Tunable stiffness flexural bearings

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 139-140).Compressed flexures have a downwards-tunable stiffness in their compliant directions; their stiffness can theoretically be reduced by up to four orders of magnitude. The compression-stiffiness relation is linear for most of the loading, and this behavior can be taken advantage of to use the flexure as a tunable spring, opening up new design possibilities. Compressed flexures present the possibility of developing more sensitive flexure-based instruments such as accelerometers. The purpose of this research was to characterize the behavior of compressed flexures and develop guidelines for their design. Tradeoffs were assessed when substituting compressed flexures for conventional flexures and their suitability for use in a precision system. An experimental setup was designed and built to test a stage guided by four compressed flexure bearings. Compression was applied to the test flexures via a displacement input that was deamplified by a wedge pair to increase preload resolution. The motion in each of the stage's six degrees of freedom in response to flexure compression and in response to a voice coil actuator acting in the flexure's compliant direction was measured by seven capacitance probes arranged around the test stage, and a seventh capacitance probe measured the input displacement. With the experimental setup it was found that a stiffness reduction of a factor of 7 was possible. The magnitude of parasitic motions in the test stage were found to increase linearly with flexure compression. When being actuated in the compliant direction, parasitic motions were evident with magnitudes of the same order of magnitude as the sensor noise. Compressed flexures are highly sensitive to thermal variations; a 0.5 C temperature increase resulted in an 11% increase in stiffness. A model developed in this thesis predicts that deviations from column straightness of 2% of the flexure thickness limit the stiffness reduction to a factor of 1.2, while a deviation of 0.2% allows for a stiffness reduction of 10,000.by Aaron E. Ramirez.S.M

Model-Based Design Optimization of Soft Polymeric Domes Used as Nonlinear Biasing Systems for Dielectric Elastomer Actuators

Due to their unique combination of features such as large deformation, high compliance, lightweight, energy efficiency, and scalability, dielectric elastomer (DE) transducers appear as highly promising for many application fields, such as soft robotics, wearables, as well as micro electromechanical systems (MEMS). To generate a stroke, a membrane DE actuator (DEA) must be coupled with a mechanical biasing system. It is well known that nonlinear elements, such as negative-rate biasing springs (NBS), permit a remarkable increase in the DEA stroke in comparison to standard linear springs. Common types of NBS, however, are generally manufactured with rigid components (e.g., steel beams, permanent magnets), thus they appear as unsuitable for the development of compliant actuators for soft robots and wearables. At the same time, rigid NBSs are hard to miniaturize and integrate in DE-based MEMS devices. This work presents a novel type of soft DEA system, in which a large stroke is obtained by using a fully polymeric dome as the NBS element. More specifically, in this paper we propose a model-based design procedure for high-performance DEAs, in which the stroke is maximized by properly optimizing the geometry of the biasing dome. First, a finite element model of the biasing system is introduced, describing how the geometric parameters of the dome affect its mechanical response. After conducting experimental calibration and validation, the model is used to develop a numerical design algorithm which finds the optimal dome geometry for a given DE membrane characteristics. Based on the optimized dome design, a soft DEA prototype is finally assembled and experimentally tested

Contribution au micro-actionnement multi-stable piloté par radiations optiques

In this work, a bistable mechanism based on antagonistic pre-shaped double beams was proposed. Employing the proposed bistable mechanism, a quadristable micro-actuator was designed. ln order to validate the quadristability of the device, a meso-scaled prototype was fabricated from MDF by laser cutting. After the quadristability was experimentally confirmed, a quadristable micro-actuator was realized on SOl wafer using DRIE technique. Strokes for inner row and outer row were reduced to 300 µm and 200 µm respectively. For the actuation of the quadristable micro-actuator,laser heated SMA elements with deposited Si02 layer were used to realize the optical wireless actuation. With the help of a laser beam steering micro-mirror, both inner row and outer row were successfully actuated. ln order to further reduce the stroke, a bistable actuator with stroke reducing structure was designed and a prototype eut from MDF was tested. Bistability was validated and a stroke of 1µm was experimentally achieved. Based on this bistable module, a multistable nano-actuator, which contains four parallel coupled bistable modules,was designed and simulated. The simulated result have indicated that it was capable of outputs 16 discrete stable positions available from 0 nm to 150 nm with a step of 10 nm between two stable positions.Cette thèse traite le sujet du micro-actionnement multistable employant des radiations optiques pour atteindre les différentes positions offertes par le micro-actionneur. Dans le cadre des travaux réalisés, un mécanisme bistable reposant sur un principe de doubles poutres préformées situées en position antagoniste est proposé, et, sur cette brique élémentaire, un micro-actionneur quadristable a été conçu. Afin de valider le principe de fonctionnement de micro-actionneur, des procédés de fabrication Laser (sur le matériau « médium - MDF») puis DRIE (sur un wafer SOI de silicium) ont été utilisés. Sur le prototype en silicium, permettant une réduction des courses du rang interne et du rang externe du micro-actionneur, celles-ci ont été fixées à 300 µm et 200 µm respectivement. L’actionnement à distance de ce micro-actionneur a été prouvé en utilisant le chauffage laser d’un élément actif en Nitinol structuré par un dépôt de SiO2, ceci générant un effet « deux sens » de l’élément actif permettant d’annuler la charge sur les poutres du micro-actionneur une fois celui-ci déclenché puis en position stable. L’utilisation d’un banc expérimental incluant une membrane MEMS de balayage laser a permis de démontrer la quadristabilité du micro-actionneur sur 90 000 cycles. Afin de réduire davantage la course de ce micro-actionneur, des concepts de dispositifs de réduction de course ont été développés pour démontrer, à partir de prototypes fabriqué en MDF par usinage laser, la capacité à atteindre une course de 1 µm. Enfin, à la suite de ces travaux de réduction de course, un concept de nano-actionneur multistable a été proposé. Ce nano-actionneur est composé de quatre modules bistables liés et disposés en parallèle pour offrir 16 positions discrètes sur une course rectiligne. Les simulations de cet actionneur montrent la possibilité d’atteindre les 15 positions espacées de 10 nm sur une course de 150 nm

Programmable Multistable Mechanisms: Design, modeling, characterization and applications

Multistable Mechanisms are mechanical devices having more than one stable state. Since these mechanisms can maintain different deformations with zero force, they are advantageous for low power environments such as wristwatches and medical devices. In this thesis, I introduce programmable multistable mechanisms (PMMs), a new family of multistable mechanisms where the number, position, and stiffness of stable states can be controlled by programming inputs modifying the boundary conditions. PMMs can be synthesized by combining bistable mechanisms. This method was used to produce the T-mechanism, a PMM consisting of two double parallelogram mechanisms (DPMs) connected orthogonally where each DPM consists of two parallel beams connected centrally by a rigid block and axially loaded by programming input. An analytical model based on Euler-Bernoulli beam theory was derived to describe qualitatively the stability behaviour of the T-mechanism. The model approximates the mechanism's stiffness by a sixth order polynomial from which the reaction force and strain energy expressions can be estimated. These explicit formulas provide analytical expressions for the number, position, and stiffness of stable and unstable states as functions of the programming inputs. The qualitative stability behavior was represented by the programming diagram, bifurcation diagrams and stiffness maps relating the number, position and stiffness of stable states with the programming inputs. In addition, I showed that PMMs have zero stiffness regions functioning as constant-force multistable mechanisms. Numerical simulations validated these results. Experimental measurements were conducted on the T-mechanism prototype manufactured using electro-discharge machining. An experimental setup was built to measure the reaction force of the mechanism for different programming inputs. I verified the possible configurations of the T-mechanism including monostability bistability, tristability, quadrastability, zero stiffness regions, validating my analytical and numerical models. Compared to classical multistable mechanisms which are displaced between their stable states by imposing a direct displacement, PMMs can be displaced by modifying mechanism strain energy. This property increases the repeatability of the mechanism as the released energy is independent of the driving parameters, which can be advantageous for mechanical watches and medical devices. Accurate timekeepers require oscillators having repeatable period independent of their energy source. However, the balance wheel spiral spring oscillator used in all mechanical watches, suffers from isochronism defect, i.e., its oscillation period depends on its amplitude. I addressed this problem by introducing novel detached constant force escapements for mechanical wristwatches based on PMMs. In the medical domain, I applied PMMs to construct a retinal vein cannulation needle for the treatment of retinal vein occlusion. PMMs based needles produce sufficient repeatable puncturing energy with a predefined stroke independent of the operator input. Numerical simulations were used to model and dimension our proposed tool and satisfy the strict requirements of ophthalmologic operations. The tool was manufactured using 3D femto-laser printing of glass. An experimental setup was built to characterize the tool's mechanical behavior and to verify my computations. The tool was applied successfully to cannulate retinal veins of pig eyes

Dielectric elastomer actuators for binary robotics and mechatronics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections."February 2006."Includes bibliographical references (p. 145-153).Future robotics and mechatronics applications will require systems that are simple, robust, lightweight and inexpensive. A suggested solution for future systems is binary actuation. Binary actuation is the mechanical analogy to digital electronics, where actuators "flip" between two discrete states. Systems can be simple since low-level feedback control, sensors, wiring and electronics are virtually eliminated. However, conventional actuators, such as DC motors and gearbox are not appropriate for binary robotics because they are complex, heavy, and expensive. This thesis proposes a new actuation technology for binary robotics and mechatronics based on dielectric elastomer (DE) technology. DE actuators are a novel class of polymer actuators that have shown promising low-cost performance. These actuators were not well understood and, as a result, faced major reliability problems. Fundamental studies conducted in this thesis reveal that reliable, high performance DE actuation based on highly viscoelastic polymers can be obtained at high deformation rates, when used under fast, intermittent motion.(cont.) Also, analytical models revealed that viscoelasticity and current leakage through the film govern performance. These results are verified by an in-depth experimental characterizion of DE actuation. A new DE actuator concept using multi-layered diamond-shaped films is proposed. Essential design tools such as reliability/performance trade-offs maps, scaling laws, and design optimization metrics are proposed. A unit binary module is created by combining DE actuators with bistable structures to provide intermittent motion in applications requiring long-duration stateholding. An application example of binary robots for medical interventions inside Magnetic Resonance Imaging (MRI) systems illustrates the technology's potential.by Jean-Sébastien Plante.Ph.D