Calibrating Modular Instruments

Publish Date: Apr 11, 2012 | 28 Ratings | 3.04 out of 5 |  PDF

Overview

This paper examines a modular instrumentation platform and when and how to calibrate modular instrumentation. The fundamental principles of metrology and calibration are the same regardless of whether the instrumentation platform is based on modular instruments or traditional instruments. However, the flexibility of the modular platform and the software-defined interface allow for a variety of options for calibrating these instruments.

Table of Contents

  1. Modular Instruments
  2. Measurement Systems Designed for Accuracy
  3. Calibration Fundamentals
  4. Calibrating Modular Instruments
  5. NI Calibration
  6. Additional Resources

1. Modular Instruments

As the move toward modularity becomes more popular, test systems can increasingly be categorized into one of two types of instrumentation: traditional and software-defined. Figure 1 illustrates the architectures of these different types of systems.

Figure 1. Traditional and Software-Defined Instrument Architectures

Figure 1 shows that the hardware architecture for the two approaches is similar; however, the software interfaces and component packaging differ significantly. A traditional instrument is a stand-alone device with a fixed functionality and user interface. The instrument vendor controls the process for updating the embedded software, which defines the device’s capabilities, so you are limited to the measurements defined in this software. In contrast, software-defined modular instruments combine the flexibility of software you can customize with the functionality of modular hardware options. With this combination, you can make custom measurements, adjust for new and evolving standards, and scale system capabilities as requirements grow and change.

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2. Measurement Systems Designed for Accuracy

To better understand the similarities of virtual and software-defined instruments, consider the measurement subsystems of both. Figure 2 shows the block diagram for the measurement component of most modern instruments, which represents the measurement subsystem of the traditional instrument referenced previously or the block diagram for one of the modular hardware components of the software-defined instrument.

Figure 2. Block Diagram for the Measurement Subsystem of Most Modern Instruments

First, the analog signal is acquired into the analog front end, which includes signal scaling and conditioning. Once the analog signal passes through the front end, it is digitized by the analog-to-digital converter (ADC). The digital values are then stored in onboard memory for processing and analysis. This postprocessing can include digital signal processing (DSP) in the measurement system, post-acquisition processing in the processing component of the system, or logging for offline analysis.

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3. Calibration Fundamentals

Now that you have reviewed how the measurement components of modular hardware work, it is important to understand the fundamentals of calibration so you can see how they apply to the idea of a software-defined modular instrument. Measurement system calibration requires two core actions: the verification of performance according to some standard and, if necessary, the adjustment of the measurements to meet those standards.

Verification is the key step in calibrating an instrument. In this step, an instrument measures a known value, which is most often provided by a high-accuracy calibration standard. The results of these measurements are then compared to the required performance of the device, which is typically defined by the manufacturer’s specifications. By comparing the actual measurements to the listed specifications, you can demonstrate that the stated performance properties of a measurement system are achieved.

Once you have verified the current performance of your system, you may be able to adjust the measurements capability, if needed, to meet your specifications. This adjustment can include changes to the actual measurement circuitry itself, or it can be as simple as scaling the acquired data based on an offset determined during verification. The adjustment step of calibration modifies measured values so that they correspond to the expected value; however, it is important to determine the values you expect and whether adjustment is actually required to meet your needs.

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4. Calibrating Modular Instruments

Calibrating the measurement components of your virtual instrument is just as important as calibrating a traditional instrument. However, because the measurement system is a separate modular component, you have several options for verifying and adjusting your instrument.

Verification

You can choose from several methods to verify the performance of your modular instrument, though the theory of comparing your measurement against the expected value is consistent in all of them. The first method is to use test panel software that simulates the interface to a traditional instrument, like the Soft Front Panel application that is included in most drivers for NI modular instruments. Figure 3 shows the interface to the NI-FGEN Soft Front Panel. The Soft Front Panel may be the most familiar way to verify performance if you are used to traditional instruments. However, it is also the least efficient way because you must manually adjust the settings of the Soft Front Panel to verify different test points. This method fails to take advantage of the speed and flexibility of the modular instrumentation platform.

Figure 3. You can use the NI-FGEN Soft Front Panel to verify measurement performance.

Another way to verify the performance of your instrument is to automate the measurement process in software. Both traditional and modular instrumentation vendors typically release a calibration procedure that includes a methodology for verifying performance according to specification. These procedures include test points, measurement techniques, and test limits. Manual calibration procedures from NI include descriptions of the specific driver software functions to use to verify performance. By programmatically calling the functions necessary to verify performance, you can leverage the benefits of the virtual instrumentation platform to automate the measurement process.

Figure 4. With calibration procedures from NI, you can programmatically verify your measurements.

Finally, you can use several off-the-shelf software programs to automate the verification of your measurements even further by automating both the measurements taken for verification and the control of the calibration standards that are providing the references. An example of this software is NI Calibration Executive, which takes advantage of NI LabVIEW, NI-VISA, IVI, and NI TestStand technologies to create an integrated, stand-alone environment for calibrating NI measurement devices. Procedures to calibrate these devices can run automatically with Calibration Executive using NI GPIB to communicate with external calibration standards. Automating both your measurements and the control of calibration standards saves you even more time when verifying your products. In addition, Calibration Executive provides the collected data in calibration reports that are stored in an ODBC-compliant database for easy access from other programs.

Figure 5. Using NI Calibration Executive, you can automate the entire calibration process to be even more efficient.

Adjustment

The modular nature of a software-defined instrument provides several options for adjusting measurements after you have compared them to your specifications during verification. Because the digital values acquired by the measurement system are typically stored and processed onboard, you can often adjust the performance of your instrument by modifying calibration coefficients stored in the memory. For example, the block diagram for the NI PXI-5652 RF signal generator is shown in Figure 6. The portion of the memory that stores calibration coefficients is highlighted.

Figure 6. NI PXI-5652 calibration coefficients are stored in onboard EEPROM.

You should determine the calibration coefficients based on the verification data and your required performance. For NI products, an adjustment API is included in most hardware drivers. These APIs provide access to function calls that can read and write the calibration coefficients and give you the ability to lock the coefficients to prevent unauthorized access. Both Calibration Executive and the manual calibration procedures developed by NI provide instructions for adjusting coefficients to meet published NI specifications. An example of the calibration API for the NI-SCOPE driver is shown in Figure 7.

Figure 7. The Calibration API for NI-SCOPE

In addition to adjusting measurements in firmware on the measurement system, the software-defined nature of modular instruments helps you make your own adjustment. You can program the software that runs the user interface on the instrument controller to compensate for differences between the measured data and known standards. Typically, individual measurement modules are calibrated separately using the onboard calibration coefficients. However, the user-defined software can implement scaling to adjust for differences between measurement modules, like in an RF multiple input, multiple output (MIMO) system, or to compensate for end-to-end error that accumulates in the entire measurement system, including terminal blocks, cables, and transducers.

Calibration is an important part of maintaining any measurement instrument, whether it is a traditional instrument or a software-defined one based on modular hardware components. Many of the features that make a modular approach more flexible and scalable also offer a number of options for calibrating your measurement hardware. The software-defined approach makes it easier to verify your measurement performance and adjust when necessary.

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5. NI Calibration

To help you meet your calibration needs, NI provides calibration support that includes services to calibrate your products, manual calibration procedures, and automated calibration software specifically designed for use by metrology laboratories.

Calibration Services

NI provides the following calibration services to help keep your NI products performing at their best:

Traceable Calibration Service
With traceable calibration, you can track how your instruments are performing as well as minimize the time and costs associated with unscheduled downtime and quality issues. This service includes verification of measurement performance and adjustment when possible. It also features "As Found" and "As Left" measurement data to show the measurements for every test point every time. This data can help you characterize the performance of your device and lets you know precisely what you are measuring. This service level also helps you meet the requirements of quality programs like ISO 9001.

Compliant Calibration Service
Compliant calibration service helps you meet the needs of more advanced quality standards. It is performed in a lab that is accredited to ISO 17025 and the service complies with ANSI/NCSL Z540-1-1994. This service includes verification of measurement performance and adjustment when possible, as well as provision of full "As Found" and "As Left" measurement data and an evaluation of measurement uncertainty.

ISO 17025 Accredited Calibration Service
NI Calibration Services Austin is accredited to ISO/IEC 17025:2005 by A2LA to meet all your needs for accredited calibration service. In addition, NI Certified Calibration Centers located throughout the world can meet local needs and provide the same levels of service no matter where you are. This service includes full calculated measurement uncertainty and the logo of the local accrediting body so that you can be confident that the service was performed exactly according to the standard.

Additional Calibration Options

Along with standard calibration offerings, NI can assist you with additional calibration services tailored to your specific needs. These services for NI products include, but are not limited to, express calibration services, on-site calibration services, system calibration, and custom calibration procedures. Contact your local NI services representative for a customized calibration service agreement.

In addition to service offerings, NI provides several options if you want to calibrate NI products yourself. Manual calibration procedures including test points and step-by-step instructions are online. However, these procedures are often time-consuming and require programming steps to adjust the measurement capabilities of the products. To meet some of the challenges of a manual procedure, NI provides NI Calibration Executive automated calibration software, which makes calibrating NI products even faster and easier. You can visit the additional resources below to learn more about manual procedures or Calibration Executive.

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6. Additional Resources

Obtain Additional Information Regarding NI Calibration

Learn More about NI Modular Instrumentation Platforms

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