The use of exoskeletons by the elderly, disabled people, heavy labor workers, and soldiers can have great social and economic benefit. However, limitations in usability are impeding the widespread adoption of exoskeletal devices. Kinematic compatibility, comfort, volume, mass, simplicity, expandability, and the ability to transmit forces, relative angles between the exoskeleton and the human, and the donning and doffing procedure need to be considered. Over the last decades, a large number of exoskeletons have been developed, to assert kinematic compatibility and compensate for misalignment. To such a degree, that it has become difficult to keep an overview of the different strategies. Therefore, this review article presents an extensive overview of different misalignment compensation strategies existing in the literature. Further, these strategies are organized in nine categories, evaluated and discussed around the exoskeleton's application domain and its specific requirements and needs.

Misalignment Compensation for Full Human-Exoskeleton Kinematic Compatibility: State of the Art and Evaluation

Piezoelectric energy harvesters (PEHs) promote the construction of a smarter world through powering electric devices with energy scavenged from environmental vibrations by means of piezoelectric effect. To enable the piezoelectric effect, piezoelectric materials are usually attached to mechanical structures (e.g. flexible beams) that can deform due to mechanical vibration and produce strain in piezoelectric material. The mechanical structure for energy harvesting in nature is a Compliant Mechanism (CM). A large variety of structural solutions have been proposed aiming to expend the working frequency range and maximizing the energy output of PEHs. To advance PEHs, a comprehensive review on existing structural solutions and materials is necessary. According to structural characteristics of current PEHs from the aspect of CMs, designs in state of the art are analyzed and categorized into five configurations, mono-stable, multi-stable, multi-degrees-of-freedom, frequency up-conversion and stress optimization. For each configuration, working principles and compatibilities with miniaturization to MEMS scale are analyzed and assessed. Additionally, several CMs are first proposed for PEHs in different configurations as inspirations and references to prompt the development of PEHs. Piezoelectric materials are also important factors in enhancing the energy harvesting performance. Characters of several widely adopted piezo-materials are summarized and compared. The metric of Normalized Power Density (NPD) is introduced to compare and assess the energy generation capability of PEHs with several widely-used piezoelectric materials and in different scales. A NPD-Volume graph is first presented based on the data collected in literature. It shows that PEHs with PZT have the highest NPD and stable energy generation performance in a wide volume range. Both the structural categorization and NPD-Volume graph provide guidance and reference for design and optimization of PEHs.

A review on vibration-based piezoelectric energy harvesting from the aspect of compliant mechanisms

Fabrication of complex and multi-articulated mechanisms is often seen as a time consuming and demanding process. The development of functional multi-articulated mechanisms that could be fabricated in a single step without the need for post-manufacturing assembly is therefore very attractive. Additive manufacturing (AM) has been pointed out as a feasible solution due to its numerous advantages and high versatility in comparison to other manufacturing techniques. Nevertheless, AM techniques also present different shortcomings that limit the complexity of the mechanism for single step fabrication. Here, we review the applications of AM techniques in fabrication of non-assembly multi-articulated mechanisms and highlight the involved challenges, thereby providing a perspective regarding the advantages and limitations of current AM techniques for production of complex mechanical devices. The paper starts off with basic joint elements in rigid-body and compliant configurations and proceeds with presenting an overview of multiple arrangements of joints and assemblies with embedded mechanical components. For every case of non-assembly fabrication, the limitations of the applicable AM processes are presented and further discussed. This work concludes with a discussion of the major shortcomings found in current non-assembly mechanisms fabricated by AM and recommending alternative techniques and future developments on AM.

Additive manufacturing of non-assembly mechanisms

Compliance Synthesis of CSFH MEMS-Based Microgrippers

Large stroke flexure mechanisms inherently lose stiffness in supporting directions when deflected. A systematic approach to synthesize such hinges is currently lacking. In this paper a new building block based spatial topology optimization method is presented for optimizing large stroke flexure hinges. This method consists of a layout variation strategy based on a building block approach combined with a shape optimization to obtain the optimal design tuned for a specific application. A derivative free shape optimization method is adapted to include multiple system boundaries and constraints to optimize high complexity flexure mechanisms in a broad solution space. To obtain the optimal layout, three predefined 3-D “building blocks” are proposed which are consecutively combined to find the best layout with respect to specific design criteria. More specifically, this new method is used to optimize a flexure hinge aimed at maximizing the frequency of the first unwanted vibration mode. The optimized topology shows an increase in frequency of a factor ten with respect to the customary three flexure cross hinge, which represents a huge improvement in performance. The numerically predicted natural frequencies and mode shapes have been verified experimentally.

Building block based spatial topology synthesis method for large stroke flexure hinges

A Review on Compliant Joints and Rigid-Body Constant Velocity Universal Joints Toward the Design of Compliant Homokinetic Couplings

This paper presents the concept and fabrication of a large deflection compliant Constant Velocity universal joint (CV joint). A novel compliant structure is proposed based on the 6R Hybrid spatial overconstrained linkage. Due to symmetry, its kinematic properties are such that can transfer rotational motion between two angled shafts with true constant velocity. The kinematic of the mechanism and the Pseudo-Rigid-Body model of its compliant configuration are studied and analyzed. A prototype was manufactured and experimentally evaluated. It was verified that the experimental results are consistent with the theoretical expectations.Copyright © 2015 by ASME

A Fully Compliant Constant Velocity Universal Joint

Vibration energy harvesting can be used as a sustainable power source for various applications. Usually, the generators are designed as devices with a single degree of freedom (SDoF) along the direction of the driving motion. In this research, harvesting from multi-directional (translational) motion sources will be investigated. Three strategies are assessed: a reference SDoF generator, a SDoF generator using an orientation strategy, and a Multi Degree of Freedom (MDoF) system. This led to the development of a design metric by which any 2D design problem can be described by two dimensionless parameters: the relative strength of vibrations, $p_{v}$, and the relative dimension of the design space, $p_{l}$. It was shown that the relative power density (RPD) of a 2DoF system compared to a reference SDoF system only depends on the product $p^{\ast}=p_{v}p_{l}$, and has a maximum of 1.185 for $p^{\ast}=1$. The application of powering a hearing aid is investigated as a case study. It was found that the vibrations in the area of the human head while walking can be represented by a two-directional vibration source with $p_{v}=0.55$. Three different design spaces are assessed for a miniaturized generator and three different optimal embodiments are found. For one of the considered situations where $p^{\ast}=1.1$, a 2DoF system was found to have a 16% higher power output compared to a SDoF reference. The aim of future work will be the validation of the developed metric.

Vibration Energy Harvesting from Multi-Directional Motion Sources

Static balancing is used to reduce the actuation stiffness in a translational stage compliant mechanism The planar and monolithic compliant mechanism is preloaded using a buckling beam of which the top part is guided by a double folded flexure Hooks, that lock the top part of the beam in place, ensure a permanent static balancing of the entire device The preloading action is caused by an external shaking or shock to the device A theoretical micro electromechanical system (MEMS) model with a radius of 186 mm is developed and a scale 6:1 prototype is fabricated and tested for static balancing, first eigenfrequency (EF) and eigenmode (EM) Finite element modelling is used to predict static balancing and EM behaviour Experiments on two equally fabricated prototypes show a reduction of −123 % and −126% actuation stiffness where −1045% was predicted The expected reduction for the designed MEMS device is 984% The experimental first EF of the prototype is 310±025 Hz against a theoretical value of 309 Hz The theoretical first EF of the MEMS model is 218 HZ The prototypes are successfully preloaded by applying shaking or shock by hand The predicted minimal energy requirement for this is found to be 49e-3 J, while 64e-3 J and 59e-3 J were calculated based on experimental results The expected minimal energy required for preloading the designed MEMS device is 10e-5 J

Permanent Preloading by Acceleration for Statically Balancing MEMS Devices

A new fully compliant rotational power transmission mechanism is presented. The design is based on the Pseudo-Rigid-Body Model (PRBM) of the Oldham constant velocity coupling. It can be fabricated as a single piece device with planar materials which make it suitable for micro scale applications. The internal stiffness of the proposed structure is eliminated by static balancing technique. Therefore, the compliance and zero stiffness behavior compensate for the structural error and poor efficiency inherent in rigid-body Oldham coupling, resulting in high mechanical efficiency power transmission system. The device is designed and its motion, torsional stiffness, and torque-angular displacement relations are predicted by the PRBM and finite element modeling. A large/macro scale prototype was manufactured and measured to evaluate the concept. This high efficient power transmission system can be applied in different applications in precision engineering and the relevant field such as micro power transmission system.

D. Farhadi Machekposhti

Papers

A Review on Compliant Joints and Rigid-Body Constant Velocity Universal Joints Toward the Design of Compliant Homokinetic Couplings

A Fully Compliant Constant Velocity Universal Joint

Vibration Energy Harvesting from Multi-Directional Motion Sources

Permanent Preloading by Acceleration for Statically Balancing MEMS Devices

Monolithic and Statically Balanced Rotational Power Transmission Coupling for Parallel Axes