Hybrid additive manufacturing with MIG-deposit of aluminium alloy enhanced by friction stir processing

Hybrid additive manufacturing (HAM) is an additive manufacturing (AM) process that integrates multiple metal processing/shaping techniques. This thesis work focuses on the development of a HAM process that combines the MIG welding technique, to produce the initial AM via multi-layer deposit, with friction stir processing (FSP) technique, to enhance the properties of the deposited layers. In order to validate this hybrid concept, filler wire of aluminium alloy AA5183, with diameter of 1.2 mm was deposited on a base plate of aluminium alloy AA5083, with thickness of 6 mm. The initial AM component was produced with three overlapped layers, resulting in a plate of about 400 x 130 x 9 mm, over the base plate. Each layer was produced with parallel and partially overlapping string passes with MIG. The resulting AM component was then processed by FSP, in parallel passes aligned with the initial MIG passes. The effect of the HAM process on the strength and microstructure of the final component was then investigated. It was observed that the initial AM microstructure was refined, with evident dynamic recrystallization in the stirred region by the probe of the FSP tool. There are evidences that the porosities produced by MIG were removed by the FSP. In terms of mechanical properties, the ductility increased in comparison to the initial AM material, in both transversal and longitudinal directions. Concerning the strength, the ultimate tensile (UTS) and yield strength 〖(σ〗_y) are higher than the initial AM material in the longitudinal direction, but lower than the initial AM material in the transversal direction. This fact is mainly due to the overlap ratio between the FSP passes, along the transversal direction, which did not reach continuous overlapping of the stirred zones. Based on a global analysis, encompassing several mechanical properties, the overall quality of the HAM sample improved in comparison to the initial AM

Controllable and reversible tuning of material rigidity for robot applications

Tunable rigidity materials have potentially widespread implications in robotic technologies. They enable morphological shape change while maintaining structural strength, and can reversibly alternate between rigid, load bearing and compliant, flexible states capable of deformation within unstructured environments. In this review, we cover a range of materials with mechanical rigidity that can be reversibly tuned using one of several stimuli (e.g. heat, electrical current, electric field, magnetism, etc.). We explain the mechanisms by which these materials change rigidity and how they have been used for robot tasks. We quantitatively assess the performance in terms of the magnitude of rigidity, variation ratio, response time, and energy consumption, and explore the correlations between these desired characteristics as principles for material design and usage

Variable Stiffness Actuator for Soft Robotics Using Dielectric Elastomer and Low-Melting-Point Alloy

A novel variable stiffness actuator composed of a dielectric elastomer actuator (DEA) and a low-melting-point-alloy (LMPA) embedded silicone substrate is demonstrated. The device which we call variable stiffness dielectric elastomer actuator (VSDEA) enables functional soft robots with a simplified structure, where the DEA generates a bending actuation and the LMPA provides controllable stiffness between soft and rigid states by Joule heating. The entire structure of VSDEA is made of soft silicones with an elastic modulus of less than 1 MPa providing a high compliance when the LMPA is active. The device has the dimension of 40 mm length × 10 mm width × 1 mm thickness, with mass of ~1 g. We characterize VSDEA in terms of the actuation stroke angle, the blocked force, and the reaction force against a forced displacement. The results show the controllable actuation angle and the blocked force up to 23.7 ° and 2.4 mN in the soft state, and 0.6 ° and 2.1 mN in the rigid state. Compared to an actuator without the LMPA, VSDEA exhibits ~90× higher rigidity. We develop a VSDEA gripper where the mass of active parts is ~2 g, which is able to successfully hold an object mass of 11 g, exhibiting the high performance of the actuator

Modeling Residual Stress Development in Hybrid Processing by Additive Manufacturing and Laser Shock Peening

The term “hybrid” has been widely applied to many areas of manufacturing. Naturally, that term has found a home in additive manufacturing as well. Hybrid additive manufacturing or hybrid-AM has been used to describe multi-material printing, combined machines (e.g., deposition printing and milling machine center), and combined processes (e.g., printing and interlayer laser re-melting). The capabilities afforded by hybrid-AM are rewriting the design rules for materials and adding a new dimension in the design for additive manufacturing paradigm. This work focuses on hybrid-AM processes, which are defined as the use of additive manufacturing (AM) with one or more secondary processes or energy sources that are fully coupled and synergistically affect part quality, functionality, and/or process performance. Secondary processes and energy sources include subtractive and transformative manufacturing technologies, such as machining, re-melting, peening, rolling, and friction stir processing. Of particular interest to this research is combining additive manufacturing with laser shock peening (LSP) in a cyclic process chain to print 3D mechanical properties. Additive manufacturing of metals often results in parts with unfavorable mechanical properties. Laser shock peening is a high strain rate mechanical surface treatment that hammers a work piece and induces favorable mechanical properties. Peening strain hardens a surface and imparts compressive residual stresses improving the mechanical properties of the material. The overarching objective of this work is to investigate the role LSP has on layer-by-layer processing of 3D printed metals. As a first study in this field, this thesis primarily focuses on the following: (1) defining hybrid-AM in relation to hybrid manufacturing and classifying hybrid-AM processes and (2) modeling hybrid-AM by LSP to understand the role of hybrid process parameters on temporal and spatial residual stress development. A finite element model was developed to help understand thermal and mechanical cancellation of residual stress when cyclically coupling printing and peening. Results indicate layer peening frequency is a critical process parameter and highly interdependent on the heat generated by the printing laser source. Optimum hybrid process conditions were found to exists that favorably enhance mechanical properties. With this demonstration, hybrid-AM has ushered in the next evolutionary step in additive manufacturing and has the potential to profoundly change the way high value metal goods are manufactured. Advisor: Michael P. Seal

An Overview of Legged Robots

The objective of this paper is to present the evolution and the state-of-theart in the area of legged locomotion systems. In a first phase different possibilities for mobile robots are discussed, namely the case of artificial legged locomotion systems, while emphasizing their advantages and limitations. In a second phase an historical overview of the evolution of these systems is presented, bearing in mind several particular cases often considered as milestones on the technological and scientific progress. After this historical timeline, some of the present day systems are examined and their performance is analyzed. In a third phase are pointed out the major areas for research and development that are presently being followed in the construction of legged robots. Finally, some of the problems still unsolved, that remain defying robotics research, are also addressed.N/

Shape-changing segments of load-bearing elements of robotic systems

Robotické systémy mají v dnešních výrobních procesech pevné spojovací segmenty mezi klouby. Tyto prvky jsou velmi často vyrobeny z běžně dostupných polotovarů, jako jsou trubky, tyče, odlitky apod. Tyto prvky jsou převážné rovné a jejich tvar a rozměry vymezují pracovní prostor robotického systému. Robotický systém s těmito prvky má pevnou kinematickou strukturu. To přináší vzhledem k požadavkům výhody jako je, dobrá opakovatelnost polohování, nosnost atd. Nevýhodou je, že tyto systémy nejsou flexibilní a většinou slouží jednoúčelově. V případě změny pracovního úkolu může nastat stav, kdy stávající kinematika systému nebude dostatečná nebo optimální. Jedním ze způsobů řešení tohoto problému je změna konfigurace robotického systému a tím pracovního prostoru manipulátoru zvýšením stupně volnosti (DoF) přidáním modulu. Stále častěji se objevují systémy, které nenavyšují počet stupňů volnosti, ale dochází u nich ke tvarové změně spojovacích segmentů. Při vhodné změně tvaru spojovacího segmentu může systém dosáhnout změny pracovního prostoru tak, aby byl dostatečný a optimální pro novou úlohu. Předkládaná disertační práce se zabývá tématem tvarově měnitelných segmentů jako nosných prvků robotických systémů. Úvodní část této disertační práce se věnuje možnostem zvyšování flexibility pomocí přídavných modulů do stávajících struktur. Jsou zde popsány typy modulů a jejich možnosti spojování. Dále se úvod práce zabývá systémy s variabilní tuhostí a schopnosti měnit tvar. Jsou zde popsány principy, jak systémy dosahují změny tuhosti a tím i tvaru. Následně jsou stanoveny cíle práce, které vycházejí z rešerše aktuálního stavu problematiky tvarově měnitelných segmentů. Vlastní část práce je rozdělena na dílčí kapitoly, dle jednotlivých definovaných cílů práce. V práci je popsáno určení možnosti ohybu segmentu tvarově měnitelného segmentu a metoda definice jeho délky. Jsou zde pomocí simulace představeny možnosti robotických systémů se zakřivenými prvky. Dále je představen experimentální segment s variabilní tuhostí na základě použití materiálu s nízkou teplotou tání. Na experimentální segmentu byli provedeny tři experimenty pro zjištění jeho mechanických parametrů. Následně se práce zabývá nutnou změnou materiálu s nízkou teplotou táním a způsobem změny jeho tuhosti. V předposlední a poslední kapitole jsou shrnuty výsledky a definovány přínosy pro vědní obor, praxi a doporučení pro další výzkum.Robotic systems in today's manufacturing processes have fixed connecting segments between joints. These elements are very often made from commonly available semi-finished products such as tubes, rods, castings, etc. These elements are mostly straight and their shape and dimensions define the working envelope of the robotic system. A robotic system with these elements has a rigid kinematic structure. This brings advantages with respect to the requirements such as, good repeatability of positioning, load capacity, etc. The disadvantage is that these systems are not flexible and mostly serve a single purpose. In case of a change in the work task, a situation may arise where the existing kinematics of the system is not sufficient or optimal. One way to solve this problem is to reconfigure the robotic system and thus the manipulator workspace by increasing the degree of freedom (DoF) by adding a module. Increasingly, systems are emerging that do not increase the number of degrees of freedom, but do change the shape of the connecting segments. With a suitable change in the shape of the connecting segment, the system can achieve a change in the working envelope that is sufficient and optimal for the new task. The present thesis deals with the topic of shape changing segments as supporting elements of robotic systems. The introductory part of this thesis explores the possibilities of increasing flexibility by adding modules to existing structures. The types of modules and the possibilities of connecting them are described. Furthermore, the introduction of the work deals with systems with variable stiffness and shape changing capabilities. The principles of how systems achieve variation in stiffness and thus shape are described. Then, the objectives of the thesis are stated, which are based on a survey of the current state of the art of shape changeable segments. The actual part of the thesis is divided into subchapters, according to the individual defined objectives of the thesis. The thesis describes the determination of the bending possibility of a shape-changing segment and the method of defining its length. The possibilities of robotic systems with curved elements are presented by simulation. Furthermore, an experimental segment with variable stiffness based on the use of a low melting point material is presented. Three experiments were performed on the experimental segment to determine its mechanical parameters. Subsequently, the paper discusses the necessary variation of the low melting temperature material and the method of changing its stiffness. In the penultimate and last chapter, the results are summarized and the contributions to science, practice and recommendations for further research are defined.354 - Katedra robotikyvyhově

Design of a Variable Stiffness Passive Layer Jamming Structure for Anthropomorphic Robotic Finger Applications

Soft robots can effectively mimic human hand interface characteristics and facilitate collaborative operations with humans in a safe manner. This dissertation research concerns the design and fabrication of a low cost variable stiffness structure for applications in compliant robotic fingers. A conceptual design of a compact multi-layer structure is proposed for realizing variable stiffness, when applied to underactuated fingers of an anthropomorphic robotic hand. The proposed design comprises thin material layers with clearance that permits a progressive hardening feature while grasping and added design flexibility and tuning of the fingers’ compliance. The design permits stiffness variations in a passive manner in the soft contact regions. The design is realized to ensure ease of scalability and cost-effective fabrication by the ’Additive Manufacturing (AM)’/3D-printing technology. Both the multi-layer structures and the fingers could be fabricated as a single entity, and from a single base material with relatively low elastic modulus. The proposed design also exhibits finite degrees-of-freedom representative of the human finger - The feasibility of the design and its manufacturability are verified through prototype fabrication using a readily available 3D-printing material, namely; 'Thermoplastic PolyUrethane (TPU)' with Young’s Modulus of 25MPa. The chosen material permitted low stiffness of the multi-layer structure in the contact interface under relatively small deformations, while ensuring sufficient rigidity on the non-contact regions of the finger. A finite element (FE) model is formulated considering 3D tetrahedral elements and a nodal-normal contact detection method together with the augmented Lagrange formulation. The model is analyzed to determine the force-displacement characteristics of the structure subject to linearly increasing compressive load, under the assumption of low interface friction. A simplified analytical model of the multi-layer structure is also formulated considering essential boundary and support conditions for each individual layer. The model revealed progressive hardening characteristics of the multilayer structure during compression due to sequential jamming of individual layers. The force-displacement characteristics of the design could thus be varied by varying the multi-layer structure parameters, such as number of layers, thickness of individual layers, material properties, and clearance between the successive layers. It is shown that the simplified analytical model could provide reasonably good estimate of the force-deflection properties of the structure in a computationally efficient manner. The analytical model is subsequently used to investigate the influences of variations in the multilayer structure parameters in a computationally efficient manner. It is shown that the proposed design offers superior tuning flexibility to realize desired force-displacement characteristics of the structure for developing scalable anthropomorphic robotic fingers of a compliant robotic hand, in addition to the cost-effective manufacturability

Vertical ladder climbing by the HRP-2 humanoid robot

International audienceWe report the results obtained from our trials in making the HRP-2 humanoid robot climb vertical industrial-norm ladders. We integrated our multi-contact planner and multi-objective QP control as basic components. First, a set of contacts to climb the ladder is planned off-line and provided as an input for a finite state machine that sequences tasks to be realized by our multi-objective model-based QP in closed-loop control. The trials we made revealed that hardware changes are to be made on the HRP-2, and the software has to be made more robust. Yet, we confirmed that HRP-2 has power capability to climb real industrial ladders, such as those found in nuclear power plants and large scale manufacturings (e.g. airliners, shipyards and buildings)