Robotic design, especially in underwater robots, is critical to research, national defense, deep sea exploration and sea disaster rescue. Developing an advanced underwater robot, however, is complicated, as it involves propulsion, depth regulation, motion between propellers and other auxiliary system coordination, as well as sensing and feedback signals synchronization. Additionally, it is more challenging to manage the aquatic environment and guarantee the robotic design. In particular, the propulsion system could fit well in this environment and allow for efficient swimming. These challenges make the development of an underwater robot much more difficult, and finding the best solutions to building a robot in a standard and robust manner is critical to satisfying the large amount of requirements of the underwater robots in different perspectives.
Aquatic creatures have developed swimming capabilities far superior in many ways to what has been achieved by nautical science and technology and have inspired alternative ideas of developing smart and advanced novel robotic mechanisms for propulsion in different fluid environments. Many bioinspired aquatic robots mimic the structure design, locomotion behaviors and even control method of their counterparts in nature and achieved great swimming performance. The further development of a more general design methodology for bioinspired underwater robots, however, has been impeded due to the diversity of biological sources for underwater propulsion. Consequently, there have been several studies attempting to understand basic propulsion principles to synchronize the biological diversity.
In this dissertation, we first review the current stages and challenges of design of underwater robots. Afterwards, we provided a methodology for the design of efficient underwater robots from a biological perspective at multiple scales. To achieve this goal, we introduced the unique propulsion features of aquatic species in terms of locomotion mechanism as the swimmer increased in size from the micro/nanoscale to the macro-scale. Then, we discussed the biological propulsion principles for aquatic robotic design, including design of propeller, body, propulsion appendages, locomotion control and auxiliary system. In addition, we introduced the method for the implementation of bioinspired robots, including mechanical design, electronic engineering and system integration (Chapter 1). The following chapters show that four aquatic robots from the micro/nanoscale to the macro-scale were designed by learning unique features from biology and providing specific investigation of propulsion principle for robotic design at each scale. We validated and demonstrated the design of each robot using both mathematical model based simulation and hardware implemented robot experiments.
In chapter 2, propulsion was investigated at micro/nanoscale (body length<10-2m). Due to the constraints imposed at micro/nanoscale which has low Reynolds number (Re < 0.1), the design of efficient propulsive systems for nanorobots has proven challenging. An approach for the design of an efficient nanorobotic propulsive system was proposed. First, resistive force theory was used to develop a dynamic model for the propulsion of nanorobots, accounting for the fluid dynamics generated by the propeller (flagellum). Second, an optimal control problem was formulated to balance the trade-off between energy utilization and tracking efficiency. Finally, simulations were conducted to analyze the effect of different body to flagellum ratios (BFR) on propulsive efficiency. It was found that the optimal flexural rigidity of the nanorobot propeller was 5.8×10-19 N·m2, within the range of sperm flagellum, 0.7×10-19 -74.0×10-19 N·m2. Further, simulations of multiples BFRs demonstrated that multipoint actuation of the nanopropeller was more efficient at BFRs of less than 1.0, while single actuation was only effective for nanorobots with a BFR >0.2. The results from this study provide useful insight for the design of nanorobotic propulsive systems, in terms of energy efficiency and trajectory tracking accuracy.
In chapter 3, propulsion was investigated at transition scale by using example of whirligig beetle inspired robot. The whirligig beetle, claimed to be one of the most energy-efficient swimmers in the animal kingdom, has evolved a series of propulsion strategies that may serve as a source of inspiration for the design of propulsion mechanisms for energy-efficient surface swimming. In this study, we introduce a robot platform that was developed to test an energy-efficient propulsion mechanism inspired by the whirligig beetle. A propeller-body-fluid interaction dynamics model is proposed and based on this model, the propeller flexural rigidity and beating patterns are optimized in order to achieve energy-efficient linear swimming and turning. The optimization results indicate that a propeller with decreasing flexural rigidity enhances vortex shedding and improves thrust generation. It has also been found that an alternating asymmetrical beating sequence and optimal beating frequency of 0.71 Hz improves propulsion efficiency for linear swimming of the robot. The alternating beating of the outboard propellers and the unfolded inboard propellers working as brakes results in efficient turning with a smaller turning radius. Both simulation and experimental studies were conducted and the results illustrate that decreasing flexural rigidity along the propeller length, an oscillating body motion, and an S-shaped trajectory are critical for energy-efficient propulsion of the robot.
In chapter 4, a generic propulsion method, undulatory locomotion was investigated by comparing the propulsion principles across scale, expecting to come out a guidance for the robot design at multiple scales. In nature, swimmers commonly utilize undulation for propulsion. The Undulatory locomotion patterns, in fluid environments, at different Reynolds (Re) numbers (i.e., scale) vary as a result in variation among aspects that affect undulation patterns. Aspects include actuation. Swimmer’s inertia, damping, stiffness, and fluid viscosity. Here, we investigated the natural propulsion principles driving anguilliform and carangiform undulation using spermatozoa, eels, alligators, and trout fish as a means to identify universal aquatic propulsion principles and enhance underwater robotic design. Through biological observations of these species, we identified that as propulsion area stiffness increased, wave number decreases and mass center shifts away from the propulsion area, indicating a conserved biological trend for undulation based swimming that could be applied to designing bio-inspired swimming robotics. To quantitatively test and investigate the mechanistic aspects of this biological trend, a hydrodynamics model, combining resistive force and reactive force theory across scales, was formulated. Using this model, simulations were used to determine the material and kinematic features for effective propulsion. We found that for material features, simulation results showed mass had a diminishing effect as Re increased, while elasticity demonstrated the opposite trend. For the kinematics parameters, simulation results showed that a larger Re usually corresponded to a smaller optimal wavenumber, an increased amplitude, but the amplitude has larger frequency dependent behavior. These results were experimentally validated using a modular robotic platform built to allow robot disassemble and reassemble as a means to mimic undulation modes of the four biological swimmers and controlled by a Central Pattern Generators (CPGs) based algorithm and a PD control. Experimental results validated our simulation and biological findings; as well as, demonstrated a conserved aquatic propulsion principle for underwater swimming that could be translated to the design of future autonomous underwater vehicles with optimal propulsion mechanisms.
In chapter 5, an autonomous underwater vehicle was designed by integrating several propulsion mechanism to allow efficient swimming. Underwater propulsion using flexible propeller is usually observed in aquatic species. Unique propulsion features, such as three dimensional (3D) propulsion surface and the manipulation of the fluid through the coordination of multiple propellers allow energy-efficient swimming with high maneuverability. In this study, propulsion features from four aquatic animals, including batoidea fish, diving beetle, alligator and box fish, were used to inspire an autonomous under vehicle (AUV). A 1.3 meter long robot was built to implement the AUV locomotion. Modular design method was employed. Five propulsion modules and one central control module with independent power, communication and control system were integrated to the AUV body. This design significantly increased the operation robustness of the AUV. Five propellers that actuated by 15 motors were designed to allow three propulsion pattern, including flapping, rowing and undulating motion, provided big potentials for agile swimming. A 3D hydrodynamics model that incorporate resistive and reactive force theory was constructed for the quantitatively characterize the AUV’s underwater swimming. A hybrid control method that combines the adaptive control, Central pattern generators based control and PD control were developed to achieve optimal synchronization of the multiple propellers. Finally, simulation and experiments were conducted, and the results show the effectiveness of the proposed AUV design. This insights dawn from this paper provided a guidance for the next generation of AUV using flexible propellers
To conclude, we proposed and demonstrated a design methodology for aquatic robotics from biological perspective. We identified and extracted biological principles for efficient propulsion and derived the robotic design after theoretical optimization. Experiment results from four types of robotic platform demonstrated the effectiveness of the proposed aquatic robotic design at multiple scales.