Design of Multi-channel Temperature Measurement System Based on LabVIEW

In order to measure multi-point temperature in various application environments, a multi-channel temperature measurement system based on LabVIEW is designed. The system is based on the LabVIEW graphical development environment, using RTD as a temperature sensor, continuously collecting sensor signals, and performing signal conditioning through the N19219 four-channel RTD input module. It is connected to the computer through USB to continuously collect and measure signals, display each channel signal in real time, and analyze and process temperature data. The system test results show that the accuracy of the measurement system is 0.01℃, and the effective measurement range is 0～+300℃, which verifies its effectiveness and feasibility.

Temperature is one of the common process parameters in industrial production and scientific experiments, and temperature indicators are also an indispensable and important parameter in many engineering projects. For example, the reaction rate of iron carbide rises and falls with the changes during operation. The high and low operating temperature during the reaction not only affects the time required for the reaction to complete, but also affects the size of the conversion rate. Therefore, it is particularly important to obtain temperature data accurately and conveniently. In the fields of hydrology and meteorology, power environment monitoring of computer rooms, granaries, soil, farms, mining, smart home accessories, etc., temperature monitoring and measurement are required at multiple monitoring points. Therefore, the design of multi-point temperature monitoring and measurement systems is of great significance.

1 System Working Principle

According to the characteristics of multi-point temperature measurement, a multi-channel temperature measurement system based on the virtual instrument platform LabVIEW is designed. The Pt1000 platinum resistor is selected as the temperature sensor, and the data is collected through the NI9219 data acquisition card. The hardware filtering and software filtering techniques are used to improve the anti-interference ability of the multi-channel temperature measurement system. The temperature change of each channel in the whole measurement process is displayed in real time in the form of a waveform chart on the host computer software interface. After the measurement, the original data results of the whole measurement process are recorded and saved.

The multi-channel temperature measurement system consists of 4 Pt1000 platinum resistors, NI9219 data acquisition card, NI USB-9162 module box connector, and a computer.

Pt1000 is a platinum thermal resistor, and its resistance value changes with temperature. The number 1000 after Pt means that its resistance value is 1000Ω at 0℃, and its resistance value is about 2 120.515 Ω at 300℃, and the resistance value of Pt1000 increases linearly with the temperature rise. The lead wire of Pt1000 platinum resistor adopts three-wire system, which reduces the additional error caused by the wire resistance; NI9219 data acquisition card is a 24-bit universal analog input data acquisition module, which can collect and condition RTD signals, and connect to the computer for data acquisition through the NI USB~9162 module outer box connector. The entire measurement system can collect 4 temperature signals at the same time. The sampling mode, sampling rate and sampling number, the start time and end time of sampling can be set on the upper computer software interface. During the entire measurement process, the interface can use waveform charts to display the temperature measurement change value of each channel in real time, as well as the maximum, minimum and average temperature values ​​during the entire measurement process. After the measurement process is completed, the original data of the measurement can be recorded and saved for subsequent data processing. The structure block diagram of the multi-channel temperature measurement system is shown in Figure 1.

Figure 1 Multi-channel temperature measurement system block diagram

2 Overall design of multi-channel temperature measurement system

2.1 Hardware Circuit Design

Each channel of NI 9219 is isolated from each other, and four 24-bit analog-to-digital converters (ADCs) can sample four analog input channels simultaneously. Since the platinum thermal resistor Pt1000 outputs a low-voltage signal, and its signal is easily interfered by noise, the NI9219 data acquisition card must condition and filter the low-voltage signal output by Pt1000. The input circuit of a channel of NI9219 is shown in Figure 2.

NI9219 can collect 4 temperature signals at the same time. Each EX+ and EX- port corresponds to the pin of Pt1000. The LO port is the common ground terminal of each channel and is isolated from other modules in the system. After the channel is filtered, it is sampled by a 24-bit analog-to-digital converter. In 3-wire RTD mode, NI9219 provides excitation current, and the current value varies with the load value between EX+ and EX- terminals. In this mode, if all wires have the same resistance, linear impedance errors can be compensated. NI 9219 provides 2x voltage gain for the negative terminal. The ADC uses this voltage value as the negative terminal reference voltage to eliminate linear errors between positive and negative terminals. The excitation circuit of NI 9219 has overvoltage protection and overcurrent protection. When overvoltage and overcurrent occur, the module automatically disables the circuit. After the fault is eliminated, the channel can automatically recover. The module supports low-power sleep mode. It cannot communicate with other modules in sleep mode. The system power consumption is low in sleep mode, and the heat dissipation is also lower than the normal working mode.

Figure 2. Input circuit of a channel of the NI 9219 and 3-wire RTD mode

2.2 Software Process Design

The software flow chart of the multi-channel temperature measurement system based on LabVIEW is shown in Figure 3.

Figure 3 Multi-channel temperature measurement system software flow chart

The host computer software interface can set various parameters of the multi-channel temperature measurement system, including the configuration of the acquisition physical channel and the resistor type, the setting of the current excitation source and the current excitation value, the sampling mode, the sampling rate and the number of samples per channel, the target temperature range to be measured, the measurement start time and end time and other parameter settings. [page]

During the measurement process, the upper computer waveform chart can monitor the temperature changes of the four channels in real time, and the temperature data of each channel is marked with different colors, and the maximum, minimum and average values ​​of the collected data of each channel are displayed in real time, so as to quickly draw preliminary measurement conclusions at the measurement site. At the end of the measurement, all the original data of the measurement will be saved for later analysis and processing. The software interface is shown in Figure 4.

Figure 4 Multi-channel temperature measurement system host computer software interface

The design of a multi-channel temperature measurement system can be divided into four stages: system configuration, data acquisition, data processing, and data storage.

The system configuration link mainly involves the configuration of the physical channel and resistance type of the NI9219 data acquisition card, the setting of the current excitation source and current excitation value, the target temperature range to be measured, the measurement start time and end time and other parameter settings.

In the data acquisition link, the system sets the sampling mode, sampling rate and number of samples per channel according to the measurement, and the NI9219 data acquisition card acquires the four waveform data in the analog input channel task.

In the data processing stage, the waveform chart of the host computer reads the temperature data in the data buffer in real time. The temperature data of each channel is marked with different colors, and the maximum, minimum and average values ​​of the collected data of each channel are displayed in real time, which is convenient for the measurer to intuitively view and preliminarily analyze. Although the entire system uses the DAQmx driver of NI9219 to configure the data acquisition module, avoiding the mathematical calculation process of converting voltage data to temperature data, and can reduce signal interference to a certain extent, in the process of resistance-temperature data acquisition, electromagnetic interference or zero drift will cause the voltage to fluctuate up and down, so that the measured temperature value will fluctuate in a small range, resulting in a decrease in the accuracy of the measurement result. In the upper computer software part of this system, formula nodes are used in the program flowchart of LabVIEW to continuously collect 1,000 temperature values ​​within 1 second, calculate their arithmetic mean, and use the average value as the sampling result. This can effectively suppress the jump of temperature values, and improve the accuracy of measurement results by increasing the sampling rate of the data acquisition card and the number of samples per channel.

The data storage link implements the original data storage function and writes it into the TDMS file to facilitate subsequent data viewing, extraction, and processing.

3 Experimental results and data analysis

The multi-channel temperature measurement system based on LabVIEW was placed in a high-precision constant temperature bath for stability experiments. The high-precision constant temperature bath was established by the Methods Department of Guangzhou Marine Geological Survey in 2009 according to the work needs of the research group. The equipment consists of a high-precision constant temperature bath, first-class platinum thermocouples , high-precision temperature measurement bridges and AC Wenle equipment, with an accuracy of 0.01°C, as shown in Figure 5.

The parameters of the thermostatic bath were adjusted and set, and the probes of the four RTDs were placed in the thermostatic bath for testing. The number of sampling points was set to 500, and the sampling frequency was set to 1 Hz. The test was repeated many times, and the experimental data obtained are shown in Table 1.

Figure 5 High-precision temperature calibration constant temperature bath

Table 1 Measurement results of multi-channel temperature measurement system in a constant temperature bath

From the measurement results of the multi-channel temperature measurement system in the constant temperature bath, it can be seen that the maximum temperature difference of the measured points of the four channels is 0.02℃, and the maximum difference in the entire constant temperature bath is 0.028℃, achieving the preset purpose. Multiple experimental data show that the stability of the measurement system is very good.

4 Conclusion

The multi-channel temperature measurement system based on LabVIEW introduced in this paper has a measurement accuracy of 0.01℃, and the effective measurement range supported by experimental data is 0～+300℃. The system adopts the function of real-time monitoring of the temperature of the measured object, realizes the function of automatic measurement and data acquisition of PC, and also realizes the real-time display and storage function of data. The measurement process is easy to operate and does not require human intervention. It has high reliability and can run multi-task synchronously in real time, better ensuring the real-time, reliability and scalability of the processing and display system of multi-point temperature measurement data. In addition, by using the standard data acquisition module and LabVIEW graphical development environment, secondary development can be quickly carried out on its basis, which improves the development efficiency and reflects the broad prospects of virtual instruments in the field of multi-point temperature measurement and monitoring.