Kinematic analysis of the 3-RPR parallel manipulator

The aim of this paper is the kinematic study of a 3-RPR planar parallel manipulator where the three fixed revolute joints are actuated. The direct and inverse kinematic problem as well as the singular configuration is characterized. On parallel singular configurations, the motion produce by the mobile platform can be compared to the Reuleaux straight-line mechanism

Optimal dimensional synthesis of force feedback lower arm exoskeletons

This paper presents multi-criteria design optimization of parallel mechanism based force feedback exoskeletons for human forearm and wrist. The optimized devices are aimed to be employed as a high fidelity haptic interfaces. Multiple design objectives are discussed and classified for the devices and the optimization problem to study the trade-offs between these criteria is formulated. Dimensional syntheses are performed for optimal global kinematic and dynamic performance, utilizing a Pareto front based framework, for two spherical parallel mechanisms that satisfy the ergonomic necessities of a human forearm and wrist. Two optimized mechanisms are compared and discussed in the light of multiple design criteria. Finally, kinematic structure and dimensions of an optimal exoskeleton are decided

A Pair of Measures of Rotational Error for Axisymmetric Robot End-Effectors

International audienceThis paper deals with the problem of representing the rotational error of spatial robots with three orientational degrees of freedom (DOF). Typically, the errors on each of three Euler angles defining the orientation of an end-effector are analysed separately. However, this is wrong since an accuracy measure should depend only on the "distance" between the nominal pose and the actual one, and not on the choice of reference frame in which these are represented. Several bi-invariant metrics for rotational error exist but are single-parameter and, by definition, disregard the shape of the robot end-effector. Yet, robot end-effectors are typically axisymmetric. Therefore, we propose a two-parameter measure of rotational errors that is better suited for such robot end-effectors

Singularity Analysis of Zero-Torsion Parallel Mechanisms

International audienceThis paper presents the singularity analysis of four 3-DOF symmetric zero-torsion parallel mechanisms. These mechanisms are composed of three identical legs ending with a spherical joint that is constrained to move in one of three equally spaced plane intersecting at one line. The computation of the singularity loci is based on the degeneracy of the system of screws applied on the platform by the legs. The whole study is based on the use of a special orientation representation, previously introduced under the name of Tilt-and-Torsion angles. This representation is briefly introduced. Then the interdependence between the Cartesian coordinates of the general class of parallel mechanisms is derived. Finally, the singularity loci are derived and the size of the workspace taking into account all singular configurations is shown

Resolution of spherical parallel Manipulator (SPM) forward kinematic model (FKM) near the singularity

One of the advantages of parallel manipulators is their good accuracy but near the singularity their dexterity and accuracy is poor. In this paper, we propose an approach to eliminate the disturbance caused by the presence of singularities in the workspace of a parallel manipulator. The effect of parallel singularity on the resolution of the forward kinematics model (FKM) is eliminated by adding a redundant sensor in one passive joint of the parallel manipulator. A study is made to choose the passive joint that gives the maximum of accuracy on the resolution of FKM. The proposed approach is applied to a spherical parallel manipulator (SPM) for tele-manipulation with haptic feedback. An experimental comparison is made to prove the benefit of the redundant sensor on the FKM. A singularity and kinematics study of the SPM is developed

Design, implementation and control of self-aligning, bowden cable-driven, series elastic exoskeletons for lower extremity rehabilitation

We present AssistOn-Leg, a modular, self-aligning exoskeleton for robotassisted rehabilitation of lower extremities. AssistOn-Leg consists of three selfaligning, powered exoskeletons targeting ankle, knee and hip joints, respectively. Each module can be used in a stand-alone manner to provide therapy to its corresponding joint or the modules can be connected together to deliver natural gait training to patients. In particular, AssistOn-Ankle targets dorsiflexion/ plantarflexion and supination/pronation of human ankle and can be configured to deliver balance/proprioception or range of motion/strengthening exercises; AssistOn-Knee targets flexion/extension movements of the knee joint, while also accommodating its translational movements in the sagittal plane; and AssistOn- Hip targets flexion/extension movements hip joint, while allowing for translations of hip-pelvis complex in the sagittal plane. Automatically aligning their joint axes, modules of AssistOn-Leg ensure an ideal match between human joint axes and the exoskeleton axes. Self-alignment of the modules not only guarantees ergonomy and comfort throughout the therapy, but also significantly shortens the setup time required to attach a patient to the exoskeleton. Bowden cable-driven series elastic actuation is utilized in the modules located at the distal (knee and ankle) joints of AssistOn-Leg to keep the apparent inertia of the system low, while simultaneously providing large actuation torques required to support human gait. Series elasticity also provides good force tracking characteristics, active back-driveability within the control bandwidth and passive compliance as well as impact resistance for excitations above this bandwidth. AssistOn-Hip is designed to be passively back-driveable with a capstan-based multi-level transmission. Thanks to passive compliance of the distal modules and passive backdriveability of the hip module, the overall design ensures safety even under power losses and robustness throughout the whole frequency spectrum