A new attitude controller is proposed for spacecraft whose actuator has variable input saturation limit. There are three identical flywheels orthogonally mounted on board. Each rotor is driven by a brushless DC motor ...A new attitude controller is proposed for spacecraft whose actuator has variable input saturation limit. There are three identical flywheels orthogonally mounted on board. Each rotor is driven by a brushless DC motor （BLDCM）. Models of spacecraft attitude dynamics and flywheel rotor driving motor electromechanics are discussed in detail. The controller design is similar to saturation limit linear assignment. An auxiliary parameter and a boundary coefficient are imported into the controller to guaran- tee system stability and improve control performance. A time-varying and state-dependent flywheel output torque saturation limit model is established. Stability of the closed-loop control system and asymptotic convergence of system states are proved via Lyapunov methods and LaSalle invariance principle. Boundedness of the auxiliary parameter ensures that the control objective can be achieved, while the boundary parameter＇s value makes a balance between system control performance and flywheel utilization efficiency. Compared with existing controllers, the newly developed controller with variable torque saturation limit can bring smoother control and faster system response. Numerical simulations validate the effectiveness of the controller.展开更多
The levels of heavy metals that accumulated in stemwood of mature trees grown for 20 years in a plantation in an abandoned peat quarry in areas that were fertilized with different amounts of domestic sewage sludge(180...The levels of heavy metals that accumulated in stemwood of mature trees grown for 20 years in a plantation in an abandoned peat quarry in areas that were fertilized with different amounts of domestic sewage sludge(180,360,and720 Mg ha-1on a dry basis) were compared with trees grown in a reference nonfertilized area.Included in the study was a hybrid poplar(Populus tremula x Populus tremuloides) developed for use as an energy crop,three local tree species and one introduced tree species.The concentrations of Cd,Cr,Cu,Ni,Pb and Zn in the stemwood of the trees grown in the fertilized and nonfertilized fields were determined,and found to be significantly lower than their respective concentrations in the soil.Cd and Cr were found only in several wood samples at concentrations close to the limits of detection or qualification; therefore,they were not analyzed further.A correlation analysis suggested that 75%of the correlations between the concentrations of heavy metals in the stemwood and the concentrations in the soil were negative.The ability of trees to accumulate the metals from soil in most cases decreased for Cu and Ni; however,the correlations were not as clear for Pb and Zn.The followingsequence for the levels of heavy metals found in the stemwood of the analysed trees was Zn [ Pb [ Ni [ Cu [(Cr,Cd).The results of this study showed that the levels of heavy metals in the studied wood would not exceed the permitted limits of heavy metal pollution in the air and ash when used for energy production.展开更多
基金National Natural Science Foundation of China(10902003)
文摘A new attitude controller is proposed for spacecraft whose actuator has variable input saturation limit. There are three identical flywheels orthogonally mounted on board. Each rotor is driven by a brushless DC motor （BLDCM）. Models of spacecraft attitude dynamics and flywheel rotor driving motor electromechanics are discussed in detail. The controller design is similar to saturation limit linear assignment. An auxiliary parameter and a boundary coefficient are imported into the controller to guaran- tee system stability and improve control performance. A time-varying and state-dependent flywheel output torque saturation limit model is established. Stability of the closed-loop control system and asymptotic convergence of system states are proved via Lyapunov methods and LaSalle invariance principle. Boundedness of the auxiliary parameter ensures that the control objective can be achieved, while the boundary parameter＇s value makes a balance between system control performance and flywheel utilization efficiency. Compared with existing controllers, the newly developed controller with variable torque saturation limit can bring smoother control and faster system response. Numerical simulations validate the effectiveness of the controller.
文摘The levels of heavy metals that accumulated in stemwood of mature trees grown for 20 years in a plantation in an abandoned peat quarry in areas that were fertilized with different amounts of domestic sewage sludge(180,360,and720 Mg ha-1on a dry basis) were compared with trees grown in a reference nonfertilized area.Included in the study was a hybrid poplar(Populus tremula x Populus tremuloides) developed for use as an energy crop,three local tree species and one introduced tree species.The concentrations of Cd,Cr,Cu,Ni,Pb and Zn in the stemwood of the trees grown in the fertilized and nonfertilized fields were determined,and found to be significantly lower than their respective concentrations in the soil.Cd and Cr were found only in several wood samples at concentrations close to the limits of detection or qualification; therefore,they were not analyzed further.A correlation analysis suggested that 75%of the correlations between the concentrations of heavy metals in the stemwood and the concentrations in the soil were negative.The ability of trees to accumulate the metals from soil in most cases decreased for Cu and Ni; however,the correlations were not as clear for Pb and Zn.The followingsequence for the levels of heavy metals found in the stemwood of the analysed trees was Zn [ Pb [ Ni [ Cu [(Cr,Cd).The results of this study showed that the levels of heavy metals in the studied wood would not exceed the permitted limits of heavy metal pollution in the air and ash when used for energy production.
