Abstrict A magnetic flow meter includes a cylindrical vessel within which
electrodes are placed directly opposite each other about a magnetically
permeable core. In that way, two flow channels are obtained in the
vessel. A common magnetic field is applied across both flow channels.
The magnetic flow meter is used in conjunction with a controller
which monitors amplitude modulated signals representative of applied
magnetic field and induced electric field. These amplitude modulated
signals are passed to Sigma-Delta modulators, which directly digitise
the amplitude modulated AC signals into digital pulses. The controller
is capable of monitoring reverse flow through the meter, and to
store results until forward flow resumes. Moreover, the controller
generates a driving signal for coils which generates a magnetic
field, the coil being placed in conjunction with a capacitor which
causes resonance. This reduces power consumption by the coil in
driving the meter.
Claims What is claimed is:
1. A magnetic flow meter comprising: a conduit operable to guide
a flow along a flow path; a magnetic field generator operable to
generate an alternating magnetic field at a drive frequency across
the flow path; a flow sensor operable to sense electromagnetic fields
generated across the flow path and to output a corresponding flow
sensor signal including: (a) an electric field component at the
drive frequency and at a first phase, which electric field component
varies with an electric field generated by the interaction of the
magnetic field generated by said magnetic field generator and said
flow; and (b) an interfering component at the drive frequency and
at a second different phase; a magnetic field sensor operable to
sense the magnetic field generated by said magnetic field generator
and to output a corresponding magnetic field sensor signal at said
drive frequency at a phase related to said second phase; a calibration
processor operable to process said magnetic field sensor signal
to generate an alternating calibration control signal at said drive
frequency whose phase depends upon the phase of said magnetic field
sensor signal; a first processing circuit responsive to the calibration
control signal and operable to process the flow sensor signal to
determine a measure of the strength of the electric field component
thereof; a second processing circuit operable to process the magnetic
field sensor signal to determine a measure of the strength of the
magnetic field sensor signal; and a measurement processor operable
to determine a measure of the flow in said conduit in dependence
upon the determined measure of strength of said electric field component
and the determined measure of strength of said magnetic field sensor
signal.
2. A magnetic flow meter according to claim 1 wherein said conduit
includes a flow separator for separating the flow along the conduit
into first and second flow paths.
3. A magnetic flow meter according to claim 2 wherein said magnetic
field generator is operable to generate said magnetic field across
both said first and second flow paths and wherein said flow sensor
is operable to sense electromagnetic fields generated across at
least one of said flow paths.
4. A magnetic flow meter according to claim 2 wherein said separator
is a magnetically active element and is arranged to condition the
magnetic field generated by said magnetic field generator to act
substantially orthogonally to the direction of flow through the
flow paths.
5. A magnetic flow meter according to claim 2 wherein said flow
sensor is operable to sense electromagnetic fields generated across
both flow paths and to output a common flow sensor signal.
6. A magnetic flow meter according to claim 2 comprising first
and second flow sensors each operable to sense electromagnetic fields
generated across a respective one of the first and second flow paths.
7. A magnetic flow meter according to claim 1 wherein said conduit
comprises an elongate vessel defining the flow path, wherein the
magnetic field generator comprises a magnetically permeable core
positioned internally longitudinally of the vessel and magnetically
permeable poles external to the vessel.
8. A magnetic flow meter according to claim 7 wherein the flow
sensor comprises first and second electrodes that are substantially
magnetically impermeable and arranged on substantially diametrically
opposed sides of the core, interposed between the core and an internal
surface of the vessel, with two flow paths being defined in the
vessel which are separated by the first and second electrodes and
the core in combination.
9. A magnetic flow meter according to claim 8 wherein the elongate
vessel is substantially cylindrical, and wherein the core is substantially
cylindrical and is aligned substantially axially of the vessel.
10. A magnetic flow meter according to claim 1 wherein said magnetic
field generator comprises a coil and a drive circuit for applying
an alternating drive signal to the coil.
11. A magnetic flow meter according to claim 10 wherein said drive
circuit and coil are resonant.
12. A magnetic flow meter according to claim 11 further comprising
an energy source operable to supply energy intermittently to said
resonant drive circuit and coil.
13. A magnetic flow meter according to claim 1 wherein said magnetic
field sensor comprises a sensor coil.
14. A magnetic flow meter according to claim 1 wherein said first
processing circuit comprises a circuit operable to receive the flow
sensor signal and for generating therefrom a series of pulses whose
frequency depends upon the measure of strength of the electric field
component of the flow sensor signal.
15. A magnetic flow meter according to claim 14 wherein said first
processing circuit includes a counter for counting the pulses within
said series of pulses.
16. A magnetic flow meter according to claim 15 wherein said first
processing circuit comprises an up/down control circuit operable
to cause said counter to increment the count with each pulse in
the series or to decrement the count with each pulse in-the series,
and wherein said up/down control circuit is responsive to said calibration
control signal.
17. A magnetic flow meter according to claim 16 wherein said up/down
control circuit is operable to cause said counter to increment the
count during a first half-cycle of said calibration control signal
and is operable to cause said counter to decrement the count during
a second half-cycle of said calibration control signal.
18. A magnetic flow meter according to claim 1 wherein said second
processing circuit comprises a circuit operable to receive the magnetic
field sensor signal and for generating therefrom a series of pulses
whose frequency depends upon the measure of strength of the magnetic
field sensor signal.
19. A magnetic flow meter according to claim 18 wherein said second
processing circuit includes a counter for counting the pulses within
the series of pulses generated from said series of pulses.
20. A magnetic flow meter according to claim 19 wherein said second
processing circuit comprises an up/down control circuit operable
to cause said counter to increment the count with each pulse in
said series or to decrement the count with each pulse in said series,
and wherein said up/down control circuit is responsive to said calibration
control signal.
21. A magnetic flow meter according to claim 20 wherein said up/down
control circuit is operable to cause said counter to increment the
count during a first half-cycle of said calibration control signal
and is operable to cause said counter to decrement the count during
a second half-cycle of said calibration control signal.
22. A magnetic flow meter according to claim 1 wherein said measurement
processor is operable to calculate a ratio of said measure of strength
of said electric field component and the measure of strength of
the magnetic field sensor signal and is operable to determine said
measure of the flow using the calculated ratio.
23. A magnetic flow meter according to claim 8 further comprising
a first elongate electrode support including an elongated portion
positioned adjacent the inner surface of the flow tube and at least
one flow divider extending inward toward the core, the first electrode
support having at least one rail, said rail being circumferentially
aligned with the first electrode; and a second electrode support
including an elongated portion positioned adjacent the inner surface
of the flow tube and at least one flow divider extending inward
toward the core, said second electrode support having at least one
rail, said rail being circumferentially aligned with the second
electrode.
24. A magnetic flow meter according to claim 23 wherein the at
least one rail of the first electrode support comprises first and
second flow dividers axially spaced from each other, the first electrode
being positioned between the first and second flow dividers; and
the at least one rail of the second electrode support comprises
third and fourth flow dividers axially spaced apart from each other,
the second electrode being positioned between the third and fourth
flow dividers.
25. A magnetic flow meter according to claim 23 wherein the core
includes a through-bore extending from a side of the core adjacent
a radially inner side of the first electrode to a side of the core
adjacent a radially inner side of the second electrode, wherein
the first electrode includes a through-bore and further comprising
an electrode lead which passes through the through-bore of the core
and the through-bore of the first electrode and which is connected
to the second electrode for connecting the second electrode to the
first processing circuit.
26. A magnetic flow meter according to claim 25 wherein the first
electrode support includes an axially extending electrode passage,
and wherein the electrode lead for the second electrode extends
within said electrode passage to an exterior of the conduit together
with an electrode lead for connecting the first electrode to the
first processing circuit.
27. A magnetic flow meter according to claim 23 wherein at least
a portion of the at least one flow divider of the first electrode
support contacts the core and at least a portion of the at least
one flow divider of the second electrode support contacts the core,
thereby holding the core in position.
28. A magnetic flow meter according to claim 8 wherein said first
and second electrodes are formed from a graphite material.
29. A magnetic flow meter according to claim 1 having a periodic
measurement cycle, wherein said first and second processing circuits
are operable to process the respective signals continuously during
each measurement cycle and to accumulate the respective sensor signal
strengths in the measurement cycle and wherein said measurement
processor is operable to determine a measure of the flow along said
flow path at the end of each measurement cycle using the strength
measures accumulated during that cycle.
30. A domestic water meter comprising: a conduit operable to guide
water along a flow path; a magnetic field generator operable to
generate an alternating magnetic field at a drive frequency across
the flow path; a flow sensor operable to sense electromagnetic fields
generated across the flow path and to output a corresponding flow
sensor signal including: (a) an electric field component at the
drive frequency and at a first phase, which electric field component
varies with an electric field generated by the interaction of the
magnetic field generated by said magnetic field generator and the
water flowing along said flow path; and (b) an interfering component
at the drive frequency and at a second different phase; a magnetic
field sensor operable to sense the magnetic field generated by said
magnetic field generator and to output a corresponding magnetic
field sensor signal at said drive frequency at a phase related to
said second phase; a calibration processor operable to process said
magnetic field sensor signal to generate an alternating calibration
control signal at said drive frequency whose phase depends upon
the phase of said magnetic field sensor signal; a first processing
circuit responsive to the calibration control signal and operable
to process the flow sensor:signal to determine a measure of the
strength of the electric field component thereof; a second processing
circuit operable to process the magnetic field sensor signal to
determine a measure of the strength of the magnetic field sensor
signal; and a measurement processor operable to determine a measure
of the flow of water in said conduit in dependence upon the determined
measure of strength of said electric field component and the determined
measure of strength of said magnetic field sensor signal.
31. A process flow control meter comprising: a conduit operable
to guide a flow along a flow path; a magnetic field generator operable
to generate an alternating magnetic field at a drive frequency across
the flow path; a flow sensor operable to sense electromagnetic fields
generated across the flow path and to output a corresponding flow
sensor signal including: (a) an electric field component at the
drive frequency and at a first phase, which electric field component
varies with an electric field generated by the interaction of the
magnetic field generated by said magnetic field generator and said
flow; and (b) an interfering component at the drive frequency and
at a second different phase; a magnetic field sensor operable to
sense the magnetic field generated by said magnetic field generator
and to output a corresponding magnetic field sensor signal at said
drive frequency at a phase related to said second phase; a calibration
processor operable to process said magnetic field sensor signal
to generate an alternating calibration control signal at said drive
frequency whose phase depends upon the phase of said magnetic field
sensor signal; a first processing circuit responsive to the calibration
control signal and operable to process the flow sensor signal to
determine a measure of the strength of the electric field component
thereof; a second processing circuit operable to process the magnetic
field sensor signal to determine a measure of the strength of the
magnetic field sensor signal; and a measurement processor-operable
to-determine a measure of the flow in said conduit in dependence
upon the determined measure of strength of said electric field component
and the determined measure of strength of said magnetic field sensor
signal.
32. A method of measuring a flow rate of a fluid comprising: providing
a conduit operable to guide the fluid along a flow path; generating
an alternating magnetic field at a drive frequency across the flow
path; a first sensing step of sensing electromagnetic fields generated
across the flow path and outputting a corresponding flow sensor
signal including: (a) an electric field component at the drive frequency
and at a first phase, which electric field component varies with
an electric field generated by the interaction of the generated
magnetic field and said flow; and (b) an interfering component at
the drive frequency and at a second different phase; a second sensing
step of sensing the magnetic field generated in said magnetic field
generating step and outputting a corresponding magnetic field sensor
signal at said drive frequency at a phase related to said second
phase; using a calibration processor to process said magnetic field
sensor signal to generate an alternating calibration control signal
at said drive frequency whose phase depends upon the phase of said
magnetic field sensor signal; using a first processing circuit responsive
to the calibration control signal, to process the flow sensor signal
to determine a measure of the strength of the electric field component
thereof; using a second processing circuit to process the magnetic
field sensor signal to determine a measure of the strength of the
magnetic field sensor signal; and determining a measure of the flow
in said conduit in dependence upon the determined measure of strength
of said electric field component and the determined measure of strength
of said magnetic field sensor signal.
33. A magnetic flow meter comprising: a conduit operable to guide
a flow along a flow path; a magnetic field generator operable to
generate a magnetic field across the flow path; an electric field
sensor operable to sense an electric field generated across the
flow path by the interaction of the magnetic field generated by
said magnetic field generator and the flow in said flow path and
to output a corresponding electric field sensor signal; a magnetic
field sensor operable to sense the magnetic field generated by said
magnetic field generator and to output a corresponding magnetic
field sensor signal; a first processing circuit operable to process
the electric field sensor signal to determine a measure of the strength
of the electric field sensor signal; a second processing circuit
operable to process the magnetic field sensor signal to determine
a measure of the strength of the magnetic field sensor signal; and
a measurement processor operable to determine a measure of the flow
in said conduit in dependence upon the determined measure of strength
of said electric field sensor signal and the determined measure
of strength of said magnetic field sensor signal; wherein said magnetic
flow meter has a periodic measurement cycle, wherein said first
and second processing circuits are operable to process the respective
sensor signals continuously during each measurement cycle and to
accumulate the respective sensor signal strengths in the measurement
cycle and wherein said measurement processor is operable to determine
a measure of the flow along said flow path at the end of each measurement
cycle using the strength measures accumulated during that cycle.
Description The present invention is concerned with magnetic flow meters, and
particularly, but not exclusively, with magnetic flow meters for
the metering of water consumption.
A magnetic flow meter can be used to measure the flow of an electrically
conductive fluid along a flow path. The meter is operative to apply
a magnetic field across the flow path, on the principle that a conductor
moving in a magnetic field causes the induction of a voltage across
the conductor. In the case of the fluid flow meter, the conductor
in question is the fluid (being water, or an aqueous solution).
The direction of the induced voltage is mutually perpendicular to
the direction of flow and the direction of the magnetic field.
U.S. Pat. Nos. 3911742 and 4716769 describe radial field magnetic
flow meters. The device described in U.S. Pat. No. 3911742 is
for use with a catheter tube. As such, it consists of an elongate
wand containing a device from which a magnetic field emanates radially.
The voltage, or electrostatic field, induced by the flow of water
(or other electrically conductive fluid) parallel with the longitudinal
axis of the wand, will circulate around the wand. Two electrodes
are placed on the surface of the wand, and an electrically insulating
septum extending radially from the wand is positioned between the
electrodes. The electrostatic field induced about the wand is interrupted
by the septum and its magnitude is then measured.
U.S. Pat. No. 4716769 describes another meter, which is of similar
construction to that described in U.S. Pat. No. 3911742 except
that it further comprises a body surrounding the wand and septum
previously described.
A first aspect of the invention provides a magnetic flow meter
including two flow passages, means for applying a magnetic field
across both passages, and means for measuring induced voltage in
at least one of the passages, including means for shaping the magnetic
field applied in use such that the field is substantially perpendicular
to the direction of flow and to the direction in which induced voltage
is measured.
The invention provides a meter capable of measuring flow with a
degree of immunity to asymmetric flow profiles. In that way, flow
need not be conditioned upstream of the meter, for example by means
of a long stretch of straight piping.
A second aspect of the invention provides a magnetic flow meter
including a flow passage, means external of the flow passage for
applying a magnetic field across the flow passage, and means for
measuring induced voltage in said flow passage, further comprising
means within the flow passage for shaping the applied magnetic field
in use.
A third aspect of the invention provides a magnetic flow meter
comprising a body, a vessel having first and second ends, the vessel
defining a flow channel between said ends, and measurement means
supported on an anchor of said body for measuring flow within said
channel and wherein said anchor extends within said vessel from
one or both of said first and second ends.
A fourth aspect of the invention provides a magnetic flow meter
including a flow chamber of annular cross-section, and means for
defining a magnetic field radially of the flow chamber.
A fifth aspect of the invention provides a magnetic flow meter
comprising a flow chamber, means for generating a magnetic field
in said chamber and means for measuring voltage induced thereby,
wherein the means for generating a magnetic field is at least partially
resonant.
A sixth aspect of the invention provides a measurement device including
means for receiving an amplitude modulated AC signal and means for
converting a signal directly received by the receiving means into
a series of coded pulses for digital processing thereof.
In accordance with that sixth aspect, the measurement device preferably
comprises means for receiving an amplitude modulated signal representative
of an applied magnetic field in a magnetic flow meter and means
for receiving an amplitude modulated signal representative of an
induced electrostatic field in the magnetic flow meter, and wherein
first and second signal conversion means are provided, each being
operative to separately directly encode a respective one of said
received signals into a stream of coded pulses.
In accordance with that sixth aspect, the measurement device preferably
includes analogue to digital conversion means operable to convert
an amplitude modulated signal to a digital signal and digital integration
means operable to integrate said digital signal.
The digital integration means may include means for rejecting crossover
distortion in the output thereof.
A seventh aspect of the invention provides a water meter comprising
means defining a water flow channel, means for applying a magnetic
field across the channel, and means for measuring a voltage induced
in water in said channel in use, wherein said meter includes means
for shaping said magnetic field within a volume of the flow channel
identified for measurement, such that said field is substantially
perpendicular to the direction of induced voltage measured by said
measuring means, in use.
Specific and preferred embodiments of the invention will now be
described with reference to the accompanying drawings, in which:
FIG. 1 is a perspective view of a magnetic flow meter in accordance
with a first specific embodiment of the present invention;
FIG. 2 is a perspective view of the magnetic flow meter illustrated
in FIG. 1 exploded for illustration of its component parts;
FIG. 3 is a side view of the magnetic flow meter illustrated in
FIG. 1;
FIG. 4 is a cross-section of the magnetic flow meter illustrated
in FIG. 1 in the direction of arrows IV--IV indicated in FIG. 3;
FIG. 5 is an axial sectional view of the magnetic flow meter illustrated
in FIG. 1 in the direction of arrows V--V indicated in FIG. 4;
FIG. 6 is a perspective view of an electrode of the magnetic flow
meter illustrated in FIG. 1;
FIG. 7 is a detailed cross-section of the magnetic flow meter illustrated
in FIG. 1 illustrating the distribution, in use, of a magnetic
field therein;
FIG. 8 is a schematic diagram showing the magnetic flow meter illustrated
in FIG. 1 arranged with a meter controller;
FIG. 9 is a schematic circuit diagram of an analogue sense amplifying
unit of the controller illustrated in FIG. 8;
FIG. 10A is a graph against time of a coil drive voltage signal
generated by the controller of FIG. 8 during operation;
FIG. 10B is a graph against time of a current resultant at a coil
drive output of the controller on output of the coil drive voltage
signal illustrated in FIG. 10A;
FIG. 10C is a graph against time of a magnetic field pick-up voltage
signal received by the controller in use;
FIG. 10D is a graph against time of an induced voltage signal generated
-across the electrodes of the magnetic field flow meter in use;
FIG. 11 is a schematic circuit diagram of a second order Sigma-Delta
modulator of the analogue sense amplifying unit illustrated in FIG.
9;
FIG. 12A is a graph against time showing a signal input in use
to the Sigma-Delta modulator illustrated in FIG. 11;
FIG. 12B(i) is a graph against time on a time scale expanded relative
the scale in FIG. 12A showing a first intermediate demodulated signal
for a first sample time period indicated in FIG. 12A;
FIG. 12B(ii) is a graph against time on a time scale expanded relative
the scale in FIG. 12A showing a first intermediate demodulated signal
for a second sample time period;
FIG. 12B(iii) is a graph against time on a time scale expanded
relative the scale in FIG. 12A showing a first intermediate demodulated
signal for a third sample time period;
FIG. 12C(i) is a graph against time on the same time scale as in
FIG. 12B (ii), showing a second intermediate demodulated signal
for the first sample time period;
FIG. 12C(ii) is a graph against time on the same time scale as
in FIG. 12B(ii), showing a second intermediate demodulated signal
for the second sample time period;
FIG. 12C(iii) is a graph against time on the same time scale as
in FIG. 12B(iii), showing a second intermediate demodulated signal
for the third sample time period;
FIG. 12D is a graph against time on the same time scale as in FIG.
12A showing the second intermediate demodulated signal as illustrated
in FIGS. 12C(i), 12C(ii) and 12C(iii) for the time period represented
in FIG. 12A;
FIG. 12E is a graph against time on the same time scale as in FIG.
12A showing a third intermediate demodulated signal for the time
period represented in FIG. 12A;
FIG. 13 is a circuit diagram of a first integrator of the Sigma-Delta
modulator illustrated in FIG. 11;
FIG. 14 is a circuit diagram of a second integrator of the Sigma-Delta
modulator illustrated in FIG. 11;
FIG. 15 is a circuit diagram of a microcontroller of the controller
illustrated in FIG. 8;
FIG. 16 is a schematic diagram showing the configuration of the
microcontroller illustrated in FIG. 15;
FIG. 17 is a schematic diagram of a coil drive of the controller,
illustrated in FIG. 8 shown in conjunction with coils of the magnetic
flow meter illustrated in FIG. 1; and
FIGS. 18 and 19 are flow diagrams illustrating routines for performance
in the counter unit of the configuration shown in FIG. 16;
FIG. 20 is a flow diagram illustrating a routine for performance
in the totaliser unit of the configuration shown in FIG. 16;
FIG. 21 is a perspective view of a magnetic flow meter in accordance
with a second specific embodiment of the present invention;
FIG. 22 is a schematic cross section of the magnetic flow meter
illustrated in FIG. 21; and
FIG. 23 is a process diagram of an industrial application of the
meter illustrated in FIG. 1.
As illustrated in FIGS. 1 to 5 a flow meter 20 comprises two complementary
body portions 22. The body portions 22 should be of a magnetically
non-permeable material, and preferably non-conductive. Common engineering
plastics, and some relatively non-conductive metals, are appropriate
materials. Each body portion 22 is substantially cylindrical, with
upper and lower parallel flat faces 23 for location of the meter
against mounting fixtures (not shown).
Each body portion 22 has an axially extending bore 24 through its
length. The bore 24 is of generally uniform diameter, except for
a portion 26 at one end of the bore 24 which is wider. That wider
portion 26 is internally threaded to co-operate with an external
thread of a connecting pipe (not shown), or with a fitting for connection
to a connecting pipe. A shoulder 28 between the wider portion 26
of the bore 24 and the remainder of the bore 24 is arranged so that
a pipe can abut against it. The difference in diameter of the wider
portion 26 from the remainder of the bore 24 may be selected so
that the remainder of the bore 24 has the same internal diameter
as a connecting pipe connected to the meter, thereby limiting drag
in the flow of fluid within the flow path.
Each body portion 22 has upper and lower assembly lugs 30 located
on the opposite end thereof from the threaded portion 26 of the
bore 24. Each assembly lug 30 extends away from the body portion
22 in an axial direction, and then radially thereof to define an
"L" shape. The radially extending portion of each assembly
lug 30 includes an assembly bore 32 extending therethrough in a
direction parallel to the bore 24 of the body portion.
The two body portions 22 are axially aligned so that the assembly
lugs 30 abut each other, and the assembly bores 32 align with each
other. Assembly bolts 31 are passed through the aligned assembly
bores 32 and engaged with nuts 33 to fix the body portions 22 together.
Before fixing, a tubular pressure vessel 34 is assembled within
the bore 24 of the body portions 22. The tubular pressure vessel
34 fits tightly in the bores 24. Alternatively or in addition to
the tight fit, sealing means such as O-rings, sealant compound,
glue or the like could be interposed between the pressure vessel
34 and each of the body portions 22 so as to avoid the escape of
water from the flow path through the meter.
The pressure vessel 34 is constructed of titanium. Titanium is
selected as it has relatively high strength, and negligible magnetic
permeability. The high strength of titanium means that strain in
the wall of the vessel due to fluid pressure within the meter can
be maintained, under normal operating conditions, within a predetermined
(low) range. Moreover, titanium has sufficiently high electrical
resistivity that eddy currents in the pressure vessel walls are
kept acceptably low. It will be understood that other materials
which meet these criteria would also be acceptable for use in the
construction of the pressure vessel 34.
Fixing rings 36 are placed at either end of the pressure vessel
34 for supporting two support members 38 within the pressure vessel
34. The support members 38 extend throughout the length of the pressure
vessel 34. The support members 38 are supported on the fixing rings
36 so as to be diametrically opposed across the pressure vessel
34. Each support member 38 has two axially spaced flow dividers
40. Each flow divider 40 is streamlined at an end distal the other
flow divider, and is defined by two radial planes of the pressure
vessel 34 which subtend an angle of approximately 45.degree. at
the longitudinal central axis of the pressure vessel 34. The flow
divider 40 extends into the tube a distance of about half of the
radius of the pressure vessel 34 from the inner wall thereof.
Each support member 38 supports an electrode 42 interposed between
the two flow dividers 40. Each electrode 42 is of graphite material.
The graphite electrodes 42 are of the same cross-section as the
flow dividers 40. The end faces 43 i.e. those faces generally transverse
to the pressure vessel 34 of each electrode 42 are substantially
perpendicular to the longitudinal axis of the pressure vessel 34.
Other arrangements for supporting the electrodes and for dividing
the flow in the meter could be provided. An advantage of the arrangement
illustrated in FIG. 5 is that the supports extend axially beyond
the pressure vessel 34 removing the requirement for fixings in
the metrologically most sensitive part of the meter which could
have a deleterious effect on the performance of the meter.
Graphite is chosen because it has suitably low surface impedance,
it is relatively soft so as to enhance manufactureability, it is
durable and it is suitably electrically non-noisy on the other hand,
platinum has been found to be acceptable (but is somewhat more expensive
than graphite), and conductive ceramic electrodes, such as Ebonex
or the like, could also be considered.
In the region of the electrodes 42 the interior of the pressure
vessel 34 is coated with an electrically insulating coating, such
as Parylene, in order to reduce the effect of the conductivity of
the pressure vessel 34 on electrostatic field distribution near
the electrodes 42.
A cylindrical core 44 of magnetically permeable material is placed
between the two support members 38 and associated electrodes 42
aligned axially and centrally of the titanium tube 38. The material
for the core is selected to have high magnetic permeability, low
remanence, and low conductivity to reduce eddy currents. Materials
which are relatively chemically inert, which provide increased resistance
to corrosion, are preferred; alternatively, a protective coating
could be employed. Standard barium or zinc ferrites, or powdered
iron, could be employed.
At one end of the magnetic core 44 there is a conical flow smoothing
cap 46 and at the other end a generally hemispherical flow smoothing
cap 48 is placed. In use, the meter is intended to be connected
so that the conical flow smoothing cap 46 is at the normally downstream
end of the meter, and the hemispherical flow smoothing cap 48 is
at the normally upstream end of the meter. In this way, turbulence
can be avoided or at least minimised, thereby causing drag in the
meter to be minimised.
A pair of pole pieces 60 are arranged either side of the body portions
22. The pole pieces 60 engage with the portion of the pressure vessel
34 between the two body portions 22 and with the lugs 30. The pole
pieces 60 are generally cuboidal in shape, with part cylindrical
cutouts to correspond with the outer surface of the pressure vessel
34. The pole pieces 60 are placed either side of the pressure vessel
34 and are arranged with a yoke 62 with upper and lower coils 64
for the application of a magnetic field between the two pole pieces
60.
The pole pieces 60 are generally precision machined, to fit accurately
against the pressure vessel 34. A pick-up coil 66 is interposed
between one of the pole pieces 60 and the pressure vessel 34. The
pick-up coil 66 is arranged to measure the magnetic field applied
by the pole pieces 60 in use. In one arrangement, the pole pieces
60 are precision machined to fit accurately against the lugs 30.
In that way, asymmetry brought about by the presence of a pick-up
coil 66 can be avoided. Alternatively or additionally, a further
pick-up coil could be interposed between the other pole piece 60
and the pressure vessel 34; this further pick-up coil could either
be connected in series for additional magnetic flux linkage, or
left disconnected as a dummy coil, merely to enhance the symmetry
of the meter.
The magnetic field applied by the pole pieces 60 is best illustrated
by example in FIG. 7. The presence of the magnetic core 44 distorts
the magnetic field between the pole pieces 60 such that the magnetic
field (as illustrated by lines of flux) generally extends radially
between the wall of the pressure vessel 34 and the magnetic core
44 and then from the magnetic core 44 to the opposite wall of the
pressure vessel 34.
The flow channel defined between the pressure vessel 34 and the
magnetic core 44 is divided into two generally equal parts by the
diametrically opposed electrodes 42. In each part of the flow channel,
the meter in operation has a magnetic field whose lines of flux
extend radially between the pressure vessel 34 and the magnetic
core 44.
Any electrostatic field induced in the part of the flow channel
in question will be in a direction perpendicular to the magnetic
field flux lines. Accordingly, the electrostatic field lines will
generally circulate about the core 44.
Therefore, it is clearly convenient to dispose the faces of the
electrodes, in contact with the fluid in the flow channel, radially
of the part-annular flow channel.
By supporting the pressure vessel 34 the electrodes 42 and the
core 44 directly from the body portions 22 rather than supporting
the electrodes 42 and core 44 from the pressure vessel 34 the pressure
vessel 34 need not be designed to accommodate the function of supporting
other components. Instead, the design of the pressure vessel 34
is governed by its ability to withstand pressure differentials without
substantial strain, and by its substantial lack of impact on the
magnetic and electrical circuits of the meter. This is advantageous,
since the pressure vessel can be designed to match those criteria
more closely if other factors can be removed from consideration.
The geometry of the components described above is preferably constrained
within predetermined ranges., so as to enhance performance of the
meter. Suitable constraints will now be described.
A first constraint preferably applied to the geometry concerns
an angle .theta., defined as the angle subtended at the centre of
the tube by one of the two flow paths through the meter. Generally,
maximising .theta. is advantageous in that it provides a maximum
signal to noise ratio, and minimises pressure drop in the meter.
However, if .theta. is too large, then the electrodes do not present
a wide enough geometric barrier to magnetic flux, causing flux leakage
through the electrodes to become unacceptably high. This can cause
non-linearities in the behaviour of the meter, which can lead to
errors.
A practical limitation on .theta. can be expressed as follows:
##EQU1##
where g is the gap between the wall of the pressure vessel 34 and
the core 44 also known as the working flow gap, and R is the inner
radius of the pressure vessel 34. However, it will be appreciated
that this is an empirical formula; the maximum value of .theta.
should be determined by reference to the effect described above
which is to be avoided.
A second constraint preferably applied to the geometry concerns
L.sub.f, which is the length, in the axial direction of the meter,
over which the magnetic field is applied. This length is equal to
the length of the permeable blocks, or pole pieces 60 placed on
the outside of the pressure vessel 34 which is equal to the length
of the permeable core 44. L.sub.f is limited to ensure that the
meter does not apply a significant pressure drop to fluid flowing
therethrough. However, in practice, the field applied by the pole
pieces 60 tends to bulge outwards at the ends of the pole pieces
60 and core 44. Therefore, L.sub.f must be sufficiently long that
the magnetic field applied, in the region in which the electrical
current flows between the electrodes 42 is substantially uniformly
radial. That constraint can best be represented by the following
relation:
where L.sub.e is the length of the electrodes at the inner wall
of the pressure vessel, in the axial direction of the pressure vessel
34. However, it should be appreciated that this is an empirical
formula, and that arrangements not satisfying the formula may still
deliver the desired effect that the magnetic field in the region
between the electrodes is substantially radial.
A third constraint applied to the geometry of the magnetic flow
meter concerns the thickness t of the pressure vessel 34 in combination
with the magnetic and mechanical properties of the material used
in the pressure vessel. The side wall of the pressure vessel 34
presents a radial magnetic reluctance in series with the magnetic
gap between one magnetic pole 60 and the core 44 and from the core
44 to the other pole 60. It is desirable to maintain that series
reluctance at a minimum in order to obtain a highly magnetically
efficient flow meter. This can be achieved by means of a thin walled
pressure vessel, a pressure vessel constructed of a material with
high permeability, or a combination of both.
However, both thickness and permeability are otherwise constrained.
The thickness of the pressure vessel is constrained by the fact
that the side wall of the pressure vessel may have to withstand
high differential pressures, while exhibiting little strain, and
while avoiding mechanical failure. With reference to the required
sensitivity and accuracy of the meter, and the expected pressure
differentials across the side wall of the pressure vessel, a minimum
wall thickness t.sub.min can be defined in terms of yield strength
and elastic modulus of the material selected for the pressure vessel,
such that unacceptably high strain, or mechanical failure of the
pressure vessel, can be avoided. By selecting a material with high
elastic modulus, t.sub.min can be set at a low level, so that the
actual wall thickness t can be selected so as to minimise the contribution
of the pressure vessel wall to the magnetic reluctance of the meter.
The permeability of the selected material should not be so high
as to provide a low reluctance pathway for flux between the two
magnetic poles 60 which would divert flux away from the gap between
the pressure vessel wall and the core 44 and distort the field
within the meter. This latter effect has more influence on the performance
of the meter than the influence of the reluctance of the side wall
in series with the magnetic pathway between the poles 60 and the
core 44. The following relation is provided as a simple constraint
on the geometry of the system having regard to the selected material:
##EQU2##
where .mu. is the relative permeability of the material of the
pressure vessel 34 and g the radial distance between the interior
face of the side wall of the pressure vessel 34 and the surface
of the core 44. If a non-permeable material is used, i.e. .mu. is
approximately equal to 1 as in the case of titanium, then the leakage
flux between the poles 60 is most dependent on the separation of
the poles 60. This distance is clearly related to the values of
.theta. and t.
A bore 50 extends radially through the magnetically permeable core
44 in the vertical direction as shown in FIG. 5 from adjacent the
lower electrode 42 illustrated therein and a corresponding bore
52 extends vertically through the upper electrode 42. A connecting
wire 54 leads from the lower electrode 42 through those two bores
50 52 and along the inner wall of the pressure vessel 34. Another
connecting wire 56 is led alongside the aforementioned connecting
wire 54 from the upper electrode 42. The two connecting wires are
led out of one end of the pressure vessel 34 and through bores
58 in the side wall of one of the body portions 22 adjacent each
other. This is an advantageous arrangement in that the two connecting
wires 54 56 can be arranged sufficiently close together that flux
linked by the two electrode connecting wires 54 56 can be minimised.
Such flux linkage could lead to erroneous readings of voltage detected
at the electrodes 42.
Moreover, by leading the connecting wires out of one end of the
pressure vessel 34 there is no need to place holes in the pressure
vessel wall. The placement of holes in the pressure vessel wall
could weaken the wall, and so the present arrangement allows a thinner
walled pressure vessel 34 to be employed than would be possible
if the pressure vessel 34 included holes for the passage of connecting
wires therethrough.
The meter 20 is connected to a controller 100 as illustrated in
FIG. 8. The coils 64 of the meter are connected in parallel to the
controller 100 by means of two connecting wires 102. The pick-up
coil 66 of the meter 20 is connected to the controller 100 by means
of two connecting wires 106. The electrodes 42 of the meter 20 are
connected to the controller 100 by means of two connecting wires
108.
The controller 100 comprises an analog sense amplifying unit 200
which receives signals from each of the electrodes 42 and the pick-up
coil 66 carried on the connecting wires 108 106 respectively.
The pick-up coil signal is proportional to the magnetic field strength
generated between the pole pieces 60 and directed through the flow
channels defined between the pressure vessel 34 and the core 44.
A reference voltage generator 202 is operative to generate a substantially
constant reference voltage for use in the analog sense amplifying
unit 200.
The analog sense amplifying unit 200 is operable to generate two
digitized signals, one being representative of the induced voltage
detected at the electrodes 42 the other being representative of
the magnetic field strength detected by the coil. The two signals
are passed to a microcontroller 300 which is operative to send signals
to a display 320 for the display of information concerning the flow
of water through the flow meter 20.
The controller 100 includes a port 110 which allows connection
of a programmer 112 thereto, for communication with the microcontroller.
Connection may be effected by wireless means, allowing transmission
of data to and from the controller as appropriate.
The programmer 112 may be a detachable unit, used in maintenance,
repair or inspection of the meter 20 and may not need to be physically
connected but may be capable of communicating with the microcontroller
300 remotely such as by infrared communications link. The microcontroller
300 and the programmer 112 could be configured such that establishment
of a communications link therebetween may involve authorisation,
and/or encryption and decryption, to prevent unauthorised access
to the microcontroller 300. This is advantageous when the meter
20 is to be used for the billing of a utility such as water, where
a meter reading should not be capable of being accessed, and potentially
changed, by anybody other than an authorised representative of the
utility supplier.
The controller 100 further includes a coil drive 400 and the microcontroller
300 also produces drive signals for the operation of the coil drive
400. The coil drive 400 is operative to generate a signal to be
passed to the coils 64 of the meter 20 to generate a magnetic field
through the yoke 62 and through the pole pieces 60.
FIG. 10 shows the analog sense amplifying unit 200 in more detail.
The amplifying unit 200 includes an electrode sense amplifier 204
which receives signals, via the connecting wires 108 from the electrodes
42. The electrode sense amplifier 204 amplifies the difference between
the two electrode voltages through a gain of approximately 200.
This gain is substantial because the amplitude of the voltage detected
at the electrodes 42 is likely to be of the order of a millivolt.
Therefore, substantial amplification is needed to obtain a signal
which can be processed accurately.
The output of the electrode sense amplifier 204 passes through
a high pass filter 206 to remove any DC effects which might be introduced
by the electrode sense amplifier 204.
The amplifying unit 200 further comprises a B field sense amplifier
208 which receives a signal, via the connecting wires 106 from
the pick-up coil 66. The B field sense amplifier 208 has a low frequency
gain of approximately 10 and has a low pass cut-off of about 77
Hz, to substantially eliminate any noise picked up by the coil 66.
The output of the B field sense amplifier 208 passes to a first
order Sigma-Delta modulator 210 which is clocked by a clock signal
CLK received from the microcontroller 300. The output of the first
order Sigma-Delta modulator 210 comprises a train of digital pulses
hereinafter referred to as B.sub.MEAS.
The output of the B field sense amplifier 208 is also fed through
a further high pass filter 212 which rejects DC components which
might be introduced by the B-field sense amplifier 208. The outputs
of the high pass filters 206 212 are passed to an analog switch
214 switched by a signal CAL received from the microcontroller
300. The signal CAL is normally set so that the analog switch 214
passes the output from the high pass filter 206 from the electrode
sense amplifier 204 therethrough.
The output of the analog switch 214 is passed to an active low
pass filter 216 having low frequency gain of approximately 5. The
low pass filter 216 is constructed of a 3 pole Butterworth filter.
The low pass filter 216 is designed such that at 15 Hz its gain
is 3 dB lower than at DC and 50 Hz it is 30 dB lower. The filter
216 therefore rejects high frequency noise, including any artefact
of the supply line voltage (normally at 50-60 Hz). Considering that
the meter 20 may be used in a water supply and therefore buried
in the ground, the rejection of supply voltage is advantageous since
the meter 20 may be inadvertently placed close to a high voltage
cable at 50-60 Hz, which might otherwise reduce the accuracy of
the meter 20.
The output of the active low pass filter 216 is passed to a second
order Sigma-Delta modulator 218. The second order Sigma-Delta modulator
218 is clocked by the same clock signal CLK as the first order Sigma-Delta
modulator 210 previously described. The output of the second order
Sigma-Delta modulator 218 comprises a sequence of digital pulses
representing the voltage received from the electrodes 42. The output
of the Sigma-Delta modulator 216 is hereinafter referred to as E.sub.MEAS.
In practice, the low pass filter 216 as described not only has
reduced gain at higher frequency, but also introduces phase shift
at higher frequency. For example, the output from the filter 216
at 8 Hz will be about 60.degree. ahead of its input. This is undesirable,
because any extraneous signal picked up from the magnetic field
pick-up coil 64 will already be in quadrature to (i.e. leading by
90.degree.) the signal from the electrodes 42.
This is demonstrated most effectively in FIGS. 10A to 10D of the
drawings. FIG. 10A shows a trace of the voltage V.sub.D output by
the coil drive 400. The second graph, FIG. 10B, shows a trace of
current I.sub.C through the coil. This current I.sub.C is in phase
with-the drive voltage V.sub.C because the coil drive 400 drives
the coil 64 and resonating capacitor 81 at their resonating frequency,
i.e. the coil and capacitor behave as if they were a pure resistance.
At resonance, the power consumption of the coil has been found to
be minimised, as described in more detail later. The third graph,
FIG. 10C, shows a trace of the voltage V.sub.F picked up by the
pick-up coil 66. This voltage is in phase with the voltage across
the coil 64 and so is 90.degree. ahead of the voltage V.sub.D output
by the coil drive 400. Finally, the fourth graph, FIG. 10D, shows
a trace of the voltage V.sub.E between the electrodes 42. This voltage
is in phase with the coil flux, which is in phase with the coil
current I.sub.C (FIG. 10B).
The signal received at the active low pass filter 216 may include
an artifact of the signal picked up by the pick-up coil 216 even
if the analog switch is set so that the filter 94 receives the amplified
electrode signal.
Therefore, the existence of further phase shift in the measured
signal will reduce the ability of the meter to distinguish between
the signals derived from the voltage V.sub.E at the electrodes 42
and an artefact of the voltage driving the magnetic field. The analog
switch 214 is provided so that the output from the B field sense
amplifier 208 can be selectively passed, via high pass filter 212
through the active low pass filter 216 so as to nullify any artefact
from that signal in the measured electrode signal. That process
is carried out by means of the microcontroller under the control
of software implemented therein, to be described later.
With reference to FIG. 11 the internal structure of the second
order Sigma-Delta modulator 218 will now be described. The modulator
218 comprises first and second integration stages 220 222 sending
an output to a comparator 224. The comparator 224 comprises an operational
amplifier receiving in its inverting input a reference voltage (V.sub.REF)
generated at the reference voltage generator 202. The operational
amplifier receives into its non-inverting input the signal received
from the second integration stage 222 and to be compared with V.sub.REF.
The comparator 224 sends an output to the data input of a D type
latch 226 clocked by the clock signal CLK received from the microcontroller
300. The non-inverted output Q of the D type latch 226 is sent as
a reset signal RESET2 to the second integration stage 222 and the
inverted output -Q of the D type latch 226 is sent as a reset signal
RESET1 to the first integration stage 220. The inverted output -Q
is also fed to an input of a two input AND gate 228 the other input
of the AND gate 228 being fed from the clock signal CLK. The output
of the AND gate 228 is the output E.sub.MEAS of the Sigma-Delta
modulator 218.
The operation of the second order: Sigma-Delta modulator 218 will
be understood from the graphs illustrated in FIGS. 12A to 12E. FIG.
12A is a graph of the analog input E.sub.SIG to the Sigma-Delta
modulator over time. The Sigma-Delta modulator converts the input
voltage into a series of pulses at the inverted output -Q of the
D type latch 226. Those pulses represent charge balancing pulses
supplied to the integration stages 220 222 through the reset inputs
thereof. FIGS. 12B(i), 12B(ii) and 12B(iii) illustrate three cases-of
possible signals in the Sigma-Delta modulator, over an expanded
time axis relative FIG. 12A, corresponding to respective periods
of time .delta.t.sub.1 .delta.t.sub.2 .delta.t.sub.3 illustrated
in FIG. 12A indicated by broken lines.
In FIG. 12B(i), corresponding to a time period .delta.t.sub.1
near a peak of the signal illustrated in FIG. 12A, the inverting
output -Q of the D type latch 226 is mostly digital LOW with infrequent
pulses of digital HIGH voltage. In contrast, as shown in FIG. 12B(ii),
in the period .delta.t.sub.2 at a minimum of the input voltage
illustrated in FIG. 12A, the inverted output -Q of the latch 226
is mostly digital HIGH with infrequent digital LOW pulses. In the
period .delta.t.sub.3 when the input voltage is on or around cross-over,
the inverted output -Q of the latch 226 has a mark-to-space ratio
of approximately 1 as shown in FIG. 12B(iii).
The inverted output -Q of the latch 226 is used as a gate for controlling
the transmission of the 32 kHz clock signal CLK, received from the
microcontroller 300 through the AND gate 228. The output of the
gate 228 corresponding with each of the three time periods described
above is illustrated in FIGS. 12C(i), 12C(ii) and 12C(iii). In period
.delta.t.sub.1 the infrequent HIGH pulses are translated into infrequent
samples of the CLK pulses; in period .delta.t.sub.2 the gate 228
admits most CLK pulses through to the output, with infrequent suppression
of pulses; in period .delta.t.sub.3 the gated output comprises
pulses corresponding to approximately every other CLK pulse.
FIG. 12D illustrates the gated clock output E.sub.MEAS corresponding
with the input voltage illustrated in FIG. 12A, and in the same
time axis as FIG. 12A. This graph shows how pulses are concentrated
in regions of high input voltage, and how concentration of pulses
diminishes as voltage reduces.
FIG. 13 illustrates in further detail the first integrator 220
included in the second order Sigma-Delta modulator 218.
The first integrator 220 comprises an operational amplifier 230
which receives reference voltage V.sub.REF in its non-inverting
input and which has an integrating capacitor 232 arranged in negative
feedback across it in the conventional manner. The operational amplifier
230 receives through summing input resistors 234 236 input currents
generated by the application on the one hand of an input voltage
E.sub.SIG from the analog switch 214 and an the other hand the inverted
output -Q of the D type latch 226. The summing resistors 234 236
are selected to be in the order of 1-10 M.OMEGA. in order to minimise
current through the Sigma-Delta modulator.
A resistor 238 is arranged between the output of the operational
amplifier 230 and the reference voltage. The resistor 238 is generally
about three orders of magnitude smaller than the summing input resistors
234 236. Despite the fact that this resistor 238 will increase
the power consumption of the modulator, it allows for the rejection
of cross-over distortion generated as a result of imperfections
in the output stage of the operational amplifier 230. This is because,
since the summing input resistors 234 236 are relatively large,
the integrating capacitor 232 will charge and discharge relatively
slowly. The resistor 238 will assist in movement of charge onto
or off the capacitor 232 which will assist with balancing the integrator
220 at crossover.
The second integrator 222 is illustrated in FIG. 14 and, as far
as it is identical to the first integrator its corresponding components
are allocated the same reference numerals distinguished by a prime
mark ('). In addition, in series with the integrating capacitor
232', a noise shaping resistor 233' is provided. This has the effect
of shaping the noise spectrum of the sigma delta modulator to be
substantially the inverse of the frequency response of the operational
amplifiers 230 230'. This is useful in reducing the noise on the
output of the sigma delta modulator.
The analog circuits described above are particularly advantageous
in that conversion from amplitude modulated AC signals to trains
of digital pulses can be effected without the intermediate conversion
of the analog signal into a baseband (DC) signal. In that way, there
is no opportunity for DC offsets in the demodulated signal to affect
the final measurement of volumetric flow.
A second order sigma-delta modulator is used in the modulation
of the signal received from the electrode sense amplifier 82 in
order to take advantage of enhanced noise shaping capabilities of
such a device. A first order Sigma-Delta modulator in that case
would probably generate some sampling tones, which might introduce
errors into subsequent processing. However, a first order Sigma-Delta
modulator is suitable for use in measuring the magnetic field, since
in that case only long term drift of the magnetic field value is
of importance; there is little likelihood of the amplitude of the
magnetic field signal varying rapidly over time.
It is desirable that the hardware of the meter be symmetrical about
a plane containing the axis of the pressure vessel 34 and perpendicular
to the magnetic field applied by the pole pieces 60. For example,
the magnetic pole pieces 60 should be aligned perpendicular to the
axis of flow through the meter, to avoid unbalanced signals and
magnetic pick-up. The manufacturing tolerances of the components
of the meter, and especially of the fixing rings 36 and the electrode
support members 38 should be set such that alignment is sufficiently
accurate for the desired level of accuracy of the meter.
With reference to the FIGS. 15 and 16 of the drawings, the microcontroller
300 will now be described in further detail. The microcontroller
comprises a central processing unit (CPU) 302 which is capable of
receiving input signals E.sub.MEAS and B.sub.MEAS from the analog
sense amplifying unit 200 via input lines 304. The CPU 302 is further
configured to produce three control signals on output lines 306
for controlling the coil drive 80 to be described later. Further
outputs 308 are provided in order to present the CLK and CAL signals
to other parts of the apparatus. The microcontroller 300 further
comprises a memory unit 126 for the storage of processor implementable
instructions and data and an I/O interface 312 by which the microcontroller
300 can be connected to the programmer 112 linked to the port 110.
The microcontroller 300 further comprises a display buffer 314 for
the storage of data to be displayed at the display 320.
The microcontroller 300 can be programmed by connecting a programmer
112 to the I/O interface 312 and by writing computer implementable
instructions to the memory 310. In that way, the microcontroller
300 can be configured as the particular situation dictates.
An example will now be given of the manner in which the microcontroller
can be configured in order to present data to the display 320 representative
of the consumption of water passing through the magnetic flow meter
20.
In FIG. 16 the microcontroller 300 includes a counter 316 which
counts pulses input on the E.sub.MEAS and B.sub.MEAS signals, to
produce integrated count signals based on those inputs. The signals
are passed to a totaliser 318 which processes those count signals
to produce display data to be stored in the display buffer 314.
The totaliser 318 further produces a mixer switch control signal
which is passed back to the counter 316. The mixer switch control
signal controls the direction in which the counter 316 counts each
of E.sub.MEAS and B.sub.MEAS pulses.
In FIG. 12E, the mixer switch control signal is shown as a digital
signal which can take either "DOWN" or "UP"
values. The mixer switch control signal is assigned the "DOWN"
value when the input voltage in FIG. 13A is in the positive half
of the cycle, and the "UP" value when the input voltage
is in the negative half of the cycle. FIG. 12E illustrates the situation
where fluid is flowing through the meter 20 in a predetermined forward
direction. In that way, pulses are concentrated in the parts of
the signal for which the mixer switch control signal is assigned
the "UP" value. Therefore, the integrated counts for E.sub.MEAS
and B.sub.MEAS will both gradually rise as the number of pulses
in the "UP" sections of the signal exceeds the number
of pulses in the "DOWN" sections of the signal. Towards
the end of the signal, it can be seen that the flow falls to zero,
and the concentration of the pulses reaches a steady state. In that
case, the counter will count up the same number of pulses as it
counts down.
If, over a whole number of cycles of the 8 Hz input signal, the
integrated count .SIGMA.E relating to the E.sub.MEAS signals falls,
it can be ascertained that the fluid being measured through the
meter is flowing in the reverse direction. Rather than allowing
this reversal of flow to result in a totalised value at the display
320 being allowed to fall, the reduction in the integrated count
is stored in a buffer in the totaliser 318. The accumulation of
this reverse flow value will gradually be taken into account as
flow returns to the normal forward direction.
Of course, in a domestic supply, reverse flow would be very rare,
and would only occur in certain situations, such as if flow has
been interrupted after a period of consumption, allowing water to
fall back through a supply pipe.
The totaliser 318 further processes the integrated count by scaling
the count to a value for consumption, expressed in recognised units
such as m.sup.3 or litres, following which the final volumetric
consumption value is stored in the display buffer 314. The volumetric
consumption value can be stored in the display buffer 314 as the
data becomes available, but it is convenient to only update the
display 320 itself very slowly. This is because rapidly updating
digital displays are very difficult to read. Accordingly, an update
period of one second is suggested to be appropriate.
The scaling function may include some rounding of the previously
derived values, and the remainders of those rounding functions are
stored in a remainder buffer until they reach a level significant
that they can be added to the volumetric consumption total.
The totaliser 318 may also include the facility to transmit messages
as display data, such as messages indicating low flow or a reverse
flow indicator.
The mixer switch control signal can be considered to be a demodulating
control signal. This should be in phase with the received signal
from the electrodes, so as to obtain an accurate count. If the phase
of the received signal drifts, then the phase of the mixer switch
control signal should be adjusted accordingly. Otherwise, errors
may be incorporated into the integrated counts, in particular DC
drift from stray magnetic field pick-up.
Configuration of the microcontroller 300 to perform functions as
described above may be by means of processor implementable instructions,
stored in the memory unit 310. Preferably, the memory unit 310 includes
read only memory, and the processor implementable instructions can
be programmed into that read only memory prior to operation of the
meter 20.
Processor implementable instructions are illustrated by way of
example in FIGS. 18 to 20 attached hereto.
FIG. 18 illustrates a routine which can be performed by the counter
316 on receipt of a pulse input on the E.sub.MEAS signal. In step
S10 the value of the mixer switch control signal received from
the totaliser 318 is checked. Then, in step S12 an enquiry is made
as to whether the mixer switch control signal is UP. If it is not
UP (i.e. it is DOWN) then the routine passes to step S14 where a
sum value .SIGMA.E is decreased by 1. Alternatively, if the mixer
switch control signal is UP, then the routine passes to step S16
where the sum .SIGMA.E is increased by 1. After either step S14
or step S16 the new value of .SIGMA.E is returned to memory for
use by other routines.
FIG. 19 illustrates a routine which can be performed by the counter
316 on receipt of a pulse input on the E.sub.MEAS signal. In step
S20 the value of the mixer switch control signal received from
the totaliser 318 is checked. Then, in step S22 an enquiry is made
as to whether the mixer switch control signal is UP. If it is not
UP (i.e. it is DOWN) then the routine passes to step S24 where a
sum value .SIGMA.B is decreased by 1. Alternatively, if the mixer
switch control signal is UP, then the routine passes to step S26
where the sum .SIGMA.B is increased by 1. After either step S24
or step S26 the new value of .SIGMA.B is returned to memory for
use by other routines.
FIG. 20 illustrates a totaliser routine which starts the instant
the microcontroller initialises on start-up. In step S30 which
is an initialisation step, four variables, namely .SIGMA.E.sub.NEW,
.SIGMA.E.sub.PREV, .SIGMA.B.sub.NEW, .SIGMA.E.sub.PREV, are set
to initial values. Initial values for the .SIGMA.E.sub.NEW and .SIGMA.E.sub.PREV
variables may be 0 but the initial values for .SIGMA.B.sub.NEW
and .SIGMA.B.sub.PREV cannot be 0. This is because in later division
calculations, .SIGMA.B.sub.NEW and .SIGMA.B.sub.PREV are used as
the divisor. A suitable initial value for each of .SIGMA.B.sub.NEW
and .SIGMA.B.sub.PREV could be 1.
In step S32 values of .SIGMA.E.sub.NEW and .SIGMA.B.sub.NEW are
received from the counter 316. Then, in step S34 a test is made
to ascertain whether the following condition holds: ##EQU3##
Each of the variables represents a value for the strength of the
electric field (E) and the magnetic field (B) as the case may be,
at a particular sampling time. Therefore, by dividing values of
the E field by values of the B field, it is possible to obtain a
value representative of volumetric flow through the meter. Step
S34 ascertains whether volumetric flow has increased or decreased
through the meter since the last test. If the inequality holds,
then the routine passes to step S36.
In step S36 a consumption value is defined which represents the
difference between the value representative of volumetric flow at
the present time and volumetric flow previously, that difference
being representative of the amount of liquid which has flowed through
the meter since the last test.
In step S38 the consumption value is tested against a value contained
in a back flow buffer. The back flow buffer contains a value representative
of a volume of liquid which might have flowed back through the meter,
and must be accounted for in later totals. If consumption is greater
than the value in the back flow buffer then the routine passes to
step S40 wherein the value in the back flow buffer is subtracted
from the consumption value, and in step S42 the back flow buffer
is set back to zero.
If, in step S38 the consumption value is not greater than the
value held in the back flow buffer, then the consumption value measured
at this time is not great enough to clear the back flow buffer.
In that case, in step S44 the consumption value is subtracted from
the back flow buffer, so as to clear as much of the contents of
the back flow buffer as is possible at this time. In step S46 the
consumption value is set to zero.
Referring back to step S34 if the inequality does not hold, then
it can be concluded that fluid has passed in the reverse direction
through the meter. This is known as back flow as previously noted.
A back flow value is calculated in step S48 being the difference
between the ratio of E field to B field values as previously described
but taking account of the fact that the previous ratio will be larger
than the new ratio. Thereafter, the back flow value is added to
the contents of the back flow buffer in step S50. Following step
S50 the routine resumes, with step S46 resetting consumption to
zero (in case it had in some way not been at zero).
Following either step S42 or step S46 step S52 scales the consumption
value (which after step S46 will be zero), to produce a value for
volumetric consumption in recognised units, such as cubic metres
or litres. Thereafter, that scale consumption value is displayed
in step S54. Display may take place on only a sample of possible
consumption values, so as to retain a value on the display for a
sufficiently long period (such as one second) that the display can
be read easily.
Following step S54 the received summation values .SIGMA.E.sub.NEW
and .SIGMA.B.sub.NEW are moved into variables .SIGMA.E.sub.PREV
and .SIGMA.B.sub.PREV in step S56 in preparation for receipt of
new values of .SIGMA.E.sub.NEW and .SIGMA.B.sub.NEW. Thereafter,
in step S58 the routine waits for the next cycle. That wait step
is advantageous in that it prevents excessive power consumption
by the totaliser 318 thereby limiting demand on the power supply.
This allows the power supply to be in the form of a battery which
can have a long lifetime. This allows use of the present meter as
a domestic water meter, which requires very little maintenance.
Following the wait step S58 the routine resumes with the receipt
of new values of .SIGMA.E.sub.NEW and .SIGMA.B.sub.NEW from the
counter, in step S32.
The coil drive 400 will now be described in further detail with
reference to FIG. 17. As mentioned previously, the CPU is capable
of producing drive signals for the coil drive on 3 lines. However,
in the present implementation, the CPU only has the facility to
produce a single output with accurately timed edges. In order to
drive the two coils 64 illustrated, it is necessary to have two
signals with accurately timed edges. Therefore, the circuit of FIG.
17 has been devised which only requires one accurately timed signal
to be provided, along with two less accurate switching pulses.
The coil drive 400 receives 3 signals from the CPU. The first and
second signals DRA and DRB are 8 Hz square waves with generally
poorly timed edges. The signals are inverses of each other. If these
signals were alone to be used to drive the coil, the result would
be an extremely jittery coil signal. This is undesirable, since
it would result in extremely noisy and inaccurate meter responses.
Instead, a third signal, hereinafter referred to as DRIVE PULSE,
is also provided. DRIVE PULSE is an accurately timed 16 Hz square
wave which can be used to time the coil drive 400.
The coil drive 400 as illustrated in FIG. 17 is provided with
two AND gates 402 404 by which signals DRA, DRB are respectively
gated by the DRIVE PULSE SIGNAL. In that way, DRA and DRB may be
given accurately timed edges. The accurately timed gated DRA, DRB
signals are input to enable inputs ENA, ENB of a dual tristate gate
406. The dual tristate gate 406 has two inputs DA, DB, which are
passed to corresponding outputs OA, OB if the corresponding enable
input ENA, ENB is logically LOW. Otherwise, the output is tristate
(open circuit).
Practically, a gate device is provided for the tristate gate 406
which has four DA inputs, four DB inputs and four of each of the
corresponding outputs OA, OB. In that arrangement, the Boolean sum
of the four inputs is passed to each of the four corresponding outputs
if the respective enable input is low. The present embodiment makes
use of that device by tying together the groups of four inputs and
the groups of four output, specifically to obtain outputs with advantageously
low impedance.
The inputs DA, DB are all tied to supply voltage. Therefore, depending
on the enable inputs, the outputs can either be logically high or
tristate.
The gated DRA and DRB signals are also connected to gates of respective
field effect transistors 408 410. The drains of the field effect
transistors 408 410 are connected to outputs of respective tristate
gates 406. The sources of the field effect transistors are held
to ground.
An inductive capacitive network is connected between the drains
of the field effect transistors 408 410. The inductive capacitive
network comprises the drive coils 64 connected in series, on one
side of which is the resonating capacitor 104. A resistor 412 is
connected between the resonating capacitor 104 and the drive coils
64 and to supply voltage. A capacitor 414 on the other side of
the coils 64 provides AC coupling.
By this arrangement, the drive coils 64 can be caused to resonate
by the provision of accurately timed gated DRA and DRB signals,
which repeatedly switch on the field effect transistors 408 410
to pull the connections to the inductive capacitive network to ground,
following which the field effect transistors 408 410 are switched
off and the tristate gates 406 switched on. When the tristate gates
406 are switched on, the connections to the inductive capacitive
network are sent to supply voltage. This is conducted at 8 Hz, and
the oscillation causes sufficient energy to be stored in the network
to generate a semi-resonant oscillation. Once the gated DRA and
DRB signals are removed, the field effect transistors 156 158 are
switched permanently on and the gates 154 are rendered tristate.
In that case, the only significant loss in the system is internal
to the reactive components 64 and any resistance of the field effect
transistors 408 410. In fact, the circuit can be designed to be
sufficiently efficient that firing pulses need only be sent to the
coil drive 80 periodically. The need to send coil drive signals
DRA, DRB and DRIVE PULSE, can be determined by monitoring the magnitude
of the B.sub.MEAS signal received from the analog sense amplifying
unit 200.
It will be appreciated that the natural resonant frequency of the
network may be other than 8 Hz. However, the present embodiment
accommodates variations from the resonant frequency, by means of
the circuit having a Q factor of at least 10.
The invention is not limited to the embodiments illustrated in
the drawings and described above. For example, it would be possible
to shape the flow path if it was found that other shapes were advantageous
or provided further structural strength. Moreover, a rectangular
flow tube including selectively large pole pieces to define a uniform
magnetic field across the flow path could be designed, as long as
the construction was capable of withstanding the recognised stresses
without excessive strain.
The invention has application in the field of commercial water
metering, such as for domestic properties. A single 3 volt battery
can be provided to power the meter, which can have a total power
requirement of less than 0.30 mW. An advantage of using digital
components to drive the coils 64 in such circumstances is that analog
switches would have distorted outputs at reduced supply voltages
near the end of the battery life. Also, software demodulation allows
sensing circuitry to be provided without a need for analog switches.
Despite the fact that the meter has very low power consumption (about
40000 times less than typical meters presently on the market) it
provides a means of measuring volumetric flow with comparable measuring
performance.
As an alternative to the magnetic flow meter in accordance with
the first specific embodiment of the invention, a second embodiment
is illustrated in FIGS. 21 and 22. This magnetic flow meter is a
differential meter, providing the facility for measuring flow through
two different flow channels, either measuring the absolute value
of both flows or the difference between the two flows. In this case,
the flow meter 20' is provided with many components which are identical
to those provided in the first flow meter 20. The components illustrated
in FIG. 21 which are identical to those illustrated in FIG. 1 are
given the same reference numerals.
Moreover, the flow meter 20' has body portions 22' which are modified,
in that they have two parallel through bores 24'. Each through bore
24' is in fluid communication with one side of the electrode/magnetic
core assembly within the pressure vessel 34. In order to keep the
two flow channels separate, the support members 38' are slightly
modified from those provided in the first embodiment, in that they
include extended portions so as to define a fluid divider, which
abuts the body portions 22' suitably to maintain separation of the
flow paths.
As illustrated in FIG. 22 the pressure vessel 34 is of the same
construction as that in the first embodiment.
The arrangement of electrodes 42' and a magnetic core 44 is the
same as in the first embodiment. However, the construction of the
electrodes is somewhat different. In this case, the electrodes 42'
are of electrically insulating material. The surfaces of the electrodes
42' which are in contact with fluid flowing through the meter in
use have a surface coating 42A applied thereto. The surface coating
42A is electrically conductive. Electrical connections 42B are made
from each of the surface coatings 42A; those electrical connections
are guided out of the pressure vessel 34 in the same manner as in
the first embodiment.
The differential flow meter 20' so described provides the facility
for measurement of two flow rates. Measurement can be effected by
short circuiting the two faces of the lower electrode, and measuring
the potential difference between the two faces of the upper electrode.
This can be carried out using the controller circuit illustrated
in FIG. 8 connecting the two faces of the upper electrode to points
E1 and E2 in FIG. 9. Alternatively, two analog sense amplifier units
200 could be provided, each monitoring potential difference developed
between the face of the upper electrode and a corresponding face
of the lower electrode, the signals produced by those units 200
being processed by a microcontroller adapted to receive signals
from two analog sense amplifier units simultaneously.
FIG. 23 illustrates a further application of the meter 20 of the
first embodiment. The meter 20 is installed in a chemical processing
plant 500 comprising a chemical processing unit 510. The chemical
processing unit 510 has an inlet 512 for a fluid which is processed
by the chemical processing unit to generate a product at an outlet
514.
A supply of the fluid to be processed is stored in a reservoir
516 and the meter 20 is installed in the path between the reservoir
516 and the inlet 512 in order to monitor delivery of the fluid
to the chemical processing unit 510.
A valve 518 is provided upstream of the meter, controlled by a
valve controller 520. The valve controller is operable in response
to display data generated by the controller 100 as illustrated in
FIG. 8. In order to facilitate monitoring of the display data by
the valve controller 520 the controller 100 may be provided with
a facility for producing a voltage output, or a current output (4
to 20 milliamps) representative of the display value. The valve
controller 520 is configured to operate the valve 518 in response
to display data, so as to control delivery of fluid to the inlet
512 of the chemical processing unit 510. In that way, automatic
delivery of fluid to a chemical processor unit 510 can be effected,
leading to convenient operation of a chemical process.
In the implementation shown in FIG. 23 the meter 20 and the controller
100 can be installed such that a mains power supply is available.
Therefore, some of the features of the controller 100 which are
provided for limiting power consumption may not be essential in
the implementation. For instance, it may not be necessary to resonate
the coils of the meter. Moreover, more powerful computation means
might be provided in the controller 100 to facilitate better analysis
of data. This could provide more convenient data presentation for
the valve controller 520 leading to more accurate control of fluid
delivery.
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