An electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit

Abstract

The invention discloses an isolated output circuit for an electromagnetic flowmeter and an ultrasonic flowmeter. A signal processing power supply provided by a main switch power supply is used to power the isolated output circuit. The flowmeter main control MCU converts a required current output signal or a pulse output signal into a low-frequency PWM signal. The low-frequency PWM signal is modulated with a high-frequency PWM signal and used as a driving signal for a micro-power isolated switch power supply. A pulse output control signal or a current output control signal, a pulse output power supply, and a current output power supply whose voltage is proportional to the current output are obtained after demodulation at the output end of the micro-power isolated switch power supply by a related circuit. Thus, an isolated output circuit is formed, and a high-precision, dynamically powered 4-20mA current output or pulse output is realized. The two output modes are converted through a set of circuit programming, and the circuit has the characteristics of secondary isolation, strong anti-interference, low power consumption, low cost, and simple control.

Description

Technical Field

[0001] The present invention relates to the technical field of flowmeter signal output, and in particular to an isolation output circuit for an electromagnetic flowmeter and an ultrasonic flowmeter. Background Art

[0002] Electromagnetic flowmeters and ultrasonic flowmeters are widely used in industrial manufacturing and control, water metering and other industries. These flowmeters are metering instruments that need to be verified or calibrated. When verifying or calibrating, the flowmeter needs to output a pulse signal that represents the flow signal. When used on site, analog (4-20) mA output is generally used, so the instrument has two sets of circuits: pulse output and current output. These two sets of circuits are powered by the primary isolation fixed voltage of the flowmeter main power supply, and require pulse output and current output signal isolators and digital-to-analog conversion chips, which increases the circuit cost. In addition, the current output driver tube powered by the primary isolation fixed voltage of the flowmeter main switch power supply has large losses when outputting small currents, and the (4-20) mA output is easily affected by various electromagnetic interferences, resulting in unstable output. Many sites need to configure a (4-20) mA isolator with secondary isolation power supply.

[0003] Chinese patent document CN113110671A discloses a "variable power consumption analog current output circuit for control systems". It adopts an adjustable voltage output module, a constant current module, and a feedback module. The adjustable voltage output module is connected to the constant current module and the feedback module respectively. The constant current module is connected to the feedback module. The constant current module is used to convert the output voltage of the adjustable voltage output module into current and perform constant current regulation. The feedback module is used to detect the current change and voltage change of the constant current module, perform calculation processing on the detection results, obtain feedback results, and transmit the feedback results to the adjustable voltage output module. The adjustable voltage output module is used to obtain power from an external power supply output, and adjust the output voltage according to the feedback results, and change the output power according to the peripherals. The above technical solution is difficult to adapt to multiple output modes, the cost is high, and the isolation effect is limited. Summary of the invention

[0004] The present invention mainly solves the technical problems that the original technical solution is difficult to adapt to multiple output modes, has high cost and limited isolation effect, and provides an electromagnetic flowmeter and an ultrasonic flowmeter isolation output circuit. The signal processing power provided by the main switch power supply is used to power the isolation output circuit. The flowmeter main control MCU converts the required current output signal or pulse output signal into a low-frequency PWM signal. The low-frequency PWM signal is modulated with the high-frequency PWM signal as a driving signal of a micro-power isolation switch power supply. After demodulation at the output end of the micro-power isolation switch power supply through a related circuit, a pulse output control signal or a current output control signal, a pulse output power supply, and a current output power supply whose voltage is proportional to the current output are obtained. Thus, an isolation output circuit is formed, and high-precision, dynamically powered (4-20) mA current output or pulse output is realized. The two output modes are converted through a set of circuit programming, and have the characteristics of secondary isolation, strong anti-interference, low power consumption, low cost and simple control.

[0005] The above-mentioned technical problems of the present invention are mainly solved by the following technical solutions: the present invention includes a signal modulation and drive output circuit unit, an isolation transformer, the isolation transformer is connected to a trigger shaping circuit unit through a low-frequency detection circuit unit, the trigger shaping circuit unit is respectively connected to a pulse output circuit unit and a constant current output circuit unit, and a pulse width-voltage conversion circuit unit is provided between the trigger shaping circuit unit and the constant current output circuit unit.

[0006] Preferably, the signal modulation and drive output circuit unit includes a chip U1, the chip U1 model is SN74LVC1G08, the chip U1 pin 1 inputs a high-frequency PWM signal F1, the chip U1 pin 2 inputs a low-frequency PWM signal F2, pin 3 is grounded, pin 4 is connected to the gate of the MOS tube Q1 through a resistor R1 and is grounded through a resistor R2, the source of the MOS tube Q1 is grounded, the drain of the MOS tube Q1 is connected to the positive electrode of the system power supply +5VD through a diode D1 and a voltage regulator diode DW1 in sequence, and the drain of the MOS tube Q1 is connected to +5VD through a coil N1, and the chip U1 pin 5 is connected to +5VD and is grounded through parallel capacitors C1 and C2.

[0007] The signal modulation and drive output circuit unit is mainly composed of an AND gate circuit, a switch drive MOS tube, and a switch transformer. The input signals of the AND gate circuit are high-frequency PWM signal F1 and low-frequency PWM signal F2. F1 and F2 are programmed and output by the flow meter MCU.

[0008] As a preferred option, in current output mode, the duty cycle of F1 is determined by the instantaneous flow rate Q and the full-scale flow rate Qmax. The duty cycle of F1 increases with the increase of current output, and the output power supply voltage also increases accordingly, realizing dynamic power supply and reducing the loss of the current output driver tube. When the output is 4mA, the duty cycle of F1 is D1, generally D1 ≥ 5%, and when the output is 20mA, the duty cycle of F1 is D2, generally D2 ≤ 50%. When the instantaneous flow rate is Q, the duty cycle of F1 is programmed as:

[0009]

[0010] In active pulse output mode, the F1 duty cycle is fixed at 50%, and in passive pulse output mode, the F1 duty cycle is fixed at 5%;

[0011] When the low-frequency PWM signal F2 is in current output mode, the current output control signal is linearly related to the duty cycle of F2; when pulse output is in progress, the frequency of F2 represents the relevant amount of pulse output. The high-frequency PWM signal F1 is used for micro-power isolation switch power supply to transmit power. The frequency range is a fixed value between 100kHz and 1MHz. The current output mode and the pulse output mode use the same frequency.

[0012] As a preferred embodiment, in the current output mode, F2 is selected at (300-500) Hz, and according to the characteristics of the current output circuit, the duty cycle corresponding to (4-20) mA is matched, corresponding to between 15% and 85%. When F1 outputs a driving signal with a duty cycle of D1, the MCU programs the duty cycle D3 of F2 to make the current output 4mA. When F1 outputs a driving signal with a duty cycle of D2, the duty cycle D4 of F2 is programmed to make the current output 20mA. The duty cycle of F2 is programmed as follows at the instantaneous flow rate Q:

[0013]

[0014] The AND gate circuit modulates the programming input signals F1 and F2 and outputs the high-low frequency modulation signal F3. The low frequency envelope frequency of the modulation signal F3 is F2, and F2 contains the F1 frequency signal; F3 drives the switch tube of the switching power supply, which is transmitted through the micro-power isolation high-frequency transformer, and the control signal voltage and the power supply voltage are output at the secondary of the micro-power isolation high-frequency transformer respectively. When the low-frequency PWM signal F2 is in the current output mode, the current output control signal is linearly related to the duty cycle of F2; when the pulse is output, the frequency of F2 represents the relevant amount of the pulse output. The micro-power isolation switching power supply can be directly powered by the instrument signal processing system power supply without the need for a separate winding power supply.

[0015] As a preferred embodiment, the invention further comprises a rectifier filter circuit and a voltage stabilizing output circuit connected to the isolation transformer, wherein the rectifier filter circuit comprises an inductor L1, one side of the inductor L1 is respectively connected to the power supply end and the ground through capacitors C7 and C8 connected in parallel, and the other side of the inductor L1 is connected to the coil N3 through capacitors C6 and C5 connected in parallel, and a diode D3 is connected between the coil N3 and the positive side of the capacitor C5. When the output voltage VCC after rectification and filtering is in the current output mode, the output voltage VCC increases with the increase of the (4-20) mA output current, realizing the dynamic power supply of the current output drive tube, when in the active pulse output mode, the higher output voltage is used to power the pulse output drive tube, and when in the passive pulse output mode, the lower output voltage is used to power the signal processing circuit.

[0016] Preferably, the voltage-stabilized output circuit includes a chip U2, wherein pin 1 of chip U2 is grounded, the positive electrode of the output power supply +VCC is connected to the input terminal 2, and after voltage stabilization, +VCC outputs a signal processing power supply +3VC from pin 3, and the output +3V voltage of +3VC after filtering by C9 is used to power the signal processing circuit.

[0017] Preferably, the low-frequency detection circuit unit includes a coil N2, the same-name end of the coil N2 is connected to the VS signal end through a diode D2 and a resistor R3, a resistor R4 and a capacitor C4 are connected between VS and the ground, and the other end of the coil N2 is grounded through a resistor R5 and connected to the cathode of the diode D2 through a capacitor C3. The same-direction output of the micro-power isolation high-frequency transformer is rectified by a diode, filtered by a capacitor, and filtered by an RC first-order low-pass filter to obtain a low-frequency signal VS with the same frequency and direction as the low-frequency PWM control signal F2. The rising edge of VS is the same as F2. The resistance and capacitance values ​​of the first-order low-pass filter circuit are selected so that the peak voltage at the top of the slope part of the falling edge of VS is less than 1V.

[0018] Preferably, the trigger shaping circuit unit includes an operational amplifier U3, wherein pin 1 of the operational amplifier U3 outputs a shaped signal VF, pin 2 is connected to +3VC through a resistor R7, and pin 2 is grounded through a parallel resistor R8 and a capacitor C10, pin 3 is connected to the VS signal terminal through a resistor R6, and pin 3 is grounded through a diode DW2, pin 4 is grounded, and pin 8 is connected to +3VC. A bias voltage is connected to the reverse end of the operational amplifier U3, and the bias voltage is higher than the peak voltage at the top of the slope portion of the VS falling edge. After the VS signal is triggered and shaped, the output signal VF is output, and the pulse width of VF is the same as the pulse width of F2, and the peak voltage is +3V, thereby demodulating a control signal at the output end of the micro-power isolation high-frequency transformer to control the current output or pulse output.

[0019] Preferably, the pulse output circuit unit includes a MOS tube Q2, the source of which is grounded and connected to the pulse output negative terminal DO-signal terminal, the gate of which is connected to the VF signal terminal through a resistor R10, and the gate of which is grounded through a resistor R11, the drain of which is connected to the pulse output positive terminal DO+signal terminal and connected to the output power positive terminal +VCC through a switch SW and a resistor R9 in sequence. The signal VF after triggering and shaping controls the conduction and shutoff of the pulse output tube, and obtains a pulse output signal between the drain and source of the pulse output tube.

[0020] The beneficial effects of the present invention are as follows: the signal processing power provided by the main switch power supply is used to power the isolated output circuit, the output winding and the rectifier filter circuit of the main switch power supply are omitted, the flow meter main control MCU converts the required current output signal or pulse output signal into a low-frequency PWM signal, the low-frequency PWM signal is modulated with the high-frequency PWM signal and used as the driving signal of the micro-power isolated switch power supply, and the pulse output control signal or current output control signal, the pulse output power supply, and the current output power supply whose voltage is proportional to the current output are obtained after demodulation at the output end of the micro-power isolated switch power supply by a related circuit, thereby forming an isolated output circuit, realizing high-precision, dynamic power supply (4-20) mA current output or pulse output, and the two output modes are converted through a set of circuit programming, which has the characteristics of secondary isolation, strong anti-interference, low power consumption, low cost and simple control. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a flow meter circuit structure comparison diagram of the present invention.

[0022] Figure 2 It is a circuit principle connection structure diagram of the present invention.

[0023] Figure 3 It is a circuit diagram of the present invention.

[0024] Figure 4 The invention discloses a signal modulation and drive output circuit diagram.

[0025] Figure 5 The present invention is a rectifier and filter circuit diagram.

[0026] Figure 6 The invention discloses a voltage-stabilizing output circuit diagram.

[0027] Figure 7 The invention discloses a low-frequency detection circuit diagram.

[0028] Figure 8 The invention discloses a trigger shaping circuit diagram.

[0029] Fig. 9 It is a pulse output circuit diagram of the present invention.

[0030] Fig.10 It is a pulse width-voltage conversion function diagram of the present invention.

[0031] Fig.11 This is a 4-20mA constant current output function diagram of the present invention.

[0032] Fig.12 It is a schematic diagram of a control signal waveform of the present invention.

[0033] Fig.13 It is a measured 4mA control waveform diagram of the present invention.

[0034] Fig.14 It is a measured 12mA control waveform diagram of the present invention.

[0035] Fig.15 It is a measured 20mA control waveform diagram of the present invention.

[0036] Fig.16 It is a measured 4mA demodulation waveform diagram of the present invention.

[0037] Fig.17 It is a measured 12mA demodulation waveform diagram of the present invention.

[0038] Fig.18 It is a measured 20mA demodulation waveform diagram of the present invention. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further specifically described below through embodiments and in conjunction with the accompanying drawings.

[0040] Embodiment: An electromagnetic flowmeter and an ultrasonic flowmeter isolation output circuit of this embodiment, such as Figure 2 As shown, it includes a signal modulation and drive output circuit unit, a rectifier and filter circuit unit, a voltage stabilization output circuit unit, a low-frequency detection circuit unit, a trigger shaping circuit unit, a pulse output circuit unit, a pulse width-voltage conversion circuit unit, and a (4-20) mA constant current output circuit unit.

[0041] like Figure 3 , Figure 4As shown, the signal modulation and drive output circuit unit includes a chip U1, the chip U1 model is SN74LVC1G08, the chip U1 pin 1 inputs a high-frequency PWM signal F1, the chip U1 pin 2 inputs a low-frequency PWM signal F2, the pin 3 is grounded, the pin 4 is connected to the gate of the MOS tube Q1 through the resistor R1 and is grounded through the resistor R2, the source of the MOS tube Q1 is grounded, the drain of the MOS tube Q1 is connected to the positive electrode of the system power supply through the diode D1 and the diode DW1 in sequence, and the drain of the MOS tube Q1 is connected to the positive electrode through the coil N1, and the chip U1 pin 5 is connected to the positive electrode of the system power supply and is grounded through the parallel capacitor C1 and capacitor C2. The signal modulation and drive output circuit unit is mainly composed of an AND gate circuit, a switch drive MOS tube, and a switch transformer. The input signals of the AND gate circuit are respectively the high-frequency PWM signal F1 and the low-frequency PWM signal F2, and F1 and F2 are programmed and output by the flow meter MCU.

[0042] The high-frequency PWM signal F1 is used for power transmission of the micro-power isolation switch power supply. The frequency range is a fixed value selected between 100kHz and 1MHz. The current output mode and the pulse output mode use the same frequency.

[0043] In current output mode, the duty cycle of F1 is determined by the instantaneous flow rate Q and the full-scale flow rate Qmax. The duty cycle of F1 increases with the increase of current output, and the output power supply voltage also increases accordingly, realizing dynamic power supply and reducing the loss of the current output driver tube. When the output is 4mA, the duty cycle of F1 is D1, generally D1 ≥ 5%, and when the output is 20mA, the duty cycle of F1 is D2, generally D2 ≤ 50%. When the instantaneous flow rate is Q, the duty cycle of F1 is programmed as:

[0044]

[0045] In active pulse output mode, the F1 duty cycle is fixed at 50%, and in passive pulse output mode, the F1 duty cycle is fixed at 5%;

[0046] When the low-frequency PWM signal F2 is in current output mode, the current output control signal is linearly related to the duty cycle of F2; when pulse output is in pulse output, the frequency of F2 represents the relevant amount of the pulse output.

[0047] In current output mode, F2 is selected at (300-500) Hz. According to the characteristics of the current output circuit, the duty cycle corresponding to (4-20) mA is matched, corresponding to 15%-85%. When F1 outputs a driving signal with a duty cycle of D1, the MCU programs the duty cycle D3 of F2 to make the current output 4mA. When F1 outputs a driving signal with a duty cycle of D2, the duty cycle D4 of F2 is programmed to make the current output 20mA. The duty cycle of F2 is programmed as follows at the instantaneous flow rate Q:

[0048]

[0049] The AND gate circuit modulates the programming input signals F1 and F2 and outputs the high-low frequency modulation signal F3. The low frequency envelope frequency of the modulation signal F3 is F2, and F2 contains the F1 frequency signal; F3 drives the switch tube of the switching power supply, which is transmitted through the micro-power isolation high-frequency transformer, and the control signal voltage and the power supply voltage are output at the secondary of the micro-power isolation high-frequency transformer. The micro-power isolation switching power supply can be directly powered by the instrument signal processing system power supply without the need for a separate winding power supply.

[0050] like Figure 5 As shown, it also includes a rectifier filter circuit and a voltage-stabilized output circuit connected to the isolation transformer, wherein the rectifier filter circuit includes an inductor L1, one side of the inductor L1 is connected to the power supply end and the ground respectively through parallel capacitors C7 and C8, and the other side of the inductor L1 is connected to the same-name end of the coil N3 through parallel capacitors C6 and C5, and a diode D3 is connected between the other end of the coil N3 and the positive side of the capacitor C5. The rectifier filter circuit is composed of a reverse winding of the secondary of a micro-power isolation high-frequency transformer, a rectifier diode, a filter capacitor, and a common mode filter to form an ordinary rectifier filter circuit. When the output voltage VCC after rectification and filtering is in the current output mode, the output voltage VCC increases with the increase of the (4-20) mA output current, realizing the dynamic power supply of the current output drive tube. When in the active pulse output mode, the higher output voltage is used to power the pulse output drive tube, and when in the passive pulse output mode, the lower output voltage is used to power the signal processing circuit.

[0051] like Figure 6 As shown, the voltage regulated output circuit includes chip U2, pin 1 of chip U2 is grounded, pin 2 is connected to the positive electrode of the output power supply +VCC, and pin 3 is connected to +VCC and grounded through capacitor C9. The voltage regulated output circuit is composed of a low voltage difference and low power consumption linear voltage regulator. The output voltage VCC after rectification and filtering is outputted as +3V voltage +3VC after passing through the voltage regulator. +3VC is the power supply for the signal processing circuit.

[0052] like Figure 7 As shown, the low-frequency detection circuit unit includes a coil N2, the same-name end of the coil N2 is connected to the VS signal end through a diode D2 and a resistor R3, the other end of the coil N2 is grounded through a resistor R5, and is connected to the VS signal end through a parallel resistor R4 and a capacitor C4, and the same-name end of the coil N2 is connected to the other end of the coil N2 through a diode D2 and a capacitor C3.

[0053] The low-frequency detection circuit is composed of the same-direction winding of the secondary of the micro-power isolation high-frequency transformer, a rectifier diode, a filter capacitor, and an RC first-order low-pass filter circuit. The same-direction output of the micro-power isolation high-frequency transformer is rectified by a diode, filtered by a capacitor, and filtered by an RC first-order low-pass filter to obtain a low-frequency signal VS with the same frequency and direction as the low-frequency PWM control signal F2. The rising edge of VS is the same as that of F2. The resistance and capacitance values ​​of the first-order low-pass filter circuit are selected so that the peak voltage at the top of the slope part of the falling edge of VS is less than 1V.

[0054] like Figure 8 As shown, the trigger shaping circuit unit includes an operational amplifier U3, wherein pin 1 of the operational amplifier U3 is connected to the VF signal terminal, pin 2 is connected to the positive electrode +3VC through a resistor R7, and pin 2 is grounded through a parallel resistor R8 and a capacitor C10, and pin 3 is connected to the VS signal terminal through a resistor R6, and pin 3 is grounded through a diode DW2, pin 4 is grounded, and pin 3 is connected to the positive electrode +3VC. The trigger shaping circuit is composed of an operational amplifier U3, wherein the reverse end of the operational amplifier U3 is connected to a bias voltage, and the bias voltage is higher than the peak voltage at the top of the slope portion of the VS falling edge. After the VS signal is triggered and shaped, the output signal VF is output, and the pulse width of VF is the same as the pulse width of F2, and the peak voltage is +3V, thereby demodulating a control signal at the output end of the micro-power isolation high-frequency transformer to control the current output or pulse output.

[0055] like Fig. 9 As shown, the pulse output circuit unit includes a MOS tube Q2, the source of which is grounded and connected to the pulse output negative terminal DO-signal terminal, the gate of which is connected to the VF signal terminal through a resistor R10, and the gate of which is grounded through a resistor R11, the drain of which is connected to the pulse output positive terminal DO+signal terminal, and is connected to the positive electrode of the output power supply through a switch SW and a resistor R9 in sequence. The pulse output circuit is composed of a pulse output tube, a load resistor, and a dial switch. The signal VF after triggering and shaping controls the conduction and shutoff of the pulse output tube, and a pulse signal is output between the pulse output terminals DO+ and DO-.

[0056] The pulse output mode can provide active or passive pulse output through the selection of the DIP switch SW. In the current output mode and passive pulse output, SW is disconnected. In passive pulse output, the pulse output terminal outputs pulse signals through the user's external power supply and load resistor. In active pulse output, SW is connected to the machine power supply voltage +VCC through the pulse output load resistor, and the pulse output terminal outputs pulse signals.

[0057] The demodulated signal VF is converted into a voltage signal VI which is linearly proportional to (4-20) mA by pulse width-voltage conversion, and the (4-20) mA constant current output is controlled by VI.

[0058] The (4~20)mA constant current output driver tube supply voltage +VCC increases with the (4~20)mA output current, thus achieving dynamic power supply constant current output and reducing the power consumption of the output tube. Specific embodiment:

[0060] A control method for an isolated output circuit of an electromagnetic flowmeter and an ultrasonic flowmeter comprises a signal modulation and drive output circuit unit, a rectifier and filter circuit unit, a voltage stabilization output circuit unit, a low-frequency detection circuit unit, a trigger shaping circuit unit, a pulse output circuit unit, a pulse width-voltage conversion circuit unit, and a 4-20mA constant current output circuit unit.

[0061] A control method for an electromagnetic flowmeter and an ultrasonic flowmeter isolation output circuit, comprising a signal modulation and drive output circuit unit: mainly composed of an AND gate circuit U1, a switch drive MOS tube Q1, a switch transformer T, etc. Preferably: U1 is SN74LVC1G08, Q1 is a common N-MOS tube, the magnetic core of the switch transformer T is EP7, and the primary-secondary turns ratio N1:N2:N3=1:1:3.

[0062] +5VD is the power supply voltage for the flow meter signal processing system, and DGND is the ground for the system power supply.

[0063] +5VD is connected to one end of C1 and C2 and then filtered to power pin 5 of the AND gate circuit U1. F1 and F2 are connected to input terminals 1 and 2 of U1 respectively. Output terminal 4 of U1 outputs modulation signal F3. F3 is connected to the gate of MOS tube Q1 through resistor R1. The source of Q1 is connected to DGND. The gate of Q1 is connected to DGND in parallel with resistor R2.

[0064] The other path of +5VD is connected to the same-name end of the primary winding N1 of the switching transformer T, and the other end of N1 is connected to the drain of Q1. The secondary D1 is connected in series with the voltage regulator DW1 and then connected to both ends of N1.

[0065] The input signals of U1 are high-frequency PWM signal F1 and low-frequency PWM signal F2. F1 and F2 are programmed and output by the flow meter MCU.

[0066] The high-frequency PWM signal F1 is used for the switching power supply to transmit power. The preferred F1 is 350kHz, and the current output mode and the pulse output mode use the same frequency.

[0067] In current output mode, the duty cycle of F1 is determined by the instantaneous flow rate Q and the full-scale flow rate Qmax. The pulse width of F1 increases with the increase of current output, and the output power supply voltage also increases accordingly, realizing dynamic power supply and reducing the loss of the current output drive tube. When the output is 4mA, the duty cycle of F1 is D1, preferably, D1 is 20%, and when the output is 20mA, the duty cycle of F1 is D2, preferably, D2 is 50%. When the instantaneous flow rate is Q, the duty cycle of F1 is programmed as:

[0068]

[0069] In active pulse output mode, preferably, the F1 duty cycle is fixed at 50%. In passive pulse output mode, in order to reduce circuit loss, preferably, the F1 duty cycle is fixed at 5%.

[0070] The low-frequency PWM signal F2 is used to transmit a current output or pulse output control signal. Preferably, the frequency of F2 is 400 Hz in the current output mode. In the pulse output mode, the frequency of F2 represents the relevant amount of the pulse output, which is generally 1 Hz-10 KHz.

[0071] In the current output mode, F2 matches the duty cycle corresponding to (4-20) mA according to the characteristics of the current output circuit. Preferably, when F1 outputs a 20% driving signal with a duty cycle, the MCU programs the duty cycle D3 of F2, which is generally around 20%, so that the current output is 4 mA. When F1 outputs a 50% driving signal with a duty cycle, the MCU programs the duty cycle D4 of F2, which is generally around 80%, so that the current output is 20 mA. When the instantaneous flow rate Q is, the duty cycle of F2 is programmed as follows:

[0072]

[0073] AND gate circuit U1 modulates programming input signals F1 and F2 and outputs high and low frequency modulation signal F3.

[0074] The low frequency envelope frequency of the modulation signal F3 is F2, and F2 includes the frequency signal of F1.

[0075] F3 drives the switch tube Q1 of the switching power supply, and transmits it through the high-frequency transformer T in isolation. The secondary N2 of the high-frequency transformer T outputs the control signal, and N3 outputs the power supply voltage.

[0076] A control method for an isolated output circuit of an electromagnetic flowmeter and an ultrasonic flowmeter includes a rectifier and filter circuit unit. The reverse output of a winding N3 is rectified and filtered to produce an output voltage VCC, where VCC is the power supply of the output circuit.

[0077] The reverse end of N3 is connected to the anode of diode D3, the cathode of D3 is connected to the filter capacitors C5, C6 and one input end of the common mode filter, the same-name end of N3 is connected to the other end of the filter capacitors C5, C6 and another input end of the common mode filter, one output end of the common mode filter is connected to one end of the filter capacitors C7, C8 and the output voltage +VCC, and the other output end of the common mode filter is connected to the other end of the filter capacitors C7, C8 and CGND.

[0078] In current output mode, since the duty cycle of the switching power supply changes with the change of current output, the voltage +VCC after rectification and filtering is about 8V~22V when the output is 4mA~20mA, thus realizing dynamic power supply.

[0079] +VCC is about 4.5V when passive pulse output is used, and +VCC is about 22V when active pulse output is used.

[0080] A control method for an electromagnetic flowmeter and an ultrasonic flowmeter isolated output circuit includes a voltage stabilization output circuit unit, using an ultra-low quiescent current voltage stabilizer U2, preferably, U2 is SGM7130, U2 stabilizes the output voltage +VCC and outputs a +3V voltage +3VC to provide power for the output circuit chip, the input terminal 2 pin of U2 is connected to +VCC, the 1 pin of U2 is connected to CGND, and the output terminal 3 pin of U2 is connected to +3VC. The filter capacitor C9 is connected between +3VC and CGND.

[0081] A control method for an isolated output circuit of an electromagnetic flowmeter and an ultrasonic flowmeter, comprising a low-frequency detection circuit unit: a secondary N2 of a high-frequency transformer T is a unidirectional output, a unidirectional end of N2 is connected to an anode of a diode D2, a cathode of D2 is connected to a resistor R3 and one end of C3, the other end of R3 is connected to a resistor R4 and one end of a capacitor C4 to output a signal voltage VS, the other ends of R4 and C4, and one end of R5 are connected to CGND, and the other end of R5 and C3 is connected to a reverse end of N2.

[0082] The same-direction output of the high-frequency transformer N2 is rectified by diode D2, filtered by C3, and then integrated by R3, R4, R5, and C4 to obtain a low-frequency signal VS with the same frequency and direction as F2. The rising edge of VS is the same as that of F2. The resistance and capacitance values ​​of the integration circuit are selected so that the pulse width of the VS falling edge at a level above about 1V is the same as the pulse width of F2. Preferably, R3 and R5 are 100Ω, R4 is 680Ω, and C3 and C4 are 0.01uF.

[0083] A control method for an electromagnetic flowmeter and an ultrasonic flowmeter isolated output circuit, comprising a trigger shaping circuit unit: a trigger shaping circuit is formed by an operational amplifier U3, preferably, U3 is TPA5512, and a low-frequency signal VS is output as a signal VF after shaping, so that the pulse width of VF is the same as the pulse width of F2 and the peak voltage is +3V.

[0084] The VS signal is connected to one end of resistor R6, and the other end of R6 is connected to the same-direction input terminal 3 of U2A and the cathode of the voltage regulator DW2. The anode of the voltage regulator DW2 is connected to CGND, which is used for overvoltage protection of the U2A input terminal. One end of +3VC is connected to one end of resistor R7, and the other end of resistor R7, resistor R8, and one end of capacitor C10 are connected to the reverse input terminal 2 of U3. The other ends of resistor R8 and capacitor C10 are connected to CGND, the power supply terminal 8 of U3 is connected to the power supply +3VC, and the ground terminal 4 of U3 is connected to CGND. The values ​​of R7 and R8 are selected so that the reverse terminal voltage of U3 is greater than the peak voltage at the top of the slope portion of the falling edge of the VS signal. Preferably, R7 and R8 are 150KΩ. After the VS signal is shaped by the comparator, the signal VF is output through the output terminal 1 of U3.

[0085] A control method for an isolated output circuit of an electromagnetic flowmeter and an ultrasonic flowmeter, comprising a pulse output circuit unit, which is composed of a MOS tube Q2, a load resistor R9, input resistors R10 and R11, and a dial switch SW. A signal VF after triggering and shaping is connected to one end of the resistor R10, the other end of R10 is connected to the gate of the MOS tube Q2, a resistor R11 is connected between the gate of Q2 and CGND, the source of Q2 is connected to CGND and serves as the negative end D0- of the pulse output, the drain of Q2 is connected to one end of SW and serves as the positive end DO+ of the pulse output, the other end of SW is connected to the resistor R9, and the other end of R9 is connected to the power supply +VCC.

[0086] The pulse signal VF controls the on and off of the MOS tube Q2, and outputs a pulse signal between the drain and source of Q2. Through the selection of the DIP switch SW, active or passive pulse output can be provided. In the current output mode, SW is disconnected, and the pulse output terminals DO+ and DO- are not connected to the load; in active pulse output, SW is connected to +VCC through the load resistor R9, and DO+ and DO- output pulse signals; in passive pulse output, SW is disconnected, and DO+ and D0- output pulse signals through the external power supply and load resistor.

[0087] A control method for the isolated output circuit of an electromagnetic flowmeter and an ultrasonic flowmeter, including a pulse width-voltage conversion circuit, a signal VF after trigger shaping passes through the pulse width-voltage conversion circuit, and outputs a DC voltage signal VI that is linear with (4-20) mA, and a constant current circuit is controlled by VI to output a (4-20) mA current, and +3VC is used to power the internal reference power supply, trigger, and operational amplifier of the circuit. The measured waveform is shown in the figure below: Figure 13-Figure 18 shown.

[0088] A control method for an electromagnetic flowmeter and an ultrasonic flowmeter isolated output circuit, including a (4-20) mA constant current output circuit, a current control signal being a DC voltage signal VI that is linear with (4-20) mA, +3VC being a power supply for a comparator inside the constant current output circuit, and +VCC being a power supply for a constant current output transistor, where +VCC is approximately 8-22 V and changes with (4-20) mA, thereby realizing dynamic analog output power supply and improving circuit efficiency. The current output terminals IO+ and I0- have a load capacity of 0-900 ohms. In pulse output mode, the current output terminals IO+ and IO- are not connected to a load. The current output test data and the pulse output test data are shown in the following table.

[0089] Dual resistor 500Ω current output test F1: 30kHz F2: 400Hz

[0090] Theoretical current value mA 4 6 8 10 12 14 16 18 20 Actual measured value mA 4.001 6.002 8.002 10.003 12.003 14.004 16.004 18.005 20.006 Relative error % 0.026 0.0333333 0.025 0.03 0.025 0.028571 0.025 0.02778 0.03 400Hz pulse width us 520 710 900 1090 1280 1470 1660 1850 2040 400Hz Duty Cycle % 20.8 28.4 36 43.6 51.2 58.8 66.4 74 81.6 350kHz Duty Cycle % 20 23.75 27.5 31.25 35 38.75 42.5 46.25 50 NCC voltage V 7.95 9.63 11.2 13.6 15.22 16.8 19.44 21.2 21.6 +5VD input current mA 11.2 15.5 24.5 37.3 53.2 71.2 94.2 117.5 133.6

[0091] Table 1 Current output test data table

[0092] Pulse output test (passive)

[0093] F1 F1 Duty Cycle F2 F2 duty cycle VCC Input current +5VD 350kHz 5％ 1Hz-10kHz 20％ 4.5V 3.17mA max

[0094] Pulse output test (active)

[0095] F1 F1 Duty Cycle F2 F2 duty cycle NCC Input current +5VD 350kHz 50％ 1Hz-10kHz 50％ 21.6V Max 65mA

[0096] Table 2 Pulse output test data table

Claims

1. An electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit, characterized in that: It includes a signal modulation and drive output circuit unit, an isolation transformer, the isolation transformer is connected to a trigger shaping circuit unit through a low-frequency detection circuit unit, the trigger shaping circuit unit is respectively connected to a pulse output circuit unit and a constant current output circuit unit, and a pulse width-voltage conversion circuit unit is arranged between the trigger shaping circuit unit and the constant current output circuit unit; the input high-frequency PWM signal F1 is modulated with the low-frequency PWM signal F2 to output a high- and low-frequency modulation signal F3, which is used as a drive signal of a micro-power isolation switch power supply to output a control signal voltage and a power supply voltage respectively; In the current output mode, the duty cycle of the high-frequency PWM signal F1 is determined by the instantaneous flow rate Q and the full-scale flow rate Qmax to achieve dynamic power supply. The current output control signal is linearly related to the duty cycle of the low-frequency PWM signal F2. In pulse output mode, the frequency of the low-frequency PWM signal F2 represents the relevant amount of the pulse output. When active, the duty cycle of the high-frequency PWM signal F1 is fixed at 50%, and when passive, the duty cycle of the high-frequency PWM signal F1 is fixed at 5%.

2. The electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit according to claim 1, characterized in that: The signal modulation and drive output circuit unit includes a chip U1, the model of the chip U1 is SN74LVC1G08, the pin 1 of the chip U1 inputs a high-frequency PWM signal F1, the pin 2 of the chip U1 inputs a low-frequency PWM signal F2, the pin 3 is grounded, the pin 4 is connected to the gate of the MOS tube Q1 through a resistor R1 and is grounded through a resistor R2, the source of the MOS tube Q1 is grounded, the drain of the MOS tube Q1 is connected to the positive electrode of the system power supply through a diode D1 and a voltage-stabilizing diode DW1 in sequence, and the drain of the MOS tube Q1 is connected to the positive electrode of the system power supply through a coil N1, and the pin 5 of the chip U1 is connected to the positive electrode of the system power supply and is grounded through parallel capacitors C1 and C2.

3. The electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit according to claim 2, characterized in that: In current output mode, the duty cycle of high-frequency PWM signal F1 is determined by instantaneous flow Q and full-scale flow Qmax. The duty cycle of high-frequency PWM signal F1 increases with the increase of current output, and the output power supply voltage also increases accordingly, realizing dynamic power supply and reducing the loss of current output drive tube. When 4mA output, the duty cycle of high-frequency PWM signal F1 is D1, D1 ≥ 5%, when 20mA output, the duty cycle of high-frequency PWM signal F1 is D2, D2 ≤ 50%, and when the instantaneous flow is Q, the duty cycle of high-frequency PWM signal F1 is programmed as: In active pulse output mode, the duty cycle of high-frequency PWM signal F1 is fixed at 50%, and in passive pulse output mode, the duty cycle of high-frequency PWM signal F1 is fixed at 5%; When the low-frequency PWM signal F2 is in current output mode, the current output control signal is linearly related to the duty cycle of the low-frequency PWM signal F2; when pulse output is performed, the frequency of the low-frequency PWM signal F2 represents the relevant amount of the pulse output.

4. The electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit according to claim 2, characterized in that: In current output mode, the low-frequency PWM signal F2 is selected at 300-500Hz. According to the characteristics of the current output circuit, the duty cycle corresponding to 4-20mA is matched, corresponding to 15%-85%. When the high-frequency PWM signal F1 outputs a driving signal with a duty cycle of D1, the MCU programs the duty cycle D3 of the low-frequency PWM signal F2 to make the current output 4mA. When the high-frequency PWM signal F1 outputs a driving signal with a duty cycle of D2, the duty cycle D4 of the low-frequency PWM signal F2 is programmed to make the current output 20mA. The duty cycle of the low-frequency PWM signal F2 at the instantaneous flow rate Q is programmed as: The AND gate circuit modulates the programmed input signal high-frequency PWM signal F1 and the low-frequency PWM signal F2 and outputs a high-low frequency modulation signal F3. The low-frequency envelope frequency of the modulation signal F3 is the low-frequency PWM signal F2, and the low-frequency PWM signal F2 contains the frequency signal of the high-frequency PWM signal F1; the modulation signal F3 drives the switch tube of the switching power supply, is transmitted through a micro-power isolation high-frequency transformer, and the control signal voltage and the power supply voltage are output at the secondary of the micro-power isolation high-frequency transformer respectively.

5. The electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit according to claim 1 or 2, characterized in that: It also includes a rectifier filter circuit and a voltage stabilizing output circuit connected to the isolation transformer, the rectifier filter circuit includes an inductor L1, one side of the inductor L1 is connected to the power supply end and the ground respectively through parallel capacitors C7 and C8, the other side of the inductor L1 is connected to the coil N3 through parallel capacitors C6 and C5, and a diode D3 is provided between the coil N3 and the positive side of the capacitor C5.

6. The electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit according to claim 5, characterized in that: The voltage-stabilized output circuit includes a chip U2, the model of the chip U2 is SGM7130, the pin 1 of the chip U2 is grounded, the pin 2 is connected to the positive electrode of the output power supply, and the pin 3 is connected to the positive electrode of the signal power supply and grounded through a capacitor C9.

7. The electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit according to claim 1, characterized in that: The low-frequency detection circuit unit includes a coil N2, one end of the coil N2 is connected to the VS signal end through a diode D2 and a resistor R3, the other end of the coil N2 is grounded through a resistor R5, and is connected to the VS signal end through a parallel resistor R4 and a capacitor C4, and one end of the coil N2 is connected to the other end of the coil N2 through a diode D2 and a capacitor C3.

8. The electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit according to claim 1 or 2, characterized in that: The trigger shaping circuit unit includes an operational amplifier U3, the model of which is TPA5512. Pin 1 of the operational amplifier U3 is connected to the VF signal terminal, pin 2 is connected to the positive electrode of the signal power supply through a resistor R7, and pin 2 is grounded through a parallel resistor R8 and a capacitor C10. Pin 3 is connected to the VS signal terminal through a resistor R6, and pin 3 is grounded through a diode DW2, pin 4 is grounded, and pin 8 is connected to the positive electrode of the signal power supply.

9. An electromagnetic flowmeter and ultrasonic flowmeter isolation output circuit according to claim 1 or 2, characterized in that: The pulse output circuit unit includes a MOS tube Q2, the source of which is grounded and connected to the pulse output negative terminal DO-signal terminal, the gate of which is connected to the VF signal terminal through a resistor R10, and the gate of which is grounded through a resistor R11, the drain of which is connected to the pulse output positive terminal DO+signal terminal and is connected to the positive electrode of the output power supply through a switch SW and a resistor R9 in sequence.

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