查看更多>>摘要：This paper presents two generalized models that can quickly formulate the closed-form equations of compliance and precision for multiple-axis flexure hinges including symmetric and hybrid types under small-deflection occasions, and introduces new types of multiple-axis flexure hinges. Notch contours of multiple-axis flexure hinges are divided into multiple-axis base and mirror segments with multiple segments. Solving closed-form compliances and precisions of multiple-axis flexure hinges is transformed into several definite integrals associated with the notch shape functions of designated base segments and matrix operations. The closed-form compliances for the conic-section, elliptical-arc and truncated-cone base segments are derived. Utilizing the pre-designed base segments, mirroring operations and serial combination design, more than two hundred multiple-axis flexure hinges are designed and their closed-form compliances can be formulated with the presented models, and tedious integral operations are avoided. The presented models are verified by means of the finite element results and existing equations. The design domain is introduced and the performances of the designed hinges are investigated. The presented models and designed hinge configurations are helpful in improving the integrated performances of spatial compliant mechanisms.
查看更多>>摘要：This work complements and continues efforts towards developing a complete numerical integration method for multibody dynamic systems with frictional impacts. Specifically, the new method is a time-stepping scheme, which is applicable to mechanical systems subject to a set of bilateral and a single unilateral motion constraint. During an impact free phase of the motion, a suitable augmented Lagrangian methodology is applied. When an impact is detected during a time step, the post-impact state is determined by employing a special process, which addresses properly the inherent numerical stiffness associated with the impact events. The basic contribution of the present study is the extension to cover cases where the contact between the interacting bodies is possible to exist for a finite period of time after an impact. This phenomenon occurs frequently in machines and mechanisms and is known as persistent contact. First, a complete strategy is developed for establishing a general numerical procedure for capturing and treating both activation of contact and loss of contact between two bodies. Then, the accuracy and efficiency of this procedure is tested and demonstrated by applying it to a selected set of mechanical examples.
查看更多>>摘要：The transient impulses are one of the most important features for the defective bearing diagnosis. Many methods like the envelope analysis are created to analyze the fault-induced impulse. The situation is however different for high-speed bearing-rotor systems whose rotor runs in flexible bearing supports such as aero-engines and gas-turbines, as the impulsive responses are affected by resonance characteristics and rotor eccentric excitation. Few scholars pay attention to the phenomena, since one speed is usually chosen for dynamics or diagnosis in the state-of-the-art research. In this work, a symmetrical rigid bearing-rotor system running in flexible bearing supports was modeled. The influence of the resonance characteristics and rotor eccentric excitation on the fault characteristic frequencies was analyzed via envelope analysis performing on the dynamic response from the numerical simulation at different speeds. Besides, the contrast of the fault characteristic frequencies demonstrated the favorable and unfavorable influence of the resonance characteristics and the rotor eccentric excitation on the quality of the bearing fault vibration signal in different speed ranges, respectively. Accordingly, the mechanisms were explained by analysis of the resonance characteristics and bearing contact force of the system. Finally, the influence of the resonance characteristics was qualitatively verified by the experimental results.
查看更多>>摘要：Compliant Mechanisms (CMs) present several desired properties for mechanical applications only depending on elastic deformation of the involved compliant beams/flexures. As reported in the current literature, most CM designs utilize straight beams and initially curved beams (ICBs) as the fundamental flexible members. In CM research community, many great contributions regarding modeling these elementary flexible members have been achieved. In this paper, a comprehensive modeling methodology, based on beam theory, has been established to characterize the static planar deflection of slender beam. Then such a methodology has been applied to solve 8 loading scenarios of large beam-deflection problems that exist in the design of CMs. Essentially speaking, all these beam-deflection problems are treated as a type of boundary value problems (BVPs) of an ordinary differential equation (ODE) and solved by a modified collocation method. After that, this methodology has been used to model some representative CMs with large-deflection strokes, such as compliant parallelograms.
J. L. Torres-MorenoN. C. CruzJ. D. AlvarezJ. L. Redondo...
19页
查看更多>>摘要：Four-bar mechanisms are widespread in industrial and quotidian applications, but their design requires deep knowledge and experience in Mechanisms and Machine Theory. In this area, path synthesis is a common problem. It consists in calculating the length of the bars defining a mechanism so that a particular point of its structure follows the desired trajectory. This paper presents SALAR Mechanism Synthesizer, a new open-source software package that aims to be an easy-to-use yet powerful tool for addressing this kind of problem. This tool offers four different optimizers from the literature, one of them specially designed for mechanism synthesis. It also includes a new method for expressing and comparing paths, and hence, to define the optimization criterion. This method, referred to as normalized shape-descriptor vector(s) (NSDV(s)) and presented in this paper, allows us to handle paths without being affected by translation, rotation, and scale changes. The software package has been used for addressing six well-known problems from the literature. According to the results achieved, it arises as a valuable tool in terms of accuracy, convenience, and availability.
查看更多>>摘要：This paper focuses on the minimum-time trajectory planning in a structured environment for a wheeled mobile robot with a trailer. The constraints which are accounted for include: obstacle avoidance, boundary conditions on the position/orientation of both the tractor and the trailer, maximum articulation angle between the two platforms, kinodynamic capacities of the actuators, and dynamic stability of the system. The proposed technique is based on the random-profile approach, which uses a versatile master-slave optimization scheme. It is robust enough to handle different scenarios (free or imposed path) and different structures of the system (on-axle or off-axle articulation). Simulation results show that it converges, in reasonable runtimes, towards good-quality solutions which are saturating the constraints. A preliminary experimental test suggests that the planned trajectories are realistically achievable.
查看更多>>摘要：This paper presents an effective trajectory generation algorithm to shorten and smooth the jerky paths obtained from the sampling-based planners in cluttered environments. We utilize the cubic smoothing B-spline curves to represent the final trajectories that are collision-free, strict C~2 continuity and respect velocity and acceleration constraints. Our algorithm mainly contains two core parts: a two-layer local adjustment strategy to avoid collisions and a fast trajectory pruning algorithm by introducing the Douglas-Peucker algorithm into the Path pruning technique to remove unnecessary and jerky motions. The two-layer local adjustment strategy takes advantage of B-splines' local support properties and geometry operations, adaptively selecting the trajectory adjustment method according to the complexity of the environment. We emphasize that our trajectory adjustment strategy only needs to adjust the segments around the colliding sections, and it is even feasible for environments with narrow passages. Furthermore, the pruning algorithm decimates the initial path with fewer dominant points, significantly accelerating the pruning speed. Simulation and actual experimental results show that our algorithm can rapidly generate smooth, collision-free, and natural-looking trajectories.
查看更多>>摘要：Tendon-driven continuum robots have increasingly attracted attention these years. Conventionally, such kind of robots utilizes elastic central backbones to hold the structure, which makes the robots inextensible, as well as reduces the dexterity and the workspace. Inspired by the reconfigurable feature of origami structures, this paper presents the design, analysis, and validation of a hybrid-driven continuum robot without an elastic backbone. The fabric-based, soft, and unstretchable origami pneumatic chamber holds the continuum structure and makes the robot exhibit a high extension ratio, low input pressure, and no radial expansion. With the antagonistic actuation of tendon-pulling and air-pushing, the robot can perform 3DoF motion with variable stiffness. The kinetostatics modeling and analysis are developed based on a discretization-based approach to predict the motion behavior and control the proposed robot. To validate the proposed design principle and the modeling method, a prototype is built, on which a series of experiments have been conducted. The results show that, with the proposed kinetostatics model, the prototype possesses acceptable positioning accuracy and tracking performances, while the structural stiffness can also be effectively adjusted.
查看更多>>摘要：In order to realize the safe taking-off and landing of aircrafts on unstructured terrains, the landing gear on the aircraft is required to have a terrain-adaptive function. As a kind of aircraft onboard equipment, adaptive landing gear puts forward higher requirements for its design. This article proposes a virtual equivalent parallel mechanism (VEPM) modeling and performance analysis approach for the landing gear mechanism with three kinematic branch chains. In this approach, the unstructured terrain data were mapped to a moving platform. The moving platform, virtual restrained chains and the original mechanism of the landing gear mechanism combines the VEPM. In this way, the performance analysis object of the landing gear can be converted from a single kinematic branch to the entire system. The new approach can more effectively evaluate the overall motion performance and stability performance of the landing gear, and can help improve the structural design and control algorithm design of the spacecraft landing gear.
查看更多>>摘要：Planar parallelogram mechanisms (PPMs) have been used in many applications either as standalone pieces or as compositional joints of mechanisms, such as parallel mechanisms, remote center mechanisms and variable-DOF mechanisms. An n-RR PPM is composed of congruent base and moving platform connected with n RR links of equal link lengths. Although it is well known that an n-RR PPM has one 1-DOF circular translation mode, whether an n-RR PPM has structure/motion modes in addition to the 1-DOF motion mode and how the link parameters affect the number of extra motion/structure modes of the n-RR PPMs have not been well investigated. This paper is about the motion/structure mode analysis and classification of n-RR PPMs. Firstly, the motion/structure mode analysis of 3-RR PPMs is carried out. Then conditions for an n(n > 3)-RR PPM to have the same motion/structure modes as a 3-RR PPM are revealed. Finally, n(n ≥ 3)-RR PPMs are classified into three types. In addition to one 1-DOF motion mode, Types I, II and III PPMs have two, one or zero structure mode respectively. This work provides a foundation for the construction and analysis of several variable-DOF mechanisms composed of PPMs.
NSTL
Elsevier