Miniature Underwater Research Articles

An open source acoustic modem for communication between autonomous underwater vehicles

Miniature underwater robots (MUR) are becoming popular for underwater robotics research. Existing acoustic modems are typically designed for larger vehicles, which requires relatively high power and high cost. We present the Bluebuzz, a open source acoustic modem with reduced size and low power consumption aiming for the applications of MURs. We will report the detailed technical solutions and the experimental results collected and presents future improvements. The modems have also been tested to support higher level networking functions among multiple MUR.

Communication and Cooperation for Spherical Underwater Robots by Using Acoustic Transmission

The application of miniature underwater robots in the marine area is becoming more extensive. Occasionally, the ability of the single miniature underwater robot is limited by its size, configuration, function or propulsion method. At this moment, if a group of robots can work together in a division of labor, that can solve more problems underwater. For autonomous underwater vehicles, the support of underwater wireless communication equipment is required to achieve communication and collaboration. Acoustic communication equipment has generally been used in large underwater vehicles due to its large size and power. This article employs a small acoustic communication module to enable underwater communication in a spherical robot with a diameter of only 45 cm. The fourth-generation spherical underwater robot is exploited as the prototype. The article also verifies the accuracy, efficiency and timeliness of the proposed underwater communication systems. Finally, a cooperative mode is established based on the base station, master robot and slave robots, and some cooperation motion experiments are carried out to verify its effectiveness. Early work on this article was presented at the IEEE ICMA 2018 conference S. Gu et al., 2018. In this conference article, the authors only provided theoretical research and a small number of experiments. On this basis, this article has conducted in-depth research, proposed a base station-mater-slave robot communication mode, and established an instruction set for further experiments.

A 5 cm‐Scale Piezoelectric Jetting Agile Underwater Robot

Existing miniature underwater robots, which use electromagnetic actuators or soft actuators, function in shallow waters. However, in the deep sea, the robots face challenges, such as the miniaturization of the pressure protection unit and the driving method for rapid and agile motions. Herein, a jet‐driven method for a high‐pressure underwater environment is proposed by utilizing the high‐frequency vibration of a piezoelectric vibrator. Thereafter, an antihydropressure miniature robot with a body length of less than 5 cm is designed, which can perform the highly agile actions of floating, sinking, hovering, straight driving, and turning under a water pressure of 20 MPa (equal to the pressure under a water depth of 2000 m). A vertical velocity of 2.95 BH/s and a horizontal velocity of 3.22 BL/s are realized by the prototype, and it achieves faster motions than existing miniature underwater robots. Some potential applications have been realized, including small multicorner pipe exploration by carrying a camera, seaweed epidermal cell sampling in designated areas, and large object transportation by swarming, which proves the high maneuverability and agility of the developed robot. These merits make the robot ideal for multitasking operations in narrow environments with high water pressure, such as multiobstacle seabed.

Underwater self-sustaining combustion and micro-propulsion properties of Al@FAS-17/PTFE-based direct-writing nanothermite

Micro-propulsion underwater technology has been an extensively concerned topic in advanced marine energy systems because of its wide prospects in high-precision prospecting and positioning. Wherein, the design and selection of propellant in energy-supply systems are the key issues to greatly explore its future application. Here, the hydrophobic thermite sticks which contain hydrophobic polytetrafluoroethylene nanoparticles and modified aluminum coated with 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane shells, were fabricated via a simple direct-writing approach. The modified thermite sticks exhibit stable underwater combustion and self-sustained propagation with a small diameter size (0.94 ± 0.05 mm). In-situ observation shows that the underwater reaction process could be divided into three steps: laser-induction, organic decomposition-dissolution, and thermite reaction. The aging results indicate that the thermite sticks demonstrate an excellent storage stability wherever in air or underwater environments. Simultaneously, it is found that its critical energy required to sustain stable underwater combustion is higher than that in air. Most notably, it not only can be used as the fuels for controllable miniature underwater propulsion, but also can be applied as the energy-delivery materials to expand thrust capacity of vehicles by igniting higher impulse explosives, such as CL-20.

A Multi-Sensor Fusion Self-Localization System of a Miniature Underwater Robot in Structured and GPS-Denied Environments

Aiming to deal with underwater localization for small-size robots in GPS-denied and structured environment, this paper proposed a novel multi-sensor fusion-based self-localization system using low-cost sensors. Based on multi-sensor information fusion, an Extended Kalman Filter (EKF) is utilized to synthesize the multi-source information from an Inertial Measurement Unit (IMU), optical flow, pressure sensor and ArUco markers, which enables the robot obtain a highly precise positioning. This method also can reduce the location drift over time owing to the loss of markers in pure markers-based localization. Specially, a velocity correction model is proposed using the angle information obtained by IMU, which can compensate optical flow-based velocity estimation errors caused by robot posture changes. Finally, to validate the performance of the proposed self-localization system, simulations are conducted using Gazebo simulator on the robot operating system (ROS). Moreover, a series of experiments in an indoor swimming pool are presented. Results of the proposed method and dead reckoning are compared in simulation and experiment to demonstrate the robustness and feasibility of the proposed localization system.

Design and Experiments with a Robot-Driven Underwater Holographic Microscope for Low-Cost In Situ Particle Measurements

Microscopic analysis of microparticles in situ in diverse water environments is necessary for monitoring water quality and localizing contamination sources. Conventional sensors such as optical microscopes and fluorometers often require complex sample preparation, are restricted to small sample volumes, and are unable to simultaneously capture all pertinent details of a sample such as particle size, shape, concentration, and three-dimensional motion. This paper proposes a novel and cost-effective robotic system for mobile microscopic analysis of particles in situ at various depths which are fully controlled by the robot system itself. A miniature underwater digital in-line holographic microscope (DIHM) performs high-resolution imaging of microparticles (e.g., algae cells, plastic debris, sediments) while movement allows measurement of particle distributions covering a large area of water.

Implicit coordination for 3D underwater collective behaviors in a fish-inspired robot swarm.

Many fish species gather by the thousands and swim in harmony with seemingly no effort. Large schools display a range of impressive collective behaviors, from simple shoaling to collective migration and from basic predator evasion to dynamic maneuvers such as bait balls and flash expansion. A wealth of experimental and theoretical work has shown that these complex three-dimensional (3D) behaviors can arise from visual observations of nearby neighbors, without explicit communication. By contrast, most underwater robot collectives rely on centralized, above-water, explicit communication and, as a result, exhibit limited coordination complexity. Here, we demonstrate 3D collective behaviors with a swarm of fish-inspired miniature underwater robots that use only implicit communication mediated through the production and sensing of blue light. We show that complex and dynamic 3D collective behaviors-synchrony, dispersion/aggregation, dynamic circle formation, and search-capture-can be achieved by sensing minimal, noisy impressions of neighbors, without any centralized intervention. Our results provide insights into the power of implicit coordination and are of interest for future underwater robots that display collective capabilities on par with fish schools for applications such as environmental monitoring and search in coral reefs and coastal environments.

Performance evaluation of a two-directional energy harvester with low-frequency vibration

This study evaluates the power harnessing performance of a two-directional inertial pendulum energy harvester for miniature underwater mooring platforms (UMPs). This novel harvester can achieve a dominant rotational motion under different parametrical excitations. The two-directional energy harvesting performance is investigated through vibration platform experiments over roll-pitch coupling excitations. The frequency ratio between the pitch and roll motion is about 2 to 1, the roll excitation frequency is over the range of 0.2–0.3 Hz, and the pitch amplitude is much lower than the roll amplitude. Experimental results indicate that the inclination configuration of the harvester can improve two-directional energy harvesting performance significantly with low-frequency vibration. Geometrical tunability through adjusting inclination configuration for the harvester can enhance the two-directional energy harvesting apparently. Four prominent phenomena of the two-directional energy harvesting performance are observed with the increasing load resistance. The motion state transformation of the inertial pendulum can result in dramatical jumps or drops in two-directional energy harvesting.

A low-profile wall shear comparator to mount and test surface samples

Accurate measurement of shear stress on a solid surface is a crucial but challenging task in fluid mechanics. Different sensors are usually used for different experimental settings: water channel, wind tunnel, towing tank, watercraft, aircraft, etc. This paper presents a direct shear sensor designed to work for varying test objects and flow conditions. Designed to compare two different sample surfaces, the shear sensor is comprised of two floating elements, whose displacement is proportional to the shear stress they experience, and two optical encoders, which measure the displacements precisely, right under the floating elements. The main plate includes two identical sets of floating elements and flexure beams machined monolithically from a thick piece of metal, allowing displacements in only one in-plane direction. The side-by-side arrangement allows the two floating elements to experience essentially the same flow conditions, regardless of test condition, enabling the comparative sensing. The method of machining these folded-beam flexures, whose width is on the scale of micrometers, while thickness and length are in millimeters and centimeters, respectively, is presented. The main plate is designed with the help of finite element analysis to ensure dynamic response of the floating elements is appropriate for target flow conditions. The utility of the shear sensor is verified in three different flow settings, i.e., water tunnel, boat in open water, and wind tunnel. A miniature underwater camera system is also developed to observe the sample surfaces during testing on a moving object, such as a boat.

Design of a Miniature Underwater Angle-of-Attack Sensor and Its Application to a Self-Propelled Robotic Fish

This article proposes a miniature underwater angle-of-attack (AoA) sensor appropriate to self-propelled fin-actuated underwater robots, featuring real-time measurement of AoA relative to the incoming flow. Specifically, the developed sensor adopts a vane-rotating structure with a high-performance Hall chip that is responsible for detection of the angular deviation. Meanwhile, the Hall-effect-based acquisition scheme endows the device with several features, including low rotational damping, to satisfy underwater usage requirements. Another contribution is that an AoA model is derived to allow the real-time acquisition of AoA at the trailing edge of the caudal fin assumed in a quiescent flow. Based on this model, the AoA profile is obtained as theoretical reference with only the control laws required. Finally, the performance of the numerical model and the developed sensor are investigated through aquatic experiments assisted by a global visual measurement system. The experimental results demonstrate that the acquired AoA possesses over 0.97 Parson correlation coefficient with the theoretical data, verifying the effectiveness of the proposed mechatronic design. The developed AoA sensor and its application to robotic fish could provide potential assistance for the flow control and hydrodynamic optimization of fin-actuated underwater robots.

Layout optimization of an inertial energy harvester for miniature underwater mooring platforms

Comprehensive analyses of the layout influence on power performance of a low-frequency miniature horizontal-pendulum-type inertial energy harvester for underwater mooring platforms are presented in this article. The mathematical models are obtained utilizing the Newton-Euler method. The power performance is evaluated by simulations over different parameters, such as damping, excitation frequency, and layout position. Simulation results indicate that the harvester can extract energy from low-frequency excitations over the range of 0.1 Hz–0.4 Hz, and maximum output power can reach to 0.7 W under excitation frequency f = 0.4 Hz. Different layout positions influence the energy harvesting performance of the harvester dramatically. Changing the layout position to positive value can enhance and broaden the power performance. The amplitude responses with different layout positions do not show resonant characteristics, even near the natural frequency of the harvester, due to large amplitude vibrations and strong nonlinear characteristic. As excitation frequency f = 0.4 Hz, there is a zero output power point appears at layout position d = −1.3 m. Moreover, the harnessed power has a quadratic relationship with the layout position value.

Modeling and Characterization of Bio-Inspired Hydro-Acoustic Sensor

A biomimetic miniature underwater acoustic sensor is proposed and analyzed for the measurement of directivity of underwater sound propagation. Unlike a hydrophone array, which detects propagation direction by the arrival time of sound waves, this novel sensor is based on a mechanically coupled mechanism, which amplifies the time delay of the arriving sound wave. In this paper, a mathematical model of the sensor is developed based on the finite element (FE) modeling technique, and it is used to study performance characteristics of the sensor. Effects of the fluid–structure interaction are examined through simulation of the sensor model and the results are compared with those obtained by a full scale FE model developed in a commercial software package.

Online High-Precision Probabilistic Localization of Robotic Fish Using Visual and Inertial Cues

This paper focuses on the development of an online high-precision probabilistic localization approach for the miniature underwater robots equipped with limited computational capacities and low-cost sensing devices. The localization system takes Monte Carlo localization (MCL) as the main framework and utilizes the onboard camera and low-cost inertial measurement unit (IMU) for information acquisition to provide a decimeter-level precision with 5-Hz refreshing rate in a small space with several artificial landmarks. Specifically, a novel underwater image processing algorithm is introduced to improve the underwater image quality; two key parameters, including a distance factor and an angle factor, are finally calculated to serve as the criteria to MCL. Meanwhile, the accurate orientation and rough odometry of the robot are acquired by onboard IMU. Moreover, a Kalman filter is adopted to filter the key information extracted from the sensors' data processing. In principle, when visual and inertial cues are both obtained, visual information with higher reliability has the priority to be used in the algorithm, which finally results in rapid convergence to the real pose of the robot. A series of relevant experiments are systematically conducted on the robotic fish, which prove that the online localization algorithm herein is highly accurate, robust, and practical for the miniature underwater robots with limited computational resources.

Development of a three-dimensional guidance system for long-range maneuvering of a miniature autonomous underwater vehicle

The present paper introduces a three-dimensional guidance system developed for a miniature Autonomous Underwater Vehicle (AUV). The guidance system determines the best trajectory for the vehicle based on target behavior and vehicle capabilities. The dynamic model of this novel AUV is derived based on its special characteristics such as the horizontal posture and the independent diving mechanism. To design the guidance strategy, the main idea is to select the desired depth, presumed proportional to the horizontal distance of the AUV and the target. By connecting the two with a straight line, this strategy helps the AUV move in a trajectory sufficiently close to this line. The adjacency of the trajectory to the line leads to reasonably short travelling distances and avoids unsafe areas. Autopilots are designed using sliding mode controller. Two different engagement geometries are considered to evaluate the strategy’s performance: stationary target and moving target. The simulation results show that the strategy can provide sufficiently fast and smooth trajectories in both target situations.

Towards docking for small scale underwater robots

This article presents a novel docking system developed for miniature underwater robots. Recent years have seen an increased diffusion of robots for ocean monitoring, exploration and maintenance of underwater infrastructures. The versatility of those vehicles is extremely affected and limited by energetic constraints and difficulties in updating their mission parameters. Submerged docking stations are a promising solution for providing energy sources and data exchange, thus extending autonomy and mission duration of underwater robots. Furthermore, the docking capability is a novel, but promising approach to enable modularity and reconfigurability in underwater robotics. The authors here propose a hybrid docking system composed of a magnetic alignment unit and a mechanical connection. The former passively aligns and guides the underwater vehicle facilitating a subsequent mechanical connection. The reliability of the system is both analytically investigated and experimentally validated. Finally, the mechanical design of the docking system of two miniature underwater robots is described in detail.

Tail-Enabled Spiraling Maneuver for Gliding Robotic Fish

Gliding robotic fish, a new type of underwater robot, combines both strengths of underwater gliders and robotic fish, featuring long operation duration and high maneuverability. In this paper, we present both analytical and experimental results on a novel gliding motion, tail-enabled three-dimensional (3D) spiraling, which is well suited for sampling a water column. A dynamic model of a gliding robotic fish with a deflected tail is first established. The equations for the relative equilibria corresponding to steady-state spiraling are derived and then solved recursively using Newton's method. The region of convergence for Newton's method is examined numerically. We then establish the local asymptotic stability of the computed equilibria through Jacobian analysis and further numerically explore the basins of attraction. Experiments have been conducted on a fish-shaped miniature underwater glider with a deflected tail, where a gliding-induced 3D spiraling maneuver is confirmed. Furthermore, consistent with model predictions, experimental results have shown that the achievable turning radius of the spiraling can be as small as less than 0.4 m, demonstrating the high maneuverability.

The Design of an Miniature Underwater Robot for Hazardous Environment

An miniature underwater robotic system for hazardous environment has been developed. The system consists of an underwater robot, a robot control station and a cameral control station. The underwater robot is installed two cameras for inspection, one is a camera of radiation resistant with two-freedom PTZ in the front of the robot, and the other is a fixed camera in the back of the robot. A miniature manipulator is equipped under the fore-camera to catch the small parts like bolts and nuts in the pools. The movement of the underwater robot is controlled by the master control station and the cameral control station controls the rotation and focus of the fore-camera. Besides, the underwater robot is equipped with the sensors, as MEMS inertial measurement unit, magnetometers, side scan sonar, water-depth gauges, which are integrated to determine the orientation and location of the robot. Meanwhile the navigation information is displayed in the virtual environment, which is modeled upon the real pools of the nuclear power plant. The underwater robotic system is easy to operate and will be applied to the hazardous environment like nuclear environment in future.

Microautonomous Robotic Ostraciiform (MARCO): Hydrodynamics, Design, and Fabrication

Boxfish with multiple fins can maneuver in confined spaces with a near zero turning radius, and it has been found that its unusual boxy shape is responsible for a self-correcting mechanism that makes its trajectories immune to water disturbances. The microautonomous robotic ostraciiform aims to apply these features in a novel underwater vehicle design. Miniature underwater vehicles with these characteristics have a variety of applications, such as environmental monitoring, ship wreck exploration, inline pipe inspection, forming sensor networks, etc. This paper presents the research leading to the design and fabrication of a robotic ostraciiform. Tail fin hydrodynamics have been investigated experimentally using robotic flapper mechanisms to arrive at a caudal fin shape with optimal-shape-induced flexibility. Fluid simulation studies were utilized to arrive at the body shape that can result in a self-correcting vorticity generation. Finally, the robotic ostraciiform prototype was designed based on the previous results. The ostracifform locomotion is implemented with a pair of 2 DOF pectoral fins and a single DOF tail fin. The finalized body shape of the robot is produced by 3-D prototyping two separate halves.

Optical Design for Underwater Television Viewing Systems

The design of underwater television viewing systems is dictated to a large extent by the optical properties of the water and by the mechanical and dynamic constraints imposed on the sensor package by the application. The system design is usually also limited in lens selection to available commercial lenses. When the system designer includes the lens design as an integral part of the viewing system development, significant improvements in system performance can be achieved. This paper describes three lens designs which have been produced for three different underwater remote viewing applications: a remote controlled underwater inspection vehicle for offshore operations; a miniature underwater camera for internal inspection of water cooled nuclear reactors; and an underwater camera for submersibles, search systems, and work vehicle application. A unique feature of these lenses is the ability to orient the line of sight by rotatio of the optics rather than the camera. The added complexity of the optics in each case is more than offset by resulting advantages in the overall viewing system performance. The paper emphasizes the necessity for a system approach. which coordinates the design of the optics, sensor and illumination with other elements of the system to achieve optimum performance.