Image-based Three-beam Laser Ranging For Underwater Distance Measurement

In the exploration of deep-sea polymetallic mineral resource,the height of the submarine video earrier from the seabed is an important measurement parameter for the evaluation of deep-sea polymetallie mineral resources.At present,the altimeter is mainly used for the measurement.The measurement frequency of altimeter is 1 Hz,which can not measure the real change of height from the seabed in time during the operation of the submarine video carrier.Therefore,there is a problem that the height from the seabed does not match the video image synchronously.Meanwhile,the size,coverage area and coverage rate of deep-sea polymetallic minerals are unable to be effectively measured due to altimeter does not provide image scale for video image.In view of the above problems,the image-based three-beam laser ranging system is designed and developed based on the requirement of real-time measurement of height information from the seabed and providing image scale.The underwater imaging system and underwater laser g:roup with fixed pitching angle are used together innovatively.The software and hardware of the laser module,imaging module,control system,deep sea sealed cabin structure and sealing system are designed.Considering the effect of angle on ranging accuracy,theoretical models have been built for the three different angles(vertical,with rotating angle,with pitching angle)between the target and the laser beams.Mathematical relationships have been established between underwater distance and geometric parameters of the laser spot pattern in the underwater image.The three-beam laser ranging calibration experiment is conducted in the air.On this basis,the three-beam laser ranging calibration formulas are derived,the feasibility of three-beam laser ranging model under three angles is verified.In order to improve the accuracy of three-beam laser ranging when the angle type is unknown,the unified three-beam laser ranging calibration formula is developed.Experiment shows that within the range of 10 m,the mean absolute measurement error is 1.5 cm,and the mean relative error is 0.3%using the optimized underwater three-beam laser ranging algorithm.The water turbidity has little effect on the accuracy of underwater three-beam laser ranging system,which has high system stability.Compared with the underwater single-beam laser ranging and altimeter,the underwater three-beam laser system can improve the real-time performance of the system,effectively remove the influence of water turbidity and reduce the complexity of system structure.At the same time,the three-beam laser ranging system has the function of image scale,which can measure the size and area of the target based on ranging,providing a new measuring method for underwater target detection.Finally,the three-beam laser ranging system is deployed to the deep-sea polymetallic mineral resource exploration missions of the Ocean 44B voyage in July 2017 and 51 voyage in September 2018,achieving effective real-time measurement of the height of the submarine video carrier from the seabed,the size,the coverage area and coverage ratio of mineral.It provides effective technical support for quantitative exploration and evaluation of deep-sea polymetallie mineral resourees.