Autonomous Underwater Vehicle Docking 16.3 Sensors For Homing 391
Part B | 16.3
of an acoustic array for homing is dictated by its
aperture compared to the acoustic wavelength at the
operating frequency, with smaller arrays having poorer
directionality.
However, small size has benefits as well, for example, generally allowing impacts at higher speeds.
Consider the docking of a torpedo-like vehicle weighing 200 kg, impacting a dock at 1:5 m=s. The dock
and vehicle must absorb 225 J of kinetic energy, not
counting added mass of entrained water. If the linear dimension of the system is scaled up by a factor
of 2, the vehicle will weigh 1600 kg, and the dock
must now dissipate eight times the energy. If, on the
other hand, the approach speed can be dropped by
half, then the quadratic dependence of kinetic energy
on velocity drops the energy dissipated by a factor of
4. Consequently, as vehicle size increases, the speed
of docking needs to decrease, and this, in turn, drives
larger vehicles to multi-thruster configurations that can
be controlled at lower speeds.
The vehicle’s ability to apply useful force to the
docking process using its propulsion system is likely
also to be a function of size. Use of the vehicle’s propulsion system to create a force that can complete physical
elements of docking is attractive because it minimizes
the number of additional actuators that are required.
A useful metric, therefore, is the thrust to weight ratio for a given vehicle. Propulsion systems are typically
sized to overcome vehicle drag and drag scales with the
area of the vehicle. In contrast, mass scales as the vehicle volume. Consequently, if all other factors are held
constant, then shrinking a vehicle by two in length will
double the vehicle’s thrust to mass ratio. Thus smaller
vehicles have higher accelerations and are able to apply
significantly more force as compared to their mass in
the docking process.
16.3 Sensors For Homing
The homing sensor provides the AUV with the information necessary to find and connect to the dock. The ideal
homing sensor would allow the AUV to reliably sense
the relative location of the dock at any distance with
high accuracy, and would update that information many
times a second and with no latency. In practice, there
is no such technology. Consequently, homing systems
for docking are either a combination of different technical approaches with complementary characteristics, or
have non-ideal characteristics that must be mitigated by
other aspects of the docking system design.
The primary information provided by the homing
sensor is the relative position of the two platforms. This
provides the vehicle with the information required to
approach the dock in a controlled manner. While the
AUV presumably has some knowledge of its own location and that of the dock in Earth coordinates, this
knowledge is unlikely to be sufficiently accurate for the
AUV to achieve physical connection to the dock. Thus
the approach and terminal homing phases of docking
require accurate relative position information.
Drivers for the homing system key characteristics
are as follows:
Acquisition distance: how far away can the AUV
acquire information on the relative location of the
dock? A homing system that operates at a great distance greatly minimizes the chance that the AUV
will not be able to find the dock and/or provides
a larger margin for maneuvering the vehicle to position for final approach.
Accuracy: how accurately can the vehicle measure
the relative position of the dock? The importance
of accuracy increases as the vehicle gets closer to
the dock. The simplest homing sensors provide only
left–right, up–down cues, without providing accurate relative bearing, elevation, and range. While
these systems are adequate for head-on approaches
to a homing target, they can be less attractive in
more complex situations, for example, when there
are currents or when the homing target is offset from
the dock.
Environmental sensitivity: many sensing modalities
in the ocean have highly variable performance. The
range of a particular optical sensor might vary from
sub-meter to greater than 100 m, depending on water properties. Methods depending on acoustic signals are sensitive to acoustic conditions, including
refraction due to sound speed variations and reflections from the sea surface and seafloor. The severity
of performance variability is highly location dependent, and, consequently, the choice of homing
system can be heavily influenced by the nature of
the environment in which the dock will be deployed.
Update rate: the more frequent the update rate, the
more rapidly the vehicle can correct a non-optimal
approach. For example, a vehicle traveling 1:5 m=s
traverses 3 m between updates given an update rate
of 2 s. Under ideal conditions, this might not be
a problem; however any cross-currents, dock motion, or errors in the homing system measurement
can cause a missed approach.
Part B | 16.3
of an acoustic array for homing is dictated by its
aperture compared to the acoustic wavelength at the
operating frequency, with smaller arrays having poorer
directionality.
However, small size has benefits as well, for example, generally allowing impacts at higher speeds.
Consider the docking of a torpedo-like vehicle weighing 200 kg, impacting a dock at 1:5 m=s. The dock
and vehicle must absorb 225 J of kinetic energy, not
counting added mass of entrained water. If the linear dimension of the system is scaled up by a factor
of 2, the vehicle will weigh 1600 kg, and the dock
must now dissipate eight times the energy. If, on the
other hand, the approach speed can be dropped by
half, then the quadratic dependence of kinetic energy
on velocity drops the energy dissipated by a factor of
4. Consequently, as vehicle size increases, the speed
of docking needs to decrease, and this, in turn, drives
larger vehicles to multi-thruster configurations that can
be controlled at lower speeds.
The vehicle’s ability to apply useful force to the
docking process using its propulsion system is likely
also to be a function of size. Use of the vehicle’s propulsion system to create a force that can complete physical
elements of docking is attractive because it minimizes
the number of additional actuators that are required.
A useful metric, therefore, is the thrust to weight ratio for a given vehicle. Propulsion systems are typically
sized to overcome vehicle drag and drag scales with the
area of the vehicle. In contrast, mass scales as the vehicle volume. Consequently, if all other factors are held
constant, then shrinking a vehicle by two in length will
double the vehicle’s thrust to mass ratio. Thus smaller
vehicles have higher accelerations and are able to apply
significantly more force as compared to their mass in
the docking process.
16.3 Sensors For Homing
The homing sensor provides the AUV with the information necessary to find and connect to the dock. The ideal
homing sensor would allow the AUV to reliably sense
the relative location of the dock at any distance with
high accuracy, and would update that information many
times a second and with no latency. In practice, there
is no such technology. Consequently, homing systems
for docking are either a combination of different technical approaches with complementary characteristics, or
have non-ideal characteristics that must be mitigated by
other aspects of the docking system design.
The primary information provided by the homing
sensor is the relative position of the two platforms. This
provides the vehicle with the information required to
approach the dock in a controlled manner. While the
AUV presumably has some knowledge of its own location and that of the dock in Earth coordinates, this
knowledge is unlikely to be sufficiently accurate for the
AUV to achieve physical connection to the dock. Thus
the approach and terminal homing phases of docking
require accurate relative position information.
Drivers for the homing system key characteristics
are as follows:
Acquisition distance: how far away can the AUV
acquire information on the relative location of the
dock? A homing system that operates at a great distance greatly minimizes the chance that the AUV
will not be able to find the dock and/or provides
a larger margin for maneuvering the vehicle to position for final approach.
Accuracy: how accurately can the vehicle measure
the relative position of the dock? The importance
of accuracy increases as the vehicle gets closer to
the dock. The simplest homing sensors provide only
left–right, up–down cues, without providing accurate relative bearing, elevation, and range. While
these systems are adequate for head-on approaches
to a homing target, they can be less attractive in
more complex situations, for example, when there
are currents or when the homing target is offset from
the dock.
Environmental sensitivity: many sensing modalities
in the ocean have highly variable performance. The
range of a particular optical sensor might vary from
sub-meter to greater than 100 m, depending on water properties. Methods depending on acoustic signals are sensitive to acoustic conditions, including
refraction due to sound speed variations and reflections from the sea surface and seafloor. The severity
of performance variability is highly location dependent, and, consequently, the choice of homing
system can be heavily influenced by the nature of
the environment in which the dock will be deployed.
Update rate: the more frequent the update rate, the
more rapidly the vehicle can correct a non-optimal
approach. For example, a vehicle traveling 1:5 m=s
traverses 3 m between updates given an update rate
of 2 s. Under ideal conditions, this might not be
a problem; however any cross-currents, dock motion, or errors in the homing system measurement
can cause a missed approach.