Abstrict A flow meter for measuring the flow in a dielectric pipe is disclosed.
Two microwave signals are transmitted at measurement points via
at least two transmitter antennae through the dielectric pipe and
a flowing material. The distance between the measurement points
is known. The signals that have passed through are received by receiver
antennae and signal cross correlation is formed by a correlator,
the flow rate being determined on the basis of the best correlation
result. The measurement result is improved by filtering and/or directing
the signals.
Claims We claim:
1. In a method of measuring the flow rate of a material with poor
electrical conductivity in a dielectric conduit, the improvement
to the method comprising:
transmitting at least two different microwave signals from respective
different measurement points a known distance apart through the
conduit and material;
receiving a combination of the signals transmitted from the respective
measurement points through the conduit and material;
separating the signals received in the combination from each other
on a basis of the respective measurement points from which the signals
were transmitted; and
forming cross correlations between at least some of the separated
signals for the measuring of the flow rate on a basis of a best
one of the cross correlations.
2. A method as claimed in claim 1 wherein the signals are separated
from each other by bandpass filtering upon reception.
3. A method as claimed in claim 1 wherein the first signal is
low-pass filtered and the second signal is high-pass filtered upon
reception.
4. A method as claimed in claim 1 wherein the separation of he
signals is improved by selectively directing the transmitted signals
in different directions.
5. A method as claimed in claim 1 wherein the measured flow rate
is determined as an average of respective measured flow rates determined
by more than one iteration of the method.
6. A method as claimed in claim 1 wherein, the cross-sectional
area of the conduit being known, the flow rate is used to determine
the volume flow rate, allowing the production volume to be defined.
7. A method as claimed in claim 1 wherein the dielectric conduit
is a rubber tube in a soft tissue machine.
8. The method claim 1 wherein the respective microwave signals
are transmitted by respective pairs of transmitter and receiver
antennae across the diameter of the dielectric pipe.
9. The method of claim 1 wherein the different signals have the
same frequency.
10. In a method of measuring the flow rate of a material with poor
electrical conductivity in a dielectric conduit, the improvements
to the method comprising:
transmitting at least two microwave signals having the same frequency
but different polarizations from respective different measurement
points a known distance apart through the conduit and material;
receiving a combination of the signals transmitted from the respective
measurement points through the conduit and material;
separating the signals received in the combination from each other
by the polarizations; and
forming cross correlations between at least some of the separated
signals for the measuring of the flow rate on a basis of a best
one of the cross correlations.
11. A method as claimed in claim 10 wherein the separation of
the signals is improved by transmitting modulated signals so that
the signals are also separated from each other by demodulation.
12. The method of claim 10 wherein the respective microwave signals
are transmitted by respective pairs of transmitter and receiver
antennae across the diameter of the dielectric pipe.
13. In a method of measuring the flow rate of a material with poor
electrical conductivity in a dielectric conduit, the improvements
to the method comprising:
transmitting at least two microwave signals having the same frequency
but modulated in different ways from respective different measurement
points a known distance apart through the conduit and material;
receiving a combination of the signals transmitted from the respective
measurement points through the conduit and material;
separating the signals received in the combination from each other
by demodulation; and
forming cross correlations between at least some of the separated
signals for the measuring of the flow rate on a basis of a best
one of said correlations.
14. A method as claimed in claim 13 wherein the separation of
the signals is improved by using different polarization in the signals
transmitted, whereby the signals are also separated from each other
by polarization.
15. The method of claim 13 wherein the respective microwave signals
are transmitted and received by respective pairs of transmitter
and receiver antennae across the diameter of the dielectric pipe.
16. In a flow meter for measuring the flow rate of material with
poor electrical conductivity in a dielectric conduit, the flow meter
comprising at least two transmitter antennae, a receiver antenna,
and a correlator, wherein the improvement comprises:
means for transmitting at least two different microwave signals
from respective ones of the transmitter antennae at respective different
measurement points a known distance apart through the conduit and
material, the receiver antenna receiving a combination of the signals
transmitted from the respective different measurement points through
the conduit and material; and
means for separating signals from the received combination of signals
so that the correlator forms cross correlations between two separated
signals from respective ones of the different measurement points
and uses a best one of the cross correlations to determine the measured
flow rate.
17. A flow meter as claimed in claim 16 wherein the flow meter
is arranged to separate the signals from each other by using a low-pass
filter and a high-pass filter.
18. A flow meter as claimed in claim 9 wherein the transmitting
antennae and the receiving antenna are polarization antennae.
19. A flow meter as claimed in claim 16 wherein the flow meter
is further arranged to separate signals transmitted by respective
transmitter antennae in respective different directions.
20. A flow meter as claimed in claim 16 wherein the flow meter
is arranged to determine the flow rate as an average value on the
basis of more than one measurement.
21. A flow meter as claimed in claim 16 wherein, the cross-sectional
area of the pipe being known, the flow meter is arranged to determine
the volume flow rate by means of the flow rate, allowing the production
volume to be defined.
22. A flow meter as claimed in claim 16 wherein the transmitting
and receiving antennae are adapted to measure the flow rate in a
rubber tube of a soft tissue machine.
23. A flow meter as claimed in claim 16 wherein the flow meter
comprises a transmitter antenna and a receiver antenna adapted to
be pressed to a pipe, the receiver antenna and transmitter antenna
being repeatedly attachable and detachable without damage to the
pipe, and easily movable from place to place on a pipe and from
one pipe to another.
24. The flow meter of claim 23 further comprising a clamp adapted
to mount the transmitter and receiver antennae on opposite sides
of a pipe without opening the pipe or the interrupting a flow of
material in the pipe.
25. The flow meter of claim 16 wherein the different signals are
transmitted by the respective antennae towards corresponding antennae
across the diameter of the pipe.
26. The flow meter of claim 16 wherein the different signals have
the same frequency.
27. A flow meter for measuring the flow rate of a material with
poor electrical conductivity in a dielectric conduit, the flow meter
comprising at least two transmitter antennae, a receiver antenna,
and a correlation, wherein the improvement comprises:
a modulator arranged to modulate microwave signals transmitted
from respective ones of the transmitter antennae in different ways,
the transmitter antennae being arranged to transmit the microwave
signals from respective different measurement points a known distance
apart through the conduit and material, the receiver antenna receiving
a combination of the signals from the respective different measurement
points through the conduit and material;
a demodulator arranged to separate signals from the received combination
of signals by demodulation so that the correlator forms cross correlations
between the separated signals from respective ones of the different
measurement points and uses a best one of the cross correlations
to determine the measured flow rate.
28. The flow meter of claim 27 wherein the different signals are
by the respective transmitter antennae towards corresponding receiver
antennae across the diameter of the pipe.
29. A flow meter for measuring the flow rate of a material with
poor electrical conductivity in a dielectric conduit, the flow meter
comprising at least two transmitter antennae, a receiver antenna,
and a correlator, wherein the improvement comprises:
means for transmitting microwave signals polarized in different
ways from respective ones of the transmitter antennae at respective
different measurement points a known distance apart through the
conduit and material, the receiver antenna receiving a combination
of the signals from the respective different measurement points
that have passed through the conduit and material; and
means for separating signals from the received combination of signals
by the ways of the polarizations so that the correlator forms cross
correlations between the separated signals from respective ones
of the different measurement points and uses a best one of the cross
correlations to determine the measured flow rate.
30. A flow meter as claimed in claim 29 wherein the flow meter
also comprises modulators, and the flow meter is arranged to transmit
the signals modulated in different ways and to separate the signals
from each other by using a demodulator.
31. The flow meter of claim 29 wherein the different signals are
transmitted by respective transmitter antennae towards corresponding
receiver antennae across the diameter of the pipe.
Description FIELD OF THE INVENTION
The invention relates to a method of measuring the flow in a dielectric
pipe or the like.
The invention also relates to a flow meter, arranged to measure
the flow in a dielectric pipe or the like.
The invention further relates to a flow meter, arranged to measure
the flow in a dielectric pipe or the like, the flow meter comprising
a transmitter and a receiver.
BACKGROUND OF THE INVENTION
Flow meters are usually employed for measuring the flow of a liquid
material in a pipe. Prior art meters are of several types and they
are typically based on such physical phenomena as the Coriolis force,
pressure difference and voltage induced by the movement of a conductive
material in the magnetic field. Furthermore, ultrasound techniques
have been used to implement meters based on correlation and the
Doppler effect. Microwave techniques have also been used to implement
flow meters based on the Doppler effect. Solutions utilizing microwave
correlation by using a metallic process pipe are also known, e.g.
U.S. Pat. Nos. 4423623 4888547 and WO patent publication 94/17373.
In the solutions according to U.S. Pat. Nos. 4423623 and 4888547
the process pipe is used as a wave guide and the variations in the
cut-off frequency of the wave guide act as correlating signals.
In the solution of WO patent publication 94/17373 the correlation
of signals at the same frequency or at least in the same frequency
band is utilized after the signals have passed through a flowing
material.
These techniques involve a plurality of drawbacks. For example,
the problem in ultrasound measurement is that it is difficult to
make ultrasound move from the transmitter sensor to the dielectric
material of the tube wall and further to the material flowing inside
the tube. However, it is particularly difficult to apply the above
methods to pneumatic pipelines, since a magnetic flow meter, for
instance, requires an adequately high electric conductivity of the
material to be measured; a property which materials flowing in pneumatic
transfer lines usually lack. Prior art solutions based on microwave
techniques typically (U.S. Pat. Nos. 4423623 and 4888547) do
not allow measurement of e.g. the flow of pulp in dielectric conduits,
such as a rubber tube. In the solution according to WO patent publication
94/17373 in turn, the signals of different transmitter antennae
interfere with each other upon receiving.
BRIEF DESCRIPTION OF THE INVENTION
It is thus an object of the invention to provide a method and an
apparatus according to the method so as to be able to measure the
flow of materials with poor electric conductivity even materials
flowing in dielectric conduits without breaking pipes and without
interference between the signals received.
This is achieved by a method of the type presented in the introduction,
characterized by transmitting at least two microwave signals at
different frequencies through the dielectric pipe from measurement
points a known distance apart, the signals that have passed through
being received, and the cross correlation or the like between at
least two signals being formed, the flow rate being determined on
the basis of the best correlation result.
The method of the invention is also characterized by transmitting
at least two microwave signals at the same frequency but polarized
in different ways, through the dielectric pipe from measurement
points a known distance apart, the signals that have passed through
being separated upon reception from each other by polarization antennae,
and the cross correlation or the like between said at least two
signals being formed, the flow rate being determined on the basis
of the best correlation result.
The method of the invention is further characterized by transmitting
at least two microwave signals at the same frequency but modulated
in different ways, through the dielectric pipe from measurement
points a known distance apart, the signals that have passed through
being received, separated from each other by demodulation, and the
cross correlation or the like between said at least two signals
being formed, the flow rate being determined on the basis of the
best correlation result.
The flow meter of the invention, in turn, is characterized in that
the flow meter comprises at least two transmitters at different
frequencies and a receiver, at least two transmitter antennae and
two receiver antennae, the transmitter antennae being arranged to
transmit at least two microwave signals at different frequencies
from at least two different measurement points through the dielectric
pipe, and the receiver antennae being arranged to receive signals
that have passed through the dielectric pipe, and the flow meter
comprising a correlator for forming the cross correlation or the
like between at least two signals, whereby, the distance between
the measurement points being known, the flow meter is arranged to
use the best correlation result to determine the flow rate.
The flow meter of the invention is also characterized in that the
flow meter comprises at least two transmitter antennae, at least
two receiver antennae, modulators arranged to modulate signals in
different ways, and demodulators, the transmitter antennae being
arranged to transmit at least two microwave signals at the same
frequency but modulated in different ways from at least two different
measurement points through the dielectric pipe, and the receiver
antennae being arranged to receive the signals that have passed
through the dielectric pipe, the demodulators being arranged to
separate the received signals by demodulation, and the flow meter
comprising a correlator for forming the cross correlation or the
like between said at least two signals, whereby, the distance between
the measurement points being known, the flow meter is arranged to
use the best correlation result to determine the flow rate.
The flow meter of the invention is further characterized in that
the flow meter comprises at least two polarization transmitter antennae
and two polarization receiver antennae, the polarization transmitter
antennae being arranged to transmit at least two microwave signals
polarized in different ways from at least two different measurement
points through the dielectric pipe, the polarization receiver antennae
being arranged to receive substantially only the signal that is
polarized suitably for that particular antenna and has passed through
the dielectric pipe, and the flow meter comprising a correlator
for calculating the cross correlation or the like between at least
two signals, whereby, the distance between the measurement points
being known, the flow meter is arranged to use the best correlation
result to determine the flow rate.
The method and flow meter of the invention provide several advantages.
By using two different frequencies, polarizations and/or modulations,
a microwave is prevented from passing crosswise between the transmitter
and receiver antennae, ensuring an unambiguous measurement result.
The result can be further improved by directing different microwave
signals in different ways. The flow can be measured from outside
the pipe or the like without breaking the pipe. Installing the meter
in the pipe does not cause a risk of leakage. The meter can be installed
in the measurement site any time, even with the process going on.
The only requirement set by the measurement on the material to be
measured is that it is permeable to microwave signals and dielectric
in a way that modulates the microwave signal. The inventive solution
enables the flow rate to be measured even if the pipe is not full.
BRIEF DESCRIPTION OF THE FIGURES
In the following the invention will be described in greater detail
in association with preferred embodiments with reference to the
attached drawings, in which
FIG. 1 shows the principle of a measuring arrangement;
FIG. 2 shows a measuring arrangement in which signals are separated
from each other by filtering;
FIG. 3 shows a measuring arrangement in which the signals are modulated;
FIG. 4 shows a measuring arrangement in which signals have different
directions;
FIG. 5 shows the fastening of a flow meter to a pipe;
FIG. 6 shows the fastening of a flow meter to a pipe, and
FIG. 7 shows a preferred structure of a flow meter.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a flow meter solution according to the invention,
comprising a microwave transmitter 10 and 11 two transmitter antennae
12 and 13 two receiver antennae 14 and 15 a receiver 16 and a
correlator 17. The measuring arrangement also comprises a pipe 18
and a material 19 flowing in the pipe. In the solution of the invention,
the pipe 18 or the like does not have to be full of the flowing
material 19 but it is sufficient that the pipe 18 contains at least
some material. In this solution, signals from the transmitters 10
and 11 are transmitted via the antennae 12 and 13 through the pipe
18 and the flowing material 19. The measurement points are within
the travel range of the signals 20 and 21 and the distance between
the measurement points is known. When the signals 20 and 21 are
transmitted in the direction of the normal of the surface of the
pipe 18 through the pipe 18 the distance is the same as the distance
between the transmitter antennae 12 and 13 which is easy to measure.
The receiver antennae 14 and 15 receive the measurement signals
20 and 21 and the receiver 16 amplifies and indicates the signals
in accordance with prior art. The correlator 17 then compares the
similarity of the signals 20 and 21 by means of correlation. The
signals resemble each other, i.e. correlate best, at such a delay,
i.e. after such a time, that the flowing material 19 uses to travel
the distance between the antennae 12 and 13 and/or 14 and 15 since
the flowing material 19 modulates the signals 20 and 21. When the
flowing material 19 flows from left to right in FIG. 1 and the
flow rate is such that the material within the measurement range
between the antennae 12 and 14 moves in .tau. seconds to the measurement
range between the antennae 13 and 15 the correlation between the
signals 20 and 21 is at its highest when the signal 20 is delayed
by .tau. seconds in relation to the signal 21. When the time spent
by the flowing material for the distance between the measurement
points is known, the flow rate can be easily formed. The flow rate
is v=s/.tau., where v is the flow rate, s is the distance between
the measurement points (=the distance between the signals 20 and
21), and .tau. is the time spent by the flowing material 19 for
the distance between the antennae, measured by the correlation.
In an ideal case, the receiver antenna 14 receives the signal of
the transmitter 12 only, and similarly the receiver antenna 15 receives
the signal of the transmitter antenna 13 only. However, in practice
the measurement is interfered with by crosstalk, whereby some 22
power of the transmitter 10 is received at the receiver antenna
15 and some 23 power of the transmitter 11 is received at the receiver
antenna 14. This is particularly disturbing in prior art solutions
in which both transmitters 10 and 11 transmit at the same frequency
or in the same band. In the solution of the invention these interferences
can be avoided since the transmitters 10 and 11 operate at different
frequencies.
FIG. 2 shows a flow meter solution according to the invention,
comprising microwave transmitters 10 and 11 two transmitter antennae
12 and 13 two receiver antennae 14 and 15 a receiver 16 and a
correlator 17. The measuring arrangement also comprises a pipe 18
and a material 19 flowing in the pipe. In this inventive solution,
the transmitters 10 and 11 transmit signals at different frequencies,
the signals being filtered with filters 24 and 25 associated with
the receiver 16. In this way the received signals can be separated
from each other and hence the effect of crosstalk interferences
22 and 23 can be removed from the measurement result. The filters
24 and 25 can be bandpass filters, whereby the filter 24 only passes
the signal 20 and the filter 25 only the signal 21 through to the
actual receiver 16. The filters 24 and 25 can also be low-pass and
high-pass filters. Let us assume that the frequency of the signal
20 is below that of the signal 21. In this case the filter 24 can
be a low-pass filter whose upper frequency limit exceeds the frequency
of the signal 20 but is below the frequency of the signal 21. The
filter 25 in turn, can be a high-pass filter whose lower limiting
frequency slightly exceeds the frequency of the signal 20 but is
lower than the frequency of the signal 21.
The inventive solution, in which the signals 20 and 21 have different
frequencies, can be further improved by polarizing the signals 20
and 21 in different ways. On the other hand, when the signals 20
and 21 are polarized in different ways in the inventive solution,
the signals 20 and 21 can also have the same frequency, and the
transmitters 10 and 11 can then be replaced with one transmitter.
In this case the transmitter antenna 12 and the receiver antenna
14 operate at the same polarization level, and, similarly, the transmitter
antenna 13 and the receiver antenna 15 operate at the same polarization
level. With the polarization level angle of the signals 20 and 21
being 90.degree., the best possible separation is achieved for the
signals. With more than two measurement signals 20 and 21 the angle
between the polarization levels remains below 90.degree., and hence
polarization-based separation of the signals 20 and 21 is at its
best with two measurement signals. Instead of level polarization,
right and left hand circular polarization may also be used to implement
the polarization.
FIG. 3 shows an inventive solution in which the signals 20 and
21 are separated from each other by modulation. In this case the
signals 20 and 21 can have either different or the same frequencies.
If two or more signals 20 and 21 at the same frequency are used,
the transmitters 10 and 11 can be replaced with one transmitter.
A modulator 26 modulates the signal 20 of the transmitter 10 in
one manner. A modulator 27 in turn, modulates the signal 21 of
the transmitter 11 in a different manner. The various prior art
modulation methods are known to persons skilled in the art, and
are therefore not explained in any greater detail herein. In this
way the signals 20 and 21 can be separated from each other in the
receiver 16 which comprises demodulators 28 and 29. The demodulator
28 indicates the modulation of the modulator 26 from the signal
20 simultaneously indicating the interference effect caused by
the signal 21 and received by the antenna 14. Similarly, the demodulator
29 removes the modulation of the modulator 27 from the signal 21
and removes the interference effect caused by the signal 20 and
arrived via the antenna 15.
FIG. 4 shows a third way of limiting the interference between the
signals 20 and 21. In this solution, the antennae 12 and 13 are
directed away from one another towards the receiver antennae 14
and 15.
In the inventive solution all different alternatives can be combined
or used alone as needed. The signals 20 and 21 with different frequencies
can be simultaneously polarized, modulated and directed advantageously
and separated one from another upon reception by filtering (bandpass
filtering or high-pass/low-pass filtering), polarization, modulation
and steerable antennae.
Once the cross-sectional area A of the pipe is known, the flow
rate can be used to determine also the volume flow rate P by multiplying
the flow rate v with the cross-sectional area A of the pipe 18
i.e. P=V/.tau.=A.multidot.s/.tau.=A.multidot.v. This also allows
the production volume to be defined. The production volume can be
further specified by measuring the consistency of the flowing material
and the fill rate of the pipe, which can be measured by radioactive
consistency measurement, for example. In a flow meter solution shown
in FIGS. 1 to 4 the flow meter also preferably determines the flow
rate and, if necessary, the volume flow rate. This takes place e.g.
in the correlator 17. In the solution of the invention, the flow
rate is preferably determined as an average value from several measurements.
This allows the measurement result to be averaged from several measurements
performed with two transmitter antennae 12 and 13 and two receiver
antennae 14 and 15 and/or still more transmitter antennae and receiver
antennae, whereby the measurement is carried out at several measurement
points, and the flow rate in the pipe 18 is determined as the average
value of the measurement results of all these measurement points.
The accuracy of the measurement increases as the number of single
measurement results taken into account in determining the final
value increases.
FIG. 5 shows a possible structure of a flow meter. Elements 31
to be fastened around the pipe 18 comprise a hinge 32 by means of
which the elements 31 can be easily fastened and detached. The elements
31 comprise at least the transmitter antennae 12 and 13 and the
receiver antennae 14 and 15. FIG. 6 shows the elements 31 in the
direction of the pipe 18. On the opposite side of the hinge 32
the elements 31 comprise a locking element 34 with which the antennae
in the elements 31 are locked in place to the pipe 18 for the measurement.
In FIGS. 5 and 6 an element 35 stands for the receiver 16 and the
correlator 17 which are combined to the antennae 12 to 15 by a
cable 33. In FIG. 6 an element 36 stands for the transmitter antennae
12 and 13 and an element 37 for the receiver antennae 14 and 15.
FIG. 7 shows a preferred structure of the inventive solution. The
inventive meter is a clamp-shaped means 38 comprising the elements
31 and the transmitter and receiver antennae 36 and 37. The clamp-like
means 38 of the invention can easily be pressed against the pipe
18 for purposes of measurement. The solution is easy to fasten and
detach repeatedly.
The correlator 17 forms the correlation C(.tau.) in a general case
according to the formula (1)
where x(t) is the signal 21 and Y(t-.tau.) is the signal 20 delayed
by .tau.. The value of the correlation is calculated with several
delay values, so that .tau..di-elect cons.[t.sub.0 . . . , t.sub.n
], where t.sub.0 is the shortest possible delay and t.sub.n the
longest possible delay. The shortest and longest possible delays
determine the highest and lowest measurable flow rate. The measurement
range .tau..di-elect cons.[t.sub.0 . . . , t.sub.n ] is determined
case-specifically. In the inventive solution, the correlator 17
operates electronically and can be analog or digital.
In the inventive solution the flowing material 19 can be any liquid
material permeable to microwave radiation. The inventive solution
is particularly suitable for use e.g. in measuring the flow of pulp
in the rubber tube system of a soft tissue machine. The tubes, typically
totaling about 40 are used to transfer pulp with a consistency
of about 1% from the bypass manifold to the head box. When a paper
machine is taken into use, the flows in the tubes have to be adjusted
and for this purpose the flows need to be measured. It is practical
to carry out the measurement with the device of the invention, since
the measurement device can be easily moved from one tube and place
to another.
Even though the invention has been described above with reference
to the example according to the attached drawings, it is obvious
that the invention is not restricted thereto, but can be modified
in various ways within the scope of the inventive idea disclosed
in the attached claims. |