Design Con 2015

Lowering the total cost of ownership of PXI instruments

Mark Catelani -February 08, 2013

The calibration of an individual PXI module ensures the specified accuracy of the resulting measurements. This concept takes on greater importance when applied to PXI-based instruments that contain multiple modules.

Out of the box, a preassembled multi-module instrument (MMI) can be a time-saver: the chassis is already loaded, the modules are configured, and the instrument is ready to use. Beyond convenience, confidence in measurement results is also important—and confidence comes from calibration of the preassembled MMI as a unified instrument. After the MMI has been deployed, service and repair capabilities can save time, ensure ongoing confidence, and enhance instrument uptime while also reducing the total cost of ownership.

This article explores the meaning and value of calibration before offering service-related evaluation criteria that will be useful in the evaluation of MMIs. These criteria will be helpful to organizations seeking to minimize downtime, and will be of special interest to companies committed to robust ongoing quality programs. This article also explains, by way of example of one way to speed repairs, Agilent Technologies’ core exchange assembly program.

Understanding the extent of calibration
There are important differences between the calibration of an individual module and that of an MMI. With a single module, specifications are ensured through calibration of that individual unit. Traceability of the calibration is established through the properties of the measurement results whereby the results can be related to a reference (i.e., the test standards) through a documented, unbroken chain of calibrations, each contributing to the measurement uncertainty.

There is a caveat. Once a system creator or end user adds that module to a chassis and connects it to other modules, the resulting instrument may not have warranted performance specifications. To obtain warranted performance specifications, procedures must exist to calibrate the instrument as a whole. If the original equipment manufacturer (OEM) doesn’t provide instrument-level testing or documented test methods, then the user must invest time and money in developing and documenting an in-house calibration procedure.

The situation is more concrete with an MMI that has been calibrated as a unit. In effect, the calibration binds the modules together and measures their collective performance as an instrument. When the MMI is calibrated and shipped as a unit, it carries traceable, warranted performance. As long as the MMI is used as a set, the calibration and warranted performance remain valid.


Getting more than just a sticker
For some organizations, having a “calibrated” sticker on the front panel is sufficient reassurance of specified performance. For those that are pursuing a rigorous quality program, a sticker is necessary but insufficient documentation.

The meaning—and the testing—behind the sticker is what matters most. The following elements are the foundation of a meaningful calibration:

  • Performance tests check all warranted specifications
  • Performance tests are automated to save time, reduce cost and ensure consistent results
  • Measurement uncertainty is consistent with the ISO Guide to Uncertainty in Measurement (ISO GUM)
  • Compliance certification includes ISO 17025, ILAC-G8, ILAC-P14, Z540-1 and Z540.3
  • Adjustments are performed when an instrument is measured out of spec
  • The calibration laboratory’s scope of accredited parameters match the requirements of the performance tests

Documentation should be provided in the form of a calibration certificate and measurement report. For an MMI, the certificate should identify the model number and serial number of each module that is part of the instrument. The measurement report should contain the same identifying numbers, and it should list the tests performed, the test standards, and the measured data and the uncertainties for every measurement point.

For organizations with national or international operations, it is also useful to know if the service provider uses the same set of automated calibration procedures in all of its service facilities around the world. Identical procedures can help ensure consistent instrument calibrations and measurement results, wherever service is provided.

Calibration management is a final item worth considering. Some modular instruments now include an electronic “calibration sticker” that is stored inside the modules. This enables the user to easily check the calibration status of an MMI using the programmatic interface or, with a browser, the soft front panel (see Figure 1). Some modules also let the user set preferred calibration intervals, “calibration due” reminders, and password-protected access.


Figure 1. Storing calibration information and preferences inside a module helps simplify calibration management.

This electronic sticker capability may also enable storage of calibration information for multiple configurations of an MMI. If so, then a system could be calibrated with different combinations of spare modules. This is especially useful if spare modules are purchased along with an MMI to ensure greater uptime. The OEM may not provide multiple-combination calibration as a free service, but it could be a worthwhile expense when maximum availability is crucial on a high-volume production line or in a mission-critical aerospace or defense application.

Continue reading page two (on tmworld.com)>>

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