Lockable Joints Research Articles

Application of finite screw method for multi-mode mechanism classification and determination

To enrich the theoretical system of multi-mode mechanisms, a classification method and a determination method are proposed in this paper. From the perspective of configuration transformation, the multi-mode mechanisms are divided into two types: one based on lockable joints and the other based on the principle of bifurcated motion. Furthermore, the mechanisms based on the principle of bifurcated motion are categorized into two types: one based on the variable mobility branch and the other based on constraint singularity generated by branches. The principles of classification are expounded and the determination method is developed. The proposed classification and the determination methods of the multi-mode mechanism provide new insights for their analysis.

Development and investigation for rigid-flexible coupling dynamic performances of the morphing wing with clearance joints

Morphing wings have enjoyed much research interest due to their advantages in improving aircraft adaptability to various service environments. A single drive and continuous variable bending morphing mechanism with multiple configurations is proposed. The morphing wing designed based on this mechanism can have the advantages of lightweight, multiple configurations and smooth morphing shapes. Transient aerodynamic loads will destroy the dynamic performance of the wing during the multi-mode morphing process. Therefore, it is crucial to analyze the vibration characteristics for the reliability of the morphing wing. However, it is difficult to obtain its dynamic model and solution because of the complex configuration and the multi-mode morphing. In this paper, a mathematical model for the multi-mode morphing of the morphing mechanism based on lockable joints is described using the constraint equation firstly. Then, a rigid-flexible coupling dynamic model of the morphing wing with clearance joints is established. Finally, the influence of the infective joint and the flexible link on the vibration properties of the morphing wing is analyzed in detail. Moreover, differences in the dynamic parameters between rapid and regular actuation are discussed. The results obtained in this paper have an important engineering value for the design and manufacture of the morphing wing.

Design and Motion Principle Analysis of new parallel mechanisms with fewer active inputs than the degrees of freedom

AbstractIn this paper, two new parallel mechanisms (PMs) with fewer active inputs than the degrees of freedom (DOFs)are proposed: (i) an nSPS (n = 7, 8, 9) six‐DOF PM with n‐6 active inputs and six lockable joints. and (ii) a 3RPS‐SPS 3‐DOF PM with one active input and three lockable joints. Compared with the traditional PMs, the difference is that the proposed PMs can achieve the same mobility by using a minimal number of active joints. Moreover, the load‐carrying capacity is also improved compared with the original standard mechanisms, since the new PMs become statically redundant when all the branches are locked. For this purpose, a sequential motion control principle is introduced that requires both inverse and forward kinematics of PMs. Kinematic modeling, dimensional optimization, and structural design are carried out for the 7SPS and 3RPS‐SPS mechanisms, and the two prototypes are constructed for experimental validation. Both simulation and experiment results have shown that the proposed hybrid PM can achieve the original mobility while significantly reducing the number of actuators.

Elastostatic analysis of a module-based shape morphing snake-like robot

This paper describes the stiffness analysis of a module-based shape morphing snake-like robot. Snake-like robots have the characteristic of adapting to unstructured environments by exploiting their ability to reconfigure their body’s shape. However, the excellent mobility contrasts with the ability to transmit high loads, precluding its application in manufacturing operations. This article presents a hybrid structure based on reconfigurable modules equipped with lockable joints. The use of multiple modules in series allows for a large workspace. Furthermore, the parallel structure of the single modules provides for transferring or sustaining high loads. First, the reliability and precision of the theoretical model has been verified using finite element analysis (FEA). The relative errors are less than 5%. Then, a morphing module has been constructed as a physical demonstrator for the kinematic parameters and stiffness parameters used in elastostatic analysis. Finally, a five-segment prototype has been manufactured and tested. There is a deviation between the experimental results and the theoretical results due to manufacturing errors of the prototype but the trend of displacement change shown in the experimental results is basically consistent with the theoretical results.

Development and evaluation of a light-weight flexure-based lockable joint for morphing wings

Single degree of freedom (single-DOF) morphing wings have the advantages of light-weight and multiple morphing shapes, which can enable aircraft to adapt to uncertain flight environments and various mission types. A single-DOF morphing wing with four morphing shapes designed based on the reconfigurable mechanism is introduced firstly in this paper. Then, a light-weight compliant locking mechanism (CLM) based on flexure hinges is proposed for lockable joints in the reconfigurable mechanism. The CLM has two types depending on the locking method, namely the “plug-in” compliant locking mechanism (PCLM) and the “friction” compliant locking mechanism (FCLM). The working principles of the two CLMs are described. The mathematical models are established to assess the static and dynamic characteristics of the proposed CLM, which are also verified by the finite element simulation. The influence of the flexure hinge dimensions on the working performance of the CLM is analyzed, and its dimensions are optimized considering both the locking performance and the maximum stress. Finally, a PCLM and a FCLM actuated by SMA wires are fabricated based on the optimized dimensions. The experiment is conducted to assess the locking torque of the CLM and the feasibility of the proposed CLM used for morphing wings is confirmed.

A new reconfigurable parallel mechanism using novel lockable joints for large scale manufacturing

A new reconfigurable parallel mechanism using novel lockable joints for large scale manufacturing

Origami-Inspired Wearable Robot for Trunk Support

We present a wearable “exo-shell” device inspired by the human spine for improving the gait of elderly people during obstacle avoidance tasks. This device—designed and fabricated with origami-inspired techniques—features a serial chain of lockable joints that can be stiffened using a braking system inspired by laminar jamming concepts. Current related work has identified that the trunk plays a crucial role in obstacle avoidance tasks. In this article, we thus propose an affordable wearable system that can be quickly fabricated and whose design can be adjusted to fit the individual wearer. The design leverages switchable, passive systems, in combination with lightweight materials that remain as “transparent” to the user as possible when inactive. This article focuses on translating human requirements into a tangible design that addresses the current state of our biomechanics knowledge. We describe the kinematics and forces of our proposed device, describe the performance of our system in a locked and unlocked state, discuss the integration of various sensors into our device, and characterize the performance of the device when locked and unlocked.

Research on configuration synthesis method of multi-mode parallel mechanism based on lockable joints

Research on configuration synthesis method of multi-mode parallel mechanism based on lockable joints

A compliant-mechanism-based lockable prismatic joint for high-load morphing structures

Lockable joints are widely used in robotic systems and adaptive structures for energy management and/or topology reconfiguration. However, it is still challenging to design a joint with desired properties, including high locking load, infinite locking positions, short switching time, energy-efficient control, and a compact and lightweight structure. This paper aims at this open problem by presenting a novel piezoelectric (PZT) actuated lockable prismatic joint. This joint is a compliant mechanism (CM) consisting of a compound bridge-type compliant mechanism (CBCM) and a pair of compound multibeam parallelogram mechanisms (CMPMs). It can produce the required input/output stiffness to transmit large forces for high-load locking. It can also provide a desired input/output motion range for PZT actuation-based unlocking and for facilitating preloading adjustment. An analytical model is presented based on a compliance matrix method and the nonlinear model of the CMPM to predict the joint's static characteristics under various input/output conditions. A two-step optimization framework is proposed for locking applications. The theoretical study and nonlinear FEA/experimental verification confirm the feasibility of the design and the accuracy of the proposed model.

Deployable space grids with lockable joints

The use of deployable structures in architecture began in the 1960s, based on the pioneering work of Emilio Pérez Piñero. It has had an interesting development using bundle or scissor modules, but with solutions that are always based on through-bolted joints. In this document, a new system for deployable space grids is proposed. This new system is based on the use of self-lockable joints, opening up a new and interesting field of design possibilities. The resulting space grids can be deployed by means of joints arranged on the bars that have a self-locking mechanism which is engaged in the final open position. This configuration overcomes the significant buckling that is characteristic of deployable structures with through-bolted joints. Different solutions are proposed for various uses, and the data from the theoretical and experimental analysis are provided.

Semi‐active modal control of structures with lockable joints: general methodology and applications

In this study, a novel modal control strategy by means of semi-actively lockable joints is proposed. The control strategy allows for a directed flow of energy between vibrational modes, which makes it suitable not only for vibration attenuation purposes but also for energy scavenging driven by electromechanical energy harvesters. The proposed control strategy is an extension of the prestress-accumulation release (PAR) technique; however, it introduces also new concepts that increase the efficiency of the overall control system. Contrary to the PAR, the proposed method requires measurement of both strains in the vicinity of the semi-active joints and translational velocities that provide global information about system behavior. The latter aspect requires the control system to be organized within a hierarchical feedback architecture. The benefit from this higher complexity of the control system is its better performance compared to the PAR. The proposed semi-active modal control not only attenuates structural vibration faster, but it also achieves this goal with a smaller number of switches implemented in the joints. The effectiveness of the proposed methodology has been demonstrated on structures equipped with two lockable joints. Two practical examples have been investigated: one employs the concept of vibration-based energy harvesting for a two-story frame structure, while the second one reduces vibration of an eight-story frame structure subjected to kinematic excitation.

Design and Analysis of a Multisegment Shape Morphing Mechanism

Abstract This article presents a novel design of a multisegment shape morphing mechanism that combines a lockable reconfigurable variable geometry truss manipulator (VGTM) with an active parallel compliant mechanism. The structure of the VGTM is in a parallel-serial structure, and its hyper-redundant degree-of-freedom (DOF) can be fully controlled by using two active flexible panels and some lockable joints. This mechanism is suitable for aerospace applications that require light and compact structure with high load-carrying ability as well as achieve multiple DOFs for large-scale shape deformation. To make shape morphing process simple and efficient, the mobility and topological configuration of the mechanism are analyzed first. Then, a control strategy combining the approximate motion mode and the exact motion mode is proposed. The kinematic models for different motion modes are established and solved analytically. It has been found that, under the exact motion mode, two approaches could be realized for the pose control under external loads for each segment. The one with the shorter moving path is selected in this article. Finally, a prototype was constructed to demonstrate the feasibility of this structure and to verify the proposed kinematic model.

Design and analysis of an active 2-DOF lockable joint

This article presents a new design of a 2-DOF active lockable joint, which can be applied in parallel reconfigurable manipulators as well as in other applications such as haptic devices. The direct and inverse kinematic problems of the proposed joint were considered in detail, and the kinematic features of the input cranks are presented. Velocity equations, Jacobians, and singular positions of the joint were thoroughly considered. The workspace of the joint was analyzed by utilizing performance indices. A quasi-static analysis was carried out, providing the output force distribution along the workspace. The finite element simulation was carried out in Ansys Workbench, in order to identify the most loaded parts of the joint and verify its strength. A prototype, manufactured using 3D printing, has proven the functionality of the concept. The prototype was used as a haptic device in order to control a five-bar flexible parallel manipulator, which was being developed by the CompMech research group.

Design and analysis of a configuration-based lengthwise morphing structure

The ability of changing aircraft wing shape or geometry has drawn great attention of researchers in the past decades. This paper proposes a new approach to design a lengthwise morphing structure for an airfoil morphing wing. The structure is composed of n planar mechanism units. Each unit is designed to have 3 degrees-of-freedom (DOFs) that can be fully controlled by two actuators using lockable joints. This structure is reconfigurable through dual mode switching by locking specific joints. The end-effector of the mechanism can reach any desired position and the morphing wing can change to any desired shape. When locking three or four actuators, the structure turns into a statically determinate or indeterminate truss structure to withstand required loads. Force analysis for the structure during morphing motions is conducted based on statics, and an optimization method is used to determine the appropriate number of actuators and their placements. Prototypes with or without skins are fabricated to verify the feasibility of the design.

Reconfiguration for the Maximum Dynamic Wrench Capability of a Parallel Robot

In this paper, a Stewart-platform robot with sliding lockable base joints is proposed for reconfiguration, and it addresses the determination of the optimal configuration for the prescribed motion with maximum allowable dynamic wrench capability subject to the constraints imposed by the kinematics and dynamics of the proposed reconfigurable architecture. The numerical results from the hierarchical optimization process allow us to investigate the effects of the base point locations on the maximum dynamic wrench capability. The effectiveness of the proposed algorithm is demonstrated in the improvement of the maximum allowable dynamic wrench capability of the reconfigurable Stewart-platform robot.

Assist On-Ankle: a reconfigurable ankle exoskeleton with series-elastic actuation

We present the kinematics, optimal dimensional synthesis, series-elastic actuation, control, characterization and user evaluation of AssistOn-Ankle, a reconfigurable, powered exoskeleton for ankle rehabilitation. AssistOn-Ankle features reconfigurable kinematics for delivery of both range of motion (RoM)/strengthening and balance/proprioception exercises. In particular, through lockable joints, the underlying kinematics can be configured to either a self-aligning parallel mechanism that can naturally cover the whole RoM of the human ankle, or another parallel mechanism that can support the ground reaction forces/torques transferred to the ankle. Utilizing a single device to treat multiple phases of treatment is advantageous for robotic rehabilitation, since not only does it decrease the device cost and help with the space requirements, but also shorten the time it takes for patients to familiarize with the device. Bowden cable-based series-elastic actuation of AssistOn-Ankle allows for a remote placement of the motors/drivers to result in a compact design with low apparent inertia, while also enabling high-fidelity force/impedance control and active backdriveability of the device.

Type Synthesis of 3-DOF multi-mode translational/spherical parallel mechanisms with lockable joints

A 3-DOF (degrees-of-freedom) multi-mode translational/spherical PM (parallel mechanism) with lockable joints is a novel reconfigurable PM. It has both 3-DOF spatial translational operation mode and 3-DOF spherical operation mode. This paper presents an approach to the type synthesis of translational/spherical PMs with lockable joints. Using the proposed approach, several 3-DOF translational/spherical PMs are obtained. It is found that these translational/spherical PMs do not encounter constraint singular configurations and self-motion of sub-chain of a leg during reconfiguration. The approach can also be used for synthesizing other classes of multi-mode PMs with lockable joints, multi-mode PMs with variable kinematic joints, partially decoupled PMs, and reconfigurable PMs with a reconfigurable platform.

Details on the Design of a Lockable Spherical Joint for Robotic Applications

The paper proposes the mechanical design of a lockable spherical joint, which is designed to be manually or automatically configured in different kinematic solutions. The device is conceived for being used as a conventional spherical joint or converted in a universal joint, or still downgraded to a revolute pair. Therefore different configurations can be chosen according to user needs. In particular, two of the three axes of revolution, arranged in the typical roll-pitch-roll sequence of robot spherical wrists, can be locked alternately in order to provide two differently arranged universal joints. It can be demonstrated that such behavior allows to activate different mobilities of a class of reconfigurable parallel kinematics manipulators and for this task the device has been dimensioned. The transition between such mobilities occurs exploiting the concept of over-constrained kinematics, which is realized by the lockable joint during the switching phase in order to avoid an instantaneous mobility of the robot.

Inverse and forward dynamics of N-3RPS manipulator with lockable joints

SUMMARYThe N-3 Revolute-Prismatic-Spherical (N-3RPS) manipulator is a kind of serial-parallel manipulator and has higher stiffness and accuracy compared with serial mechanisms, and a larger workspace compared with parallel mechanisms. The locking mechanism in each joint allows the manipulator to be controlled by only three wires. Modeling the dynamics of this manipulator presents an inherent complexity due to its closed-loop structure and kinematic constraints. In the first part of this paper, the inverse kinematics of the manipulator, which consists of position, velocity, and acceleration, is studied. In the second part, the inverse and forward dynamics of the manipulator is formulated based on the principle of virtual work and link Jacobian matrices. Finally, the numerical example is presented for some trajectories.

A head and transducer holding technique for TCD monitoring of the vertebrobasilar circulation

For monitoring the Doppler spectrum of the vertebral and basilar arteries the authors have used a microball vacuum cushion for fixing the head of the patient in lateral, moderately anteflected position, and a three lockable joint type instrument holder allowing adjustment of the probe suboccipitally. This setting was used for transforaminal vertebrobasilar emboli monitoring in 5 patients having prosthetic heart valves. As an alternative technique a microball filled vacuum collar was tried out for immobilisation of the patient's head, and a snake neck type surgical instrument holder for guiding the probe.