Theses - Department of Mechanical Engineering
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Item Open Access A miniature, foldable, collision resilient quadcopter(Bilkent University, 2023-06) Bakır, AlihanIn the fields of surveillance, mapping and security, the use of unmanned aerial vehicles (UAVs), is becoming inevitable day by day, especially with their au-tonomous movement capabilities. The main reason for the increasing use of un- manned aerial vehicles is their ability to map and survey unknown and dangerous places such as caves without risking human life. At the same time, the ability to conduct aerial surveillance in full autonomy for public safety and health is also an important factor. Today, UAVs are used for many purposes, such as cave mapping and surveying, detection and intervention of forest fires, and inspection of outdoor and crowded areas for security purposes, often accompanied by an operator. Quadcopters are also used for missions that do not require long duration flight, as they are able to both hold position and ensure a highly stable flight. In addition to their many advantages, these UAVs are very sensitive to even the slightest impact, so their fully autonomous flight is still limited to controlled areas. Various studies are being conducted to increase the crash resistance of the quadcopters. Among these researches, there are different ideas such as protective shells and bumpers that surround the UAV and absorb the impact in case of collisions. The spherical cases that surround the UAV are usually in a mesh structure to be lightweight and not obstruct the airflow. Bumpers designed to protect the most sensitive parts of the UAV, such as the motors and propellers, are insufficient to protect the body. For these reasons, making different parts of the UAV from more flexible materials will eliminate the vulnerability of the UAV and increase its resistance to collisions. In this thesis, in order to increase the impact resistance of quadcopters and to ensure that they do not break, robots with some flexible and some rigid parts were tested and the results of these tests were evaluated in detail. During these tests, the effects of the compliance of the robot’s arms and the compliance of the bumpers protecting the propellers upon the impact were analyzed. In order to make this comparison, flexible and rigid robot bodies with dimensions as close as possible to each other, as well as rigid and flexible bumpers of similar size and structure were designed. The flexible bumper and body were produced by cutting PET sheets via a laser cutter and folding them in an origami-inspired pattern. This production method adds flexibility thanks to the thinness of the PET sheets and structural rigidity thanks to the origami-inspired folding technique. To in- crease the flexibility of the robot in the event of a collision and the stability of the motors, inserts made of TPU are inserted into the body. In addition to impact resilience, this thesis also discusses a soft sensor that can be attached to collision-resiliant drones. This sensor, made of conductive TPU, allows the robot to sense its surroundings by giving it a sense of touch. Thanks to the flexible sensor, robots can detect when a collision occurs and react accordingly. This sensor works entirely based on the flexibility of the UAV’s bumpers and senses the bending of these bumpers. Therefore, such a sensor cannot be used on rigid hulls and bumpers as they do not bend.Item Open Access Active lubrication interfaces with tunable micro-textures(Bilkent University, 2023-07) Pekol, SenaThis thesis investigates a homogenization-based space-time optimization framework in the context of hydrodynamic lubrication in order to design micro-textures which can be actively controlled through external stimuli. The response at the interface is established via the Reynolds equation to describe the physics of the lubrication for a small film thickness. Subsequently, the interface is subjected to multiscale analysis and effective macroscopic parameters are derived via homogenization method. In order to calculate the macroscopic parameters, Finite Element (FE) formulation is employed and the implementation of the parameters in the in-house FE code is demonstrated. For the suboptimality problem due to typically employed fixed unit cell in FE analysis, a geometry optimization scheme is developed. Thereafter, a sensitivity based topology optimization framework is introduced with the aim of identifying the spatial distribution and temporal variation of the micro-texture, and the shape of the unit cell which together help achieve the targeted lubrication response. The performance of the employed framework is assessed through objectives which ultimately determine the macroscopic flux at the interface as well as the frictional traction that is associated with the macroscopic dissipation at the interface. Finally, three-dimensional realizations are constructed for active micro-textures by adopting a readily deployable experimental architecture.Item Open Access Adaptive control of cyberphysical human systems(Bilkent University, 2021-08) Tohidi, Seyed ShahabaldinThis dissertation focuses on the control of cyberphysical human systems in the presence of actuators’ redundancy and constraints. A novel adaptive control tech-nique is proposed to allocate control signals among redundant actuators in the presence of uncertainty and actuator saturation. The proposed method does not require any uncertainty identification or persistency of excitation assumption. The stability of the proposed method is guaranteed using Lyapunov stability analysis. In addition, a modified projection operator that can be implemented to the adaptive control allocation is proposed. This operator enables the allo-cator to handle both magnitude and rate limits of actuators. A novel sliding mode controller with time-varying sliding surface is designed to complement the adaptive allocator and guarantee stability and reference tracking in the presence of uncertainty and actuator saturation. This controller is robust to both adap-tive control allocation error and external disturbance. Furthermore, an adaptive human model is proposed to mimic the human control response in the presence of uncertainty. The proposed structure is based on the model reference adaptive control, and the adaptive laws are obtained using the Lyapunov-Krasovskii stabil-ity criteria. To validate this model, an experimental setup is employed to collect data and a statistical analysis is conducted to measure the predictive power of the pilot model. Finally, the stability limits of a human-in-the-loop closed loop control system, where the plant to be controlled has redundant actuators with uncertain dynamics, are demonstrated. Various human models with and without time delays are investigated. Simulation results are provided to demonstrate the effectiveness of the proposed methods in each chapter.Item Open Access Aspects of constitutive modeling in continuum-kinematics-inspired peridynamics(Bilkent University, 2022-10) Ekiz, EkimContinuum-kinematics-inspired Peridynamics (CPD) has been recently proposed as a geometrically exact formulation of peridynamics (PD) that is also thermo- dynamically and variationally consistent. Unlike the original formulation of PD, CPD can accurately capture the Poisson effect. CPD consists of one-, two- and three-neighbor interactions. The isotropic CPD formulation of non-local elasticity therefore involves three material constants associated with length, area and volume. This manuscript aims to establish the relationships between the material parameters of CPD and isotropic linear elasticity for two- and three-dimensional problems. Two alternatives for the CPD energy density are introduced. Analytical solutions of the energy densities for affine deformations are derived. It is shown that the three material parameters of CPD reduce to two independent pa- rameters in the linearized framework, and can be expressed in terms of any pairs of isotropic linear elasticity constants, such as Lame parameters. The analysis here provides a physical interpretation for the first Lame constant. Finally, the admissible ranges for CPD material parameters are established.Item Open Access Bow tie shaped coplanar waveguide microwave resonators for single cell detection, flow rate measurements, and nanopore sensing of viruses(Bilkent University, 2020-09) Seçme, ArdaMeasurement sensitivity of different biosensing applications can be enhanced by using the microwave resonators. In the first application, microwave sensors based on bow tie shaped coplanar waveguide (CPW) resonator was designed to detect single cells in real-time. While the resonator was kept at its resonance frequency, cells/particles were made a pass through the sensing electrodes and their frequency shift statistics were obtained. For each cell, the geometrical size that is obtained from the optical microscope was correlated to the electrical volume of the cell which was measured by the microwave signals. A linear relationship was observed between the electrical and geometrical volume of a cell. Dispersion caused by the device geometry was elucidated using the standard sized polystyrene microparticles. To observe the single-cell dynamic, a target cell was trapped around the sensing region, and its microwave response was continuously recorded. Then cells were treated with dimethyl sulfoxide (DMSO), a chemical accelerating dehydration, and a decline in the resonance shifts by time was observed as the cell lost total content. Secondly, the same microwave design was patterned on a low-stress thin film membrane and used for flow rate measurements. When the flow is on, there were certain shapes continuously formed on the membrane and after a critical point pulsation of the membrane cause a shift in the resonance frequency. When the flow rate was increased, it was observed that these shapes formed faster so does frequency shifts in the resonance. Therefore, the effective flow rate could be correlated to the pulsation frequency of the membrane. Then, devices with different membrane size and different channel geometry were fabricated to span different flow rate values. As a secondary sensing mechanism, the flow was given from the reset condition where there was no flow. In this case, the amount of frequency shift was related to the flow rate and a monotonically increasing relation was obtained. As a next step, instead of liquid, the air was pressurized to measure the flow rate. Airflow measurements have become important during the COVID19 pandemic as the flow rate sensors are the most essential component of the ventilation machines. Using the secondary mechanism, frequency shifts induced by airflow were recorded and a linear relation was observed between the applied air pressure and frequency modulations. In the last application, the sensing electrodes were patterned down to hundreds of nanometer apart to detect nanoparticles and biological samples such as polystyrene nanoparticles or viruses. A nanopore having a diameter around 400 nm was drilled on the membrane using focused ion beam (FIB) and analytes were translocated using electrokinetic motion. Since the events would be quick in electrokinetic motion, data were collected with CompactRIO (cRIO), however, when the PLL was running, there were spikes in cRIO for this reason after the resonator was locked to zero degrees, LabVIEW was stopped. Yet, since the resonator has low quality factor (≈100), the phase of the resonator dwells around zero degrees and still sensitive to translocations through the pore. In the control run, there were no precipitous jumps, however, when the particles were added sudden jumps induced by the particles were recorded. Therefore, can be optimized and proposed as a biophysical sensor to characterize single viruses.Item Open Access Collision resilient foldable micro aerial robot(Bilkent University, 2019-09) Dilaveroğlu, LeventCollision management strategies are integral part of micro air vehicles for the reliability of their operation. Collision avoidance strategies require enhanced environmental and situational awareness for generating evasive maneuver trajectories. Simpler and more adaptable option is to prepare for collisions and design the physical frame around predicted collision patterns. In this work, a mechanically compliant frame design collaborating origami-inspired foldable robotics methods with protective shock absorbing or guiding elements has been proposed for a collision resilient quad-rotor UAV. General workings and mathematical model of quadrotor has been explained to inform the reader further about the quadrotor mechanics. 2D design of the foldable structure and the manufacturing process, including electronic hardware elements and software has been discussed. Control scheme, communication and operation is explained in detail to be an informative guideline for the future air vehicle projects of the Bilkent Miniature Robotics Lab.Item Open Access Concurrent design of energy management and vehicle stability control algorithms for a parallel hybrid vehicle using dynamic programming(Bilkent University, 2012) Dokuyucu, Halil İbrahimConcurrent design of controllers for a vehicle equipped with a parallel hybrid powertrain is studied. Our work focuses on simultaneously solving two automotive control problems, energy management and vehicle stability, which are traditionally considered separately. The optimal actions for the controllers are obtained by applying dynamic programming using pre-determined drive cycles. By analyzing these actions rule-based controllers are designed so that the results can be implemented on real vehicle controllers. These control algorithms calculate the desired values for the state-of-charge and the wheel slip for the vehicle and this information together with the actual data are used to supervise the subsystem controllers. Our control strategy is based on minimizing the fuel consumption and the wheel slip concurrently. The controller design problems are solved separately also and compared to the concurrent solution. Results show that promising benefits can be obtained from the concurrent approach for designing hybrid vehicles which display better fuel economy and vehicle stabilityItem Open Access A control allocation technique to recover from driver-induced oscillations(Bilkent University, 2019-12) Sarwar, AyeshaThe focus of this thesis is the study of driver induced oscillations. When the rear tires of a car are force saturated due to aggressive driver behaviour, high velocity on a slippery road, sudden steering action or heavy braking, rear end of the car tends to lose traction on the road and starts skidding. At the same time, the driver, having the direct steering control over the front two tires, feels a time delay at the response of the rear tires to the steering actions. The delay between the driver's action and the vehicle's response may eventually instigate the swinging of the rear end of the vehicle, which is generally referred to as \fishtailing". In the literature, few studies exist regarding fishtailing motion and in those studies, the theoretical background of this motion is not studied in detail. In this thesis, fishtailing motion dynamics are investigated in detail by employing a non linear vehicle model, and this motion is recreated for a specific vehicle configuration. Then, a control allocation technique is presented to recover from this driverinduced fishtailing motion. The proposed control allocation method helps the vehicle recover from these undesired oscillations by minimizing the phase shift between the commanded and realized forces and moments. The simulation results demonstrate that using the proposed method, it is possible to make the vehicle recover from driver induced oscillations even at high velocities where conventional approaches fail. For the cases of actuator failure, for example loss of tire inflation pressure, an adaptive version of the control allocation is also proposed that can stabilize the vehicle even when the actuator effectiveness is reduced significantly.Item Open Access DC-electrokinetic motion of colloidal cylinder(s) in the vicinity of a wall(Bilkent University, 2021-07) Atay, AtakanDC-electrokinetic behavior of colloidal particles in the vicinity of a conducting/non-conducting planar boundary is investigated using an inhouse boundary element method (BEM) based solver in MATLAB R environment. In the model, contribu-tion of hydrodynamic drag, electrokinetic (electrophoretic and dielectrophoretic), and colloidal forces (van der Waals and EDL) to over-all particle velocity is com-puted. The electrokinetic and colloidal forces are calculated using prescribed relations obtained from the literature. These forces are then included in the model as external forces acting on the particles. The electrokinetic (EK) forces are obtained by integrating Maxwell stress tensor (MST) over particles’ surfaces. Throughout this work, a thin EDL assumption is made. Position and velocities of the particles along with resulting flow and electric fields are computed. Over-all, results are compared with experimental observations and a general discussion regarding colloidal behavior is made.Item Open Access Design and characterization of a micro mechanical test device(Bilkent University, 2016-08) Altıntepe, ElifDevices with micro- and nano- scale components are becoming more commonplace and demand for better quantification of the properties such as Young's modulus, stiffness, and damping of small-scale components is increasing. Since these properties can differ significantly from their bulk values, their direct measurements using a micro mechanical test device is offered in the thesis. The micro-scale test device described in this thesis consists of a platform that also includes subsystems to measure stress and strain, actuation, sample fabrication and grippers to mount the samples. A notch- exure based monolithic structure is used for the device platform to provide high-resolution precise motion. A piezoelectric actuator, a force transducer, and a vibrometer are used for actuation, force measurement and velocity measurement, respectively. Finite element analyses and experiments are carried out in order to characterize the apparatus as a micro mechanical test device. Static, time-dependent cases are analyzed and its eigenfrequencies are determined. Required calibrations and drift analysis of instruments are conducted. Force and velocity relations are obtained, and results are evaluated for linearity and repeatability. Finally, operating range of proposed device is determined for use as a micro mechanical test device.Item Open Access Design and fabrication of micro end mills for the machining of difficult-to-cut materials(Bilkent University, 2016-08) Oliaei, Samad Nadimi BavilMicromilling is a cost-e ective method of fabricating miniaturized components with complex, three-dimensional features made from di cult-to-cut materials. Microcutting tools are exposed to harsh conditions during machining of such materials, which leads to short tool life and thus a ects the economics of the process. The aim of this thesis is to develop a systematic approach to the design and fabrication of high-precision micro-cutting tools. Machining characteristics of three di erent di cult-to-cut materials–stainless steel, titanium alloy, and silicon–have been investigated using experimental techniques. The results reveal the importance of interaction between tool micro geometry and work material mechanical properties. This observation leads to the development of tailored micro-end mills which are designed and fabricated based on the requirements of the specific machining task. This study also examines in detail built-up edge, an important but usually overlooked issue in micromachining of ductile materials, which a ects the process forces, tool wear, and tool deflections. The protective e ect of built-up edge has been exploited by creating micro-dimples on the tool surface using electrical discharge machining. Its positive influence on tool performance has been demonstrated. As for the micromachining of silicon, the flow of cut material around the cutting edge is paramount in tool design. A novel tool design for machining of silicon has been proposed and its e ectiveness has been validated through experiments. It has been shown that the selection of proper process parameters together with tailored tool design may increase the productivity of micromachining and improve surface quality and dimensional accuracy of micro-scale parts.Item Open Access Design of a droplet based microfluidic reactor to synthesize chitosan coated iron oxide nanoparticles(Bilkent University, 2019-06) Wahab, Malik AbdulNanoparticles possess unique structural, mechanical, thermal, optical and chemical properties which are highly dependent on their size; therefore it is important to be able to synthesize them uniformly. In general they are synthesized using conventional batch-wise techniques; however microfluidic platforms are also used because they provide precise control over reaction conditions like mixing time, temperature, concentration and improved reaction kinetics. This work is the first study where coating of magnetic nanoparticles with chitosan is realized by utilizing a microfluidic platform. These particles have potential application in targeted drug delivery due to their magnetic behavior and the possibility of carrying drug in the chitosan layer. In the past, this synthesis reaction was performed by using batch wise techniques. In this work we demonstrate the synthesis of chitosan coated nanoparticles using a droplet based microfluidic platform. PDMS devices are fabricated using conventional soft lithography technique. Droplets from two different reagents are generated using double T junction with tapered geometry. The taper angle is optimized such that both reagents generate droplets alternatively with efficiency of more than 95%. Viscosity and surface tension of both droplet phase and continuous phase is taken into account to optimize the geometry. As both reagents need to be mixed in equal proportion, flow rates are adjusted to make the spacing and size of droplets identical. Later, two consecutive droplets are merged in a pillar structure by using the fact that increasing the width of channel will slow down the droplets. Dimensions of channels are optimized so that only two consecutive droplets are merging while pillars avoid accumulation of droplets at that location. Olive oil and silicon oil are used as the continuous phase while chitosan solution and iron chloride solution are used as dispersed phases to form alternating droplets. Then ammonia solution is added as dispersed phase and it forms another droplet at a T-junction and this droplet is merged with the upcoming droplet to initiate the reaction. Synthesized nanoparticles are characterized using transmission electron microscopy (TEM) and fourier-transform infrared spectroscopy (FTIR). As a side study, hydroxyapatite nanoparticles were also synthesized using this droplet-based microfluidic system at various concentration of reactants and results are analyzed using SEM imaging.Item Open Access Design of a droplet-based microfluidic system for hybrid polymer nanoparticle synthesis(Bilkent University, 2021-12) Şahinoğlu, Osman BerkayDroplet microfluidics is advantageous in synthesizing microparticles for both confining their size to the physical dimensions of the droplet and providing a monodisperse result due to rapid mixing inside the droplets. Thusly named microreactors became the focus of the microfluidics community in the recent decade due to their superior ability to control the reaction environment. In this study, for the application of microreactors, a hybrid organic-inorganic material that became prominent in last years named polyhedral oligomeric silsesquioxane (POSS) is chosen. POSS is a polymer that, beside its hybrid nature, shows heat resistance property which made its use in protective painting applications and high radical group affinity that can be utilized to further configure its material properties. This study proposes two microreaction systems for monomer POSS with ther-mal and photopolymerization methods that aim to increase monodispersity and solve the clogging problem encountered in previous studies by introducing the oil phase in the system. Feasibility of systems was investigated numerically using COMSOL Multiphysics and analyses showed adequate heating of and mixing in microreactors. A robust post-processing procedure is proposed to remove excess oil from the sample. Measurements showed microdroplet and sub-micron particle generation where the size distribution of these particles are quantified using MATLAB. Though the use of oil in the system proved to be another challange, hexane based substitute materials are proposed for future work.Item Open Access Design, characterization, and applications of soft 3D printed strain gauges(Bilkent University, 2023-07) Özbek, DoğaThe development of soft sensors for integration into untethered miniature robots is significant for improving their environmental perception in physically challenging scenarios, such as collapsed buildings after an earthquake. The primary objective is to design and manufacture reliable soft sensors that serve as structural and sensing elements within the robots, eliminating the need for post-processing methods like data-driven learning and optimization. The soft sensors employ resistive sensing, similar to strain gauges, and are implemented on a Wheatstone bridge to convert resistive changes into voltage changes under me-chanical actuation or deformation. The study explores two categories of soft sensor designs: sheet-type and 3D shaped sensors. Sheet-type sensors are embedded in the C-legs of a soft quadruped robot (SQuad), enabling gait control, while 3D shaped sensors are structurally integrated into the robots to enhance environmental perception. Manufacturing of the soft sensors is made accessible and efficient through 3D printing technology, using conductive Thermoplastic Polyurethane (cTPU) as the printing material. Challenges arise in integrating the soft sensors into the robots while preserving their soft nature, locomotion, and agility. The thesis addresses these challenges by implementing the soft sensor concept in various robots and their parts, including the C-leg of SQuad, Modular Soft Quadruped (M-SQuad), Suspensionized Soft Quadruped (S-SQuad), Sensorized Collision Resilient Robot (SCoReR), and a tail for Reconfigurable Miniature Modular Robot (ReMBot). The soft sensors enable different functionalities to these robots, such as gait control feedback, obstacle detection, inclination detection, and collision detection, enhancing the adaptability of the robots in physically challenging environments. The thesis highlights the potential of soft 3D printed strain gauges. The ease of manufacturing and cost-efficiency of these sensors make them promising for applications in wearable robots and human-computer interfaces. Future directions are highlighted, emphasizing the need for detailed sensor characterization experiments and the development of detection algorithms to improve reliability. Additionally, a dynamic model of the coil-shaped sensors is proposed to simulate resistance changes, streamlining the design process without repetitive manufacturing iterations. As a result, this thesis presents a reliable soft sensor design, manufacturing, and integration into untethered miniature robots. The outcome of this work demonstrates the effectiveness of soft sensors in enhancing environmental perception, paving the way for innovative solutions in force measurement applications and human-computer interactions.Item Open Access Design, control, fabrication and maneuverability analysis of an untethered miniature soft robot(Bilkent University, 2019-09) Aygül, CemAs the robotics field grows, it searches for new potential workspaces to be interacting with. The conventional robotics, which involves utilization of robots made out of hard materials like metals and hard plastics, has helped humankind automatebmany different sorts of labor and such robots have been assisting the humans in various tasks. Nevertheless, some environments require very delicate interactions and adaptability of the robots to unstable elements and obstacles. A fairly new sub-field of robotics, soft robotics, arises as a very alluring area of research as it promises advancements in these particular premises beyond the conventional hard robotics. A major point of dispute arises around the term 'softness' as some specific robots achieve softness via the use of soft materials (specific polymers like PDMS) whereas although some others are made out of hard materials, they behave in a soft manner thanks to clever use of the mechanisms involved in them. The robot in this work is fully made out of soft structural materials and uses a exible circuit board. The electronics, actuators and several little connection parts are hard. The robot that is designed and constructed is a soft-hybrid robot that can be considered as a preliminary standpoint for autonomous robots to be operating in challenging working environments such as earthquake zones, pipelines, rough terrain military areas and so on. Miniature sized, it can be fitted with various sensors and communication devices in order to be used for search and rescue, surveillance and patrol missions. Its soft legs, body, and circuit enables it to overcome obstacles that conventional hard miniature robots tend to be tackled by The work I did involves mostly iterative sequences due to the challenges of modeling the soft components' behavior. Different leg mechanisms with different types of actuators are evaluated. Some of the evaluated mechanisms utilize a kinematic chain whereas the final version does not. These can be considered the novel aspects of the work done as virtually no examples in the literature use kinematic chains in soft robots. Thus, my work can be considered to be a multi-disciplinary study that involves design and fabrication of different soft locomotion mechanisms, body designs, and exible circuit boards. Finite element analyses are conducted in order to estimate differences between different leg mechanisms and soft joints. Finally, by the help of specific sensors and microcontrolling elements, the whole robot is controlled to maneuver in the desired behavior.Item Open Access Design, control, modeling, and gait analysis in miniature foldable robotics(Bilkent University, 2018-09) Askari, MohammadMiniature or micro robotic platforms are perfect candidates for accomplishing tasks such as inspection, surveillance, and hazardous environment exploration where conventional macro robots fail to serve. Such applications require these robots to potentially traverse uneven terrain, implying legged locomotion to be suitable for their design. However, despite the recent advances in the nascent eld of miniature robotics, the design and capabilities of these robots are very limited as roboticists favor legged morphologies with low degrees of freedom. This limits small robots to work with a single gait set during the design phase, as opposed to legged creatures which bene t from e cient gait modi cation during locomotion. MinIAQ, a 23 g origami-inspired miniature foldable quadruped with individually actuated legs, is designed to address such limitations. The design of the robot is unique in which a high structural integrity is achieved by transforming a single exible thin sheet into a rigid mechanical system through folding. MinIAQ's design novelties help modulate and extend the design standards of origami robots. The actuation independency of MinIAQ enables gait modi cation and exhibits maneuvering capabilities which is another novel quality for a robot at this scale. The design of the compliant four-bar legs is optimized for better foot trajectory in a newer version of the robot, MinIAQ{II, through dimensional synthesis of mechanisms. The resulting robot demonstrates signi cant improvements over its predecessor. For characterization and synchronization of the motors, custom encoders are designed to estimate speed and phase of each leg. Accordingly, a closed-loop feedback control algorithm is applied to follow an envisioned gait pattern. Towards understanding these gaits in robots with passive closed-chain legs, a comprehensive mathematical model is developed to describe the 6-DOF rigid body dynamics of MinIAQ. The proposed dynamics employs a nonlinear viscoelastic spring-damper model to estimate the feet-ground interactions. An interactive GUI is developed based on the model in MATLAB to simultaneously visualize the e ects of design parameters on performance. 3D simulation results closely match with the experiments and e ectively predict locomotion trends on at terrain. Since there is no control on foot placement in such underactuated robots, the model has given an insight into analyzing how close the actual locomotion is to the envisioned gait. This suggests that a comprehensive locomotion study with the model can lead to optimizing the gait and improve performance of miniature legged robots.Item Open Access Design, control, modeling, and locomotion analysis of a multi-legged modular miniature robot with soft backbones(Bilkent University, 2020-07) Mahkam, NimaSoft Modular Legged roBot (SMoLBot) is a legged, foldable, modular, miniature robot with soft backbones. SMoLBot’s body and locomotion mechanisms are folded out of acetate sheets and its compliant connection mechanisms are molded from Polydimethylsiloxane (PDMS). High maneuverability and smooth walking pattern can be achieved in miniature robots if high stiffness kinematic parts are connected with compliant components, providing the robot structural compliance and better adaptability to different surfaces. SMoLBot is exploiting features from origami-inspired robots and soft robots, such as low weight and low cost foldable rigid structures and adaptable soft connection mechanisms made of PDMS. Every single module in SMoLBot is actuated and controlled by two separate DC motors, that enable gait modification and a higher degree of freedom on controlling the motion and body undulation of the robot in turning and rough terrain locomotion. Each module has 44.5 mm width, 16.75 mm length, and 15 mm height, which is approximately the same size as two DC motors and a Li-Po battery. The dynamic formulation of SMoLBot is obtained using Newton-Euler formulation and it depends on the physical parameters of the contact and closed-chain kinematic analysis of the feet. The dynamic model framework is proposed by determining the dynamic locomotion parameters of each module as an individual system, as well as, considering the dynamics of the whole robot; i.e. the robot is modeled as one system and modules are considered to be set of flexible links connected to each other, within this system. Kinematic constraints among these modules are obtained by considering the types of backbones integrated in the robot. Various types of backbones are used within the experiments that are classified into two groups: rigid, and compliant backbones. Experimental results of SMoLBot running/walking with different symmetrical and asymmetrical gates validate the dynamic model presented in this thesis. Additional to the dynamic model, the effect of the backbone stiffness on the locomotion of the legged miniature modular robots with multiple numbers of modules is studied. Analyses comparing the velocity of SMoLBot with different numbers of modules and different types of backbones are presented using the proposed dynamic model. The results indicate that there is an optimum torsional stiffness of the backbone for a legged miniature modular robot that maximizes the robot’s translational velocity. Additionally, we can show that, for a given backbone stiffness or a specific range of compliance between the modules, there is an optimum number of feet for the miniature robots. Furthermore, in this thesis, a locomotion study that investigates the motion patterns of the running/walking multi-legged modular miniature robots with soft module connections, is conducted. The locomotion study is done using the presented dynamic model and results are verified using SMoLBot. The optimum feet sequence and the optimum stride length of a multi-legged robot are derived using the locomotion analyses, and the dynamic and kinematic formulations. The optimum gait analysis of the multi-legged SMoLBots represents different but unique feet contact sequence patterns for each robot with a different module number and diverse ranges of compliance between the modules. Furthermore, analysis considering the effect of various feet failure cases on the locomotion of a multilegged robot with soft/rigid backbones, is conducted. This study investigates the locomotion behavior of a legged miniature robot with different combinations of the non-functioning feet. Additionally, a case-sensitivity study of an n-legged SMoLBot’s locomotion on its individual modules during the operation, is also conducted. This study investigates the modular robot’s locomotion with multiple different failure cases where each particular case only considers the effect of an individual module failure on the overall motion of the robot, while the gate is not altered.Item Open Access Design, development and performance evaluation of a three-axis miniature machining center(Bilkent University, 2011) Korkmaz, EmrullahThere is a growing demand for highly accurate micro-scale parts from various industries including medical, biotechnology, energy, consumer, and aerospace. Mechanical micro-machining which is capable of fabricating three dimensional micro-scale features on a wide range of engineering materials such as metals, polymers, ceramics and composites is a viable micro-manufacturing technique to effectively address this demand. Miniature machine tools (MMTs) are developed and used in mechanical micro-machining since their small size improves the accuracy and efficiency of the process. The output quality of the final product manufactured on an MMT depends on choosing the optimum machining parameters. However, the full potential of micro-machining can not be achieved due to challenges that reduce the repeatability of the process. One of the most significant challenges in micro-machining is the deterioration of output quality due to the MMT vibrations. This thesis demonstrates the development of a threeaxis miniature machine tool, the performance evaluation of its micro-scale milling process, and the characterization of its dynamic behaviour using finite element simulations and experiments. The MMT is designed and constructed using precision three-axis positioning slides (2 micrometers positioning accuracy, 10 nanometers positioning resolution, 60 mm x 60 mm x 60 mm workspace), miniature ultra-high speed spindles (ceramic bearing electrical spindle with maximum 50,000 rpm rotational speed and air bearing air turbine spindle with maximum 160,000 rpm rotational speed), a miniature force dynamometer, and a microscope. Three dimensional finite element simulations are performed on the developed MMT to obtain the static and dynamic characteristics of the spindle side. A maximum static deflection of 0.256 µm is obtained on the designed base when 20 N forces in three directions are applied to the center of the spindle. Dynamic finite element analysis predicts the first three natural frequencies as 700 Hz, 828 Hz and 1896 Hz; hence corresponding spindle speeds should be avoided for successful application of micro-machining. To demonstrate the capability of MMT for manufacturing three dimensional (3D) features, micro-milling is proposed as a novel method for fabricating Poly(methyl methacrylate) (PMMA) and poly(lactic-co-glycolic acid) (PLGA) polymer micro-needles. The micro-machinability of PMMA and PLGA polymers is investigated experimentally by machining a group of 3 mm length and 100 µm depth slots using 50,000 and 100,000 rpm spindle speeds with different feedrates (5, 10, 15, and 20 µm/flute). The micro-machinability study concludes that PLGA has better machinability than PMMA. It is also observed that the machining parameters of 50,000 rpm spindle speed and 20 µm/flute feedrate give better output quality. Using these machining parameters, micro needles with different geometries are successfully manufactured from PMMA and PLGA polymers. During this study, it is observed that polymer pillars bend due to machining forces and vibrations, which causes dimensional errors. To address the deterioration of the output quality due to vibrations stemming from machining forces and high-speed-rotations, MMT vibrations particularly focusing on the spindle side dynamics are investigated experimentally using runout (spindle axis offset) measurements and experimental modal analysis techniques. The results are compared with those from three-dimensional finite element simulations. The investigation of MMT vibrations indicates that the developed MMT is convenient for accurate applications of micro-machining using air-turbine air bearing spindle. However, the selection of the operation frequencies for electrical spindle is challenging at certain speeds with this design because most of the critical natural frequencies of the developed MMT appear in the operating frequency range of electrical spindle. Runout measurements using two laser doppler vibrometer (LDV) systems and experimental modal analyis which utilizes an impact hammer and accelerometer are conducted to obtain spindle side dynamics. Runout measurements performed on the miniature ultra-high speed ceramic bearing electrical spindle show that both magnitude and shape of the runout errors vary considerably with spindle speed. A peak of 1.62 µm synchronous runout is observed at 15,000 rpm. Asynchronous runout errors become significant between spindle speeds of 40,000 and 50,000 rpm and reach to a maximum of 0.21 µm at 45,000 rpm. On the other hand, experimental modal analysis is conducted to obtain both the steady-state and speed dependent frequency response functions (FRFs) of the mechanical structures. Steady state FRFs indicate that 750 Hz and 850 Hz are two important natural frequencies for successful application of micro-machining. Compared to the three dimensional finite element simulations, there is 7 % difference for the first mode and 3 % difference for the second mode. Both steady-state experimental modal analysis and finite element simulations could not consider the speed-dependent dynamics. Therefore, experimental modal analysis at different spindle speeds is also performed and it is concluded that natural frequencies of the mechanical structures change significantly depending on spindle speed. Speed-dependent FRFs show that the maximum response of about 0.35 µm/N is obtained while the spindle is rotating at 16,000 rpm but the peak occurs at 24,000 rpm (400 Hz). In addition, the vibration amplitude grows between the spindle speed of 40,000 rpm and 50,000 rpm. Experiments and finite element simulations provide a machine operation frequency selection guide. It is suggested to avoid two different spindle speed ranges (15,000- 25,000 rpm and 40,000-50,000 rpm) to prevent vibration related inaccuracies. In addition, structural modifications can be achieved to further optimize the design based on the experimental data obtained in this work. The obtained experimental data can be used to derive mathematical model of the MMT and to perform stability studies to increase the productivity of the micro-machining processes. Overall, the novel micro-machining technique tested on the developed MMT highlights the quality and ranges that can be achieved in micro-manufacturing.Item Open Access Design, fabrication, and applications of electrostatically buckled nanomechanical systems(Bilkent University, 2018-08) Erbil, Selçuk OğuzBuckling is an important resource for memory and sensing applications at the micro- and nano-scale. Although di erent approaches have been developed to access buckling, such as the use of pre-stressed beams or thermal heating, none of them can dynamically and precisely control the critical bifurcation parameter |the compressive stress on the nanobeam| while keeping the heat generation and power dissipation at levels acceptable for real-life applications. Here, we develop an all-electrostatic architecture to control the compressive force, as well as the direction and amount of buckling, without heat generation. The devices, consisting of contact pads, comb-drive and beam, have been fabricated on Silicon on Insulator (SOI) chip by using micro-/nano-fabrication techniques. With this architecture, we demonstrated fundamental aspects of device function and dynamics. By applying signal voltages as low as 0.5 V, we controlled the direction of buckling to store binary information. Lateral de ections as large as 12% of the beam length were achieved, allowing nanomechanical manipulations at large deformations. We performed fatigue tests on the device which showed no discernible damage even after 10,000 buckling cycles. By modulating the compressive stress and lateral electrostatic force acting on the beam, we tuned the potential energy barrier between the post-bifurcation stable states and observed persistent transitions between the states. The proposed architecture, in this work, opens avenues for developing DC-controlled multibit nanomechanical logic gates, nano-manipulators, switches, and for studying the relationship between entropy and information.Item Open Access Design, fabrication, and applications of multi-mode nanoelectromechanical systems(Bilkent University, 2017-07) Arı, Atakan BekirMiniaturization of systems allowed wide spread consumer use of microelectronics, integrated circuits and MEMS based sensors. Thanks to the advancement in microfabrication methods, it is possible to build structures with submicron dimensions. The integration of electronic control to these submicron structures started the NEMS eld. Due to their minuscule dimensions and very high frequency response, NEMS can sense external perturbations with unprecedented sensitivity. This made NEMS excellent candidates for sensor applications. NEMS are starting to evolve from academic research tools to become mass produced and large scale integrated sensing devices. Information extracted from the higher order modes further increase the capabilities of NEMS. In order to attain this extra information, we fabricated NEMS that can reach higher order mechanical modes. Every step of fabrication was done at Bilkent University research facilities such as UNAM and ARL. To pattern the submicron feature sizes, we relied on electron beam lithography. Thermal and electron beam evaporators were deployed for metallization of contacts and etch mask. In order to suspend the doubly clamped beams, we developed anisotropic silicon nitride and isotropic silicon dry etch recipes. At each step of the fabrication, tools such as SEM and stylus pro lometer was utilized for characterization. Fabricated NEMS were wirebonded to printed circuit boards for detection. Electrothermal actuation, an integrated method, was chosen to drive the nanomechanical resonator to its higher order modes. Piezoresistive down-mixing, another integrated method to complement the actuation, was used to detect the resulting nanomechanical motion. We used high frequency electronic equipment to detect RF range responses of our NEMS. Using these NEMS, we studied two novel applications on intermodal and mechanical coupling. First, we investigated intermodal coupling e ect of doubly clamped beams in order utilize this coupling e ect in higher order mode detection. When a doubly clamped beam is excited at its resonance frequency, every other mode of the device gets tuned. This occurs due to the clamping on both sides preventing longitudinal elongation and causing a stress on the beam. Using intermodal coupling method, we probed higher order modes of a nanomechanical resonator while tracking the fundamental frequency at the same time. We were able to detect mechanical modes up to 840 MHz, well out of the detection limit of our setup. We propose intermodal coupling as a novel detection method to acquire frequency response of NEMS at higher order modes which can not be detected with conventional methods. Finally, we studied nano scale energy sinks that absorb energy from a another structure. Energy sinks are linear oscillators that can trap the energy of a nearby structure within their phase space. When the natural frequency of these sinks are distributed optimally, nite number of sinks can mimic absorption of in nite sinks. We envisioned a real time dissipation controlled NEMS platform by deploying energy sinks. In order to test energy sink performance at nano scale, we devised an experimental setup, comparing identical nanomechanical resonators with and without energy sinks. We have shown that energy sinks successfully absorb energy of a resonator at nanoscale.