查看更多>>摘要：This paper presents a robust density-based topology optimization approach for synthesizing pressure-actuated compliant mechanisms. To ensure functionality under manufacturing inaccuracies, the robust or three-field formulation is employed, involving dilated, intermediate and eroded realizations of the design. Darcy's law in conjunction with a conceptualized drainage term is used to model the pressure load as a function of the design vector. The consistent nodal loads are evaluated from the obtained pressure field using the standard finite element method. The objective and load sensitivities are obtained using the adjoint-variable approach. A multi-criteria objective involving both the stiffness and flexibility of the mechanism is employed in the robust formulation, and min–max optimization problems are solved to obtain pressure-actuated inverter, gripper, and contractor compliant mechanisms with different minimum feature sizes. Limitations of the linear elasticity assumptions while designing mechanisms are identified with high pressure loads. Challenges involved in designing finite deformable pressure-actuated compliant mechanisms are presented.
查看更多>>摘要：Buckled beams have become key building blocks in compliant mechanisms to achieve nonlinear behaviors, such as bistability, constant-force and stiffness tuning. However, designing and modeling such components is challenging due to their highly nonlinear characteristics and existing models typically lack accuracy or rely on numerical computations. This paper aims at giving closed-form formulas to efficiently characterize the snap-through behavior and facilitate the design process of rotationally actuated pinned–pinned and fixed–pinned bistable buckled beams. A new generic analytical model for precompressed beams based on Euler–Bernoulli beam theory is first established and then applied to these two configurations. Finite element and experimental validations are performed, showing excellent agreement with the model. The results show that the angular input at the pinned beam extremity is significantly decoupled from the angular or moment output at the other extremity, until snap-through. Additionally, we show that the stable output values can be controlled by adjusting the precompression displacement of the buckled beam. Finally, we demonstrate the practicality of the studied building blocks and their modeling on a novel bistable gripper design.
查看更多>>摘要：The centrifugal pendulum dampers are widely used to isolate drivetrain torsional vibrations. The kinematic structure of these devices includes one or more rollers meshing with slots carved on the absorbers and on the rotor. Such a kinematic higher-pair has been termed caged-roller joint. This investigation focuses on the analysis of rollers dynamics influence on modern centrifugal dampers architectures. To specialize the caged-roller joint equations for each kind of pendulum, the absorber motion is described in terms of kinematic invariants. As a result, more refined linear tuning conditions and nonlinear design procedures are disclosed. Finally, numerical tests are carried out to assess the sensitivity of rollers design parameters on the system dynamics. These results show that the rollers inertia effect cannot be neglected in an accurate design, due to the introduction of an undesired detuning of the devices.
查看更多>>摘要：To solve the problems of narrow contact area and small contact ratio of the involute cylindrical worm helical gear drive, this paper proposed a point-contact conjugate hourglass worm drive (PCHW drive) based on the meshing theory of conjugate tooth surfaces generated by two generating surfaces. The hourglass worm and worm gear are generated by the helix surface of an involute helical gear and that of an involute cylindrical worm. A mathematical model of the drive is established and a method for solving contact points based on involute pitch point is given. The instantaneous transmission ratio is proved theoretically to be constant and transmission error is zero. The formation mechanism of the conjugate surfaces of the drive is revealed. The results of the tooth contact analysis for a numerical example show that he contact area of this drive is wider, the contact ratio is slightly increased compared to the involute cylindrical worm helical gear drive. Furthermore, the drive is insensitive to manufacturing and assembly errors. A prototype is manufactured and meshing test is performed to verify the theoretical correctness.
查看更多>>摘要：In order to enlarge the proportion of constant-torque output, a new type of constant-torque flexure hinge based on line-arc-line segment (LAL-CTFH) is designed. The LAL-CTFH has the characteristics of good constant-torque output performance, simple structure and easy manufacturing. The improved pseudo-rigid-body model (PRBM) method named four-bar linkage PRBM (FB-PRBM) for the LAL-CTFH is proposed to analyze the torque-angle characteristics of LAL-CTFH. The method is verified by finite element analysis (FEA). Furthermore, the constant-torque output characteristics of LAL-CTFH are optimized. Theoretical analysis, FEA, physical prototype production and experimental test of the optimized model are carried out respectively. The results show that in the range of 0–40°, the proportion of constant-torque output can reach more than 65%, the error between theoretical value and FEA value is less than 1%, and the average error between theoretical value and test value is less than 5%, which verifies the feasibility of the design scheme and the validity of the theoretical analysis.
查看更多>>摘要：The solutions to small deflections of uniform cantilever beams have been obtained and widely used. However, when the deflections become large, it is quite challenging to derive the solutions due to the geometric nonlinearity, especially for non-uniform beams. To handle that, in this paper, a novel and an efficient method called being gradually softened approach (BGSA) is proposed to predict the large deflections of uniform and non-uniform cantilever beams subjected to distributed and tip loads. The cantilever beam is initially regarded as a rigid beam and then is gradually softened from the free end to the fixed end to solve the deflection of the cantilever beam. As new rigid units of the cantilever beam are softened, the direction of the force load acting on the previous softened units changes. Because of that, the deflections of the previous softened units need to be updated accordingly. Experimental results show that this method can solve the large deflections of uniform and non-uniform cantilever beams that experience distributed and tip loads with high accuracy.
查看更多>>摘要：A unified framework is established for general balance stability criteria of biped systems. The stability regions of balanced and steppable states are partitions of the center-of-mass (COM) state space that is augmented for general biped systems and tasks. Their boundaries are the set of maximum allowable COM velocity perturbations, under which a specified contact can be maintained (balanced) or a desired step can be made (steppable). Whole-body models with full-order dynamics and system-specific kinematic and actuation limits are established to quantify the effects of momenta and stepping on the system's balance stability under contact interactions. The generality of the framework with respect to systems and tasks is verified with a humanoid robot and a human for walking and standing. Validity and implications of the computed stability regions are demonstrated in the sagittal plane through comparative analyses across strategies and against existing criteria (capturability and zero-moment point), reduced-order models, and simulations. Distinct stability characteristics of experimental walking for the robot (overly balanced) versus humans (unbalanced but steppable) are validated, and the walking principle is analyzed. The partitions also provide inter/intra-region analyses of multi-level momentum and stepping strategies for balancing. A partition-aware controller is implemented for the robot to monitor the stability of the current state and selectively exploit stabilizing actions during simulated standing push recovery.
查看更多>>摘要：Deployable mechanisms have attracted much attention because of their potential in the fields of aerospace. In this paper, novel basic units with good composability and deployable performance are proposed to construct regular polygonal prisms and ring truss deployable mechanisms. Structure designs of basic units and their constructed regular polygonal prisms are first presented to describe the mechanism principles. To study the mobility of deployable mechanisms, the basic unit and its sub-mechanism are separated to analyze. The degree-of-freedom (DOF) of the sub-mechanism is carried out by screw theory and then verified by line geometry theory. According to the number of independent parameters, all the basic units, polygonal prisms, and ring truss deployable mechanisms have one DOF. Afterward, a class of ring truss deployable mechanisms for satellite antennas is presented as candidates. The kinematic models of the basic unit and typical deployable mechanism are established to show the deployable performance. The deployable mechanisms proposed in this paper provide a well theoretical reference for the structure design of satellite antennas, which have a good application prospect in the field of deep space exploration.
查看更多>>摘要：Chaotic dynamic behavior in clearance joints is the prominent nonlinear factor for multibody systems. This study presents a passive chaos suppression method to stabilize the planar slider-crank mechanism with a clearance joint. The strategy resorts to a vibro-impact oscillator attached to the slider. Primarily, the impact principle is employed to characterize the dynamics in clearance joints. The mathematical model of the integral system is conducted subsequently. The chaotic system behavior is evaluated by bifurcation analysis varying clearance size and crank speed. When the controller is activated, parametric optimizations of the stiffness, clearance coefficient, and particle mass are conducted to achieve optimum chaos elimination. Phase portraits, the largest Lyapunov exponent, and time-frequency distribution are evaluated. Numerical results illuminate that an adjusted oscillator can suppress the chaotic high-frequency composition of system response and help to maintain continuous contact between the joint elements. The results suggest applying the identical clearance size in the revolute joint and vibro-impact oscillator for fast optimization. Moreover, the method is still technology valid when considering different contact principles. Ultimately, a test platform is established to achieve some experimental validations.
查看更多>>摘要：The ideology of non-specific mismatched drive is expounded, and its meshing theory is investigated by taking a new-type worm gearing composed of planar enveloping hourglass worm and spur involute gear as an example. The contact equations for determining the instantaneous contact points are established and its equivalent form containing only one unknown are obtained by artificial elimination, thereby realizing the datum point, the contact endpoints can be calculated more concisely. A comprehensive design method is proposed to avoid edge contact and guarantee the center locating of bearing contact area. On this basis, an effective mismatched scheme for the worm gearing is obtained. Simulation of tooth contact indicates that the scheme can simultaneously ensure: (a) no curvature interference; (b) respectable carrying capacity; (c) reasonable kinematic errors. Additionally, the influence of some mismatched parameters on meshing property is discussed, which provides references for the matching and selection of parameters. The developed mismatched strategies and meshing theory discussed in this paper can be extended to other types of complicated point-contact spatial gearing as well.
NSTL
Elsevier