A non-contact laser sensor based on the ring optical cutting image method to measure the size and profile of the inner wall of a pipe is developed. The sensor consists of a laser diode light source, an optical ring pattern generator and a CCD camera. The optical ring pattern from the optical ring pattern generator is projected onto the inner wall of the pipe, which is then viewed by the CCD camera. Reasonable parameters according to the theory of light scattering and reflection are chosen to simulate and optimize the measurement system. The adaptive weighted average value filter and the subpixel technique on several-step computing and half Gaussian fitting are deployed to obtain the edge and the centre of the ring image in order to filter the noise of the image and raise the resolution of the measurement system. The experimental results show that the principle is correct and the techniques are realizable.

http://iopscience.iop.org/article/10.1088/0957-0233/9/9/004/pdf

Non-contact laser inspection for the inner wall surface of a pipe

Phase-shifting digital holography is applied to the measurement of the surface profile of the inner surface of a pipe for the detection of a hole in its wall. For surface contouring of the inner wall, a two-wavelength method involving an injection-current-induced wavelength change of a laser diode is used. To illuminate and obtain information on the inner surface, a cone-shaped mirror is set inside the pipe and moved along in a longitudinal direction. The distribution of a calculated optical path length in an experimental alignment is used to compensate for the distortion due to the misalignment of the mirror in the pipe. Using the proposed method, two pieces of metal sheet pasted on the inner wall of the pipe and a hole in the wall are detected. This shows that the three-dimensional profile of a metal plate on the inner wall of a pipe can be measured using simple image processing.

Digital holographic profilometry of the inner surface of a pipe using a current-induced wavelength change of a laser diode.

A new noncontact laser-inspection technique based on a position-sensitive detector (PSD) to inspect the inner surface of minidiameter pipes is proposed, and the corresponding sensor is developed. A light spot is projected onto the pipe inner wall after the laser beam is reflected by two mirrors. At the same time, four current signals are produced when the light spot is read by the 2-D PSD. According to the magnitude of the current signals and the structure parameters of the sensor, the spot position on the inner wall can be calculated in a local 3-D coordinate system. With a micromotor, the surface of the whole inner wall can be scanned by the laser beam. This way, the coordinates of all the sample points in the section are obtained. After that, several reasonable data-processing methods are used, such as data segmentation and least squares fitting. Finally, the section curve can be reconstructed. The radius and defects of the section can also be obtained. Driven by a micropipe robot, the sensor can inspect a long curved pipe. The inspection system based on this technique can detect the inner surface of minidiameter pipes, which has an inner diameter of 9.5-10.5 mm and a curvature radius of more than 100 mm. The measurement accuracy of this inspection system for the inner diameter and surface defect reaches plusmn0.1 mm.

Noncontact Laser Inspection Based on a PSD for the Inner Surface of Minidiameter Pipes

A qualitative approach for microwave imaging multilayered cylindrical structures (e.g., dielectric pipes) is proposed in this paper. This approach relies on a modified circular synthetic aperture imaging technique that exploits closed-form Green’s function to account for the different propagation delays, internal reflections, and refractions. The image can then be computed by employing a matched filter expressed in terms of efficient Fourier Transforms. Consequently, a high-resolution and contactless approach for the inspection of this kind of structures is achieved. Moreover, the computational resources to render the images are negligible. Practical aspects of the technique such as sampling criteria, resolution, and limitations of the technique are discussed. The efficacy of the method is illustrated via several examples including imaging of objects and anomalies inside different types of dielectric pipes.

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Nondestructive Evaluation of Microwave-Penetrable Pipes by Synthetic Aperture Imaging Enhanced by Full-Wave Field Propagation Model

This paper describes a noncontact laser sensor for pipe inner wall inspection using the circular optical cutting method. The sensor, consists principally of a laser diode light source, an optical ring pattern generator, and a CCD camera. The generator forms the light from the laser diode into a circular pattern to project on the pipe inner wall. The light reflected or scattered by the pipe inner surface is imaged on the CCD camera. It is possible to select the proper parameters by funda- mental performance analysis and simulation according to the theory of light reflection and scattering. Experiments show that with the sensor mounted on an automated moving system, pipes of interior diameter between 80 and 140 mm can be inspected with resolution of 100 m mi n radius. Analysis of the pipe interior wall surface using wavelet transforms is also described. © 1998 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(98)03205-X)

Masaru Watanabe

Papers

Optical inspection system for the inner surface of a pipe using detection of circular images projected by a laser source