The Development of Neural Network Based System Identification ...

188 CHAPTER 7 FLIGHT CONTROL SYSTEM DESIGN: RESULTS AND DISCUSSION
The response of the NNAPC controller with the recursive NN training is studied
under the variation of r w values. The NNAPC controller validation test is made under
the coupled pitch-roll controller action. First, the helicopter is operated manually with
the yaw axis servomechanism locked. The throttle is then increased gradually to the
value of 0.25 as the initial rotor speed (Full transmitter’s stick deflection is 1). Next,
the controller action in pitch and roll is activated after the main rotor gains sufficient
rotation speed. During this stage, the roll-pitch controller drives the helicopter to level
orientation in the roll and pitch channels respectively. All other control channels such
as the yaw and altitude channels are under manual control. The yaw servomechanism
is then released after the straight and level condition is achieved. The throttle is then
increased to 0.60 causing a small increase in the altitude channel. Next, a step response
in pitch channel is activated, stepping up from 0 ◦ to 10 ◦ (positive pitch indicates nose
down) while maintaining level orientation in the roll channel. The constraints on the
amplitude of lateral and longitudinal cyclic inputs are set between −1 ≤ u(k) ≤ 1,
while the prediction horizon length N p is set to a sufficiently long value at 10. The step
response is maintained for 275 samples, which equals approximately 9 s at a sample
period time of 33 ms.
Figure 7.2 shows the result for comparing different values of r w with the summary
of controller performance given in Table 7.3. The MSE value is calculated in the period
after the steady state value has been obtained. The settling time is calculated when
the response settles within the 15 % settling time threshold, starting after initiation of
the step response. The overshoot value is given as the percentage of the highest peak
compared with the settling point, and the rise time is determined as the time taken by
the response signal to move between 10 % and 90 % of the step value. The result shows
that r w = 1.5 produces the best response compared with other r w values. When r w is
assigned with a low value, r w = 1, the rate of change of the control signal u is not highly
penalised in the cost function which results in an active controller response as can be
seen in the results. The MSE for r w = 1 is high compared with other values of r w in
both dynamic channels signalling that the controller produces an unstable response with
poor compensation performance. Contrary to this, setting r w = 2 means that controller