Design and Implementation of Virtual Multimeter Based on LabVIEW

1 Introduction

LabVIEW is a graphical programming language launched by National Instruments ( NI ) in the United States. It is also a famous virtual instrument development platform. It plays the leading role in the key concept of virtual instruments, "software is instrument". It combines the high performance and flexibility of graphical programming with high-end performance and configuration functions designed for test measurement and automated control applications. It can provide necessary development tools for various applications such as data acquisition , instrument control, measurement analysis and data display. The expression and function of LabVIEW are similar to actual instruments, but the LabVIEW program can easily change settings and functions. Therefore, LabVIEW is particularly suitable for laboratories and occasions where the parameters and functions of instruments and equipment need to be changed frequently. This design is based on the LabVIEW platform environment to build a virtual multimeter. The knob of the virtual multimeter is designed according to the real DT9205 mode.

The external signal is input into the computer using the data acquisition card, and the measurement of various parameters is realized in the software interface of the virtual multimeter.

2. Virtual multimeter design

2.1 Display Panel Design

Use the [Controls/New Style/Value/Dial] command to place a dial control on the front panel and set its properties: set the data type to [Unsigned Long Integer]; double-click the option in the [Text Label] column in the [Text Label] tab, write the name corresponding to the knob, and then click the [Insert] button, repeat multiple times, and write the name of each item. The knob interface is shown in Figure 1.

 

Place a string on the front panel to display the measurement results. Place a numeric input control to control the measurement accuracy. Users can select the measurement accuracy through keyboard input or drop-down menu. Also place a working indicator light to indicate the working status of the instrument. Place a text display box to display time and information.

2.2  Software Design of Virtual Multimeter

In the rear panel, place a conditional structure to instruct the multimeter to perform different actions for different scales of the knob. Set 32 ​​branches in the conditional structure so that each branch corresponds to 32 different actions. Connect the knob to the [Branch Selector] of the conditional structure. Branches 0-31 correspond to the text label values ​​of the knob.

Branch 0 displays the on/off status of the system. The default is off, the indicator light is off, and the display shows [Off].

Branch 1 is responsible for detecting diodes. When the red test lead is connected to the anode of the diode, the circuit in the system is connected, and the display shows "the red test lead is connected to the anode of the diode at this time". When it is connected in reverse, it will show "the red test lead is connected to the cathode of the diode at this time". When the test lead is not connected to the diode, it will show "no diode is detected". The program flowchart is shown in Figure 2.

 

Branches 2 to 6 are responsible for the detection of capacitance, and each branch has a different range. During the measurement, the size of the data is first detected to determine whether the measured value exceeds the range. If it does not exceed the range, the measurement accuracy is read next; if it exceeds the range, "Out of measurement range, please select a high position" is displayed in the text box to prompt the user to switch to a high position.

The program flowchart is shown in Figure 3. 　　

Branches 7 to 10 are responsible for measuring AC current, and branches 11 to 14 are responsible for measuring DC current. The measurement principles are the same, but the ranges are different. When designing the measurement software, first determine the size of the measured value, and then select and display the corresponding accuracy. The program flowchart is shown in Figure 4.　　 Branches 15 to 21 are responsible for resistance measurement. The resistance range is quite wide, and the multimeter can only measure correctly within the range. If it exceeds the measurement range, the front panel will display "Out of measurement range, please select a high position". The program flowchart is shown in Figure 5. 　　 Branches 22 to 31 are responsible for voltage measurement. The design of the measurement branch is the same as the various design methods described above, only the units and sizes are different. The flowchart is shown in Figure 6.　　 Set a time function, and then use a string connection control to connect the information to be displayed and the time string together for display. The program flowchart is shown in Figure 7.　　 After the program design is completed, a loop program must be added to enable the multimeter to work continuously. Use the While loop to control the operation of the program. The entire program flowchart is shown in Figure 8. At this point, the design of the virtual multimeter is complete.　　 3. Conclusion
Experiments are the bridge between theory and practice. In recent years, traditional laboratories have many limitations, such as outdated experimental equipment, slow update speed, high cost, etc., which are increasingly unable to meet the needs of teaching and scientific research. With the rapid development of virtual simulation technology and network technology, building network virtual laboratories and virtual instruments will become the first economical and efficient solution.