The usually high eigenfrequencies of miniaturized oscillators can be significantly lowered by reducing the stiffness through stiffness compensation. In this work, a mechanical design for a compliant ortho-planar mechanism is proposed in which the stiffness is compensated to such a degree that it can be identified as statically balanced. The mechanism was fabricated using laser micro-machining and subsequently preloaded through packaging. The statically balanced property of the mechanism was experimentally validated by a measurement of the force-deflection relation. A piezoelectric version of the design was fabricated for the purpose of energy harvesting from low-frequency motion. For a sub 1 Hz excitation, the device demonstrated an average power output of 21.7 μW and an efficiency that compares favorably to piezoelectric energy harvesters reported in the literature. Therefore, it was found that stiffness compensation is a promising method for the design of piezoelectric energy harvesters for low-frequency motions.

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There is a high demand for novel flexible micro-devices for energy harvesting from low-frequency and random mechanical sources. The research of new functional designs is required to strategically enhance the performances and to increase the control on mechanical flexibility. In this work we report the fabrication and characterization of bi-stable and statically balanced thin-film piezoelectric transducers based on Aluminum Nitride (AlN). The device consists of a piezoelectric layer sandwiched between two thin Molybdenum electrodes that were deposited on a Kapton substrate by reactive sputtering and patterned by UV lithography. In order to improve the out-of-plane flexibility, the mechanical design is distinguished by a post-buckled flexure that introduces a negative stiffness to compensate the otherwise positive stiffness of the system. The buckling was introduced by a new method, called Package-Induced Preloading (PIP) where the mechanisms are laminated over a package with a geometry extending out-of-plane. The induced buckling resulted in bi-stable and statically balanced mechanisms which demonstrated an enhanced voltage output during a triggered snapping step. A preliminary study shows potential for the statically balanced designs and the PIP method for wind energy harvesting, revealing prospective applications and future improvements for the development of energy harvesters.

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Cantilever piezoelectric energy harvesting from ambient vibrations is a viable solution for powering wireless sensors and low-power electronics. However, the greatest issue preventing these systems from being widely used is their poor reliability. With the aim to maximise their power output, the devices are often operated close the fracture strength, which results in cracks in the brittle piezoceramic layer. Tapered cantilevers are suggested to improve the mechanical reliability. A relative comparison is made between tapered piezoelectric cantilevers and conventional rectangular cantilevers in terms of reliability and power output. Tensional strains causing fractures show a serious reduction of power output and eigenfrequency. Experiments show that tapered cantilevers have a higher power output per unit area.@en

In this paper a method is demonstrated for tuning the stiffness of building blocks for statically balanced compliant ortho-planar mechanisms. Three post-buckled mechanisms are proposed where the flexural rigidity can be manipulated over a part of their length in order to tune the ratio between the first two critical loads. A sensitivity analysis using finite element simulation showed that the best balancing performance is obtained in these mechanisms when this ratio was maximized. The results were validated experimentally by capturing the force-deflection relations.

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In this paper, a novel design concept and manufacturing method for the compliant bistable structure is proposed. The pulsed laser technique is utilized as the manufacturing method for both the fabrication and the introduction of desired pre-stresses, simultaneously. Based on this concept, a novel bistable structure consisted of one pre-compressed main beam, and a pair of supporting beams is designed and fabricated. The deformation difference between the main beam and the supporting beams induced by laser heating residual stress make the main beam to buckle under the constraints of two supporting beams and possess a bistable feature. The bistable structures can be implemented into other devices in the form of cantilevers thanks to the internal integration of the buckled beam and the boundary conditions. The characteristics of this new bistable structure, including its stable shape and snap-through response, are investigated both experimentally and numerically. During the snap forth and back process with the snapping load of 19 mN and the required energy of 77 mN·mm, an impressive energy dissipation with a loss factor value of 0.3 exists. Finally, a parametric study was carried out to find the critical performance parameters.

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Stiffness in compliant micro mechanisms can negatively affect performance. Current methods for stiffness reduction in micro electro mechanical systems (MEMS) consume power, have a large footprint or are relatively complex to manufacture. In this paper stiffness is reduced by static balancing. A building block commonly used for stiffness reduction in large scale compliant mechanisms is made compatible with MEMS. Preloading required to create negative stiffness is obtained from residual film stress by thermal oxidation of silicon. Instead of buckling a plate spring by moving its end points, a SiO 2 film 1900 nm to 2500 nm thick will stretch micro-beams 24 μ m wide, while the end points are fixed. To show efficacy of our method, the building block is coupled with a simple linear stage. However, the building block can readily be combined with other compliant micro mechanisms to reduce their stiffness. Statically balanced MEMS will enable novel designs in low-frequency sensor technology, low-frequency energy harvesting and pave the way to autonomous micro-robotics. We show a stiffness reduction of a factor 9 to 46. The balancing effect remained after SiO 2 removal, due to plastic deformation of the beams. [2019-0023]. @en

Although motion energy harvesting at the small scales has been a research topic for over 20 years, the implementation of such generators remains limited in practice. One of the most important contributing factors here is the poor performance of these devices under low-frequency excitation. In this research, a new metric is proposed to evaluate the performance and bandwidth of generators at low frequencies. For that, a classification based on the dynamics was made. It was found that the highest efficiencies were found in single-degree-of-freedom resonators where a large motion amplification was achieved. Smaller generators can be designed by limiting the motion through end-stops at the cost of a reduced efficiency. Moreover, it was argued that upon miniaturization, resonators could be outperformed by generators using a frequency up-conversion principle.

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Microtransmission mechanisms made of elastic materials present an opportunity for exploring scalable mechanical systems integrated with sophisticated functionalities. This paper shows how the fundamentally limited range of motion in elastic mechanisms can be circumvented to create a frequency doubling functionality analog to angular velocity doubling in classical gears. The proposed mechanism utilizes the elastic deformation of its internal architecture and buckling of microflexures to perform frequency doubling kinematics. We demonstrate this by the fabrication of a microtransmission device for application in mechanical wrist watches. A key benefit of the proposed method is that such a transmission system can be integrated and fabricated as an embedded part of microarchitected materials to boost the frequency characteristics of energy storage, actuators, and inertial sensors to perform adequately for different applications.

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Residual stress from thermal oxidation can cause plastic deformation in silicon microelectromechanical systems (MEMS). This paper presents a novel method to distinguish elastic and plastic strain in silicon beams, by removing the oxide layer to show the plastic strain. A lever mechanism is used as a mechanical amplifier. The plasticity model by Alexander and Haassen (AH) is used in a numerical model to predict the elastic and plastic strain. Experiments in epitaxially grown silicon show significantly less plastic strain than predicted by the model. We conclude that the AH model is not valid for epitaxially grown silicon with very little initial dislocations. Since epitaxially grown silicon generally has less dislocations compared to floating zone silicon we recommend using the former when plastic deformation is to be avoided.

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Although motion energy harvesting at the small scales has been a research topic for over 20 years, the implementation of such generators remains limited in practice. One of the most important contributing factors here is the poor performance of these devices under low-frequency excitation. In this research, a classification of miniaturised generators is proposed based on the dynamics of the nonlinear systems. This provides insight in the performance of different types of designs, which can be used to develop new designs with better efficiencies under realistic conditions.@en

Vibration energy harvesters based on piezoceramics can provide a sustainable source of energy for low-power electronics. The greatest issue preventing these systems from being widely used is their poor reliability. With the aim to maximise their power output, the devices are often operated close the point of yielding, which results in microcracks and fatigue in the piezoceramic layer. This paper offers a comparative review of design principles that aim to improve the reliability of piezoelectric vibration energy harvesters. Three different design principles are investigated with the focus on strain limitation. The results show that strain homogenisation, strain limitation and compressive strains can be effective design principles to increase reliability without sacrificing efficiency.

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This work presents a novel design, model and prototype of a motion energy harvester based on bi-stability and frequency up-conversion. The Parametric Frequency up-converter Generator (PFupCG). The PFupCG was designed to harvest energy under conditions where the amplitude of the driving motion is larger than the internal displacement limit. Instead of an impact member, the PFupCG uses a compliant suspension mechanism that combines a bi-stable characteristic with a strong stiffening behavior as a result of geometric effects. This resulted in a prototype of the PFupCG with an internal-to-applied motion amplitude ratio of 0.2. A case study was carried out where the PFupCG was analyzed by simulation and experiment for vibration conditions representative of human walking motion (2Hz, 25 mm).

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This paper presents a method for the design of compliant micro transmission mechanisms which multiply the motion frequency of a cyclic input motion. Compliant embodiments are generated based on exploiting the singularity in a double-slider mechanism, which provides building blocks with a frequency multiplication factor of two. It is shown that the proposed building blocks can be concatenated for higher frequency multiplication ratios, and that the maximum number of blocks is limited by the desired travel range of the output. The input-output kinematics and the different stiffness characteristics of the building blocks are discussed based on pseudo-rigid-body model (PRBM). To validate the building block approach, a compliant micro transmission mechanism is presented which quadruples the frequency of a cyclic rectilinear input motion. Furthermore, a micro scale prototype was dimensioned and fabricated out of silicon using deep reactive ion etching to evaluate the design experimentally and validate the PRBM and finite element models. [2017-0318]@en

A method is introduced to design compliant micro transmission mechanisms which double the motion frequency of a cyclic input motion. Compliant embodiments are generated based on exploiting the singularity in a double-slider mechanism, which provides building blocks with a frequency-multiplication factor of two. It is shown that the proposed building blocks can be concatenated for higher frequency-multiplication ratios. To validate the building block approach, a compliant micro transmission mechanism is presented which quadruples the frequency of a cyclic rectilinear input motion.@en

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^∗=p_vp_l, and has a maximum of 1.185 for p^∗=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^∗=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.

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This paper presents a fully compliant, potentially monolithic, power transmission mechanism which can rectify a large lateral offset between two parallel rotational axes. The planar nature of the design makes it ideal for manufacturing-limited applications such as micro/meso-scale power transmissions. The proposed compliant transmission is generated based on the Oldham Coupling and its equivalent Pseudo-Rigid-Body Model (PRBM). Normally, a compliant transmission mechanism cannot achieve a high efficiency due to the internal stiffness, i.e. actuation stiffness is not zero. However, in the proposed design, the internal stiffness is removed by static balancing and results in a statically balanced compliant transmission mechanism, i.e. with zero actuation stiffness. Therefore, the monolithic embodiment and static balancing features compensate for backlash, friction, assembly errors and poor mechanical efficiency inherent in conventional Oldham coupling, resulting in a transmission mechanism with high mechanical efficiency. Possible compliant design configurations based on the importance of different design criteria are discussed. Further, a compliant device based on the Paired Double Parallelogram (DP-DP) linear flexure bearing
is designed and dimensioned. Moreover, the transmission stiffness, i.e. input-output rotational stiffness within the maximum allowable stress, and the actuation stiffness, i.e. minimum required actuation torque for certain angular displacement, of the designed device are predicted by the theoretical model and finite element modeling. Besides, the result shows the device is providing a constant transmission stiffness through a full cycle rotation. To prove the concept, a macro scale prototype is constructed and evaluated experimentally.
It is shown that the results from the experiment are in agreement with the
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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 18.6 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 -104.5% was predicted. The expected reduction for the designed MEMS device is 98.4^%. The experimental first EF of the prototype is 3.10±0.25 Hz against a theoretical value of 3.09 Hz. The theoretical first EF of the MEMS model is 21.8 HZ. The prototypes are successfully preloaded by applying shaking or shock by hand. The predicted minimal energy requirement for this is found to be 4.9e-3 J, while 6.4e-3 J and 5.9e-3 J were calculated based on experimental results. The expected minimal energy required for preloading the designed MEMS device is 1.0e-5 J.

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This paper introduces a homokinetic coupling, a constant velocity universal joint (CV joint), which is fully compliant and potentially monolithic. The proposed compliant design can accommodate high misalignment angles between the input and the output rotational axes. Additional kinematic constraints are applied to well-known Double Hooke’s universal joint, to guarantee a one-to-one constant velocity rotation transmission for all different misalignment angles. The influence of the extra constraints on degrees-of-freedom (DOF) of the mechanism is studied using screw theory. Furthermore, it was shown that the mechanism is yet a 1DOF linkage for rotation transmission and a 2DOF rotational joint as all universal joints. The kinematics of the mechanism is studied, and constant velocity conditions are identified. The pseudo-rigid-body model (PRBM) of the new angled arrangement of the Double Hooke’s universal joint is created, and the input–output torque relationship is then studied. The different possible compliant embodiments based on the PRBM model were discussed and illustrated. Moreover, one of the proposed compliant counterparts is dimensioned as a power transmission coupling for a high misalignment angle, up to 45 deg. Further, a prototype was manufactured for the experimental evaluation, and it is shown that the results are consistent with the PRBM
and the finite element model.@en

Usage of compliant micromechanical oscillators has increased in recent years, due to their reliable performance despite the growing demand for miniaturization. However, ambient vibrations affect the momentum of such oscillators, causing inaccuracy, malfunction, or even failure. Therefore, this paper presents a compliant force-balanced mechanism based on rectilinear motion, enabling usage of prismatic oscillators in translational accelerating environments. The proposed mechanism is based on the opposite motion of two coplanar prismatic joints along noncollinear axes via a shape-optimized linkage system. Rigid-body replacement with shape optimized X-bob, Q-LITF, and LITF joints yielded a harmonic (R>0.999), low frequency (f=27 Hz) single piece force-balanced micromechanical oscillator (Ø 35 mm). The experimental evaluation of large-scale prototypes showed a low ratio of the center of mass (CoM) shift compared to the stroke of the device (≈ 0.01) and proper decoupling of the mechanism from the base, as the oscillating frequency of the balanced devices during ambient disturbances was unaffected, whereas unbalanced devices had frequency deviations up to 1.6%. Moreover, the balanced device reduced the resultant inertial forces transmitted to the base by 95%.@en