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FIELD OF THE INVENTION
The invention relates to a method of determining the mass flow speed of a
granular material through a channel by means of electromagnetic waves,
microwaves from a transmitter to a receiver. The invention relates in
particular to measurements performed in moving combine harvesters, where
it is important to know the quantity of grains conveyed to the grain tank
of the combine harvester. The invention is, however, related to the
measuring of a grain flow or a material flow in general.
BACKGROUND OF THE INVENTION
Several systems have been suggested for such a measuring of the flow, such
as by means of yielding guard plates being pushed more or less backwards
in response to the force they are subjected to by an incoming falling flow
of the granular material. It has, however, been recognized that such
mechanical measuring methods are too uncertain and that it is possible to
employ a more advanced measuring technique based on radioactive radiation.
Thus it has been found that it is possible to achieve a well-defined
expression of the mass flow of a granular material by said flow passing a
measuring area, where a radioactive radiation is emitted from one side of
said measuring area towards an opposing side where a receiver detects the
radiation and continuously detects the amount of radiation absorbed by the
grain flow. In this manner it is possible to determine the mass flow.
Although it is thereby possible to employ radioactive sources which are in
fact of a neglectable size, the Authorities have declared that such
sources should be avoided because they require so much inspection that an
inspection of thousands of moving units would be completely unrealistic.
According to the invention it has been recognized that it is possible to
use an officially acceptable type of radiation-based mass determination.
In other words it is possible to use electromagnetic microwaves in a
frequency area where a predetermined quantity of radiation has been
allowed. Besides it is by the present invention sufficient to employ a
power of the magnitude of 1 W, whereby suitable shieldings may secure that
the radiation problems are reduced to an acceptable level.
It is known from U.S. Pat. No. 4,628,830 to perform a continuous
determination of the mass flow in a flow of granular material by means of
microwaves. However, this publication deals only with absorption of the
wave energy caused by the water content in a coal powder fed to a burner
in a power station. The measuring is performed on a falling flow of the
material in a pipe where a microwave generator is placed opposite a
receiver. In this manner it is possible to measure the amount of energy
absorbed in the material, or rather to obtain an expression of the
variations applying to the mass flow and thereby to calibrate said
variations into an expression of the mass flow. A measuring of the water
flow is aimed at, where said water flow in a predetermined material
represents the flow of the material itself for a predetermined water
content.
The above is possible as long as the material in question is almost
homogeneous. A fundamental condition applies, however, to combine
harvesters, namely, that the measuring device must be able to operate with
various types of material which in no way is homogeneous. The major
advantage obtained by the use of the above radioactive radiation is indeed
that it is possible to operate with a well-defined calibration of the
equipment for various types of grains and seeds.
A substantially analogous use of microwaves does not provide a similar
result. Tests performed on microwaves of the type being commercially used
exactly for emission of energy into wet substances turned out to be
extremely unfortunate, for instance in connection with absorption of
energy for heating products in microwave ovens, as it turned out to be
impossible by means of one and the same equipment to obtain merely
tolerably correct measurements of various mass flows of various granular
materials.
It has, however, nevertheless become possible by the invention to base the
measurings on the use of microwaves. A method of determining the mass flow
speed of a granular material, such as grains, through a channel by means
of electromagnetic waves, microwaves from a transmitter to a receiver is
characterised in that the attenuation and/or the phase-shift for the main
signal and optionally the reflection are measured, the amplitude and the
phase shift being measured by comparing the main signal through the
material with a reference signal of the same frequency, the reference
signal being provided by comparing the output of the transmitter with an
injection signal, said compared signal being transmitted through a
separate connection to the receiver.
The increased frequency, such as 10 GHz compared to 2 to 3 GHz, results in
a considerably higher radiation reflection without considerably
influencing the radiation absorption in the material. In other words, the
attenuation can be predominantly ascribed to the reflection. At the same
time the undesired reflection from the walls of the chamber is increased,
and under unchanged conditions the latter renders it almost impossible to
obtain useful results.
The associated minimizing of the wall reflection can be obtained in several
ways optionally in combination. The chamber can be structured so as not
exactly to facilitate reflections towards the measuring window, and it can
be coated with radiation-absorbing material, such as sheet material of
plastics with carbon powder cast therein. A preferred, although rather
complicated possibility is to structure the transmitter aerial system in
such a way that the radiation is directed sharply towards the measuring
window, whereby only a minor amount of primary radiation causes wall
reflections. Good results are obtained by means of slot aerials and
focusing parabolic reflectors.
Furthermore, it is important to arrange the measuring chamber in a steady
environment. Combine harvesters comprise many metal parts moving relative
to one another, and as metal is a good conductor for microwaves, such
parts can cause disturbances in the measuring field adjacent the measuring
chamber. Although the measuring field adjacent the measuring chamber is
shielded, the external forces may, however, manifest themselves to such an
extent that a high measuring accuracy aimed at is reduced in case
significant vibrations apply. It has surprisingly been found that the
measuring chamber is most suitably placed on the location where the
radioactive measuring system was previously placed.
On this measuring location, namely, at the top of a pipe bending on a grain
channel hoop, the radioactive system aimed at an almost homogeneous
distribution of the material transverse to the grain flow, and this is
another advantageous aspect of the technique using said measuring location
according to the invention because this technique also turned out to
operate in the best possible manner with a homogeneous distribution of
material. The latter would be of no importance or at least far less
importance in connection with measurings based on absorption.
A further incentive for increasing the frequency of the microwaves by the
invention is that in order to obtain the desired reflection effect from
the various types of grains and seeds it is necessary to take into account
that some of these products, such as grass seeds, are of such a small
grain size that the grain diameter is smaller than the wavelength of
ordinary microwaves for heating purposes and for measuring absorption
attenuation, respectively. In view thereof it is according to the
invention preferred to operate with a frequency of approximately 22 GHz,
i.e. approximately 10 times higher than the frequency for ordinary
microwaves, and consequently it is additionally obvious that one should
concentrate on attenuation measurings based on reflection rather than
absorption.
It is, of course, correct that it is impossible to ignore the attenuation
caused by an absorption of the microwave energy in the passing material
due to the water content thereof. The importance thereof can indeed be
weakened by the use of higher frequencies, but the absorption effect is
still a significant factor. Accordingly, it has been accepted that for a
good measuring accuracy it is advantageous to perform a supplemental
determination of the water content in the measuring mass by means of an
independent measuring equipment in or close to the measuring site for the
mass flow.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is explained in greater detail below with reference to the
accompanying drawings, in which
FIG. 1 illustrates the position of the measuring site for the flow
measuring,
FIG. 2 illustrates an electronic circuit for carrying out the flow
measuring and comprising a transmitter and a receiver,
FIG. 3 illustrates the transmitter of the electronic circuit for flow
measuring, and which is particularly suited for measuring a flow of
oil-containing grains, and
FIGS. 4a and 4b illustrates the receiver mating the transmitter of FIG. 3.
DESCRIPTION OF PREFERRED EMBODIMENTS
Reference is made to FIGS. 1A and 1B. The measuring site for the flow
measuring is advantageously located at the pipe bending 1 on a channel
pipe of a square cross section and with a slot-shaped transmitter window 2
of for instance 10 times 80 mm arranged transverse to the pipe in the
lower curved pipe wall, as well as a slightly wider, but several times
longer receiver window 3 arranged in the upper pipe wall. The windows
should be made of a suitable non-absorbing material, such as teflon or a
suitable ceramics.
The microwave frequency used can be in a relatively large range, such as 5
to 50 GHz, preferably approximately 20 to 25 GHz. Although the measuring
system can be efficiently shielded, and although a power of no more than
approximately 1 W is employed, it can be practical to use a frequency
released for industrial use, viz. 22.6 GHz, whereby it is easy to observe
the official radiation limits.
In principle, it is possible to perform a differential measuring between
the transmitted and the received signal strength directly on the
high-frequency signals with the result that an expression of the quantity
of grains present in the measuring area at the measuring moment can be
obtained. A frequent reading of the measuring value, such as every msec.
or .mu.sec. renders it possible to determine the flow of grain mass 6 when
the advancing speed of said grains is known. The measuring result
corresponds to the signal attenuation caused by both the absorption in and
the reflection from the grains.
The invention has, however, recognized that it is possible to obtain
further information by a further signal processing by means of an
amplitude-modulated transmitter signal having a modulation frequency of
for instance 100 kHz. The detection of nothing but the modulation
frequency can be performed by mixing the transmitter signal before it is
modulated with the modulated receiver signal, cf. FIG. 2.
The modulation signal deviates in amplitude as function of the total
attenuation caused by the dielectricity constant and the water content of
the grains.
The original modulation signal has per se been amplitude-modulated by a
frequency depending on the speed of the grains, namely, based on the grain
passages through the measuring field, and this superposed modulation is a
result of the reflection from the individual grains due to the difference
between the dielectricity constants of the grains and the air,
respectively. This modulation is called secondary modulation.
This secondary modulation can be measured by detection of the primary
modulation signal. The amplitude of the secondary signal is proportional
to the reflection of the high-frequency carrier wave. As a result it is
possible to determine the reflection when the total transmission loss is
known, cf. the measuring of the primary modulation signal. It turned out
that the resulting possibility of determining both the reflection and the
absorption can be used for determining the type of grain involved because
the proportion of these values is characteristic of the various types.
Moreover, an advanced signal processing allows the accuracy of the
measuring of the grain mass flow to be increased.
While passing the measuring field before and after the central field of
said measuring field the grains are advanced along a path diverging from
perpendicular relative to said central field. As a result, a
Doppler-effect applies which manifests itself in the frequency of the
secondary modulation varying by a frequency change .DELTA. proportional to
the speed of the grains. As a result, two frequency bands apply with a
secondary modulation, namely, the grains are carried into the central
field and leave said field. The associated information on the grain speed
can be used for verifying the instantaneous speed and consequently for
making the measuring of the mass flow very accurate. Otherwise, the speed
is set to be proportional to the speed of rotation of the grain conveyor,
but fluctuations may apply with various mass distribution, which in
unfortunate situations can cause measuring errors.
FIG. 2 illustrates the circuit in question. The transmitter is shown which
transmits a non-modulated signal to a mixing step. The modulated signal is
transmitted through the transmitter aerial, and after passing the grain
flow and reaching the receiver aerial said signal is transmitted to said
mixing step where it is mixed with the non-modulated transmitter signal.
As a result, a signal is transmitted at a frequency corresponding to the
modulation frequency through the band-pass filter, and furthermore a
signal is transmitted which deviates therefrom by a frequency change
.DELTA. (through the band-stop filter) proportional to the speed of the
grain. A signal processor provides the attenuation and the speed of the
grains, respectively.
It should be mentioned that a further possibility of determining the
reflecting radiation applies, namely, to use a cross-polarized receiver
aerial or to remove two signals from a cross-polarized receiver aerial,
respectively. As a result a signal can be provided which only applies at
reflection from the grains. As the reflection depends on the grain size,
it is consequently possible to provide information on the type of the
grains involved.
As already indicated, the transmitter aerial can be a parabolic aerial
focused in one plane so as to meet the demand for operating with a
parallel field of a specific size. The receiver aerial can be elliptic
focussed on the longitudinal direction of the transmitter aerial. The
feeding unit of the receiver aerial is preferably displaced for an optimum
utilization of the measuring field.
A particularly advantageous embodiment for measuring oil-containing grains,
such as rape, is shown in FIGS. 3 and 4. Oil-containing grains influence
the dielectricity constant and thereby the phase of the transmitted signal
in such a manner that the phase-shift is substantially proportional to the
flow of oil-containing grains. However, the attenuation is not influenced
in an unambiguous manner, and it cannot be used for measuring the flow.
Like previously, the flow measuring is performed by means of a
high-frequency electromagnetic radiation, in this case at a frequency of
22 GHz. A generator 10 transmits a signal of 11 GHz through two amplifiers
11 and 12, a driver 13 and a frequency doubler 14. An adjustment signal is
transmitted both to the driver 13 and to the frequency doubler 14 for
stabilizing the signal signal amplitude. The frequency-doubled signal of
22 GHz is transmitted to one end of a U-shaped waveguide 15 through a
rod-shaped aerial inserted in said waveguide. A small rod-shaped aerial 16
is accommodated in the centre of the U-shaped waveguide 15. The rod-shaped
aerial is preferably of a length corresponding to a quarter of a
wavelength. At the opposite end of the waveguide 15 a further rod-shaped
aerial 18 is provided, said rod-shaped aerial serving as a reference
aerial. This aerial 1 8 receives a signal partly being mixed (at 20) with
an injection signal of 7.4 GHz from the receiver and used as a reference
signal in said receiver and partly being used for running the driver 13
and the frequency doubler 14 (through a filter- and adapting unit 21 and a
DC/HF splitter 22). The mixing in the mixing step 20 uses the third
harmonic of the injection signal of 7.4 GHz. The mixing step 20 results in
a signal of 125 MHz. This signal is transmitted through the filter- and
adapting unit 21 to the DC/HF splitter 22. Subsequently, the 125 signal is
returned through a filter 23 to the receiver. The signal transfer to the
receiver is performed through a semi-stiff cable 28. The 7.4 GHz signal
from the receiver is also transmitted through said semi-stiff cable 28.
Furthermore, a DC-voltage is transferred from the receiver for running the
transmitter. The DC-voltage is fed to a voltage regulator generating the
necessary supply voltages.
A chart of the receiver is shown in FIG. 4. The signal transmitted by the
aerial 16 of 22 GHz is received at a slot aerial 25. The signal received
is mixed in a mixing step 26 with the above injection signal of 7.4 GHz
(from 27) after a suitable amplification. The signal of 7.4 GHz is, as
previously mentioned, also transmitted to the transmitter through the
semi-stiff cable 28. The DC-voltage for running the transmitter is also
transmitted through the semi-stiff cable 28. The supply of the DC-voltage
is transmitted through a filter. The above reference signal of 125 MHz is
also received from the semi-stiff cable 28. This signal is transmitted to
a DC/HF-splitter 30 and subsequently to an amplifier 32 and a variable
delay 34 (including a varactor diode) for the initial phase setting. The
variable delay 34 is set in a specific position. From the variable delay
34 the signal is transmitted to an amplifier and subsequently split into
three portions. The first and the second portion is transmitted through a
further variable delay 36. Now the signal is transmitted through an
amplifier 37 to a phase detector 38 (REFMIX). One of the output signals
from the phase detector 38 is returned through a feedback loop to the
variable delay 36, which automatically moves to equilibrium position.
Moreover, the signal from the amplifier 37 is transmitted to a phase
detector 40 (COSMIX). The signal from the variable delay 34 is furthermore
transmitted directly to a third phase detector 39 (SINMIX).
Now the main signal is discussed. The signal from the mixing step 26 is a
signal of 125 MHz deviating from the previously mentioned reference signal
of 125 MHz. The deviations mean that the main signal includes information
on the material passed by the signal of 22 GHz. The main signal is
transmitted through a filter 42 to a variable attenuator and amplifier 43.
The latter amplifier includes a PIN-diode, which in connection with an
amplitude circuit ensures that the output signal has a constant amplitude
allowing phase comparisons. This output signal is transmitted through an
amplifier 45 to SINMIX 39 and COSMIX 40, respectively, and is compared
with the reference signal of 125 MHz, whereby a phase is obtained both
with respect to the sinus and the cosinus function.
The amplitude circuit comprises an amplitude detector 46 rectifying the
signal from the variable attenuator 43. The signal from the amplitude
detector 46 is transmitted to a linear logarithmic converter 47, the
output signal of which is used as a reference in the variable attenuator
43, which in turn ensures that the output signal has a constant amplitude.
This output signal is then the mass flow signal, which by a phase
comparison with the reference signal provides both the phase-shift caused
by the mass flow and consequently the mass flow.
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