Study on an optical design of the laser-beam scanning sensor for pipe inspection

The viewing angle used in a previously developed laser-beam pipe-scanning sensor has been optimized to improve measurement accuracy. The viewing angle is the angle between the optical paths - that of the beam from the laser diode (LD) and that of the light scattered to the viewing lens. The trade-off between the scattering characteristics of the measured surface and the magnifying power of the viewing lens was then used to determine the best viewing angle. The results of this method agreed with the angles determined experimentally by manufacturers of displacement meters. In the previous sensor, the laser power was constant. However, the reflection ratio on the inner wall of a conduit varies substantially, so the incident light scattered from the surface to the position sensor varies significantly. The output signal thus becomes saturated, and the S/N ratio is degraded, making it impossible to measure the radii accurately. It is now changed appropriately when the reflectivity of the scanned pipe wall changes by using analog feedback control circuits, in which the laser power signal and the output signal of the position sensor are fed back in parallel. With these two improvements, the laser-beam pipe-scanning sensor can more accurately measure the inner radii of conduits.