Abstrict In an inductive flow meter, the magnetic field generator (11),
provided for generating a magnetic field within a tube (10), is
connected to a alternating current voltage source via an electronic
switch (13). The switch (13) is controlled in such a manner that
it becomes conducting only at a zero crossing of the alternating
current, respectively, and subsequently remains in the conducting
state until the next zero crossing of the exciting current (i) occurs.
The useful voltage received at the electrodes (16a, 16b) is integrated
in an integrator (19) whereby the noise voltage, coupled into the
electrode loop by self-induction, is zero because of ##EQU1##
Claims I claim:
1. An inductive flow meter, comprising:
a magnetic field generator connectable to an alternating current
voltage,
a pair of electrodes,
an evaluating circuit connected to said electrodes,
a control circuit provided with a zero crossing detector for detecting
the zero crossings of the alternating current voltage, and
an electronic switch connecting the magnetic field generator to
the alternating current voltage, which electronic switch is controlled
by the control circuit in such a manner that it becomes conducting
only at a zero crossing of the alternating current voltage and is
switched out at a zero crossing of the exciting current,
characterized in that the electronic switch supplies the alternating
current voltage to the magnetic field generator each time for less
than one period length and that the evaluating circuit includes
an integrator which, starting with the conducting state of the switch,
integrates the electrode voltage over one period length of the alternating
current or a multiple thereof.
2. The inductive flow meter according to claim 1 wherein the electronic
switch is controlled to assume the conducting state at each n-th
zero crossing of the alternating current voltage, n being an adjustable
integer larger than 1 and preferably larger than 2.
3. The inductive flow meter according to claim 1 wherein the electronic
switch is controlled to assume the conducting state at each n-th
zero crossing of the alternating current voltage, n being an odd
number or an even number.
4. The inductive flow meter according to claim 3 wherein n is
an odd number and wherein there are provided a first sample and
hold circuit for receiving the positive output signals of the integrator
and a second sample and hold circuit for receiving the negative
output signals of the integrator, the sample and hold circuits storing
the respective time integral of the directly preceding exciting
current, and wherein a difference former is provided for generating
the difference between the stored signals of the two sample and
hold circuits.
5. The inductive flow meter according to claim 1 wherein a detector
for detecting the presence of liquid between the electrodes positively
locks the electronic switch if no liquid is present.
6. The inductive flow meter according to claim 1 wherein the electronic
switch is a triac or a thyristor.
7. The inductive flow meter according to claim 1 wherein there
is provided at least one auxiliary coil which is magnetically coupled
to the magnetic field generator, the output signal of the auxiliary
coil being supplied to a double integrator, and wherein a divider
divides the output signal of the integrator by that of the double
integrator so as to generate the output signal of the evaluating
circuit.
8. An inductive flow meter, comprising:
a magnetic field generator,
a pair of electrodes,
a switch for connecting the magnetic field generator to a source
of alternating current voltage, whereby the alternating current
voltage is supplied to the magnetic field generator and an exciting
current is supplied to the magnetic field generator,
a control circuit, the control circuit including a zero crossing
detector for detecting a zero amplitude of the alternating current
voltage, means for bringing the switch to a conductive state at
a zero amplitude of the alternating current voltage, and means for
bringing the switch to a non-conductive state at a zero amplitude
of the exciting current,
means for controlling the switch to supply the alternating current
voltage to the magnetic field generator for a duration less than
a period of the alternating current voltage, and
an evaluating circuit connected to the pair of electrodes, the
evaluating circuit including an integrator for integrating a voltage
at the electrodes over a period of time beginning when the switch
is brought to the conductive state and corresponding to at least
a period of the alternating current voltage.
9. The inductive flow meter according to claim 8 wherein the switch
is brought to a conductive state after the alternating current voltage
has reached zero amplitude n number of times, where n is equal to
an integer larger than 1.
10. The inductive flow meter according to claim 8 wherein the
switch is brought to a conductive state after the alternating current
voltage has reached zero amplitude n number of times, where n is
equal to an integer larger than 2.
11. The inductive flow meter according to claim 8 wherein the
switch is brought to a conductive state after the alternating current
voltage has reached zero amplitude n number of times, where n is
equal to an odd integer.
12. The inductive flow meter according to claim 8 wherein the
switch is brought to a conductive state after the alternating current
voltage has reached zero amplitude n number of times, where n is
equal to an even integer.
13. The inductive flow meter according to claim 8 further comprising
detection means for detecting the presence of liquid between the
electrodes and for locking the switch if the presence of liquid
between the electrodes is not detected.
14. The inductive flow meter according to claim 8 wherein the
switch comprises a triac.
15. The inductive flow meter according to claim 8 wherein the
switch comprises a thyristor.
16. The inductive flow meter according to claim 8 wherein the
integrator generates positive output signals and negative output
signals corresponding to the time integral of the exciting current,
comprising:
a first sample and hold circuit for receiving and storing the positive
output signals of the integrator,
a second sample and hold circuit for receiving and storing the
negative output signals of the integrator, and
a difference generator for generating a signal corresponding to
a difference between the signal stored by the first sample and hold
circuit and the signal stored by the second sample and hold circuit.
17. The inductive flow meter according to claim 8 wherein the
integrator generates an output signal, comprising:
an auxiliary coil for generating an output signal, the auxiliary
coil being magnetically coupled to the magnetic field generator,
a double integrator,
means for supplying the output signal of the auxiliary coil to
the double integrator, and
a divider for dividing the output signal of the integrator by the
output signal of the double integrator to thereby generate an output
signal of the evaluating circuit.
Description BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is directed to an inductive flow meter.
2. Description of Related Art
Inductive flow meters have a magnetic field generator, e.g. a coil,
for generating a laterally directed magnetic field in a tube. Further,
the tube is provided with two electrodes the voltage of which is
supplied to an evaluating circuit. When an electrically conductive
liquid is flowing through the tube transversely to the magnetic
field, a voltage is generated at the electrodes, the level of this
voltage being proportional to the magnetic field strength and to
the flow speed.
The first inductive flow meters have been devices operated by alternating
fields, wherein the magnetic field generator is connected directly
to the voltage of the power supply, for example 220 V/50 Hz.
An advantage of such an alternating-field device resides in that,
without requiring too complex switching and control means, a strong
magnetic field can be generated so that a large signal amplitide
is obtained at the electrodes and thus a large signal-to-noise ratio
can be reached. By the comparatively high frequency of the power
supply voltage, an inductive flow meter of the described type has
a fast response characteristic. However, the systems of alternating-field
devices have the inherent disadvantage that the useful signal is
subjected to a zero drift. The useful signal generated at the electrodes
has a phase-shift of about 90.degree. with respect to the alternating
voltage of the power supply. Since the evaluating circuit, along
with the lines coming from the electrodes, is very susceptible to
interferences, noise voltages originating directly in the power
supply voltage as well as self-induced noise voltages are carried
into the evaluating circuit. According to usual practice, the noise
signals contained in the useful signal are eliminated by phase selection.
However, since the phase relation between the noise signal and the
useful signal varies with time in dependence of the temperature
of the device and other influences, a phase-selective elimination
of the noise signals necessitates frequent adjustment and re-adjustment.
A further disadvantage consists in a considerable reactive-power
consumption of alternating-field devices.
More recent inductive flow meters are operated with a clocked DC-current
field. In these flow meters, the magnetic field generator is excited
by a clocked DC-current, while the polarity of the DC-current excitation
may alternate. Devices of this type have excellent zero stability
and low power consumption. Nevertheless, it is a massive disadvantage
of these devices that, using electronic means merely to a reasonable
extent, only relatively small exciting currents can be generated
so that the amplitude of the obtained useful signals is also small.
The clock frequency of the clocked DC-current field is comparatively
low so that the reaction speed of the flow metering is reduced.
Both, the relatively low useful voltage and the low clock frequency,
cause a small signal-to-noise ratio.
Further, an inductive flow meter is known from German Patent Application
20 54 624. In this flow meter, the exciter coil generating the magnetic
field is contained in a bridge circuit consisting of four electronic
switches and being connected to the supply voltage. At each time,
two diagonally arranged electronic switches of the bridge circuit
are in the conducting state. By switching the pairs of switches,
the polarity of the exciter coil can be reversed with respect to
the supply voltage. This reversal of polarity is performed respectively
at a zero crossing of the alternating current. Since the electronic
switches are triacs, they are always extinguished at the zero crossing
of the exciting current and then are immediately ignited again.
Each of the two pairs of switches remains active over a specific
number of periods which is detected by a counter. Then, the other
pair of switches is excited. The periodical reversal of polarity
of the exciter coil with respect to the supply voltage is provided
to compensate those noise voltages which, with respect to their
amplitude and their phase length, undergo a slow change over time.
The evaluating circuit is provided with a rectifier which receives
a phase reference signal and admits only that component of the electrode
signal which is in phase with a phase reference signal. Also this
arrangement involves the above-mentioned disadvantages of alternating-field
devices, i.e. this method has no inherent zero accuracy (autozero).
German Patent Application 27 25 026 discloses also a alternating-field
device, wherein the magnetic field exciter is connected to the alternating
current voltage via an electronic switch. The electronic switch
is clocked by a clock-pulse generator having a fixed clock ratio,
the frequency of the clock-pulse generator being a multiple of the
supply current frequency. The clockpulse generator is operated independently
of the signals of the evaluating circuit so that the timing of the
switch actuation is in no predetermined phase relation to the frequency
of the alternating current. The electrode signals, having been amplified,
are supplied to sample and hold circuits the output signals of which
are subtracted from each other. Also the system of this method does
not provide zero accuracy because the noise component, generated
by self-induction, is not eliminated by the subtraction.
It is the object of the invention to provide an inductive flow
meter which combines the advantages of the device operated by an
alternating-field and the devices using a clocked DC-current field
without taking on the disadvantages of those two types of devices.
This means that the inductive flow meter is to be operated using
AC-voltage and without excessive control efforts while, at the same
time, avoiding the danger of zero drift, i.e. in this device zero
accuracy (autozero) shall be obtained as a feature inherent to the
system (and not by compensation or adjustment).
SUMMARY OF THE INVENTION
In the inductive flow meter of the invention, the magnetic field
generator is connected to the AC-voltage via an electronic switch.
The switch is controlled by a zero crossing detector in such a manner
that the exciting current, generating the magnetic field, is switched
on at the zero crossing of the AC-voltage. The switch is opened
again when the exciting current passes through zero. This is followed
by a pause of an adjustable number of half-waves of the AC-voltage,
and subsequently the exciting current is switched on as mentioned
above.
It is a substantial advantage of the inventive flow meter that
the useful signal, which is proportional to the exciting current,
need not be freed from noise signals by a critical phase selection
but can be obtained by mere integration, with all of the noise signals
entering into the evaluating circuit from the outside being eliminated
within half a clock period, inclusive of the noise voltages induced
by self-induction into the electrode circuit. At the end of a exciting-current
pulse, this exciting current has become zero. Thus, at this point
of time (or, respectively, a short time afterwards due to existing
eddy currents) the magnetic field B.sub.2 is equally large as the
field B.sub.1 at the beginning of the excitation. The useful signal
can be obtained by integration of the electrode voltage over one
or a plurality of periods of the frequency of the exciting voltage
(alternating current voltage). However, the integral over one or
a plurality of periods of a power-supply noise voltage is zero,
independently of the phase position of this noise voltage. Further,
the noise voltage component, coupled into the circuit by self-induction
into the electrode loop, due to B.sub.1 = B.sub.2 (see above) is
positively zero in order to obtain also ##EQU2##
This results in that the flow meter of the invention is insensitive
to self-induced or externally induced noise voltages. Moreover,
according to the relevant teaching, electric transient-oscillation
processes lead to a current amplitude and thus also to an amplitude
of the field and the useful signal which exceeds the amounts of
the quasi-stationary values almost by the factor two.
The inventive flow meter offers the advantages of the alternating-field
devices, i.e. a large signal amplitude, because the magnetic field
generator is directly connectable (via the electronic switch) to
the supply voltage, but avoids the disadvantage of the vagabonding
zero point. On the other hand, there is much less control effort
required for generating the magnetic field than with flow meters
using a clocked DC-current field.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will be explained hereunder in greater
detail with reference to the drawings.
In the drawings
FIG. 1 is a schematic circuit diagram of the inductive flow meter,
and
FIG. 2 is a diagram of a plurality of signals occurring in the
circuit of FIG. 1.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The flow meter is provided, in a known manner, with a tube 10 made
of a non-conductive material and serving for the passage of a liquid
the flow rate of which is to be measured. The tube 10 has the magnetic
field generator 11 mounted thereto which, in the present case, consists
of two coils 11a and 11b being arranged coaxially to each other
at opposite sides of the tube and being connected in series. An
exciting current, flowing through the two coils 11a, 11b, generates
a magnetic field in the tube 10. This magnetic field extends transversely
through the interior of the tube. The magnetic field generator 11
is further provided with reference coils 12a, 12b the function of
which will be explained later.
The series connection of the coils 11a and 11b is connected in
series with the electronic switch 13 and via lines 14 and 15 to
the supply network so that the series arrangement consisting of
switch 13 and coils 11a, 11b is directly connectable to the supply
network of, for instance, 220 V and 50 Hz.
At opposite positions at the inner wall of the tube 10 there are
arranged two electrodes 16a, 16b which are connected to the evaluating
circuit 17.
The evaluating circuit 17 includes an amplifier 18 connected to
the electrodes 16a, 16b, the signal of the amplifier 18 being transmitted
to an integrator 19. The output signal of the integrator 19 is supplied
to two sample and hold circuits 20a and 20b having their outputs
connected to the two inputs of a difference amplifier 21. The output
signal of the difference amplifier 21 is supplied to the counter
input of a divider 22. The output signal of the divider circuit
22 at the output 23 is proportional to the flow rate of the liquid
within tube 10.
The amplitude of the alternating current voltage in line 14 is
supplied, via line 24 to the electronic control means 25. This
electronic control means is provided with a zero crossing detector
which generates an impulse with each zero crossing of the AC-voltage.
In FIG. 2 along the uppermost line a), there is shown the development
in time of the AC-voltage, indicated by U, of lines 14 and 15. The
vertical markings along line b) represent the needle pulses 26 which,
by the zero crossing detector, are generated respectively at the
zero crossings of the AC-voltage U.
Along the uppermost line a) of FIG. 2 the development in time
of the exciting current flowing through the magnetic field generator
11 is indicated by i. The electronic switch 13 being provided as
a triac, is controlled, by the electronic control means 25 and under
use of pulses Q.sub.0 supplied to the gate of the switch, in such
a manner that it is brought into the conducting state with each
zero crossing of the AC-voltage. The triac remains in its conducting
state until the current i, having a phase shift with respect to
the AC-voltage U, has become zero. As known in the art, a thyristor
or a triac are controlled to assume the non-conducting state if
the main current becomes zero with no control voltage existing at
the gate. According to FIG. 2c), the signal Q.sub.0 extends over
a half-wave of the AC-voltage U. Therefore, the triac can enter
the non-conducting state in the subsequent halfwave when the current
i becomes zero.
In the present embodiment, the signals Q.sub.0 which are supplied
to the gate of the triac, are generated respectively after n=5 zero
crossings of the AC-voltage U so that the current i, since n is
an odd number in this case, is generated alternately as a positive
and a negative current. If n were an even number, the exciting currents
i would be generated to be exclusively unipolar, i.e. with the same
polarity. Also in this case the flow meter would work properly.
Generally, this operating mode is to be preferred to the bipolar
generation of the exciting current i according to FIG. 2(a) because,
in this manner, a still better compensation of the noise voltages
is possible, i.e. in unipolar operation also the relatively small
interferences of the residual induction are eliminated.
In dependence of the zero crossing signals 26 the electronic control
means 25 generates the pulses Q.sub.1 respresented along line d)
of FIG. 2 which pulses Q.sub.1 extend over a period of the AC-voltage
U, i.e. over a positive and a negative half-wave. Each pulse Q.sub.1
starts with that zero crossing of the AC-voltage U at which the
triac 13 is controlled to assume the conducting state. The pulses
Q.sub.1 drive the integrator 19 in such a manner that the integrator
19 performs integration only for the duration of the pulses Q.sub.1
and, subsequently, its output signal is positively reset. Since
occurring noise voltages have the frequency of the AC-voltage U
(or a multiple thereof), the integral of this noise voltage becomes
zero over a full period of the AC-voltage. On the other hand, the
integral of the exciting current i within the respective period
assumes the value I, the amount of which corresponds to the hatched
area in FIG. 2. The useful voltage generated at the electrodes 16a
and 16b is in proportion to the area I. At the end of each signal
Q.sub. 1 a value given at the output of the integrator 19 has an
amount corresponding to the time integral I of the immediately preceding
exciting current i. This output value of the integrator 19 while
being clocked by the signal Q.sub.2 generated by the electronic
control means, is supplied to a sample and hold circuit 20a or 20b.
The signal Q.sub.2 is a short pulse generated immediately subsequent
to termination of the signal Q.sub.1. The succession of the signals
Q.sub.2 is such that alternately one of these signals, respectively,
activates the sample and hold circuit 20a and the next signal activates
the sample and hold circuit 20b. In this manner, the positive time
integrals I, which in the diagram of FIG. 2(a) are arranged above
the time axis t, are taken into the sample and hold circuit 20a
while the negative time integrals, arranged below the time axis
t, are taken into the sample and hold circuit 20b. The function
of the sample and hold circuits consists in that, upon occurrence
of a pulse Q.sub.2 they take over and store the respective output
signal of the integrator 19 and keep it stored until occurrence
of the subsequent pulse Q.sub.2.
The plus input of the difference amplifier 21 is connected to the
sample and hold circuit 20a, and the minus input is connected to
the sample and hold circuit 20b. Since the value contained in the
sample and hold circuit 20b is negative, the values of the two contents
of the sample and hold circuits are added by the difference amplifier
21. At the output of the difference amplifier 21 a voltage is generated
which is proportional to the voltage between the electrodes 16a
and 16b and which, at the same time, depends on the value of the
AC-voltage U supplied to the lines 14 and 15.
In order to eliminate the dependence of the output signal from
the value of the AC-voltage, an additional coil arrangement, consisting
of auxiliary coils 30a and 30b, is provided at the magnetic field
generator 11. These auxiliary coils are connected in series with
each other and form secondary coils for the coils 11a and 11b. The
auxiliary coils 30a and 30b are connected to the inputs of amplifier
31 the output of amplifier 31 being connected to the input of a
double integrator 32. The output signal of the double integrator
32 is transmitted to a sample and hold stage 33 having its output
connected to the denominator input of divider 22.
The double integrator 32 consists of two integrators, connected
in series, the first of them generating the time integral over the
voltage induced into the auxiliary coils. This generating of integrals
corresponds to the generating of integrals according to U.sub.N
=k.intg.i dt, with U.sub.N representing the voltage of the useful
signal between the electrodes 16a, 16b, k representing a constant
and i representing the exciting current. Thus, by the first integrator
of the double integrator 32 the voltage at the input of amplifier
18 is imitated. The second integrator of the double integrator 32
has the same function as integrator 19 and is also controlled by
the signal Q.sub.1. The sample and hold circuit 33 has the function
of the sample and hold circuits 20a and 20b and is also controlled
by the signal Q.sub.2. This means that the circuits 31 32 and 33
perform the same signal processing as the usable-signal circuit
consisting of the elements 18-21 except, however, for the fact
that the voltage at the output of the sample and hold circuit 33
depends exclusively from the value of the AC-voltage supplied to
the lines 14 and 15 and not from the flow within tube 10. By the
dividing of two voltages in the divider 22 with both of the voltages
depending on the value of the AC-voltage, the influence of the alternating
current amplitude is eliminated. Thereby, the output signal at line
23 becomes independent from the value of the alternating current.
If no conductive liquid is present between the electrodes 16a and
16b, the exciting current i should be switched off for preventing
unnecessary heat-accumulation in the magnetic field generator 11.
To this purpose, an auxiliary current source (not shown) is connected
to the electrodes 16a and 16b; when no current flows through the
electrodes, this auxiliary current source signals this condition
to the electronic control means 25. When no electrode current is
flowing, the electronic control means 25 effects positive blocking
of the switch 13. In this manner, simple switch-off of the magnetic
field generator 11 can be accomplished without additional circuit
elements. |