The design of unmanned underwater vehicle (UUV) was inspired by the form and functionality of Jellyfish. These natural organisms were chosen as bio-inspiration for a multitude of reasons including: efficiency, good room for payload, and a wide range of sizes and morphology. Shape memory alloy (SMA) actuators were selected as the primary source of actuation for the propulsion of the artificial jellyfish node. These actuators offer high power density which enables a compact system size and silent operation which is preferred for surveillance. SMA wires mimic the form and function of natural muscles; allowing for a wider range of applications than conventional actuators. Commercial SMA wires (100 um in diameter) can exhibit a 4% deformation of the initial actuator length with a blocking stress of over 200 MPa.
The deformation of SMA wire is not enough to mimic the bell contraction of jellyfish. In order to resolve this problem, a beam-shape composite actuator using SMA wires as the active component, termed as BISMAC, was designed to provide large curvature. The BISMAC design was inspired by rowing jellyfish bell contraction. Characterization of maximum deformation in underwater conditions was performed for different actuator configurations to analyze the effect of design parameters that include silicone thickness, flexible steel thickness and distance between SMA and flexible steel. A constant cross-section (CC) BISMAC of 16 cm in length was found to achieve deformation with a radius of curvature of 3.5 cm. Under equilibrium conditions, the CC-BISMAC was found to achieve 80% of maximum deformation consuming 7.9 J per cycle driven at 16.2 V/0.98 A and frequency of 0.25 Hz. Using the a developed analytical model, an actuator design was fabricated mimicking the maximum deformation profile of the A. aurita. The optimized AA-BISMAC achieved a maximum curvature of 0.428 1/cm as compared to 0.438 1/cm for the A. aurita with an average squared root error of 0.043 (1/cm), 10.2% of maximum A. aurita curvature.
BISMAC actuators are unidirectional flexible actuators capable of exhibiting high curvature. To extend the application range of these actuators, they were modified to achieve bidirectional deformation. The new bidirectional actuators termed as "BiFlex" actuators had the capability to achieve large deformation in two directions. The FlexLegs consist of six segments which can be actuated individually. Two different sets of legs were constructed to determine the effect of size. The small legs measured 35.8 mm in height and 63.2 mm in width and the large legs were 97.4 mm in height and 165.4 mm in width. The small FlexLegs achieved a maximum deformation of 12 % and 4 % in the x- and y-direction respectively using a power of 0.7 W while producing a maximum force of 0.023 N. They were also able to withstand a load of 1.18 N. The large FlexLegs had a maximum deformation of 57 % and 39 % in the x- and y-direction respectively using a power of 3 W while producing a force of 0.045 N. They were able to withstand a load of 0.25 N. The legs were also able to perform several walking algorithms consisting of stepping, crabbing and yawing.
In order to reduce the power consumption and contraction time of SMA wires, a feedback control scheme using wire resistance was developed. The controller required the knowledge of threshold resistance and safe current inputs which were determined experimentally. The overheating effect of SMA wires was analyzed for BioMetal Fiber (BMF) and Flexinol 100 "m diameter wires revealing an increase in resistance as the wires overheated. The controller was first characterized on a SMA wire with bias spring system for a BMF 100 using I_hi=0.5 A and I_low=0.2 A, where hi corresponds to peak current for fast actuation and low corresponds to the safe current which prevents overheating and maintains desired deformation. A contraction of 4.59% was achieved in 0.06 s using the controller and the deformation was maintained for 2 s at low current. The BISMAC actuator was operated using the controller with I_hi=1.1 A and I_low=0.65 A achieving a 67% decrease in contraction time compared to using a constant driving current of I_low=0.2 A and a 60% decrease in energy consumption compared to using constant I_hi=0.5 A while still exceeding the contraction requirements of the Aurelia aurita.
Two fundamental parameters at the composition level were associated with the power consumption of SMA: i) martensite to austentite phase transition temperature and ii) thermal hysteresis. Ideally, one would like to reduce both these quantities and for this purpose an equiatomic Ni-Ti alloy was modified with Cu. Replacing nickel with 10 at% copper reduces the thermal hysteresis by 50% or more. For Ni-Ti alloys with nickel content greater than 50 at%, transition temperature decreases linearly at a rate of 100 "C/Ni at%. Given these two power reducing factors, an alloy with composition of Ni40+xTi50-xCu10 was synthesized with x = 0, ±1, ±2, ±3, ±4, ±5. Metal powders were melted in an argon atmosphere using an RF induction furnace to produce ingots. All the synthesized samples were characterized by differential scanning calorimetric (DSC) analysis to reveal martensite to austenite and austenite to martensite transition temperatures during heating and cooling cycles respectively. Scanning electron microscopy (SEM) was conducted to identify the density and microstructure of the fractured samples. The results show the possibility of achieving low power consuming high performance SMAs.
Using the BISMAC actuator and feedback control system, a robotic jellyfish called Robojelly that mimics the morphology and kinematics of the Aurelia aurita species was created. A systematic fabrication technique was developed to replicate the essential structural features of A. aurita. Robojelly's body was fabricated from RTV silicone having a total mass of 242 g and bell diameter of 16.4 cm. Robojelly was able to generate enough thrust in static water conditions to propel itself and achieve a proficiency of 0.19 s-1 while the A. aurita achieves a proficiency of around 0.25 s-1. A thrust analysis based on empirical measurements for natural jellyfish was used to compare the performance of the different robotic configurations. The configuration with best performance was a Robojelly with segmented bell and a passive flap structure. Robojelly was found to consume an average power on the order of 17 W with the actuators not having fully reached thermal steady state. A comparative kinematics analysis was conducted between a natural Aurelia aurita and Robojelly. The resistance feedback controller was implemented to tailor the deformation profile of BISMAC actuators embedded in Robojelly. Robojelly's performance was quantified in terms of thrust production and power consumption during vertical swimming experiments. A maximum average instantaneous thrust production of 0.006 N was achieved at a driving current (Ihi) of 1.5 A with 35% duty cycle. Rapid heating of SMA wires was found to reduce power consumption and increase thrust. The bell kinematic analysis revealed resemblance and differences in bell deformation trajectories of the biomimetic and natural jellyfish. The inflexion point of the A. aurita was found to convert an inner bell trajectory into an outer one during contraction which assists the thrust production.
A biomimetic robot inspired by Cyanea capillata, termed as "Cyro", was developed to meet the functional demands of underwater surveillance in defense and civilian applications. The design of Cyro required kinematics of large C. capillata which are elusive creatures. Obtaining accurate kinematic data of animals is essential for many biological studies and bio-inspired engineering applications. Many animals such as the C. capillata however, are either too large or too delicate to transport to controlled environments where accurate kinematic data can easily be obtained. Often, in situ recordings are the only means available but are often subject to multi-axis motion and relative magnification changes with time, which lead to large discrepancies in animal kinematics. In Chapter 5, techniques to compensate for magnification and body rotation of animal footage were developed. A background reference point and animal dimensions were used to account for magnification. A linear fit of body points were used to measure body rotation. These techniques help resolve animal kinematics from in situ video footage. The techniques were applied to a large jellyfish, Cyanea capillata, swimming in ocean waters. The bell kinematics were captured by digitizing exumbrella profiles for two full swimming cycles. Magnification was accounted for by tracking a reference point on the ocean floor and by tracking the C. capillata exumbrella arclength in order to have a constant scale through the swimming cycles. A linear fit of the top bell portion was used to find the body angle with respect to the camera coordinate system. Bell margin trajectories over two swimming cycles confirm the accuracy of the correction techniques. The corrected profiles were filtered and interpolated to provide a set of time-dependent points along the bell. The ability to use in situ footage with significant multi-axis motion provides an opportunity to analyze previously impractical footage for gaining a better understanding of large or delicate organisms.
The swimming kinematics of the C. capillata were analyzed after extracting the required kinematics from the in situ video. A discrete model of the exumbrella was developed and used to analyze the kinematics. The exumbrella discretization was done using three different methods. The first method consists of analyzing the animal anatomy for structural and mechanical features. The second method consists of analyzing the bell kinematics for areas of highest deformation over time. The third method consists of optimizing node locations that can provide minimal error with comparison to the digitized profiles.
Two kinematic models of the C. capillata swimming motion were developed by fitting Fourier series to the discretized segments and angles formed by each segment. The four-segment anatomical kinematic model was used to analyze the bell kinematics of the C. capillata. It was found that the bell does not deform uniformly over time with segments lagging behind others. Hysteresis between contraction and relaxation was also present through most of the exumbrella. The bell margin had the largest hysteresis with an outer path during contraction and inner path during relaxation. The subumbrella volume was approximated based on the exumbrella kinematics and was found to increase during contraction.
Cyro was designed to mimic the morphology and swimming mechanism of the natural counterpart. The body of the vehicle consists of a rigid support structure with linear DC motors which actuate eight mechanical arms. The mechanical arms in conjunction with artificial mesoglea create the hydrodynamic force required for propulsion. The full vehicle measures 170 cm in diameter and has a total mass of 76 kg. An analytical model of the mechanical arm kinematics was developed. The analytical and experimental bell kinematics were analyzed and compared to the C. capillata. Cyro reached the water surface untethered and autonomously from a depth of 182 cm in five actuation cycles. It achieved an average velocity of 8.47 cm/s while consuming an average power of 70 W. A thrust stand was developed to calculate the thrust directly from a single bell segment yielding an average thrust of 27.9 N for the whole vehicle. Steady state velocity during Cyro's swimming test was not reached but the measured performance during its last swim cycle resulted in a cost of transport of 10.9 J/kg m and total efficiency of 3%.
It was observed that a passive flexible margin or flap, drastically increases the performance of the Robojelly. The effects of flap length and geometry on Robojelly were analyzed using PIV. The flap was defined as the bell section which is located between the flexion point and bell margin. The flexion point was established as the location where the bell undergoes a significant change compliance and therefore in slope. The flap was analyzed in terms of its kinematics and hydrodynamic contribution. An outer trajectory is achieved by the flap margin during contraction while an inner trajectory is achieved during relaxation. The flap kinematics was found to be replicable using a passive flexible structure. Flaps of constant cross section and varying lengths were put on the robotic vehicle to conduct a systematic parametric study. Robojelly's swimming performance was tested with and without a flap. This revealed a thrust increase 1340% with the addition of a flap. Velocity field measurements were performed using planar Time Resolved Digital Particle Image Velocimetry (TRDPIV) to analyze the change in vortex structures as a function of flap length. The robot input parameters stayed constant over the different configurations tested thus maintaining a near constant power consumption. Non-dimensional circulation results show a dependence on flap kinematics and geometry. The robot was approximated as a series of pitching panels circularly oriented around its apex. The first circulation peak of the pitching panel approximation revealed a normalized standard deviation of 0.23. A piston apparatus was designed and built to test different flexible margin configurations. This apparatus allow the isolation of the flap parameters and remove the uncertainties coming from the robotic vehicle. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/50639 |
| Date | 08 May 2013 |
| Creators | Villanueva, Alexis A. |
| Contributors | Mechanical Engineering, Priya, Shashank, Costello, John H., Vlachos, Pavlos P., Kurdila, Andrew J., Mueller, Rolf |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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