The control circuits for the electronic flow meter which are the
subject of this invention and whose purpose is to meter with precision
the flow through a pipe, eliminating disturbances caused by the
metering procedure use, consist of an excitation block (2) of coils
(B1 and B2) for the magnetic circuit and a processing block (3)
for the metering signal which is captured through sensors (S) arranged
orthogonally to the magnetic circuit pole components.
1. Control circuits for an electromagnetic flow meter which include
an excitation block (2) and a metering signal processing circuit,
said excitation block comprising an excitation control signal generating
circuit which generates a base control wave of zero continuous level
and a working cycle of 50%, to provide a signal at the input to
an integrated circuit (6) which excites coils B1 and B2 the excitation
control signal generating circuit consists of a generator (4) followed
by a continuous level suppressor and an amplifier (5), while the
metering signal processing circuit consists of a capacitive conduction
suppressor (7) and an amplifier (8), the output from which runs
through high- and low-pass filters (9) and (10) formed by operational
amplifiers with adaptation modules inserted between them using the
same type of circuitry and thence through an attenuator unit (11)
to a true effective value convertor (12).
2. Control circuits for an electromagnetic flow meter as set forth
in claim 1 wherein the excitation control signal generator circuit
and the continuous level suppressor and amplifier take the form
of an oscillator, a high-pass filter and an amplifier implemented
by means of an operational amplifier.
3. Control circuits for an electromagnetic flow meter as set forth
in claim 1 wherein the integrated circuit (6) is an input tension-controlled
transadmittance integrated LSI circuit which can supply a current
with gain loop adjustment through end components.
4. Control circuits for an electromagnetic flow meter as set forth
in claims 1 wherein the excitation control signal generating circuit
preferably generates a 6.5 Hz base control wave of zero continuous
level, a 50% work cycle, and providing a signal of approximately+11
V at the input to the integrated circuit for the coils (B1) and
5. Control circuits for an electromagnetic flow meter as set forth
in claim 1 wherein the amplifier (8) of the metering signal processing
circuit is a high-precision instrumentation amplifier which can
operate in extreme conditions.
6. Control circuits for an electromagnetic flow meter as set forth
in claim 1 wherein the true effective value convertor (12) is based
on an integrated circuit closely linked to the attenuator unit (11)
at whose output an amplifier (13) is installed to amplify the conversion
circuit signal (12) to a continuous value providing a signal which
is easily processed and analyzed.
SUBJECT OF THE INVENTION
As stated in the heading to these Specifications, this invention
refers to an electro-magnetic flow meter designed for the accurate
metering of the flow through a pipe, to the specific coil excitation
circuits and those for the detection and processing of the metering
signal, and making it possible to eliminate perturbations generated
by the metering technique employed.
STATE OF THE ART
An electromagnetic flow meter is a system for measuring fluids
in piping, and consists of a cylindrical tube through which the
fluid flows. Said tube is under the influence of a magnetic field
generated by two coils, one each over and under the tube. There
are two electrodes, diametrically opposite, arranged on the system's
The flow meter's operation is based on Faraday's Law according
to which the electromotive force induced in a flux moving within
a magnetic field is proportional to the speed of the flux. In electromagnetic
flow meters, the flux is the actual fluid in movement, which must
have a minimum conductivity threshold (2-20 microsiemens/sec). The
magnetic field produced by the currents flowing through the excitation
coils induces an electromotive force in the flux which is proportional
to its speed, and at right angles to it and to the magnetic field
If the flow is to be measured by sampling the electromotive force
signal, the magnetic induction value of the field created by the
coils must remain constant throughout the sampling period.
The key to a magnetic flow meter is the control of the magnetic
field, this being the content of the proposed invention.
Usually, in commercial units, the coil windings are fed from a
square-wave voltage source with a constant half-cycle value. Main
features are, on the one hand, the square wave switching frequency,
with a value associated with the mains frequency while, on the other,
a change in the environmental conditions (temperature, etc.) produces
a change in the intensity flowing through the coil windings.
The electromotive force detected in the electrodes of these units
must be processed in special comparison circuits, which remove the
Perturbations in the power supply.
Changes in environmental conditions.
Units do exist in which the coils are excited by superimposing
a number of voltage supplies with square waves of differing frequencies,
or by using a sine AC voltage supply.
DESCRIPTION OF THE INVENTION
The control circuits of an electromagnetic flow meter as proposed
in the invention feed the induction coils from a square-wave constant
half-cycle current generator.
This is obtained with a transadmittance amplifier and an oscillator.
The main features of this power supply are, firstly the use of
an unsampled mains switching frequency which is independent of the
mains to prevent harmonic interference and, secondly, its very high
half-cycle stability, irrespective of environmental conditions (temperature
The main advantages of this approach are that a harmonic-free and
highly stable magnetic field enables an electromotive force signal
to be captured at the electrodes without any perturbations. In the
first instance, this simplifies subsequent processing of the electromotive
force signal: however, in addition, at very low fluid speeds, the
factor limiting the flow meter's sensitivity is the signal/signal-noise
ratio of the electromotive force signal. A perturbation-free stable
magnetic field is essential for accurate flow metering when fluid
movement is very slow.
To attain these ends, the flow meter magnetic circuit consists
of two coils, each of n loops, connected in series, through which
a current is circulated. Said coils are installed on a magnetic
structure made of iron plate. The magnetic field lines are closed
through it and the fluid. To complete the structure, two electronic
blocks form the control circuits. They may be referred to as the
coil excitation block and the metering signal detection, conditioning
and processing block.
The excitation block, consisting of the coil excitation circuit,
includes an oscillator or excitation control signal generator circuit
followed by an amplifier which supplies the input signal to an integrated
input-tension-controlled LSI transadmittance circuit supplying constant
current to the excitation coils.
The second block, which handles the signal which eventually provides
the flow measurement, captures, processes and amplifies the signal,
which flows through a component to eliminate the capacitive effects
of conduction, through an amplifier, high-pass and low-pass filters,
an attenuation unit, a true efficient value convertor and an amplifier
which establishes a suitable signal level.
All the sections of the circuit are fed from a multiple power supply
in which account is taken of the necessary polarization tensions
for all the circuit elements and which is able to supply the intensity
required by each section.
Such a structure meets the objectives established for the design
of a flow meter able to meter accurately and without errors the
flow of a fluid through a pipe, by overcoming or eliminating all
agents which may cause perturbations in the measurement of the induced
tension and those arising from the flow speed which is the element
which, in the long run, determines the flow.
The proposed control circuits can be used to meter flow in circular
systems operating under load, and in circular systems operating
in free-flow conditions.
They can also be used to control magnetic flow meters in canals
or rivers, in which case the single magnetic field generating coil
is located over the canal or river, with the electrodes in the bottom
of each housing.
DESCRIPTION OF THE DRAWINGS
To complete this description and to aid in a better understanding
of the features of the invention, these Specifications are accompanied
by a set of drawings, forming an integral part hereof which, by
way of illustration only and without limitation, show the following:
FIG. 1 a general diagram of all the elements making up the flow
meter, the coil support structure, the excitation block and the
signal amplification and processing components.
FIG. 2 shows the electronic diagram of the coil excitation circuit.
FIG. 3 is a block diagram of the metering signal processing signal
following capture of the signal.
FIG. 4 is the electronic chart for the block diagram in the previous
figure, from signal-capture to its output as continuous tension
value at a maximum level of two volts.
A PREFERENTIAL EMBODIMENT OF THE INVENTION
From the figures, it can be seen that the flow meter control circuits
are associated with a magnetic circuit that has a support structure
(1) for the coils (B1) and (B2) to create the constant magnetic
field, and the induced tension sensors or electrodes (5).
The control circuits advocated here comprise the excitation block
(2) which supplies the coil excitation current, and the metering
signal amplifier and processing block (3).
In one design of the invention, the support structure (1) takes
the form of a 40 cm diameter pipe with 600 loops per coil, to give
total resistance of 33.4 ohms. The pole components are 9 cm deep,
radius R1 is 24 cm, while R2 is 31 cm, with an angel b of 52.degree.
and an angel of 21.degree. angles. The total experimentally-measured
inductance of the winding (two coils in series) is 0.8 henrys, and
the L/R winding time constant is 0.024 seconds, 31.16% of the excitation
signal period. The sensors (S) are stainless steel, to resist attack
from the fluid as well as possible deposits. As can be seen from
FIG. 1 sensors (S) are arranged orthoginally respect to the pole
The excitation block (2) whose circuit is shown in FIG. 2 comprises
an excitation control signal generator circuit designed to generate
a 6.5 Hz control base wave of zero continuous level and a 50% working
cycle, to provide a +11 V signal at the input to a integrated circuit
for the excitation of the coils (B1 and B2). The excitation control
signal generator circuit consists of a 6.5 Hz generator (4) followed
by a continuous level suppressor and amplifier (5) which takes the
form of a 6.5 Hz oscillator based on the CI 555 a high-pass band
filter and an amplification unit implemented by means of an operational
The coil excitation circuit (6) is an integrated LSI transadmittance
circuit controlled by the input tension, and able to supply a maximum
2 A current with gain loop adjustment by means of external components.
In the signal processing circuit shown in block diagram form in
FIG. 3 the part for amplification and processing of the signal,
developed in FIG. 4 consists of a capacitive conduction suppressor
(7), an amplifier based on an AD 524 circuit (8) in the form of
a high-precision instrumentation amplifier operable in extreme conditions.
In all data-capture systems, this circuit combines characteristics
enabling fully simplified designs merely by control of the differential
amplification, with a high degree of linearity, a very high common-mode
rejection factor, low drift level and very low noise level.
The amplifier output (8) is made to flow through two high-pass
(9) and low-pass (10) filters designed using operational amplifiers
with adaptation modules 15 16 inserted between them and using the
same type of circuits, then flowing through an attenuator unit (11)
to a true effective value convertor (12).
The attenuator unit (11) adapts the signal levels to the values
of the true effective value conversion circuit since the maximum
tension which can be applied to it is 200 mVef, and the maximum
peak rate is related to that value.
The effective value convertor (12) is based on an AD 636 integrated
circuit which carries out this function in close relation with the
attenuator unit (11), at the output from which an amplifier (13)
is fitted to amplify the conversion circuit signal (12) to a continuous
value providing an easily processed and analyzed signal. For these
purposes, a gain amplifier 10 is required, with very low noise level
and a maximum output tension of 2 volts. At the output of the amplifer
(13) is a stand alone voltage to frequency converter (14), which
is basic configuration of the LM 131 integrated circuit.
In practice, the circuit sections described are assembled on a
printed circuit board. The plate where the excitation block is situated
also contains the electrode signal capture and treatment elements.
All this provides a reliable flow metering structure which is compact
elements and control components such as a microprocessor, to facilitate
not only the exchange of orders within the system but also the configuration
of the visualization section. This gives a clear idea of the versatility
of the proposed electromagnetic flow meter.