12.6. AUTOMATIC LIMIT AVOIDANCE FOR UAVs

Figure 12.9 is a block diagram representation of the integration of the limit
detection and limit avoidance algorithms with the low-level controller. The
limit detection block receives the current values of the slow states and the
current control inputs, which are fed into the limit detection algorithms.
The neural net predictions, which lead the actual response, are used to
calculate control margin vector (for details, see reference 17).

The control margin vector (Δδ) provides the maximum / minimum allowable
control input travel before a limit is encountered. The maximum and
minimum allowable control inputs therefore are computed using the limit
avoidance algorithm:

can be viewed as the upper and lower bounds of
‘‘control soft limits,’’ in order to protect the vehicle from exceeding a
performance limit. In order to reduce the undesirable effects associated with
control saturation in the context of an adaptive controller, a technique called
‘‘pseudo-control hedging’’ is implemented. The pseudo-control hedging avoids
the false adaptation that could result from artificially ‘‘saturating’’ the controls,
while protecting the vehicle from exceeding a limit. In order to prevent

wrong adaptation, the reference model, labeled as command filter in Figure
12.9, is ‘‘hedged,’’ so that the adaptive law would not see these system
characteristics [23].

The performance of the automatic limit avoidance algorithms is tested in
reference 23 through a series of simulations using the TMAN helicopter
model. High-g pull-up and push-over maneuver simulations are used to
demonstrate satisfactory performance of the automatic limit avoidance system.
In the example presented here, the unmanned helicopter is commanded
to perform high-g push-up and pull-over maneuvers. The controller is in rate
command controller mode. It receives a pitch rate command of 0.4 rad/s at
85 knots forward flight. Figure 12.10 shows the variation of the load factor

response with and without limit avoidance. The load factor limits for this
aircraft were set as + 2g and - 0.5g. The load factor clearly exceeds the
prescribed limit boundaries for this maneuver without a limit avoidance
system. However, with the limit avoidance system, the actuator commands
are modified, forcing the vehicle to stay within the limit boundaries.