IMPROVED POSITIVE DISPLACEMENT FLOW METER

DEBITMETRE DE DEPLACEMENT POSITIF AMELIORE

English Abstract

There is a positive displacement flowmeter which includes at least two primary

measuring elements rotatable on respective spaced apart axes within a housing
between a fluid inlet and a fluid outlet. The elements are linked to rotate in

accordance with volumetric flow through the flowmeter from said inlet to said
outlet. There are at least two opposed pole magnets disposed at separated
positions on one of the elements and a pole sensitive sensor located in a
rotational
path of said magnets whereby said sensor is adapted to emit an output signal
upon activation by said at least two of said opposed pole magnets.

Claims

The claims:


1. A positive displacement flowmeter including at least two primary measuring
elements rotatable on respective spaced apart axes within a housing between
a fluid inlet and a fluid outlet, said elements being linked to rotate in
accordance with volumetric flow through the flowmeter from said inlet to
said outlet, at least two opposed pole magnets disposed at separated
positions on one of the elements and a pole sensitive sensor located in a
rotational path of said magnets whereby said sensor is adapted to emit an
output signal only upon activation by said at least two of said opposed pole
magnets.


2. The positive displacement flowmeter as claimed in claim 1 wherein said at
least two of said opposed pole magnets are positioned at about 180 degrees
apart on said one of the elements.


3. The positive displacement flowmeter as claimed in claim 1 wherein said
pole sensitive sensor is of a type to adapt a first electrical state upon
passing
of one of said at least two opposed pole magnets and retain said first
electrical state until another of other of said at least two opposed pole
magnets passes whereby said sensor adopts a second electrical state to emit
said output signal.


4. The positive displacement flowmeter as claimed in claim 1 wherein said
pole sensitive sensor is a pole sensitive latching sensor.



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5. The positive displacement flowmeter as claimed in claim 4 wherein said
pole sensitive latching sensor is a bi-polar hall effect device.


6. The positive displacement flowmeter as claimed in claim 1 wherein said
primary measuring elements are oval rotors with rotor lobes.


7. The positive displacement flowmeter as claimed in claim 6 wherein said
rotors are offset by 90 degrees and are meshed together to rotate around said
spaced apart axes.


8. The positive displacement flowmeter as claimed in claim 7 wherein said
opposed pole magnets are disposed one each on respective ones of said rotor
lobes.


9. The positive displacement flowmeter as claimed in claim 1 wherein said
pole sensitive sensor is fixed to said housing.



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Description

CA 02752078 2011-09-01
IMPROVED POSITIVE DISPLACEMENT FLOW METER

FIELD OF THE INVENTION

This invention relates to apparatus for flow measurement. More particularly,
although not exclusively it discloses improvements to positive displacement
flowmeters to achieve improved electrical signal generation.

BACKGROUND OF THE INVENTION
Positive displacement flowmeters typically include oval rotor, rotary vane
and gear wheel flowmeters. Oval rotor flowmeters for example comprise a
paii' of oval shaped rotors as the primary measuring elements. With those
meters having direct pulse generation magnets are fitted to the rotors. These
rotors rotate in accordance with fluid flow passing through the meter. One or
more fixed sensors or signal generating means such as reed switches, hall
effect devices, coils or proximity switches etc. are excited to generate an
electrical pulse each time one of the rotor mounted magnets passes. An
output train or series of such pulses is necessary for the propose of flow
rate
integration and calculation.

One disadvantage with such prior art flowmeters is that transient flow
reversals adversely affect the quality and point accuracy of the flowmeter
output. Such flow reversals occur for example in internal combustion engine
fuel supply systems and pulsating flow lines. They can cause substantial
inaccuracies in the measurement of fluid flow in the forward direction. More
specifically, with prior ait devices such as that shown schematically in
figure
1, magnets 1 are mounted on respective lobes 2 of rotors 3. As the rotors
rotate these magnets each pass a single fixed sensor 4 which is thereby
activated to generate one pulse. After the magnetic field from each magnet
passes out of the sensor activation zone the sensor automatically resets
itself
for the next pass. If at any time during this process the fluid flow direction
and rotor rotation briefly reverses, the last activating magnet after leaving

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CA 02752078 2011-09-01

can re-enter the sensor activation zone to cause generation of an additional
aberrant pulse. This can produce a point error of as much as 50% to 100%.
This condition can apply to a magnet either approaching or leaving the
sensor activation zone. Generally the greater the order of reverse flow the
larger is the potential for erroneous pulses. These pulses result in
inconsistent and inaccurate readings being transmitted to the receiving
instrument.
SUMMARY OF THE INVENTION

It is therefore an object of this invention to ameliorate the aforementioned
disadvantage and accordingly a positive displacement flowmeter is disclosed
which includes at least two primary measuring elements rotatable on
respective spaced apart axes within a housing between a fluid inlet and a
fluid outlet, said elements being linked to rotate in accordance with
volumetric flow through the flowmeter from said inlet to said outlet, at least
two opposed pole magnets disposed at separated positions on one of the
elements and a pole sensitive sensor located in a rotational path of said
magnets whereby said sensor is adapted to emit an output signal only upon
activation by said at least two of said opposed pole magnets.

Preferably said opposed pole magnets are positioned at about 180 degrees
apart on one of said elements.

It is further preferred that said pole sensitive sensor is a bi-polar hall
effect
device or other pole sensitive status latching sensor.

It is further preferred that primary measuring elements are rotors and said
opposed pole magnets are disposed on opposite lobes of one of said rotors.
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CA 02752078 2011-09-01
BRIEF DESCRIPTION OF THE DRAWING

One currently preferred embodiment of the invention will now be described
with reference to the attached figure 2 which shows a schematic cross-
sectional view of a flowmeter constructed in accordance with said invention.

DESCRIPIION OF THE PREfER RED IIvIBODIMENT

There is a flowmeter housing 5 from which the cover (not shown) has been
removed to show a pair of oval rotors 6, 7 within an operating chamber 8.
The rotors are offset by 90 degrees as shown and meshed together to rotate
around spaced apart parallel axis 9, 10 in directions indicated for example by
arrows 11, 12 as fluid passes through the meter. In accordance with the
invention there is a pole sensitive status latching sensor such as a bi-polar
hall effect device 13 fixed in the housing closely adjacent the rotational
path
of the rotor lobes 14, 15. These lobes have opposed pole magnets 16, 17
mounted in them which upon rotor rotation alternately pass their facing
north and south magnetic poles closely over the sensor. The sensor is of a
type to adopt a first electrical state upon the passing of one magnetic pole
and retain that state until the other opposite pole passes whereby it adopts a
second electrical state to emit an output pulse. The effect of this novel
arrangement is that the rotational operating window required for the sensor
pulse generation is substantially increased to the extent that typical
transient
flow reversals and associated rotor reversals are not registered by the meter.
Forward flow measurement of the meter is thus not compromised by
transient flow reversals as with prior art meters.

It will thus be appreciated that this invention at least in the form of the
embodiment disclosed provides a novel and useful improvement in the
construction of positive displacement flowmeters. Clearly however the
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CA 02752078 2011-09-01

example described is only the currently preferred form of the invention and a
wide variety of modifications may be made which would be apparent to a
person skilled in the art. For example the shape, configuration and number
of rotors, the placement of the magnets and the type and positioning of the
sensor may change following further development work by the inventors.

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Representative Drawing