Dept. of Mechanical Engineering - Master's degree
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Item Open Access Development of a supervisory controller for energy management problems(Bilkent University, 2011) Akgün, EmreMulti energy source systems, like hybrid electric vehicles in automotive industry, started to attract attention as a remedy for the greenhouse gas emission problem. Although their environmental performances are better than conventional technologies such as the case of gasoline vehicles versus hybrid electric vehicles in automotive industry, their operational management can be challenging due to their increased complexity. One of these challenges is the operational management of the energy flow among these multiple sources and sinks which in this context referred as the energy management problem. In this thesis, a supervisory controller is developed to operate at a residential environment with multiple energy sources. First, dynamic optimization techniques are applied to the available mathematical models of the multi-energy sources to create a non-causal optimal controller. Then, a set of implementable rules are extracted by analyzing the optimal trajectories resulted from the dynamic optimization to create a causal supervisory controller. Several simulations are conducted with Matlab/Simulink to validate the developed controller. The supervisory controller achieves not only a daily cost reduction between 6-7.5% compared to conventional energy infrastructure used in residential areas but also performs 2% better than heuristic control techniques available in the literature. Another simulation study is conducted, with different demand cycles, for verification of the controller. Although its performance reduces as expected, it still performs 1% better than heuristic control strategies. In the final part of this thesis, the formulation used in the residential problem which was originally adopted from an example in automotive industry, is generalized so that it can be used in all types of energy management problems. Finally, for exemplary purposes, a formulation for energy management problem in mobile devices is created by using the developed generic formulation.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 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 Mechatronic design of a modular three-axis slider system for high-precision positioning applications(Bilkent University, 2012) Ulu, ErvaFollowing the recent improvements in precision engineering related technology, interest for micro/nano-engineering applications are increased and various micro/nano-scale operations and products are developed. For micro/nano-scale applications, high-precision equipment including micro/nano-positioning devices with high accuracy and precision are required. In this thesis, mechatronic design of a three axes micro/nano-positioning device is discussed in detail. In order to satisfy nanometer level precision, an adaptive method to increase the available measurement resolution of quadrature encoders is presented. Performance characteristics of micro/nano-positioning devices usually include positioning accuracy of their each individual axis, operation range, maximum velocity and maximum acceleration. For this reason, permanent magnet linear motors (PMLM) are chosen as actuators in the presented design due to their outstanding characteristics. Moreover, in order to provide high-flexibility in terms of applications and simplify the control of the system, modularity is one of the main concerns while designing the micro/nano-positioning system presented here. Building the modular single axis slider system, three axes positioning device is constructed by assembling three of them perpendicularly. In this design, linear optical encoders are used as feedback sensors. Movement range of the designed system is 120mm in each direction. Since the available linear optical encoders have measurement resolution of 1µm, resolution of them is to be improved in software for sub-micron level positioning applications. For this purpose, a new method to increase the available measurement resolution of quadrature encoders is presented in this thesis. This method features an adaptive signal correction phase and an interpolation phase. Imperfections in the encoder signals including amplitude differences, mean offsets and quadrature phase shift errors are corrected by using recursive least squares (RLS) with exponential forgetting and resetting. Interpolation of the corrected signals is accomplished by a quick access look-up table calculated offline to satisfy linear mapping from available sinusoidal signals to higher order ones. With the conversion of the high-order sinusoids to binary pulses, position information is derived. By using the presented method, 10nm measurement resolution is achieved with an encoder with 1µm off-the-shelf resolution. Experiment results demonstrating the effectiveness of the proposed method are presented. Validation of the method is accomplished for several cases including the best resolution obtained. Practical constraints limiting the maximum interpolation number are also discussed in detail.Item Open Access Development of a modular control algorithm for high precision positioning systems(Bilkent University, 2012) Ulu, Nurcan GeçerIn the last decade, micro/nano-technology has been improved significantly. Micro/nano-technology related products started to be used in consumer market in addition to their applications in the science and technology world. These developments resulted in a growing interest for high precision positioning systems since precision positioning is crucial for micro/nano-technology related applications. With the rise of more complex and advanced applications requiring smaller parts and higher precision performance, demand for new control techniques that can meet these expectations is increased. The goal of this work is developing a new control technique that can meet increased expectations of precision positioning systems. For this purpose, control of a modular multi-axis positioning system is studied in this thesis. The multiaxis precision positioning system is constructed by assembling modular single-axis stages. Therefore, a single-axis stage can be used in several configurations. Model parameters of a single-axis stage change depending on which axis it is used for. For this purpose, an iterative learning controller is designed to improve tracking performance of a modular single-axis stage to help modular sliders adapting to repeated disturbances and nonlinearities of the axis they are used for. When modular single-axis stages are assembled to form multi-axis systems, the interaction between the axes should be considered to operate stages simultaneously. In order to compensate for these interactions, a multi input multi output (MIMO) controller can be used such as cross-coupled controller (CCC). Cross-coupled controller examines the effects between axes by controlling the contour error resulting in an improved contour tracking. In this thesis, a controller featuring cross-coupled control and iterative learning control schemes is presented to improve contour and tracking accuracy at the same time. Instead of using the standard contour estimation technique proposed with the variable gain cross-coupled control, presented control design incorporates a computationally efficient contour estimation technique. In addition to that, implemented contour estimation technique makes the presented control scheme more suitable for arbitrary nonlinear contours and multi-axis systems. Also, using the zero-phase filtering based iterative learning control results in a practical design and an increased applicability to modular systems. Stability and convergence of the proposed controller has been shown with the necessary theoretical analysis. Effectiveness of the control design is verified with simulations and experiments on two-axis and three-axis positioning systems. The resulting controller is shown to achieve nanometer level contouring and tracking performance.Item Open Access Elastic impact of a pendulum on frictional surface(Bilkent University, 2012) Birlik, Seyit CanConstrained impacts with friction frequently exist in mechanical systems such as robotic arms, hard disk drives and other mechanisms. Such discontinuous contacts, if not designed and analysed properly, can lead to malfunctions. In particular, for the analysis of problems that involve eccentric collisions and reversal of friction force, use of stereomechanical impact theory with coefficient of restitution can produce paradoxical energy increase. Alternatively, continuum models, which provide more detailed analysis for such problems, can be used, however they are computationally tedious. Instead, here, contact is described by compliant elements with friction and applied to a physical pendulum. In this thesis, impact-momentum relations for general three-dimensional free collisions are modified for a pendulum which exemplifies an impact with friction and constraint. Inclusion of tangential compliance to model enables the model to demonstrate tangential force reversals and their transition between stick and slip, which is demonstrated using a sphere and a slender rod obliquely colliding with a rough massive plane. Use of compliant elements to describe impact by a planar pendulum produces differences in the behavior of a constrained system compared with free impacts. For instance, in free collisions an impact that starts with an initial sticking, is always followed by sliding. However, in a pendulum if the contact begins by sticking, it continues to stick throughout the duration of impact. Another difference appears when contact starts with an initial sliding. In free impact, sliding is followed by sticking and sliding, then the body rebounds unless the collision is inelastic. However, in the constrained case wedging of the pendulum is observed if initial angle of collision is below a critical value for a specified friction coefficient.Item Open Access Injection molding of polymeric microfluidic devices(Bilkent University, 2013) Koska, Arif KorayMass-production of microfluidic devices is important for fields in which disposable devices are widely used such as clinical diagnostic and biotechnology. Injection molding is a well-known, promising process for the production of devices on a mass-scale at low-cost. The major objective of this study is to develop a technique for repeatable, productive and accurate fabrication of integrated microfluidic devices on a mass production scale. To achieve this, injection molding process is adapted for the fabrication of a microfluidic device with a single microchannel. During the design procedure, numerical experimentation was performed using Moldflow® simulation tool. To increase the product quality, high-precision mechanical machining is utilized for the manufacturing of the mold of the microfluidic device. A conventional injection molding machine is implemented for the injection molding process of the microfluidic device. Injection molding is performed at different mold temperatures. The warpage of the injected pieces is characterized by measuring the part deformation. The effect of the mold temperature on the quality of the final device is assessed in terms of part deformation and the bonding quality. From the experimental results, one-to-one correspondence between the warpage and the bonding quality of the molded pieces is observed. As the warpage of the pieces decresases, the bonding quality increases. A maximum point for the breaking pressure of the bonding and the minimum point for the warpage was found at the same mold temperature. This mold temperature was named as the optimum temperature for designed microfluidic device. The experimental results are also used to discuss the assessment of the simulation results. It was observed that although Moldflow® can predict many aspects of the process, all the physics of the injection molding process cannot be covered.Item Open Access Modeling of cutting forces in micro milling including run-out(Bilkent University, 2014) Kanlı, MuammerMicro milling is widely used in precision manufacturing industry which is suitable for producing micro scale parts having three dimensional surfaces made from engineering materials. High material removal rate is its main advantage over other micro manufacturing technologies such as lithography, micro EDM, laser ablation etc. Modeling of micro milling process is essential to maximize material removal rate and to obtain desired surface quality at the end of the process. The first step in predicting the performance of micro milling process is an accurate model for machining forces. Machining forces are directly related to machine tool characteristics where the process is performed. The spindle and the micro milling tool affects the machining forces. In this thesis, the influence of runout of the spindle system on micro milling forces is investigated. Two different spindle systems with different levels of runout are considered and necessary modifications are introduced to model the trajectory of the tool center for better prediction of process outputs in the presence of runout. A modified mechanistic force modeling technique has been used to model meso/micro scale milling forces. Detailed micro milling experiments have been performed to calculate the cutting and edge force coefficients for micro end mills having diameters of 2, 0.6, and 0.4 mm while machining titanium alloy Ti6AL4V. Good agreements have been observed between the predicted and measured forces. It is found that statically measured runout values do not translate into dynamic machining conditions due to machining forces acting on the end mill.Item Open Access Microfluidic device with 3D electrode structure for high throughput dielectrophoretic applications(Bilkent University, 2014-10) Zeinali, SoheilaMicrofluidics is the combination of micro/nano fabrication techniques together with knowledge of fluid behavior at the microscopic level to pursue powerful techniques in controlling, manipulating and measuring chemical, physical and biological processes at micro/nano scale. Sorting and separation of bio-particles are highly considered in diagnostics and biological analyses. By implementing the characteristics of microscale flow phenomenon, dielectrophoresis (DEP) has offered unique advantages for microfluidic devices. In DEP devices asymmetric pair of planar or three dimensional (3D) electrodes could be employed to generate non-uniform electric field. In DEP applications, facing 3D sidewall electrodes is considered to be the key solution of increasing device throughput because of producing homogeneous electric fields along the height of microchannels. Despite all advantages, fabrication of 3D vertical electrodes requires considerable challenge. In this thesis, in order to highlight the advantage of 3D electrodes over planar electrodes, the simulations are performed. Based on the developed computational model, the design parameters are decided. For the fabrication of the device, two different fabrication techniques have been proposed. In the first method, both the mold and the electrodes are fabricated using high precision machining. In the second method, the mold is fabricated with tilted sidewalls using high precision machining and the electrodes are deposited on the sidewall using sputtering together with a shadow mask fabricated using wire electric discharge machining (WEDM). The both techniques are assessed as highly repeatable and robust methods. Only the manipulation of particles with negative-DEP has been demonstrated in the experiments, and the throughput values up to 105 particles/min have been reached in a continuous flow.Item Open Access Nanotribological properties of graphene grown by chemical vapor deposition and transferred onto silicon oxide substrates(Bilkent University, 2015) Demirbaş, TunaTo extend the lifespan of mechanical systems, wear and friction must be minimized with the utilization of lubricants. On the other hand, traditional fluid-based lubrication schemes fail in nano- and micro-scale systems due to increasing surface-to-volume ratios and associated physical effects. As such, research efforts in recent years have been aimed at characterizing the structure and mechanical properties of various candidates for solid lubricants. Due to its outstanding electronic and mechanical properties, the two-dimensional “wonder material” graphene has been the focus of a large variety of experiments in the past decade. Based on its promise as a single-layer solid lubricant suitable for use in nano- and micro-scale systems, the nanotribological properties of graphene have been investigated in several studies in the literature. While frictional characteristics of mechanically exfoliated graphene samples as a function of layer number have been related to the effect of puckering, the nanotribological behavior of graphene samples grown by chemical vapor deposition (CVD) is still under investigation. Considering that high quality graphene of sufficient dimensions for practical applications is currently grown by CVD and requires transfer from metal foils onto various substrates, the need for an extensive understanding of the nanotribological properties of such graphene samples arises. Based on the discussion above, this M.S. thesis presents a comprehensive structural and nanotribological characterization of CVD-grown graphene transferred onto oxidized silicon substrates (SiO2/Si). In particular, the processes of sample preparation and post-preparation transfer onto SiO2/Si substrates are optimized via a series of experiments. Advanced microscopy techniques are utilized for the structural and morphological characterization of the obtained graphene films. In particular, optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) are used to inspect graphene coverage on the substrate and associated structural features. On the other hand, Raman spectroscopy is employed to confirm the single-layer character of CVD-grown samples. The nanotribological properties of CVD-grown graphene samples on SiO2/Si are studied by AFM in the friction force microscopy (FFM) mode under ambient conditions by measuring the evolution of friction force with increasing normal load. The effect of using different probe tips, growth conditions, and post-transfer cleaning procedures on frictional behavior is evaluated. A comparison of lubrication performance with mechanically-exfoliated graphene is also performed. Results indicate that CVD-grown graphene acts as a very good solid lubricant on SiO2/Si, reducing coefficients of friction by ~90% for all investigated samples. It is shown that as-transferred CVD-grown graphene exhibits the highest mean lubrication performance and that the associated values drop slightly with post-transfer cleaning. Finally, the effect of wrinkles associated with CVD-grown graphene on measured friction values are quantitatively evaluated, with results revealing a substantial increase in friction on these structural defects.Item Open Access Modeling and fabrication of silicon micro-grooved heat pipes(Bilkent University, 2015-04) Taze, SerdarMicro heat pipes (MHPs) are of current interest in the cooling of electronic components due to their high heat removal capacity as a result of the phase change mechanism. This thesis work focuses on finite element modeling and fabrication of a silicon micro-grooved heat pipe system. The computational model is developed to design an MHP system which consists of cooling and heating units and micro-grooves. The 3-D computational model is developed by using the phase change results of a detailed computational model on a unit cell as a boundary condition. Finite element modeling is also used for the design of the cooling channels and the heaters of the MHP system. The 3-D temperature distribution on an MHP system is obtained, and the effects of multiple channels, which cannot be captured by the unit cell analysis are reported. Two different main fabrication techniques, namely lithography-based and mechanical-based, have been assessed for the fabrication of micro-groove structures. For the lithography-based fabrication, deep reactive ion etching together with photo-lithography is used. Many process parameters are tested and optimized to achieve the desired micro-groove structure. According to the tested parameters, a final recipe is prepared and tested on a < 100 > Si wafer. Square micro-grooves with a width and a depth of 200 µm are obtained for 580 cycle dry etching with grassing formation which is below 5% (acceptable) of the micro-groove height. For the mechanical fabrication, cutting with an automated dicing saw, and high-precision machining with a diamond tool and a PCD tool have been assessed. Satisfactory results have been achieved by the dicing saw. A drawback of the dicing saw technique is the presence of a curve-shaped profile at the beginning and end of the grooves. This study showed the dicing saw to be a fast and cost effective alternative to other techniques. On the other hand, the results of high-precision machining are found to be unsatisfactory for the fabrication of micro-grooves. Moreover, the machining time and the cost of this technique turns out to unfeasible for the fabrication of a MHP system. The cooling channels are fabricated using PDMS molding, and the chromium heaters are fabricated using photolithography and sputtering. The bonding of the layers of the MHP system is accomplished by plasma treatment. The lithography-based fabrication and the dicing saw techniques are performed at the Bilkent University National Nanotechnology Research Center, and high-precision machining is performed at the Bilkent University Micro System and Design Center.Item Open Access Droplet-based microfluidic systems for silica coating and synthesis of conjugated polymer nanoparticles(Bilkent University, 2015-07) Özkan, AlicanNanoparticles have unique electronic, optic and magnetic properties due to their large area to volume ratio. In order for them to preserve their properties for longer times, some of them need to be coated with a protective layer such as silica (silicon dioxide) layer. This coating has to be made uniformly to obtain monodisperse size distributions, which is essential to obtain uniform properties for all nanoparticles. Obtaining monodisperse size distribution relies on the control over reaction conditions such as residence time, concentration and temperature. This thesis presents a microuidic reactor that can achieve strict control over reaction conditions by utilizing a meandering geometry of microchannels and droplet-based ow. Meandering channels reduce the time needed for mixing due to the reduced diffusion lengths; whereas droplet-based flow provides uniform residence time inside the reactor due to the circulating flow profile of droplets as opposed to parabolic ow profile in straight channels. Before fabricating the device, the mixing performance of droplets at different channel cross-sections and meandering geometries were simulated by using Comsol Multiphysicsr. As a result, it is concluded that the channel cross-section and meandering dimensions should be as small as possible for faster mixing. Based on these simulation results, the microuidic device was designed and later fabricated in polydimethyl siloxane (PDMS) by using the soft lithography technique. This system was used to understand the effect of solvent concentrations and residence time on silica formation in order to be able to control the coating thickness compared to batchwise methods. Initially silica nanoparticle formation inside droplets were tested; and 102 nm ± 4 nm diameter of silica nanoparticles were obtained; which is a significant improvement compared to the bath-wise synthesis methods. Additionally, experimental studies on the synthesis of green Conjugated Polymer Nanoparticles (CPN) was also conducted. By using three different methods, bulk solution, continuous ow and droplet-based ow, nanoparticles were synthesized. From the results, it was acquired that droplet-based ow provided higher quality of nanoparticles in terms of nanoparticle size, uniformity and monodispersity.Item Open Access Mechanical and controller design of a modular mechatronic device - mechacell(Bilkent University, 2015-08) Ristevski, StefanSince ancient times people have been building tools to aid them in their life. Robots evolved from being purely mechanical to mechatronic, from immobile to mobile and became smaller in scale. As the technology in building robots matured researchers, began working to build robotic systems that cooperate similar to the ones in nature. Ability of ants to accomplish tasks beyond the capability of a single ant intrigued scientists in robotics society to mimic that feature of ants and develop simple modules that alone cannot accomplish much, but together can complete complex assignments. Our motivation is to develop a miniaturizable mechatronic module{MechaCell. Mechanical design focuses on a novel locomotion system having a mechanism that converts vibrations into translational motion. Two independent controllers, one for steering and one for translational speed control are designed such that MechaCell can follow a complex path and group of MechaCells can guide an object to follow a complex path. Simulation results from the model of the MechaCell developed in SimMechanics are presented. Experimental setup comprising of a Bluetooth enabled PC, a platform, an overhead camera and four MechaCells is set up and simulation results are experimentally veri ed. Possible application of coordinated object manipulation is in manufacturing systems that have limited xture capabilities and desired precision in sub{centimeter levels.Item Open Access Process modeling for projection based stereo lithography(Bilkent University, 2015-08) Zulfiqar, AliStereo lithography is a widely used additive manufacturing process, where a three dimensional object is fabricated directly from a solid computer model. This thesis develops a projection type SLA (PSLA) test bed using a digital micro mirror device. The goal is to improve the dimensional accuracy and surface quality of the polymer parts through detailed process modeling and gain predictive ability about the duration of the printing process. For that purpose; (i) process parameters of the PSLA system have been analyzed, (ii) material properties of different polymers have been identiffed through experimental techniques, and a curing process model has been established, and (iii) some case studies have been conducted. The information deduced from the system is used to set the continuous movement speed of the vertical axis to obtain "layerless printing" of parts where the surface quality is signiffcantly improved compared to conventional layer-by-layer printing. The results show that the process planning approach used in this thesis can produce highly accurate parts. Experiments on more challenging part designs such as high aspect ratio and micro scale parts have also been conducted, and limits of the three dimensional printing system have been determined.Item Open Access Influence of interface structure on the nanotribological properties of exfoliated graphene(Bilkent University, 2016-07) Balkancı, ArdaOn the nano- and micro-scale, conventional liquid-based lubrication cannot be utilized to minimize friction due to excessive surface tension and related effects. To overcome this limitation, solid lubricants suitable for use in nano- and microscale systems are needed. Being a two-dimensional material with outstanding mechanical properties, graphene emerges as a promising candidate for this purpose. Motivated as above, this M.S. thesis presents a comprehensive investigation of the nanotribological properties of mechanically-exfoliated graphene conducted via atomic force microscopy (AFM), whereby special emphasis is placed on the effect of interface structure. Graphene samples ranging from single- to few-layers were fabricated using the mechanical exfoliation method and transferred onto Si/SiO2 substrates. By utilizing optical microscopy and Raman spectroscopy, graphene akes exhibiting single- and bi-layer regions were located and identified. Furthermore, using topographical maps and associated profiles obtained via AFM, 3-, 4-layer and bulk graphite regions were found. Moreover, AFM probes were calibrated both for accurate normal force readings, and for obtaining quantitative friction force data from lateral force measurements conducted via contact-mode AFM under ambient conditions. Following sample preparation, identification and probe calibration, experiments aimed at measuring the effect of applied load on friction of single- and 2-, 3-, 4-layers of graphene were performed, confirming previous results reported in the literature as explained by the puckering phenomenon. Additionally, the effect of tip radius and thus, contact area, on the frictional behavior of graphene was quantitatively measured. In particular, thermal evaporation- and PECS (precision etching coating system)-based coating of gold onto AFM probes were utilized to modify tip radii. Results led to the determination of a new parameter affecting friction on graphene: interface roughness. In collaboration with scientists from UC Merced who performed molecular dynamics simulations complementing the experiments presented here, the effect of substrate roughness, which may be in addition to, or dominant over, the puckering phenomenon, was analyzed in terms of the frictional behavior of graphene. Presented experimental results provide a new perspective towards the layer-dependent frictional behavior of graphene, underlining the in uence of substrate roughness in addition to the phenomenon of puckering that is well-studied in the literature.Item Open Access Development of an iterative learning controller for polymer based micro-stereolithography prototyping systems(Bilkent University, 2016-08) Türeyen, Erkan BuğraAdditive manufacturing systems provide fast and accurate fabrication opportunities for micro-scaled structures. Various methods of processing are used for fabrication of different materials. Stereolithography is an important technique for rapid prototyping of photo-reactive polymer based materials. Similar to the other additive manufacturing methods, DLP based projection micro-stereolithography also includes limitations in terms of dimensions, minimum feature sizes and material properties. For advanced and precise micro-sized structure fabrications, process needs to be defined with a complex control scheme. In order to develop a scheme for increasing the fabrication quality, nature of the complex chemical and physical phenomena behind the resin solidification process is investigated. A complete mathematical model for the pixel based photopolymerization process is developed. According to the parameters included in the solidification model, measurements and observations are made for understanding of the resin, optical system and positioning system. Problems of over-curing and under-curing caused by the attenuation nature of the light inside the liquid resin are observed in the simulations made based on the model which is also supported by the previous fabrication experiences for varying structures. These problems creating structural irregularities are dependent on the process parameter of exposure applied on the fabrication surface. An iterative learning based parameter control algorithm is developed for overcoming these errors decreasing the fabrication quality. Continuous fabrication platform movement instead of step-by-step movement which is one of the main features of the established system is used to define a solution. Main fabrication parameter of platform speed is adjusted for each layer according to the error amount calculated on iterations. Use of an optimized gain for parameter control, decreased the dimensional error calculated by the count of the wrongly cured pixels up to 80% in the simulations and 75% in the real life fabrication trials with the application of algorithm. These improvement ratios and proposed algorithm provide a new perspective for the possible future work about online exposure measurement and in-situ parameter control of the stereolithography process.Item Open Access Homogenization-based microscopic texture design and optimization in hydrodynamic lubrication(Bilkent University, 2016-08) Waseem, AbdullahThe aim of this thesis is to develop an optimization framework for the texture optimization in hydrodynamic lubrication using multi-scale homogenization technique. In hydrodynamic lubrication the asperities do not come into contact due to fluid film present between the surfaces and normal load is carried by the viscous fluid. The Reynolds equation can be used with confidence for such problems. For two-scale separation, a basis for optimizing the surface textures is established through an asymptotic expansion based homogenization scheme, which delivers a macroscopic Reynolds equation containing homogenized coefficients. These homogenized coefficients depend on the fluid film thickness directly and by controlling these coefficients a desired macroscopic response can be obtained. Design variables are introduced to control the fluid film thickness indirectly through an intermediate filtering stage. Both microscopic and macroscopic objectives are defined for texture optimization. The quality of the designed textures are evaluated numerically as well as aesthetically and optimization parameters are selected accordingly. Isotropic and anisotropic textures can be designed by using the proposed optimization scheme. For both microscopic and macroscopic objectives optimization surface textures are reconstructed as a sanity check. Texture optimization for prescribed load bearing capacity and maximum load bearing capacity in temporal and spatial variations are then carried out for squeeze film flow and wedge problem, respectively. Finally, to reduce the computational cost, Taylor’s expansion is proposed for the optimization problem. Overall, the methodology developed in this thesis froms a basis for a comprehensive micro-texture design framework for computational tribology.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 Development of a vector sensor using piezoelectric support beams(Bilkent University, 2016-08) Tiftikçi, Alper YasinUnderwater listening has attracted great interest since the twentieth century. Formerly, scalar hydrophones were used to detect sounds underwater by measuring only the pressure changes. These sensors needed to form an array to detect the direction of an incoming wave. They were positioned with intervals and the time delays of the sound among hydrophones gave the information of direction. However, vector hydrophones have become an attractive alternative since they can provide both scalar and vector information on their own. An Acoustic Vector Sensor (AVS) is a device used to detect acoustic signals by converting mechanical energy of an incident wave to a detectable electrical energy. Its design is based on two parallel piezoelectric beams attached to a mechanical sensing tip using a rotary joint component. The sensor uses piezoelectric elements to convert mechanical strain into sensible electrical signal. Design of the sensor is modular since each individual sensing element could be easily changed, tested and calibrated. All parts of sensor has been fabricated by mechanical micromachining and precisely assembled, which increases design and manufacturing exibility. In this thesis, the mechanical design of the sensor that uses the piezoelectric e ect for detection of sound is presented. First, a computational analysis method which predicts the dynamic response of the designed sensor is developed. Then, a tigthly controlled vibration experiment platform is set up to emulate the e ect of the motion of the acoustic particle. The computational simulations are then validated with initial experiments and the design of the sensor is nalized.Item Open Access Development of multi-axis laser micromachining system suitable for machining non-linear contoured surfaces(Bilkent University, 2016-08) Kerimoğlu, SerhatIn recent years, studies on manufacturing systems have proved the importance of cooperation of positioning systems with laser cutting technology. The performance of the manufacturing system can be improved by utilizing both laser and positioning systems together. In this study, design and development of a micromachining system which can perform on non-linear contoured surfaces is presented. Laser micromachining system is designed and assembled including a nanosecond Q-switched pulsed ber laser, a 6-DOF hexapod manipulator, a granite table in order to absorb vibrations and an external cabin system to isolate the whole system for safety and health issues. Performance characteristics of micromachining devices mainly determined by precision and characteristics of each individual components of the system. Therefore, the studies to improve the performance of the laser micromachining system are related with laser, isolation and positioning systems. A dynamic model of the positioning system is derived to obtain the control parameters for the actual positioning system. By using these parameters, the performance of the laser micromachining system on nonlinear contoured surface is improved and discussed in detail. The simulation environment MATLAB/Sim- Mechanics is used to model the dynamics of the positioning system. With the kinematic and dynamic model of the manipulation system simulations, signi cant performance enhancements are obtained on non-linear contoured surfaces.