Part B | 16.6
400 Part B Autonomous Ocean Vehicles, Subsystems and Control
16.5.2 Power Transfer
The primary objective of many docking scenarios is
to extend AUV deployment times by recharging AUV
batteries. Consequently the ability to transfer power
through the dock to the AUV is a key feature of many
docks. Being able to recharge batteries quickly allows a higher operational tempo of the AUV, and thus
the power capacity of the dock is a critical characteristic. The total energy capacity of a smaller AUV
might be only a few kilowatt-hours, while larger 21
00
diameter AUVs might have battery packs with capacities on the order of 10 kilowatt-hours or more. Other
considerations include reliability, alignment, mating
force, complexity, power capacity, efficiency, size and
weight.
There are a number of ways to establish a power
connection between the vehicle and the dock. These
include underwater mateable electrical connectors, inductive electrical coupling, transfer of material, or mechanical coupling. Docking systems to date use the
first two approaches, direct electrical connection or
inductive coupling. In principle, transfer of material
might include physically swapping batteries or fueling
chemical reactants. These might be attractive for faster
recharge of a vehicle, but have not been implemented.
Mechanical coupling might be as simple as letting the
vehicle’s propeller windmill in a current, and using the
resulting power generated by the spinning propulsion
motor to charge the battery. No mechanical power transfer methods have been reported in the literature.
Wet mateable electrical connections are conceptually simple, but require careful alignment, significant
mating forces, and can have a limited lifetime. A survey of six wet mateable connectors [16.50] reports
mating forces ranging from approximately 755 N per
circuit. Most connectors were rated for 100 mate cycles,
with two commercial connectors claiming greater than
a thousand cycles. A unique wet mateable connector described by Stokey et al. [16.8] takes advantage of the
fact that only two connections are required. The connector establishes a two-contact electrical connection
between the vehicle and the dock, but only insulates one
of those connections from seawater. Electrical isolation
of the power system from the vehicle hull prevents anodic dissolution of the hull.
Inductive power transfer systems have the advantage of no exposed electrical contacts and can be designed to be less sensitive to alignment, and have low
mating force. Reliability is typically limited by the lifetime of the electronics associated with the inductive
process, rather than the number of mate/demate cycles. Power capacity and size and weight are related,
as efficient high power transfer requires a larger inductor, and the high-permeability materials that make high
efficiency power coupling possible are quite dense. Systems developed by Feezor et al. [16.48] and Granger
et al. [16.51] used inductive coupling for both communication and power transfer. The Florida Atlantic
University (FAU) inductive power system developed by
Coulson et al. [16.52] was used for the Dorado docking
system.
Autonomous recharging requires a sophisticated
battery management system that can maximize energy
stored in the batteries while simultaneously minimizing
battery failures [16.53]. Examples of battery failures include the destruction of the advanced swimmer delivery
system by a battery fire. Lithium-ion battery fires on the
Boeing 787 grounded that aircraft’s commercial operations for months. In the undersea environment, where
batteries might be in a pressure vessel, a battery fire is
a catastrophic event.
16.6 AUV Control Considerations
In most AUV control systems, AUV control is segregated by the level of abstraction. At the lowest level,
dynamic control systems command actuators to achieve
desired orientation, speed, and position. At a higher
level, algorithms attempt to accomplish a particular task
by commanding the dynamic control system. Above
the task level, software monitors the state of tasks,
and manages sequencing of activities. While a variety of software architectures are used on operational
AUVs, most conform to some semblance of this hierarchy. In principle, even more capable systems might
include yet another level that could create plans to
achieve user defined goals, and replan when circumstances change; however, in practice this level of autonomy has not been used in docking AUVs thus
far.
16.6.1 Navigation, Currents,
and Arriving at the Dock
Control algorithms for bringing an AUV to a dock
have diverse levels of sophistication, depending on
the demands of the docking process. Perhaps the
simplest are pure pursuit algorithms, which simply
point the vehicle directly at the homing target. These
are appropriate for docking systems that can be ap-
400 Part B Autonomous Ocean Vehicles, Subsystems and Control
16.5.2 Power Transfer
The primary objective of many docking scenarios is
to extend AUV deployment times by recharging AUV
batteries. Consequently the ability to transfer power
through the dock to the AUV is a key feature of many
docks. Being able to recharge batteries quickly allows a higher operational tempo of the AUV, and thus
the power capacity of the dock is a critical characteristic. The total energy capacity of a smaller AUV
might be only a few kilowatt-hours, while larger 21
00
diameter AUVs might have battery packs with capacities on the order of 10 kilowatt-hours or more. Other
considerations include reliability, alignment, mating
force, complexity, power capacity, efficiency, size and
weight.
There are a number of ways to establish a power
connection between the vehicle and the dock. These
include underwater mateable electrical connectors, inductive electrical coupling, transfer of material, or mechanical coupling. Docking systems to date use the
first two approaches, direct electrical connection or
inductive coupling. In principle, transfer of material
might include physically swapping batteries or fueling
chemical reactants. These might be attractive for faster
recharge of a vehicle, but have not been implemented.
Mechanical coupling might be as simple as letting the
vehicle’s propeller windmill in a current, and using the
resulting power generated by the spinning propulsion
motor to charge the battery. No mechanical power transfer methods have been reported in the literature.
Wet mateable electrical connections are conceptually simple, but require careful alignment, significant
mating forces, and can have a limited lifetime. A survey of six wet mateable connectors [16.50] reports
mating forces ranging from approximately 755 N per
circuit. Most connectors were rated for 100 mate cycles,
with two commercial connectors claiming greater than
a thousand cycles. A unique wet mateable connector described by Stokey et al. [16.8] takes advantage of the
fact that only two connections are required. The connector establishes a two-contact electrical connection
between the vehicle and the dock, but only insulates one
of those connections from seawater. Electrical isolation
of the power system from the vehicle hull prevents anodic dissolution of the hull.
Inductive power transfer systems have the advantage of no exposed electrical contacts and can be designed to be less sensitive to alignment, and have low
mating force. Reliability is typically limited by the lifetime of the electronics associated with the inductive
process, rather than the number of mate/demate cycles. Power capacity and size and weight are related,
as efficient high power transfer requires a larger inductor, and the high-permeability materials that make high
efficiency power coupling possible are quite dense. Systems developed by Feezor et al. [16.48] and Granger
et al. [16.51] used inductive coupling for both communication and power transfer. The Florida Atlantic
University (FAU) inductive power system developed by
Coulson et al. [16.52] was used for the Dorado docking
system.
Autonomous recharging requires a sophisticated
battery management system that can maximize energy
stored in the batteries while simultaneously minimizing
battery failures [16.53]. Examples of battery failures include the destruction of the advanced swimmer delivery
system by a battery fire. Lithium-ion battery fires on the
Boeing 787 grounded that aircraft’s commercial operations for months. In the undersea environment, where
batteries might be in a pressure vessel, a battery fire is
a catastrophic event.
16.6 AUV Control Considerations
In most AUV control systems, AUV control is segregated by the level of abstraction. At the lowest level,
dynamic control systems command actuators to achieve
desired orientation, speed, and position. At a higher
level, algorithms attempt to accomplish a particular task
by commanding the dynamic control system. Above
the task level, software monitors the state of tasks,
and manages sequencing of activities. While a variety of software architectures are used on operational
AUVs, most conform to some semblance of this hierarchy. In principle, even more capable systems might
include yet another level that could create plans to
achieve user defined goals, and replan when circumstances change; however, in practice this level of autonomy has not been used in docking AUVs thus
far.
16.6.1 Navigation, Currents,
and Arriving at the Dock
Control algorithms for bringing an AUV to a dock
have diverse levels of sophistication, depending on
the demands of the docking process. Perhaps the
simplest are pure pursuit algorithms, which simply
point the vehicle directly at the homing target. These
are appropriate for docking systems that can be ap-