Fig 4 - uploaded by Mingxi Zhou
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A simulated ray intersects with the terrain. The sound propagation inside the beamwidth is displayed in a red fan shape.
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Autonomous Underwater Vehicles (AUVs) are commonly used in oceanographic applications such as seafloor survey and underwater iceberg profiling. During the surveys, the vehicles are intended to follow the variation of the seafloor or iceberg surface at a constant stand-off distance in order to maintain a consistent sensor footprint. Mechanical scann...
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... contrast, directivity (DI) and target strength (TS) vary for individual rays due to the beam pattern of the transducer and different incident angles when the sound collides with the terrain. Therefore, P DI (α) and P T S (α) are the terms left when differentiating the P EI (α) with respect to α (see Eq. 5). For the two dimensional space shown in Fig. 4, P DI (α) is expressed in Eq. 6, where k is the wave number and h is the length of the transducer. The target strength, P T S (α), for a large object, i.e. seafloor, is shown in Eq. 7 according to [7], where µ is the scattering constant which can be determined empirically through experiments, and θ i is the incident angle. Figure 4 ...
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... target strength, P T S (α), for a large object, i.e. seafloor, is shown in Eq. 7 according to [7], where µ is the scattering constant which can be determined empirically through experiments, and θ i is the incident angle. Figure 4 shows the propagation path of a ray which intesects with the terrain at an incident angle of θ i with the sonar oriented forward with an angle of β. With the known forward-looking angle of the sonar (β), the angle of the ray (α), and terrain slope relative to the vehicle, θ v s , the incident angle θ i is calculated as shown in Eq. 8. ...
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Three-dimensional reconstruction is a crucial technique for mapping and object-search tasks, but it is challenging in sonar imaging because of the nature of acoustics. In underwater sensing, many advanced studies have introduced approaches that have included feature-based methods and multiple imaging at different locations. However, most existing m...
Citations
... Meanwhile, the guidance model computes the desire heading for tracking the waypoints, and the AUV model makes maneuvers. A ray-tracing sonar model [28] is implemented to simulate the sonar propagation and measurements. The sound propagation is simulated with rays, which are distributed every 1.8 • in a 120 • section. ...
Bathymetric mapping with Autonomous Underwater Vehicles (AUVs) receives increased attentions in recent years. AUVs offer a lower operational cost and smaller carbon footprint with reduced ship usage, and they can provide higher resolution data when surveying the seabed at a closer distance if compared to ships. However, advancements are still needed to improve the data quality of AUV-based surveys. Unlike mobile robots with deterministic mapping performance, multibeam sonars used in AUV-based bathymetric mapping often yields inconsistent swath width due to the varied seabed elevation and surficial properties. As a result, mapping voids may exist between planned lawnmower transects. Although this could be solved by planning closer lawnmower paths, mission time increases proportionally. Therefore, an onboard path planner is demanded to assure the defined survey objective, i.e., coverage rate. Here in this paper, we present a new data-driven coverage path planning (CPP) method, in which the vehicle automatically updates the waypoints intermittently based on an objective function constructed using the information about the exploration preference, sonar performance, and coverage efficiency. The goal of the proposed method is to plan a cost-effective path on-the-fly to obtain high quality mapping result meeting the requirements in coverage rate and uncertainty. The proposed CPP method has been evaluated in a simulated environment with a 6DOF REMUS AUV model and a realistic seafloor topography. A series of trials has been conducted to investigate the performance affected by the parameters in the objective function. We also compared the proposed method with traditional lawnmower and spiral paths. The results show that the weight assignment in the objective function is critical as they affect the overall survey performance. With proper weight settings, the AUV yields better survey performance, coverage rate and coverage efficiency, compared to traditional approaches. Moreover, the proposed method can be easily adjusted or modified to achieve different coverage goals, such as rapid data gathering of the entire region, survey of irregular workspace, or maintaining real time path planning.
... is the probability of occupancy based on a single sonar measurement, R j , that is determined using the dynamic inverse-sonar model (Zhou, Bachmayer, & deYoung, 2016b). Since the log-odds are ...
... Compared to the inverse-sonar model build for ultrasonic range finders (Elfes, 1987), the dynamic inverse-sonar model further accounts for the incident angle of the acoustic signal on the target which has a significant effect on the return acoustic signal. Here, we use a dynamic inverse-sonar model (Zhou et al., 2016b) for computing the single measurement probability. We define ...
... In contrast, the ensonified cells with relative ranges equal to R s are assumed occupied. The probability of occupancy of these cells, ranging from 0.5 to 0.999, is defined as a function of the ray angle (δ) and the trend of the target profile ( v s ψ ; Zhou et al., 2016b). ...
Icebergs pose many challenges to offshore operations in the Arctic Ocean and sub‐arctic regions. They could damage underwater infrastructure such as pipelines, and disrupt marine transportation. The below‐water shape of an iceberg is a key factor for iceberg management in the North Atlantic Ocean because it affects the iceberg towing plans and iceberg drift patterns. In recent years, unmanned platforms have been proposed as potential candidates for underwater iceberg mapping. Compared to a conventional ship‐based iceberg survey, using unmanned platforms is more efficient and safer. In this paper, we present research using a hybrid underwater glider to measure the underwater shape of an iceberg. The vehicle is equipped with a mechanical scanning sonar for range sensing and iceberg mapping, and a guidance system is designed to use the sonar measurements for guiding the vehicle to circumnavigate an iceberg at the desired standoff distance. Several field experiments have been conducted on an iceberg to evaluate the system performance. With repeated observations, the underside of the target iceberg was successfully reconstructed, and iceberg shape comparisons are presented.
(Paper available at: https://onlinelibrary.wiley.com/toc/15564967/2019/36/6)
... The object detection challenge is thougher with single-beam sonars, but can be solved with occupancy grids [4] or with a potential field method [7]. In this paper, occupancy grids are populated using the dynamic inverse-sonar model presented in [9]. The detected obstacles are then used as input to an online re-planning algorithm. ...
... Once new measurements are available from the sonar the grid is updated using one of two possible methods. If there is an obstacle present, the grid cells intersecting the sonar cone is updated according to the dynamic inverse-sonar model presented by [9]. When no obstacles are present, the grid is updated with an inverse-sonar model described thoroughly in [8]. ...
... Therefore, an uncertainty in resolving the location of the target with the measured ranges will be introduced. A dynamic inverse-sonar model [11] is now used in Equation 5, while a static inverse-sonar model is optional from [19]. ...
... The forwardlooking distance, ∆, in the controller is calculated based on the sonar configuration. As shown in Figure 2, the maximum forward-looking distance at desired standoff distance (S d ) can be calculated in Equation 11. In our scenario, the standoff distance S d is 40 meters, β is 35 degrees, and max(δ) is 17.5 degrees for the sonar integrated on the Slocum glider. ...
A Slocum underwater glider is been modified to map the underside of icebergs for monitoring iceberg deterioration off the coast of Newfoundland, Canada. The vehicle is equipped with a mechanical scanning sonar to map the iceberg surface, and a thruster for level-flight at a higher surging speed. In this paper we are presenting a profile-following controller that uses the sonar ranges to compute desired headings guiding the Slocum glider traveling safely around icebergs. A vehicle-attached occupancy map (VOM) is updated using sonar measured ranges with a dynamic inverse-sonar model. A desired path is then generated from the VOMby applying polynomial regression on the occupied cells. The line-of-sight guidance law is implemented to compute the desired heading to follow the desired path. The algorithm is initially evaluated in a simulation environment. The vehicle operation is simulated on a real-time hardware simulator, while the sonar is modeled in ray-tracing method. The iceberg is derived from an iceberg database with additional translational and rotational motion emulating a floating iceberg. After that, the guidance system is applied on a set of field data collected in 2015. During the trial, the Slocum glider was deployed to profile an underwater ramp feature in Conception Bay, Newfoundland, Canada. The feasibility of the porposed controller is indicated by the outcomes from this paper.
Operating an AUV in close proximity to terrain typically relies solely on the vehicle sensors for terrain detection, and challenges the manoeuvrability of energy efficient flight style autonomous underwater vehicles. This paper gives new results on altitude tracking limits of such vehicles by using the fully understood environment of a lake to perform repeated experiments while varying the altitude demand, obstacle detection, and actuator use of a hover capable flight style AUV. The results are analysed for mission success, vehicle risk, and repeatability, demonstrating the terrain following capabilities of the overactuated AUV over a range of altitude tracking strategies and how these measures better inform vehicle operators. A major conclusion is that the effects of range limits, bias, and false detections of the sensors used for altitude tracking must be fully accounted for to enable mission success. Furthermore it was found that switching between hover and flight style actuation based on
speed, whilst varying the operation speed, has advantages for performance improvement over combining hover and flight style actuators at high speeds.
