This paper presents a metamorphic parallel mechanism with controllable rotation center in its pure rotation topology. Based on reconfiguration of a reconfigurable Hooke (rT) joint, the rotational center of the mechanism can be altered along the central line perpendicular to the base plane. A unified Dixon resultant based method is proposed to solve the forward kinematics analytically by covering all configurations with variable rotation centers while the rotation motion is expressed using Cayley formula. Then singularity loci are derived and represented in a new coordinate system with the three Rodrigues-Hamilton parameters assigned in three perpendicular directions. Limb-actuation singularity loci are also obtained from row vectors of the Jacobian matrix. By using Cayley formula, analytical workspace boundaries are expressed by including the mechanism structure parameters and input actuation limits. Finally, singularity-free workspace of configurations with variable rotation centers is demonstrated in the proposed coordinate system.
A branch of robotics, variable impedance actuation, along with one of its subfields variable stiffness actuation (VSA) targets the realization of complaint robotic manipulators. In this paper, we present the modeling, identification, and control of a discrete variable stiffness actuator (DVSA), which will be developed for complaint manipulators in the future. The working principle of the actuator depends on the involvement of series and parallel springs. We firstly report the conceptual design of a stiffness varying mechanism, and later the details of the dynamic model, system identification, and control techniques are presented. The dynamic parameters of the system are identified by using the logarithmic decrement algorithm, while the control schemes are based on linear quadratic control (LQR) and computed torque control (CTC), respectively. The numerical simulations are performed for the evaluation of each method, and results showed the good potentialities for the system. Future work includes the implementation of the presented approach on the hardware.
A new parallel mechanism 3-CCC which has distance and angle constraints is described in this paper. A closed-form solution to the forward displacement analysis of it is presented. Sylvester resultant is used with the angle equations and three distance equations are treated with by replacing the variables, from which 64 different locations of the platform can be derived, numerical example confirms these theoretical results. Velocity and acceleration equations and Jacobean matrix are obtained by taking the derivative of the constraint equations.
Abstract Mechanical components in a robotic system were used to provide body structure and mechanism to achieve physical motions following the commands from electronic controller. This kind of robotic system utilizes complex hardware and firmware for sensing and planning. To reduce computational cost and increase reliability for a robotic system, employing mechanical components to fully or partially take over control tasks is a promising way, which is also referred to as “mechanical intelligence” (MI). This paper proposes a shape memory alloy driven robot capable of using a reciprocating motion to crawl over a surface without any use of electronic controller. A mechanical logic switch is designed to determine the activation timing for a pair of antagonistic shape memory alloy (SMA) actuators. Meanwhile, a compliant pre-strain bistable mechanism is introduced to cooperate with the SMA actuators achieving reliable reciprocating motion between the two stable positions. The SMA actuator is modeled base on a static two-state theory while the bistable mechanism is described by combining a pseudo-rigid-body model (PRBM) with a Bi-beam constraint model (Bi-BCM). Following this, the design parameters of the bistable mechanism and SMA actuators are determined according to theoretical simulations. Finally, a robotic prototype is fabricated using anisotropic friction on its feet to convert the reciprocating motion of the actuator to uni-directional locomotion of the robot body over a surface. Experiments are carried out to validate the presented design concept and the modeling methods.
Conventional slicing algorithms for additive manufacturing (AM) processes slice the designed model into a set of planar layers, due to the simplicity, robustness, and generality of most geometries. However, such planar-layer-based slicing significantly limits the performance of the AM system with stair-stepping surface finishing, massive supporting structures, non-conformable to curved substrates, and reduced strength for thin shell structures. To mitigate these drawbacks of planar layer slicing, we presented a curved layer slicing method by utilizing the isothermal surfaces in heat transfer simulation. The designed part is virtually placed on a heated substrate, and the heat spread out through the part, which establishes a temperature field. The isothermal surfaces of this temperature field naturally create curved layers for the printing process. Our method successfully generated curved layers and tool paths for additive manufacturing processes with three-axis and multi-axis 3D printing. A multi-axis motion 3D printing machine is developed based on fused decomposition modeling (FDM). Several test cases were performed to verify and demonstrate our slicing method's capabilities. A discussion of future development on our general non-planar slicing system was also given.
Aerial manipulation has direct application prospects in environment, construction, forestry, agriculture, search, and rescue. It can be used to pick and place objects and hence can be used for transportation of goods. Aerial manipulation can be used to perform operations in environments inaccessible or unsafe for human workers. This paper is a survey of recent research in aerial manipulation. The aerial manipulation research has diverse aspects, which include the designing of aerial manipulation platforms, manipulators, grippers, the control of aerial platform and manipulators, the interaction of aerial manipulator with the environment, through forces and torque. In particular, the review paper presents the survey of the airborne platforms that can be used for aerial manipulation including the new aerial platforms with aerial manipulation capability. We also classified the aerial grippers and aerial manipulators based on their designs and characteristics. The recent contributions regarding the control of the aerial manipulator platform is also discussed. The environment interaction of aerial manipulators is also surveyed which includes, different strategies used for end-effectors interaction with the environment, application of force, application of torque and visual servoing. A recent and growing interest of researchers about the multi-UAV collaborative aerial manipulation was also noticed and hence different strategies for collaborative aerial manipulation are also surveyed, discussed and critically analyzed. Some key challenges regarding outdoor aerial manipulation and energy constraints in aerial manipulation are also discussed.
Advanced robotic systems are expected to be smartly reconfigurable to adapt to new needs for versatile operations in rapidly changing unknown, uncertain environments. The evolution has led to worldwide research interest in developing reconfigurable mechanisms and robots as intelligent-mechanical systems which have the ability to change their mobility, configurations, kinematics, and dynamics performance for various application scenarios in industrial automation, healthcare, space, field exploration, maintenance, domestic operations, human assistance, and augmentation. This Special Section features 14 papers that highlight the latest theories, designs, analyses, and deployments of reconfigurable mechanisms and robots with contributions from papers presented in the IEEE/IFToMM International Conference on Reconfigurable Mechanisms and Robots (ReMAR 2021) [1] as well as an Open Call for Papers.Mechanism and robot reconfiguration stems from geometric constraint changes based on innovative design and control operation. Systematic design synthesis theory and use-oriented design methodologies are critical to generating more reconfigurable mechanism and robot concepts. Reconfiguration has been extended from basic reconfigurable joints and linkages, to various mechanisms, and to various robot functions including ground mobile motion, flying navigation, manipulation, soft robotics, and sensing interaction. Fundamental modeling and analysis of those newly developed designs are the basis to verify the reconfiguration process and guide application-based development with experimental validation. In this special issue, reconfiguration highlights numerous exemplars in the form of modular manipulators, deployable linkages, reconfigurable grippers, origami mechanisms, and reconfigurable parallel mechanisms with both rigid links and cable-driven designs.Modular design enables flexible manipulator design and reconfiguration. Ju et al. present a cable-driven manipulator with a lightweight and expandable structure based on a modular U-joint unit for flexible environment adaptability. A fast heuristic inverse kinematics model is developed for the hyper-redundant system and provides a reference solution for other reconfigurable redundant designs. A similar modular cable-driven continuum arm is developed by Sitler and Wang for free-floating underwater manipulation onboard a remotely operated vehicle (ROV). In addition to the flexible arm design based on the modular unit, a reconfigurable dual arm configuration is also realized for crawling gait, dexterous, and seafloor manipulation. Modular units-based serial chain arm could be redundant and self-reconfigurable, requiring intensive dynamic calculations in simulation. To reduce the computational load, Fass et al. present a general novel analytical approach to formulate the Newton–Euler dynamics of self-reconfigurable chains in a single vectorized differential equation which enables efficient parallel computing.Reconfigurable linkages provide a fundamental basis for mechanism reconfiguration in a wide range of application scenarios. Tang et al. present a novel quadruped robot using a single-loop metamorphic mechanism to enable the ability of transforming between different working modes. This paves the way for developing versatile mobile robots using reconfigurable linkages. By applying Hoberman’s linkage as the modular design, Zhang et al. present a snake-inspired swallowing robot that can synchronously deploy and fold both axially and radially. The work creatively demonstrates an application of reconfigurable linkages in bio-inspired robot designs. To match demands of rapid development in the automotive industry, Lyu et al. present a reconfigurable modular fixture with high modularity and flexibility. The design method has a potential in generating more flexible fixture systems in industrial applications with frequent object size changes.Robot grasping requires high flexibility and adaptability in interacting with objects of various sizes and geometry. Sun et al. propose a reconfigurable robotic gripper based on a metamorphic finger mechanism which can have both expanding and grasping functions. The work expands reconfigurable mechanism applications into robotic grippers aiming at improving object grasping performance. To have an adaptable grasping function, soft materials are applied to gripper designs. However, their grasping force is generally low. To solve this problem, Cheng et al. introduce a limiting fiber into a soft finger design which largely increases the grasping force capacity and bending response speed. The method can be applied to similar soft robotic designs.Origami designs are a kind of reconfigurable mechanisms based on their folding/unfolding functions. Combining with a hydraulic power source, an origami actuator is developed by Liu et al. and integrated into a 6DOF Stewart-Gough parallel mechanism to realize translational and rotational motions. The design shows potential to be implemented in constructing dexterous and lightweight soft robotic applications. To quantify and improve folding reliability of origami systems, Liu et al. propose a biasing method to model the folding process through the origami hyperbolic paraboloid (hypar) when folding into one of two possible configurations. The results show an increased folding accuracy from 50% to 70% and provide insights for folding reliability analysis in more complex origami patterns with various reconfigurations.Reconfigurable parallel mechanisms can change their output motion types through constraint singularity, reconfigurable joints, re-assembly, or reconfigurable base/platform and links. The first three methods are demonstrated by three papers in this Special Section. Ye et al. present a new reconfigurable parallel mechanism that can reconfigure into 1R2T and 2R1T operation modes through a constraint singularity when the platform is parallel to the base. Zhao et al. present unified kinematics and dynamics modeling of a n(3RR1S) reconfigurable manipulator based on the principle of recursive virtual power. The work provides a method for solving high-redundant series-parallel systems with reconfigurable parallel modules for potential space object grasping applications. Through module re-assembly, Feng et al. investigate all possible non-isomorphic configurations of a reconfigurable hexapod robot using Pólya enumeration theorem. The method is applicable for other similar reconfigurable robot and mechanism designs using modular combinations.Cable-driven parallel robots (CDPRs) with movable anchor points can reconfigure into infinite configurations for flexible manipulation tasks with the anchor points attached to mobile ground or aerial vehicles. To solve their real-time planning, Xiong et al. present a dynamic control method through a reconfiguration value function, which is defined to value possible RCDPR configurations for optimal selection and planning. The developed method creatively treats the planning problem as a reconfiguration model and can be extended to other similar mobile robot planning tasks.We hope this Special Section will contribute to the research on reconfigurable mechanisms and robots as a key trend in mechanisms and robotics. We would like to show our great thanks to the general chair of ReMAR 2021, Professor Fengfeng Xi, and to the Editor-in-Chief, Professor Venkat Krovi, for his guidance and huge support throughout the whole process. We are also grateful to the journal administrative team and all the authors and reviewers for their valuable support and contributions.
Self-alignment mechanisms for exoskeletons alleviate critical issues arising from misalignment between exoskeleton and human joints. In this paper we present a method for kinematic synthesis of a passive mechanism for aligning joint axes of exoskeletons with human anatomical joint axes. Planar parallel manipulators are known for their inherent stiffness and compactness; we propose a novel parallel manipulator design to achieve self-alignment. The base platform of the parallel manipulator is attached to the human arm and the moving platform is attached to the exoskeleton. Three PRR chains connect the base and moving platforms. An optimal design problem is formulated to find the smallest manipulator that spans the specified workspace, which represents the possible area of misalignment.
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