In view of a dynamic obstacle environment with motion uncertainty, we present a dynamic collision avoidance method based on the collision risk assessment and improved velocity obstacle method. First, through the fusion optimization of forward-looking sonar data, the redundancy of the data is reduced and the position, size and velocity information of the obstacles are obtained, which can provide an accurate decision-making basis for next-step collision avoidance. Second, according to minimum meeting time and the minimum distance between the obstacle and unmanned underwater vehicle (UUV), this paper establishes the collision risk assessment model, and screens key obstacles to avoid collision. Finally, the optimization objective function is established based on the improved velocity obstacle method, and a UUV motion characteristic is used to calculate the reachable velocity sets. The optimal collision speed of UUV is searched in velocity space. The corresponding heading and speed commands are calculated, and outputted to the motion control module. The above is the complete dynamic obstacle avoidance process. The simulation results show that the proposed method can obtain a better collision avoidance effect in the dynamic environment, and has good adaptability to the unknown dynamic environment.

In order to realize recovery of an autonomous underwater vehicle (AUV), the L-shaped light array installed at the bottom of the fork-carrying-pole docking device is designed, and the visual positioning algorithm based on the L-shaped light array is proposed. The L-shaped light array is identified by a camera installed on the abdomen of the AUV, the relative position deviation from the AUV and the light array is calculated, and the AUV is docked according to the obtained positional deviation. The monocular fourdegree-of-freedom positioning algorithm for AUV solves the problem of missing part of the target light source. Judging the heading angle is improved when the AUV has a longitudinal inclination (heeling angle). The docking experiment of AUV recovery is carried out to verify the feasibility of the L-shaped light array as visual guidance and the stability of the positioning algorithm. INDEX TERMS Autonomous underwater vehicle, L-shaped light array, vision guidance, positioning algorithm.

The optimal treatment strategy for femoral neck fractures remained controversial, especially the Pauwels type III femoral neck fracture of young patients was a challenge. Femoral neck system (FNS) was a newly developed internal fixation for treating femoral neck fracture and this study aimed to compare the biomechanical advantages and disadvantages between FNS and 3 cannulated configuration screws (CCS) with or without an additional medial buttress plate (MBP).In this study, Pauwels type III femoral neck fracture model with an angle of 70° was constructed and 3 different fixation models, FNS, CCS + MBP, CCS alone, were developed. A vertical force of 2100N was applied on the femoral head, then the maximum von Mises stress of whole model, distal femur, femoral head, and internal fixation was recorded, as well as the stress distribution of whole model, proximal fracture section, and internal fixation of the 3 models. Moreover, the maximum displacement of the whole model, distal femur, femoral head, internal fixation, and the relative displacement of the proximal and distal portion was also compared.The maximum von Mises stress value was 318.302 MPa in FNS, 485.226 MPa in CCS + 1/3 plate, and 425.889 MPa in CCS. The FNS showed lowest maximum von Mises stress values in distal part, femoral head, and internal implant. All fixation configurations were observed stress concentrated at the posteroinferior area of cross-section of femoral head and at the fracture section area of implant; however, FNS had more uniform stress distribution. For displacement, the maximum displacement value was 8.5446 mm in FNS, 8.2863 mm in CCS + 1/3 plate, and 8.3590 mm in CCS. However, FNS had higher maximum displacement in femoral head and internal implant, but lower maximum displacement in the distal part of fracture model. The FNS represented a significantly higher relative displacement between the femoral head and distal femur when compared with the other 2 fixation configurations.The newly developed FNS could achieve the dual effect of angular stability and sliding compression for the treatment of Pauwels type III femoral neck fractures, which provided superior biomechanical stability than CCS alone and CCS with additional MBP.

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