Patent Application
20200217700
The present invention relates generally to the control and operation of sensors for measuring fluids and more particularly to magnetic flowmeters for performing fluid flow measurements.
Magnetic flow meters measure the velocity of conductive fluids passing through pipes by generating a magnetic field and measuring the resultant voltage. These flowmeters rely upon Faraday's Law in which the flow of a conductive fluid through a magnetic field causes a voltage signal which is sensed by electrodes and the sensed voltage is proportional to the fluid's velocity.
Magnetic flowmeter assemblies are generally either insertion mag-meters or full bore mag-meters. Insertion mag-meters typically include a sensor body which is placed into a fluid flow and electrodes which are disposed at the distal end of the sensor body. The sensor can include conductive coils that generate a magnetic field which, in combination with the fluid flow creates an electromotive force (voltage), which is then sensed by the electrodes. Full bore mag-meters typically include a tubular body disposed inline along a fluid conduit, with electrodes molded into the walls of the body and conductive coils disposed on the tubular body that generates a magnetic field across the body.
Although these flowmeters are generally effective, shortfalls exist. For example, an important feature of current magnetic flowmeters is the meter's “cutoff flow” value. The “cutoff flow” value represents the lowest fluid velocity that a magnetic flowmeter can measure. The closer the cutoff flow value is to zero, the greater the utility and value of the magnetic flowmeter. However, current magnetic flowmeters suffer from various measurement errors that prevent the accurate measurement of fluid velocities that approach zero. Moreover, by measuring and tracking these measurement errors, important information regarding the fluid and measurement process can be obtained.
It should, therefore, be appreciated that there remains a need for a magnetic flowmeter assembly that addresses these concerns. The present invention fulfills these needs and others.
Briefly and in general terms, the present invention provides a system and related method for operating a magnetic flowmeter. The flowmeter measures the velocity of a conductive fluid in a flow path by driving first and second coil assemblies with an alternating current which generates an alternating magnetic field. During the change of the drive currents polarity, the current is set to zero for a specified duration which makes the magnetic field B equal zero (see
The system comprises a first coil driver for providing a first current through a first coil assembly, a second coil driver for providing a second current through a second coil assembly and a corresponding sensor for measuring an electrical property associated with the first and the second coil assemblies respectively. The system is further configured to provide feedback to a first and second voltage regulator in response to the measured electrical property. As such, the first and the second voltage regulators can modify the current through the first and the second coil assemblies in response to the feedback signal, and thereby enable a prescribed magnetic field to be created within the fluid flow path.
More specifically, by example and not limitation, the system includes a tubular body with opposing ends configured to enable fluid flow therebetween. A pair of measuring electrodes is attached to the tubular body to measure the voltage induced by a conductive fluid flowing through the magnetic field created by the coil assemblies. By using the measured voltage, the fluid velocity and flow rate can be calculated by using a processor electrically coupled to the measuring electrodes.
In a detailed aspect of an exemplary embodiment, the system comprises an instrumentation amplifier for scaling the electrical property measured by the corresponding sensor, an analog-to-digital converter for converting the electrical property, and an error amplifier for detecting a difference between the measured electrical property and a targeted (desired) electrical property. Based on the detected error, a proportional-integral-derivative (PID) controller provides a pulse-width-modulator (PWM) with an output for minimizing the error detected by using a system tuning process. The PWM output is filtered, buffered and summed into a control feedback for the voltage regulators, which then controls the voltage output, and thus current, passing through a coil driver and the respective coil assembly.
In another detailed aspect of an exemplary embodiment, the coil drivers are configured to alternate the direction of the current passing through each coil assembly, wherein one cycle in a given time period represents the current flowing in a first direction through each coil assembly, followed by flowing in a second direction through each coil assembly.
In yet another detailed aspect of an exemplary embodiment, the corresponding sensor is embodied as a shunt resistor.
For purposes of summarizing the invention and the advantages achieved over the prior art, certain advantages of the invention have been described herein. It is to be understood that not necessarily all such advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example, those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.
All of these embodiments are intended to be within the scope of the invention disclosed herein. These and other embodiments of the present invention will become readily apparent to those skilled in the art from the following detailed description of the preferred embodiments having reference to the attached figures, the invention not being limited to any particular preferred embodiment disclosed.
Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings:
In certain embodiments of the present invention, the magnetic flowmeter assembly can be configured as described and claimed in Applicant's co-pending patent applications, entitled “FULL BORE MAGNETIC FLOWMETER ASSEMBLY,” U.S. application Ser. No. 16/146,090, filed Sep. 28, 2018, and entitled “MAGNETIC FLOWMETER ASSEMBLY HAVING INDEPENDENT COIL DRIVE AND CONTROL SYSTEM,” U.S. application Ser. No. 16/243,868, filed Jan. 9, 2019, which are hereby incorporated by reference for all purposes.
Referring now to the drawings, and in particular
With continued reference to
A housing is coupled externally to the tubular body 12 and is configured to hold at least one processor that is electrically coupled to the electrodes (as seen in
The magnetic flow meter assembly 10 relies upon Faraday's Law of Electromagnetic Induction to measure the velocity of the conductive fluid in the tubular body. Specifically, Faraday's Law states that the voltage induced across any conductor that moves at right angles through a magnetic field is proportional to the velocity of the conductor.
Ue is proportional to v×B×L
Where:
Alternatively, the average fluid velocity v is proportional to
The flow of the conductive liquid through the magnetic field B creates a voltage signal Ue which can be sensed by the pair of measuring electrodes (26) and which in-turn can be used to calculate the velocity v of the conductive fluid. Magnetic flow meters are generally very accurate (e.g., <1% measurement error). As Faraday's equation illustrates, variations in the magnetic field strength B can cause significant errors in the fluid velocity v measurement.
As aforementioned, the magnetic field can be created by a pair of coil assemblies coupled to a magnetic flow assembly. Moreover, it is known that for a Helmholtz coil pair, the magnetic field B is proportional to the current I and the number of loops (e.g., number of turns), and the direction of the magnetic field is determined by the current polarity based on the right-hand rule. It is also known that the greater the magnetic field, the greater the induced voltage as measured by the electrodes.
Accordingly, variations in the magnetic field B strength can introduce significant errors into the fluid flow measurement. These variations can be caused by a number of factors including temperature changes of the coils, differences in the coil's electrical properties, drift of the drive electronics, and dimensional changes of the fluid flow path, among others. For example, the coil resistance Rcoil is a function of the coil's temperature.
R
coil
=R
coil@25° c.
+R
T(T−25° c.)
From the equation we see that the coil resistance Rcoil changes linearly with temperature change, increasing when the temperature increases and decreasing when the temperature decreases.
With reference to
Referring again to
The two coil assemblies (18, 20) are driven by the two independently controlled and operated coil drivers (32, 34) which provide a time-varying drive current (I1, I2). The coil drivers (32, 34) are each electrically coupled to a respective voltage source (39, 41) with a voltage output that is regulated by a respective voltage regulator (52, 54) in real-time based upon feedback from the sensors (36, 38) which measure an electrical property of the coil assemblies. This ensures that a constant drive current (I1, I2) is provided by the coil drivers (32, 34) to the coil assemblies (18, 20) regardless of environmental and physical changes to the coil assemblies. The drive current through the coils can have a frequency range of near zero (0 Hz) to the kilohertz range, and a magnitude generally less than ten (10) amps.
With reference to
As aforementioned, the drive systems (30, 31) each include a pair of coil drivers (32, 34) for driving the first and second coil assemblies (18, 20), sensors which are embodied as shunt resistors (36, 38) for measuring the current driving the coil assemblies, a pair of instrumentation amps (40, 42) for scaling the measured drive current (I1, I2), a pair of A to D converters (ADC) (44, 46) for converting the analog signal to a digital signal, and a pair of proportional-integral-derivative (PID) controllers (48,49) for controlling the output voltage to each coil driver circuit via a pair of pulse width modulators (PWM) (50, 51) and a pair of voltage regulators (52, 54). The voltage regulators (52, 54) regulate the voltage from each respective voltage source (39, 41) to a desired output voltage, which is provided to each coil driver circuit.
The two coil drivers (32, 34) are energy management integrated circuits (IC) which provide an active power pulse output. The coil drivers can be a fully integrated IC or a bridge built using discrete components (i.e. power MOSFETS). In an exemplary embodiment, the coil drivers can be embodied as H bridge drivers with very low resistance and low voltage drop. As such, the coil drivers are capable of alternating the direction of the current passing through each coil assembly, and switching the direction of the magnetic field emitted from each coil. The direction of the drive current is alternated, along with the magnetic field, so as to avoid the electrochemical phenomenon of electrode migration.
As illustrated in
Referring again to
In addition, and/or alternatively, the resistance of the shunt resistors (36, 38) can be manipulated to stabilize the drive current and magnetic field faster. Considering that the drive current is time-varying, obtaining a stable drive current more quickly will stabilize the magnetic field as well (as depicted in
As illustrated in the equation below, current (I) can be defined as a function of voltage (V), resistance (R), inductance (L) and time (t):
I=V/R*[1−e{circumflex over ( )}(−)R/L*t]
The resistance across the coil driver system can be based on the resistance across the shunt resistors (36, 38), and the resistance within each coil assembly (18, 20) (i.e. coil resistance). The inductance is based on the coil assembly's configuration (i.e. coil inductance). Specifically, the coil resistance and coil inductance are unique for each of the coil assemblies (18, 20), and are determined by the number of coil turns and the coil geometry within the respective coil assembly (18, 20). Referring again to the equation for current (I), it is apparent that a larger resistance R will provide a stable current faster. Thus, the resistance of the shunt resistors can be optimized to provide a stable current faster without significantly impacting the magnetic field due to power dissipation losses due to the higher voltage drop.
Referring again to
In an exemplary embodiment, the digital data from the two A/D converters (44, 46) represent the drive current (I1, I2) measured by the shunt resistors (36, 38). An error amplifier (56, 58) identifies the difference between a reference current (Iref) (60,62) and the measured current (I1, I2), and outputs an error signal. The reference current (Iref) is optimized based upon a desire for 1) a strong magnetic field (B) and 2) low power consumption. Higher power is inefficient due to excessive heat generation and other effects. On the other hand, higher power output may be necessary since that the magnetic field strength, as applied to a fluid, decreases as the pipe size increases.
With continued reference to
In an alternate embodiment, a micro-processor receives the digital data from the two A/D converters (44, 46), process the data using instructions stored in the respective memory, and provides the results as an output. Such data processing can include comparing the measured electrical property (I1, I2) against a target electrical property, wherein the processor then computes the PID output, and subsequently outputs a series of commands for the pulse width modulators (50, 51). The micro-processor can be any embedded microcontroller capable of performing the aforementioned tasks. An example of a micro-processer is a Raspberry Pi single board computer developed by the Raspberry Pi Foundation. However, it should be appreciated that current measurements (I1, I2) could be processed by a general-purpose processor (e.g., x86 device), a micro-controller, a gate array, an application specific integrated circuit (ASIC), or other type of computer processor.
Pulse width modulation (PWM) is used to control the current supplied to the two coil drivers (32, 34) via the output from the PID controller (or micro-processor, micro-controller, etc.). The pulse width modulators (50, 51) control the average voltage output, (and thereby regulate the current passing through each respective coil assembly (18, 20), by switching between supply and load rapidly based upon commands received from the PID controllers (48, 49). Each PWM (50, 51) can be configured with a duty cycle ranging from 0 to 100% and frequencies that can range up to kHz, in order to bring the ERROR to zero. Moreover, the use of PWM ensures that the power loss in the switching device is low and that there is minimal current being delivered to the coil assemblies when the switch is “off”.
The PWM signal is subsequently filtered (64, 66) and converted into a DC voltage that is buffered (68, 70), which is then used to control the voltage regulators (52, 54) to output a voltage to the respective coil driver circuit in a manner that enables the current through the coil assemblies to remain constant (or as close as possible) to the targeted current (Iref). As such, a prescribed magnetic field (B) will be generated, overcoming the presence of factors that may influence said magnetic field, and thus enable for a more accurate flow measurement.
The voltage sources (39, 41) coupled to the respective voltage regulators (52, 54) are a dependent or controlled voltage source which maintains a fixed voltage generally independent of the output current. In an alternate embodiment, the voltage sources (39, 41) are controlled in real time based on the control signal received from the respective pulse width modulator (50, 51).
An advantage in using individual coil drivers for each coil assembly is that the required voltage output can be reduced by as much as half as compared to using a single coil driver to pass current through the coil assemblies in series. The Voltage Headroom needed when using individual coil drivers is calculated based on:
V
1=(RCoil1m+RSensor)*I1
Where:
By contrast, a common coil driver with coil assemblies in arranged in series would require a Voltage Headroom based on:
V=(RCoil1+RCoil2+RSensor)*I
As such, a voltage source with a lower output voltage for each coil driver can be used. For example, the voltage required can potentially be lower than that required for a car battery, such as 10.8V.
Referring to
Referring now to
U
E
=U
E1
−U
E2
With reference to
With reference to
With reference now to
With reference now to
Two sensors measure an electrical property in one or both coil assemblies (Step 82) which is then scaled by an instrumentation amplifier, and converted from an analog to digital signal using the A/D converter. Using an error amplifier, the digital signal is compared against a target value (desired). If the digital signal is identical to the target value, (e.g., Iref=I1 and/or I2), then the drive current passing through the coil assemblies are generating the desired magnetic field, and the voltage regulators continue operating with the same output voltage (“No” branch). Alternatively, if an error signal is detected between the target value and digital signal (e.g., Iref=and/or I2), the output voltage is modified (“Yes” branch) (Step 84).
Using a PID controller, a system tuning process is applied (Step 86) to minimize the detected error signal, the tuning process provides an output that efficiently addresses any measured error. The voltage regulators are provided with an output from a PWM that modifies the respective drive voltage (Step 88). As such, the drive currents and corresponding drive voltages are modified in order to generate the desired magnetic field (Step 81) and minimize the error between the actual and target drive currents.
With reference now to
The brace 21 further serves as magnetic circuitry for the magnetic field generated by the coils (18, 20), which conducts the magnetic field going outwards the pipe to be added to the magnetic field going inwards. The brace has a generally octagonal shape, which benefits assembly and operation of the assembly 10. More particularly, the brace 21 is formed of two, generally c-shaped components 29 that slidably mate with each other about the pipe, to couple to each other. In this manner, the brace 21 can be used on pipes having different diameters. Attachments (e.g., bolts) couple the coils to the brace along the axis (Az).
The assembly 10 is configured to generate a strong alternating magnetic field (flux) B that is distributed evenly over the pipe cross-section. Utilizing an alternating magnetic field avoids electrode material migration. Configuration of the brace 21, e.g., including shape and materials, facilitates the resulting magnetic field (flux) B within the pipe 12. In the exemplary embodiment the brace 21 is formed of “soft” magnetic materials, such as soft ferrous material, which refers to relative permeability, meaning it has no remnant magnetization, when shut down. As such, magnetic losses are minimized for the magnetic field going outwards from the pipe and added to the magnetic field going inwards to the pipe.
With reference now to
As noted earlier, magnetic flowmeter relies upon Faraday's law of induction to determine the velocity of conductive fluids. The induced voltage Ue is proportional to the average velocity of the conductive fluid v, the magnetic field strength B and the distance between the sensing electrodes L.
Ue˜v×B×L
The “zero-flow” value is the induced voltage Ue when the fluid velocity is zero. In other words, when the fluid media is stationary within the flowmeter's magnetic field Faraday's law tells us that the induced voltage Ue should be zero when the fluids velocity is zero. However, this is typically not the case and there are many reasons for a Ue not to be zero at a “zero-flow” condition. These include measurement errors, calibration errors, and the fluid media having an electrical potential, among others. By correcting for these measurement errors from a fluid velocity measurement, the measurements will not only be more accurate, but it will be possible to measure lower velocity fluid flows.
The “zero-flow” value can be approximated by (UE1+UE2)/2 where UE1 and UE2 are the induced voltages sensed by the pair of electrical electrodes. This approximation assumes that UE1=UE2 which implies that the coil assemblies are perfectly matched and media conditions are uniform across the flow path. Referring to
Accordingly, a magnetic flowmeter's “zero-flow” value can be monitored and recorded continuously by programming the flowmeter to measure UE when B=0. These “real zero” flow values can then be used to correct for fluid velocity measurement errors, track calibration errors over time, identify process control issues, and/or track the fluid media's electrical potential, among others.
With reference to
U
E correct
=U
E−Zero-flow Value
Referring again to
Finally, it should also be noted that the “zero-flow” measurements can be recorded in computer memory and analyzed to detect trends, process variations, material variations and potential problems as they occur.
The statistical indications of an out of out of control condition may include: a single data point outside the upper control limit (UCL) or lower control limit (LCL), two out of three successive data points beyond the 2 Sigma limitation i.e., (first example), or four out of five successive points beyond the 1 Sigma limitation (i.e., second example). The two illustrative out of control conditions are then investigated to determine if they're indicators of real problems that should be addressed. For example, the large swing in the zero-flow value maybe indicative of a faulty grounding ring, variations in the fluid's electrical potential or differences in the magnetic ring's performance.
The present invention has been described above in terms of presently preferred embodiments so that an understanding of the present invention can be conveyed. However, there are other embodiments not specifically described herein for which the present invention is applicable. Therefore, the present invention should not to be seen as limited to the forms shown, which is to be considered illustrative rather than restrictive.
Although the invention has been disclosed in detail with reference only to the exemplary embodiments, those skilled in the art will appreciate that various other embodiments can be provided without departing from the scope of the invention, to include any and all combination of features discussed herein.
