The planetary roller screw mechanism (PRSM) is a novel precision transmission mechanism that realizes the conversion between linear and rotary motions. The contact characteristics of helical surfaces directly determine PRSM’s performance in load-carrying capacity and transmission accuracy. Therefore, studying the contact characteristics of PRSM forms the fundamental basis for enhancing its transmission performance. In this study, a three-dimensional parametric analysis method of contact characteristics is proposed based on the PRSM meshing principle and PyVista (a high-level API to the Visualization Toolkit). The proposed method considers the influence of machining errors among various thread teeth. The effects of key machining errors on contact positions and axial clearance, as well as their sensitivities, are analyzed. With excellent solution accuracy, this method exhibits higher calculation efficiency and stronger robustness than the analytical and numerical meshing models. The influence of nominal diameter and pitch errors of the screw, roller, and nut on the axial clearance follows a linear relationship, whereas flank angle errors have negligible effects on the axial clearance. The corresponding influence coefficients for these three machining errors on the axial clearance are 0.623, 0.341, and 0.036. The variations in contact positions caused by individual errors are axisymmetric. Flank angle errors and roller diameter errors result in linear displacements of the contact points, whereas pitch errors cause the contact points to move along the arc of the roller diameter. Based on the proposed three-dimensional parametric contact characteristics analysis method, the Fuzzy C-Means clustering algorithm considering error sensitivity is utilized to establish a component grouping technique in the selective assembly of critical PRSM components, ensuring the rational and consistent clearances based on the given component’s machining errors. This study provides effective guidance for analyzing contact characteristics and grouping in selective assembly for PRSM components. It also presents the proposed method’s potential applicability to similar calculation problems for contact positions and clearances in other transmission systems.
Natural mechanical materials, such as bamboo and bone, often exhibit superior specific mechanical properties due to their hierarchical porous architectures. Using the principle of hierarchy as inspiration can facilitate the development of hierarchical mechanical metamaterials (HMMs) across multiple length scales via 3D printing. In this work, we propose self-similar HMMs that combine octet-truss (OCT) architecture as the first and second orders, with cubic architecture as the third or more orders. These HMMs were fabricated using stereolithography 3D printing, with the length sizes ranging from approximately 200 µm to the centimeter scale. The compressive stress–strain behaviors of HMMs exhibit a zigzag characteristic, and the toughness and energy absorption can be significantly enhanced by the hierarchical architecture. The compressive moduli are comparable to that of natural materials, and the strengths are superior to that of most polymer/metal foams, alumina hollow/carbon lattices, and other natural materials. Furthermore, the flexural stress–strain curves exhibit a nonlinear behavior, which can be attributed to the hierarchical architecture and local damage propagation. The relatively high mechanical properties can be attributed to the synergistic effect of the stretch-dominated OCT architecture and the bending-dominated cube architecture. Lastly, an ultralight HMM-integrated unmanned aerial vehicle (HMM-UAV) was successfully designed and printed. The HMM-UAV is ~85% lighter than its bulk counterpart, remarkably extending the flight duration time (~53%). This work not only provides an effective design strategy for HMMs but also further expands the application benchmark of HMMs.
Robots with transformable tracked mechanisms are widely used in complex terrains because of their high adaptability, and many studies on novel locomotion mechanisms have been conducted to make them able to climb higher obstacles. Developing underactuated transformable mechanisms for tracked robots could decrease the number of actuators used while maintaining the flexibility and obstacle-crossing capability of these robots, and increasing their cost performance. Therefore, the underactuated tracked robots have appreciable research potential. In this paper, a novel tracked robot with a newly proposed underactuated revolute‒revolute‒prismatic (RRP) transformable mechanism, which is inspired by the sit-up actions of humans, was developed. The newly proposed tracked robot has only two actuators installed on the track pulleys for moving and does not need extra actuators for transformations. Instead, it could concentrate the track belt’s tension toward one side, and the unbalanced tension would drive the linkage mechanisms to change its configuration. Through this method, the proposed underactuated design could change its external shape to create support points with the terrain and move its center of mass actively at the same time while climbing obstacles or crossing other kinds of terrains, thus greatly improving the climbing capability of the robot. The geometry and kinematic relationships of the robot and the crossing strategies for three kinds of typical obstacles are discussed. On the basis of such crossing motions, the parameters of links in the robot are designed to make sure the robot has sufficient stability while climbing obstacles. Terrain-crossing dynamic simulations were run and analyzed to prove the feasibility of the robot. A prototype was built and tested. Experiments show that the proposed robot could climb platforms with heights up to 33.3% of the robot’s length or cross gaps with widths up to 43.5% of the robot’s length.
Passive vibration isolation systems have been widely applied due to their low power consumption and high reliability. Nevertheless, the design of vibration isolators is usually limited by the narrow space of installation, and the requirement of heavy loads needs the high supporting stiffness that leads to the narrow isolation frequency band. To improve the vibration isolation performance of passive isolation systems for dynamic loaded equipment, a novel modular quasi-zero stiffness vibration isolator (MQZS-VI) with high linearity and integrated fluid damping is proposed. The MQZS-VI can achieve high-performance vibration isolation under a constraint mounted space, which is realized by highly integrating a novel combined magnetic negative stiffness mechanism into a damping structure: The stator magnets are integrated into the cylinder block, and the moving magnets providing negative-stiffness force also function as the piston supplying damping force simultaneously. An analytical model of the novel MQZS-VI is established and verified first. The effects of geometric parameters on the characteristics of negative stiffness and damping are then elucidated in detail based on the analytical model, and the design procedure is proposed to provide guidelines for the performance optimization of the MQZS-VI. Finally, static and dynamic experiments are conducted on the prototype. The experimental results demonstrate the proposed analytical model can be effectively utilized in the optimal design of the MQZS-VI, and the optimized MQZS-VI broadened greatly the isolation frequency band and suppressed the resonance peak simultaneously, which presented a substantial potential for application in vibration isolation for dynamic loaded equipment.
Piezoelectric actuators are a class of actuators that precisely transfer input electric energy into displacement, force, or movement outputs efficiently via inverse piezoelectric effect-based electromechanical coupling. Various types of piezoelectric actuators have sprung up and gained widespread use in various applications in terms of compelling attributes, such as high precision, flexibility of stoke, immunity to electromagnetic interference, and structural scalability. This paper systematically reviews the piezoelectric materials, operating principles, representative schemes, characteristics, and potential applications of each mainstream type of piezoelectric actuator. Herein, we intend to provide a more scientific and nuanced perspective to classify piezoelectric actuators into direct and indirect categories with several subcategories. In addition, this review outlines the pros and cons and the future development trends for all kinds of piezoelectric actuators by exploring the relations and mechanisms behind them. The rich content and detailed comparison can help build an in-depth and holistic understanding of piezoelectric actuators and pave the way for future research and the selection of practical applications.
Given the limited operating ability of a single robotic arm, dual-arm collaborative operations have become increasingly prominent. Compared with the electrically driven dual-arm manipulator, due to the unknown heavy load, difficulty in measuring contact forces, and control complexity during the closed-chain object transportation task, the hydraulic dual-arm manipulator (HDM) faces more difficulty in accurately tracking the desired motion trajectory, which may cause object deformation or even breakage. To overcome this problem, a compliance motion control method is proposed in this paper for the HDM. The mass parameter of the unknown object is obtained by using an adaptive method based on velocity error. Due to the difficulty in obtaining the actual internal force of the object, the pressure signal from the pressure sensor of the hydraulic system is used to estimate the contact force at the end-effector (EE) of two hydraulic manipulators (HMs). Further, the estimated contact force is used to calculate the actual internal force on the object. Then, a compliance motion controller is designed for HDM closed-chain collaboration. The position and internal force errors of the object are reduced by the feedback of the position, velocity, and internal force errors of the object to achieve the effect of the compliance motion of the HDM, i.e., to reduce the motion error and internal force of the object. The required velocity and force at the EE of the two HMs, including the position and internal force errors of the object, are inputted into separate position controllers. In addition, the position controllers of the two individual HMs are designed to enable precise motion control by using the virtual decomposition control method. Finally, comparative experiments are carried out on a hydraulic dual-arm test bench. The proposed method is validated by the experimental results, which demonstrate improved object position accuracy and reduced internal force.
Robots are playing an increasingly important role in engineering applications. Soft robots have promising applications in several fields due to their inherent advantages of compliance, low density, and soft interactions. A soft gripper based on bio-inspiration is proposed in this study. We analyze the cushioning and energy absorption mechanism of human fingertips in detail and provide insights for designing a soft gripper with a variable stiffness structure. We investigate the grasping modes through a large deformation modeling approach, which is verified through experiments. The characteristics of the three grasping modes are quantified through testing and can provide guidance for robotics manipulation. First, the adaptability of the soft gripper is verified by grasping multi-scale and extremely soft objects. Second, a cushioning model of the soft gripper is proposed, and the effectiveness of cushioning is verified by grasping extremely sharp objects and living organisms. Notably, we validate the advantages of the variable stiffness of the soft gripper, and the results show that the soft robot can robustly complete assemblies with a gap of only 0.1 mm. Owing to the unstructured nature of the engineering environment, the soft gripper can be applied in complex environments based on the abovementioned experimental analysis. Finally, we design the soft robotics system with feedback capture based on the inspiration of human catching behavior. The feasibility of engineering applications is initially verified through fast capture experiments on moving objects. The design concept of this robot can provide new insights for bionic machinery.
Owing to their excellent performance and large design space, curvilinear fiber-reinforced composite structures have gained considerable attention in engineering fields such as aerospace and automobile. In addition to the stiffness and strength of such structures, their stability also needs to be taken into account in the design. This study proposes a level-set-based optimization framework for maximizing the buckling load of curvilinear fiber-reinforced composite structures. In the proposed method, the contours of the level set function are used to represent fiber paths. For a composite laminate with a certain number of layers, one level set function is defined by radial basis functions and expansion coefficients for each layer. Furthermore, the fiber angle at an arbitrary point is the tangent orientation of the contour through this point. In the finite element of buckling, the stiffness and geometry matrices of an element are related to the fiber angle at the element centroid. This study considers the parallelism constraint for fiber paths. With the sensitivity calculation of the objective and constraint functions, the method of moving asymptotes is utilized to iteratively update all the expansion coefficients regarded as design variables. Two numerical examples under different boundary conditions are given to validate the proposed approach. Results show that the optimized curved fiber paths tend to be parallel and equidistant regardless of whether the composite laminates contain holes or not. Meanwhile, the buckling resistance of the final design is significantly improved.
