Preventing String Instability in Altitude Control System of UAV by Nonlinear Correction

The paper is devoted to maintaining a flight formation with cross-information links between agents while controlling the altitude of nine unmanned aerial vehicles through a standard proportional-integral-differential controller. With an increase in the number of agents, the system oscillations may appear due to the "string instability" phenomenon. Other factors associated with changes in the dynamics of the nonlinear actuator of the UAV control surface can also negatively affect the formation stability. The simulation results show that the actuator saturation for one agent can lead to oscillatory processes of other agents, which can eventually destroy the formation structure. In the paper, it is proposed to introduce a nonlinear corrective device into the actuator control loop to improve the system stability and prevent dangerous oscillations. In the nominal mode, when the saturations of actuators are not active, this corrective device does not affect the operation of the controller and passes the control signal without changes, but it makes an effect when the controlling values of actuator positions and velocities reach the bounds. In the paper, for a flight altitude control problem of the UAVs formation, the corrective device was tested and its efficiency was confirmed by a series of simulations.