Haiping Liu

In order to characterize the characteristics of pathological tremor of human upper limb, a simulation system of pathological tremor of human arm was provided and its dynamic response was analyzed. Firstly, in this study, a two-degree-of-freedom human arm dynamic model was established and linearized according to the arbitrary initial angle of joints. After solving the analytical solutions of steady-state responses of the joints, the numerical solution was used to verify it. The results of theoretical analysis show that the two natural frequencies of the developed dynamic model are 2.9 Hz and 5.4 Hz, respectively, which meet the characteristic frequency range of pathological tremors. Then, combined with the measured parameters of human arm, a tremor simulation system was built, and the measured results of joint responses are in good agreement with the theoretical and simulation analysis results, which verifies the effectiveness of the theoretical model. The results show that the human arm pathological tremor simulation system designed in this paper can characterize the frequency and response amplitude of the human upper limb pathological tremor. Moreover, the relevant research lays a theoretical foundation and experimental conditions for the subsequent development of wearable tremor suppression devices.

This article presents a novel fixed-time safe-by-design control for uncertain active vehicle suspension systems (AVSSs) with matched/unmatched disturbances, high energy consumption, input saturations, and asymmetric time-varying constraints on both displacement/velocity. Several asymmetric time-varying barrier Lyapunov functions are carefully constructed to deal with displacement/velocity constraints under the fixed-time convergence framework. Furthermore, a new fixed-time auxiliary state system is proposed to compensate for the saturation effect. It is then rigorously demonstrated that the convergence time is independent of initial state conditions and both the displacement/velocity are always restricted within the asymmetric time-varying ranges. Importantly, by employing beneficial nonlinearities of a bioinspired X-shaped reference model, the burden of high control energy cost can be relaxed compared with traditional controllers. As far as we know, this should be the first attempt to achieve a fixed-time safe-by-design control for AVSSs, which simultaneously considers practical issues including meeting the safety requirements for input saturations and displacement/velocity, achieving fixed-time convergence, maintaining energy efficiency, and rejecting matched/unmatched disturbances. Experimental testing results validate the effectiveness, advantage, and feasibility of the proposed control algorithm, compared to other existing methods.

In order to effectively mitigate the micro-vibration energy from the on-orbit flywheel and high-level vibration transmitted to flywheel system in the launching stage, a nonlinear vibration absorber (NVA) with Euler-buckled beams is developed and investigated in this paper. The NVA is composed of a linear supporting spring and a set of parallel Euler-buckled beams which is used as a negative stiffness corrector. In order to evaluate vibration reduction performance and stability characteristics of this developed nonlinear coupled dynamic system with NVA. First of all, a multi-degree-of-freedom compound dynamic model, including the flywheel system, the NVA and the supporting structural plate in the satellite platform, is built. Then, based on the systematic dynamic equations, the approximate steady-state solutions are derived by using the complexification-averaging (CX-A) method under on-orbit and launching loads, respectively. The analytical method and solutions are validated by using the numerical method. The proposed NVA can realize excellent vibration mitigation performance for the three-degree-of-freedom (3-DOF) coupled nonlinear system under the launching and on-orbit excitations. Next, the slow-variable manifold equation is analyzed to address the dynamic bifurcation behaviors of the nonlinear compound system and the influences of the excitation amplitude on the bifurcation characteristics are focused on. The results show that a 3-DOF system is more prone to instability and requires larger external excitation to escape the instability region. The calculation results also show that the stiffness ratio and excitation amplitude have significant effects on the bifurcation characteristics, and a proper stiffness ratio can be chosen to avoid bifurcations. Finally, compared with typical pure cubic stiffness NVA, the proposed NVA exhibits greater adaptability, and can work efficiently for a relatively larger range of excitation amplitude.

This study develops a new class of Euler-buckled beam based nonlinear energy sink (EBNES) with three configurations, which is expected to attenuate the disturbance effects and further enhance vibration suppression under launching and on-orbit loads simultaneously. The effects of different arrangements on amplitude-frequency responses of the primary system are derived and analyzed through complexification-averaging method, and the approximate solutions are verified by fourth-order Runge Kutta method. Comparison results exhibit that the EBNES-I is much more effective with enhanced vibration reduction performance and stability in a broad frequency range. Furthermore, a two-degree-of-freedom dynamic model of the flywheel system, which integrates the EBNES-I and the supporting structure in satellite, is established. The vibration reduction and bifurcation behaviors of the proposed EBNES-I are investigated, and the efficiency of the proposed EBNES-I in vibration reduction of the flywheel is compared to that of a traditional cubic-stiffness-type NES. It is found that the EBNES-I exhibits a good vibration reduction performance on the dynamic responses of the flywheel system in launching and on-orbit stage simultaneously. Additionally, the bifurcations of the coupled system are studied in order to investigate the influences of gravity and excitation amplitudes on the stability of the EBNES. Calculation results provide conditions for occurrence of the saddle-node (SN) and Hopf bifurcations.

This study develops a new class of Euler-buckled beam nonlinear energy sink (EBNES) with three configurations, which is expected to attenuate the disturbance effects and further enhance vibration suppression under launching and on-orbit loads simultaneously. The effects of arrangements on amplitude-frequency responses of the primary system are analyzed, and the approximate solutions are verified by numerical results. Comparisons with results exhibit that the EBNES-I is much more effective with significantly enhanced vibration reduction performance and stability in a broad frequency range. Furthermore, a two-degree-of-freedom dynamic model of the flywheel system, which integrates the EBNES-I and the satellite platform, is established. The vibration attenuation and bifurcation behaviors of the proposed EBNES-I is investigated, and the efficiency of the proposed EBNES-I in vibration reduction of the flywheel is compared to that of a cubic-stiffness NES. It is found that the EBNES-I exhibits a good vibration reduction performance on the dynamic responses of the flywheel system in launching and on-orbit stage simultaneously. Additionally, the bifurcations of the system are studied in order to find influences of gravity, excitation amplitudes and geometrical parameters on the stability of the EBNES. Calculation results provide suitable conditions for occurrence of the SN and Hopf bifurcations.