Design of Virtual Circuit Experiment Based on LabVIEW

Abstract: This paper uses LabVIEW to design common experimental modules in circuit courses. Each module gives a specific program design plan and is finally successfully run and debugged. The introduction of virtual instruments can not only save costs, but also update and adjust experimental methods and means to keep the laboratory teaching equipment advanced, improve the quality and efficiency of experimental teaching and scientific research, and is a new development direction of experimental teaching, which promotes the progress of laboratory technology.

Traditional experimental teaching equipment is old and backward, and can no longer keep up with the needs of educational development. It has largely restricted the development of experimental teaching and the improvement of talent training quality. In addition, ordinary laboratories involve instrument debugging, management, and easy damage. LabVIEW programming language is flexible, open, and uses software to replace instrument functions. It has the advantages of good user interface and simple operation. Having a virtual instrument system is equivalent to having a personal laboratory. Circuit courses are an important part of the learning of students majoring in electrical engineering. The course is highly theoretical. By establishing a virtual laboratory on the computer and using the virtual experimental instruments designed by the LabVIEW development platform, common experiments in the course can be realized, including the design of modules such as branch current method, first-order dynamic circuit analysis, and second-order dynamic circuit analysis. This can improve students' interest in learning, make up for the shortcomings of experimental teaching in the hardware environment, improve the teaching quality of circuit course experimental teaching, improve teaching effects, expand students' practical platform, and provide a new auxiliary means for experimental teaching of basic electrical courses, that is, an experimental teaching method that combines actual operation with computer simulation with virtual instruments as the core.

1 Virtual Instrument Programming Platform

Virtual instruments are a new achievement that has been bred together with the close combination of testing technology and instrumentation technology in the process of intensive penetration of increasingly developed computer hardware, software and bus technology into other technical fields. According to the World Instrumentation and Automation magazine, virtual instruments will become the development direction of electrical measuring instruments in this century.

LabVIEW is a laboratory virtual instrument engineering platform. Its full name is Laboratory Virtual Instrument Engineering Workbeneh (laboratory virtual instrument integrated environment). It is a virtual instrument development tool based on G language (Graphies Language) developed by National Instruments of the United States. It is the world's first 32-bit compiled program development system for instruments that uses graphical programming technology. It is currently the most widely used, fastest-growing and most powerful graphical software development integrated environment.

The core of LabVIEW to create virtual instruments is VI, which includes the program front panel (Front Panel), block diagram program (Diagram) and icons/connectors.

2 Experimental design of branch current method

2.1 Front panel design

The front panel of the circuit is built through custom controls, as shown in Figure 1. The five resistors and three power supply components are all numerical input controls, and the values ​​are adjustable; the five numerical display controls are used to represent the five branch currents.

2.2 Flowchart Design

Apply Kirchhoff's law to the circuit and write the equations into a linear equation system in matrix form, that is,

Use the Unit Conversion node to convert between unit data and unitless data, use the MathScript node to generate the coefficient matrix and known vectors of the linear equations, and use the Solve Linear Equations.vi to solve the linear equations.

The flowchart of the branch current method circuit analysis program written according to the matrix equation group is shown in Figure 2.

Run the program and the result is shown in the front panel of Figure 1.

3 First-order dynamic circuit design

When a dynamic circuit switches from one steady state to another, a dynamic circuit transition occurs. The purpose of dynamic circuit analysis is to analyze the circuit operation rules during the transition process. Using LabVIEW to simulate the dynamic process of the circuit can display the parameter change curve, which helps to deepen the understanding of the transition process rules.

3.1 Front panel design

Build the circuit diagram, and display the currents IL, I1, and I2 in an XY diagram. The front panel is shown in Figure 3.

3.2 Block Diagram

Write the expressions for each current according to the three-element method:

According to the above formula, IL, I1 and I2 are calculated respectively using formula nodes. The flowchart is shown in Figure 4.

The running result is shown in Figure 3. The variable in the formula node is R2. You can change the value of R2 and re-observe the program running result.

4 Second-order dynamic circuit design

The zero-input response of the RLC series circuit is taken as an example to discuss the transient process of the second-order circuit.

Given L=0.4 H, C=0.1 F, uC(0-)=1 V, iL(0-)=0, and the switch is closed at t=0, discuss the zero input response of uC at different resistance values ​​and draw waveforms for comparison.

4.1 Front panel design

Create a circuit diagram and send the result to the waveform display. The front panel of the RLC second-order dynamic circuit is shown in Figure 5.

4.2 Block Diagram

After the switch is closed, write the second-order differential equation for the capacitor voltage uC.

The second-order differential equation (5) is Laplace transformed and the difference equation is solved considering the initial conditions.

If the denominator polynomial has two unequal roots p1 and p2, we have

Call the partial fraction expansion VI, PaaiM Fraction Expansion.vi, in the function palette "Math-Polynomial" to solve the root P and the coefficient A.

When writing a program, you first need to determine the multiplicity of the roots. The multiplicity of the roots can be obtained from the output parameters of the partial fraction expansion VI. The "residue" array is a two-dimensional array. When there are two unequal roots, the coefficient array obtained is an array with two rows and one column. The elements in the first row and the second row are the coefficients A1 and A2 corresponding to the two roots respectively; when there are two repeated roots, the coefficient array obtained is an array with one row and two columns. The elements in the first column and the second column are the first-order and second-order coefficients A1 and A2 of the root respectively.

Write the program flowchart for unequal roots and double roots based on equations (7) and (9).

Take different R values ​​and run the program respectively, and the corresponding waveforms under various damping conditions can be observed, as shown in Figure 5. It can be seen that when R>4 Ω, it is an oscillating waveform, when R4 Ω, it is a monotonically decaying waveform, and when R=4 Ω, it is the critical case of the two, which is consistent with the theory.

5 Conclusion

In this paper, combined with the existing conditions, virtual instrument technology is used to realize the design of circuit experiments based on LabVIEW, providing a new solution to the shortage of experimental teaching equipment in colleges and universities, making the experimental teaching methods more advanced and the experimental results more intuitive. With the continuous updating of the software, the performance and functions of the experimental system can be improved, which has broad application prospects in the future development of laboratories.