Phase-Shifting And Shearing Speckle Pattern Interferometry Used In Tire Bubble Detection

Tire bubble is an important influence for safe driving of vehicle,in order to measure the bubble in the tire ,digital phase shiftting shear speckle pattern interferometry is used generally. Digital Speckle Pattern Interferometry (DSPI) is the NDT technique which can obtain the displacement and deformation of the object observed. The tire is loaded in vacuum chamber to expand the bubbles in the tire, deformation can be detected on the surface. By subtracting the two phase map before and after loading,the interferogram can be obtained to reveal the deformation of tire surface.This paper design a phase shifting speckle shear interferometry system satisfied the practical requirements including large viewing filed phase shifting speckle shear interferometer, illumination source, four channels piezo driver supplier and acquisition& processing software. Large viewing filed phase shifting speckle shear interferometer use the structure of Michelson interferometer and inverted telephoto optical circuit: battery of plano-concave lens or large viewing filed TV lens is used as front lens, a 25mm TV lens is used as back lens. This arrangement can enlarge the viewing filed to 60 degree or 90 degree. Using a multibeam illumination method can illuminate the area of tire in the viewing filed. TracePro software is used to calculate the parameters as the number of lasers and the angle between lasers. The probability density function of visibility is used to evaluate the illumination uniformity. A power driver is developed for four channels in parallel phase shifting. This driver is connected to computer using parallel port or serial port. Image is transported by IEEE 1394-b fire ware and connected to the computer which can use 8 megapixels camera parallel at most. The soft is compiled by Visual C# 2008 and is designed for four probes working at the same time. Dynamic speckle correlation is used to detect the macro-displacement of the piezo, this method is easy and high-precision as experiment results shown.Testing progress use multiple probes for detecting in real time: using two probes for car and truck tire,and four probes for giant tire. Experiments show that 2mm bubble under the surface 3-5mm can be detected when the differential pressure is 80milibar using the probe designed in this paper. The defect phase map is despeckled by a statistic filter, and using least squares method to unwrap it can obtain a high definition phase map. Curve fitting is used to parameterize the bubble defect, a method using phase height and width can grade the bubble.