University of Macau

About: University of Macau is a education organization based out in Macao, Macau, China. It is known for research contribution in the topics: Population & Control theory. The organization has 6636 authors who have published 18324 publications receiving 327384 citations. The organization is also known as: UM & UMAC.

Development and Active Disturbance Rejection Control of a Compliant Micro-/Nanopositioning Piezostage With Dual Mode

Abstract: In the atomic force microscope (AFM) scanning system, the piezoscanner is significant in realizing high-performance tasks. To cater to this demand, a novel compliant two-degrees-of-freedom (2-DOF) micro-/nanopositioning stage with modified lever displacement amplifiers is proposed in this paper, which can be selected to work in dual modes. Moreover, the modified double four-bar P (P denotes prismatic) joints are adopted in designing the flexible limbs. The established models for the mechanical performance evaluation in terms of kinetostatics, dynamics, and workspace are validated by finite-element analysis. After a series of dimension optimizations carried out via particle swarm optimization algorithm, a novel active disturbance rejection controller, including the components of nonlinearity tracking differentiator, extended state observer, and nonlinear state error feedback, is designed for automatically estimating and suppressing the plant uncertainties arising from the hysteresis nonlinearity, creep effect, sensor noises, and other unknown disturbances. The closed-loop control results based on simulation and prototype indicate that the two working natural frequencies of the proposed stage are approximated to be 805.19 and 811.31 Hz, the amplification ratio in two axes is about 4.2, and the workspace is around 120 ×120 μm2, while the cross-coupling between the two axes is kept within 2%. All of the results indicate that the developed micro-/nanopositioning system has a good property for high-performance AFM scanning.

Properties improvement of fly ash cenosphere modified cement pastes using nano silica

Abstract: In this study, the effect of nano silica (NS) on the properties of cement pastes incorporated with of fly ash cenospheres (FACs) has been investigated. A fixed amount of 1 % NS was added to cement pastes containing various proportions of FAC. It was found that NS was beneficial in improving the mechanical strength of the FAC modified cement pastes. At the 7 – day age, the strength increase for NS modified pastes was about 17 % as compared to pastes without NS. However, the strength enhancement effect decreased with increasing age and FAC content. The dense microstructure of the NS modified pastes compensated for the higher porosity associated with FAC incorporation, leading to the reduced total porosity. With NS, there was better bonding of the FAC particles in the matrix, due to the enhanced interface. The reduced Ca(OH) 2 content and partially consumed FAC spherical shells at the early age showed that NS increased the pozzolanic activity of FAC in the cement pastes.

Strong ferromagnetism in hydrogenated monolayer MoS 2 tuned by strain

Abstract: The electronic and magnetic properties of hydrogenated monolayer MoS${}_{2}$ subject to equibiaxial tensile strain are systematically investigated using first-principles calculations. It is shown that at the strain-free state, the hydrogenated MoS${}_{2}$ is an $n$-type semiconductor with no spontaneous magnetism. As the tensile strain increases, however, a ground-state transition from nonmagnetism to ferromagnetism (FM) occurs with simultaneously enhanced magnetic moment and stability. The maximum FM state is achieved at a strain of 6.6$%$, which corresponds to Curie temperature ${T}_{c}$ of 232 K. As the strain increases further, both strength and stability of the ferromagnetic state weaken and eventually vanish due to the enhanced ionic character in Mo-S bonds and the resulting delocalization of Mo $d$ orbitals. Similar behavior is also predicted in hydrogenated monolayer MoSe${}_{2}$. The present work by combining chemical functionalization and strain engineering provides a route to harness the magnetic properties of two-dimensional transition metal dichalcogenides for spintronic applications.