Invesigation Of Flight Controller Design Methods For Aircraft At Low-Altitude

To protect flight safety at low altitude,the flight controllers with special functions should be switched to adapt the variations of flight states and flight tasks.Aircraft flying at low altitude are fragile to uncertain disturbances brought by ground effect,wind shear,crosswind and so on.Especially,aircraft without thrust vectoring system or large auxiliary lifting equipment are vulnerable to nonlinear aerodynamics,sudden changes of inertial parameters,constrained flight states and control inputs,while performing high angle of attack maneuvering,airdropping heavy cargoes at extremely low attitude,and short takeoff and landing.These aforementioned factors threaten flight safety severely.Therefore,by extensively considering the flight tasks mentioned above and the aim at protecting the flight safety at low altitude,the flight controller design methods for aircraft at low-altitude are investigated.The potential unstable high angle of attack flight motions are systematically predicted by bifurcation analysis of the aircraft nonlinear dynamics,and wide robust controllers are designed to stabilize the unstable motions,hold the altitude and attitude during airdropping heavy cargoes and track short takeoff and landing trajectories robustly.The related simplified controller design methods are validated by freescale smart vehicle's autonomous driving test and ATUH-10 unmaned aerial vehicle(UAV)autonomous cruise test.The main work and innovations are as follows.Considering the difficulty in systematical prediction and suppression of the potential unstable high angle of attack longitudinal motions,a bifurcation analysis based unstable motions analysis and stabilization method is proposed for aircraft longitudinal motion with high angle of attack.Firstly,a methodology combining differential geometry,center manifold reduction principle,reverse time transformation and bifurcation theory is developed to identify the stationary bifurcation and dynamic bifurcation for aircraft longitudinal motion.Numerical simulations for F-8 longitudinal motion show the generation,development and disappearance for saddle node bifurcation and Hopf bifurcation with respect to the airspeed and tail deflection.Meanwhile,the characteristics of the almost-periodic unstable oscillations are observed around the Hopf bifurcation.To stabilize the unstable motions,a flight controller based on linear quadratic gaussian(LQG)and polynomial fitting is presented to stabilize F-8 longitudinal motion under the conditions of large variations of airspeed or large deviations of initial states from the trimmed points.To solve the prediction and suppression problem of the post-stall maneuvering caused three dimensional unstable motions,a compound method including complex unstable motion analysis and wide robust stabilizer design is proposed for the three dimensional aircraft motions with high angle of attack.Due to the exponential explosions of the complexity of the analytical bifurcation representation with respect to the system order,a numerical approach is presented for the bifurcation analysis of aircraft three dimensional motion with high angle of attack.Numerical studies for F/A-18 model demonstrate the relationships between the bifurcation diagram and the control surfaces.Meanwhile,certain unstable motions including falling leaf motion as well as inertial coupled jump behavior are observed around the generalized Hopf(GH)bifurcation and the Bogdanov Takens(BT)bifurcation,respectively.To guarantee the flight safety,a wide robust stabilizer for the unstable motions is investigated.Considering the parametric uncertainties for aerodynamics,sliding mode observers are presented to approximate the compound uncertainties within finite time.To avoid iterative calculation of the high order virtual command derivatives in back stepping scheme,series of second order command filters are introduced.Then the airspeed,the flight path angle and the ground track angle are stabilized by a compound controller synthesized by block diagonal back-stepping and adaptive PID control.The closed loop system achieves ultimately asymptotically bounded stability under weak uncertainties.Considering the flight safety problem of the transport plane at extremely low altitude suffering from severe threats,including the cargo's motion,ground effect and crosswind caused uncertain disturbances and airdropping heavy cargoes caused catastrophic seesaw effect,a safety controller design method is proposed for the transport plane airdropping heavy cargoes at extremely low altitude.Considering the variations of the center of gravity,moment of inertia and mass during the process of airdropping heavy cargoes at super low-altitude,a generic three dimensional airdropping model is established.Taking the crosswind and the ground effect into account,the model is converted into an airdropping state error dynamics including catastrophes and uncertainties.To protect flight safety,the three dimensional controller is proposed by employing forth-order compound approach and inserting robust terms.The virtual commands for position loop are given gradually to cause the errors of the position,the flight path angle and the attitude to propagate toward the inner loop.Subsequently,the control surface and the throttle setting are designed to eliminate the loop errors of position and airspeed.The closed loop system achieves ultimately asymptotically bounded stability under weak disturbances,such as different ground effect,airdropping mass,crosswind and so on.Aiming at achieving the tracking performance and robustness simultaneously during the aircraft autonomous short takeoff and landing,a wide robust controller for aircraft automatic short takeoff and landing is investigated.To make the aircraft without thrust vectoring system or large auxiliary lifting equipment achieve short takeoff and landing performance,the high angle of attack maneuvering trajectory commands are designed.By confining the input of the command filter,the flight states are subject to the flight safety constraints at low altitude.To guarantee the tracking performance under input and state constraints,anti-windup compensators with PID form are constructed by the input and state saturation errors.Synthesized with sliding mode observer,block diagonal back stepping and adaptive PID control,the autonomous taking-off and landing controller is proposed to counteract the compound uncertainties of the ground effect and wind shear within finite time.Taking the measurement noise and bias,unmodelled dynamics and uncertainty disturbances into account,the related controller design methods are validated by simulating the practical flight environment with F/A-18 mathematic models.Based on the outputs of GPS/IMU/Magnetic sensor and the kinematics equations,hybrid extended kalman filter(EKF)is employed to output the linear least squares estimations for all the flight states.By stimulating the control inputs,the aerodynamic model of F/A-18 is identified via recursive least squares estimation(RLSE)and hybrid EKF.Considering modeling errors and aerodynamic variations,four nonlinear disturbance observers with exponential convergent performance are introduced to approximate the compound uncertainties.With the estimations of the flight states and compound uncertainties,the wide robust flight controller is formulated to track the flight commands robustly.Finally,the test bench is constructed and mounted on a smart vehicle and an UAV subsequently to validate the simplified controller design methods via path following test.