In a hydrodynamic journal bearing, the lubricant is pressurized by the relative movement of the bearing surfaces, creating a load-carrying capacity. The load-carrying capacity can be increased by using a lubricant with higher viscosity, at the cost of increased friction. A proposed method to increase the maximum load-carrying capacity, while not increasing the friction at lower loads (or at higher speeds), is by using magnetorheological (MR) fluid as lubricant and electromagnets to generate a magnetic field. MR fluid consists of a carrier fluid with micron-sized magnetic particles suspended. Under the influence of an external magnetic field, the viscosity of the fluid increases as the particles form chain-like structures. Using electromagnets, the magnetic field strength can be adjusted based on the operating conditions of the bearing and the bearing can be semi-actively controlled. The objective of this thesis is to create a visual demonstrator setup, intended for educational purposes, that can be used to investigate the pressure distribution and generated forces in a hydrodynamic half journal bearing using MR fluid and electromagnets. Furthermore, the research objective is to increase the maximum load capacity for a specified minimum film thickness, while not increasing the friction in the hydrodynamic lubrication regime. The test setup was based on an existing setup made by GUNT which is able to show the pressure distribution in a half journal bearing using pressure tubes for various eccentricity ratios. Adaptations to this setup were made to measure the generated forces (horizontal force, vertical force and friction force, defined as the frictional moment divided by the radius of the bearing) and to implement an electromagnet. First, experiments using hydraulic oil were performed to test and improve the performance of the setup. Afterwards, experiments using MR fluid were performed with varying magnetic field strengths. Furthermore, a numerical model based on the two-dimensional Reynolds equation was created and validated with the experimental results. Both the numerical and experimental results showed comparable trends of increasing pressure, horizontal force and vertical force, meaning an increasing load capacity, for increasing eccentricity ratio and applied current. The numerical friction force also increased with increasing eccentricity ratio and applied current, while the experimental friction force only increased with increasing applied current and did not show a clear trend with increasing eccentricity ratio. The magnitude of the forces differed significantly between the experimental and numerical results, where the largest differences were observed for the vertical force and the friction force. Many plausible causes for the differences are identified, such as bearing tolerances, uncertainties in eccentricity and modeling simplifications. A combination of these causes is believed to lead to the observed differences. The increase in pressure, horizontal force and vertical force due to an increase in applied current was found to be much smaller experimentally than numerically. This was thought to be caused by a larger temperature increase in the experimental tests than was modeled and by simplifications used in the numerical model such as neglecting the shearthinning characteristics of MR fluids. The friction force differed even more significantly between the numerical and experimental results. Additionally, this is caused by the small magnitude of the friction force in combination with a test setup that is not accurate enough to measure small forces consistently. The created setup can show the change in pressure distribution when the applied current or eccentricity is changed. It therefore serves as a visual demonstrator setup for an MR lubricated hydrodynamic half journal bearing which can be used for educational purposes. However, demonstrator performance was found to be far from ideal as the changes in fluid column height were very slow because of the high MR fluid viscosity. Performance can potentially be improved by using a different or diluted MR fluid. Despite the large differences between the numerical and experimental results, both can be used to display the potential of semi-active control using MR fluid and electromagnets. It is shown that the maximum load capacity can be increased while not increasing the friction coefficient in the hydrodynamic lubrication regime. Several recommendations have been made for changes to the demonstrator setup to improve performance while keeping the setup simple and elegant. The main recommendations include improving the bearing tolerances, testing with a different or diluted MR fluid, implementing capacitive distance sensors and performing experiments at a more consistent ambient temperature.

Neuromuscular diseases often result in elevated passive joint impedance (PJI), impacting the daily lives of affected individuals. To be able to apply assistive devices that compensate for PJI, it is essential to identify the PJI correctly. This study aims to identify and model the PJI of the elbow joint, to improve the control of force-based controlled active-assistance exoskeleton devices by distinguishing the voluntary from the passive forces. We used an elbow device to measure the PJI in twelve healthy males with the shoulder abducted at a 90˚ angle. The study investigates both static and dynamic conditions, encompassing various contraction velocities (≤ 0.20 rad/s) and shoulder flexion positions (0˚ and ±45˚). The analysis estimates the hysteresis, equilibrium position, and elastic property. Subsequently, based on the average elbow PJI, we developed a regression model, with the statistical analysis revealing no significant differences between the conditions, except for the observed hysteresis under varying velocities. Based on the tested conditions, the findings indicate that a single low-velocity dynamic measurement can serve as the basis for deriving a general elbow PJI regression model.

A growing energy demand has sparked interest into harvesting energy from the human body. Often, a rotational proof mass is used to harness energy for a wearable from kinetic motion of its user. The addition of a spring has the potential to significantly increase efficiency for these low-power rotational energy harvesters. In this research a system characterisation is performed from which a dimensionless ratio is formulated to determine optimal spring stiffness for maximisation of average power output as a function of harvester design parameters and excitation inputs. Creating a system that is optimized for the entirety of its operational range rather than for one specific excitation input. Implementation is investigated in the field of horology, where rotational energy harvesters are widely implemented. To increase harvesting efficiency of existing rotational energy harvesters without requiring significant design alterations a compliant design is proposed, prototyped and tested.

Flexure mechanisms are widely utilized in precision mechanisms, but their practical use is limited by three major factors, namely restricted range of motion, parasitic motion, and significant reduction in support stiffness when deflected. To address this limitation, in this research the potential of variable thickness flexures in minimizing the aforementioned issues is evaluated. Two novel parametrizations are utilized to optimize these variable thickness flexures, which are then compared to their commonly used constant thickness counterparts. Furthermore, initially curved flexures have been found to be promising in optimizing mechanisms with high performance in terms of parasitic motion and stiffness tuning. Three novel designs are proposed and optimized in this research for improved performance. As variable thickness showed promising results, these newly designed mechanisms are further optimized by incorporating variable thickness into the initially curved flexure mechanisms. The outcomes of this study demonstrate a significant enhancement in terms of support stiffness and range of motion without compromising other important factors.

This report is written for the thesis of my master Mechanical Engineering that was conducted at the department Precision and Microsystems Engineering of the Delft University of Technology. The goal of the project was to design a compliant hipback support of an exoskeleton while researching how to achieve desired directional compliance on this application. This work starts with an introduction to exoskeletons and explaining their working principles. The problem is that the reduction of range of motion for wearing an exoskeleton reduces the adoption of exoskeletons in industry. The objective of this study was to solve this by creating a hipback support with compliant behaviour in torsion and lateral bending while remaining stiff in the supporting forward bending direction. For inspiration, the state of the art of current hipback supports and research projects was reviewed. Also, the characterization methods of compliant mechanisms were reviewed for inspiration, to come up with a suitable design method for developing a compliant structure for a hipback support. By tuning stiffness of the structure in distinct directions, a conventional synthesis method for compliant mechanisms could be applied, namely the Freedom and Constraint Topologies method. This was used to come up with a preliminary design. This design was extended to improve stiffness behaviour for coupled motions by researching how to change stiffness on demand. The only categories to change stiffness are shape, material, prestress and boundary condition. For each category, a promising highlevel design was worked out with improved stiffness behaviour for coupled motions. To generate an even more satisfying solution, optimization of the beam shape was applied to handle the defined rather complex kinetostatic task and to develop a lightweight structure. The objective function was to reach specified deflections where shaperelated parameters are optimized. These parameters were the spine shape of the beam, the crosssection and orientation of the crosssection. To generate a design, a selfdeveloped optimizer using beam modelling was extended and applied. The extensions were to optimize a doubleclamped beam that was loaded in the middle point and optimizing for angular deformations as a set objective function. This extended optimizer generated the final design which was a thin rectangularshaped beam with threedimensional curves. This design was not straightforward to fabricate into a prototype. Multiple manufacturing methods were evaluated and one final method was selected to fabricate a close approximation of the final design. This final manufacturing method was to produce the prototype by roll bending and manually applying a twist on a curved lasercut strip which was extracted from flattening of the spatial shape. This prototype was verified by simulations and experiments. Simulation tests were run on an original design of the back support and compared with the new optimized shape based on the requirements. The results, for similar loadings and material, are 27 times more lateral flexibility, 1.25 more torsional flexibility and a design that is four times lighter in comparison to the original design. The original relative stiffness of 1 to 1 for forward relative to lateral bending improved to 2 to 1. An important remark to note is that the beam model generated a thin section which was less suitable for 1D beam modelling of the implemented FEM. Creating an optimizer based on 2D shell modelling code might give more accurate results and can be considered for future work.