GB2078938A - Measurement of moisture content - Google Patents

Abstract

The moisture content of biscuits (5) on a conveyor is measured in terms of the attenuation of IR radiation from a source (56) at approximately 1.2 mu m by transmission through the biscuits to a detector (22). Radiation over a broader wavelength band from the same source or a second source (52) is also detected (24) and the detector output used for compensation in respect of biscuit density and/or thickness variations and/or for reducing the intensity of the radiation sources during intervals between biscuits. Optical guides carry the radiation to the detectors (22,24). Narrow wavelength radiation is provided by a laser (56) or by placing a filter over the detector (22) when a common source is used. <IMAGE>

Description

SPECIFICATION Measurement of moisture content The invention relates to a method of and an apparatus for measuring the moisture content of a food article.

The invention provides a method of measuring the moisture content of a food article, the method having the steps of transmitting radiation through the article and measuring the attenuation of the radiation by the article to obtain a measure of the moisture content.

The invention also provides an apparatus for measuring the moisture content of spaced food articles being carried through the apparatus by conveyor means, the apparatus comprising source means of radiation of such a wavelength as to be preferentially absorbed by moisture, radiation detector means positioned to receive the radiation after transmission through the articles or through the space therebetween, means for comparing the outputs of the detector means in the presence and absence of an article between the source means and the detector means to obtain a measure of the attenuation of the radiation by the article as a measure of its moisture content.

To avoid undesired heating of the food article due to radiation of sufficient intensity to penetrate through the food article, the radiation can be selected to have a wavelength preferentially absorbed by moisture; for example, a wavelength of 1.2 lim in the near infrared wavelength range may be employed, but any radiation capable of transmission through the food article concerned for absorption by water can be used.

The invention is readily applied to on-line moisture content measurement in a plant for manufacturing such food articles as biscuits. The food articles and the measurement apparatus can be relatively moved so that the infrared radiation beam is transmitted to a detector successively through each of a series of spaced articles; the beam intensity sensed by the detector when the beam traverses the intervals between the articles provides a zero reference level. The difference between the sensed intensities in the presence and absence of an article between the beam source and the detector is a measure of the attenuation by the article and thus of its moisture content.

The result obtained is independent of moisture distribution within the articles. Given consistent article thickness and thus a consistent beam path length through the articles, a signal can be derived from the detector outputs by which the measured moisture content is expressed as a percentage content of the article.

To sense the transmitted radiation, the detector requires to be quite sensitive and to avoid the saturation that would otherwise take place when no food article is present to attenuate the beam, the invention provides means to modify the beam intensity at such times. Conveniently, a second detector, responsive to a broader wavelength band than the first detector, is provided for this purpose.

The radiation source can be modulated by the second detector output so that this detector experiences a beam intensity which is substantially constant. Compensation for variations in such parameters as the characteristics of the radiation source and the radiation paths involved is derived from the measurement of this broad band radiation by the second detector. The output of the second detector is a measure of the thickness or density of the articles and can be employed to compensate for vatiations in these characteristics.

Instead of or as well as providing a visual indication of moisture content, and detector output can be recorded and/or used for control purposes. For example, the apparatus may be positioned at the outlet of a baking oven and the output used to control baking conditions therewithin. The detector or detectors can be temperature-compensated or mounted in a temperature controlled environment.

The invention is further described below, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a schematic side view of an apparatus in accordance with the invention; Figure 2 is a sectional plan view on the line ll-ll of Figure 1; Figure 3 is a timing diagram showing typical voltage patterns developed in use within the apparatus; and Figure 4 is a view similar to that of Figure 1 but showing a modified form of apparatus.

The apparatus illustrated in Figures 1 and 2 is suited for the on-line measurement of the moisture content of biscuits, and is associated with biscuits production apparatus including first and second conveyors 2,4 of any suitable kind, having a space between their adjacent ends across which the biscuits 5 move on a dribble board 6 or other appropriate means providing an aperture 8 or a portion otherwise transparent to infrared radiation.

On one side of the aperture 8, the upper side in the illustrated apparatus, there is located a housing 10 having an infrared radiator 12 mounted therein. The radiator 12 can be a laser source, a solid state emitter or an incandescent source. The radiator 12 is backed by a reflector 14 so arranged that infrared radiation from the radiator is directed through a lens 13 and a protective window 17 in the housing wall in a substantially parallel beam substantially normal to the plane of, and of at least the same area as, the aperture 8.

On the other side of the aperture 8, an array of light guides is mounted to receive the infrared radiation transmitted through the aperture. The light guides are located, as shown in Figure 2, so that six light guides 16 have their ends surrounding the end of a central light guide 18. The light guide 18 carries the radiation it receives through a narrow band filter 20 which passes substantially only radiation with a wavelength within one of the moisture absorption bands in the infrared, for example, that at 1.2 Fm, to a first detector 22 which provides an electrical output dependent on the intensity of the radiation it receives through the filter. The six light guides 16 are located so as to carry the radiation they receive collectively to a second detector 24 which provides an electrical output dependent on the intensity of the radiation received.A filter 26 between the light guides 16 and the detector 24 passes radiation over a broader band than the filter 20, preferably from 0.8 Fm to 2.5,um.

So that the measurement apparatus can be used in a high ambient temperature, for example, at the outlet end of a baking oven, the detectors 22,24 are preferably mounted in a device 25 by which their temperature is stabilized during operation.

The output of the detector 22 is amplified by an amplifier 28 and supplied to circuitry 30 preferably constituted by a micro-processor, in which the moisture content of the biscuits is computed by comparison of the outputs of the amplifier 28 respectively with and without a biscuit over the aperture 8. The variation with time of this amplifier voltage output is shown in Figure 3 by line 29. On arrival of a biscuit at the aperture 8 at time tl, the output initially plunges and settles to a generally constant level VIA which is sampled and stored in the circuitry 30 after a predetermined delay, at time t2.After the article has left the aperture 8 at time t3, the voltage again settles to a generally constant level V1 B which is sampled and stored after a second predetermined delay, attimet4.

The infra-red radiator 12 is energized by power from a source 32 through a thyristor or other power modulating device 34. The output of the broad band detector 24 is amplified by an amplifier 36 and taken to circuitry 38 which is responsive to the differences in this output due to the presence or absence of a biscuit at the aperture 8 to modify the intensity of the radiation from the radiator 12 so that such differences are reduced substantially to zero. This is done by control of the power supplied to the radiator 12, by way of the device 34. The voltage output of the amplifier 36 is shown in Figure 3 by line 37 and the power supplied by the device 34 is shown by line 35.

So that this power variation can be taken into account by the circuitry 30 in the treatment of V1 A & BR< V1 B, the power supplied by the device 34 also is sampled and stored by the circuitry as PA & PB at times t2 & t4 respectively.

The output of the amplifier 36 is also used to compensate the outputs of the amplifier 28 for variations in the thickness and density of the biscuits 5, of which the former output is a measure. For this purpose, the amplifier output is sampled and stored by the circuitry 30 at times t2 and t4 as V2A & V2B respectively. The moisture content is then calculated from a function of V1A and V1 B, with correction factors which are functions of PA, PB, V2A & V2B.

In the modified form of apparatus shown in Figure 4, the biscuit conveying arrangements are as in Figure 1, but a housing 50 has mounted therein an infrared radiator 52, backed by a reflector 54, and also a source 56 comprising a laser and an associated light guide for focussing the laser beam. The radiator 52 can correspond to the radiator 12 of Figure 1 in that it provides radiation of relatively broad band width, at least from 0.8 ym to 2.5 Stem, but the source 56 provides approximately monochromatic radiation, preferably within a moisture absorption band for example at 1.2 Fm. A lens 53 directs radiation from the radiator 52 through a window 57 in the housing wall in a substantially parallel beam onto the aperture 8.Below the aperture 8, the radiation collecting arrangements correspond to those of Figure 1 except that the filter 20 is omitted.

The beam from the source 56 is directed through the window 57 to impinge on the light guide 18 only.

The circuit arrangements of Figure 4 resemble those of Figure 1 except in that the output of the power modulating device 34 is supplied to the source 56 through a comparator 58 having a second input from a control element 59 which is selectively variable, so that the laser source 56 is energized on when the output of the device 34 rises above a lever set by the control element.

The apparatus of Figure 4 can instead be operated with only the intensity of the laser source 56 being adjusted in dependence on the presence or absence of a biscuit at the aperture 8, the source 52 being then continuously energized.

In either form of apparatus, the output of the circuitry 30 can be shown digitally as percentage moisture content in a display device 40 andlor supplied to recording and/or control equipment 42.

Claims (16)

1. A method of measuring the moisture content of a food article, the method having the steps of transmitting radiation through the article and measuring the attenuation of the radiation by the article to obtain a measure of the moisture content.

2. A method as claimed in claim 1, wherein the radiation is at least predominantly of a wavelength in the near infrared wavelength range.

3. A method as claimed in claim 1 or 2 wherein the radiation is at least predominantly of a wavelength preferentially absorbed by moisture.

4. A method of measuring the moisture content of spaced food articles carried on a conveyor, the method comprising the steps of directing infrared radiation comprising radiation of a wavelength preferentially absorbed by moisture onto the path of the articles on the conveyor from one side thereof, detecting on the other side of the path the radiation transmitted through the articles or the spaces therebetween, and providing an indication of the attenuation of the radiation by the articles as a measure of the moisture content.

5. A method as claimed in claim 4 having the step of reducing the intensity of the radiation in the absence of food articles to receive it.

6. A method as claimed in claim 4 or 5 having the step of compensation for variations in the thickness and/or density of between successive food articles.

7. A method as claimed in claim 3,4,5, or 6 wherein the radiation includes radiation of wavelength in a broader band than the preferentially absorbed wavelength, the broader wavelength band radiation being detected separately to provide a control and/or compensation signal.

8. A method of measuring the moisture content of food articles substantially as herein described with reference to Figures 1, 2 and 3 or Figure 4 of the accompanying drawings.

9. An apparatus for measuring the moisture content of spaced food articles being carried through the apparatus by conveyor means, the apparatus comprising source means of radiation of such a wavelength as to be preferentially absorbed by moisture, radiation detector means positioned to receive the radiation after transmission through the articles or through the spaces therebetween, means for comparing the outputs of the detector means in the presence and absence of an article between the source means and the detector means to obtain a measure of the attenuation of the radiation by the article as a measure of its moisture content.

10. An apparatus as claimed in claim 9 wherein the source means provides radiation over a broad wavelength band including the preferentially absorbed wavelength, the detector means comprises a first detector receiving substantially only radiation ofthe preferentially absorbed wavelength and a second detector receiving radiation in a broader wavelength band, and means is provided for applying the output of the second detector for controlling the intensity of the source means in dependence on the presence or absence of a food article between the source means and the detector means and/or for correcting the output of the first detector in respect of a property of the articles other than moisture content.

11. An apparatus as claimed in claim 10 wherein the source means comprises a single source and the detector means comprises a filter transmitting substantially only the preferentially absorbed wavelength to the first detector.

12. An apparatus as claimed in claim 10 wherein the source means comprises a first source of substantially only the preferentially absorbed wavelength and a second source of broader wavelength band.

13. An apparatus as described in claim 12 wherein the output of the second detector is applied to control the intensity of the first source only, the second source being energised continuously.

14. An apparatus as claimed in any one of claims 9 to 13 wherein the detector means is received in a temperature controlled housing.

15. An apparatus as claimed in any one of claims 9 to 14 having radiation guides between the detector means and the side of the article receiving the radiation remote from the source means.

16. An apparatus for measuring the moisture content of food articles substantially as herein described with reference to Figures 1,2 or 3 or Figure 4 of the accompanying drawings.