WO2000036401A1

WO2000036401A1 - Method and device for evaluating spectroscopic measurements on solid materials with spatially and/or time-variable surfaces

Info

Publication number: WO2000036401A1
Application number: PCT/DE1999/003866
Authority: WO
Inventors: Herbert Furumoto; Uwe Lampe; Jens MÜHLSTEFF
Original assignee: Siemens Aktiengesellschaft
Priority date: 1998-12-15
Filing date: 1999-12-02
Publication date: 2000-06-22
Also published as: DE19857896C1

Abstract

If materials of spatially and/or time-variable surfaces are measured by spectroscopic measurement, the varying distance to the surfaces results in substantial error rates when the signal intensities are detected and evaluated on the basis of said spectrums. According to the invention, the derivations in terms of the wavelength of continuously detected spectrums are evaluated instead of their intensities or absorptions, thereby removing the effects of the spatially and/or time-variable distance of the material surface to the source of radiation. Additionally, a series of mechanical measures are provided which minimize the unavoidable varying distances between sensor and sample already before the measurements are taken.

Description

description

Method and device for evaluating spectroscopic measurements on solid materials with spatially and / or temporally varying surfaces

The invention relates to a method for evaluating spectroscopic measurements on solid materials with spatially and / or temporally varying surfaces, wherein electromagnetic radiation in the predetermined wavelength range is radiated into the material by a measuring device with at least one radiation source and after interaction of the radiation with the material the intensity of the interaction signal is measured over at least one detector over a continuous, predetermined wavelength range. In addition, the invention also relates to associated devices for carrying out the method.

Spectroscopy is increasingly used for industrial applications. In terms of measurement technology, there is a radiation source with which electromagnetic radiation is radiated into a measurement sample and at least one detector as a sensor with which the radiation reflected by the sample or transmitted by the sample is evaluated with regard to intensity or absorption by the sample. necessary.

In practice, the distance between the sample surface and the radiation source naturally fluctuates in spectroscopic measurements on samples made of solid material. Since the radiation intensity of electromagnetic radiation decreases quadratically with the distance, the measured intensities depend not only on the nature of the sample that is actually to be determined, but also on the spatially and / or temporally variable distance between the sample and the sensor. This leads to the so-called flutter amplitude problem known in practice. The latter problem is particularly relevant in the case of spectroscopic measurements on material beds on conveyor belts, where there is naturally a widely varying surface. This is the case, for example, when filling wood chips on a moving conveyor belt

Feeding a stove in the production of pulp according to DE 195 10 008 C2. As a further example, there are also described spectroscopic measurements on waste paper, wherein the measurement can be carried out either on bales or a bed of the waste paper. In both cases, the waste paper is transported in a pulper, where it is broken down into a pulp from which recycled paper is made.

However, the above problem also arises in spectroscopic measurements on material webs in which the material surface, which is in itself flat, oscillates in time in the sense of vibrations or at least varies in height. An example of this phenomenon is a paper web running fast in a paper machine, on which spectra are to be recorded, for example in accordance with DE 198 30 323 A1.

The object of the invention is therefore to propose methods of the type mentioned at the outset and to provide associated devices with which material-specific results are measured in the case of spatially and / or temporally changing distances between the radiation source or detector and sample surface and in which the disturbing metrological boundary conditions are thus eliminated .

According to the invention, the object is achieved with regard to the method by the measure of claim 1 and with respect to the associated device by the entirety of the features of claim 12. Further training is specified in the respective subclaims. With the invention it is achieved in particular by means of evaluation technology that the problem of “flattera plitude *”, which has so far not been solved satisfactorily, can be neglected. It has surprisingly been found that by using the derivatives of intensity according to the wavelength, ^ out-of-focus' positions can be compensated for, since the relative intensity ratios remain the same. In addition, other mathematical methods, such as vector normalization, minimum-maximum normalization, as well as scaling and / or centering measures and also the use of so-called wavelets can be used. Such measures known from the prior art prove to be effective for the claimed solution to the problem.

In addition to the evaluation measures, it is also proposed to improve the setup for the measurement, including the material guidance, by appropriate mechanical means. In particular, if there is material on a moving guide - such as conveyor belts or the like. - Measured, the material management can be designed so that undesirable sources of error are already minimized. Likewise, unwanted surface movements of material webs running through can be minimized by suitable mechanical guidance of the material web, for example by rolling in front of and behind the measuring device.

Further details and advantages of the invention result from the following description of the figures of exemplary embodiments with reference to the drawing in conjunction with further subclaims. Show it

FIG. 1 shows a measuring arrangement for measuring optical spectra on a bulk material on conveyor belts,

FIG. 2 measurement curves of continuous NIR spectra of two different wood materials,

FIG. 3 shows the second derivatives of the absorption curves derived from the spectra from FIG. 2 according to the wavelength, FIG. 4 shows a top view of an arrangement especially for measuring wood chips,

FIG. 5 shows a schematic illustration with a transmission array on one side and a measuring array on the other side of a paper web for transmission measurement in the transverse direction of the running paper web,

6 shows an alternative measuring device to FIG. 5 for traversing measurement over a paper web and

FIG. 7 shows an intensity spectrum of paper in the NIR range with an additional laser intensity peak at a discrete wavelength

In the figures, the same or corresponding parts are provided with the same reference numerals. Some of the figures are described together.

In FIG. 1, a material fill is denoted by 1, which is located on a transport device, for example conveyor belt 4, which moves perpendicular to the paper plane. The individual parts of the material bed 1 on the conveyor belt 4 can e.g. Wood chips that are to be transported to a pulp cooker.

The material bed 1 has, after the filling onto the conveyor belt 4, a spatial formation caused by the bed and / or material properties and thus a locally varying surface. Since the material bed 1 is moved with the conveyor belt 4, the material surface, in particular the distance to a measuring device, can also change over time.

Spectroscopic measurements are to be carried out on the bulk material 1 in order to obtain response signals which provide information about the material properties of the bulk material 1. For this purpose there are, for example, two light sources 10 and 20, with which light emitted in a predetermined spectral range via lenses 11 and 21 into the bulk material 1 is irradiated. After interaction with the material of the bulk material 1, the emitted radiation is detected. For this purpose, a lens 31 is provided, with which the radiation reflected from the material is fed to a spectrometer 50 for outputting continuous absorption spectra in the predetermined wavelength range, for example via a glass fiber 30. The spectrometer 50 in FIG. 1 contains detectors (not shown in detail) for measuring the radiation intensity, from which the absorption caused by the material results as a complementary measurement variable. A computer, for example a PC 100, can be present for evaluating the spectra.

In FIG. 2, absorption spectra 22 and 23 of two different types of wood are plotted as a function of the wavelength in the infrared range between 1 and 2 μm: with 22 there is a spectrum of eucalyptus (“eucalyptus globulus *) and with 23 a spectrum of pine (“ pinus pinaster * ) reproduced. It can be seen that the individual spectra 22 and 23 each have a significant profile over the applied wavelength. This course is specific for the organic components of the wood and is therefore similar in the rough structure for both spectra 22 and 23.

By examining the fine structure over a certain wavelength range of the two absorption spectra 22 and 23, a distinction can only be made between the types of wood from the spectra. It is essential to continuously record the spectra.

However, the absorption spectra 22 and 23 of FIG. 2 also show that the specific differences between the individual types of wood can be very small. However, this has in particular the consequence that the metrological boundary conditions have a determining influence on the measurement results and that different, possibly inaccurate results can be obtained in the event of unstable external measurement conditions. In particular due to the quadratic dependence of the absorption tion of the distance between the measuring point and the radiation source or radiation detector of the sensor can result in considerable falsifications.

FIG. 3 specifically shows the second derivatives of the absorption spectra 22, 23 from FIG. 2 according to the wavelength λ, i.e. the quotients of the differential absorption dA after the differential wavelength change dλ (so-called differential quotient). The individual derivatives in FIG. 3 gave the reference numerals 32, 32 ', 32 ″ corresponding to spectrum 22 from FIG. 2 and 33, 33 ′, 33 ″ corresponding to spectrum 23 from FIG. 2 for the respective materials, despite different absolute intensities of the output spectra especially in the second derivatives of curves 32, 32 ', 32' ', ... and 33, 33', 33 '', ... the same maxima or minima, which are characteristic of the wood types, even with different boundary conditions.

It can be seen from the comparison of FIG. 2 and FIG. 3 that the problem of flutter amplitude is avoided if the intensities themselves are evaluated, rather than the first, second or higher derivatives. Since the intensities at the different wavelengths are weakened in the same way by the distance, the differentiated signals in particular no longer depend on the distance between the sample and the sensor.

The results obtained in this way can in particular be used to model the wood texture or wood quality in a woodchip mixture in order to derive control variables for the process control, for example of a pulp cooker. In principle, the evaluation of the intensities or absorptions can be completely dispensed with. If necessary, the intensities can be used as a sensitive distance signal: probe - sensor in addition to the leads. The second derivative of the measured spectra is particularly well suited for modeling the wood properties, since in Figure 3 the differences between the types of wood can be seen particularly clearly. Modeling with the second derivative delivers better results in practice than modeling with the original spectra.

In addition to the sensors and the related evaluation using chemometric methods, the material management is also of particular importance in the measurement problem described. For this purpose, m FIG. 4 shows how work can be carried out specifically for proper measurement of bulk goods, such as wood chips.

In the plan view in FIG. 4, a woodchip fill 40 is shown in particular, which after filling is carried on a first conveyor belt 41 of a predetermined width. The fill naturally has a varying width and height profile with a characteristic surface, which is only indicated in FIG. 4.

It has been shown that it is useful first of all to widen the bulk material 40 m transverse to the direction of locomotion. For this purpose, there is a second conveyor belt 42, wider than the conveyor belt 41, on which a plurality of V-shaped wipers 43 and 44, each with a differently adjustable height and orientation, are attached. Scraper 43 is, for example, in one piece, while in the case of scraper 44 two parts are arranged relatively close behind one another. The material supplied is thus distributed over the width and any existing surface fluctuations are compensated for to a considerable extent.

One or more rollers 45, 45 ',... Follow on the conveyor belt 42, intensify the above effect and further level the surface of the fill. The continuous spectra are then measured in a measuring device 46, at which advantageously have several light sources and sensors according to FIG. 1 in the direction transverse to the conveyor belt 42.

With the arrangement according to FIG. 4 it is achieved that at

Bulk goods, the wipers 43 and 44 well in front of the actual measuring point 46 level the material to be measured with a defined material height and cover the conveyor belt 42 evenly. In order to achieve uniform coverage of the conveyor belt 42 with the bulk material, the wipers 43 and 44 are arranged in a V-shape opposite to the material flow. In order to adapt the transport performance to the production requirements in practice, it is still possible to specify the height variation of the material wipers 43 and 44. If several wipers 43 and 44 are used, the wiping heights can be different. It is favorable to provide a decreasing height in the direction of the material flow.

As already mentioned, the conveyor belt 42 is wider in the area of the measuring point 46 than along the rest of the transport route 41 in order to provide the required transport performance. It is thus achieved that in the area of the measuring point 46 the material web is covered as completely and evenly as possible. In addition to the roller 45, rollers can also be present behind the scrapers 43 and 44, which run at the same speed as the conveyor belt 42.

In a modification of FIG. 4, it is also possible to dispense with the widening of the conveyor belts. For example, for the application that for the production of recycled paper

Waste paper can be processed, the waste paper is available as a bale or bulk of flat particles. Here, only smoothing and / or pressing takes place, which is carried out in the same way as described above.

Another approach to the measurement problem described above is spectroscopic measurements through a window made of scratch-resistant glass that is transparent to the spectrum, for example in the side wall of a funnel or in the side boundary walls of a conveyor belt. The construction and the position of the window must ensure that the bulk material covers the window and that the distance between the object to be measured and the sensor fluctuates only slightly.

The latter embodiment is particularly advantageous for the measurement of granular goods with small dimensions relative to the size of the window, such as wood chips or flat goods, such as waste paper, which are compressed by their own weight.

To optimize the measuring device and the detectors, it makes sense to illuminate the material with several light sources with high intensities. 1, the light is to be guided in parallel, for which purpose light sources with converging lenses are used as condensers. In order to achieve a high light intensity at the detector, the largest possible proportion of the light reflected by the bulk material is evaluated.

In addition, it is also possible to carry out an optical distance measurement in the sensor. For this purpose, the measuring surface is illuminated selectively with a laser with a wavelength at which there are no or only negligible absorptions. The radiation intensity of the laser is measured directly. The change in the intensity of the incident wavelength on

The detector is then a direct measure of the distance between the detector and the bulk material. Such a signal can be used to correct the measured spectra 22 and 23.

The latter is particularly useful in an alternative application of the procedure described with reference to FIGS. 1 to 3 for spectroscopic measurements on rapidly moving material webs. For example, the surface of the paper web running fast in a paper machine, which is designated 70 in FIG. 5 and FIG. 6, swings in the vertical direction. In order to enable distance-sensitive measurements on such tracks 70, it is common to use a plurality of rollers before and after the measuring device to minimize the vibration or so-called flutter of the material surface. Such rollers correspond to roller 45 from FIG. 4, as shown in detail in FIGS. 6 and 7.

In FIGS. 5 and 6, 71, 71, 71 ^{, λ} ,... Mean radiation sources for electromagnetic radiation, with which the paper web 70 is illuminated in particular with parallel light radiation. To record the interaction signals, there is an arrangement of individual light-sensitive detectors 72, 72, 72 ^{, λ} ,... Behind the radiation sources 71, 71 ^x , 71, with which an intensity or absorption spectrum is recorded. Each detector 72, 72, 72 ^λ ... is assigned a laser 73, 73, 73 ^λλ , ... for ^measuring the intensity at the characteristic wavelength. Such a laser 73 can be, for example, a laser diode in which a device for intensity measurement is integrated (monitor diode). The laser should expediently irradiate at a wavelength at which there is no appreciable absorption by the material to be measured.

In particular, the measuring device in FIG. 5 is designed as a fixed array arrangement with a transmitting array on one side and a measuring array on the other side of the paper web 70. This enables transmission measurements on the running paper web with an array fixed in a measuring frame over the entire web width. In FIG. 6, on the other hand, the measuring device is designed as a measuring head for reflection measurements traversing over the paper web 70. Nothing changes in the actual signal recording or signal evaluation.

FIG. 7 shows an intensity spectrum 81 measured with the measuring arrangements corresponding to FIG. 5 or FIG. 6, which additionally has a characteristic laser intensity peak 82. In addition to the characteristic curve of the spectrum 81 for paper, from which the absorption spectrum with the In this regard, derivations according to the wavelength for the purpose of switching off the “flutter amplitude *” can be used to correct the distance, the laser intensity peak 82, the height of which is directly proportional to the distance between the sample and the sensor.

In the evaluation, in addition to the formation of the derivatives of the measured intensities and the associated normalizations, it is also possible to aim for the spectra to be recorded several times in the shortest possible time and within a predetermined time interval and for the signals to be averaged before the derivatives are formed according to the wavelengths . Other mathematical measures that can be used in the evaluation to compensate for changes in intensity due to distance fluctuations are individually or in combination:

- vector standardization,

- minimum-maximum normalization,

- centering, - centering and scaling,

- Use of wavelets.

These methods for the mathematical processing of measurement data are known per se from the prior art and have proven to be effective in the modeling of properties in connection with the evaluation of continuous spectra.

Claims

claims

1. Method for evaluating spectroscopic measurements on solid materials with spatially and / or temporally varying surfaces, wherein electromagnetic radiation in the predetermined wavelength range is radiated into the material by a measuring device with at least one radiation source and, after interaction of the radiation with the material, at least a detector, the intensity of the interaction signal is measured over a continuous, predetermined wavelength range, characterized in that only the derivatives of the intensity according to the wavelength are used for evaluation instead of the signal intensity of the continuously recorded spectra, whereby the effects of the spatially and / or temporally varying distance of the Material surface to be removed from the radiation source.

2. The method of claim 1, d a d u r c h g e k e n n - z e i c h n e t that derivatives of the specific absorption formed from the intensity are used according to the wavelength.

3. The method according to claim 1 or claim 2, d a d u r c h g e k e n n z e i c h n e t that as derivatives of

Intensity or specific absorption are formed according to the wavelength of the first, second and / or higher differential quotients.

4. The method of claim 2 or claim 3, d a d u r c h g e k e n e z e i c h n e t that the second derivative of the intensity according to the wavelength is used to model properties especially of wood.

5. The method according to any one of the preceding claims, characterized in that the first and / or the second for modeling properties Derivation of the intensities according to the wavelength can be standardized using the following mathematical measures:

- vector normalization and / or

- minimum-maximum normalization and / or - centering and / or

- centering and scaling.

6. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that so-called wavelets are used for modeling properties.

7. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that multiple radiation sources that emit parallel radiation are used.

8. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that at least one laser with discrete wavelength is additionally used for distance measurement.

9. The method according to any one of the preceding claims, that the continuous spectrum is measured in the infrared (IR) range, in particular in the near infrared range (NIR).

10. The method according to any one of the preceding claims, g e k e n n z e i c h n e t in the application with material fillings, in particular of wood chips fillings on conveyor belts.

11. The method according to any one of the preceding claims, characterized in the application to running material webs, in particular paper webs.

12. The method according to any one of the preceding claims, g e k e n n z e i c h n e t m the application to flat goods, in particular on waste paper bales and waste paper bulk on conveyor belts.

13. The method according to any one of the preceding claims, d a d u r c h g e k e n n z e i c h n e t that m is measured several times and that the measured intensities and / or the absorptions derived therefrom are averaged before the evaluation.

14. Device for performing the method according to claim 1 or one of claims 2 to 11, with a measuring device consisting of a spectrometer (50) with at least one radiation source (10, 20; 71, 71, ...) and at least one detector (30, 72, 72 ...) and an associated computer (100) with means (41 to 44, 45) for evaluating the measurement signals in such a way that spatially and / or temporally varying distances of the material surface are compensated for by the measuring device.

15. The apparatus according to claim 14, characterized in that the spectrometer (50) is assigned a plurality of radiation sources (10, 20, 71, 71 ^x , ...) which emit parallel radiation

16. The apparatus according to claim 14, characterized in that the spectrometer (50) is assigned at least one laser (73, 73 ^λ , ..) with integrated Inten- sitatsmeßemπchtung.

17. The apparatus according to claim 16, characterized in that the laser is a laser diode (73, 73 ..j, which irradiates with a wavelength in the material in which the material to be measured has no or only negligible absorptions.

18. The apparatus according to claim 16 or claim 17, characterized in that j Ede detector (72, 72 \ 72 ^λ \ ...) is assigned its own laser (73, 73 ^73, ...).

19. The apparatus of claim 14, d a d u r c h g e k e n n z e i c h n e t that the means (41 to 44, 45) for compensating the spatially and / or temporally varying distance of the material surface from the measuring device are mechanical type.

20. The apparatus of claim 14, wherein the bulk material is measured on at least one running conveyor belt, so that a second, wider conveyor belt (42) is assigned to a first conveyor belt (41) for the bulk material (42).

21. The apparatus of claim 20, so that in the direction of the material movement on the second conveyor belt (42) V-shaped wipers (43, 44) are arranged, the orientation and / or height of which is adjustable.

22. The apparatus of claim 19, d a d u r c h g e - k e n n z e i c h n e t that as a mechanical means for

Compensation for the spatially and / or temporally varying distance of the material surface from the measuring device, there is at least one height-adjustable roller (45, 45 ...).

23. Performing according to claim 22, wherein measurement is carried out on flat material webs that vary in time with their surface in height, characterized in that a plurality of rollers (45, 45 ', ...) are present before and after the measuring device (46) minimize vibration of the material surface at the measuring location.

24. The device according to claim 19, characterized in that the mechanical means for keeping the distance between the material surface and the measuring device constant from a spectral line-permeable window, which is accommodated in the side wall of a funnel or in the side wall on a conveyor belt on which the flow of the measured material in is passed appropriately.