Passive Revolute Research Articles

An Aerial Robot Achieving Bimodal Flight and Independent Attitude Control through Passive Morphing

Enhancing the adaptability and versatility of unmanned micro aerial vehicles (MAVs) is crucial for expanding their application range. In this article, a bimodal reconfigurable robot capable of operating in both regular quadcopter flight mode and a unique revolving flight mode is presented, which allows independent control of the vehicle's position and roll‐pitch attitude. This design incorporates passive revolute joints that enable seamless mid‐air transitions between the two modes, facilitated by centrifugal forces, without the need for additional actuators. In the regular mode, the robot operates with four vertical propellers, similar to conventional designs. In the revolving mode, the configuration shifts to two horizontal and two vertical propellers, achieving five degrees of freedom control with four actuators. This underactuated flight capability is made possible by exploiting the robot's fast revolving motion and the resulting cycle‐averaged forces and torques. Experimental validations confirm the feasibility and enhance the maneuverability of this design. The revolving flight mode provides significant advantages in applications such as mapping and confined space navigation.

Kinetostatics Modeling and Analysis of a Spherical Parallel Continuum Manipulator

Abstract In this paper, a spherical parallel continuum manipulator (SPCM) which is the flexible version of the 3-RRR“Agile Eye” mechanism is proposed and analyzed. The SPCM consists of three parallel flexible limbs, each limb is formed by compliant truncated cone elements, and the moving platform connects each limb with a passive revolute joint. Three servo motors are used to control the manipulator actively, and the spherical motion is realized by the coupled large deflections of the flexible links. An equivalent compliance analysis method of the element is developed based on finite element analysis and principal axis decomposition. By combining all three limbs, the kinetostatics model of the whole manipulator is derived, and a gradient iteration algorithm is developed to solve the forward and inverse kinetostatics. Finally, a prototype of the manipulator is constructed using 3D-printing technology, and the accuracy for element equivalence and end-effector characteristics is validated by experiments. The results show that the derived kinetostatics model can accurately describe the force–deflection behavior of the SPCM.

Design Optimization of a Variable Stiffness Actuator for Knee Exoskeleton Application

The field of wearable devices and exoskeletons for rehabilitation and assistance is one of the main fields where variable stiffness actuators (VSAs) are continuously incorporated. This is due to the need for such devices to adapt to erratic environments and mimic human motions. Recently, a passive revolute joint design with controllable variable stiffness was proposed by the authors. However, the previous design was only introduced as a proof of concept, with no further investigation of the design stiffness change capabilities for application utilization. In this work, we introduce an extended analysis and comprehensive parametric study of the system’s main design parameters. Based on the performed analysis, we propose an optimization framework for this design concept that allows implementation in knee exoskeletons. The main design parameters that affect the stiffness performance are defined. The proposed method allows the identification of these design parameters, which enables the redesign of the system according to the predefined user requirements, such as size, desired stiffness range, and stiffness change rate. A bench-top experimental setup is constructed to validate the obtained optimal design parameters. Results show the ability to achieve the desired stiffness performance for the intended application.

Design of a Flexure-Jointed Linkage in a Quadruped Walking Robot

Flexure-jointed linkages (FJLs) have distinct advantages, such as ease of manufacturing and assembly, no backlash, and light weight designs, over their rigid counterparts. However, the synthesis and analysis of FJLs are more complicated, especially when the compliant flexures have large deflections. This article addresses the optimal design of a large-deflection FJL, which is used as the leg of a compliant quadruped robot. The proposed FJL originates from a rigid six-bar linkage to mimic the foot path of walking mammals by replacing the passive revolute joints with two-segment combined flexible beams. Its kinematic and strain energy models are derived and verified by numerical simulations. Subjecting to material failures and geometrical constraints, the optimization of the FJL is carried out by minimizing the path deviation and strain energy to reduce needed input power, leading to the development of a quadruped walking robot. Experimental tests on the output path of the FJL and the motion capability of the robot are implemented. The results demonstrate the effectiveness of the derived models in predicting the deflections of the FJL and the feasibility of applying compliant linkages for legged robots.

Mathematical modelling and virtual decomposition control of heavy-duty parallel–serial hydraulic manipulators

This paper proposes a novel modelling approach for a heavy-duty manipulator with parallel–serial structures connected in series. Each considered parallel–serial structure contains a revolute segment with rigid links connected by a passive revolute joint and actuated by a linear hydraulic actuator, thus forming a closed kinematic loop. In addition, prismatic segments, consisting of prismatic joints driven by hydraulic linear actuators, also are considered. Expressions for actuator forces are derived using the Newton–Euler (N–E) dynamics formulation. The derivation process does not assume massless actuators decoupled from manipulator links, which is common in the Lagrange dynamics formulation. Actuator pressure dynamics are included in the analysis, leading in total to a third-order system of ordinary differential equations (ODEs). With fewer parameters than its predecessors, the proposed model in the N–E framework inspires revision of the virtual decomposition control (VDC) systematic process to formulate a control law based on the new model. The virtual stability of each generic manipulator revolute and prismatic segment is obtained, leading to the Lyapunov stability of the entire robot.

Steering Control of a Legged Walking Machine Composed of Many Leg Mechanism Units with 1DoF

Centipede-type multi-legged robots can traverse rough terrain and are expected for disaster rescue operations. However, too many actuators make robots large and heavy, and make their control systems complicated. To solve these problems, the authors have developed a centipede-type robot which is composed of many leg mechanism units with a planar 4-bar link mechanism connected with passive revolute joints and have proposed a new control scheme to steer the robot with only phase difference between units. However, quantitative experimental validation has not been carried out. Thus, in this paper, the turning motion of the robot is accurately measured with an angular velocity sensor and an optical motion capture system. The measured results reveal that the proposed control scheme is correct on the turning direction while the measured turning radius is larger than that calculated with the proposed control scheme. Resultantly, this error of turning radius is caused by the variation of leg contact position and static friction due to the rolling motion of units.

Workspace analysis of 3-DOF U-shape base planar parallel robotic motion stage using shape memory alloy restoration technique (SMART) linear actuators

The applications of lightweight planar parallel robotic manipulators are increasing enormously because of its various desirable characteristics such as low weight, lower inertia and higher stiffness. Higher accelerations and accuracies can be achieved in planar parallel manipulators. Also, shape memory alloy restoration technique (SMART)-based linear actuators are replacing huge and bulky linear actuators. This study presents the kinematic design of smart linearly actuated family of U-shape base planar parallel robotic manipulator. With the aid of solid modelling software, different available configurations were modelled and their workspace was analysed. The developed 3-DOF motion stages (18 unique configurations) were fabricated using fused deposition modelling process, and the top three configurations having higher workspace were further experimented. It is interesting to observe that the actual or experimental workspace of a particular manipulator configuration is further minimised from the predicted or feasible workspace. It is due to the presence of passive links, singularities, friction between the parts, heat dissipation, force distribution, stiffness, etc. The present study depicts the experimental workspace of the top three configurations, namely PPR-PRP-PRR, PRP-PPR-PRP and PRP-PPR-PRR. Since none of the experimental workspace observed is equal or higher than the model workspace, an efficiency loss in terms of workspace reduction was calculated to understand the acceptability of the configurations in different domains. Apart from the loss, the result disclosed that the actual workspace of all the manipulators was within the feasible workspace domain of mobile platform. The PPR-PRP-PRR manipulator was found to possess highest experimental workspace than other configurations. Note: P, P, and R refer to active prismatic, passive prismatic and passive revolute joints respectively.

Development and Workspace Analysis of Smart Actuation based Planar Parallel Robotic Motion Stage

The applications of light weight planar parallel robotic manipulators are increasing enormously because of their various desirable characteristics such as low weight, lower inertia, higher stiffness, energy efficiency etc. Higher accelerations and accuracies can be achieved using planar parallel manipulators (PPMs). Also, Shape Memory Alloy (SMA) based actuators are replacing huge and bulky actuators as SMA actuators provide higher work per unit mass as compared to other actuators and also undergoes silent actuation by simple actuation process. In the present study, three configurations (3PRP, PPR-2PRP and PRR-2PRP) of 3 degrees of freedom U-shaped base planar parallel robotic manipulator have been presented using SMA based smart actuators. The manipulator constitutes of one fixed base and one moving platform. The SMA spring acts as linear actuators mounted on the base which is connected to the end effector with the help of three limbs. A detailed experimental study was carried out to understand the workspace associated with each configuration. The movement of manipulators was investigated with the various combinations associated with the input SMA translation joints. And the study showed that the various configurations acquire different areas of workspace in the manipulator region. This study also shows the essence of SMA springs in PPMs for precise and accurate positioning of the end-effector and also its tracking ability for the development of future micro-motion stage. Note: P refers to Passive Prismatic Joint, R refers to Passive Revolute Joint and P refers to Active Prismatic Joint.

PDMeter: A Wrist Wearable Device for an at-Home Assessment of the Parkinson’s Disease Rigidity

This work focuses on the design and the validation of a wearable mechatronic device for an at-home assessment of wrist stiffness in patients affected by Parkinson's Disease (PD). The device includes one actuated joint and four passive revolute joints with a high overall intrinsic backdriveability. In order to allow the user to freely move the wrist during activities of daily living, we implemented a transparent controller on the basis of the interaction force sensed by the embedded load cell. Conversely, in order to provide perturbations for estimating the wrist flexion-extension rigidity, we implemented a torque controller. Firstly, we report a pilot study that aimed at characterizing the device in terms of range of motion (ROM) allowed, transparency perceived and torque-tracking capability. Then, we present a case study in which we tested our device with seven PD patients in both drug-OFF and drug-ON conditions and we compared the measured stiffness with the one measured in fourteen healthy controls and with the outcome of the most used clinical scale (MDS-UPDRS). The device allowed to successfully estimate the stiffness as different depending on the movement direction. Indeed, extension stiffness was higher than the flexion one, accordingly to the literature. Moreover, the device allowed to discriminate both Healthy subjects from PD subjects, and PD subjects in OFF condition from PD subjects in ON condition. In conclusion, we demonstrate the feasibility of the device in measuring wrist rigidity, thus enabling the possibility to implement an at-home assessment of the PD rigidity.

On Using Inertial Measurement Units for Solving the Direct Kinematics Problem of Parallel Mechanisms

In this paper, we investigate the accuracy and the computational efficiency of an IMU-based approach for solving the direct kinematics problem of parallel mechanisms with length-variable linear actuators under dynamic conditions. By avoiding to measure the linear actuators’ lengths and by using orientations instead, a comprehensive, low-cost sensor structure can be obtained that provides a unique solution for the direct kinematics problem. As a representative example, we apply our approach to the planar 3-RPR parallel mechanism, where P denotes active prismatic joints and R denotes passive revolute joints, and investigate the achievable accuracy and robustness on a specially designed experimental device. In this context, we also investigate the effect of sensor fusion on the achievable accuracy and compare our results with those obtained from linear actuators’ lengths when the Newton-Raphson algorithm is used to compute the manipulator platform’s pose iteratively. Finally, we discuss the applicability of inertial measurement units (IMUs) for solving the direct kinematics problem of parallel mechanisms.

A tri-state prismatic modular robotic system

Strut-type modular robots adopt an idealized truss mechanism composed of linear extensible struts (i.e., robotic modules) jointed by connector nodes. Conceptually, each strut is capable of passively rotating around its connected node, and all the struts linked by a same node share and intersect at a common center of rotation. Unfortunately, these two requirements pose difficulties in mechanically implementing such an ideal point-like compliant connector node. Meanwhile, motions of extensible struts composed of off-the-shelf prismatic actuators can be constrained in a parallel or networked robotic structure, leading to the stuck of the robotic structure and the failure of a given task. To tentatively tackle these issues, a tri-state (three-state) modular prismatic robotic system is designed with the ability to be rigid, back-drivable to external forces and to undertake self-powered motion. Two modular robotic concepts of (i) rigid node attachment and (ii) neighbor actuation are introduced and demonstrated. Specifically speaking, modular robotic structures are constituted of prismatic actuators linked by rigid connector nodes. Instead of placing revolute joints on the nodes, a passive revolute joint is in the middle of two prismatic actuators to deliver compliance. This arrangement removes many of the inherent limitations of complex connection joints and avoids the difficulty of designing and implementing point-like complaint connector nodes. In terms of neighbor actuation, modules are demonstrated that they have the ability to be self-powered or passively controlled from external forces applied by their neighboring modules. This allows robotic modules in a parallel or networked structure to change states to release physical constraints and prevent the whole structure from getting stuck. In this paper, the tri-state capability of the prismatic modules and the neighbor actuation of parallel modules are first experimentally demonstrated. Then, two physical modular robots are built and control algorithms are implemented. Experimental results validate the efficacy of the control strategy and substantiate communication and locomotion capabilities of the modular robots. Finally, a more complex robotic structure is established in a robot simulator to demonstrate that the tri-state prismatic actuators can be applied to release physical constraints of the modular robot during a locomotion task.

Performance Evaluation of a Sensor Concept for Solving the Direct Kinematics Problem of General Planar 3-RPR Parallel Mechanisms by Using Solely the Linear Actuators’ Orientations

In this paper, we experimentally evaluate the performance of a sensor concept for solving the direct kinematics problem of a general planar 3-RPR parallel mechanism by using solely the linear actuators’ orientations. At first, we review classical methods for solving the direct kinematics problem of parallel mechanisms and discuss their disadvantages on the example of the general planar 3-RPR parallel mechanism, a planar parallel robot with two translational and one rotational degrees of freedom, where P denotes active prismatic joints and R denotes passive revolute joints. In order to avoid these disadvantages, we present a sensor concept together with an analytical formulation for solving the direct kinematics problem of a general planar 3-RPR parallel mechanism where the number of possible assembly modes can be significantly reduced when the linear actuators’ orientations are used instead of their lengths. By measuring the orientations of the linear actuators, provided, for example, by inertial measurement units, only two assembly modes exist. Finally, we investigate the accuracy of our direct kinematics solution under static as well as dynamic conditions by performing experiments on a specially designed prototype. We also investigate the solution formulation’s amplification of measurement noise on the calculated pose and show that the Cramér-Rao lower bound can be used to estimate the lower bound of the expected variances for a specific pose based exclusively on the variances of the linear actuators’ orientations.

A Novel Kinematically Redundant Planar Parallel Robot Manipulator With Full Rotatability

This paper presents a novel kinematically redundant planar parallel robot manipulator, which has full rotatability. The proposed robot manipulator has an architecture that corresponds to a fundamental truss, meaning that it does not contain internal rigid structures when the actuators are locked. This also implies that its rigidity is not inherited from more general architectures or resulting from the combination of other fundamental structures. The introduced topology is a departure from the standard 3-RPR (or 3-RRR) mechanism on which most kinematically redundant planar parallel robot manipulators are based. The robot manipulator consists of a moving platform that is connected to the base via two RRR legs and connected to a ternary link, which is joined to the base by a passive revolute joint, via two other RRR legs. The resulting robot mechanism is kinematically redundant, being able to avoid the production of singularities and having unlimited rotational capability. The inverse and forward kinematics analyses of this novel robot manipulator are derived using distance-based techniques, and the singularity analysis is performed using a geometric method based on the properties of instantaneous centers of rotation. An example robot mechanism is analyzed numerically and physically tested; and a test trajectory where the end effector completes a full cycle rotation is reported. A link to an online video recording of such a capability, along with the avoidance of singularities and a potential application, is also provided.

Passive Discrete Variable Stiffness Joint (pDVSJ-II): Modeling, Design, Characterization, and Testing Toward Passive Haptic Interface

In this paper, the modeling, design, and characterization of the passive discrete variable stiffness joint (pDVSJ-II) are presented. The pDVSJ-II is an extended proof of concept of a passive revolute joint with discretely controlled variable stiffness. The key motivation behind this design is the need for instantaneous switching between stiffness levels when applied for remote exploration applications where stiffness mapping is required, in addition for the need of low-energy consumption. The novelty of this work lies in the topology used to alter the stiffness of the variable stiffness joint. Altering the stiffness is achieved by selecting the effective length of an elastic cord with hook's springs. This is realized through the novel design of the cord grounding unit (CGU), which is responsible for creating a new grounding point, thus changing the effective length and the involved springs. The main features of CGU are the fast response and the low-energy consumption. Two different levels of stiffness (low, high) can be discretely selected besides the zero stiffness. The proposed physical-based model matched the experimental results of the pDVSJ-II in terms of discrete stiffness variation curves, and the stiffness dependency on the behavior of the springs. Two psychophysiological tests were conducted to validate the capabilities to simulate different levels of stiffness on human user and the results showed high relative accuracy. Furthermore, a qualitative experiment in a teleoperation scenario is presented as a case study to demonstrate the effectiveness of the proposed haptic interface.

Dexterous Workspace Optimization for a New Hybrid Parallel Robot Manipulator

In this paper, a new hybrid parallel robot (HPR) manipulator is introduced. First three kinematic limbs of six-legged general Stewart platform (6DOF GSP) manipulator are disconnected. Afterward, each passive universal joint of remaining three-legged parallel manipulator (three-UPS) is mounted at the center of each second passive revolute joint of RPR planar parallel manipulator (3DOF PPM) where underlined letters present active joints. Active actuators of PPM mounted between base platform of GSP and ground perform translations along x and y-axes, and rotation about z-axis. Remaining three limbs of GSP mechanism provide translation z-axis, and rotation about x- and y-axes only. Thus HPR can perform motion with full dimensions (translation and rotation about x-, y-, and z-axes). Optimizations are performed by using particle swarm optimization algorithm. Optimization results demonstrated that HPR provides better dexterity and singularity-free workspace characteristics than GSP.

Design of A Novel Passive Binary-Controlled Variable Stiffness Joint (BpVSJ) Towards Passive Haptic Interface Application

In this paper we present the design, development and experimental validation of a novel Binary-Controlled Variable Stiffness Joint (BpVSJ) towards haptic teleoperation and human interaction manipulators applications. The proposed actuator is a proof of concept of a passive revolute joint, where the working principle is based on the recruitment of series-parallel elastic elements. The novelty of the system lies in its design topology, including the capability to involve an (n) number of series-parallel elastic elements to achieve ( $2^{n}$ ) levels of stiffness, as compared to current approaches. Accordingly, the level of stiffness can be altered at any position without the need to revert to the initial equilibrium position. The BpVSJ has low energy consumption and short switching time, and is able to rotate freely at zero stiffness without limitations. Further smart features include scalability and relative compactness. This paper details the mathematical stiffness modeling of the proposed actuator mechanism, as well as the experimentally measured performance characteristics. The experimental results matched well with the physical-based modeling in terms of stiffness variation levels. Moreover, Psychophysical experiments were also conducted using (20) healthy subjects in order to evaluate the capability of the BpVSJ to display three different levels of stiffness that are cognitively realized by the users. The participants performed two tasks: a relative cognitive task and an absolute cognitive task. The results show that the BpVSJ is capable of rendering stiffness with high average relative accuracy (Relative Cognitive Task relative accuracy is 97.3%, and Absolute Cognitive Task relative accuracy is 83%).

Two-axis Gimbal with Passive Revolute Joints for Motion Isolation of a Stabilizing Mechanism

Two-axis Gimbal with Passive Revolute Joints for Motion Isolation of a Stabilizing Mechanism

Micropositioning and Control of an Underactuated Platform for Microscopic Applications.

For automation of biological experiments at the micro-scale, highly precise manipulator equipped with a microscope is required. However, current micropositioning stages have several limitations, such as: 1) manual operation, 2) lack of rotational capability, 3) incompatibility with a microscope, and 4) small range of motion (RoM). This research aims to develop a microscope compatible XYθ micropositioning stage with large RoM for phenotyping multiple biological samples rapidly for various microscopic applications. An underactuated planar mechanism, kinematic analysis, and control of the XYθ stage are presented in this paper. The planar mechanism consists of two piezoelectric linear actuators for translational motion capability and two passive revolute joints at the tip of each linear actuator for rotational capability. Based on the kinematic analysis of the stage, controllability and control strategy of the underactuated stage is described. Finally, the feasibility of the micropositioning stage for a general positioning and orienting task is verified by both simulation and tissue core experiments.

Dynamics and Control of a Planar Multibody Mobile Robot for Confined Environment Inspection

In this paper, we study the dynamics of an articulated planar mobile robot for confined environment exploration. The mobile vehicle is composed of n identical modules hitched together with passive revolute joints. Each module has the structure of a four-bar parallel mechanism on a mobile platform. The dynamic model is derived using Lagrange formulation. Computer simulations illustrate the model by addressing a path following problem inside a pipe. The dynamic model presented in this paper is the basis for the design of motion control algorithms that encode energy optimization and sensor performance maximization.

Approximate analytical solution for vibration of a 3-PRP planar parallel robot with flexible moving platform

SUMMARYIn this research, using an approximate analytical method, vibration analysis of a 3-PRP (active prismatic—P, passive revolute—R, passive prismatic—P) planar parallel robot having a flexible moving platform is presented. A specific architecture of the 3-PRP parallel robot, also known as the ST (Star-Triangle) parallel robot, is considered. The moving platform of the robot, called the star, is assumed to be made of three flexible beams shaped like a star. For analytical modeling, each of the three beams of the star is assumed to be a discrete Euler–Bernoulli beam with a passive prismatic joint. Continuity equations at the center of the star are used to relate vibrations of the three beams. The vibration behavior of each beam is modeled using previously developed constrained motion equations for a planar Euler–Bernoulli beam having a prismatic joint. In this paper, previously presented “constrained assumed modes method” is further developed to solve the constrained motion equation for the ST parallel robot. The solution method is used to obtain the vibration of the robot for the inverse dynamics problem and simultaneously provides generalized constraint forces. Furthermore, the solution method can be used for the direct dynamics problem of the ST robot. Several input trajectories are considered to investigate the different behavior for the center of the star. For each of the trajectories, three different groups of mode shapes are considered and their vibrational responses are compared. In this research, for the first time, effects of the passive prismatic joint parameters such as mass, rotational moment of inertia, and its actual length are considered in an analytical model. Finally, the analytical solution and a FEM (Finite Element Method) software solution are compared.