Renzhen Chen

To address the low-frequency vibration isolation at the load-bearing and operating ends of robots and thereby enhance precision, this paper proposes a quasi-zero-stiffness (QZS) vibration isolation joint based on disc springs. The joint employs reverse-stacked disc springs to provide low stiffness in the horizontal direction and negative stiffness in the vertical direction. By adjusting the pre-compression of the disc spring s, the stiffness can be actively tuned to achieve ultra-low-frequency vibration isolation. Mathematical models are established to analyse its mechanical properties and vibration isolation performance in various directions. The results show that the joint attains quasi-zero stiffness near the equilibrium position, exhibits excellent vibration isolation performance in multiple directions, and can adapt to different excitations through active control. The proposed structure is compact and offers high load-bearing capacity, making it suitable for a wide range of applications such as high-precision surgical robots and aerospace instruments.

A power line inspection robot has to overcome many kinds of obstacles in inspection processes. The strain clamp is the obstacles difficult for inspection robots to overcome. The inspection robot needs to have a particular climbing ability to overcome the strain clamp. Therefore, the ability to climb power lines is the key point of the inspection robot’s design. An inspection robot with retractable double serial manipulators is proposed to improve the climbing and obstacle-crossing ability. Besides, this paper shows that the inspection robot is more suitable for climbing from static analysis and dynamic evaluation index. Firstly, the obstacle-crossing processes and structures of the inspection robot are introduced. Next, the static analysis is carried out when inspection robot climbs the power lines. The static analysis shows that the new inspection robot has a smaller driving torque. What’s more, the dynamic model of the inspection robot is established by Lagrange’s dynamical equations. By constructing the dynamic evaluation indexes, the inspection robot with retractable arms performs better dynamic characteristics. Finally, a prototype robot is carried out to cross obstacles and climb up power lines. The results show that the inspection robot can overcome different obstacles and has a good climbing performance.

Disk springs are widely used as preload and isolation due to their unique mechanical properties. In the prior research, the effect of linear friction on the disk spring was considered, but contact stiffness, another nonlinear contact factor, is ignored. Accordingly, in this paper, the asymmetric displacements of the contact edges are first derived, and the accurate friction dissipations are obtained, as a way to evaluate the effect of friction on the system. Then the velocity of the edges was obtained to establish a dynamic friction model. Meanwhile, the contact displacement and contact stiffness of the edge are obtained by fractal contact theory. Then the nonlinear static and dynamic models of disk spring with friction and contact stiffness are established by the energy method. The load–deflection relationship, stiffness, and hysteresis of disk spring are studied with different contact states. The results show that the model considering contact stiffness and asymmetric friction dissipation can effectively evaluate the static properties of the disk spring. Friction reinforces the nonlinear behavior of the system, while contact stiffness weakens the nonlinearity of the system. And due to the influence of nonlinear contact factors, the transmissibility curves produce multiple resonance peaks.

According to the working characteristics of a 1.5 MW wind turbine planetary gear system under complex and random wind load, a two-parameter Weibull distribution model is used to describe the distribution of random wind speed, and the time-varying load caused by random wind speed is obtained. The nonlinear dynamic model of planetary gear transmission system is established by using the lumped parameter method, and the relative relations among various components are derived by using Lagrange method. Then, the relative relationship between the components is solved by Runge Kutta method. Considering the influence of random load and stiffness ratio on the planetary gear transmission system, the nonlinear dynamic response of cyclic load and random wind load on the transmission system is analyzed. The analysis results show that the variation of the stiffness ratio makes the planetary gear have abundant nonlinear dynamics behavior and the planetary gear can get rid of chaos and enter into stable periodic motion by changing the stiffness ratio properly on the premise of ensuring transmission efficiency. For the variable pitch wind turbine, the random change of external load increases the instability of the system.

The object of this fundamental research will be to reveal the effects of variable coefficient of static friction on the static behavior of disc spring. On account of the complex relationship between friction and normal pressure, based on contact mechanics and fractal theory, a model for predicting variable coefficient of static friction of 3 D fractal surface is induced. Then the method proposed by Curti is improved by using the fractal coefficient of static friction to replace the Coulomb's law. The entire deflection range is considered under quasi-static loading condition. The model of disc spring is developed to predict the load-deflection and hysteresis characteristics while contacting with different fractal surfaces. The primary object of this study is to survey the variable static friction coefficient and its effect on the disc spring. Different fractal dimension, constant static friction coefficient and free height of disc spring are considered, and the relative error of the two methods are calculated. The investigation has revealed that the effect of fractal static friction coefficient on the load-deflection and hysteresis characteristics of disc spring is concentrated in the small deflection stage. And the method is universal for disc springs of different dimension on the same surface.

In this paper, the influence of tooth surface wear on dynamic characteristics of the gear-bearing system is discussed. The gear-bearing system considering the factors of tooth surface friction, time-varying meshing stiffness, backlash and et al. is established. Based on the Fractal theory and the Archard theory, the tooth surface wear is calculated, and it is substituted into the calculation of backlash and stiffness to obtain the time-varying backlash and the time-varying meshing stiffness. The Runge–Kutta method is used to solve the dynamic differential equation of the gear system, then the phase diagrams, the Poincare section diagrams and the bifurcation diagrams of the system are obtained. The results show that compared with the constant backlash and normal-distribution backlash under the wear condition, the wear backlash calculated by the Fractal theory and the Archard theory can better show the influence of wear on the system response. With the increasing of the accumulated wear on the tooth surface, the decrement of the time-varying meshing stiffness increases, and the system is more unstable. As the friction coefficient decreases, the regions of periodic and quasi-periodic motion state increase, and the boundaries between the periodic and quasi-periodic motion state become clear. Therefore, friction mainly plays a hysteretic role on the gear-bearing system.