There should be flaws and defects on the inner surface during the producing period of a pipe, as well as contaminations and corrosions during the using period of it. A corresponding panoramic optical annular staring inspection system has been developed. It requires no rotating mechanism to exam the whole circumference of a cross section of the inner pipe surface at once, which results high speed inspection.
There are two main subsystems in this inspection system, the panoramic optical annular staring imaging subsystem and driving robot subsystem. The Flat Cylinder Perspective (FCP) is the principle to image a panoramic annular view to a flat imagery, i.e. a cylinder of vision imaged is flat. Our imaging subsystem includes a panoramic annular lens (PAL), which is critical and used to implement the FCP, a series of image rotation lenses, a charge-coupled device (CCD) camera, and an illuminating light-emitting diode (LED) ring. The CCD camera sending the signal to a personal computer (PC) via VGA signal results a real time inspection.
The driving robot subsystem is a fine designed complicated mechanism including a subassembly of stepper motor. It can drive the inspection system forward and backward continuously in the pipe along the axial direction. The experimental system reported in this paper has the following specifications: average detection resolution of 0.5 mm at the circumference direction and 1.0 mm at the axial direction of a pipe, and inspection speed of 15 mm/s.
Although there are various methods to measure the modulation transfer function (MTF) of charge-coupled devices
(CCD), the interferometric fringe pattern method has advantages over others, such as canted slit sources, bar targets,
knife-edge, laser-speckle patterns, random noise pattern, etc. Our interferometric method is relatively simple and
versatile: It requires no critical optics and no focusing or precision alignment, the entry array is tested, the contrast ratio
of the test pattern is high enough, the spatial frequency of the fringe pattern can vary continuously.
Our method generates the formation of a sinusoidal intensity fringe pattern by the interference of two monochromatic
plane waves, and straightforward projects it onto the CCD array under test. The construction of the experimental device
is based on the Fresnel Double-Mirror structure. A 2.5 mw He-Ne laser with the wavelength of 632.8 nm is used as the
light source, the laser beam is spatially filtered by a 10 μm pinhole and expanded to a diameter of 30 mm, and the
resulting wave front is divided by two mirrors, which incline to each other at a small angle, and interfered. One of the
mirrors is rotatable to vary the frequency of the pattern. The CCD array is mounted on a stage, which is also rotatable to
make that the CCD array takes different angle with the fringe pattern direction, to receive the patterns.
With the method we provided, the spatial frequency can be extended to some 2 times the Nyquist frequency of the CCD
array to study the aliasing effect. In the Cartesian coordinates, the x- and y- axis MTFs (at angle 0° and 90°) of the CCD
array were measured, the other three MTFs (at angle 26.56°, 45° and 63.44°), which nobody has done before, were also
tested offering a more comprehensive characterization of the CCD array.