Abstrict The inductive flow meter and method of exciting a coil includes
a coil with unipolar excitation, which generates exciting pulses
having a pulse interval therebetween. The length of the pulse interval
is shorter than that of an exciting pulse. The associated evaluation
circuit includes two alternately clocked channels whose evaluation
phases overlap each other in time, the measuring voltage being obtained
at the ends of the signal paths by alternately sampling the channels.
Claims That which is claimed is:
1. An inductive flow meter comprising:
at least one coil for generating a magnetic field in a flow region
of a liquid, said coil being positioned for a periodical and unipolar
excitation so that a pulse interval between two exciting pulses
is less than that of an exciting pulses;
a plurality of electrodes positioned in the flow region; and
an evaluation circuit connected to electrodes for generating a
measured signal from the electrical field induced in the liquid,
said evaluation circuit including two channels whose evaluation
phases overlap each other in time, a measuring voltage being obtained
from the channels by alternately sampling outputs of the channels.
2. An inductive flow meter as defined in claim 1 further comprising
a rectifier bridge connected to said coil and having a plurality
of rectifiers connected as a Graetz circuit, two rectifiers of said
plurality of rectifiers conducting current in the same direction
and being arranged diagonally to each other rin said rectifier bridge
circuit.
3. An inductive flow meter a defined in claim 1 further comprising
detecting means connected to at least one channel of said evaluation
circuit for detecting an error amount caused by eddy currents in
the pulse intervals, and wherein the error amount is subtracted
from the signal of this channel after being multiplied by a factor,
to thereby reduce transformative noises.
4. An inductive flow meter as defined in claim 3 wherein said
detecting means comprises a charging circuit having a charging time
constant.
5. An inductive flow meter as defined in claim 1 further comprising
an input signal connected to an inverter which is switched on outside
the evaluation phases of the two channels and switched off in the
evaluation phases, and whose output signal is coupled into both
channels to thereby reduce drift-like noises.
6. An inductive flow meter as defined in claim 1 wherein the exciting
pulses each have a length of about one a.c. voltage period and succeed
each other with a period duration of 1.5 times the a.c. voltage
period.
7. An inductive flow meter comprising:
at least one coil for generating a magnetic field in a flow region
of a liquid, said coil being positioned for a periodical and unipolar
excitation so that a pulse interval between two exciting pulsed
is less than that of an exciting pulse;
a rectifier bridge connected to said coil and having a plurality
of rectifiers connected as a Graetz circuit, two rectifiers of said
plurality of rectifiers conducting current in the same direction
and being arranged diagonally to each other in said rectifier bridge
circuit;
a plurality of electrodes positioned in the flow region;
an evaluation circuit connected to electrodes for generating a
measuring signal from the electrical field induced in the liquid,
said evaluation circuit including two channels whose evaluation
phases overlap each other in time, the measuring voltage being obtained
from the channels by alternately sampling the channel outputs; and
detecting means connected to at least one channel of said evaluation
circuit for detecting an error amount caused by eddy currents in
the pulse intervals, and wherein the error amount is subtracted
from the signal of this channel after being multiplied by a factor,
to thereby reduce transformative noises.
8. An inductive flow meter as defined in claim 7 wherein said
detecting means comprises a charging circuit having a charging time
constant.
9. An inductive flow meter as defined in claim 8 further comprising
an input signal connected to an inverter which is switched on outside
the evaluation phases of the two channels and switched off in the
evaluation phases, and whose output signal is coupled into both
channels to thereby reduce drift-like noises.
10. An inductive flow meter as defined in claim 9 wherein the
exciting pulses each have a length of about one a.c. voltage period
and succeed each other with a period duration of 1.5 times the a.c.
voltage period.
11. A method of exciting a coil of an inductive flow meter comprising:
providing a periodical unipolar excitation of a coil so as to generate
a magnetic field in a flow region of a liquid, the length of the
pulse interval between two exciting pulses being shorter than that
of an exciting pulse;
providing a plurality of electrodes positioned in the flow region;
and
generating a measuring signal from the electrical field induced
in the liquid by an evaluation circuit connected to the plurality
of electrodes, the evaluation circuit including two channels whose
evaluation phases overlap each other in time, a measuring voltage
being obtained from the channels by alternately sampling outputs
of the two channels.
12. A method as defined in claim 11 further comprising detecting
an error amount caused by eddy currents in the pulse intervals,
and wherein the error amount is subtracted from the signal of this
channel after being multiplied by a factor, to thereby reduce transformative
noises.
Description FIELD OF THE INVENTION
The present invention relates to the field of flow meters.
BACKGROUND OF THE INVENTION
The invention relates to a magnetic-inductive flow meter with clocked
excitation.
Inductive flow meters described in the U.S. Pat. Nos. 5018391
and 5400660 both granted to Doll, or in U.S. Pat. No. 3783686
operate in the form of clocked alternating field devices generating
a magnetic field in the flowing liquid by supplying exciting pulses
derived from the primary frequency to a coil. The clock frequency
of the exciting pulses of the coil is about 1/3 to 1/10 of the primary
frequency. The exciting current pulses generate pulses of magnetic
field strength in the flowing medium. At electrodes which are in
contact with the medium, an electric potential is generated whose
magnitude is proportional to the field strength and the flow velocity.
Thus, a measuring signal is obtained which is a measure of the flow
velocity of the flowing medium. The reaction time of the flow meter
depends on the clock frequency by which the coil is excited. Further,
it has to be considered that numerous noise components occur, falsifying
the measuring signal and impairing the measuring accuracy and stability
of the flow signal.
SUMMARY OF THE INVENTION
In view of the foregoing background, it is therefore an object
of the present invention to provide an inductive flow meter with
an optimum noise suppression and a fast response time, with the
exciting power being high.
In a flow meter according to the invention, the excitation of the
coil is unipolar. This means that the coil sequentially generates
field strength pulses of the same polarity. The interval length
between two exciting pulses is smaller than the duration of one
rapid succession, noises can be suppressed quickly and thus effectively
and the flow meter has a short response time, i.e., changes in the
measured value lead to a change in the measuring signal within a
very short time.
The evaluation circuit includes two alternately clocked channels
whose evaluation phases mutually overlap. Thereby, it is possible
to update the measuring result in rapid succession although the
evaluation phases are relatively long because of the required error
compensation. With the response time being short, the flow meter
supplies a stable output signal, without there being a necessity
for compromises with respect to measuring accuracy, particularly
with respect to the zero point.
The exciting power for the coil can be taken either from a power
system, a low-voltage source operating with primary frequency or
a d.c. voltage source. When a.c. voltage is used, the coil can be
operated through four controlled rectifiers (thyristors) arranged
as a Graetz circuit. In the case of a d.c. voltage source, transistors
are suitable for controlling the coil.
When using a Graetz circuit, controllable rectifiers (e.g., thyristors)
are used, which conduct current in one direction only. Thereby,
the unipolar excitation of the coil is effected. Using a bridge
of rectifiers conducting current in only one direction has the advantage
of a greater security against primary short circuit because each
of the bridge arms is provided with two diodes arranged in opposite
directions, which can never conduct current simultaneously.
Within the scope of the subsequent description and the claims,
the term "coil" does not only mean a single coil but also
a coil array of several electrically connected coils.
BRIEF DESCRIPTION OF THE DRAWINGS
Hereinafter, an embodiment of the invention is explained in detail
with reference to the drawings, in which:
FIG. 1 is a circuit diagram of the exciting circuit for a coil
generating the magnetic field according to a flow meter of the present
invention;
FIG. 2 shows a diagram for explaining the function of the circuit
illustrated in FIG. 1 according to a flow meter of the present invention;
FIG. 3 is a schematic circuit diagram of an evaluation circuit
of a flow meter according to the present invention;
FIGS. 4(a-e) illustrates schematic diagrams for explaining the
operation of an evaluation circuit of a flow meter according to
the present invention;
FIGS. 5 and 6 illustrate further embodiments of an exciting circuit
of a flow meter according to the present invention; and
FIG. 7 is a schematic diagram of exciting circuits illustrated
in FIGS. 5 and 6 of a flow meter according to the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
The present invention will now be described more fully hereinafter
with reference to the accompanying drawings, in which preferred
embodiments of the invention are shown. This invention may, however,
be embodied in many different forms and should not be construed
as limited to the illustrated embodiments set forth herein. Rather,
these embodiments are provided so that this disclosure will be thorough
and complete, and will fully convey the scope of the invention to
those skilled in the art. Like numbers refer to like elements throughout,
and prime and double prime notation are used to indicate similar
elements in alternative embodiments.
As illustrated in FIG. 1 a Graetz circuit of four thyristors SCR1-SCR4
is connected to an a.c. voltage source supplying an a.c. voltage
U. The coil L, in which a current I is generated, is connected to
the bridge arm of the Graetz circuit. The thyristors SCR1 SCR2
being diagonally opposite one another are connected in series and
polarized in the same sense. They are controlled in synchronism
with each other. The two other thyristors SCR3 SCR4 diagonally
arranged in the bridge circuit are also connected in series, with
current direction being the same, and controlled in synchronism
with each other.
FIG. 2 illustrates the time history of the a.c. voltage U. This
voltage is, e.g., the primary voltage with a frequency of 50 Hz,
the period being T=20 ms (milliseconds).
Further, ignition pulses a and b are shown. The ignition pulses
a are those pulses by which the thyristors SCR1 SCR2 are controlled,
SCR3 SCR4 are controlled. The ignition pulses a always begin in
a positive half-wave of the voltage U, while the ignition pulses
b begin at a negative zero passage of the voltage U and are offset
with respect to the ignition pulses a by 3/2 T.
The coil current I generated is also shown in FIG. 2. It consists
of unipolar current pulses, each beginning upon ignition of a pair
of thyristors, rising first and then falling again. When the current
pulse has fallen to zero, the respective pair of thyristors is put
into the off-state. The exciting current pulses have a period of
1.5 T. This leads to the intended relatively fast clock frequency
of 2/3 of the primary frequency. Thus, the clock frequency is higher
by the factor of 2 to 10 than that of the known flow meters. Due
to the high clock frequency, the signal-to-noise ratio is improved
and the response time reduced.
Each of the pulses of the exciting current I has a length which
is slightly shorter than the period length T of the supply voltage.
This means that the duration of the exciting current pulses is shorter
than the primary period. With a primary frequency of 50 Hz, the
clock frequency of the magnetizing pulses generated by the coil
L is 33.33 Hz. The time interval between the exciting current pulses,
which is 0.5 times the primary period, is used to eliminate the
substantial noise components occurring at the electrodes.
The evaluation circuit is shown in FIG. 3. It comprises two electrodes
E1 and E2 which are in contact with the flowing medium. The voltage
V.sub.E to be measured is generated between these electrodes. The
electrode E1 is connected to an input of a differential amplifier
V3 via an amplifier V1. The electrode E2 is connected to the other
input of the differential amplifier V3 via an amplifier V2.
The output of the differential amplifier V3 is connected to the
input of an amplifier V4 via a subtracter 10. The output voltage
U.sub.E of this amplifier V4 is fed back to the subtraction input
of the amplifier V5 the resistor R1 and the capacitor C1. Hence,
the output signal of the amplifier V4 is forced to be at zero whenever
the switch S1 is controlled to be conductive.
The voltage U.sub.E is supplied to the two channels A and B, which
are different signal paths in the analog technique which is described
herein. In digital technique, the channels can be realized by software.
Channel A includes a switch S2 an integrating amplifier V7 a subtracter
12 and a latch V10. Channel B includes a switch S4 an integrating
amplifier V8 a subtracter 13 and a latch V11. The outputs of the
two channels A and B are alternately sampled by an alternating switch
S7 and supplied to an amplifier V12 providing the measuring signal
V.sub.A.
Further, the voltage U.sub.e is supplied to an inverter V6 changing
its sign. Connected in series with the inverter V6 is a switch S3
being adapted to connect the output of the inverter with the inputs
of the two integrating amplifiers V7 and V8. The outputs of the
integrating amplifiers V7 V8 are connected, via one switch S5
S6 each, with a charging circuit 14 consisting of a resistor R2
and a capacitor C2. The charging signal of the capacitor C3 is connected
to the subtraction inputs of the subtracters 12 13 via an amplifier
V9. The amplifier V9 amplifies the signal applied to its input by
a constant factor which here has a value of "3" and will
be explained hereinafter.
The evaluation circuit of FIG. 3 will be explained by the diagrams
of FIG. 4. In FIG. 4a, the intervals of 0-5 are illustrated, which
are cyclically repeated. Each interval has half the period length
T/2 of the mains voltage U. FIG. 4b shows the time history of the
field strength B in the measuring volume. With the flow being constant,
this results in a corresponding time history of the signal voltage
or flow voltage U.sup.N which is proportional to the field strength
B.
In practice, the signal voltage is superposed by numerous noise
quantities. The substantial noise quantities are the following:
(1) electrolytic d.c. voltage potential cause: barrier potential
between liquid and electrode
(2) coupled-in mains frequency cause: vagrant earth currents and
line couplings
(3) drift of the electrode potential cause: media change, contact
of the electrode surface with solid particles or the like
(4) transformative noise voltage cause: the voltage induced into
the electrode loop and the measuring liquid by field change.
Moreover, there are also capacitive and ohmic error influences
which, however, mostly play a minor part and can be suppressed by
known measures such as shielding and use of high-quality insulating
materials. The above-mentioned noise influences (1) through (4),
however, are always present in a more or less distinctive form,
and in the evaluation circuit of FIG. 3 they are suppressed by
a combination of different measures.
The sum of flow-proportional signal voltage UN and different noise
components, of which the drift is shown in FIG. 4c and the transformative
noise voltage in FIG. 4d, is applied to the electrodes E1 E2. Via
the input amplifiers V1 and V2 the electrode voltage is supplied
to the differential amplifier V3 whose amplification can be adapted
to the desired measuring range. This is followed by the amplifier
V4 the integrator 11 being arranged in the feedback path thereof.
At the beginning of the time intervals 1 and 4 of the time pattern,
switch S1 of said integrator is momentarily (order of 0.2 ms) switched
to be conductive. As a consequence, the output voltage U.sub.e of
V4 is at zero at the beginning of each period of useful signals
or excitation, as shown in FIGS. 4b and 4d.
Because the time constant of the switched integrator 11 suitably
amounts to some seconds, this step compensates the interfering d.c.
voltage of the input signal and the long-time drift thereof. As
a whole, the amplified input signal V.sub.E is now applied to the
output of amplifier V4 the momentary value being zero at the beginning
of each exciting phase (intervals 1 and 4). Thus, the interfering
d.c. voltage is eliminated, which is required in connection with
the zero correction treated later on. Coupled-in primary voltages
are suppressed in the known way by the fact that the integration
times of the integrating amplifiers V7 V8 correspond to the primary
period T.
Subsequent to the amplifier V4 a division into the two channels
A and B is effected, each signal path evaluating only every second
useful signal pulse. This means that signal path A evaluates, for
example, the useful signal phase in the time intervals 1 and 2
while signal path B evaluates the useful signal phase in the intervals
4 and 5. Dividing the signal paths is necessary because the evaluation
times of-the two channels including the drift correction extend
over four time intervals each and do thus overlap. This becomes
clear from the following explanation:
The integrators V7 and V8 mainly function to integrate the useful
signal pulses. Integrator V7 for example, integrates the useful
signal appearing in the time intervals 1 and 2 i.e., in this time,
switch S2 is closed. Switch S4 of integrator V8 is closed during
the time intervals 4 and 5. Now, these integrators are supposed
to not only add up the useful signal, but also to suppress rapidly
passing noises. For the sake of simplicity, such a noise is shown
in FIG. 4c as a linear voltage variation or drift over the two primary
periods of the intervals 0 to 3. This drift is eliminated as follows:
V6 inverts the output signal of V4. Switch S3 is closed during
the time intervals 0 and 3. This means that the areas integrated
during the intervals 0 and 3 are subtracted from the areas of the
superposing the signal voltage is eliminated by means of this subtraction
the result of which is "zero".
This means that in this operation, the evaluation phase in channel
A was completed with the time interval 3 while the completely analog
operation in channel B starts with time interval 3. This means that
the evaluation phases of the two channels with a length of four
time intervals overlap each other in the time interval 3 and 0
respectively. As a consequence, the integrators in channels A and
B provide current measuring values freed from slow and fast noises
every three time intervals (=30 ms in case of 50 Hz primary).
The previously described method of fast noise compensation by V6
however, is not suitable for eliminating a zero error caused by
the eddy current component in the transformative noise voltage.
On the contrary, this method even increases the zero error based
on the eddy current effects.
This fact is explained in FIG. 4: FIG. 4b shows the course of magnetic
field B with eddy current appendix 15 (hatched), FIG. 4d shows the
course of the transformative voltage as it occurs at the output
of amplifier V4. The curve of FIG. 4d is the derivation of the curve
of FIG. 4b, i.e., U.sub.T =dB/dt, where U.sub.T is the transformative
noise voltage. The eddy current appendix 16 is hatched. FIG. 4e
shows the integration result of the noise voltage at the output
of the integrators V7 and V8.
As can be seen in FIG. 4e, the integration result of the transformative
noise signal is an error amount proportional to "3"(B2-B1).
B2 and B1 are the field strengths at the beginning and end of the
eddy current appendix. If there is no eddy current, as approximately
is the case with small nominal widths of the pipe of the flow channel,
it applies that B2=B1 so that the above-mentioned error amount
disappears. Generally, however, eddy currents do appear, which requires
a correction of the error amount of 3k(B2-B1). Here, k is a constant.
According to the invention, the quantity k(B2-B1) available at the
output of the integrators V7 and V8 at certain times, is averaged
by the charging circuit 14 over a long time and multiplied by a
value amounting to about 3 by the amplifier V9. The multiplication
result is subtracted from the above integration result of the integrating
amplifiers V7 and V8.
Mathematically, the error quantity 3k(B2-B1) can be explained through
the dB/dt noise added up over the evaluation interval 0 to 3 according
to the following calculation:
______________________________________ time interval 0 (inverted):
- .intg.dB/dt dt = k(B2 - B1) time intervals 1 and 2: .intg.dB/dt
dt = k(B2 - B1) time interval 3 (inverted): - .intg.dB/dt dt = k(B2
- B1) sum = 3k (B1 - B1) ______________________________________
From FIG. 3 the function of the noise suppression measure can
be seen. At the end of time interval 0 switch S5 closes and thus
takes over the quantity k(B2-B1) from V7. The same happens when
switch S6 closes at the end of the time interval 3. Here, too, the
quantity k(B2-B1) is taken over, from V8 this time. Now, the capacitor
C2 is slowly charged--depending on the time constant R2C2 and the
closing duration of the switches S5 and S6 --to the value k(B2-B1),
rapid interfering actions having no influence due to the inertia
of this operation. The inert (slow) average value of k(B2B1) now
obtained is multiplied by the amplifier V9 by the factor of 3 and
then subtracted in the latches V10 and V11 from the output signals
of the integrators V7 and V8. Thus, the error amount disappears
and the zero correction is completed.
The useful signals now stored in the latches V10 and V11 with
zero correction and freed from noise signals, are identical and
are connected to the output stage V12 via the switch V7. Switch
S7 switches to and from between channel A and B with a period of
1.5 T. It connects channel A with the output stage during the time
intervals 450 and channel B with the output stage during the time
intervals 123 so that the measuring value updated every 1.5 T
is transferred to the output. It goes without saying that the analog
circuit design illustrated in FIG. 3 can also be realized in digital
technique.
The method described can be applied virtually unchanged even if
the magnet is not supplied with primary voltage but with a low voltage
of mains frequency--about 24 VAC. The magnet may also be supplied
from a d.c. voltage source, e.g., 24 VDC. In this case, however,
the thyristor bridge has to be replaced by a transistor bridge.
Such a concept is shown in FIG. 5. Here, a d.c. voltage source
is provided supplying a d.c. voltage U to a Graetz circuit consisting
of four transistors To, T2 T3 and T4. The coil L, through which
the coil current I flows, is connected into the bridge arm of the
Graetz circuit.
The circuit according to FIG. 6 has the same configuration as that
according to FIG. 5 except that the transistor T3 is replaced by
a diode Do and the transistor T4 by a diode D2.
FIG. 7 shows the time histories of the coil current I and the magnetic
field strength B with reference to the intervals. The coil current
(exciting current) and the magnetic strength no longer have a sinusoidal
course (as in FIGS. 2 and 4), but a sawtoothed course which results
in a sawtooth excitation.
Many modifications and other embodiments of the invention will
come to the mind of one skilled in the art having the benefit of
the teachings presented in the foregoing descriptions and the associated
drawings. Therefore, it is to be understood that the invention is
not to be limited to the specific embodiments disclosed, and that
modifications and embodiments are intended to be included within
the scope of the appended claims. |