Abstrict An electromagnetic fluid flow meter includes a probe or spool type
transducer whose output is coupled through a preamplifier to a sampling
switch. The output of the sampling switch is processed by a signal
processing amplifier and then coupled through a phase sensitive
demodulator to utilization circuits. To reduce the effect of spurious
signals, a storage device is connected at the output of the phase
sensor demodulator, such as a capacitor. The effect of the spurious
voltages is eliminated by first detecting the presence of a spurious
voltage or occurrence of an event capable of producing a spurious
voltage and then disabling the phase sensitive demodulator so that
the signal previously applied to the utilization circuits, which
is also stored on the storage device, is maintained during the presence
of the spurious voltages. In one form of the invention in which
the electromagnetic flow meter is of the spool type, spurious voltages
may be induced by triboelectric effects. Such voltages are sensed
by comparing the absolute value of the signal against a reference.
In another embodiment in which the flow meter may be of a probe
type suspended in an open channel, spurious signals may be induced
by vertical motion of the probe. Vertical velocity, when sensed,
produces a signal to disable the phase sensitive demodulator. The
disablement may be for either a predetermined period of time or
only so long as the spurious voltage exists.
Claims What is claimed is:
1. An electromagnetic flow meter for measuring fluid flow velocities
by use of electromagnetically induced voltages having a signal processing
chain including sensing electrodes and an amplifier coupled thereto,
a sampling switch coupled to said amplifier for sampling said amplifier,
signal processing means coupled to said sampling switch, a phase
sensitive demodulator coupled to said signal processing means and
at least one utilization circuit coupled to said phase sensitive
demodulator, wherein the improvement comprises means for disabling
said demodulator in the presence of spurious signals and a storage
device for providing to said utilization circuit a signal representative
of flow velocity prior to the disablement of said phase sensitive
demodulator.
2. The apparatus of claim 1 in which said means for disabling said
demodulator comprises a signal responsive means having an input
coupled to said signal processing chain.
3. The apparatus of claim 2 in which said signal responsive means
is coupled to a wide band portion of said signal processing chain.
4. The apparatus of claim 1 in which said means for disabling said
demodulator in the presence of spurious signals disables said demodulator
for at least a predetermined time.
5. The apparatus of claim 1 which further includes
a magnet drive for producing a magnetic field,
said means for disabling including logic means having a clock and
coupled to and driving said magnet drive,
said logic means also coupled to and controlling said phase sensitive
demodulator.
6. The apparatus of claim 5 in which said logic means includes:
counter means driven by said clock for producing plural pulse outputs,
each pulse output displaced from others of said pulse outputs,
bistable device responsive to a one of said pulse output for sequentially
changing its condition,
a pair of gating means, each coupled to a different output of said
bistable device,
a further gating means inhibited under conditions generating spurious
signals, said further gating means providing an input for each of
said pair of gating means,
said pair of gating means coupled to and controlling said phase
sensitive demodulator.
7. The apparatus of claim 6 in which said phase sensitive demodulator
comprises:
a first switching means coupled to said signal processing means,
an inverter coupled to said signal processing means,
a second switching means coupled to said inverter,
a first of said pair of gating means coupled to said first switching
means and a second of said pair of gating means coupled to said
second switching means.
8. The apparatus of claim 7 in which said means for disabling also
includes:
an absolute value circuit coupled to said signal processing chain
with an output and a comparator for comparing said output to a reference
level, said comparator controlling inhibiting of said further gating
means when said output exceeds said reference level.
9. The apparatus of claim 8 in which delay means is coupled between
said comparator and said further gating means for delaying termination
of said inhibiting.
10. The apparatus of claim 9 in which said delay means includes:
a second counter normally held in a reset state and preset to a
predetermined count by said comparator, said clock coupled to said
second counter for counting said second counter from said preset
to said reset state.
11. The apparatus of claim 8 in which said absolute value circuit
is connected to said amplifier.
12. The apparatus of claim 11 in which said sensing electrodes
are located on a spool transducer for sensing flow in a closed conduit.
13. The apparatus of claim 7 in which said sensing electrodes are
mounted on a probe transducer subject to spurious vertical movement
in which said means for disabling includes:
a vertical velocity sensor and a comparator having as one input
a signal related to vertical velocity and another input a reference
level, said comparator controlling inhibiting of said further gating
means when said one input exceeds said reference level.
14. The apparatus of claim 13 in which said delay means is coupled
between said comparator and said further gating means for delaying
termination of said inhibiting.
15. The apparatus of claim 14 in which said delay means includes:
a second counter normally held in a reset state and preset to a
predetermined count by said comparator, said clock coupled to said
second counter for counting said counter from said preset to said
reset state.
16. An electromagnetic flowmeter for measuring fluid flow velocities
by use of electromagnetically induced voltages having a signal processing
chain including sensing electrodes and an amplifier coupled thereto,
a sampling switch coupled to said amplifier for sampling said amplifier,
signal processing means coupled to said sampling switch, a memory
means connected through a connect/disconnect switch to said signal
processing means and at least one utilization circuit coupled to
said memory means, wherein the improvement comprises means disconnecting
said memory in the presence of spurious signals, the memory then
continuing to provide utilization circuits with a signal representative
of flow velocity extant prior to disconnection of the said memory
means.
Description FIELD OF THE INVENTION
The present invention relates to the field of electromagnetic flow
meters, either of the spool or probe type, and more particularly,
such apparatus in which the effect of spurious voltages may be reduced.
BACKGROUND OF THE INVENTION
The electromagnetic flow meter is a device with a well developed
body of art in which techniques are available for measuring flow
velocity of various types of fluids in various environments. In
one environment in which the flow is confined to a conduit, the
flow meter includes what is known as a spool type transducer in
which an electromagnetic field is generated by a coil surrounding
the conduit, and electrodes in contact with the flow have voltages
induced therein representative of the flow velocity. Another type
of known electromagnetic flow meter is of the probe type in which
the flowing fluid is not confined to a conduit. In the probe type
flow meter the transducer may take the form of a cylinder. The cylinder
includes therein an electromagnetic coil for producing an electromagnetic
field which is uniformly disposed about the periphery of the cylinder.
Located in the vicinity of the surface of the cylinder are electrodes
which have voltages induced therein representative of the flow velocity.
A constant source of problem in this field of technology is the
low level of signals induced in the electrodes. For example, the
spool type electrode is capable of generating signals on the order
of 300 microvolts per foot per second of fluid flow velocity. The
probe type transducer generates signals on the order of 25 microvolts
per foot per second of fluid flow velocity. Obviously, voltages
of this order of magnitude cannot be used directly and therefore
it is conventional in the art to employ signal processing to increase
the magnitude of the signal so that it may be utilized. A typical
form of signal processing is shown in my U.S. Pat. Nos. 3759097
and 3885858.
Generally stated, the function of the signal processing circuitry
is to focus on the flow induced voltages, and separate them from
the effects of extraneous noise and amplify the flow induced voltages
while ignoring the extraneous noise. While the signal processing
techniques known in the art are quite effective, nevertheless, the
extraneous voltages or noise voltages are still reflected in the
output of the signal processing chain. The signal processing circuitry
is effective to focus on the voltages periodically induced on the
electrodes such that random noise which is of the same order of
magnitude or even slightly greater than the flow induced voltages,
is acceptedly rejected. However, when the noise signals are much
greater than the flow induced voltages, they tend to saturate the
active elements in the signal processing chain. The effect of saturation
is to lose the output signal until the active elements come out
of saturation. As a result, noise voltages of relatively large orders
of magnitude cause signal dropouts which are disturbing to the user.
The triboelectric noise effects associated with flow measurements
in dielectric fluids has been reduced, in the prior art, by operating
the transducer at relatively high frequencies inasmuch as this noise
spectrum decreases with frequency. However, low frequency operation
of the transducer is desirable from a number of other standpoints.
Therefore, it is important to be able to reduce the triboelectric
noise or reduce its effects, with apparatus capable of relatively
low frequency operation.
SUMMARY OF THE INVENTION
This problem is overcome in accordance with the teachings of the
invention by adding several elements to the conventional signal
processing chain for the flow meter. Conventionally, the flow meter
includes a preamplifier coupled to the transducer electrodes, and
a sampling switch at the output of the preamplifier. The sampling
switch, in turn, is coupled to active signal processing elements,
for example, an integrator and buffer, the output of which is then
provided to a phase sensitive demodulator, the output of which is
then available for utilization circuitry. To this apparatus the
present invention adds a storage device such as a capacitor coupled
across the output line so as to maintain the demodulator output
available to the utilization circuits. In addition, a disturbance
detector is provided which is able to sense extraneous noise voltages
or the conditions which would lead to extraneous noise voltages,
and the disturbance detector is provided to disable the phase sensitive
demodulator upon occurrence of an extraneous noise voltage or a
condition which would lead to such an extraneous noise voltage.
The storage device or capacitor, coupled to the utilization circuits,
maintains the output of the phase sensitive demodulator which was
produced prior to the disturbance event and disablement of the phase
sensitive demodulator prevents the noise voltages from affecting
the utilization circuitry. Upon the occurrence of a disturbance
event the phase sensitive demodulator may be disabled for a predetermined
duration or, on the other hand, may be disabled for only so long
as the disturbing event is sensed.
In one form of the invention, applicable to spool type transducers,
especially those arranged to sense flow velocity in dielectric fluids,
triboelectric noise effects may result in voltages three to ten
times greater (or more) than flow induced voltages. Accordingly,
a threshold detector is arranged in the signal processing chain
to compare the absolute value of the signal in the signal processing
chain with a threshold. When the threshold is exceeded, the phase
sensitive demodulator is disabled.
In another form of the invention, in which a probe type electromagnetic
flow meter is suspended from the surface of an open/body of fluid,
extraneous noise voltages may be induced as a result of vertical
motion of the probe due to surface effects such as waves, etc. The
probe transducer cannot distinguish between voltages induced by
reason of the flow past the probe due to fluid flow from flow velocities
induced by reason of probe motion, and the probe merely provides
an output representative of the vector sum of these components.
Probe transducer motion due to waves is cyclic and thus its velocity
varies periodically. Desirably, the phase sensitive demodulator
is disabled only when the probe velocity exceeds some threshold.
This can be sensed by, for example, providing an accelerometer in
the probe arranged to sense vertical acceleration, integrating the
output of the accelerometer and comparing the signal corresponding
to vertical velocity with a reference. When the vertical velocity
exceeds the reference, the phase sensitive demodulator is disabled.
Alternatively, an auxiliary flow meter can be provided arranged
to sense only vertical flow velocity. When this vertical flow velocity
exceeds a threshold, the phase sensitive demodulator in the main
flow meter signalling chain is disabled.
Once disabled, the phase sensitive demodulator can be maintained
disabled for a predetermined duration; determined empirically depending
upon the application. On the other hand, the demodulator may be
maintained disabled only for so long as the disturbing event is
present.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be disclosed to enable those skilled
in the art to make and use the same in connection with the attached
drawings in which like reference characters identify identical apparatus
and in which:
FIG. 1 is a block diagram illustrating the inventive components;
FIG. 2A is a block diagram of one embodiment of the invention,
FIG. 2B is a timing chart showing the signal present during a typical
cycle of operation;
FIG. 2C is a block diagram illustrating the variations in FIG.
2A to provide a second embodiment of the invention;
FIG. 3 illustrates the square wave representing operation of the
magnet drive and also the preferable sampling times.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1A shows a typical signal processing chain for an electromagnetic
flow meter enhanced in accordance with the present invention. More
particularly, a preamplifier 10 has an input coupled to a probe
or spool type transducer. Specifically, the amplifier 10 input is
coupled to sensing electrodes 5 on the transducer. The output of
amplifier 10 is coupled to a sampling switch 11 which enables the
output of amplifier 10 to be periodically coupled to the remaining
signal processing components. These include an integrator 12 in
the form of an operational amplifier coupled to a buffer 13. The
use of an integrator and buffer are not essential and other signal
processing elements can be substituted in lieu thereof. The output
of the buffer 13 is provied to a phase sensitive demodulator which
comprises an inverter 14 coupled to the output of the buffer 13
which in turn provides an input to a sampling switch 15. A second
sampling switch 16 is coupled to the output of the buffer. The output
of the sampling switches 15 and 16 are tied together and are provided
to the utilization circuits 6. As thus far explained, the signal
processing is entirely conventional.
In accordance with the invention, a signal storage device is coupled
to the output of the phase demodulator. Such a signal storage device
is represented in FIG. 1 by the capacitor 17. In order to operate
the sampling switches 11 15 and 16 a clock 18 is provided. A logic
circuit 20 is coupled to the output of clock 18 and provides signals
for operating sampling switches 11 15 and 16 synchronous with
magnet drive 19 which is also driven by the clock 18. FIG. 3 illustrates
the square wave representing operation of the magnet drive 19 and
also the preferable sampling times which are in effect the times
at which the sampling switch 11 is closed. As is conventional, the
sampling switches 15 and 16 are closed at a time in the cycle subsequent
to the closure of the switch 11 and switches 15 and 16 operate alternately
in different half cycles of the magnet drive.
Finally, as shown in FIG. 1 a disturbance detector 21 is provided
which provides an output to the logic circuit 20. At times during
which a disturbance is detected, the signal provided by the disturbance
detector 21 to the logic circuit 20 disables the phase sensitive
demodulator by inhibiting operation of the switches 15 and 16.
In operation, assuming no disturbing event has been detected, the
sampling switch 11 is operated twice per cycle of the magnet drive,
and the switches 15 and 16 are operated once per cycle of the magnet
drive in phase opposition with each other. The output of the phase
sensitive demodulator is provided to the utilization circuits and
also stored on the capacitor 17. Each closure of one of the switches
15 or 16 adjusts the voltage stored on the capacitor 17 to reflect
real time conditions. When a disturbing event is detected, namely
an event such as to produce voltages which could seriously disturb
or saturate the signal processing circuitry and possibly cause signal
dropout, the disturbance detector 21 so signals the logic circuit
20. As a result of this, the logic curcuit 20 inhibits closure of
either of the switches 15 or 16. This prevents adjustment of the
voltage on capacitor 17 and the signal level stored on the capacitor
17 just prior to detection of such a disturbing event is maintained.
The signal level is maintained until the phase sensitive demodulator
is allowed to once again operate. In accordance with the invention,
the phase sensitive demodulator may be inhibited for a predetermined
duration following detection of a disturbing event or may be inhibited
only during the processing of a signal resulting from a disturbing
event.
FIG. 2A illustrates a specific embodiment of the invention which
can be applied with spool type transducers arranged to sense flow
velocity in a dielectric medium. Such media exhibit triboelectric
effects in which random voltages, many times the order of magnitude
of the flow induced voltages, can be sensed by the electrodes of
the transducer. Much of the circuitry shown in FIG. 2A is identical
to that shown in FIG. 1 and accordingly, a description thereof will
not be repeated here. As shown in FIG. 2A, however, the disturbance
detector 21 comprises a circuit coupled to the output of the preamplifier
10. More particularly, the disturbance detector 21 comprises a circuit
to sense the absolute value of the voltage in the signal processing
chain, compare that value with a reference, and if the detected
value exceeds the reference, signal the logic circuit 20 to disable
the phase sensitive demodulator.
Since the preamplifier is adequately wide band, it faithfully reproduces
noise, and its output is the best point to sense for disturbing
voltage--particularly saturating noise pulses. However, those skilled
in the art will understand that under certain circumstances it may
be preferable to sense for excess noise at the preamplifier and/or
other points (including the input to the preamplifier--in our discussion
of noise voltage sensing, the input and output of the preamplifier
are the same; only the impedence level is different) in the processing
chain which are in front of the phase sensitive demodulator.
In the embodiments shown here we have chosen to store or remember
prior flowmeter signal information in the hold capacitor attached
to the output of the phase sensitive demodulator. One skilled in
the art will realize that other methods of storage or memory can
be used; the important matter being that the signal information
is not conveyed to memory (e.g. by closure of demodulator switches
15 or 16) until after the signal channel has been disconnected (e.g.
by opening of sampling switch 11) from the source of signal and
noise. Disturbance information available up to the time of disconnect
(opening of switch 11) is used by the system's logic to decide whether
to admit the most recent signal information to memory, for conveyance
to the utilization circuits.
Returning to the disturbance detector 21 it comprises an absolute
value circuit including a diode 23 whose anode is coupled to the
output of the preamplifier 10 and whose cathode is coupled to the
cathode of a diode 24. An inverter 25 has an input also coupled
to the output of the preamplifier 10 and an output coupled to the
anode of the diode 24. The junction of the cathodes of diodes 23
and 24 is coupled to the input of a comparator 26. The other input
to the comparator is provided by a reference level. So long as the
absolute value of the flow induced voltages do not exceed the reference,
the output of comparator 26 will be nominally at zero. However,
if the absolute value of the flow induced voltages exceeds the reference,
comparator 26 will provide an output to the logic circuit 20.
As shown in FIG. 2A, the logic circuit 20 includes a counter 30
coupled to the output of the clock 18. Counter 30 divides the clock
output by a suitable quantity, for example/8. The counter 30 also
includes a decoder such that its outputs correspond to different
portions of the cycle established by the count to which the counter
30 is capable of counting. Different ones of these outputs are provided
to different elements. For example, FIG. 2B illustrates the clock
output, and also illustrates an A output thereof. It will be seen
that the A output is repeated twice per cycle. The A output is provided
the driving input for the sampling switch 11. A flip-flop 32 is
provided which is clocked by the B output of the counter 30. The
B output is also shown in FIG. 2B as occurring somewhat later in
the cycle from the A output. Flip-flop 32 provides two outputs C
and C. The C output is illustrated in FIG. 2B, and the C output
is the inverse. An AND gate 33 receives as one input the C output
of the flip-flop 32 and AND gate 34 receives the C output. The
D output of the counter 30 also shown in FIG. 2B, is provided as
one input to an AND gate 31. Another input to AND gate 31 is provided
by a second counter 35. Counter 35 is also clocked by the clock
18 but is normally maintained in the reset position whence it provides
an output to the AND gate 31. The output of AND gate 31 is provided
as the second input to each of the AND gates 33 and 34. Accordingly,
the output of AND gate 33 is the E signal, shown in FIG. 2B and
the output of AND gate 34 is the F signal, also shown in FIG. 2B.
The E signal drives sampling switch 16 and the F signal drives sampling
switch 15. Accordingly, one of the sampling switches 15 or 16 is
operated during each half cycle of the clock, and on the next following
half cycle of the clock, the other sampling switch is operated;
the operation of the sampling switch occurs at the time of the D
signal.
Upon the occurrence of a disturbing event, resulting in an output
from the comparator 26 the counter 35 is preset to a predetermined
count by the signal G. The presetting of counter 35 removes the
H signal and thus the AND gate 31 is inhibited from producing its
output and accordingly, the output signals E and F are also inhibited.
This inhibition of the output signals E and F, and thus the inhibition
of the phase sensitive demodulator, continues until the counter
35 counts back to its reset position. Since the clock period 18
is fixed, the period of the inhibition is determined by the count
to which the counter is preset upon the G signal. When the counter
has counted back to its reset position, the H signal again occurs,
the inhibition is terminated and the sampling switches 15 and 16
can then continue to operate.
Accordingly, the counter 35 and more particularly, the count to
which it is preset, determines the period of time during which the
phase sensitive demodulator is inhibited. When the counter has counted
back to its reset position, allowing the H signal to be produced
to terminate the inhibition of the phase sensitive demodulator,
the G signal, if still present, will cause the counter to again
be preset and prevent operation of the phase sensitive demodulator
for an additional period of time. This operation will continue until
the G signal terminates.
With reference to FIG. 2B, any disturbance deemed excessive during
the one half cycle terminating in t.sub.o (the A strobe) will activate
the processing chain's logic so as to inhibit the demodulator switch's
drive or strobe which terminates at time t.sub.mo (the E strobe);
and disturbance deemed excessive in the one half cycle terminating
in time t.sub.1 (the A strobe) will activate the logic to inhibit
the sampling drive or strobe terminating in time t.sub.m1 (the F
strobe). As discussed above, the auxiliary counter 35 provides the
necessary delay to perform this function. Indeed, the auxiliary
counter 35 may preferably be set to maintain inhibition for one
or more complete cycles of the electromagnetic flowmeter's magnet
alternation in order to allow ample time for the electromagnetic
flowmeter's and signal processing chain's recovery from the excessive
disturbance. Less preferably, instead of a clocked auxiliary counter
35 an alternative inhibition persistance means is to replace the
counter 35 with a one-shot having predetermined time-delay. The
one-shot's time delay depends on the application; indeed if it is
short enough, one can omit the one-shot entirely, in which case
inhibition will last only for the duration of the excessive disturbance.
In another embodiment of the invention, a probe type transducer
is supported along a mooring line beneath a buoy in an open body
of water such as the ocean. The probe type transducer is intended
to sense the horizontal components of ocean current. Typically,
wave motion will cause the buoy to surge or translate vertically.
Since the probe type transducer is supported from the buoy, the
probe will undergo a surge component of motion. Were the probe type
transducer ideal and thus completely insensitive to the surge component
of velocity, it could correctly measure the horizontal components
of ocean current during surge. In practice, however, the probe type
transducer measures most accurately when the surge component of
velocity is small compared with the horizontal flow. Hence, the
present invention can be used to sense the surge component of velocity,
and disable the flow instrument's demodulators (i.e. withhold flow
signal information from memory) when the surge velocity is deemed
excessive. Since the surface wave motion (and hence the surge motion)
is periodic, the times during which the probe is subjected to large
surge velocity (let us, for simplicity, call it vertical velocity)
is limited.
The invention can be employed to reduce the effect of the large
vertical velocity component as follows. In the circuit of FIG. 2A,
the diodes 23 24 and the inverter 25 are removed and a vertical
velocity sensor 40 (seen in FIG. 2C) is employed. The output of
the vertical velocity sensor is provided as an input to the comparator
26 where it is compared with a reference voltage. If the vertical
velocity component exceeds, in absolute value, the reference, then
the comparator 26 produces the G signal which is coupled to the
counter 35 in FIG. 2A. The vertical velocity sensor 40 may comprise
an accelerometer mounted in the probe transducer arranged to sense
vertical acceleration. The output of the accelerometer is provided
to an integrator, whose output is therefore representative of vertical
velocity. An absolute value circuit, conventional in the art, may
be employed to eliminate signal polarity. As an alternative to the
accelerometer, an auxiliary electromagnetic flow meter is included
in the probe arranged to sense vertical motion of the probe or vertical
flow velocity. Such auxiliary flow meter requires an auxiliary coil
to provide the necessary electromagnetic field. The electrodes on
the probe transducer can be time shared between the main and auxiliary
channels and signal processing circuitry may be included such as
that shown in my prior patents. The output of such an auxiliary
flow meter is a vertical velocity signal which can be coupled to
the comparator 26 as shown in FIG. 2C. Instead of employing the
absolute value of vertical velocity, a ratio can be taken between
vertical velocity and total flow velocity and inhibiting the demodulator
only when the absolute value of the ratio exceeds the reference
value. A signal related to this ratio can be obtained with a conventional
divider circuit.
In accordance with this embodiment of the invention, the disturbing
event is sensed separate and apart from the signal voltage in the
signal chain.
The application of the invention is not restricted to the applications
described herein, and the signals generated by the apparatus of
FIG. 2A, for example, can be employed for other purposes as well.
In my co-pending application Ser. No. 807288 I disclosed a debris
shedding electromagnetic flow transducer. As mentioned in that disclosure,
the transducer may include a scrubbing jet to provide a source of
cleaning fluid such as water, air or other fluid directed at the
surface of a probe type transducer mounted at an angle to the direction
of flow. As mentioned in that application, the scrubbing jet may
be energized periodically. In such an arrangement, the energization
of a scrubbing jet may comprise a disturbing event and the signal
energizing the scrubber may be employed as the G signal of FIG.
2A. Alternatively, the apparatus shown in FIG. 2A may be employed,
and when a G signal is produced, it may, in addition to operating
the components shown in FIG. 2A, also be used to energize the scrubbing
jet. |