Sensor Quickly Inspects Inner Pipe Walls

Aaron J. Hand

Researchers at Tianjin University and Xi'an Jiaotong University have developed a noncontact laser sensor for inspecting the inner walls of pipes with 80- to 140-mm interior diameters.

A HeCd laser illuminates selectively fluorescent dyes, revealing a cross section of chaos in the mixing tank. The elongated dark sections engulfed in the red dye are a single "regular island" that has circled the tank four times before reconnecting with itself. Some regular islands have lasted 17 hours without blending with surrounding dyes.

Such industries as chemical and petroleum inspect pipes to find scratches or other deformations. Pipes in the pharmaceutical industry, for example, carry sterile-grade liquids such as water, solvents and liquid nitrogen. The integrity of the pipes is important. Inspecting long, thin pipes presents some difficulty, however, with inspection systems typically trading off between measurement accuracy and speed.

As reported in Optical Engineering (May 1998, pp. 1643-47), the circular optical cutting method can inspect pipes with a resolution of 100 µm in radius while increasing measuring speed over previous systems. The sensor consists mainly of a laser diode, an optical ring pattern generator and a charge-coupled device (CCD) camera. It travels down a pipe on a motorized carriage. The light is projected on the pipe's inner wall in a circular pattern and reflects through a focusing lens to the CCD.

Researchers led by professor Bao Hua Zhuang used a 691-nm, 10-mW laser diode and a 524 × 580-pixel CCD camera. Image processing and wavelet transforms assisted in analysis of the pipe's inner surface. The engineers used aluminum foil to create a defect in the pipe and showed that their system detected the defect accurately.