Structural testing and calibration

-October 01, 2006

Richard Bono
Application Specialist
The Modal Shop
Courtesy of the Modal Shop 

Test data from the shop floor has become an important indicator of part quality, process integrity, and machine health. The Modal Shop is a PCB Group company that provides structural testing on vibration and acoustic sensing systems. In a recent interview, Richard Bono, application specialist at The Modal Shop, discussed accelerometer calibration.

Q: What is driving the implementation of automated calibration systems?

A: Evolving standards, along with growing inventories of accelerometers, have driven the demand for companies to invest in automated calibration systems. The cost of outsourcing calibration begins to justify a calibration system purchase return on investment once a user crosses the 100-calibrations-per-year threshold.

Additionally, uniform quality standards like ISO 9001 and ISO 17025 evolve as measurement technology advances. What was considered state-of-the-art a decade ago in terms of quality does not necessarily meet standards today. For example, total harmonic distortion measurements are now required in the ISO 16063-21 standard for vibration calibration.

As vibration and shock tests have become more ubiquitous and testing in general has become less expensive on a per-channel basis, sensor inventories have grown to meet expanding channel count capabilities. All accelerometers need regular calibration, and automation speeds the process, lowers cost, and reduces uncertainty.

Q: What should engineers know about accelerometer calibration and standards?

A: Often, standards can be technically and administratively intimidating. The first thing an engineer can do to reduce anxiety associated with learning a new calibration standard is to speak with a professional from a calibration service provider or a calibration system manufacturer.

In most cases, calibration is covered broadly in the standards section on the control of measuring and test equipment. The calibration professional can distill the essence of the standard into its technical and procedural components. It is remarkable how quickly a complicated standard can be tamed with a little bit of helpful advice.

Armed with representative data plots, example procedures, and recommended calibration intervals, a typical engineer can quickly and knowledgeably present and recommend a course of action for his team to move through compliance or certification to the various technical and quality standards. For example, our application engineers routinely help new calibration customers with performance characterizations of our precision air-bearing calibration shaker as compared to the ISO16063-21 vibration calibration standard.

Q: What is meant by calibration “traceability” and why is it important?

A: Traceability is achieved through an unbroken link of reference calibrations back to a physical constant or national standard. In many cases, this may be to the National Institute of Standards (NIST) or in Europe to the UK National Physical Laboratory (NPL) or Germany’s national calibration authority (PTB). Traceability is important to ensure the validity of the overall calibration system results. Typical calibration techniques use a back-to-back or reference sensor with which the test accelerometer signal is compared. In calibrating the system reference accelerometer, each step in the link of these reference calibrations adds more uncertainty into the end calibration performed. To provide the lowest possible uncertainty in our customers’ calibration data, PCB Group operates our own in-house laser interferometer and is accredited for primary calibration of accelerometers for test and reference use.

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