Rolling Biped Polynomial Motion Planning

This work discloses a kinematic control model to describe the geometry of motion of a two-wheeled biped’s limbs. Limb structure is based on a four-bar linkage useful to alleviate damping motion during self-balance. The robot self-balancing kinematics geometry combines with user-customized polynomial vector fields. The vector fields generate safe reference trajectories. Further, the robot is forced to track the reference path by a model-based time-variant recursive controller. The proposed formulation showed effectiveness and reliable performance through numerical simulations.

Kinematics study of a 10 degrees-of-freedom lower extremity exoskeleton for crutch-less walking rehabilitation

BACKGROUND: Wearable lower extremity exoskeletons can provide walking assistance for the physical rehabilitation of paralyzed individuals. However, most of the existing exoskeletons require crutches to maintain balance, thus a self-balancing type is needed to improve applicability. OBJECTIVE: The purpose of this work is to study the kinematic characteristics of a novel lower extremity exoskeleton for crutch-less walking rehabilitation, and evaluate the movement performance through practical experiments. METHODS: Based on the human lower limb structure and movement characteristics, a fully actuated 10 degrees-of-freedom (DoF) lower extremity exoskeleton was proposed. The kinematic characteristics of the exoskeleton were analyzed by the D-H method and geometric method, and the model validity was verified through simulations and experiments. RESULTS: The closed-form solutions for both forward and inverse kinematics models were obtained. The consistent results of theoretical calculation and numerical simulation have shown the accuracy of the established models. The practical experiments regarding six trials have demonstrated the movement performance of the proposed exoskeleton, including sit, stance, leg extension/flexion, and left/right swing. CONCLUSIONS: The kinematic characteristics of the proposed 10-DoF lower extremity exoskeleton are similar to the human lower limb, and it could meet the motion demands of crutch-less walking rehabilitation.

A novel class of generalized parallel manipulators with high rotational capability

A conventional parallel manipulator is characterized by connecting one moving platform with two or more serial kinematic limbs. Since each limb is independently supporting one moving platform, the moving platform must be a rigid body with several kinematic pairs fixed on it. However, for generalized parallel manipulators with articulated moving platforms, the moving platforms are not limited to rigid bodies but including serial kinematic chains or internal kinematic joints. The introduction of articulated moving platforms allows for improving the kinematic performance of generalized parallel manipulators, especially for rotational capability. On account of the structural characteristics of the moving platforms, it also poses a significant challenge in the construction of the structures of manipulators. This research raises a new method for the type synthesis of generalized parallel manipulators with novel articulated moving platforms. The proposed method introduces a striking shortcut for the limb structure analysis of mechanisms with high rotational capability. In this paper, a class of generalized parallel manipulator with different degrees of freedom from 3 to 6 are constructed by using the constraint synthesis method, and several examples are provided to demonstrate the feasibility of the advocated method. At last, the 3T3R generalized parallel manipulator is taken as an example to analyze the inverse kinematics, and the evaluation of the workspace is conducted to verify the rotational capacity.

Conceptual design and comparative stiffness analysis of an Exechon-like parallel kinematic machine with lockable spherical joints

This article proposes two types of lockable spherical joints which can perform three different motion patters by locking or unlocking corresponding rotational axes. Based on the proposed lockable spherical joints, a general reconfigurable limb structure with two passive joints is designed with which the conceptual designs of two types of Exechon-like parallel kinematic machines are completed. To evaluate the stiffness of the proposed Exechon-like parallel kinematic machines, an expanded kinetostatic model is established by including the compliances of all joints and limb structures. The prediction accuracy of the expanded stiffness model is validated by numerical simulations. The comparative stiffness analyses prove that the Exe-Variant parallel kinematic machine claims competitive rigidity performance to the Exechon parallel kinematic machine. The present work can provide useful information for further investigations on structural enhancement, rigidity improvement, and dynamic analyses of other Exechon-like parallel kinematic machines.

Screw-System-Variation Enabled Reconfiguration of the Bennett Plano-Spherical Hybrid Linkage and Its Evolved Parallel Mechanism

This paper presents the Bennett plano-spherical hybrid linkage and proposes a novel metamorphic parallel mechanism consisting of this plano-spherical linkage as part of limbs. In light of geometrical modeling of the Bennett plano-spherical linkage, and with the investigation of the motion-screw system, the paper reveals for the first time the reconfigurability property of this plano-spherical linkage and identifies the design parameters that lead to change of constraint equations, and subsequently to variation of the order of the motion-screw system. Arranging this linkage as part of limbs, the paper further investigates the reconfiguration property of the plano-spherical linkage evolved parallel mechanism. The analysis reveals that the platform constraint-screw system varies following both bifurcation and trifurcation with motion branch variation in the 6R linkage integrated limb structure. Consequently, this variation of the platform constraint-screw system leads to reconfiguration of the proposed metamorphic parallel mechanism. The paper presents a way of analyzing reconfigurability of kinematic structures based on the screw-system approach.

Motion Pattern Planning of a Quadruped Robot Based on Parallel Kinematics

In this paper, kinematic analysis and motion planning of a quadruped robot are presented by regarding the robot as an equivalent parallel platform-type mechanism with RRRS limb structure. Based on screw theory, the mobility of the quadruped robot with different touchdown legs is analyzed, and proves that the design for degree of freedom (DOF) is available. Base on the established kinematic model of a single leg in terms of POE formula of screw theory, several typical patterns of walking motion planning are implemented and verified by the ADAMS-based simulation. In order to show a good maneuverability and potential manipulation capability of the quadruped robot as a parallel manipulator, the gait planning reflecting two rotating motion patterns (including the rotating motion and walking motion) is modeled and simulated by kinematics of the equivalent parallel manipulators. Finally, a prototype of the quadruped robot has been built. Experimental test shows the practical walking and rotating ability as desired.

Geometry and Constraint Based Design of Metamorphic Parallel Mechanisms

This paper presents two general metamorphic kinematic units which employ only revolute joints. The two kinematic units with reconfigurability are mapped from independent set of line vectors with different geometry. The underlying principle of the metamorphosis of this kind of reconfigurable kinematic units is unraveled by investigating the dependency of corresponding screw system comprised with line vectors. Limb structure that can be used for constructing new parallel mechanisms is proposed. Subphases of the limb are enumerated accompanying to the phase change of the embodied metamorphic units and constraints corresponding to each phase are analyzed based on reciprocity of screws. Two metamorphic parallel mechanisms which can change their configuration from fully 6-DOF phase to 3-DOF translational subphase and 3-DOF spherical subphase are proposed according to geometric conditions that must be met by limbs. The mobility changes of the platform induced by phase change of limbs are addressed.