Circuit and calibration method based on PWM wave control 4-20mA current loop

Abstract

The invention relates to the technical field of instrument control, and specifically provides a circuit and a calibration method for controlling a 4-20mA current loop based on a PWM wave. The circuit comprises a central processing module, a PWM to voltage module, a 4-20mA current loop module, and a power management module; the calibration method comprises: S1, recording a duty cycle D1 of a PWM wave and a current I1 output by an instrument; S2, recording a duty cycle D2 of the PWM wave and a current I2 output by the instrument; S3, compensating the current loop by a linear method; S4, a single-chip microcomputer U3 records the values ​​of k and b in an internal flash; S5, the instrument outputs a specific current; the invention can output a constant voltage, calibrate and test the current output by the current loop, thereby improving the versatility of the circuit and reducing the cost.

Description

Technical Field

[0001] The invention relates to the technical field of instrument control, and in particular to a circuit and a calibration method for controlling a 4-20mA current loop based on a PWM wave. Background Art

[0002] Many existing industrial instruments have 4-20mA current output function. 4-20mA current output can transmit the measured physical quantity over long distances, and also has strong anti-interference ability, which is convenient for real-time monitoring of the working status of the instrument. Therefore, 4-20mA current loop is widely used in industrial instruments.

[0003] In existing instruments, there are generally two solutions for 4-20mA current loop control:

[0004] 1. Use a dedicated chip to convert digital signals into current, which is very expensive;

[0005] 2. Use the DAC module of the microcontroller U3 or a dedicated DAC chip to transfer the voltage signal, and then convert the voltage signal into 4-20mA current. However, the price of the microcontroller U3 with a DAC module or a dedicated DAC chip is often high, which will increase the cost of the instrument. In addition, due to the difference in power supply and the influence of various chip batches, the accuracy of 4-20mA is not high. Summary of the invention

[0006] In view of the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide a circuit and calibration method based on PWM wave control of a 4-20mA current loop, which is used to solve the problem of high cost of using a dedicated chip for converting digital signals into current and using a single-chip microcomputer U3 with a DAC module or a DAC dedicated chip in the prior art.

[0007] To achieve the above-mentioned purpose and other related purposes, the present invention provides a circuit calibration method based on PWM wave control of 4-20mA current loop, adopting a current loop calibration system, wherein the current loop calibration system communicates with the instrument through the modbus protocol; comprising a circuit based on PWM wave control of 4-20mA current loop, wherein the circuit comprises:

[0008] Central processing module, the central processing module adopts a single chip microcomputer U3, and the single chip microcomputer U3 outputs pwm wave through the I / O port;

[0009] A pwm-to-voltage module, which is composed of an NMOS tube Q1, an operational amplifier U1, a resistor R1, a resistor R2, a resistor R3, a resistor R4, a capacitor C1, a capacitor C2, and a capacitor C3, and outputs a voltage signal Vout;

[0010] 4-20mA current loop module, Vout is connected to the 4-20mA current loop module; the 4-20mA current loop module is composed of a current loop control chip U2, a transistor Q2, a resistor R5, a resistor R6, a resistor R8, a resistor R9, a resistor R10, a resistor R11, a resistor R12, a resistor R13, a capacitor C4, a capacitor C5, a capacitor C6, a PMOS tube Q3, and a diode D1;

[0011] A power management module is used to provide a power supply voltage VCC; the power management module uses a charge and discharge management chip U4 to manage the power supply of the entire circuit;

[0012] The central processing module and the power management module are both provided with a chip internal working voltage VDD, and the pwm voltage conversion module and the 4-20mA current loop module are both provided with a chip internal working voltage VDD2;

[0013] The I / O port of the single chip microcomputer U3 outputting the PWM wave is connected to the gate of the NMOS tube Q1 and grounded through the resistor R4, the drain of the NMOS tube Q1 is connected to one end of the resistor R2 and connected to the voltage VDD2 through the resistor R1, and the source is grounded;

[0014] The other end of the resistor R2 is connected to one end of the capacitor C1 and one end of the resistor R3, the other end of the capacitor C1 is grounded, the other end of the resistor R3 is connected to one end of the capacitor C2 and pin 1 of the operational amplifier U1, and the other end of the capacitor C2 is grounded;

[0015] Pin 2 of the operational amplifier U1 is grounded, pin 5 of U1 is connected to voltage VDD2 and grounded via capacitor C3, and pin 3 and pin 4 of the operational amplifier U1 are connected;

[0016] The current loop control chip U2 has 10 pins, the first pin of the current loop control chip U2 is connected to the voltage VCC and is grounded through capacitors C5 and C6; the second pin of the current loop control chip U2 is connected to the emitter of the transistor Q2 and one end of the resistor R12; the base of the transistor Q2 and the other end of the resistor R12 are connected to the source of the PMOS tube Q3; the collector of the transistor Q2 is connected to the third pin of the current loop control chip U2 and the gate of the PMOS tube Q3, the drain of Q3 is connected to the anode of the diode D1, and the cathode of the diode D1 outputs a 4-20mA current loop output signal;

[0017] One end of the resistor R9 and one end of the resistor R8 are connected to the 4th pin of the current loop control chip U2, and the other end of R9 is grounded; the 5th pin of the current loop control chip U2 and the other end of R8 are connected to the voltage VDD2 and grounded through the capacitor C4; one end of the resistor R6 and one end of the resistor R5 are connected to the 6th pin of U2, and the other end of the resistor R6 is connected to the voltage signal output by the pwm wave conversion voltage module.

[0018] The 7th pin of the current loop control chip U2 is grounded through a resistor R13; the 8th pin of the current loop control chip U2 is connected to the voltage VDD through a resistor R10; the 9th pin of the current loop control chip U2 is connected to an I / O port of the single-chip computer U3 and is grounded through a resistor R11. When the I / O port of the single-chip computer U3 outputs a high level, the current loop can be closed, so that the output current of the current loop is 0; the 10th pin of the current loop control chip U2 is grounded;

[0019] The other end of the resistor R5 is set to be floating without connecting any signal or connected to an FSK signal for the current loop to output a HART waveform;

[0020] The model of the current loop control chip U2 is XTR111AIDRCT;

[0021] The method comprises the following working steps:

[0022] S1, the current loop calibration system sends a command to the instrument, the I / O port of the single-chip computer U3 outputs the theoretical PWM wave corresponding to 4mA, the duty cycle of the PWM wave is recorded as D1, the current output by the instrument is collected by the calibration system and the value is recorded as I1;

[0023] S2, the current loop calibration system sends a command to the instrument, and the I / O port of the single-chip computer U3 outputs the theoretical PWM wave corresponding to 12mA. The duty cycle of the PWM wave is recorded as D2. The current output by the instrument is collected by the calibration system and the value is recorded as I2.

[0024] S3. Use the linear method to compensate the current loop, the compensation coefficient k = (D2-D1) / (I2-I1), the offset b = D2-k*I2;

[0025] S4, the single-chip microcomputer U3 records the values ​​of k and b in the internal flash. When the output current of the instrument is I, the duty cycle of the pwm wave output by the single-chip microcomputer U3 is D = k*I+b;

[0026] S5. The instrument outputs a specific current according to the requirements of the current loop calibration system;

[0027] S6. The current loop calibration system compares the specific current output by the instrument with the actual current collected, and controls the error between the specific current output by the instrument and the actual current collected to within 0.3%.

[0028] As described above, the circuit and calibration method of the 4-20mA current loop controlled by PWM wave of the present invention have the following beneficial effects:

[0029] The present invention adopts a pwm wave to control a 4-20mA current loop module, and obtains different 4-20mA current signals by changing the duty cycle of the pwm wave; therefore, only one control port on the single-chip microcomputer U3 is needed to realize the control of the 4-20mA current loop module, and there is no need to carry a DAC module on the single-chip microcomputer U3, nor is there a need to use a DAC dedicated chip, thereby improving the versatility of the circuit and reducing the material cost. In the calibration method, the present invention can significantly improve the accuracy of the 4-20mA current loop through calibration and output comparison of the current loop calibration system, and increase the reliability of the 4-20mA current loop output. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 Shown is a schematic diagram of a circuit for controlling a 4-20mA current loop based on a PWM wave disclosed in the present invention.

[0031] Figure 2 Display as Figure 1 A magnified view of the central processing module.

[0032] Figure 3 Display as Figure 1 Enlarged view of the pwm-to-voltage module.

[0033] Figure 4 Display as Figure 1 An enlarged view of the 4-20mA current loop module.

[0034] Figure 5 Display as Figure 1 A magnified view of the power management module.

[0035] Component number description

[0036] Central processing module 1; pwm to voltage module 2; 4-20mA current loop module 3; power management module 4. DETAILED DESCRIPTION

[0037] The following is a description of the implementation of the present invention by means of specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification.

[0038] See also Figures 1 to 5. It should be noted that the structures, proportions, sizes, etc. illustrated in the drawings in this specification are only used to match the contents disclosed in the specification for people familiar with this technology to understand and read, and are not used to limit the limiting conditions for the implementation of the present invention, so they have no substantial technical significance. Any modification of the structure, change in the proportional relationship, or adjustment of the size should still fall within the scope of the technical content disclosed by the present invention without affecting the effects and purposes that can be achieved by the present invention. At the same time, the terms such as "upper", "lower", "left", "right", "middle" and "one" quoted in this specification are only for the convenience of description, and are not used to limit the scope of the implementation of the present invention. Changes or adjustments in their relative relationships should also be regarded as the scope of the implementation of the present invention without substantially changing the technical content.

[0039] Example 1, please refer to Figure 1-Figure 5 , this embodiment provides a circuit based on pwm wave control 4-20mA current loop, including a central processing module 1, a pwm voltage conversion module 2, a 4-20mA current loop module 3, and a power management module 4. The central processing module 1 adopts a single-chip microcomputer U3, and the single-chip microcomputer U3 outputs a pwm wave through an I / O port; the pwm voltage conversion module 2 is composed of an NMOS tube Q1, an operational amplifier U1, a resistor R1, a resistor R2, a resistor R3, a resistor R4, a capacitor C1, a capacitor C2, and a capacitor C3. The pwm voltage conversion module 2 outputs a voltage signal Vout, and Vout is connected to the 4-20mA current loop module 3; the 4-20mA current loop module 3 is composed of a current loop control chip U2, a transistor Q2, a resistor R5, a resistor R6, a resistor R8, a resistor R9, a resistor R10, a resistor R11, a resistor R12, a resistor R13, a capacitor C4, a capacitor C5, a capacitor C6, a PMOS The power management module 4 is used to provide a power supply voltage VCC; the power management module 4 adopts a charge and discharge management chip U4 to manage the power supply of the entire circuit; the central processing module 1 and the power management module 4 are both provided with a chip internal working voltage VDD, and the pwm voltage conversion module 2 and the 4-20mA current loop module 3 are both provided with a chip internal working voltage VDD2.

[0040] The I / O port of the single-chip computer U3 outputting the PWM wave is connected to the gate of the NMOS tube Q1 and is grounded through the resistor R4. The drain of the NMOS tube Q1 is connected to one end of the resistor R2 and is connected to the voltage VDD2 through the resistor R1, and the source is grounded. The other end of the resistor R2 is connected to one end of the capacitor C1 and one end of the resistor R3. The other end of the capacitor C1 is grounded. The other end of the resistor R3 is connected to one end of the capacitor C2 and the pin 1 of the operational amplifier U1, and the other end of the capacitor C2 is grounded. The pin 2 of the operational amplifier U1 is grounded, the pin 5 of U1 is connected to the voltage VDD2 and is grounded through the capacitor C3, and the pins 3 and 4 of the operational amplifier U1 are connected.

[0041] The model of the current loop control chip U2 is XTR111AIDRCT. This chip has the function of a voltage-to-current converter. Its power supply voltage range is 7.00V-44V, its output voltage range is 3V-15V, and its maximum output current is 25mA. The current loop control chip U2 has 10 pins. The first pin of the current loop control chip U2 is connected to the voltage VCC and grounded through capacitors C5 and C6; the second pin of the current loop control chip U2 is connected to the emitter of the transistor Q2 and one end of the resistor R12; the base of the transistor Q2 and the other end of the resistor R12 are connected to the source of the PMOS tube Q3; the collector of the transistor Q2 is connected to the third pin of the current loop control chip U2 and the gate of the PMOS tube Q3, the drain of Q3 is connected to the anode of the diode D1, and the cathode of the diode D1 outputs 4- 20mA current loop output signal; one end of the resistor R9 and one end of the resistor R8 are connected to the 4th pin of the current loop control chip U2, and the other end of R9 is grounded; the 5th pin of the current loop control chip U2 and the other end of R8 are connected to the voltage VDD2, and are grounded through the capacitor C4; one end of the resistor R6 and one end of the resistor R5 are connected to the 6th pin of U2, and the other end of the resistor R5 is set to float without connecting any signal or connecting the FSK signal for the current loop to output the HART waveform, which is convenient and intuitive to monitor the output state of the current loop; the other end of the resistor R6 is connected to the voltage signal output by the pwm wave conversion voltage module, and the 7th pin of the current loop control chip U2 is grounded through the resistor R13; the 8th pin of the current loop control chip U2 is connected to the voltage VDD through the resistor R10; the 9th pin of the current loop control chip U2 is connected to an I / O port of the single-chip microcomputer U3, and is grounded through the resistor R11. The I / O port of the single-chip microcomputer U3 outputs a high level to close the current loop, so that the output current of the current loop is 0; the 10th pin of the current loop control chip U2 is grounded.

[0042] The present invention adopts a pwm wave to control the 4-20mA current loop module 3, and obtains different 4-20mA current signals by changing the duty cycle of the pwm wave; therefore, only one control port on the single-chip microcomputer U3 is required to realize the control of the 4-20mA current loop module 3, and there is no need to carry a DAC module on the single-chip microcomputer U3, nor to use a DAC dedicated chip, which improves the versatility of the circuit and reduces the material cost.

[0043] Embodiment 2, this embodiment provides a circuit calibration method based on PWM wave control 4-20mA current loop, adopts current loop calibration system, and the current loop calibration system communicates with the instrument through modbus protocol; including the circuit based on PWM wave control 4-20mA current loop described in embodiment 1, the calibration working steps are as follows:

[0044] S1, the current loop calibration system sends a command to the instrument, the I / O port of the single-chip computer U3 outputs the theoretical PWM wave corresponding to 4mA, the duty cycle of the PWM wave is recorded as D1, the current output by the instrument is collected by the calibration system and the value is recorded as I1;

[0045] S2, the current loop calibration system sends a command to the instrument, and the I / O port of the single-chip computer U3 outputs the theoretical PWM wave corresponding to 12mA. The duty cycle of the PWM wave is recorded as D2. The current output by the instrument is collected by the calibration system and the value is recorded as I2.

[0046] S3. Use the linear method to compensate the current loop, the compensation coefficient k = (D2-D1) / (I2-I1), the offset b = D2-k*I2;

[0047] S4, the single-chip microcomputer U3 records the values ​​of k and b in the internal flash. When the output current of the instrument is I, the duty cycle of the pwm wave output by the single-chip microcomputer U3 is D = k*I+b;

[0048] S5. The instrument outputs a specific current according to the requirements of the current loop calibration system;

[0049] S6. The current loop calibration system compares the specific current output by the instrument with the actual current collected, and controls the error between the specific current output by the instrument and the actual current collected to within 0.3%.

[0050] This method can significantly improve the accuracy of the 4-20mA current loop and increase the reliability of the 4-20mA current loop output through calibration and output comparison of the current loop calibration system in the calibration method.

[0051] Example 3, based on Example 2, the frequency of the PWM wave of this example is set to 5000HZ, the adjustable range of the duty cycle is 0~100%, and the adjustment accuracy of the duty cycle is 0.01%; in this embodiment, VIN uses +24v, VDD is 5v, and VDD2 is 5.46v; the current loop calibration system communicates with the instrument through the modbus protocol; the working steps of the current loop calibration system are as follows:

[0052] S1, the current loop calibration system sends a command to the instrument, the I / O port of the single-chip computer U3 outputs the theoretical PWM wave corresponding to 4mA, the duty cycle of the PWM wave is recorded as D1, and the current output by the instrument is collected by the calibration system and the value is recorded as I1;

[0053] S2, the current loop calibration system sends a command to the instrument, and the I / O port of the single-chip computer U3 outputs the theoretical PWM wave corresponding to 12mA, and the duty cycle of the PWM wave is recorded as D2. The current output by the instrument is collected by the calibration system and the value is recorded as I2;

[0054] S3. Use the linear method to compensate the current loop: compensation coefficient k = (D2-D1) / (I2-I1); offset b = D2-k*I2;

[0055] S4, the single-chip microcomputer U3 records the values ​​of k and b in the internal flash. When the instrument wants to output a current of I, the PWM duty cycle D output by the single-chip microcomputer U3 = k*I+b;

[0056] S5. The instrument outputs specific currents such as 0mA, 4mA, 8mA, 12mA, 16mA, and 20mA according to the requirements of the current loop calibration system. The current loop calibration system will compare these specific currents with the actual current collected. This example requires that the error must not exceed 0.3%, which can significantly improve the accuracy of the 4-20mA current loop and increase the reliability of the 4-20mA current loop output.

[0057] In summary, the present invention outputs a pwm wave through the I / O port of the single-chip microcomputer U3, the pwm wave is connected to the pwm voltage conversion module 2, and a constant voltage is output. The constant voltage is input to the 4-20mA current loop module 3, and the current loop calibration system calibrates and tests the current output by the current loop. The single-chip microcomputer U3 with a DAC module or a DAC dedicated chip is not required, which improves the versatility of the circuit and reduces the cost. After the current loop is calibrated, the output accuracy of the 4-20mA current loop can be significantly improved. Therefore, the present invention effectively overcomes the various shortcomings in the prior art and has a high industrial utilization value.

[0058] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A circuit calibration method based on PWM wave control of 4-20mA current loop, using a current loop calibration system, the current loop calibration system and the instrument communicate through the modbus protocol; A circuit for controlling a 4-20mA current loop based on a pwm wave is provided, the circuit comprising: Central processing module, the central processing module adopts a single chip microcomputer U3, and the single chip microcomputer U3 outputs pwm wave through the I / O port; A pwm-to-voltage module, which is composed of an NMOS tube Q1, an operational amplifier U1, a resistor R1, a resistor R2, a resistor R3, a resistor R4, a capacitor C1, a capacitor C2, and a capacitor C3, and outputs a voltage signal Vout; 4-20mA current loop module, Vout is connected to the 4-20mA current loop module; the 4-20mA current loop module is composed of a current loop control chip U2, a transistor Q2, a resistor R5, a resistor R6, a resistor R8, a resistor R9, a resistor R10, a resistor R11, a resistor R12, a resistor R13, a capacitor C4, a capacitor C5, a capacitor C6, a PMOS tube Q3, and a diode D1; A power management module is used to provide a power supply voltage VCC; the power management module uses a charge and discharge management chip U4 to manage the power supply of the entire circuit; The central processing module and the power management module are both provided with a chip internal working voltage VDD, and the pwm voltage conversion module and the 4-20mA current loop module are both provided with a chip internal working voltage VDD2; The I / O port of the single chip microcomputer U3 outputting the pwm wave is connected to the gate of the NMOS tube Q1 and grounded through the resistor R4, the drain of the NMOS tube Q1 is connected to one end of the resistor R2 and connected to the voltage VDD2 through the resistor R1, and the source is grounded; The other end of the resistor R2 is connected to one end of the capacitor C1 and one end of the resistor R3, the other end of the capacitor C1 is grounded, the other end of the resistor R3 is connected to one end of the capacitor C2 and pin 1 of the operational amplifier U1, and the other end of the capacitor C2 is grounded; Pin 2 of the operational amplifier U1 is grounded, pin 5 of U1 is connected to voltage VDD2 and grounded through capacitor C3, and pin 3 and pin 4 of the operational amplifier U1 are connected; The current loop control chip U2 has 10 pins. The first pin of the current loop control chip U2 is connected to the voltage VCC and grounded through capacitors C5 and C6; the second pin of the current loop control chip U2 is connected to the emitter of the transistor Q2 and one end of the resistor R12; the base of the transistor Q2 and the other end of the resistor R12 are connected to the source of the PMOS tube Q3; the collector of the transistor Q2 is connected to the third pin of the current loop control chip U2 and the gate of the PMOS tube Q3, the drain of Q3 is connected to the anode of the diode D1, and the cathode of the diode D1 outputs a 4-20mA current loop output signal; One end of the resistor R9 and one end of the resistor R8 are connected to the 4th pin of the current loop control chip U2, and the other end of R9 is grounded; the 5th pin of the current loop control chip U2 and the other end of R8 are connected to the voltage VDD2 and grounded through the capacitor C4; one end of the resistor R6 and one end of the resistor R5 are connected to the 6th pin of U2, and the other end of the resistor R6 is connected to the voltage signal output by the pwm wave conversion voltage module. The 7th pin of the current loop control chip U2 is grounded through a resistor R13; the 8th pin of the current loop control chip U2 is connected to the voltage VDD through a resistor R10; the 9th pin of the current loop control chip U2 is connected to an I / O port of the single-chip computer U3 and is grounded through a resistor R11. When the I / O port of the single-chip computer U3 outputs a high level, the current loop can be closed, so that the output current of the current loop is 0; the 10th pin of the current loop control chip U2 is grounded; The other end of the resistor R5 is set to be floating without connecting any signal or connected to an FSK signal for the current loop to output a HART waveform; The model of the current loop control chip U2 is XTR111AIDRCT; Characterized in that the method comprises the following working steps: S1, the current loop calibration system sends a command to the instrument, the I / O port of the single-chip computer U3 outputs the theoretical PWM wave corresponding to 4mA, the duty cycle of the PWM wave is recorded as D1, the current output by the instrument is collected by the calibration system and the value is recorded as I1; S2, the current loop calibration system sends a command to the instrument, and the I / O port of the single-chip computer U3 outputs the theoretical PWM wave corresponding to 12mA. The duty cycle of the PWM wave is recorded as D2. The current output by the instrument is collected by the calibration system and the value is recorded as I2. S3. Use the linear method to compensate the current loop, the compensation coefficient k = (D2-D1) / (I2-I1), the offset b = D2-k*I2; S4, the single-chip microcomputer U3 records the values ​​of k and b in the internal flash. When the output current of the instrument is I, the duty cycle of the pwm wave output by the single-chip microcomputer U3 is D = k*I+b; S5. The instrument outputs a specific current according to the requirements of the current loop calibration system; S6. The current loop calibration system compares the specific current output by the instrument with the actual current collected, and controls the error between the specific current output by the instrument and the actual current collected to within 0.3%.

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