WO1985003575A1 - Optical analysis instrument having rotating optical standards - Google Patents

Optical analysis instrument having rotating optical standards Download PDF

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Publication number
WO1985003575A1
WO1985003575A1 PCT/HU1985/000007 HU8500007W WO8503575A1 WO 1985003575 A1 WO1985003575 A1 WO 1985003575A1 HU 8500007 W HU8500007 W HU 8500007W WO 8503575 A1 WO8503575 A1 WO 8503575A1
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WO
WIPO (PCT)
Prior art keywords
optical
standard
radiation
filter wheel
analyzer according
Prior art date
Application number
PCT/HU1985/000007
Other languages
French (fr)
Inventor
Gábor KEMÉNY
Tibor Pokorny
Erzsébet NAGY
Gyula ISTÓK
Endre Doromby
Original Assignee
Kemeny Gabor
Tibor Pokorny
Nagy Erzsebet
Istok Gyula
Endre Doromby
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 Kemeny Gabor, Tibor Pokorny, Nagy Erzsebet, Istok Gyula, Endre Doromby filed Critical Kemeny Gabor
Publication of WO1985003575A1 publication Critical patent/WO1985003575A1/en

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Classifications

    • 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
    • G01N21/84—Systems specially adapted for particular applications
    • G01N21/85—Investigating moving fluids or granular solids
    • 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
    • G01N21/27—Colour; Spectral properties, i.e. comparison of effect of material on the light at two or more different wavelengths or wavelength bands using photo-electric detection ; circuits for computing concentration
    • G01N21/274—Calibration, base line adjustment, drift correction
    • G01N21/276—Calibration, base line adjustment, drift correction with alternation of sample and standard in optical path
    • 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
    • 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
    • G01J2003/1226—Interference filters
    • G01J2003/123—Indexed discrete 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
    • G01N2021/3129—Determining multicomponents by multiwavelength light
    • 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
    • G01N21/314—Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry with comparison of measurements at specific and non-specific wavelengths
    • G01N2021/317—Special constructive features
    • G01N2021/3174—Filter wheel
    • 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
    • G01N21/47—Scattering, i.e. diffuse reflection
    • G01N2021/4704—Angular selective
    • G01N2021/4709—Backscatter
    • 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
    • G01N21/47—Scattering, i.e. diffuse reflection
    • G01N21/4738—Diffuse reflection, e.g. also for testing fluids, fibrous materials
    • G01N21/474—Details of optical heads therefor, e.g. using optical fibres
    • 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
    • G01N21/47—Scattering, i.e. diffuse reflection
    • G01N21/4785—Standardising light scatter apparatus; Standards therefor

Definitions

  • the invention relates to optical analyzing instruments, more particularly, to reflective optical analyzing instruments having interference filters, to determine the composition of the samples by their nearinfrared (NIR) and/or visible reflective densities, with special respect to industrial on-line applications.
  • NIR nearinfrared
  • the background of the invention is the nearinfrared reflectance analysis method first suggested and elaborated by Karl H.Norris of the USDA, Beltsville, MD, USA, in the mid sixties. It was observed, that if different wavelength near-infrared radiation is incident on the surface of a sample it is absorbed or reflected to certain extents depending on the characteristics, and thus the concentrations of the constituents of the sample.
  • the reflected radiation can be collected by suitable optics and the intensity of the radiation can be measured by a suitable detector arrangement.
  • the concentrations of the material to be measured can be calculated from the said intensities measured at different wavelengths.
  • interference filter instruments A special mention among interference filter instruments is deserved for a system of prior art, described in U.S. Pat. No.4, 285, 596.
  • IREDs infrared light emitting diodes
  • all other instruments in the prior art utilize wide wavelength band halogen light sources.
  • the advantage of the special light source is, that it dissipates only a fraction of what the conventional light sources do.
  • the wavelength region is constrained to the special wavelength region of the IREDs (about 850-1050 nm). In said region a very sensitive Si diode can be used as a detector.
  • the precision of the analysis of the chemical composition is influenced by the wavelength drift of the monochromator.
  • the wavelength reproducibility is mainly an issue of the alignment and setting, it depends on the opticalmechanical construction and on the external vibrations.
  • the wavelength is more temperature dependent.
  • the filters have to be kept at controlled temperature.
  • Most commercial NIR analyzers are of single beam optical arrangement. During the measurement of a sample for one part the fluctuations have to be eliminated, and also, a relative reflective value is needed for the calculation of the concentration, therefore the NIR analyzers are able to correct for different optical standard conditions.
  • Apparatus comprising interference filter disclosed in U.S.Pat.No.4,286,327 does not contain optical reference standard.
  • tilting filter instruments disclosed in U.S.Pat No.3,861,788 and U. S.Pat.No.4,082,464 contain for solid samples a two-position drawer.
  • interference filter NIR optical analyzers either span only a narrow wavelength range or because of their complexity or the way they are optically standardized are not particularly suited for on-line industrial composition measurement.
  • the present invention is an improvement over prior optical reflective analyzers, the improvements make the instrument more simple, more precise and more stable.
  • the stability makes it possible to use the instrument according to this invention as an on-line concentration monitor in industrial processes.
  • a plurality of interference filters are mounted circularly in a wheel which is rotated at high speed.
  • the flat filter wheel is placed in a compact housing which is relatively small, and thus can efficiently be kept at constant temperature.
  • the constant temperature is vital to avoid the wavelength shifts of the interference filters.
  • On the filter wheel there are open ings f or the va r io s interference filters one or more of these are closed, these can block the radiation beam. In this state the instrument can correct for the residual light and the actual state of the detector, this is usually called "dark current compensation".
  • a reflective standard can advantageously be moved automatically and periodically into the light beam within the instrument body.
  • the device holding this standard can be moved in a circular path and at a steady rotation speed periodical reference signals are provided.
  • a further recognition in this invention is, that the aforementioned rotation can be actuated from the axis of the filter wheel, thus, using the same and only one motor by a reducing gear.
  • the reducing gear transmission is simple and has the further advantage that because of the rigid transmission there is no slip and thus the individual filters are coming into the light beam above exactly the same position of the otherwise continuously moving optical standard.
  • the individual detector signal values for each filter is used for the standardisation, thus the error caused by even the smallest spacial or optical inhomogeneity of the optical standard is eliminated.
  • the object of the invention is to provide an improved optical analyzer, which is stable enough to measure the concentrations of the material moving along before its optical window, over a long period of time without human surveillance or intervention.
  • the stable operation in case of an on-line process equipment has to be periodically tested.
  • an additional reflective standard can be mounted on the same rotating sector. This latter reflective standard should have different reflective characteristics.
  • this latter standard should not diffusely reflect the total near-infrared and visible flux, but on which the radiation suffers a measurable absorption.
  • the criterions concerning this latter absorption standard is its stable optical behaviour in time having a possible smallest temperature dependence.
  • the two standards come into the radiation beam periodically, the comparison and thus the stability check can be executed with the frequency of the rotation of the blade, holding the two standards.
  • the difference or ratio of the reflective values are measured at a given time for example at the time of the calibration of the analyzer, these values are stored in the memory of the instrument.
  • the said values serve for advantageous reference numbers which are periodically compared to the actual measured values to detect any change in the stable operation of the instrument.
  • a still further recognition in this invention is, that the measurement is more precise and more components can be measured if the light, reflected from the sample is detected not only by one type of detector, but by detector combinations simultaneously.
  • all detector types have characteristic ranges of wavelengths within which they are able to detect radiation, a plurality of different detectors span a much wider wavelength range. It is advantageous to arrange these detectors symmetrically around the test surface incorporating them into a single detector head. Thus according to the present invention it is possible to extend the detection wavelength range without any moving parts.
  • These detectors can be any combinations of any visible or near-infrared detectors, like lead sulphide, lead selenide, silicon, germanium or cadmium sulphide.
  • the radiation source 1 is a quartz halogen lamp emitting in a wide wavelength range spanning from the UV. region through the visible and in the nearinfrared region.
  • Mirror 2 depending on the application is a so called “cold” or “warm” mirror reflecting or transmitting the larger part of the infrared radiation.
  • Radiation from radiation source 1 is focused by mirror 2 onto aperture 3.
  • Radiation travelling through the opening of aperture 3 is collimated by optical lense 4, and the collimated beam is directed by plane mirror 5 towards filter wheel housing 7 covered by cylindrial cover 6.
  • optical window 8 On cover 6 is optical window 8 which transmits the radiation thus traversing narrow band transmittance interference filters 9, and leaves filter housing 7 via optical window 10.
  • a plurality of interference filters 9 are located in a circular arrangement on filter wheel 11 is rotated by motor 12 through transmission 13 with axle 14.
  • bearings 15a and 15b The easy rotating and play free operation is provided by bearings 15a and 15b.
  • optical sensor 16 provides a signal to the signal processing electronics at each slit of disc 17, which in turn is rigidly mounted relative to filter wheel 11.
  • Good wavelength reproducibility is provided by the filters 9, as the whole filter wheel 11 is rotating in a thermost-atted chamber.
  • a bifilarly wound heating coil 18 is heating filter wheel housing 7 to a constant elevated temperature elevated temperature according to temperature sensor 19.
  • the uniform temperature distribution within the filter wheel housing is provided by the air circulating in said housing as said filter wheel rotates.
  • Axle 14 of filter wheel 7 is connected to reducing gear 20.
  • axle 21 of reducing gear 20 is in the same axis as axle 14.
  • Blade 22 is fixed to axle 21.
  • the axle of the blade 22 is hold by ball bearing 23.
  • Optical reference standard 24 and optical absorption standard 25 are mounted on the two arms of blade 22.
  • Bearing 23 is mounted on plate 26.
  • Optical window 27 is fixed in an opening on plate 26, through which the sample to be measured 28 is illuminated.
  • the sample is in contact with optical window 27, light scattering starts at the interface of the external surface of the window and the sample bulk.
  • the instrument is not in contact with the sample, the reflected radiation from the illuminated sample surface is collected by spherical, elliptical or paraboloid mirrors onto the sensitive area of the detector.
  • Detectors 29 can be large surface detectors or detectors with built-in collection optics of lead sulphide, silicon diode or germanium diode, cadmium sulphide or lead selenide type.
  • Detectors are mounted in detector head 30 which is very precisely thermostatted with the help of Peltier elements 31 mounted on the large heat capacity filter wheel housing 7.
  • Fig.2 is a cutaway wiew of the optical compartment of the preferred embodiment.
  • the complete optical unit is fixed in instrument housing 32 by bolts 33.
  • Staybolts 34 are mounted on plate 26 which is of round shape.
  • Staybolts are holding the filter wheel housing 7.
  • Support plate for the motor 35 is fixed on the cover plate 6 of the filter housing, the motor is mounted on support plate by bolts.
  • Support unit 36 with cooling ribs is mounted also on plate 6 and is holding light source units 1 and 2 and also aperture 3, collimating lense 4 and mirror 5.
  • the signal from detectors 29 has to be amplified and processed but the unprocessed signal leads must be the shortest possible to avoid the enhancement of noise, therefore printed circuit signal board 37 is embracing detector head 30, thus detector can be directly connected via very short leads to the printed circuit.
  • Fig.3 is the schematic of a prefereble embodiment of the signal processing.
  • the constant temperature of filter wheel housing 7 is ensured by temperature control circuit 39 on the basis of temperature sensor 19 signal and reference signal from circuit 38.
  • the temperature control of detector head 30 is provided by Peltier cooler driver circuit 42 based on the signals from temperature sensor 40 and reference signal circuit 41.
  • the voltage of light source 1 is kept constant by regulated power supply 43.
  • a very precisely regulated voltage is supplied to detectors 29 by power supply 64. Dark current of said detectors is automatically corrected in every filter wheel rotation by circuit 45. When the beam of radiation is blocked by the filter wheel in position 46, dark current compensation circuit 45 is activated by the signal from the microcomputer 48, and detector output 47 is zeroed.
  • the signal from the detector is amplified by variable gain amplifier circuit 50. The factor of amplification is selected by the microcomputer for each filter in the filter wheel.
  • the signal from the other types of detectors is amplified by a separate amplifier 51, and the signals are selected by analog switch circuit 52 and introduced into an analog-to-digital converter 53 through a microcomputer controlled analog switch circuit 52.
  • Signal from temperature sensor 54 is amplified by amplifier 55 and also connected to the analog switch circuit 52. This additional temperature sensor 54 is placed in the wall of the instrument body.
  • the analog-to-digital converter 53 is connected to the microcomputer 48.
  • the inputs of the microcomputer 48 are - apart from analog-to-digital converter output 56 filter wheel position signal 57, reference holder blade position signal 58, and keyboard input 59.
  • the outputs of the microcomputer are: analog signal output 61 produced by digital-to-a ⁇ alog converter 60, upper concentration limit alarm 63, concentration display outputs 65a, 65b, 65c, parallel printer output 66, microcomputer bus output 67, serial RS 232C output 68, and the controller outputs for the analog circuits of the analyzer.
  • These controller outputs in the present preferred embodiment are: dark current circuit controller signal 49, variable gain controller signal 69 and selection signal 70 for the analog switch circuit.
  • the 15th opening was closed by a metal disc.
  • the wheel was rotated by a sinchronous motor type FS 7343 (AKAI, Japan) with a 5:1 reducing gear from 1500 rev/min.
  • the diamater of the interference filters was 20 mm, the effective diameter of the collimated beam was 9 mm, the optical bandwidth of the filters varied from 10 to 35 nm.
  • the transmittance of the individual filters varied from 20 to 50 % .
  • Three lead sulphide detectors (HIKI, Hungary) were connected in parallel to increase the overall signal. Symmetrically in the fourth position a silicon PIN diode (United Detector Technology Inc, USA) was fixed.
  • a reducing gear of two consecutive 4:1 ratio resulted in an overall 6:1 reduction to the standard holder blade compared to the filter wheel.
  • the position of the filter wheel is given by 15 radial slits in a 1 mm thick disc fixed to the filter wheel axle.
  • the slits are sensed by TIL 138 (Texas Instruments Inc, USA) optical sensor.
  • TIL 138 Texas Instruments Inc, USA
  • the signal of the detector was digitalized by AD 574, 12bit analog-to-digital converter, the output analog signal is provided by AD 561 (both from Analog Devices, USA).
  • the microcomputer is based on 8085 (Intel, USA) microprocessor. The complete measurement cycle in this example consists of 16 filter rotations.
  • the reflection standard and the absorption standard once move into the radiation beam.
  • the measures of the standard discs is such that they keep the radiation beam intercepted for more than one complete filter wheel rotation, thus it is possible to measure the signal at each filter for each standard.
  • the signal intensities at different wavelengths are , and I M, ⁇ with blocked beam.
  • the composition, in th is example let it be the protein content of soymeal, is calculated by the microcomputer:
  • the long term stability is also controlledby measuring the temperature of the instrument. In any case, if set limits are exeeded, an alarm status is generated and error message displayed.
  • the analyzer consists of two instrument units, one comprising the optical unit, the microcomputer and power supplies, the other is a unit to display the concentration.
  • the analyzer was used to measure raw protein, fat and moisture content in soymeal, corn and crushed sunflower.
  • Fig.4 shows three possible application examples.
  • the optical analyzer 71 is mounted on silo 72 in such a manner, that plate 26 with its built in optical window 27 is in direct contact with the product to be measured.
  • the silo there can be fine grain or coarse grain products as extracted soymeal, ground, corn, meat or fishmeal.
  • the product is covering optical window 27 through which the optical measurement is carried out.
  • the concentration of a stationary product can be determined within acceptable error limits. If the average diameter is greater, the diameter distribution is wider, the measurement is precise enough only if the sample is moving. According to Fig. 4a coarse grain product can be analyzed if there is a constant output of material from the silo, and thus it is flowing slowly before the optical window, and the instrument is "viewing" different patterns for every measurement. Thousands of measurements and averaging at each wavelength is feasible because of the fast rotation of the filter wheel.
  • the analyzer according to Fig.4a is mounted on silos containing the principal components used in a feed mixing plant, and in an off-line mode the protein and moisture content of the actual product is measured and displayed. In an on-line mode the analyzer is connected to the central process control computer, and thus an optimised recipe can be calculated for each batch before mixing, refined by the actual composition data.
  • Fig.4b the material to be measured 73 is delivered by conveyor 74.
  • Baffle 75 is built into the stream of material, and optical analyzer 71 is mounted on this.
  • the baffle has to be aligned in a way that optical window 27 should be covered and that the material is streaming at the same time.
  • the measurement procedure and connections are the same as in the case described for Fig.4b.
  • the optical analyzer 71 is mounted on holder plate 76. Material to be measured 73 is delivered continuously by conveyor 77, during the measurement cycle onto optical window 27. The window is again completely covered by the material, which then slips from holder plate 76 into balance 78.
  • the concentration analyzer is connected to the central process control logic and in this case the microcomputer of the analyzer has to be informed, what type of material is being measured, that is, which calibration equation is to be used.

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Abstract

A near-infrared and/or visible optical analyzer which is able to measure the concentrations of material in industrial processes. The analyzer is equipped with a number of interference filters (9), mounted in a rotating filter wheel (11). The monochromatic radiation is reflected by the material moving along the optical window (27) of the analyzer. More than one type of detectors (29) are mounted in a detector head (30) to measure the reflected radiation in a wide wavelength range. The amplified and dark current compensated signal is processed by a microprocessor controlled circuit. The reflection optical standard (24) used as a reference in the analyzer periodically intercepts the radiation beam and radiation reflected from the standard is also impinging upon the same detectors. The reflection standard is mounted on a blade (22) rotated by a reduction gear mechanism (20). On the other side of the blade a so called absorption optical standard (25) is mounted, with the help of which the stable operation of the analyzer can be checked.

Description

OPTICAL ANALYSIS INSTRUMENT HAVING ROTATING OPTICAL STANDARDS
BACKGROUND OF THE INVENTION The invention relates to optical analyzing instruments, more particularly, to reflective optical analyzing instruments having interference filters, to determine the composition of the samples by their nearinfrared (NIR) and/or visible reflective densities, with special respect to industrial on-line applications.
The background of the invention is the nearinfrared reflectance analysis method first suggested and elaborated by Karl H.Norris of the USDA, Beltsville, MD, USA, in the mid sixties. It was observed, that if different wavelength near-infrared radiation is incident on the surface of a sample it is absorbed or reflected to certain extents depending on the characteristics, and thus the concentrations of the constituents of the sample. The reflected radiation can be collected by suitable optics and the intensity of the radiation can be measured by a suitable detector arrangement. The concentrations of the material to be measured can be calculated from the said intensities measured at different wavelengths. For the history and background of the NIR measurement techniques see the article by L. Allen Butler published in the Cereal Foods World, 28, 238 (1983).
In the prior art an instrument with interference filters is described in U.S. Pat.No.4,236,076. Other monochromator systems are also known in the near-infrared region, a tilting interference filter system is disclosed in U.S.Pat.No.4,082,464, and a holographic grating fast analyzer is described in U. S.Pat. No.4,285,596.
A special mention among interference filter instruments is deserved for a system of prior art, described in U.S. Pat. No.4, 285, 596. As a light source infrared light emitting diodes (IREDs) are used, whereas all other instruments in the prior art utilize wide wavelength band halogen light sources. The advantage of the special light source is, that it dissipates only a fraction of what the conventional light sources do. In return the wavelength region is constrained to the special wavelength region of the IREDs (about 850-1050 nm). In said region a very sensitive Si diode can be used as a detector.
The precision of the analysis of the chemical composition is influenced by the wavelength drift of the monochromator. In the case of the optical grating monochromators the wavelength reproducibility is mainly an issue of the alignment and setting, it depends on the opticalmechanical construction and on the external vibrations. In the case of the interference filter monochromators the wavelength is more temperature dependent. For precise measurements the filters have to be kept at controlled temperature. Most commercial NIR analyzers are of single beam optical arrangement. During the measurement of a sample for one part the fluctuations have to be eliminated, and also, a relative reflective value is needed for the calculation of the concentration, therefore the NIR analyzers are able to correct for different optical standard conditions.
Apparatus comprising interference filter disclosed in U.S.Pat.No.4,286,327 does not contain optical reference standard. In the prior art, tilting filter instruments disclosed in U.S.Pat No.3,861,788 and U. S.Pat.No.4,082,464 contain for solid samples a two-position drawer.
In one position of the drawer an optical standard comes into the light beam, the sample in the other. In this prior art standardisation occurs at the beginning and at the end of the measurement, thus a longer time may lead to drifts and increased measurement errors.
From among NIR analyzers in the prior art in U.S. Pat. No.4, 236,076 the beam of the radiation is directed onto the sample surface by a mirror. By the displacement of this mirror the radiation beam is periodically diverted, and impinges upon an optically permanent surface. According to said patent the radiation beam is modulated in three different ways, the slowest is by the rotation of the disc holding the filters, a more rapid modulation is by vibrating the mirror between sample and reference positions and most rapidly by a rotating sector type light chopper. As a consequence to the triple modulation and to the vibrating parts the equipment is more complicated and requires a complex signal processing system.
For the above reasons interference filter NIR optical analyzers either span only a narrow wavelength range or because of their complexity or the way they are optically standardized are not particularly suited for on-line industrial composition measurement.
All routine analyzers use one type of detector in the above mentioned instruments usually lead sulphide photoresistor or silicon photodiodes. In larger, more expensive laboratory research equipments there is option provided to change detectors usually by alternating the light path in a mechanical manner by movable mirrors.
SUMMARY OF THE INVENTION
The present invention is an improvement over prior optical reflective analyzers, the improvements make the instrument more simple, more precise and more stable. The stability makes it possible to use the instrument according to this invention as an on-line concentration monitor in industrial processes. A plurality of interference filters are mounted circularly in a wheel which is rotated at high speed. The flat filter wheel is placed in a compact housing which is relatively small, and thus can efficiently be kept at constant temperature. The constant temperature is vital to avoid the wavelength shifts of the interference filters. On the filter wheel there are open ings f or the va r io s interference filters one or more of these are closed, these can block the radiation beam. In this state the instrument can correct for the residual light and the actual state of the detector, this is usually called "dark current compensation".
An important recognition in the present invention is that a reflective standard can advantageously be moved automatically and periodically into the light beam within the instrument body. The device holding this standard can be moved in a circular path and at a steady rotation speed periodical reference signals are provided.
Thus, the direction of the beam of the radiation is not diverted and also the same detectors are used for sample and reference measurements. This combination provides a very stable and precise means for optical standardisation.
A further recognition in this invention is, that the aforementioned rotation can be actuated from the axis of the filter wheel, thus, using the same and only one motor by a reducing gear. The reducing gear transmission is simple and has the further advantage that because of the rigid transmission there is no slip and thus the individual filters are coming into the light beam above exactly the same position of the otherwise continuously moving optical standard. The individual detector signal values for each filter is used for the standardisation, thus the error caused by even the smallest spacial or optical inhomogeneity of the optical standard is eliminated.
The object of the invention is to provide an improved optical analyzer, which is stable enough to measure the concentrations of the material moving along before its optical window, over a long period of time without human surveillance or intervention. The stable operation in case of an on-line process equipment has to be periodically tested. In order to achieve this goal, a further recognition in the present invention is that in addition to the above mentioned optical standard an additional reflective standard can be mounted on the same rotating sector. This latter reflective standard should have different reflective characteristics.
In contrast to the first standard which sould be a near ideal diffuse reflector, this latter standard should not diffusely reflect the total near-infrared and visible flux, but on which the radiation suffers a measurable absorption. The criterions concerning this latter absorption standard is its stable optical behaviour in time having a possible smallest temperature dependence.
The two standards come into the radiation beam periodically, the comparison and thus the stability check can be executed with the frequency of the rotation of the blade, holding the two standards.
The difference or ratio of the reflective values are measured at a given time for example at the time of the calibration of the analyzer, these values are stored in the memory of the instrument. The said values serve for advantageous reference numbers which are periodically compared to the actual measured values to detect any change in the stable operation of the instrument.
A still further recognition in this invention is, that the measurement is more precise and more components can be measured if the light, reflected from the sample is detected not only by one type of detector, but by detector combinations simultaneously. As all detector types have characteristic ranges of wavelengths within which they are able to detect radiation, a plurality of different detectors span a much wider wavelength range. It is advantageous to arrange these detectors symmetrically around the test surface incorporating them into a single detector head. Thus according to the present invention it is possible to extend the detection wavelength range without any moving parts. These detectors can be any combinations of any visible or near-infrared detectors, like lead sulphide, lead selenide, silicon, germanium or cadmium sulphide.
DESCRIPTION OF PREFERRED EMBODIMENTS
In Fig.1 the radiation source 1 is a quartz halogen lamp emitting in a wide wavelength range spanning from the UV. region through the visible and in the nearinfrared region. Mirror 2 depending on the application is a so called "cold" or "warm" mirror reflecting or transmitting the larger part of the infrared radiation. Radiation from radiation source 1 is focused by mirror 2 onto aperture 3. Radiation travelling through the opening of aperture 3 is collimated by optical lense 4, and the collimated beam is directed by plane mirror 5 towards filter wheel housing 7 covered by cylindrial cover 6. On cover 6 is optical window 8 which transmits the radiation thus traversing narrow band transmittance interference filters 9, and leaves filter housing 7 via optical window 10. A plurality of interference filters 9 are located in a circular arrangement on filter wheel 11 is rotated by motor 12 through transmission 13 with axle 14.
The easy rotating and play free operation is provided by bearings 15a and 15b. The exact location of the individual filters is sensed by optical sensor 16, which provides a signal to the signal processing electronics at each slit of disc 17, which in turn is rigidly mounted relative to filter wheel 11. Good wavelength reproducibility is provided by the filters 9, as the whole filter wheel 11 is rotating in a thermost-atted chamber. A bifilarly wound heating coil 18 is heating filter wheel housing 7 to a constant elevated temperature elevated temperature according to temperature sensor 19. The uniform temperature distribution within the filter wheel housing is provided by the air circulating in said housing as said filter wheel rotates. Axle 14 of filter wheel 7 is connected to reducing gear 20.
In the present preferred embodiment axle 21 of reducing gear 20 is in the same axis as axle 14. Blade 22 is fixed to axle 21. The axle of the blade 22 is hold by ball bearing 23. Optical reference standard 24 and optical absorption standard 25 are mounted on the two arms of blade 22. Bearing 23 is mounted on plate 26. Optical window 27 is fixed in an opening on plate 26, through which the sample to be measured 28 is illuminated. In a present embodiment of the present invention the sample is in contact with optical window 27, light scattering starts at the interface of the external surface of the window and the sample bulk. In another preferred embodiment the instrument is not in contact with the sample, the reflected radiation from the illuminated sample surface is collected by spherical, elliptical or paraboloid mirrors onto the sensitive area of the detector.
The reflected radiation - after reflection, absorption or scatter in the sample - impinges upon the detectors 29 situated adjacent to optical window 27. Detectors 29 can be large surface detectors or detectors with built-in collection optics of lead sulphide, silicon diode or germanium diode, cadmium sulphide or lead selenide type.
Detectors are mounted in detector head 30 which is very precisely thermostatted with the help of Peltier elements 31 mounted on the large heat capacity filter wheel housing 7.
Fig.2 is a cutaway wiew of the optical compartment of the preferred embodiment.
The complete optical unit is fixed in instrument housing 32 by bolts 33. Staybolts 34 are mounted on plate 26 which is of round shape. Staybolts are holding the filter wheel housing 7. Support plate for the motor 35 is fixed on the cover plate 6 of the filter housing, the motor is mounted on support plate by bolts. Support unit 36 with cooling ribs is mounted also on plate 6 and is holding light source units 1 and 2 and also aperture 3, collimating lense 4 and mirror 5. The signal from detectors 29 has to be amplified and processed but the unprocessed signal leads must be the shortest possible to avoid the enhancement of noise, therefore printed circuit signal board 37 is embracing detector head 30, thus detector can be directly connected via very short leads to the printed circuit.
Fig.3 is the schematic of a prefereble embodiment of the signal processing. The constant temperature of filter wheel housing 7 is ensured by temperature control circuit 39 on the basis of temperature sensor 19 signal and reference signal from circuit 38. The temperature control of detector head 30 is provided by Peltier cooler driver circuit 42 based on the signals from temperature sensor 40 and reference signal circuit 41.
The voltage of light source 1 is kept constant by regulated power supply 43. A very precisely regulated voltage is supplied to detectors 29 by power supply 64. Dark current of said detectors is automatically corrected in every filter wheel rotation by circuit 45. When the beam of radiation is blocked by the filter wheel in position 46, dark current compensation circuit 45 is activated by the signal from the microcomputer 48, and detector output 47 is zeroed. The signal from the detector is amplified by variable gain amplifier circuit 50. The factor of amplification is selected by the microcomputer for each filter in the filter wheel. The signal from the other types of detectors is amplified by a separate amplifier 51, and the signals are selected by analog switch circuit 52 and introduced into an analog-to-digital converter 53 through a microcomputer controlled analog switch circuit 52. Signal from temperature sensor 54 is amplified by amplifier 55 and also connected to the analog switch circuit 52. This additional temperature sensor 54 is placed in the wall of the instrument body. The analog-to-digital converter 53 is connected to the microcomputer 48. The inputs of the microcomputer 48 are - apart from analog-to-digital converter output 56 filter wheel position signal 57, reference holder blade position signal 58, and keyboard input 59. The outputs of the microcomputer are: analog signal output 61 produced by digital-to-aπalog converter 60, upper concentration limit alarm 63, concentration display outputs 65a, 65b, 65c, parallel printer output 66, microcomputer bus output 67, serial RS 232C output 68, and the controller outputs for the analog circuits of the analyzer. These controller outputs in the present preferred embodiment are: dark current circuit controller signal 49, variable gain controller signal 69 and selection signal 70 for the analog switch circuit.
In an example of the preferred embodiment there were 14 interference filters and the 15th opening was closed by a metal disc. The wheel was rotated by a sinchronous motor type FS 7343 (AKAI, Japan) with a 5:1 reducing gear from 1500 rev/min. The diamater of the interference filters was 20 mm, the effective diameter of the collimated beam was 9 mm, the optical bandwidth of the filters varied from 10 to 35 nm. The transmittance of the individual filters varied from 20 to 50 % . Three lead sulphide detectors (HIKI, Hungary) were connected in parallel to increase the overall signal. Symmetrically in the fourth position a silicon PIN diode (United Detector Technology Inc, USA) was fixed. A reducing gear of two consecutive 4:1 ratio resulted in an overall 6:1 reduction to the standard holder blade compared to the filter wheel. The position of the filter wheel is given by 15 radial slits in a 1 mm thick disc fixed to the filter wheel axle. The slits are sensed by TIL 138 (Texas Instruments Inc, USA) optical sensor. The two optical standards were
50 mm diameter, 2 mm thick ceramic disc as reflective optical standard and a pressed cellulose powder disc of the same measures as absorption optical standard. The light source was 12 V, 50 W quartz halogen lamp (Tungsrau, Hungary), the collimating lense was of focal length f= 15 mm, the optical windows were 1,1 and 4 mm thick made from hard optical glass. The signal of the detector was digitalized by AD 574, 12bit analog-to-digital converter, the output analog signal is provided by AD 561 (both from Analog Devices, USA). The microcomputer is based on 8085 (Intel, USA) microprocessor. The complete measurement cycle in this example consists of 16 filter rotations. Due to the 16:1 transmission, in this cycle the reflection standard and the absorption standard once move into the radiation beam. The measures of the standard discs is such that they keep the radiation beam intercepted for more than one complete filter wheel rotation, thus it is possible to measure the signal at each filter for each standard. Let the detector signal intensity at filter "i" of λi wavelength be % at the reflection standard and
Figure imgf000012_0002
Figure imgf000012_0003
at the absorption standard. In the 15th position the filter wheel blocks the radiation beam, the measured signal intensity is IR,Ø and IA, Ø respectively. The measurements at partially blocked radiation positions are discarded, and in all other cases the sample is measured. The signal intensities at different wavelengths are , and IM,Ø with blocked beam. The composition, in th
Figure imgf000012_0004
is example let it be the protein content of soymeal, is calculated by the microcomputer:
Figure imgf000012_0001
where Ki are calibration constants. The dark current compensation, that is the substraction of Ix,Ø , is done automatically via an electronic circuit. Independent from the sample the intensity difference measured on the two standards is characteristic to the stability of the instrument. L
Figure imgf000013_0001
In the preferred embodiment the long term stability is also controlledby measuring the temperature of the instrument. In any case, if set limits are exeeded, an alarm status is generated and error message displayed.
In this example the analyzer consists of two instrument units, one comprising the optical unit, the microcomputer and power supplies, the other is a unit to display the concentration. The analyzer was used to measure raw protein, fat and moisture content in soymeal, corn and crushed sunflower.
Fig.4 shows three possible application examples. On Fig.4a the optical analyzer 71 is mounted on silo 72 in such a manner, that plate 26 with its built in optical window 27 is in direct contact with the product to be measured. In the silo there can be fine grain
Figure imgf000013_0003
or coarse grain
Figure imgf000013_0002
products as extracted soymeal, ground, corn, meat or fishmeal. The product is covering optical window 27 through which the optical measurement is carried out.
If the grain size is negligibly small compared to the illuminated surface, which can be 10-20 mm diameter, the concentration of a stationary product can be determined within acceptable error limits. If the average diameter is greater, the diameter distribution is wider, the measurement is precise enough only if the sample is moving. According to Fig. 4a coarse grain product can be analyzed if there is a constant output of material from the silo, and thus it is flowing slowly before the optical window, and the instrument is "viewing" different patterns for every measurement. Thousands of measurements and averaging at each wavelength is feasible because of the fast rotation of the filter wheel. The analyzer according to Fig.4a is mounted on silos containing the principal components used in a feed mixing plant, and in an off-line mode the protein and moisture content of the actual product is measured and displayed. In an on-line mode the analyzer is connected to the central process control computer, and thus an optimised recipe can be calculated for each batch before mixing, refined by the actual composition data.
In Fig.4b the material to be measured 73 is delivered by conveyor 74. Baffle 75 is built into the stream of material, and optical analyzer 71 is mounted on this.
The baffle has to be aligned in a way that optical window 27 should be covered and that the material is streaming at the same time. The measurement procedure and connections are the same as in the case described for Fig.4b. In application example 4c the optical analyzer 71 is mounted on holder plate 76. Material to be measured 73 is delivered continuously by conveyor 77, during the measurement cycle onto optical window 27. The window is again completely covered by the material, which then slips from holder plate 76 into balance 78. In this arrangement the concentrations of various materials to be weighed and mixed can be determined. The concentration analyzer is connected to the central process control logic and in this case the microcomputer of the analyzer has to be informed, what type of material is being measured, that is, which calibration equation is to be used.

Claims

WHAT WE CLAIM IS:
1. An optical analyzer comprising a radiation source(1), means to collimate said radiation (4), a plurality of interference filters (9) mounted in a filter wheel(11) to select specific monochromatic wavelengths from said radiation, optical window(27), through which said monochromatic radiation comes into optical cantact with sample to be measured, a plurality of detectors(29) arranged in a manner to collect part of the radiation reflected from said sample, characterized in that means is provided for optical standard (24) to periodically intercept said monochromatic radiation in a manner that radiation reflected from said optical standard (24) is incident upon the same plurality of detectors(29).
2. An optical analyzer of claim 1, characterized in that reflection standard(24) is moving in a circular path.
3. An optical analyzer according to any of claims 1. or 2., characterized in that filter wheel (11) and holder blade of said standard(22)are connected by a rigid transmission system.
4. An optical analyzer according to any of claims 1. -3., characterized in that said rigid transmission system is geared drive(20).
5. An optical analyzer according to any of claims 1. -4., characterized in that the axle of the filter wheel(14) is in the same axis as axle of the holder blade(21) of said reflection standard(24).
6. An optical analyzer according to any of claims 1. -5., characterized in that the filter wheel(11) is blocking the radiation path at least one time in every rotation of said filter wheel(11).
7. An optical analyzer according to any of claims 1. -6., characterized in that a plurality of different types of detectors(29) are mounted in any combinations in a common detector head(30), spanning a wider wavelength range of the visible and near-infrared region without diverting the radiation path.
8. An optical analyzer according to any of claims 1. -7.,characterized in that the sample to be measured is in physical contact with the optical window(27) of the instrument or is streaming before said window.
9. An optical analyzer according to any of claims 1. -8., characterized in that electronic means(50) is provided to amplify the signal of the detectors (29) by different gain factors belonging to individual filters(9) or groups of filters.
10. An optical analyzer according to any of claims
1. -9., characterized in that the dark current of the detector (29) is at least one time compensated in every rotation of filter wheel (11) by analog electronic circuitry (47).
11. An optical analyzer according to any of claims 1. -10., characterized in comprising an additional reflective optical standard (25) having different optical properties and means to compare said optical properties of optical standards (24) and (25) in order to check the long term stability of said analyzer.
12. An optical analyzer of claim 11, characterized in that said additional reflective optical standard(25) is on the same holder blade(22) as firstoptical standard(24).
PCT/HU1985/000007 1984-02-13 1985-02-13 Optical analysis instrument having rotating optical standards WO1985003575A1 (en)

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