FI91564C - Sensor - Google Patents

Description

915 64

Sensor

The invention relates to a sensor, in particular to a gas sensor which, in the infrared spectroscopic measurements of the kSytetSS, can be used to analyze one or more gases contained in the sample space from the external radiation source. a filter 10 consisting of two parallel and at least partially reflective coated at least partially coated window panes and a piezoelectric actuator by means of which the distance between the window panes can be adjusted by means of a voltage applied across the actuator. absorption peaks in the wavelength range determined by the filter delimiting the wavelength range.

The present invention relates, for example, to concentration measurements of gas components in flue gases, and kek-20 sintbS can be utilized e.g. in measurements of energy production and measurements of air pollution concentrations. In smoke-gas measurements, the infrared method is hampered by the high water vapor content of the flue gas, and in order to eliminate it, the gas sample to be measured has to be dried in many cases before the measurement. Measurement directly from the flue has significant advantages, but it places special demands on the characteristics of the infra-red analyzer, the most important of which is good spectral resolution. Of the known analyzer types, FTIR spectrometers and Fabry-Perot spectrometers have sufficient resolution.

Fabry-Perot has a simpler structure, is cheaper and is more suitable for measuring a single gas component.

The present solution is a Fabry-Perot type 35. The central part of the Fabry-Perot spectrometer consists of 2 91564 two window panes separated by a narrow air gap. Ik-kunal plates are transparent and their inner surfaces are partially coated with a reflective coating. The transmission spectrum of such a structure consists of a plurality of evenly spaced passbands in the wavenumber-5 region, which are displaced by moving one of its windows. Known spectrometer solutions utilize only one passband at a time and the flow range corresponding to the distance between two consecutive passbands is delimited by 10 separate filters. A piezoelectric actuator is commonly used to move a window, which elongates due to the voltage applied to it and moves the windows closer together.

One known solution is the method and apparatus described in DE-15 37 06 833 for Fabry-Perot spectroscopic measurement. The device disclosed in that publication comprises a piezoelectric actuator between the window panels, with which the distance between the panels is changed. However, in the solution presented in that publication, the length of the actuator 20 is considerably longer than the distance between the reflective inner surfaces of the window panels in order to obtain a sufficiently long travel for the filter, since the travel of the actuator is proportional to its length.

. In the structure disclosed in said publication, the actuator 25 is placed in the edge area of the window panels in a space which is thus considerably longer, i.e. longer than the distance between the reflecting surfaces, and it is thus clear that the present invention does not accomplish the present invention. significant benefits related to the shortness of the required business trip. This structure therefore does not represent a pure Etalon structure.

DE-39 39 359, on the other hand, discloses a Fabry-Perot type spectrometer, the filter part of which comprises piezoelectric actuators for changing the distance 35 of the window panes, but these actuators are placed on the plate plate.

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3 91564.

There are several problems with known Fabry-Perot spectrometer designs. Good spectral resolution combined with a wide range of conditions requires good surface evenness from the windows 5 and good parallelism from the inner surfaces of the windows. Since in known constructions the windows do not support each other, the parallelism requirement requires high dimensional accuracy from the spectrometer body, windows and piezo actuator and low thermal expansion from the materials used. In addition, a wide range of settings requires a large piezo actuator, which makes it difficult to integrate the structure into a small sensor.

The object of the present invention is to provide a new-15 type sensor design, which is mainly suitable for gas measurements, by means of which the problems associated with known designs can be significantly reduced. for reflective coatings to a spacer whose strength substantially corresponds to the distance between the reflective coatings, the piezoelectric spacer together with the coated window panes forming a beam Etalon filter.

The solution according to the invention is based on a new way of positioning the piezo actuator of a Fabry-Perot spectrometer and on a new way of performing the flattening of a certain spectral range by utilizing the periodicity of the spectral peaks of the 30 gas components.

: The sensor solution according to the invention achieves several significant advantages. The use of several (n) passbands (instead of one) for flattening a certain spectral range increases the distance between the windows n-fold. When the required 35 Travel distance remains the same, the relative change in thickness required from the piezo actuator placed between the windows is reduced to 1 / n part, which makes it possible to measure several gas components with already known piezoelectric materials. When the same bandwidth is used for the 5 passbands in both cases, the required finesse value is reduced to 1 / n part, which increases the permissible errors for the surface smoothness of the windows and the parallelism of the inner surfaces by a factor of n. In addition, even the relaxed dimensional accuracy requirements only apply to the piezo-10 tuner, because the windows support each other through it. Another embodiment of the invention, which is possible piezoelectric when the material is transparent, is even simpler in structure. The windows are omitted and the partially reflective coatings, which in this case are also electrically conductive, are located on the surfaces of the piezo actuator, whereas in other embodiments according to the invention the partially reflective coating is on the inner surfaces of both window panels. Reduced tolerance requirements facilitate the fabrication of the structures of the invention and reduce manufacturing costs. In addition, the constructions can be made small, so that their integration becomes a compact sensor together with the components closely related to them. possible.

The invention will now be described in more detail with reference to the accompanying drawings, in which Fig. 1 shows the structure and location of a sensor in a measuring environment, Fig. 2 -matt embodiment, 35 Figure 5 shows the infrared spectrum of carbon monoxide

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5 91564 and the blade spectrum of an Etalon filter adapted for carbon monoxide measurement, Figure 6 shows a further integrated sensor solution.

Figure 1 schematically shows the structure and location of a sensor according to the invention in a measuring environment. The sensor 1 is in particular a gas sensor which is used in infrared spectroscopic concentration measurements to analyze the incoming rain from the external rain source 2 and the sample space 3 and 10. The sample space 3 can be, for example, a flue gas duct which contains one or more gases. The sensor 1 encapsulates a Fabry-Perot type Etalon filter 7, 15 available in addition to a wavelength-limiting filter 4, a collimator lens 5 and an infrared detector 6. from the actuator 12, with which the distance d of the window panels can be obtained by means of a voltage U connected across the actuator 12.

20 The tension can be connected to the actuator, for example by coating its upper and lower surfaces with a metal foil. According to the basic idea of the invention, the piezoelectric actuator 12 is a spacer placed between the window panels 10 and 11, which unambiguously determines the silence distance '25d of the window panels 10 and 11 and together with the window panels 10 and 11 forms an available Etalon filter 7. Figure 5 shows in this case, the infrared spectrum of carbon monoxide (CO) and the transmission spectrum of a Etalon filter adapted for carbon monoxide measurement, wherein with further reference to Figures 1 and 30 5, the fast The structure of the delimiting filter 4 in the wavelength range S-35 The structure-35 shown in Fig. 1 further encounters a rain collector 2 and a second collimator lens 15 interposed between the wavelength delimiting filter 4, with which the radiation from the radiation source 2 can be combined.

According to Fig. 1, the most important part of the sensor 1 is a passable Etalon filter 7, which consists of two unidirectional window plates 10 and 11 and a spacer separating them, thus operating a piezoelectric actuator 12. The sensor 1 itself accommodates a wideband infrared from the wavenumber range, the desired -10 tu can be delimited from the absorption peaks 14a to 14j of the transmission spectrum of the gas to be examined shown in Fig. 5. Thus, the sensor further comprises an adjustable Etalon filter 7, a collimator lens 5 and an infrared-sensitive detector 6, all of which, in a preferred embodiment of the invention, are housed in the same housing, which is represented by the dashed line 1.

This implementation forms a very compact and durable whole.

With the longer integrated sensor solution shown in Fig. 6, the structure of the sensor 1 can be further simplified by manufacturing a wideband infrared filter 4 on the free outer surface of the radiation source 2 side window plate 10 and combining the function of the collimator lens 5 with the detector 6 side window 11. The filter 4 is formed as a thin film on the surface of the window panel 10. In addition, as before, an external radiation source 2, which emits broadband radiation in the infrared range, is required in the measurement arrangement. The sample space 3, such as a flue, is located between the radiation source 2 and the sensor 1.

The sensor 1 is used in spectroscopic measurements to measure and analyze the radiation coming from the external radiation source 2 to the detector 6 through a sample space 3 containing gases. The measurement is based on the absorption of the radiation into the gas components of the sample space 3, which can be seen in the attenuation of the incoming radiation to the detector 35 6. Different gas components

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7 91564 la strong absorption bands are usually located in different wavelength ranges and even in overlapping parts the distances of the periodic peak series differ from each other. This makes the sensor a very selective concentration meter.

For example, the transmission spectrum of carbon monoxide (CO) taken from the region of the strongest absorption band is shown at the top of Figure 5, and the transmission spectrum of an Etalon filter adapted to measure the left branch is shown at the bottom of Figure 5. In the gas spectrum, downward peaks 14a to 14j represent regions of strong absorption. The vfil distance of the peaks increases slowly to the right in the mentfies, but is very even in the narrow-bound, filter-bounded region S, whereby the passages 13a to 13j of the Etalon filter correspond in pairs to the gas absorption peaks 15a to 14j. In the initial situation of the measurement period, the piezoelectric actuator is de-energized and its thickness is dimensioned so that the passbands 13a to 13j occur at the intervals of the absorption peaks 14a to 14j and the radiation to the detector is at its strongest. As the control voltage is gradually increased by 20 degrees, the distance between the windows increases and the passbands move to the right. At the same time, the radiation received by the detector is reduced and reaches a minimum when the passbands 13a to 13j and the absorption peaks 14a to 14j are aligned. With a maximum value of the control voltage corresponding to the transmission of the passage peaks between some two adjacent passage peaks, the passbands 13a to 13j again enter the low absorption region. The same change in the detector signal is repeated when the control voltage is gradually lowered towards the initial situation. In a preferred embodiment of the invention, the distance adjustment range dd of the window panels for the passage of the limited wavelength range S substantially corresponds substantially to the distance between the two passing passes of the Etalon filter, e.g.

8,91564

The change in the signal (I) depends on the concentration (p) of the gas component, respectively k k = ln (Iæajt / Imin) where k is a factor describing the absorption strength. Since the concentration depends only on the ratio of the signals, no special requirements are placed on the stability of the sedimentation beam when the duration of the measurement period is also relatively short.

The spectral blade of the standard depends primarily on the distance d between the windows and the reflectance R 10 of the coatings. -15 kolla fasting lanes are evenly spaced V „= l / lamdam = m / (2nd) with a silence distance of l / (2nd) and Etalon's finesse F, which describes the ratio of the fasting distance of the fasting lanes to the width of the fasting lane, is determined from 20 FR = 3.14 R1 / 2 / (1-R).

In use, the value of finesse can be reduced by non-idealities of the structure, such as unevenness of the reflecting surfaces and deviation of the parallelism of the surfaces. The woman is described as the second finesse with a value of 25 Ff = M / 2, where M represents the ratio of the idealities to the wavelength, so that the smoothness of the surface is lamda / M. The total finesse is now obtained from the formula 1 / F2 = 1 / Fr2 + 1 / Ff2.

For example, in the solution according to the invention, manufacturing tolerances 10 times higher than in previously known constructions can be allowed for the selected CO sensor.

The suitability of the construction according to the invention for measuring the various gas components can be examined by means of Table 91564 9 kon 1. The table shows the most important design parameters for some of the gases of interest, adapted to the measurement of the strongest absorption velocity of each gas (v = position of the belt on the wave reading scale). Although not all of the gases considered are as regular as carbon monoxide, groups of absorption peaks can be found to be sufficiently evenly distributed. The parameters in the table are the order m of the middle 1M band of the Etalon filter, the thickness d of the piezo-10 actuator and the change dd of the piezo actuator thickness, which corresponds to the shift according to the fast (l / 2nd) of the fasting bands in the filter flattening spectrum.

Table 1 Dimensioning of the resulting Etalon filter for the measurement of different gas components.

vmd dd [1 / cm] [mm] [pm] 20 CO 2170 610 1.4 2.3 C02 2330 1300 2.8 2.2 CH4 3100 330 0.53 1.6 NO 1900 620 1.6 2.6 25 N02 1630 550 1.7 3.1 N20 2210 2300 5.2 2.3 NH3 1080 57 0.26 4.6 S02 1350 950 3.5 3.7 H2S 1120 110 0.49 4.4 30 HC1 2800 120 0.21 1.8 HF 4100 120 0.15 1.3

The suitability of the solution according to the invention for the measurement of different gases depends essentially on the relative thickness change required from the piezo actuator 35 (dd / d = 10 91564 1 / m). For commonly used piezo actuators made of PZT-type materials, this is of the order of 0.1 to 0.15%, which is good enough for measuring carbon dioxide (CO2), nitrous oxide (N2O) and sulfur dioxide (SO2).

5 Some new materials (eg PLZT and PMN) have achieved 0.2% thickness changes, which makes it possible to measure carbon monoxide (CO), nitrogen monoxide (NO) and nitrogen dioxide (NO2) in addition to the above. The other gases considered call for even more efficient actuators. Of the actuator materials mentioned above, PLZT is transparent at wavelength values higher than 1500 1 / cm, in addition to allowing the measurement of the same gas components with the simpler, plate-type Etalon, except for SO2. From the parameters in Table 1, the thickness (d) of the actuator and its change (dd) must be divided by the refractive index of PLZT (approx. 2.4).

In a preferred embodiment of the invention, according to Figure 2, the spacer plate acting as a piezoelectric actuator consists of a disc 12, the central part of which comprises an opening 16 through which the radiation can pass. This solution is simple in construction and does not place any restrictions on the choice of material for the window panels 10 or 11. The embodiment shown in Fig. 2 is similar to that in Fig. 1, i.e. one in which the central part of the piezoelectric actuator 12 covers the opening.

In a preferred embodiment of the invention, according to Figure 3, the spacer consists of at least three actuators 12a to 12c arranged symmetrically between the windows 10 and 11. Sensors 17a to 17c (3 pcs.) Are placed between the actuators, which measure the distance between the window panes. This solution is more complicated than the previous one, but it facilitates the construction of the construction with regard to the parallelism requirement of the windows, since the Piet actuators 12a to 12c can be controlled separately so that small errors are corrected. The distance measuring sensors 17a to 17c are, for example, capacitive sensors.

In a preferred embodiment of the invention, as shown in Figure 4, the entire Etalon filter 7 consists of a speZS-5 capable piezoelectric disk 12, on the surfaces of which are located partially reflective coatings 8 and 9, which in this case are also electrically conductive. vfilityksellS. The spacer of the TS1-15, i.e. the actuator 12, forms a plate-like Eta-10 lon filter. This solution is the simplest in structure, but it imposes a variety of requirements on the materials used. In the embodiments according to Figures 1-3, the partially reflective coatings 8 and 9 are placed on the inner surfaces of the window panels 10 and 11. In the embodiment according to Fig. 15 4, no actual separate window panels are present, but a partially reflective coating is placed on the surface of the piezoelectric actuator 12. Thus, the definition of the existence of window panels in the wording of the main claim for the embodiment 20 according to Figure 4 should be understood as meaning that the & partially reflective coating comprises both the coating and the window panel, although in the construction, the actual separate window panels do not occur.

Although the invention has been described above with reference to the examples according to the accompanying drawings, it is clear that the invention is not limited only to them, but can be modified in many ways within the scope of the inventive idea set out in the appended claims.

Claims (4)

An encoder, especially a gas sensor, which is used in infrared spectroscopic measurements for analyzing radiation from an outer radiation shaft (2) through a sample space (3) containing one or more gases, the encoder comprising a filter (4), which delimits a waveguide range, a collimator lens (5) and an infrared detector (6), and a Fabry-Perot type adjustable filter (7), which consists of two parallel and partially reflective coatings (8.9 ) at least partially coated window plates (10,11) and a piezoelectric actuator (12), by which the distance (d) between the window plates can be controlled by a voltage coupled over the actuator (12), and the passband of the filter (7) (13a-13j) are arranged to correspond in pairs to absorption peaks (14a-14j) in the gas fed within the wave range (S) determined by the wave range bounding filter (4), characterized by the fact that the piezoelectric actuator (12 ) are placed between n the reflective coatings (8.9) of the window plates (10 and 11) as an intermediate plate, the thickness of the spring substantially corresponds to the distance between the reflective coatings (8.9), the piezoelectric intermediate plate along with those of the coating (8.9). ) the provided window plates (10, 11) form an adjustable Eta-lon filter. 2. Sensor according to claim 1, characterized in that the intermediate plate is constituted by a disc, the middle part of the spring comprising an opening (16). 3. Sensor according to claim 1, characteristic; as a result, the intermediate plate is made up of at least three separate symmetrically between the disks (12a-12c) and that the disks (17a-17c) are placed between the disks which measure the distance (d) between the disks (10,11). 4. Sensor according to claim 1 or 2, characterized in that the piezoelectric material in the intermediate plate is transparent within the applied waveguide range and that the partially reflecting coatings (8, 9), as in this case in addition, it is conductive, located on the surfaces of the intermediate plate in the steel of the windows, the actuator (12) forming a disc-shaped Etalon filter.