Showing papers on "Revolute joint published in 2022"

Bioinspired Knee Joint of a Lower-Limb Exoskeleton for Misalignment Reduction

Abstract: Misalignment between an exoskeletal robot and the wearer’s body must be minimal to prevent injury and discomfort during active assistance. It is particularly difficult for the robot to imitate the knee joint motion of a human because the human knee joint has a complex structure, which cannot be realized with a single revolute joint. In this article, a knee joint mechanism that closely realizes the human knee joint motion using the curved guide rail and bearings is proposed. For the optimal design of the proposed mechanism, the motions of the tibia and the femur are captured, and the guide rail of the proposed mechanism is designed to realize the captured human knee joint motion. A simulation study is performed based on kinematic calculations, and the shape of the guide rail is optimized for the proposed device to precisely imitate the human motion. This article also introduces an experimental method for the quantitative evaluation of the misalignment; the pressure inside the brace is measured using an air-pressure sensor pad, and the pressure measurements are utilized for objective comparison of the misalignment with respect to the joint mechanism. An additional experiment is performed to verify the noninterfering of the human walking motion.

Switch controllers of an n-link revolute manipulator with a prismatic end-effector for landmark navigation

Abstract: Robotic arms play an indispensable role in multiple sectors such as manufacturing, transportation and healthcare to improve human livelihoods and make possible their endeavors and innovations, which further enhance the quality of our lives. This paper considers such a robotic arm comprised of n revolute links and a prismatic end-effector, where the articulated arm is anchored in a restricted workspace. A new set of stabilizing switched velocity-based continuous controllers was derived using the Lyapunov-based Control Scheme (LbCS) from the category of classical approaches where switching of these nonlinear controllers is invoked by a new rule. The switched controllers enable the end-effector of the robotic arm to navigate autonomously via a series of landmarks, known as hierarchal landmarks, and finally converge to its equilibrium state. The interaction of the inherent attributes of LbCS that are the safeness, shortness and smoothness of paths for motion planning bring about cost and time efficiency of the controllers. The stability of the switched system was proven using Branicky’s stability criteria for switched systems based on multiple Lyapunov functions and was numerically validated using the RK4 method (Runge–Kutta method). Finally, computer simulation results are presented to show the effectiveness of the continuous time-invariant velocity-based controllers.

Elongatable Gripper Fingers With Integrated Stretchable Tactile Sensors for Underactuated Grasping and Dexterous Manipulation

Abstract: The ability to grasp a wider range of objects in size and shape directly relates to the performance of robotic grippers. Adapting to complex geometries of objects requires large degrees of freedom to allow complex configurations. However, complexity in controlling many individual joints leads to introduction of underactuated mechanisms, in which traditional finger designs composed of revolute joints allow only flexion/extension motions. In this article, we propose a length-adjustable linkage mechanism in the underactuated finger controlled by an antagonistic tendon pair. The resulting gripper can elongate the fingers for an increased task space or shorten them for a finer spatial resolution. For tactile sensing, hyperelastic soft sensors are used to stretch with finger elongation. Contact pressures measured by the soft sensors are used in force-feedback control for which either the joint angles or the link lengths are adjusted. Lastly, a multimodal control scheme that combines elongation and flexion modes is demonstrated with tasks of dexterous manipulation.

Dynamic modeling and responses investigation of spatial parallel robot considering lubricated spherical joint

Abstract: Lubricated spherical joint is widely used in spatial parallel robots, which significantly influences the dynamic response of the mechanism. Current research of mechanism dynamics considering lubricated kinematic joint mainly focuses on the plane mechanism with revolute joint , while that involves lubricated spherical joint of spatial mechanism is very few, especially, spatial parallel robot has not been reported. Dynamic modeling and responses investigation of spatial parallel robot considering lubricated spherical joint is realized in this paper. Combined with Reynolds' equation and Gumbel's boundary condition, dynamic modeling method of spatial parallel robot with lubricated spherical joint is proposed. The dynamic model of 4-U P S/R P S parallel robot with lubricated spherical joint is developed. Lubrication force model has been improved to provide better numerical stability for spatial parallel robots. The influences of lubricated joint and dry contact joint on dynamic response of the parallel robot are simulated by adjusting clearance size and driving speed. Results demonstrate that contact-impact at clearance joint is closely related to impact response of moving platform, and lubricated joint effectively weakens this effect. The deviation degree of acceleration is weak when clearance size is less than 0.4 mm, which means that the robot has good performance. It is of great significance to investigate influences of lubricated spherical joint on performance of parallel robots for dynamic prediction and structural optimization design of the system.

Dynamic modeling and responses investigation of spatial parallel robot considering lubricated spherical joint

Abstract: Lubricated spherical joint is widely used in spatial parallel robots, which significantly influences the dynamic response of the mechanism. Current research of mechanism dynamics considering lubricated kinematic joint mainly focuses on the plane mechanism with revolute joint , while that involves lubricated spherical joint of spatial mechanism is very few, especially, spatial parallel robot has not been reported. Dynamic modeling and responses investigation of spatial parallel robot considering lubricated spherical joint is realized in this paper. Combined with Reynolds' equation and Gumbel's boundary condition, dynamic modeling method of spatial parallel robot with lubricated spherical joint is proposed. The dynamic model of 4-U P S/R P S parallel robot with lubricated spherical joint is developed. Lubrication force model has been improved to provide better numerical stability for spatial parallel robots. The influences of lubricated joint and dry contact joint on dynamic response of the parallel robot are simulated by adjusting clearance size and driving speed. Results demonstrate that contact-impact at clearance joint is closely related to impact response of moving platform, and lubricated joint effectively weakens this effect. The deviation degree of acceleration is weak when clearance size is less than 0.4 mm, which means that the robot has good performance. It is of great significance to investigate influences of lubricated spherical joint on performance of parallel robots for dynamic prediction and structural optimization design of the system. • The kinematic and force model of spatial revolute clearance joint is derived. • The dynamic model of 3-RRRRR parallel mechanism with spatial revolute clearance joints is developed. • The displacement, velocity, acceleration and contact force of the mechanism are analyzed and verified.

Collision-free Inverse Kinematics of Redundant Manipulator for Agricultural Applications through Optimization Techniques

Abstract: This article presents an optimization-based technique for solving the inverse kinematics (IK) of spatially redundant manipulators in agricultural environments (workspaces). A kinematic configuration of 9 degrees of freedom (DOF) manipulator with eight revolute and one prismatic joint has been modelled to improve the accessibility in complex workspaces. The proposed manipulator has been simulated for harvesting fruits and vegetables. To perform the desired task in the working environment, the IK solution of the robot needs to be determined. The IK problem has been formulated as a constrained optimization problem with the objective of minimizing the positional and orientational errors by avoiding obstacles. A 3D CAD environment with different fruits and vegetable plants has been modelled in Solidworks. A target location in this environment has been chosen to pluck the fruit/vegetables. The trunk, branches, and leaves are considered as obstructions. The collision avoidance technique was implemented using a bounding box approach by including a collision detection algorithm. IK simulations of the spatial redundant manipulator in a cluttered environment were performed and results are reported. The joint trajectories of the robot while reaching desired task-space location has been depicted using Simscape Multibody. The results demonstrate that the end-effector of the robot has been reached desired task location successfully with an accurate IK solution. The approach is adaptable in a wide range of working environments based on the simulation results of the IK solution of robots.

Identification and Control of a 3-X Cable-Driven Manipulator Inspired From the Bird’s Neck

Abstract: This paper is devoted to the control and identification of a manipulator with three anti-parallelogram joints in series, referred to as X-joints. Each X-joint is a tensegrity one-degree-of-freedom mechanism antagonistically actuated with cables and springs in parallel. As compared to manipulators built with simple revolute joints in series, manipulators with tensegrity X-joint offer a number of advantages, such as an intrinsic stability, variable stiffness, and lower inertia. This design was inspired by the musculosleketon architecture of the bird’s neck, which is known to be very dextrous. A test-bed prototype is presented and used to test computed torque control laws. Friction and cable elasticity are modeled and identified. Their effect on the performance of control laws is analyzed. It is shown that in the context of antagonistic actuation and lightweight design, friction plays a leading role and the significance of modeling cable elasticity is discussed.

A Novel Pressure-Controlled Revolute Joint with Variable Stiffness

Abstract: The compliance and deformability of soft robotics allow human-machine interactions in a safe manner without the need of sophisticated control systems inherent in rigid-body robotic devices. However, these advantages introduce controllability and predictability challenges. In this study, we propose a novel fluidic-driven variable stiffness revolute joint (VSRJ) based on hybrid soft-rigid approach to achieve adjustable compliance while addressing the abovementioned challenges. The VSRJ is composed of a silicone rubber cylinder as a pressure chamber and two identical rigid links. The soft cylinder is positioned in a fully closed compartment created by the assembly of the two rigid links, thus constraining its expansion when pressure is applied. By applying pressure, the stiffness of the joint increases accordingly for the following reasons: (1) increasing the friction force between the cylinder and the compartment walls and (2) creating a locking mechanism through the expansion of the cylinder into space between rigid links in a "bump" formation. Experimental results show that the VSRJ can achieve up to 8-fold rotational stiffness enhancement from 0 to 5 bar input pressure within -30° to +30° rotation angle. The modular design of the rigid link allows the assembly of multiple VSRJs to build a variable stiffness structure in which each VSRJ has an independent stiffness and relative position. The VSRJ was characterized in terms of repeatability, torque, and stiffness. The experimental results were validated by finite element analysis. This approach can provide opportunities for the use of this new variable stiffness concept as an efficient alternative to traditional variable-stiffness linkages.

On the Actuation Modes of a Multiloop Mechanism for Space Applications

Abstract: A symmetric, double-tripod multiloop mechanism (DTMLM), intended for grabbing objects in outerspace, is the subject of this article. Actuation modes are analyzed while introducing a novel tool applicable to space mechanisms. The key issue here lies in establishing the criteria for selecting the optimum mode from multiple actuation possibilities. The evaluation procedure includes generalized-force values, power requirement, and actuation-strategy models, along with their optimization. Accordingly, the optimization procedure targets the mode(s) with 1) uniformity of generalized-force distribution, 2) uniformity of power-requirement distribution, and 3) the fewest working actuators in a given maneuver. In this way, a rather complex problem is formulated in a simple manner, whereby the optimum actuation mode is found by list-lookup, from a reduced number of candidates. The DTMLM, which carries three compound hinges plus three prismatic, and six revolute joints, has three degrees of freedom. To analyze the actuation modes of the mechanism, the dynamics model of the DTMLM is established; as well, five representative modes are selected from 84 possibilities. It turns out that the 3 R -type (three revolute actuators) shows a uniform power-requirement distribution among the three actuators in the bending motion mode; in the 3 P -type (three prismatic actuators), one single actuator is operational, and hence, takes all the load. Thus, the 3 R -type is the best from the power-distribution viewpoint, thereby providing strong power support, but complex from the actuation-strategy viewpoint, as illustrated in the article. Simulation results and prototype experiments of the DTMLM are reported, thereby verifying the analysis results.

Determination of modes of vibration for accurate modelling of the flexibility effects on dynamics of a two link flexible manipulator

Abstract: In this paper, the effect of flexibility on the dynamics of a two-link planar manipulator having two revolute joints is investigated. For this purpose, a mathematical model validated through experiments is obtained for a Two-link planar manipulator having two revolute joints . The mathematical model takes into account the continuous change of Eigen value due to change of configuration of the manipulator. A general expression for the frequency equation for the vibrating links has been obtained which is found to be time-dependent. The effect of this time-dependency of Eigen values of links on Joint and Tip responses is included in the assumed modes method. In this paper, the types of boundary conditions to be used for the flexible links are also researched upon and it is proposed that first four modes of vibration should be used for exactly modelling the flexible links .

Generalized Swing-Up Control of Underactuated Mechanical Systems

Abstract: Time-reversal symmetry, which is one of the main characteristics of mechanical systems without frictional forces, is applied to underactuated mechanical systems to generate desired trajectories for the generalized swing-up control. A novel necessary and sufficient condition is provided to examine whether a system trajectory is time-reversible. A desired trajectory for the generalized swing-up control is experimentally generated by a simple swing-down controller. The time-reverse of the swing-down trajectory is used as the desired trajectory for the generalized swing-up control. With the help of this method, desired swing-up trajectories can be generated also for the popular underactuated mechanical systems with pendulum-like zero dynamics such as the Acrobot, the Pendubot, the Furuta pendulum or the cart-pendulum system. Moreover, it is argued heuristically that a planar robot with an arbitrary number of revolute joints can always be swung up by only one driven joint. The theoretical results are supported by simulations with a planar robot with three revolute joints whose only one joint is actuated.

Multiscale Graph-Guided Convolutional Network With Node Attention for Intelligent Health State Diagnosis of a 3-PRR Planar Parallel Manipulator

Abstract: The data-driven intelligent health state diagnosis strategy has been successfully applied in modern industrial equipments. However, many of existing research works suffer from two major deficiencies: First, the sample independence assumption is widely adopted, so the influences of the relationship between samples on the overall performances are not explored. Second, most of the abovementioned application objects are typical key functional components (such as bearings, gearboxes, etc.), and research works on the intelligent health state diagnosis of planar parallel manipulator are rarely reported. To address these issues, a novel intelligent health state diagnosis method, termed multiscale graph-guided convolutional network with node attention (MSGCN-NA), is proposed for a 3-PRR (P and R represent prismatic and revolute pairs, respectively) planar parallel manipulator. Specifically, the developed MSGCN-NA model mainly contains the following two parts: first, the one is an unsupervised convolutional autoencoder, which is employed for the extraction of deep representation features, and then combined with Pearson metric to establish the adjacency matrix. Second, The other part is the constructed multiscale graph convolutional network, in which the node attention mechanism is adopted to achieve cross-scale fusion of different neighborhood information. The effectiveness of the proposed MSGCN-NA method is fully verified based on the simulation and experimental scenarios, the results show that MSGCN-NA can achieve superior diagnosis performances.

Denavit-Hartenberg Notation-based Kinematic Constraint Equations for Forward Kinematics of the 3-6 Stewart Platform

Abstract: This paper presents a new forward kinematic approach for the 3-6 Stewart platform. In the 3-6 Stewart platform, the position and orientation of the moving platform can be determined through kinematic constraint equations that indicate the possible trajectories of the three joints on the platform from the base coordinates. In most previous studies, the constraint equations were obtained through vector equations, which involves many complicated steps and multiple variables. Based on Denavit-Hartenberg (D-H) notation, the proposed approach allows a constraint equation representation with only four parameters. In addition, to implement the forward kinematics with the fewest possible D-H parameters, the 3-6 Stewart platform is reconfigured into three virtual revolute joints sharing base coordinates. Consequently, the constraint equations can be intuitively derived, and the computation process becomes more concise. The feasibility of the proposed approach is verified through an iterative numerical analysis and a simulation comparison with the general inverse kinematics.

A Novel Semi-Supervised Graph-Guided Approach for Intelligent Health State Diagnosis of a 3-PRR Planar Parallel Manipulator

Abstract: As one of the most important operation and maintenance approaches, health state diagnosis technology plays a crucial role in ensuring the safety and reliability of mechanical equipment. The planar parallel manipulator, as a typical actuator, is widely employed in the field of precision manufacturing due to its advantages of high stiffness, large load support capability, and high precision. However, compared with common key functional components (such as bearings and gearboxes), planar parallel manipulators have more complicated operating mechanisms and failure behaviors. To satisfy the health state diagnosis demands of planar parallel manipulators in the scenario of insufficient label information, a novel intelligent health state diagnosis approach, termed semi-supervised graph-guided network with perception attention (SGN-PA), is developed for a 3-PRR (P and R represent prismatic and revolute pairs, respectively) planar parallel manipulator. Specifically, an improved multiorder graph perception module is constructed to extract multiscale feature information, and achieve feature fusion by combining perceptual attention mechanism, which enables the proposed SGN-PA model to have adaptation adjustment capabilities. Following that, local and nonlocal feature constraint strategies are employed with pseudo-label technology to reduce intraclass differences and maximize interclass differences, and then to fit the demands of health state diagnosis tasks. Eventually, based on the simulation and experimental scenarios of a 3-PRR planar parallel manipulator, the effectiveness and feasibility of the proposed SGN-PA model is extensively confirmed, and the diagnosis results show that it can significantly relax the constraints of label information while maintaining superior performances.