The Development of Neural Network Based System Identification ...

2.4 AUTOMATIC FLIGHT CONTROL SYSTEM 51
approach taken by the above-mentioned approaches. Norgaard [2000] proposes that the
high computation effort involving MPC implementation with non-linear NNARX model
can be reduced with the so-called ‘instantaneous linearisation’ technique. This technique
is employed to extract the linear transfer function models at every time step from the
non-linear NNARX model. No modifications are made to the NNARX model structure,
in contrast to the approach taken by Kuure-Kinsey et al. [2006a]. Similar to abovementioned
approaches, the extraction process of the linear models from the NNARX
model enables the execution of linear MPC strategy, which is simpler in terms of finding
the minimum of the controller’s optimisation criterion. The extracted linear models
from the NNARX can also be applied for various linear controller techniques such as
pole placement method and minimum variance design [Norgaard, 2000, Ab Wahab et al.,
2009]. Furthermore, the usage of the instantaneous linearisation principle provides more
valuable information about the dynamics of the system compared with the black-box
nature of the NNARX model.
The transfer function of MPC introduced in Norgaard [2000] is often regarded to be
less effective in handling multi-variable dynamic processes [Wang, 2009a]. Transformation
of the transfer function form into a non-minimal state space (NMSS) formulation
should improve the proposed NN-MPC formulation in terms of simpler design framework
and analysis. Furthermore, the NMSS form still retains the basic features of the transfer
function model which only requires state variables chosen from the set of measured
outputs, inputs and their time lag values. This modification to original transfer function
form can avoid the use of an observer to access the state information.
Since the dynamic of the helicopter based UAS is part of a highly complex system
with strong non-linearity, designing an automatic flight control for such a dynamic
system is difficult and presents significant challenges. Moreover, the dynamic system of
the helicopter is also known to be coupled and time varying, which requires necessary
compensation through the use of adaptive type controllers. Numerous types of direct
adaptive NN controllers have been reported in the literature with the majority of
the control techniques being only validated in numerical simulations. More extensive
experiments need to be performed to evaluate the effectiveness of these approaches in a