ObjectiveAiming at the insufficient load-bearing capacity of existing inverted planetary roller screws under high-load operating conditions, an inverted planetary roller screw with concave-convex contact was proposed for parametric design and load performance analysis.MethodsFirstly, the mathematical model of the concave-convex contact inverted planetary roller screw was established, and parametric design software was developed to realize rapid modeling. Secondly, orthogonal tests were designed and finite element analyses were carried out to investigate the effects of five key parameters on the load performance, namely the arc radius ratio of nut–roller thread, the arc radius ratio of screw–roller thread, the thread arc radius of roller, thread flank angle and lead. Finally, the optimal parameter combination was obtained via range analysis and matrix analysis.ResultsThe results show that the thread arc radius of roller is the dominant factor affecting the load performance of the concave-convex contact inverted planetary roller screw, followed by the thread flank angle and lead. The optimal parameter combination for load performance is 2.9∶1, 2.8∶1, 12.27 mm, 44.8° and 5.1 mm. The research findings can provide a reference for the design of inverted planetary roller screws.
ObjectiveFrequent damage and failure of bearing rings in vibrating screen exciters are observed in coal washing plants, and wear and fatigue damage are difficult to be measured directly. The competitive mechanism between wear and rolling contact fatigue damage on the raceway surface of bearing rings was investigated.MethodsFirstly, a zonal division analysis method for bearing rings was proposed, the dynamic model was optimized based on Abaqus software, and 4 target regions with complex stress under special working conditions were determined to support accurate calculation of wear amount. Secondly, combined with the rainflow counting method and Miner’s linear cumulative damage criterion, the model under complex random load spectrum was converted into an equivalent contact model under pulsating cyclic load with the aid of Matlab software to support fatigue crack propagation analysis. Then, the crack propagation characteristics were solved based on fracture mechanics, and refined simulation of multi-stage crack propagation was completed. Finally, a competition criterion for wear and fatigue crack propagation damage was established to analyze the influence laws of three key factors, including rotational speed, eccentric block mass and radial clearance, on the competition mechanism.ResultsThe results show that wear accelerates the fatigue crack propagation rate. Fatigue damage is dominant throughout the whole process under the working conditions of 750 r/min and 960 r/min, while wear damage is dominant under the working condition of 1 500 r/min. The influence of eccentric block mass on the competitive mechanism is significantly higher than that of rotational speed and radial clearance, and the working condition with 210 μm radial clearance is the most unfavorable to the safe operation of bearings. This method can provide reference for fatigue life evaluation of parts under complex working conditions.
ObjectiveKnot-tying operations of flexible surgical robots are faced with long time consumption, and traditional path planning algorithms are restricted by low efficiency. The research on time-optimal trajectory optimization for surgical knot-tying was carried out.MethodsFirstly, the kinematic mapping relations between drive space and joint space, as well as joint space and end-effector space of flexible surgical robots were established to clarify the corresponding mechanism among driving rope length, joint angle and end-effector pose. Secondly, the 3-5-3 hybrid polynomial interpolation method was introduced to construct the piecewise joint trajectory for the suture ring formation process. Then, dynamic adjustments were performed on the inertia weight and learning factors of particle swarm optimization algorithm, and the global search strategy of whale optimization algorithm was integrated to construct an improved particle swarm optimization algorithm. Finally, the minimum total formation time of suture ring was taken as the fitness function, and the time-optimal trajectory solution was obtained under the constraints of joint velocity and acceleration.ResultsThe results show that the improved algorithm shortens the suture ring formation time by 6% and reduces the number of algorithm iterations. Laparoscopic wound knot-tying tests verify the effectiveness of the proposed algorithm and the feasibility of the knot-tying method, which can provide references for the optimization of wound knot-tying in robot-assisted surgery.
ObjectiveIn order to achieve the integration of wheels and legs, as well as structural-functional integration design, a robot based on dual closed chain five-bow-shaped-bar linkages was proposed. The robot was composed of a spherical cap body and bow-shaped limbs, and it possessed the capacity to reconfigure into a dual closed-chain rolling body for tumbling motion by means of controlling joint movements. The steering motion when the spherical cap body contacts the ground was pivotal in facilitating the robot’s flexible movement.MethodsFirstly, the kinematic model of the steering when the spherical cap body was grounded was established, followed by the derivation of equations for the robot’s center of mass through D-H coordinate transformation. Secondly, the strategy of decomposing the steering motion into forward rolling and lateral swinging enabled the establishment of kinematic constraints, thereby determining the motion range of the robot’s center of mass. Thirdly, the inverse kinematics was solved using a numerical method to obtain the joint motion trajectories, under the condition of a given target trajectory for the contact point of the spherical cap body. Finally, motion simulations were performed using Adams, and a prototype was constructed for testing.ResultsThe results of the simulations and tests validate the effectiveness of the kinematic model, the constraint conditions and the trajectory planning method, demonstrating the feasibility of the steering motion when the spherical cap body is contact with the ground.
ObjectiveTo reveal the coupled vibration mechanism of a precision cycloidal reducer under the combined effects of error excitations and motor input, and to provide a theoretical basis for error control and coordinated optimization of electromechanical parameters.MethodsFirstly, a standard three-crank double-cycloid precision cycloidal reducer was investigated. Based on the small-vibration coordinate method and Lagrange equations, an 11-degree-of-freedom planar bending-torsional coupled dynamic model incorporating the input shaft, crankshafts, cycloidal gears, and output carrier was established. The incremental relation among servo-motor voltage, current, and electromagnetic torque were further introduced to formulate an electromechanical coupled dynamic model. Then, time-varying meshing of the input-stage gear pair, two-dimensional elastic support of the eccentric bearings, and unilateral contact of the cycloid-pin tooth pairs were considered to investigate the effects of error excitations, load fluctuations, and motor parameters on the dynamic responses.ResultsTorsional vibration at the input side is converted into planar vibration of the cycloidal gears through the crankshafts and eccentric bearings and is subsequently transmitted to the output carrier through eccentric-support reaction forces, forming a coupled transmission path of input torsion-cycloidal-gear planar motion-output torsion. The input-stage gear error mainly affects the torsional responses of the input shaft and crankshafts, and its influence gradually attenuates toward the output side, whereas the cycloid-pin tooth error directly changes the multi-tooth contact state and instantaneous load distribution and therefore has a greater influence on the output dynamic transmission error. Compared with the baseline condition, increases in the input-stage gear error, cycloid-pin tooth error, and load fluctuation increase the peak-to-peak output dynamic transmission error by 12.37%, 30.55%, and 7.96%, respectively. Input-shaft speed fluctuations and current disturbances form a bidirectional electromechanical coupling through back electromotive force and electromagnetic torque. Increasing mechanical damping effectively suppresses input-side vibration, whereas reducing winding inductance intensifies current ripple.
