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The Development of Neural Network Based System Identification ...

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190 CHAPTER 7 FLIGHT CONTROL SYSTEM DESIGN: RESULTS AND DISCUSSION<br />

Table 7.3<br />

<strong>The</strong> control performance comparison with various r w values.<br />

Tuning parameter MSE Settling Time (s) Overshoot (%) Rise Time (s)<br />

θ<br />

φ<br />

r w = 1.0 1.9219 unstable unstable unstable<br />

r w = 1.5 0.1934 1.32 23.02 0.36<br />

r w = 2.0 0.1975 3.80 47.45 0.59<br />

r w = 1.0 10.26 n/a n/a n/a<br />

r w = 1.5 0.07 n/a n/a n/a<br />

r w = 2.0 0.75 n/a n/a n/a<br />

action is penalised too much in the cost function resulting in higher overshoot value.<br />

Next, the controllers’ performance is tested under the effect <strong>of</strong> prediction horizon N p<br />

variation. <strong>The</strong> experiment procedures for this test are similar to the previous experiment<br />

procedures. <strong>The</strong> only difference is that the test is performed under active roll, pitch<br />

and yaw controllers while the altitude channel is under manual control. Figure 7.3 and<br />

7.4 show the results for comparing the controller performances under different values <strong>of</strong><br />

N p . Both <strong>of</strong> the figures show the controller performance results for roll, pitch and yaw<br />

channels. <strong>The</strong> control penalty factor <strong>of</strong> r w = 1.5 is used in each <strong>of</strong> MPC controller setup.<br />

<strong>The</strong> same input constraints are used for roll and pitch controller as in the previous test,<br />

whereas, the constraint on the yaw rate controller is set up based on the rate <strong>of</strong> the<br />

control input, −0.015 ≤ ∆u(k) ≤ 0.015.<br />

Since the RC helicopter dynamics is unstable, several stability augmentation devices<br />

such as the mechanical stabiliser bar and active yaw rate feedback controller have been<br />

introduced to the RC helicopter system to enable human pilots to control the RC<br />

helicopter with ease. <strong>The</strong> stabiliser bar acts as a rate lagged feedback mechanism that<br />

dampens the roll and pitch control sensitivity due to the lateral and longitudinal cyclic<br />

inputs [Mettler, 2003]. <strong>The</strong> feedback mechanism would ease the manual control <strong>of</strong> the<br />

helicopter and at the same time reduce the destabilising effect on the helicopter due to<br />

wind gust or turbulence.<br />

In addition, the yaw rate feedback controller and a gyro sensor are also included<br />

in all RC helicopter models to improve the stabilisation <strong>of</strong> the helicopter in the yaw<br />

axis. Figure 7.5 shows the illustration <strong>of</strong> the link between the yaw dynamics <strong>of</strong> the<br />

helicopter and the component <strong>of</strong> the stability augmentation systems. Ideally, the yaw

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