two adjacent transmis¬ sion peaks whatever, the passbands 13a to 13 come to the area of weak absorption again. The same change in a detector signal is repeated when the control vol¬ tage is gradually lowered towards the starting situa¬ tion. In a preferred embodiment of the invention, the adjustment range dd of the distance between the win¬ dow panes for going through the defined wave number range S corresponds substantially to the distance between two adjacent passbands of the etalon filter, the passbands 13a and 13b, for instance, which makes the use of the sensor structure of the invention as appropriate and effective as possible. A change in a signal (I) depends on the concen- tration (p) of a gas component according to the equa¬ tion
k p = lnCI^/I^
where k is a coefficient representing the intensity of absorption. Since the concentration depends only on the ratio of the signals, no special demands are made on the stability of the source of radiation, especially as the duration of a measuring period is relatively short.
The spectral transmission of etalon depends primarily on the distance d between the windows and on a reflection coefficient R of the coatings. The average wavelengths lambda-,- of the passbands are determined by the equation
m lambda,-. = 2 n d,
where m is an order of the passband and n is the in¬ dex of refraction of a substance between the windows, n(air) = 1. On a wave number scale the passbands are located at equal distances
Vm = l/lambdam = m/(2nd)
the distance being l/(2nd), and a finesse F of eta¬ lon, representing the ratio of the distance between the passbands to the width of the passband, is deter- mined by the equation
FR = 3,14 R12 /• (1-R).
In practice, the finesse value can be decreased by non-idealities of the structure, such as an uneven- ness of the reflective surfaces and a deflection of the surfaces from the parallel direction. These are described by another finesse value
FF = M/2,
where M represents the ratio of the non-idealities to the wavelength in such a manner that the smoothness of the surface is lambda/M. The total finesse is now obtained from the formula
1/F2 = 1/FR 2 + 1/FF 2.
In the solution of the invention, the CO sensor chosen as an example can be allowed to have 10 times looser manufacturing tolerances than the prior art structures have.
The applicability of the structure of the in- vention to the measurement of various gas components can be examined by means of Table 1. The table shows the most important dimensional parameters for a few interesting gases, arranged for the measurement of the strongest absorption belt of each gas (v = posi- tion of belt on a wave number scale). Though the spectrum peak systems of all gases studied are not as regular as those of carbon monoxide, groups located at sufficiently equal distances can be found by se¬ lecting from the absorption peaks. The parameters of the table are the order m of the midmost transmission band of the etalon filter, the thickness d of the piezo actuator and the change dd in the thickness of the piezo actuator, the change corresponding to a displacement measuring the distance l/2nd) between the passbands in the transmission spectrum of the ilte .
Table 1
Dimensioning of an adjustable etalon filter for the measurement of various gas components.
CO
C02
CH4
NO
N02
N20
NH3
S02
H2S
HC1
HF
Figure imgf000014_0001
The applicability of the solution of the inven¬ tion to the measurement of various gases depends sub¬ stantially on the relative change in thickness (dd/d = 1/m) required of the piezo actuator. In generally used piezo actuators manufactured of PZT type mate¬ rials, this is of the order 0.1 to 0.15 %, which suf¬ fices well for the measurement of carbon dioxide (C02), laughing gas (N20) and sulphur dioxide (S02). By means of some newer materials (e.g. PLZT and PMN) changes in thickness of 0.2 % have been achieved, which makes it possible to measure carbon monoxide (CO), nitrogen monoxide '(NO) and nitrogen dioxide (N02), in addition to the previous gases. The rest of the gases studied require actuators even more effec- tive than this. Of the actuator materials mentioned, PLZT is transparent at wave number values higher than 1500 1/cm, which makes it possible to measure the same gas components, except for S02, also with a sim- pier etalon of plate type. Of the parameters of Table 1, the actuator thickness (d) and its change (dd) shall then be divided by the index of refraction (about 2.4) of PLZT.
In one preferred embodiment of the invention according to Figure 2, the spacer functioning as a piezoelectric actuator is formed of a disc 12, the middle part of which comprises an opening 16, through which radiation can propagate. The structure of this solution is simple and does not place any restric- tions on the selection of the material of the window panes 10 or 11. The embodiment shown in Figure 2 is similar to that of Figure 1, i.e. an embodiment in which the middle part of the piezoelectric actuator 12 comprises an opening. In one preferred embodiment of the invention according to Figure 3, the spacer is formed of at least three actuators 12a to 12c positioned symmetri¬ cally between the windows 10 and 11. Between the ac¬ tuators are positioned detectors 17a to 17c (3 detec- tors) measuring the distance between the window panes. This solution is more complicated than the previous one, but it facilitates the manufacture of the construction as far as the demand on the windows being parallel is concerned, for the piezo actuators 12a to 12c can be controlled separately in such a manner that small errors will be amended. The dis¬ tance measuring detectors 17a to 17c are for instance capacitive detectors.
In one preferred embodiment of the invention according to Figure 4, the whole etalon filter 7 con- sists of a transparent piezoelectric disc 12, the surfaces of which are provided with partially re¬ flective coatings 8 and 9, which are in this case also electrically conductive, since the control vol- tage is connected to the actuator through them. Then the spacer, i.e. the actuator 12, forms a plate-like etalon filter. This solution has the simpliest struc¬ ture of all, but makes various demands on the materi¬ als used. In the embodiments according to the Figures 1 to 3, the partially reflective coatings 8 and 9 are positioned on the inner surfaces of the window panes 10 and 11. The embodiment of Figure 4 does not show any actual separate window panes, but the partially reflective coating is positioned on the surface of the piezoelectric actuator 12. Consequently, as to the embodiment of Figure 4, the definition of the existence of the window panes in the wording of the main claim shall be understood in such a way that the partially reflective coating includes both the coat- ing and the window pane, even though this structure does not actually show any separate window panes. The solution of Figure 4 has a good producibility, for the demands on dimensional accuracy can be met more easily, since there are no separate window panes, but the surfaces of the piezoelectric spacer are provided with the coatings 8 and 9.
Even though the invention has been described above with reference to the examples according to the enclosed drawings, it is clear that the invention is not restricted to them only, but it can be modified in many ways within the scope of the inventive idea set forth in the enclosed claims.

Claims

Claims :
1. A sensor, especially a gas sensor, which is used for infrared spectroscopic measurements of con- centration, for analyzing a radiation coming from an external source (2) of radiation through a sample space (3) containing one gas or several gases, the sensor comprising, besides a filter (4) defining a wave number range, a collimator lens (5) and an in- frared detector (6), also an adjustable filter (7) of Fabry-Perot type, which filter is formed of two par¬ allel window panes (10 and 11) at least partially coated with a partially reflective coating (8 and 9) and of a piezoelectric actuator (12), by means of which the distance (d) between the window panes can be adjusted by a voltage switched over the actuator (12), and passbands (13a to 13 ) of which filter (7) are arranged in pairs to correspond to absorption peaks (14a to 14 ) of a gas to be measured within a wave number range (S) determined by the filter (4) defining the wave number range, c h a r a c t e r i z e d in that the piezoelectric actuator (12) is located between the reflective coat¬ ings of the window panes (10 and 11) to form one spacer or several spacers with a thickness essential¬ ly corresponding to the distance between the reflec¬ tive coatings (8 and 9), the piezoelectric spacer forming together with the window panes the adjustable etalon filter. 2. A sensor according to claim 1, c h a r a c¬ t e r i z e d in that the piezoelectric spacer is formed of a disc, the middle part of which comprises an opening (16).
3. A sensor according to claim 1, c h a r a c- t e r i z e d in that the piezoelectric spacer is formed of at least three separate discs (12a to 12c) positioned symmetrically between the windows and that detectors (17a to 17c) measuring the distance (d) between the windows (10 and 11) are positioned be- tween the discs.
4. A sensor according to claim 1, c h a r a c¬ t e r i z e d in that the piezoelectric material of the spacer is transparent within the used wave number range and that the partially reflective coatings (8 and 9), which in this case are also electrically con¬ ductive, are located on the surfaces of the spacer instead of those of the windows, and then the actu¬ ator (12) itself forms the plate-like etalon filter.
5. A sensor according to one of the foregoing claims 1 to 3, c h a r a c t e r i z e d in that the broad-band infrared filter (4) forms a thin film on the free outer surface of the window pane (10) facing the source (2) of radiation and the function of the collimator lens (5) is combined with the win- dow (11) facing the detector (6) by giving the outer surface of the window a curved form.
AMENDED CLAIMS
[received by the International Bureau on 22 March 1993 (22.03.93); original claim 1 amended; remaining claims unchanged (2 pages)]
1. A sensor, especially a gas sensor, which is used for infrared spectroscopic measurements of con¬ centration, for analyzing a radiation coming from an external source (2) of radiation through a sample space (3) containing one gas or several gases, the sensor comprising, besides a filter (4) defining a wave number range, a collimator lens (5) and an in¬ frared detector (6) , also an adjustable filter (7) of Fabry-Perot type, which filter is formed of two par¬ allel window panes (10 and 11) at least partially coated with a partially reflective coating (8 and 9) and of a piezoelectric actuator (12) , by means of which the distance (d) between the window panes can be adjusted by a voltage switched over the actuator
(12), and passbands (13a to 13j) of which filter (7) are arranged in pairs to correspond to absorption peaks (14a to 14j) of a gas to be measured within a wave number range (S) determined by the filter (4) defining the wave number range, c h a r a c t e r i z e d in that the piezoelectric actuator (12) is located between the reflective coat- ings of the window panes (10 and 11) to form one spacer or several spacers with a thickness essential¬ ly corresponding to the distance between the reflec¬ tive coatings (8 and 9) , the piezoelectric actuator spacer forming together with the window panes the etalon filter being piezoelectrically adjustable in the area between the reflective coatings (8, 9).
2. A sensor according to claim 1, c h r a c¬ t e r i z e d in that the piezoelectric spacer is formed of a disc, the middle part of which comprises an opening (16) . T8
3. A sensor according to claim 1, c a r a c¬ t e r i z e d in that the piezoelectric spacer is formed of at least three separate discs (12a to 12c) positioned symmetrically between the windows and that detectors . (17a to 17c) measuring the distance (d) between the windows (10 and 11) are positioned be¬ tween the discs.
4. A sensor according to claim 1, c h a r a c¬ t e r i z e d in that the piezoelectric material of the spacer is transparent within the used wave number range and that the partially reflective coatings (8 and 9) , which in. this case are also electrically con¬ ductive, are located on the surfaces of the spacer instead of those of the windows, and then the actu- ator (12) itself forms the plate-like etalon filter.
5. A sensor according to one of the foregoing claims l o 3, c h a r a c t e r i z e d in that the broad-band infrared filter (4) forms a thin film on the free outer surface of the window pane (10) facing the source (2) of radiation and the function of the collimator lens (5) is combined with the win¬ dow (11) facing the detector (6) by giving the outer surface of the window a curved form.
PCT/FI1992/000294 1991-10-31 1992-10-30 Sensor WO1993009422A1 (en)

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GB2286041A (en) * 1994-01-20 1995-08-02 Marconi Gec Ltd High resolution infared spectroscope
GB2314617A (en) * 1996-06-24 1998-01-07 Graviner Ltd Kidde Gas detection system employing a comb filter matched to target gas absorption
EP0883791A1 (en) * 1995-06-07 1998-12-16 Northrop Grumman Corporation Interferometer
ES2155355A1 (en) * 1998-11-04 2001-05-01 Univ Madrid Carlos Iii Motor vehicle exhaust contaminants concentration meter consists of an infra red spectral absorption assembly with a receiver distinguishing multiple contaminants
WO2001046740A2 (en) * 1999-12-23 2001-06-28 Michael Scalora Apparatus and method for controlling optics propagation based on a transparent metal stack
WO2005047647A1 (en) * 2003-11-10 2005-05-26 Baker Hughes Incorporated A method and apparatus for a downhole spectrometer based on electronically tunable optical filters
US7957004B2 (en) 2005-04-15 2011-06-07 Sinvent As Interference filter
CN102735273A (en) * 2012-06-29 2012-10-17 中国科学院半导体研究所 Optical fiber sensor based on Fabry-Perot cavity
EP2770319A1 (en) * 2013-02-25 2014-08-27 Sick Ag Gas measuring device
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US20180164215A1 (en) * 2016-12-13 2018-06-14 Infineon Technologies Ag Gas analyzer
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EP0608049A3 (en) * 1993-01-13 1995-01-18 Vaisala Oy Single-channel gas concentration measurement method and apparatus.
EP0608049A2 (en) * 1993-01-13 1994-07-27 Vaisala Oy Single-channel gas concentration measurement method and apparatus
GB2286041B (en) * 1994-01-20 1998-04-08 Marconi Gec Ltd High resolution infrared spectroscope
GB2286041A (en) * 1994-01-20 1995-08-02 Marconi Gec Ltd High resolution infared spectroscope
EP0930481A2 (en) * 1995-06-07 1999-07-21 Northrop Grumman Corporation Interferometer
EP0883791A1 (en) * 1995-06-07 1998-12-16 Northrop Grumman Corporation Interferometer
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EP0930481A3 (en) * 1995-06-07 1999-09-15 Northrop Grumman Corporation Interferometer
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GB2314617A (en) * 1996-06-24 1998-01-07 Graviner Ltd Kidde Gas detection system employing a comb filter matched to target gas absorption
GB2314617B (en) * 1996-06-24 2000-08-23 Graviner Ltd Kidde High sensitivity gas detection
ES2155355A1 (en) * 1998-11-04 2001-05-01 Univ Madrid Carlos Iii Motor vehicle exhaust contaminants concentration meter consists of an infra red spectral absorption assembly with a receiver distinguishing multiple contaminants
WO2001046740A2 (en) * 1999-12-23 2001-06-28 Michael Scalora Apparatus and method for controlling optics propagation based on a transparent metal stack
WO2001046740A3 (en) * 1999-12-23 2001-12-13 Michael Scalora Apparatus and method for controlling optics propagation based on a transparent metal stack
US6339493B1 (en) 1999-12-23 2002-01-15 Michael Scalora Apparatus and method for controlling optics propagation based on a transparent metal stack
WO2005047647A1 (en) * 2003-11-10 2005-05-26 Baker Hughes Incorporated A method and apparatus for a downhole spectrometer based on electronically tunable optical filters
US7362422B2 (en) 2003-11-10 2008-04-22 Baker Hughes Incorporated Method and apparatus for a downhole spectrometer based on electronically tunable optical filters
US7957004B2 (en) 2005-04-15 2011-06-07 Sinvent As Interference filter
CN102735273A (en) * 2012-06-29 2012-10-17 中国科学院半导体研究所 Optical fiber sensor based on Fabry-Perot cavity
CN102735273B (en) * 2012-06-29 2014-11-05 中国科学院半导体研究所 Optical fiber sensor based on Fabry-Perot cavity
EP2770319A1 (en) * 2013-02-25 2014-08-27 Sick Ag Gas measuring device
US9360417B2 (en) 2013-02-25 2016-06-07 Sick Ag Gas measurement device
CN107024443A (en) * 2015-09-25 2017-08-08 德尔格制造股份两合公司 Method for the signal detection of gas analysis system
CN107024443B (en) * 2015-09-25 2019-12-10 德尔格制造股份两合公司 Method for signal detection of a gas analysis system
US10466174B2 (en) * 2016-12-13 2019-11-05 Infineon Technologies Ag Gas analyzer including a radiation source comprising a black-body radiator with at least one through-hole and a collimator
US20180164215A1 (en) * 2016-12-13 2018-06-14 Infineon Technologies Ag Gas analyzer
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FI915153A0 (en) 1991-10-31
FI915153A (en) 1993-05-01
FI91564C (en) 1994-07-11

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