Abstrict A flow meter comprises a conduit adapted to conduct a flow of a
fluid, a stop in the conduit, and a body that is movably received
in the conduit, is normally kept in engagement with the stop by
the fluid flow, and has at least a portion of a magnetically responsive
material. An electromagnet is located outside the conduit in a position
upstream from the stop, relative to the direction of the fluid flow,
and has its poles located such as to impose a magnetic force on
the magnetically responsive material of the body when it is energized.
A ramp current generator energizes the electromagnet so as to progressively
increase the magnetic force that it produces until the magnetic
force is sufficient to move the body against the fluid flow and
attract the body to a position between the poles. A multi-turn sensor
coil or a magnetic field sensor detects changes in the magnetic
flux field of the electromagnet when the body moves away from the
stop and toward the electromagnet and outputs a sensing signal,
which in turn initiates the capture of a measurement of the current
that resulted in attraction of the body away from the stop against
the force of the fluid flow.
Claims We claim:
1. A flow meter comprising a conduit adapted to conduct a flow
of a fluid, a stop in the conduit, a body movably received in the
conduit, normally kept in engagement with the stop by the fluid
flow, and including at least a portion of a magnetically responsive
material, an electromagnet located outside the conduit in a position
upstream from the stop relative to the direction of the fluid flow
and having poles at a location such as to impose a magnetic force
on the magnetically responsive material of the body when it is energized,
means for energizing the electromagnet so as to progressively increase
the magnetic force that it produces until the magnetic force is
sufficient to move the body against the fluid flow and attract the
body to a position between the poles, means for detecting changes
in the magnetic flux field of the electromagnet when the body moves
away from the stop and toward the electromagnet and producing a
sensing signal indicative thereof, and means responsive to the sensing
signal for detecting the magnitude of the current supplied to the
electromagnet and producing a measurement signal indicative thereof.
2. A flow meter according to claim 1 wherein the detecting means
includes a multi-turn coil of electrically conductive material wound
around a portion of the electromagnet so as to have a voltage induced
therein by the change in flux.
3. A flow meter according to claim 1 wherein the detecting means
includes a Hall effect sensor mounted on the electromagnet.
4. A flow meter according to claim 3 wherein the Hall effect sensor
is mounted in a recess in a pole of the electromagnet and is oriented
such that its poles are intersected by different numbers of magnetic
flux lines when the body is seated on the stop and when the body
is moved from the stop by the magnetic force of the electromagnet.
5. A flow meter according to claim 3 wherein the Hall effect sensor
is mounted in a recess in a portion of the electromagnet remote
from the poles of the electromagnet so as to detect changes in the
flux density when the body is moved from the stop by the magnetic
force of the electromagnet.
6. A flow meter according to claim 1 wherein the conduit is a bore
through a housing that is adapted be received within a tube through
which the fluid is flowing, and the electromagnet and sensor means
are mounted in the housing.
Description BACKGROUND OF THE INVENTION
The flow rate of a fluid flowing in a flow system at slow to moderate
rates can be measured by placing a magnetically responsive body
in a flow conduit having a stop against which the body rests under
the influence of the fluid flow (and gravity in a vertically downward
or sloping mode) and determining the force required to be exerted
on the body by an electromagnet located upstream from the stop to
move the body away from the stop. With the body resting on the stop,
the current supplied to the electromagnet is increased from zero
(or a level below that which causes the body to leave the seat).
When the current reaches a certain level, the magnetic force applied
to the body becomes large enough to move the body upstream against
the flow. Inasmuch as the magnetic force exerted on the body rapidly
increases as soon as the body starts moving toward the electromagnet,
the body rapidly moves against the flow into a position between
the magnetic poles. By measuring accurately the current supplied
to the electromagnet when the body moves away from the stop, one
has a value that can be converted into a flow rate value based on
calibration data. Movement of the body away from the stop has previously
been detected by optical sensors. U.S. Pat. No. 4003255 (Spencer,
Jan. 1977 "the '255 patent") describes and shows a flow
measuring system of the type just described. The '255 patent is
incorporated by reference into the present disclosure.
Other fluid flow measuring systems based on magnetic attraction
of magnetically responsive bodies and optical sensing of movements
of the bodies have been proposed, such as those of U.S. Pat. Nos.
4041723 (Head et al., October 1977); 3662598 (Spencer, May 1972);
3605741 (Spencer, September 1971); and 4167115 (Stoever, September
1979).
Optical sensing requires that the flow conduit and the fluid be
transparent and is also subject to malfunction due to false indications
of ball movement caused by ambient light incident on the light detector.
Ambient light conditions can, for example, cause inaccuracies in
the measurements due to variations in the position of the body along
the path it takes between the stop and the magnet when a signal
indicative of the movement of the body is generated.
SUMMARY OF THE INVENTION
One object of the present invention is to provide a flow meter
that does not require the use of a transparent conduit and that
can be used to measure flows of non-transparent fluids. Another
object is to provide a flow meter that is not subject to inaccurate
measurements due to ambient light. It is also an object to provide
a flow meter detection unit that can be retrofitted into existing
flow meters of other types, such as rotameters, in a manner that
permits the existing meter to continue in use as a back-up or corroboratory
measurement device.
The foregoing and other objects are attained, in accordance with
the present invention, by a flow meter comprising a conduit adapted
to conduct a flow of a fluid, a stop in the conduit, and a body
movably accommodated in the conduit and normally engaging the stop
under the influence of the fluid flow, the body including at least
a portion of a magnetically responsive material. An electromagnet
is located outside the conduit in a position upstream from the stop
relative to the direction of fluid flow and is positioned with its
poles at a location such as to impose a magnetic force on the magnetically
responsive material of the body when energized so as to move the
body away from the stop. A controllable power supply energizes the
magnet so as to progressively increase the magnetic force output
of the electromagnet until the magnetic force is sufficient to move
the body against the fluid flow and attract the body to a position
between the poles. Movement of the body away from the stop by the
magnetic force is sensed by detecting the change in magnetic flux
in the electromagnet when the body moves to a position between the
poles, and a sensing signal indicative of such movement is generated.
The magnitude of the current supplied to the electromagnet is detected
in response to the sensing signal, and a measurement signal indicative
of that current is generated.
The present invention makes use of the fact that when the body
is attracted away from the stop and toward the gap between the poles
of the electromagnet, the flux density of the magnet changes at
a rate significantly different than the rate at which it changes
when the current is increased but the body remains against the stop.
The change in the rate of change in the flux density is due to the
fact that the magnet becomes "short-circuited" or shunted
when the body moves between the poles. By detecting the change in
the rate of change in the flux density of the electromagnet, movement
of the body away from the stop toward the gap between the magnetic
poles is clearly indicated. Such detection is free of the limitations
of optical detection referred to above.
The change in the rate of change in the flux density of the electromagnet
during a measurement cycle may be detected by various means. One
such means is a multi-turn coil of electrically conductive material
wound around a portion of the electromagnet. The change in flux
density induces a voltage in the coil, and that voltage is a suitable
sensing signal. Another suitable device for detecting the change
in the rate of change in flux density resulting from movement of
the body away from the stop is a Hall effect sensor mounted on the
electromagnet. The voltage output of the Hall effect sensor changes
when the flux field cutting through it changes, and the change in
voltage is a suitable sensing signal.
When a Hall effect sensor is used in the device, it may be mounted
in a recess in a pole of the electromagnet in an orientation such
that its poles are intersected by different numbers of magnetic
flux lines when the body is seated on the stop and when the body
is moved from the stop by the magnetic force of the electromagnet.
Alternatively, the Hall effect sensor may be mounted in a recess
in a portion of the electromagnet remote from the poles so as to
detect the increase in the density of the flux flow within the core
of the electromagnet when the body is moved from the stop by the
magnetic force and more flux lines pass through the recess in which
the sensor is mounted.
The flow meter may be configured to be installed in a fluid flow
pipe or in an existing flow meter of another type, such as a rotameter,
by providing a housing with the meter flow conduit extending through
it, the housing being externally configured to be accommodated within
a pipe or flow meter passage through which the fluid is flowing.
The electromagnet and sensor of the flow meter are mounted in the
housing.
For a better understanding of the invention, reference may be made
to the following description of an exemplary embodiments, taken
in conjunction with the accompanying drawings.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of an embodiment in which changes
in the rate of change of flux density in the electromagnet are detected
by a multi-turn coil of an electrically conductive material;
FIG. 2 is a schematic diagram of an embodiment in which changes
in the rate of change of flux density in the electromagnet are detected
by Hall effect sensor;
FIGS. 3A and 3B are diagrams illustrating one way of locating a
Hall effect sensor to detect the position of a body and showing
the manner in which the position of the body is sensed by the sensor;
FIGS. 4A to 4C are diagrams illustrating another way of locating
a Hall effect sensor, FIG. 4A being a top plan view of the core
of the electromagnet and FIGS. 4A and 4B being detail plan views
of a part of the core; and
FIG. 5 is a generally schematic side cross-sectional view of a
fluid flow-detecting unit suitable for installation in a pipe section
or within a flow meter of another type.
DESCRIPTION OF THE EMBODIMENTS
Many of the elements of the embodiment shown in FIG. 1 and the
manner in which they function are described and shown in the '255
patent, and reference may be made to that patent for a further understanding
of the present invention. A conduit 10 which may oriented vertically,
horizontally, or on a slope, receives a flowing fluid, the flow
rate of which is to be measured. The conduit 10 is of a material
that is not responsive to magnetic fields, such as certain metals
(e.g., 304 stainless steel or aluminum or Monel metal), glass, quartz,
a non-porous ceramic or a plastic, is of sufficient strength to
contain the fluid and is of a composition that can endure the physical
(e.g., temperature) and chemical (e.g., corrosive) conditions to
which it is exposed by the fluid and the environment. The inside
of the conduit may, alternatively, be coated with an inert material,
such as "Teflon," to make it corrosion resistant. A body
12 of a magnetically-responsive material, such as 440 stainless
steel, low-carbon steel or a metallic-glass alloy (e.g., "Metglas"),
is accommodated in the conduit with a clearance from the conduit
walls for fluid to flow past the body. The body 12 normally--under
the influence of the fluid flow --rests on a non-ferromagnetic stop
14 having a seat 141 in by which the body is supported in the center
of the conduit. The stop 14 is attached to the conduit walls by
one or more radially extending support arms 142 which leave most
of the annular flow path around the stop open for fluid flow. The
body 12 and stop 14 may also be coated for corrosion resistance.
An electromagnet 16 having a core 161 with poles 162 and windings
18 is located outside the conduit. The poles 162 are located a few
millimeters above the top of the stop 14 and about 0.5 millimeters
from the outside wall of the conduit 10. During each measurement
cycle, the windings 18 of the electromagnet are energized by a current
supplied through leads 181 182 by a current ramp generator 20
which generates a linear or fine-stepped ramp-up of the current,
starting from nil or from a value below that which will produce
a magnetic field sufficient to attract the body 12 with a force
sufficient to lift the body from the stop against the force applied
to it by the fluid flow (and the force due to gravity in a vertically
oriented or sloped installation). The supplying of current to the
electromagnet is initiated, and the ramp-up of current is controlled,
by a pulse generator 22. The pulses from the pulse generator are
counted by a counter 24. Inasmuch as the current supplied to the
electromagnet and the pulses that control the current supply are
related, the pulse count is a function of the current. The force
exerted by the electromagnet on the body 12 is, in turn, a function
of the current supplied to it. Accordingly, the count on the counter
24 is related to the force exerted by the electromagnet on the body
12.
When the force exerted by the electromagnet 16 on the body 12 exceeds
the force applied to the body by the fluid flow (and by gravity
and buoyancy, where applicable), the body is accelerated toward
the electromagnet and away from the stop. As the body moves toward
the gap between the poles 162 the reluctance of the flux path between
the poles is progressively and rapidly decreased due to the short-circuiting
of the gap by the magnetically responsive body. The resulting change
in the flux field of the electromagnet is, according to one aspect
of the present invention, detected and provides a sensing signal
that is analyzed in a signal processor 28 as described below, so
as to provide a stop signal that stops the pulse generator 22 and
latches the counter 24.
In the embodiment shown in FIG. 1 the change in the magnetic field
of the electromagnet caused by short-circuiting of the magnetic
circuit when the body 12 is attracted into the gap between the poles
162 of the electromagnet 16 is detected by an electrically conductive
wire coil 26 of several hundred turns wound around the core 16 of
the electromagnet. The change in flux density in the core that occurs
when the current is ramped up during a measurement cycle induces
a voltage (EMF) in the coil 26 the magnitude of which is substantially
proportional to the rate of increase in the magnetic field flowing
in and adjacent to the core of electromagnet. The short-circuiting
of the electromagnet upon movement of the body 12 away from the
stop produces a change in the rate of increase in flux flow in the
magnetic circuit 16. Suitable circuitry in the signal processor
28 connected by leads 261 262 to the coil 26 filters and analyzes
the voltage induced in the coil and outputs a stop signal to the
pulse generator 22 when the increased rate of change in flux flow
is detected.
In the embodiment of FIG. 2 the change in the flux field indicative
of the movement of the body 12 away from the stop 14 and toward
the poles 162 is detected by a Hall effect sensor 32 interposed
in the magnetic circuit such as to produce a change in the voltage
output of the sensor that is consistent and measurable when the
body reaches a certain point in the path between the rest position
of the body against the stop and a rest position between the magnetic
poles. Inasmuch as a Hall effect sensor detects changes in the flux
lines that cut through its poles, the response of the sensor depends
on its location and orientation in the magnetic circuit.
In the embodiment of FIG. 2 the Hall effect sensor 32 is located
in a notch in one of the poles 162 of the electromagnet at a location
and orientation such that the sensor's poles are cut by approximately
equal numbers of flux lines as the flux density of the electromagnet
increases during a measurement cycle, as shown in FIG. 3A. Thus,
the Hall effect voltage remains essentially constant as long as
the body remains at rest against the stop, even though the flux
density and the size of the flux field between the electromagnet's
poles is increasing. When the flux field between the poles 162 is
short-circuited upon movement of the body 12 to a position close
to the poles, the flux field between the poles is deflected, as
shown in FIG. 3B, thus changing the pattern of flux across the sensor
26 and producing a change in the voltage output of the sensor.
Another location for the Hall effect sensor 32 as shown in FIGS.
4A to 4C, is in a notch 163 in the core of the electromagnet remote
from the poles. At lower currents and flux densities, relatively
few flux lines pass through the notch (FIG. 4B). As the magnetic
field strength in the circuit increases, increasing numbers of flux
lines cross the gap, and the voltage in the hall effect sensor increases.
When the gap between the poles 162 is short circuited by movement
of the body away from the stop and into the gap between the poles,
the rate of increase in the magnetic flux field in the core of the
magnet changes sharply, and the flux flow across the notch, and
thus through the sensor 32 increases correspondingly (FIG. 4C).
The Hall effect sensor 32 responds by outputting a higher voltage
signal. Locations and orientations for the Hall effect sensor other
than those described above are possible.
Circuitry in a signal processor 28' (FIG. 2) filters and evaluates
the voltage output of the sensor 32 and detects a change indicative
of the arrival of the body at a certain point in the flux field,
such as a peak sensor output voltage or a change in the rate of
change in the sensor output voltage of a certain duration. When
the indicated change in the Hall sensor voltage is detected by the
processor, a signal is sent to the pulse generator 22 to stop generating
pulses and latch the counter 24. The latching of the counter initiates
the generation by display driver circuitry of a display 30 of a
flow rate measurement. The pulse count signal from the counter can
also, of course, be recorded and can be used to control a valve
or pump to adjust the flow being measured. Pulse count signals from
the counter for many measurement cycles can be stored in a memory,
and average flow rates computed, displayed, used in controlling
flow, and recorded, as desired.
Measurement cycles can be initiated manually by a push button (not
shown) or automatically on a timed basis by a reset signal (reset
36) generated automatically by a timer after a time delay long enough
for the body to move with the fluid flow back onto the stop and
supplied to the pulse generator. Actual instrument output will,
of course, be correlated to flow rates based on calibration data
for particular fluids, temperatures, ranges of flow rates, and other
variables. Conversion of measured counts of the counter may be made
using calibration charts or by supplying the count data from each
measurement cycle to a computer or microprocessor in which calibration
data are stored and the count data are processed in accordance with
the calibration data to produce actual flow rates for display, recording,
and use. Where fluid temperatures or other conditions are significant
variables in the measured flow rate, such conditions may be measured
and input along with the count rate signals from the counter 24
to the computer or microprocessor.
As is known per se, the apparatus of the present invention can
also be used in conjunction with other measurement devices and a
microprocessor or computer to measure viscosities. By providing
stops on opposite sides of an electromagnet in a horizontal tube,
the apparatus can be used to measure flows in opposite directions
(at different times).
FIG. 5 shows a measuring unit 38 that is suitable for installation
in line in a tube 40 through which a fluid, the flow rate of which
is to be measured, is flowing. The tube may, for example, be a portion
of a tube of a rotameter, which is modified by shortening the flow
tube section to accommodate the unit. The unit 38 includes a housing
42 of a non-ferromagnetic material having an outer wall 421 in sliding
clearance with the wall of the passage and sealed to the passage
wall by O-rings 44. The housing may be configured to seat at its
lower end against an existing abutment or a spacer ring 46 or may
be fastened in place in the tube 40 in any suitable manner. A cavity
or notch 422 extends radially into and circumferentially part-way
around the housing 42 and receives an electromagnet 48 the poles
of which are on diametrically opposite sides of a flow conduit 50
in the form of a bore through the housing 42. A stop 52 supported
by arms 521 receives a body 54 of the measuring unit. Movement of
the body away from the stop is detected by a multi-turn coil or
Hall effect sensor 54 as described above. The leads (not shown)
of the coils of the electromagnet and the sensor may be routed through
passages (not shown) leading to the bottom of the housing and out
through a suitable sealed hole (not shown) in the tube 40 downstream
from the unit. |