Design of Temperature Measurement System Based on LabVIEW

Temperature is a very important parameter in mechanical industrial production and scientific research experiments. Many systems work within a certain temperature range, and there are many occasions where temperature measurement and temperature control are needed. The current temperature measurement control system often uses single-chip microcomputer control. This technology is widely used, but its programming is complex, the control is unstable, and the system accuracy is not high. The temperature measurement system developed and designed using virtual instruments uses an ordinary PC as the host and the graphical visual software hbVIEW as the software development platform to monitor temperature changes, collect data, and process, store, and display it. The equipment has low cost, is easy to use, and is flexible.

1 Introduction to Virtual Instrument Technology and LabVIEW
Virtual technology, computer communication technology and network technology are the three core technologies of information technology, among which virtual instruments are an important part of virtual technology. In the virtual instrument system, flexible and powerful computer software is used to replace some hardware of traditional instruments, and human intellectual resources are used to replace many material resources. In particular, the computer is used in the system to directly participate in the generation of test signals and the analysis of measurement characteristics, so that some hardware in the instrument or even the entire instrument "disappears" from the system, and the computer's software and hardware resources complete their functions.
LabVIEW is a virtual instrument software development tool based on G language launched by NI of the United States. It is one of the most widely used virtual instrument software platforms in the world. It is mainly used in instrument control, data acquisition, data display and other fields, and can be applied to multiple operating system platforms such as Windows, Macintosh, and UNIX. Unlike traditional programming languages, LabVIEW uses a powerful graphical language for programming, which is aimed at test engineers rather than professional programmers. It is easy to program and the human-computer interaction interface is intuitive and friendly. Designers can easily build measurement systems
and construct their own instrument panels like building blocks without having to write any tedious computer codes. Even if users do not have much programming experience, they can still use LabVIEW to develop their own applications.

2 System Design Scheme
The virtual temperature tester converts the temperature of the object to be measured into analog signals such as voltage or current, and after power amplification and filtering by the signal conditioning circuit, it is converted into a standard voltage signal that can be collected by the data acquisition card. The analog signal is converted into a digital signal in the data acquisition card, and sent to the computer bus under the data acquisition instruction. The collected data is processed in various ways required by the installed virtual instrument software in the PC. Its overall framework is shown in Figure 1.

To design an instrument, we must first consider its function, and then design the front panel and flowchart according to its function. In the virtual instrument, the panel of the "instrument" needs to be displayed on the computer screen, and can be modified at any time as needed. The virtual temperature tester designed in this paper should realize the following functions: 1) Set control buttons and display windows to display the temperature in real time, and control the acquisition process; 2) Set warning signals. When the temperature exceeds a preset temperature value, the warning light will light up; 3) The collected temperature signal can be displayed, stored and
printed, and the collected temperature can be called for analysis and processing and waveform playback; 4) The temperature change process can be intuitively seen in the form of a real-time trend chart. In the real-time trend chart, new data is continuously extended behind the existing data, and the waveform continues to move forward.
2.1 Selection of sensor
For temperature measurement, the selection of temperature sensor is the first step of the entire system and one of the important factors that directly affect the system performance. Because the thermal resistor has good linearity. It is widely used in the temperature range of -200 to +500℃, so the thermal resistor temperature sensor is selected. Its working principle is: Thermistor thermometer measures temperature based on the characteristics of the resistance of metal conductors changing with temperature. Commonly used platinum resistors are characterized by high precision, stable and reliable performance, and are specified by international organizations for temperature measurement of -259 to +500°C. The relationship between its resistance and temperature can be expressed as:

where RT and R0 are the resistance values ​​at T°C and 0°C respectively; A and B are constants, A=3.908x10-3°C, B=5.802x10-7°C.
Thermistor sensors require an external power supply to convert the resistance value into a voltage value for measurement. Usually, the temperature change of thermistor is converted into a voltage change output through a balanced bridge. Then it is amplified and the temperature value is obtained by measuring the change in the bridge output voltage. [page]

The temperature variation range measured in this article is: -20～+120℃, and the accuracy requirement is 0.5 level. The system is calibrated by the curve fitting method, and the temperature corresponding to any voltage within the temperature measurement range can be obtained.
2.2 Signal conditioning of temperature test system
This temperature sensor uses a temperature transmitter for signal conditioning. The working principle of the temperature transmitter is: a thermal resistor is used as a temperature measuring element, and the signal output from the temperature measuring element is sent to the transmitter module. After the voltage stabilization filter, operational amplification, nonlinear correction and reverse protection processing circuits, it is converted into a 4～20 mA current signal output that is linearly related to the temperature. A 220 Ω resistor is added to the signal output end to convert it into a 0.88～4.4 V voltage signal output.
2.3 Data acquisition of temperature test system
Modular design of data acquisition. The design of data acquisition module has a direct impact on the subsequent data display and analysis results and the realization of the whole system function. This module is designed by using NI's DAQ (Data Acquisition) card and its driver. The integrated DAQ function library and subVI with comprehensive functions are fully utilized to design a data acquisition module that can realize the control of data acquisition, including trigger control, channel control, etc.
2.4 Program flowchart of temperature test system
When performing temperature test, first determine which channel to collect temperature signals, and then debug the system. After debugging, start data acquisition, storage and backup. When the temperature exceeds the limit value set by the user, the temperature test system will alarm. When the temperature is within the allowable range, the test system will filter and analyze the collected signals, display waveforms, and adjust waveforms. The program design includes two parts: the front panel and the program flowchart. The system front panel consists of parameter settings and function buttons. There are corresponding program modules in the background. The running state of each program module corresponds to a loop structure. The user uses the buttons or controls on the front panel to select the state. After running the program, the background executes the corresponding state. At the same time, the front-end dialog box provides an interface for testers to fill in parameters or select functions. In order to facilitate subsequent personnel to make small-scale modifications according to their own requirements, the back panel program flowchart is also designed in an intuitive and concise way. The specific flow chart and program flowchart are shown in Figures 3 and 4. [page]

In the flowchart of Figure 4, the case loop is used to determine whether to execute the temperature test program, which filter to select, and whether to alarm if the limit is exceeded.

3 Conclusions
By setting different parameters on the left side of the front panel, including the upper and lower temperature limits and filter settings, the right side is the waveform display part, including the original temperature waveform display and the adjusted waveform display, as well as the local refinement and amplification of the graph, and the numerical display of the index value. The measurement results shown in Figure 5 show that the measurement method has the advantages of high measurement accuracy, good linearity, and short time.
The virtual temperature measurement system is realized by LabVIEW software, which can display and control the temperature in real time on the computer, improve the working conditions, improve the accuracy, save time, and reduce the cost. The system has strong scalability, and can change the function according to its own requirements for the instrument, and easily realize the operations required by the user, such as realizing remote temperature measurement and control.