NMI has two highly specialised laboratories dedicated to maintaining and developing the Australian standards for acoustics and vibration. An additional laboratory is also maintained for the development of an ultrasound standard. Our main facilities are described below. Click here for details of our NATA accreditation.
Click here for our acoustics, ultrasound and vibration calibration services.
Acoustics
Vibration
Acceleration standard
Acceleration is regarded as a secondary unit, being derived from the fundamental units of time and length. The method that is used for the most precise measurement of absolute acceleration is to measure the oscillatory displacement of a test artefact using laser interferometry. Velocity and acceleration are then derived from the displacement by differentiation with respect to time. Traceability to the fundamental units of length and time is achieved through the known wavelength of the laser and the NMI atomic clock.
The Acceleration Laboratory is purposely located at ground level, with the floor of the room resting directly on bedrock, this provides the laboratory with exceptionally low levels of ambient vibration. With the use of very heavy isolation blocks and optical tables, highly controlled temperature and humidity provides the perfect environment for high precision calibration work.
Primary acceleration reference
The primary acceleration standard at NMI is a single-phase Michelson configuration interferometer, operating with a stabilised He-Ne laser with a wavelength of 632 nm. With this instrument, three methods can be employed to accurately measure acceleration:
(1) Fringe counting (method 1 in ISO 16063:11) is by far the simplest method and is used when the vibrational displacement being measured is much greater than the wavelength of the laser. With a conventional calibration shaker, it is possible to calibrate instrumentation up to a few kHz with fringe counting. A variation on fringe counting uses modulation of the reference mirror to introduce a DC velocity in the interferometer that can aid in obtaining measurements at higher frequencies.
(2) Zero's of Bessel functions or minimum point (method 2 in ISO 16063:11) is used with smaller vibrational displacements . This method uses the zeros of ordinary first order Bessel functions (typically J1) as precise indicators of particular discrete displacement amplitudes and is usable in the frequency range ~1 kHz up to 10 kHz using a conventional calibration shaker.
(3) Ratios of Bessel functions is a modified version of the minimum point method where the ratios of odd (e.g. J1/J3) or even (e.g. J2/J4) Bessel functions are used to calculate displacement.
With the use of a dual-phase quadrature interferometer, a modified Michelson or a Mach-Zehnder configuration, a fourth method can be employed that is generally referred to as sine approximation (method 3 in ISO 16063:11). This method recreates the absolute displacement of the test item and can be used to calibrate the absolute acceleration and phase of the test item.
NMI's Acceleration Laboratory has an experimental dual-phase modified Michelson homodyne quadrature interferometer, and a number of Mach-Zehnder heterodyne quadrature interferometers, however these are not currently used for calibrations.
Performing an absolute calibration by interferometry can be a lengthy process. This is mainly due to the fact that an interferometer measures the acceleration (after integration from displacement) at the exact spot on the transducer's surface where the laser beam makes contact. However the transducer, say a piezo electric accelerometer, measures the relative acceleration of its seismic mass that is located somewhere within the accelerometer's housing. These two measurements of acceleration may not necessarily be the same. To overcome this problem a number of techniques have been developed that use statistical methods and modelling techniques to estimate the true sensitivity of the accelerometer.
Reference accelerometers
In order to facilitate time and cost savings in the dissemination of the acceleration standard throughout Australia, NMI maintains a number of reference grade accelerometers that are regularly calibrated using absolute techniques. In turn these references are used to calibrate client accelerometers and velocity transducers using comparison techniques based on ISO 5347:3 (ISO 16063:21).
The accelerometers maintained by NMI include several Endevco 2270 back-to-back references, Bruel and Kjaer 8305 back-to-back references, and Kistler 8002 single-end mounted references. For low frequency comparison calibrations down to 1 Hz, several additional references can be used including a number of Sundstrand Q-Flex and Q-2000 servo accelerometers.
For use with the charge-type reference accelerometers, NMI maintains a number of precision charge amplifiers. These include Bruel and Kjaer models 2650, 2651 and 2694.
Shakers/vibration exciters
NMI has a number of specialised calibration exciters for use in calibrations.
The main shaker used for comparison calibrations is a modified Bruel and Kjaer 4801, electrodynamic shaker with an assortment of Bruel and Kjaer 4811 (high g) and 4812 (general purpose) heads. This shaker is generally used for frequencies between 20 Hz and 10 kHz, although calibration at lower frequencies is possible. With the attachment of tuned resonators, accelerations up to 5000m.s-2 are attainable on this system.
NMI has developed its own specialised vertical air-bearing shaker for high precision calibrations of reference accelerometers with one of the group's interferometers. This shaker is capable of delivering about 70 m.s-2 for a 20 gm load from 80 Hz up to 10 kHz, with lower frequencies attainable at lower amplitudes.
Low frequency calibrations down to 1 Hz can be performed with a custom made horizontal air-bearing electro-dynamic shaker. This shaker has a maximum displacement of about 50 mm and can deliver up to 10 m.s-2 for frequencies greater than 10 Hz.
For very low frequency calibrations down to 0.05 Hz, a custom made air-bearing mounted flywheel exciter can be employed. This 'shaker' uses gravity as the acceleration source and a sinusoidal excitation is achieved by varying the accelerometer's vertical orientation.
References
LP Dickinson and NH Clark (1999) Accelerometer Calibration with Imperfect Exciters (Shakers). Proceedings of the Australian Acoustical Society Conference, Melbourne
ISO 5347 (1993) Methods for the Calibration of Vibration and Shock Pick-ups, Part 3 Secondary Vibration Calibration
ISO 16063 (1999) Methods for the Calibration of Vibration and Shock Transducers, Part 11 Primary Vibration Calibration by Laser Interferometry
NH Clark (1983) An Improved Method for Calibrating Reference Standard Accelerometers. Metrologia 19, 103–107
NH Clark (1989) An Interferometric Method to Measure Oscillatory Displacements, 2 nm to 255 nm. Metrologia 26, 127–133
LP Dickinson (2002) High Standards for Australia — A Report on the Recent BIPM Key Comparison for Vibration. Proceedings of the Australian Acoustical Society Conference, Adelaide