Navigation Functionalities for an Autonomous UAV Helicopter

50 CHAPTER 4. PATH FOLLOWING CONTROL MODE
Y AWyacs = rff + K y
1 δψ
P IT CHyacs = K p
1 δpx + K p
2 δvx + K p
3
d
dt δvx + K p
�
4
δvxdt
ROLLyacs = φff + K r 1δpy + K r 2δvy (4.24)
T HRyacs = K t 1δpz + K t 2δvz + K t �
3 δvzdt
where the K’s are the control gains, rff and φff are the feed-forward control
terms resulting from the model in 4.13. The other two terms θff and Tff
relative to the pitch and throttle channels have not been implemented.
These channels are controlled by the feedback loop only. δψ is the heading
error, δp is the position error vector (difference between the control point
and helicopter position), δv is the velocity error vector (difference between
target velocity and helicopter velocity).
Adding the feed forward control terms, especially on the roll channel,
results in a great improvement in the tracking capability of the control system
compared to a PID feedback loop only. Results of the control approach
are shown in the next section.
4.4 Experimental results
This section presents experimental results of the PFCM implemented on the
RMAX helicopter. In Fig. 4.9, a 3D path is flown starting from an altitude
of 40 meters and finishing at 10 meters. The path describes a descending
spiral and the velocity vcruise given by the path planner was set to 10 m/s.
In Fig. 4.10, the velocity profile of the path is represented and it can be
observed that as soon as the helicopter reaches 10 m/s it slows down in
order to make the turn with the compatible velocity. This was an early
test, where the roll angle limitation was quite strict (around 8 deg). This
explains the consistent decreasing velocity. In addition, the acceleration
phase was missing. In fact, the commanded target velocity, represented by
the dashed line, starts at 10 m/s. This resulted in an abrupt pitch input at
the beginning of the flight. Results of several trials of the same path flown
with different wind conditions are shown in Paper I.