Abstract
One form of cooperative behavior for a group
of AUVs is to fly in formation while performing tasks.
A necessary component for formation-flying is that the
vehicles must sense their relative positions.
Assuming that each vehicle is capable of sensing its
inertial position using an acoustic long-baseline
ranging system, the relative vehicle position can be
determined by exchanging this data. The penalty for
this approach is that exchanging inertial position data
consumes communications bandwidth. Alternately,
relative position may be obtained by intercepting
acoustic ranging signals used to determine inertial
position, obviating the need for exchange of position
data. We explore the use of a two-hydrophone sensor
to measure relative heading of two vehicles in a
formation. It is assumed that a broad-band navigation
signal emanating from one vehicle is intercepted by
another vehicle containing the sensor. Relative
heading is extracted from the time delay between the
two hydrophones. Cross-correlation is used to
determine time delay. A model is proposed that
predicts stochastic precision and bias for the sensor.
For a fixed ranging waveform, precision and bias are
dependent upon signal-to-noise ratio, relative range
and relative heading. This dependence means that the
sensor will be most useful for certain combinations of
range and heading. Measurements were performed to
determine the precision of the two-hydrophone
arrangement as a relative heading sensor.
Simulations were used to explore the performance of
formation-flying controllers that employ the
two-hydrophone sensor. The controller used a
saturating linear output feedback control law to
simultaneously follow inertial waypoints and
maintained formation. The simulations showed that
this controller would be able to use relative heading
provided by the two-hydrophone sensor to maintain
formations in which the vehicles are approximately
abreast.
I. INTRODUCTION
Formation-flying is an elemental cooperative behavior
that may be used by groups of Autonomous Underwater
Vehicles (AUV’s) while performing tasks. A requirement
for maintaining formation is that each vehicle controller must
have some information about the relative positions of one or
more other vehicle(s) in the group. For example, it can be
shown that a follower can maintain formation with a leader if
the follower can sense the distance and heading to the
leader [1]. In addition, if the formation must follow a
specified path, at least one vehicle must have knowledge of
its inertial position. Relative position can be sensed by
exchanging the inertial position of each vehicle via an
acoustic page link [2], or by sensors that can determine the range
to another vehicle directly, like those hypothesized in [3].
Because of the severe constraints on acoustic bandwidth in
the underwater environment, it would be preferable for
vehicle controllers to obtain relative position data by
sensors that do not rely upon acoustic communications.
A hybrid control approach is to require that each vehicle
maintain formation and follow inertial waypoints
simultaneously [4]. Each vehicle obtains its position with
an acoustic Long BaseLine (LBL) sensor. The formation is
maintained by requiring a leader vehicle to broadcast its
position in parallel to the other vehicles in the formation via
an acoustic link. This control scheme has certain
advantages. It is adaptable to 1-D, 2-D and 3-D formations.
A formation can tolerate the loss of a leader, in that another
vehicle in the formation can replace the leader. In the case
of a total loss in communication, each vehicle can revert to
the fall-back case of independent navigation. A further
advantage is that the hybrid algorithm only requires that the
followers know the angular heading of the leader relative to
the follower, instead of both relative heading and distance.
In this paper, a potential approach to sense relative
vehicle position using the navigation ping(s) from another
vehicle in the formation is described. The sensor would
consist of two hydrophones, located at the bow and stern of
an AUV. With this arrangement, the hydrophones would
be separated by a distance of approximately 1 meter. A
navigation ping from another vehicle in the formation is
intercepted by the two hydrophones, and the hydrophone
voltage signals are used to determine the heading and
range to the vehicle that issued the navigation ping.
Heading would be determined by extracting the difference in
arrival time by cross-correlating the hydrophone signals,
and range would be determined by measuring the
difference in received amplitude. Presently, we focus on
the determination of relative heading and its application in
the hybrid formation controller.


Download full report
http://googleurl?sa=t&source=web&cd=1&ve...r_AUVs.pdf&ei=8iwxTrTgG6PTiAK1m4WSBg&usg=AFQjCNFVb3753AcudOlDijJx6FvZjamEBg