Autonomous Underwater Vehicle Docking 16.4 Capture and Connection Mechanisms 395
Part B | 16.4
is ambiguous. To resolve this ambiguity, an additional
dipole source was introduced to transmit a timing signal, allowing synchronization of the homing signal. The
source coil for the timing dipole field was oriented vertically at the dock, so that a vehicle approaching in the
plane of the dock would see a vertically oriented timing magnetic field. The homing dipole was oriented so
that its axis aligned with the axis of the docking funnel. The vertical source coil generated a dipole timing
field for the AUV oscillating at 1 kHz, while the homing
dipole field oscillated at 2 kHz. The two signals were
locked such that a frequency doubled signal generated
by the timing field would be synchronized to the homing dipole field.
A three-axis magnetic sensor on an Odyssey IIb
AUV was employed to detect and home on magnetic
signals generated by the dipole fields. On the AUV,
a vertical detection coil was used to measure the magnetic field generated by the vertical source dipole,
following which a phased-lock-loop captured the phase
information in the timing signal. The frequency-double
output of the phase-locked-loop provided a reference
for a lock-in amplifier style detection of the homing
signal. The provision of a reference signal, generated
from the timing signal, allowed determining both the
amplitude and the direction of the homing field. Because the AUV had a three-axis sensor, the homing field
could be completely resolved in the coordinate frame of
the AUV. The AUV homing algorithm commanded the
vehicle heading to align the vehicle with the homing
field. In effect, the AUV followed the field lines into
the dock. Tests of this system on Odyssey II AUVs in
Buzzards Bay, Massachusetts, were highly successful
with the caveat that the vehicle needed to approach to
a range of approximately 30 m to detect the magnetic
dipole fields.
16.3.4 Constraining Vertical Position
Homing can be reduced to a two-dimensional horizontal homing problem if the vehicle knows the vertical
location of the dock. This can be particularly attractive
when there is the potential that the homing signal may
suffer from degradation in the vertical plane, for example, when the dock is near a reflecting surface such as
the sea surface or seafloor. In general, the vehicle will
have at least a rough knowledge of the dock depth, for
example, that it is near the seafloor or in a particular
depth range in the water column. However, to use depth
as a cue for the terminal phase of homing requires the
vehicle to know the dock depth to an accuracy better
than the vertical capture aperture of the docking system.
Pressure can be used as a cue for vertical plane homing, effectively allowing homing at a constant depth,
if the vehicle knows the depth of the dock. However,
pressure as a cue can be complicated by factors such as
tidal variations in water depth, changing atmospheric
pressure at the surface, inaccuracies and hysteresis in
pressure sensors, and changes in the density of the water column. For example, tidal variability can cause the
depth of the dock to change as local sea level rises
and fall if the dock is secured relative to the seafloor.
Environmental factors can be mitigated by providing
a method for the dock to communicate its pressure
depth to the vehicle, but at the cost of added complexity.
Altitude is a particularly robust vertical plane cue if
the dock is mounted in a way that keeps its height over
the seafloor constant. For example, the docking system
in Fig. 16.1 uses a rigid tripod, holding the dock a constant 3 m over the seafloor. For this system, the AUV
uses its altimeter to maintain a constant 3 m altitude during the final homing phase, resulting in highly accurate
homing in the vertical plane.
16.4 Capture and Connection Mechanisms
Capture is the process whereby the AUV becomes physically attached to the dock. Connection describes the
activity of securing the vehicle and making any links,
for example, for power or communications. Typically,
the capture mechanism guides the vehicle into alignment, brings the vehicle into its final docked position,
and latches the vehicle and dock together to ensure that
the vehicle will stay connected. The connection mechanisms establish electrical or optical connection between
the vehicle and the dock, and may secure the vehicle
within an enclosure that protects the vehicle against the
environment while docked.
Chief among the goals associated with design of
the capture mechanism is to simplify the homing problem. Considerations include providing a large capture
aperture in the horizontal and vertical planes, and reducing constraints on approach direction and attitude.
This is simplest to understand in the context of a dock
shaped like a funnel, into which the vehicle will attempt to drive. If the funnel is very small, then both
homing sensors and vehicle control must be very accurate to ensure that the vehicle impacts the interior
of the funnel. The funnel also requires that the vehicle approach the mouth of the funnel along the axis of
the funnel. Thus, the funnel dock requires the vehicle
to know the orientation of the funnel and be able to
achieve an orientation to make a final approach along
the funnel axis.
Part B | 16.4
is ambiguous. To resolve this ambiguity, an additional
dipole source was introduced to transmit a timing signal, allowing synchronization of the homing signal. The
source coil for the timing dipole field was oriented vertically at the dock, so that a vehicle approaching in the
plane of the dock would see a vertically oriented timing magnetic field. The homing dipole was oriented so
that its axis aligned with the axis of the docking funnel. The vertical source coil generated a dipole timing
field for the AUV oscillating at 1 kHz, while the homing
dipole field oscillated at 2 kHz. The two signals were
locked such that a frequency doubled signal generated
by the timing field would be synchronized to the homing dipole field.
A three-axis magnetic sensor on an Odyssey IIb
AUV was employed to detect and home on magnetic
signals generated by the dipole fields. On the AUV,
a vertical detection coil was used to measure the magnetic field generated by the vertical source dipole,
following which a phased-lock-loop captured the phase
information in the timing signal. The frequency-double
output of the phase-locked-loop provided a reference
for a lock-in amplifier style detection of the homing
signal. The provision of a reference signal, generated
from the timing signal, allowed determining both the
amplitude and the direction of the homing field. Because the AUV had a three-axis sensor, the homing field
could be completely resolved in the coordinate frame of
the AUV. The AUV homing algorithm commanded the
vehicle heading to align the vehicle with the homing
field. In effect, the AUV followed the field lines into
the dock. Tests of this system on Odyssey II AUVs in
Buzzards Bay, Massachusetts, were highly successful
with the caveat that the vehicle needed to approach to
a range of approximately 30 m to detect the magnetic
dipole fields.
16.3.4 Constraining Vertical Position
Homing can be reduced to a two-dimensional horizontal homing problem if the vehicle knows the vertical
location of the dock. This can be particularly attractive
when there is the potential that the homing signal may
suffer from degradation in the vertical plane, for example, when the dock is near a reflecting surface such as
the sea surface or seafloor. In general, the vehicle will
have at least a rough knowledge of the dock depth, for
example, that it is near the seafloor or in a particular
depth range in the water column. However, to use depth
as a cue for the terminal phase of homing requires the
vehicle to know the dock depth to an accuracy better
than the vertical capture aperture of the docking system.
Pressure can be used as a cue for vertical plane homing, effectively allowing homing at a constant depth,
if the vehicle knows the depth of the dock. However,
pressure as a cue can be complicated by factors such as
tidal variations in water depth, changing atmospheric
pressure at the surface, inaccuracies and hysteresis in
pressure sensors, and changes in the density of the water column. For example, tidal variability can cause the
depth of the dock to change as local sea level rises
and fall if the dock is secured relative to the seafloor.
Environmental factors can be mitigated by providing
a method for the dock to communicate its pressure
depth to the vehicle, but at the cost of added complexity.
Altitude is a particularly robust vertical plane cue if
the dock is mounted in a way that keeps its height over
the seafloor constant. For example, the docking system
in Fig. 16.1 uses a rigid tripod, holding the dock a constant 3 m over the seafloor. For this system, the AUV
uses its altimeter to maintain a constant 3 m altitude during the final homing phase, resulting in highly accurate
homing in the vertical plane.
16.4 Capture and Connection Mechanisms
Capture is the process whereby the AUV becomes physically attached to the dock. Connection describes the
activity of securing the vehicle and making any links,
for example, for power or communications. Typically,
the capture mechanism guides the vehicle into alignment, brings the vehicle into its final docked position,
and latches the vehicle and dock together to ensure that
the vehicle will stay connected. The connection mechanisms establish electrical or optical connection between
the vehicle and the dock, and may secure the vehicle
within an enclosure that protects the vehicle against the
environment while docked.
Chief among the goals associated with design of
the capture mechanism is to simplify the homing problem. Considerations include providing a large capture
aperture in the horizontal and vertical planes, and reducing constraints on approach direction and attitude.
This is simplest to understand in the context of a dock
shaped like a funnel, into which the vehicle will attempt to drive. If the funnel is very small, then both
homing sensors and vehicle control must be very accurate to ensure that the vehicle impacts the interior
of the funnel. The funnel also requires that the vehicle approach the mouth of the funnel along the axis of
the funnel. Thus, the funnel dock requires the vehicle
to know the orientation of the funnel and be able to
achieve an orientation to make a final approach along
the funnel axis.