Thermistor and thermocouple compatible high-precision temperature measurement method

Abstract

The invention discloses a thermistor and thermocouple compatible high-precision temperature measurement method, relates to a high-precision temperature measurement method, and constructs a hardware temperature measurement circuit. The temperature measuring circuit comprises a temperature sensor, a digital-to-analog conversion chip, a digital isolation chip and a single-chip microcomputer. The temperature sensor is composed of a thermistor and a thermocouple which are connected to the two ends of the digital-to-analog conversion chip respectively. The single-chip microcomputer is responsible for taking out temperature data in the digital-to-analog conversion chip and then taking out corresponding temperature data by matching different data processing algorithms according to the types of accessed sensors, a cold junction compensation part related to the thermocouple can be realized by using an internal temperature sensor of the digital-to-analog conversion chip, and the thermocouple and the thermal resistor are adaptive to different precision and range scenes; according to the compatible temperature measurement method provided by the invention, convenience is provided for industrial scene use, the same interface can be compatible with different types of sensors, disconnection detection and power frequency suppression functions are provided, and the compatible temperature measurement method can be applied to complex industrial high-precision temperature measurement scenes, and is high in precision and strong in anti-interference capability.

Description

Technical Field

[0001] The invention relates to a high-precision temperature measurement method, in particular to a high-precision temperature measurement method compatible with a thermistor and a thermocouple. Background Art

[0002] With the advancement of Industry 4.0, industrial production processes are becoming more and more automated and intelligent. High-precision temperature measurement is one of the key links to achieve precise control and optimization of production processes. For example, in advanced manufacturing fields such as semiconductor manufacturing, precision machining, and in the steel and metallurgical industry, temperature measurement in high-temperature furnaces needs to be able to withstand extreme temperatures and harsh environments while ensuring measurement accuracy to ensure product quality and production efficiency. These industrial scenarios usually have complex environmental conditions, such as high temperature, high pressure, strong corrosion, strong electromagnetic interference, etc., which puts higher requirements on the reliability and accuracy of temperature measurement equipment. Summary of the invention

[0003] The purpose of the present invention is to provide a high-precision temperature measurement method that is compatible with thermistors and thermocouples. The present invention is compatible with two different sensors. The digital communication interface of a single-chip microcomputer collects temperature data of a temperature sensor transmitted from a digital isolation chip. The thermistor uses a four-wire constant current source measurement method, and an external high-precision reference resistor enables the system measurement accuracy to reach ±0.05; an external high-precision reference source facilitates system calibration and measurement.

[0004] The purpose of the present invention is achieved through the following technologies: A high-precision temperature measurement method compatible with thermistors and thermocouples, comprising: constructing a hardware temperature measurement circuit; the temperature measurement circuit comprises: a temperature sensor, a digital-to-analog conversion chip, a digital isolation chip and a single-chip microcomputer; the temperature sensor is composed of a thermistor and a thermocouple, which are respectively connected to the two ends of the digital-to-analog conversion chip; the single-chip microcomputer is responsible for taking out the temperature data in the digital-to-analog conversion chip, and the cold end compensation part of the thermocouple can be realized by using the temperature sensor inside the digital-to-analog conversion chip; The ADC interface of the temperature measurement system collects the temperature data of the temperature sensor and then sends it to the single-chip microcomputer for data processing; then the temperature measurement method is determined according to the sensor type to perform accurate temperature measurement; The relationship between the resistance and temperature of the RTD in the temperature data collected by the ADC interface of the single-chip microcomputer and the temperature sensor is not completely linear, especially in the high and low temperature areas, the nonlinear error will be more obvious. In order to improve the accuracy of the measurement, it is necessary to compensate for the nonlinear error. The nonlinear error compensation can be achieved by establishing a temperature-resistance characteristic curve model of the RTD, and then using the model to calculate the corresponding temperature value based on the measured resistance value. A commonly used method is to establish a temperature-resistance characteristic curve model by polynomial fitting. For example, a second-order or third-order polynomial can be used to fit the temperature-resistance characteristic curve of the RTD.

[0005] A further improvement of the above scheme is that a protection module and a digital communication isolation module are further connected between the thermocouple and the thermocouple digital converter.

[0006] A further improvement of the above scheme is that the protection module includes a TVS tube and an ESD tube, and its purpose is to protect the ADC sampling channel. The TVS tube can be quickly turned on at the moment of electrostatic discharge, clamping the high voltage generated by static electricity within a safe voltage range.

[0007] A further improvement of the above solution is that the TVS (transient voltage suppression diode) model is MT9820.

[0008] A further improvement to the above scheme is that the digital communication interface of the single-chip microcomputer collects temperature data from the temperature sensor transmitted from the digital isolation chip, wherein the thermal resistor uses a four-wire constant current source measurement method, which requires a constant current source, and the system measurement accuracy can reach 0.05°C through an external high-precision reference resistor. The external high-precision reference source facilitates system calibration and measurement. The thermocouple generates a very small change in the hot end voltage difference signal, and an external or built-in PGA is required to amplify the signal so that the system can obtain a higher resolution. The bias voltage part of the thermocouple can be realized using the ADC built-in bias source and disconnection detection function.

[0009] A further improvement of the above scheme is that a filter, a follower and an amplifier are connected between the temperature sensor and the single chip microcomputer.

[0010] A further improvement of the above solution is that the single chip microcomputer can output signals through a digital isolator and SPI.

[0011] A further improvement of the above solution is that the digital isolator is an MT9820 chip.

[0012] A further improvement of the above solution is that the digital converter is an AD7124 chip.

[0013] The advantages and positive effects of the present invention are: 1. The digital communication interface of the single-chip microcomputer of the present invention collects temperature data from the temperature sensor transmitted by the digital isolation chip, wherein the thermal resistor uses a four-wire constant current source measurement method, which requires one constant current source. The system measurement accuracy can reach ±0.05°C through an external high-precision reference resistor, and the external high-precision reference source facilitates system calibration and measurement.

[0014] 2. The present invention can be widely used in complex industrial high-precision temperature measurement scenarios. The same interface is compatible with different types of sensors, and provides disconnection detection and power frequency suppression functions. It has high temperature measurement accuracy and strong anti-interference ability. At the same time, the system has good compatibility and can meet the use requirements of different occasions. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a schematic diagram of the circuit structure of a preferred embodiment of the present invention; Figure 2 The present invention is a preferred high-precision temperature measurement method flow chart of a thermistor and a thermocouple compatible method. DETAILED DESCRIPTION

[0016] In the thermocouple high-precision temperature measurement method of this embodiment, first construct Figure 1 The circuit shown includes: thermistor and thermocouple compatible circuit design, single chip microcomputer and digital isolator; the thermistor and thermocouple are connected to both ends of AD7124 through a low-pass filter, and the differential capacitor design in the sampling circuit is to transplant differential mode interference, relying on the functional diversity of the analog-to-digital conversion chip to provide design convenience for temperature data sampling data.

[0017] The positive end of the temperature sensor is connected to the digital-to-analog conversion chip through a resistor, and the negative end of the temperature sensor is connected to the AD7124 digital-to-analog converter through a resistor. Filter capacitors are arranged between the nodes between the AD7124 digital-to-analog converters. The filter capacitors are used to eliminate and weaken differential-mode interference and common-mode interference. The external temperature circuit has a four-wire RTD setting circuit and an external precision reference resistor circuit.

[0018] Taking into account the special environment corresponding to the industrial environment, the signal of the thermocouple is easily affected by external electromagnetic interference. These interference signals will also be amplified, which can easily cause the temperature acquisition signal to fluctuate. The filtering module can effectively eliminate interference, suppress fluctuations, and make temperature measurement more accurate.

[0019] In this embodiment, the digital-to-analog converter uses the AD7124 chip, the internal structure of which includes a programmable gain amplifier (PGA), an analog-to-digital conversion module (ADC), a constant current source, and a bias voltage module; it can collect thermocouple signals and convert them into digital signals of temperature, and thanks to the 24-bit high-precision ADC, the AD7124 has multiple analog input channels, for example, the AD7124-4 has 4 differential or 7 single-ended input channels, and the AD7124-8 has 8 differential or 15 single-ended input channels. This allows it to connect multiple temperature sensors at the same time, and is suitable for multi-region or multi-point temperature measurement application scenarios, such as temperature monitoring of different parts of large industrial equipment, distributed temperature sensing systems, etc., which can greatly improve the coverage and efficiency of temperature monitoring.

[0020] AD7124 has a measurement accuracy of ±0.1% and also has multiple functions such as protection and fault detection. The chip has a temperature sensor inside, which can be used to perform cold-end compensation for thermocouples. The chip provides a variety of filtering options, which can effectively filter the temperature signal, remove high-frequency noise and interference signals, and make the measurement results more stable and reliable. For example, in the presence of various electromagnetic interferences in industrial sites, the filtering function of AD7124 can ensure the accuracy of temperature measurement.

[0021] The AD7124 chip in this implementation has rich diagnostic functions, which can detect whether the cable connection is normal, whether there is a short circuit or other fault conditions, and promptly discover problems in the temperature measurement system, improve the reliability and stability of the system, and reduce temperature measurement errors or system downtime caused by equipment failure. Data verification: The converted data can be verified to ensure the accuracy and integrity of the data, providing reliable temperature data support for the temperature monitoring system.

[0022] The temperature sensor circuit uses a four-wire RTD. In the wiring method of the four-wire RTD, two wires are used to provide a constant current, and the other two wires are used to measure the voltage on the RTD. Since no current passes through the two wires for measuring the voltage, there will be no voltage drop due to the resistance of the wire itself, thereby eliminating the influence of the lead resistance on the temperature measurement. Compared with the two-wire and three-wire RTDs, the four-wire system can more accurately measure the change in the resistance value of the RTD, thereby improving the accuracy of temperature measurement. In laboratories, scientific research, precision machining and other fields that require extremely high temperature measurement accuracy, the high-precision advantage of the four-wire RTD is particularly prominent. The four-wire system can reduce measurement errors caused by factors such as contact resistance and line aging. In the long-term temperature monitoring process, even if the contact state or resistance characteristics of the wire change slightly, the impact on the measurement result is very small, which can ensure the stability and reliability of the measurement.

[0023] The microcontroller has a commonly used communication interface and ADC interface, which are used to process the temperature information of the temperature sensor obtained by the analog-to-digital conversion chip. The ADC port is used to process the temperature measurement analog signal of the cold end compensation point. The internal calculation process can obtain the high-precision temperature measurement of the thermocouple. Finally, the temperature measurement value is output to the microcontroller through the digital isolation chip. The MT9820 used in the digital isolator has a high isolation level and can meet the GB 9706.1-2020 standard.

[0024] The high-precision temperature measurement method of this embodiment is as follows: Figure 2 The ADC interface of the single-chip microcomputer collects the temperature data of the temperature sensor. The power frequency suppression function of the analog-to-digital conversion chip can effectively suppress the interference of the power frequency signal. In addition, the AD7124 chip provides a disconnection detection function, which can effectively detect the disconnection of the sensor.

[0025] The digital communication interface of the single-chip microcomputer collects temperature data from the temperature sensor transmitted by the digital isolation chip, wherein the thermal resistor uses a four-wire constant current source measurement method, which requires a constant current source. The system measurement accuracy can reach 0.05°C through an external high-precision reference resistor. The external high-precision reference source facilitates system calibration and measurement. The thermocouple generates a very small change in the hot end voltage difference signal, and an external or internal PGA is required to amplify the signal so that the system can obtain higher resolution. The bias voltage part of the thermocouple can be realized using the ADC built-in bias source and disconnection detection function.

Claims

1. A high-precision temperature measurement method compatible with thermistors and thermocouples, characterized in that: The method comprises constructing a hardware temperature measurement circuit; the temperature measurement circuit comprises: a temperature sensor, a digital-to-analog conversion chip, a digital isolation chip and a single-chip microcomputer; the temperature sensor is composed of a thermistor and a thermocouple, which are respectively connected to two ends of the digital-to-analog conversion chip; the single-chip microcomputer is responsible for taking out the temperature data in the digital-to-analog conversion chip, and the cold end compensation part of the thermocouple is realized by using the temperature sensor inside the digital-to-analog conversion chip; (1) The digital-to-analog conversion chip is responsible for collecting temperature sensor data. The constant current source required for thermistor temperature measurement is also provided by this ADC chip. The high-precision characteristics of the external reference source are used to accurately measure the sensor data. (2) The digital communication interface of the single-chip microcomputer collects temperature data from the temperature sensor transmitted by the digital isolation chip. The thermistor uses a four-wire constant current source measurement method and is equipped with a constant current source. The external high-precision reference resistor enables the system measurement accuracy to reach 0.05°C. The external high-precision reference source facilitates system calibration and measurement. The voltage difference signal at the hot end generated by the thermocouple has a very small change, and an external or internal PGA is required to amplify the signal so that the system can obtain higher resolution. The bias voltage part of the thermocouple is implemented using the ADC's built-in bias source and wire break detection function.

2. A high-precision temperature measurement method compatible with thermistors and thermocouples according to claim 1, characterized in that: An anti-aliasing filter circuit is provided between the thermocouple and the thermal resistor to cut off the signal and restore the original signal.

3. A high-precision temperature measurement method compatible with thermistors and thermocouples according to claim 1, characterized in that: The front sampling circuit end is provided with a protection circuit, which is composed of a TVS tube and an ESD tube to prevent static electricity from damaging the ADC channel and causing irreversible damage.

4. A high-precision temperature measurement method compatible with thermistors and thermocouples according to claim 1, characterized in that: A digital isolation chip is provided between the digital-to-analog conversion chip and the main control chip to prevent digital circuit interference from affecting the accuracy of the analog chip.

5. A high-precision temperature measurement method compatible with thermistors and thermocouples according to claim 1, characterized in that: The filtering module is a first-order low-pass filtering circuit, and a first-order low-pass filter composed of a resistor and a capacitor is arranged at one end of the temperature sensor and the end of the single-chip computer.

6. A high-precision temperature measurement method compatible with thermistors and thermocouples according to claim 1, characterized in that: A filter is connected between the temperature sensor and the single-chip microcomputer, and a programmable operational amplifier module is used inside the digital-to-analog conversion chip.

7. A high-precision temperature measurement method compatible with thermistors and thermocouples according to claim 1, characterized in that: The signal output by the temperature sensor has no preamplifier circuit to follow.

8. A high-precision temperature measurement method compatible with thermistors and thermocouples according to claim 1, characterized in that: The single chip microcomputer performs SPI communication via a digital isolator.

9. A high-precision temperature measurement method compatible with thermistors and thermocouples according to claim 1, characterized in that: The digital isolator is the MT9820 chip.

10. A high-precision temperature measurement method compatible with thermistors and thermocouples according to claim 1, characterized in that: The analog-to-digital converter is an AD7124 chip.

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