Part B | 16.4
398 Part B Autonomous Ocean Vehicles, Subsystems and Control
the current, provided the vehicle can measure current
direction. Pole docks can also support a larger vertical aperture fairly easily by simply making the docking
segment of the pole long. This feature significantly
simplifies homing in the vertical plane. However, pole
docks have the disadvantage that the vehicle must carry
some sort of mechanism for attaching itself to the pole,
thus adding to vehicle size and weight, and possibly to
drag.
Although omnidirectional in the horizontal plane,
pole docks can be more complicated homing targets in
the vertical plane, despite their large capture aperture.
It is not simple to place a homing target like an acoustic
beacon or a light source in the middle of a pole. Singh
placed the homing beacon at the top of the docking
pole. In such a configuration, the AUV must home on
a location offset from the homing signal emitter. This
complicates vertical plane homing, as a simple deadahead homing strategy does not work. Options include
constraining vertical plane homing by pressure or altitude, or using a homing system that provides good
off-axis navigation.
Ensuring a pole dock stays vertical can be difficult
or impossible, depending on the structure hosting the
dock. For example, if the pole is part of a mooring, then
a current can introduce a tilt into the mooring and by
extension to the dock. If the dock is not upright, the
vertical offset between the pole and the homing target
introduces a horizontal offset for approach directions
perpendicular to the tilt. Further, a tilted dock can complicate engagement of the vehicle with the dock.
While pole docks simplify the AUV approach
phase, they complicate making power and communications links between the dock and the vehicle. The
ability of the vehicle to attach to the dock from any
direction means that any communication or power connection to the dock must either be omnidirectional,
or the vehicle and the dock must be aligned. In the
case of the Woods Hole Oceanographic Institution
(WHOI) dock, a mechanism nicknamed the juicer was
used to clamp the vehicle into place and align the
vehicle with the dock’s communications and power
transformer [16.5]. However, clamping the AUV to
the pole also creates potential problems. The clamped
AUV is potentially subject to rotating currents imposing a winding motion of the AUV around the mooring
axis. One could allow the AUV to twist the entire
mooring, or put some sort of electrical slip-ring somewhere between the mooring and the AUV, but such
additions increase system complexity and compromise
reliability.
16.4.3 Soft Docking Approaches
Soft docking methodologies use a low approach speed
during the final approach, and thus require a vehicle equipped to be controllable at low and no speed.
Motivations for soft docking include removing constraints on the nature of the docking structure, achieving
greater control of alignment of the vehicle and dock
during capture, and minimizing docking forces. For example, AUV servicing of subsea infrastructures may
require that the vehicle attach itself to a subsea control
panel [16.44] similar or identical to those designed for
servicing by an ROV. Thus the AUV must be able to approach the panel in a similar manner to a ROV. Another
motivation for soft docking is to allow simpler dock design. For example, some docking work has simply used
a cradle on which the AUV lands [16.43].
Soft docking implies a vehicle with control over
four or more degrees of freedom at zero and near zero
speed. For example, ROVs can control surge, heave,
sway, and yaw, but usually not roll or pitch. Thus,
soft-docking AUVs are typically multi-thruster vehicles. Examples of vehicles used for soft docking work
include the Australian Starfish and the SAAB Double
Eagle SAROV in Fig. 16.2.
16.4.4 Other Docking Mechanisms
In the methods described above, the vehicle is the active
element that achieves a mechanical connection between
the vehicle and the dock. At most, the dock actuates
mechanisms to engage to the vehicle after the initial connection has already been established. However, there is
a class of approaches that involves the dock actively
grappling the vehicle. Examples include a vehicle recovery scheme in which a towed vehicle homes and grapples with a free swimming AUV [16.45, 46]. An underice docking scheme [16.19, 47] involved a multi-step
process whereby an AUV (the Explorer) autonomously
homed to the vicinity of an ice-mounted dock and came
to rest on the under-ice surface, where a small ROV attached lines used to pull the AUV into the dock.
Yet another approach is discussed by Smith and
Kronen [16.37], who evaluated a tail-hook docking
system suitable for bottom mounting and full speed approaches. The tail hook deployed by the AUV contained
a puck at its end, which was captured by a slotted capture mechanism. The capture mechanism allowed entry
of the puck from any of four directions. Like the cone
and pole docks, the tail-hook concept in principle allows a vehicle to approach at full speed.
398 Part B Autonomous Ocean Vehicles, Subsystems and Control
the current, provided the vehicle can measure current
direction. Pole docks can also support a larger vertical aperture fairly easily by simply making the docking
segment of the pole long. This feature significantly
simplifies homing in the vertical plane. However, pole
docks have the disadvantage that the vehicle must carry
some sort of mechanism for attaching itself to the pole,
thus adding to vehicle size and weight, and possibly to
drag.
Although omnidirectional in the horizontal plane,
pole docks can be more complicated homing targets in
the vertical plane, despite their large capture aperture.
It is not simple to place a homing target like an acoustic
beacon or a light source in the middle of a pole. Singh
placed the homing beacon at the top of the docking
pole. In such a configuration, the AUV must home on
a location offset from the homing signal emitter. This
complicates vertical plane homing, as a simple deadahead homing strategy does not work. Options include
constraining vertical plane homing by pressure or altitude, or using a homing system that provides good
off-axis navigation.
Ensuring a pole dock stays vertical can be difficult
or impossible, depending on the structure hosting the
dock. For example, if the pole is part of a mooring, then
a current can introduce a tilt into the mooring and by
extension to the dock. If the dock is not upright, the
vertical offset between the pole and the homing target
introduces a horizontal offset for approach directions
perpendicular to the tilt. Further, a tilted dock can complicate engagement of the vehicle with the dock.
While pole docks simplify the AUV approach
phase, they complicate making power and communications links between the dock and the vehicle. The
ability of the vehicle to attach to the dock from any
direction means that any communication or power connection to the dock must either be omnidirectional,
or the vehicle and the dock must be aligned. In the
case of the Woods Hole Oceanographic Institution
(WHOI) dock, a mechanism nicknamed the juicer was
used to clamp the vehicle into place and align the
vehicle with the dock’s communications and power
transformer [16.5]. However, clamping the AUV to
the pole also creates potential problems. The clamped
AUV is potentially subject to rotating currents imposing a winding motion of the AUV around the mooring
axis. One could allow the AUV to twist the entire
mooring, or put some sort of electrical slip-ring somewhere between the mooring and the AUV, but such
additions increase system complexity and compromise
reliability.
16.4.3 Soft Docking Approaches
Soft docking methodologies use a low approach speed
during the final approach, and thus require a vehicle equipped to be controllable at low and no speed.
Motivations for soft docking include removing constraints on the nature of the docking structure, achieving
greater control of alignment of the vehicle and dock
during capture, and minimizing docking forces. For example, AUV servicing of subsea infrastructures may
require that the vehicle attach itself to a subsea control
panel [16.44] similar or identical to those designed for
servicing by an ROV. Thus the AUV must be able to approach the panel in a similar manner to a ROV. Another
motivation for soft docking is to allow simpler dock design. For example, some docking work has simply used
a cradle on which the AUV lands [16.43].
Soft docking implies a vehicle with control over
four or more degrees of freedom at zero and near zero
speed. For example, ROVs can control surge, heave,
sway, and yaw, but usually not roll or pitch. Thus,
soft-docking AUVs are typically multi-thruster vehicles. Examples of vehicles used for soft docking work
include the Australian Starfish and the SAAB Double
Eagle SAROV in Fig. 16.2.
16.4.4 Other Docking Mechanisms
In the methods described above, the vehicle is the active
element that achieves a mechanical connection between
the vehicle and the dock. At most, the dock actuates
mechanisms to engage to the vehicle after the initial connection has already been established. However, there is
a class of approaches that involves the dock actively
grappling the vehicle. Examples include a vehicle recovery scheme in which a towed vehicle homes and grapples with a free swimming AUV [16.45, 46]. An underice docking scheme [16.19, 47] involved a multi-step
process whereby an AUV (the Explorer) autonomously
homed to the vicinity of an ice-mounted dock and came
to rest on the under-ice surface, where a small ROV attached lines used to pull the AUV into the dock.
Yet another approach is discussed by Smith and
Kronen [16.37], who evaluated a tail-hook docking
system suitable for bottom mounting and full speed approaches. The tail hook deployed by the AUV contained
a puck at its end, which was captured by a slotted capture mechanism. The capture mechanism allowed entry
of the puck from any of four directions. Like the cone
and pole docks, the tail-hook concept in principle allows a vehicle to approach at full speed.