Effects of Currents on Human Freestyle and Breaststroke Swimming Analyzed by a Rigid-Body Dynamic Model

Swimming is a kind of complex locomotion that involves the interaction between the human body and the water. Here, to examine the effects of currents on the performance of freestyle and breaststroke swimming, a multi-body Newton-Euler dynamic model of human swimming is developed. The model consists of 18 rigid segments, whose shapes and geometries are determined based on the measured data from 3D scanning, and the fluid drags in consideration of the current are modeled. By establishing the interrelations between the fluid moments and the swimming kinematics, the underlying mechanism that triggers the turning of the human body is uncovered. Through systematic parametric analyses, the effects of currents on swimming performance (including the human body orientation, swimming direction, swimming speed, and propulsive efficiency) are elucidated. It reveals that the current would turn the human body counterclockwise in freestyle swimming, while clockwise in breaststroke swimming (which means that from the top view, the human trunk, i.e., the vector pointing from the bottom of feet to the top of the head, rotates counterclockwise or clockwise). Moreover, for both strokes, there exists a critical current condition, beyond which, the absolute swimming direction will be reversed. This work provides a wealth of fundamental insights into the swimming dynamics in the presence of currents, and the proposed modeling and analysis framework is promising to be used for analyzing the human swimming behavior in open water.

Design, analysis, control simulation, and experiment of novel electromagnetic hybrid vibration absorber

This paper presents a novel type of electromagnetic hybrid vibration absorber (EHVA) with a double-ring series permanent magnetic circuit applied to improve the two-stage vibration isolation system of marine power machinery. The EHVA comprises a mover with several groups of coils having a series connection, as well as a stator with internal permanent magnets and external permanent magnets, which form a novel magnetic circuit structure based on an improved Halbach array. In this magnet circuit, two adjacent groups of magnetic induction lines overlap at the coil position, thereby strengthening the magnetic field intensity. When the mover moves axially, the coil cuts the magnetic induction line to produce Lorentz force and magnetic damping force. First, the dynamic parameters of the EHVA were calculated based on multi-rigid-body dynamic theory and fixed-point theory. Second, the mechanical and electromagnetic structures of the EHVA were designed. Finally, a test stand was constructed, the system control channel was identified, and the simulation as well as experiment for vibration control using an [Formula: see text] controller were performed. Results indicated that the hybrid vibration reduction system under control reduced the acceleration amplitude by 12.3 dB at 30.6 Hz and achieved better vibration absorption effects between 27 and 33 Hz compared with the uncontrolled system.

Dynamic error of multiaxis machine tools considering position dependent structural dynamics and axis coupling inertial forces

During the working process of high-speed multiaxis machine tools, inertial forces can cause vibration and deformation of mechanical structure, which lead to the dynamic error of tool center point (TCP) relative to worktable and can adversely affect the machining performance. Considering the varying feed positions and accelerations during machining, a parameter-varying multi-rigid-body dynamic model of a 3-axis gantry machine tool is proposed. This model represents the position dependent structural dynamics and inertial forces, which can simulate the dynamic error of TCP relative to worktable within the entire workspace. The results show that the dynamic error in one direction is affected by the feed motions of multiple feed axes. The magnitudes of the dynamic error significantly vary with the position of Z-axis. And the dynamic errors in Y- and Z-direction show different varying trends. Then the theoretical model is used to discuss the dynamic error and position dependency. The expressions of TCP dynamic response and inertial forces reveal the reason why the dynamic errors in Y- and Z-direction show different varying trends.

Lagrange-method-based dynamic analysis of multi-stage planetary roller screw mechanism

A rigid-body dynamic model of multi-stage planetary roller screw mechanism (multi-stage PRSM) is proposed in this paper. The structure of multi-stage PRSM is introduced and the motion analysis is presented. The total kinetic energy of the mechanism is calculated. The rotation of the screws and carriers is chosen as generalized degrees of freedom. The generalized forces and motion equations of multi-stage PRSM are derived using the Lagrange method. The transient and steady-state behaviours of multi-stage PRSM are simulated, followed by an analysis of the influence of friction coefficients and thread pitches on the motion and forces acting on the multi-stage PRSM. Taking a two-stage PRSM as an example, the simulation results show that the friction coefficient between screw #1 and screw #2 has a slight effect on efficiency and rotational velocity ratios of carriers to screws. When the sum of the pitches of screws is a constant, the axial component of contact force between screw #1 and roller #1 decreases with the increase in the pitch of screw #1.

On New Modifications of Some Perturbation Procedures

In this paper, we present new modifications for some perturbation procedures used in mathematics, physics, astronomy, and engineering. These modifications will help us to solve the previous problems in different sciences under new conditions. As problems, we have, for example, the rotary rigid body problem, the gyroscopic problem, the pendulum motion problem, and other ones. These problems will be solved in a new manner different from the previous treatments. We solve some of the previous problems in the presence of new conditions, new analysis, and new domains. We let complementary conditions of such studied previously. We solve these problems by applying the large parameter technique used by assuming a large parameter which inversely proportional to a small quantity. For example, in rigid body dynamic problems, we take such quantity to be one of the components of the angular velocity vector in the initial instant of the rotary body about a fixed point. The domain of our solutions will be depending on the choice of a large parameter. The problem of slow (weak) oscillations is considered. So, we obtain slow motions of the bodies instead of fast motions and find the solutions of the problem in present new conditions on both of center of gravity, moments of inertia, and the angular velocity vector or one of these parameters of the body. This study is important for aerospace engineering, gyroscopic motions, satellite motion which has the correspondence of inertia moments, antennas, and navigations.

Dynamic Simulation-Guided Design of Tumbling Magnetic Microrobots

Design of robots at the small scale is a trial-and-error based process, which is costly and time-consuming. There are few dynamic simulation tools available to accurately predict the motion or performance of untethered microrobots as they move over a substrate. At smaller length scales, the influence of adhesion and friction, which scales with surface area, becomes more pronounced. Thus, rigid body dynamic simulators, which implicitly assume that contact between two bodies can be modeled as point contact are not suitable. In this paper, we present techniques for simulating the motion of microrobots where there can be intermittent and non-point contact between the robot and the substrate. We use these techniques to study the motion of tumbling microrobots of different shapes and select shapes that are optimal for improving locomotion performance. Simulation results are verified using experimental data on linear velocity, maximum climbable incline angle, and microrobot trajectory. Microrobots with improved geometry were fabricated, but limitations in the fabrication process resulted in unexpected manufacturing errors and material/size scale adjustments. The developed simulation model is able to incorporate these limitations and emulate their effect on the microrobot's motion, reproducing the experimental behavior of the tumbling microrobots, further showcasing the effectiveness of having such a dynamic model.

Rigid-Body Dynamic Analysis of Multi-Stage Planetary Roller Screw Mechanism

Based on the structural characteristics of the multi-stage Planetary Roller Screw Mechanism (PRSM), the motion and force among the different stages are analyzed. In terms of the Newton's second law, the rigid-body motion equations of the multi-stage PRSM without considering the manufacturing and assembly errors are derived. Then, the method for solving the motion equations is given. The forces acting on the parts in the multi-stage PRSM and the motion of the mechanism can be obtained from the present rigid-body dynamic model. The influence of the friction coefficients among the different stages on the dynamic characteristics of the multi-stage PRSM is discussed. The results show that the forces acting on the first-stage PRSM are larger than that acting on the second-stage PRSM, although the nominal radius of the screw in the first-stage PRSM is smaller. The friction coefficient between the nut and the screw in the different stages has the great influence on the efficiency of multi-stages PRSM with small helix angles, while that among the screws in the different stages has the slight effect on the efficiency.

Dynamic Analysis and Control Research of a 3-DOF Hydraulic Driven Parallel Mechanism

Background: The parallel mechanism is widely used in motion simulators, parallel machine tools, medical equipment and other fields. It has advantages of high rigidity, stable structure and high carrying capacity. However, the control strategy and control method are difficult to study because of the complexity of the parallel mechanism system. Objective: The purpose of this paper was to verify the dynamic model of a hydraulic driven 3-DOF parallel mechanism and propose a compound control strategy to broaden the bandwidth of the control system. Methods: The single rigid body dynamic model of the parallel mechanism was established by the Newton Euler method. The feed forward control strategy based on joint space control with inverse kinematic was designed to improve the bandwidth and control precision. The co-simulation method based on MATLAB / SIMULINK and ADAMS was adopted to verify the dynamics and control strategy. Results: The bandwidth of each degree of freedom in the 3-DOF parallel mechanism was used to expand about 10Hz and the amplitude error was controlled below 5%. Conclusion: Based on the designed dynamic model and composite control strategy, the controlled accuracy of the parallel mechanism is improved and the bandwidth of the control system is broadened. Furthermore, the improvements can be made in aspects of control accuracy and real-time performance to compose more patents on parallel mechanisms.