Describes the development and verification of a six degree of freedom, non-linear simulation model for the REMUS AUV, the first such model for this platform. In this model, the external forces and moments resulting from hydrostatics, hydrodynamic lift and drag, added mass, and the control inputs of the vehicle propeller and fins are all defined in terms of vehicle coefficients. The paper briefly describes the derivation of these coefficients. The equations determining the coefficients, as well as those describing the vehicle rigid-body dynamics, are left in non-linear form to better simulate the inherently non-linear behavior of the vehicle. Simulation of the vehicle motion is achieved through numeric integration of the equations of motion. The simulator output is then verified against vehicle dynamics data collected in experiments performed at sea. The simulator is shown to accurately model the motion of the vehicle. The paper concludes with recommendations for future model validation experiments.

Development of a Six-Degree of Freedom Simulation Model for the REMUS autonomous Underwater Vehicle

Biological fish can create high forward swimming speed due to change of thrust/drag area of pectoral fins between power stroke and recovery stroke in rowing mode. In this paper, we proposed a novel type of folding pectoral fins for the fish robot, which provides a simple approach in generating effective thrust only through one degree of freedom of fin actuator. Its structure consists of two elemental fin panels for each pectoral fin that connects to a hinge base through the flexible joints. The Morison force model is adopted to discover the relationship of the dynamic interaction between fin panels and surrounding fluid. An experimental platform for the robot motion using the pectoral fin with different flexible joints was built to validate the proposed design. The results express that the performance of swimming velocity and turning radius of the robot are enhanced effectively. The forward swimming velocity can reach 0.231 m/s (0.58 BL/s) at the frequency near 0.75 Hz. By comparison, we found an accord between the proposed dynamic model and the experimental behavior of the robot. The attained results can be used to design controllers and optimize performances of the robot propelled by the folding pectoral fins.

Dynamic Analysis of a Robotic Fish Propelled by Flexible Folding Pectoral Fins

The work in this paper focuses on the examination of the effect of variable stiffness distributions on the kinematics and propulsion performance of a tuna-like swimmer. This is performed with the use of a recently developed fully coupled fluid-structure interaction solver. The two different scenarios considered in the present study are the stiffness varied along the fish body and the caudal fin, respectively. Our results show that it is feasible to replicate the similar kinematics and propulsive capability to that of the real fish via purely passive structural deformations. In addition, propulsion performance improvement is mainly dependent on the better orientation of the force near the posterior part of swimmers towards the thrust direction. Specifically, when a variable body stiffness scenario is considered, the bionic body stiffness profile results in better performance in most cases studied herein compared with a uniform stiffness commonly investigated in previous studies. Given the second scenario, where the stiffness is varied only in the spanwise direction of the tail, similar tail kinematics to that of the live scombrid fish only occurs in association with the heterocercal flexural rigidity profile. The resulting asymmetric tail conformation also yields performance improvement at intermediate stiffness in comparison to the cupping and uniform stiffness.

https://iopscience.iop.org/article/10.1088/1748-3190/abb3b6/pdf

The effect of variable stiffness of tuna-like fish body and fin on swimming performance

In this paper, we numerically investigate the propulsive performance of three-dimensional pitching flexible plates with varying flexibility and trailing edge shapes. We employ our recently developed body-conforming fluid-structure interaction solver for our high-fidelity numerical study. To eliminate the effect of other geometric parameters, only the trailing edge angle is varied from 45 ° (concave plate), 90 ° (rectangular plate) to 135 ° (convex plate) while maintaining the constant area of the flexible plate. For a wide range of flexibility, three distinctive flapping motion regimes are classified based on the variation of the flapping dynamics: (i) low bending stiffness K B low, (ii) moderate bending stiffness K B moderate near resonance, and (iii) high bending stiffness K B high. We examine the impact of the frequency ratio f * defined as the ratio of the natural frequency of the flexible plate to the actuated pitching frequency. Through our numerical simulations, we find that the global maximum mean thrust occurs near f * ≈ 1 corresponding to the resonance condition. However, the optimal propulsive efficiency is achieved around f * = 1.54 instead of the resonance condition. While the convex plate with low and high bending stiffness values shows the best performance, the rectangular plate with moderate K B moderate is the most efficient propulsion configuration. To examine the flow features and the correlated structural motions, we employ the sparsity-promoting dynamic mode decomposition. We find that the passive deformation induced by the flexibility effect can help in redistributing the pressure gradient, thus, improving the efficiency and the thrust production. A momentum-based thrust evaluation approach is adopted to link the temporal and spatial evolution of the vortical structures with the time-dependent thrust. When the vortices detach from the trailing edge, the instantaneous thrust shows the largest values due to the strong momentum change and convection process. Moderate flexibility and convex shape help to transfer momentum to the fluid, thereby improving the thrust generation and promoting the transition from drag to thrust. The increase in the trailing edge angle can broaden the range of flexibility that produces positive mean thrust. The role of added mass effect on the thrust generation is quantified for different pitching plates and the bending stiffness. These findings are of great significance to the optimal design of propulsion systems with flexible wings.

A high-fidelity numerical study on the propulsive performance of pitching flexible plates

Numerical study on the hydrodynamics of C-turn maneuvering of a tuna-like fish body under self-propulsion

Oily sludge on the floor of the tank is a significant problem for petrochemical industries and floor inspection robots. Oily sludge is a hazardous material containing a complex mixture of hydrocarbon, water, sand, and minerals deposited on the floor of the oil storage tanks. Sludge accelerates corrosion, reduces storage capacity, sticks to floor inspection robots and disrupts further tank operations Industries have started deploying robots in a tank to automate and replace the hazardous manual tank tasks. This paper presents the design of a screw conveyor based sludge cleaning mechanism to clean the sticky sludge from the floor of aboveground oil storage tanks and interface effectively with tank inspection robots to perform cleaning and inspection synchronously. The cleaning mechanism consists of a screw conveyor mounted on a 'C' shaped case with a bearing on both sides, a waterproof motor connected to the screw conveyor with a worm-wheel gear. A Rheometer is used for measuring sludge properties to understand its flow behavior. Computational fluid dynamics (CFD) based numerical simulation is performed to visualize the flow of oily sludge through the proposed cleaning mechanism.

Design and Simulation of a Tank Floor Cleaning Mechanism for Mobile Robots used in Storage Tanks

2D based bimetallic MOFs have attracted significant attention for energy conversion and storage due to their synergistic effects and high electrocatalytic performance. However, the limited active sites and stability inhibit their multi-functional applicability in water electrolysis and supercapacitor applications. Herein, we report the 2D bimetallic Fe/M-BDC ((M-Ni, Zn, Co and Cu), BDC:1,4 benzene dicarboxylic acid) framework prepared by solvothermal and exfoliation condition, which exhibits excellent electrochemical performance. Electrochemical measurements and spectroscopic analysis establish that 2D Fe/M-BDC (M-Ni, Zn) contains a high surface area and abundant active sites that can accelerate the reaction kinetics. The as-prepared catalyst exhibits very low overpotentials of 78 mV for the hydrogen evolution reaction and 160 mV for the oxygen evolution reaction @ 10 and 20 mA cm−2, respectively. The bifunctional electrode assembly cell using anodic Fe/Zn-BDC and cathodic Fe/Ni-BDC couple provides the very low cell voltage of 1.35 V to drive current density of 10 mA cm−2. In addition, Fe/Zn-BDC and Fe/Ni-BDC exhibit high specific capacitances of 633.21 F/g and 1190.88 F/g at 1 mVs−1, respectively, with good stability when they are used as supercapacitor electrodes. 

Two-dimensional bimetallic Fe/M- (Ni, Zn, Co and Cu) metal organic framework as efficient and stable electrodes for overall water splitting and supercapacitor applications


Periodic inspection is of paramount importance to assess structural integrity and safety. Conventional concrete inspection procedures ranging from rebound hammer, to state-of-the-art ground penetrating radar and ultrasonic tomography focus on superstructures. Often, the part of the structure underwater, is subject to significant levels of corrosion. Today, qualitative visual inspection of the submerged surface can be performed by diving teams, wherein internal defects often go undetected. Furthermore, divers are limited to shallow depths and face several risks, especially when visibility is poor and operating in confined spaces. Presently, there are few commercially available quantitative NDT techniques for submerged structures.

This paper presents an alternate solution to inspection of submerged concrete structures — using lightweight remotely operated vehicles (ROV) manufactured by Planys Technologies to perform concrete NDT underwater. Rebound hammer and ultrasonic pulse velocity techniques, although elementary, have been redesigned for use on a submersible drone. Several challenges were addressed including long-range communication and data acquisition, marinizing transducers and pulser-receivers, obtaining measurements in water currents and over deformed surfaces. Planys’ ROVs carry payloads including high-definition cameras, lasers, sonar, and underwater positioning systems to improve data-acquisition quality and location tagging of defects. 

Key results from experimental studies in a laboratory on canonical specimens are presented. Furthermore, the technology has been successfully tested in a port — details of the structure, operational arrangement and sample results from the field trial are also presented. Following calibration procedures, ROV based underwater concrete NDT was used to acquire data from the structure in grids and subsequently, identify zones of potential non-homogeneity. Numerous challenges, including interpretation of the time traces obtained, and correlation of results to structural integrity and to structures above water, are discussed in the paper. This work will be of interest to asset owners in marine, water management and power generation industries.


Quantitative Non-Destructive Testing (NDT) of Submerged Civil Concrete Structures Using Remotely Operated Robotic Drones

Thrusters are an integral part of the propulsion system of electric underwater vehicles, and they are loaded in very harsh conditions. The current commercial market segment for electric thrusters possesses a steep increment in cost with increasing reliability and operational lifetime. As a result, a major void exists in the category of commercial thrusters with high thrust output, high power to thrust ratios and affordability for mini observation class remotely operated underwater vehicles (ROV). This is due to the rapid escalation of various challenges in thruster design related to overall size, tight tolerances in manufacturing, electrically accelerated corrosion, pressure compensated encapsulation, seal design, thermal effects and so on. The challenge during an optimal thruster design process is further propagated by the limitations in performing accelerated testing in the laboratory with conditions simulating hostile loading environments. Also, given that such thrusters are designed for a very long lifetime, it is challenging to test them for their full life. In addition, their loading patterns are also highly dynamic in nature, further making the simulation of environmental conditions even more challenging. Given such challenges in product design and testing, a minimalistic design methodology and test results are presented for a thruster developed at Planys Technologies targeted specifically for mini observation class ROVs.There are many models available for accelerated testing of commercial products. However, conventional accelerated testing methods most often use statistical methods to analyse the test data. This leads to the necessity of testing a large number of samples to get a reliable life estimation data. Also, the overall test design is further complicated by the fact that the number of variables affecting the life of the product are very high. In view of the said complications, and also since underwater marine robotic products are expensive to manufacture, the proposed test method uses a minimum number of samples for life estimation. This is achieved by utilizing a test strategy centered around the anticipated failure modes of a thruster. This methodology not only saves the overall cost, but also the overall time required to release the product to the market. In addition, the test setup and the results are also presented for the particular model that was tested using this methodology.In this article, the test design philosophy with key considerations is explained followed by a deeper emphasis on the test methodology which has been prepared with the aid of the experience gained by operating indigenously designed ROVs in tough underwater conditions across the globe. It is to be noted that the proposed solution may not be as accurate as the conventional statistical test methods in all cases, nevertheless, is very much useful in giving an insight into the product lifetime range to an acceptable degree. The proposed test methodology can also be extended to other product life testing in marine robotics and beyond.

Accelerated testing and results of underwater electric thrusters for mini observation class ROVs

This paper presents the design optimization and testing of a novel screw conveyor based system to scoop crude oil sludge from the floor of oil storage tanks This proposed new system consists of a screw conveyor mounted on a ‘C’ shaped casing with a bearing on both sides driven by a waterproof motor through a worm drive A novel mathematical model is developed to help the design of a screw conveyor for a maximum amount of sludge scooping per turn, and numerical simulations are performed using computational fluid dynamics to visualize the flow of material particles in various possible designs of the system The proposed mechanism was 3D printed, and laboratory tests were conducted to quantify the amount of sludge removal by the different designs of the screw Optimized design of a mechanism screw with a radius ratio of 040 and a pitch ratio of 015 scoops up to a maximum of 588 kg hr of material at 110 rev min 

Santhosh Ravichandran

Papers

Design and Simulation of a Tank Floor Cleaning Mechanism for Mobile Robots used in Storage Tanks

Two-dimensional bimetallic Fe/M- (Ni, Zn, Co and Cu) metal organic framework as efficient and stable electrodes for overall water splitting and supercapacitor applications

Quantitative Non-Destructive Testing (NDT) of Submerged Civil Concrete Structures Using Remotely Operated Robotic Drones

Accelerated testing and results of underwater electric thrusters for mini observation class ROVs

Design optimization of a novel screw conveyor based system to scoop oil sludge from floor of storage tanks