TEST & MEASUREMENT FEATURE
A GOOD MEASURE of vibration
A laser vibrometry testing rig has been installed at ASDEC, the Advanced Structural Dynamics Evaluation Centre, enabling highly accurate measurement of vibration. Tim Stubbs, general manager, looks into how this is opening up new possibilities for measurement and testing
A
SDEC, the Advanced Structural Dynamics Evaluation Centre (ASDEC),
is built on the expertise of the University of Leicester in 3D laser vibrometry and modal analysis, capabilities which allow the detailed evaluation of the structural dynamics of a wider range of objects, both large and small. The business has, in fact, recently been recognised by the Research Council UK and PraxisUnico as making the most important contribution to UK business of any research commercialisation or knowledge exchange activity in 2015. Experimental modal analysis is the study of the natural characteristics of structures. Through testing, ASDEC can derive the natural frequencies (the frequencies a structure will naturally vibrate at when excited, like the note of a wine glass when you tap it, or a tuning fork or bell) and the mode shapes (the bending and twisting that a structure does when vibrating at its natural frequencies). Understanding both the natural frequency and mode shape helps
coating to increase reflectivity. The front faces of the plates are covered with a thin layer of aluminium foil (60nm) spanning the pores to reduce the thermal load and to block visible light. Testing MCPs is critical in the development of satellite systems, where engineers need to understand the structural characteristics to ensure durability through launch and into the expected life of the system. Even the slightest touch can, however, ruin the device, damaging the structure or removing the film. It is also a requirement of all space hardware that the behaviour under structure borne vibration and acoustic excitation must be characterised. By offering this capability of
measurements on delicate precision instruments, ASDEC brings understanding of structural dynamics to test objects which were previously out of reach of traditional methods. The scalable offering also allows the use of a portable system for situations where clients need the test work to be done on their site, which is critical in many applications.
The possibilities of laser vibrometry is being demonstrated by a recent project with the Space Research Centre of the University of Leicester
beams as they strike a vibrating object; with the reflected laser revealing the deflection of the target. Reducing vibration is the aim of design engineers across multiple industries, a goal often obstructed by the difficulty of obtaining a good measure of vibration as it happens. Laser vibrometry allows testing
without distortion by observer effect, which is particularly important during vibration measurement and analysis of
“Laser vibrometry allows testing without distortion by observer effect, which is particularly important during vibration measurement and analysis of extremely small and lightweight systems”
to design structural systems for noise, vibration and durability applications. ASDEC uses modal analysis to help clients design automotive structures, aircraft structures, spacecraft and computers, and even tennis rackets, golf clubs, bicycles.
LASER VIBROMETRY TESTING At the facility there is a laser vibrometry testing rig, one of only three in the world that are commercially available. Here, laser Doppler vibrometry (LDV), a new and effective way of measuring vibration, is used. The technique depends on very accurate measurement of the deflection of laser
extremely small and lightweight systems that would be impossible with traditional accelerometer based techniques. The possibilities of such a capability
is demonstrated by a recent project with the Space Research Centre of the University of Leicester.
TESTING MCPS The task involved testing the Glass Microchannel Plates (MCPs) used as X-ray optics in space science instrumentation by the Space Research Centre. These are typically 40mm x 40mm x 1.2mm glass plates with 20 micron square pores with an Iridium
NON-CONTACT MEASUREMENT At the other end of the size scale, the vibration analysis equipment at ASDEC is proving valuable for the power generation industry’s drive to utilise modern and novel lightweight and composite materials for increased efficiency, in part driven by the demands of legislation. Not only are these materials difficult
to measure in traditional ways, but the simulation models are not as advanced as for traditional materials. By measuring through a non-contact technique, the ASDEC facilities ensure that there is no observer effect on even the lightest material. Furthermore, the unit’s CAE and
FE correlation capabilities support customers in the composites manufacturing field to improve their modelling techniques, so improving their design process and reducing costs. Physical test is an essential cost in design, however ASDEC can provide customers with more accurate virtual models so they can run more design cycles at low cost through simulation, leading to fewer physical prototypes. ASDEC’s static and portable LDV
facilities open the door for UK engineering to utilise the latest expertise in the evaluation of structural dynamics, allowing assessment of objects and materials which would otherwise have been impossible.
ASDEC
http://asdec.co
INSTRUMENTATION | DEC/JAN 2016 29
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