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. ... Read more

Individuals suffering from neuromuscular conditions, such as Duchenne muscular dystrophy (DMD), experience a progressive weakening of muscles. During the progression, the affected patients require supporting devices like orthoses or exoskeletons to perform their daily activities. Although immense efforts are put into the development of exoskeletons, due to the challenge involved in balancing the weight on the hand, where the torque at the wrist changes with the position of the hand, there are limited wrist support devices available. Existing solutions either rely on rigid body mechanisms or active power sources, which come with drawbacks like bulkiness and discomfort. This research introduces an innovative approach to passively counterbalance hand weight using a simplified torque profile for a single degree of freedom in hand movement, namely flexion and extension. By employing parametric optimization, a compliant mechanism has been developed that can compensate for the weight of the hand against gravity. The resultant compliant mechanism design is compact, lightweight, and safe, rendering it an optimal choice for supporting the wrist’s degrees of freedom and for the application of the device. Through the experimental validation of the model, the results are interpreted as the mechanism’s usefulness in wrist flexion and extension support. In short, this study underscores the utility of employing compliant mechanisms in wrist support orthoses, particularly for individuals with neuromuscular conditions like DMD. ... Read more

This work aims to achieve a softening moment-angle response by utilizing compliant parts. The approach uses the principle of contact release, which involves initially prestressed torsional bars arranged in series, separated by rigid bodies. By stepwise activation of the torsional bars, a softening behavior is achieved. Mechanical stops are employed to maintain the initial prestress.
A Pseudo Rigid Body Model (PRBM) is developed to calculate the moment-angle behavior of the complete prototype. The optimization of the PRBM is performed using a cost function based on the least square error. The optimization parameters are the stiffness of the torsional bars and their initial prestress. Additionally, a Finite Element Analysis (FEA) is conducted to analyze the behavior of an individual I-profile torsional bar. Experimental validation of both the PRBM and the FEA models is carried out using a prototype. The prototype is constructed based on the results obtained from the PRBM optimization to follow the desired graph closely. The results from both FEA and PRBM align closely with the predicted curves, confirming the effectiveness of the proposed approach. ... Read more

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. ... Read more

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%. ... Read more

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. ... Read more

People in healthcare, warehousing, and the agriculture sector all have one thing in common. They require labour which can be demanding on the human body, this could lead to problems in the long term. A way to alleviate these problems is to use a passive exoskeleton. While passive exoskeletons can have a variety of different use cases and types of support, this thesis will focus on a wearable passive back support. A company specialising in these passive wearable back supports is Laevo. They have their own version of such a mechanism using torsional springs which are attached to the upper torso and legs. This ensures that when bending the mechanism provides support. However when walking, these springs are also activated and make walking more difficult and require more energy. A way to solve this problem is to use a differential mechanism. Such a mechanism can be quite complex and bulky and comprised of a lot of parts. A solution could be found in the world of compliant mechanisms.

The goal of this project is to create and analyse a compliant differential mechanism for use in passive exoskeleton design. As a basis for a design, the proposed mechanism by Maurice Valentijn was used. The two main challenges were the location of the rotational axis and a relatively low stiffness ratio between the bending and walking scenario. While this mechanism showed potential as a compliant differential mechanism, it did have some problems which needed to be overcome before the use in a passive exoskeleton would be feasible.

The proposed design to solve these challenges consists of a thin-walled beam, with an H-shaped cross section, which has two curves forming a U-shape. By applying constraints on the sides the rotational axis of the mechanism could be changed to align with the rotational axis of the hip joint. This mechanism in combination with reintroduction of potential energy using springs to lower the stiffness of the mechanism when walking. This allowed for a design which could be used as a compliant differential mechanism in the use of exoskeleton design. To investigate the proposed design for a compliant differential mechanism, the mechanism first needs to be modelled and optimised to meet the requirements needed for the use in an exoskeleton. For the optimisation, a framework is proposed which integrates the use of a simulated Ansys model in combination with a Matlab optimisation problem. With this optimised model the behaviour of the mechanism can be analysed. The behaviour of the mechanism is investigated in the paper in Chapter 4. In this paper the simulated model is validated using an experimental setup. This paper is the main contribution of this thesis.

Findings of the paper show that the stiffness of the mechanism can be significantly reduced by reintroducing potential energy into the system to compensate the stored potential elastic energy in the material during walking. This caused the mechanism to have different types of behaviour: positive stiffness, zero stiffness, and negative stiffness. These stiffnesses depend on the initial preload of the springs, more initial preload means more energy is stored in the springs and releases more energy for the same displacement. This changes the overall potential energy to have these three aforementioned stiffness states. Zero stiffness is the most interesting for exoskeleton design, this minimises the amount of work required while walking without having bistable behaviour in the mechanism.

Finally, As a proof of concept for the mechanism a wearable exoskeleton prototype has been created to get a practical understanding of the mechanism and to find problems with the implementation of the mechanism for future works. The wearable exoskeleton prototype showed a lot of potential, the behaviour of the mechanism in the experimental setup was transferred to the prototype and low stiffness while walking was achieved without effecting the bending support. ... Read more

Lateral torsional buckling (LTB) of I-beams is an established buckling phenomenon in civil and structural engineering known for causing structural failures. However when a buckling instability is present the structure contains a form of negative stiffness. Due to the observed tendency of I-beams subject to LTB to twist about their longitudinal axis it is suspected that such I-beams contain a form of negative rotational stiffness about their longitudinal axis. Rather than considering LTB as a failure mode for structures, this work aims to investigate the potential of cantilever I-beams subject to LTB having a reduced rotational stiffness about their longitudinal axis with minimal lateral deflection on demand by subjecting the beam to LTB. By minimising the lateral deflection the longitudinal twisting behaviour approaches the behaviour of a compliant rotational joint. The occurrence of negative rotational stiffness in the buckled state is verified and a parameter study is executed to evaluate the stiffness reduction and co-occurring lateral deflection by means of 1-dimensional finite element modelling. Finally an optimisation is performed to obtain a beam with minimal lateral deflection and a significant reduction in longitudinal rotational stiffness. The simulated behaviour is experimentally validated. The simulation and experiment showed that rotational stiffness can be significantly reduced through LTB. Additionally the experiment verified the significant reduction of lateral deflection for the optimised beam. Overall this work forms an initial step towards the use of reduced rotational stiffness of beams by exploiting lateral torsional buckling. ... Read more

This report proposes a concept for an I-profile curved beam compliant transmission system. The application of this system can be as a hip connection within a passive exoskeleton in which it transfers the walking motion from one leg to another. To obtain the desired behavior the torsion stiffness is reduced and the amount of work needed to move the system is minimized, resulting in a reduction of 80%. Thereby, it provides a bending stiffness that is almost ten times larger than the torsional stiffness, offering support during bending and lifting movements. Replacing classical subsystems with compliant mechanisms offers opportunities for parts and weight reduction. A proof of concept prototype is obtained out of PA12 and the behavior is demonstrated in a series of experiments. These experiments are supported by a Finite-element analysis, which is also used to predict the behavior for different materials, geometries and applications. The validated concept is applied in a design case for the back support system of Laevo Exoskeletons. ... Read more

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. ... Read more

Especially in the car industry, spot and projection resistance welding are commonly used to weld thin sheets of steel and aluminium. Spot welding leaves visible indentations after welding, whereas the company Arplas Systems uses a projection welding method resulting in almost invisible welds. For spot welding, C- and X-type welding machines are available, but for projection welding machines no X-type variant exists as of yet. This is because projection welding need high precision, high electrode forces and most importantly, fast follow-up characteristics.

In this thesis, the current C-type machine is analyzed and based on those machine characteristics, a design of a robot mounted X-type machine was developed and tested. The prototype of the X-type design was successfully built and is capable of welding both steel and aluminum samples. The design implements the projection welding method of Arplas systems and can be used manually or mounted on a robotic arm. ... Read more

Elastic neutral stability involves elastic deformation without stiffness or loads. It is a remarkable appearance, since the deformation of materials is normally associated with an increase of potential energy and a resulting opposing reaction force or moment. Neutrally stable mechanisms can be used to overcome unnecessary actuation which makes them an interesting topic for the aerospace industry, space exploration and the development of wearable assistive devices. This last group benefits from the use of spatially-curved thin-walled elastic structures, called compliant shell mechanisms. A literature review aims to give an overview of occurrences of elastic neutral stability and aims to find methods for creating neutrally stable compliant shell mechanisms. Single element mechanisms are herein further emphasized and a division between the application of pre-stress and the application of geometrical boundary conditions is proposed. So far, all neutrally stable (shell) mechanisms require either of the two conditions to be imposed. A new type of compliant shell structure, featuring a neutrally stable deformation mode without requiring one of the aforementioned conditions, is presented. The structure is composed of two initially flat compliant facets that are connected via a curved crease. It can be reconfigured into a second zero-energy state without apparent effort via propagation of a transition region. Both the structure’s local width and the local crease curvature turn out to be effective parameters for tuning the behavior regarding stability during transition. This structure shows potential for combining geometric simplicity with complex and highly tune-able behavior. However, its discontinuity obstructs physical realization. Therefore, a monolithic variant of this structure is investigated. The transition of a double-curved distributed compliant shell towards its second equilibrium configuration forms the basis of this investigation. A varying material thickness profile, described by an ideal set of design parameters, is obtained using an optimization procedure. Numerical analysis of the resulting optimized shell structure predicts a significant region of near-constant energy and associated near-zero loads within this unique deformation mode. Prototypes are manufactured using a 3D-printing process and demonstrate the validity of the modelled results by featuring a continuous equilibrium within a significant range of motion. These results lay the foundation for compliant beam elements with an internal statically balanced bending degree of freedom. Finally, a different deformation mode, involving crease actuation of the same type of structure, is examined. Curved creases are characterized by the coupled facet deformation upon actuation. The forces exerted by the facets can be used to oppose the effects of crease stiffness during actuation to achieve an overall stiffness decrease. An analytical approach, based on a combination of a pseudo-rigid-body model (PRBM) and plate theory, predicts the potential for constant-force actuation around its second stable, or ‘inverted’ state. The accuracy of the model is validated by numerical simulations. However, due to prototype inaccuracies, the experimental results do not feature the desired constant force behavior. Nevertheless, a stiffness decrease, or ‘softening’ is experienced, verifying the concept and marking a first step towards statically balanced curved creases. ... Read more

Micrometer thin, braided wires, is a niche requirement that has applications in beam instrumentation. Due to the rare use-case of extremely thin braided wires there are no established or industrial braiding methods developed. For this reason, a novel method for braiding thin wires was developed. In order to achieve a repeatable result a prototype braiding machine was designed, built and tested. Stainless steel, nylon and carbon wires were tested and evaluated. The aim of this research was to prove that braiding extremely small yarns is possible. Also, the proposed machine parameters have been opti- mized to ensure repeatability and good quality wires production. By proving that braiding thin wires, in the order of a few tens of microns, we open the way for braiding extremely thin wires for wire scanners. ... Read more

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. ... Read more