Optimal Design of Beam-Based Compliant Mechanisms via Integrated Modeling Frameworks

Beam-based Compliant Mechanisms (CMs) are increasingly studied and implemented in precision engineering due to their advantages over the classic rigid-body mechanisms, such as scalability and reduced need for maintenance. Straight beams with uniform cross section are the basic modules in several concepts, and can be analyzed with a large variety of techniques, such as Euler-Bernoulli beam theory, Pseudo-Rigid Body (PRB) method, chain algorithms (e.g.~the Chained Beam-Constraint Model, CBCM) and Finite Element Analysis (FEA). This variety is unquestionably reduced for problems involving special geometries, such as curved or spline beams, variable section beams, nontrivial shapes and, eventually, contacts between bodies. 3D FEA (solid elements) can provide excellent results but the solutions require high computational times. This work compares the characteristics of modern and computationally efficient modeling techniques (1D FEA, PRB method and CBCM), focusing on their applicability in nonstandard problems. In parallel, as an attempt to provide an easy-to-use environment for CM analysis and design, a multi-purpose tool comprising Matlab and modern Computer-Aided Design/Engineering (CAD/CAE) packages is presented. The framework can implement different solvers depending on the adopted behavioral models. Summary tables are reported to guide the designers in the selection of the most appropriate technique and software architecture. The second part of this work reports demonstrative case studies involving either complex shapes of the flexible members or contacts between the members. To improve the clarity, each example has been accurately defined so as to present a specific set of features, which leads in the choice of a technique rather than others. When available, theoretical models are provided for supporting the design studies, which are solved using optimization approaches. Software implementations are discussed throughout the thesis. Starting from previous works found in the literature, this research introduces novel concepts in the fields of constant force CMs and statically balanced CMs. Finally, it provides a first formulation for modeling mutual contacts with the CBCM. For validation purposes, the majority of the computed behaviors are compared with experimental data, obtained from purposely designed test rigs

A Practical Method for Determining the Pseudo-rigid-body Parameters of Spatial Compliant Mechanisms via CAE Tools

Compliant Mechanisms (CMs) are employed in several applications requiring high precision and reduced number of parts. For a given topology, CM analysis and synthesis may be developed resorting to the Pseudo-Rigid Body (PRB) approximation, where flexible members are modelled via a series of spring-loaded revolute joints, thus reducing computational costs during CM simulation. Owing to these considerations, this paper reports about a practical method to determine accurate PRB models of CMs comprising out-of-plane displacements and distributed compliance. The method leverages on the optimization capabilities of modern CAE tools, which provide built-in functions for modelling the motion of flexible members. After the validation of the method on an elementary case study, an industrial CM consisting of a crank mechanism connected to a fully compliant four-bar linkage is considered. The resulting PRB model, which comprises four spherical joints with generalized springs mounted in parallel, shows performance comparable with the deformable system

Design and Virtual Prototyping of a Variable Stiffness Joint via Shape Optimization in a CAD/CAE Environment

During the latest decade, collaborative robots, namely machines specifically designed for the physical interaction with humans, have been gradually making their transition from laboratories to real-world applications [1]. Naturally, whenever the envisaged task would benefit form physical human-machine interaction, safety and dependability become issues of paramount importance [2]. Nonetheless, especially when dealing with collaborative operations in the manufacturing industry, safety regulations may lead the plant designer to face opposite goals. On one hand, robots should indeed be designed so as to never cause harm to people (both during regular functioning or in case of failure). On the other hand, the wide-spread use of industrial manipulators traditionally leverages on their capabilities to carry rather high payloads, while achieving a very fast and precise positioning of the end-effector. These requirements are usually pursued by coupling powerful actuation systems with extremely rigid mechanical structures, which hardly comply with safety needs whenever the workers are supposed to enter the robot workspace. Therefore, the engineering challenge when designing collaborative robotics systems, which have to be safe and efficient at the same time, is usually tackled via the following strategies: i) by enhancing the robot sensory apparatus; ii) by adopting active control strategies; iii) by reducing the inertia of any moving part employing lightweight materials whenever possible. In parallel, as previously proven by several researchers [3], another way to actually implement safe machines for collaborative tasks is to increase (rather than minimize) the inherent compliance of their mechanical structure [4], simultaneously introducing the possibility to actively vary such compliance during the robot movements. This capability can be implemented, for instance, by means of Variable Stiffness Joints (VSJ), namely particular actuation systems which allow to independently control the position of an output link along with the transmission stiffness. In light of this consideration, the present talk describes the design of a novel VSJ architecture, depicted in Fig. 1a. The VSJ can achieve stiffness modulation via the use of a pair of compliant mechanisms with distributed compliance, which act as nonlinear springs with proper torque-deflection characteristic. These elastic elements are composed of slender beams whose neutral axis is described by a spline curve with non-trivial shape. The beam geometry is determined by leveraging on a CAD/CAE framework that allows for the shape optimization of complex flexures. In particular, the design method makes use of the modeling and simulation capabilities of a parametric CAD seamlessly connected to a FEM tool. For validation purposes, proof-concept 3D printed prototypes of both elastic elements (Fig. 1a) and overall VSJ (Fig. 1b) are finally produced and tested (Fig. 1c). Experimental results fully confirm that the VSJ behaves as expected. BIBLIOGRAFY [1] Heyer, C., 2010. \u201cHuman-robot interaction and future industrial robotics applications\u201d. Proceeding of the IEEE/RSJ International Conference on Intelligent Robots and Systems, pp. 4749\u20134754. [2] Fryman, J., and Matthias, B., 2012. \u201cSafety of industrial robots: From conventional to collaborative applications\u201d. Proceeding of ROBOTIK, 7th German Conference on Robotics, May, pp. 1\u20135. [3] Bicchi, A., and Tonietti, G., 2004. \u201cFast and soft arm tactics: Dealing with the safety-performance trade-off in robot arms design and control\u201d. IEEE Robotics and Automation Magazine, 11(2), pp. 22\u201333. [4] Berselli, G., Guerra, A., Vassura, G., and Andrisano, A. O., 2014. \u201cAn engineering method for comparing selectively compliant joints in robotic structures\u201d. IEEE/ASME Transactions on Mechatronics, 19(6), pp. 1882\u20131895

An Overview of Industrial Robots Control and Programming Approaches

Nowadays, manufacturing plants are required to be flexible to respond quickly to customer demands, adapting production and processes without affecting their efficiency. In this context, Industrial Robots (IRs) are a primary resource for modern factories due to their versatility which allows the execution of flexible, reconfigurable, and zero-defect manufacturing tasks. Even so, the control and programming of the commercially available IRs are limiting factors for their effective implementation, especially for dynamic production environments or when complex applications are required. These issues have stimulated the development of new technologies that support more efficient methods for robot control and programming. The goal of this research is to identify and evaluate the main approaches proposed in scientific papers and by the robotics industry in the last decades. After a critical review of the standard IR control schematic, the paper discusses the available control alternatives and summarizes their characteristics, range of applications, and remaining limitations

Engineering Method and Tool for the Complete Virtual Commissioning of Robotic Cells

Intelligent robotic manufacturing cells must adapt to ever-varying operating conditions, developing autonomously optimal manufacturing strategies to achieve the best quality and overall productivity. Intelligent and cognitive behaviors are realized by using distributed controllers, in which complex control logics must interact and process a wide variety of input/output signals. In particular, programmable logic controllers (PLCs) and robot controllers must be coordinated and integrated. Then, there is the need to simulate the robotic cells’ behavior for performance verification and optimization by evaluating the effects of both PLC and robot control codes. In this context, this work proposes a method, and its implementation into an integrated tool, to exploit the potential of ABB RobotStudio software as a virtual prototyping platform for robotic cells, in which real robots control codes are executed on a virtual controller and integrated with Beckhoff PLC environment. For this purpose, a PLC Smart Component was conceived as an extension of RobotStudio functionalities to exchange signals with a TwinCAT instance. The new module allows the virtual commissioning of a complete robotic cell to be performed, assessing the control logics effects on the overall productivity. The solution is demonstrated on a robotic assembly cell, showing its feasibility and effectiveness in optimizing the final performance

Path Approximation Strategies for Robot Manufacturing: A Preliminary Experimental Evaluation

Industrial Robots (IRs) are increasingly adopted for material subtraction or deposition functions owing to their advantages over machine tools, like cost-effectiveness and versatility. Unfortunately, the development of efficient robot manufacturing processes still faces unsolved issues related to the IRs poor positioning accuracy and to the tool path generation process. Novel engineering methods and tools are needed for CAD based programming of accurate paths and continuous robot motions to obtain the required manufacturing quality and tolerances. Within this context, to achieve smoothness along the tool path formed by linear G-code segments, the IR controllers’ approximation strategies, summarily reported in the manufacturer’s manuals, must be considered. The aim of this paper is to present the preliminary work carried out to identify the approximation algorithms of a Kuka IR when executing linear moves. An experimental study is conducted by varying the controller settings and the maximum translational velocity. The robot behavior has been acquired thanks to the controller tracing function and then processed to yield relations readily employable for the interpretation of G-Code commands and the subsequent generation of proper robot motion instructions. The obtained formulas allow to accurately predict the robot geometric path and kinematics within the corner transition between two linear segments

Quasi-static models of a four-bar quick-release hook

Quick-Release Hooks (QRHs) are connection devices for chains or metal ropes, which can be unfastened under full-load conditions by using a limited opening force. Despite their widespread use, the scientific literature about the mechanical behaviour of QRHs is rather limited. This paper deals with the kinematic and quasi-static analysis of a class of QRHs, based on a spring-loaded four-bar mechanism operating in the proximity of a singularity configuration. The quasi-static analysis allows to estimate the opening force as a function of the mechanism geometry and of the safety spring's features. At last, a multibody model of the system is developed, in order to validate the analytical model and to evaluate the influence of friction in revolute joints

An Integrated Approach for Motion Law Optimization in Partially Compliant Slider-Crank Mechanisms

none4mixedBaggetta, Mario; Bilancia, Pietro; Pellicciari, Marcello; Berselli, GiovanniBaggetta, Mario; Bilancia, Pietro; Pellicciari, Marcello; Berselli, Giovann

Design of a beam-based variable stiffness actuator via shape optimization in a CAD/CAE environment

Industrial robots are commonly designed to be very fast and stiff in order to achieve extremely precise position control capabilities. Nonetheless, high speeds and power do not allow for a safe physical interaction between robots and humans. With the exception of the latest generation lightweight arms, purposely design for human-robot collaborative tasks, safety devices shall be employed when workers enter the robots workspace, in order to reduce the chances of injuries. In this context, Variable Stiffness Actuators (VSA) potentially represent an effective solution for increasing robot safety. In light of this consideration, the present paper describes the design optimization of a VSA architecture previously proposed by the authors. In this novel embodiment, the VSA can achieve stiffness modulation via the use of a pair of compliant mechanisms with distributed compliance, which act as nonlinear springs with proper torque-deflection characteristic. Such elastic elements are composed of slender beams whose neutral axis is described by a spline curve with non-trivial shape. The beam geometry is determined by leveraging on a CAD/CAE framework allowing for the shape optimization of complex flexures. The design method makes use of the modeling and simulation capabilities of a parametric CAD software seamlessly connected to a FEM tool (i.e. Ansys Workbench). For validation purposes, proof-concept 3D printed prototypes of both non-linear elastic element and overall VSA are finally produced and tested. Experimental results fully confirm that the compliant mechanism behaves as expected

A CAD/CAE integration framework for analyzing and designing spatial compliant mechanisms via pseudo-rigid-body methods

Compliant Mechanisms (CMs) are currently employed in several engineering applications requiring high precision and reduced number of parts. For a given mechanism topology, CM analysis and synthesis may be developed resorting to the Pseudo\u2013Rigid Body (PRB) method, in which the behavior of flexible members is approximated via a series of rigid links connected by spring-loaded kinematic pairs. From a CM analysis standpoint, the applicability of a generic PRB model requires the determination of the kinematic pairs\u2019 location and the stiffness of a set of generalized springs. In parallel, from a design standpoint, a PRB model representing the kinetostatic behavior of a flexible system should allow to compute the flexures\u2019 characteristics providing the desired compliance. In light of these considerations, this paper describes a Computer-Aided Design/Engineering (CAD/CAE) framework for the automatic derivation of accurate PRB model parameters, on one hand, and for the shape optimization of complex-shape flexures comprising out-of-plane displacements and distributed compliance. The method leverages on the modelling and simulation capabilities of a parametric CAD (i.e. PTC Creo) seamlessly connected to a CAE tool (i.e. RecurDyn), which provides built-in functions for modelling the motion of flexible members. The method is initially validated on an elementary case study taken from the literature. Then, an industrial case study, which consists of a spatial crank mechanism connected to a fully-compliant four-bar linkage is discussed. At first, an initial sub-optimal design is considered and its PRB representation is automatically determined. Secondly, on the basis of the PRB model, several improved design alternatives are simulated. Finally, the most promising design solution is selected and the dimensions of a flexure with non-trivial shape (i.e. hybrid flexure) is computed. This technique, which combines reliable numerical results to the visual insight of CAD/CAE tools, may be particularly useful for analyzing/designing spatial CMs composed of complex flexure topologies