An optimal controller in Soft Walls is one which always pushes away from the nofly zone. To blend th

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Adam Cataldo Edward Lee Shankar Sastry
Soft Walls
http//softwalls.eecs.berkeley.edu
Introduction Soft Walls is a technological
response to the September 11, 2001 hijackings.
The Soft Walls strategy is to store a 3D database
of no-fly zones, or restricted airspace,
on-board each aircraft and enforce these no-fly
zones using an avionics control system. Each
aircraft will have its own Soft Walls system.
Also, the database will require a digital
signature to update the no-fly zones so that the
system is non-hackable.

• Objections to Soft Walls
• Reducing pilot authority is dangerous.
• Pilots need the authority to respond to
  emergencies, including unexpected weather
  conditions, possible collisions with other
  aircraft or other obstacles, turbulence, on-board
  equipment failures, fires, or other problems. A
  pilots responsibility, however, extends beyond
  the craft, crew and passengers to the people on
  the ground. No on-board emergency is severe
  enough to justify endangering large numbers of
  people on ground.
• The crew should have an override.
• The surest way to make the Soft Walls system
  effective is to prohibit override in any form.
  Manual override on the aircraft is certainly out
  of the question.
• GPS is vulnerable to attacks.
• GPS signals are vulnerable to jamming, where a
  malicious party transmits a radio signal that
  swamps the one of interest, making it impossible
  to receive reliably, and spoofing, where a
  malicious party transmits a radio signal that
  masquerades as the radio signal of interest,
  hoping that it will be picked up instead of the
  legitimate signal. GPS signals currently contain
  encrypted channels that make spoofing extremely
  difficult. Since GPS cannot be spoofed, jamming
  can be reliably detected.
• The Research Problem
• Soft Walls is an example of a collision avoidance
  problem, where a collision occurs if the
  aircraft enters the no-fly zone, and we try to
  design a controller which will prevent
  collisions. Safety is critical in this
  application, so we must be able to prove that no
  collision can occur. For any controller, there
  may be some states for which we cannot prevent a
  collision. We must also be able to identify these
  dangerous states.

Previous Results For simple models of the
aircraft dynamics, we have applied analytical
methods to derive provably safe control laws, as
well as numerical which approximate provably safe
control laws.
A video game simulation of Soft Walls using
Ptolemy II
Hybrid System Challenges
Design Objectives The Soft Walls controller does
not remove pilot input when the aircraft
approaches a no-fly zone. Instead the controller
adds a bias to the pilot input. A pilot who
approaches a no-fly zone and holds steady will be
turned away from the no-fly zone until it is safe
to let the aircraft fly straight. A pilot who
chooses to turn away faster can do so. A pilot
who tries to fly into the no-fly zone will be
unsuccessful. Through this, Soft Walls will
maximize pilot authority subject to the
constraint that no-fly zones are enforced. This
will give the pilot more maneuverability in an
emergency.
An optimal controller in Soft Walls is one
which always pushes away from the no-fly zone.
To blend the control in gradually, we would like
to increase the control bias from zero to the
optimal bias when as the aircraft approaches the
no-fly zone. This can be dangerous a pilot who
fights the control input while it is between zero
and the maximum bias can move the aircraft to a
position where the optimal bias suddenly changes
direction. This can cause chatter along a
switching boundary, and may destabilize the
aircraft.
Pilot holds steady
In this particular case, we can add states to
ensure that this switching never occurs, while at
the same time guaranteeing that collision
avoidance. The end result is a smooth control
law which guarantees safety, without much
additional complexity added to the controller. An
open research question is how to generalize this
to other collision avoidance systems. Another
important hybrid systems concept in Soft Walls is
that of abstraction. That is, can we abstract
our system model into a form over which it is
easier to design a safe control law?
Pilot turns away from no-fly zone
Pilot tires to fly into no-fly zone
Control Applied
September 30, 2009