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In this paper a new friction model for the pre-sliding regime of dry friction is proposed. Current highly accurate friction models that incorporate analysis of the pre-sliding friction regime are all dynamic models. In these models the pre-sliding regime seems to be underdeveloped. The main focus of the models lies upon the analysis of the sliding regime and therefore they are velocity dependent. In the pre-sliding regime frictional behaviour appears to be a function of displacement rather than velocity. Therefore, the proposed model is a static, position dependent model specifically formulated for the pre-sliding regime of friction. Furthermore, the model is formulated in such a way that it corresponds to the physical nature of friction currently assumed to be caused by adhesion. Therefore, the model is a modified Bristle model where clusters of molecular bonds formed due to adhesion are represented as pliable bristles with a maximum deflection and certain stiffness. For parameter estimation and validation of the model, experiments have been performed by means of a measurement setup. In the experiments, values have been obtained for the system’s initial stiffness and the maximum deflection of the bristles. The experiments show that the order of magnitude of the parameters found, correspond to the expected values. The validation experiments point out that the expected relation between the pre-sliding displacement and relaxation displacement could be confirmed. Furthermore, an expected proportional relation between the systems initial stiffness and normal load on the surface could not be invalidated. The effectiveness of the model was tested in simulations of the experimental data and by comparison to simulations made by other models. The results showed that the new model is more accurate and faster in modelling the pre-sliding regime of friction than the LuGre and Dahl model.

Lifting patients is a demanding task for care providers. In addition, the number of patients is rising and the number of people with obesity is expanding. Current lifting aids are single purpose and time consuming to use. Exoskeletons can fulfill the demand for a versatile and easy to use lifting aid. A drawback of a typical exoskeleton is that in order to function correctly the axes need to be aligned to the human joints, which is time consuming. Furthermore, an exoskeleton for healthcare must reduce the reaction forces in the user while lifting. It is chosen to design an exoskeleton, which requires no adjustment and can cope with power enhancement. The goal of this paper is to design an exoskeleton that is fast, easy to use and reduces the reaction forces in the user. A design is proposed which easily fits different sized users. In addition the reaction forces in the human skeleton are eliminated. The model is tested by means of a demonstrator. Elastic tension elements are used as a gravity compensator. By reducing the potential energy fluctuation the required external input is reduced. Optimizing a number of design parameters leads to a calculated moment reduction of 98.8% and moment fluctuation reduction of 96.8%. This is the first exoskeleton to combine fast to use design with high-energy efficiency gravity compensation.

The particular category of the underactuated grippers is chosen for the automation of the item picking in distribution centers. The underactuated grippers have fewer degrees of actuation than degrees of freedom, so they are mechanically simpler than the fully-actuated grippers, and they are able to adapt to objects regardless of their shapes. However, the existing underactuated grippers found in the literature are regarded overdesigned because more than enough passive elements are included. This paper is aimed to design and build a more simplified but still workable underactuated gripper for the item picking. The designed gripper contains a cable-pulley driven underactuated finger which has two phalanges, and an opposite fixed finger. Moreover, the fingertip of the underactuated finger is intended to move along the ground where the target object is laid. The dimensions of the gripper are selected in order to achieve the following two tasks: picking the cylindrical objects from the ground and retaining the grasp during a lifting transportation. The experiment setup fails to drive the fingertip of the underactuated finger moving along the ground, but it is shown that the designed gripper is still able to fulfill these two tasks, except for the case that when the initial spacing between the moving underactuated finger and the object is rather large.

Meniscectomy is a medical procedure where ruptured meniscal tissue is removed within the knee joint. The con- ventional cutters fail to reach the entire meniscus. Therefore, the focus of this study is to create a cutter with a steerable tip, which allows sideway steering to increase the reacha- bility within the knee joint. Additionally, this steerable joint is required to be robust to transmit a cutting force of up to 190N. The mechanism design is divided into the functions: steering and actuating cutting mechanism. The most promis- ing solution of each function was combined and resulted in the use of a crossed configuration of a Compliant Rolling- Contact Element for the instrument joint. Flexural Steering Beams actuate the rotation of the joint using the principle of a parallelogram mechanism. A prototype of this mechanism has a range of motion of +25 and -22 degrees with a steering stiffness at the handle side of 18Nmm/rad. An axial load of 200N on the tip corresponded with a parasitic deflection of 4 degrees. This unique type of a steerable joint shows poten- tial to be functional in a meniscectomy cutter due to its great robustness towards compression, yet allowing the instrument tip to deflect.

The majority of existing problems within conventional prosthetic fingers are related to the use of conventional rigid links and kinematic joints and to the lack of adaptability of the finger. In this paper these problems are solved by the design of a fully compliant under-actuated prosthetic finger. At first a basic structure was defined. Subsequently a Pseudo Rigid Body method was used for a type synthesis and rough dimensional analysis in order to determine the topology of the conceptual design. In order to evaluate the grasping behavior of the conceptual design, four mock-ups were created. Detailed dimensioning design was performed by semi automatic numerical analysis using a finite element method in which the conceptual design was used as an initial input. A prototype based on this final design was manufactured and experimentally evaluated. It was found that utilizing the concepts of under actuation and compliance solved the identified problems within conventional prosthetic fingers. As a result of the design process and the use of a predefined structure a fully compliant underactuated finger with a monolithic structure and distributed compliance was obtained. In addition to the application field of prosthetics the design shows potential of being applied in the field of robotics and graspers.

Fatigue tests are a very time consuming process in engineering, in which also a lot of energy is required. Speeding up fatigue tests and making them less energy consuming, would lead to major profits. The aim of this study is to obtain these profits by the development of a new bending fatigue test. In addition, the objective is to apply this new method to the bending fatigue test of rings from a CVT-Pushbelt. A new bending fatigue testing method is developed and is successfully applied to the bending fatigue test of the CVT-pushbelt rings. The new method makes use of spring capabilities of the ring for displacement amplification via resonance. A concept model is created with which tests are done to validate the new method. It is concluded that the speed is increased by almost 60% and the energy requirement is reduced by 95%. Tests until specimen failure show similar results as current tests, while the conditions of the fatigue test remain the same.

A gravity balancer is a mechanism that compensates the weight of a mass over a range of motion. When no friction is present, this gives an energy efficient mechanism and little effort is required to move an object. Conventional mechanisms have drawbacks due to the use of conventional rigid joints. Compliant joints do not have these disadvantages, can be made from fewer parts and can increase performance compared to rigid body joints. The goal of this paper is to develop a single degree of freedom gravity balancer where all the rigid joints are replaced with compliant joints. To reach this goal a new method has been developed. The method is based on connecting rigid links with compliant joints. With a constant potential energy as objective, the method allows new gravity balancers to be designed. It can be concluded that for the first time a gravity balancers has been constructed where all the rigid joints are replaced with compliant joints. The gravity balancer had a peak moment reduction of 93%. The presented method is extensible and allows others to understand and to further develop gravity balancers with compliant joints for other applications.

Static balancing is a useful concept to reduce operating effort in mechanisms. A statically balanced system which is designed to counterbalance a mass, is referred to as a gravity equilibrator. The potential energy in a gravity equilibrator is constant, which in most of the times is achieved by mechanical springs. Often helical springs are used, although these springs take a lot of space within the workspace of the mechanism. This paper presents the design of an adjustable gravity equilibrator using torsion bars, which saves space in the working area. Static balancing is achieved with a non-constant transmission (NCT). A new NCT design, and a general method to calculate the design parameters are presented. The stiffness of the torsion bars can be adapted by changing the active length. In this way it is possible to balance different masses with the same system.

This thesis brings together, for the first time, the fields of energy harvesting and static balancing. The proposal of two new architectures for the design of mechanical oscillators is supported by an extensive review on the existing energy harvesters. For the first one, a statically balanced oscillator, an analytically study proved it to be ineffective. This pushed for the development of a statically balanced frequency up-converter, that can integrate an energy harvester capable of coping with low frequencies vibrations of broadband nature. On the static balancing ground, a new mechanism is proposed, with the balancing of the folded suspension, a traditional mechanism of precision engineering. Numerical analysis suggests that high quality balancing is achieved for a large amplitude of motion. A preliminary study is also executed, introducing bond graph modeling to the field of energy harvesting. Bond graphs are a natural representation for the cross-domain nature of energy harvesters, allowing an integrative view.

A gravity equilibrator is a statically balanced system which is designed to counterbalance a mass such that any preferred position is eliminated and thereby the required operating effort to move the mass is greatly reduced. Current spring-to-mass gravity equilibrators are limited in their range of motion as a result of design limitations. An increment of the range of motion is desired to expand the field of applications. The goal of this paper is to develop a compact one degree of freedom mechanical gravity equilibrator that can statically balance a rotating pendulum over an unlimited range of motion. Static balance over an unlimited range of motion is achieved by a coaxial gear train that uses non circular gears. These gears convert the continuous rotation of the pendulum into a reciprocating rotation of the torsion bars. The shape of these gears are specifically designed to balance a rotating pendulum. Our gear train design and the method to calculate the parameters and the shape of the non circular gears are presented. A prototype is designed and built to validate that the presented method can balance a pendulum over an unlimited range of motion. Experimental results show that a work reduction of 86.8 \% is achieved compared to an unbalanced pendulum and the hysteresis in the mechanism is 36.3 \%.

The Minimally Invasive Manipulator (MIM) copies the movements of the hands of the surgeon to the tip of the instrument inside the operating area. The movement is mechanically copied using cables and pulleys. When the surgeon grasps and holds an object, the friction inside the MIM increases and distorts the force feedback in the remaining degrees of freedom. The increased friction is the result of the reaction forces on the cables and pulleys due to the required pinch force to hold the object. The goal of this report is to present two designs for a bi-stable compliant grasper. The grasper will have two stable positions: open and closed. In those positions there is no need for an actuation force from the surgeon. When the grasper is closed, it will produce enough pinch force to hold an object. The first design is a compliant grasper combined with bi-stable beam. Both the grasper and bi-stable element will be a planer design. A large scale prototype is presented and evaluated. The prototype showed bi-stable behavior over a range of 8 mm. Using finite element modeling, dimensions for a bi-stable beam suitable for the MIM are found. The second design uses a disc spring as bi-stable element. This design is a 3D design, and is on the scale of the MIM. An ANSYS model and a database were validated. Using the model and database disc spring with suitable dimensions for the MIM scale was found. Two disc springs in parallel were needed to make a bi-stable compliant grasper. Both the bi-stable beam and the disc spring can be used in the design for a bi-stable grasper. The bi-stable beam is more effectively creating negative stiffness inside a circular space. As a result, it should be able to produce more force and/or stroke in the same volume as a disc spring. This makes the bi-stable beam more suitable for use in a bi-stable grasper than a disc spring.

There is an emerging need to apply adaptive robotic hands to substitute humans in dangerous, laborious, or monotonous work. The state-of-the-art robotic hands cannot fulfill this need, because they are expensive, hard to control and they consist of many vulnerable motors and sensors. It is aimed to develop simple, adaptive hands that are capable of grasping and holding a large variety of objects. To achieve these properties, the concept of underactuation (i.e. having fewer actuators than independently moving fingers) is applied. First new metrics are defined which quantify the range of object sizes that underactuated hands can grasp and hold. Furthermore, a new method is developed to dimension the main design parameters of underactuated hands, such that the fingers can envelope and stably grasp the required range of objects. The new performance metrics and design method are applied to the design and evaluation of a new robotic hand that consists of a minimum number of motors (i.e. one) and sensors (i.e. zero). The innovation of this hand is that it mechanically decides whether to hold an object in a precision grasp or a power grasp configuration. No sensors, auxiliary actuation mechanisms, motors or control are needed to convert between these two distinct grasp configurations. It is concluded that the principle of underactuation and the proposed design method are effective to achieve self-adaptive, robust and cheap hands that are capable to grasp and hold a large range of different objects.

The fields of static balancing and tensegrity structures are combined into statically balanced tensegrity mechanisms. This combination results in a new class of prestressed structures that behave like mechanisms: although member lengths and orientations change, they can be deformed into a wide range of positions, while continuously remaining in equilibrium; in other words, the structures have zero stiffness. The key to these structures is the use of zero-free-length springs as tension members. The tools of structural engineering were used to search for, and understand, zero-stiffness modes in the tangent stiffness matrix of prestressed pin-jointed bar frameworks. To this end the recently uncovered parallels between structural engineering and mathematical rigidity theory were exploited. Mathematical literature described that affine transformations preserve the equilibrium of a tensegrity structure; these findings gained value when translated from a mathematical concept into the engineering terms rigid-body motions, shear and dilation. Not only did these transformations prove to be instrumental for describing zero stiffness, but it also provided new insight in the form-finding methods for tensegrity structures: the minimum nullity requirement for the stress matrix is formed by the affine transformations. In this research it was shown that affine transformations of the structure that preserve the length of conventional members are zero-stiffness modes valid over finite displacements: these are statically balanced zero-stiffness modes. What is more, for prestress stable structures with a positive semi-definite stress matrix of maximal rank -- meaning there are only affine transformations in its nullspace -- those are the only possible zero-stiffness modes. The length-preserving affine transformations exist if and only if the directions of the conventional members lie on a conic at infinity. If all conventional member directions lie on a conic, the number of independent length-preserving affine transformations can then be found with a simple counting rule. A systematic analysis of the zero-stiffness modes in the tangent stiffness matrix of a prestressed pin-jointed bar framework yielded several interesting scenarios that warrant further attention, as they cannot be fully described within the currently developed framework. Finally, a demonstration prototype was designed and constructed to illustrate the properties of statically balanced tensegrity mechanisms; the prototype serves as a proof of concept, not as a practically applicable design. Prior to construction, the range of motion of the tensegrity used for the prototype was extensively analysed using the analytic equilibrium conditions. The results were instrumental in dimensioning the prototype.

This paper presents the first zero stiffness six degrees of freedom (DoF) compliant precision stage. To deal with problems like backlash, friction and lubrication for performing ultra-precise positioning in a vacuum environment, a novel compliant structure is proposed. All six degrees of freedom are statically balanced (i.e. near zero stiffness) to balance the gravity force and cancel out the stiffness due to the compliant design of the structure. Cooperative action of post-buckling behavior of bi-stable beams and constant stiffness of v-shaped beams, arranged in three units in a triangular configuration, are proposed for out-of-the-horizontal-plane motions. The in-plane motions are achieved by three flexible rods loaded near their buckling load. An investigation on adjusting the design parameters to minimize the residual actuation force is also performed. A prototype was manufactured and finite element modeling was performed to evaluate the concept. Experimental evaluation showed that the design is successful: for the case study a gravity force of 34.4N was balanced with a residual stiffness of 1.75N/mm in a domain of 2mm for the out-of-plane translation, while the out-of-plane rotational stiffness was less than 18.5Nm/rad, caused by parasitic torsion of the bi-stable beams and v-shaped beams. The stiffness for in-plane translations and rotation was 0.4N/mm and 2Nm/rad, respectively. Near zero stiffness 6DoF positioning can thus be achieved. The novel mechanism or the principle may be extensively applied in several applications in precision engineering or in other relevant fields, such as vibration isolation.

Patients with foot deformation, cartilage wear or a badly healed calcaneus fracture often suffer from severe pain in the foot. After fixation of the subtalar joint, which stabilizes motion, most patients can walk without pain again. Because the curtent operation is an open approach, it is difficult to reach the entire joint space and to remove all bone and cartilage. To improve the operation technique, a minimally invasive technique was developed. Because the joint is curved and the current shavers are straight, the instrument has to allow for this. To this end, a new instrument was developed: The Tuijthof flexible instrument. The Tuijthof flexible instrument has anisotropic stiffness. It is flexible in the bending direction and is stiff in lateral direction, so the surgeon can stir the instrument. Tissue is removed by a drill. The instrument is promising, but still too flexible. In bending direction the stiffness is 10 N/m, a value between 83 N/m and 167 N/m is desirable. In lateral direction the stiffness is 1.90-10^ N/m, it should be larger than 2.5-103 N/m. Also during tests, glued connections between the wires and rings broke loose. The goal of this master's thesis is to design a single-piece compliant mechanism and to optimize the dimensions of the shaft.

Precision engineering impacts every one, every day, by enabling the fabrication techniques and functions required for 21st technological revolutions. Various applications, from display technologies to sensor systems to optical networks make use of so called microelectromechanical systems (MEMS). Motions at micro level with monolithic structures are realized by using slender elements that deform elastically. The use of compliant elements comes with great advantages such as; simplified fabrication, increased precision and elimination of maintenance. However, the monolithic nature of compliant elements has a major drawback: the elastic deformation of the monolithic structure requires significant force and energy. The thesis contributes to a solution to make MEMS more energy efficient and thereby enhancing the feasibility of smaller MEMS devices. In particular, new stiffness behaviors, revolutionary preloading mechanisms and new fabrication methods have lead to an extraordinary reduction of the stiffness in the direction of motion of MEMS. The proposed concepts create room for a new avenue in the field of precision engineering.