The minimum number of bodies required to form a closed kinematic chain capable of motion is four. A planar fourbar mechanism has one degree of freedom (DOF), whereas in three-dimensional space, it becomes over-constrained, reducing its DOF to negative two. While the kinematic idealization remains unaffected, its physical realization introduces conflicting constraints. This study deliberately incorporates misaligned overconstraints in four-bar mechanisms to achieve tailorable load displacement characteristics by utilizing a compliant coupler link. Such tunable characteristics could potentially eliminate the need for traditional springs and dampers in mechanical systems. A positioning strategy for misaligned over-constraints is developed. An Euler-Bernoulli beam model with superposition is employed to analyze how misalignments influence the loaddisplacement response. Additionally, a numerical model using Simscape Multibody is implemented to verify the analytical results. To further validate the findings, a modular physical prototype is constructed and tested to compare real-world behavior with computational models. The numerical simulations effectively capture the response of the four-bar mechanism, considering elastic deformations in the compliant coupler link. The results from all models exhibit strong agreement. Parameter relaxation is introduced to account for manufacturing tolerances and geometric imperfections. Furthermore, a parametric study is conducted to examine the influence of individual design parameters on the load-displacement characteristics. To facilitate design exploration, a graphical user interface (GUI) is developed, enabling users to tailor four-bar mechanisms based on specific load-displacement requirements. Two distinctive behaviors are presented: extended regions of nearly constant torque and sinusoidal load-displacement characteristics, both of which have potential applications in precision engineering and motion control.

This paper introduces a fully compliant spherical joint with an optimized stiffness profile specifically for balancing a pendulum. The design builds on previous work that has successfully created a fully compliant spherical joint using tetrahedron-shaped elements connected in series. To efficiently compute and optimize the balancing behaviour of these compliant joints, a novel algorithm is developed and presented. Using this algorithm, optimizations are conducted to obtain simulated pendulum balancers under five different conditions. The performances of these results are analysed to assess the potential and limitations of the algorithm and these spherical joints. Based on one of the optimized results, a prototype is fabricated and expermentally validated, achieving a moment reduction of 90.5%. The deformation calculated by the TetraFEM tool closely matches the prototype’s deformation with an accuracy of 89.6%, demonstrating its potential for application in the development of shoulder exoskeletons.

Neutrally stable metamaterials can maintain differ- ent shapes without any energy input, making it a key innovation in the quest for more energy-efficient technologies. Despite this intriguing property, the research in this area is scarce. This study proposes a method for achieving neutral stability in metama- terials. This method is validated with a novel unit cell design that utilizing two identical beam elements that are mirrored. Each element displays a constant force characteristic. By pre- tensioning these elements, we align their constant force regions, thereby inducing a state of neutral stability. Through finite element method (FEM) simulations and geometrical optimisation, the beam of this design is optimised to achieve the optimal constant force response. A prototype is made and a test setup is constructed to validate the accuracy of the simulations and the feasibility of the method for achieving neutral stability. Results indicate that while perfect neutral stability was not fully achieved, this method can be applied on other constant force mechanisms to create neutrally stable metamaterials.

Meta-materials provide many possibilities with their specially tailored properties on the macro level caused by their micro level structure in the form of unit cells. These unit cells often take the form of Compliant Mechanisms (CM), which have been widely researched. CM are often made neutrally stable to improve their energy efficiency, making it plausible that meta-materials can be made neutrally stable as well. This research aims to create such a neutrally stable meta-material using constant force (zero-stiffness) unit cells. This was achieved by optimizing the geometry of coiled spatially curved shell structures such that their buckling mode under compression facilitated a constant force region under compression. Simulations of the optimized structures within a meta-material lattice are compared to experimental tests performed on 2 different prototypes of such a meta-material. The experimental results show that the force-displacement curves indeed have a constant force trend line under compression. However, the magnitude of the trend line for one experiment is significantly lower than expected in the simulations. Despite this magnitude difference, the shapes of the experimental force-displacement curves are similar to the curves of the simulations. Small regions of neutral stability are observed for the deformed unit cells of the meta-material, but sawtooth-like behaviour of the force-displacement curves around the trend line caused these regions to be small and unstable.

This thesis focuses on the design, prototyping, and testing of a single-use bistable microneedle applicator that is optimized for consistent, and user-friendly skin penetration. The applicator integrates a bistable dome mechanism which is designed to deliver a certain value of force after stretching the skin. Design criteria—manufacturability, user-friendliness, durability, and cost— help the selection process through lots of concepts. Polypropylene (PP) and Polyamide 12 (PA12) are chosen for prototyping. However, the final production material is not the same type of PP as it is used for the prototypes. These models are made by using 3D printing, and are analyzed by finite element simulations in ANSYS to evaluate force-displacement responses and bistable behavior. Experimental testing is done to validate the simulations. However, differences appear that are caused by various reasons such as limitations on 3D printing and testing method, applying boundary conditions, and ....

The creation of neutrally stable compliant mechanisms is a balancing act between knowledge on the system and the application of a stiffness countering prestress. This paper presents a novel prestressing method for polymeric compliant zero-stiffness mechanisms, utilizing stress relaxation as an actor for imprinting a permanent elevated internal stress state. The method in questiondoes not require a complex mechanical setup for application of the prestress. A sensitivity analysis is performed for the variables of relaxation time and imposed deformation. Two mechanisms with different origins for zero-stiffness behaviour are presented, and made neutrally stable. The simulations for one of the two examples, after correcting for a stretch in the force-displacement relation ship, match with the experimental results for both the variation in relaxation time and imposed deformation. For the second mechanism, the simulation for relaxation time matches that of the experiment, but for theimposed displacement this does not seem to be the case.

Compliant mechanisms, particularly helicoidal shell joints, present intriguing possibilities in mechanical design with applications in medical devices, robotics, automotive, and aerospace engineering. This research focuses on the synthesis of nonlinear torque-angle profiles using a compliant helicoidal shell mechanism such as gravity-balancing profiles. This study required a thorough exploration of the mechanism’s diverse design variations through Finite Element Modeling (FEM) and more specifically, Isogeometric Analysis (IGA). Subsequently, a targeted optimization process is utilized, incorporating both global geometric parameter adjustments and localized modifications by using splines. The prominent challenge addressed is the synthesis of gravity balancing torque-angle profile, achieved by tailoring the output profile of a compliant shell mechanism through optimization. Considering the inherent sine function output of a pendulum during gravitational equilibrium, an algorithm is developed to optimize the mechanism’s behavior to align with a sine function, hence enabling gravity balancing. Additionally, experimental validation was undertaken through manufacturing prototypes and conducting measurements to provide a crucial link between simulations and real-world behavior. The results of this research, encompassing optimized geometry and experimental data, are presented, and comprehensively discussed. This research contributes a numerical methodology that utilizes isogeometric analysis and optimization algorithm within the framework of finite element analysis for achieving nonlinear torque-angle profiles in complaint helicoidal shell mechanisms, such as gravity balancing profiles, offering valuable insights for possible applications in various engineering domains.

This study proposes a novel design of monolithic compliant variable torsional stiffness (VTS) mechanism whose stiffness can be tuned continuously. This mechanism is then used to create a compliant variable stiffness (VS) ball joint. This compliant VS ball joint can be beneficial in various applications such as exoskeletons, prostheses, and bio-mimicking due to their high adaptability to environmental changes and the capability for multitasking.

This thesis report presents research on small walking robots. The report begins with an introduction that explains the project’s origin, outlines the project’s goal, and provides an overview of the report’s contents. Following the introduction, a literature study is conducted to investigate the current state-of- the-art steering mechanisms for walking robots.
After the literature study, the research paper is presented, detailing the design process and the robot’s performance results. The paper demonstrates the successful design of a frequency-actuated resonance robot with both forward locomotion and steering capabilities, achieved by using only one actuator.
In conclusion, the literature study provides a novel classification and overview of state-of-the-art walking robots. Furthermore, the research paper showcases the successful design of a frequency-actuated resonance robot with a forward locomotion and steering mechanism, all accomplished with the use of a single actuator.

The bending stiffness of a structure is highly dependent on its shape, and this relation can be utilized to allow for a structure to morph into different shapes during deformation. This paper focuses on shape-morphing into multiple shapes (Multi Shape-Morphing), and presents a design method for a parametric
structure consisting of beams, that is able to approximate a set of objective shapes during deformation by exploiting its geometry. Several designs are generated with this design method and experimental validation has been performed on several physical models verify the multi shape-morphing performance. A sensitivity analysis has been conducted on relevant design vari-
ables of generated designs to identify their relative importance concerning the structure’s multi shape-morphing performance. To demonstrate the feasibility of the proposed design method for use with exoskeletons, a human-scaled prototype has been designed, and validated.

Compliant mechanisms are a popular alternative to conventional mechanisms because of their advantages on mass reduction, friction, backlash and lubrication. However, the major drawback in the use of compliant mechanisms is the stiffness is the desired direction of motion. The advantages of conventional and compliant mechanisms can be combined if the stiffness can be reduced or ideally removed, resulting in a zero-stiffness compliant mechanism. In current designs of zero-stiffness mechanisms, a preload is applied in the stiffest (compression) direction of a flexible beam. This working principle is based on Euler buckling. In this work, compliant mechanisms with zero-stiffness behaviour are obtained using lateral torsional buckling as their working principle. For this purpose, two compliant joints are modelled in a FEA, a translational and a rotational joint. The results of the FEA are verified by experiments. Also, a sensitivity analysis is performed on the cross-sectional dimensions of the flexible beams to optimize the range of zero-stiffness. Both types of joints showed zero-stiffness behaviour for the optimal preload and a good agreement is found between simulations and experiments. From the sensitivity analysis is found that a rectangular cross-section results in the largest range of zero-stiffness.

Shoulder exoskeleton is a popular solution to work-related shoulder disorders and muscle fatigue. With a wide range of exoskeletons designed, a comprehensive report on how the use of shoulder exoskeletons changes shoulder biomechanics is still missing. In this project, the impact of exoskeletons on shoulder biomechanics was investigated with the musculoskeletal simulation OpenSim. This study proposed a "human-in-the-loop" optimization-based design tool for shoulder exoskeletons. This design tool incorporates the predicted biomechanical effects of a shoulder exoskeleton from musculoskeletal simulations into design considerations. This design tool was validated with a case study designing a shoulder exoskeleton based on a compliant beam and testing the design in the musculoskeletal simulation and experiments.

The exoskeleton design tool is a coupling of finite element analysis and OpenSim. OpenSim calculates the deformation of the exoskeleton with human motion, and the finite element analysis calculates the force exerted from the exoskeleton upon deformation. Then OpenSim computes muscle activities under the external force from the exoskeleton. By merging muscle activities and the resultant glenohumeral joint reaction force to an objective function, the optimization-based design loop is closed by looking for the best objective value iteratively.

Several exoskeletons were designed by the new design tool to assist different types of tasks. The design tool exhibited good ability in finding optimal solutions for a range of design choices and design requirements. Simulated tests of designed exoskeletons showed significant effects on reducing muscle activities and good robustness in resisting the influence of perturbed motions in arm-elevated tasks. An exoskeleton was selected to be tested with an experiment set up in the same way as the simulated test. Experiment results supported the performance of the exoskeleton predicted in the simulated test.

This project established a method to comprehensively predict the effect of an exoskeleton on shoulder biomechanics and provided a more comprehensive understanding of biomechanical effects of shoulder exoskeletons. This facilitated the “human-in-the-loop” design process of shoulder exoskeletons which could greatly save money and time investments into prototyping, testing, and validation.

The exploration of compliant mechanisms, especially those providing linear motion, has become a central focus in recent engineering research. These mechanisms, characterized by their monolithic bodies and reduced component interactions, present an attractive alternative to traditional rigid mechanisms, having reduced friction, decreased energy losses, and cost efficiencies. Despite these advantages, designing and analyzing compliant mechanisms remain difficult tasks, with particular challenges arising when aiming for large-range linear motion with high support stiffness and minimal parasitic displacement. This study presents a unique design: the combination of a flexure-based linear guide with the features of a bistable compliant switch mechanism. Because a combination of these mechanisms has not been looked into while still offering a long stroke, high support stiffness and minimal parasitic motion. Through Finite Element Analysis (FEA) and subsequent iterations, an optimized model was conceived, prioritizing stiffness and minimizing undesired motions. This optimized design was turned into a 3D printed prototype. The experiments with the prototype confirmed the results of our computational insights, showcasing the parasitic motion of the prototype to be within 1mm, and its minimum stiffness at least being 10kN/m while still having a stroke of 70mm. A case study has been set up, of which the proof of concept works and is validated.

This project presents a compliant slender mechanism with a low axial-bending stiffness ratio. This can be used in passive exoskeletons. Passive exoskeletons can assist people with their work by preventing injuries. Currently exoskeletons uses sliders to account for the extension of the spine when bending, but this could be uncomfortable. The proposed mechanism tries to solve this problem by having a low axial stiffness while maintaining its bending stiffness. It consist of vertical rigid bodies connected by horizontal flexures. The distance between the flexures is found to be a variable almost independent of the axial stiffness, thus able to easily influence the ratio. To help with future designs a PRBM-model is proposed that describes the proposed mechanism. This is then used in a dimensional optimization algorithm. The found design is then produced as a prototype and tested. The axial-bending stiffness ratio that has been found was 8.5 and the maximum error of the PRBM model 18%.

This thesis presents the design of a quasi-compliant spherical joint which is neutrally stable in two DoF and has a remote center of rotation. Such a joint can be used, for example, as an exoskeleton's shoulder joint. The foundation of this joint design lies in previous work that has succeeded in designing a compliant spherical joint with a remote center of rotation, but without neutral stability. The existing joint is simulated and its energy properties are analysed. Thereafter it is adapted and optimized for an axi-symmetrical energy field. A spring is introduced to the joint and optimized such that the combined energy field of both spring and joint, is neutrally stable. Experimental verification of the simulation was achieved with a prototype for which a moment reduction of 83.69% was achieved through the addition of the spring.

Increasing demand for higher precision machines is arising in the high-tech industry. Linear guides are often used to establish this highly accurate and highly repeatable motion. The amount and variation in preload play an important role in the total motion errors. The objective of this paper is to investigate the relation between preload and repeatability for both the ball linear guide and the cross-roller linear guide by performing experimental tests. Three preload cases at three positions are investigated and both longitudinal repeatability and rotational repeatability are measured. In the first case, the preload will be applied uniformly by adjusting two adjustment screws on one side. In the second case, only one screw will be adjusted and therefore generates an angular preload. In the third case, an additional mass is added resulting in external preload. The experiments are performed at 0-25mm, 25-50mm, and 50-75mm, in which zero represents the position both rails are in one line. Performances of the roller guide have a smaller spread at different positions compared to the ball guide. Also, after averaging the different positions for both rails, the repeatability of the roller guide tends to be generally lower than the ball guide, suggesting that the performances of the roller guide are better than those of the ball guide. For the ball guide, the longitudinal repeatability has an optimal point for uniform applied preload, while for the roller guide the performance slightly improves as the preload increases. The longitudinal repeatability generally increases for both the ball and roller guide with the angular applied preload, with the exception of an adjustment of 10 degrees for the ball guide. The longitudinal repeatability stays constant with the external preload. The rotational repeatability decreases for both the ball and roller guide with both uniform and angular applied preload. The rotational repeatability remains constant for both the ball and roller guide with external preload.

In this work, the possibilities to approach various nonlinear moment-angle characteristics with a kinematically indeterminate rigid body balancer with torsion springs are examined. These torsion springs are mounted on the axes that intersect the rigid bodies. The rigid body balancer is coupled to an inverted pendulum. Although the kinematic indeterminate nature of the system enables the balancer to rotate non-proportionally along with the pendulum, the kinematics should correspond with the equilibrium configurations of the system. The required system parameters as spring stiffnesses and element lengths are obtained by optimization with a genetic algorithm. In addition to the standard optimization case, the effects of prestressed springs with contact release, nonlinear springs, optimizable initial configuration and an extra segment on the approximations are studied as well. Moreover, an extra objective function that concerns the distribution of energy among the springs is introduced. Eventually, the results that are obtained by the proposed method are verified with an experimental setup that contains a prototype of the system. The experimental results show agreement with the model with 93.47% work reduction. The corresponding model reduces the required work with more than 99%, which is higher than found in the state of the art.

Duchenne Muscular Dystrophy (DMD) patients suffer from a severe form of progressive muscular weakness. Consequently, as the disease progresses these patients become more and more dependent on assistive devices for their daily activities. For instance lifting their arms against gravity already becomes challenging. So far, a considerable amount of effort has been put in the development of assistive devices. However, compensating the weight of the hand remains challenging as the required balancing torques are dependent on the position of the hand and the orientation of the forearm.

In this research a solution is proposed to passively compensate the weight of the hand by making use of a simplified torque profile (a constant torque). The effectiveness of this profile was assessed experimentally in a group of healthy subjects. For this the wrist muscle activity required to lift the hand against gravity was measured through surface electromyography, for several types of compensation. From this experiment the conclusion can be drawn that a constant torque profile performs similar to the theoretically ideal type of balancing, showing a similar reduction in the anti-gravity muscle activity.

Based on these findings, a mechanism has been developed capable of providing an adjustable constant force, by making use of a combination of positive and negative stiffness springs, for implementation in a wrist support for DMD patients.

A broad range of bistable, multi-stable and neutrally stable compliant mechanisms have been developed over the years. These non-monostable systems show great potential for deployment in e.g. the medical, the manufacturing and the precision industry. Often the presence of prestress is a requirement for obtaining non-monostability within the realm of compliant mechanisms. This prestress is for example a strict necessity for neutrally stable behaviour, but also various types of bistable systems need it for correct operation. Systematic methods to induce prestress to these mechanisms do not yet exist. Currently, the application of prestress is most often achieved by manual assembly steps. The manual nature of these assembly steps poses problems onto the dimensional scalability, the production scalability and the design freedom of these mechanisms. In this research the possibilities of imposing prestress onto compliant mechanisms during their fabrication, solely by utilizing inherent features of the manufacturing method, are explored. With such a technique at hand, an accurate, scalable and tailorable method for prestressing is obtained. In this work the fused filament fabrication technique is used to create PLA monostable compliant mechanisms. By making use of the heat-induced shrinkage property of PLA, a distributed and variable material shrinkage field can be acquired. By the virtue of this variable shrinkage, elements within the system can selectively be prestressed. Results show that the mechanical stability behaviour of a demonstrator mechanism can permanently be changed from monostable to bistable. The influence of various design parameters on the shrinkage yield and the force-delivering potential of PLA is examined to tailor and maximize both factors. The result of this work provides the onset for the further creation of multi and neutrally stable compliant mechanisms in a systematic way.

Active materials are a type of material that can respond to external stimuli and change their physical properties as a response. Neutrally stable compliant mechanisms are a class of compliant mechanisms that use the flexibility of their members to deform and maintain that deformed configuration without requiring external work. In order to make a compliant mechanism neutrally stable, pre-stress is required. In most cases, this pre-stress is applied when the neutrally stable compliant mechanisms are being made. Therefore, neutrally stable compliant mechanisms are always in a zero-stiffness state. In this research, a new Compliant Active Neutrally Stable joint (CANS-J) is developed in which this pre-stress can be turned on and off using active materials. This means that the joint can switch between a stiff state and a zero-stiffness state. This change in the stiffness is realized by a Flexinol SMA wire. A Flexinol wire can contract when it is heated. This active behavior of the Flexinol wire is used to turn on and off the pre-stress when required. Results show that the CANS-J shows behavior which is close to neutral stability in the active state with a significant range of motion. The CANS-J can for instance be used to transport a satellite with solar panels to its orbit around the world. During transportation the solar panels are folded for which the joints are required to be in a stiff state. In orbit, the solar panels are unfolded for which the joints are required to have zero stiffness.