This paper proposes the systemic dynamic modeling and analysis of a 2PRU-UPR parallel robot with two rotations and one translation based on screw theory, where P, R and U denote prismatic, revolute and universal joints, respectively. Compared with existing parallel robots having two rotations and one translation, the two actuated prismatic joints of the 2PRU-UPR parallel robot are mounted on a fixed base to reduce the movable mass and improve the dynamic response. First, the inverse kinematics are presented. Next, adopting the screw-based method, the velocity and acceleration of joints and limbs of the 2PRU-UPR parallel robot are analyzed in detail. The actuated forces of the three actuators are then obtained according to the principle of virtual work. Additionally, a numerical simulation is conducted using ADAMS software to investigate the dynamic model of the 2PRU-UPR manipulator and to verify the correctness of the theoretical results. Finally, distributions of the dynamic manipulability ellipsoid index are used to evaluate the dynamic translational and rotational performances of the 2PRU-UPR parallel robot. A prototype based on the dynamic analysis has been fabricated. The dynamic modeling and evaluation provide a basis for the efficient and precise control of the 2PRU-UPR parallel robot in actual machining manipulations. The 2PRU-UPR parallel robot has great potential in machining workpieces with curved surfaces.

Large workpieces are important components of core equipment in aerospace and other fields, where the machining mainly focuses on the surfaces and inner cavities. However, it may be unsuitable for existing machining robots to directly achieve integrated machining, that is, not only the high-precision surface machining but also the machining of different inner cavities in a limited space. To satisfy these machining requirements, a new reconfigurable hybrid robot (RHR) is proposed, called the 3PRR-3PSS-UPU RHR, for machining the surface and inner cavity of large workpieces (where P, P, R, S, and U stand for the actuated prismatic joint, passive prismatic joint, revolute joint, spherical joint, and universal joint, respectively). The proposed RHR consists of two parallel manipulators (PMs), in which one is a spatial 3PRR PM with one translational degree-of-freedom (DOF) and the other is a 3PSS-UPU reconfigurable PM (RPM) with different configurations of two rotational and one translational (2R1T) DOFs using locking equipment, which is the main advantage of the designed robot. The inverse kinematics and singularities of two PMs are analyzed. The stiffness performance of the spatial 3PRR PM is compared with that of a moving slider with one translational DOF. By evaluating the workspace and motion/force transmissibility, the kinematic performance of two PMs is presented using several local and global indices, followed by the dimensional optimization of link parameters. Based on the structural characteristics and excellent performance, it can be inferred that the 3PRR-3PSS-UPU RHR has great potential for machining large workpieces.

Redundantly actuated parallel manipulators with two rotations and one translation (2R1T RAPMs) have the potential for machining complex surfaces, where a large orientation workspace and high stiffness are required. Considering the advantages of an offset moving platform, such as an enlarged orientation workspace and improved stiffness, a novel 2R1T (2PRR)R-PRS-PSS RAPM with an offset moving platform is proposed in this paper, called M2. Compared with the existing 2R1T RAPM with an offset moving platform, the main advantage of the proposed RAPM is that the heavy motors of four limbs are mounted on the base to reduce the movable mass and improve dynamic response. The kinematical analysis is investigated, including mobility, inverse, forward kinematics, and singularity analysis. Meanwhile, comprehensive evaluations of the properties of the offset moving platform and actuation redundancy are carried out. Compared with M2 RAPM’s form without an offset in moving platform, i.e., no auxiliary platform, and M2 RAPM’s nonredundantly actuated form, the proposed M2 RAPM can achieve a larger orientation workspace and higher stiffness. Particularly, the maximum stiffness of the proposed M2 is 68.8% larger than its form without an auxiliary platform. Finally, the dimensional parameters of the proposed M2 are optimized to obtain an improved satisfactory workspace.

In this paper, we propose a novel passive parallel continuum ankle exoskeleton that can provide assistive torque during ankle plantar flexion. Due to the flexible branches arranged in compliance with ankle motion and shape, the compact design can also offer some vertical support. The proposed parallel mechanism consists of two types of branches. The first type is a pre-bent flexible rod, mainly used to apply assistive force/torque during ankle plantar flexion. The second type of branch consists of a bounded sphere joint, flexible rod, and bounded sphere joint (BFB), which is mainly used for support. We formulate the kinetostatic model of the BFB branch as a series of parallelizable unconstrained optimization problems to ensure efficient solvability. After that, we derive the kinetostatic model of the proposed mechanism. After calibration, the wrench error of the kinetostatic model is 9.07%. Simulation analysis based on the calibrated model shows that the designed mechanism has high supporting stiffness and low rotational stiffness. The assistive torque caused by the nonlinear rotational stiffness in the sagittal plane is similar to that of passive clutch-like mechanisms. These properties can still be maintained when the joint center changes within a small range. Besides, a walking experiment was conducted, and the results show that the proposed design can reduce gastrocnemius activity.

Redundantly actuated parallel manipulators (PMs) have attracted a great deal of attention since they generally have better stiffness than non-redundantly actuated ones. This paper presents an analytical elastostatic stiffness modeling and performance study of a 2UPR–2PRU PM with actuation redundancy, which has two rotational and one translational degrees of freedom (U: universal joint; P: prismatic joint; R: revolute joint). First, the inverse displacement is reviewed and verified briefly. Second, the stiffness matrices of UPR and PRU limbs are deduced by using the principle of strain energy, followed by the overall stiffness matrix of the 2UPR–2PRU PM. Combined with the ANSYS software, the finite element analysis method is then used to verify the correctness and universality of the stiffness models by calculating the deformations of four selected configurations. Finally, the stiffness index based on the virtual work is used to evaluate the performance of the 2UPR–2PRU PM, and the influence of different external loads and operational heights on the stiffness performance is discussed. The relationship between singular configurations and the stiffness index is also presented. The stiffness models and performance distributions of the 2UPR–2PRU PM with actuation redundancy can provide references for the actual applications.

Overconstrained parallel manipulators (PMs) without actuation redundancy have been widely used in various applications, such as the parallel module of the Exechon robot for workpiece machining. However, existing methods are inadequate for describing the motion/force constraint performance of nonredundant and overconstrained PMs (NOPMs) because they do not reasonably consider the influence of the overconstrained structure. This performance is crucial for evaluating the capability to resist external loads, which is of great significance for applications requiring high stiffness and precision. To address this issue, this paper proposes a new evaluation method for the motion/force constrainability of NOPMs. First, based on the four wrench and twist systems of limbs, the output constraint performances of all the single-degree-of-freedom (DOF) mechanisms under different cases can be obtained. These single-DOF mechanisms are constructed by selectively “losing” some constraint wrench screws and “locking” all the transmission wrench screws of the original overconstrained PM, which is the key step in the establishment of this method and achieved using the ergodic method. Then, the output and input constraint performance indices in a certain configuration can be calculated, followed by the definition of a local constraint index to evaluate the motion/force constrainability of the NOPM. Notably, all the proposed indices are characterized by coordinate independence. Finally, two PMs and their corresponding overconstrained structures are simulated to demonstrate the correctness of the proposed evaluation method. The evaluation strategy can also be used for the optimal design of the NOPMs in the future.