Laser power control in a pipe inner wall inspection system with high-speed laser beam scanning

The high-speed, rotational scanning of laser beam was applied to the pipe inner wall inspection system that measures the cross-sectional shape of the pipes based on triangulation. The system is mainly composed of the laser diode (LD) and a position sensitive device (PSD) that goes through inside the pipe to obtain three-dimensional cross-sectional image. Since the reflectivity of the light from LD varies with pipe's inner conditions, simultaneous control of the PSD's receiving light level and the LD light maximum intensity is required for accurate measurement and the LD protection. Moreover, high-speed responding of the control is essential for time-effective inspection. Therefore, we designed an analog circuit that has two feedback loops in this study. One is to keep the PSD's receiving light intensity at a constant level and the second is to keep the maximum output of LD lower than the maximum ratings by feeding back the output of built-in photodiode of the LD. We experimentally showed that the proposed control system keeps the receiving light amount of the PSD at a constant level against a sudden reflectivity change by about 15 times.