  Stabilization and trajectory control of a quadrotor carrying a suspended load
with a fixed known mass has been extensively studied in recent years. However,
the load mass is not always known beforehand or may vary during the practical
transportations. This mass uncertainty brings uncertain disturbances to the
quadrotor system, causing existing controllers to have worse stability and
trajectory tracking performance. To improve the quadrotor stability and
trajectory tracking capability in this situation, we fully investigate the
impacts of the uncertain load mass on the quadrotor. By comparing the
performances of three different controllers -- the proportional-derivative (PD)
controller, the sliding mode controller (SMC), and the model predictive
controller (MPC) -- stabilization rather than trajectory tracking error is
proved to be the main influence in the load mass uncertainty. A critical motion
mass exists for the quadrotor to maintain a desired transportation performance.
Moreover, simulation results verify that a controller with strong robustness
against disturbances is a good choice for practical applications.
