FR2733051A1 - DEVICE FOR MEASURING THE MOISTURE RATE OF A MATERIAL, IN PARTICULAR AGRICULTURAL PRODUCTS PRESENT IN THE FORM OF GRAINS OR SEEDS - Google Patents

Abstract

A device for measuring the moisture content of a material, particularly agricultural produce in the form of grains or seeds. The device includes a sample holder with a recess (29) for the material of interest, a microwave emitting system with a metal emitting system horn (10) and an anti-echo chamber, and a reception system for receiving the radiation passing through the sample, including a metal reception horn (20) and an anti-echo chamber. The emitting system and the reception system each form an integral assembly supported by a frame (1) via mechanical shock-absorbing means. The measurement device enables non-destructive moisture content measurements to be performed in hostile environments, particularly in agricultural locations, and provides high quality (accurate, sensitive) measurements regardless of any disruptions caused by its use in the field.

Description

DEVICE FOR MEASURING THE MOISTURE RATE OF A MATERIAL,
PARTICULARLY AGRICULTURAL PRODUCTS
IN THE FORM OF GRAINS OR SEEDS
The invention relates to a device for measuring the moisture content of a material. It applies in particular but not exclusively to measure the moisture content of agricultural products in the form of grains or seeds.

 By "humidity" is meant any parameter providing an image of the quantity of water contained in a given quantity of materials; this humidity can in particular be expressed by the ratio of the weight of water contained in a sample of material to the total weight of the material and of water. The humidity levels indicated below are represented by this report.

In the agricultural sector, several devices are available on the market to measure the humidity of a granular material. These devices listed below operate according to different principles but all have their own drawbacks.
- measurement in an oven (long, heavy and expensive implementation),
- measurement by means of high frequency radiation (accuracy dependent on the humidity of the medium and becoming very poor beyond 25%),
- measurement by means of infrared radiation (expensive and fragile equipment, providing only surface measurements),
- NMR measurement (very heavy and very expensive equipment, fragile, usable only in the laboratory).

 In addition, US Pat. No. 5,039,947 describes a new measurement method consisting in irradiating the material to be studied by means of microwaves, in measuring the energy dissipated in the material, and in deducing therefrom the humidity level of that -this. However, to the knowledge of the inventors, this document was not followed by a practical realization allowing measurements to be carried out on site. This lack of development probably stems, on the one hand, from the very severe constraints to which a measuring device which works on site in the agricultural sector is subject: aggressive agents present in the environment (dust, chemical agents), electromagnetic disturbances of the environment, vibrations in on-board mode, ... on the other hand, development difficulties coming from the very principle of measurement by microwaves (risks of parasitic reflections). These factors cause significant disturbances of the measurements and a defect of reproducibility of these when performed outside a laboratory.

 The present invention proposes to provide a device for measuring humidity by microwave, which is capable of performing non-destructive measurements in a hostile environment, in particular on an agricultural site, and provides a good quality of measurement (precision, sensitivity). independent of disturbances linked to on-site implementation.

To this end, the device targeted by the invention for measuring the moisture content of a material comprises a frame extending along an axis XX ′, a sample holder, disposed in the frame along a plane substantially orthogonal to the axis XX 'and adapted to contain a predetermined thickness of the material to be studied, a microwave emission system of predetermined wavelength Mo, arranged opposite the sample holder to irradiate the sample, a system of reception of the radiation passing through the sample, arranged opposite the sample holder opposite the emission system and adapted to deliver an electrical signal representative of the power of the radiation received, and of the processing means, arranged to receive the signal electric from the reception system and a signal representative of the power emitted and adapted to deliver an exploitable signal representative of the moisture content of the material according to the present invention tion, the device is characterized in that
(a) the emission system includes
a metal emission horn in the shape of a pyramid trunk extending substantially along the axis XX ', said horn having a small base connected by a microwave chain to a microwave source and a large base located opposite the door -sample, said large base being closed by a plate substantially transparent to microwaves separated from the axial plane (pp ') of the sample holder by a distance D1 substantially between 1.70 Ag and 3 Ag, said emission horn having in the plane of the electric field E an opening angle a1 substantially between 200 and 350, and a height H1 between its large base and its small base substantially between 8 Ag and 10 Ag,
an anti-echo chamber, arranged between the emission horn and the sample holder and comprising, on the one hand, side walls absorbing with respect to the microwaves, distant from the axis XX 'by a distance (1, Aw1) at least equal to 4 Ag, on the other hand, inclined absorbent panels with respect to microwaves, connecting the large base of the emission horn to the side walls and rising towards the latter to form with these an acute angle (ss1 or 'i) between 290 and 550,
(b) the sample holder is made of a material which is substantially transparent to microwaves and comprises a cavity of thickness -e- of value greater than A0 / 2 and less than Mo for the placement of the material to be studied,
said cavity extending around an axis YY ′ on a surface at least equal to that of a circle of diameter Dp = 3 Ag,
said cavity being surrounded by a peripheral wall that absorbs microwaves,
said cavity being equipped with a removable closure cover which is substantially transparent with respect to microwaves,
positioning means being associated with the frame and with the sample holder in order to make the latter removable and allow it to be arranged so that the axis YY 'of its cavity substantially coincides with the axis
XX 'of the device,
(c) the receiving system includes
a metal receiving horn in the form of a pyramid trunk extending substantially along the axis XX ', said horn having a small base connected by a microwave chain to a microwave probe adapted to the wavelength Ag and a large base located opposite the sample holder, said large base being closed by a plate substantially transparent to microwaves separated from the sample holder by a distance D2 substantially between λ0 and 2.5 Ag, said receiving horn having in the plane of the electric field E an opening angle a2 substantially between 200 and 350 and a height H2 between its large base and its small base substantially between 8 Ag and 10 Ag,
an anti-echo chamber arranged between the receiving horn and the sample holder and comprising, on the one hand, absorbent side walls with respect to microwaves, distant from the axis XX 'by a distance (A2 , axis2) at least equal to 4 Ag, on the other hand, inclined absorbent panels with respect to microwaves, connecting the large base of the receiving horn to the side walls and rising towards the latter to form therewith ci an acute angle (B2, B'2) between 290 and 550.

 It should be emphasized that, to simplify the description of the device, only the wavelength Ag of the source has been mentioned, wavelength which serves as a reference to the various parameters of the device. We know that this wavelength Ag undergoes variations in the guides, horns and other organs, the guided wavelength λg in these organs being greater than the wavelength Ag emitted by the source. In the following, for the sake of simplification, reference will also be made and only to the wavelength Ag.

According to other advantageous characteristics of the device
the emission horn, the associated microwave chain and the source of the emission system are mechanically assembled so as to form a unit in one piece carried by the frame by means of mechanical damping means,
the reception horn, the associated microwave chain and the reception system probe are mechanically assembled so as to form a unit in one piece carried by the frame by means of mechanical damping means.

 The transmission horn and reception horn may in particular have a rectangular cross section suitable for working in TEo1 fundamental mode at the small base and in TEM (free propagation) mode at the large base, with a gradual evolution of 'one mode to another along the cornet. The frame of the device then preferably has the general shape of a rectangular parallelepiped closed on its faces, the space between said frame and the transmission horn as well as the space between said frame and the reception horn being at least partially lined with a block of synthetic connecting material capable of acting as a mechanical damper.

 Thus, the measuring device according to the invention combines a plurality of characteristics which allow it to work in a hostile environment under difficult conditions, while providing precise and faithful measurements. As will be better understood later, the stray reflections are reduced to a minimum and trapped so as to prevent them from disturbing the measurements; external electromagnetic interference is stopped so that it cannot reach the measurement area; aggressive environmental agents can not enter the transmission system or microwave reception system which are immune to misalignment or deterioration; in addition1 in on-board mode, the vibrations which are essentially of the very low frequency type are mechanically attenuated and absorbed and the fragile organs (microwave source, probe, etc.) are protected.

According to a preferred embodiment, the frame comprises
- on a front face, a light for introducing the sample holder,
- internally, slides for placing said sample holder in the manner of a drawer,
- Positioning stops provided for wedging said sample holder in the position where the axis YY 'of its cavity substantially coincides with the axis XX' of the device.

 It is thus easy, even in a situation where the implementation is delicate, to place the sample suitably in relation to the transmission and reception systems in order to eliminate the risks of error. It should be noted that taking into account the structure of the device (dimensions of the sample, opening of the microwave beam, etc.), the transverse positioning of the sample need not be very precise.

 The processing means delivering the exploitable electrical signal representative of the moisture content of the material are of a type known per se. These means preferably include means for storing calibration curves establishing, by the nature of the materials to be studied, correspondences between the values of the humidity rate and the values of the attenuation of the power received by the reception in relation to the power transmitted by the transmission system.

 The wavelength io of the microwaves used can be advantageously chosen in a range between 2.4 cm and 3.6 cm corresponding to frequencies between 8.2 and 12.4 Hz; in particular it is possible to use a wavelength Ag substantially equal to 3.17 cm corresponding to a frequency 9.45 GHz. This choice increases the sensitivity of the device due to the proximity of this frequency with the Debye resonant frequency.

Other characteristics, objects and advantages of the invention will emerge from the description which follows, with reference to the appended drawings, which present by way of nonlimiting example an embodiment of the invention; on these drawings
FIG. 1 is an overall perspective view of the device with a partial cutaway, the sample holder and its cover being extracted and located opposite the introduction light,
FIG. 2 is a block diagram showing the microwave chain of the device and its electronic processing means,
FIG. 3 is an axial section of the device through a plane A, the sample holder being removed,
FIG. 4 is a partial axial section similar to that of FIG. 3 with the sample holder in place but empty,
FIG. 5 is an axial section of the device through a plane B perpendicular to the plane A, the sample holder being placed opposite its insertion light,
FIG. 6 is a detailed view of the upper fixing member of the reception system on the frame, which member fulfills a mechanical damping function,
FIG. 7 is a schematic perspective view of the reception cone (or of the emission cone which is similar),
- Figure 8 is a detail view of a connecting flange used to fix two successive members of the microwave chain.

 The device for measuring the humidity level shown by way of example in the figures comprises a frame 1 in the general shape of an elongated parallelepiped extending along an axis XX ', in the vertical example. This frame consists of four walls parallel to the axis XX ': two side walls la, lb, a front wall lc and a rear wall id. The front wall lc has an inclined upper part in which are integrated the command or control means of the device. The frame is also closed by an upper wall 1f and a lower wall le.This frame can be made of reinforced resin (fiberglass or other) or any other suitable electrically insulating material; if necessary, reinforcing members may be provided in particular at the edges.

 As illustrated in block diagram 2, the device comprises a microwave emission system -Se- supplied by electrical supply means 2, a reception system -Sr-, power measurement means 3 comprising a Nanowattmeter and connected to the power supply means 2 and the processing means -T- of the signal from the measuring means 3.

 Between the emission system Se and the reception system Sr is disposed a sample holder P through which the radiation passes. Each of these assemblies is described below in detail.

 The source of the emission system Se comprises a Gunn 4 diode oscillator set to the frequency corresponding to the wavelength Mo, in the example frequency of 9.45 GHz corresponding to Ag = 3.17 cm. This oscillator is connected via a conventional microwave chain (isolator 5, transition guide 6, calibrated attenuator 7, transition guide 8) to a quadrupole frequency filter 9 of the type with 3 adaptation plungers (stubs), which is set to ensure unloaded (in the absence of sample) one-way guidance of guided microwave waves corresponding to the emission wavelength Ag (absence of reflected waves). This filter is connected to an emission horn 10 in the form of a pyramid trunk of rectangular cross section.

 This transmission horn 10, which is similar to the reception horn shown diagrammatically in perspective in FIG. 7, is made of aluminum alloy "Alpax" (registered trademark) and structured to work in TEo1 mode on its small base and in TEM (free propagation) on its large base. It has a small base connected by a flange 11 to the output waveguide of the filter 9 and a large base closed by a plate 12 of extruded polystyrene. The height H1 between large base and small base is in the example equal to 31 , 2 cm or 9.84 Ag. The opening angle al of the horn in the plane of the electric field E (in the example vertical plane B passing through the perpendicular bisectors of the front faces lc and rear ld of the frame) is equal to 260 (FIG. 5), however that the opening angle a'1 in the perpendicular plane (plane containing the largest component of the magnetic field H) is greater than the angle a1 and in the example equal to 340.

 The emission horn 10, the microwave chain 5-9 and the source 4 are mechanically assembled to form a unit in one piece which is carried by the frame 1 by means, on the one hand, of legs 13 provided damping blocks 14 of the "Silent Block" type, on the other hand, a damping block 15 constituted by a synthetic bonding material such as extruded polystyrene, disposed between the horn 10 and the faces 1a, 1b, lc and ld of the frame. These means make it possible to maintain the emission system in the frame with a mechanical damping function ensuring absorption of low frequency vibrations.

 The elements of the microwave chain called to guide the microwaves are assembled by means of flanges such as 11 (FIGS. 7 and 8) with the interposition of a conductive seal 16 (rubber seal charged to be electrically conductive). ). An external bead 17 of rubber or elastomer of the "Rubson" type guarantees absolute sealing.

Furthermore, between the sample holder P and the plate 12 for closing the horn 10, an anti-echo chamber is arranged comprising absorbent side walls with respect to microwaves such as 18 and inclined absorbent panels such as 19. These walls made of absorbent panels are in particular made of foam loaded with carbon (for example material "Ecosorb type
AN73 "(registered trademark).

 The side walls 18 are glued to the inside of the walls of the frame 1 and have external faces, on the frame side, which have reflective properties with respect to microwaves, so as to at least partially reflect the electromagnetic disturbances from the medium. surrounding in the microwave range (the non-reflected fraction being absorbed by the absorbent material).

 The absorbent side walls 18 are at a distance A1 or 1 from the axis xx 'at least equal to 4 Ag, or 12.7 cm. In the example, the distance A1 of the two absorbent walls glued to the front faces ic and rear id of the frame is approximately 14 cm, while the distance 1 of the two absorbent walls glued to the lateral faces la, lb of the frame is about 17 cm.

 In addition, the inclined panels 19 which connect these absorbent walls to the large base of the horn 10 tilt towards said walls; in the axial plane B, these inclined panels form with these walls an acute angle B1 equal to about 350; in the axial plane A, the inclined panels form with these walls an acute angle B'1 equal to about 350.

 In addition, the horn 10 is positioned so that its distance D1 from the axial plane pp 'of the sample holder is equal to 1.73 au, or 5.5 cm.

 Opposite the emission system with respect to the sample holder, the reception system consists of symmetrical elements which will not be described again: reception horn 20 and anti-echo chamber and its lateral absorbent walls and panels absorbent inclined. The parameters of distances and angles relative to these elements (assigned to the index 2) are in the example substantially equal to those of the corresponding elements of the emission system.

 The reception horn 20 is connected by a conventional microwave transmission chain (adaptation quadrupole 21, coaxial transition waveguide 22, SMA-N type adapters of diameter 23) to a probe 24 of the diode probe type, which delivers to the measuring means 3 an electrical signal representative of the power of the radiation received.

 In the same way as for the transmission system, the reception horn 20, the elements of the microwave chain 21-23 and the probe 24 of the reception system form an assembly which is fixed to the frame, on the one hand, by an arm 25 associated with a damping block 26, of the "Silent Block" type, on the other hand, by a damping block 27 providing the connection between the horn 20 and the frame 1.

 As before, the elements of the microwave chain are assembled together by means of flanges fitted with seals.

 The sample holder P consists of a frame 28 made of extruded polystyrene (Figure 1), delimiting a central cavity 29 of thickness e = 0.95 au, or 3 cm.

This cavity, called to come to center on the axis XX 'of the frame when the sample holder is in place, is intended to receive the material to be studied. This cavity extends around its axis YY 'over an area at least equal to that of a circle with a diameter Dp = 3lo. In the example, it has a rectangular shape with a long side equal to 7.9 au, or 25 cm and a short side equal to 7.3 ai or 23 cm. This dimensioning associated with the opening angles of the microwave brush from the emission horn guarantees that the entire brush passes through the material of the sample without reflections or noticeable parasitic edge effects. To reduce possible disturbances at the edge, the cavity 29 is surrounded by a peripheral wall 30 which absorbs microwaves.

 A cover 31 in the same material as the frame 28 closes the latter and gives it a uniform thickness.

 The frame 1 is internally provided with slides 32 which allow the sample holder to be placed in the frame in the manner of a drawer. A light 33 formed in the front face of the frame allows the introduction of the sample holder. Positioning stops realize at the end of introduction a setting of the sample holder in the position where the axis YY 'of its cavity substantially coincides with the axis XX' of the frame. In the example, these stops are constituted by shoulders 34 which the frame forms to come into abutment against the edges of the insertion opening 33 of the frame.

 Furthermore, the processing means T conventionally comprise data transmission means 35 which are adapted to make them compatible with a processing unit 36 which calculates the humidity level and controls its display on display means 37 The processing unit 36 comprises calculation means and means for storing calibration curves establishing, by the nature of the materials to be studied, correspondences between the values of the humidity rate and the values of the attenuation of the power received by the reception system with respect to the power transmitted by the transmission system.

 The device described makes it possible to carry out precise and faithful measurements on site of the moisture content of a material, in particular agrifood materials such as grains or cereal seeds. The humidity is deduced from the value of the measured power attenuation, from the calibration curves. A preliminary measurement in vacuum eliminates the influence of the sample holder and the low attenuation of the radiation generated by it.

 Experiments have made it possible to demonstrate that the device described above, placed in a dusty environment, on a support subjected to low frequency vibrations (simulating those of an engine of agricultural equipment) makes it possible to carry out measurements with precision. significant on the order of 1% in a humidity range from 3% to 70%; the tests were carried out using corn seeds as material.

Claims (12)

 1 / - Device for measuring the moisture content of a material, comprising a frame (1) extending along an axis XX ', a sample holder (P), arranged in the frame along a plane substantially orthogonal to the axis XX 'and adapted to contain a predetermined thickness of the material to be studied, a microwave emission system (Se) of predetermined wavelength Ag, arranged opposite the sample holder to irradiate the sample, a system (Sr) for receiving the radiation passing through the sample, arranged opposite the sample holder opposite the emission system and adapted to deliver an electrical signal representative of the power of the radiation received, and processing means (T), arranged to receive the electrical signal from the reception system and a signal representative of the power transmitted and adapted to deliver an exploitable signal representative of the moisture content of the material, said device being characterized in that  (a) the emission system (Se) includes  a metal emission horn (10) in the shape of a pyramid trunk extending substantially along the axis XX ', said horn having a small base connected by a microwave chain (5-9) to a microwave source ( 4) and a large base located opposite the sample holder, said large base being closed by a plate (12) substantially transparent to microwaves separated from the axial plane (pp ') of the sample holder by a distance D1 substantially between 1 , 70 Ag and 3 Ag, said emission horn having in the plane of the electric field E an opening angle a1 substantially between 200 and 350, and a height H1 between its large base and its small base substantially between 8 Fo and 10 Ag,  an anti-echo chamber, arranged between the emission horn (10) and the sample holder (P) and comprising, on the one hand, side walls absorbing against microwaves (18), distant from the axis XX 'of a distance (t Ih 1) at least equal to 4 Ag, on the other hand, inclined panels (19) absorbing with respect to the microwaves, connecting the large base of the emission horn to side walls and inclining towards the latter to form with them an acute angle (B1 or B'1) of between 290 and 550,  (b) the sample holder (P) is made of a material which is substantially transparent to microwaves and comprises a cavity (29) of thickness -e- of value greater than A0 / 2 and less than Ag for positioning material to study,  said cavity extending around an axis YY ′ on a surface at least equal to that of a circle of diameter Dp = 3 A0,  said cavity being surrounded by a peripheral wall (30) absorbing with respect to microwaves,  said cavity being equipped with a removable closure cover (31), substantially transparent with respect to microwaves,  positioning means (32, 34) being associated with the frame and with the sample holder in order to make the latter removable and allow it to be arranged so that the axis YY 'of its cavity substantially coincides with the axis XX' of the device,  (c) the reception system (Sr) includes  a metal receiving horn (20) in the shape of a pyramid trunk extending substantially along the axis XX ', said horn having a small base connected by a microwave chain (21-23) to a microwave probe (24) adapted to the wavelength Ag and a large base located opposite the sample holder, said large base being closed by a plate substantially transparent to microwaves separated from the sample holder by a distance D2 substantially between Ag and 2.5 jo, said receiving horn having in the plane of the electric field ff an opening angle a2 substantially between 200 and 350 and a height H2 between its large base and its small base substantially between 8 Ag and 10 Ag,  an anti-echo chamber arranged between the receiving horn (20) and the sample holder (P) and comprising, on the one hand, side walls absorbing against microwaves, distant from the axis XX ' by a distance (2 2) at least equal to 4 Ag, on the other hand, inclined absorbent panels with respect to microwaves, connecting the large base of the receiving horn to the side walls and rising towards the latter to form with them an acute angle (ss2 ss'2) between 290 and 550.  2 / - Measuring device according to claim 1, characterized in that the transmission horn (10) and the reception horn (20) have a rectangular cross section suitable for working in TEo1 mode at its small base and in free propagation TEM mode at its large base, the opening angle a1 (or a2) of the horn in the plane of the electric field E being less than the opening angle a'1 (or a'2) of the horn in the perpendicular plane, this opening angle a'1 (or a'2) being substantially between 300 and 400.  3 / - Measuring device according to claim 2, characterized in that  - the emission horn (10), the associated microwave chain (5-9) and the source (4) of the emission system are mechanically assembled so as to form a unit in one piece carried by the frame (1 ) by means of mechanical damping means (14, 15),  - the reception horn (20), the associated microwave chain (21-23) and the reception system probe (24) are mechanically assembled so as to form a whole in one piece carried by the frame (1) by through mechanical damping means (26, 27).  4 / - Measuring device according to claim 3, wherein the frame (1) has the general shape of an elongated rectangular parallelepiped closed on its faces, the space between said frame and the emission horn as well as the space between said frame and the receiving horn being at least partially filled by means of a block of synthetic connecting material (15, 27) capable of acting as a mechanical damper.  5 / - Measuring device according to claim 4, characterized in that the frame (1) comprises  - on a front face (10), a light (33) for introducing the sample holder,  - internally, slides (32) for positioning said sample holder in the manner of a drawer,  - positioning stops (34) provided for wedging said sample holder in the position where the axis YY 'of its cavity substantially coincides with the axis XX' of the device.  6 / - Measuring device according to one of claims 1, 2, 3, 4 or 5, wherein the absorbent side walls of the anti-echo chambers (18) have external faces, on the frame side, having reflective properties to the with regard to microwaves.  7 / - Measuring device according to one of the preceding claims, in which, in the transmission system as in the reception system, the horn and the elements of the associated microwave chain are fixed by flanges (11) with interposition conductive seals (16) and sealing beads (17).  8 / - Measuring device according to one of the preceding claims, in which the microwave source (4) comprises a Gunn diode oscillator, associated with a quadrupole frequency filter (5) with three adaptation plungers ("stubs") .  9 / - Measuring device according to one of the preceding claims, in which the microwave probe (24) comprises a diode probe associated with a coaxial transition waveguide (22).  10 / - Measuring device according to one of the preceding claims, in which the processing means (T) comprise means for storing calibration curves establishing, by nature of the materials to be studied, correspondences between the values of the rate d humidity and the attenuation values of the power received by the reception system in relation to the power transmitted by the transmission system.  11 / - Measuring device according to one of the preceding claims, in which the transmission system and the reception system are adapted on a frequency substantially equal to 9.45 GHz corresponding to a wavelength jo = 3.17 cm.  12 / - Application of the device according to one of claims 1 to 11, for measuring the moisture content of agricultural products in the form of grains or seeds.