Bogazici University Digital Archive
http://digitalarchive:80
The DSpace digital repository system captures, stores, indexes, preserves, and distributes digital research material.2024-03-29T15:43:36ZResponse analysis and damage mitigation of buried continuous pipes subjected to faulting actions
http://digitalarchive.boun.edu.tr/handle/123456789/21488
Response analysis and damage mitigation of buried continuous pipes subjected to faulting actions
Perdibuka, Dardan.
Buried continuous steel pipelines are critical lifelines failure of which under fault rupture incidents may lead to significant and deteriorating environmental and socio economic outcomes. Proper understanding and estimation of the mechanical behavior of buried steel pipes under such geohazards and investigation of means of mitigating these deleterious effects is of paramount importance. This thesis aimed at developing rigorous and simplified numerical models of the problem to realistically simulate the behavior of buried continuous pipes under strike-slip fault rupture-induced permeant ground deformations. The response of buried pipe cases under the fault load was investigated with respect to the variation of fault crossing angle (β) and pipe wall thickness (t). The second phase of this dissertation involved the investigation of the effect of four mitigation techniques to protect the buried pipe against fault rupture-induced damages. Lastly, a case study involving the evaluation of the effect of using CFRP wraps on the response of Thames Water Pipe which suffered great damage during the devastating 1999 Izmit is presented. The outcomes of this thesis indicate that the performance of the pipeline is sensitive to the variation of fault crossing angle and pipe wall thickness, increasing both parameters lead to overall improved pipe performance. Results indicate that all mitigation approaches offer certain degrees of improvement, where most effective mitigation approach is the wrapping of the pipeline surface with CFRP wraps while the use of controlled-low strength material was the least effective approach. Comparison of simplified and rigorous numerical models revealed that a good agreement exist between the approaches. Lastly, evaluation of the response of Thames Water Pipe protected using CFRP indicates that despite the considerable reduction in stresses and strains complete avoidance of failure for this particular case does not seem to be attainable.
2022-01-01T00:00:00ZModular safety-critical control of legged robots
http://digitalarchive.boun.edu.tr/handle/123456789/21487
Modular safety-critical control of legged robots
Tosun, Berk.
With recent performance improvements, legged robots will soon enter our lives to stay. Various control algorithms are already employed in deploying existing robots, and many more algorithms are still in the making. Safety concerns during the operation of legged robots must be addressed to enable their widespread use and ease their development. Especially machine learning- based control methods would benefit from model-based constraints to improve their safety. This thesis presents a modular safety filter to improve safety, i.e., reduce the chance of a fall of a legged robot. The availability of a robot capable of locomotion is assumed, i.e., a nominal controller exists. During locomotion, terrain properties around the robot are estimated through machine learning which uses a minimal set of proprioceptive signals. A novel deep-learning model utilizing an efficient transformer architecture is used for terrain estimation. A quadratic program combines the terrain estimations with inverse dynamics and a novel control barrier function constraint to filter and certify nominal control signals. The result is an optimal controller that acts as a filter and the filtered control signal allows the safe locomotion of the robot. The resulting approach is general and could be transferred with low effort to any other legged system.
2022-01-01T00:00:00ZA reinforcement learning based controller to minimize forces on the crutches of a lower-limb exoskeleton
http://digitalarchive.boun.edu.tr/handle/123456789/21486
A reinforcement learning based controller to minimize forces on the crutches of a lower-limb exoskeleton
Utku, Aydın Emre.
The majority of the metabolic energy consumption of a lower-limb exoskeleton user comes from the upper body effort, since the lower body can be considered to be passive. However, the upper body effort of lower limb exoskeleton users is ignored during motion controller development process in the literature. In this thesis study, deep reinforcement learning is used to develop a locomotion controller that minimizes the ground reaction forces (GRF) on crutches. The rationale for minimizing the ground reaction forces is to minimize the upper body effort of the user. A model of the human-exoskeleton system with crutches is created in URDF and XML formats. Reward functions that encourage the forward displacement of the center of mass of the exoskeleton-human system without falling and extreme joint torques are shaped. The state-of-the-art methods, Twin Delayed Deep Deterministic Policy Gradient (TD3) and Proximal Policy Optimization (PPO), are employed with the RaiSim and MuJoCo physics simulators and with different algorithm specific parameters in multiple training trials. The employed networks generate the joint torques based on the joint angle and velocities along with the ground reaction forces on feet and crutch tips. These generated joint torques are directly sent to the exoskeleton model and a new state is observed after implementing the action that the deep RL framework provides. Policies trained by the TD3 and PPO methods on RaiSim are observed to fail to generate proper control commands for a stable and natural looking gait. In general, it is observed that the PPO method generated higher rewards than the TD3 method on RaiSim. After failing to develop a desired policy with RaiSim, MuJoCo is employed as the simulator. Eventually, a policy that can generate a reasonable gait with a desired crutch usage and with 35% minimization in GRFs with respect to the baseline policy is developed.
2022-01-01T00:00:00ZHigh school students' meanings of cartesian coordinate system
http://digitalarchive.boun.edu.tr/handle/123456789/21483
High school students' meanings of cartesian coordinate system
Güven, Semra.
The aim of this study was to investigate high school students’ reasoning of spatial and quantitative Cartesian coordinate systems, their meanings of a point on a graph, outputs of a function and graphing within spatial and quantitative coordinate systems. The study was conducted at a private high school in Istanbul towards the end of Spring semester in 2021- 22 academic year. Participants consisted of 229 high school students from different grade levels who learned different kinds of functions and graphing in Cartesian coordinate system. The instrument with nine open ended questions was developed by the researcher based on the literature. Data were categorized by coded analysis and descriptively analyzed mainly according to framework for reasoning about graphs in spatial and quantitative Cartesian coordinate system and framework for representing a multiplicative object in the context of graphing. Analysis of the results showed that students had critical difficulty in conceiving axes as frame of reference to represent quantities as horizontal and vertical directed distances from origin in describing location of a point and coordinating quantities. Additionally, results showed that significant number of student students viewed x and y- coordinates and output of a function as a point on a graph hence carried non-normative meanings for point in terms of multiplicative object. Besides, students’ difficulties with function and function notation were found to be an issue in their reasoning about points. Relatedly, results regarding graphing in Cartesian coordinate system pointed to students’ difficulties in envisioning a graph as an emergent trace of multiplicative object representing changes in two quantities simultaneously. Students either focused on gross variation of the quantities and sketched a memorized graph, sketched a discrete graph, could not envision how the graph behaves between landmark points or sketched two separate graphs considering time as a secondary variable. NOTE Keywords : Cartesian coordinates, Graphical representation, Mathematics education, Mathematics teaching, High schools students, Spatial coordinate system, Quantitative, coordinate system, Multiplicative object.
2022-01-01T00:00:00Z