Digital Multimeter Automatic Calibration System

Instrument calibration is a technical means to restore and maintain equipment performance indicators and ensure accurate and reliable measurements. Therefore, it is necessary to calibrate regularly in a national authorized calibration agency. However, with the rapid development of computer technology, large-scale integrated circuit technology and communication technology, the field of instrument technology has undergone tremendous changes, from the initial analog instruments to the current digital instruments, embedded instruments and intelligent instruments. Injecting new vitality into the field of modern measurement technology and instrument control, new test fields, methods and new instrument structures continue to emerge, breaking through the concept of traditional instruments in many aspects, and the functions and roles of electronic instruments have undergone qualitative changes. In this context, the relationship between instruments and test systems becomes more important. Without the support of automatic or intelligent test systems, some complex experiments cannot obtain ideal test results at all. The application of virtual instrument technology in the calibration of digital multimeters in this project, that is, the automatic calibration system of digital multimeters, is based on the needs of laboratory metrology work and the requirements of the current corresponding verification procedures. The user interface and control program are developed using the LabVIEW software environment. This software system ensures the reliability of the calibration process and the accuracy of the calibration data by automatically calibrating the digital multimeter and automatically processing the data.

Overall design of the system

A complete LabVIEW program mainly consists of three parts: the front panel, block diagram program, icon and connector pane. The front panel is an interactive graphical user interface used to set input values ​​and observe output quantities. The block diagram program is a graphical source code that defines the VI function, and uses graphical language to control the front panel control quantities and indication quantities. The chart and connector pane are used to define the program as a subroutine so that it can be called in other programs.

The general steps of calibration are:

• Preheat the instrument (including the instrument to be calibrated and the standard source);
• Set the instrument status, perform measurements and record data;
• Determine the data results and draw conclusions;
• Calibration certificates and original records are automatically generated.

This system is divided into modular programming, mainly including:

• Initialize the settings module
• Data acquisition dynamic display module
• Certificate generation module
  

Figure 1 Structure diagram of the digital multimeter automatic calibration system

Specific implementation process of the automated calibration system

First, the standard source and digital multimeter are turned on and preheated as required, and the hardware devices (GPIB card, 488 cable, etc.) are connected. After the hardware connection is completed, start the computer and search for the physical address allocation of the entire test system. According to the addresses of each instrument found, set the correct address configuration when the calibration software is running.

1 Initialization setting module

Double-click the corresponding automatic calibration program icon, the system starts, and enters the main interface of the test system. The style of the main interface is simple and practical, and the function buttons are on the left. The first thing to enter is the initialization setting module. The front panel of the initialization module is shown in Figure 2. The initialization module needs to set the calibration items of the device under test, set the GPIB address of the calibrated instrument and the standard source, and select whether it is the first test. The purpose of this function is to save the measured data to prevent the loss of measured data due to accidents, and the need to re-test. Select Chinese and English language, select calibration, verification, and select the name of the device under test. The initialization setting is completed.

Figure 2: Front panel of the initialization module

The main functions of this module include: initializing the instrument, setting the instrument status, measuring values, data bit control, dynamic display of data, data result judgment, data storage, etc. The automated data acquisition process completely simulates the manual measurement process for measurement. The initialization configuration of the instrument and the modules such as range, display bit, accuracy, sampling rate, sampling time, measurement value, function selection, etc. are downloaded from the NI website. Programmers can also write corresponding modules according to the SCPI language commands provided in the instrument programming manual. The data display bit, data range, upper and lower limits, etc. in this module are automatically generated according to the requirements of the test and measurement for the instrument, and the data result judgment is also completed automatically. The program displays the unqualified data in red font, so that the metrologist can easily find that the data is unqualified after the measurement. The front panel of the data acquisition dynamic display module is shown in Figure 3.

Figure 3: Front panel of the data acquisition dynamic display module

3 Certificate and original record generation module

Automatically generating certificates and original records greatly facilitates the work of metrologists, eliminates errors that are prone to occur in human operations, and liberates labor. The metrologist only needs to enter the relevant instrument information and calibration information, calibration items, and select the corresponding certificate template on the front panel of the certificate generation module, and the program can automatically generate the corresponding calibration certificate and original records. The front panel of the certificate module is shown in Figure 4.

Figure 4 Front panel of the certificate and original record module

Conclusion

This system has been put into use in the electrical room of the 514th Institute of Aerospace, the Hebei Electric Power Research Institute and other metrology departments. The labor force has been liberated, the efficiency of calibration has been greatly improved, and the accuracy and reliability of calibration have been better guaranteed.

This system is developed based on LabVIEW, with strong operability and fast speed, and can imitate the process of manual measurement to realize automated test measurement. The system has a clear structure, easy to transplant and update modules, and can directly issue certificates and original records, which not only saves manpower and material resources, but also has a fast measurement speed, avoids contact between people and high voltage and high current, and protects personnel safety. At the same time, it also has the advantages of many measurement data points, accurate and reliable, and can eliminate errors that may occur in manual counting and arithmetic errors in manual calculations.