Fault Tolerant Margins for Unmanned Aerial Vehicle Flight Safety

Consider an unmanned aerial vehicle (UAV) operation from the time it is launched until the time it is put into an autonomous flight control mode. The control input u(t) during this time duration is modeled sigma u(t) and assumed healthy with sigma = 1. In practice, however, the control inputs are less effective with a sigma-value in the interval [0, 1) (Fan et al. 2012; Jayakumar and Das 2006; Hu et al. J. Guid. Control. Dyn. 34(3), 927-932, 2011). Complete damaged condition is inferred from sigma = 0. Given a stabilizing controller, a range of sigma-values in the interval [0,1) for which the closed loop system would remain stable is referred as the fault-tolerant margin (FTM). Flight operations with control effectiveness beyond the FTM are catastrophic. Further, when aerodynamic parameter variations due to an uncertain atmospheric condition in an UAV are present, it is shown that the FTMs are extremely sensitive to these parameter perturbations. In this paper, the FTMs are presented. The effects of control effectiveness factor on the nonlinear UAV when it is in the stable range are investigated. An UAV model in Ashokkumar and York (2016) is considered to illustrate the FTMs as a threshold to guarantee the safe operation.