GB2062219A - Contactless measurement for substance concentration - Google Patents

Abstract

Contactless measurement of substance concentration, (e.g. humidity and fat content) in compacted surface 14 of a sample is performed by measurement of the absorption of diffusely reflected IR radiation. Non-diffusely reflected radiation and extraneous illumination are excluded. Wavelengths for measurement and reference radiation beams are produced by metal interference filter 7 and coloured and/or silicon or germanium glass filter 10. Irradiation of the surface is at an angle different from 90 DEG , over a large area, mostly using measurement radiation, and only occasionally reference radiation. Scanning is made with an optical system of large solid angle, the output of which is subjected to one- channel signal processing. Calibration standards are used to check and compensate for ageing of system components. Details of the optical components and circuitry for the signal processing are disclosed. <IMAGE>

Description

SPECIFICATION A method and an apparatus for contactless measurement of substance concentration The invention serves for contactless measurement of the concentration of a substance (e.g. fat or protein), particularly humidity on the principle of selective absorption of infrared radiation, with a high accuracy. The method according to the invention, and the apparatus for carrying it out, may be used for laboratory or process measurement of the substance concentration for instance in the textile, woodworking, paper and food industries.

It is known to measure substance concentrations using the method of absorption of infrared radiation, which is based on the physical effect that infrared radiation of different wavelengths is absorbed differently.

It is known that the impinging radiation is reflected partly diffusely and partly non-diffusely.

However, substantially only the diffusely reflected portion of the radiation contains measurement information, while the non-diffusely reflected radition depends mostly on the quality of the surface of the sample and is therefore subject to disturbance which is not dependent on concentration. Known methods and apparatus, particularly those described in German Auslegeschrift Nos. 1 598467,1 917628 and 1 934 919 use perpendicular irradiation of and reflection from the sample and detect therefore both the portions of the radiation.

The sample is not screened from extraneous light. In all the known solutions the infrared radiation of the wavelength of greater absorption dependent on the substance (measurement radiation) or smaller absorption (reference radiation) are produced by different narrow band filters, which are arranged alternately on a rotating filter wheel, so that the surface of the sample is constantly alternately spot-illuminated by measurement or reference radiation. The reflected measurement or reference radiation is directed by a hollow mirror to a lead sulphide resistor situated in the focus of the mirror, and is converted to an electric signal.

This signal is by means of a complicated synchro-control fed to a two-channel evaluation electronic means, which produces, by division or subtraction, the measuring signal for the indicator.

The absorption of the radiation in dependence on the substance concentration proceeds exponentially. Linear indication of the measurement value, which is needed for accurate measuring results, is not available.

As wavelengths for the reference radiation or the measuring radiation are used wavelengths of 1.6 to 1.8 ,um for the former and 1.9 and 1.95 ym for the latter. Means for compensation for the component aging, for the adjustment of the voltage level for the reference radiation, or for widening or shifting of the scope of measurement are not given. The illumination and the scanning of the untreated surface of the sample are punctiform.

Known solution work relatively inaccurately because they register both the diffuse and the non-diffuse portion of the reflection and there is no optical screening from the disturbing extraneous illumination.

The selection of the reference radiation within 1.6 to 1.8 ,um has the disadvantage that particularly when 1.7 ,um is used for a humidity measurement, the fat portion present in the sample has a disturbing effect.

The selection of the measurement radiation at 1.9 ,um or 1.95 yrn does not ensure maximum absorption difference between the measurement and reference radiations.

The complicated two-channel evaluation electronics which due to the constant filter change works necessarily practically without inertia is due to the great number of components expensive and works inaccurately, which is caused by the nonideal transmission properties of components.

The rotation of the filters causes that they are highly mechanically loaded and light passes through the whole diameter. Due to the nature of the optics (faults in the plane-parallelity of the surfaces; the usable diameter is only 80-90% of the diameter of the filter) inaccuracies in measurements are unavoidable, and in addition mechanical inaccuracies of the filter wheel considerably influence measurement. The narrow band interference filters are expensive and cannot be exposed to high mechanical loading.

Because the known solutions have no elements for the checking and compensation of component aging, they work with the increasing time of operation more and more inaccurately. No optical precision adjustment is available and the illumination and scanning of the untreated surface of the sample are punctiform, which leads to inaccuracies in the establishment of the measuring signal, because the roughness and compaction of the surface of the sample strongly influence absorption, and inhomogeneities of the substance are constantly detected which results in the loss of accuracy. The non-linear indication of the measuring signals increases the reading errors.

The aim of the invention is to devise a method and apparatus for its carrying out which are more accurate and reliable than known solutions, are of simpler design and may therefore be made more economically.

The aim of the invention is to discard the non-diffusely reflected portion of radiation and extraneous illumination, to use economical and stable filter combinations which, as to their wavelength, are scientifically so chosen that, in dependence on the substance concentration which must always be determined, the measurement radiation ensures maximum and the reference radiation ensures minimum infrared absorption.

So as to be able to realise the complicated twochannel evaluation electronics, which is needed to obtain measuring signals dependent on substance concentration, more accurately reliably and simply, the change between the measuring and reference radiations must be made only for the purpose of comparison and not all the time.

Widening and shifting of the scope of measurement must be ensured as accurately as the checking and compensation of component aging, defined smoothing and compacting of the surface of the sample and scanning of a larger area of the sample in order to even out inhomogeneities of the substance.

The filters are subject only to a small mechanical and thermal loading.

This problem is solved according to the invention in that by the choice of the angle of irradiation such that it differs from 90 and by screening of the space between the sample and the optics, preferably by a screen (tubus), the nondiffused portion of the reflected radiation and also extraneous illumination are discarded.

In order to produce for the measurement and reference radiations optimum wavelengths which are obtained by receiving reference spectra in near infrared, metal interference filters combined with coloured and/or silicon or germanium glass filters are used. Preferably for the measurement radiation wavelengths of 1.94 ym are used for humidity measurement, 1.725 ,um for fat contents measurement and 2.18 Jum for protein measurement, and 1.82 ,um for the reference radiation.

As a holder for the filters for the measurement and reference radiations is used a simple and exactly adjustable device, preferably a slidable holder, no constant change of the filters being undertaken. An optical precision adjustment, which is preferably obtained with the use of a grey wedge, ensures by the change of intensity adaptation of the reference radiation to any possible scope of measurement.

The widening and/or shifting of the scope of measurement is obtained, apart from the optical precision adjustment, by a combination of a preamplifier, which preferably contains a field effect transistor, and a selective alternating voltage amplifier with variable resistance feedback. Aging of components may be ascertained by a standard (non-hygroscopic substance having a defined absorption) and may be compensated for in combination with the optical and electronic possibilities of adjustment. Logarithm of the measuring signal is taken downstream of the indicator.

The needed constancy of the thickness of the layer of the sample to be measured (the minimum thickness of the layer is 10 mm) and the smoothing of the surface of the sample is obtained by mechanical means, preferably by a squeegee.

Compacting of the sample is obtained by a roller with a fixed gap following the squeegee. The depth of the gap is 2 to 5 mm (according to the thickness of the sample) below the level of the squeegee. By using a lens the surface of the sample is scanned on a large area.

The diffusely reflected radiation is received by optical means having a large solid angle, preferably a Fresnel lens.

One embodiment of an apparatus according to the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a diagrammatic illustration of the optical and mechanical parts of the apparatus, Figure 2 shows a modulating disc, Figure 3 shows a filter holder, and Figure 4 is a simplified wiring diagram of the electronic evaluation part of the apparatus.

As shown in Figure 1 riadiation, emitted by a source 2 of light, which is arranged in the focus of a reflector 1, passes through a plano-convex lens 4, while a diaphragm 3 selects cooler marginal rays of the light source 2. The radiation then passes through a modulating disc 5 and a filter holder 9 to a deflection mirror 12 and through a lens 36 arrives at the surface 14 of a sample to be measured, is there diffusely reflected and passes through a Fresnel lens 15 and via a hollow mirror 1 6 to a radiation receiver 17, which is preferably a lead sulphide transistor. The radiation receiver 1 7 supplies, according to the modulation process, a voltage pulse train a for the measurement or reference radiation.This is according to Figure 4 fed, via a capacitor 18, to a pre-amplifer the output signal of which is amplified by a selective alternating voltage amplifier 20, is rectified by phase-sensitive rectifier 24-29, is linearized by a logarithmic unit 31 and indicated by the voltmeter 32. The modulating disc 5, driven by a motor 8, chops the parallel luminous radiation and for this purpose is provided with holes 5a (Fig. 3). Their mark-tospace ratio is 1 :1. By the phase-bound peripheral portions 5b of light and darkness a control pulse train b is formed, via a light barrier consisting of a lamp 22 and receiver 23. As shown in Figure 4 the control pulse train b arrives at a switch 24.

The filter holder 9 according to Figs. 1 and 3 may be moved either manually or automatically by a time-controlled mechanism, which is not illustrated, into the path of the beam; the holder 9 is equipped with metal interference filters 7 and 11 and coloured glass filters 6 and/or silicon or germanium glass filters 10. By these two filter combinations are determined the wavelengths of the measurement and reference radiations, while the intensity of the reference radiation may be influenced by the optical precision adjustment means 13, preferably a grey wedge. The selection of the measurement and reference radiations depends on the substance to be measured and on the sample.

The evaluation of reflection spectres in near infrared the suitable wavelengths for the measurement and reference radiations may be determined, which ensure maximum or minimum absorption of the infrared radiation. For organic substances, such as milk powder or chocolate, are provided wavelengths for the measurement or reference radiation of 1.94 jum for humidity, 1.725 ,um for fat and 2.18 ym for protein or 1.82 um as reference radiation for both kinds of substances.

According to Figure 4 the pre-amplifier 1 9 preferably contains as an active element a field effect transistor. In the feedback branch of the selective alternating voltage amplifier 20 is a combination 21 of resistors which serves for coarse or accurate regulation of sensitivity.

The control pulse train b is fed via the switch 24 to transistors 25 and 26. These short circuit alternately to mass the negative or positive half waves which appear downstream of the Lresistors 27 and 28. This ensures that on the noninverting input of the amplifier 29 are only positive half waves and on the inverting input of the amplifier 29 only the negative half waves.

As a consequence on the output of the amplifier 29 is a positive DC voltage which is smoothed by a precision capacitor which is not illustra.ed and is situated in the feedback branch of the amplifier 29.

Further treatment of the signal is performed by the circuit 34 (which is not shown in detail) a resistor 30, and the logarithmic unit 31, and is indicated on the voltmeter 32. The combination and variation of the elements 13, 21 and 30 which are determined by the scope of measurement or by sensitivity, enable setting of the desired scopes of measurement and of the reference radiation to a freely selectable value so that the voltmeter 32 may be directly linearly calibrated for the respective substance concentration.

The defined smoothing and compacting of the surface of the sample is performed by a mechanical device, preferably a squeegee 37 and a roller 33 situated downstream thereof. The screening from extraneous illumination is achieved by a screen (tubus) 35. By two non-illustrated calibration standards (non-hygroscopic material with a defined high or low absorption of infrared radiation) may be established calibration curves which are specific for the product and their deviation due to component aging may be checked. The compensation of the deviations is obtained by adjustment of the corresponding optical and electronic elements. By the circuit 34, which is not described in detail, is obtained, in connection with the standards, storing of the measuring signals which are determined for specimens with a low or high substance concentration.

The invention has the advantage that by the described method and apparatus for its carrying out and by exclusion of the non-diffused portion of the reflected infrared radiation and of the extraneous illumination, by smoothing and compacting of the surface of the sample, by the illumination and scanning of a large area of the sample, by the selection of scientifically found wavelengths for the measurement and reference radiations, by the simple design of the onechannel evaluation electronics, with a small amount of components, and by the linear indication a substantial improvement of the accuracy and reliability is obtained. Due to the small amount of components, simple design and the use of metal interference filters arranged in the parallel path of the radiation, combined with coloured and/or silicon/ germanium glass filters, the apparatus may be made economically. The optical and electronic adjustments ensure, together with the storer and standards, a high degree of comfort in measurement, which contributes to increased accuracy of measurement.

Claims (9)

1. A method for contactless measurement of substance concentration, such. as humidity and fat contents, making use of the principle of selective absorption of infrared radiation and using measurement and reference radiations, wherein the desired wavelengths of the measurement and reference radiations are obtained by at least one metal interference filter combined with at least one coloured and/or silicon/germanium glass filter, a large area of a mechanically smoothed and compacted surface of the sample to be measured is irradiated at an angle different from 900, while the sample is being simultaneously screened from extraneous illumination, the measurement takes place mostly with measurement radiation and only temporarily for a controlled period of time with reference radiation for checking the constancy of the reference signal, the radiation diffusely reflected from the sample is scanned by an optical system of a large solid angle whereupon onechannel electronic processing follows, the substance concentration is indicated directly by a measuring apparatus, the checking and compensation of the aging of components is performed by calibration standards or by the setting of corresponding optical and/or electronic function elements, changes of the width of measurement are obtained by optical and/or electronic precision adjustment with alternately applied measurements and reference radiations, and measurement signals of various samples of higher and lower substance concentration are selectively stored for the establishing and checking of the calibration curves.

2. A method according to Claim 1 wherein the measurement radiation wavelengths of 1.94,um are used for humidity and 1.725 ym for fat, and for reference radiation wavelengths of 1.82 ,um are used for both humidity and fat.

3. An apparatus for carrying out a method according to Claim 1 or 2 comprising an optical part, and an electronic part, wherein the optical part includes a reflector, a source of light, a diaphragm, a planoconvex lens, a modulating disc driven by a motor, combined with a light barrier and a filter holder, a precision optical adjustment means, a deflection mirror, a lens for diffuse irradiation of the sample, a screen for screening off extraneous illumination, a Fresnel lens for receiving the diffusely reflected radiation, a hollow mirror and a transducer for converting optical signals reflected onto it from the hollow mirror to electrical signals, and the electronic part includes a capacitor, which blocks all unmodulated constant voltage signals, a pre-amplifier, a selective alternative voltage amplifier, a phasesensitive rectifier, a circuit for the storing of the calibration curves, a resistor for the adjustment of sliding voltage, a logarithmic unit and a voltmeter.

4. An apparatus according to Claim 3 wherein the pre-amplifier comprises a field-effect transistor.

5. An apparatus according to Claim 3 or 4 wherein the filter holder is in the form of a slidable holder having two filter combinations, including a metal interference filter and a glass filter.

6. An apparatus according to Claim 3, 4 or 5 wherein the precision optical adjustment means includes a grey wedge.

7. An apparatus according to Claim 3, 4, 5 or 6 including a mechanical part incorporating means for smoothing the sample, for ensuring a constant thickness of the sample and for compacting the sample.

8. A method for contactless measurement of substance concentration substantially as herein described with reference to the drawings.

9. An apparatus for carrying out the method according to Claim 1, 2 or 8 constructed, arranged and adapted to operate substantially as herein described with reference to, and as shown in, the accompanying drawings.