A Novel Disturbance Observer Based Fixed-Time Sliding Mode Control for Robotic Manipulators With Global Fast Convergence

This paper proposes a new global fixed-time sliding mode control strategy for the trajectory tracking control of uncertain robotic manipulators.First,a fixed-time disturbance observer(FTDO) is designed to deal with the adverse effects of model uncertainties and external disturbances in the manipulator systems.Then an adaptive scheme is used and the adaptive FTDO(AFTDO) is developed,so that the priori knowledge of the lumped disturbance is not required.Further,a new non-singular fast terminal sliding mode(NFTSM) surface is designed by using an arctan function,which helps to overcome the singularity problem and enhance the robustness of the system.Based on the estimation of the lumped disturbance by the AFTDO,a fixed-time non-singular fast terminal sliding mode controller(FTNFTSMC)is developed to guarantee the trajectory tracking errors converge to zero within a fixed time.The settling time is independent of the initial state of the system.In addition,the stability of the AFTDO and FTNFTSMC is strictly proved by using Lyapunov method.Finally,the fixed-time NFESM(FTNFTSM) algorithm is validated on a 2-link manipulator and comparisons with other existing sliding mode controllers(SMCs) are performed.The comparative results confirm that the FTNFTSMC has superior control performance.

Preparation of unidirectional porous AlN ceramics via the combination of freeze casting and combustion synthesis

Unidirectional porous AlN ceramics(UP-AlNs)have attracted great attention for their wide applications in catalyst supports,filters and composite reinforcements.However,traditional fabrication processes usually require high temperature and long production cycle.Herein,UP-AlNs were successfully fabricated via the combination of tertiary butyl alcohol(TBA)-based freeze casting and combustion synthesis route using Al and AlN powders as raw materials.The microstructure,open porosity,thermal conductivity and compressive strength of UP-AlNs can be manipulated by synergistically regulating the AlN diluent content and Al/AlN solid loading of the freezing slurries.The optimal UP-AlNs exhibited controllable structure wavelength(λ)and open porosity in a wide range of 21.1-47.1μm and 54.2%-86.0%,respectively.In addition,the corresponding products also possessed anisotropic thermal conductivity and compressive strength.This novel route for the fabrication of UP-AlNs has the advantages of low cost,energy-saving and high efficiency,which shows significant promise for industrial applications.

Continuous SiC skeleton reinforced highly oriented graphite flake composites with high strength and specific thermal conductivity

Highly oriented graphite-based composites have attracted great attention because of their high thermal conductivity(TC),but the low mechanical properties caused by the inhomogeneous distribution and discontinuity of reinforcements restrict the wide applications.Herein,continuous SiC ceramic skeleton reinforced highly oriented graphite flake(SiC/GF)composites were successfully prepared by combining vacuum filtration and spark plasma sintering.The effect of SiC concentration on the microstructure,flexural strength,and thermophysical properties of the composites was investigated.The GF grains in the composites exhibited high orientation with a Lotgering factor of>88%when the SiC concentration was≤30 wt%,and the SiC skeleton became continuous with the SiC concentration reaching 20 wt%.The formation of continuous SiC skeleton improved the flexural strength of the composites effectively while keeping the TC in a high level.Especially,the composites with 30 wt%SiC exhibited the flexural strength up to 105 MPa,and the specific TC reaching 0.118 W·m^(2)·K^(−1)·kg-1.The composites with excellent flexural strength and thermophysical properties showed significant promise for thermal management applications.