SENSORS & SENSING SYSTEMS FEATURE
SENSING TECHNOLOGY in the automotive industry
Within the automotive industry, high resolution measurement technology using fibre optics as sensors helps to mitigate risk in designs using new polymer composites. Luna Innovations explains how
I
n the automotive industry, engineers are exploring the use of new
lightweight materials for the vehicle’s structural components and drive train; and comprehensive end-to-end testing combined with finite element modelling is critical here. If finite element models were always
100% accurate, then model validation through testing would not be necessary. However, even when using materials with well understood characteristics, there is a need to validate and calibrate finite element models. The use of polymer composites adds a whole new level of complexity given their lack of homogeneity when compared to metals normally used in automobile design. Traditional point sensing using strain
gauges is inadequate and may fail to accurately reflect the distribution of strain throughout a structure comprised of these new composite materials. This is especially true of areas of large strain gradients, which strain gauges are not capable of measuring.
AN ADVANCED SYSTEM To help, Luna Innovations has developed an advanced system that uses fibre optic cables as a distributed sensor to measure either strain or temperature along a continuous length of fibre optic cable. When illuminated, the cables have the equivalent of an optical fingerprint and this will change,
in a predictable and repeatable way, in response to changes in temperature and to fibre elongation when bonded to a structure experiencing strain. The cable replicates a virtually continuous line of strain gauges with just mm spacing between sensing points. The fibre optic cable functioning as
a sensor is immune to EMI, is inherently a dielectric and can be bonded or embedded within a structure under test. A single cable can be laid out along the structure in a grid pattern in order to provide a full picture of the distribution of strain. For cylindrical objects the fibre can be wrapped in a helical fashion and can reflect strain resulting from either bending moments or torsion. Strain or temperature data can be displayed versus length, or individual points can be
Luna Innovations has developed an advanced system that uses fibre optic cables as a distributed sensor to measure either strain or temperature along a continuous length of fibre optic cable
selected anywhere along the fibre and displayed versus time. The location of these measurement points can be changed within minutes versus the hours it would take to reposition a strain gauge. The application of the fibre optic distributed sensor is similar to the traditional strain gauge in that the surface is sanded and cleaned prior to the sensor being epoxied in place. With a fibre optic sensor, however, you are able to install a virtually continuous line or array of sensors in one application. In addition, this line of sensors is connected to the processing unit through the same fibre optic cable, so the cable functions as both the sensing elements and the signal path for these elements back to the processing unit. This provides an improvement by several orders of magnitude over traditional strain gauges in the simplicity of the wiring and size of the harness connecting the strain sensors to the processing unit. As well as offering significant advantages during the initial test set-up, this will aid greatly in subsequent test set-ups to accommodate any design iterations.
VALIDATING DESIGNS Even for the design and testing of structures using traditional metallic materials, distributed fibre optic technology offers a quantum leap in set-up time and simplification of the wiring and harness. Luna Innovations’ distributed sensing technology therefore helps to validate twenty first century designs for automotive, aerospace and industrial applications.
Luna Innovations
www.lunainc.com Enter 210
/ DESIGNSOLUTIONS
DESIGN SOLUTIONS | MAY 2015
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