查看更多>>摘要：In this paper, we present a dynamics modeling approach for lithographic machines. These machines consist of N-open chains with multiple stages in each chain. A stage can suppress or exert motion in specific directions. The stages affect each other through the structural connections available within a chain or various chains. We want to capture the energy transfer in the machine to better realize the design requirements. A specific application will be the optical lithography wafer-scanner machine. In such machines, step-and-scan motion profiles demand synchronization between the chains end-effectors. The rapid production rates requirements are projected onto the design space as high-acceleration profiles that will cause vibration based on the energy paths available. Various positioning errors will take place during the production of one complete substrate. Characterizing the machine dynamics due to jointly motion and vibration helps in providing better ways to synthesis the controllers for each stage. To minimize the positioning errors, we also propose replacements of specific stages where actively controlled fine positioning stages are used. The presented machine characterization provides a means to design and optimize such stages as will be illustrated through simulation examples.
查看更多>>摘要：To achieve positioning of nanometer or even sub-nanometer resolution from traditional piezoelectric actuators and giant magnetostrictive actuators, a bridge-type differential displacement reducer, which shows the capability of obtaining very large reduction ratio, is proposed. The reducer contains two bridge-type flexure mechanisms connected in a way that the output of the reducer equals the difference of the outputs of the two mechanisms. If the two bridge-type flexure mechanisms are identical, no matter how large the input is, the output will always be 0, indicating an infinite reduction ratio theoretically. Therefore, introducing a slight difference between the two mechanisms can yield a very large reduction ratio. A kinetostatic model for the reducer is developed, base on which a reducer exhibiting a reduction ratio of 100 is designed and prototyped. The results of the kinetostatic model, the finite element model and the experiment agree well with each other, validating the effectiveness of the proposed reducer and the correctness of the kinetostatic model.
查看更多>>摘要：In this study, the Absolute Nodal Coordinate Formulation (ANCF) is examined in the framework of the Arbitrary Lagrangian–Eulerian (ALE) formulation for cable simulation. The objective is to determine the stability, efficiency, and accuracy in different frequency domains for two different numerical time integration methods. The methods being compared are the Runge–Kutta method RK4, and the Generalized-α method. For simulations where the lower frequency domain is of interest, the Generalized-α method is stable, accurate and can be used with large time-steps; while where the high frequency domains is of interest, the RK4 procedure can be a more efficient method. The penalty method, the Lagrange multiplier method and linear coupling are tested as constraint methods for connecting ALE-ANCF cable elements. For RK4, the penalty constraint method highly affects the maximum time-step, which again affects the total solution time. For the Generalized-α method, the constraint methods all provide accurate results, and they have similar solution time. ALE-ANCF cable elements can be connected by different constraint techniques without impeding the ability to exhibit large deformations.
查看更多>>摘要：As wearable mechanisms, exoskeletons are highly coupled with limbs, which makes it necessary to carefully consider the compliances of the human-machine system. In this paper, the compliances are divided into kinematic, static, and dynamic compliances which should be met simultaneously during design. However, most of the current designs only focused on one of them but ignored the other two. This paper proposes planar gravity-balanced exoskeletons with linkage-belt hybrid transmissions. Each exoskeleton includes a locating part, an anti-gravity torque generation part, and a torque transmission part. The locating part achieves the kinematic compliance when misalignments between human-machine joints occur, the anti-gravity torque generation part realizes the static compliance by balancing the human-machine system, and the torque transmission meets the dynamic compliance by placing the heavy part of the mechanism at the proximal end to reduce the distal inertia, thereby achieving comprehensive wearability.
查看更多>>摘要：Strain wave gearing is widely applied in robotics and aerospace because of its zero-backlash and high gear ratio. Its circular spline (CS) is a rigid internal gear engaged with a flex spline (FS), driven by the wave generator's rotation. Obtaining the CS's tooth profile is difficult because the FS has a deformed ellipse shape caused by the wave generator. Furthermore, most CSs are manufactured by gear shaping and thus are lower precision and productivity comparing to external gears produced by gear hobbing. Power skiving is recently applied to overcome this problem. Mathematical models of CS and its skiving tool are established here. Moreover, a cylindrical tool is submitted to make tool manufacturing easier than a conical tool. A novel design for the error-free cylindrical tool and the skived gear is eventually derived. By modifying machine settings of the skiving process, it provides a relief angle to avoid the cylindrical tool rubbing with the internal gear. This manufacturing process is conducted on a six-axis CNC bevel gear cutting machine. An error analysis of the VERICUT cutting simulation result confirms the correctness of the proposed models.
查看更多>>摘要：A brush type contactor has been proposed to detumble non-cooperative space targets. Current researches mainly focused on the control schemes and their validations, whereas the dynamic modeling is rarely investigated. In this paper, both the analytical formulation and the numerical approach are employed to establish the dynamic model of the detumbling system. An analytical formula is proposed to evaluate the detumbling efficiency from the aspect of energy. The numerical simulation is conducted using flexible multibody dynamics and the contact theory. The analytical results and the numerical results are compared and the influence of various model parameters on dynamic behaviors is studied. It shows that the detumbling efficiency can be significantly affected by several factors, including the stiffness and length of the brush, the end velocity of the contactor, the contact angle, and the contact position. The detachment phenomenon of the brush from the solar panel is investigated. A criterion for the detachment is proposed and verified by numerical simulations.
查看更多>>摘要：This paper presents the design of an underactuated gripper with an embedded bistable mechanism. This gripper is specifically targeted at seizing soft objects while minimizing the deformation of the latter. Decreasing the magnitudes of these deformations is key in many practical applications to reduce potential damages due to excessive contact pressure. For instance, for both food processing and medical applications bruising the manipulated item is usually unacceptable. The gripper presented in this work aims at solving this issue by taking advantage of a recently published new method to accurately model the interactions between a compliant gripper and a deformable object. The method is also extended to include contact with the palm of the gripper. The main advantage of this technique compared to existing traditional approaches such as finite element analyses (FEA) is its computational efficiency which allows its use in demanding optimization algorithms, as will be shown. In the first part of this paper, the overall design of the gripper is proposed and the corresponding model is described. The optimization process is then presented and completed with the experimental testing of a prototype.
查看更多>>摘要：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.
查看更多>>摘要：The orientational workspace of parallel mechanisms is known to be restricted due to singular configurations of type II. Recently, a (6+3)-degree-of-freedom (DOF) kinematically redundant parallel mechanism was proposed based on the well-known Gough–Stewart platform. It was shown that, for the specific architecture proposed, a minimum of three redundant DOFs is necessary to guarantee the existence of a non-singular configuration for any pose of the platform. This work presents a different architecture with two redundant DOFs instead of three, and has for primary objective to derive the singularity locus for zero-torsion configurations. The results indicate that the mathematically possible singularities are outside of the reachable workspace, suggesting that for zero-torsion trajectories, two kinematically redundant DOFs are sufficient to greatly enhance the orientational workspace of the proposed architecture. An example path with large tilting angle is presented in a multimedia extension of the article in order to demonstrate the capability of the mechanism to reach such orientations without encountering inevitable singularities.
查看更多>>摘要：The computerized design of non-generated double circular-arc helical gear drives with different combinations of transverse pressure angle based on the active design of the meshing line function is presented. The application of different transverse pressure angles for the double circular-arc tooth profiles is discussed and the parametric equations for the driving and driven gear tooth surfaces are derived. The meshing performance and mechanical behavior are studied in terms of the achieved contact patterns and the evolution of the contact and bending stresses for nine cases of design. The results show that one of the cases of design of double circular-arc helical gear drives considering a combination of transverse pressure angles of 25 and 15 degrees for the convex and concave circular-arc profiles of the pinion shows better mechanical performance with respect to the other combinations of different transverse pressure angles. The proposed geometry of non-generated gears is appropriate for manufacturing by injection molding or additive manufacturing for plastic, ceramic, metal as well as nanocomposite gears.
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Elsevier