RU2631012C1 - Method of control of fixed electromagnetic flowmeter and electromagnetic flowmeter with self-control function - Google Patents

Abstract

FIELD: measuring equipment.

SUBSTANCE: in addition to measuring the amount of fluid flow, the time of the transient is measured when the current is turned on or off in the inductor and, from that time, the flowmeter is in good working order and there are no external disturbances affecting the accuracy of the measurements. Several variants of ways of monitoring the serviceability of an electromagnetic flowmeter and several variants of an electromagnetic flowmeter implementing the proposed methods are proposed.

EFFECT: increase of reliability of recognition of failures of components of a flowmeter and influence of external interferences on the accuracy of measurements and increase of accuracy of measurements, expansion of a range of measurements and a spectrum of application of the device.

16 cl, 4 dwg

Description

The invention relates to instrumentation, in particular to electromagnetic devices for measuring the flow rate (flow meters) of electrically conductive media.

The known prototype US 9163968 (B2) "Electromagnetic flowmeter with the diagnosis of the signal of the inductor."

A method for diagnosing failures in an electromagnetic flowmeter, which consists in applying current to the winding through the first and second circuit, measuring current through the first and second circuit, and performing diagnostics based on transients of the current when the current switches from the first circuit to the second. This method allows you to identify all kinds of failures in the circuit, including a parallel current circuit, failures of the switching keys and short circuits inside the coil.

Additionally, an asymmetric change in the transient is determined when the current flows from the first circuit to the second and vice versa.

The disadvantages of this method are: it does not provide for the diagnosis of changes in the inductance of the coil associated with the action of an external magnetic field, and when implementing this method for recording the transient, high-speed analog-to-digital converters are required. And also the fact that the transition process actually consists of two: attenuation of the current through the coil in one direction and growth in the other. In the absence of a dead time, these two processes merge in time and it is almost impossible to separate them. This makes it impossible to diagnose some types of failures, for example, related to the asymmetry of magnetic circuits.

Known electromagnetic flowmeter with a dead time pause US 3783687. The purpose of introducing a dead time pause in this invention is to reduce the requirements for key performance.

The disadvantage of this method is that there is no monitoring of health.

Closest to the proposed method and device is the method and device according to RU 2529598 C1 "Electromagnetic flowmeter and method for controlling the measurement of fluid flow".

The essence of the known method is to measure the voltage proportional to the current through the inductor, and the voltage at the inductor and determine the values of active and inductive resistances, deviations of the current values from the preset in the memory of their reference values.

When implementing this method, the current value of the active resistance of the inductor R is calculated by the formula:

where V _{l is the} voltage across the inductor and I is the current through the inductor.

And the inductance (inductance) of the inductor L by the formula:

where dI / dt is the current slew rate immediately after the current is turned on.

Since, under the influence of an external magnetic field, the active and inductive resistance of the inductor change relative to the values obtained during the production of the flowmeter, then, upon recording the presence of changes in the compared values, it is concluded that there is an external magnetic field that distorts the measurement results.

Also, according to RU 2529598, a device is known that implements this method, the essence of which is that the control circuit contains an additional ADC, one input of which is capable of measuring voltage proportional to the current through the inductor, and the second input with the ability to measure voltage proportional to the voltage at the inductor while the output of the additional ADC is connected to the microcontroller with the possibility of transmitting the measured voltage values from each input to the microcontroller.

The disadvantages of this method are: to determine the slew rate of the current in the inductor, the ADC imposes stringent requirements on the time it takes to make the changes. There is also a need to measure the slew rate of the current in the coil, and for this it is necessary to make at least two measurements after a short period of time and noise in adjacent changes has a big impact on accuracy.

The objective of the invention is to identify a malfunction in the magnetic system and the electronic circuit of the electromagnetic flowmeter and the influence of the environment on its performance and accuracy.

The invention is illustrated by drawings:

Figure 1 shows the waveforms of voltage and current during the transient process when the current is turned on to the inductor and when the current supply is turned off and the polarity of the connection of the inductor coil is changed.

T1 - moment of inclusion of the current stabilizer;

T2 is the point in time when the current through the coil reaches a given stabilization current;

T3 - the moment the current stabilizer is turned off and the polarity of the inductor coil is changed;

T4 is the point in time when the current through the coil stops;

ΔТн - time of the transient process when the current is turned on;

ΔТс - time of the transient process when the current is turned off;

U1 is the voltage across the inductor during the transition process when the current is turned on;

U2 is the voltage at the inductor in steady state;

U3 is the voltage across the inductor during the transient process when the current is turned off.

Until time T1, a dead-end pause continues. The voltage on the inductor coil and the current through the coil are zero.

Between time instants T1 and T2, a transient process occurs when the current is turned on.

Between times T2 and T3, the current stabilizer operates in steady state. The magnitude of the current is equal to the specified stabilization current and the voltage is equal to the stabilization current multiplied by the ohmic resistance.

Between times T3 and T4, the transient occurs when the current is turned off. The energy of the magnetic field from the coil is returned to the power circuit. The current through the coil gradually decreases, and the magnitude of the voltage is determined by the supply voltage.

After time T4, a dead time pause begins again.

First, consider the use case when the transient, which judges the health of the flowmeter, is a transient when the current is turned on.

At the same time, it is proposed to use two different criteria for the end of the transient process in voltage and current.

In the first embodiment, the voltage at the inductor is compared with a threshold value that exceeds the steady-state value of the voltage at the inductor, and the transient time is determined by the time the threshold voltage is exceeded.

When the current stabilizer is turned on, the duration of the transition process is determined by the inductance of the coil, the resistance of the coil and the supply voltage. At times T1, the current source is turned on. Until the current through the coil reaches the set stabilization current, the current source supplies the maximum possible voltage U1 determined by the voltage of the power source to the inductor coil.

The current stabilizer contains a circuit for comparing the current with a given one. After reaching the specified current, at time T2, the current stabilizer goes into steady state and the voltage across the inductor decreases to U2. A sharp change in voltage across the inductor coil from U1 to U2 signals the end of the transition process. To measure the duration of this transient, the simplest device is needed - a comparator comparing the voltage across the inductor with the threshold voltage, and a timer. It is also possible to use the fact of achieving a current value with a given stabilization current as a criterion for the end of the transient process. Moreover, in contrast to the method proposed in the prototype, there is no need to measure the current value. It is enough only to compare the current with the threshold.

Consider the second option, when the transient, which judges the health of the flowmeter, is a transient when the current is turned off. At the same time, it is also proposed to use two different criteria for the end of the transient process in voltage and current. The voltage at the inductor is compared with a threshold value that exceeds the steady-state value of the voltage at the inductor, and the transient time is determined by the time the threshold voltage is exceeded. It should be noted that the mechanism for the occurrence of excess voltage on the inductor coil is completely different than when the current is turned on.

At time T3, the current stabilizer turns off and the switch keys change the polarity of the coil to the current source. An inductor coil has an inductance and the current through it cannot change instantly. Since at time T3, the coil is connected in the opposite polarity, during inductive discharge, current from the coil flows to the power source.

In order for this inductive surge to appear, it is not enough just to turn off the current source. It is also necessary to switch the inductor coils to the opposite polarity. The voltage at the inductor during inductive surge U3 is determined by the supply voltage of the current stabilizer circuit. Inductive ejection time is determined by the magnitude of the magnetic induction, the voltage of the inductive ejection and the ohmic resistance of the coil. Inductive release occurs at the beginning of a dead time pause when the current source is turned off, therefore its influence on the time of inductive release is excluded. At time T4, the inductive surge ends.

The waveform according to Figure 1 has steep edges and this makes it easy to measure the transient time when the current ΔТс is turned off.

It is also possible to use the fact of the termination of the passage of current through the coil as a criterion for the end of the transient process. In this case, the transient time is defined as the time the current flow stops the inductor.

Various criteria are also proposed for determining the health of a flowmeter over a transient process.

In one embodiment, during the calibration of the flow meter, it is necessary to remember the time of the transient process as a reference. During operation of the flowmeter, time is measured and in the event of a deviation of the measured time from the reference by more than a certain amount, give an error signal.

In this case, you can use any of the measured transient times: on and off. And you can also use both of these times together.

The supply voltage during operation may deviate from the one at which the calibration was carried out. Accordingly, the duration of the transition process will change in this regard.

Also, the ohmic resistance of the coils can vary widely. For example, the resistance of copper, the most common material for coil windings, is almost proportional to the temperature expressed in degrees Kelvin. This change will also lead to a change in transient time. In order that this does not cause false alarms, an option is proposed for the performance monitoring of the flowmeter. Measuring the voltage across the inductor during the transient during calibration of the flow meter and storing it as a reference. During operation, the expected transition time must be recalculated relative to the reference in accordance with the difference between the voltage and the reference. By what specific formula the recount is made, is not a significant difference. You can use the exact formulas, but you can also approximate, approximating the dependence on voltage.

It is also proposed to measure the voltage across the inductor in steady state to evaluate the ohmic resistance of the coils. Remember this voltage as a reference during calibration of the flow meter and recalculate the expected transient time.

If you do not take into account the differences between the real voltages at the inductor, to exclude false alarms, it is necessary to expand the tolerance to the difference in the transient times. The proposed option will allow to detect errors with better accuracy.

Figure 2 explains the occurrence of asymmetry of magnetic induction in the presence of an external magnetic field. The figure shows the magnetization curve of the magnetic circuit.

Nvn - external magnetic field;

ΔH1; ΔН2 is the magnetomotive force caused by the current flowing through the inductor coil of positive and negative polarity;

ΔB1; ΔВ2 is the change in magnetic induction caused by a current flowing through the inductor coil of positive and negative polarity;

The presence of an external magnetic field Нвн leads to an inequality of changes in the magnetic induction ΔВ1 and ΔВ2.

As can be seen from the figure, in the presence of an external magnetic field, the magnitude of the change in the magnetic induction ΔB1 and ΔB2 will not be equal. Therefore, even if the currents are equal, the magnitude of the magnetic induction will be distorted, which will lead to inequality of transients with positive and negative current polarity.

In addition to the external magnetic field, all kinds of failures can also lead to inequality of transient times for different current directions. In case of failure or degradation of the parameters of the switch keys, the coil currents of the inductor of positive and negative polarity will cease to be equal. Also, when leaks appear on the housing, it is highly unlikely that they will be symmetrical about the different ends of the coil. This will also lead to non-symmetry currents.

All these reasons lead to the same result - the inequality of the magnitude of the magnetic induction with positive and negative polarity of the current through the coil and can be detected in the same way.

Accordingly, an embodiment of a method for monitoring the operability of an electromagnetic flow meter is proposed.

Comparing the magnitude of the transient process with positive and negative polarity of the current through the coil, you can detect all of these defects. Since the magnitude of the supply voltage and the resistance of the coils are the same in both directions of current, there is no need to measure voltages.

Two versions of the method for monitoring the operability of an electromagnetic flowmeter are proposed for the equality of the transient time with a positive and negative direction of current in the inductor. One uses transient time when the current is turned on. In another, they use the transient time after turning off the current supply and changing the polarity of the inductor connection.

An electromagnetic flowmeter is also proposed that implements a check of its health by measuring the transient time by exceeding the voltage at the inductor

Figure 3 shows the functional diagram of the proposed flow meter, which implements the control of its health by measuring the transient time by exceeding the voltage at the inductor.

1 - primary electromagnetic flow transducer, including a pipeline with electrodes;

2 - inductor with coils;

3 - measuring circuit;

4 - stabilizer current supply coils of the inductor;

5 - keys of the switch, providing a change in the direction of the current;

6 - comparator;

7 - timer;

8 - control microcontroller;

9 - ADC.

All work is controlled by a microcontroller. The current stabilizer 4 has a control input that allows you to turn it on and off. During a dead time pause, the stabilizer is turned off. The keys of the switch, providing a change in the direction of the current, is controlled by a microcontroller. The current flowing through the inductor causes a magnetic field. This magnetic field is applied to the measuring pipeline and, according to the Faraday law, an EMF proportional to the flow rate appears on the electrodes located in the measuring pipeline. This EMF goes to the measuring circuit containing the ADC. Based on the measured EMF, the microcontroller calculates the flow rate of the fluid flowing through the measuring pipeline. After completing the EMF measurement at one current polarity, the microcontroller turns off the current stabilizer 4 and at the same time switches the state of the switches 5 to change the polarity of the inductor coil.

The difference from the previously known technical solutions is that the proposed flowmeter contains a comparator 6, which compares the voltage across the inductor (see Fig. 1) when the current is turned on with a certain threshold voltage, the value of which lies between the voltage across the inductor during the transient process U1 and voltage across the inductor in steady state U2. The comparator can be connected to the inductor coils directly or through switch 5, as shown in Fig. 2. This is not a significant difference, as the result will be the same.

The signal from the output of the comparator is fed to a timer 7, which measures the transition process ΔT between the moment the current stabilizer T1 is turned on and the moment when the current through the coil reaches the preset stabilization current T2. Implementation is also possible when timer 7 consists of two timers: one of which tracks the time T1 and the other T2 and the difference ΔT is calculated by the microprocessor. The microcontroller contains a storage device that stores a reference value of the transient time. As the transient time ΔТ, one can use both ΔТн - the transient time when the current is turned on in accordance with Figure 1, and ΔТс - the transient time when the current is turned off. The proposed flow meter can control either any of them, or both at the same time.

There are commercially available microcontrollers containing comparators, timers and ADCs: for example, the MSP430 series manufactured by Texas Instruments.

Also, the flow meter may contain a number of additional devices, such as an indicator, a communication channel in a serial code, frequency and current outputs.

A flowmeter option is also proposed that additionally contains an ADC 9. This ADC will allow you to measure the voltages U1, U2 and U3 and in the microcontroller to recalculate the difference between the expected ΔТ from the reference.

Figure 4 shows the functional diagram of the proposed flow meter that implements the control of its health by measuring the time of the transient current.

1 - primary electromagnetic flow transducer, including a pipeline with electrodes;

2 - inductor with coils;

3 - measuring circuit;

4 - stabilizer current supply coils of the inductor;

5 - keys of the switch, providing a change in the direction of the current;

6 is a diagram for comparing current values;

7 - timer;

8 - control microcontroller;

9 - ADC.

The operation of this flowmeter is completely similar to that described above, except that the flowmeter contains a circuit for comparing current values.

The comparison circuit can be connected anywhere in the current path: between the stabilizer 4 and the keys of the switch 5 as shown in Fig. 3, between the keys of the switch 5 and the common wire or between the power source and the stabilizer 4. This is not a significant difference, since the result will be the same.

In the case when the transient time is measured when the current is turned on, the circuit compares the current through the coils with a given stabilization current and gives a signal to the timer when they are equal. In the case where the transient time is measured when the current is turned off, the circuit compares the current through the coils with zero.

Many microcircuits designed for power sources incorporate a device for comparing the actual output signal with the given one and capable of generating an external signal indicating the end of the transient process. This can be used in the proposed flow meter.

As you know, the principle of operation of an electromagnetic flowmeter is based on the Faraday law and EMF induced on electrodes, and is determined by the magnitude of the magnetic induction, the speed of the medium, and geometric dimensions. To distinguish the EMF induced at the electrodes from the electrochemical EMF, alternating current is used in the inductor coils. To increase the accuracy of the flowmeter, the current passing through the inductor coil is stabilized by a current stabilizer.

The main source of measurement error is the instability of magnetic induction. This explains the need for a simple and reliable way to control the difference in the magnitude of the induction from the one at which the flowmeter was calibrated. Magnetic induction can change for a variety of reasons:

a) failure of the power supply current of the inductor coils, leading to a change in the stabilization current;

b) failure or degradation of the parameters of the switch keys, providing a change in the direction of the current;

c) in the presence of leakage currents parallel to the inductor coils;

d) in the presence of leakage currents from the inductor coils to the device body;

e) inter-turn short circuits inside the coils;

f) the effect of a strong external magnetic field, leading to a change in the parameters of the magnetic circuit.

Failures of parts of an electromagnetic flowmeter including.

A change in magnetic induction also leads to a change in the duration of the transient current in the inductor. Thus, measuring the time of this transient process reveals a wide range of failures of the electromagnetic flowmeter. This eliminates the need for current measurement during the transient process.

It is especially necessary to explain the role of a dead time pause in the implementation of the proposed technical solution. Known electromagnetic flow meters without dead time. In these flowmeters, the transient process when the current is turned off, which is associated with the flux of magnetic induction in one polarity and the transient when the current is turned on in the other polarity merge. In these flow meters there is no way to easily separate them.

In relation to the proposed method, by introducing a dead time, an unexpected technical result is achieved for such an addition: separation of transients when the current is turned off and when turned on in time. By the time the current stabilizer is turned on, the previous transition process has ended and the current in the inductor coils will be zero. This will allow you to measure the duration of these transients separately and diagnose a larger number of faults with greater reliability.

Thus, there are two options for monitoring the health of an electromagnetic flowmeter: in one embodiment, it is proposed to use the transient time when the current is turned on after a dead time. In another embodiment, it is proposed to use the time of the transition process after turning off the current supply and changing the polarity of the inductor after the steady-state operation of the current stabilizer before the start of a dead time.

Claims (16)

1. A method for monitoring the operability of an electromagnetic flowmeter, which consists in exciting a stabilized alternating current with an uninterrupted pause in the inductor and determining by the transient current in the inductor the health of the flowmeter, characterized in that the transient time is used as the basis for determining the health of the flowmeter when the current is turned on. 2. The method according to p. 1, characterized in that the time of the transient process when the current is turned on is defined as the time that the voltage on the inductor exceeds the voltage in the current stabilization mode. 3. The method according to p. 1, characterized in that the time of the transient process when the current is turned on is defined as the time to reach the current through the inductor of the specified stabilization current. 4. The method according to PP. 2, 3, characterized in that the health of the flow meter is determined by the equality of the transition process with a reference value. 5. The method according to p. 4, characterized in that they measure the voltage on the inductor during the transient process and, when the steady state is used, when determining the operability of the flow meter, the differences in the voltage on the inductor from those that were used to determine the reference value are taken into account. 6. The method according to PP. 2, 3, characterized in that the health of the flow meter is determined by the equality of the transient time with positive and negative directions of the current in the inductor. 7. A method for monitoring the operability of an electromagnetic flowmeter, which consists in exciting a stabilized alternating current with an uninterrupted pause in the inductor and determining by the transient current in the inductor the health of the flowmeter, characterized in that the transient time after switching off the current supply is used as the basis for determining the health of the flowmeter and change the polarity of the inductor. 8. The method according to p. 7, characterized in that the time of the transition process is defined as the time that the voltage on the inductor exceeds the voltage in the current stabilization mode. 9. The method according to p. 7, characterized in that the time of the transition process is defined as the time to stop the flow of current through the inductor. 10. The method according to p. 8, 9, characterized in that the health of the flow meter is determined by the equality of the transition process with a reference value. 11. The method according to p. 10, characterized in that they measure the voltage on the inductor during the transition process and when the steady state and when determining the health of the flow meter take into account the differences in the voltage on the inductor from those that were when determining the reference value. 12. The method according to p. 8, 9, characterized in that the health of the flow meter is determined by the equality of the transition process with positive and negative directions of current in the inductor 13. An electromagnetic flowmeter with a self-monitoring function, comprising a primary electromagnetic flow transducer, including an inductor with coils, in the gap of which there is a pipeline with electrodes connected to the measuring circuit, a current stabilizer of the power supply of the inductor coils, controlled by a microcontroller, a switch that provides a change in the direction of the current in the inductor coils controlled by a microcontroller, a microcontroller, characterized in that it contains a voltage comparator, which compares the voltage to and a threshold inductor, the input of which is connected to the inductor, and the output with a timer, the timer output is connected to the microcontroller. 14. An electromagnetic flowmeter with a self-monitoring function according to claim 13, characterized in that it contains an ADC, the input of which is connected to the inductor, and the output is connected to the microcontroller. 15. An electromagnetic flowmeter with a self-monitoring function, comprising a primary electromagnetic flow transducer, including an inductor with coils, in the gap of which there is a pipeline with electrodes connected to the measuring circuit, a power supply current stabilizer for the inductor coils controlled by a microcontroller, a switch that provides a change in the direction of the current in the inductor coils controlled by a microcontroller, a microcontroller, characterized in that it contains a circuit comparing the actual and set current values and you od this comparison circuit connected to the timer, and the timer output is coupled to the microcontroller. 16. An electromagnetic flowmeter with a self-monitoring function according to claim 15, characterized in that it contains an ADC, the input of which is connected to the inductor, and the output is connected to the microcontroller.

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