Accepted Manuscripts

Fugui Xie, Xin-Jun Liu, Jinsong Wang and Markus Wabner
J. Mechanisms Robotics   doi: 10.1115/1.4037254
This paper deals with the kinematic optimization of a five degrees of freedom (DoFs) spatial parallel mechanism with three kinematic chains. Due to the potential advantages, this mechanism is used as a movable plug-in module in a multi-axis machine center to process large-scale parts with rotary contour surfaces. To derive its optimal parameters, kinematic optimization based on the motion/force transmissibility is carried out. The parameter design space (PDS) is generated first. Then the performance evaluation index (i.e., local transmission index, LTI) is derived sequentially. On this basis, the good transmission positioning workspace (GTPW) for a given orientaion is defined by constraining the value of LTI with a certain metric. Thereafter, the atlases of the GTPW and the optimal region satisfying the workspace constraint are derived in the PDS. Within this region, a set of optimal parameters without dimension are selected. Consequently, the cuboid workspaces within GTPWs are identified in detail. By using the ratio between required workspace in application and the derived cuboid workspaces, optimal geometric parameters with dimension are derived. Workspace analysis results show that, for an arbitrary orientation between the vertical and horizontal direction, there is always a cuboid workspace within GTPW larger than required workspace. In addition, the orientational capability of the mechanism can reach more than 90° and the flexible two DoFs rotations can also be realized. The work in this paper is very helpful to the development of a mobile machining module.
TOPICS: Kinematics, Optimization, Parallel mechanisms, Dimensions, Degrees of freedom, Design, Machinery, Machining, Kinematic chains, Performance evaluation
Daniel Lemus, Jan van Frankenhuyzen and Heike Vallery
J. Mechanisms Robotics   doi: 10.1115/1.4037255
Fall-related injuries are among the most serious and common medical problems in aging societies. However, existing wearable robotic devices do not primarily target balance, but rather other subtasks of locomotion. Particularly support for weight bearing requires bulky and heavy exoskeletons, which are impractical in daily life. We recently proposed the theoretical idea of a wearable balancing aid, consisting of a set of Control Moment Gyroscopes (CMGs) contained into a backpack-like orthopedic corset. In this paper, we describe the design and evaluation of a a hardware test setup, consisting of a single Control Moment Gyroscope mounted to an inverted pendulum. The design considers aerodynamic characteristics of the flywheel as well as dynamic effects of the wearer's motion on the CMG. The developed system was capable of tracking a gyroscopic moment up to 70 Nm with a total CMG mass of about 10 kg. Moreover, the system was capable of emulating several virtual stiffnesses while keeping the pendulum balanced.
TOPICS: Design, Pendulums, Wounds, Biomedicine, Exoskeleton devices, Weight (Mass), Hardware, Flywheels, Bearings, Robotics, Orthopedics
Damien Six, Sebastien Briot, Abdelhamid Chriette and Philippe Martinet
J. Mechanisms Robotics   doi: 10.1115/1.4037256
Parallel robots present singular configurations that divide the operational workspace into several aspects. It was proven that Type 2 and Leg Passive Joint Twist System singularities can be crossed with a trajectory respecting a given dynamic criterion. However, the practical implementation of a controller able to track such trajectories is up to now limited to restrictive cases of Type 2 singularities crossing. Analyzing the structure of the inverse dynamic model, this paper proposes a global solution allowing the tracking of trajectories respecting the general criterion for any singularity that leads to potential issues of dynamic model degeneracy. The tracking is operated in the robot joint space. Experimental results on a five-bar mechanism showed the controller ability to successfully cross Type 2 singularities.
TOPICS: Control equipment, Robots, Dynamic models, Trajectories (Physics)
Technical Brief  
Na Li, Hai-Jun Su and Xianpeng Zhang
J. Mechanisms Robotics   doi: 10.1115/1.4037186
Dynamic characteristics analysis is very important for the design and application of compliant mechanisms, especially for dynamic and control performance in high speed applications. Although pseudo-rigid-body models have been extensively studied for kinetostatic analysis, their accuracy for dynamic analysis is relatively less evaluated. In this paper, we first evaluate the accuracy of PRB model by comparing against the continuum model using dynamic simulations. We then investigate the effect of mass distribution on dynamics of pseudo-rigid-body model for compliant parallel guided mechanisms. We show that when the beam mass is larger than 10% of the motion stage, the error is significant. We then propose a new PRB model with a corrected mass distribution coefficient which significantly reduces the error in prediction of natural frequency. Moreover, to verify the effectiveness of the new modified PRB model for large deflection dynamic analysis, we compared the dynamic response of three (continuum, original PRB and modified PRB) models subject to the same external excitation. The results show that the corrected PRB model is very effective in predicting responses in large deflection dynamic analysis. At last, a compliant double parallel guiding mechanism is used as a case study for validation of the new PRB model for dynamics of compliant mechanisms.
TOPICS: Dynamic analysis, Dynamics (Mechanics), Deflection, Errors, Compliant mechanisms, Excitation, Dynamic response, Simulation, Design, Engineering simulation
Marius Stuecheli, Marianne Schmid Daners and Mirko Meboldt
J. Mechanisms Robotics   doi: 10.1115/1.4037114
The VariLeg is an exoskeleton allowing a paraplegic to walk. It was used for competing on an obstacle course at the first Cybathlon. It integrates an adjustable stiffness in the knee joint to improve the walking performance. However, the adjustable stiffness mechanism (ASM) of the VariLeg is bulky and heavy, which hampers the handling of the exoskeleton. Hence, the choice of an ASM concept that only needs small springs is essential. This study benchmarks six state-of-the-art ASMs regarding their needed energy-storage capacity, thus their potential for a high compactness. The benchmark is performed with the requirements of the VariLeg and a second requirements set, which can be fulfilled by all six ASMs. The benchmark can be transferred to other requirements as well. It is based on models of the ASMs with their design parameters optimized for the given requirements set. The benchmark reveals large differences between the performances of the investigated ASM concepts of up to a factor of five in the energy-storage capacity. This compactness benchmark is a useful design tool to choose a suitable mechanism to realize a compact implementation. More compact ASMs will improve the handling of assistive robots with a physically adjustable stiffness, such as the VariLeg, to support handicapped people in everyday life.
TOPICS: Stiffness, Design, Energy storage, Exoskeleton devices, Robots, Springs, Knee
Xianwen Kong
J. Mechanisms Robotics   doi: 10.1115/1.4037111
Although kinematic analysis of conventional mechanisms is a well-documented fundamental issue in mechanisms and robotics, the emerging reconfigurable mechanisms and robots pose new challenges in kinematics. One of the challenges is the reconfiguration analysis of multi-mode mechanisms (including kinematotropic mechanisms and mechanism with bifurcation or multifurcation), which refers to finding all the motion modes and the transition configurations of the multi-mode mechanisms. Recent advances in mathematics, especially algebraic geometry and numerical algebraic geometry, make it possible to develop an efficient method for the reconfiguration analysis of reconfigurable mechanisms and robots. This paper first presents a method for formulating a set of kinematic loop equations for mechanisms using dual quaternions. Using this approach, a set of kinematic loop equations of spatial mechanisms is composed of six polynomial equations. Then the reconfiguration analysis of a novel multi-mode 1-DOF (degree-of-freedom) 7R spatial mechanism is dealt with by solving the set of loop equations using tools from algebraic geometry. It is found that the 7R multi-mode mechanism has three motion modes, including a planar 4R mode, an orthogonal Bricard 6R mode, and a plane symmetric 6R mode. Three (or one) R joints of the 7R multi-mode mechanism lose their DOF in its 4R (or 6R) motion modes. Unlike the 7R multi-mode mechanisms in the literature, the 7R multi-mode mechanism presented in this paper does not have a 7R mode in which all the seven R joints can move simultaneously.
TOPICS: Kinematics, Robots, Degrees of freedom, Robotics, Bifurcation, Geometry, Polynomials, Kinematic analysis, Mathematics, Algebra
Wei Li and Jorge Angeles
J. Mechanisms Robotics   doi: 10.1115/1.4037112
A novel parallel robot, dubbed the SDelta, is the subject of this paper. The robot is a simpler alternative to both the well-known Stewart-Gough platform (SGP) and current three-limb, full-mobility parallel robots, as it contains fewer components and all its motors are located on the base, which greatly reduces the inertial load on the system, making it a good candidate for high-speed operations. SDelta features a symmetric structure; its forward-displacement analysis leads to a system of three quadratic equations in three unknowns, the robot direct-kinematics thus admitting up to eight solutions, or half the number of those admitted by the SGP. The kinematic analysis, undertaken with a geometrical method based on screw theory, leads to two Jacobian matrices, whose singularity conditions are investigated. Instead of using the determinant of a 6×6 matrix, we derive one simple expression that characterizes the singularity condition. This approach is also applicable to a large number of parallel robots whose six actuation wrench axes intersect pairwise, such as the SGP and three-limb parallel robots whose limbs include, each, a passive spherical joint. The workspace is analyzed via a geometric method, while the dexterity analysis is conducted via a discretization method. Both show that the given robot has the potential to offer both large workspace and good dexterity with a proper choice of design variables.
TOPICS: Kinematics, Robots, Mechanical admittance, Motors, Screws, Stress, Design, Displacement, Jacobian matrices, Kinematic analysis
Bruno Belzile and Lionel Birglen
J. Mechanisms Robotics   doi: 10.1115/1.4037113
The sense of touch has always been challenging to replicate in robotics but it can provide critical information when grasping objects. Nowadays, tactile sensing in artificial hands is usually limited to using external sensors which are typically costly, sensitive to disturbances, and impractical in certain applications. Alternative methods based on proprioceptive measurements exist to circumvent these issues but they are designed for fully actuated systems. Investigating this issue, the authors previously proposed a tactile sensing technique dedicated to underactuated, a.k.a. self-adaptive, fingers based on measuring the stiffness of the mechanism as seen from the actuator. In this paper, a procedure to optimize the design of underactuated fingers in order to obtain the most accurate proprioceptive tactile data is presented. Since this tactile sensing algorithm is based on a one-to-one relationship between the contact location and the stiffness measured at the actuator, the accuracy of the former is optimized by maximizing the range of values of the latter, thereby minimizing the effect of an error on the stiffness estimation. The theoretical framework of the analysis is first presented, followed by the tactile sensing algorithm, and the optimization procedure itself. Finally, a novel design is proposed which includes a hidden proximal phalanx to overcome shortcomings in the sensing capabilities of the proposed method. This paper demonstrates that relatively simple modifications in the design of underactuated fingers allow to perform accurate tactile sensing without conventional external sensors.
TOPICS: Design, Stiffness, Sensors, Actuators, Algorithms, Touch (physiological), Grasping, Optimization, Robotics, Errors
Peter Steinkamp
J. Mechanisms Robotics   doi: 10.1115/1.4037077
In Appendix B, Hopper Technical Details, the leaf spring thickness was erroneously stated as 0.045 mm. The correct thickness is 0.45 mm.
Shuguang Huang and Joseph M. Schimmels
J. Mechanisms Robotics   doi: 10.1115/1.4037019
This paper addresses the passive realization of any selected planar elastic behavior with a parallel or a serial manipulator. Sets of necessary and sufficient conditions for a mechanism to passively realize an elastic behavior are presented. These conditions completely decouple the requirements on component elastic properties from the requirements on mechanism kinematics. The restrictions on the set of elastic behaviors that can be realized with a mechanism are described in terms of acceptable locations of realizable elastic behavior centers. Parallel-serial mechanism pairs that realize identical elastic behaviors (dual elastic mechanisms) are described. New construction-based synthesis procedures for planar elastic behaviors are developed. Using these procedures, one can select the geometry of each elastic component from a restricted space of kinematically allowable candidates. With each selection, the space is further restricted until the desired elastic behavior is achieved.
TOPICS: Elasticity, Construction, Manipulators, Kinematics, Geometry

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