The cost of computer-based virtual measuring instruments is lower than that of box-type measuring instruments, and their application in recent years has been widespread. It is the same as the traditional box-type measuring instrument. The instrument has a calibration validity period, so regular calibration is required to ensure the measurement accuracy. This article introduces the internal and external calibration methods for computer-based measuring instruments.
Computer-based measuring instruments have great flexibility, and applications are becoming increasingly popular. By controlling the function of the instrument, a measurement system that meets special requirements can be developed. For any measurement system, cost is the first consideration. The cost of developing a computer-based measuring instrument is often several times cheaper than buying a stand-alone instrument. This is because the hardware cost is low, the software can be reused, and one test instrument can often replace several independent measuring instruments.
Computer-based measuring instruments are closely linked to the computer industry. They benefit from advances in computer technology, including open communication standards, web servers, and simple interfaces for electronic tabulation and word processing between instruments and desktop applications. These measuring instruments also benefit from the stability of the computer's performance and the reduction in price, so that the performance of computer-based measuring instruments can be continuously improved without price increases.
Use calibration for precise measurements
Most measuring instruments provide information about the accuracy of the measuring circuit of a measuring instrument in the form of an accuracy table. The accuracy specification table helps determine the total uncertainty of the measuring instrument. However, these accuracy specifications only apply to the circuit board that has been successfully calibrated. Therefore, you must use these specifications to verify the operation of the board before and after measurement adjustment.
The ability of a measuring instrument to accurately measure changes in physical quantities changes according to certain factors. Service life, temperature, humidity, exposure to the external environment, and misuse all affect the accuracy of the measurement. By comparing the obtained test results with known standards, calibration quantifies the measurement uncertainty. It is to verify whether the measuring instrument is working within the specified index range. If the measured value of the instrument exceeds the published uncertainty, the measuring circuit must be adjusted to comply with the published specifications.
After a period of time, users need to calibrate traditional measuring instruments, as do computer-based measuring instruments. Users should select computer-based test instruments with internal calibration (also called automatic calibration) and external calibration tools.
Internal calibration
If you use an instrument such as an oscilloscope, then you have completed the internal calibration. In fact, when you change the vertical range setting, most oscilloscopes have completed internal calibration. Basically, the instrument digitizes the high-precision and on-board voltage source and compares its reading with the known value, and then saves the calibration factor in the electrically erasable read-only memory carried by the instrument itself. The upper voltage source is also calibrated to a well-known standard such as NIST. The main purpose of internal calibration is to compensate for changes in working environment, changes in internal calibration temperature, and other factors that may affect the measurement.
Like traditional measuring instruments, computer-based measuring instruments should support internal calibration. The internal calibration of computer-based measuring instruments is initiated by calling the software function of the calibration measuring circuit. Since the measurement can be performed immediately, and there is no need to wait for this internal calibration to adjust the vertical range whenever possible, the internal calibration technology controlled by the software can save test time.
Computer-based measuring instruments are installed in environments such as desktop computers, PXI / CompactPCI chassis, or VXI / VME chassis. Because computer-based measuring instruments are installed in many different computer environments, designers should remember Measuring instruments will be affected by electromagnetic interference and changes in power supply voltage, but also work in a wide temperature range. Traditional measuring instruments are facing similar challenges due to their increasingly tight integration with personal computers.
The most basic solution to eliminate electromagnetic interference includes: separating the ground planes of digital and analog signals, performing local filtering on power signals, and shielding sensitive components. In order to compensate for changes in the voltage source, a DC-DC converter can be used to increase the power supply voltage, a voltage regulator is used to control the voltage of the power supply on the board, and a large capacitor is used to eliminate harmonics of the power supply on the board. The on-board temperature sensor and internal calibration can be used to complete the calibration of different temperatures in the operating environment. For information on the above design techniques, check the white paper on the NI website titled "Accurate Measurement with PC-based Data Acquisition Hardware".
Many users are wondering whether computer-based measuring instruments can be moved from one computer to another without affecting the calibration. The answer is yes, if this measuring instrument is designed using the above standards, the calibration is still valid. Computer-based measuring instruments, like traditional instruments, are usually calibrated in a metrology laboratory. The operating temperature of this laboratory is likely to be different from the operating temperature of the production workshop or design laboratory. Installing a computer-based measurement instrument on a new computer is no different than moving a traditional oscilloscope in one environment to another. Because the design of the instrument is oriented to different usage environments, all calibrations are still valid. The above white paper contains test results that clarify this reason.
When selecting a measuring instrument, make sure that the computer-based measuring instrument supports internal calibration. For ease of use, the internal calibration process should be automated, that is to say, this process does not need to adjust the voltage divider and jumper in the device. For example, the internal calibration of NI's 12-bit data acquisition products can automatically complete the full-scale internal calibration in about 10-20 seconds.
External calibration
After a period of time, usually one year later, the on-board voltage source used to complete the internal calibration needs to be calibrated to a known standard. The calibration process of this on-board power supply is an example of external calibration.
External calibration uses high-precision external standards. During external calibration, the on-board calibration constants should be adjusted with reference to external standards. As with internal calibration, external calibration does not require adjustment of the potentiometer or moving jumpers. External calibration is usually reserved for metrology laboratories or other organizations with proven sources. Once the external calibration is completed, the new calibration constants are saved in the protected area of ​​the measuring instrument's memory and cannot be obtained by the user. This protects the calibration integrity due to accidental adjustments. Regardless of any measuring instrument, the manufacturer must provide the corresponding calibration process and the calibration software necessary for external calibration on a computer-based measuring instrument device.
Manual and automatic calibration process
In order to meet the needs of engineers for calibrating computer-based measuring instruments, manual and automatic calibration schemes can be used. The automatic calibration system is fast, requires no human intervention and can provide detailed calibration records for compliance with standards like ISO-9000. The manual calibration process provides detailed information for users who want to embed the calibration function directly into the measurement system, which avoids the trouble of moving computer-based measurement instruments back to the metrology laboratory.
The manual calibration process tells you how to externally calibrate the measuring instrument. This process is usually sold as part of the maintenance manual for the measuring instrument or can be downloaded from the manufacturer's website.
The disadvantage of manual process calibration is that it is time-consuming and laborious. This is not due to the troublesome adjustment of the measuring instrument, but due to a long measurement verification process. To meet the requirements of the calibration guidelines, the performance of the measuring instrument must be verified before and after calibration. Only in this way can it be determined whether the measuring instrument is working within the specifications before and after calibration. For example, to perform external calibration on an E-series data acquisition device, three measurements of gain, dynamic range, and polarity are required. This requires hundreds of measurements.
Products such as Fluke MET / CAL and the NICalibration Executive tool contain a description of how to automate this process in the metrology laboratory, thereby greatly reducing the time required for external calibration. Through GPIB communication with external standards, the calibration software can set and read the external voltage values ​​from the instrument. These values ​​are then used to verify and calibrate the adjusted instrument. At the end of the calibration process, the instrument technology can be automatically read from the configuration file Indicators and generate a detailed calibration report (Figure 1). Using automatic calibration software, you can measure several measuring instruments at the same time within the time required to calibrate a traditional measuring instrument, especially when there is no voltmeter in the device under test. The purpose of designing these tools is to meet the metrology laboratory Strict requirements for calibration.
For large companies that have their own metrology laboratories, they are equipped with manual processes and calibration software products. For companies that do not have metrology laboratories, data collection and measurement instrument companies usually have to work with metrology service companies around the world.
There are usually 2 types of external calibration certificates. One is the basic calibration certificate, which is usually generated after the product is manufactured and provided by the measuring instrument manufacturer. This kind of certificate makes it possible for NIST or local standards verification agencies to trace the source of the instrument and measure the environmental status during the calibration validity period. Accredited metrology laboratories usually provide detailed calibration certification. In addition to providing the same information contained in the basic certification, this certification will also improve the data before and after each measurement. The detailed calibration certification should meet the requirements of specific guidelines such as ANSI-Z540-1. These guidelines are mainly adopted in the United States, or more in ISO Guide 25. These guidelines ensure the continuity of calibration, and most ISO-9000 certified companies meet the requirements of these guidelines.
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