The Ongoing Development of Dynamic Calibration

Your calibration program is key to promoting control and confidence in your measured data.  It is also a cornerstone to your organization’s measurement competency.  As with organizations embracing the quality benefits of ISO 9001, those companies achieving accreditation to the ISO 17025 and/or ANSI 540Z standards are ensuring the rigor, discipline, and competency of the test and calibration scope. They are also stepping up to ensure quality and excellence in results.  Most often, the normative references and procedures in the area of vibration calibration refer to another technical standard, ISO16063-21 Vibration Calibration by Comparison to a Reference Transducer. Within the industrial marketspace the American Petroleum Institute Standard 670 for Machinery Protection systems provides vendors and reliability teams with accuracy requirements and guidelines for field testing critical vibration alarms and instrumentation.  Regardless of marketspace, state-of-the-art dynamic calibration systems no longer rely on proprietary, black block analyzers and algorithms.  Modular, commercial-off-the-shelf (COTS) components are integrated for easiest operation, best-in-class uncertainties as well as maintainability and lowest lifetime costs.

The beryllium mount precision calibration air bearing shaker has become the de facto standard for laboratory vibration calibration. When reading the verbiage of the ISO16063-21 vibration calibration standard, it becomes readily apparent that the standard helps to provide guidelines for techniques and measurement system selection.  This is achieved through use of an uncertainty calculation which balances the real world of measurements, including sensor imperfections / noise / environment, and dozens of other electronic specifications.  Specifically, two of the largest contributors to the overall system uncertainty are: 1. The imbedded uncertainty in the calibration of the reference transducer (i.e. was it calibrated compared to another reference sensor or was it calibrated by a primary means such as a laser, providing the lowest possible reference uncertainty?) and 2. The cross axis motion allowed by the calibration exciter.  One of the enhancements in the 2003 revision of the standard was the recommendation for minimum transverse vibration level over the standard accelerometer frequency range.  This extra measurement scrutiny has been responsible for the beryllium mount air bearing shaker becoming, by far, the most utilized calibration exciter in modern accredited vibration calibration systems.

In the dynamic calibration space, specialty actuators continue to forge ahead in capability and performance. Similar to the de facto standardization of the precision calibration grade air bearing shaker above, the pneumatic shock exciter has also gained broad acceptance for shock calibration up to 10,000 g as described in ISO16063-22. Within the last few years, the adaptation and introduction of industrial linear actuators (consisting of segmented strips of alternating pole magnets) has allowed for unprecedented stroke lengths in low frequency calibration actuators down to 0.1 Hz.  The optical encoder for displacement reference also enhances both the lowest frequency uncertainties and the ability to provide a shaker control reference signal.

The last frontier for ICP sensing is metrology laboratories.  While many National Metrology Labs still operate legacy charge mode reference accelerometers, the progress to industry standard ICP reference sensors is happening.  Most vibration reference accelerometers used commercially are now low impedance quartz sensing element.  A quartz sensing element provides the best long term stability in calibrated sensor output.  These quartz vibration reference sensors are embedded internally within a calibration grade precision air bearing shaker.  By mounting the quartz reference sensor on the opposing face of the sensor-under-test beryllium mounting surface the design optimizes for the frequency range which will have no relative motion between test sensor and reference sensor.   Beryllium is chosen not only for decreased mass with enhanced stiffness, but also for it’s pedigree as the performance choice in decades of air bearing shakers across many manufacturers.  The selection of an ICP powered reference further reduces variance due to environment and cabling.  Ultimately, this simplifies user operation and reduces random uncertainties.  Even in shock references, the ICP back-to-back reference is now prolific in pneumatic actuators useful in the 10,000 g range.

Advances in dynamic reference sensors have moved beyond the standard back-to-back ICP reference sensors specifically in the area of low frequency calibration.  A patented optical technique now utilizes a precision optical encoder (displacement reference) for frequencies below a few Hertz to a fraction of a Hertz.  The optical technique provides constant resolution with fixed stroke. This is a change from traditional piezoelectric accelerometer references which suffer ill conditions at lower frequencies. Rising resistor noise in the internal electronics coupled with the 1/f decay of acceleration level for a fixed stroke exciter at declining frequencies cause these issues.  A multi-head industrial homodyne laser has created a novel advance in the laser primary calibration of vibration sensors by reducing noise and improving uncertainty.  The multi-head configuration allows for multiple point measurement around the base of the sensor-under-test (SUT), reducing the standardized measurement cycle time from rotating to three measurement points to a single measurement.  Laser primary accuracies are now available at a reduced price point.

Portable vibration calibrators (PVC), or shaker tables as they are sometimes called in the industrial market, are now more rugged than ever.  These portable solutions allow users to confirm vibration alarms in the field for a myriad of instruments from proximity probes to vibration switches.  Previously with a reputation for temperamental operation and a fragile package, the modern PVC offers reliable operation and an elegant user interface.  The built-in calibration shaker is ruggedized and contains a precision quartz reference sensor for long-term stability.  Features now include on-board signal conditioning for test sensors, closed loop control and digital data acquisition for test sensor measurement, and reference comparison, as well as simplified GUI control through only two push button knobs with proportional control.  Calibration data useful for safety audits and compliance programs can be stored locally and moved to a standard USB flash drive for archiving or remote hard copy print out. Furthering the PVC as a platform product, low frequency operation of the device has been extended down to 0.7 Hz for slow speed machinery applications in power generation or manufacturing plant, on-site confirmation of seismic activity shutdown systems, as well as exposure and human vibration monitoring.

Continued Growth and Expansion of Dynamic Calibration Standards

Within the International Standards Organization, the Technical Committee 108 Sub Committee 3, there is always progress toward expanding and improving standards in dynamic calibration.  Currently there is a working group formed for the draft of a dynamic pressure calibration standard.  There are at least three pressure actuators on the market for low (1,000 psi), mid (15,000 psi) and high (80,000 psi) calibration pressure generation and dynamic reference measurements.  These calibration techniques are poised to deliver a new level of measurement confidence by providing sensor calibration results based on a dynamic, rather than a static, pressure event.  This is of distinct importance to the energy transport industry and biomedical research. The high frequency pressure actuator has found broad acceptance in the market for high pressure calibration of conformal ballistics pressure sensors. Two distinct measurement events are used, impulse and step. Further research is contributing to a dynamic primary calibration technique deploying a shock tube.

Summary

In summary, both dynamic sensors and dynamic calibration continue to progress toward easier operation and more consistent quality data in measurements.  The simplicity in sensing broadens the application possibilities. The diligence in calibration ensures competency, control, and confidence.  Having observed the last 50 years, the horizon shows the impending explosion in smart sensing and intelligent systems fueling the Industry 4.0 emergence.  The next 50 years should provide an unprecedented growth in our market, measured not in multiples but rather in orders of magnitude.  I believe it is going to be a great time to be in the sensors and measurement industry.