The Development of Neural Network Based System Identification ...

4 CHAPTER 1 INTRODUCTION
is then designed for each different flight condition such as hovering flight or forward
flight at different velocities. Since we have multiple controllers that provide satisfactory
control for different operating points, gain scheduling approach is used to determine
the current flight operating region and to activate the appropriate linear controller.
Several measured variables from the on-board instrumentation such as airspeed, dynamic
pressure or altitude can be used to trigger a specific linear controller relative to the
current flight operating region. However, such control technique suffers performance
degradation when performing large amplitude manoeuvres [Mettler, 2003, Kendoul,
2012, Valavanis, 2007]. Furthermore, a scheduling control method can results in a
control signal that jumps to different values at each model transition. This effect needs
to be reduced in practice as the sudden increase in the control action can result in
dangerous sudden movement of the helicopter [Joelianto et al., 2011]. In addition to
limitation of linear approaches, the control problem becomes much more challenging
to solve as the operation of the helicopter also exposes them to varying atmospheric
disturbance.
Numerous advanced non-linear controllers such as feedback linearisation, adaptive
control and non-linear model based approaches have been suggested in the literature to
overcome the limitation of linear approaches with successful implementation in real flight
tests [Kendoul, 2012, Cai et al., 2010]. Kendoul [2012] further argues that even though
numerous advanced non-linear control approaches were proposed to improve the AFCS
performance, significant improvement in flying capabilities has not been achieved with
the non-linear controllers when compared with standard linear controllers. This is due to
the fact that the linear controllers such as the widely used PID, LQR and H ∞ methods
are robust and mature enough for the helicopter based control application. However,
Kendoul [2012] in his review proposed that certain aspects of AFCS development can be
further investigated and improved. The lists of AFCS developments that are required
for further improvement are given as follows:
• The development of a general purpose, flexible and self-tunable flight controller
that can be deployed into different UAV airframes in a shorter development time.
Such a controller design should include an adaptation mechanism to the internal