Electromagnetic flowmeter excitation control circuit and method

Abstract

The invention discloses an electromagnetic flowmeter excitation control circuit and method, and relates to the technical field of electromagnetic flowmeter excitation driving, the electromagnetic flowmeter excitation control circuit comprises a main control MCU, the main control MCU is connected with a dynamic filtering control circuit, and the dynamic filtering control circuit outputs a control signal K1 and a control signal K2; the dynamic filtering control circuit is connected with the excitation driving detection circuit and the feedback reference switching circuit, and the control signal K2 controls the excitation driving detection circuit and the feedback reference switching circuit; the excitation driving detection circuit is connected with the feedback voltage control circuit and the feedback reference switching circuit; and the feedback voltage control circuit adjusts the feedback voltage FB to control the output voltage of the SEPIC excitation power supply output circuit.

Description

Technical Field

[0001] The present invention relates to the technical field of electromagnetic flowmeter excitation drive, and in particular to an electromagnetic flowmeter excitation control circuit and method. Background Art

[0002] Electromagnetic flowmeters require different excitation frequencies when measuring different media. When measuring low conductivity or granular media, it is necessary to increase the excitation frequency and improve the signal-to-noise ratio of the electrode signal. High-frequency excitation requires a higher excitation drive voltage to allow the excitation coil to quickly reach a constant current state, so as to ensure that the constant current state time of the excitation coil and the duty cycle of the excitation pulse width are within an appropriate range. The existing technology mostly uses DC / DC step-down conversion to power the excitation circuit. This method has a large ripple in the excitation coil when the constant current is constant, which affects the stability of the excitation current and interferes with the motor signal. In addition, this method generally controls the DC / DC output by detecting the excitation current threshold or the differential pressure threshold of the constant current drive tube. These thresholds are generally single-pole controls. This single-pole control DC / DC circuit will produce jitter interference at the output end.

[0003] For example, Chinese patent CN108539984B, published on April 14, 2020, provides a PFWM control system for a switching power supply circuit, which controls the output voltage, output current or output power of the switching power supply circuit. The PFWM control system for the switching power supply circuit includes a duty cycle control unit, a frequency control unit and a PFWM signal generation module; the switching power supply circuit includes a Boost conversion circuit and a DC-DC converter; the duty cycle control unit samples the output voltage, output current or output power of the DC-DC converter, and calculates the working duty cycle of the switching device of the switching power supply circuit according to the sampling results; the frequency control unit samples the input voltage or output voltage of the Boost conversion circuit, and calculates the working frequency of the switching device of the switching power supply circuit according to the sampling results; the PFWM signal generation module synthesizes the PFWM drive signal according to the working duty cycle and the working frequency; the PFWM drive signal drives the switching devices of the Boost conversion circuit and the DC-DC converter to control the output voltage, output current or output power provided by the DC-DC converter to the load. This solution has the above problems. Summary of the invention

[0004] The technical problem to be solved by the present invention is that the existing electromagnetic flowmeter excitation power supply cannot well match the working modes of high-frequency measurement and low-frequency measurement. An electromagnetic flowmeter excitation control circuit and method are proposed, which adopt a group of low-voltage power supplies to power the SEPIC lifting power supply, dynamically control the output filter time constant of the SEPIC power supply in different excitation states, sample a smaller filter time constant at the initial stage of excitation to reduce the influence on the excitation constant current duty cycle, adopt a larger output filter time constant during the constant current period to reduce the ripple of the excitation current and improve the measurement accuracy.

[0005] In order to achieve the above technical problems, the technical solution adopted by the present invention is: an electromagnetic flowmeter excitation control circuit, including a main control MCU, the main control MCU is connected to a dynamic filtering control circuit, the dynamic filtering control circuit outputs a control signal K1 and a control signal K2; the dynamic filtering control circuit is connected to an excitation drive detection circuit and a feedback reference switching circuit, the control signal K2 controls the excitation drive detection circuit and the feedback reference switching circuit; the excitation drive detection circuit is connected to a feedback voltage control circuit and the feedback reference switching circuit; the feedback voltage control circuit adjusts the feedback voltage FB to control the output voltage of the SEPIC excitation power supply output circuit.

[0006] An electromagnetic flowmeter excitation control circuit, the main control MCU input terminal detects the constant current state signal C_CS, determines the excitation state and the duty cycle of the constant current, and outputs control signals K1 and K2. The MCU outputs a pulse width modulation signal PWM1 by judging the constant current duty cycle. PWM1 outputs a reference voltage V1 after being demodulated inside the feedback reference switching circuit. At the initial stage of excitation, the control signal K2 connects V1 to the reference REF_VEE of the feedback voltage control circuit. The feedback voltage control circuit adjusts the resistance between the feedback voltage FB and the output voltage VEE of the SEPIC excitation power supply output circuit according to the reference REF_VEE; thereby changing the SEPIC excitation power supply output circuit to output a suitable voltage at the initial stage of excitation, so as to meet the constant current duty cycle with the lowest voltage. During the excitation constant current period, the main control MCU input terminal detects the constant current drive tube high voltage drop alarm signal VDH and the constant current drive tube low voltage drop alarm signal VDL of the excitation drive detection circuit, and outputs the pulse width modulation signal PWM2. After being demodulated inside the feedback reference switching circuit, PWM2 outputs the reference voltage V2. During the excitation constant current period, the control signal K2 connects V2 to the reference REF_VEE of the feedback voltage control circuit. The feedback voltage control circuit adjusts the resistance between the feedback voltage FB and the output voltage VEE of the SEPIC excitation power supply output circuit according to the reference REF_VEE, thereby changing the SEPIC excitation power supply output circuit to output a suitable voltage during the excitation constant current period, so that the excitation constant current drive tube voltage drop is within a suitable range. The control signals K1 and K2 output by the main control MCU control the dynamic filtering control circuit, so that the output filter time constant of the SEPIC excitation power supply output circuit can achieve dynamic filtering at different stages of excitation.

[0007] Preferably, the SEPIC excitation power supply output circuit includes a SEPIC control chip, the SEPIC control chip is connected to the gate of the MOS tube, the drain of the MOS tube is connected to the positive electrode of the diode, a capacitor is arranged between the drain of the MOS tube and the diode, the capacitor is connected in parallel with the transformer, the SEPIC control chip inputs a feedback voltage FB, and the cathode of the diode outputs a working voltage VEE. The internal reference voltage terminal of the SEPIC control chip is connected to the feedback voltage FB, the output terminal of the SEPIC control chip is connected to the gate of the MOS tube, the drain of the MOS tube is connected to the same-name end 1 of the coil of the coupling inductor and one end of the capacitor, the other end 3 of the coupling inductor coil is connected to the supply voltage, the other end of the capacitor is connected to the same-name end 2 of the other coil of the coupling inductor and the positive electrode of the output diode, the other end 4 of the other coil of the coupling inductor is grounded, and the negative electrode of the output diode outputs the excitation voltage VEE, and the output voltage VEE is controlled between 3.3V and 120V by adjusting the input voltage of the feedback input terminal FB of the SEPIC control chip.

[0008] Preferably, the dynamic filtering control circuit includes a MOS transistor Q1 and a MOS transistor Q2, wherein the drain of the MOS transistor Q1 is connected to the negative electrode of the capacitor C1, the drain of the MOS transistor Q2 is connected to the negative electrode of the capacitor C2, the positive electrodes of the capacitors C1 and C2 are connected to the output VEE of the SEPIC excitation power supply with the capacitor CO, the gate of the MOS transistor Q1 is connected to the control circuit K1, and the gate of the MOS transistor Q2 is connected to the control circuit K2. From the initial stage of excitation to the constant current period of excitation, the access of the filter capacitors C1 and C2 is controlled by K1 and K2, and the output filter time constant of the SEPIC excitation power supply is gradually increased to realize dynamic filtering of the excitation voltage VEE, reduce the influence of the filter capacitor on rapid magnetization, and reduce the interference caused by the sudden change of the excitation voltage. At the same time, the charges stored in the high-voltage filter capacitor and the low-voltage filter capacitor are saved by turning off Q1 and Q2, and high and low voltages can be quickly provided for excitation in the next excitation cycle.

[0009] Preferably, the excitation drive detection circuit includes a comparator U2A, which compares the voltage drop of the constant current sampling resistor R_EXC with the reference voltage REF_I and outputs a high-level constant current signal C_CS. The excitation drive detection circuit detects the voltage drop of the constant current sampling resistor R_EXC and compares it with the reference voltage REF_I. When the excitation current reaches the set value, a high-level constant current signal C_CS is output, and C_CS is connected to the main control MCU of the flow meter for excitation state detection.

[0010] Preferably, the excitation drive detection circuit also includes a constant current drive tube voltage drop detection alarm output circuit, and the constant current drive tube voltage drop detection alarm output circuit includes a window comparator composed of a comparator U2B and a comparator U2C, and the window comparator is connected to the differential amplifier circuit through an analog switch S, and the analog switch is controlled by the control signal K2. The differential amplifier circuit inputs a working voltage VEE, the comparator U2B inputs a reference voltage REF_L, and the comparator U2C inputs a reference voltage REF_H, and the comparator U2B outputs a high-level alarm signal VDL, and the comparator U2C outputs a high-level alarm signal VDH. The other side of the excitation drive detection circuit differentially amplifies the excitation voltage VEE and the voltage across the excitation coil. The amplified signal is connected to the window comparator through the analog switch S. The analog switch S is turned on by the flow meter main control MCU output signal K2 to control the analog switch S. The amplified signal amplitude is the difference between the excitation supply voltage VEE and the voltage across the excitation coil during the excitation constant current. The difference is approximately equal to the voltage drop of the constant current drive tube. The voltage drop and the two reference voltages REF_H and REF_L form a window comparator to form a constant current drive tube voltage drop detection alarm output circuit. When the constant current drive tube voltage drop is higher than the set value REF_H, a high-level alarm signal VDH is output, indicating that the power consumption of the constant current drive tube is too large. When the constant current drive tube voltage drop is lower than the set value REF_L, a high-level alarm signal VDL is output, indicating that the constant current drive tube voltage drop is too small and the constant current drive tube adjustment rate is too low. The alarm signals VDH and VDL are connected to the main control MCU of the flow meter, which are used to monitor the voltage drop of the constant current drive tube. The main control MCU of the flow meter adjusts the pulse width of the feedback reference switching circuit PWM2 according to the above alarm signal to make the voltage drop of the constant current drive tube within the designed range. The alarm signal VDH is also connected to the feedback voltage control circuit, which is used to control the output voltage of the SEPIC excitation power supply to protect the constant current drive tube after the pulse width control circuit of PWM2 loses control.

[0011] Preferably, the feedback voltage control circuit includes a comparator U1A, the in-phase end of the comparator U1A is connected to the working voltage VEE after voltage division, the reverse end of the comparator U1A is connected to the feedback reference switching circuit, the output end of the comparator U1A is connected to the positive electrode of the diode D1, the negative electrode of the diode D1 is connected to the feedback voltage output loop, the feedback voltage output loop includes a transistor Q3 and a transistor Q4, the feedback voltage output loop outputs a feedback voltage FB, the negative electrode of the diode D1 is connected to the overvoltage protection circuit, and the overvoltage protection circuit inputs the high level alarm signal VDH. The feedback voltage control circuit is composed of a comparator, an overvoltage protection circuit, and a feedback voltage output circuit. The same-direction end of the comparator is connected to the excitation voltage VEE after voltage division, and the reverse end of the comparator is connected to the reference voltage REF_VEE. REF_VEE is connected to the feedback reference switching circuit. The reference voltage REF_VEE has a higher voltage value at the initial stage of excitation and a lower voltage value during the constant current period of excitation. The output end of the comparator is connected to the feedback voltage output circuit through an isolation diode to control the conduction current of transistors Q4 and Q3 in the feedback voltage output circuit and adjust the value of the feedback voltage FB. FB is connected to SEPI The feedback input terminal FB of the SEPIC control chip of the C excitation power supply output circuit is used. When FB is greater than the reference voltage REF_FB inside the feedback input terminal of the SEPIC control chip, the output voltage of the SEPIC excitation power supply is reduced. When FB is less than the reference voltage REF_FB inside the feedback input terminal of the SEPIC control chip, the output voltage of the SEPIC excitation power supply is increased, thereby controlling the SEPIC excitation power supply output to output different supply voltages in the initial stage of excitation and the constant current period, controlling the constant current excitation duty cycle and the differential pressure of the excitation constant current drive tube within the set range, and matching different sensors and excitation frequencies. The overvoltage protection signal VDH is connected to the excitation drive detection circuit. When the feedback reference REF_VEE control fails and causes the VDH to continuously alarm at a high level, the VDH signal is delayed by RC and followed by the output to turn on the transistors Q4 and Q3 of the feedback voltage output loop, the FB voltage increases, and the output voltage of the SEPIC excitation power supply decreases, thereby protecting the excitation constant current drive tube.

[0012] Preferably, the feedback reference switching circuit includes a first pulse width demodulation circuit and a second wide demodulation circuit, the first pulse width demodulation circuit inputs a pulse width modulation signal PWM1, the second wide demodulation circuit inputs a pulse width modulation signal PWM2, the first pulse width demodulation circuit outputs a voltage V1, the second pulse width demodulation circuit outputs a voltage V2, the first pulse width demodulation circuit and the second pulse width demodulation circuit are connected to an analog control switch S1, and the analog control switch S1 is connected to the feedback voltage control circuit. The main control MCU of the flow meter outputs a pulse width modulation signal PWM1, and PWM1 outputs a voltage V1 after pulse width demodulation. V1 is controlled by the analog switch S1. When the MCU does not detect the excitation constant current signal, the MCU outputs a control signal K2, connects V1 to REF_VEE through S1, and controls the output voltage of the SEPIC excitation power supply at the initial stage of excitation through V1. The main control MCU of the flow meter outputs another pulse width modulation signal PWM2. After pulse width demodulation, PWM2 outputs voltage V2. V2 is controlled by analog switch S1. When MCU detects the excitation constant current signal, MCU outputs control signal K2, connects V2 to REF_VEE through S1, and controls the output voltage of SEPIC excitation power supply during the excitation constant current period through V2, thereby controlling the output voltage of SEPIC excitation power supply in the initial excitation period and the excitation constant current period, and automatically matching various sensors, excitation frequencies, measurement media and measurement methods.

[0013] An electromagnetic flowmeter excitation control includes the following steps: S1: The main control MCU outputs pulse width modulation signals PWM1 and PWM2; S2: The main control MCU outputs the excitation drive signal, and the control circuit starts working; S3: When the excitation current setting value is not reached, the main control MCU controls the feedback reference switching circuit to output the voltage V1. When the excitation current setting value is reached, the main control MCU controls the feedback reference switching circuit to output the voltage V2. S4: When the excitation constant current is applied, the main control MCU calculates the constant current duty cycle, compares the constant current duty cycle with the set value, and adjusts the duty cycle D1 of the pulse width modulation signal PWM1 according to the comparison result; S5: adjusting the pulse width D2 of the pulse width modulation signal PWM2 to obtain the value D2_H of the pulse width D2 when the excitation drive detection circuit outputs the high level alarm signal VDH and the value D2_L of the pulse width D2 when the excitation drive detection circuit outputs the high level alarm signal VDL; S6: Set the final continuous output value of D2 according to D2_H and D2_L.

[0014] S7: During the excitation current rising period and the excitation constant current period, the main control MCU outputs control signals K1 and K2 according to different states of the excitation current, and gradually increases the filter time constant of the SEPIC power supply output.

[0015] The invention discloses an electromagnetic flowmeter excitation control. The main control MCU of the flowmeter adjusts two pulse width modulation signals PWM1 and PWM2 according to the excitation frequency and sensor characteristics, and controls the output voltage of the SEPIC excitation power supply in real time.

[0016] Preferably, the specific method of comparing the constant current duty cycle and the set value in step S4 is as follows: when D is greater than the set value, the duty cycle D1 of the pulse width modulation signal PWM1 is reduced, and when D is less than the set value, the duty cycle D1 of the pulse width modulation signal PWM1 is increased, and the main control MCU saves the duty cycle D1 of the pulse width modulation signal PWM1 when the constant current duty cycle D runs at the set value and maintains the output of D1. Duty cycle D=T3 / T, D set value is 80%, and the main control MCU controls the SEPIC excitation power supply output voltage VH to maintain a constant current duty cycle of 80%.

[0017] Preferably, when the temperature change causes the DC resistance of the excitation coil to change and the voltage of the constant current driving tube changes in the REF_L-REF_H interval, the main control MCU does not adjust the duty cycle D2 of the pulse width modulation signal PWM2; when the tube voltage drop of the constant current driving tube exceeds the REF_L-REF_H interval, the main control MCU adjusts the value of the duty cycle D2 of PWM2 according to the alarm signal to maintain the tube voltage drop of the constant current driving tube in the REF_L-REF_H interval.

[0018] The substantial effect of the present invention is as follows: the present invention adopts the excitation constant current duty cycle to control the output voltage of the SEPIC lifting and lowering output circuit at the initial stage of excitation, and reduces the filtering time constant of the output circuit, samples the upper and lower limit threshold range of the constant current driving tube differential pressure during the excitation constant current period to control the output voltage of the SEPIC lifting and lowering output circuit, and increases the filtering time constant of the output circuit, thereby providing the excitation circuit with efficient and stable driving current, reducing power consumption and interference, having low cost and simple control. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is the SEPIC excitation power supply output circuit of the first embodiment.

[0020] Figure 2 This is the dynamic filtering control circuit of embodiment 1.

[0021] Figure 3 This is the excitation drive detection circuit of embodiment 1.

[0022] Figure 4 This is the feedback voltage control circuit of embodiment 1.

[0023] Figure 5 This is the feedback reference switching circuit of the first embodiment.

[0024] Figure 6This is a schematic diagram of the control waveform of Example 1. DETAILED DESCRIPTION

[0025] The specific implementation of the present invention will be further described below through specific examples in conjunction with the accompanying drawings.

[0026] Embodiment 1: An electromagnetic flowmeter excitation control circuit and method, such as Figure 1 As shown, the SEPIC excitation power supply output circuit is a DC / DC conversion circuit of SEPIC topology structure. The DC / DC of SEPIC topology structure has a simple structure, high efficiency, and can increase or decrease output. The MOS tube is driven by the SEPIC control chip, and the working voltage VEE required by the excitation drive circuit is output after coupling inductance, capacitance and diode. The output voltage VEE is controlled between 3.3V and 120V by adjusting the input voltage of the feedback input terminal FB of the SEPIC control chip. When the excitation output is short-circuited, VEE is controlled to the minimum output of 3.3V to protect the relevant circuits. When the excitation frequency is the highest value, the initial maximum voltage of 120V excitation is output. In this embodiment, the SEPIC control chip is UC2842, the maximum duty cycle is controlled to 100%, the feedback reference voltage REF_FB is 2.5V, and the maximum working voltage is 30V. The supply voltage of the SEPIC excitation power supply output circuit is 24V, which is similar to the constant current drive voltage of most sensors, thereby improving the power supply efficiency.

[0027] like Figure 2As shown, the dynamic filtering control circuit is composed of a high-frequency filter capacitor C0, a high-voltage filter capacitor C1, a low-voltage filter capacitor C2, a grounding control MOS tube Q1 of the high-voltage filter capacitor C1, and a grounding control MOS tube Q2 of the low-voltage filter capacitor C2. C0 in this embodiment is a 2.2uF / 200V capacitor, C1 is a 22uF / 160V electrolytic capacitor, and C2 is a 470uF / 50V electrolytic capacitor. Q1 and Q2 are 200V10A NMOS tubes. The positive ends of the high-frequency filter capacitor C0, the high-voltage filter capacitor C1, and the low-voltage filter capacitor C2 are connected to each other and then connected to the power output VEE of the SEPIC excitation power supply output circuit, and the other end of the high-frequency filter capacitor C0 is grounded. The other end of the high-voltage filter capacitor C1 is connected to the drain of the MOS tube Q1, the source of Q1 is grounded, and the gate of Q1 is connected to the flow meter main control MCU and the control circuit K1. The other end of the low-voltage filter capacitor C2 is connected to the drain of the MOS tube Q2, the source of Q2 is grounded, and the gate of Q2 is connected to the flow meter main control MCU and the control circuit K2. The flowmeter main control MCU and the control circuit are in the period T0 at the beginning of the excitation cycle T, T0 is 10%T, the output control signals K1 and K2 are pulled to the ground, Q1 and Q2 are controlled to be turned off, and the SEPIC excitation power supply output is filtered by the 2.2uF high-frequency filter capacitor C0; the flowmeter main control MCU and the control circuit are in the initial non-constant current period T1 period, the output control signal K1 is 12V, Q1 is controlled to be turned on, the output control signal K2 is pulled to the ground, Q2 is controlled to be turned off, and the SEPIC excitation power supply output is filtered by the 2.2uF high-frequency filter capacitor C0 and the 22uF high-voltage filter capacitor C1; the main control MCU and the control circuit are in the excitation constant current period T3 period, K1 is pulled to the ground, and the 12V control signal K2 is output to control Q1 to be turned off, Q2 to be turned on, and the SEPIC excitation power supply output is filtered by the 2.2uF high-frequency filter capacitor C0 and the 470uF low-voltage filter capacitor C2. Through the above control, the output filter time constant of the SEPIC excitation power supply is gradually increased to achieve dynamic filtering of the excitation voltage VEE. By gradually increasing the output filter capacitor, the effect of the capacitor on the rapid magnetization of the excitation coil in the early stage of excitation is reduced, and the interference caused by the sudden change of the excitation voltage is reduced. During the excitation constant current period, a larger output filter time constant is used to keep the excitation current stable, reduce the effect of DC / DC ripple on the excitation current, and improve the measurement accuracy.

[0028] like Figure 3As shown, the excitation drive detection circuit detects the voltage drop of the constant current sampling resistor R_EXC and compares it with the reference voltage REF_I. When the excitation detection current reaches the set value, a high-level constant current signal C_CS is output. C_CS is connected to the main control MCU of the flow meter for excitation state detection. The set value of the excitation detection current is 99% of the excitation constant current. The constant current sampling resistor R_EXC is 1Ω. When the excitation current is 200mA, the reference voltage REF_I is 1*200*99%=198mV. When the excitation current reaches 198mA, a high-level signal C_CS is output. The main control MCU of the flow meter determines the excitation current in advance. After the main control MCU of the flow meter detects the high-level signal C_CS for a period of time, the main control MCU of the flow meter and the control circuit output a 12V control signal K2.

[0029] The other side of the excitation drive detection circuit differentially amplifies the excitation voltage VEE and the voltage across the excitation coil. When the excitation current reaches 198mA, the flowmeter main control MCU continuously detects the high-level constant current signal C_CS and then outputs the control signal K2 to control the analog switch S to turn on the output. The output signal amplitude is the difference between the excitation supply voltage VEE and the voltage across the excitation coil during the excitation constant current. The difference is approximately equal to the voltage drop of the constant current drive tube. The voltage drop and the two reference voltages REF_H and REF_L form a window comparator to form a constant current drive tube voltage drop detection alarm output circuit. When the constant current drive tube voltage drop is higher than the set value REF_H, a high-level alarm signal VDH is output, indicating that the constant current drive tube power consumption is too large. When the constant current drive tube voltage drop is lower than the set value REF_L, a high-level alarm signal VDL is output, indicating that the constant current drive tube voltage drop is too small and the constant current drive tube adjustment rate is too low. The reference voltage REF_H is 3V. When the voltage drop of the constant current drive tube is higher than 3V, a high-level alarm signal VDH is output, indicating that the power consumption of the constant current drive tube is too large. The reference voltage REF_L is 1V. When the voltage drop of the constant current drive tube is lower than 1V, a high-level alarm signal VDL is output, indicating that the voltage drop of the constant current drive tube is too small and the adjustment rate of the constant current drive tube is too low. The alarm signals VDH and VDL are connected to the main control MCU of the flow meter for monitoring the voltage drop of the constant current drive tube. The main control MCU of the flow meter adjusts the pulse width of WM2 according to the above alarm signal to make the voltage drop of the constant current drive tube in the range of 1V-3V. The alarm signal VDH is also connected to the feedback voltage control circuit, which is used to control the output voltage of the SEPIC excitation power supply to protect the constant current drive tube after the pulse width control circuit of PWM2 loses control.

[0030] like Figure 4As shown, the feedback voltage control circuit consists of a comparator circuit, an overvoltage protection circuit, and a feedback voltage output loop. The comparator circuit consists of resistors R1, R2, a comparator U1A, and an isolation diode D1. The same-direction end of the comparator U1A is connected to one end of the resistor R1 and the resistor R2, the other end of the resistor R1 is connected to the output voltage VEE of the SEPIC excitation power supply output circuit, and the other end of the resistor R2 is grounded. The reverse end of the comparator U1A is connected to the reference voltage REF_VEE, and REF_VEE is connected to the feedback reference switching circuit. The output end of the comparator U1A is connected to the positive electrode of the diode D1. The negative electrode of the diode D1 is connected to one end of the resistor R4 of the feedback voltage output loop. The comparator circuit compares the divided voltage of the output voltage VEE of the SEPIC excitation power supply output circuit with the reference voltage REF_VEE output by the feedback control circuit, and outputs a control signal to the feedback voltage output loop through an isolation diode to control the conduction current of transistors Q3 and Q4 in the feedback voltage output loop, thereby controlling the voltage divided ratio of the SEPIC excitation power supply output voltage and resistor R7 to change the voltage of FB.

[0031] The reference voltage REF_VEE is higher at the initial stage of excitation and lower during the constant current period of excitation. REF_VEE is connected to the reverse end of the comparator U1A. The output voltage VEE of the SEPIC excitation power supply is connected to the same direction end of the comparator after being divided by resistors R1 and R2. The output end of the comparator is connected to the feedback voltage output loop after passing through an isolation diode. When the output voltage VEE of the SEPIC excitation power supply does not reach the set value, VEE is lower than the reference voltage REF_VEE through the voltage divided by resistors R1 and R2, and the comparator outputs a low level. When the output voltage VEE of the SEPIC excitation power supply exceeds the set value, VEE is higher than the reference voltage REF_VEE through the voltage divided by resistors R1 and R2, and the comparator outputs a high level.

[0032] The resistor R1 is 470KΩ, the resistor R2 is 12KΩ, U1A is a common op amp, and the input voltage of the same direction end of U1A is VEE*R2 / (R1+R2). When the output voltage VEE of the SEPIC excitation power supply output circuit is the maximum value of 120V, the input voltage of the same direction end of U1A is about 3V. When the output voltage VEE of the SEPIC excitation power supply output circuit is the minimum value of 3.3V, the input voltage of the same direction end of U1A is about 82mV. The reference voltage REF_VEE output by the feedback control circuit is selected to be between 80mV-3V, and the output of the comparator is controlled. Through the output of the comparator, the output voltage FB of the feedback voltage output loop is controlled, thereby controlling the output voltage of the SEPIC excitation power supply output circuit to be between 3.3V-120V.

[0033] The overvoltage protection is composed of a resistor R3, a capacitor C3, an operational amplifier U1B, and an isolation diode D2. The constant current tube high voltage drop alarm signal VDH output by the excitation drive detection circuit is connected to one end of the resistor R3, and the other end of the resistor R3 is connected to one end of the capacitor C3 and the same direction end of the follower U1B. The output end of the follower U1B is connected to the positive electrode of the isolation diode D2, and the negative electrode of the isolation diode D2 is connected to one end of the input resistor R4 of the feedback voltage output loop.

[0034] The resistor R3 is 3MΩ, the capacitor C3 is 2.2uF, and U1B is an ordinary op amp. When the excitation drive detection circuit outputs the constant current tube high voltage drop alarm signal VDH, the flow meter main control MCU will respond in time and adjust the feedback signal PWM pulse width during constant current to quickly eliminate the alarm. When the flow meter main control MCU and related control circuits are abnormal, the control signal of the output voltage VEE of the SEPIC excitation power supply output circuit is abnormal, and the VEE voltage is too high. The continuous high voltage will damage the constant current control tube and other related devices in the excitation output circuit. For this reason, after the overvoltage protection is delayed by the resistor R3 and the capacitor C3, when the main control signal still cannot eliminate the alarm signal, the constant current tube high voltage drop alarm signal VDH output by the excitation drive detection circuit is delayed by RC and output through the follower to control the output voltage FB of the feedback voltage output circuit, so that the SEPIC excitation power supply output circuit outputs a minimum voltage of 3.3V to protect the excitation drive components.

[0035] The feedback voltage output loop is composed of resistor R4, resistor R5, resistor R6, resistor R7, capacitor C1, NPN transistor Q4, and PNP transistor Q3. One end of resistor R4 is connected to the isolation diode D1 output by the comparator and the negative electrode of the isolation diode D2 output by the overvoltage protection circuit. The other end of resistor R4 is connected to the base of transistor Q4. The emitter of transistor Q4 is grounded. The collector of transistor Q4 is connected to the base of transistor Q3. The emitter of Q3 is connected to one end of resistor R5. The other end of resistor R5 is connected to the SEPIC excitation power supply output voltage VEE together with resistor R6 and one end of compensation capacitor C1. The other ends of resistors R6 and C1 are connected to the collector of Q3, one end of resistor R7, and feedback voltage control FB. The other end of resistor R7 is grounded.

[0036] When the output voltage VEE of the SEPIC excitation power supply is lower than the set value, the comparator U1A and the follower U1B output a low level, Q3 and Q4 are cut off, and the feedback voltage FB is output after the voltage is divided by VEE and R6 and R7. The FB voltage decreases, and FB is connected to the feedback input terminal FB of the SEPIC control chip of the SEPIC excitation power supply output circuit. Since FB is less than the reference voltage REF_FB inside the feedback input terminal of the SEPIC control chip, the output voltage of the SEPIC excitation power supply increases.

[0037] When the output voltage VEE of the SEPIC excitation power supply exceeds the set value, VEE is higher than the reference voltage REF_VEE through the voltage division of resistors R1 and R2, the comparator outputs a high level, Q3 and Q4 are turned on, and the feedback voltage FB is connected in series with R5 and the equivalent on-resistance of Q3 from VEE and then in parallel with R6 as the upper bias. The upper bias is output after the voltage division of R7, and the upper bias is reduced, the FB voltage increases, FB is connected to the feedback input terminal FB of the SEPIC control chip of the SEPIC excitation power supply output circuit, FB is greater than the reference voltage REF_FB inside the feedback input terminal of the SEPIC control chip, the output voltage of the SEPIC excitation power supply decreases, and the on-resistance of the triode Q3 is controlled by the REF_VEE voltage, and the bias resistance value of FB is changed, thereby adjusting the feedback voltage FB and controlling the output voltage VEE of the SEPIC excitation power supply.

[0038] During the excitation constant current period, when the SEPIC excitation power supply output voltage VEE exceeds the set value, the excitation drive detection circuit outputs the alarm signal VDH. The flow meter main control MCU detects the VDH signal and reduces the pulse width of PWM2. When the flow meter main control MCU, comparator and input control fail, the alarm signal VDH cannot be eliminated. After the alarm signal VDH is delayed by R3 and C3, the protection circuit U1B continues to output a high level, Q3 and Q4 continue to be turned on, and the FB voltage increases. FB is connected to the feedback input terminal FB of the SEPIC control chip of the SEPIC excitation power supply output circuit. FB is greater than the reference voltage REF_FB inside the feedback input terminal of the SEPIC control chip. The SEPIC excitation power supply output voltage is as low as 3.3V.

[0039] Through the above control, the voltage divided by resistors R1 and R2 of the SEPIC excitation power supply output is equal to the reference voltage REF_VEE, and the upper bias resistor of the SEPIC excitation power supply feedback voltage is adjusted by the reference voltage REF_VEE, so that the output voltage of the SEPIC excitation power supply is controlled at the set value. In the initial stage of excitation and the constant current period, the main control MCU outputs different duty cycle control signals PWM1 and PWM2 to control the reference voltage REF_VEE, thereby controlling the SEPIC excitation power supply to output different supply voltages in the initial stage of excitation and the constant current period, controlling the constant current excitation duty cycle and the differential pressure of the excitation constant current drive tube within the set range, and matching different sensors and excitation frequencies.

[0040] The resistor R4 is 1KΩ, the resistor R5 is 3.3KΩ, the resistor R6 is 470KΩ, the resistor R7 is 10KΩ, C1 is 220pF, Q4 is an NPN transistor, and Q3 is a PNP transistor. The highest voltage of REF_VEE is 3V, and the lowest voltage is 0.082V. The reference voltage REF_FB fed back by the SEPIC control chip UC2842 is 2.5V. The output voltage of the SEPIC excitation power supply is: The maximum voltage output by the SEPIC excitation power supply is: (R6+R7)*REF_FB / R7≈120V The minimum voltage of SEPIC excitation power supply output is: (R5*R6 / (R5+R6)+R7)*REF_FB / R7≈3.3V The real-time value of SEPIC excitation power supply output is: (R1+R2)*REF_VEE / R2 When the maximum voltage of REF_VEE is 3V, SEPIC excitation power supply output is: (R1+R2)*REF_VEE / R2≈120V When the REF_VEE maximum voltage is 0.082, SEPIC excitation power supply output is: (R1+R2)*REF_VEE / R2≈3.3V like Figure 5 As shown, the feedback reference switching circuit is composed of two pulse width demodulation circuits and an analog switch. The main control MCU of the flow meter outputs a pulse width modulation signal PWM1. After pulse width demodulation, PWM1 outputs a voltage V1. V1 is controlled by an analog switch S1. When the MCU does not detect the excitation constant current signal, the MCU outputs a control signal K2, connects V1 to REF_VEE through S1, and controls the output voltage of the SEPIC excitation power supply in the initial stage of excitation through V1. The main control MCU of the flow meter outputs another pulse width modulation signal PWM2. After pulse width demodulation, PWM2 outputs a voltage V2. V2 is controlled by an analog switch S1. When the MCU detects the excitation constant current signal, the MCU outputs a control signal K2, connects V2 to REF_VEE through S1, and controls the output voltage of the SEPIC excitation power supply in the excitation constant current period through V2. The pulse width demodulation circuit is composed of a Schmitt trigger and a follower, and S1 is an ordinary analog switch. The flow meter main control MCU controls the pulse width of the two pulse width modulation signals PWM1 and PWM2 to control the output voltage of the SEPIC excitation power supply at the initial stage of excitation and the constant current period of excitation, and can automatically match various sensors, excitation frequencies, measurement media and measurement methods. Figure 6 shown.

[0041] The flow meter main control MCU controls the output voltage of the SEPIC excitation power supply: 1. When the power is turned on, the flow meter main control MCU outputs PWM1 and PWM2 pulse width modulation signals. The duty cycle D1 of the PWM1 pulse width modulation signal is 75%. After pulse width demodulation, it outputs a voltage reference V1 of about 2.5V. The duty cycle D2 of the PWM2 pulse width modulation signal is 2.5%. After pulse width demodulation, it outputs a voltage reference V2 of about 82mV.

[0042] 2. The flow meter main control MCU outputs the excitation drive signal, and the excitation drive circuit starts working.

[0043] 3. At the initial stage of excitation, the flow meter main control MCU does not detect the high-level constant current signal C_CS during the excitation T2 period. The flow meter main control MCU outputs a low-level signal K2, controls the analog switch S1 to connect REF_VEE to V1, and connects the 2.5V reference voltage V1 to the feedback voltage control circuit. The output voltage of the SEPIC excitation power supply is 100V at the initial working voltage of excitation.

[0044] 4. When the excitation current reaches 99% of the set value, the excitation drive detection circuit outputs a high-level signal C_CS. After the flow meter main control MCU detects the high-level trigger signal C_CS, it continues to output a high-level signal K2 until the end of the excitation cycle T. During this period, K2 controls the analog switch S1 to connect REF_VEE to V2, and connects the 82mV reference voltage V2 to the feedback voltage control circuit. The SEPIC excitation power supply outputs 3.3V to prevent the flow meter main control MCU from outputting too high a SEPIC excitation power supply when detecting the constant current duty cycle.

[0045] 5. From the excitation cycle T to the triggering of the constant current signal C_CS, during the period T3, the flow meter main control MCU calculates the constant current duty cycle D=T3 / T according to the excitation cycle, and the flow meter main control MCU makes a judgment based on the set value of D. Preferably, the set value of D is 80%. When D is greater than 80%, the duty cycle D1 of PWM1 is reduced. When D is less than the set value, the duty cycle D1 of PWM1 is increased. The main control MCU saves the duty cycle D1 of the PWM1 control signal when the constant current duty cycle D is running at the set value and keeps the output of D1. Maintain a constant current duty cycle of 80%.

[0046] 6. Continue to increase the pulse width D2 of PWM2. When the excitation drive detection circuit outputs a high-level alarm signal VDH, save the current value D2_H of the pulse width D2 of PWM2. At this time, the voltage drop of the constant current tube of the excitation drive circuit is about 3V.

[0047] 7. The pulse width D2 of PWM2 is gradually reduced from D2_H until the excitation drive detection circuit outputs a high-level alarm signal VDL, and the current pulse width D2 of PWM2 is saved as D2_L. At this time, the voltage drop of the constant current tube of the excitation drive circuit is about 1V.

[0048] 8. Set the pulse width D2 of PWM2 to (D2_H+D2_L) / 2, and keep D2 output. At this time, the voltage drop of the constant current tube of the excitation drive circuit is about 2V.

[0049] 9. When temperature changes cause the DC resistance of the excitation coil to change, and the voltage of the constant current drive tube changes in the range of 1V-3V, the flow meter's main control MCU does not adjust the duty cycle D2 of PWM2 to avoid frequent adjustments causing interference to the circuit. Only when the tube voltage drop of the constant current drive tube exceeds the range of 1V-3V, the flow meter's main control MCU adjusts the value of the duty cycle D2 of PWM2 according to the alarm signal to maintain the tube voltage drop of the constant current drive tube in the range of 1V-3V.

[0050] The invention adopts a group of low-voltage power supplies to supply power to the SEPIC lifting power supply, controls the output of the SEPIC power supply through the excitation constant current duty cycle, and controls the output voltage at the initial stage of excitation through the constant current duty cycle when different sensors and excitation frequencies are used, so that the excitation high voltage is within a reasonable range, which not only satisfies the constant current duty cycle, but also does not increase differential interference by being too high. During the constant current period, the output of the SEPIC power supply is controlled by the upper and lower limit thresholds of the differential pressure between the input and output of the excitation constant current driving tube. Within the range of the upper and lower limit thresholds of the differential pressure of the constant current driving tube, the PWM of the SEPIC power supply works in a fixed duty cycle mode, controls the output voltage within a reasonable range, reduces the power consumption of the constant current tube and the jitter interference of the power supply output; at the same time, in different excitation states, dynamically controls the output filter time constant of the SEPIC power supply, adopts a smaller filter time constant at the initial stage of excitation, reduces the influence on the excitation constant current duty cycle, adopts a larger output filter time constant during the constant current period, reduces the ripple of the excitation current, and improves the measurement accuracy. By using the electromagnetic flowmeter converter of the present invention, various sensors, excitation frequencies, measuring media and measuring methods can be automatically matched without classification design, thus reducing factory costs and expanding the user's measurement range.

[0051] The above embodiments only express several implementation methods of the present invention, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention patent. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, and these all belong to the protection scope of the present invention.

Claims

1. An electromagnetic flowmeter excitation control circuit, characterized in that: The invention comprises a main control MCU, wherein the main control MCU is connected to a dynamic filtering control circuit, and the dynamic filtering control circuit outputs a control signal K1 and a control signal K2; the dynamic filtering control circuit is connected to an excitation drive detection circuit and a feedback reference switching circuit, and the control signal K2 controls the excitation drive detection circuit and the feedback reference switching circuit; the excitation drive detection circuit is connected to a feedback voltage control circuit and the feedback reference switching circuit; and the feedback voltage control circuit adjusts a feedback voltage FB to control the output voltage of a SEPIC excitation power supply output circuit.

2. The electromagnetic flowmeter excitation control circuit according to claim 1, characterized in that: The SEPIC excitation power supply output circuit includes a SEPIC control chip, the SEPIC control chip is connected to the gate of the MOS tube, the drain of the MOS tube is connected to the anode of the diode, a capacitor is arranged between the drain of the MOS tube and the diode, the capacitor is connected in parallel with the transformer, the SEPIC control chip inputs a feedback voltage FB, and the cathode of the diode outputs a working voltage VEE.

3. The electromagnetic flowmeter excitation control circuit according to claim 1, characterized in that: The dynamic filtering control circuit includes a MOS transistor Q1 and a MOS transistor Q2, wherein the drain of the MOS transistor Q1 is connected to the negative electrode of the capacitor C1, the drain of the MOS transistor Q2 is connected to the negative electrode of the capacitor C2, the positive electrode of the capacitor C1 and the positive electrode of the capacitor C2 are connected to the capacitor CO, the gate of the MOS transistor Q1 is connected to the control circuit K1, and the gate of the MOS transistor Q2 is connected to the control circuit K2.

4. The electromagnetic flowmeter excitation control circuit according to claim 1, characterized in that: The excitation drive detection circuit includes a comparator U2A, which compares the voltage drop of the constant current sampling resistor R_EXC with the reference voltage REF_I and outputs a high-level constant current signal C_CS.

5. The electromagnetic flowmeter excitation control circuit according to claim 4, characterized in that: The excitation drive detection circuit also includes a constant current drive tube voltage drop detection alarm output circuit, and the constant current drive tube voltage drop detection alarm output circuit includes a window comparator composed of a comparator U2B and a comparator U2C. The window comparator is connected to the differential amplifier circuit through an analog switch S, and the analog switch is controlled by the control signal K2. The differential amplifier circuit inputs a working voltage VEE, the comparator U2B inputs a reference voltage REF_L, the comparator U2 inputs a reference voltage REF_H, the comparator U2B outputs a high level alarm signal VDL, and the comparator U2 outputs a high level alarm signal VDH.

6. The electromagnetic flowmeter excitation control circuit according to claim 5, characterized in that: The feedback voltage control circuit includes a comparator U1A, wherein the in-phase end of the comparator U1A is connected to the working voltage VEE after voltage division, the reverse end of the comparator U1A is connected to the feedback reference switching circuit, the output end of the comparator U1A is connected to the positive electrode of the diode D1, the negative electrode of the diode D1 is connected to the feedback voltage output loop, the feedback voltage output loop includes a transistor Q3 and a transistor Q4, the feedback voltage output loop outputs a feedback voltage FB, the negative electrode of the diode D1 is connected to the overvoltage protection circuit, and the overvoltage protection circuit inputs the high level alarm signal VDH.

7. An electromagnetic flowmeter excitation control circuit according to claim 1 or 2 or 3 or 4, characterized in that: The feedback reference switching circuit includes a first pulse width demodulation circuit and a second pulse width demodulation circuit, the first pulse width demodulation circuit inputs a pulse width modulation signal PWM1, the second pulse width demodulation circuit inputs a pulse width modulation signal PWM2, the first pulse width demodulation circuit outputs a voltage V1, the second pulse width demodulation circuit outputs a voltage V2, the first pulse width demodulation circuit and the second pulse width demodulation circuit are connected to an analog control switch S1, and the analog control switch S1 is connected to the feedback voltage control circuit.

8. An electromagnetic flowmeter excitation control method, applicable to an electromagnetic flowmeter excitation control circuit as claimed in any one of claims 1 to 7, characterized in that: The following steps are involved: S1: The main control MCU outputs pulse width modulation signals PWM1 and PWM2; S2: The main control MCU outputs the excitation drive signal, and the control circuit starts working; S3: When the excitation current setting value is not reached, the main control MCU controls the feedback reference switching circuit to output the voltage V1. When the excitation current setting value is reached, the main control MCU controls the feedback reference switching circuit to output the voltage V2. S4: When the excitation constant current is applied, the main control MCU calculates the constant current duty cycle, compares the constant current duty cycle with the set value, and adjusts the duty cycle D1 of the pulse width modulation signal PWM1 according to the comparison result; S5: adjusting the pulse width D2 of the pulse width modulation signal PWM2 to obtain the value D2_H of the pulse width D2 when the excitation drive detection circuit outputs the high level alarm signal VDH and the value D2_L of the pulse width D2 when the excitation drive detection circuit outputs the high level alarm signal VDL; S6: Set the final continuous output value of D2 according to D2_H and D2_L; S7: During the excitation current rising period and the excitation constant current period, the main control MCU outputs control signals K1 and K2 according to different states of the excitation current, and gradually increases the filter time constant of the SEPIC power supply output.

9. The electromagnetic flowmeter excitation control method according to claim 8, characterized in that: The specific method of comparing the constant current duty cycle and the set value in step S4 is as follows: when D is greater than the setting, the duty cycle D1 of the pulse width modulation signal PWM1 is reduced; when D is less than the setting, the duty cycle D1 of the pulse width modulation signal PWM1 is increased; the main control MCU saves the duty cycle D1 of the pulse width modulation signal PWM1 when the constant current duty cycle D runs at the set value and maintains the output of D1.

10. The electromagnetic flowmeter excitation control method according to claim 8, characterized in that: When temperature changes cause the DC resistance of the excitation coil to change and the voltage of the constant current driver tube changes in the REF_L-REF_H range, the main control MCU does not adjust the duty cycle D2 of the pulse width modulation signal PWM2; when the tube voltage drop of the constant current driver tube exceeds the REF_L-REF_H range, the main control MCU adjusts the value of the duty cycle D2 of PWM2 according to the alarm signal to maintain the tube voltage drop of the constant current driver tube in the REF_L-REF_H range.

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