Mechanism and Machine Theory最新文献

The restitution coefficient serves as a critical parameter to evaluate the energy loss during the contact/impact process. Its in–depth researches are beneficial to accurately describing the contact/impact phenomenon. Given the limitations of existing restitution coefficient models, a novel variable restitution coefficient model, which considers the material properties and the initial relative contacting velocity between two colliding bodies, is proposed in this work. Firstly, the function relationship between the restitution coefficient and the equivalent plastic strain can be obtained based on the energy equivalence principle. After that, multi–condition FEM numerical simulation cases are conducted to explore the mapping relationship among the equivalent plastic strain, material properties and initial relative contacting velocity, with which the new normal variable restitution coefficient model can thus be established. Finally, various comparisons with several existing restitution coefficient models are conducted to showcase its superior performance, with the low–speed experimental data and high–speed FEM results acting as reference values. Furthermore, the new restitution coefficient model is extended to describe interaction process between two spheres, and also employed for the establishment of a new continuous contact force model.

This study aims to address the issue of vibration modeling and coupling vibration mechanism analysis of multistage blisk-rotor (MBR) systems with blade cracks. First, a comprehensive coupling vibration model of the MRB system containing the flexibility of shaft and blade, the swing motion of offset blisk structure, and the nonlinear effect of blade breathing crack is developed. Then, the coupling vibration mechanism of the MRB system is systematically investigated through analytical and numerical analysis, based on which the vibration indicators for the presence of blade cracks are extracted. At last, the effects of blade crack and operating condition parameters on the coupling vibration of the MRB system are comprehensively analyzed. The comparative results indicate that the crack depth and aerodynamic force may significantly affect the crack indicators in shaft bending and torsional vibrations, while the disc unbalance mainly affects the crack indicators in shaft bending vibrations. It is also suggested that the BHM and crack detection of rotating blades should comprehensively consider different crack indicators and perform continuous condition monitoring to obtain more robust and accurate results.

A curved bistable micro-beam, subjected to electrostatic loading from an electrode facing its concave side, may produce a snap-through response with voltages as low as 54%, when compared to actuation from a convex facing electrode. Such actuation has been dubbed “bow actuation” due to the similarity of preloading an arrow onto a bow, and the resulting equilibrium shift, as “bow snap-through”. Under a certain elevation-to-thickness ratio, a bistable beam will also become latchable, allowing the beam to maintain itself in its second stable state under zero load/voltage. In the current work, necessary conditions are found for static and dynamic bow snap-through, which can be used as a tool to design and produce bow snap-through responses, promoting efficient non-volatile and low-power consumption bistable based devices. The conditions are found using an undamped dynamic single degree-of-freedom (DoF) reduced-order (RO) model, attained via Galerkin’s decomposition. Subsequent numerical calculations, conducted in the presence of ambient damping, show that the condition is necessary to attain bow snap-through responses, while also disclosing the snapping behaviour of the model.

To broaden the methods of configuration synthesis, this study proposes a new method for configuration synthesis and screening of multiple closed-loop unit tandem mechanisms. Firstly, some closed-loop configurations are taken as basic units, and the closed-loop basic units are synthesized in series under the condition of considering the basic unit types, contacts, and input and output relationships. Secondly, the connectivity of the mechanism is obtained by weighting the adjacency matrix, and the entropy weight method is used for the first time to establish a comprehensive evaluation index coefficient to assess the quality of the mechanism. Then, an improved simulated annealing-greedy algorithm is proposed and an algorithm software screening platform is built to screen mechanisms. Finally, the multi-fingered grasper is used as design case to verify the practicality of the proposed configuration synthesis and screening method. This study can provide a reference for theoretical research on the configuration synthesis and screening methods of multiple closed-loop unit tandem mechanisms.

Precision operations, such as micro-injection and micro-assembly tasks, require accurate and constant torque output. Compliant constant torque mechanisms (CCTMs) can be considered as a potential alternative due to the characteristic of accurate and constant torque output without the need for complex control algorithms and structures. Therefore, it is essential to propose a general, designer-friendly, and efficient design methodology for the design of CCTMs. In this paper, we propose a methodology for designing compliant constant torque mechanisms based on a co-design process between Cartesian coordinate system and Frenet frame. We use geometrically exact beam theory to model the static deflection of the built-in flexible beams so as to analyze the overall mechanism. We transform the design of this mechanism into a typical constrained optimization problem, and solve it via highly efficient numerical methods to achieve the desired design objectives given the user-defined constraints and satisfy some specific engineering requirements. Our optimized design is thoroughly evaluated and validated through finite element method and experimental testing, demonstrating a significantly improved performance compared to existing designs.

The overhead conductor rail system (OCR) is an important current-transmitting structure for electric trains in tunnels. As the train speed increases, the wave propagation behaviour in the OCR plays an ever-increasingly important role in affecting the current collection quality. This paper is the first endeavour to numerically and experimentally explore wave behaviours in the OCR. With the help of a finite element model, the spatial propagation and frequency-domain characteristics of the wave propagation are investigated. Based on the time-space distribution of waves, the wave speed of the OCR is identified. Subsequently, a full-scale experimental test is conducted to identify a real-life OCR's wave speed for the first time. The relative error between the simulated and experimental speed is only 5.50 %, highlighting the effectiveness of the presented model. Then, the influence of wave propagation on the interaction performance of pantograph-OCR is analysed. A significant reduction of the interaction performance is observed when the train speed approaches the wave speed. Through sensitivity analysis, the bending stiffness, the linear density, and the span length are identified as sensitive parameters affecting the wave speed.

Compliance capability is one of the most important properties of suspension systems for rough-terrain robots. This imposes specific requirements on system stiffness design, which directly determines the platform's performance in response to external stimuli. To address the challenge of stiffness optimization design in active and passive compliant systems, this paper proposes a novel and practical method for optimizing stiffness for a terrain-adaptive wheel-legged rover. Firstly, the kinematic model of this multi-degree-of-freedom platform is established. Secondly, the deformation capability coefficient, load capacity coefficient, energy efficiency coefficient, and dynamic stability coefficient are derived as performance indices to assess the behavior of the system. By establishing the relationship between joint configuration variation and system stiffness, stiffness parameters could be evaluated through these performance indices. Ultimately, the global optimum stiffness parameters are selected from the refined intersection of the individual performance optimal domains. Thirdly, the optimum parameters are calculated and applicability verified numerically. The designed parameters are verified experimentally on a wheeled-legged rover. The experimental results demonstrate that the proposed algorithm can find the parameter combination that achieves optimal system performance, thereby enhancing the system's terrain adaptability.

Meshing impact is one of the three major internal excitations that cause vibration and noise in gear systems. Ingress impact is the most significant component of meshing impact. Current research typically considers the ingress impact within a meshing period while neglecting the different impacts of the integrated systematic errors of each gear pair on it. However, in practical scenarios, it is unrealistic to presume that the systematic errors of each tooth are identical. Base pitch errors, for instance, are crucial metrics that reflect the quality of gear manufacturing, and empirical measurements have confirmed that these errors are indeed non-uniform. Therefore, the current calculation methods for ingress impact limit the accurate representation of the dynamic behavior of gear systems. Based on this, a model that captures the ingress impact throughout the full-tooth meshing period is established. When compared with the traditional calculation method, which solely considers the ingress impact in a single meshing period, the results demonstrate that the method proposed in this paper offers a superior description of the dynamic behavior of gear systems.

3-RRR spherical parallel mechanisms (SPMs) have been extensively studied due to their numerous applications. Substantial effort has been devoted to their forward kinematics (FK), which is essential for their calibration and feedback control. However, despite their simple architecture, rather few 3-RRR SPMs with closed-form FK solutions (CFFKS) have been reported; iterative procedures are thus required in most cases. This paper presents three classes of 3-RRR SPMs with CFFKS, with the univariate polynomials for their FK being linear, quadratic, or quartic. These classes include a large set of designs, thereby enhancing the flexibility in selecting their architecture parameters. Moreover, they cover the majority of 3-RRR SPMs with special geometries that have been reported, while encompassing 3-RRR SPMs with certain special geometries yielding exceptional features such as unlimited rotation capacity about certain directions. Notably, these formulations are also applicable to many SPMs with alternative topologies and certain parallel mechanisms of other types. This work expands the family of SPMs with CFFKS, highly desirable in many practical applications.

This paper presents an innovative method to construct reconfigurable Generalized Parallel Manipulators (GPMs) through the integration of kinematotropic single-loop linkages and configurable platforms. The approach puts forward, for the first time, the development of reconfigurable manipulators by introducing kinematic redundancy. Firstly, this study analyzes the geometric configurations of the twofold-symmetric 8-bar single-loop linkage across various working phases. Secondly, by combining the kinematotropic linkage with moving platforms in GPMs, reconfigurable manipulators with kinematic redundancy are achieved. To validate the kinematic constraints and reconfigurable characteristics of the derived mechanisms, the algebraic structure properties of Lie groups are investigated. Then, a novel class of reconfigurable GPMs with kinematic redundancy is synthesized. The ability to shape the configurations of the platforms enables the resulting manipulators to exhibit reconfigurability. Consequently, these manipulators can execute multiple motion models corresponding to different working phases of the 8-bar linkages. Finally, this research highlights the potential applications of deployable stage by using the kinematic redundancy. The idea presented in this paper explores new possibilities for the development of reconfigurable manipulators with adaptive capabilities, and extend the potential applications of GPMs.