Continuously Adaptive Digital Coil Driver for Magnetic Flow Meters

Abstract

A magnetic flowmeter for measuring fluid flow, comprising a flow tube assembly for receiving flow, the flow tube assembly having a coil for receiving a coil current to generate a magnetic field in the fluid, the coil having a first coil winding and a second coil winding. This generates an EMF in the fluid representing the flow. The EMF sensor is arranged to sense the EMF and generate an output related to the flow. A current supply circuit provides a coil current to the first coil winding and the second coil winding of the coil in response to a command signal. A digital control circuit provides a command signal to the current supply circuit according to a control algorithm. In one aspect, the control algorithm adapts to changes in electrical parameters of the coil. A method for implementing a magnetic flowmeter is also provided.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application is based upon and claims the benefit of priority of U.S. Provisional Patent Application No. 62 / 896,120, filed on September 5, 2019, the entire contents of which are incorporated herein by reference. Technical Field

[0003] Embodiments of the present disclosure relate to magnetic flow meters, and more particularly, to techniques for controlling current used to generate a magnetic field for flow measurement. Background Art

[0004] Precise and accurate flow control is critical to a wide range of fluid handling applications, including bulk fluid handling, food and beverage preparation, chemicals and pharmaceuticals, water and air distribution, hydrocarbon extraction and processing, environmental control, and a range of manufacturing technologies that utilize materials such as thermoplastics, films, glues, resins, and other fluids. The flow rate measurement technology used in each specific application depends on the fluids involved and the associated process pressures, temperatures, and flows.

[0005] Exemplary flow measurement technologies include: turbine devices that measure flow based on mechanical rotation; pitot tube sensors and differential pressure devices that measure flow based on the Bernoulli effect or pressure drop across a flow restriction; eddy current and Coriolis devices that measure flow based on vibration effects; and mass flow meters that measure flow based on thermal conductivity. Magnetic flow meters are distinguished from these technologies by characterizing flow based on Faraday's Law, which depends on electromagnetic interactions rather than mechanical or thermodynamic effects. In particular, magnetic flow meters rely on the conductivity of the process fluid and on the electromotive force (EMF) induced when the fluid flows through a magnetic field region.

[0006] Conventional magnetic flowmeters include a sensor portion and a transmitter portion. The transmitter portion includes a coil driver that drives current through a coil of the sensor portion to generate a magnetic field on the pipe portion. The magnetic field induces an EMF or potential difference (voltage) on the flow, which is proportional to the flow rate. The magnetic flowmeter measures the flow rate based on the voltage difference detected by the sensor portion.

[0007] A current supply circuit in the flow meter is used to apply alternating current to the electromagnetic coil. The supply circuit includes an H-bridge transistor circuit having a first switch and a second switch, which couple one of the first coil winding and the second coil winding to a supply conductor. A third switch and a fourth switch of the bridge circuit couple the other of the first coil winding and the second coil winding to the second supply conductor. A control circuit periodically alternates the on and off of the first switch, the second switch, the third switch, and the fourth switch to reverse the polarity of the coil current. Alternating current applied to an inductive load may be difficult to control and may introduce errors in flow measurement. Summary of the invention

[0008] A magnetic flowmeter for measuring fluid flow, comprising a flow tube assembly for receiving flow, the flow tube assembly having a coil for receiving a coil current to generate a magnetic field in the fluid, the coil having a first coil winding and a second coil winding. This generates an EMF in the fluid representing the flow. The EMF sensor is arranged to sense the EMF and generate an output related to the flow. A current supply circuit provides a coil current to the first coil winding and the second coil winding of the coil in response to a command signal. A digital control circuit provides a command signal to the current supply circuit according to a control algorithm. In one aspect, the control algorithm adapts to changes in electrical parameters of the coil. A method for implementing a magnetic flowmeter is also provided.

[0009] This summary is provided to introduce a selection of concepts further described below in the detailed description in a simplified form. This summary is not intended to indicate the key features or essential features of the claimed subject matter, nor is it intended to help determine the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all of the shortcomings noted in the background technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 is a simplified diagram of an exemplary industrial process measurement system according to an embodiment of the present disclosure.

[0011] Figure 2 is a simplified circuit diagram of a prior art coil driver for a magnetic flow meter.

[0012] Figure 3 is a block diagram of a current source for a magnetic flowmeter current driver.

[0013] Figure 4A and Figure 4B is a simplified schematic diagram of a coil drive circuit for driving a coil in a magnetic flow meter using pulse width modulation (PWM).

[0014] Figure 5A and 5Bis a diagram illustrating exemplary control signals from a microcontroller to switches of an H-bridge according to an embodiment of the present disclosure.

[0015] Figure 6 is a block diagram of the adaptive coil drive digital circuit. DETAILED DESCRIPTION

[0016] Embodiments of the present disclosure are described more fully below with reference to the accompanying drawings. Elements identified with the same or similar reference numerals refer to the same or similar elements. However, various embodiments of the present disclosure can be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. On the contrary, these embodiments are provided so that the present disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art.

[0017] Specific details are given in the following description to provide a thorough understanding of the embodiments. However, it should be understood by those of ordinary skill in the art that the embodiments may be practiced without these specific details. For example, circuits, systems, networks, processes, frameworks, supports, connectors, motors, processors, and other components may not be shown or may be shown in block diagram form to avoid obscuring the embodiments in unnecessary detail.

[0018] Magnetic flowmeters are used to measure the flow rate of fluids. Magnetic flowmeters operate based on Faraday's law of electromagnetic induction to measure the flow rate of a conductive liquid through a pipe. In a magnetic flowmeter, a coil is used to apply a magnetic field to the conductive fluid. According to Faraday's law, the movement of the conductive fluid through the magnetic field generates an EMF (voltage) which can be sensed using sensing electrodes. The magnitude of this voltage is proportional to the flow rate of the conductive process fluid.

[0019] In magnetic flowmeters, high frequency current reversal is required to increase the signal to noise ratio (1 / f). The coil driver needs to control the current in a way that minimizes / no overshoot and provides a fast settling time before measuring the electrode voltage. A tight optimal current control loop is critical to improve system performance. Magnetic flow tube coil drivers need to be used with flow tubes of different sizes. Flow tube parameters change with time and temperature. The coil driver system identifies the impedance of the attached flow tube at power-up, continuously monitors any parameter changes and provides optimal current control.

[0020] Figure 1 is a simplified diagram of an exemplary industrial process measurement system 98 according to an embodiment of the present disclosure. The system 98 can be used in the processing of materials (e.g., process media) to transform the material from a less valuable state into a more valuable and useful product, such as petroleum, chemicals, paper, food, etc. For example, the system 98 can be used in a refinery that performs industrial processes that can process crude oil into gasoline, fuel oil, and other petrochemical products.

[0021] System 98 includes a pulsed direct current (DC) magnetic flow meter 100 configured to sense flow of a process fluid flow 101, such as through a pipe or flow tube 102. Magnetic flow meter 100 includes an electromotive force (EMF) sensor 316 (see Figure 4A , Figure 4B ) and flow meter electronics 106. Sensor 316 is generally configured to measure or sense the flow rate of fluid stream 101. Electronics 106 is generally configured to control the applied magnetic field to measure the flow rate, and optionally communicate the measured flow rate to an external computing device 111 (e.g., a computer control unit), which may be located remotely from flow meter 100, such as in a control room 113 of system 98.

[0022] The electronic device 106 can communicate with the external computing device 111 via a suitable process control loop. In some embodiments, the process control loop includes a physical communication link (e.g., a two-wire control loop 115) or a wireless communication link. The communication between the external computing device 111 and the flow meter 100 can be performed on the control loop 115 according to conventional analog and / or digital communication protocols. In some embodiments, the two-wire control loop 115 includes a 4-20 mA control loop, wherein the process variable can be determined by the loop current I flowing through the two-wire control loop 115. L Exemplary digital communication protocols include, for example, The communication standard modulates the digital signal onto the analog current level of the two-wire control loop 115. Other purely digital technologies may also be used, including FieldBus and Profibus communication protocols. Exemplary wireless versions of process control loops include, for example, wireless mesh network protocols (e.g., (IEC 62591) or ISA 100.11a (IEC 62734)), or another wireless communication protocol (e.g., WiFi, LoRa, Sigfox, BLE), or any other suitable protocol.

[0023] The magnetic flow meter 100 may be powered from any suitable power source. For example, the magnetic flow meter 100 may be powered entirely by the loop current I flowing through the control loop 115. L Power. One or more power sources may also be used to power the process magnetic flow meter 100, such as internal or external batteries. A generator (e.g., a solar panel, a wind turbine, etc.) may also be used to power the magnetic flow meter 100 or to charge a power source used by the magnetic flow meter 100.

[0024] Figure 21 is a simplified block diagram of a coil driver circuit 120. The flow tube 102 includes a coil 104, which is electrically connected to the driver circuit 120. The driver circuit 120 includes an analog current source 108 connected to an H-bridge circuit 110. The H-bridge circuit 110 includes four switches (typically transistors) driven by a polarity command signal source 112. The output of the signal source is fed through a switch driver 114 so that at any given time, the current along Figure 2 The direction indicated by the arrow or the opposite direction is applied to the coil 104.

[0025] In a conventional magnetic flowmeter coil driver, a current source 108 and an H-bridge 110 are used to inject a square wave current into the coil 104 of the flow tube 102. The magnetic flowmeter controls the current set point, and the H-bridge 110 is used to set the polarity (direction) of the current applied to the coil 104. Since one coil drive circuit design can be used to power flow tubes of different sizes, after identifying the impedance of the attached flow tube, the driver circuit 120 can be configured by selecting one of several different control schemes to control the current flowing into the coil 104, which changes the current distribution to compensate for parameter variations between the flow tubes 102. Each control scheme can control the current flowing to a group of flow tubes. However, the control scheme may not be optimized for any given flow tube. In addition, when the current is reversed, the current source 108 may become unstable due to the freewheeling current in the induction coil 104. During this period, the operating voltage range of the controller may be exceeded until the coil driver recovers. In addition, the coil driver circuit 120 may not work with the newly designed flow tube configuration, so the circuit needs to be redesigned.

[0026] Figure 3 The configuration of a typical conventional current source 108 used in a magnetic flow meter coil driver for generating a desired current profile is shown. The control scheme is selected based on knowledge of the flow tube impedance. Figure 3 , controller 200 uses corresponding switches 208-1, 208-2, 208-3...208-N to select the desired current control scheme 202-1, 202-2, 202-3...202-N. The control scheme can be implemented as an analog circuit. A current set point 204 is used, and the current is applied to the coil of the flow tube 102 through a regulator 206. A feedback current is applied to the control scheme for use by the selected control algorithm. The specific control scheme can be based on known control algorithm techniques. For example, a PID (proportional integral derivative) algorithm can be used, including those using feedback and / or feedforward signals. In general, a good controller is one that minimizes response time and / or tracks the command signal as closely as possible.

[0027] When the flow tube is installed, the specific impedance and the desired current drive characteristics are used to select the appropriate control scheme 202-1 to 202-N. The controller 200 may include a microcontroller or other circuits as desired. However, such configurations are limited to predetermined control schemes. In addition, since the electrical characteristics of the coils of the flow tube 102 change over time, typical conventional control schemes cannot adapt to these changes.

[0028] Unlike conventional coil drivers in which the current through an inductive load is controlled primarily by analog circuits, a fully digital coil driver is provided, such as Figure 4A and Figure 4B shown. Figure 4A A magnetic flow meter 300 using a digital coil driver circuit is shown according to an example embodiment. Figure 4A and Figure 4B Zhongyu Figure 2 Components that are similar to those shown retain their numbering. In contrast to the predetermined fixed control scheme implemented by conventional controller 200 that cannot adapt to changing flow tube parameters, with the digital coil driver described herein, the current distribution can be controlled by controlling the signals used to operate the H-bridge circuit. More specifically, a PWM (pulse width modulation) command input signal can be used to control the switches in the H-bridge.

[0029] like Figure 4A As shown, the H-bridge driver 110 is connected between the bus voltage and the electrical ground and includes four switches 302A, 302B, 302C and 302D, which are, for example, field effect transistors. Switch 302B is complementary to switch 302A, and switch 302D is complementary to switch 302B. The switch driver 114 is shown as four analog driver circuits 304A, 304B, 304C and 304D. Each switch driver 304A to 304D receives a PWM command from a microcontroller 308. Figure 4B A slightly different configuration of flow meter 300 is shown, in which switch drivers 304A and 304C are anti-phase drivers. Figure 4A This configuration is in contrast to the configuration of , which allows the microcontroller 308 to apply two PWM command signals.

[0030] The H-bridge 110 is configured to receive unfiltered current from a bus voltage or power supply. The microcontroller 308 controls the switch pairs 302A and 302C and 302B and 302D to generate high frequency (e.g., 10-100 kHz) current pulses from the unfiltered current, which are transmitted to a low pass filter (LPF) 312 via conductors 320 and 322. The low pass filter (LPF) 312 operates to attenuate the high frequency current pulses output from the H-bridge 110 on conductors 320A and 322A to form low frequency (e.g., 5-100 Hz) coil current pulses on corresponding lines 320B and 322B, which form the coil current transmitted through the coil 104.

[0031] The microcontroller 308 controls the direction of the filtered coil current flowing through the coil 104 by modulating the duty cycle of the switch 302 . Figure 5A and Figure 5B 312 is a diagram showing exemplary control signals from microcontroller 308 to switch 302 that cause coil current to flow in the opposite direction through coil 104. Typically, a series of narrow (short duration, low duty cycle) pulses for switch 302A or switch 302B will result in corresponding short current pulses in line 320A or line 322A. When these high frequency short current pulses pass through LPF 312, a low DC voltage is caused on the corresponding line 320B or line 322B. Similarly, a series of wide (long duration, high duty cycle) pulses will cause a high DC voltage to be applied to the corresponding line 320A or 322A. When these high frequency long current pulses pass through LPF 312, a high DC voltage is caused on the corresponding line 320B or 322B. For example, when switches 302A to 302D are actuated by microcontroller 308, as shown in FIG. Figure 5A As shown, the duty cycle of switch 302A is greater than the duty cycle of switch 302B, and the duty cycle of switch 302C is less than the duty cycle of switch 302D. This makes the average voltage in line 320B greater than the average voltage in line 322B, causing the coil current to flow along Figure 4A When the control signal is Figure 5B As shown in FIG. 1 , the duty cycle of switch 302A is less than the duty cycle of switch 302D. This makes the average voltage in line 322B greater than the average voltage in line 320B, causing the coil current to flow along the same Figure 4A Flow in the opposite direction as shown.

[0032] Thus, by controlling the PWM command signal applied to the H-bridge 110, the microcontroller 308 can control the amplitude, rate of change, and shape of the current signal applied to the coil 104. This technique for regulating the direction and amplitude of the coil current differs from conventional power amplifiers of the magnetic flowmeter 102, which use an H-bridge to simply direct current from a power source through the coil of the flow tube assembly in alternating directions.

[0033] The current sensor 310 is used to sense the current applied to the coil 104. The sensed current provides a feedback signal to an analog-to-digital converter (ADC) of the microcontroller 308. The current sensor 310 may be based on any suitable technology, such as, for example, a voltage drop measured across a series resistor.

[0034] Figure 4A Also shown is an EMF sensor 316 which is electrically coupled to the fluid in the flow tube 102 and which may include, for example, electrodes. A differential amplifier 318 receives the output signal from the EMF sensor 316 and provides an amplified differential signal to an analog-to-digital converter (ADC) of the microcontroller 308. As previously described, the magnitude of the voltage between the EMF sensor 316 is related to the flow rate of the process fluid flowing through the flow tube 102.

[0035] exist Figure 4A , Figure 4B In the digital coil driver configuration of FIG. 3 , the microcontroller 308 uses the PWM command applied to the H-bridge circuit 110 to control the current flowing into the flow tube 102. In addition, the PWM command can be determined based on the current feedback sensed by the current sensor 310, and the control algorithm is implemented digitally in the microcontroller 308. Figure 6 It is shown in Figure 6 is a diagram of a continuously adaptive digital coil driver 400 . Figure 6 The diagram of FIG. 4 shows the microcontroller 308 coupled to a "load" 402, which is the effective load from the coil 104 and other components in the current path, as seen by the microcontroller circuit 308 at the switch driver 114. The current through the load 402 is sensed by a sensor 310 that provides a current feedback 404. The sensed current feedback 404 is converted to a digital value using an analog-to-digital converter 406 and applied to the control algorithm 408 and the impedance identification algorithm 410. The control algorithm 408 operates according to the selected current profile 412 to control the operation of the switches 302A to 302D in the H-bridge 110, which, together with the LPF 312, are connected to the H-bridge 110. Figure 6416. The control algorithm provides a command signal 414 to the PWM 416 and the impedance identification algorithm 410. The impedance identification algorithm 410 determines the impedance of the load 402 by comparing the digitized current feedback 404 with the command signal 414 provided by the control algorithm 408. Based on the impedance identified by the impedance identification algorithm 410, the impedance identification algorithm adjusts the control parameters used by the control algorithm 408 to adapt to the specific impedance provided by the load 402.

[0036] In operation, when power is applied, the load impedance connected to the coil driver may be unknown. In order to identify the electrical characteristics (impedance) of the connected load, the microcontroller 408 first excites the load in an open loop (based on control without feedback) using a small excitation signal (voltage) and measures the current feedback. In the non-ideal case where there is a non-linear relationship between voltage and current (most inductor core materials exhibit hysteresis characteristics, and the impedance of the coil 104 is a function of its operating current), the microcontroller 308 increases the applied voltage and measures the current until the desired operating current set point is reached. The impedance identification algorithm 410 then calculates the "load" impedance using known techniques, after which the microcontroller calculates the optimal parameter setting for the control loop. For example, a step voltage input can be applied to the coil 104. By monitoring the rise time of the resulting current using the current sensor 110, the value of the inductance of the coil 104 and any parasitic resistance can be determined. The microcontroller 308 then closes the current loop using the setting of the parameters initially calculated. The impedance of the flow tube 102 will change over time due to the environment (e.g., the temperature of the flow tube 102), and the impedance identification algorithm 410 continuously measures the impedance of the flow tube while operating in a closed loop. After the new impedance passes through a low pass filter, the microcontroller calculates new optimal control loop parameters for the control scheme that has been implemented. In this way, the magnetic flowmeter coil driver continuously operates at optimal performance. This can reduce or substantially eliminate overshoot and can also provide fast settling time, thereby achieving fast coil frequency response.

[0037] Thus, the microcontroller uses the knowledge of the current feedback to calculate the impedance of the flow tube and derive the optimal tuning parameters (for the applied control law) after power-up. Through continuous impedance measurement, the microcontroller can maintain this optimal performance by adjusting the setting of the parameters as the parameters (impedance) of the flow tube change due to, for example, changing environmental conditions (e.g., temperature) and aging effects. Since the control law is implemented digitally inside the microcontroller, the microcontroller can also decode the appropriate PWM signals or commands and apply them directly to the switching of the H-bridge switches to control the current through the flow tube. In addition, the magnetic flowmeter can be adapted to operate with new flow tubes automatically without the need for a predetermined control scheme as with conventional magnetic flowmeters.

[0038] Although the present invention has been described with reference to preferred embodiments, those skilled in the art will recognize that modifications may be made in form and detail without departing from the spirit and scope of the present invention. This configuration may provide optimal performance for any attached flow tube. In addition, the system adapts to changes in flow tube parameters due to temperature changes and aging. The microcontroller may be implemented in a microcontroller according to any technology, for example. In the magnetic flowmeter described herein, a square (or trapezoidal) current waveform is created by a coil driver. It is expected that the system reaches a steady state as soon as possible after the current / magnetic field transition. In a steady-state magnetic field, the flow tube electrode voltage is sensed and the flow rate is calculated. It is assumed that the magnetic field is known when the current is in a steady state (or shortly thereafter). The higher the frequency of the flow measurement, the lower the "1 / f" noise (pink noise). Therefore, it is desirable to optimize the magnetic field (or current) settling time (or minimize its settling time). In addition, if the current overshoots in the coil, the magnetic field may settle to a different magnetic field strength in some magnetic materials than in the case of no overshoot. Since the magnetic field strength is measured in flow tubes based on the current amplitude, current overshoot can cause measurement errors in some systems. The digital control circuit described in this article can be used to control the current applied to the coil to optimize the above factors.

Claims

1. A magnetic flowmeter for measuring fluid flow, comprising: a flow tube assembly receiving the flow rate, the flow tube assembly having a coil for receiving a coil current, the coil having a first coil winding and a second coil winding, the coil current generating a magnetic field, the magnetic field generating an EMF in the fluid representative of the flow rate; an EMF sensor arranged to sense the EMF and generate an output indicative of the flow rate; a current supply circuit configured to provide the coil current to a first coil winding and a second coil winding of the coil in response to a command input; as well as a digital control circuit that provides the command input to the current supply circuit according to a control algorithm, The command input includes a pulse width modulation (PWM) signal, and the pulse width modulation (PWM) signal controls the direction of the coil current passing through the coil.

2. The magnetic flowmeter according to claim 1, wherein: The current supply circuit includes a current source and at least one switch for selectively coupling the current source to the coil.

3. The magnetic flowmeter according to claim 2, wherein: The PWM signal is applied to the at least one switch.

4. The magnetic flow meter of claim 1 further comprising a low pass filter connecting the current supply circuit to the coil to provide a DC coil current to the coil.

5. The magnetic flowmeter according to claim 1, wherein: The control algorithm is configured to control the command input according to a current profile.

6. The magnetic flowmeter according to claim 5, wherein: The current distribution includes at least one of amplitude, frequency, waveform and overshoot.

7. The magnetic flowmeter according to claim 1, wherein: The digital control circuit includes an impedance identification algorithm configured to determine an impedance of the coil, and the command input is a function of the determined impedance.

8. The magnetic flowmeter according to claim 7, wherein: The impedance identification algorithm receives a current feedback signal related to the current through the coil for determining the impedance of the coil.

9. The magnetic flowmeter according to claim 1, wherein: The current supply circuit includes four switches configured to control current through the coil.

10. The magnetic flowmeter according to claim 9, wherein: The command input includes four signals that are applied individually to each of the four switches.

11. The magnetic flowmeter according to claim 9, wherein: The command input includes two signals applied to two pairs of switches among the four switches.

12. The magnetic flowmeter according to claim 1, wherein: The control algorithm is configured to generate the command input based on a sensed current flowing through the coil, and the digital control circuit includes an impedance identification algorithm, The impedance identification algorithm is configured to identify the impedance of the coil based on the sensed current and the command input and responsively control parameters of the control algorithm.

13. The magnetic flowmeter according to claim 1, wherein: The parameters of the control algorithm adapt to changes in the electrical parameters of the coil.

14. A method for measuring process fluid flow using a magnetic flow meter, comprising: receiving a flow rate of a process fluid through a flow tube assembly having a coil for receiving a coil current, the coil having a first coil winding and a second coil winding, responsively generating a magnetic field and generating an EMF in the fluid representative of the flow rate; sensing the EMF with a sensor and generating an output indicative of flow; In response to a command input, providing the coil current to a first coil winding and a second coil winding of the coil using a current supply circuit; generating said command input comprising a pulse width modulation (PWM) signal; as well as Controlling the direction of the coil current through the coil includes applying the command input to a current supply circuit.

15. The method according to claim 14, wherein: The command input is controlled according to the current profile.

16. The method according to claim 15, wherein: The current distribution includes at least one of amplitude, frequency, waveform and overshoot.

17. The method according to claim 14, wherein: An impedance of the coil is determined using an impedance identification algorithm, and the command input is a function of the determined impedance.

18. The method according to claim 17, wherein: The impedance identification algorithm receives a current feedback signal related to the current through the coil for determining the impedance of the coil.

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