Abstrict A flow meter having electrical signal outputs and a digital display
is described. A transducer uses a magnet inside the flow meter to
position a ball on a resistor bar and a contact bar and, thus, generate
a position signal. The position signal provides an accurate electrical
indication of flow to the circuitry driving the display.
Claims Having thus described our invention, we claim:
1. A fluid flow meter comprising:
a body portion having an inlet port and an outlet port;
a core tube within said body portion having an inlet in flow communication
with said inlet port and having an outlet in flow communication
with said outlet port;
a movable stem within said core tube comprising a piston, a connecting
rod and a magnet holder, said piston adapted to allow flow through
a variable portion of said core tube outlet dependent upon piston
position, whereby piston position is dependent on fluid flow through
said meter;
a magnet contained within said magnet holder;
an envelope fixed to said body portion enclosing said magnet holder;
a raceway fixed to said envelope and isolated from said fluid flow,
said raceway comprising first and second parallel bars;
said first parallel bar comprises a resistor having a first end
and a second end, said first end connected to a first source of
reference voltage, said second end connected to a second source
of reference voltage;
said second parallel bar being connected to a signal output line;
contact means movable in said raceway, said contact means adapted
to be attracted to said magnet and being electrically conductive
whereby said raceway and contact means function as a voltage divider
potentiometer having an output on said signal output line indicative
of contact means position;
a signal carrying means accepting said signal and conveying it
to a display means, said display means accepting said signal from
said signal carrying means and creating a visually perceptible representation
based upon said signal.
2. The meter of claim 1 wherein said contact means is a ball and
said first and second parallel bars have generally circular cross
section.
3. The meter of claim 1 wherein said contact means is visible from
the exterior of said meter and a scale is provided adjacent the
range of positions of said contact means whereby flow rate may be
visually perceived.
4. A transducer for a fluid flow meter having a stem moving vertically
in response to changes in flow, said stem having a magnet rigidly
attached thereto, said transducer comprising:
a first bar of generally uniform cross section having an axis parallel
to the direction of movement of said magnet, said first bar being
an electrically conductive polished cylindrical rod of non-magnetic
metal;
a second bar of generally uniform cross section having a first
end, a second end and an axis parallel to the axis of said first
bar, said second bar being non-magnetic and comprising a tight helical
winding of fine electrically resistive wire disposed on a non-magnetic
core;
an electrically conductive, magnetically attractable metallic spherical
contact means movably engaging said first bar and said second bar
whereby said first bar and said second bar are in electrical contact
with one another through said contact means;
power supply means providing a first reference voltage to said
second bar first end and a second reference voltage different from
said first reference voltage to said second bar second end; and
signal carrying means electrically connected to said first bar
and obtaining a signal therefrom indicative of contact means position.
5. A fluid flow meter comprising:
a body portion having an inlet port, an outlet port and a chamber
connecting the inlet port to the outlet port;
a movable stem within said chamber, said stem adapted to change
position in response to changes in flow through said chamber such
that the position of said stem is indicative of flow rate;
a magnet fixed to said stem; and
a transducer outside of said chamber comprising a resistor bar
having first and second ends, a contact bar and indicator means,
said resistor bar being connected to a source of reference voltage
at said first end and a source of reference ground at said second
end, said contact bar being connected to a position signal line,
and said indicator means adapted to be attracted to said magnet
and making electrical contact between said resistor bar and said
contact bar at a point adjacent said magnet thereby creating a position
signal on said position signal line.
6. The meter of claim 5 wherein said indicator is a steel ball
having a ball diameter and adapted to be attracted by said magnet.
7. A fluid flow meter comprising:
a body portion having an inlet port, an outlet port and a chamber
connecting the inlet port to the outlet port;
a movable stem within said chamber, said stem adapted to change
position in response to changes in flow through said chamber such
that the position of said stem is indicative of flow rate;
a magnet fixed to said stem; and,
a transducer comprising a resistor bar having a circular cross
section, a first diameter and first and second ends, a contact bar
having a circular diameter and a second diameter different from
said first diameter, and steel ball indicator means having a ball
diameter and adapted to be attracted by said magnet, said resistor
bar being connected to a source of reference voltage at said first
end and a source of reference ground at said second end, said contact
bar being connected to a position signal line, and said indicator
means adapted to be attracted to said magnet and making electrical
contact between said magnet thereby creating a position signal on
said position signal line;
an analog to digital converter adapted to receive said position
signal and create a digital position signal dependent on said position
signal;
a memory means containing correction data, said memory means adapted
to output one correction datum corresponding to said digital position
signal;
a digital to analog converter adapted to receive said correction
datum and create an analog correction signal based on said correction
datum; and,
an analog adder means adapted to add said analog correction signal
and said position signal and create a corrected position signal
based thereon.
8. The meter of claim 7 wherein said signal processing unit additionally
comprises:
a digital adder means adapted to add said digital position signal
and said correction datum and create a digital corrected flow rate
signal; and
a display driver and digital display adapted to display a digital
representation of said digital corrected flow rate signal.
9. The meter of claim 8 wherein said signal processing unit additionally
comprises:
a totalizer means adapted to receive said digital corrected flow
rate signal and a clock signal and generate a total flow signal
based thereon; and,
a display driver and digital display adapted to display a digital
representation of said total flow signal.
10. The meter of claim 9 wherein said indicator means is visible
from the exterior of said meter and a scale is fixed to said meter
adjacent the range of positions of said indicator whereby flow rate
may be perceived by comparing the position of said indicator to
said scale.
11. A fluid flow meter comprising:
a body portion having an inlet port and an outlet port;
a core tube within said body portion having an inlet in flow communication
with said inlet port and having an outlet in flow communication
with said outlet port;
a movable stem within said core tube comprising a piston, a connecting
rod and a magnet holder, said piston adapted to allow flow through
a variable portion of said core tube outlet dependent piston position,
whereby piston position is dependent on fluid flow through said
meter;
a magnet contained within said magnet holder;
an envelope fixed to said body portion enclosing said magnet holder;
a raceway fixed to said envelope, said raceway comprising first
and second parallel bars;
said raceway first parallel bar having a first circular cross section,
a first diameter and comprises a resistor having a first end and
a second end, said first end connected to a first source of reference
voltage, said second end connected to a second source of reference
voltage;
said second parallel bar having a second cross section and a second
diameter different from said first diameter and is connected to
a signal output line;
a ball contact means movable in said raceway, said contact means
adapted to be attracted to said magnet and being electrically conductive
whereby said raceway and contact means function as a potentiometer
having an output on said signal output line indicative of contact
means position;
an electrical means sensing the position of said contact means
in said raceway and generating a signal indicative of said position;
and
a signal carrying means accepting said signal and conveying it
to a display means, said display means accepting said signal from
said signal carrying means and creating a visually perceptible representation
based upon said signal.
12. The meter of claim 11 wherein the diameters of said first and
second bars are related to one another by approximately a factor
of two.
13. The meter of claim 11 wherein the diameters of said first and
second bars are related to one another by a factor of five-thirds.
14. The meter of claim 13 wherein the diameter of said first bar,
the diameter of said second bar and the diameter of said contact
ball are related by the ratio 12 to 20 to 25.
15. The meter of claim 14 wherein said first bar comprises a rod
having fine wire wound thereon in a tight helical pattern.
16. A fluid flowmeter comprising:
a body portion having an inlet port and an outlet port;
a core tube within said body portion having an inlet in flow communication
with said inlet port and having an outlet in flow communication
with said outlet port;
a movable stem within said core tube comprising a piston, a connecting
rod and a magnet holder, said piston adapted to allow flow through
a variable portion of said core tube outlet dependent piston position,
whereby piston position is dependent on fluid flow through said
meter;
a magnet contained within said magnet holder;
an envelope fixed to said body portion enclosing said magnet holder;
a raceway fixed to said envelope, said raceway comprising first
and second parallel bars;
said first parallel bar comprises a resistor having a first end
and a second end, said first end connected to a first source of
reference voltage, said second end connected to a second source
of reference voltage;
said second parallel bar being connected to a signal output line;
contact means movable in said raceway, said contact means adapted
to be attracted to said magnet and being electrically conductive
whereby said raceway and contact means functions as a potentiometer
having an output on said signal output line indicative of contact
means position;
said signal output line conveying a contact position signal to
a signal processing unit comprising:
an analog to digital converter adapted to receive said contact
position signal and create a digital position signal dependent on
said contact position signal;
a memory means containing correction data, said memory means adapted
to output one correction datum corresponding to said digital position
signal;
a digital to analog converter adapted to receive said correction
datum and create an analog correction signal based on said correction
datum; and,
an analog adder means adapted to add said analog correction signal
and said contact position signal and create a corrected position
signal based thereon.
17. The meter of claim 16 wherein said signal processing unit additionally
comprises:
a digital adder means adapted to add said digital position signal
and said correction datum and create a digital corrected flow rate
signed; and,
a display driver and digital display adapted to display a digital
representation of said digital corrected flow rate signal.
18. A transducer for a fluid flow meter having a stem moving vertically
in response to changes in flow, said stem having a magnet rigidly
attached thereto, said transducer comprising:
a first bar of generally uniform circular cross section having
an axis parallel to the direction of movement of said magnet and
a first diameter generally constant over the length of the first
bar, said first bar being electrically conductive and non-magnetic;
a second bar of generally uniform circular cross section having
a first end, a second end and an axis parallel to the axis of said
first bar and a second diameter generally constant over the length
of said second bar, said second bar being non-magnetic and electrically
resistive along its length;
said first and second bar diameters being related to one another
by a factor of approximately two;
an electrically conductive, magnetically attractable contact means
movably engaging said first bar and said second bar whereby said
first bar and said second bar are in electrical contact with one
another through said contact means;
power supply means providing a first reference voltage to said
second bar first end and a second reference voltage different from
said first reference voltage to said second bar second end; and,
signal carrying means electrically connected to said first bar
and obtaining a signal therefrom indicative of contact means position.
19. A transducer for a fluid flow meter having a stem moving vertically
in response to changes in flow, said stem having a magnet rigidly
attached thereto, said transducer comprising:
a first bar of generally uniform circular cross section having
an axis parallel to the direction of movement of said magnet and
a first diameter generally constant over the length of the first
bar, said first bar being electrically conductive and non-magnetic;
a second bar for generally uniform circular cross section having
a first end, a second end and an axis parallel to the axis of said
first bar and a second diameter generally constant over the length
of said second bar, said second bar being non-magnetic and electrically
resistive along its length;
said first and second bar diameters being related to one another
by a factor of approximately five-thirds;
an electrically conductive, magnetically attractable contact means
movably engaging said first bar and said second bar whereby said
first bar and said second bar are in electrical contact with one
another through said contact means;
power supply means providing a first reference voltage to said
second bar first end and a second reference voltage different from
said first reference voltage to said second bar second end; and,
signal carrying means electrically connected to said first bar
and obtaining a signal therefrom indicative of contact means position.
20. The transducer of claim 19 wherein said contact means is a
steel ball having a diameter related to the smaller of said first
and second bars approximately by the ratio two to one.
21. The transducer of claim 20 wherein said ratio is approximately
twenty-five to twelve.
22. The transducer of claim 21 wherein said second bar comprises
a core upon which a tightly wound helix of fine wire is wound.
23. A fluid flow meter comprising:
a body portion having an inlet port, an outlet port and a chamber
connecting the inlet port to the outlet port;
a movable stem within said chamber, said stem adapted to change
position in response to changes in flow through said chamber such
that the position of said stem is indicative of flow rate;
a magnet fixed to said stem; and,
a transducer comprising a resistor bar having a circular cross
section, a first diameter and first and second ends, a contact bar
having a circular cross section and a second diameter different
from said first diameter, and steel ball indicator means having
a ball diameter and adapted to be attracted by said magnet, said
resistor bar being connected to a source of reference voltage at
said first end and a source of reference ground at said second end,
said contact bar being connected to a position signal line, and
said indicator means adapted to be attracted to said magnet and
making electrical contact between said resistor bar and said contact
bar at a point adjacent said magnet thereby creating a position
signal on said position signal line.
24. The meter of claim 23 wherein said second diameter is related
to said first diameter by approximately a factor of two.
25. The meter of claim 23 wherein said second diameter is related
to said first diameter by approximately a factor of five-thirds.
26. The meter of claim 23 wherein said second diameter is approximately
slightly less than twice said first diameter and said ball diameter
is approximately slightly more than twice said first diameter.
27. The meter of claim 23 wherein said first diameter, said second
diameter and said ball diameter are approximately related by the
ratio 12 to 20 to 25.
28. The meter of claim 23 wherein said resistor bar is comprised
of non-conductive core and a coil of fine wire wound on said core.
29. The meter of claim 28 wherein said coil comprises more than
six hundred turns per inch.
Description BACKGROUND OF THE INVENTION
This invention relates to flow meters for measuring the rate of
flow of a fluid media, either liquid or gaseous. More particularly,
the invention relates to a flow meter having an output signal capable
of being transmitted to a remote or proximate receiver for display
and/or further manipulation.
This invention relates to flow meters in general but will be described
with particular reference to variable area flow meters. A first
type of variable area flow meter normally comprises a housing having
an inlet port and an outlet port, a cylindrical core tube receiving
flow into its interior from the input port of the housing and an
output slot along the side of the core tube. A movable piston within
the core tube responds to the fluid flowing through the core tube
by opening the slot sufficiently to allow the flow demanded. An
indicator attached to the movable piston is indexed against a scale
showing the position of the piston and thereby the flow through
the meter. Variable area flow meters are rugged, reliable, accurate
and fluidtight. The construction of such flow meters and their theory
of operation are described in U.S. Pats. Nos. 3691834; 4361051;
and 4489614 all of which are incorporated hereinby reference.
A second type of variable area flow meter, generally known as a
rotameter comprises an inlet port, a vertically disposed, upwardly
expanding conical chamber and an outlet port. A piston disposed
in the conical chamber forms a restriction in the cross-sectional
area of the conical chamber. The piston rises or falls within the
chamber, thus varying the area of the restriction, in response to
changes in flow. An indicator attached to the piston is indexed
against a scale and shows flow rate.
Prior art flow meters, including those described in the above-referenced
U.S. patents, provide reliable, accurate readings of flow rate at
the point at which a meter is disposed. Moreover, such prior art
devices could be adapted to provide electrical signals generally
indicative of flow rate. However, such electrical flow meters are
expensive. Moreover, the means used to produce electrical signals,
such as differential transformers, required that the overall flow
meter be much larger than a non-electrical flow meter.
The present invention contemplates a flow meter providing electrical
output usable by process control equipment and/or capable of operating
a remote display which overcomes the above referred to problems
and others and provides a compact, accurate flow meter which is
simple to install and use and economical to manufacture.
In accordance with the present invention, there is provided a flow
meter comprising a housing, a core tube, a piston slidably retained
within the core tube which changes its position in accordance with
flow through the core tube, a magnet fixed to the piston and a potentiometer
comprised of two bars and a contact positioned such that the contact
moves along the bars in lock step with the magnet thereby generating
a signal indicative of magnet position and flow.
Further in accordance with the invention, the potentiometer provided
is comprised of two parallel bars, one bar being a solid metallic
bar and the other bar comprising a central core surrounded by a
tightly helically wound fine wire.
Still further in accordance with the invention, the contact used
in the potentiometer is a metallic ball electrically contacting
both bars.
Yet further in accordance with the invention, the poteniometer
bars are both circular in cross-section and have a ratio of diameters
of 0.075 to 0.125 and the diameter of the ball is 2.08 times the
diameter of the smaller bar.
Still further in accordance with the invention, the poteniometer
comprised of the ball and two bars is enclosed in a transducer assembly
having a transparent outwardly facing surface to which is fixed
a scale whereby flow may be visually read at the location of the
flow meter in addition to visually read in an electronic display
signal or otherwise at the remote location.
Yet further in accordance with the invention, a transducer assembly
is provided with mounting rings allowing the transducer to be mounted
on the mechanical portions of a flow meter by simply sliding the
transducer into place.
Still further in accordance with the invention electronic circuitry
is provided processing the signal generated by the poteniometer.
The potentiometer signal representative of magnet position is digitized.
The digitized position signal is used to access an erasable programmable
read only memory (EPROM) location containing a digital correction
datum for that particular position. The digitized position figure
and the digital correction datum are added to produce a digital
flow signal for display. The digital correction figure is also converted
to an analog correction signal and added to the potentiometer signal
to create a corrected analog flow signal. An analog variable voltage,
an analog variable current signal and a digital signal, all directly
indicative of flow are thereby created. Additionally, digital integration
of flow rates over time to provide total flow and/or other signal
processing is also performed allowing display of such signals or
the creation and delivery of such signals to a digital computer
or other process control equipment for control of an overall process.
It is the primary object of the present invention to provide a
reliable, accurate, economical and rugged flow meter having an output
which is usable by process control equipment and easily carried
to a remote location for further signal processing and/or display.
It is another object of the present invention to provide a flow
meter having an output signal available at a remote location and
also a local reading of flow at the location of the meter.
It is another object of the invention to provide a flow meter having
electrical output signals usable by process control equipment and
no electrical parts exposed to fluid being measured.
It is yet another object of the present invention to provide a
flow meter having an electrical output and/or other easily transmittible
output available at a remote location and a mechanically driven
output signal, such as a ball and scale, available at the position
of the flow meter even during periods of electrical failure or electronic
malfunction.
It is yet another object of the present invention to provide a
flow meter having electronic signal processing capabilities such
that the flow meter may contribute data and/or signal processing
capabilities to larger process control equipment.
It is another object of the invention to provide a transducer assembly
which can be slipped over a mechanically complete flow meter and
be immediately operable without the need to open the mechanical
portions of the flow meter for adjustment or interconnection.
It is a further object of the invention to provide a flow meter
having an electrical output, no dynamic seals and complete isolation
between the fluid being measured and the meter's electrical parts.
It is still another object of the present invention to provide
an electronic flow meter which is very precise and provides repeatable
signals.
The invention may take physical form in certain parts and arrangements
of parts, a preferred embodiment of which will be described in detail
in the specification and illustrated in the accompanying drawings
which form a part hereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an elevation of a flow meter, transducer and remote display
unit in accordance with the present invention;
FIG. 2 is an elevational sectional view of the flow meter of FIG.
1 taken along line 2--2 of FIG. 1;
FIG. 3 is a cross-sectional view taken along line 3--3 of FIG.
1;
FIG. 4 is a cross-sectional view taken along line 4--4 of FIG.
1;
FIG. 5 is a plan view of the transducer printed circuit board used
to support the poteniometer in the preferred embodiment;
FIG. 6 is an exploded view showing how the two bars of the potentiometer
are assembled to the printed circuit board;
FIG. 7 is an exploded view of the transducer assembly including
the potentiometer, used in the preferred embodiment;
FIG. 8 is a partially exploded view showing how the preferred transducer
assembly is connected to a mechanical flow meter;
FIG. 9 is a partially exploded view showing how the preferred transducer
assembly is assembled to a second type of mechanical flow meter;
and
FIG. 10 is an electrical schematic showing the electrical characteristics
of the transducer, the signal processing equipment and the display
unit.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawings, wherein the showings are for the
purposes of illustrating the preferred embodiment of the invention
only and not for the purposes of limiting same, FIG. 1 shows a flow
meter and transducer assembly A and an electronic display assembly
B interconnected by a cable C. While the flow meter and transducer
assembly A and the electronic display assembly B are shown here
in close proximity, they can be separated by a great distance. The
cable C can be of any reasonable desired length. Long length cables
must, of course, be properly shielded.
The flow meter and transducer assembly A is comprised of a transducer
assembly D and a mechanical assembly E shown assembled in FIG. 1
and disassembled in FIG. 8. Visible at the bottom of the flow meter
mechanical assembly E is a body 10 having an inlet port 12 and an
outlet port 14. Both the inlet port 12 and the output port 14 are
conventionally internally threaded or provided with other fastening
means such that the mechanical assembly E can be easily connected
to fluid carrying pipes or tubing.
The mechanical aspects of the flow meter mechanical assembly E
are best shown in FIG. 2. The inlet port 12 of the body portion
10 feeds fluid to the interior of a core tube 40. The core tube
40 has a slot 42 in its side which allows fluid to exit the interior
of the core tube and flow into the outlet port 14 of the body 10.
A stem assembly 44 is disposed within the core tube 40. The stem
assembly 44 is comprised of a piston 46 a connecting rod 48 and
a magnet carrier 50. The piston 46 is circular in cross section
and slightly smaller in diameter than the interior diameter of the
core tube 40. The piston 46 rides freely vertically within the core
tube 40 and by its motion adjusts the open area of the slot 42 to
accommodate flow through the flow meter. As explained in U.S. Pat.
Nos. 3691834; 4361051 and 4489614 incorporated herein by
reference, the piston position is determined by flow rate and indicative
of flow rate. The magnet carrier 50 is rigidly fixed with respect
to the piston 46 by the connecting rod 48. Hence, the position of
the magnet carrier 50 and the magnet 52 contained therein are also
indicative of flow through the flow meter. Thus, the magnet carrier
50 moves up and down above flow meter body 10 in response in variations
in flow through the flow meter. The magnet carrier 50 vertical position
is therefore directly indicative of flow rate.
A closed cylindrical envelope 57 surrounds the path of travel of
the magnet carrier 50. The envelope is fabricated from brass, 18-8
non-magnetic stainless steel, plastic, or other non-magnetic material
and is closed at its top end. The bottom end of the envelope 57
is received in an opening in the top of body 10. The joint between
the envelope 57 and the body 10 is sealed with an O-ring 58. The
envelope 57 is held in place on the body 10 by a retaining ring
59 and a retaining spacer 60. A vertical guide rod 56 is fixed inside
the envelope 57. A notch 54 in the magnet carrier 50 engages the
guide rod 56 and prevents the carrier from rotating with respect
to the envelope 57. This assures alignment of the magnet 52 with
transducer.
The body 10 and the envelope 57 form a sealed housing for the mechanical
assembly E of the flow meter. No moving parts penetrate the housing
and there are no dynamic seals. A reliable fluid-tight structure
is provided.
The transducer assembly D is shown assembled in FIG. 8 disassembled
in FIG. 7 and in section in FIG. 2. A printed circuit board 62 is
provided with a terminal block 63 which is connected to four current
paths on the printed circuit board. A vertical non-magnetic stainless
steel contact bar 64 is connected to one of the current paths and
fixed to the printed circuit board. A resistor bar 65 is fixed to
the printed circuit board 62 parallel to and in close proximity
to the contact bar. The resistor bar 65 is connected at its upper
end to one current path on the printed circuit board and on its
lower end to another current path. Both the contact bar 64 and the
resistor bar 65 are mounted on the printed circuit board 62 by means
of soldering or the like. The fourth current path on the printed
circuit board is a grounded strip surrounding the outside of the
board and is not active in the circuit except to provide isolation.
Indicator ball 66 contacts both resistor bar 65 and contact bar
64 and electrically connects them. The indicator ball 66 is enclosed
in a raceway recess 67 provided in plastic or ceramic or laminated
panel which completely covers the printed circuit board 62 thereby
protecting it. The raceway recess accommodates the contact bar 64
to move freely along the bars. The raceway panel is sealed to the
printed circuit board by a layer of adhesive sealant 69 which is
applied to the printed circuit board prior to assembly. A transparent
cover 70 is fixed over the portion of the raceway panel 68 in which
the raceway recess is disposed. The cover 70 is made form glass,
acrylic, polycarbonate or another material as conditions require.
An elastomeric sealing ring 71 in a recess in the raceway panel
surrounding the raceway recess seals the joint between the raceway
panel 68 and the transparent cover 70. The raceway recess 67 is
thereby completely sealed and isolated from the surrounding environment.
A scale 72 is fixed to the outside of the transparent cover 70
by scale screws 72a. The scale 72 is positioned such that indicator
ball 66 position can be read against it and flow through the meter
discerned.
A terminal box 73 is disposed near the top of the printed circuit
board 62 on the side of the board opposite the raceway panel 68.
The box 73 is fixed to the printed circuit board 62 and raceway
panel by screws 74 and is sealed against the printed circuit board
62 by a layer of adhesive sealant 75. The terminal box encloses
the terminal block 63 and provides connections of the current paths
on the printed circuit board 62 of conductors in the cable C.
A cover 76 is fixed to the terminal box 73 by means of screws 77.
The joint between the cover 76 and the terminal box 73 is sealed
by a gasket 78.
An upper mounting ring 79 and a lower mounting ring 80 are fixed
to the side of the printed circuit board 62 opposite the raceway
panel 68 by means of transducer assembly screws 81. The screws pass
through the transparent cover 70 the raceway panel 68 the printed
circuit board 62 and are threaded into the mounting rings 79 80.
The two mounting rings have circular central apertures slightly
larger than the outside diameter of the envelope 57. As seen in
FIGS. 8 and 9 the transducer assembly is mounted on the mechanical
assembly by sliding it over the envelope 57 until a positioning
screw 82 encounters the body 10 of the mechanical assembly E. The
positioning screw is adjusted for proper reading and two set screws
83 one in each mounting ring, are then tightened, fixing the transducer
assembly in place.
FIG. 9 shows a mechanical assembly E in which the envelope 57 and
magnetic carrier are disposed below the body 10. Such a structure
is fully described in U.S. Pat. Nos. 4361051 and 4489614 incorporated
herein by reference. The transducer assembly is fixed to such a
structure by means of the set screws already described and positioned
by means of a threaded aperture 84 in the body 10 of the mechanical
assembly.
The heart of the transducer assembly D is a potentiometer 85 comprised
of the contact bar 64 the resistor bar 65 and the indicator ball
66 best seen in FIGS. 3 and 6. The electrical connection of the
potentiometer is shown in FIG. 10. One end of the resistor bar 65
is connected to a source of a reference positive voltage through
the printed circuit board 62 and a reference positive voltage terminal
86. The other end of the resistor bar 65 is connected to a reference
ground terminal 87. The contact bar is connected through the printed
circuit board to a position signal terminal 88.
The resistor bar 65 is a tight helix of very fine wire 65a wound
on a non-conducting core. The winding is sufficiently tight such
that adjacent coils are spaced fifteen-ten thousanths (0.0015) of
an inch, center to center. The resistance of the resistor bar, from
one end to the other, is from about forty thousand ohms to about
eighty thousands ohms. This high resistance makers other resistances
in the circuit negligible in comparison and allows cable C to be
long. In effect, the resistor bar presents a range of voltages from
reference positive voltage to reference ground over its length.
The indicator ball 66 rides along the resistor bar 65 and the contact
bar 64 in lock step with the magnet 52. The indicator ball 66 carries
the voltage present on the resistor bar 65 at the point of contact
to the contact bar 64. The voltage at the point of contact is thereby
connected to the position signal terminal 88 for transmission over
cable C to the electronics and display assembly B. As resistor bar
65 has over six hundred turns of wire per inch of length, the voltage
level signal at terminal 88 is a very precise indication of ball
position. For this signal to be an accurate representation of flow,
it is necessary that the ball position must closely follow the magnet
position. A number of factors interact to assure that the ball 66
accurately follows the magnet 52 in the present invention.
A high strength Neodymium Iron Boron or Samarium Cobalt rare earth
magnet is used. All such magnets can be energized to a high energy
product. The particular magnetic material is selected for proper
temperature characteristics. The magnet is positioned in the carrier
50 near its outer edge and, thus, near the ball 66. The contact
bar is fabricated from 18-8 non-magnetic stainless steel and given
a very smooth 20 to 40 microinch surface finish. The indicator ball
66 is fabricated from 400 stainless steel and is given a mirror
smooth surface finish. The contact bar and the indicator ball may
be plated with gold, silver or other material. Importantly, the
resistor bar 65 has an outside diameter of approximately 0.075 inches,
the contact bar 64 has a diameter of 0.125 inches and the indicator
ball 66 has a diameter of 0.1562 inches. It has been found that
use of a contact bar with a larger diameter than the resistor bar
improves accuracy. The use of two bars of different diameters smaller
than the diameter of the indicator ball also improves accuracy.
The ball 66 has been found to move along the two bars, so sized,
with a minimum of resistance and a minimum of lagging. The ball
66 therefore, puts a minimum load on the magnet 52 and the stem
assembly 44. The ball also closely follows the magnet providing
very accurate position signals at position signal terminal 88. Importantly,
hysterysis errors are minimized even when an increasing flow becomes
a decreasing flow.
As described above, the ball 66 responds to movement of a magnet
52 within the flow meter by moving along the two bars thereby carrying
the signal through the position signal terminal 88. This position
signal is carried by cable C to a conventional bridge circuit 92.
The bridge circuit 92 contains trimming poteniometers to adjust
the zero reference level and the "span" or "range"
of the position signal and amplifies this signal in a conventional
manner. An output signal, the analog magnet position signal, is
generated and applied to an analog to a digital converter 93. The
analog to digital converter is conventional and creates an eleven
bit position signal representative of the analog signal received
as input. The digital position signal is applied to an EPROM (erasable
programmable read only memory) circuit 94. The EPROM circuit 94
is factory programmed with an eight bit correction datum for each
possible digital position signal. The EPROM is programmed for each
model of meter produced and for special applications by testing
the flow meter under the conditions in which it will be used, noting
inaccuracies and building a correction curve which is programmed
into the EPROM in a conventional manner. In the preferred embodiment
of the invention, the EPROM is programmed by setting up to approximately
2000 correction points to form a correction curve which precisely
compensates for nonlinearities and inaccuracies in the mechanical
components of the flow meter. As is conventional, the EPROM is programmed
once at the factory and not reprogrammed unless a major change in
metering conditions is encountered.
The output of the EPROM circuit 94 a digital correction datum,
is added to the digital position signal in a digital adder 95 producing
a corrected digital flow signal. The corrected digital flow signal
is processed in a display driver 96 and the resulting display signals
drive a digital display 97 which digitally displays the flow rate.
The corrected digital flow signal from the digital adder 95 is
also applied to a totalizer 98 where it is combined with signals
from a clock circuit 99. A signal representing total flow over a
selected period is generated and applied to a second display driver
100 which creates display signals driving a second digital display
101. The second digital display shows total flow over a period on
the face of the electronics and display assembly B.
The digital correction datum from the EPROM circuit 94 is also
applied to a digital to analog converter 104 which creates an analog
correction signal. The analog correction signal is added to the
analog position signal from the bridge circuit 92 in an analog adder
105. The analog adder produces a variable current output having
a range of four to twenty milliamperes and a variable voltage output
having a range of zero to ten volts, both output signals representing
flow rate. These output ranges are conventional in the process control
industry.
The clock circuit 99 and a power supply 107 provide timing signals
and power to the elements already described in a conventional manner.
From the above, it can be seen that a very precise mechanical flow
meter provides a very precise mechanical indication of flow. Additionally,
the transducer and electronic circuit provided very precisely track
the mechanical indication of flow and presents an extremely precise
representation of this signal at a visual display and in conventional
process control signal ranges. To maintain the level of precision
seen in the flow meter and in the electronic circuitry, it is necessary
to provide a highly precise transducer capable of accurately describing
the position of the magnet within the flow meter to the electronic
circuitry. This is accomplished by the interaction of the elements
described above.
The invention has been described with reference to a preferred
embodiment. Obviously, modifications and alterations will occur
to others upon the reading and understanding of this specification.
Thus, one could use an internal battery to supply power to the transducer
assembly D and replace cable C with a fiber optics cable driven
by a digitizer and a diode laser within the junction box 73 for
operation in environments where electrical cables are unacceptable.
It is out intention to include all such modifications and alterations
insofar as they come within the scope of the appended claims or
the equivalents thereof. |