Automated Precision Impedance Analysis System Based on Labview

0 Introduction

With the advancement of science and technology, the objects and methods of various experimental studies are becoming more and more complex, and the complexity of the corresponding experimental systems and experimental platforms is also growing exponentially. Traditional manual operations are far from meeting the requirements in terms of accuracy and repeatability. Therefore, the high degree of integration and automation of instruments and equipment has become an indispensable feature of contemporary instruments. However, the measurement parameters and steps required by different experimental systems are different, so the required experimental instrument system should also be modular and autonomously integrated. The concept of virtual instruments (VIs) makes it possible to use traditional instruments and equipment to build different experimental platforms. And because VI is programmable and programmable, the design of the experimental platform can be changed according to different experimental requirements. Compared with commercial integrated platforms, VI-based platforms have significant advantages: a. Complex systems can be built using various traditional instruments; b. The functions of the platform can be simply changed by changing the program; c. While avoiding complex repetitive measurement work, the accuracy of the experiment has also been significantly improved.

1 Construction of precision impedance analysis system

Agilent 4294A precision impedance analyzer is mainly used to measure various parameters such as resistance, reactance, and capacitance of materials. Its high precision, multiple scans and multi-parameter settings make it widely used in the characteristic tests of various semiconductor thin film materials, nanomaterials and engineering materials. However, it is difficult to be integrated in the construction of experimental systems. The main reasons are: a. Lack of convenient storage carriers; b. Scanning measurement requires manual and repeated data recording; c. Single scan cannot be repeated automatically. However, 4294A has integrated the IBASIC language system developed by HP and multiple shift registers for state storage and identification. It is also equipped with a GPIB interface, so the PCI to GPIB card can be used to enable 4294A to communicate and store data with a personal computer.

In the design of VI, Labview software developed by NI was selected. As the first VI design software launched by NI, Labview has been widely used in the construction and design of various experimental instrument platforms due to its high versatility and efficient human-computer interaction interface. Labview 8.0 is used to select GPIB port addresses, transmit and receive commands, set instrument parameters, trigger measurement commands, draw waveforms and realize data storage functions.

In terms of connection, ADLINK PXI-3488 GPIB card and GPIB dedicated communication data line are used. Therefore, the construction of the experimental system is shown in Figure 1.

2 Writing of Labview Experimental Program

Labview is needed to perfectly realize the various functions of Agilent 4294A. The GPIB port can be used to connect the PC to the 4294A, and the SCPI (Standard Commands for Programmable Instruments) instructions can be used to query the instrument status and program control. The 4294A has a complete set of status registers to store the instrument status. Therefore, by calling the values ​​of different bits of the query register, it is possible to determine whether the measurement parameter setting is completed, whether the scan is started or stopped, and whether the data is read in. The register structure of 4294A is a typical hierarchical structure, as shown in Figure 2. The detailed operating status of the instrument can be known by querying the corresponding bits of the event register and the status bit register.

The programming of the instrument control part is mainly divided into the following parts: a. Instrument parameter setting part; b. Graphic drawing and data display part; c. Data recording and storage part. [page]

2.1 Instrument parameter setting program

For the automatic control of 4294A, the first thing to do is to realize the program control of its panel parameters. Therefore, it is necessary to build a virtual front panel to simulate the instrument control panel, and at the same time, realize the setting and adjustment of various parameters in the program design part. The parameter setting virtual front panel and its corresponding program flowchart are shown in Figures 3 and 4.

As can be seen from Figure 4, first of all, for 4294A instruments connected to different GPIB addresses, we can use the address search bar on the lower right of the panel to find the required instrument address. After that, different input variable types and SCPI conversion of ordinary strings are used to convert obscure machine-read commands into easily identifiable command options and present them in the parameter setting front panel. Users only need to select simple commands on the front panel to send SCPI command codes to the instrument. However, since the 4294A needs to consider the delay when reading data from the GPIB port, a display box for identifying SCPI commands is designed. After confirming that the command input is correct, you can click the "Write Command to 4294A" button to complete the parameter setting.

After clicking this button, since a large number of SCPI commands written to the GPIB port require a certain amount of time to complete, and the delay of the Labview program itself must also be considered, we must ensure that these written commands have been executed before subsequent operations. To check whether the input command is completed, we need to access the read-only status register layer in SRQ and read the status of the corresponding position. When the return value of the bit is 1, it proves that the data is written and the next step can be executed. The corresponding flowchart of this process is shown in Figure 5.

It can also be seen from Figure 5 that after using the SCPI command "*OPC?" to read the value of the command write status bit in the status register and judging it as 1, the number of scans will continue to be set, and the conditional structure and While loop will be used to realize the selection and switching between single scan or multiple scans. Similarly, in the module for setting scan parameters, a SCPI command display window is also designed. After judging that the command is correct, you can click the "START" button to manually send a trigger signal to the instrument through the GPIB port to start data measurement.

2.2 Graphic drawing and data display program

An important performance of 4294A is to display the measured data, such as real-time curves of characteristics such as resistance, capacitance, and dielectric loss. Therefore, while recording instrument data, we also need to add a graphic drawing module, which can not only verify the accuracy of the program, but also increase the practicality of the program. The front panel and program of this part are shown in Figures 6 and 7.

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2.3 Data recording and storage procedures

After the measurement is completed and the graph is drawn, all the data needs to be saved for subsequent analysis of the results, such as the calculation of the dielectric constant. The flowchart of the experimental data saving part is shown in Figure 8.

It can be seen that using a film structure, data A, data B, and scanning parameters are stored separately. Because the data read out is an array type of double-precision real numbers, we need to use a While loop and an array conversion to dynamic data units to write the data one by one. After writing, all the data are saved as a .1vm format file. This file can be opened and modified by various document editing programs, which is very convenient for post-processing.

3 Verification and analysis of experimental results

In order to confirm the feasibility and accuracy of the experimental procedure, we compared the graphics drawn by the virtual front panel (Figure 9), the graphics drawn by post-processing of recorded data (Figure 10), and the graphics displayed on the instrument panel itself (Figure 11), as shown in the figure.

It can be seen that the three figures are very consistent in the waveform and data accuracy of data A and data B. This confirms the accuracy and practicality of the automatic control program.

4 Summary

Using Labview program and GPIB interface communication, the construction of an automated precision impedance analysis system was successfully realized. The results show that the precision and accuracy of this platform perfectly retain the performance of the original instrument; at the same time, it also realizes the automation of repeated measurements of a large amount of data. The GPIB card in this platform also has 15 external interfaces, which means that 15 other instruments can be connected to build a more complex automated experimental platform. At the same time, using the Web function of the Labview program, we can even use the Internet or LAN to achieve remote control of the system.