Browsing by Author "Kara, Ahmet Burhan"

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A comparative evaluation of sepiolite and nano-montmorillonite on the rheology of cementitious materials for 3D printing
(Elsevier, 2022-10-03) Aydın, Eylül Mina; Kara, Ahmet Burhan; Bundur, Zeynep Başaran; Özyurt, N.; Bebek, Özkan; Gulgun, M. A.; Civil Engineering; Mechanical Engineering; BUNDUR, Zeynep Başaran; BEBEK, Özkan; Aydın, Eylül Mina; Kara, Ahmet Burhan
Through the last decade, methods of digital manufacturing of concrete gained a significant interest compared to conventional concrete. The main challenge in additive manufacturing (3D printing) is to design a highly thixotropic cementitious system. This study aims to investigate the use of sepiolite as a rheology modifier as a novel approach to improve the thixotropic behavior and adapt cementitious systems to 3D printing. To understand the influence of sepiolite on rheological properties, a comparative evaluation with nano-montmorillonite was established. The effectiveness of clay addition was also investigated in fly-ash amended cement-based materials. The rheological analysis was done on cement-paste samples containing both clays in terms of their effects on thixotropy, structural build-up, and recovery. A preliminary printability assessment was done with a lab scale printer having a ram extruder. The results show that the incorporation of clays increased the dynamic yield stress and time-dependent evolution of static yield stress. Moreover, the addition of clays improved the thixotropic behavior of cement-based systems, particularly those containing fly-ash. Herein, the sepiolite was found to be more effective compared to nano-montmorillonite in terms of improving thixotropy, structural build-up and recovery. The results showed that use of fly-ash enhances the printability of the mix for the specified extruder and the samples containing 1% nano-montmorillonite or 0.5% sepiolite can be printed. The positive effects of sepiolite were attributed to opposing surface charges of the clay layers and its micro-fibrous microstructure. The findings in this study enabled an in-depth understanding of the rheology and printability of fly-ash amended clay containing printable cement-based mortars.
Metadata only
End-effector compliance index for optimizing cable anchor points in redundant cable-driven parallel robots : design, construction, and control for cement based additive manufacturing
Kara, Ahmet Burhan; Bebek, Özkan; Bundur, Zeynep Başaran; Bebek, Özkan; Ünal, Ramazan; Erbatur, K.; Department of Mechanical Engineering
This thesis presents a Cable-Driven Parallel Robot (CDPR) for additive manufac- turing that has been designed, constructed, and controlled based on an introduced methodology for cable anchor point selection. The design and construction procedure of the CDPR was explained. The designed CDPR is a suspended type and redun- dantly actuated system. The robot comprises various components, including a frame for building the robot, cables connected to the end-effector, pulleys redirecting the cables to the end-effector from winches, winches for controlling cable length, an end- effector for printing, and an electrical panel to house electrical equipment. The cable tensions and stiffness of the end-effector were analyzed to achieve the desired size of the robot. To achieve this, the end-effector compliance index (ECI) was proposed to assess the stiffness of the end-effector within the workspace. This novel index uses cable direction vectors and cable lengths to determine the compliance of the given robot pose. Simulation results led to the development of a relation for obtaining an improved CDPR frame size, applicable to both suspended and constrained type CDPRs. Based on these simulations, a redundantly actuated suspended cable-driven parallel robot was constructed for a 1 m base length print part. Motion tracking ex- periments were conducted to assess the mobility and accuracy of the designed robot. The robot's working accuracy was evaluated by testing various printing paths. The experimental results verified that the developed system can achieve printing with an accuracy margin of less than 0.2%.
Open Access
Mass flow rate control of solenoid-based injectors
(Sage, 2023-09) Qureshi, Muhammad Sarmad; Kara, Ahmet Burhan; Ertunç, Özgür; Bebek, Özkan; Mechanical Engineering; BEBEK, Özkan; ERTUNÇ, Özgür; Qureshi, Muhammad Sarmad; Kara, Ahmet Burhan
This paper presents a mass flow rate control method for solenoid-based injectors in open-loop, i.e. without physical sensor feedback. Measuring the mass flow rate in injectors is important because the performance of the injector determines fuel consumption and emissions. While it is possible to perform this control using different types of sensors, this would increase the cost and reduce the reliability. With this study, it was aimed to control the mass flow rate of solenoid-based injectors using continuous switching control to regulate the injector’s needle displacement without using sensors. For this purpose, a solenoid-actuated injector prototype was created. Model-based estimators were used to determine the states of the injector, while a super-twisting sliding mode controller was used to track the desired mass flow rate references. The results of the experiments showed that tracking for sinusoidal, triangular, and square trajectories yielded a mass flow rate percent error of less than 1.06. The proposed open-loop control algorithm can be used to track different mass flow rate references in internal combustion engines.