Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第18947号 / 工博第3989号 / 新制||工||1614 / 31898 / 京都大学大学院工学研究科航空宇宙工学専攻 / (主査)教授 泉田 啓, 教授 琵琶 志朗, 教授 西脇 眞二 / 学位規則第4条第1項該当

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18947号 / 工博第3989号 / 新制||工||1614(附属図書館) / 31898 / 京都大学大学院工学研究科航空宇宙工学専攻 / (主査)教授 泉田 啓, 教授 琵琶 志朗, 教授 西脇 眞二 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

With a look towards the future, which involves a push towards utilizing renewable energy sources and cementing energy independence for future generations, the design of more efficient aircraft and novel energy systems is of utmost importance. This dissertation looks at leveraging some of the benefits offered by inflatable wings for use in tethered kite-like systems towards the goal of designing a High Altitude Aerial Platform (HAAP). Uses of such a system include Airborne Wind Energy Systems (AWES), among others. The key bene- fit offered by such wings is their lightweight construction and durability, but challenges to aerodynamic performance arise out of their flexible nature and non-standard airfoil profile. Studying the aerodynamic behavior of such wings forms the critical focus of this research. This effort primarily encompasses an experimental investigation of two swept, tethered, inflatable wings conducted in the Virginia Tech Stability Wind Tunnel, and numerical CFD computations of these wings. The experiment was conducted in the modular wall configuration of the anechoic test section at speeds ranging from 15 − 32.5 m/s for three different tether attachment configurations and wings constructed out of two different fabric materials. Along with static aeroelastic deformation data using a 3D photogrammetry system, aerodynamic measurements were taken in the form of Pitot and static pressure measurements in the wake of the wing, force and moment measurements at the base of the mount, and tension measurements at the tether attachment locations. This provides a data set for validating static aeroelastic modeling approaches for such a system and highlights the dramatic effect of the variability in test configuration on the wing's aerodynamics. In addition to the wind tunnel tests, 3D steady RANS CFD computations of the rigid 3D scanned inflatable wing geometry were conducted in the wind tunnel environment for these configurations to validate the CFD modeling approach and highlight the level of detail necessary to accurately characterize the wing aerodynamic performance. Static aeroelastic deformation data from the 3D photogrammetry system, at a speed of 27.5 m/s, were also used to deform the 3D scanned inflatable wing geometry, and RANS CFD computations of this deformed inflatable wing were conducted at a wind tunnel speed of 27.5 m/s. Several turbulence models were investigated and comparisons were made with the wind tunnel test data. Good agreement was found with experimental data for the forces and moments and wake Pitot pressure coefficient contours. Comparisons were also made with the rigid wing CFD computations at the same tunnel speed of 27.5 m/s to illustrate the effect of static aeroelastic deformations on the aerodynamic performance, wake Pitot pressure coefficient contours and wing-tip vortex structures, of these flexible inflated wings. In effect, this research utilizes the synergy be- tween wind tunnel experiments and numerical CFD computations to study the flow behavior over inflatable wings and provide a comprehensive verification and validation approach for modeling such complex systems. / Doctor of Philosophy / With a look towards the future, which involves a push towards utilizing renewable energy sources and cementing energy independence for future generations, the design of more efficient aircraft and novel energy systems is of utmost importance. This dissertation looks at leveraging some of the benefits offered by inflatable wings for use in tethered kite-like systems towards the goal of designing a High Altitude Aerial Platform (HAAP). Uses of such a system include Airborne Wind Energy Systems (AWES), among others. The key benefit offered by such wings is their lightweight construction and durability, but challenges to aerodynamic performance arise out of their flexible nature and non-standard airfoil profile. Studying the aerodynamic behavior of such wings forms the critical focus of this research. This effort primarily encompasses an experimental investigation of two swept, tethered, inflatable wings conducted in the Virginia Tech Stability Wind Tunnel, and computer simulations of the aerodynamic flow over these wings. The experiment was conducted in the modular wall configuration of the anechoic test section at speeds ranging from 15 − 32.5 m/s for three different tether attachment configurations and wings constructed out of two different fabric materials. Along with measurements of the wing deformations using a 3D photogrammetry system, aerodynamic measurements were taken in the form of pressure measurements in the wake of the wing, force and moment measurements at the base of the mount, and tension measurements at the tether attachment locations. This provides a data set for validating static aeroelastic modeling approaches for such a system and highlights the dramatic effect of the variability in test configuration on the wing's aerodynamics. In addition to the wind tunnel tests, detailed computer simulations of the scanned inflatable wing geometry were conducted in the wind tunnel environment for these configurations to validate the computational modeling approach and highlight the level of detail necessary to accurately characterize the wing aerodynamic performance. The wing deformation data from the 3D photogrammetry system, at a speed of 27.5 m/s, were also used to deform the scanned inflatable wing geometry, and computer simulations of this deformed inflatable wing geometry were conducted at a wind tunnel speed of 27.5 m/s. Good agreement was found between the experimental and computational forces and moments and wake Pitot pressure coefficient contours. Comparisons were also made with the undeformed wing computations at the same tunnel speed of 27.5 m/s to illustrate the effect of wing flexibility on the aerodynamic performance. In effect, this research utilizes the synergy between wind tunnel experiments and numerical CFD computations to study the flow behavior over inflatable wings and provide a comprehensive verification and validation approach for modeling such complex systems.

Increasing costs for space shuttle missions translate to smaller, lighter, and more flexible satellites that maintain or improve current dynamic requirements. This is especially true for optical systems and surfaces. Lightweight, inflatable structures, otherwise known as gossamer structures, are smaller, lighter, and more flexible than current satellite technology. Unfortunately, little research has been performed investigating cost effective and feasible methods of dynamic analysis and control of these structures due to their inherent, non-linear dynamic properties. Gossamer spacecraft have the potential of introducing lenses and membrane arrays in orbit on the order of 25 m in diameter. With such huge structures in space, imaging resolution and communication transmissibility will correspondingly increase in orders of magnitude. A daunting problem facing gossamer spacecraft is their highly flexible nature. Previous attempts at ground testing have produced only localized deformation of the structure's skin rather than excitation of the global (entire structure's) modes. Unfortunately, the global modes are necessary for model parameter verification. The motivation of this research is to find an effective and repeatable methodology for obtaining the dynamic response characteristics of a flexible, inflatable structure. By obtaining the dynamic response characteristics, a suitable control technique may be developed to effectively control the structure's vibration. Smart materials can be used for both active dynamic analysis as well as active control. In particular, piezoelectric materials, which demonstrate electro-mechanical coupling, are able to sense vibration and consequently can be integrated into a control scheme to reduce such vibration. Using smart materials to develop a vibration analysis and control algorithm for a gossamer space structure will fulfill the current requirements of space satellite systems. Smart materials will help spawn the next generation of space satellite technology. / Master of Science

Satellite antenna design requirements are driving the satellite size to proportions that cannot be launched into space using current technology. In order to reduce the launch size and mass of satellites, inflatable structures, also known as gossamer structures, are being considered. Inflatable space-based structures are susceptible to vibration disturbance due to their low stiffness and damping. This thesis discusses the structural dynamics and vibration suppression via piezoelectric actuators, using active control of an inflatable torus. A commercial finite element package, ANSYS, is used to model the inflated torus. The effect of torus aspect ratio and inflation pressure on the vibratory response of the structure is investigated. The interaction with the torus of the surface-mounted piezoelectric patches, made of PVDF, is modeled using Euler-Bernoulli beam theory. A state space representation is created of the model in modal space and modal truncation is performed. Traditional control tools are used to suppress vibration in the structure. First observer-based full state feedback is used, then direct output velocity feedback is explored. The aspect ratio of the torus is found to significantly influence the mode shapes. Toroids of small aspect ratios, skinny toroids, act like rings, but the mode shapes of toroids with large aspect ratios are much more complicated. For toroids of small aspect ratios, increasing the inflation pressure simply results in stiffening the ring, thereby increasing the natural frequencies. Increasing the pressure in toroids with large aspect ratios changes both the mode shapes and natural frequencies. The passive effect of PVDF on the dynamics of the torus is small, the mode shapes do not change and the frequencies are only slightly reduced. Active control of toroids with small aspect ratios using piezoelectric devices is effective. It may be more difficult to control toroids with large aspect ratios because the mode shapes are much more complicated than the simple ring modes found in toroids with small aspect ratios. / Master of Science

The research presented in this thesis uses the macro-fiber composite (MFC) actuator that was recently developed at the NASA Langley Research Center for two major themes, sensing and actuation for vibration control, and power harvesting. The MFC is constructed using piezofibers embedded in an epoxy matrix and coated with Kapton skin. The construction process of the MFC affords it vast advantages over the traditionally used piezoceramic material. The MFC is extremely flexible, allowing it to be bonded to structures that have curved surface without fear of accidental breakage or additional surface treatment as is the case with monolithic piezoceramic materials. Additionally the MFC uses interdigitated electrodes that capitalize on the higher d33 piezoelectric coupling coefficient that allow it to produce higher forces and strain than typical monolithic piezoceramic materials. The research presented in this thesis investigates some potential applications for the MFC as well as topics in power harvesting. This first study performed was to determine if the MFC is capable of being used as a sensor for structural vibration. The MFC was incorporated into a self-sensing circuit and used to provide collocated control of an aluminum beam. It was found that the MFC makes a very accurate sensor and was able to provide the beam with over 80% vibration suppression at its second resonant frequency. Following this work, the MFC was used as both a sensor and actuator to apply multiple-input-multiple-output vibration control of an inflated satellite component. The control system used a positive position feedback (PPF) controller and two pairs of sensors and actuators in order to provide global vibration suppression of an inflated torus. The experiments found that the MFC and control system was very effective at attenuating the vibration of the first mode but ineffective at higher modes. It was found the positioning of the sensors and actuators on the structure contributed heavily to the controller's performance at higher modes. A discussion of the reasons for the controller's ineffectiveness is supply and a solution using self-sensing techniques for collocated vibration suppression was investigated. Subsequent to the research in vibration sensing and control, the ability to use piezoelectric materials to convert ambient vibration into usable electrical energy was tested and quantified. First, a model of a power harvesting beam is developed using variational methods and is validated on a composite structure containing four separate piezoelectric wafers. It is shown that the model can accurately predict the power generated from the vibration of a cantilever beam regardless of the load resistance or excitation frequency. The damping effects of power harvesting on a structure are also demonstrated and discussed using the model. Next, the ability of the piezoelectric material to recharge a battery and a quantification of the power generated are investigated. After determining that the rechargeable battery is compatible with the power generated through the piezoelectric effect, the MFC was compared with the traditional monolithic PZT for use as a power harvesting material. It was found that the MFC produces a very low current, making it less efficient than the PZT material and unable to charge batteries because of their need for relatively large current. Due to the MFC being incapable of charging batteries, only the PZT was used to charge batteries and the charge times for several nickel metal hydride batteries ranging from 40 to 1000mAh are supplied. / Master of Science

To enable a crewed mission to Mars, precision landing capabilities of Entry, Descent, and Landing (EDL) systems must be improved. The need for larger payloads, higher landing sites, and controllability has motivated the National Aeronautics and Space Administration (NASA) to invest in new technologies to replace traditional rigid aeroshell systems, which are limited in size by the payload envelope of existing launch vehicles. A Hypersonic Inflatable Aerodynamic Decelerator (HIAD) is an emerging technology that provides an increased drag area by inflating the aeroshell to diameters not possible with rigid aeroshells, allowing the vehicle to decelerate higher in the atmosphere, offering access to higher landing sites with more timeline margin. To enable a crewed mission to Mars, future entry vehicles will require precision landing capabilities that go beyond heritage EDL guidance strategies that utilize fuel-intensive and error-prone bank reversals. A novel Direct Force Control (DFC) approach of independently controlling the lift and side force of a vehicle that utilizes a HIAD with an aerodynamic shape morphing capability is proposed. To date, the mechanisms and controls required to morph an inflatable structure to generate lift have not been explored. In this dissertation, novel morphing HIAD concepts are investigated and designed to satisfy mission requirements, aerodynamic tools are built to assess the aerodynamic performance of morphed blunt body shapes, and a structural feasibility study is performed using models correlated to test data to determine the forces required to generate the desired shape change based on a crewed mission to Mars. A novel control methodology is introduced by applying a unique DFC strategy to a morphing HIAD to enhance precision landing capabilities of EDL systems, and the ability of a morphing HIAD to safely land a vehicle on Mars is assessed by performing a closed-loop feedback simulation for a Mars entry trajectory. Finally, a control mechanism is demonstrated on a small-scale inflatable structure. Conclusions and contributions of this research are presented along with a discussion of future research opportunities of morphing HIADs. / PHD / A Hypersonic Inflatable Aerodynamic Decelerator (HIAD) is a reentry vehicle designed to inflate the aeroshell to diameters outside of the payload shroud to decelerate the vehicle higher in the atmosphere, offering access to higher landing sites with more timeline margin. To enable a crewed mission to Mars, the landing accuracy of a HIAD must be significantly improved beyond heritage bank angle control approaches that are fuel-intensive and prone to errors. A novel Direct Force Control (DFC) approach is proposed that permits direct control of the angle of attack and sideslip by morphing the inflatable shape of the HIAD to enable its precision landing capabilities. A morphing HIAD concept is proposed in this dissertation to satisfy the requirements of landing humans successfully on Mars. Aerodynamic tools are built to assess the aerodynamic performance of morphed blunt body shapes, and structural models correlated with test data are created to determine the forces required to generate the desired shape change. Novel DFC methodologies are introduced and applied to a morphing HIAD system, a motor sizing study is performed to compare the total energy usage and cost and weight estimates of the morphing HIAD to heritage control systems, and a Mars entry trajectory simulation is performed to assess the capability of a morphing HIAD to safely land a crewed vehicle on Mars. Finally, a control mechanism is demonstrated on a small-scale inflatable structure. Conclusions and contributions of this research are presented along with a discussion of future research opportunities of morphing HIADs.

Inflatable Aerodynamic Decelerators (IADs) are a candidate technology NASA began investigating in the late 1960’s. Compared to supersonic parachutes, IADs represent a decelerator option capable of operating at higher Mach numbers and dynamic pressures. IADs have seen a resurgence in interest from the Entry, Descent, and Landing (EDL) community in recent years. The NASA Space Technology Roadmap (STR) highlights EDL systems, as well as, Materials, Structures, Mechanical Systems, and Manufacturing (MSMM) as key Technology Areas for development in the future; recognizing deployable decelerators, flexible material systems, and computational design of materials as essential disciplines for development. This investigation develops a multi-scale flexible material modeling approach that enables efficient high-fidelity IAD design and a critical understanding of the new materials required for robust and cost effective qualification methods. The approach combines understanding of the fabric architecture, analytical modeling, numerical simulations, and experimental data. This work identifies an efficient method that is as simple and as fast as possible for determining IAD material characteristics while not utilizing complicated or expensive research equipment. This investigation also recontextualizes an existing mesomechanical model through validation for structures pertaining to the analysis of IADs. In addition, corroboration and elaboration of this model is carried out by evaluating the effects of varying input parameters. Finally, the present investigation presents a novel method for numerically determining mechanical properties. A sub-scale section that captures the periodic pattern in the material (unit cell) is built. With the unit cell, various numerical tests are performed. The effective nonlinear mechanical stiffness matrix is obtained as a function of elemental strains through correlating the unit cell force-displacement results with a four node membrane element of the same size. Numerically determined properties are validated for relevant structures. Optical microscopy is used to capture the undeformed geometry of the individual yarns.

Soft robots and particularly inflatable robots are of interest because they are lightweight, compact, robust to impact, and can interact with humans and their environment relatively safely compared to rigid and heavy traditional robots. Improved safety is due to their low mass that results in low-energy collisions and their compliant, soft construction. Inflatable robots (which are a type of soft robot) are also robust to impact and have a high torque to weight ratio. As a result inflatable robots may be used for many applications such as space exploration, search and rescue, and human-robot interaction. One of the potential problems with inflatable or pneumatically actuated robots is air leaking from the structural or actuation chambers. In this thesis methods are demonstrated to detect leaks in the structural and actuation chambers of inflatable and pneumatically actuated robots. It is then demonstrated that leaks can be slowed by lowering a target pressure which affects joint stiffness to prolong the life of the system. To demonstrate the effects of lowering the target pressure it is first shown that there exists a trade-off between the commanded target pressures at steady-state and the steady-state error at the robot end effector under normal operation. It is then shown that lowering the target pressure (which is related to stiffness) can extend the operational life of the system when compressed air is a limited resource. For actuator leaks a lower target pressure for the leaking joint is used to demonstrate the trade-off between slowing the leak rate and system performance. For structural leaks a novel control algorithm is demonstrated to lower target pressure as much as possible to slow the leak while maintaining a user specified level of accuracy. The method developed for structural leaks extends the operational life of the robot. Long-term error during operation is decreased by as much as 50% of the steady-state error at the end effector when compared to performance during a leak without the control algorithm. For actuation leaks in a joint with a high-torque load the possibility of a 30% increase in operation time while only increasing steady-state error by 2 cm on average is demonstrated. For a joint with a low-torque load it is shown that up to a 300% increase in operation time with less than 1 cm increased steady-state error is possible. The work presented in this thesis demonstrates that varying stiffness may be used to extend the operational life of a robot when a leak has occurred. The work discussed here could be used to extend the available operation time of pneumatic robots. The methods and principles presented here could also be adapted for use on other types of robots to preserve limited system resources (e.g., electrical power) and extend their operation time.

Soft robotic platforms are becoming increasingly popular as they are generally safer, lighter, and easier to manufacture than their more rigid, heavy, traditional counterparts. These soft platforms, while inherently safer, come with significant drawbacks. Their compliant components are more difficult to model, and their underdamped nature makes them difficult to control. Additionally, they are so lightweight that a payload of just a few pounds has a significant impact on the manipulator dynamics. This thesis presents novel methods for addressing these issues. In previous research, Model Predictive Control has been demonstrably useful for joint angle control for these soft robots, using a rigid inverted pendulum model for each link. A model describing the dynamics of the entire arm would be more desirable, but with high Degrees of Freedom it is computationally expensive to optimize over such a complex model. This thesis presents a method for simplifying and linearizing the full-arm model (the Coupling-Torque method), and compares control performance when using this method of linearization against control performance for other linearization methods. The comparison shows the Coupling-Torque method yields good control performance for manipulators with seven or more Degrees of Freedom. The decoupled nature of the Coupling-Torque method also makes adaptive control, of the form described in this thesis, easier to implement. The Coupling-Torque method improves performance when the dynamics are known, but when a payload of unknown mass is attached to the end effector it has a significant impact on the dynamics. Adaptive Control is needed at this point to compensate for the model's poor approximation of the system. This thesis presents a method of layering Model Reference Adaptive Control in concert with Model Predictive Control that improves control performance in this scenario. The adaptive controller modifies dynamic parameters, which are then delivered to the optimizer, which then returns inputs for the system that take all of this information into account. This method has been shown to reduce step input tracking error by 50% when implemented on the soft robot.