U.S. patent application number 09/820057 was filed with the patent office on 2002-05-02 for magnetic flow sensor and method.
Invention is credited to Feller, Murray F..
Application Number | 20020050175 09/820057 |
Document ID | / |
Family ID | 46277448 |
Filed Date | 2002-05-02 |
United States Patent
Application |
20020050175 |
Kind Code |
A1 |
Feller, Murray F. |
May 2, 2002 |
Magnetic flow sensor and method
Abstract
A magnetic flow sensor is configured to compensate for electrode
related drifts by connecting the electrodes either to each other or
to a reference voltage during most of an operating duty cycle. Some
versions of the invention use multiple sensing heads in a single
sensor to increase the magnitude of the flow related signal, either
by interconnecting ones of the electrodes or by externally summing
the signals. Additionally, some versions of the sensor can be used
with weakly electrically conducting flow conduits, such as blood
vessels.
Inventors: |
Feller, Murray F.;
(Micanopy, FL) |
Correspondence
Address: |
DAVID KIEWIT
5901 THIRD ST SOUTH
ST PETERSBURG
FL
33705
US
|
Family ID: |
46277448 |
Appl. No.: |
09/820057 |
Filed: |
March 28, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09820057 |
Mar 28, 2001 |
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09704913 |
Nov 2, 2000 |
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Current U.S.
Class: |
73/861.12 |
Current CPC
Class: |
G01F 1/588 20130101;
G01F 1/586 20130101; G01F 1/584 20130101; G01F 1/60 20130101 |
Class at
Publication: |
73/861.12 |
International
Class: |
G01F 001/58 |
Claims
1) A flow sensor for measuring a flow rate component, along a
selected direction, of a fluid flowing in a flow direction relative
to a sensing head, the sensor comprising: at least one magnet
arranged to have its magnetic axis generally orthogonal to the flow
direction and to the selected direction; at least one pair of
electrodes adapted to be wetted by the flowing fluid, the
electrodes of the at least one pair thereof spaced apart from one
another along a line generally orthogonal to both the selected
direction and to the magnetic axis; at least one switching device
having at least two states, the switching device, when in the first
state, directly electrically connecting each electrode to a
respective reference voltage; the switching device, when in a
second state, connecting a voltage measurement circuit between an
electrode of the at least one pair thereof electrodes and that
other electrode with which it is paired; wherein the voltage
measurement circuit is adapted to measure a voltage between the
electrodes of the at least one pair thereof and to provide
therefrom an output representative of the flow rate component.
2) The flow sensor of claim 1 wherein the reference voltage for
each of the electrodes is an electric ground.
3) The flow sensor of claim 1 further comprising means to vary at
least one respective reference voltage responsive to a flow
output.
4) The flow sensor of claim 1 wherein the at least one switching
device has a third state wherein it connects an AC signal source
between the two electrodes of the at least one pair thereof.
5) The flow sensor of claim 1 wherein the at least one pair of
electrodes comprises one pair of arrays of electrodes spaced apart
from each other, and wherein the switching device is adapted to
sequentially connect the voltage measurement circuit between a
selected electrode and each of the other electrodes from which it
is spaced apart.
6) The flow sensor of claim 1 further comprising a timing generator
adapted to control the at least one switching device to repeatedly
switch between the first and the second states so that the
switching device is in the first state most of the time.
7) The flow sensor of claim 1 wherein the at least one magnet
comprises at least one pair of magnets respectively associated with
each pair of electrodes.
8) The flow sensor of claim 1 comprising at least two pairs of
electrodes spaced apart along the flow direction and aligned with
respect to the at least one magnet so that a magnetic field from
the at least one magnet is in the same direction adjacent each of
the pairs of electrodes and wherein one of the electrodes of each
pair thereof is connected to a respective electrode of another pair
thereof whereby the voltage measurement circuit is adapted to
measure a sum of the voltages associated with the respective
electrode pairs.
9) The flow sensor of claim 1 comprising two pairs of electrodes
spaced apart from each other along the flow direction and aligned
with respect to the at least one magnet so that a magnetic field
from the at least one magnet is in a first direction adjacent a
first of the electrode pairs and in the opposite direction adjacent
the second of the electrode pairs, and wherein the voltage
measurement circuit is adapted to separately measure the voltage
between each of the two pairs of electrodes and to provide the
output representative of the flow rate responsive to a sum of the
measured voltages.
10) A flow sensor for measuring a flow rate at which a fluid flows,
the sensor comprising: at least one magnet arranged to have a
magnetic axis generally orthogonal to a direction in which the
fluid flows adjacent the at least one magnet; at least two pairs of
electrodes adapted to be wetted by the flowing fluid, the two
electrodes of each of the at least two pairs spaced apart from one
another along a respective line generally orthogonal to both the
direction in which the fluid flows between them and to the magnetic
axis; the two pairs of electrodes spaced apart from each other
along the flow direction; at least one switching device adapted to
switch each of the at least two pairs of electrodes between a
respective open circuit state in which a respective voltage between
the electrodes of the respective pair thereof is representative of
the fluid flow and a closed circuit state in which each of the
electrodes of the respective pair thereof is connected to one of a
respective reference voltage and that electrode with which the each
of the electrodes is paired; and a voltage measurement circuit
adapted to be connected by the at least one switching device to
each of the pairs of electrodes when that pair of electrodes is in
its respective open circuit state.
11) The flow sensor of claim 10 wherein, in the respective closed
state, each of the electrodes is shorted to that other electrode
with which it is paired.
12) The flow sensor of claim 10 wherein, in the respective closed
state, each of the electrodes is connected to a respective
reference potential.
13) The flow sensor of claim 10 wherein the at least one magnet
comprises at least one pair of permanent magnets respectively
associated with each pair of electrodes.
14) The flow sensor of claim 10 wherein the at least two pairs of
electrodes are aligned with respect to the at least one magnet so
that a magnetic field from the at least one magnet is in the same
direction adjacent each of the pairs of electrodes, and wherein one
electrode of the first pair thereof is connected to a respective
electrode of the second pair thereof so that the voltage
measurement circuit is adapted to measure a sum of the voltages
associated with the respective electrode pairs.
15) The flow sensor of claim 10 comprising two pairs of electrodes
aligned with respect to the at least one magnet so that a magnetic
field from the at least one magnet is oppositely directed adjacent
respective electrode pairs, and wherein the voltage measurement
circuit is adapted to separately measure the voltage between each
of the respective pairs of electrodes and to provide the output
representative of the flow rate responsive to a sum of the measured
voltages.
16) The flow sensor of claim 10 wherein the switching device is
adapted to switch all of the electrode pairs into the respective
open circuit state simultaneously.
17) The flow sensor of claim 10 wherein the switching device is
adapted to switch each of the respective electrode pairs into the
respective open circuit state sequentially.
18) A method of operating a magnetic flow sensor in which a fluid
flows in a flow direction orthogonal to a magnetic field, the flow
sensor comprising at least one pair of electrodes spaced apart from
each other along a selected direction orthogonal to the magnetic
field, the flow sensor further comprising at least one voltage
measurement circuit for measuring an electric voltage between the
at least one pair of electrodes, the electric voltage proportional
to a component of the flow rate that is orthogonal to both the
selected direction and to the magnetic field, the method comprising
the steps of: operating at least one electric switching device to
switch, for a first selected interval, each of the at least one
pair of electrodes into a closed circuit state in which each of the
electrodes is connected to one of a respective reference voltage
and that electrode with which the each of the electrodes is paired;
operating the at least one electric switching device to connect,
for a second selected interval, each of the at least one pair of
electrodes to a voltage measurement circuit; and measuring the
electric voltage during the second selected interval.
19) The method of claim 18 further comprising a step of operating
the at least one electric switching device to connect, for a third
selected interval, an AC signal source between each of the at least
one pair of electrodes.
20) The method of claim 18 wherein the at least one pair of
electrodes comprises a pair of arrays of electrodes and wherein
during the second interval the switching device sequentially
connects an electrode of a first array thereof to each of the
electrodes of the second array thereof.
21) The method of claim 18 wherein each of the electrodes is wetted
by the fluid.
22) The method of claim 18 wherein the fluid flows in a tube having
a weakly electrically conductive wall and wherein each of the
electrodes is in electrical contact with an outer surface of the
wall.
23) The method of claim 18 wherein the selected direction is
orthogonal to the flow direction, and wherein the flow sensor
comprises at least two pairs of electrodes spaced apart along the
flow direction, and wherein one electrode of the first pair thereof
is connected to a respective electrode of the second pair thereof
so that the step of measuring the electric voltage comprises
measuring a sum of voltages between each of the pairs of
electrodes.
24) The method of claim 18 wherein the selected direction is
orthogonal to the flow direction, and wherein the flow sensor
comprises two pairs of electrodes spaced apart from each other
along the flow direction, and wherein the step of measuring the
respective voltage during the respective second interval is
followed by a step of adding the measured voltages from the two
pairs of electrodes.
25) A flow sensor for measuring a flow rate at which a fluid flows
through a tube having a weakly electrically conductive wall, the
sensor comprising: a housing having an external concave surface
shaped so as to receive the tube; at least one permanent magnet
disposed in the housing so that its magnetic axis is generally
orthogonal to a direction of flow through the tube when the tube is
received in the housing; at least one pair of electrodes spaced
apart on the external concave surface of the housing so as to
contact an external surface of the tube when the tube is received
in the housing and so that a line between the two electrodes making
up the at least one pair thereof is generally orthogonal to both
the direction of flow in the tube and the magnetic axis; at least
one switching device disposed in the housing, the switching device
adapted to switch each of the pairs of electrodes between a
respective open circuit state in which a voltage between the
electrodes of the respective pair thereof is representative of the
fluid flow and a closed circuit state in which each of the
electrodes is connected to one of a respective reference voltage
and that electrode with which the each of the electrodes is paired;
and a voltage measurement circuit disposed in the housing, the
voltage measurement circuit adapted to measure the respective
voltage representative of the flow rate.
26) The flow sensor of claim 25 further comprising a power supply
disposed in the housing, the power supply connected so as to
provide electric power to the switching device and the measurement
circuit.
27) The flow sensor of claim 25 further comprising a communication
circuit disposed in the housing, the communication circuit adapted
to receive an output from the measurement circuit.
28) The flow sensor of claim 25 comprising at least two pairs of
electrodes spaced out along the tube.
29) The flow sensor of claim 25 wherein the housing comprises a
major member and a cap member, the cap member having at least one
cap permanent magnet disposed therein, the cap permanent magnet
arranged so that the cap and the major member are magnetically
attracted so as to capture the tube between them.
30) Apparatus for measuring a flow rate of a fluid in a tube, the
apparatus comprising: at least two pairs of electrodes spaced apart
a selected distance along the tube, each of the electrodes adapted
to be wetted by the fluid in the tube, each of the electrodes
spaced apart from that other electrode with which it is paired
along a line generally orthogonal to the tube; at least two magnets
spaced apart by the selected distance along the tube so that at
least one magnet is respectively adjacent each pair of electrodes,
each of the at least two magnets adapted to provide a respective
magnetic field orthogonal both to the tube and to the respective
line along which the two electrodes of the adjacent pair thereof
are spaced; a piece of ferromagnetic material extending the
selected distance along the tube so as to extend between two of the
at least two magnets; and at least one voltage measurement circuit
adapted to measure a voltage difference between each of the
electrodes and that other electrode with which it is paired.
31) The apparatus of claim 30 wherein each of the at least two
magnets is a respective permanent magnet and wherein the at least
two permanent magnets are arranged so that the magnetic field from
a first of the at least two magnets has a polarity opposite to that
of the magnetic field from a second of the at least two
magnets.
32) The apparatus of claim 30 wherein each of the at least two
magnets is a respective electromagnet and wherein the at least two
electromagnets are wound so that the polarity of the magnetic field
from a first of the at least two magnets is opposite to that of the
magnetic field from a second of the at least two magnets.
33) The apparatus of claim 30 further comprising a switching device
adapted to switch each of the at least two pairs of electrodes
between a respective open circuit state in which a voltage between
the electrodes of the respective pair thereof is representative of
the fluid flow and a closed circuit state in which each of the
electrodes is connected to one of a respective reference voltage
and that electrode with which the each of the electrodes is paired.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation in part of U.S.
application Ser. No 09/704,913, filed on Nov. 2, 2000
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
[0002] This invention relates to apparatus and method for
determining the rate of flow of a fluid by measuring an electrical
potential difference developed in the fluid as the fluid moves
through a magnetic field.
BACKGROUND INFORMATION
[0003] In a magnetic flow meter an electrical potential difference
developed in the fluid is sensed by at least one pair of electrodes
contacting the liquid and spaced apart from each other along a line
that is generally orthogonal to both the direction in which the
flow is being measured and a magnetic field produced by a magnet.
The measured potential difference has a magnitude proportional to
the flow rate of the fluid. As is known to those skilled in the
art, the overall potential difference between two such electrodes,
usually termed a voltage difference, has two major components: a) a
flow-related voltage due to the flow of the fluid when acted upon
by the magnetic field; and b) a net `drift voltage`, which is the
sum of voltages due to all other factors, such as electrode
polarization.
[0004] In prior art flow sensors of this type, alternating magnetic
fields from electromagnets have generally been used to provide an
alternating magnetic field. The alternating magnetic field
facilitates signal amplification and processing that accepts
flow-related electrode signals while rejecting electrode drift
signals which would otherwise introduce serious measurement errors.
However, generating those fields and processing the measured
voltage signals requires sophisticated circuits and techniques
which raise the cost of such sensors and limit their
application.
[0005] An example of a radical departure from the prior art is
found in my U.S. Pat. No. 6,085,599 in which I teach mechanical
means to alternate the polarity of the magnetic fields. Those
techniques provide practical ways of simplifying magnetic flow
sensors and reducing their costs. However, the use of mechanical
means to alternate the field polarity, even though this may be
performed with a high degree of ruggedness and reliability, reduces
the marketability of such an instrument. The disclosure of U.S.
Pat. No. 6,085,599 is incorporated herein by reference.
[0006] Another problem encountered in prior art magnetic flow
sensors is that of entrapment of ferromagnetic debris. This is
particularly true of arrangements using permanent magnets as in my
U.S. Pat. No. 6,085,599. Such debris can change the magnetic flux
distribution and thereby alter the calibration of the flow meter.
Moreover, pieces of ferromagnetic debris can sometimes bridge the
electrodes, which are normally electrically insulated from each
other, producing a conductive path that may partially short out the
electrode signals and thereby reduce the output voltage. Fine
particles of debris can also form a film on normally insulating
portions of the structure surrounding the electrodes and thereby
shunt the electrode signals.
[0007] It is therefore an object of the invention to provide a
practical magnetic flow sensor using stationary permanent
magnets.
[0008] It has also been discovered that the methods of the present
invention can be used with conventional magnetic flow sensors using
electromagnets to improve their performance and such is therefore a
further objective of the invention.
BRIEF SUMMARY OF THE INVENTION
[0009] The above and other objects are attained by magnetic flow
sensors in accordance with various preferred embodiments of the
present invention. In preferred embodiments the magnetic axis
(i.e., the line extending from the south to the north pole) of a
permanent magnet is oriented generally perpendicular to a direction
of flow of a fluid. As is known in the magnetic flow metering art,
the flux from a magnet arranged in this fashion generates, in the
fluid, a voltage difference proportional to the flow rate of the
fluid. In various embodiments of the invention this voltage
difference is sensed by the use of a sensing head comprising a pair
of electrodes (which preferably have the same size and shape and
are made of the same material) which are spaced apart from each
other along a line that is generally orthogonal to both a direction
of flow and the magnetic axis.
[0010] The voltage indicative of flow rate is measured when the two
electrodes of a pair are in an open-circuit state in which they are
externally electrically connected to a high impedance voltage
measurement circuit. In this open circuit state the electrode
potentials are electrically influenced by electrode polarization
and other measurement error-inducing factors that develop
relatively slowly. In order to minimize measurement errors with
these factors, sensors of some embodiments of the invention provide
an operating cycle in which the two electrodes of a pair thereof
are in a closed circuit state for most of the time, and are placed
in an open circuit state only during a brief measurement interval
portion of the operating cycle. When in the closed circuit state
the electrodes may be short circuited to each other, connected to
respective reference voltage sources (typically zero to a few tens
of microvolts) or connected to a common potential such as ground. A
major purpose of the closed circuit state, reducing drifts, is
served by connecting the two electrodes together. Connection to
other selected potentials, including ground, can provide
compensation for minor drifts. The reference voltage sources
include voltage levels which may be different for each electrode
and which may even vary with the output flow rate signal from the
flow sensor. In the closed circuit state, particularly during
installation and set-up, the electrodes may be connected to
alternating potentials having magnitudes as high as several volts
and frequencies of several kilohertz in order to drive the
electrodes quickly into a steady state condition. Periodically,
each electrode pair may be switched from its closed circuit to its
open circuit state for a brief time interval so that the
flow-generated voltage difference then appearing at the electrodes
may be detected and processed to provide an output signal
representative of the flow rate of the fluid. During the open
circuit portion of this duty cycle, drift inducing factors begin to
cause drift signals to develop. However, they develop relatively
slowly compared to the brief time interval required to detect the
flow rate signal and thereby enable electronic processing to
discriminate between the two. This method of flow rate detection
thereby enables an extremely simple magnetic flow sensor to be
made. In other cases, in which the flow rate signal is found to
change slowly with respect to drift signals, the closed circuit
state may comprise a smaller portion of the operating duty cycle
and the open circuit state a correspondingly large portion of the
duty cycle so as to allow the full magnitude of the flow rate
signals to be detected.
[0011] As will be disclosed in greater detail hereinafter, the flow
rate of a fluid can be sensed by arrays of sensing heads comprising
two or more pairs of electrodes and at least one magnet having its
magnetic axis oriented perpendicular to a direction of flow. Each
of die sensing heads in an array, as recited above, comprises a
pair of electrodes spaced apart from each other along a line
generally orthogonal to both the direction of flow at that sensing
head and to the magnetic flux. The sensing heads in an array
thereof are spaced apart from each other along the flow path of the
fluid. For example, two sensing heads can be spaced out along a
section of pipe or tubing. The flow rate voltages from the
plurality of heads can be polarized to be additive in the
associated signal processing circuitry, which may be adapted to
measure all the heads simultaneously, or which may measure the
voltages one at a time in a sequential, scanning, fashion.
Furthermore, because more than one pair of electrodes may be used
with a single or with cooperative magnetic fields, the sensor can
be configured as comprising paired arrays of electrodes that can be
momentarily externally connected in differing combinations so as to
provide a statistical sampling base from which the output signal is
derived. For example, if two arrays of four electrodes each are
paired, sixteen different combinations of individual electrode
pairs can be sampled. Because DC drift voltages at the various
electrodes would have a random distribution of magnitudes and of
polarities, the drift voltages thus tend to average out to zero
when the overall electrode voltages are summed or sampled and
averaged. The magnitude of the flow related signal can thus be made
relatively high compared to the error related drifts, thereby
improving sensor performance. Series connection of the electrodes
between more than one sensing head is also applicable and similarly
advantageous and enables the direct addition of the flow related
signals to be obtained. The present invention is well adapted to
such configurations because of the low cost of the components that
are used.
[0012] In addition to improving the ratio of flow-related signals
to drift signals, a two-headed sensing configuration comprising an
upstream head and a downstream head can be used to detect the
presence of ferromagnetic debris, most of which is likely to be
trapped by the permanent magnet portion of the upstream sensing
head. This debris can alter the magnetic flux distribution and
shunt the flow-related voltage of the upstream head, thereby
reducing the magnitude of its flow-related voltage. Thus, if one
compares the flow-related signals from identical upstream and
downstream sensing heads and finds that those signals differ by
more than some predetermined threshold value, one can conclude that
at least the upstream head is contaminated with ferromagnetic
debris and that cleaning of the wetted portions of the sensor is
required.
[0013] Although various numbers of sensing heads can be used in the
invention, in preferred methods of operation the paired electrodes
of each sensing head are in the closed circuit state during a
relatively long portion of an operating duty cycle. During a
relatively short portion of the duty cycle a switching device can
be used to sequentially open circuit pairs of electrodes and
connect each open circuited pair to a common measurement circuit in
order to measure its flow-related open circuit voltage. A switching
device can also open circuit pairs of the electrodes and connect
them to separate inputs of a common measurement circuit to measure
the flow related voltages. Those skilled in the signal processing
arts will realize that with these and other arrangements for
aggregating open circuit voltages one can obtain a simple average
of the output voltages, an average of the sum of the individual
output voltages, or various other selected statistical
measures.
[0014] Generally speaking, the flow-generated component of the open
circuit voltage will appear quickly (i.e., it can be measured after
a predictable rise time that depends primarily on the resistivity
and dielectric constant of the flowing fluid) after an electrode
pair is switched from a closed circuit state to an open circuit
state. Electrode pair drift voltages, by contrast, depend on
electrode polarization and other generally much more slowly acting
effects and can thus generally be effectively excluded by making
the open circuit voltage measurement quickly. Thus, one can readily
determine a fluid-dependent operating duty cycle comprising a first
period in which all electrode pairs are connected together in a
closed circuit state for a long enough interval for polarization
and other drift effects to reach an acceptably stable condition;
and a second readout period in which appropriate switching devices
and voltage measurement circuitry are used to detect the open
circuit voltages from all the electrode pairs used in the sensor.
In a preferred embodiment the first period is substantially longer
then the second. Other relationships between the lengths of the
first and second periods are also workable.
[0015] In some embodiments multiple permanent magnets are used with
an internal streamlined body and a flow tube, both of which are
electrically insulating and in contact with the fluid. Each such
section has its own pair of electrodes. Both the magnetic flux,
which is orthogonal to the fluid flow, and the fluid itself are
thus concentrated to provide a relatively large flow-related
signal.
[0016] In another preferred embodiment of the present invention,
the flux from two permanent magnets reinforce each other across an
orthogonally oriented passage through which a fluid flows. Various
other preferred embodiments including probe configured flow sensors
are included.
[0017] In some embodiments, the present invention is applied to
conventional magnetic flow sensors which use a pulse of electrical
energy through a coil of wire to produce a pulsed magnetic field.
After the pulsed magnetic field stabilizes, the electrodes are
placed in the open circuit state so that the flow generated voltage
difference can be detected and processed to provide a flow signal
representative of the flow rate of the fluid. Operation is
therefore essentially the same as when the permanent magnet is
used.
[0018] Those skilled in the arts of magnetic flow sensing will
appreciate that although relative motion between a liquid and a
sensing head is essential in instruments of this sort, there is no
requirement that the sensing head be stationary in an inertial
frame of reference. One can equally well use the invention for
measuring the rate of progress of a sensing head through a
stationary fluid, as is done when measuring the speed of a ship
having a sensing head mounted to or projecting outwardly from its
hull. Moreover, one can configure a sensor having two pairs of
mutually orthogonally disposed electrodes (e.g. as depicted in FIG.
1) in which each of the pairs is responsive to a component of fluid
flow orthogonal to the line along which that pair is spaced. A
sensor of this sort can be used to determine the direction of flow,
as well as for measuring the magnitude of the flow rate.
[0019] Although it is believed that the foregoing recital of
features and advantages may be of use to one who is skilled in the
art and who wishes to learn how to practice the invention, it will
be recognized that the foregoing recital is not intended to list
all of the features and advantages. Moreover, it may be noted that
various embodiments of the invention may provide various
combinations of the hereinbefore recited features and advantages of
the invention, and that less than all of the recited features and
advantages may be provided by some embodiments.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
[0020] FIG. 1 is a schematic view of the sensing head portion,
i.e., the signal generating components, of a flow sensor of the
invention.
[0021] FIG. 2 is a schematic block diagram of an electronics
circuit usable to control and process signals generated by a
sensing head of the invention.
[0022] FIG. 3 is a partly cut away side elevational view of a
preferred sensing head mounted in a pipe or tube.
[0023] FIG. 4 is a cross sectional view of the sensing head of FIG.
3, the section taken as indicated with the double headed arrow 44
in FIG. 3.
[0024] FIG. 5 is a partly cut-away schematic side elevational view
of another in-line flow sensing head arrangement of the
invention.
[0025] FIG. 5a is variation on the sensing head arrangement of FIG.
5 in which the number of magnets is reduced.
[0026] FIG. 6a is a cross-sectional view of a sensing head similar
to that of FIG. 5, but from which the bridging bar is omitted, the
section taken as indicated by the double-headed arrow 6-6 in FIG.
5.
[0027] FIG. 6b is a cross-sectional view of the sensing head of
FIG. 5, the section taken as indicated by the double-headed arrow
6-6 in FIG. 5.
[0028] FIG. 6c is a cross-sectional view of a sensing head similar
to those of FIGS. 6a and 6b, except that the magnets are depicted
as being electromagnets, and the bridging bars of FIG. 5 are
replaced with a bridging tube surrounding the flow sensing
head.
[0029] FIG. 7 is a schematic end view of a flow probe in which
three permanent magnets supply magnetic flux to two flow
channels.
[0030] FIG. 8a is a schematic side view of a sensing head
configured as a flow probe.
[0031] FIG. 8b is a longitudinal section taken though a flow probe
similar to that of FIG. 8a, but comprising two magnets and two
pairs of electrodes.
[0032] FIG. 8c is an end view of the flow probe of FIG. 8b.
[0033] FIG. 9 is a schematic side view of a flow probe in which two
permanent magnets are used with three pairs of electrodes.
[0034] FIG. 10a is a schematic cross-sectional view of a sensor
arranged to sum the flow generated signal from two sensing
heads.
[0035] FIG. 10b is a schematic cross-sectional view of a sensor
using an array of two of the sensors heads of FIG. 10a.
[0036] FIG. 11 is a schematic cross-sectional view of a sensing
head arrangement using two arrays, each having four sensing heads,
wherein the composite sensor arrangement sums the flow generated
signals from each array.
[0037] FIG. 12 is a partly cut away elevational view of a sensing
head arrangement which alternates both the direction of the fluid
flow past the heads and the magnetic polarity in order to enable
the flow generated signals from all of the heads to be summed.
[0038] FIG. 13 is a schematic cross-sectional view of the head of
FIG. 10a, the section taken as indicated with the double-headed
arrow 13-13 in FIG. 10a.
[0039] FIG. 14 is a sectional view of the head of FIG. 11, the
section taken as indicated with the double-headed arrow 14-14 in
FIG. 11.
[0040] FIG. 15 is a sectional view of the head of FIG. 12, the
section taken as indicated with the double-headed arrow 15-15 in
FIG. 12.
[0041] FIG. 16 is a partial elevational view of a sensing
arrangement using a flow loop in which fluid sequentially flows by
each of a plurality of heads, wherein all the heads have a common
magnetic orientation, and wherein a magnetic subassembly comprising
four permanent magnets and a ferromagnetic disk has been removed to
show the flow loop, electrodes, and second magnetic
subassembly.
[0042] FIG. 17 is a partly cut-away exploded view of a sensing head
assembly comprising a two-part electrically insulating housing that
can be fitted about a fluid-carrying tube without first breaking or
severing the tube.
[0043] FIG. 17a is a partly schematic cross-sectional view of the
sensing head of FIG. 17a, the section taken as indicated by the
double-headed arrow 17a-17a in FIG. 17.
[0044] FIG. 18 is a schematic view of sensing head portion of a
flow sensor of the invention having arrays of electrodes.
DETAILED DESCRIPTION OF THE INVENTION
[0045] FIG. 1 schematically illustrates the basic signal generating
components, or sensing head 10, used in a variety of flow sensors
of the present invention. A magnet 12, which is preferably a
permanent magnet but which may be an electromagnet, is aligned so
that its magnetic axis 14 provides magnetic flux generally
orthogonal to a plane in which fluid 18 is flowing along a
direction of flow 16. When the direction of flow is known, a pair
of electrodes 20, 22 are spaced apart along a line 24 that is
generally orthogonal to both the magnetic axis 14 and to the
direction of flow 16 to sense the flow-responsive signal generated
in the fluid 18. When the direction of flow is not known and a
component of the flow rate along a selected direction is to be
measured, the electrodes are spaced apart along a line orthogonal
to both the magnetic axis and to the selected direction. Those
skilled in the art of magnetic flow meters, or Faraday flow meters,
will understand that although an ideal measuring arrangement of
this sort is discussed in terms of mutually orthogonal axes,
deviations from perpendicularity may occur in practice and result
in corresponding degradation of performance that is explicable by
the well known cosine relationships that are used to deal with
circumstances in which the direction of fluid flow is not exactly
perpendicular to the line along which the electrodes are spaced
apart. Hence, the term "orthogonal", as used hereinafter denotes
generally orthogonal relationships as ideal, but encompasses
deviations from that ideal arrangement.
[0046] As depicted in FIG. 1, one may provide a second pair of
electrodes 20a, 22a spaced apart along a line orthogonal to both
the magnetic axis and to the line along which the first pair 20, 22
of electrodes is spaced apart. In a sensor of this sort, each pair
of electrodes generates a signal proportional to the cosine of the
angle between the line along which the two electrodes of the
respective pair are spaced apart and the direction of flow 16. In
other words, each pair of electrodes is responsive to a component
of fluid flow orthogonal to the line along which the two electrodes
of the pair thereof are spaced apart. In cases in which the
direction of flow is not known a priori, or in which that direction
can vary, a sensor comprising two mutually orthogonal pairs of
electrodes can be used to determine both the magnitude and
direction of flow.
[0047] A preferred sensing head 10 comprises at least one permanent
magnet 12 and at least one pair of electrodes 20, 22 arranged as
described above. As will be described in greater detail
hereinafter, additional magnets 12 or electrode pairs 20, 22 may be
used in some sensing heads 10. These additional components are
schematically indicated in FIG. 1 by black dots 26.
[0048] Turning now to FIG. 2, one finds a simplified block diagram
of preferred electronic circuitry used with the preferred sensing
head 10. A switching device 30, which is preferably a CMOS switch,
but which may be an electromechanical relay or other suitable
switching element, is arranged so that it either places the two
electrodes 20, 22 of a pair in the closed circuit state; or it
places them in the open circuit state in which they are connected
to circuitry adapted to measure the open circuit voltage difference
between them. In a preferred embodiment, the switching device 30
operates under control of an appropriate timing circuit 31 to
provide an operating duty cycle comprising a relatively long period
during which the electrodes 20, 22 are in a closed circuit state
and a relative short interval during which the voltage measurement
is made. The voltage measurement circuit 28 can comprise a
plurality of amplifiers 32, 34, and 36 that can amplify and detect
the difference voltage between the electrodes, and store the
measured voltage in a sample and hold circuit 38 for final
amplification by an output amplifier 40. Such an amplifier chain
would typically incorporate high pass filtering to pass the short
duration voltage pulse present during the measurement interval
while the relay 30 is open, while rejecting any slow changing
voltage typical of electrode drifts. The preferred amplifier chain
incorporates capacitive coupling to the sample and hold 38 so that
its own DC voltage drift will be eliminated from the stored signal.
The output amplifier 40 would typically have a low pass filter to
attenuate noise and ripple resulting from the sampling operation of
the sample and hold 38. The amplifier chain 32, 34, 36 is also
ground referenced to the sensing components 10 so that they operate
within their dynamic range. The ground is preferably a direct
connection to the fluid and typically involves an electrode
attached to the sensing head, a conductive portion of the sensing
head mechanical assembly or connection to electrically conductive
pipes or tubing through which the fluid may be flowing. The ground
may also be obtained by connecting the average voltage of the
electrodes 20, 22 through a high impedance to a ground connection
of the electronics 28.
[0049] The circuitry may include other refinements as apparent to
skilled practitioners in the electronic arts. For example,
amplifiers 32, 34 and 36 need relatively high speed response to
amplify the short pulse of flow rate related signal along with low
noise, and this requires relatively high amplifier operating power.
This power may be reduced by removing operating power except during
their amplification of those pulses. The sample and hold 38, output
amplifier 40 and timing circuits typically operate at very low
power levels so that the output signal would be maintained.
[0050] In one preferred embodiment of the present invention, a
switching device 30 periodically connects the electrodes 20, 22
together during a relatively long period after which the connection
is broken for a much shorter period during which the difference in
their voltages is detected and processed to yield a signal
indicative of flow rate. The electrodes 20, 22 could instead have
been connected to the electrical ground or some other electrical
potential, for example. Should a sustained high rate of fluid 18
flow result in a significant residual electrode 20, 22 voltage
difference due to electrode polarization, this polarization may be
neutralized by connecting the electrodes during at least a portion
of the period that they would have been connected together, to a
source of an opposing voltage. One way to provide polarization
neutralization which is self regulating is to use a portion of the
signal from a voltage divider to ground which is supplied from the
output amplifier 40, as the voltage source to which electrode 22 is
shorted to while connecting the other electrode to the same voltage
magnitude but with opposite polarity as provided by an inverting
amplifier. During the closed circuit state, and at other times, the
paired electrodes may be connected to an AC signal source 27 in
order to aid in attaining stability. The AC source 27 may, for
example, have an output of several volts at a frequency of several
kilohertz.
[0051] The sensing head 10 of FIG. 1 and basic switching and
measurement circuitry of FIG. 2 can be viewed as building blocks
for various embodiments of the present invention and may be used
more than once, as indicated in both figures by the black dots 26.
Because the relay 30 opens for only a very short interval compared
to the drift rate normally associated with the electrodes 20, 22,
the resultant drift voltage will be relatively small. Furthermore,
the electrode signals have a consistent flow rate related magnitude
and polarity which enables those signals to be summed to increase
those magnitudes. However, if the electrodes are of the same
substance, have similar surface areas, and are exposed to the same
fluid environment, they will experience voltage drifts of a random
nature that will tend to average out to zero. The extent of the
neutralization provided depends on the materials used for the
various electrodes as well as on the electrical properties of the
fluid, all of which must be compatible to enable the electrodes to
recover from the closed circuit state quickly enough to sense and
convey the flow generated signals from the fluid to the input
amplifiers 32, 34 while the sample and hold circuit 38 is in its
sampling state. Under some operating conditions the flow rate
signals may change slowly with respect to the drift signals. In
these situations the paired electrodes 20, 22 may be placed in the
closed circuit state for a relatively shorter portion of the
operating duty cycle in order to allow a larger flow generated
voltage to be detected by the sample and hold circuit 38. Moreover,
under some operating conditions, it may be advantageous to operate
the sample and hold 38 multiple times during a single dosed circuit
state portion of an operating duty cycle. In particular, this may
be true when combining the outputs from multiple sensing electrode
pairs.
[0052] In a typical operation the contacts of the relay 30 are
closed and connect the electrodes together almost continuously. For
example, they cyclically close for ninety nine milliseconds and
open for only a one millisecond interval during which the signal
processing occurs. With this method of operation, electrical
currents between the electrodes will tend to equalize their
residual offset voltages to enable instability problems due to
electrode polarization and other factors to be sufficiently reduced
so that a practical flow sensor can be realized without the need to
alternate the polarity of the magnetic flux, thereby enabling a
relatively simple and low cost flow sensor to be produced.
[0053] FIGS. 3 and 4 depict one embodiment of a sensing head in
accordance with the present invention. A tube 42 confines a flowing
fluid that passes around a streamlined member 44 elongated in the
direction of flow and retained in its position along the axis of
the tube by a suitable support that projects through the wall of
the tube and that supports the electrode pair. In a preferred
depicted embodiment, the streamlined member 44 defines a fluid flow
region that, except for the portion intercepted by the support, is
annular. Sensing electrodes 20, 22 are preferably located on
opposite sides of the support so as to define a nearly annular path
over which the flow-related voltage is measured. Magnets 12 provide
a generally uniform field of magnetic flux orthogonal to the
annulus. This use of multiple permanent magnets having their
magnetic axes aligned along radii of the tube, and a streamlined
member 44 reduces the cross sectional area of the passages for the
flowing fluid 18, increases the voltage generating distance between
the electrodes to be nearly equal to an inner circumference of the
tube 42, and increases the magnetic flux in those passages, thereby
increasing the magnitude of the flow-generated signal.
[0054] In FIGS. 3 and 4, a plurality of permanent magnets 12 spaced
apart along a circumference of the outside of a tube 42 are used to
provide the magnetic field. A single magnet magnetized with a
radial orientation of its flux could similarly be used. One or more
magnets with other flux orientations which may use magnetic
materials to direct and concentrate the flux are also usable. A
moving coil loudspeaker, for example, uses a similar radially
oriented flux to activate its voice coil. The magnets may
additionally or entirely be located in the streamlined member 44.
Moreover, the streamlined member 44 may be modified to be a flat
plate to provide the equivalent isolation between the electrodes.
The relative polarity of the magnetic fields may be changed as long
as the flow generated voltages in the fluid do not short circuit
each other.
[0055] FIGS. 5, 6a, 6b, and 6c depict variations of a preferred
embodiment of the flow sensor which is well suited for small pipe
sizes. Magnets 12 provide mutually aiding magnetic flux through the
fluid which is contained by the electrically insulating housing 54.
Electrodes 20, 22, sense the voltage signals generated in the fluid
and route them to the supporting electronics. A separate ground
electrode 50 provides a ground connection between the fluid and the
supporting electronics, and a magnetic trap 48 removes magnetic
debris from the inlet to the sensing head.
[0056] A representative flow sensor having a single sensing head
and generally configured like that of FIGS. 5 and 6a was
constructed using a Type 360 brass body into which a polysulfone
insulating liner having a one half inch bore was inserted and
sealed with O-rings. The brass body was slotted to receive two Nd
rare earth magnets and had holes drilled through it and aligned
with corresponding holes in the liner to receive the sensing
electrodes, which were electrically insulated from the body. Each
of the magnets was one half inch in diameter, one half inch long,
and had a maximum energy product rating of twenty seven
Megagauss-Oersteds. The two sensing electrodes were one quarter
inch in diameter and made of the same alloy as the body. The end of
the brass body, which contacted the fluid 18, was used as the
ground electrode 50. This sen sing head was used in conjunction
with an electronic circuit made in accordance with the depiction of
FIG. 2. A general purpose CMOS switch 30 was used to short the
electrodes 20, 22 together. A CMOS differential amplifier having a
high input impedance and enabling an overall voltage gain of one
thousand to be achieved in the circuit was used to amplify the
electrode voltages. A timing generator 31 supplied pulses with a
duration of one millisecond to the CMOS switch at approximately ten
times per second. The sample and hold circuit 38 was enabled two
hundred microseconds after the CMOS switch opened to allow the
amplified voltage to stabilize before sampling the signal. Sampling
continued for the balance of the CMOS switch open time. An output
from this circuit was approximately 0.125 volts for each gallon per
minute of flow when tap water was passed through the sensing
head.
[0057] When physical dimensions are small, the permanent magnets
used in the depiction of FIGS. 5, 6a and 6b are relatively
inexpensive and can conveniently and economically be used to
provide a medium to high intensity magnetic field throughout the
entire passage used for flow sensing. When only two electrodes and
two magnets are used in the depicted sensor, sufficient flow
related signal relative to electrode drift signal is present to
enable practical sensors to be made this way. The overall sensing
region may be elongated so that multiple sensing heads can be
located along its length, as discussed above with respect to signal
addition and to the detection of ferromagnetic debris. If a sensing
head 10a is configured with two sets of magnets which are
alternated in polarity with pieces of ferromagnetic material 52
joining them from pole to pole on the outside of the flow passage,
the magnetic fields will be mostly confined to the sensing
head.
[0058] In FIG. 5, the ground reference electrode 50 could also be
located between each pair of sensing electrodes 20, 22 rather than
off to the left side as shown. Electrode 50 would then tend to
provide some electrical isolation between the left and right
sensing heads. The effectiveness of this isolation is increased if
electrode 50 is made relatively large and also in the shape of an
annular ring. One or more such rings for isolation between the
sensor heads and also on either side of the sensor heads may be
beneficially used. Electrodes 50 may also be electrically energized
to at least partially cancel the shunting effect of the fluid 18
which occurs with a series connection between 2 or more closely
coupled sensing heads. For example, the signal from the right
sensing head is series added to that of the left sensing head so
that electrode 20 from the left head is connected to electrode 20
from the right head, while electrode 22 from the left head and
electrode 22 from the right provide the output signals.
[0059] Other sensing heads can be mechanically configured in a
parallel array to accommodate a greater flow volume. For example,
the flow probe depicted in FIG. 7 provides two flow passages 54,
two pairs of electrodes 20a, 22a, 20b, 22b and three magnets 12,
one of which is shared between the two flow passages. An
arrangement of this sort reduces the total number of magnets used.
As an additional refinement, a magnetic material in the shape of a
tube may surround the sensing head, at least in the vicinity of the
magnets, in order to complete the magnetic path between the magnets
outside of the flow passage.
[0060] FIG. 8a depicts a sensing head configured as a flow probe
10b in which a single magnet 12 provides the required magnetic
flux. As in other sensing heads described above, a pair of
electrodes 20, 22, sense the voltage signals in a fluid 18 flowing
in a direction perpendicular to the plane depicted in FIG. 8a.
FIGS. 8b and 8c depict a sensing head configured as a flow probe
comprising two closely spaced magnets having the same polarity,
where each of the magnets has a pair of electrodes adjacent it. The
choice of the magnets' polarity in this probe is such as to
minimize the risk of the magnets being bridged by ferromagnetic
debris. When space permits, the magnets may be sufficiently removed
from each other so that they can be oriented with opposite
polarities without bridging. An advantage of opposite polarities is
a shorter and more confined magnetic field outside of the region
used for generating the flow rate responsive signal.
[0061] Turning now to FIG. 9, one finds a depiction of another
sensing head arranged as a flow probe to operate with fluid flowing
perpendicular to the plane of the drawing. In this configuration
the magnets 12 generate a magnetic flux orthogonal to the flowing
fluid passing on either side of the support 44. By adding magnet
and electrode pairs, the magnitude of the flow related signal may
be further increased and averaging of the electrode signals further
improved.
[0062] FIGS. 10a through 15 depict sensing head arrangements which
channel the fluid 18 through various arrays of flow sensing heads
to facilitate the direct summing of the flow related signals. In
all of these figures the magnets are shown dotted in the side view
sections to show their orientation with respect to the flow
passages, as shown in the end view sections.
[0063] In FIG. 10a a fluid flow 18 splits into 2 paths 16 as
defined by housing 54, to pass between pairs of magnets 12 on a
vertical axis as illustrated in the cross sectional view of FIG.
13. A flow generated voltage is thereby generated on the upper and
lower surfaces of each passage which is sensed by electrodes 20 and
22. The electrical connection between the passages is provided by
electrode 23 which may be of a material like that of electrodes 20
and 22, or simply a conductive path provided by the fluid 18. It
will be noted that the electrode 23 in FIG. 10a could be grounded
to yield a balanced measurement. The magnets on the horizontal axis
are provided to complete the magnetic circuit and may also function
as magnetic traps. Ferromagnetic material 52, typically in the disc
shape, completes the magnetic circuit on each side of the sensor.
Since the flow 16 through the sensor passages is in the same
direction and experiences the same polarity of magnetic flux, the
flow generated voltages are additive. The drift voltages, when
similar electrode materials are used, are of a random nature and
will tend to average out to zero.
[0064] FIG. 10b is basically a multi-head representation of FIG.
10a. Output from the 2 pairs of electrode 20, 22, 23 signals would
be electronically combined as indicated earlier and the common
magnetic flux path between the heads may be conveniently conveyed
through ferromagnetic material 52, rather than a permanent magnet
since it would not have a magnetic trapping function.
[0065] FIGS. 11 and 14 depict another multi-head arrangement. The
sensor housing 54 splits the fluid 18 into 4 paths 16, each having
path 2 sensing heads which conveniently allow the magnetic circuit
as illustrated in FIG. 14 to be completed. Electrodes 23 connect
each vertical array of 4 sensing heads together so that the sum of
all of each array appears at electrodes 20 and 22. The outputs from
both arrays are electrically combined by the electronic processing
circuits as indicated earlier.
[0066] FIGS. 12 and 15 depict an arrangement where the fluid 18
flow direction 16 is alternated by flow passages through housing 54
while the polarity orientation of magnets 16 is also alternated.
Electrodes 23 series connect the outputs from all the sensing heads
so that the net voltage at the electrodes 20 and 22 sense the sum
of the flow generated voltage produced by each pair of magnets 12.
Since the magnets 12 alternate in polarity, a magnetic flux path
between them is easily provided by ferromagnetic material 52 as
shown in FIG. 15. The full flow is used to produce a signal at each
generating location so that a relatively high signal output is
possible. The increased pressure drop experienced by the fluid
because of its serpentine routing is tolerable in many
applications.
[0067] FIG. 16 depicts yet another arrangement using a flow loop in
which fluid sequentially flows by each of a plurality of heads. In
this embodiment, all the heads are exposed to the same magnetic
flux orientation so that the flow-generated signals are additive.
It may be noted that although only a single magnetic subassembly
comprising four permanent magnets and a ferromagnetic disk is
shown, a preferred version of this sensing arrangement has a second
magnetic subassembly on the side of the loop from which the view of
FIG. 16 is taken. It will also be noted that a second measurement
stage, similar to that shown in FIG. 16, could be used and arranged
so that the exit port of the sensing array was co-axial with the
inlet port.
[0068] A surface-mount embodiment 65 of the invention depicted in
FIGS. 17 and 17a is adapted to be used with a blood vessel or other
flow tube 55 having weakly electrically conducting walls in which
the electrical conductivity is high enough to allow electrodes
connecting an outer surface 66 of the tube 55 to measure the
flow-related voltage, but low enough, compared to the conductivity
of the fluid 18, so that the shunting effect on the flow generated
signals is tolerable. The embodiment depicted in FIGS. 17 and 17a
preferably uses an electrically insulating housing comprising two
separate parts 56 and 57, each of the housing parts having an
external concave surface 62 shaped so as to receive the tube 55.
With this configuration, the tube 55 can be brought into operative
contact with the sensing head by locating it between the housing
portions 56, 57 without requiring that the flow tube 55 be severed.
If the tube 55 is a blood vessel, it is also soft enough and
conformable enough to enable a reliable contact to be made to the
electrodes 20, 22 and 50 when the vessel is received in the
housing. Because the magnets 12 depicted in FIG. 17a are mutually
attractive, the two housing parts 56, 57 squeeze against each
other, facilitating a good electrical contact with the electrodes
20, 22, 50 as long as a snug or tight fit is provided. As discussed
previously with respect to other embodiments of the invention
(e.g., FIG. 5), the surface mount sensor 65 can employ pieces of
ferromagnetic material to complete various magnetic circuits. In
the interest of clarity of presentation, no ferromagnetic material
is shown in the depictions of FIGS. 17 and 17a.
[0069] In a preferred embodiment of the surface-mount sensor 65,
one of the two housing parts 57 is essentially a cap member that
may contain one or more cap magnets and cap ferromagnetic material,
but that does not contain electrodes or any of the related
circuitry. The major housing member 56 preferably contains the
sensing 20, 22 and ground 50 electrodes; the requisite flow
processing circuitry 28; a power supply 58, which may comprise a
battery or an inductive loop; and a communication circuit 60, which
may comprise RF, optical, acoustic, or other communication
arrangements known in the art, and which may also incorporate
memory allowing it to function as a data store and forward unit of
the type known in the datalogging arts. In this arrangement the
surface-mount sensor can be implanted in a living body for blood
flow measurements, or used in a surgical setting where it is
desirable to have a blood flow sensor that does not encumber the
surgical field with a profusion of wires. Although the cap portion
57 of the housing basically functions to provide a stronger, better
defined magnetic field by the sensing electrodes, it may be noted
that a sensor of this sort, having all the active components
located in one part of the housing, may be operated without using
the cap 57.
[0070] It may also be noted that a surface mount sensor
configuration can be made in which the electrodes 20, 22 and 50
protrude somewhat from the housing 56 to make a better contact with
tube 55. In non-biological applications, an insulating tube may be
used with a surface mount sensor having electrodes configured to
pierce its wall, or may have ports cut into a wall of the
insulating tube so as to allow contact to electrodes disposed on
the housing 56.
[0071] In all of the embodiments previously described a stable
magnetic field needs to be present so as to penetrate the fluid
during the interval when the sensing electrodes are connected to
the voltage measurement circuit to detect the flow generated
voltage difference. The field need not be present at other times.
Thus, although most of the foregoing discussion has addressed the
use of permanent magnets, one could as well choose to use
electromagnets in configuring the various exemplar sensing heads.
The sensing arrangements depicted in FIGS. 5, 5a, 6a, 6b and 6c,
for example, are particularly amenable to the use of electromagnets
(as specifically depicted in FIG. 6c) instead of permanent magnets.
For example, if the magnets 12 in FIG. 5a were electromagnets wound
to have the depicted alternating polarity, that dual sensing head
arrangement would provide a highly efficient magnetic circuit. This
arrangement would have, relative to prior art electromagnetic flow
meters, a high signal to drift voltage output, an advantage that
would accrue even to flow metering arrangements that did not use
the disclosed switching arrangements that connect the electrodes to
a voltage measurement circuit only during a very brief portion of
an operating cycle. Moreover, when an electromagnet is used, the
polarity of the magnetic field may be periodically reversed, in
which case the processing electronics would additionally
incorporate an appropriate rectifying function. Such fields and
electronic processing are commonly used with conventional magnetic
flow meters. By using the features of the present invention to
reduce electrode drift, these sensors become more stable and
tolerant of their installation environment.
[0072] A particular deficiency of prior art permanent magnet flow
meters is that by not reversing the magnetic field or at least
cyclically diminishing it to zero, ferromagnetic debris in the
fluid will be attracted to and accumulate on the sensor surfaces
opposite the pole faces of the magnets 12. Such accumulations will
affect the fluid flow through the sensor, distort its signal
generating magnetic field and shunt the flow generated signal
thereby degrading its precision of measurement in varying degrees
depending on the extent of the accumulations. In applications where
ferromagnetic debris is present in the fluid, the sensing head may
have to be removed from service and cleaned periodically.
[0073] Another approach to minimizing problems with ferromagnetic
debris is to install a magnetic trap 48 on at least the upstream
side of a sensing head. This trap 48 need only consist of a
permanent magnet providing a magnetic flux which effectively acts
as a filter to attract and retain ferromagnetic debris before it
reaches the sensing head. The trap 48 may be mounted in a pipe or
tube in direct contact with the fluid and may be in the form of a
plug which is removable for cleaning. The trap may further be
inserted and removed when isolated though a valve without the need
to stop normal fluid flow. Furthermore, a sensing head based upon
the present invention may be configured as a probe that can be
easily inserted and removed from its flow environment to facilitate
frequent cleaning.
[0074] Because the mechanical, magnetic and electrical components
of a preferred sensor are relatively simple and inexpensive, it is
practical to have the same fluid flow pass through what is
essentially a second sensing head which may also be mechanically
supported by the same housing. While the flow generated signals may
be combined for the purposes of making a flow rate measurement,
they may also be compared to determine whether they differ
substantially. If they do so differ, it would likely be an
indication of accumulation of magnetic debris. It is noted that the
flow responsive components on the inlet, upstream, side of the
sensor would then have functioned as a magnetic trap and attracted
virtually all of the debris, while those on the downstream end
would be clean. When the corresponding difference in output signals
is great enough, it can be used to activate an alarm calling for
sensor servicing. Furthermore, when an upstream sensing head
provides a flow rate signal determined to be substantially in error
because of debris, that signal may be automatically omitted so that
only the output signal produced by the downstream flow responsive
components, with a correct scaling factor, is used to provide the
output signal until the sensor is serviced. The difference
detection and compensation arrangement expressed above is also
useful for detecting any defect in operation in general and thereby
for improving the overall reliability of the sensor.
[0075] Accumulations of magnetic debris are particularly
troublesome when they are electrically conductive because they can
then substantially short circuit the flow generated signal.
However, the short circuit can be detected by periodic measurement
of the electrical resistance between the electrodes. If the
resistance should fall below a predetermined alarm threshold, an
alarm function can then be initiated to advise service personnel of
the need to clean the sensing head. The resistance measurement is
preferably made by occasionally switching a voltage difference of
0.1 volts, for example, across an electrode pair and ground during
a period when the switching device 30 is connecting the electrodes
to the measurement circuit and the sample and hold is disabled. The
electrical resistance is equal to the value of a series resistor
multiplied by the ratio of the difference voltage between the
electrodes and the resistor. This technique may further be used to
compare resistance values between different sensing heads when
making the comparisons recited above in deciding whether to
activate an alarm or to eliminate invalid signals.
[0076] Another alternate or additional approach to protecting a
sensing head from ferromagnetic debris is to periodically remove
the sensor's magnetic field by physically removing the permanent
magnets or turning off electromagnets. This will release the debris
which can then be removed by fluid flow or captured by a trap. The
sensing heads depicted in FIGS. 5, 5a, 6a and 6b are examples of
configurations compatible with removal of the permanent magnets,
which can slide horizontally out of position and then be replaced.
Ideally, the magnets would be configured as one or more modules to
facilitate such removal and replacement.
[0077] Those skilled in the art can now appreciate from the
foregoing description that the broad teachings of the present
invention can be implemented in a variety of forms. For example,
the present sensing method is applicable to revenue collecting
water meters, robotic flow sensing as in paint sprayers,
agricultural water and chemical flow sensors and industrial,
chemical and pharmaceutical flow sensors. A further application
includes biological, medical or animal raising activities because
the inherently small size, low power requirements and absence of
moving parts of the related sensors. Such sensors may be readily
implanted, and in some applications, when the highest precision is
not required, may be used with electrodes and ground connections
that do not penetrate the walls of flow passages (e.g., blood
vessels), but only contact the outside walls of these vessels.
These walls, being weakly electrically conductive, enable such flow
sensors to function. Therefore, while this invention has been
described in connection with particular examples thereof, the true
scope of the invention should not be so limited since other
modifications will become apparent to the skilled practitioner upon
a study of the drawings, specification and following claims.
[0078] Turing now to FIG. 18, one finds a schematic depiction of a
sensing head 10 having paired arrays of electrodes (20a, 20b, 20c,
20d; 22a, 22b, 22c, 22d) instead of the paired single electrodes
20, 22 depicted in FIG. 1. This arrangement permits of a
combinatorial approach to averaging out drift voltages. In this
approach each electrode of one of the two arrays is sequentially
paired with each of the electrodes in the other array, a voltage
measurement is made for each pairing, and the resultant array of
voltage values are averaged. In the depicted example, in which each
array comprises four electrodes, one could measure sixteen possible
combinations of such voltages.
[0079] Although the present invention has been described with
respect to several preferred embodiments, many modifications and
alterations can be made without departing from the invention.
Accordingly, it is intended that all such modifications and
alterations be considered as within the spirit and scope of the
invention as defined in the attached claims.
* * * * *