阅读说明：本技术 一种基于现场总线的热电偶测量模块及其测量方法 (thermocouple measuring module based on field bus and measuring method thereof ) 是由 李武库 高海涛 李涛 刘文英 于 2019-10-10 设计创作，主要内容包括：本发明提供一种基于现场总线的热电偶测量模块及其测量方法,热电偶测量模块包括处理器、以太网交换机、电源模块和总线隔离单元,处理器分别与以太网交换机、电源模块和总线隔离单元电连接,以太网交换机一路通过隔离变压器后与以太网连接器LINK1电连接,另一路通过隔离变压器后与以太网连接器LINK2电连接,总线隔离单元通过模数转换器与多路模拟开关电连接,多路模拟开关依次通过低通滤波电路、过压保护电路后与IO接口连接器电连接。本模块所有测量端口可以进行热电阻RTD和热电偶TC测量任意配置,集成标准以太网工业协议,特别适用于大数据量传输和实时控制温度测量系统使用,方便工业现场快速组网和系统扩展,满足客户不同需求。(the invention provides a thermocouple measuring module based on a field bus and a measuring method thereof, wherein the thermocouple measuring module comprises a processor, an Ethernet switch, a power supply module and a bus isolation unit, the processor is respectively and electrically connected with the Ethernet switch, the power supply module and the bus isolation unit, one path of the Ethernet switch is electrically connected with an Ethernet connector LINK1 after passing through an isolation transformer, the other path of the Ethernet switch is electrically connected with an Ethernet connector LINK2 after passing through the isolation transformer, the bus isolation unit is electrically connected with a multi-path analog switch through an analog-to-digital converter, and the multi-path analog switch is electrically connected with an IO interface connector after passing through a low-pass filter circuit and an overvoltage protection circuit in sequence. All measurement ports of the module can be configured randomly for measuring the RTD and the TC of the thermocouple, a standard Ethernet industrial protocol is integrated, the module is particularly suitable for large data volume transmission and real-time control temperature measurement systems, rapid networking and system expansion of industrial fields are facilitated, and different requirements of customers are met.)

1. A thermocouple measurement module based on field bus is characterized in that: including treater, ethernet switch, power module and bus isolation unit, the treater is connected with ethernet switch, power module and bus isolation unit electricity respectively, ethernet switch is connected with ethernet connector LINK1 electricity after passing through isolation transformer on one hand, and another way is connected with ethernet connector LINK2 electricity after passing through isolation transformer on the other hand, bus isolation unit passes through analog to digital converter and is connected with multichannel analog switch electricity, multichannel analog switch loops through behind low pass filter circuit, the overvoltage protection circuit and is connected with IO interface connector electricity.

2. the fieldbus-based thermocouple measurement module of claim 1, further comprising: the processor is also electrically connected with the multi-path analog switch through the IO isolation unit.

3. A fieldbus-based thermocouple measurement module as claimed in claim 1 or 2, in which: the power supply module is respectively electrically connected with an IN pin of an M8-4 core power supply connector and an OUT pin of an M8-4 core power supply connector.

4. The fieldbus-based thermocouple measurement module of claim 1, further comprising: the processor model is STM32F407, and the Ethernet switch model is LAN 9303.

5. the fieldbus-based thermocouple measurement module of claim 3, further comprising: the Ethernet connector is an M8-4 core Ethernet connector, and the IO interface connector is an M12-5 core IO connector.

6. The fieldbus-based thermocouple measurement module of claim 5, wherein: the control module is cascaded in the following way: the OUT interface of the upper-stage M8-4 core power connector is electrically connected with the IN interface of the lower-stage M8-4 core power connector, and the LINK2 of the upper-stage Ethernet connector is electrically connected with the LINK1 of the lower-stage Ethernet connector.

7. a method for measuring a thermocouple measuring module according to any one of claims 1 to 6, comprising the steps of:

When the RTD temperature is measured, a proportional method is adopted for measurement, and the ADC works in a bipolar differential mode:

In the formula:

CODE is ADC output CODE;

N is the resolution of the ADC;

Rref is a reference resistor;

g is the selected gain;

By the above formula: the RTD measured resistance value is only related to the reference resistance precision and the drift;

the temperature was calculated by the Callender-Van Dusen formula,

the temperature t <0 ℃ is given by the formula:

R_rtd(t)＝R₀[1+At+Bt²+C(t-100)t³]

the formula of the temperature t is more than or equal to 0 ℃ is as follows:

R_rtd(t)＝R₀(1+At+Bt²)

In the formula:

t is RTD temperature (. degree. C.)

R_rtd(t) is the resistance (Ω) of the RTD at temperature (t)

R₀RTD resistance at 0 deg.C (R when RTD sensor is PT 100)₀＝100Ω)

A. b, C coefficient of sensor for RTD

The processor MCU stores the coefficients, and the current temperature value can be obtained by measuring the resistance of the RTD and calculating.

8. The method of claim 7, wherein the measuring method comprises: when TC temperature is measured, cold end compensation is measured by adopting external PT1000, the magnitude of thermoelectric potential of a TC temperature measuring thermocouple is not only related to hot end temperature, but also related to cold end temperature, and a PT1000 temperature sensor is adopted at each measuring port to be used as cold end temperature measurement;

The cold junction temperature must be converted to a voltage, which is converted using the following polynomial

In the formula:

e is the cold junction thermoelectric voltage;

Ci is a polynomial coefficient related to the thermocouple type;

t₉₀Cold junction temperature (deg.C);

i is a polynomial order;

finally, the thermoelectromotive force E of the TC is obtained through cold end compensation measurement, and the actual temperature value of the TC is calculated on a processor MCU software by adopting an ITS-90 temperature scale coefficient according to the following formula:

t₉₀＝c₀+c₁E+c₂E²+....c_iEⁱ

In the formula:

E is the thermoelectric voltage (uV);

t₉₀actual temperature (. degree. C.) measured for the thermocouple;

C_iIs a polynomial coefficient related to the thermocouple type;

i is a polynomial order.

9. the measurement method of the fieldbus-based thermocouple measurement module according to claim 7 or 8, wherein: when the temperature measurement and calibration of the RTD and the TC are carried out, a high-precision temperature process calibrator is adopted to calibrate the resistance value and the voltage value of the TC respectively, and gain and offset errors can be eliminated for the temperature through calibration; the calibration process is to measure a plurality of resistance values, voltage values and actual values under different gains and store the values into a processor, and calculate the corrected temperature value by a piecewise linear interpolation method.

10. the method of claim 9, wherein the measuring method comprises: the calibrated resistance value and voltage value are calculated by a piecewise linear interpolation method, the working principle of the method is to divide the calibrated sample point into a plurality of different intervals, and the interval is marked as [ X ]_i,X_i+1]Within each intervalis a linear function and satisfies

Then

the resistance value and the voltage value of any point on the measuring interval can be calibrated according to the formula, so that the corrected temperature value can be calculated.

Technical Field

The invention relates to the field of industrial buses, in particular to a thermocouple measuring module based on a field bus and a measuring method thereof.

background

high precision temperature measurement is a key parameter in industrial automation applications. In the field of industrial temperature measurement application, equipment needs to adapt to severe working condition environments, and has the advantages of wide temperature measurement range, high sampling speed and high precision requirement on temperature acquisition. Different customers in field application use different temperature sensors (TC or RTD sensors) and different wiring modes (such as 2/3/4 wiring system), and the current temperature acquisition module is difficult to simultaneously meet the different measurement requirements; meanwhile, some products do not have a standard industrial bus protocol, and cannot meet the requirements of large data volume transmission and inconvenient industrial networking. Therefore, it is an urgent problem to develop a thermocouple measuring module based on a fieldbus and a measuring method thereof.

disclosure of Invention

The invention aims to solve the technical problems and provides a thermocouple measuring module based on a field bus and a measuring method thereof, which can meet the requirements of different sensors, have a standard industrial Ethernet bus protocol, and are safe, reliable and small in size.

In order to solve the technical problems, the invention adopts the technical scheme that:

the utility model provides a thermocouple measuring module based on field bus, includes treater, ethernet switch, power module and bus isolation unit, the treater is connected with ethernet switch, power module and bus isolation unit electricity respectively, ethernet switch is connected with ethernet connector LINK1 electricity after passing through isolation transformer on one hand, and another way is connected with ethernet connector LINK2 electricity after passing through isolation transformer, and ethernet switch extension two way ethernet interfaces carry out data transmission, bus isolation unit is connected with multichannel analog switch electricity through analog-to-digital converter, multichannel analog switch loops through low pass filter circuit, overvoltage protection circuit back and is connected with IO interface connector electricity.

Furthermore, the processor is electrically connected with the multi-channel analog switch through the IO isolation unit, and controls the multi-channel analog switch to realize multi-channel switching and temperature measurement and reading.

furthermore, the power supply module is respectively electrically connected with an IN pin of an M8-4-core power supply connector and an OUT pin of an M8-4-core power supply connector, the power supply module converts external DC 18-36V into an internal circuit which requires 3.3V and 5V levels, and meanwhile, the power supply OUT supplies power for the next-stage module, so that the modules are conveniently cascaded.

Further, the control module adopts IP67 protection grade.

Further, the ethernet connector LINK1 and the ethernet connector LINK2 are respectively connected to an external ethernet communication circuit. The IO interface connector collects temperature signals.

further, the Ethernet connector is an M8-4 core Ethernet connector, and the IO interface connector is an M12-5 core IO connector.

The control module is cascaded in the following way: the OUT interface of the upper-stage M8-4 core power connector is electrically connected with the IN interface of the lower-stage M8-4 core power connector, and the LINK2 of the upper-stage Ethernet connector is electrically connected with the LINK1 of the lower-stage Ethernet connector.

A measuring method adopting the thermocouple measuring module comprises the following steps:

when the RTD temperature is measured, a proportional method is adopted for measurement, and the ADC works in a bipolar differential mode:

in the formula:

CODE is ADC output CODE;

N is the resolution of the ADC;

Rref is a reference resistor;

g is the selected gain;

By the above formula: it is known that the RTD measured resistance is only related to the reference resistance accuracy and drift.

The temperature is calculated by the Callender-Van Dusen formula

The temperature t <0 ℃ is given by the formula:

R_rtd(t)＝R₀[1+At+Bt²+C(t-100)t³]

the formula of the temperature t is more than or equal to 0 ℃ is as follows:

R_rtd(t)＝R₀(1+At+Bt²)

in the formula:

t is RTD temperature (. degree. C.)

R_rtd(t) is the resistance (Ω) of the RTD at temperature (t)

R₀RTD resistance at 0 deg.C (R when RTD sensor is PT 100)₀＝100Ω)

A. b, C coefficient of sensor for RTD

The processor MCU stores the coefficients, and the current temperature value can be obtained by measuring the resistance of the RTD and calculating.

Further, when the TC temperature is measured, the cold end compensation is measured by adopting an external PT1000, and the high-precision low-temperature drift reference is adopted inside, so that the absolute voltage measurement precision is ensured.

the magnitude of the thermoelectric potential of the TC temperature measuring thermocouple is not only related to the temperature of the hot end, but also related to the temperature of the cold end. By employing a PT1000 temperature sensor at each measurement port as a cold side temperature measurement. And the accuracy of the cold end temperature is ensured.

The cold junction temperature must be converted to a voltage, which is converted using the following polynomial

in the formula:

E is the cold junction thermoelectric voltage;

ci is a polynomial coefficient related to the thermocouple type;

t₉₀cold junction temperature (deg.C);

i is a polynomial order;

And finally, obtaining the thermal electromotive force E of the TC through cold end compensation measurement, and calculating the actual temperature value of the TC by adopting an ITS-90 temperature scale coefficient on a processor MCU software according to the following formula.

t₉₀＝c₀+c₁E+c₂E²+....c_iEⁱ

in the formula:

E is the thermoelectric voltage (uV);

t₉₀Actual temperature (. degree. C.) measured for the thermocouple;

C_iis a polynomial coefficient related to the thermocouple type;

i is a polynomial order;

Further, temperature measurement calibration of RTD and TC: and a high-precision temperature process calibrator is adopted to calibrate the resistance value and the voltage value of the TC respectively, and gain and offset errors can be eliminated for the temperature through calibration. The calibration process comprises measuring multiple resistance values, voltage values and actual values under different gains, storing the values into a processor, calculating calibrated resistance values and voltage values by piecewise linear interpolation, and dividing the calibrated sample point into multiple different intervals according to the working principle of [ X ]_i,X_i+1]within each intervalis a linear function and satisfies

then

the resistance value and the voltage value of any point on the measuring interval can be calibrated according to the formula, so that the corrected temperature value can be calculated.

The invention has the advantages and positive effects that: all measurement ports of the module can be configured randomly for measuring the RTD and the TC of the thermocouple, a standard Ethernet industrial protocol is integrated, the module is particularly suitable for large data transmission and real-time control temperature measurement systems, and rapid networking and system expansion in industrial fields are facilitated; the structure is compact and small, the installation is convenient, the device is particularly suitable for being used in harsh environment of industrial field, and different requirements of customers are met.

Drawings

FIG. 1 is a functional block diagram of the present invention;

FIG. 2 is a block diagram of a module cascade system;

FIG. 3 is a schematic of a 4 wire RTD measurement;

Fig. 4 is a low-pass filter circuit diagram.

Detailed Description

The following detailed description of specific embodiments of the invention refers to the accompanying drawings.

as shown in fig. 1, a thermocouple measurement module based on a field bus includes a processor, an ethernet switch, a power module and a bus isolation unit, where the processor is electrically connected to the ethernet switch, the power module and the bus isolation unit, the ethernet switch is electrically connected to an ethernet connector LINK1 after passing through an isolation transformer on one side, and is electrically connected to an ethernet connector LINK2 after passing through an isolation transformer on the other side, the ethernet switch expands two ethernet interfaces for data transmission, the bus isolation unit is electrically connected to a multi-path analog switch through an analog-to-digital converter, and the multi-path analog switch is electrically connected to an IO interface connector after passing through a low pass filter circuit and an overvoltage protection circuit in sequence. The processor is also electrically connected with the multi-channel analog switch through the IO isolation unit, and controls the multi-channel analog switch to be switched to realize multi-channel switching and temperature measurement and reading. The power supply module is respectively electrically connected with an IN pin of an M8-4-core power supply connector and an OUT pin of an M8-4-core power supply connector, the power supply module converts external DC 18-36V into an internal circuit which needs 3.3V and 5V levels, and meanwhile, the power supply OUT supplies power for a next-stage module, so that the modules are convenient to cascade. The ethernet connector LINK1 and the ethernet connector LINK2 are respectively connected to an external ethernet communication circuit. The IO interface connector collects temperature signals. The Ethernet connector is an M8-4 core Ethernet connector, and the IO interface connector is an M12-5 core IO connector. The treater model is STM32F407, power module includes LM2596 and LM1117-3.3, the ethernet switch model is LAN 9303. The bus isolation unit is ADuM5401, the IO isolation unit is ADuM1280, and the analog-to-digital converter is AD 7193. The multi-way analog switch is an ADG 714. As shown in fig. 4, the low pass filter circuit is designed with RC filtering. Overvoltage protection ensures that the input current is limited within the operating range of the device at DC30V overvoltage by placing a current limiting resistor in series with the input loop. The low-pass filter circuit and the overvoltage protection circuit are designed by conventional circuits, and redundant description is omitted here.

as shown in fig. 2, the control module is cascaded in the following manner: the OUT interface of the upper-stage M8-4 core power connector is electrically connected with the IN interface of the lower-stage M8-4 core power connector, and the LINK2 of the upper-stage Ethernet connector is electrically connected with the LINK1 of the lower-stage Ethernet connector.

Power module LM2596 converts outside DC18 ~ 36V power input into 5V level, and LM1117-3.3 will become the internal circuit with 5V level conversion and need 3.3V level, and power OUT makes things convenient for the module cascade for next stage module power supply simultaneously.

The overvoltage protection circuit realizes the protection of the internal measurement circuit and prevents accidental damage; the low-pass filter circuit filters high-frequency interference in the measurement signal; the multi-path analog switch realizes the measurement functions of different loops and different sensors; the ADC adopts different PGA gains according to different external sensors, and is filtered by an internal SINC4 digital filter. The inside of the module adopts a high-performance ARM Cortex-M4STM32F407 processor to communicate with a three-port management Ethernet switch LAN9303 through an MII interface, the LAN9303 expands two-way Ethernet interfaces LINK1 and LINK2 to perform high-speed data exchange, and the processor also packs related network data and sends the packed related network data to the next module through a LINK2 port to realize a data exchange function.

In order to meet the requirements of industrial field application, the module circuit is specially designed for hardware protection: the power supply module is provided with an overvoltage and undervoltage protection circuit, and a power supply interface can bear 500V surge impact; the communication interface adopts the design of an isolation transformer, and the communication function is normal through 2kV rapid pulse group test; the measurement port employs an overvoltage protection circuit that can withstand up to DC30V voltages.

all measurement ports of the module can be configured at will for measuring the RTD and the TC of the thermocouple: the RTD measurement supports various sensors of PT100, PT200, PT500 and PT1000 and simultaneously supports 2/3/4 wire system arbitrary configuration, and the TC measurement supports B/E/J/K/N/R/S/T sensors; the integrated standard Ethernet industrial protocol (EtherCAT, Modbus/TCP, EtherNet/IP, ProfiNet), especially suitable for the transmission of the large data bulk and real-time control temperature measurement system use, facilitate the quick network deployment of industrial field and system expansion; the structure is compact and small, the external dimension is 155x40x35mm, and the installation is convenient; the full encapsulation structure design and the IP67 protection grade are particularly suitable for being used in the harsh environment of an industrial field.

In practical industrial application, different requirements of field application are met for customers; the cost pressure of stock can be reduced for manufacturers; in addition, the product has low cost and strong market competitiveness.

The temperature measurement is carried out on software and hardware, the basic temperature measurement circuit principle is that temperature signals pass through an overvoltage protection circuit, a low-pass filter circuit and a multi-path analog switch and enter a 24-bit high-fraction analog-digital acquisition chip AD7193, an SPI interface of a processor MCU communicates with the AD7193 through a bus isolation chip ADuM5401, GPIO of the MCU controls the multi-path analog switch to be switched to realize multi-channel and switching through an isolation IO chip ADuM1280, and the temperature measurement and reading are realized.

Different measurement principles are adopted for different RTD and TC measurements:

RTD temperature measurement: by adopting the proportional method for measurement, the precision and the drift error of the excitation current source can be eliminated, and the temperature precision is only related to the reference resistor.

figure 3 is an exemplary circuit for a 4-wire RTD ratio measurement. (2/3 wire system principle is similar)

From the figure it can be deduced that:

Vrtd＝Rrtd×Iexc

Vref＝Rref×Iexc

The ADC operates in a bipolar differential mode,

in the formula:

CODE is ADC output CODE;

N is the resolution of the ADC (24 in this circuit);

rref is a reference resistor;

g is the selected gain;

By the above formula: it is known that the RTD measured resistance is only related to the reference resistance accuracy and drift.

The temperature is calculated by the Callender-Van Dusen formula

the temperature t <0 ℃ is given by the formula:

R_rtd(t)＝R₀[1+At+Bt²+C(t-100)t³]

the formula of the temperature t is more than or equal to 0 ℃ is as follows:

R_rtd(t)＝R₀(1+At+Bt²)

In the formula:

t is RTD temperature (. degree. C.)

R_rtd(t) is the resistance (Ω) of the RTD at temperature (t)

R₀RTD resistance at 0 deg.C (R when RTD sensor is PT 100)₀＝100Ω)

A. B, C coefficient of sensor for RTD

The processor MCU stores the coefficients, and the current temperature value can be obtained by measuring the resistance of the RTD and calculating.

And (3) measuring TC (temperature coefficient): the cold junction compensation adopts external PT1000 measurement, and the inside adopts high accuracy, low temperature drift benchmark, has guaranteed absolute voltage measurement accuracy.

the magnitude of the thermoelectric potential of the TC temperature measuring thermocouple is not only related to the temperature of the hot end, but also related to the temperature of the cold end. By employing a PT1000 temperature sensor at each measurement port as a cold side temperature measurement. And the accuracy of the cold end temperature is ensured.

the cold junction temperature must be converted to a voltage, which is converted using the following polynomial

In the formula:

E is the cold junction thermoelectric voltage;

Ci is a polynomial coefficient related to the thermocouple type;

t₉₀cold junction temperature (deg.C);

i is a polynomial order;

And finally, obtaining the thermal electromotive force E of the TC through cold end compensation measurement, and calculating the actual temperature value of the TC by adopting an ITS-90 temperature scale coefficient on a processor MCU software according to the following formula.

t₉₀＝c₀+c₁E+c₂E²+....c_iEⁱ

In the formula:

E is the thermoelectric voltage (uV);

t₉₀actual temperature (. degree. C.) measured for the thermocouple;

C_iIs a polynomial coefficient related to the thermocouple type;

i is a polynomial order;

temperature measurement calibration of RTD and TC: and a high-precision temperature process calibrator is adopted to calibrate the resistance value and the voltage value of the TC respectively, and gain and offset errors can be eliminated for the temperature through calibration.

In the basic process, a plurality of resistance values, voltage values and actual values under different gains are measured and stored in a processor, a calibrated resistance value and a calibrated voltage value are calculated by a piecewise linear interpolation method, and the working principle of the method is that a calibrated sample point is divided into a plurality of different intervals which are marked as X_i,X_i+1]Within each intervalIs a linear function and satisfies

Then

The resistance value and the voltage value of any point on the measuring interval can be calibrated according to the formula, so that the corrected temperature value can be calculated.

The temperature high-precision measurement is realized through the measures, and meanwhile, the fault diagnosis functions of sensor open circuit, short circuit, overload and the like are realized.

the embodiments of the present invention have been described in detail, but the description is only for the preferred embodiments of the present invention and should not be construed as limiting the scope of the present invention. All equivalent changes and modifications made within the scope of the present invention should be covered by the present patent.