U.S. patent number RE28,989 [Application Number 05/531,418] was granted by the patent office on 1976-10-05 for electromagnetic water current meter.
Invention is credited to Vincent J. Cushing.
United States Patent |
RE28,989 |
Cushing |
October 5, 1976 |
Electromagnetic water current meter
Abstract
A body means of electrically non-conductive material supports at
least one pair of electrically conductive detecting electrodes
disposed at opposite portions of the body means. Means is supported
within the body means between the electrodes for producing an
alternating magnetic field. An electrical circuit is connected to
the detecting electrodes and includes indicating means. In one form
of the invention, electrically conductive guard means is disposed
adjacent the electrodes and means is provided for establising a
potential on the guard means which is directly proportional to the
potential on the detecting electrodes. In another form of the
invention, means is provided for driving the electromagnet to
produce an alternating magnetic field at a predetermind frequency
wherein the magnetic field is driven to a finite value for a
predetermind time interval during each cycle, and the electrical
circuit includes means for measuring the signal from the detecting
electrodes during a time delayed portion of said time interval. In
a further form of the invention, pairs of oppositely disposed
detecting electrodes are provided at right angles to one another so
as to indicate the direction of water current flow. In each form of
the invention, shield means is preferably employed in the form of
electrically conductive material disposed between the detecting
electrodes and the means for producing the magnetic field.
Inventors: |
Cushing; Vincent J.
(Kensington, MD) |
Family
ID: |
26749227 |
Appl.
No.: |
05/531,418 |
Filed: |
December 10, 1974 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
068674 |
Sep 1, 1970 |
03759097 |
Sep 18, 1973 |
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Current U.S.
Class: |
73/861.15;
73/181; 73/861.17 |
Current CPC
Class: |
G01F
1/58 (20130101) |
Current International
Class: |
G01F
1/58 (20060101); G01F 1/56 (20060101); G01F
001/58 () |
Field of
Search: |
;73/194EM,181 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ruehl; Charles A.
Attorney, Agent or Firm: Pollock, Vande Sande &
Priddy
Claims
What is claimed is:
1. An electromagnetic water current meter comprising body means
formed of non-conductive material, a pair of electrically
conductive detecting electrodes supported by said body means at
opposite portions thereof, means within said body means and
disposed between said electrodes for producing an alternating
magnetic field and including an electromagnet, drive means for
driving said electromagnet to produce an alternating magnetic field
at a predetermined frequency wherein the magnetic field is driven
to a finite value for a predetermined time interval during each
half cycle thereof, and an electrical circuit connected with said
detecting electrodes for measuring the electrical signal from said
electrodes during a time delayed portion of each said time interval
whereby undesired portions of the signal from said detecting
electrodes due to transformer effect voltage are substantially
eliminated, said electrical circuit including a single
sample-and-hold circuit gated by said drive means and having an
alternating current output, and means for processing said
alternating current output to produce an analog output signal
proportional to the peak-to-peak amplitude of said alternating
current output including a band pass filter connected with the
output of said sample-and-hold circuit.
2. Apparatus as defined in claim 1 wherein said means for producing
an alternating magnetic field is surrounded by said body means,
said body means being formed of waterproof material to prevent
water from coming into contact with said means for producing the
alternating magnetic field.
3. Apparatus as defined in claim 1 including a coating of
electrically non-conductive anti-fouling material disposed on the
outer surface of said body means.
4. Apparatus as defined in claim 1 wherein said detecting
electrodes are disposed at the outer surface of said body means and
are formed of an electrically conductive mixture of anti-foulant
and carbon.
5. Apparatus as defined in claim 1 including a second pair of
detecting electrodes supported by said body means at opposite
portions of the body means.
6. Apparatus as defined in claim 5 wherein the electrodes of said
two pairs of electrodes are disposed at 90.degree. intervals from
one another.
7. Apparatus as defined in claim 1 wherein said means for producing
an alternating magnetic field includes an electromagnet having a
coil and a core, said core being formed of substantially eddyless
material to reduce eddy currents.
8. Apparatus as defined in claim 1 including shield means for
electrically shielding said detecting electrodes from said means
for producing the alternating magnetic field.
9. Apparatus as defined in claim 8 wherein said shield means
comprises electrically conductive material disposed between said
detecting electrodes and said means for producing the alternating
magnetic field.
10. Apparatus as defined in claim 9 including lead means connected
with said means for producing the alternating magnetic field, said
shield means also including electrically conductive material around
said lead means.
11. Apparatus as defined in claim 1 wherein said means for
processing includes a phase-sensitive detector connected with the
output of said band pass filter.
12. An electromagnetic water current meter comprising body means
formed of non-conductive material, a pair of electrically
conductive detecting electrodes supported by said body means at
opposite portions thereof, means within said body means and
disposed between said electrodes for producing an alternating
magnetic field and including an electromagnet, drive means for
driving said electromagnet to produce an alternating magnetic field
at a predetermined frequency wherein the magnetic field is driven
to a finite value for a predetermined time interval during each
half cycle thereof, and an electrical circuit connected with said
detecting electrodes for measuring the electrical signal from said
electrodes during a time delayed portion of each said time interval
whereby undesired portions of the signal from said detecting
electrodes due to transformer effect voltage are substantially
eliminated, said electrical circuit including a single
sample-and-hold circuit gated by said drive means and having an
alternating current output, and means for processing said
alternating current output to produce an analog output signal
proportional to the peak-to-peak amplitude of said alternating
current output, the last recited means including a filter for
eliminating dc components of said alternating current output.
13. Apparatus as defined in claim 12 including a second pair of
detecting electrodes supported by said body means at opposite
portions thereof.
14. Apparatus as defined in claim 13 wherein the electrodes of each
of said pairs of electrodes are disposed at substantially
90.degree. intervals to one another.
15. Apparatus as defined in claim 12 wherein said means for
producing an alternating magnetic field includes a coil and a core,
said core being of eddyless construction and being formed of
ferrite material to reduce eddy currents.
16. Apparatus as defined in claim 12 including shield means for
shielding said detecting electrodes from said means for producing
the alternating magnetic field.
17. Apparatus as defined in claim 16 wherein said shield means
comprises electrically conductive material disposed between said
detecting electrodes and said means for producing the alternating
magnetic field.
18. Apparatus as defined in claim 17 including lead means connected
with said means for producing the alternating magnetic field, said
shield means including electrically conductive material surrounding
said lead means. .Iadd. 19. Apparatus as defined in claim 5 in
which said body means is a cylinder of circular cross-section.
.Iaddend..Iadd. 20. Apparatus as defined in claim 6 in which said
body means is a cylinder of circular cross-section. .Iaddend..Iadd.
21. Apparatus as defined in claim 13 in which said body means is a
cylinder of circular cross-section. .Iaddend..Iadd. 22. Apparatus
as defined in claim 14 in which said body means is a cylinder of
circular cross-section. .Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention is directed to an instrument capable of very
accurate measurement of the magnitude of water current as well as
the direction thereof. It is useful for measuring water currents in
the ocean, estuaries, tidal waters, rivers and lakes. It may also
be used to measure the flow of water in aqueducts, sewers, sluices
and flumes. Such an instrument should be linear so that it may be
employed for frequency analysis of water currents and in separating
the steady and alternating components of water flow as, for
example, the measurement of water flow near the surface where
surface waves superpose orbital water velocities on the otherwise
steady movement of water.
A water current meter may be employed in fresh, brackish or salt
water, and accordingly the instrument should be insensitive to the
electrical conductivity of the metered water so that its meter
factor is the same in various types of water. Known water current
meters are sensitive to the conductivity of the metered water
whereby the meter factor is not constant in different kinds of
water. For example, the zero-point or base line moves around
depending on the conductivity of the metered water and this
movement of the base line or zero-point generally changes with time
due to the so-called electrochemical aging effects at the interface
between the instrument and water.
It is desirable to not only measure the magnitude of the water
current but also the direction thereof. In the past, water current
meters have not successfully detected the direction of the water
current where the magnitude and direction of the water current
change with time.
Almost all ocean current measuring devices have employed a
mechanical impeller or propeller which suffers from a number of
drawbacks. The output of such meters is a non-linear function of
water velocity and the output is sensitive to the speed of water
movement regardless of the direction thereof. Additionally, such a
construction is very much subject to marine fouling so that the
meter factor can be trusted only for about 12 to 24 hours of
immersion.
Fouling generally consists of the accretion of a thin layer of
marine organisms on the surface of the instrument. The electrical
characteristics of these organisms may be such as to alter the
signal voltage thereby causing the meter to be inaccurate in
operation.
In all electromagnetic velocity meters, either an alternating or a
steady magnetic field may be used to develop the voltage. However,
the electrical noise associated with electrode electrochemical
polarization is very rich in the low frequency end of the spectrum.
At zero frequency when employing a steady magnetic field, the
polarization voltage is orders of magnitude larger than the flow
induced voltage, and accordingly alternating magnetic induction is
employed.
Alternating induction goes a long way toward solving the
electropolarization problem, but it introduces a "transformer
effect" noise voltage since the alternating flux threads various
circuit loops in the transducer circuitry. This noise is at the
same frequency as the signal produced due to the water current,
although it is 90.degree. out of phase. A phase-sensitive detector
can in principle reject this "transformer effect" voltage if it
works perfectly, and if there are no substantial phase-shifting
mechanisms in the overall transducer circuitry, including that
portion of the circuitry which passes through the metered water.
Unfortunately, such is not the case, and the phase-sensitive
detector does not function in a perfect manner.
It is also desirable to substantially eliminate the various
spurious voltages which are in phase with the signal generated by
the water current and which are indistinguishable from the water
current signal. These voltages have been permitted to exist in the
prior art and variation in these spurious voltages with time causes
a concomitant variation in the zero-point of the instrument.
Accordingly, the measurements obtained by the meter are inaccurate
by the amount the base line drifts from time to time.
The voltage applied to the electromagnet in an alternating
induction water current meter in a practical instrument is on the
order of several volts. The signal voltage generated by water
motion may be on the order of 100 microvolts. If the base line is
to be held steady, no spurious voltages on the order of perhaps 1
microvolt to 0.1 microvolt can be spuriously generated in the water
or in the signal sensing circuitry. If only one part in ten million
or one part in one hundred million of the magnet voltage is
permitted to leak into the signal circuitry, the base line of the
overall instrument may move around intolerably. It is therefore of
fundamental importance that all voltages associated with the
electromagnet be shielded so that there is substantially zero
electrical admittance between the electromagnet circuitry and the
water current sensing circuitry.
In electromagnetic water current meters, a very serious
phaseshifting mechanism is due to the time-dependent
electrochemical effects at the interface between the detecting
electrodes and the water. There is, in effect, a very large
capacitance due to electrochemical effects at the interface between
the electrodes and the water. This capacitance coupled with the
resistance of the water and/or the resistance of the electrode
itself is a serious phase-shifting mechanism that shifts the
"transformer effect" voltage partially so that it becomes
indistinguishable from the signal voltage produced by water flow.
Electromagnetic water current meters heretofore have employed a
sinusoidal alternation in the magnetic induction. This
electrochemical phase-shifting mechanism shifts part of the
"transformer effect" voltage by 90.degree. so that it is sensed by
the signal sensing circuitry which, of course, is very
undesirable.
For a given phase-shifting mechanism, such as the electrochemical
phase-shifting mechanism at the electrode-water interface, a
sinusoid is perhaps the poorest form of alternation to be employed.
In other words, the signal sensing circuitry is most sensitive to
the phase-shifting mechanisms when a sinusoidal alternating
magnetic induction is employed.
SUMMARY OF THE INVENTION
In the present invention, a body means of electrically
non-conductive material is provided and a pair of electrically
conductive electrodes are supported by the body means at opposite
portions thereof. Means such as an electromagnet is supported
within the body means and between the electrodes for producing an
alternating magnetic field. The detecting electrodes are connected
with an electrical circuit including indicating means for measuring
the water current.
In one form of the invention, guard means is supported by the body
means adjacent the electrodes and is insulated therefrom. The
electrical circuit is connected with the guard means for
establishing a potential on the guard means which is substantially
directly proportional to the potential on the detecting electrodes
and which is preferably at substantially the same potential.
This guard arrangement is particularly suited for overcoming the
fouling problem. By driving the guard means or electrodes such that
they are substantially at the same signal potential as the
detecting electrodes, the overall flow induced voltage is restored
to its correct value which makes the overall instrument less
susceptible to the effects of fouling.
This guard arrangement is additionally useful when the detecting
electrodes and the guard means are not in direct contact with water
but are separated therefrom by a sleeve formed of electrical
insulating material. This sleeve may also contain toxins for
preventing marine fouling.
In another form of the invention, the means for producing the
alternating magnetic field includes an electromagnet along with
means for driving the electromagnet to produce an alternating
magnetic field at a predetermined frequency wherein the magnetic
field is driven to a finite value for a predetermined time interval
during each cycle. The electrical circuit in this form of the
invention includes means for measuring the output signal from the
detecting electrodes during a delayed portion of the time interval
whereby "transformer effect" voltage is substantially
eliminated.
This construction employs a drive means which produces a
rectangular or square wave output. This drive means produces a
magnetic alternation wherein the time rate of change of the
magnetic induction is zero for a substantial portion of the period
of alternation.
The electrical circuit is such that the sensing electronics remains
gated closed while the magnetic induction is changing and for an
additional length of time thereafter to permit transients to die
out whereupon the sensing electronics is gated open during the
latter portions of the interval when the time rate of change of the
magnetic induction is zero.
The electromagnet as well as the leads thereto is substantially
fully shielded from the detecting electrodes and the electrical
circuit associated therewith so as to eliminate causes of base line
drift in the overall water current metering instrument.
In still another form of the invention wherein it is desired to
measure not only the magnitude but the direction of flow of the
water current, two pairs of detecting electrodes are employed, each
pair being disposed at substantially opposite sides of the body
means, the two pairs of electrodes being disposed at right angles
to one another. These two pairs of electrodes enable the
measurement of two vector components of the current flow enabling
the direction of flow to be readily determined. The meter is
capable of continuous measurement of the vector components of water
current even though the magnitude and direction of water current
may be changing with time.
The meter of the present invention has no moving parts and has a
substantially zero threshhold. It is capable of measuring both the
magnitude as well as the vector components of water current. The
instrument is useful for metering fresh or salt water and may also
be employed in other applications with any adequately conducting
liquid. The form of the invention employing the guard means may
also be employed for metering either conductive or dielectric
liquids.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a top perspective view partially broken away illustrating
a first form of the invention;
FIG. 2 is an enlarged view of the lower end of the device shown in
FIG. 1 and partially broken away for the purpose of
illustration;
FIG. 3 is a sectional view taken substantially along line 3--3 of
FIG. 2 looking in the direction of the arrows;
FIG. 4 is a flattened view of the surface of the meter adjacent the
sensing electrodes;
FIG. 5 is an enlarged vertical section through the meter shown in
FIG. 1;
FIG. 6 is a sectional view taken substantially along line 6--6 of
FIG. 5 looking in the direction of the arrows;
FIG. 7 is a sectional view similar to FIG. 5 illustrating a
modified form of the invention;
FIG. 8 is a sectional view taken substantially along line 8--8 of
FIG. 7 looking in the direction of the arrows;
FIG. 9 is flattened view of the outer surface of the meter adjacent
the detecting electrodes and the associated guard means;
FIG. 10 is a vertical longitudinal section through still another
form of the invention;
FIG. 11 is a sectional view taken substantially along line 11--11
of FIG. 10 looking in the direction of the arrows;
FIG. 12 is a schematic illustration of an electrical circuit which
is adapted to be employed with the embodiment shown in FIGS. 1-6,
inclusive;
FIG. 13 is a schematic illustration of an electrical circuit
adapted to be employed with the embodiment as shown, for example,
in FIGS. 7-11, inclusive;
FIG. 14 is a schematic illustration of an electrical circuit
adapted to be employed with the meter of the present invention if
the meter is energized by a conventional sinusoidal drive;
FIG. 15 is a schematic illustration of an electrical circuit of
still another form of the invention wherein the drive means for the
electromagnet produces a rectangular output voltage;
FIG. 16 is a waveform and timing diagram for the circuit shown in
FIG. 15;
FIG. 17 illustrates a further modification of the invention;
FIG. 18 is a sectional view taken substantially along line 18--18
of FIG. 17 looking in the direction of the arrows; and
FIG. 19 is a sectional view taken substantially along lines 19--19
of FIG. 17 looking in the direction of the arrows.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings wherein like reference characters
designate corresonding parts throughout the several views, a first
embodiment of the invention is illustrated in FIGS. 1-6, inclusive.
The meter includes a transducer portion indicated generally by
reference numeral 20 comprising a cylindrical body means 22 formed
of a relatively rigid electrically non-conductive material such as
fiberglass or the like. The body means includes an end portion 24
which closes off the end of the body means. The exterior of the
body means is covered by a coating 26 formed of non-conductive
anti-fouling paint or the like which inhibits fouling of the
apparatus. It will be noted that the transducer portion of the
meter is generally cylindrical and includes a rounded lower end to
minimize disturbances to the water velocity profile in the
neighborhood of the transducer.
As seen in FIG. 5, a first pair of detecting electrodes 30 and 32
are supported on the outer surface of the body means at
substantially diametrically opposite portions thereof. Each of
these detecting electrodes is substantially rectangular in
configuration as seen in FIG. 4, the detecting electrodes being
formed of a suitable electrically conductive material such as a
conductive mixture of anti-fouling paint and carbon. The detecting
electrodes 30 and 32 are connected with suitable leads X.sub.1 and
X.sub.2 respectively, these leads extending through holes provided
in the wall of body means 22.
A second pair of detecting electrodes 34 and 36 are supported on
the outer surface of the body means and are disposed at
substantially diametrically opposite portions thereof. The
electrodes of the two pairs of detecting electrodes are disposed at
90.degree. intervals from one another as can be seen most clearly
in FIGS. 4 and 6. Electrodes 34 and 36 are connected with leads
W.sub.1 and W.sub.2 which also extend through holes provided in the
body means. The leads connected with the detecting electrodes are
in turn connected with an electrical circuit hereinafter
described.
A ground electrode 40 of generally annular configuration is
supported on the outer surface of the body means. This ground
electrode is formed of an electrically conductive material and
includes a gap 42 in one portion thereof so as to prevent the
deleterious effect of a completely short circuited loop. A lead G
is connected to the ground electrode and extends through a suitable
hole provided in the wall of the body means. This lead is in turn
connected to instrument ground.
A means for producing an alternating magnetic field is disposed
within the body means and between the detecting electrodes. This
means comprises an electromagnet having a core 50 and a coil
winding 52 disposed therearound, this coil winding in turn being
connected with suitable leads Z.sub.1 and Z.sub.2 extending to a
portion of an electrical circuit hereinafter described. The
electromagnet is of a generally eddyless construction including a
core formed of ferrite material which substantially eliminates eddy
currents. For low frequencies of alternating magnetic induction
such as 30 Hz, it is satisfactory to wind the coil with ordinary
copper wire. If high frequency induction is used such as 1 kHz or
above, then eddy current production in the coil wire itself may be
dilatorious and instead of employing copper wire, Litz wire may be
employed in the coil.
The coil is potted so as to provide a body 54 of insulating
material around the coil. A thin layer 56 of electrically
conductive material such as silver paint is provided on the outer
surface of insulation 54 to serve as a shield for shielding the
electromagnet from the detecting electrodes. If the frequency of
magnetic alternation is sufficiently low such as 30Hz, silver
conductive paint may be employed and eddy currents produced in such
a conductive paint are tolerable at this low frequency. On the
other hand, if the frequency is large, such as 1 kHz, eddy current
production in the conductive layer 56 may become intolerable and a
conductive surface of less conductivity such as carbon conductive
paint may be employed.
Leads Z.sub.1 and Z.sub.2 are a shielded twisted pair of wires
having a cylindrical shield 60 of electrically conductive material
disposed in surrounding relationship thereto. This shield 60 is
faired into layer 56 so that all large voltages associated with the
electromagnet, whether they be on the coil windings themselves or
on the twisted pair of lead wires, are thoroughly shielded from the
signal detection circuitry including the water in contact with the
detecting electrodes. This overall shielding of the magnet
circuitry must be thorough to the extent that the electrical
admittance between the magnet circuitry and the detection
circuitry, including the water in contact with the detection
electrodes, is negligible compared with the electrical admittance
between the detecting electrodes and instrument ground.
It will be noted that the sensing portion of the meter is disposed
near one end of the transducer portion, while the signal
conditioning electronics or a portion thereof is separated from the
sensing portion so that the amount of the alternating magnetic
induction which threads the signal conditioning circuitry is at a
minimum since spurious voltages generated thereby could be
troublesome. Accordingly, the signal conditioning electronics may
be disposed at the opposite end of the transducer portion of the
apparatus or it may be preferably disposed in a separate secondary
unit at a substantial distance from the transducer portion of the
apparatus and connected thereto by a suitable underwater cable or
transmission line 64 as seen in FIG. 1. As seen in FIG. 12, the two
leads X.sub.1 and X.sub.2 are adapted to be connected to two leads
X'.sub.1 and X'.sub.2 respectively, which are in turn connected to
a differential amplifier the output of which is connected with
suitable signal conditioning circuitry. Lead G, previously
described, is connected with lead G' which in turn is connected to
ground. It should be understood that each pair of electrodes is
connected to a separate differential amplifier, and accordingly a
further differential amplifier similar to amplifier 70 would be
connected with the leads W.sub.1 and W.sub.2 as seen in FIG. 6 of
the drawings.
Referring now to FIG. 2 of the drawings, the electromagnet produces
something like a dipole magnetic induction which passes out through
the metered water. The magnetic field is indicated by the dashed
lines as shown in this figure. Motion of the water through this
alternating magnetic field induces an alternating electric field in
the water, this electric field in turn being sensed by the
detecting electrodes.
The first pair of detecting electrodes 30 and 32 senses a voltage
which is directly proportional to the component of water flow
perpendicular to the plane of this figure. The second pair of
detecting electrodes disposed at right angles to the first
mentioned pair of detecting electrodes senses a voltage which is
directly proportional to the component of the water flow parallel
to the plane of FIG. 2.
The windings of the electromagnet are energized by an alternating
voltage that is large compared with the voltage induced in the
water due to water motion. FIG. 3 illustrates the electric
potential field induced by the water motion. For a water velocity
in the direction as indicated by the arrow in this figure, the
induced electric field is indicated by the solid lines. The lines
of constant induced potential in the water are indicated by the
dotted lines. Electrodes 30 and 32 sense the electric field
developed by the water moving in the direction of the arrow in this
figure whereas electrodes 34 and 36 will sense a zero voltage since
the component of water flow velocity perpendicular to the line
connecting these latter two electrodes is zero. Each pair of
detecting electrodes senses a voltage whose waveshape is identical
to that of the alternating magnetic induction.
Because of the symmetry of the magnetic induction field, the
arrangement as shown in FIGS. 1-6 senses the two water velocity
components perpendicular to the axis of the circular cylinder. The
device is insensitive to the component of water flow parallel to
the axis of the cylinder.
This form of the invention is capable of measuring the two vector
components of water velocity which are perpendicular to the axis of
the transducer. If it is desired to measure only one of these
vector components of water velocity, one of the pairs of electrodes
shown in FIG. 6 may be eliminated. In this case, the area where the
electrodes are eliminated would be covered by anti-fouling paint
since the exterior of the body means is fully covered with
anti-fouling paint except for the electrode areas to minimize the
growth of marine fouling.
If only one pair of electrodes is employed, the cross section of
the transducer should be modified to provide a hydrofoil or
eliptical cross section. The detecting electrodes would be placed
on opposite sides of the minor axis of such an elipse. This type of
configuration is more streamlined and minimizes the disturbance of
the water flow due to flow separation and the generation of eddies.
A streamlined version of the meter employing a single pair of
electrodes can operate satisfactorily with greater water
velocity.
Referring now to FIGS. 7-9 inclusive of the drawings, a modified
form of the invention is illustrated. This construction is similar
to that previously described, the only difference being that each
of the detecting electrodes in the form of the invention is
surrounded by a guard electrode. The components in this form of the
invention, similar to those previously described have been given
the same reference numeral primed. As seen most clearly in FIG. 9,
detecting electrodes 30', 34', 32' and 36' are surrounded by guard
electrodes 80, 84, 82 and 86 respectively, each of these guard
electrodes being substantially rectangular in configuration and
including a central aperture which receives the associated
detecting electrode and provides an insulating gap between the
detecting electrode and the adjacent guard electrode. Each of the
guard electrodes is formed of a suitable electrically conductive
material.
The leads X.sub.1 and X.sub.2 to the detecting electrodes
correspond to the similar leads in the previously described
embodiments of the invention. Leads Y.sub.1 and Y.sub.2 are
connected to guard electrodes 80 and 82 respectively, as seen in
FIG. 7, these leads being of tubular construction and being
disposed in spaced surrounding relationship to leads X.sub.1 and
X.sub.2 so as to shield these latter mentioned leads. Leads Y.sub.1
and Y.sub.2 are connected to the associated guard electrodes
through suitable holes provided in the body means.
Referring now to FIG. 13, an electrical circuit is indicated
wherein the leads indicated in FIG. 7 are connected to the leads
having corresponding reference characters primed. It will be noted
that the leads X.sub.1 and X.sub.2 are connected with the input of
unit gain preamplifiers 90 and 92 the outputs of which are each
connected to a differential amplifier 94. The output of this
differential amplifier is in turn connected with suitable signal
conditioning circuitry hereinafter described.
The output of preamplifier 90 is connected to lead Y.sub.1 while
the output of preamplifier 92 is connected to lead Y.sub.2. With
this arrangement, the entire shield on the transmission line as
well as on the guard electrode to which the shield is electrically
connected is at the same flow generated potential as the detecting
electrodes.
The guard means or guard electrodes serve to establish a potential
in the neighborhood of the detecting electrode which is directly
proportional to the potential on the detecting electrode. The best
constant of proportionality is unity so that the guard electrode is
driven at the same flow generated potential as the detecting
electrode. Other constants of proportionality may be employed
simply by using something other than a unit gain preamplifier. In
an extreme case, the constant of proportionality may be zero
whereby the guard electrode may be connected to instrument ground
and preamplifiers 90 and 92 may be eliminated.
The construction shown in FIGS. 7-9 is adapted to measure two
vector components of water velocity. The circuitry shown in FIG. 13
is adapted to be connected with one set of detecting electrodes, it
being understood that an identical duplicate set of preamplifiers
and an associated differential amplifier would necessarily be
connected with the second set of detecting electrodes. If it is
only desired to measure one vector component of water velocity, one
pair of electrodes and the corresponding electrical circuitry may
be eliminated. If this is done, the cross-sectional configuration
of the device would be altered to provide a more streamlined
configuration, as discussed hereinbefore.
Referring now to FIGS. 10 and 11, still another form of the
invention is illustrated. This form of the invention is similar to
that shown in FIGS. 7-9 inclusive, and identical parts have been
given the same reference characters in FIGS. 10 and 11 as applied
to the components shown in FIGS. 7-9. In this latter modification
of the invention, the entire transducer is surrounded by a sleeve
100 which is formed of a suitable, preferably dielectric, material.
In this way, the detecting electrodes as well as the guard
electrodes and the ground electrode are all prevented from being in
direct contact with the metered water. This sleeve may be made of a
material with excellent toxic properties so as to eliminate or
minimize marine fouling and it may be of a material to minimize
electrochemical phase-shifting mechanisms that ordinarily operate
at the interface between a conductive electrode and water.
Without such a sleeve, the detecting electrode in direct contact
with the metered water has a relatively low contact resistance, or
in other words, it has a relatively high contact admittance. The
contact admittance to the water is greatly reduced when utilizing a
sleeve formed of low conductivity or dielectric material.
Accordingly, the guard electrodes must be of modified construction
so as not only to surround the detecting electrodes on the surface
of the transducer as in the previously described modification but
also to be positioned between the detecting electrodes and the
inner side thereof.
As seen in FIGS. 10 and 11, guard electrodes 80, 82, 84, and 86 are
provided with extended portions 80', 82', 84' and 86' respectively
which are disposed inwardly of the associated detecting electrodes
and which extend to the inner surface of the body means and are
connected with the respective tubular leads. In the arrangement
shown in FIGS. 10 and 11, the entire detecting electrodes as well
as the associated signal transmission leads are completely guarded.
Since the electrodes are not in direct contact with water, the
electrodes may not be made from a conductive mixture of
anti-fouling paint and carbon, but, on the contrary, ordinary
conductive carbon paint may be used.
The various electrical leads shown in FIGS. 5, 7 and 10 are
connected by means of a suitable underwater cable to a signal
processing unit which is located at some remote position.
Alternatively, the signal processing unit may be incorporated
within an oversized transducer itself so that the unit is
self-contained, including batteries for power. In the latter case,
a self-contained recorder may be employed for readout at a later
time.
If the electromagnet of the meter is energized by conventional
sinusoidal magnet drive, a signal conditioning circuit as shown in
FIG. 14 is employed, the input X to the phase-sensitive detector
110 being connected to the output of the differential amplifier of
either the circuit shown in FIG. 12 or FIG. 13, as the case may be.
The output of this phase-sensitive detector is an analog voltage
proportional to the vector component of current sensed by a pair of
detection electrodes.
An amplitude stable sinusoidal oscillator 112 energizes the power
amplifier 114 so that the power amplifier's output current is
amplitude stable. This power amplifier is connected with one of the
leads Z.sub.1 to the electromagnet, the other of the leads Z.sub.2
of the electromagnet being connected to instrument ground through a
small resistor 116. This resistor develops a voltage which is in
phase with the electromagnet current and which is used as the
reference voltage for the phase-sensitive detector. The system is
sensitive to voltage which is in phase with the magnetic induction,
or, in other words, is sensitive to the voltage developed by the
water current and is insensitive to quadrature voltage which is
proportional to the time rate of change of the magnetic induction,
i.e., the so-called "transformer effect" voltage.
Referring now to FIG. 15 of the drawings, a further modified form
of the invention is illustrated which is especially adapted to
minimize the deleterious effects of various phase-shifting
mechanisms or time delaying mechanisms such as the electrochemical
phase-shifting mechanism at the electrode-water interface discussed
hereinbefore.
An oscillator 120 operates at some multiple of the desired
operating frequency. The output of this oscillator is connected
with a digital divider, the output of which provides the desired
operating frequency and also the waveforms necessary to operate the
remainder of the signal conditioning circuitry. The output of the
digital divider is connected with a decoder 124.
A the output of the decoder 124 which is fed to the input of a
power amplifier 126 has a current waveform as illustrated in FIG.
16f. This is a rectangular or square wave voltage. The output of
the power amplifier is connected to leads Z.sub.1 and Z.sub.2
connected with the electromagnet. Since the electromagnet is an
inductive load, it is unable to faithfully follow the waveform of
the input voltage to the power amplifier, and accordingly the
output current waveform of the power amplifier is illustrated in
FIG. 16g. The important thing to note is that this output waveform
of the power amplifier includes a portion of the square wave input
waveform wherein the current or voltage is not changed with time
during each half cycle of the alternation.
Since the magnetic induction is directly proportional to the
current passing through the electromagnet, means is provided for
driving the electromagnet to produce an alternating magnetic field
at a predetermined frequency wherein the magnetic field is driven
to a finite value for a predetermined time interval during each
half cycle of operation.
A conventional sample-and-hold gate 130 is connected with the
output of one of the differential amplifiers as previously
described. The voltage waveform appearing at the input of the
sample-and-hold circuit is indicated in FIG. 16a. The larger
portions of the voltage waveform labeled V.sub.1 are due to
"transformer effect" and they represent large voltages generated
while the magnetic field is changing. When the magnetic field is
not changing, the "transformer effect" voltage substantially
disappears and only the signal voltage V.sub.s as shown in FIG. 16a
remains. During this portion of the alternation, the
sample-and-hold gate may be opened to admit the flow signal.
However, phase-shifting mechanisms or time-delay mechanisms cause a
short after-effect of the large "transformer effect" voltage.
In order to eliminate virtually all of this after-effect, the gate
is not opened immediately after the "transformer effect" voltage
disappears, but rather an additional time delay is interposed
before the signal gate is opened. The waveform from decoder 124 to
the sample-and-hold gate is shown in FIG. 16d which causes the
sample-and-hold circuit to be gated open only at the latter portion
of each half cycle of alternation of the magnetic induction.
The advantages of sampling for a short interval toward the end of
each half period of alternation of the magnetic field is that not
only is the "transformer effect" voltage rejected but also any
effects due to phase-shifting or time-delay mechanisms are allowed
to decay to a small value. Since the "transformer effect" and its
after-effects are responsible for the variation in base line of the
overall instrument, this form of the invention provides much better
results than the sinusoidal excitation generally used with
electromagnetic water current meters.
The output of the sample-and-hold circuit is fed to a band pass
filter 132, and the waveform at the output of the sample-and-hold
circuit is shown in FIG. 16b. This output voltage is substantially
a square wave with a peak-to-peak value proportional to the signal
generated by water current. The band pass filter filters this
signal so that only the fundamental sinusoidal component is
retained, and the output of the band pass filter is illustrated in
FIG. 16c. This sinusoidal output is then fed into a phase-sensitive
detector 134.
The waveform of the signal from the decoder to the phase-sensitive
detector is shown in FIG. 16e, and the final output of the
phase-sensitive detector is then fed to a suitable indicating means
such as a volt meter 136 connected with ground or any other
suitable indicating means. The output is an analog voltage
proportional to the water current components sensed by a particular
pair of electrodes. It will, of course, be understood that a
similar circuit can be employed with a second pair of
electrodes.
The phase-sensitive detector used in this circuit is required only
so that it is sensitive to the phase as well as magnitude of the
signal at its input. The phase-sensitive detector's output voltage
is positive for water current flow in one direction and the output
voltage passes through zero and changes sign as the sensed
component of water current passes through zero and reverses at the
transducer. The phase-sensitive detector is not called upon to
reject quadrature voltage or "transformer effect" voltage as is
required in the conventional sinusoidal signal conditioning circuit
as shown in FIG. 14 and, accordingly, the quadrature rejection
capabilities of the phase-sensitive detector shown in FIG. 15 are
not nearly as stringent as those shown in FIG. 14.
At this juncture, it is well to note that the sampling signal
conditioning technique described in FIG. 15 has an especial added
advantage with electromagnetic water current meters in that it can
be made capable of rejecting undesirable power frequency spurious
voltages (e.g. 60 Hz and harmonics thereof) which often times exist
where water current measurements is desired, e.g. when flow
measurement is made near an hydro-electric power plant. It is
difficult to shield the sensing circuit on electromagnetic water
current meters from these extraneous noises because its sensing
electrodes face outward into the metered medium. The key to the
signal conditioning's capability for power frequency noise
rejection is to note from FIG. 16d that the frequency of sampling
is double the frequency of the alternating magnetic induction, as
shown in FIGS. 16f and/or 16g. As an explicit example, if the
electromagnetic water current meter operates with a magnetic
induction of 30 Hz, its sampling gate in this form of the invention
is opened at a 60 Hz rate. We have already seen how a 30 Hz signal
(i.e. our water current flow signal) is handled by this system so
that there is ultimately an analog output voltage indication
proportional to water current flow. A 60 Hz signal, however, is
completely rejected for the following reasons. Since the sampling
gate is opened once every one-sixtieth of a second, the sampling
system samples an identical portion of the repetitive 60 Hz noise
signal at each sample, and the sample-and-hold system therefore
retains a constant, dc voltage at its output. The ensuing 30 Hz
filter completely rejects this dc voltage, and by this means the
system fully accepts the 30 Hz water flow signal, but completely
rejects a 60 Hz spurious noise signal.
Consideration of the harmonics of 60 Hz shows that the system also
fully rejects these spurious noises. Further consideration shows
that similar complete rejection of 60 Hz and its harmonics is
obtained if the sampling gate is operated at frequencies of 60 cps,
30 cps, 15 cps, etc. or in general is operated at the power line
frequency or any subharmonic thereof. If the sample system is
operated at these frequencies, we say that the system is
synchronously rejecting power frequency spurious noise. At the same
time, of course, the magnetic induction in the water current meter
is alternated at a frequency which is half the frequency of the
sampling system. In all cases the .[.bank.]. .Iadd.band
.Iaddend.pass filter after the sample and hold has its center
frequency at the magnetic induction frequency.
Best operation is obtained .[.in.]. .Iadd.if .Iaddend.the sampling
gate is synchronously locked to the power line frequency (whether
it be 60 Hz, 50 Hz, 25 Hz, etc.), or to a subharmonic thereof.
Quite good rejection is obtained, however, provided the gate
frequency is substantially closer to the aforedescribed synchronous
rejection frequencies.
Referring now to FIG. 17 of the drawings, yet another embodiment of
the invention is illustrated. The transducer in this form of the
invention comprises a body means including two spaced generally
parallel coil means 140 and 142 interconnected by spaced cross
members 144. The body means is symmetrical about longitudinally
extending axis A--A. Each of the coil means is annular in
configuration and lies in a plane, the two planes being disposed
parallel with one another. The two coil means are connected to a
common coil cable 146.
A pair of spaced detecting electrodes 150 and 152 are supported on
a support member 154 formed of electrically non-conductive material
and connected with a pair of cross members 144. The detecting
electrodes are in turn connected with an electrode cable 156.
The detecting electrodes are supported within the confines of the
coil means and lie in a plane which is parallel to the planes of
the two coil means and perpendicular to the longitudinal axis of
the body means.
The arrangement as shown in FIG. 17 produces a signal voltage which
is proportional to the component of water velocity parallel to the
plane of the coil means and perpendicular to a line connecting the
two detecting electrodes. Here again, large voltages are necessary
to energize the electromagnet and it is imperative that these large
voltages be thoroughly electrically shielded from the metered water
and from the detection circuitry in order to achieve an instrument
with high accuracy and substantially zero base line variation.
As seen in FIG. 18, each of the coil means includes a central coil
winding 160 surrounded by a layer of insulating material such as
fiberglass which in turn is surrounded by a layer 164 of
electrically conductive material such as conductive silver paint.
This conductive layer is in turn surrounded by a further layer 166
of electrical insulating material such as fiberglass.
If the entire surface of the coils were surrounded with a
conductive layer of silver paint or the like this would constitute
a very sizable short-circuited circuit loop which would dissipate
considerable energy, or more seriously, produce some phase-shifted
magnetic flux. It may be preferable to form this shield of a less
conductive material such as carbon paint rather than silver paint.
Another solution to the problem is illustrated in FIG. 19 wherein
the conductive layer 164 is broken at one point to provide an
overlapped break or gap 170 so as to prevent a short-circuited
conductive loop. Instead of a simple break, an interleaved brake
may be provided to maintain good electrical shielding
capability.
* * * * *