An excitation circuit and control method for an electromagnetic flowmeter

Abstract

This invention discloses an excitation circuit and its control method for an electromagnetic flowmeter, belonging to the technical field of high-frequency excitation circuits for electromagnetic flowmeters. This invention temporarily increases the voltage through a boost circuit to quickly establish the excitation current, and then reduces the voltage to decrease switching losses; it uses PWM hysteresis control to keep the average current constant; and it uses a fast recovery diode for freewheeling to reduce switching losses; thus solving the problems of high switching losses and high interference under high-frequency excitation.

Description

Technical Field

[0001] This invention relates to high-frequency excitation circuits for electromagnetic flowmeters, and more particularly to an excitation circuit and control method for an electromagnetic flowmeter. Background Technology

[0002] The excitation circuit is an important component of electromagnetic flowmeters. In most existing electromagnetic flowmeters, the excitation waveform is mostly square wave excitation. Square wave excitation involves switching. Moreover, the higher the excitation frequency (which is beneficial for measuring low conductivity and slurry), the higher the drive voltage of the excitation circuit needs to be. This is to charge the excitation coil faster and ensure a longer effective time for the flow signal. This creates a contradiction: to achieve a higher excitation frequency, a higher drive voltage is required, but at the same time, the energy consumption of the switching transistor will increase.

[0003] Currently, two types of excitation circuits are commonly used in the industry. One is the linear constant current excitation circuit, such as the excitation circuit and electromagnetic flowmeter disclosed in Chinese Patent No. CN104169691B. This type of circuit is characterized by low interference but high loss. To increase the excitation frequency, the driving voltage needs to be increased to ensure the effective time of the flow signal, but this drastically increases the energy loss of the switch and the constant current loss. The other type is the excitation circuit for electromagnetic flowmeters disclosed in Chinese Patent No. CN105651348A. Its background technology not only analyzes the shortcomings of the linear constant current excitation circuit, but also proposes a circuit that uses a switching power supply chip for constant current excitation. This circuit is characterized by a significant improvement in loss compared to the linear excitation circuit, but the increase in driving voltage is limited (depending on the input voltage limit of the switching power supply chip), which means that the charging speed of the excitation coil is limited, and it is not suitable for high-frequency excitation. In addition, in the research of low-power excitation circuits, Chinese patent CN104266700A also discloses a low-power high-low voltage power supply switching excitation control system. This circuit mainly achieves low power consumption by switching two voltages, HV and LV, to change the excitation voltage, but this also increases the complexity of the power supply board. At the same time, the technology disclosed in this patent is essentially still linear excitation. Even after the excitation stabilizes and switches to the low voltage LV, the constant current tube Q7 in the current sensing circuit still works in a linear state, and the loss is still relatively large.

[0004] In summary, current excitation circuits suffer from technical problems such as limited charging speed and high energy loss. Summary of the Invention

[0005] The purpose of this invention is to address the aforementioned problems by providing an excitation circuit and control method for an electromagnetic flowmeter. This method stabilizes the excitation current in the excitation coil at a preset current value by switching the transistor on and off. Furthermore, the charging speed of the excitation coil can be controlled by controlling the boost circuit. The freewheeling circuit, composed of fast recovery diodes, reduces the turn-on losses of the transistor.

[0006] This invention temporarily increases the voltage through a boost circuit to quickly establish the excitation current, and then reduces the voltage to reduce switching losses; it uses PWM hysteresis control to keep the average current constant; and it uses a fast recovery diode to reduce the switching losses; thus solving the problems of high switching losses and high interference under high-frequency excitation.

[0007] The technical solution adopted in this invention is as follows: an excitation circuit for an electromagnetic flowmeter, the circuit including the following structure: an excitation coil, a boost circuit, an H-bridge circuit, a freewheeling circuit, a current sampling resistor, a control logic circuit, and a timing controller MCU; The output of the boost circuit is connected to the power input of the H-bridge circuit; The H-bridge circuit includes a first switch, a second switch, a third switch, and a fourth switch. The freewheeling circuit includes a first fast recovery diode, a second fast recovery diode, a third fast recovery diode, and a fourth fast recovery diode; The fast recovery diode in the freewheeling circuit provides a freewheeling path for the excitation coil during the turn-off period of the switching transistor; The current sampling resistor is connected in series with the excitation coil, and its sampled voltage is fed back to the control logic circuit. The control logic circuit outputs a pulse width modulation (PWM) signal to control the switching of the corresponding transistor in the H-bridge circuit based on the comparison result between the sampled voltage and the preset reference voltage, so that the average value of the excitation current is stabilized at the preset current value. The timing controller MCU is connected to the enable terminal of the boost circuit and the control logic circuit respectively. During the rising phase of the excitation current, it enables the boost circuit to output a first voltage to accelerate the charging speed of the excitation coil. After the excitation current reaches a preset value, the boost circuit is disabled to output a second voltage lower than the first voltage to reduce the turn-on loss of the switching transistor.

[0008] Furthermore, the midpoint of the current sampling resistor connected in series with the excitation coil is used as the reference terminal of the control circuit, which is isolated from the reference terminal of the power circuit.

[0009] Furthermore, one end of the first switching transistor and the second switching transistor are connected to the output terminal of the boost circuit; the other end of the first switching transistor is connected to the anode of the first fast recovery diode, the cathode of the third fast recovery diode, and one end of the excitation coil; the cathode of the first fast recovery diode is connected to one end of the third switching transistor; the other end of the second switching transistor is connected to the anode of the second fast recovery diode, the cathode of the fourth fast recovery diode, and one end of the current sampling resistor. The cathode of the second fast recovery diode is connected to one end of the fourth switching transistor; The other end of the third switch, the other end of the fourth switch, the anode of the third fast recovery diode, and the anode of the fourth fast recovery diode are all connected to the reference terminal of the power circuit.

[0010] Furthermore, the control logic circuit includes a comparator circuit or a pulse width modulation circuit. The timing controller MCU selects the positive or negative sampled voltage and sends it to the control logic circuit through an analog switch or direct connection.

[0011] Furthermore, one end of the current sampling resistor is connected to the source of the fourth and third switching transistors, and the other end is connected to the common reference terminal of the high-frequency excitation circuit; the anodes of the third and fourth fast recovery diodes are both connected to the common reference terminal of the high-frequency excitation circuit.

[0012] Furthermore, a method for controlling the excitation circuit of an electromagnetic flowmeter includes the following steps: Step S100: Before the start of positive or negative excitation, the timing controller MCU enables the boost circuit to output the first voltage; Step S200: Charge the excitation coil through the corresponding switch in the H-bridge circuit to make the excitation current rise rapidly; Step S300: The current sampling resistor collects the excitation current in real time, and the sampled voltage is sent to the control logic circuit for comparison with the preset reference voltage; Step S400: When the excitation current reaches the preset value, the timing controller MCU disables the boost circuit, causing its output to drop to the second voltage; Step S500: The control logic circuit controls the switching transistor's on / off state via a comparator circuit or pulse width modulation: when the sampled voltage is higher than the preset reference voltage, the switching transistor is turned off, and the excitation coil freewheels through the fast recovery diode; when the sampled voltage is lower than the preset reference voltage, the switching transistor is turned on again, so that the average value of the excitation current is stabilized at the preset value. Step S600: During the freewheeling period, the freewheeling circuit composed of fast recovery diodes reduces the losses when the switching transistor is turned on next time; Step S700: After one excitation half-cycle ends, switch the conduction direction of the H-bridge circuit and repeat the above steps to achieve reverse excitation.

[0013] Furthermore, the first voltage is higher than the second voltage, and the first voltage is only briefly activated during the rising phase of the excitation current.

[0014] Furthermore, the components are: excitation coil, first fast recovery diode, third switching transistor, fourth fast recovery diode, current sampling resistor, and excitation coil; during negative excitation, the components are: excitation coil, current sampling resistor, second fast recovery diode, fourth switching transistor, third fast recovery diode, and excitation coil.

[0015] Furthermore, the timing controller MCU also controls the analog switch to directly sample the positive voltage of the current sampling resistor during positive excitation and to sample the signal after the voltage of the current sampling resistor is processed by an inverting amplifier during negative excitation.

[0016] Furthermore, the H-bridge circuit adopts either isolated half-bridge drive or non-isolated half-bridge drive, and whether the reference terminal of the control circuit and the reference terminal of the power circuit share a common ground is selected by the circuit's own structure.

[0017] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. During the rising phase of the excitation current, the present invention enables the boost circuit to output a higher voltage, which accelerates the charging speed and allows the excitation current to reach the preset value more quickly, thereby extending the effective sampling time of the flow signal; after the current stabilizes, the boost circuit is disabled and the output voltage drops to a lower value, which significantly reduces the switching loss of the switching transistor under high voltage. 2. During the turn-off period of the switching transistor, the excitation coil freewheels through the fast recovery diode, which has a short reverse recovery time and can significantly reduce the current spike and loss when the switching transistor is turned on again. 3. This invention controls the conduction time of the switching transistor using PWM, so that the average value of the excitation current is stabilized at a preset value, avoiding continuous energy dissipation in the linear constant current scheme and significantly reducing the overall power consumption. 4. The reference terminal of the control circuit and the reference terminal of the power circuit are isolated from each other, and the high-frequency noise of the power circuit is effectively blocked from coupling to the control circuit through the isolation half-bridge drive, thereby improving the signal-to-noise ratio of the measurement signal. Attached Figure Description

[0018] The present invention will be described by way of embodiments and with reference to the accompanying drawings, wherein: Figure 1 This is a schematic diagram of the circuit structure of the present invention; Figure 2 This is a flowchart of the control method of the present invention; Figure 3 This is a schematic diagram of the circuit operation waveforms of the present invention; Figure 4 This is a schematic diagram of the excitation current of the present invention; Figure 5This is a schematic diagram of a non-isolated high-frequency excitation circuit according to an embodiment of the present invention.

[0019] Figure label: 6, boost circuit; i=N, N is a natural number. (i=1,2,3) are the current sampling resistors. U1 is a diode, Q is a switching transistor, U2 and U3 are AND gates, U4 is an isolated half-bridge driver, and U5 is an operational amplifier. The timing signal is L1, the excitation coil is C, the capacitor is UT1, the optocoupler is R4 and R5 are unity-gain amplifiers; AGND and PGND are reference terminals. Detailed Implementation

[0020] In the description of this specification, it should be noted that if terms such as "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," or "outer" appear to indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product is in use, they are only for the convenience of describing this specification and simplifying the description, and do not indicate or imply that the device or component referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this specification.

[0021] Furthermore, the use of terms such as "horizontal" or "vertical" in this specification does not imply that the component must be absolutely horizontal or suspended, but rather that it can be slightly tilted. For example, "horizontal" simply means that its direction is more horizontal relative to "vertical," and does not mean that the structure must be completely horizontal, but can be slightly tilted.

[0022] In the description of this specification, it should also be noted that, unless otherwise expressly specified and limited, the terms “set up,” “install,” “connect,” and “link” should be interpreted broadly. For example, a link can be a fixed link, a detachable link, or an integral link; it can be a mechanical link or an electrical link; it can be a direct link or an indirect link through an intermediate medium; it can be a connection within two components.

[0023] Because the excitation circuit in an electromagnetic flowmeter requires a higher excitation frequency to be suitable for measuring low-conductivity fluids and slurries, a higher driving voltage is needed to accelerate the charging speed of the excitation coil and ensure effective sampling time of the flow signal. However, an increase in driving voltage leads to a sharp increase in the turn-on / loss of the switching transistor, enhanced electromagnetic interference, and increased overall power consumption. Therefore, this invention proposes an excitation circuit and its control method for an electromagnetic flowmeter, which effectively reduces the turn-on loss of the switching transistor, reduces electromagnetic interference, and lowers overall power consumption while achieving high-frequency excitation (to improve the performance of measuring low-conductivity and slurries).

[0024] Example 1 like Figure 1 As shown, one embodiment of the present invention is a low-interference, low-power high-frequency excitation circuit for an electromagnetic flowmeter, the circuit comprising the following structure: Excitation coil L1, boost circuit 6, H-bridge circuit, freewheeling circuit, current sampling resistor R3, control logic circuit and timing controller MCU; The H-bridge circuit includes a first switch Q1, a second switch Q2, a third switch Q3, and a fourth switch Q4; The freewheeling circuit includes a first fast recovery diode D1, a second fast recovery diode D2, a third fast recovery diode D3, and a fourth fast recovery diode D4; The output of the boost circuit is connected to the power input of the H-bridge circuit; The timing controller MCU is connected to the enable terminal of the boost circuit and the control logic circuit respectively. During the rising phase of the excitation current, it enables the boost circuit to output the first voltage V2 to accelerate the charging speed of the excitation coil. After the excitation current reaches the preset value, the boost circuit disables the output of the second voltage V1, which is lower than the first voltage V2, to reduce the turn-on loss of the switching transistor. The current sampling resistor is connected in series with the excitation coil, and its sampled voltage is fed back to the control logic circuit. The control logic circuit outputs a pulse width modulation (PWM) signal to control the switching of the corresponding transistor in the H-bridge circuit based on the comparison result between the sampled voltage and the preset reference voltage VREF, so that the average value of the excitation current is stabilized at the preset current value. The midpoint of the current sampling resistor R3 connected in series with the excitation coil is used as the reference terminal AGND of the control circuit, which is isolated from the reference terminal PGND of the power circuit.

[0025] The fast recovery diode in the freewheeling circuit provides a freewheeling path for the excitation coil during the turn-off period of the switching transistor.

[0026] Its working principle is as follows: The excitation process is divided into two stages: positive excitation and negative excitation. During positive excitation, the current flows from the boost circuit 6 through the second switch Q2, the excitation coil L1, the first fast recovery diode D1, the third switch Q3, and the current sampling resistor R3 to ground. During negative excitation, the current flows in the opposite direction, through the first switch Q1, the excitation coil L1, the second fast recovery diode D2, and the fourth switch Q4 to ground. The voltage across the current sampling resistor R3 (representing the excitation current) is compared with the preset reference voltage VREF. When the current exceeds the set value, the control logic shuts down the switching transistor (such as the second switching transistor Q2) and enters the freewheeling stage. The freewheeling path (such as the excitation coil L1, the first fast recovery diode D1, the third switching transistor Q3, the fourth switching transistor Q4, and the current sampling resistor R3) maintains the current to decrease slowly.

[0027] When the current is lower than the set value, the switching transistor is turned on again to recharge. Through high-frequency on / off control, the average value of the excitation current is stabilized at the preset value.

[0028] The boost voltage is dynamically adjusted. During the current rise phase, the timing controller MCU enables the boost circuit, outputting a higher voltage V2 to accelerate charging. After the current stabilizes, the timing controller MCU disables the boost circuit, reducing the output voltage to V1. This reduces the voltage stress when the switching transistor is turned on and off, thereby reducing switching losses and electromagnetic interference.

[0029] The freewheeling path uses fast recovery diodes (D1-D4), which have short reverse recovery time and can significantly reduce losses when the switching transistor is turned on again.

[0030] The control circuit and power circuit use different reference grounds (AGND / PGND), and the interference coupling from the power circuit to the control circuit is reduced by isolation drive (optocoupler, isolated half bridge drive).

[0031] This embodiment uses dynamic boost control to provide high voltage when fast charging is needed and reduce voltage after the current stabilizes, balancing charging speed with switching losses and electromagnetic interference. It also achieves precise control of the excitation current through sampling feedback.

[0032] Example 2 like Figures 1-4 As shown, based on Embodiment 1, a further feasible implementation method is proposed. Q is a switching transistor, D is a fast recovery diode, R is a current sampling resistor, U1 and U2 are AND gates used to control the on / off enable of the PWM signal; U3 is an isolated half-bridge driver, U4 is an isolated half-bridge driver, and U5 is an operational amplifier.

[0033] The high-frequency excitation circuit consists of a DC power supply VEX, a boost circuit 6, H-bridge switching transistors Q1, Q2, Q3, and Q4, fast recovery diodes D1, D2, D3, and D4 forming the freewheeling circuit of the excitation coil L1, isolated half-bridge drivers U3 and U4, an optocoupler UT1 for isolation control of the boost drive controller, the excitation coil L1, a current sampling resistor R3, an inverting unity-gain amplifier (U5, R4, R5), an analog switch U4, control logic circuitry (which can be a pulse width modulation circuit such as TL494, or a comparator circuit), AND gates U1 and U2, and an MCU for logic control.

[0034] The DC power supply VEX is boosted by the boost circuit 6 to obtain the voltage EX_VDD, which is then connected to the drain of the switching transistors Q1 and Q2. The source of Q1 is connected to the anode of the fast recovery diode D1, the cathode of the fast recovery diode D3, and one end of the excitation coil L1. The cathode of the fast recovery diode D1 is connected to the drain of the switching transistor Q3. The anode of the fast recovery diode D3 and the source of the switching transistor Q3 are then connected together to the power supply reference terminal PGND.

[0035] In the other half of the H-bridge, the source of Q2 is connected to the anode of fast recovery diode D2, the cathode of fast recovery diode D4, and one end of current sampling resistor R3. The cathode of fast recovery diode D2 is connected to the drain of switching transistor Q4. The anode of fast recovery diode D4 and the source of switching transistor Q3 are then connected together to the power supply reference terminal PGND.

[0036] The midpoint of the series connection between the excitation coil L1 and the current sampling resistor R3 is connected to the reference terminal AGND of the control circuit.

[0037] The signal CS1 on the sampling resistor has two paths: one directly enters analog switch U4, and the other enters analog switch U4 via resistors R4 and R5 and operational amplifier U5, which together form a unity-gain inverting amplifier. The output of analog switch U4 is then connected to the control logic circuit. The output of the pulse width modulation circuit or comparator circuit is connected to the input of AND gates U1 and U2. The outputs of AND gates U1 and U2 are connected to the gates of Q1 and Q2, respectively, to control the turn-on time of switching transistors Q1 and Q2.

[0038] like Figure 3 As shown, before reaching the positive excitation period, at time t0, the MCU outputs the timing control signals E3 and E4. The MCU enables the boost drive controller through R2, the isolation optocoupler UT1, and R1, so that before time t1, the DC power supply VEX is boosted to output EX_VDD voltage V2. This is to prepare for the EX_VDD voltage at time t1 to perform a fast charging operation on the excitation coil L1 through the switching transistors Q2 and Q3, so as to reach the preset current value faster and make the excitation current I1 at CS1 on the current sampling resistor R3 reach the preset current value faster.

[0039] At time t2, the MCU outputs a disabling signal for the boost drive controller, causing the DC power supply VEX to be output to EX_VDD through the inductor L2 and fast recovery diode D5 inside the boost circuit 6, so that the EX_VDD voltage drops to V1, thereby reducing the switching losses of the switching transistor Q2 from time t3 to t4.

[0040] During positive excitation: the timing signal E3 turns on the switch Q3 through the isolation half-bridge driver U3, and the control signal E2 turns the switch Q2 on and off through the isolation half-bridge driver U4. The charging current of the excitation coil L1 is collected by the current sampling resistor R3 and reaches the control logic circuit through the analog switch U4 controlled by the MCU. The excitation current I1 is compared with the preset current reference value VREF. When time t2 is reached, the excitation current I1 is greater than the preset current reference value VREF. At time t2, the control logic circuit outputs a signal to turn off Q2.

[0041] When the switching transistor Q2 is turned off, the excitation coil L1 passes through diode D1, switching transistor Q3, diode D4, and current sampling resistor R3 in sequence before returning to the excitation coil L1. When the freewheeling time reaches t3, the excitation current I1 is less than the preset current reference value VREF. The control logic circuit outputs a signal at t3 to turn on Q2.

[0042] Similarly, during the positive excitation period from t3 to t4, the excitation current I1, collected by the current sampling resistor R3, is sent to the control logic circuit via the analog switch U4 controlled by the MCU. The excitation current I1 is compared with the preset current reference value VREF. The comparison result is output to the isolation driver U4 through the AND gate U2 to drive the switching transistor Q2 to turn on and off. The EX_VDD voltage charges the excitation coil L1 through the switching transistors Q2 and Q3, and the excitation coil L1 passes through the fast recovery diode D1, the switching transistor Q3, the fast recovery diode D4, and the current sampling resistor R3 before returning to the excitation coil L1, so that the average value of the excitation current I1 during the positive excitation period reaches the preset current value.

[0043] During negative excitation: Similar to the positive excitation period, except that the timing signal E4 turns on the switch Q4 through the isolated half-bridge driver U4, and the control signal E1 turns the switch Q1 on and off through the isolated half-bridge driver U3. After the charging current of the excitation coil L1 is collected by the current sampling resistor R3, a reverse excitation current is generated at the CS1 terminal. This current needs to pass through the inverting unity-gain amplifier (U5, R4, R5) and then through the analog switch U4 controlled by the MCU to reach the control logic circuit. This is equivalent to comparing the absolute value of the excitation current I1 with the preset current reference value VREF. The result of the comparison by the control logic circuit is output to the isolated driver U3 through the AND gate U1 to drive the switch Q1 on and off. When the switching transistor Q1 is turned off, the excitation coil L1 passes through the current sampling resistor R3, the fast recovery diode D2, the switching transistor Q4, and the fast recovery diode D3 in sequence before returning to the excitation coil L1. When the absolute value of the excitation current I1 is less than the preset current reference value VREF, the control logic circuit outputs a signal to turn on Q1.

[0044] The excitation coil L1 is reverse-charged and freewheeled to make the average value of the absolute value of the excitation current I1 during the negative excitation period reach the preset current value.

[0045] In this case, fast recovery diodes D1 and D4 are used for forward freewheeling of the excitation coil, which can reduce the turn-on loss of Q2. Fast recovery diodes D2 and D3 are used for reverse freewheeling of the excitation coil, which can reduce the turn-on loss of Q1.

[0046] In this embodiment, the control circuit and power circuit need to use different reference terminals (AGND, PGND), so they need to be powered by different power supplies and driven in an isolated manner. This can reduce the interference of the control circuit. At the same time, by controlling the boost circuit to change the charging voltage of the excitation coil within a predetermined time, the charging time of the excitation coil can be reduced, making the effective signal time longer. It can also reduce the turn-on loss of the switching transistor and the interference of the power circuit. Since a switching excitation is used, the power consumption is also relatively low.

[0047] Example 3 like Figure 5 As shown, based on Embodiment 2, a further simplified implementation method is proposed. This embodiment proposes a non-isolated high-frequency excitation circuit.

[0048] The high-frequency excitation circuit consists of a DC power supply VEX, a boost circuit 6, H-bridge switching transistors Q1, Q2, Q3, and Q4, diodes D1, D2, D3, and D4 forming the freewheeling circuit of the excitation coil, non-isolated half-bridge drivers U6 and U7, excitation coil L1, current sampling resistor R6, control logic circuit (which can be a pulse width modulation circuit such as TL494, or a comparator circuit), AND gates U1 and U2, and an MCU for logic control.

[0049] Among them, the DC power supply VEX is boosted by the boost circuit 6 to obtain the voltage EX_VDD and is connected to the drain of the switching transistors Q1 and Q2. The source of Q1 is connected to the anode of diode D1, the cathode of diode D3 and one end of the excitation coil L1. The cathode of diode D1 is connected to the drain of switching transistor Q3. The other half of the H-bridge has the source of Q2 connected to the anode of diode D2, the cathode of diode D4, and the other end of the excitation coil L1, while the cathode of diode D2 is connected to the drain of switching transistor Q4. The sources of switching transistors Q2 and Q3 and one end of the current sampling resistor R6 are connected to CS2, while the other end of diodes D3 and D4 and the current sampling resistor R6 are connected to the circuit reference terminal GND.

[0050] During the forward excitation of the excitation coil L1, the charging current passes through the switching transistor Q2, the excitation coil L1, the diode D1, the switching transistor Q3, and the current sampling resistor R6 to the circuit reference terminal GND; during the forward freewheeling of the excitation coil L1, the freewheeling current passes through the excitation coil L1, the diode D1, the switching transistor Q3, and the current sampling resistor R6 to the circuit reference terminal GND and the diode D4. During the reverse excitation of the excitation coil L1, the charging current passes through the switching transistor Q1, the excitation coil L1, the diode D2, the switching transistor Q4, and the current sampling resistor R6 to the circuit reference terminal GND; during the reverse freewheeling of the excitation coil L1, the freewheeling current passes through the excitation coil L1, the diode D2, the switching transistor Q4, and the current sampling resistor R6 to the circuit reference terminal GND and the diode D3. The above Figure 5 Regardless of whether the current in the excitation coil is positive or negative, the voltage representing the excitation current across the current acquisition resistor R6 is always positive. Figure 4 The excitation current I2 represents the signal acquired by the current sampling resistor R6, and Example 2 is omitted here. Figure 1 The inverted unity-gain amplifier (U5, R4, R5) and analog switch U4 are included.

[0051] The Figure 5 The control reference terminal and power reference terminal are both GND. Then, the isolated half-bridge drivers U3 and U4 are replaced with non-isolated half-bridge drivers U6 and U7. The boost drive controller control signal EN is directly connected to the MCU.

[0052] In this embodiment, the control circuit and power circuit need to use the same reference terminal (GND), so the power supply and all circuits do not need to be isolated and driven. Similar to Embodiment 2, the boost circuit can be controlled to change the charging voltage of the excitation coil within a predetermined time. This can reduce the charging time of the excitation coil, making the effective signal time longer, and also reduce the turn-on loss of the switching transistor and the interference of the power circuit. Since a switching excitation is used and this embodiment does not require isolation devices, the power consumption is further reduced compared to Embodiment 2.

[0053] Example 4 like Figure 2 As shown, this embodiment proposes a low-interference, low-power high-frequency excitation circuit control method for an electromagnetic flowmeter. This method can be applied to the aforementioned Embodiments 2 and 3, and includes the following steps: Step S100: Before the start of positive or negative excitation, the timing controller MCU enables the boost circuit to output the first voltage V2. Step S200: Charge the excitation coil through the corresponding switch in the H-bridge circuit to make the excitation current rise rapidly; Step S300: The current sampling resistor R3 collects the excitation current in real time, and the sampled voltage is sent to the control logic circuit for comparison with the preset reference voltage VREF; Step S400: When the excitation current reaches the preset value, the timing controller MCU disables the boost circuit, causing its output to drop to the second voltage V1; Step S500: The control logic circuit controls the switching transistor's on / off state via a comparator circuit or pulse width modulation: when the sampled voltage is higher than the preset reference voltage VREF, the switching transistor is turned off, and the excitation coil freewheels through the fast recovery diode; when the sampled voltage is lower than the preset reference voltage VREF, the switching transistor is turned on again, so that the average value of the excitation current is stabilized at the preset value. Step S600: During the freewheeling period, the freewheeling circuit composed of fast recovery diodes reduces the losses when the switching transistor is turned on next time; Step S700: After one excitation half-cycle ends, switch the conduction direction of the H-bridge circuit and repeat the above steps to achieve reverse excitation.

[0054] This embodiment uses high-voltage V2 for rapid charging only during the short window of the rising excitation current. Once the current reaches the preset value, it immediately switches to low-voltage V1 for subsequent PWM constant current control. The number of times the switching transistor is turned on and off at high voltage is minimized (only a few times initially or even once per half-cycle), thus significantly reducing turn-on losses. A fast recovery diode is used during the freewheeling phase to reduce current spike losses during the next turn-on, thereby significantly reducing switching losses. Simultaneously, the rapid charging via high-voltage V2 allows the excitation current to reach the preset value in a very short time, thus providing a longer stable freewheeling (effective signal sampling) time within an excitation half-cycle. This is particularly important for measuring low-conductivity fluids and slurries.

[0055] This invention is not limited to the specific embodiments described above. The invention extends to any new feature or combination disclosed in this specification, as well as any new method or process step or combination disclosed herein.

Claims

1. An excitation circuit for an electromagnetic flowmeter, characterized in that, The circuit includes the following structure: excitation coil, boost circuit, H-bridge circuit, freewheeling circuit, current sampling resistor, control logic circuit, and timing controller MCU; The output of the boost circuit is connected to the power input of the H-bridge circuit; The H-bridge circuit includes a first switch, a second switch, a third switch, and a fourth switch. The freewheeling circuit includes a first fast recovery diode, a second fast recovery diode, a third fast recovery diode, and a fourth fast recovery diode; The fast recovery diode in the freewheeling circuit provides a freewheeling path for the excitation coil during the turn-off period of the switching transistor; The current sampling resistor is connected in series with the excitation coil, and its sampled voltage is fed back to the control logic circuit. The control logic circuit outputs a pulse width modulation (PWM) signal to control the switching of the corresponding transistor in the H-bridge circuit based on the comparison result between the sampled voltage and the preset reference voltage, so that the average value of the excitation current is stabilized at the preset current value. The timing controller MCU is connected to the enable terminal of the boost circuit and the control logic circuit respectively. During the rising phase of the excitation current, it enables the boost circuit to output a first voltage to accelerate the charging speed of the excitation coil. After the excitation current reaches a preset value, the boost circuit is disabled to output a second voltage lower than the first voltage to reduce the turn-on loss of the switching transistor.

2. The excitation circuit of an electromagnetic flowmeter according to claim 1, characterized in that: The midpoint of the current sampling resistor connected in series with the excitation coil is used as the reference terminal of the control circuit, which is isolated from the reference terminal of the power circuit.

3. The excitation circuit of an electromagnetic flowmeter according to claim 1, characterized in that: One end of the first and second switching transistors are connected to the output of the boost circuit; the other end of the first switching transistor is connected to the anode of the first fast recovery diode, the cathode of the third fast recovery diode, and one end of the excitation coil; the cathode of the first fast recovery diode is connected to one end of the third switching transistor; the other end of the second switching transistor is connected to the anode of the second fast recovery diode, the cathode of the fourth fast recovery diode, and one end of the current sampling resistor. The cathode of the second fast recovery diode is connected to one end of the fourth switching transistor; The other end of the third switch, the other end of the fourth switch, the anode of the third fast recovery diode, and the anode of the fourth fast recovery diode are all connected to the reference terminal of the power circuit.

4. The excitation circuit of an electromagnetic flowmeter according to claim 1, characterized in that: The control logic circuit includes a comparator circuit or a pulse width modulation circuit. The timing controller MCU selects the positive or negative sampled voltage and sends it to the control logic circuit through an analog switch or direct connection.

5. The excitation circuit of an electromagnetic flowmeter according to claim 1, characterized in that: One end of the current sampling resistor is connected to the source of the fourth and third switching transistors, and the other end is connected to the common reference terminal of the high-frequency excitation circuit; the anodes of the third and fourth fast recovery diodes are both connected to the common reference terminal of the high-frequency excitation circuit.

6. A method for controlling the excitation circuit of an electromagnetic flowmeter, applied to the excitation circuit of an electromagnetic flowmeter according to any one of claims 1 to 5, characterized in that, The method includes the following steps: Step S100: Before the start of positive or negative excitation, the timing controller MCU enables the boost circuit to output the first voltage; Step S200: Charge the excitation coil through the corresponding switch in the H-bridge circuit to make the excitation current rise rapidly; Step S300: The current sampling resistor collects the excitation current in real time, and the sampled voltage is sent to the control logic circuit for comparison with the preset reference voltage; Step S400: When the excitation current reaches the preset value, the timing controller MCU disables the boost circuit, causing its output to drop to the second voltage; Step S500: The control logic circuit controls the switching transistor's on / off state via a comparator circuit or pulse width modulation: when the sampled voltage is higher than the preset reference voltage, the switching transistor is turned off, and the excitation coil freewheels through the fast recovery diode; when the sampled voltage is lower than the preset reference voltage, the switching transistor is turned on again, so that the average value of the excitation current is stabilized at the preset value. Step S600: During the freewheeling period, the freewheeling circuit composed of fast recovery diodes reduces the losses when the switching transistor is turned on next time; Step S700: After one excitation half-cycle ends, switch the conduction direction of the H-bridge circuit and repeat the above steps to achieve reverse excitation.

7. The excitation circuit control method for an electromagnetic flowmeter according to claim 6, characterized in that: The first voltage is higher than the second voltage, and the first voltage is only activated briefly during the rising phase of the excitation current.

8. The excitation circuit control method for an electromagnetic flowmeter according to claim 6, characterized in that: The freewheeling path during positive excitation is: excitation coil, first fast recovery diode, third switch, fourth fast recovery diode, current sampling resistor, and excitation coil; during negative excitation, it is: excitation coil, current sampling resistor, second fast recovery diode, fourth switch, third fast recovery diode, and excitation coil.

9. The excitation circuit control method for an electromagnetic flowmeter according to claim 8, characterized in that: The timing controller MCU also controls the analog switch, directly sampling the positive voltage of the current sampling resistor during positive excitation, and sampling the signal after the voltage of the current sampling resistor is processed by the inverting amplifier during negative excitation.

10. The excitation circuit control method for an electromagnetic flowmeter according to claim 6, characterized in that: The H-bridge circuit can use either isolated half-bridge drive or non-isolated half-bridge drive. Whether the reference terminal of the control circuit and the reference terminal of the power circuit share a common ground is determined by the circuit's own structure.

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