DE3204573A1 - Circuit arrangement for generating a pulsating magnetic field in the exciter coil of an inductive flow meter - Google Patents

Description

FROM KREISLER SCHÖNWALD EISHOLD FUES FROM KREISLER KELLER SELTING WERNER

PATENTANWÄLTE TURBO-WERK Fritz Hammelrath Dr.-Ing. by Kreislert 1973

Gremberger Str. 1 51 _Dr _ _{| ng K Schönwdd / Kö} | _n

Dr.-Ing. K. W. Eishold, Bad Soden 5000 Cologne 91 Dr, J. F. Fues, Cologne

Dipl.-Chem. Alek von Kreisler, Cologne Dipl.-Chem. Carola Keller, Cologne Dipl.-Ing. G. Selting, Cologne Dr. H.-K. Werner, Cologne

DEICHMANNHAUS AT THE MAIN RAILWAY STATION

D-5000 COLOGNE 1

February 10, 1982 Sg-Fe

Circuit arrangement for generating a pulsating magnetic field in the excitation coil of an inductive Flow meter

The invention relates to a circuit arrangement for generating a pulsating magnetic field in the excitation coil of an inductive flow meter with a current pulse source, which contains at least one controllable electronic component that the current through the Excitation coil regulates depending on the signal from a current sensor located in the excitation circuit.

Inductive flow meters are used to measure the flow rate of a liquid through a pipe (DE-OS 29 41 383). They have at least one excitation coil, which is pulsating inside the pipeline Magnetic field generated. The charge carriers contained in the flowing liquid are created in the liquid an electrical potential gradient, which is generated by the mutually spaced electrical

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trode is measured and evaluated. The measured potential difference forms a measure of the amount of liquid flowing per unit of time.

In the known flowmeters, a current pulse source is used to feed the excitation coil, in which _: the amplitude of the current pulses is regulated. The current pulse source is supplied with an unregulated voltage, which of course must be greater than the output voltage. The voltage difference between the unregulated input voltage and the output voltage of the current pulse source multiplied by the current is converted into heat by the controllable electronic component. This means that not only do considerable energy losses occur, which make an unnecessarily high input power of the device necessary, but also that considerable effort is required for cooling the electronic components which are heated up. Such coolants increase the weight and the structural dimensions of the device.

In the known flow meters, the repetition frequency is 2.0 with which the current pulses for the excitation coil are generated. On the one hand, the pulse repetition rate should be large be to get a short response time of the measuring device and to a large frequency difference between the To receive the useful signal and the low-frequency interference; On the other hand, the repetition frequency must not be made too high, since the cable capacitance at higher frequencies and the eddy current losses lead to errors. The correct choice of the repetition frequency is also dependent on the conductivity depending on the liquid and in particular on the size and distribution of conductive particles in the Liquid.

It is known to use a linear control loop to generate the current for the excitation coil, in which the Electronic component is controlled to the extent that it is in the conductive state that a current of the desired size is generated will. However, this results in considerable heat loss. It is also known to operate two in push-pull to use electronic components, with a positive amplitude value of the excitation current in one half cycle and a reasonable amplitude value is generated in the next half-period.

The invention is based on the object of a circuit arrangement of the type mentioned in such a way,. that the power consumption compared to the known circuits is significantly reduced and that accordingly the effort for dissipating the heat loss is also reduced.

To solve this problem, the invention provides that the electronic component is controllable in pulses, only between a closed and an open state • Switchable electronic switch is that the signal of the current sensor in a comparator with a setpoint signal is compared and that the setpoint signal is a periodically changing pulse signal.

According to the invention, the electronic switch is either in the open or closed state. The current state but is not maintained over a half period of the setpoint signal, but it changes with a relative high frequency, which is significantly higher than that of the low-frequency setpoint signal. The excitation coil, the the voltage switched by the electronic switch is supplied, causes a smoothing and middle word b.il-

generation of the current. The mean value automatically adjusts itself to the desired setpoint. This is because of this achieves that the response threshold of the comparator down several times during a half period of the setpoint signal and is exceeded upwards, so that the comparator causes frequent switching of the electronic switch. The switching threshold of the comparator changes depending on the setpoint signal that the comparator programmed in a way.

A particular advantage of the circuit arrangement consists in the low power loss. The total energy supplied to the circuit arrangement becomes extremely high Part used for supplying the excitation coil. Since the excitation coil has only low ohmic losses and predominantly is inductive, the total power consumed is very low. The device is therefore also suitable for battery operation. It can even be designed as a portable hand-held device because of its heavy-weight heat dissipation plates are not needed. When the '20 electronic switch is in the conductive state, its resistance is extremely low, so that the heat loss occurring in the electronic switch in this state even with large currents is small. If, on the other hand, the electronic switch is in the open state, only a very small amount of it flows Residual current. In this state, the power loss is also very low. An intermediate stage between the open and the closed state practically does not occur.

The switching frequency with which. the comparator switches

processing of the electronic switch depends essentially on the hysteresis of the comparator. If this hysteresis is large, the switching is less frequent than with low hysteresis. It is also possible to use a comparator that has no switching hysteresis when a delay element is connected between the current sensor and the comparator. This delay element, causes the signal of the: current sensor to be fed to the input of the comparator with a certain delay, so that the switching process in the comparator always starts with a certain delay is carried out. The delay element determines in connection with the data of the excitation coil the frequency with which the electronic switch is switched.

In a preferred embodiment of the invention are two electronic switches controlled in push-pull, which are connected to opposite potentials connected to the excitation coil. In this way, with relatively small absolute values of the amplitudes, a large temporal change of the current value at the excitation coil can be achieved.

According to a further invention is for generating the low-frequency setpoint signal a frequency divider chain is provided at which different pulse frequencies can be tapped. At a such a flow meter is therefore the pulse frequency of the excitation current can be selected. The pulse frequency can be adjusted to the particular application. She · is selected to be relatively high, for example, if the electrode potential, pulsating currents, etc. are low-frequency Interfering signals arise, which then on simple

Can be filtered out with a frequency filter, and it is chosen to be relatively low if cable capacitances or other high-frequency influences are so great that they affect the measurement result at a high pulse frequency would flow. The frequency divider chain is preferably controlled with the mains frequency. This type of control has the advantage that it is very easy to carry out and that it results in a very constant pulse frequency that is synchronized with the mains leads, in which the useful signal 10 superimposed network disturbances can be suppressed in a known manner.

A pulse filter can be provided to generate the setpoint signal be that from a pulse train only lets through the η-th pulse, where η is a natural number, preferably is a power of 2. This makes it possible to take the measurement only at certain points in time, which are temporally Successive intervals in order to reduce the power consumption of the device and thus the remaining power loss adjustable in the transmitter and excitation coil (battery operation, operation at high ambient temperatures). There is also the possibility of several excitation coils of different flow meters from one common To feed circuit arrangement from one after the other staggered in time.

The frequency at which the electronic switch switches is preferably in the audio frequency range, for example on the order of 1 to 20 kHz. It is significantly greater than the frequency of the setpoint signal, which is preferably of the order of 1 Hz to 2 5 Hz. Both Frequencies should differ by at least a factor of 100.

In the following, with reference to the drawings, a Embodiment of the invention explained in more detail.

Show it:

1 shows a schematic block diagram of the circuit arrangement ,

Fig. 2 is a schematic circuit diagram of the circuit for generating the setpoint signals and

3 shows a pulse representation of the voltage and current curves at various points in the circuit according to FIG. 1.

According to Figure 1, the excitation coil 10, which is outside of a (not shown) tube is arranged and generates a magnetic field inside this tube, from an excitation current flowed through, alternately by a first electronic switch 11 or a second electronic Switch 12 is supplied. The electronic switches 11 and 12 are in the present embodiment Complementary switching transistors, the switch 11 being an npn transistor and the switch 12 being a pnp transistor is. The emitters of switches 11 and 12 are connected together and connected to one pole of the excitation coil 10. The bases of the transistors are also connected together. The collector of the switch 11 is with the positive pole and the collector of the switch 12 with connected to the negative pole of a voltage source. Instead of the switching transistors, other controllable semiconductor elements, such as thyristors can be used.

The other pole of the excitation coil 10 is via a current

sensor 13 connected to ground potential. The current sensor consists of a low resistance at which the current flowing through the excitation coil 10 has a voltage drop generated. The voltage at the current sensor 13 is fed via a delay element 14 to the actual value input of a comparator s 15 supplied. The delay element 14 has a Series resistance and a leakage capacitor connected to ground '. By changing the value of the longitudinal resistance the time constant of the delay element can be changed.

The setpoint input of the! Comparator 15 is connected to a Potentiometer 16 supplied setpoint signal that can be tapped off. This setpoint signal, the generation of which will be explained below is a pulse train of alternating positive and negative low-frequency pulses. The comparator 15 has threshold behavior, i.e. it generates a "1" signal, if the amplitude at the actual value input is greater than the amplitude at the setpoint input, and it generates in in all other cases a "0" signal. The output signal of the Comparator 15 controls directly or via other switching means the bases of the electronic switches 11 and 12. As with clocked power supplies, you can also use one here If necessary, do without the power mains transformer and the power source Connect directly to the mains via rectifiers and filter capacitors, with the bases of the power transistors can be controlled via optocouplers.

In FIG. 3, at a, the course of the setpoint signal over time is ■ nals shown. This setpoint signal consists of alternating positive and negative voltages with normalized Amplitude. The frequency of the setpoint signal is 25 Hz.

At b, the time course of the current I through the excitation coil 10 is shown. When the setpoint signal U is positive, the comparator 15 delivers a "1" signal, ■ that placed on the bases of the electronic switches 11 and 12

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will. This opens the switch 11 and closes the switch 12. The positive supply voltage, which is applied to the switch 11, generates an excitation current in the steady state, which is greater than the set one Setpoint. However, this excitation current builds up because of the Inductance of the excitation coil 10 only slowly. if the excitation current has reached the setpoint I, switches with a delay caused by the delay element 14 the comparator 15 um. The switch 11 is then closed and the switch 12 is fully opened. As a result of Storage effect of the excitation coil 10, the excitation current drops slowly to fall below the setpoint I again.

ken. This decrease in the excitation current is shown in Fig. 3 ' · 'Denoted by 17. A short time after falling below the setpoint value I, the comparator 15 switches again

around, whereby the switch 12 is switched back into the open state and the switch 11 into the closed state will. In this way, the excitation current I has the serrated profile shown in FIG. 3. The excitation current oscillates around the setpoint I, which forms the mean value. These spikes are created by the fact that the electronic Switches 11 and 12 are alternately switched between the closed and the open state will.

If the setpoint signal U is then negative, then the same processes take place, with a sawtooth shape An excitation current that changes by a negative setpoint as the mean value arises.

In Fig. 3 the voltage curve U at the exciter magnet is at c 10 shown. As long as the switch 12 is open and the switch T1 is closed, the voltage U increases

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the full positive value. If, on the other hand, switch 12 is closed and switch 11 is opened in phase 17, then the voltage U assumes the full negative value. The mean value of the excitation voltage U during the positive Half-wave of the setpoint signal U assumes a value that is able to produce exactly the desired excitation current.

If the delay time of the delay element 14 is increased, then increase the duration of the edges 17 and 18, so that the frequency with which the electronic Switches 11 and 12 are switched, is reduced. This switching frequency is practically at one. led embodiment at about 10 kHz and accordingly has more than a hundred times the frequency of the setpoint signal U.

In Fig. 2, a circuit for generating the setpoint signal U is shown.

With the mains voltage of 50 Hz, a pulse shaper stage is created 19, which delivers square-wave pulses of alternating positive and negative amplitude. This rectangular sleeve control a counter or a pulse divider chain 20. The outputs of the pulse divider chain 20 are each designated with the divisors 1, 2, 4, 8, by which the input frequency is divided. One of these at a time , 25 outputs can be connected to a line 21, which via a frequency divider stage 22 and an amplifier 23 is connected to the potentiometer 16. The other connection of the potentiometer 16 is connected to ground. The frequency divider stage 22 halves the pulse frequency coming from the line 21 at its input. You develop

also holds a frequency divider chain, the single Levels are designated with 2, 4, 8 and 16 according to the divisor. The pulse frequency at output 8 of the frequency divider occurs, is therefore 1/8 of the input frequency.

The outputs 2, 4, 8, 16 of the pulse divider 22 are via decoupling resistors 24 connected to the four inputs of a NAND gate 25. Each of these entrances is as well connected to ground potential via a normally closed switch 26, 27, 28 or 29. The switches

26 to 29 can be operated manually in such a way that the switch 26 is opened first and then remains open that then also the switch

27 is opened and so on until all switches 26 to 29 are open at the same time.

The NAND gate 25 supplies a "1" signal at its output only when all input signals are "0". Ground potential corresponds to a "0" signal. The output signal of the NAND gate 25 controls via an inverter 3 0 an electronic switch 40, which is shown schematically as a mechanical Switch is shown, and the potentiometer 16 is paraMelgeschaltet and this can short-circuit ..

The pulse divider stage 22 forms, together with the switches 26 to 29, the NAND gate 25 und.dem switch 40, a pulse filter 21, the only one of n-pulses for generating a Soilwertsignales to the potentiometer 16 passes, η is a natural power of 2 and can • thus assume the value 2, 4, 8, 16 ... Are all switches 26 to 29 closed, then the switch 40 remains closed and the potentiometer 16 produces koine

Tension. If only switch 26 is opened, the pulses arrive at output 2 of pulse divider 22 Via the NAND gate 25 and the inverter 30 for control to the switch 40. Since these pulses are half the frequency of the impulses on line 21 ', everyone will second pulse, which comes from amplifier 23, short-circuited by switch 40. Used in addition to the Switch 26 still the switch 27 is open, then only every fourth pulse from line 21 generates one Voltage pulse to potentiometer 16. Are the switches 26, 27 and 28 open together, only every eighth pulse from line 21 generates a pulse on the potentiometer 16 and if all four switches 26 to 29 are open, then only every sixteenth pulse is generated from line 21 a pulse on potentiometer 16.

In this way the frequency of the inductive flow measurement can be changed in order, for example, to save electricity by taking less frequent measurements.

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Claims (6)

ι · # ■ * ■ EXPECTATIONS Circuit arrangement for generating a pulsating magnetic field in the excitation coil of an inductive flow meter, with a current pulse source, which contains at least one controllable electronic component, which the current through the excitation coil as a function by the signal of a current sensor located in the excitation circuit, whereby g.e. that the electronic component is a pulsed controllable, only switchable between a closed and an open state electronic switch (11,12) is that the signal from the current sensor (13) in a switch (11,12) controlling comparator (15) is compared with a setpoint signal (U) and that the setpoint signal is a periodically changing pulse signal. 2. Circuit arrangement according to claim 1, characterized in that that between the current sensor (13) and the comparator (15) a delay element (14) is connected is. 3. Circuit arrangement according to claim 1 or 2, characterized characterized in that two in push-pull controlled electronic switches (11,12) which are connected to opposite Potentials are connected to the excitation coil (10) are connected. -H- 4. Circuit arrangement, in particular according to claim 1, characterized in that for generating the setpoint signal (U) a frequency divider chain (20) is provided on which different pulse frequencies can be tapped. 5. Circuit arrangement according to one of the preceding claims, characterized in that for generation of the setpoint signal (U), a pulse filter (31) is provided, which consists of a pulse train only in each case Lets η-th pulse, where η is a natural number, preferably a power number of 2. 6. Circuit arrangement according to one of the preceding 'claims, characterized in that the frequency, with which the electrical switch (11,12) switches, is in the audio frequency range and is much larger than the frequency of the setpoint signal (U).