Abstract:
The octorotor is an unmanned VTOL capable vehicle with eight motors with xed
pitch rotors. It is controlled by varying the speeds of its eight motors which are
placed around the vehicle. There is no need for a complex swashplate system, making
the vehicle low cost and dynamically simple. The increase in the number of e ectors
over the quadrotor allows for inbuilt hardware redundancy. It is this redundancy
which is of particular interest as the capabilities and applications of VTOL capable
UAVs increases and the payloads become more expensive and sensitive. It would be
unacceptable for a hardware failure to result in the loss of the vehicle and payload,
especially if operating in close proximity to people.
An operational requirement is that the operator must be able to control the vehicle's
position and yaw angle. Position reference commands are generated in an inertial
frame and these must be related to the vehicle- xed frame through a rotation matrix.
The downfall of this method is that trigonometric singularities exist for large
body angles where gimbal lock can occur. For this reason the unit quaternion attitude
representation is used. The octorotor is not open-loop stable so a PD controller
is used to provide for singularity-free, almost global asymptotic stability which is
capable of following
ightpaths as well as recovering from an initial inverted attitude.
The output of the controller is called the virtual control since this demand
is passed to the control allocation subsystem where the overall forces and moments
are generated.
A suitable control allocation method is needed since there are more e ectors than
actuated degrees of freedom. The e ectors are assumed to be linear and various
methods are used to provide constrained control allocation. If the virtual control
is constrained then the allocation problem is always the unconstrained allocation
problem and is guaranteed to be successful. By applying the constraints directly to
the e ectors it is not necessary to use complex face searching algorithms to calculate
the constrained virtual control.
An objective of this thesis is to ensure that e ector failures do not a ect the vehicle's
ight performance. This is integral to the aim of demonstrating that the hardware
redundancy is su cient to allow
ights over populated areas. E ector failures are
modelled as an instantaneous loss of thrust from an e ector. This causes an adverse
roll, pitch, and yaw disturbance as well as a drop in altitude. The recovery is
based on the fault hiding method where the virtual control remains invariant from the nominal case and the fault is hidden in the plant. If none of the remaining
e ectors are saturated then the failure-free performance is maintained and the operator
should not notice any change to the vehicle handling. Kalman controllability
analysis is done to determine the combinations of e ector failures which result in a
controllable vehicle.
Flight testing has demonstrated the suitability of the controller to the task of stabilising
the vehicle. The failure scenarios are initialised before the
ight and the
performance is invariant to the health of the e ectors. The reasons for di erences
between the simulation data and
ight data are explained. Future work will implement
an online fault detection scheme.