Calibration of Virtual Instruments

1 Introduction

With the development of science and technology, computer-based measuring instruments, which we often call virtual instruments, are increasingly closely connected with computers. Due to their flexible settings and complete data processing functions, the application range of virtual instruments covers almost all areas of testing: from data acquisition, signal conditioning, sound and vibration measurement, vision, motion, instrument control, signal generation, signal measurement, distributed I/O to CAN interface and other industrial communication fields. And with the continuous improvement of computer performance and the increasing types of virtual instrument cards, more users accept the concept of virtual instruments, which makes the application scope of virtual instruments continue to expand.

2 Composition of Virtual Instruments

Traditional test instruments are mainly composed of input/output parts, power supply parts, instrument internal core, display control unit, etc. For virtual instruments, the display control part is realized by computer instead, and the core part of the instrument is virtual instrument card with various types of bus structure. As a new concept of test instrument, virtual instrument is a real instrument in terms of its function, which has the same function as traditional test instrument; and the meaning of "virtual" in virtual instrument is to use software to realize many functions realized by hardware in traditional instruments, and its core is virtual instrument card. In order to enable hardware engineers to complete the writing of test software, NI and other companies have developed graphical programming languages ​​​​specially used for virtual instrument programming, such as LABVIEW, in addition to providing test boards for virtual instruments, and provide users with a large number of measurement algorithm software packages in the form of subVIs. Users can easily use these to complete conventional virtual instrument measurement tasks, thereby greatly reducing the complexity of virtual instrument development, making the widespread use of virtual instruments possible.

3 Necessity of virtual instrument calibration

The measurement accuracy of the virtual instrument is related to the accuracy of the virtual instrument card used and the test software. The algorithms in the test software have been taken into consideration during the design (such as multiple averaging to reduce single measurement errors). Therefore, the measurement accuracy of the virtual instrument mainly depends on the accuracy of the virtual instrument board.

As we all know, the measurement accuracy of test instruments varies according to certain factors. The aging of the electronic components used inside the measuring instrument, the instrument's usage time, environmental changes, and misuse will all affect the accuracy of the measurement results.

The purpose of calibration is to quantify the measurement uncertainty by comparing the test results of the measuring instrument with the standard values ​​of the standard instrument traceable to the national benchmark at a certain time interval to confirm whether the measuring instrument is working within the instrument technical specifications.

Like traditional measuring instruments, virtual instruments also have a calibration cycle and must be calibrated regularly to ensure their measurement accuracy. The calibration of virtual instruments is similar to that of traditional test instruments in that they also require regular external calibration using standard instruments; the difference is that the calibration of virtual instruments only calibrates the virtual instrument card.

3.1 Calibrate the virtual instrument card to see if it meets the virtual instrument calibration requirements

As mentioned above, to calibrate a virtual instrument, you only need to calibrate the virtual instrument card; however, the computer used to calibrate the virtual instrument card is the calibration computer in the calibration laboratory, not the original matching computer. Some people may ask, after calibration, when the calibrated virtual instrument card is used in the original computer, will it still maintain the technical indicators after calibration in the laboratory?

According to the verification procedures and calibration specifications, calibration of any instrument should be carried out in a laboratory that meets the environmental requirements. In actual use, the environmental conditions may be different from those in the laboratory. Since the design of virtual instruments has certain environmental conditions for use, when virtual instruments are used in an environment that meets the above conditions, they still meet the requirements of the technical specifications. There is no essential difference between installing a virtual instrument card calibrated in a calibration laboratory on a new computer to form a test instrument and moving a traditional digital multimeter from one environment to another environment. Therefore, calibrating a virtual instrument card can fully meet the calibration requirements of a virtual instrument.

3.2 Can the internal self-calibration of virtual instruments replace external calibration?

Today's traditional test instruments, such as digital oscilloscopes or spectrum analyzers, have an internal self-calibration function. Similar to traditional test instruments, virtual instruments also have an internal self-calibration function.

The purpose of internal self-calibration is to compensate for the influence of changes in the instrument's working environment, changes in the internal calibration temperature, and other factors that may affect the measurement on the virtual instrument, so as to improve the measurement accuracy of the virtual instrument. Internal calibration is to call the software of the internal calibration measurement circuit and compare it with the standard value inside the instrument.

You may ask, since the virtual instrument has an internal self-calibration function, is it still necessary to send the virtual instrument card to the calibration laboratory for regular external calibration?

The answer is yes. Although virtual instruments have internal self-calibration functions, they still need to be sent to a laboratory for external calibration regularly (usually once a year). The reasons are as follows:

First: When the virtual instrument card is self-calibrated, the result displayed is pass or fail, and the test result value of the actual parameter is not displayed.

Second: The self-calibration parameters cannot cover all the parameters of the virtual instrument card.

Third: Since self-calibration requires a standard value to be provided inside the instrument for self-calibration, whether this internal standard value meets the requirements of technical parameters also requires regular external calibration.

Fourth: Since traditional test instruments with self-calibration function also need to be calibrated outside regularly, virtual instruments with self-calibration function also need to be sent to a laboratory for calibration regularly. [page]

4 Calibration results

In order to give you an intuitive understanding of the calibration items, instruments and technical indicators of virtual instruments, the following uses NI's E series data acquisition card as an example to introduce laboratory calibration. Other boards such as oscilloscope cards, signal generator cards, frequency meter cards, and RF signal generation and analysis cards are not mentioned here.

4.1 Standard instruments used for calibration

The calibration of the data acquisition card involves the calibration of parameters such as voltage input, voltage output and time base. According to the requirements of the American NI company, the standard instruments to be selected are as follows:

Table 1 Standards required to calibrate NI's E-series data acquisition cards

Table 2 E series data acquisition card analog input calibration results

Table 3 E series data acquisition card analog output calibration results

Table 4 Calibration results at 5MHz frequency

4.3 Interpretation of calibration results

It can be seen from Table 2, Table 3, and Table 4 that the calibration results include all the parameters that need to be calibrated for the E series data acquisition card: analog input signal measurement accuracy, output signal accuracy, and 5MHz frequency accuracy.

During calibration, the calibration procedure requires the virtual instrument card to warm up for 15 minutes, followed by an internal self-calibration.

For the measurement accuracy of analog input signals, the calibration parameters are selected as the upper and lower limits of the 10V range with gains of 0.5, 1, 10, and 100, and the analog voltage measurement accuracy at zero input, especially the calibration of the value at 0 input, which is used to calibrate the zero point deviation. The standard used for this calibration is the Fluke5500 multi-function calibrator.

For the accuracy of analog output signal: the calibration parameter is selected as the output voltage accuracy of 20V range in channel 0 and channel 1. The standard used for this calibration is a 6.5-digit digital multimeter.

For the calibration of 5MHz frequency accuracy, direct measurement is performed by a frequency meter.

Before is the calibration result before adjustment, and After is the calibration result after adjustment. In addition, it can be seen that the low limit and high limit of Before given by NI are slightly larger than the Low Limit and High Limit of After. The parameter value is adjusted during the calibration process of the virtual instrument card, which can be seen from the slight difference between the Before value and the After value.

Since this data is first calibrated and then recalibrated, the difference between Before and After is not very large.

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5. Uncertainty evaluation of calibration results

The measurement uncertainty of the PCI-6024E calibration result involves voltage parameters and frequency parameters. This paper only evaluates the measurement uncertainty of 4.9V voltage. The standard instrument used for the evaluation is 5520A multi-function calibrator.

5.1 Measurement uncertainty components

Measurement uncertainty consists of the following aspects:

(1) 5520A output voltage accuracy

(2) Measurement repeatability

(3) Instrument display resolution

5.2 Evaluation of standard measurement uncertainty

5.2.1 Measurement uncertainty of standard instrument 5520A

5.3 Combination of measurement uncertainty

6 Conclusion

Like traditional measuring instruments, virtual instruments also need to be regularly sent to laboratories with calibration capabilities for calibration to ensure that the measuring instruments are always within the specified technical indicators. As a legal metrology institution established by the Shanghai Municipal Government in accordance with the law, the Shanghai Institute of Metrology and Testing Technology has an unshirkable responsibility in the metrology and calibration of virtual instruments. In order to better provide services to customers, the headquarters of the Shanghai Institute of Metrology and Testing Technology moved to No. 1500 Zhangheng Road, Zhangjiang High-tech Park, Pudong, Shanghai's microelectronics center in 2006, and its metrology and calibration capabilities have been greatly developed. The Electronic and Electrical Metrology Technology Institute under the Shanghai Institute of Metrology and Testing Technology was formed by the merger of the original electromagnetic room, radio room and optical room. The measurement coverage ranges from DC signals, low-frequency signals, high-frequency signals, pulse signals to radio, visible light, and even invisible light. It now preserves dozens of national benchmarks, the highest standards in East China, and the highest standards in Shanghai. It has two professor-level senior engineers and more than ten senior engineers. Taking various factors into consideration and after much investigation, the National Instruments Corporation (NI) of the United States decided to set up the calibration center in China at the Electronic Institute of Shanghai Institute of Metrology and Testing Technology. The calibration scope covers all its products (including data acquisition cards, signal generator cards, oscilloscope cards, frequency meter cards, RF signal generator cards, RF signal analyzer cards, downconverter cards and all other card types). We can not only provide calibration services (calibration) for virtual instrument cards, but also include adjustment services (adjustment) when they exceed their technical indicators (specifications). The number of virtual instrument cards calibrated by us for customers is also increasing year by year. We have now provided calibration services for nearly a thousand virtual instrument cards of NI China users, meeting the needs of customers. In addition, it also proves that virtual instruments, like traditional test instruments, need to be calibrated regularly.