4-20mA current output circuit for PLC (Programmable Logic Controller) system

Abstract

The invention provides a 4-20mA current output circuit for a PLC (Programmable Logic Controller) system. A control signal provided by the PLC system is converted into a corresponding digital signal through an input signal processing module; the digital signal is transmitted to the DAC conversion module through the isolation module; the digital signal is converted into a corresponding analog voltage signal through a DAC conversion module; the voltage-to-current module uses a voltage-to-current circuit to generate stable 4-20 mA current output from the voltage signal converted by the DAC, the structure is simple, the design is concise, no complex analog circuit assembly exists, the DAC conversion module is adopted to convert the digital signal into the voltage signal suitable for adjusting the current, and the voltage-to-current conversion module is used for converting the digital signal into the voltage signal suitable for adjusting the current. And current adjustment is performed through the voltage-to-current circuit, current output is accurately controlled, the requirement of a PLC system for precision is met, the design complexity is also reduced, and the circuit has the characteristics of high reliability, high precision, high stability and easiness in implementation. And long-time reliable operation in an industrial environment can be realized.

Description

Technical Field

[0001] The present invention relates to the technical field of signal transmission, and particularly to a 4-20mA current output circuit for a PLC system. Background Art

[0002] In industrial automation systems, the 4-20mA current loop, as a standard analog signal transmission method, is commonly used for communication between PLC control systems and sensors or actuators. The PLC system usually needs to convert its internal control signals (such as digital or analog signals) into 4-20mA current signals for data transmission and control with external devices.

[0003] Existing 4-20mA current output circuits usually adopt complex designs, including digital signal conversion modules or integrated circuits, which may result in high costs, complex designs, and poor reliability. To overcome these problems, there is an urgent need for a 4-20mA current output circuit with a simple structure, low cost, stable performance, and easy integration. Summary of the Invention

[0004] The purpose of the present invention is to provide a 4-20mA current output circuit with a simple structure, low cost, stable performance, and easy integration, which converts the control signal of the PLC into a standard 4-20mA current signal through a simple circuit design.

[0005] To achieve the above object, the present invention provides a 4-20mA current output circuit for a PLC system, including: Input signal processing module: Receives the control signal provided by the PLC system, converts the control signal provided by the PLC system into a corresponding digital signal, and performs adaptation processing on the input signal as required; Isolation module: Isolates external interference, isolates the external signal from the input signal processing module, protects the input signal processing module, and transmits the digital signal to the DAC conversion module; DAC conversion module: Converts the digital signal (such as a binary signal) into a corresponding analog voltage signal; Voltage-to-current module: Generates a stable 4-20mA current output using a voltage-to-current circuit based on the voltage signal after DAC conversion.

[0006] Further, the voltage-to-current circuit in the voltage-to-current module is: One end of resistor R2 is connected to the input voltage, and the other end is connected to capacitor C4; the common end of resistor R2 and capacitor C4 is connected to pin 6 of chip U1; The other end of capacitor C4 is connected to resistor R3. The other end of resistor R3 is connected to pin 7 of chip U1. The common terminal of resistor R3 and capacitor C4 is connected to pins 10 and 11 of chip U1 and is commonly connected to AGND; The common terminal where pin 4 and pin 5 of chip U1 are shorted is connected to one end of capacitor C3. The other end of capacitor C3 is grounded to AGND; Capacitor C1 and capacitor C2 are connected in parallel. One end is commonly connected to pin 1 of chip U1, and the other end is commonly connected to pin 9 of chip U1 and is directly connected to AGND together with pin 9 of chip U1; Pin 2 of chip U1 is connected to the emitter of triode T1. One end of resistor R1 is connected to the common terminal of pin 2 of chip U1 and triode T1, and the other end is connected to the base of triode T1; Pin 3 of chip U1 is connected to the collector of triode T1; The common terminal of pin 3 of chip U1 and the collector of triode T1 is connected to the gate of MOS transistor Q1; The common terminal of resistor R1 and the base of triode TI is connected to the drain of MOS transistor Q1. The drain of the drain terminal of MOS transistor Q1 is the output current source.

[0007] Further, the input signal processing module is a single-chip microcomputer MCU.

[0008] Further, the ratio of the input voltage to resistor R3 is the output current.

[0009] Further, pin 1 of chip U1 is the power supply pin, and the supply voltage is 12.5V.

[0010] Further, pin 9 of chip U1 is the control output status pin and is directly connected to AGND.

[0011] Further, it further includes an output current module. The output current module is used to output the adjusted current to an external device (such as a sensor, an actuator, an instrument, etc.) through the output terminal for the transmission and control of the PLC.

[0012] Further, the value of resistor R1 is 15Ω, the value of resistor R2 is 1kΩ, and the value of resistor R3 is 1kΩ.

[0013] Further, pin 1 of the chip is the power input terminal of the chip, which is used to supply power to the entire chip. Pin 2 of the chip is the current output pin, which is connected to an external resistor to limit the output current. Pin 3 of the chip is the gate driver of the external field effect transistor, which can prevent the short circuit of the power supply and the ground. The circuit is clamped to keep within 18V of the positive power supply. If the FET gate has the potential to exceed the specified rating, it protects the external FET. Pins 4 and 5 of the chip are used together and can supply a stable voltage source to the outside. However, in the circuit of the present invention, this function is not used. Therefore, to protect the characteristics of the device, it is selected to be connected to a capacitor and then connected to AGND. Pin 6 of the chip is the signal input pin, and the signal of the DAC conversion enters the chip. Pin 7 of the chip is the gain adjustment pin, and this pin can adjust the gain through the resistor connected to it, thereby adjusting the ratio between the input voltage and the output current. Pin 8 of the chip is the shutdown control pin of the chip, which is used to turn off the operation of the chip by pulling down the level. Since in the circuit of the present invention, there is no need to turn off the chip, this pin is in a floating state. Pin 9 of the chip is the current inhibit output pin, and this pin is generally set to a low level and is connected to AGND. Pin 10 of the chip is the ground pin, which is directly connected to AGND. Pin 11 of the chip is the pad at the bottom of the chip, and this pad is grounded and is connected to AGND.

[0014] Further, the voltage-to-current circuit includes four modules, namely the first module, the second module, the third module, and the fourth module. Among them, the first module circuit is composed of short-circuiting capacitor C3 between pins 4 and 5 of chip U1. Pins 4 and 5 of chip U1 can be used to output a stable voltage. This function is not used in this circuit. If the pins are floating, they can be interfered by the outside. Therefore, after short-circuiting them, they are connected to capacitor C3, and C3 is then connected to the ground. The main function of this capacitor C3 is to eliminate noise and is used to isolate the electromagnetic interference between the chip and the outside. The second module circuit includes capacitor C4, resistor R3, pin 6 of chip U1, pin 7 of chip U1, pin 10 of chip U1, and pin 11 of chip U1, as Figure 2As shown in the figure, the voltage signal after DAC conversion enters pin 6 of the chip through resistor R2 and capacitor C4. Resistor R2 and capacitor C4 form an RC circuit, which can reduce the steepness of the signal edge and reduce the interference of noise on the signal. Pin 7 of the chip is the setting pin for the output current, and the size of the output current can be adjusted by adjusting the size of resistor R3 connected thereto. Pin 10 of the chip is the grounding pin, and pin 11 of the chip is the pad at the bottom of the chip, and this pad is grounded, so pins 10 and 11 are shorted and connected to AGND. In the third module circuit, capacitor C1 and capacitor C2 are connected in parallel between pin 1 and pin 9 of chip U1. Pin 1 of this chip U1 is the power input terminal of the chip, that is, the power supply pin, which supplies power to the entire chip, and the supply voltage is 12.5V. Pin 9 of the chip is the current inhibit output pin, and this pin is generally set to a low level and is connected to AGND. Capacitor C1 and C2 are connected in parallel between pin 1 and pin 9, that is, in parallel between the power supply 12.5V and AGND. The bypass capacitor between the power supply and AGND can help stabilize the power supply voltage, absorb the sudden pulses in the circuit, and maintain a constant voltage level. The fourth module includes transistor T1, pin 2 of chip U1, pin 3 of chip U1, resistor R1, and MOS transistor Q1. The function of the fourth module circuit is an output current limiting circuit. Pin 2 of the chip is the current output terminal, and pin 3 of the chip is the gate driver of the field effect transistor. When the output current is small, the voltage across resistor R1 is small, and transistor T1 is not conducting. At this time, MOS transistor Q1 is conducting, and the current can be output smoothly. When the current is large (greater than 37mA), when passing through resistor R1, the voltage across resistor R1 is high, and transistor T1 is conducting. At this time, MOS transistor Q1 will turn off, and the current will stop outputting, thus achieving the effect of current limiting.

[0015] Furthermore, the working principle of the present invention is as follows: 1. The PLC system sends a control signal to the input signal processing module through communication. The input signal processing module converts the input signal into a digital signal and transmits it to the isolation module; 2. The converted digital signal is transmitted to the DAC conversion module through the isolation module; 3. The digital signal is converted into a voltage signal suitable for adjusting the current through the DAC conversion module; 4. Through the voltage-to-current circuit in the voltage-to-current module, the output current is adjusted according to the input voltage signal to ensure that an accurate 4-20mA current is output within the signal range; 5. Finally, the output current module outputs the adjusted current to an external device for the transmission and control of the PLC.

[0016] Compared with the prior art, the advantages of the present invention are as follows: 1. The structure of the present invention is simple and the design is concise. There are no complex analog circuit components. By adopting a DAC conversion module, digital signals are converted into voltage signals suitable for regulating current, and current regulation is performed through a voltage-to-current circuit to accurately control the current output. This not only meets the accuracy requirements of the PLC system but also reduces the design complexity. It has the characteristics of high integration, high reliability, high precision, strong stability, strong anti-interference ability, and easy implementation, and can operate reliably for a long time in an industrial environment. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the framework of the 4 - 20mA current output circuit for the PLC system in the embodiment of the present invention; Figure 2 It is a schematic diagram of the voltage-to-current circuit in the embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions of the present invention will be further described below.

[0019] This embodiment provides a 4 - 20mA current output circuit for a PLC system. As Figure 1 shown, the output circuit structure includes an input signal processing module, an isolation module, a DAC conversion module, a voltage-to-current module, and a current output module. Among them, the input signal processing module is a single-chip microcomputer MCU, which is used to receive the control signal provided by the PLC system, convert the control signal provided by the PLC system into a corresponding digital signal, and perform adaptation processing on the input signal as needed; The isolation module is used to isolate external interference, isolate the external signal from the input signal processing module, protect the input signal processing module, and transmit the digital signal to the DAC conversion module; The DAC conversion module is used to convert digital signals (such as binary signals) into corresponding analog voltage signals; In the voltage-to-current module, a voltage-to-current circuit is used to generate a stable 4 - 20mA current output from the voltage signal converted and processed by the DAC conversion module.

[0020] The output current module is used to output the current regulated by the voltage-to-current module to external devices (such as sensors, actuators, meters, etc.) through the output terminal for the transmission and control of the PLC.

[0021] In this embodiment, as Figure 2 shown, the voltage-to-current circuit in the voltage-to-current module is specifically: One end of resistor R2 is connected to the input voltage, and the other end is connected to capacitor C4; the common terminal of resistor R2 and capacitor C4 is connected to pin 6 of chip U1, so that the signal after DAC conversion passes through the current limiting of resistor R2 and the filtering of capacitor C4 and enters pin 6 of chip U1.

[0022] The other end of capacitor C4 is connected to resistor R3, the other end of resistor R3 is connected to pin 7 of chip U1, and the common terminal of resistor R3 and capacitor C4 is connected to pins 10 and 11 of chip U1 and is commonly connected to AGND, so that resistor R3 is connected between chip U1 and AGND. The ratio of the input voltage OUTA to resistor R3 is the output current OUT1. The conversion formula for converting the voltage value to a 4-20 mA current is: I OUT =V OUTA / R3 The common terminal where pins 4 and 5 of chip U1 are short-circuited is connected to one end of capacitor C3, and the other end of capacitor C3 is grounded to AGND; Capacitors C1 and C2 are connected in parallel. One end is commonly connected to pin 1 of chip U1, and the other end is commonly connected to pin 9 of chip U1 and is directly connected to AGND together with pin 9 of chip U1; among them, pin 1 of chip U1 is the power supply pin, and the supply voltage is 12.5 V; pin 9 of chip U1 is the control output status pin and is directly connected to AGND. Chip U1 always has an output signal when the power supply is satisfied.

[0023] Pin 2 of chip U1 is connected to the emitter of triode T1 and is also connected to one end of resistor R1, as Figure 2 shown. One end of this resistor R1 is connected to the common terminal of pin 2 of chip U1 and triode T1, and the other end is connected to the base of triode T1; in this embodiment, resistor R1 and triode T1 have a current limiting effect on the output signal.

[0024] Pin 3 of chip U1 is connected to the collector of triode T1 and is also connected to the gate of MOS transistor Q1, as Figure 2 shown. The common terminal of pin 3 of this chip U1 and the collector of triode T1 is connected to the gate of MOS transistor Q1. The main function of this MOS transistor Q1 is energy dissipation.

[0025] At the same time, the common terminal where resistor R1 is connected to the base of triode TI is connected to the drain of MOS transistor Q1. The drain of the drain terminal of this MOS transistor Q1 is the output current source.

[0026] In this embodiment, the U1 chip actually uses the XTR111AIDGQR chip of Texas Instruments. Pin 1 of the chip is the power input terminal of the chip, which is used to supply power to the entire chip. Pin 2 of the chip is the current output pin, which is connected to an external resistor to limit the output current. Pin 3 of the chip is the gate driver of the external field effect transistor, which can prevent the short circuit of the power supply and the ground. The circuit is clamped to stay within 18V of the positive power supply. If the FET gate has the potential to exceed the specified rating, the external FET is protected. Pins 4 and 5 of the chip are used together and can supply a stable voltage source to the outside. However, in the circuit of the present invention, this function is not used. Therefore, to protect the characteristics of the device, it is selected to be connected to a capacitor and then connected to AGND. Pin 6 of the chip is the signal input pin, and the signal after DAC conversion enters the chip. Pin 7 of the chip is the gain adjustment pin, and the gain can be adjusted by the resistor connected to it, thereby adjusting the ratio between the input voltage and the output current. Pin 8 of the chip is the shutdown control pin of the chip, which is used to turn off the operation of the chip by pulling down the level. Since the chip does not need to be shut down in the circuit of the present invention, this pin is in a floating state. Pin 9 of the chip is the current inhibit output pin, and this pin is generally set to a low level and is connected to AGND. Pin 10 of the chip is the ground pin, which is directly connected to AGND. Pin 11 of the chip is the pad at the bottom of the chip, and this pad is grounded and is connected to AGND.

[0027] As Figure 2 shown, in this embodiment, the voltage-to-current circuit includes four modules, namely the first module, the second module, the third module, and the fourth module. Among them, the first module circuit is composed of the short-circuit capacitor C3 between pins 4 and 5 of the chip U1. Pins 4 and 5 of the chip U1 can be used to output a stable voltage. This function is not used in this circuit. If the pins are floating, they can be affected by external interference. Therefore, after shorting them, they are connected to the capacitor C3, and C3 is then connected to the ground. The main function of this capacitor C3 is to eliminate noise and isolate the electromagnetic interference between the chip and the outside.

[0028] The second module includes the capacitor C4, the resistor R3, pin 6 of the chip U1, pin 7 of the chip U1, pin 10 of the chip U1, and pin 11 of the chip U1. As Figure 2 shown, the voltage signal after DAC conversion enters pin 6 of the chip through the resistor R2 and the capacitor C4. The resistor R2 and the capacitor C4 form an RC circuit, which can reduce the steepness of the signal edge and reduce the interference of noise on the signal. Pin 7 of the chip is the setting pin for the output current, and the magnitude of the output current can be adjusted by adjusting the magnitude of the resistor R3 connected to it. Pin 10 of the chip is the ground pin, and pin 11 of the chip is the pad at the bottom of the chip, and this pad is grounded. Therefore, pins 10 and 11 are shorted and then connected to AGND.

[0029] In the circuit of the third module, capacitor C1 and capacitor C2 are connected in parallel between pin 1 and pin 9 of chip U1. Pin 1 of this chip U1 is the power input terminal of the chip, that is, the power supply pin, which supplies power to the entire chip, and the supply voltage is 12.5V. Pin 9 of the chip is the current inhibit output pin, and this pin is generally set to a low level and is connected to AGND. Capacitor C1 and C2 are connected in parallel between pin 1 and pin 9, that is, in parallel between the power supply of 12.5V and AGND. The bypass capacitor between the power supply and AGND can help stabilize the power supply voltage, absorb sudden pulses in the circuit, and maintain a constant voltage level.

[0030] The fourth module includes transistor T1, pin 2 of chip U1, pin 3 of chip U1, resistor R1, and MOS transistor Q1. The function of the circuit of this fourth module is an output current limiting circuit. Pin 2 of the chip is the current output terminal, and pin 3 of the chip is the gate driver of the field effect transistor. When the output current is small, the voltage across resistor R1 is small, and transistor T1 is not conducting. At this time, MOS transistor Q1 is conducting, and the current can be output smoothly. When the current is large (greater than 37 mA), when passing through resistor R1, the voltage across resistor R1 is high, and transistor T1 conducts. At this time, MOS transistor Q1 will turn off, and the current stops output, thus achieving the effect of current limiting.

[0031] In this embodiment, the value of resistor R1 is 15 Ω, the value of resistor R2 is 1 kΩ, the value of resistor R3 is 1 kΩ, and the voltage value at pin 6 of chip U1 after DAC conversion is between 0.4V and 2V. The conversion formula for converting the voltage value to a 4 - 20 mA current is: I OUT =V OUTA / R3.

[0032] The chip of this embodiment can be replaced by other similar chips, such as chip INA333, MAX4008.

[0033] The above are only the preferred embodiments of the present invention and do not impose any limitation on the present invention. Any person skilled in the art within the technical field, without departing from the technical solution of the present invention, makes any form of equivalent substitution or modification and other changes to the technical solution and technical content disclosed by the present invention, all of which belong to the content within the technical solution of the present invention and still fall within the protection scope of the present invention.

Claims

1. A 4-20mA current output circuit for a PLC system, characterized in that: include: Input signal processing module: converts the control signal provided by the PLC system into a corresponding digital signal; Isolation module: isolates external interference and transmits the digital signal to the DAC conversion module; DAC conversion module: convert the digital signal into a corresponding analog voltage signal; Voltage-to-current module: Use the voltage-to-current circuit to convert the voltage signal converted by the DAC into a stable 4-20mA current output; The voltage-to-current circuit is: One end of the resistor R2 is connected to the input voltage, and the other end is connected to the capacitor C4; the common end of the resistor R2 and the capacitor C4 is connected to the 6th pin of the chip U1; The other end of capacitor C4 is connected to resistor R3, the other end of resistor R3 is connected to pin 7 of chip U1, the common end of resistor R3 and capacitor C4 is connected to pins 10 and 11 of chip U1, and are connected to AGND together; The common end of the short-circuited pins 4 and 5 of the chip U1 is connected to one end of the capacitor C3, and the other end of the capacitor C3 is grounded; Capacitor C1 and capacitor C2 are connected in parallel, with one end connected to pin 1 of chip U1, and the other end connected to pin 9 of chip U1, and directly connected to AGND together with pin 9 of chip U1; Pin 2 of chip U1 is connected to the emitter of transistor T1, one end of resistor R1 is connected to pin 2 of chip U1 and the common end of transistor T1, and the other end is connected to the base of transistor T1; Pin 3 of chip U1 is connected to the collector of transistor T1; the common end of pin 3 of chip U1 and the collector of transistor T1 is connected to the gate of MOS tube Q1; The common end of the resistor R1 and the base of the transistor TI is connected to the drain of the MOS transistor Q1 , and the drain of the MOS transistor Q1 is an output current source.

2. The 4-20mA current output circuit for a PLC system according to claim 1, characterized in that: The input signal processing module is a single chip microcomputer MCU.

3. The 4-20mA current output circuit for a PLC system according to claim 1, characterized in that: The ratio of the input voltage to the resistor R3 is the output current.

4. The 4-20mA current output circuit for a PLC system according to claim 1, characterized in that: Pin 1 of the chip U1 is a power supply pin, and the supply voltage is 12.5V.

5. The 4-20mA current output circuit for a PLC system according to claim 1, characterized in that: Pin 9 of the chip U1 is a control output status pin and is directly connected to AGND.

6. The 4-20mA current output circuit for a PLC system according to claim 1, characterized in that: It also includes an output current module, which is used to output the regulated current to an external device through an output terminal for transmission and control of the PLC.

7. The 4-20mA current output circuit for a PLC system according to claim 1, characterized in that: The value of the resistor R1 is 15Ω, the value of the resistor R2 is 1kΩ, and the value of the resistor R3 is 1kΩ.

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