Nonlinear control of spacecraft formation flying with disturbance rejection and collision avoidance

A nonlinear controller for disturbances rejection and collision avoidance is proposed for spacecraft formation flying.The formation flying is described by a nonlinear model with the J2 perturbation and atmospheric drag. Based on the theory of the state-dependent Riccati equation（SDRE）, a finite time nonlinear control law is developed for the nonlinear dynamics involved in formation flying. Then, a compensative internal mode（IM） control law is added to eliminate disturbances.These two control laws compose a finite time nonlinear tracking controller with disturbances rejection. Moreover, taking safety requirements into account, the repulsive control law is incorporated in the composite controller to perform collision avoidance manoeuvres. A numerical simulation is presented to demonstrate the effectiveness of the proposed method.Compared to the conventional control method, the proposed method provides better performance in the presence of the obstacles and external disturbances.

Nonlinear suboptimal tracking control of spacecraft approaching a tumbling target

We investigate the close-range relative motion and control of a spacecraft approaching a tumbling target. Unlike the traditional rigid-body dynamics with translation and rotation about the center of mass（CM）, the kinematic coupling between translation and rotation is taken into consideration to directly describe the motion of the spacecraft＇s sensors or devices which are not coincident with the CM. Thus, a kinematically coupled 6 degrees-of-freedom（DOF） relative motion model for the instrument（feature point） is set up. To make the chaser spacecraft＇s feature point track the target＇s, an optimal tracking problem is defined and a control law with a feedback-feedforward structure is designed. With quasi-linearization of the nonlinear dynamical system, the feedforward term is computed from a specified constraint about the dynamical system and the reference model, and the feedback action is derived starting from the state-dependent Ricca equation（SDRE）. The proposed controller is compared with an existing suboptimal tracking controller, and numerical simulations are presented to illustrate the effectiveness and superiority of the proposed method.

Aerodynamic performance of distributed electric propulsion with wing interaction

Distributed electric propulsion(DEP)uses multiple propellers driven by motors distributed along the leading edge of the wing to produce beneficial aerodynamic interactions.However,the wing will be in the sliding flow of the propeller and the lift and drag characteristics of the wing will change accordingly.The performance of the propeller will also be affected by the wing in its rear.In this paper,combined with wind tunnel tests,the low Reynolds aerodynamic properties of multiple DEP structures are numerically simulated by solving the Reynolds averaged Navier-Stokes(RANS)equation of multiple reference frames(MRF)or slip grid technology.The results demonstrate that the lift and drag of DEP increase in all cases,with the magnitude depending on the angle of attack(AOA)and the relative positions of propellers and wing.When the AOA is less than 16°(stall AOA),the change of lift is not affected by it.By contrast,when the AOA is greater than 16°the L/D(lift-to-drag ratio)of the DEP system increases significantly.This is because the propeller slipstream delays laminar flow separation and increases the stall AOA.At the same time,the inflow and the downwash effect,which is generated on both sides of the rotating shaft,result in the actual AOA of the wing being greater than the free flow AOA with a fluctuation distribution of the lift coefficient along the span.Also,for the propeller in the DEP,the blocking effect of the wing and the vortex of the trailing edge of the wing result in a significant increase in thrust.

Stereodivergent catalytic asymmetric allylic alkylation(AAA)

Chirality or handedness,i.e.,the non-superimposability of a structure(lacking a plane or center of symmetry)with its mirrorimage,strongly impacts various chemistry/biology areas,and plays an important role,e.g.,in materials chemistry[1].In medicinal chemistry it is critical to evaluate the therapeutic and toxicological properties of all stereoisomers of a drug molecule because diastereoisomers and enantiomers may exhibit distinct biological and pharmacological activities[2].