Lowering the Barriers of Laser Primary Calibration
While the ever lowering of uncertainty is the quixotic quest of the metrology industry, for certain laboratories the lowest possible uncertainty is a must.Of particular note are National Metrological Laboratories, Primary Standards Laboratories (often serving branches of the Armed Services or National Labs), Accelerometer Manufacturers (there are over 150 now) and Accelerometer Calibration System Manufacturers (where there are still only just a few…).Each of these organizations has a specialized metrology function in providing primary calibrations for accelerometers that will be used in precision accelerometer calibration systems as references or transfer standards as called out in ISO16063-21.In these cases, the best measurement uncertainties are needed because the reference, or transfer standard uncertainty, will be directly adding to the overall measurement uncertainty of the calibration system. This is critical since it is most often the single largest contributor to the system uncertainty.
While Laser Primary calibration of accelerometers is clearly the best technique for the lowest uncertainty, there have been a number of traditional barriers prohibiting some laboratories from obtaining a system.The foremost barrier is often the high cost of a laser primary accelerometer calibration system due in part to the component cost of the Laser Doppler Vibrometer at the core of the system.While some labs opt to make their own Laser Vibrometer, a commercially available one typically costs on the same order as a luxury automobile.The benefits of this type of laser include finished commercial packaging and the ability to work with non-cooperative (minimally reflective) targets. However, the single beam laser typically takes more time and labor cost in multiple positioning steps in order to follow the standard ISO16063-11.The standard recommends that the laser measurement should be aligned for 3 different measurement points around the base of the (accelerometer) sensor under test (SUT).Taking the do-it-yourself path in constructing one's own laser measurement system often saves hardware money up front, but can ultimately cost significantly more in total lifetime costs as there are complex optical setups and measurement system developments needed, as well as regular maintenance, and quality control.
As illustrated, the new laser primary accelerometer system also benefits from a multi-head and multi-pass beam operation that allows for the simultaneous measurement of four points around the base of the sensor under test.In addition to cutting the hardware costs, the simultaneous measurement operation also decreases the test time needed to laser calibrate an accelerometer by a factor of 3 compared to repositioning a single beam laser.The tradeoffs (needs cooperative surface reflection) in selecting the homodyne laser are easily overcome by mirroring the insert of a calibration grade precision air-bearing exciter.Some traditional technical papers also report that homodyne lasers have difficulties compared to heterodyne lasers (challenges like sensitivity to signal intensity and ambient light variation, inability to measure continuously and difficulty to align) but this is no longer the case in a new generation homodyne laser that has found thousands of uses in manufacturing metrology.This more economical, rugged, reliable (built from lasers designed to withstand plant floor metrology needs) system has been adapted for accelerometer calibration use and tested in proficiency comparisons to deliver uncertainties on par with traditional laser vibrometer primary calibration systems.Any National Metrology Laboratory, Primary Standards Laboratory or Accelerometer manufacturer can now step up to world class uncertainties at a fraction of the traditional costs.