Detectors and sensors
Electronic Speckle Pattern Interferometry for Layer Measurement of Racing Tyres
June 12, 2018
Motorsport tyres are the only link between the power generated by the engine and the road surface. Maximising their performance in terms of grip and wear is crucial to the success of racing teams.
Racing tyres have a complex structure made of multiple layers of different vulcanised rubber compounds, metal, synthetic (nylon) reinforcements and cables. The optimisation of the layer structure is critical to the vehicle dynamics modelling and hence achievable track speed and importantly driver safety, but assessing the layer thicknesses is difficult to obtain using standard techniques such as visual inspection or even classical microscopy.
There are a number of models in the literature for assessing tyre behaviour based on physical or empirical approaches. This whitepaper studies one such method for measuring the thickness of different layers through the tyre, since the thickness of each layer determines the thermal behaviour of the tyre, its grip and the wear characteristics.
Dr. Vito Pagliarulo and his co-authors, studied two high performance “slick” tyres, one automotive and one motorcycle using two laser based methods Electronic Speckle Pattern interferometry (ESPI) and laser scanning. Both are contactless methods for making precise engineering measurements such as displacement and vibration.
ESPI uses a highly coherent light source where the beam is split into a reference beam that is fed to the detector through a single mode fibre and an object beam that illuminates the sample, where the beam intensity ratio is controllable. The object beam is passed through an electronically tilted optical etalon that shifts the beam laterally without changing its direction of propagation, allowing for imaging of an area. A CCD/CMOS camera records the interference pattern resulting from the interaction of the reference beam and that reflected from the object. A representation of the optical scheme is shown in Figure 1. The resolution and accuracy of the measurements are related to the actual laser wavelength and the output wavelength stability.
Dr. Pagliarulo and his team used a Laser Quantum torus 532 laser (Figure 2) due to its single frequency operation with a line width of <1 MHz and TruLoQ™ technology that maintains a wavelength stability of <2 pm through a proprietary wavelength stabilising feedback loop (Figure 3).
Multiple images are taken to increase the signal-to-noise ratio. The sample then undergoes deformation in the form of heating by a 100 W halogen lamp for around 3 seconds (enough to cause a surface temperature increase of 1°C), and a new set of images taken. The software then subtracts the two images. The displacement seen by each of the tyre layers is related to the co-efficient of thermal expansion of each compound and being different for rubber mixtures, metal, nylon, cables etc. therefore clear measurements can be taken.
In optical microscopy images of a tyre cross-section, the steel and nylon wires make it difficult to identify the various layers. However, when the phase contrast map is taken by ESPI on the same samples, in contrast to the optical images, the clear demarcation between layer boundaries allows thickness measurements at sub-micron resolution.
The value of this technique can be seen by comparing the results from an optical microscope and ESPI. Since the accuracy and precision of measurement of layer thickness is related to the clarity of the experimental image of layer boundaries, Dr. Pagliarulo and his team conclude that the use of ESPI is a valuable tool in the assessment of layer profiles, especially in their study of racing tyres.