Amplitude-modulated continuous-wave laser scanner with focusing optics can realize extremely high-precision 3D measurement. Since amplitude-modulated continuous-wave scheme employs periodical modulation, the longitudinal resolution and the maximum unambiguous range are in a trade-off. Our system utilizes dual-frequency modulation compromise such trade-off. However, such an attractive laser scanner suffers from ranging ambiguity due to aliasing, which is the systematic error inherent in amplitude-modulated continuous-wave scheme. We have removed the ranging ambiguity by aliasing synthesis. Secondly, the acquired 3D point clouds contain phase jumping at the maximum unambiguous range. With leveraging the relationship between the intensity and spatial information, the phase jumping was unwrapped to recover the spatial continuity. Thirdly, the 3D point clouds in the defocused region of the amplitude-modulated continuous-wave laser scanner distort since the depth-of-focus of focusing optics is generally cm order. The 3D point clouds in the defocused area are contaminated by aliasing which can also be regarded as a ranging ambiguity problem. We have experimentally restored the 3D point clouds by aliasing synthesis with the assistance of intensity information. The ranging area can be elongated by at least ten times of the depth-of-focus with such data processing. With the above-all mentioned configuration and data processing, we have compromised the ranging ambiguity inherent in the amplitude-modulated continuous-wave laser scanner comprehensively. We expect that our results contribute to high-precision industrial inspection for Industry 4.0.
With the rise of Industry 4.0, smart factory is fast becoming a key concept in infrastructure. To realize the autonomous production system, it is necessary to ensure the parts are properly manufactured. 3D scanners are expected to play a vital role in quality assessment in smart factories. Especially, amplitude-modulated continuous-wave laser scanners benefit from high accuracy and high sensitivity which are suitable for industrial inspection. However, due to the limited dynamic range of receiver electronics, such laser scanners fail to obtain the data points in 3D measurement of highly reflective objects. This impairment deteriorates the performance of conversion of 3D point clouds to solid data for shape inspection, 3D modeling, reverse engineering etc. We coped with receiver saturation by adopting a high-speed polarizationindependent variable optical attenuator in our laser scanner. With such a lase scanner, we have succeeded in prevention of data loss due to receiver saturation.
Amplitude-modulated continuous-wave laser scanners can realize 3D measurement with high longitudinal resolution. With installation of focusing optics in the laser scanning system, the high lateral resolution can be realized, which is enhanced by the focused beam spot size <100 um. However, the depth-of-focus of the focusing optics is generally several cm. The 3D data of the defocused objects are contaminated by aliases distributed by integer times of the half cycle of the periodical modulation. Aliasing is an impairment inherent in the amplitude-modulated continuous-wave scheme. We experimentally recovered the defocused data drastically by synthesizing those aliases. The ranging area can be elongated by at least ten times with such data processing compared with the depth of focus. Our results will contribute to highprecision industrial inspection for Industry 4.0.