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 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.

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.

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%.

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.

The goal of this thesis is to look into the possibilities of making origami neutrally stable. This is wanted because the inherent stiffness of origami mechanisms introduces unwanted artifacts such as a higher actuation force, and the mechanism not following the theoretical kinematics. By making an origami mechanism neutrally stable the inherent stiffness of the pattern can be removed, and with that the unwanted artifacts as well. Two different strategies are explored, both a form of static balancing. With static balancing, two elements are balanced against each other. In the origami application, this means that two creases are balanced against each other. For the first strategy, a negative stiffness crease is combined with a positive stiffness crease. And for the second strategy two equal but opposite constant moment creases are balanced. To achieve this a negative stiffness, or constant moment crease needs to be designed. For the negative stiffness crease, a design was made where a flat sheet with a slot in the middle was prestressed into a saddle form. This showed bi-stable behavior, and with that negative stiffness. The range of negative stiffness was too short to be relevant for origami. And the prestressing proved hard to model. For the constant moment crease, a convex crease was designed, which did not need to be prestressed. This was easier to model, and three different geometries were found that showed a constant moment. By optimizing these geometries a constant moment over a range of 80 degrees was found. To check if the model is correct, a prototype experiment was performed. The optimized geometry was 3D printed and tested under the same boundary conditions that were present in the model. The results of the prototype experiment matched the results of the model, thereby validating it.

The mechanisms that move plants can serve as biological role models for engineers, designers and architects. This practice is slowly being implemented in various engineering fields, with architects often being pioneers. Various methodologies have been written about the subject, but often only from an architectural point of view.
The classification presented in this paper provides a different perspective on the
subject. It is structured like a toolbox, containing a clear classification of the technical working principles that plants use to generate motion. With the working principles abstracted, it is no longer necessary to dive deep into the inner workings of plants.
The Scopus and Web of Science databases have been systematically searched for
compliant plant movements. Plants mainly move by reallocating water, either actively via osmosis or passively via hygroscopic tissue. In compliant plant mechanisms, these basic movement initiators bring about deformations of plant parts. These movements are classified according to their goal: does the plant move quasi-static or dynamic? And does the plant only use a mechanism or does it rely on the gradual storage and fast release of elastic energy as well? Quasi-static movements are often only mechanical and reversible, while dynamic movements rely on energy storage and are often irreversible due to their failure-based release. A bilayer structure in one form or another is present in almost all mechanisms, proving its large adaptability to various circumstances. This
adaptability is achieved by the various configurations of the two layers, including
fibre-orientation and cellular set-up.
Existing bio-inspired devices are classified according to the same system. This
enables identification of plant mechanisms that are already suited for implementation and exposes plant movements that are not yet used in the human world. Additionally, a lot could be gained from copying mechanisms on a cellular level rather than on a macroscopic level. Most importantly, a change in mindset needs to happen in order to fully benefit the intricate mechanisms that the plant world has to offer.

The objective of this thesis is to show that warping can be used in a thin-walled beam to create a suitable replacement for a classical differential mechanism. This normally negated deformation of the cross section can be used to create an inverse transmission mechanism, used to create a monolithic and simple alternative to a classic differential mechanism. This warping beam differential mechanism can be used to add walking functionality to a back support passive exoskeleton. These exoskeletons are optimized to reduce muscle stress during lifting, but this comes with the downside of an increase in muscle activity during walking. To counter this increase in muscle activity, the use of a differential mechanism is suggested. To show the feasibility of the proposed solution, a characterization of the warping beam differential mechanism is made for different geometric properties. This characterization is done with respect to two functionalities, to determine the geometric advantage of the inverse transmission and the rotational compliance under torque of the beam. The warping constant and torsion constant are determined to be the geometric properties of most influence on the behaviour of the warping beam. It is also shown that within the proposed boundary conditions, different beam cross sections with the same geometric properties show the same behaviour. An increase in warping constant results in an increase in geometric advantage and a reduction of the rotational compliance. The torsion constant has a linear correlation to the stored energy for the inverse transmission functionality, having less influence on the geometric advantage and rotational compliance compared to the warping constant. This thesis shows that this new kind of differential mechanism can be used to replace the classical mechanism and the potential can be seen with an achieved theoretical geometric advantage of almost 1. A systematic overestimation of the geometric advantage and an underestimation of the compliance is made by the model with respect to the experiments, but with some improvements this theoretical goal is not far away.

Running robots at insect scale are an upcoming research field, because of their numerous potential applications like exploration of hazardous environments such as collapsed buildings, natural disaster sites and debris. To date no report exists on such a robot that either exploits its resonance, or that is monolithic. These properties, however, could improve the performance of these robotic insects in terms of efficiency, actuation complexity, manufacturing tolerance, suitability for rapid prototyping, large number reproduction and miniaturization. This article presents the monolithic design of FARbot; a Frequency Actuated Resonant robot that uses resonance to increase its stride length and consists of one single piece. To achieve the design of FARbot a novel design methodology is presented and utilized to systematically obtain the necessary compliant mechanism that resonates in a desired motion and at a desired frequency. The final design has been manufactured monolithically from the material HTM140-V2 using digital light processing 3D-printing technology. In terms of production time FARbot outperforms all other robots in this research field. An earlier produced, non-monolithic prototype shows resonance at the desired eigenfrequency at which its stride length is amplified by a maximum factor of 22.74 for constant energy demand. This proves the benefit of using resonance and also validates the proposed design methodology.

Compliant shell mechanisms are thin walled structures that achieve their motion through deformation. Shell mechanisms are of recent interest for designing exoskeletons that are inconspicuous. One of the challenges in designing shell mechanisms is getting as much compliance as possible in certain directions while keeping the other directions sufficiently stiff.

Prestressing is a technique that is used to change the stiffness of compliant mechanisms and it makes it possible to generate compliant mechanisms that have zero or negative stiffness. Seffen et al. have generated a shell with zero stiffness by rolling a metal plate in two perpendicular directions showing that it is possible to generate shell mechanisms without stiffness.

The main disadvantage of this technique is how it limits the different shapes that are possible. The goal of this thesis is to create a compliant shell with more freedom in shape that used the same physical principle to generate a degree of freedom with a much lower stiffness. The method chosen was to use a shell made out a carbon fibre composite that was prestressed with the thermal stresses that are the result of the curing process.

The thesis presents the analytical model and the finite element model that has been used to describe the effects of prestress and the experiments that have been done to verify these models.
 

A particular design choice in origami engineering is to use curved creases. By folding a curved-crease design, the sheets connected to the crease line have to bend, allowing for novel properties unachievable with straight-crease origami. This thesis explores the design of curved-crease-folding mechanisms with integrated planar bending actuators. The ability of bending actuators to transmit forces across crease lines is investigated. The influence of the shape of the crease line on the output force is studied on a simple curved-crease gripper. From our analysis we conclude that curved creases perform better, provided that the crease pattern or the surface material ensures that the moving of generators along the surface is minimized. Other curved-crease and actuation concepts are also explored.