| 1. |
- Ji, Qinglei, 1993-
(author)
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Learning-based Control for 4D Printing and Soft Robotics
- 2022
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Doctoral thesis (other academic/artistic)abstract
- Exploiting novel sensors and actuators made of flexible and smart materials becomes a new trend in robotics research. The studies on the design, production, and control of the new type of robots motivate the research fields of soft robots and 4D printed robots. 3D Printing (3DP) is an additive manufacturing technology that is widely used in printing flexible materials to fabricate soft robots. 4D Printing (4DP) combines 3DP technologies with smart materials to produce transformable devices. 4DP first prints structures with specifically designed responsive materials. When external stimuli such as temperature, voltage, or magnetic field are applied to the printed structure, it changes shape in a programmable way. The shape morphing property of 4DP makes it a novel approach to the actuators of robots.The employment of these special materials empowers these new robots with better compliance and adaptability to the working environment. However, compared with the rigid counterparts, they also have complex dynamic properties such as substantial non-linearity and time-variance. These factors make the precise modeling and robust control of these new robots challenging and thus hinder their potential applications. Focusing on soft robotic systems enabled by 3DP and 4DP approaches, this dissertation studies both traditional and Machine Learning (ML)-based approaches to the modeling, perception, and control of soft, non-linear, and time-variant robotic systems. The main contributions of this dissertation are:The scheme of Closed-Loop (CL) controlled 4DP (CL4DP) using temperature stimulated Shape Memory Polymer (SMP) is designed and validated numerically and experimentally. The feedback control system increases the precision and robustness of the shape morphing process of 4D printed SMP. Applications of CL4DP are explored.Data-driven model identification methods are applied to learn the dynamic model of the shape morphing process of CL4DP and the learned model has good quality to support model-based control design. Model-free and adaptive Reinforcement Learning (RL) controllers are developed to deal with the non-linearity and time variance of 4D printed actuators. To improve the stability and quick adaptability, a concise basis function set is selected instead of blindly using Deep Neural Networks (DNNs).A quadruped robot enabled by soft actuators and its simulation model are developed. The computation efficiency and model accuracy of the simulator are studied and optimized by comparing different simulation methods such as Finite Element Method (FEM) and lumped parameter method.The optimal walking gait pattern of a soft-legged quadruped robot is found by grid parameter search and RL with a physics based simulation model. To speed up the RL training process, modeling tricks are used to reduce the simulation time of the model and curriculum learning is used to reduce the learning time.A soft sensor made by printable conductive materials and 3DP is designed and optimally calibrated to estimate the shape of a pneumatically driven soft actuator. The geometry of the soft sensor is optimally designed for the best linearity, hysteresis and drift properties. The online estimation is based on a linear regression model learned from experimental data.A pneumatically driven soft gripper is developed by 3DP, the printable soft sensor, and pole-placement control methods. The operation of the gripper does not require an external image feedback system to measure its shape, which is estimated by the integrated soft sensor. The position feedback by the soft sensor and the controller by the pole-placement method enable the soft gripper to perform complex tasks with high precision.
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| 2. |
- Wang, Lei, 1985-
(author)
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Artificial Photosynthesis: Molecular Catalysts for Water Oxidation
- 2015
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Doctoral thesis (other academic/artistic)abstract
- Artificial photosynthesis provides a promising solution to the future sustainable energy system. Water is the only suitably sufficient protons and electrons supplier by the reaction of water oxidation. However, this reaction is both kinetically and thermodynamically demanding, leading to a sluggish kinetics unless the introduction of a catalyst.The theme of this thesis is to design, synthesize and evaluate molecular catalysts for water oxidation. This thesis consists of seven parts:The first chapter presents a general introduction to the field of homogenous catalysis of water oxidation, including catalysts design, examination and mechanistic investigation.The second chapter investigates the electronic and noncovalent-interaction effects of the ligands on the activities of the catalysts.In the third chapter, halogen substitutes are introduced into the axial ligands of the ruthenium catalysts. It is proved that the hydrophobic effect of the halogen atom dramatically enhanced the reactivity of the catalysts.Chapter four explores a novel group of ruthenium catalysts with imidazole-DMSO pair of axial ligands, in which the DMSO is proved to be crucial for the high efficiency of the catalysts.Chapter five describes the light-driven water oxidation including the three-component system and the sensitizer-catalyst assembled system. It is found that the common Ru(bpy)32+ dye can act as an electron relay and further benefit the electron transfer as well as the photo-stability of the system.In chapter six, aiming to the future application, selected ruthenium catalysts have been successfully immobilized on electrodes surfaces, and the electrochemical water oxidation is achieved with high efficiency.Finally, in the last chapter, a novel molecular catalyst based on the earth abundant metal ―nickel has been designed and synthesized. The activities as well as the mechanism have been explored.
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| 3. |
- Wang, Lei
(author)
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Experimental Studies of Separated Flow and Heat Transfer in a Ribbed Channel
- 2007
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Doctoral thesis (other academic/artistic)abstract
- The main concern of this thesis is to experimentally investigate the turbulent flow and heat transfer in a ribbed channel. Such flows are encountered in many engineering applications, e.g., gas turbine cooling. In order to highlight the physical mechanism of flow separation, only one wall of the channel is fitted with periodic ribs. The inter-rib spacing is set such that the reattachment is allowed to take place on the portion between consecutive ribs and a distinct post-reattachment-redevelopment region is introduced prior to a re-separation over the next rib. With aid of liquid crystal thermography (LCT) and particle image velocimetry (PIV), respectively, the temperature and velocity fields can be obtained with high resolution. Measurement of local heat transfer coefficients shows that the flow separation induced by ribs greatly enhances the heat transfer rate in comparison with the smooth channel. In addition, it is found that the position of the maximum Nusselt number corresponds well to the reattachment point. In the corner immediately downstream of a rib, however, the heat transfer coefficients decrease sharply because the fluid is almost stagnant and heat conduction is dominant. In the recovery region, the agreement between the Nusselt number and skin-friction coefficient is poor, which indicates that the Reynolds analogy fails to predict the heat transfer coefficients. In the re-separation region downstream reattachment, the heat transfer rates rise again due to strong turbulent momentum exchanges from mainstream flow to the inner boundary layer. The separated shear layer is dominated by the roller vortices that are generated due to the Kelvin-Holmholtz instability. Similar to a plane-mixing layer, the growth rate of the separated shear layer is linear with respect to the streamwise direction. Moreover, it is found that the inflection point plays a crucial role in the production of turbulence. Near the reattachment point three interesting features are characterized; first, the maximum shear stresses decay rapidly just downstream of reattachment; second, the anisotropy parameter is close to unity; third, the flatness factor is high. In addition, the two-point correlations are presented to give the spatial structure of the flow. In order to visualize the coherent structures, which are the backbones of turbulent motions, various decomposition methods are employed in this thesis, including Reynolds decomposition, Galilean decomposition, large eddy simulation (LES) decomposition and proper orthogonal decomposition (POD). Among these methods, the proper orthogonal decomposition is discussed in detail. The result shows that the leading edge and the separated shear layer contain most of the turbulent kinetic energy. In the less energetic eigenmodes the coherent structures disappear gradually and turbulence becomes homogeneously chaotic. This means the symmetries are restored in the less energetic POD modes. List of papers 1. Lei Wang, Jiri Hejcik, and Bengt Sundén, PIV Measurement of Separated Flow in a Square Channel with Streamwise Periodic Ribs on One Wall, accepted for publication in ASME J. Fluids Engineering, to appear July 2007, Vol. 129. 2. L. Wang, S. S. Burgers, M. M. J. F. Verbeek, and B. Sundén, Experimental Investigation of Flow Fields in a Square Channel Roughened with Various Ribs on One Wall, Proceedings of 2006 ASME International Mechanical Engineering Congress and Exposition, IMECE2006?13659. 3. Lei Wang and Bengt Sundén, (2005). Experimental Investigation of Local Heat Transfer in Square Duct with Continuous and Truncated Ribs, Experimental Heat Transfer, Vol. 18, pp. 179-197. 4. Lei Wang and Bengt Sundén, (2007). Experimental Investigation of Local Heat Transfer in a Square Duct with Various-Shaped Ribs, J. Heat Mass Transfer, Vol. 43, pp. 759-766.
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| 4. |
- Wang, Lei, 1989-
(author)
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Protein Nanomaterials: : Functionalization, Self-assembly, and Applications
- 2020
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Doctoral thesis (other academic/artistic)abstract
- As one of the major classes of materials that is relevant to biological function of our daily life, proteins are highly interesting in both biological and material science. Self-assembled protein amyloid nanostructures have been considered not only as aggregates in pathological tissue, but also as a kind of advanced one dimension materials in material science perspective due to their favorable characteristics including high aspect ratio, abundant surface charge groups, high stability, and tunable surface properties. The Protein nanofibrils (PNFs) can be self-assembly derived from a wide range of proteins, which isolated from natural and renewable sources, means it is relative cheap, environmentally friendly, and sustainable. The PNFs will contain additional functionalized properties for further applications by functionalizing with other materials such as fluorophores or conducting materials. An easy method is to utilize mechanochemistry, such as the use of a shaker mixer mill for grinding operation as well as hand grinding by mortar and pestle, helping mixing materials into fine powder thus helping the insoluble compounds and protein mixture to be water dispersible. Also, the liquid assisting ball milling exfoliation was achieved by high impact force to fracture the graphite and shear force to exfoliate the layered structure. In this thesis, interesting new properties of protein hybrids have been studied mainly focusing on two aspects: 1) by co-grinding hydrophobic dyes and proteins, a protein hydrophobic compound hybrid is obtained and following by inducing fibrils formation. The resulting functionalized PNFs thus have the fibril structure properties as well as the properties from incorporated compounds. By further self-assembly the functionalized PNFs to films, the materials transfer from micro to macrostructure. Besides, the protein act as surfactant for disperse hydrophobic probes for detection of Cu2+. 2) by milling the protein or protein nanofibrils dispersion with graphite, Graphene nanoplatelets (GNPs) is exfoliated and the GNPs ink functionalized by PNFs converted to devices and shows good properties for thermoelectrical voltage generation and water evaporation induced energy generation. Throughout the study of the thesis, we summarize how the protein hybrid materials was investigated. By demonstrating dyes functionalized PNFs and further PNFs films, as well as GNPs-PNFs hybrids acting as active materials on thermoelectrical and evaporation induced energy generating devices, we show the protein hybrid materials a promise new breakthrough in optical or energy generating aspects.
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| 5. |
- Wang, Yang, 1991-
(author)
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A Position Control-based Approach to Stiff Objects Haptic Rendering
- 2021
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Doctoral thesis (other academic/artistic)abstract
- With electronic components and computational power becoming more portable and available to general consumers, applications like virtual reality (VR) and mixed reality (MR) are on the rise and renovating the industry of training simulators. Though it is not fully accepted in surgical robots, haptic force feedback can be safely introduced into various surgical training simulators to convey the necessary haptic cues needed to develop hand-eye coordination and motor skills. However, with the current generation of haptic device hardware, it is still challenging to render high stiffness virtual objects while stability allows. Passivity-based approaches are commonly used to ensure stability but they may lead to conservative behavior. Therefore, new approaches are needed to explore other ways of rendering high stiffness without losing stability. This thesis proposes a position control-based haptic rendering approach that designs a position controller to do force computation for the rendering of high-stiffness objects. Instead of computing a penalty force signal that mimics the desired interaction and softly constrains the objects to have little penetration, this proposed approach aims to achieve no penetration with the help of a position controller and sensors. The approach can be easily adapted and applied on any haptic device since it relies on the building of an implicit simulation model which can be linearized at one or several operating points to describe the full dynamics of the system. Different user scenarios, disturbance signals, and evaluation metrics are also proposed to assess and compare the performance with other common approaches. The results show that the proposed approach can stably render a virtual wall with stiffness higher than the common way of modeling the wall as a spring-damper system. By using a piece-wise linear model generated based on multiple operating points and a gain scheduling controller with linear interpolation, the performance of the system is further improved.
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| 6. |
- Wang, Yong-Lei, 1983-
(author)
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Electrostatic Interactions in Coarse-Grained Simulations : Implementations and Applications
- 2013
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Doctoral thesis (other academic/artistic)abstract
- Electrostatic interactions between charged species play a prominent role in determining structures and states of physical system, leading to important technological and biological applications. In coarse-grained simulations, accurate description of electrostatic interactions is crucial in addressing physical phenomena at larger spatial and longer temporal scales.In this thesis, we implement ENUF method, an abbreviation for Ewald summation based on non-uniform fast Fourier transform technique, into dissipative particle dynamics (DPD) scheme. With determined suitable parameters, the computational complexity of ENUF-DPD method is approximately described as O(N logN). The ENUF-DPD method is further validated by investigating dependence of polyelectrolyte conformations on charge fraction of polyelectrolyte and counterion valency of added salts, and studying of specific binding structures of dendrimers on amphiphilic membranes.In coarse-grained simulations, electrostatic interactions are either explicitly calculated with suitable methods, or implicitly included in effective potentials. The effect of treatment fashion of electrostatic interactions on phase behavior of [BMIM][PF6] ionic liquid (IL) is systematically investigated. Our systematic analyses show that electrostatic interactions should be incorporated explicitly in development of effective potentials, as well as in coarse-grained simulations to improve reliability of simulation results.Detailed image of microscopic structures and orientations of [BMIM][PF6] at graphene and vacuum interfaces are investigated by using atomistic simulations. Imidazolium rings and alkyl side chains of [BMIM] lie preferentially flat on graphene surface. At IL-vacuum interface, ionic groups pack closely together to form polar domains, leaving alkyl side chains populated at interface and imparting hydrophobic character. With the increase of IL filmthickness, orientations of [BMIM] change gradually from dominant flat distributions along graphene surface to orientations where imidazolium rings are either parallel or perpendicular to IL-vacuum interface with tilted angles. The interfacial spatial ionic structural heterogeneity formed by ionic groups also contributes to heterogeneous dynamics in interfacial regions.
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