Modeling, Design, and Development of Soft Pneumatic Actuators with Finite Element Method

This work presents a comprehensive open-source simulation and design tool for Soft pneumatic actuators (SPAs) using finite element method, compatible and extensible to a diverse range of soft materials and design parameters. Thorough characterization of the hyperelastic and viscoelastic behavior is illustrated using a sample soft material (Ecoflex 00_30), and an appropriate material constitutive law. SPA performance (displacement and blocked-force) are simulated for two types of SPA and validated with experimental testing. Real-world case studies are presented in which SPA designs are iteratively optimized through simulation to meet specified performance criteria and geometric constraints.

Experimental and numerical studies on size and constraining effects in lead-free solder joints

John Botsis, Joël Cugnoni, Venkatesh Sivasubramaniam

Search String Advanced > Saved Searches > ARTICLE TOOLS Get PDF (641K) Save to My Profile E-mail Link to this Article Export Citation for this Article Get Citation Alerts Request Permissions More Sharing ServicesShare | Share on citeulike Share on facebook Share on delicious Share on www.mendeley.com Share on twitter Abstract Article References Cited By View Full Article (HTML) Enhanced Article (HTML) Get PDF (641K) Keywords: constrain effect;elastic-plastic behaviour;finite element;size effect;solder joint;structural optimisation ABSTRACT The durability and reliability of lead-free solder joints depends on a large number of factors, like geometry, processing parameters, microstructure and thermomechanical loads. In this work, the nature and influence of the plastic constraints in the solder due to joining partners have been studied by parametric finite element simulation of solder joints with different dimensions. The apparent hardening due to plastic constraints has been shown to strongly depend on the solder gap to thickness ratio with an inversely proportional evolution. Due to interaction of several parameters, the macroscopic stress–strain constitutive law of lead-free solder materials should be determined in the most realistic conditions. In order to identify the elasto-plastic constitutive law of Sn–Ag–Cu solders, a sub-micron resolution Digital Image Correlation technique has been developed to measure the evolution of strain in solder joints during a tensile test. Experimental results of the stress–strain response of Sn–Ag–Cu solder joints have been determined for several solder gaps. The measured load–displacement curves have been used in an inverse numerical identification procedure to determine the constitutive elasto-plastic behaviour of the solder material. The effects of geometrical constraints in a real solder joint with heterogeneous stress and strain fields are then studied by comparing the apparent (constrained) and constitutive (non-constrained) stress–strain relationships. Once the size dependant constraining effects have been removed from the stress–strain relationship, the scale effects can be studied separately by comparing the constitutive elasto-plastic parameters of joints with a variable thickness. Experimental stress–strain curves (constrained and unconstrained) for Sn–4.0Ag–0.5Cu solder in joints of 0.25–2.4 mm gap are presented and the constraining and the size effects are discussed.

2007

Caractérisation de la génération et la propagation d'ondes de pressions dans des tissus biologiques pour la conception d'appareils médicaux

Mathieu Benoit

Therapies using so called extracorporeal shock waves (Extracorporeal Shock Wave Therapy ESWT) have become current medical practice in orthopedy and traumatology. In order to understand and to optimize the effect of shock waves in clinical applications, medical results must be correlated with well characterized mechanical stimuli. This thesis has an industrial scope. It contributes to the comprehension of the generation and propagation of pressure waves in human tissues with the aim of improving existing ESWT therapies and of providing the industrial partner with tools for the design of a new generation of extracorporeal shock waves devices. The adopted general approach is based on a combination of experimental characterization and analytical and numerical modeling of wave generation and propagation phenomena in a medical treatment device and in biological tissues. Firstly, the characterization of a wave generator is based, on the one hand, on measurements of the dynamic behavior of the moving parts coupled with rigid body simulation, and on the other hand on measurements of wave propagation by means of a Hopkinson bar coupled with finite elements simulations. This characterization has shown that the generator produces very reproducible stress pulses. The simulation technique allows designing a new wave generator with a higher energy range and with well controlled operating parameters. The new design is covered by a patent. Secondly, a measurement technique for generation and propagation of pressure waves in soft animal tissues has been developed that is based on PVDF gages. The applicability of these gages has been qualitatively validated by comparative measurements with a Hopkinson bar. The perturbation effect of the gage, acting as an inclusion in the medium to be characterized, has been evaluated by means of simulations of wave propagation in water. Comparison with measurements in soft tissues suggests that it is negligible for pressure measurement in this type of materials. An independent calibration of the gage could however not be performed. Finally, measurements of wave propagation in pig skin and fat using PVDF gauges showed good reproducibility for a given sample. They highlighted the influence of the supply pressure of the wave generator on the amplitude and on the attenuation of the wave in tissues. Moreover, the dependence between the amplitude of the wave and its propagation velocity suggests a non-linear viscoelastic behavior of soft tissues as well as the need of a constitutive model for high strain rates. Simulations of wave propagation using a known hyperelastic constitutive model highlighted the difficulty of modeling such soft tissues. A viscoelastic non-linear constitutive model based on power laws was considered and is an interesting candidate for future simulations. The simulation technique for wave generation and propagation in a solid (aluminum) and a liquid (water) has been validated by comparing its results with measurements performed in these materials (strain gages and PVDF hydrophone respectively). Simulations of pressure wave propagation validated for solids and liquids showed that they can be applied to biological tissues modeled using a known constitutive model; they are a tool for any other simulation using more complex constitutive models. This work contributes to a broader study aimed at establishing and validating constitutive models for biological materials suitable for use in simulations of wave propagation.

EPFL2009

Design and Computational Modeling of a Modular, Compliant Robotic Assembly for Human Lumbar Unit and Spinal Cord Assistance

Gunjan Agarwal, Jamie Paik, Matthew Aaron Robertson, Harshal Arun Sonar

Wearable soft robotic systems are enabling safer human-robot interaction and are proving to be instrumental for biomedical rehabilitation. In this manuscript, we propose a novel, modular, wearable robotic device for human (lumbar) spine assistance that is developed using vacuum driven, soft pneumatic actuators (V-SPA). The actuators can handle large, repetitive loads efficiently under compression. Computational models to capture the complex non-linear mechanical behavior of individual actuator modules and the integrated assistive device are developed using the finite element method (FEM). The models presented can predict system behavior at large values of mechanical deformations and allow for rapid design iterations. It is shown that a single actuator module can be used to obtain a variety of different motion and force profiles and yield multiple degrees of freedom (DOF) depending on the module loading conditions, resulting in high system versatility and adaptability, and efficient replication of the targeted motion range for the human spinal cord. The efficacy of the finite element model is first validated for a single module using experimental results that include free displacement and blocked-forces. These results are then extended to encompass an extensive investigation of bio-mechanical performance requirements from the module assembly for the human spine-assistive device proposed.