Isolated 32-channel 4-20mA high-precision current acquisition system

Abstract

The invention discloses an isolated 32-channel 4-20mA high-precision current acquisition system, which comprises a power supply management module, an MCU (Microprogrammed Control Unit) control module, an RJ45 network communication module, an isolated 4-20mA current generation module, a resistance sampling module, an isolated signal conditioning module, a low-pass filtering module and an ADC (Analog to Digital Converter) measurement module, the power management module is used for providing required power rails for the MCU control module, the RJ45 network communication module, the isolation 4-20 mA current generation module, the resistance sampling module, the isolation signal conditioning module, the low-pass filtering module and the ADC measurement module in the system. According to the invention, the precision of all 32 channels + / -0.075% can be realized in a full-scale range, the completely isolated current measuring circuit is provided with an isolation power supply, and noise interference is reduced to the greatest extent while accurate measurement is realized.

Description

Technical Field

[0001] The present invention relates to the field of industrial automation technology, and in particular to an isolated 32-channel 4-20mA high-precision current acquisition system. Background Art

[0002] Currently, 4-20mA signals are widely used in the measurement and control fields of industrial automation systems, such as temperature measurement and control, pressure measurement and control, liquid level measurement and control, etc. In industrial automation and process control, it is essential to continuously and accurately monitor terminal variables of multiple channels. However, industrial sites usually require sensors to be directly in contact with grounded metal structures, while the control systems they communicate with can refer to different ground potentials. This potential difference will generate a ground loop current, which will cause unwanted noise and potential offsets in the signal due to the long distance involved, thereby significantly reducing the measurement accuracy.

[0003] To this end, this application proposes an isolated 32-channel 4-20mA high-precision current acquisition system. Summary of the invention

[0004] The purpose of the present invention is to solve the above technical problems and to propose an isolated 32-channel 4-20mA high-precision current acquisition system.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] An isolated 32-channel 4-20mA high-precision current acquisition system, including a power management module, an MCU control module, an RJ45 network communication module, an isolated 4-20mA current generation module, a resistance sampling module, an isolated signal conditioning module, a low-pass filter module, and an ADC measurement module;

[0007] The power management module is used to provide the required power rails for each part of the MCU control module, RJ45 network communication module, isolated 4-20mA current generation module, resistance sampling module, isolated signal conditioning module, low-pass filter module, and ADC measurement module in the system;

[0008] The main functions of the MCU control module are: communicating with the host computer; reading the ADC voltage value; selecting the ADC acquisition channel link to realize the time division multiplexing of ADC acquisition; selecting the calibration channel to realize the step-by-step calibration or fixed-point calibration of 1-32 channels;

[0009] The RJ45 network communication module is mainly used to transmit data signals and is used for interconnection between Ethernet devices and networks;

[0010] The isolated 4-20mA current generating module is mainly used to generate a 4-20mA current simulating an industrial site;

[0011] The resistance sampling module is connected as an independent PCB board through a male and female board-to-board connector;

[0012] The isolated signal conditioning module mainly realizes the isolation and amplification of the current acquisition voltage;

[0013] The low-pass filter module is used to filter out interference on the collected voltage.

[0014] Preferably, the power management module includes a main power supply, and the main power supply adopts a standard 24V power supply.

[0015] Preferably, the number of the isolated signal conditioning modules is 32, and the 32 isolated signal conditioning modules mainly include 32 independent precision fixed gain amplifiers AMC3330 and precision amplifiers TLV313 with fully integrated isolated DC / DC converters.

[0016] Preferably, 24 isolation operational amplifier modules are further included, and the 24 isolation operational amplifier modules adopt a 5V LDO power supply with low noise and high PSRR.

[0017] Preferably, the isolated 4-20mA current generating module simulates a 4-20mA current loop and is connected in series with an external digital multimeter, and then controlled by the MCU unit to access a calibration loop to achieve self-calibration of 32-channel currents on the board.

[0018] Preferably, the resistance sampling module is equipped with 32 precision molded metal film resistors, which serve as current sampling resistors.

[0019] Preferably, the low-pass filtering module mainly includes 4 independent low-pass filters with a cut-off frequency of 10 Hz.

[0020] Preferably, the MCU control module uses a 3.3V LDO power supply.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] All 32 channels can achieve an accuracy of ±0.075% within the full-scale range, and the on-board isolated 4-20mA current loop that meets industrial standards can be easily used for self-calibration with an external high-precision digital multimeter.

[0023] The fully isolated current measurement circuit of the present invention has its own isolated power supply, which can achieve accurate measurement while reducing noise interference to the greatest extent.

[0024] The present invention is based on TI's highly integrated isolation operational amplifier, and the integrated isolation DC / DC converter can measure non-grounded signals, which is convenient for achieving isolation between channels in multi-channel applications.

[0025] The communication between the present invention and the host computer adopts RJ45 network communication, which easily realizes the unified management of multiple systems. The highly integrated solution not only reduces the volume of the PCB board, but also greatly reduces the testing cost.

[0026] In summary, the present invention can achieve an accuracy of ±0.075% for all 32 channels within the full-scale range, and the fully isolated current measurement circuit has its own isolated power supply, which can achieve accurate measurement while minimizing noise interference. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a diagram of an isolated 32-channel 4-20mA high-precision current acquisition system proposed by the present invention;

[0028] Figure 2 This is one of the power management implementation plan diagrams;

[0029] Figure 3 This is the second diagram of the power management implementation plan;

[0030] Figure 4 This is one of the signal conditioning implementation scheme diagrams;

[0031] Figure 5 This is the second diagram of the signal conditioning implementation plan;

[0032] Figure 6 This is one of the current detection implementation scheme diagrams;

[0033] Figure 7 This is the second current detection implementation diagram;

[0034] Figure 8 This is one of the onboard self-calibration implementation schemes;

[0035] Fig. 9 This is the second diagram of the onboard self-calibration implementation plan. DETAILED DESCRIPTION

[0036] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0037] Reference Figure 1-Figure 9 , an isolated 32-channel 4-20mA high-precision current acquisition system, including a power management module, an MCU control module, an RJ45 network communication module, an isolated 4-20mA current generation module, a resistance sampling module, an isolated signal conditioning module, a low-pass filter module, and an ADC measurement module;

[0038] The power management module is used to provide the required power rails for each part of the MCU control module, RJ45 network communication module, isolated 4-20mA current generation module, resistance sampling module, isolated signal conditioning module, low-pass filter module, and ADC measurement module in the system;

[0039] The main functions of the MCU control module are: communicating with the host computer; reading the ADC voltage value; selecting the ADC acquisition channel link to realize the time division multiplexing of ADC acquisition; selecting the calibration channel to realize the step-by-step calibration or fixed-point calibration of 1-32 channels;

[0040] The RJ45 network communication module is mainly used to transmit data signals and is used for interconnection between Ethernet devices and networks;

[0041] The isolated 4-20mA current generating module is mainly used to generate a 4-20mA current simulating an industrial site;

[0042] The resistance sampling module is connected as an independent PCB board through a male and female board-to-board connector;

[0043] The isolated signal conditioning module mainly realizes the isolation and amplification of the current acquisition voltage;

[0044] The low-pass filter module is used to filter out interference on the collected voltage.

[0045] In the present invention, the power management module includes a main power supply, and the main power supply adopts a standard 24V power supply.

[0046] In the present invention, the number of the isolated signal conditioning modules is 32, and the 32 isolated signal conditioning modules mainly include 32 independent precision fixed gain amplifiers AMC3330 and precision amplifiers TLV313 with fully integrated isolated DC / DC converters.

[0047] The present invention further comprises 24 isolation operational amplifier modules, and the 24 isolation operational amplifier modules adopt a 5V LDO power supply with low noise and high PSRR.

[0048] In the present invention, the isolated 4-20mA current generating module simulates a 4-20mA current loop and is connected in series with an external digital multimeter, and then is controlled by the MCU unit to access a calibration loop to achieve self-calibration of 32-channel currents on the board.

[0049] In the present invention, the resistor sampling module is equipped with 32 precision molded metal film resistors, which serve as current sampling resistors.

[0050] In the present invention, the low-pass filter module mainly includes 4 independent low-pass filters with a cut-off frequency of 10 Hz.

[0051] In the present invention, the 3.3V LDO power supply used by the MCU control module; in order to further reduce power dissipation, the 24V is first stepped down to 5.2V by a first-level DCDC, and then input into an LDO regulator with an extremely low input and extremely low voltage drop having an enable terminal. The 3.3V LDO power input used by the MCU control unit is also provided by the 5.2V power supply. The onboard calibration module needs to be isolated from the measurement and control module, so the power supply also needs to be isolated. The present invention uses two small-volume, high-efficiency, micro-power, constant voltage input, isolated non-regulated single-channel output, DC / DC module power supplies.

[0052] In the present invention, the power input of the present invention adopts a standard 24V power supply, but is not limited to 24V. It can work normally within a wide power supply voltage range of +21.6V to 26.4V. The input interface is equipped with a transient voltage suppressor TVS, which is mainly used to protect the circuit from the impact of transient high-voltage spike pulses (static electricity or lightning surges). Most electronic components use direct current to work, and the reverse connection of the power line may burn out. In order to prevent the positive and negative input terminals of the power supply from causing unexpected situations such as burning of the load circuit, the present invention adopts an anti-reverse connection circuit. The traditional anti-reverse connection scheme is a series diode. Its unidirectional conduction characteristics can effectively prevent the reverse connection of the power supply from damaging the circuit, but the problem that comes with it is that the diode has a PN node voltage, which is usually around 0.7V. The low current has no obvious effect, but when a large current flows, such as a 1A current, it will generate 0.7W of power consumption. The 0.7W power consumption and heat generation are a very big test for the reliability of the component itself and the surrounding components. It can be seen that the biggest problem of diode reverse connection is the tube voltage drop. The lower the loss, the smaller the loss. Among transistors, the field effect tube has the lowest conduction voltage drop. In the present invention, when the input is positive at the top and negative at the bottom, the MOS parasitic diode goes to the ground through R1 and R3. After the voltage is divided by R1 and R3, the GS pole voltage is greater than the MOS conduction voltage Vgs, the MOS is turned on, and the entire circuit loop is connected. When the input is negative at the top and positive at the bottom, the current path is reversely cut off by the MOS parasitic diode, the MOS tube is cut off because there is no voltage at the GS pole, the entire circuit loop is disconnected, and the system circuit is effectively protected. TPS62933 is a high-efficiency synchronous buck converter. The device adopts a fixed-frequency peak current control mode to achieve fast transient response and excellent line and load regulation. The internal loop compensation is optimized and no external compensation components are required. Operating in forced continuous current modulation mode can maintain low output ripple under all load conditions. TPS62933O operates in Out-of-Aud io mode to avoid audible noise. The post-stage LDO circuit further reduces noise.

[0053] The current acquisition scheme in the present invention is a precision current sensing resistor + enhanced isolation amplifier + precision amplifier. The sampling resistor used in the present invention uses a 96% alumina magnetic rod, combined with precision metal film deposition technology, and laser resistance adjustment to achieve the target resistance. The lowest temperature coefficient of this series is ±5ppm / ℃, and the load stability is significantly improved compared to general metal film resistors. At an ambient temperature of 70℃, the maximum resistance value changes by ±0.05% after loading the rated power for 1000 hours. It has strong moisture resistance and the pin structure is conducive to reducing the influence of PCB stress on the resistor. It is very suitable for electronic circuits with high requirements for temperature coefficient and resistance stability. AMC3330 is a precision isolated amplifier with a fully integrated isolated DC / DC converter that can achieve single power supply operation on the low side of the device. The enhanced capacitor isolation layer is certified by VDE V0884-11 and UL1577 standards, separating parts of the system operating at different common-mode voltage levels and protecting the low-voltage domain from damage. The input of AMC3330 is optimized for direct connection to a high-impedance voltage signal source (such as a resistor divider network) to sense high-voltage signals. The integrated isolated DC / DC converter can measure non-grounded signals and makes the device a unique solution for noisy space-constrained applications. The device has excellent performance and supports accurate voltage monitoring and control. Even with a current of 20mA on a 10R sampling resistor, the voltage is only 200mV. The AMC3330 has a fixed gain of 2.0 and the output is a differential voltage signal. For ADCs with non-fully differential inputs, a differential-to-single-ended circuit needs to be designed in the post-stage of the AMC3330. In order to make the ADC have the best acquisition dynamic range, the present invention adopts the TLV313 differential amplifier circuit and designs a 6-fold voltage amplification factor. In the current range of 4-20mA, the input voltage of the ADC is 480mV-2400mV.

[0054] The present invention has a total of 32 acquisition channels. In order to reduce costs, time division multiplexing is used for polling acquisition. The four 8-input channel MUX multiplexing switches on the board can poll the 32 acquisition voltages into the four acquisition channels of the ADC. The 10Hz low-pass filter further reduces the input noise. The reference voltage of the 24-bit ADC chip uses a low-noise, low-drift and high-precision reference voltage source REF2030AIDDCR. This reference voltage source has excellent long-term stability and low noise level, and is an ideal choice for high-precision industrial applications.

[0055] This method is to ensure the accuracy of the measurement results and to ensure that the acquisition system is in good condition and high performance for a long time. The board is equipped with a 4-20mA current generating circuit. When calibration is required, the MCU switches the channel to be calibrated from the loop measurement mode to the calibration mode. The XTR1110 module on the board can be set by the MCU to output a 4-20mA current. The board has a reserved interface for connecting a high-precision digital multimeter. To ensure an accuracy of ±0.075%, a digital multimeter with more than 6 and a half digits should be used. Automatic calibration of 32 channels can be easily achieved through the upper computer calibration software.

[0056] In summary, the present invention has completely isolated current collection, and the measuring system and the measured system are independent of each other and do not interfere with each other.

[0057] With up to 32 measurement channels, it reduces both size and cost.

[0058] Each channel has an accuracy of ±0.075%, which greatly improves the measurement accuracy.

[0059] With the help of the host computer software, 32-channel self-calibration can be easily achieved.

[0060] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. An isolated 32-channel 4-20mA high-precision current acquisition system, characterized in that: Including power management module, MCU control module, RJ45 network communication module, isolated 4-20mA current generation module, resistance sampling module, isolated signal conditioning module, low-pass filter module, ADC measurement module; The power management module is used to provide the required power rails for each part of the MCU control module, RJ45 network communication module, isolated 4-20mA current generation module, resistance sampling module, isolated signal conditioning module, low-pass filter module, and ADC measurement module in the system; The main functions of the MCU control module are: communicating with the host computer; reading the ADC voltage value; selecting the ADC acquisition channel link to realize the time division multiplexing of ADC acquisition; selecting the calibration channel to realize the step-by-step calibration or fixed-point calibration of 1-32 channels; The RJ45 network communication module is mainly used to transmit data signals and is used for interconnection between Ethernet devices and networks; The isolated 4-20mA current generating module is mainly used to generate a 4-20mA current simulating an industrial site; The resistance sampling module is connected as an independent PCB board through a male and female board-to-board connector; The isolated signal conditioning module mainly realizes the isolation and amplification of the current acquisition voltage; The low-pass filter module is used to filter out interference on the collected voltage.

2. An isolated 32-channel 4-20mA high-precision current acquisition system according to claim 1, characterized in that: The power management module includes a main power supply, and the main power supply adopts a standard 24V power supply.

3. The isolated 32-channel 4-20mA high-precision current acquisition system according to claim 1, characterized in that: The number of the isolated signal conditioning modules is 32, and the 32 isolated signal conditioning modules mainly include 32 independent precision fixed gain amplifiers AMC3330 and precision amplifiers TLV313 with fully integrated isolated DC / DC converters.

4. The isolated 32-channel 4-20mA high-precision current acquisition system according to claim 1, characterized in that: It also includes 24 isolation operational amplifier modules, and the 24 isolation operational amplifier modules use a 5V LDO power supply with low noise and high PSRR.

5. The isolated 32-channel 4-20mA high-precision current acquisition system according to claim 1, characterized in that: The isolated 4-20mA current generating module simulates a 4-20mA current loop and is connected in series with an external digital multimeter, and then controlled by the MCU unit to access a calibration loop to realize self-calibration of 32-channel currents on the board.

6. The isolated 32-channel 4-20mA high-precision current acquisition system according to claim 1, characterized in that: The resistor sampling module is equipped with 32 precision molded metal film resistors, which serve as current sampling resistors.

7. The isolated 32-channel 4-20mA high-precision current acquisition system according to claim 1, characterized in that: The low-pass filter module mainly includes 4 independent low-pass filters with a cut-off frequency of 10 Hz.

8. The isolated 32-channel 4-20mA high-precision current acquisition system according to claim 1, characterized in that: The MCU control module uses a 3.3V LDO power supply.

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