U.S. patent application number 14/325187 was filed with the patent office on 2016-01-07 for enhanced accuracy for tracking tethered airborne vehicles.
The applicant listed for this patent is Google Inc.. Invention is credited to Brian Hachtmann, Kurt Hallamask, Corwin Hardham, Kenny Jensen, Rob Nelson, Elias Wolfgang Patten.
Application Number | 20160005159 14/325187 |
Document ID | / |
Family ID | 54783205 |
Filed Date | 2016-01-07 |
United States Patent
Application |
20160005159 |
Kind Code |
A1 |
Hallamask; Kurt ; et
al. |
January 7, 2016 |
Enhanced Accuracy for Tracking Tethered Airborne Vehicles
Abstract
Wind energy systems, such as an Airborne Wind Turbine ("AWT"),
may be used to facilitate conversion of kinetic energy to
electrical energy. An AWT may include an aerial vehicle that flies
in a path to convert kinetic wind energy to electrical energy. The
aerial vehicle may be tethered to a ground station with a tether
that terminates at a tether termination mount system. In one
aspect, the tether termination mount system may include a tether
termination unit configured in one or more gimbals that allow for
the tether termination unit to rotate about one or more axes while
tracking the aerial vehicle in flight. In a further aspect, the
tether termination mount system may include an imaging device
configured for imaging the aerial vehicle during flight in order to
enhance tracking accuracy over that which is performed by angular
motion of the tether termination unit.
Inventors: |
Hallamask; Kurt; (San
Carlos, CA) ; Hachtmann; Brian; (San Martin, CA)
; Nelson; Rob; (Alameda, CA) ; Hardham;
Corwin; (Mountain View, CA) ; Jensen; Kenny;
(Berkeley, CA) ; Patten; Elias Wolfgang; (Seattle,
WA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Google Inc. |
Mountain View |
CA |
US |
|
|
Family ID: |
54783205 |
Appl. No.: |
14/325187 |
Filed: |
July 7, 2014 |
Current U.S.
Class: |
244/110C ;
701/300 |
Current CPC
Class: |
B64C 39/022 20130101;
B64F 1/02 20130101; G06T 7/70 20170101; B64F 3/00 20130101; B64F
3/02 20130101 |
International
Class: |
G06T 7/00 20060101
G06T007/00; B64F 1/02 20060101 B64F001/02 |
Claims
1. An apparatus comprising: a tether termination mount system
comprising a tether termination unit configured for rotation about
a first axis of rotation and a second axis of rotation, the first
and second axes of rotation being oriented at an angle to one
another; a tether configured for coupling an aerial vehicle to the
tether termination mount system, the tether having a proximal end
emanating from the tether termination unit along a pointing axis of
the tether termination unit and distal end attached to the aerial
vehicle, wherein the pointing axis is configured for tracking
angular motion of the proximal end of the tether induced by flight
motion of the aerial vehicle; an imaging device attached to the
tether termination unit and having an image plane oriented at a
fixed angle with respect to the pointing axis, and a reference
point in the image plane corresponding to the pointing axis
direction in a field-of-view (FOV) of the image plane; and a
vehicle tracking subsystem configured for tracking a location of
the aerial vehicle with respect to the tether termination mount
system during tethered flight of the aerial vehicle by (i)
determining the pointing axis direction as a function of measured
rotation angles of the tether termination unit about the first and
second axes of rotation, (ii) determining an angular offset between
an image of the aerial vehicle in the image plane and the reference
point in the image plane, and (iii) applying the determined angular
offset as a correction to the determined pointing axis
direction.
2. The apparatus of claim 1, wherein the fixed angle is a right
angle, whereby the image plane is oriented perpendicularly to the
pointing axis.
3. The apparatus of claim 1, wherein the imaging device comprises a
device selected from one of a digital camera and a line-scan camera
synchronized to a pattern of blinking lights on the aerial
vehicle.
4. The apparatus of claim 1, wherein the image of the aerial
vehicle in the image plane is at least one of an optical image of a
physical form of the aerial vehicle, an optical image of a pattern
painted on the aerial vehicle, an optical image of a pattern of
lights emitted from the aerial vehicle, or a signal reflected from
a surface pattern on the aerial vehicle.
5. The apparatus of claim 1, wherein the vehicle tracking subsystem
is further configured for tracking the location of the aerial
vehicle with respect to the tether termination mount system during
tethered flight of the aerial vehicle by measuring the
line-of-sight distance to the aerial vehicle during tethered flight
of the aerial vehicle.
6. The apparatus of claim 1, further comprising a distance ranging
device configured for measuring a line-of-sight distance to the
aerial vehicle, and wherein the vehicle tracking subsystem is
further configured for tracking the location of the aerial vehicle
with respect to the tether termination mount system during tethered
flight of the aerial vehicle by measuring the line-of-sight
distance to the aerial vehicle during tethered flight of the aerial
vehicle with the distance ranging device.
7. The apparatus of claim 6, wherein the distance ranging device is
one of a LIDAR device or a RADAR device.
8. The apparatus of claim 6, wherein the imaging device and the
distance ranging device are configured as integral components of a
single distance-imaging device.
9. The apparatus of claim 1, wherein the first axis of rotation is
incorporated in a first gimbal device, and the second axis of
rotation is incorporated in a second gimbal device, wherein the
first gimbal device comprises a first encoder configured for
measuring a rotation angle of the tether termination unit about the
first axis of rotation, and wherein the second gimbal device
comprises a second encoder configured for measuring a rotation
angle of the tether termination unit about the second axis of
rotation.
10. The apparatus of claim 1, wherein the first axis of rotation is
orthogonal to the second axis of rotation.
11. The apparatus of claim 1, wherein the pointing axis is
configured for tracking angular motion of the proximal end of the
tether due to rotation of the tether termination unit about the
first and second axes of rotation.
12. The apparatus of claim 1, wherein the tether termination unit
comprises a spindle through which the tether is physically routed
between the tether termination mount system and the aerial
vehicle.
13. The apparatus of claim 1, wherein the reference point is at the
center of the image plane, and further corresponds to the center of
the FOV.
14. The apparatus of claim 1, wherein tracking the angular motion
of the proximal end of the tether induced by flight motion of the
aerial vehicle comprises tracking the aerial vehicle during flight
with sufficient accuracy to locate the aerial vehicle within the
FOV of the image plane.
15. The apparatus of claim 1, wherein the image plane is configured
for rotation about the pointing axis direction in response to a
twisting motion of the proximal end of the tether about the
pointing axis.
16. A method comprising: determining a pointing direction of a
pointing axis of a tether termination unit as a function of
measured rotation angles of the tether termination unit about a
first axis of rotation and a second axis of rotation, wherein the
tether termination unit is configured for terminating a proximal
end of a tether that is coupled at a distal end with an aerial
vehicle, and wherein the pointing axis is configured for tracking
angular motion of the proximal end of the tether induced by flight
motion of the aerial vehicle; determining an angular offset between
an image of the aerial vehicle in flight in an image plane of an
imaging device attached to the tether termination unit and a
reference point in the image plane, wherein the image plane is
configured at a fixed angle with respect to the pointing axis, and
the reference point corresponds to the pointing axis direction in a
field-of-view (FOV) of the image plane; and determining a location
of the aerial vehicle in flight with respect to the tether
termination unit by applying the determined angular offset as a
correction to the determined pointing axis direction.
17. The method of claim 16, wherein the fixed angle is a right
angle, whereby the image plane is oriented perpendicularly to the
pointing axis.
18. The method of claim 16, wherein the image of the aerial vehicle
in the image plane is at least one of an optical image of a
physical form of the aerial vehicle, an optical image of a pattern
painted on the aerial vehicle, an optical image of a pattern of
lights emitted from the aerial vehicle, or a signal reflected from
a surface pattern on the aerial vehicle.
19. The method of claim 16, further comprising measuring a
line-of-sight distance to the aerial vehicle during tethered flight
of the aerial vehicle.
20. The method of claim 19, wherein the tether termination unit is
a component of an apparatus that further comprises a distance
ranging device select from one of a LIDAR device and a RADAR
device, and wherein measuring the line-of-sight distance to the
aerial vehicle during tethered flight of the aerial vehicle
comprises measuring the line-of-sight distance to the aerial
vehicle during tethered flight of the aerial vehicle with the
distance ranging device.
21. The method of claim 16, wherein the first axis of rotation is
incorporated in a first gimbal device, and the second axis of
rotation is incorporated in a second gimbal device, and wherein
determining the pointing direction of the pointing axis of the
tether termination unit as a function of measured rotation angles
of the tether termination unit about the first axis of rotation and
the second axis of rotation comprises: measuring a rotation angle
of the tether termination unit about the first axis of rotation
with a first encoder of the first gimbal device; and measuring a
rotation angle of the tether termination unit about the second axis
of rotation with a second encoder of the second gimbal device.
22. The method of claim 16, wherein the first axis of rotation is
orthogonal to the second axis of rotation.
23. The method of claim 16, determining the pointing direction of
the pointing axis of the tether termination unit as a function of
measured rotation angles of the tether termination unit about the
first axis of rotation and the second axis of rotation comprises
tracking angular motion of the proximal end of the tether due to
rotation of the tether termination unit about the first and second
axes of rotation.
24. The method of claim 16, wherein the tether termination unit
comprises a spindle through which the tether is physically routed
between the tether termination mount system and the aerial
vehicle.
25. The method of claim 16, wherein the reference point is at the
center of the image plane, and further corresponds to the center of
the FOV.
26. The method of claim 16, wherein determining the pointing
direction of the pointing axis of the tether termination unit as a
function of measured rotation angles of the tether termination unit
about the first axis of rotation and the second axis of rotation
comprises tracking the aerial vehicle during flight with sufficient
accuracy to locate the aerial vehicle within the FOV of the image
plane.
27. The method of claim 16, further comprising rotating the image
plane about the pointing axis direction in response to a twisting
motion of the proximal end of the tether about the pointing
axis.
28. A non-transient computer-readable storage medium having stored
therein instructions, that when executed by one or more processors
of an apparatus comprising a tether termination unit, cause the
apparatus to perform functions comprising: determining a pointing
direction of a pointing axis of the tether termination unit as a
function of measured rotation angles of the tether termination unit
about a first axis of rotation and a second axis of rotation,
wherein the tether termination unit is configured for terminating a
proximal end of a tether that is coupled at a distal end with an
aerial vehicle, and wherein the pointing axis is configured for
tracking angular motion of the proximal end of the tether induced
by flight motion of the aerial vehicle; determining an angular
offset between an image of the aerial vehicle in flight in an image
plane of an imaging device attached to the tether termination unit
and a reference point in the image plane, wherein the image plane
is configured at a fixed angle with respect to the pointing axis,
and the reference point corresponds to the pointing axis direction
in a field-of-view (FOV) of the image plane; and determining a
location of the aerial vehicle in flight with respect to the tether
termination unit by applying the determined angular offset as a
correction to the determined pointing axis direction.
29. The non-transient computer-readable storage medium of claim 28,
wherein the fixed angle is a right angle, whereby the image plane
is oriented perpendicularly to the pointing axis.
30. The non-transient computer-readable storage medium of claim 28,
wherein the imaging device comprises a device selected from one of
a digital camera and a line-scan camera synchronized to a pattern
of blinking lights on the aerial vehicle.
31. The non-transient computer-readable storage medium of claim 28,
wherein the image of the aerial vehicle in the image plane is at
least one of an optical image of a physical form of the aerial
vehicle, an optical image of a pattern painted on the aerial
vehicle, an optical image of a pattern of lights emitted from the
aerial vehicle, or a signal reflected from a surface pattern on the
aerial vehicle.
32. The non-transient computer-readable storage medium of claim 28,
the functions further comprise measuring a line-of-sight distance
to the aerial vehicle during tethered flight of the aerial
vehicle.
33. The non-transient computer-readable storage medium of claim 32,
wherein the apparatus further comprises a distance ranging device
select from one of a LIDAR device and a RADAR device, and wherein
measuring the line-of-sight distance to the aerial vehicle during
tethered flight of the aerial vehicle comprises measuring the
line-of-sight distance to the aerial vehicle during tethered flight
of the aerial vehicle with the distance ranging device.
34. The non-transient computer-readable storage medium of claim 32,
wherein the imaging device and the distance ranging device are
configured as integral components of a single distance-imaging
device.
35. The non-transient computer-readable storage medium of claim 28,
wherein the first axis of rotation is incorporated in a first
gimbal device, and the second axis of rotation is incorporated in a
second gimbal device, and wherein determining the pointing
direction of the pointing axis of the tether termination unit as a
function of measured rotation angles of the tether termination unit
about the first axis of rotation and the second axis of rotation
comprises: measuring a rotation angle of the tether termination
unit about the first axis of rotation with a first encoder of the
first gimbal device; and measuring a rotation angle of the tether
termination unit about the second axis of rotation with a second
encoder of the second gimbal device.
36. The non-transient computer-readable storage medium of claim 28,
wherein the first axis of rotation is orthogonal to the second axis
of rotation.
37. The non-transient computer-readable storage medium of claim 28,
determining the pointing direction of the pointing axis of the
tether termination unit as a function of measured rotation angles
of the tether termination unit about the first axis of rotation and
the second axis of rotation comprises tracking angular motion of
the proximal end of the tether due to rotation of the tether
termination unit about the first and second axes of rotation.
38. The non-transient computer-readable storage medium of claim 28,
wherein the reference point is at the center of the image plane,
and further corresponds to the center of the FOV.
39. The non-transient computer-readable storage medium of claim 28,
wherein determining the pointing direction of the pointing axis of
the tether termination unit as a function of measured rotation
angles of the tether termination unit about the first axis of
rotation and the second axis of rotation comprises tracking the
aerial vehicle during flight with sufficient accuracy to locate the
aerial vehicle within the FOV of the image plane.
40. The non-transient computer-readable storage medium of claim 28,
wherein the functions further comprise rotating the image plane
about the pointing axis direction in response to a twisting motion
of the proximal end of the tether about the pointing axis.
Description
BACKGROUND
[0001] Unless otherwise indicated herein, the materials described
in this section are not prior art to the claims in this application
and are not admitted to be prior art by inclusion in this
section.
[0002] Power generation systems may convert chemical and/or
mechanical energy (e.g., kinetic energy) to electrical energy for
various applications, such as utility systems. As one example, a
wind energy system may convert kinetic wind energy to electrical
energy.
SUMMARY
[0003] In one aspect, an apparatus includes: a tether termination
mount system comprising a tether termination unit configured for
rotation about a first axis of rotation and a second axis of
rotation, the first and second axes of rotation being oriented at
an angle to one another; a tether configured for coupling an aerial
vehicle to the tether termination mount system, the tether having a
proximal end emanating from the tether termination unit along a
pointing axis of the tether termination unit and distal end
attached to the aerial vehicle, wherein the pointing axis is
configured for tracking angular motion of the proximal end of the
tether induced by flight motion of the aerial vehicle; an imaging
device attached to the tether termination unit and having an image
plane oriented at a fixed angle with respect to the pointing axis,
and a reference point in the image plane corresponding to the
pointing axis direction in a field-of-view (FOV) of the image
plane; and a vehicle tracking subsystem configured for tracking a
location of the aerial vehicle with respect to the tether
termination mount system during tethered flight of the aerial
vehicle by (i) determining the pointing axis direction as a
function of measured rotation angles of the tether termination unit
about the first and second axes of rotation, (ii) determining an
angular offset between an image of the aerial vehicle in the image
plane and the reference point in the image plane, and (iii)
applying the determined angular offset as a correction to the
determined pointing axis direction.
[0004] In another aspect, a method involves: determining a pointing
direction of a pointing axis of a tether termination unit as a
function of measured rotation angles of the tether termination unit
about a first axis of rotation and a second axis of rotation,
wherein the tether termination unit is configured for terminating a
proximal end of a tether that is coupled at a distal end with an
aerial vehicle, and wherein the pointing axis is configured for
tracking angular motion of the proximal end of the tether induced
by flight motion of the aerial vehicle; determining an angular
offset between an image of the aerial vehicle in flight in an image
plane of an imaging device attached to the tether termination unit
and a reference point in the image plane, wherein the image plane
is configured at a fixed angle with respect to the pointing axis,
and the reference point corresponds to the pointing axis direction
in a field-of-view (FOV) of the image plane; and determining a
location of the aerial vehicle in flight with respect to the tether
termination unit by applying the determined angular offset as a
correction to the determined pointing axis direction.
[0005] In yet another aspect, a non-transient computer-readable
storage medium have instructions stored therein, that when executed
by one or more processors of an apparatus comprising a tether
termination unit, cause the apparatus to perform functions
including: determining a pointing direction of a pointing axis of
the tether termination unit as a function of measured rotation
angles of the tether termination unit about a first axis of
rotation and a second axis of rotation, wherein the tether
termination unit is configured for terminating a proximal end of a
tether that is coupled at a distal end with an aerial vehicle, and
wherein the pointing axis is configured for tracking angular motion
of the proximal end of the tether induced by flight motion of the
aerial vehicle; determining an angular offset between an image of
the aerial vehicle in flight in an image plane of an imaging device
attached to the tether termination unit and a reference point in
the image plane, wherein the image plane is configured at a fixed
angle with respect to the pointing axis, and the reference point
corresponds to the pointing axis direction in a field-of-view (FOV)
of the image plane; and determining a location of the aerial
vehicle in flight with respect to the tether termination unit by
applying the determined angular offset as a correction to the
determined pointing axis direction.
[0006] These as well as other aspects, advantages, and
alternatives, will become apparent to those of ordinary skill in
the art by reading the following detailed description, with
reference where appropriate to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 illustrates an Airborne Wind Turbine (AWT), according
to an example embodiment.
[0008] FIG. 2 illustrates a simplified block diagram illustrating
components of an AWT, according to an example embodiment.
[0009] FIG. 3A illustrates a first perspective view of an example
ground station for an aerial vehicle of an AWT, according to an
example embodiment.
[0010] FIG. 3B illustrates a second perspective view of an example
ground station for an aerial vehicle of an AWT, according to an
example embodiment.
[0011] FIG. 3C illustrates a cross-sectional view of an example
tether with insulated electrical conductors, according to an
example embodiment.
[0012] FIG. 4 illustrates a perspective view of an example
embodiment of a tether termination mount system for a ground
station of an aerial vehicle of an AWT.
[0013] FIG. 5 is schematic illustration of example operation of a
tether termination mount system, according to an example
embodiment.
[0014] FIG. 6 illustrates a perspective view of an example tether
termination mount system, according to an example embodiment.
[0015] FIG. 7 is schematic illustration of an example image plane
during example operation of a tether termination mount system,
according to an example embodiment.
[0016] FIG. 8 is a flow chart illustrating an example method of
tracking an aerial vehicle, according to an example embodiment.
DETAILED DESCRIPTION
[0017] Example methods and systems are described herein. It should
be understood that the words "example," "exemplary," and
"illustrative" are used herein to mean "serving as an example,
instance, or illustration." Any embodiment or feature described
herein as being an "example," being "exemplary," or being
"illustrative" is not necessarily to be construed as preferred or
advantageous over other embodiments or features. The example
embodiments described herein are not meant to be limiting. It will
be readily understood that the aspects of the present disclosure,
as generally described herein, and illustrated in the figures, can
be arranged, substituted, combined, separated, and designed in a
wide variety of different configurations, all of which are
explicitly contemplated herein.
1. OVERVIEW
[0018] Example embodiments relate to aerial vehicles, which may be
used in a wind energy system, such as an Airborne Wind Turbine
(AWT) system. In particular, example embodiments may relate to, or
take the form of, methods and systems for facilitating an aerial
vehicle in the process of conversion of kinetic energy to
electrical energy. As used herein, the term "aerial vehicle"
generally refers to a type of vehicle that is capable of, or
configured for flight, and does not necessarily refer to an
operational state in which such a vehicle is in flight. The
operational state of an aerial vehicle may be specified herein
explicitly, such as in a "flying aerial vehicle" or "an aerial
vehicle in flight," although in some instances of the discussion
herein, the operational state may be apparent or implicit from
context.
[0019] An AWT system (or just AWT for short) may include an aerial
vehicle configured for flying in a path, such as a substantially
circular path, while converting kinetic wind energy to electrical
energy via onboard turbines. In an example embodiment, the aerial
vehicle may be connected to a ground station via a tether. While
tethered, the aerial vehicle may: (i) fly at a range of elevations
and substantially along the path, and return to the ground, and
(ii) transmit electrical energy to the ground station via the
tether. In some embodiments, the ground station may transmit
electricity to the aerial vehicle for take-off and/or landing, for
example.
[0020] In an AWT, an aerial vehicle may rest in and/or on a ground
station, for example, when the wind is not conducive to power
generation. When the wind is conducive to power generation, such as
when a wind speed may be 10 meters per second (m/s) at an altitude
of 200 meters (m), the ground station may deploy (or launch) the
aerial vehicle. In addition, when the aerial vehicle is deployed
and the wind is not conducive to power generation, the aerial
vehicle may return to the ground station.
[0021] An aerial vehicle of an AWT may be configured for hover
flight and crosswind flight. Crosswind flight may be undertaken for
travel in a motion, such as in a substantially circular motion, and
thus may be the primary technique that is used for generating
electrical energy. Hover flight in turn may be used by the aerial
vehicle to prepare and position itself for crosswind flight. In
particular, the aerial vehicle could ascend to a location for
crosswind flight based at least in part on hover flight. Further,
the aerial vehicle could take-off and/or land via hover flight.
[0022] In hover flight, a span of a main wing of the aerial vehicle
may be oriented substantially parallel to the ground, and one or
more propellers of the aerial vehicle may cause the aerial vehicle
to hover over the ground. In some embodiments, the aerial vehicle
may vertically ascend or descend in hover flight.
[0023] In crosswind flight, the aerial vehicle may be propelled by
the wind substantially along a path on which, as noted above, it
may convert kinetic wind energy to electrical energy. In some
embodiments, the one or more propellers of the aerial vehicle may
generate electrical energy by slowing down the incident wind.
[0024] The aerial vehicle may enter crosswind flight when (i) the
aerial vehicle has attached wind-flow (e.g., steady flow and/or no
stall condition (which may refer to no separation of air flow from
an airfoil)) and (ii) the tether is under tension. Moreover, the
aerial vehicle may enter crosswind flight at a location that is
substantially downwind of the ground station.
[0025] In an example embodiment, the ground station can include a
tether termination mount system for terminating a proximal end of
the tether at the ground side, and tracking and measuring angular
motion of the tether caused by flight motion of the aerial vehicle
at a distal end of the tether. In an example embodiment, the tether
can be of fixed length during a phase of generally stable flight,
such as cross-wind flight. For example, a fixed length tether may
be approximally 500 meters long and approximally 20 millimeters in
diameter. The tether may include one or more insulated conductors
to transmit electrical energy, or other electrical signals, along
the tether length. Additionally or alternatively, the tether may
include one or more optical fibers to transmit optical signals
along the tether length.
[0026] By using a fixed length tether, the tether termination mount
system can be used to measure two dimensions of tether angle and
tether velocity. A tether termination mount system can also be used
to measure the orientation of the tether, the rate of rotation of
the tether, and the precession motion and nutation angles of a
distal end of the tether. Angular orientation and motion of the
tether can, in turn, be used in monitoring the position of the
aerial vehicle with respect to the ground station during
flight.
[0027] A tether termination mount system (or just a tether
termination mount, for short) at the ground station may be
desirable for various reasons. For example, the aerial vehicle in
cross-wind flight may oscillate many times over the life of the
system (for e.g., 30 million cycles of aerial vehicle and tether
rotation), so a tether termination mount that does not wear or rub
the tether may facilitate robust and reliable operation. In the
case of rigid or semi-rigid tethers, a tether termination mount
that does not impart significant bending loads onto the tether may
similarly be desirable.
[0028] In the case of a tether with one or more conductors, a
tether termination mount that does not accumulate twists in the
tether may be desirable. Tether twisting can have an adverse effect
on performance, because a twisted tether may have reduced
conductivity due to the twisting or eventual breaking of the
conductor(s), and/or may also have reduced tensile breaking
strength. For example, the tether termination mount may either
actively or passively rotate to align the tether at the ground-side
system with the motion of the aerial vehicle. The tether
termination mount may include a servomotor or other drive mechanism
to manually rotate the tether and reduce the likelihood of
significant tether twisting. Additionally in the case of a tether
with one or more conductors, a tether termination mount that
communicates power either into the ground side system or out to the
aerial vehicle may be desirable.
[0029] In an example embodiment, the tether termination mount
system can include a tether termination unit for terminating the
proximal end of the tether. The tether termination unit can have a
pointing axis direction along which the proximal end of the tether
is aligned during the phase of generally stable flight. The tether
termination unit can be configured for rotation about two axes,
such that its angular motion induced by the tether motion
responsive to flight motion of the aerial vehicle can provide the
angular orientation and motion measurements of the tether
termination system.
[0030] In an example embodiment, the tether termination unit can
include a physical channel or tube-like structure through which the
tether is threaded, and out from which the proximal end of the
tether emerges or emanates toward the distal end where it couples
to the aerial vehicle. As such, the tether termination unit can
accommodate length adjustments of tether as necessary (e.g., for
take-off and landing), while maintaining a physical termination
that can track angular motion of the proximal end of the tether. In
this configuration, the angular orientation and motion of the
pointing axis of the tether mount can be used to track the proximal
end of the tether.
[0031] In some embodiments, a tether termination mount system of
the ground station can include a ground-side gimbal (GSG)
comprising a set of bearings that allow the angles of connection of
the tether to the ground station to change, and that help to reduce
the flex within the tether. For example, the GSG can support two
axes of rotation of the tether termination unit. In some
embodiments, the GSG can also include a bearing that allows the
tether to rotate about the tether axis, thereby allowing the tether
to de-twist in cases in which the flight path of the aerial vehicle
is topologically circular. In some embodiments, the GSG can further
comprise a slip ring that allows electrical signals to be passed
from the tether to the ground station as the GSG allows the tether
to rotate about the tether axis.
[0032] An example embodiment of a GSG can include a two-axis mount,
such as an altitude-azimuth mount, in which one axis is an altitude
axis for rotation with respect to a local horizon, and the other
axis is the azimuth axis for rotation with respect to a local
vertical axis. Each axis can include a bearing to allow for a low
overturning moment for easy rotation and one or more encoders for
measuring angular position and rotation rate about the axis.
[0033] In an example embodiment, the ground station can include an
imaging device configured for imaging the aerial vehicle in flight
within an image plane. For example, the imaging device could be a
digital camera. The ground station can further include a vehicle
tracking subsystem configured for analyzing imaging of the vehicle
in the image plane, in order to enhance the accuracy of the
vehicle's position as otherwise determined by angular measurements
of the tether measured by the tether termination mount system.
[0034] More particularly, the imaging device can be attached to the
tether termination unit in a fixed orientation such that the
pointing axis is perpendicular to the image plane and corresponds
to a reference line-of-sight (LOS) direction in the image plane.
For example, the pointing axis can correspond to the center of the
field-of-view (FOV) of the image plane. In this configuration, the
aerial vehicle can be viewed within the FOV of the image plane at
least during a phase of generally stable flight. That is, physical
tracking of the proximal end of the tether can provide a
sufficiently accurate fix on the position of the aerial vehicle
during stable flight to align the FOV with region that includes the
aerial vehicle.
[0035] In example embodiments, the position of an image of aerial
vehicle in the image plane can be analyzed in order determine an
accurate location of the aerial vehicle with respect to the ground
station (or the tether termination mount system). More
particularly, the location of the aerial vehicle in the image plane
can be rendered with respect to the center of the FOV, which can
correspond to the pointing axis, as described above. Examples of
analysis techniques for determining the location of the aerial
vehicle's image in the image plane can include detection and/or
recognition of predetermined patterns or markings on the aerial
vehicle's surface. In addition, the aerial vehicle can be equipped
with light-emitting diodes (LEDs) or other lights to aid or enhance
image-plane detection. The relative location of the aerial
vehicle's image can then be converted to a linear distance in a
rectangular coordinate system (e.g., x and y), for example by
geometry (and/or trigonometry). This linear distance can then be
used as a correction to an estimated location of the aerial vehicle
obtained from the pointing axis direction alone.
[0036] In an example embodiment, the ground station can include a
distance ranging device, such a RADAR or LIDAR, which can be used
to determine a LOS distance from the ground station to the aerial
vehicle. A measured LOS distance can then be used to further
improve a geometrically-determined (and/or
trigonometrically-determined) correction to the location of the
aerial vehicle with respect to the ground station (or the tether
termination mount system). By way of example, the aerial vehicle
could include one or more reflective spots configured for enhancing
RADAR and/or LIDAR detection and ranging. RADAR and/or LIDAR
devices can also provide imaging capabilities as well. Other
techniques for LOS distance determination can be used as well. For
example, propagation delay of a radio-frequency (RF) signal between
a ground station and an aerial can be used to determine LOS
distance. In addition to, or possibly in place of, signal-based
distance measurement, the length of an extended tether can provide
a measure of LOS distance. Tether length can be determined as the
tether is unwound from a drum or spool, for example.
[0037] In an example embodiment, the imaging device and the
distance ranging device could be integral components of single
distance-imaging device. For example, imaging could be based on
ranging data, such that LOS distance is inherent in the imagining
data.
[0038] In an example embodiment, the imaging device could be
configured for rotation of the image plane about the pointing axis,
while the image plane still remains perpendicular to the pointing
axis. In this configuration, the image plane can rotate about the
pointing axis in response to twisting and de-twisting of the
tether. This can further help alignment of the FOV of the image
plane with the observed position of the aerial vehicle, thereby
facilitating approximal siting of the aerial vehicle in the image
plane.
[0039] In an example embodiment, the tether termination unit can
include, or take the form of, a spindle. A spindle can be a
cylindrical length of housing that extends from the tether
termination mount system and through which the tether passes. The
spindle can have two ends, a proximal spindle end and a distal
spindle end. The proximal spindle end can be attached to the tether
termination mount system. The distal spindle end can then extend
towards the distal end of the tether for some distance. The distal
spindle end can be connected to a bearing or some other type of
sensor, such as a strain gauge or load cell to measure force. The
tether may slide in and out of an inner race of the bearing to help
accommodate elongation of the tether when the tether is loaded.
[0040] In a further aspect, the spindle can help to reduce bending
loads on the tether termination mount system. The spindle may also
increase the leverage the tether has on the two-axis mount, which
in turn can help to increase the ability of the mount to follow the
tether. Additionally, the spindle can help align the tether and the
two-axis mount, which in turn can help to reduce the likelihood of
binding forces on the bearings of the two-axis mount.
2. ILLUSTRATIVE SYSTEMS
[0041] a. Airborne Wind Turbine (AWT) System
[0042] FIG. 1 depicts an AWT system 100, according to an example
embodiment. In particular, the AWT 100 includes a ground station
110, a tether 120, and an aerial vehicle 130. As shown in FIG. 1,
the aerial vehicle 130 may be connected to the tether 120, and the
tether 120 may be connected to the ground station 110. In this
example, the tether 120 may be attached to the ground station 110
at one location on the ground station 110, and attached to the
aerial vehicle 130 at two locations on the aerial vehicle 130.
However, in other examples, the tether 120 may be attached at
multiple locations to any part of the ground station 110 and/or the
aerial vehicle 130.
[0043] The ground station 110 may be used to hold and/or support
the aerial vehicle 130 until it is in an operational mode. The
ground station 110 may also be configured to allow for the
repositioning of the aerial vehicle 130, for example to facilitate
deploying of the aerial vehicle 130. Further, the ground station
110 may be further configured to receive the aerial vehicle 130
during a landing. The ground station 110 may be formed of any
material that can suitably keep the aerial vehicle 130 attached
and/or anchored to the ground while transitioning between hover and
crosswind flight.
[0044] In addition, the ground station 110 may include one or more
components (not necessarily shown), such as a winch, that may vary
a length of the tether 120. Such components will be described in
greater detail later in this disclosure. For example, when the
aerial vehicle 130 is deployed, the one or more components may be
configured to pay out and/or reel out the tether 120. In some
implementations, the one or more components may be configured to
pay out and/or reel out the tether 120 to a predetermined length.
As examples, the predetermined length could be equal to or less
than a maximum length of the tether 120. Further, when the aerial
vehicle 130 lands in the ground station 110, the one or more
components may be configured to reel in the tether 120.
[0045] The tether 120 may transmit electrical energy generated by
the aerial vehicle 130 to the ground station 110. In addition, the
tether 120 may transmit electricity to the aerial vehicle 130 in
order to power the aerial vehicle 130 for takeoff, landing, hover
flight, and/or forward flight. The tether 120 may be constructed in
any form and using any material which may allow for the
transmission, delivery, and/or harnessing of electrical energy
generated by the aerial vehicle 130 and/or transmission of
electricity to the aerial vehicle 130. The tether 120 may also be
configured to withstand one or more forces of the aerial vehicle
130 when the aerial vehicle 130 is in an operational mode. For
example, the tether 120 may include a core configured to withstand
one or more forces of the aerial vehicle 130 when the aerial
vehicle 130 is in hover flight, forward flight, and/or crosswind
flight. The core may be constructed of any high strength fibers. In
some examples, the tether 120 may have a fixed length and/or a
variable length. For instance, in at least one such example, the
tether 120 may have a length of 140 meters. However other lengths
may be used as well.
[0046] The aerial vehicle 130 may be configured to fly
substantially along a path 150 to generate (or while it generates)
electrical energy. The term "substantially along," as used in this
disclosure, refers to exactly along and/or one or more deviations
from exactly along that do not significantly impact generation of
electrical energy as described herein and/or transitioning an
aerial vehicle between certain flight modes as described
herein.
[0047] The aerial vehicle 130 may include or take the form of
various types of devices, such as a kite, a helicopter, a wing
and/or an airplane, among other possibilities. The aerial vehicle
130 may be formed of solid structures of metal, plastic and/or
other polymers. The aerial vehicle 130 may be formed of any
material which allows for a high thrust-to-weight ratio and
generation of electrical energy that may be used in utility
applications. Additionally, the materials may be chosen to allow
for a lightning hardened, redundant and/or fault tolerant design
which may be capable of handling large and/or sudden shifts in wind
speed and wind direction. Other materials may be used in the
formation of aerial vehicle as well.
[0048] The path 150 may be various different shapes in various
different embodiments. For example, the path 150 may be
substantially circular. And in at least one such example, the path
150 may have a radius of up to 265 meters. The term "substantially
circular," as used in this disclosure, refers to exactly circular
and/or one or more deviations from exactly circular that do not
significantly impact generation of electrical energy as described
herein. Other shapes for the path 150 may be an oval, such as an
ellipse, the shape of a jelly bean, the shape of the number of 8
("figure eight"), among others.
[0049] As shown in FIG. 1, the aerial vehicle 130 may include a
main wing 131, a front section 132, rotor connectors 133A-B, rotors
134A-D, a tail boom 135, a tail wing 136, and a vertical stabilizer
137. Any of these components may be shaped in any form which allows
for the use of components of lift to overcome gravity and/or move
the aerial vehicle 130 forward.
[0050] The main wing 131 may provide a primary lift for the aerial
vehicle 130. The main wing 131 may be one or more rigid or flexible
airfoils, and may include various control surfaces, such as
winglets, flaps, rudders, elevators, an so on. The control surfaces
may be used to stabilize the aerial vehicle 130 and/or reduce drag
on the aerial vehicle 130 during hover flight, forward flight,
and/or crosswind flight.
[0051] The main wing 131 may be any suitable material for the
aerial vehicle 130 to engage in hover flight, forward flight,
and/or crosswind flight. For example, the main wing 131 may include
carbon fiber and/or e-glass. Moreover, the main wing 131 may have a
variety dimensions. For example, the main wing 131 may have one or
more dimensions that correspond with a conventional wind turbine
blade. As another example, the main wing 131 may have a span of 8
meters, an area of 4 meters squared, and an aspect ratio of 15. The
front section 132 may include one or more components, such as a
nose, to reduce drag on the aerial vehicle 130 during flight.
[0052] The rotor connectors 133A-B may connect the rotors 134A-D to
the main wing 131. In some examples, the rotor connectors 133A-B
may take the form of or be similar in form to one or more pylons.
In example illustrated in FIG. 1, the rotor connectors 133A-B are
arranged such that the rotors 134A-D are spaced between the main
wing 131. In some examples, a vertical spacing between
corresponding rotors (e.g., rotor 134A and rotor 134B or rotor 134C
and rotor 134D) may be 0.9 meters.
[0053] The rotors 134A-D may be configured to drive one or more
generators for the purpose of generating electrical energy. In this
example, the rotors 134A-D may each include one or more blades,
such as three blades. The one or more rotor blades may rotate via
interactions with the wind and which could be used to drive the one
or more generators. In addition, the rotors 134A-D may also be
configured to provide a thrust to the aerial vehicle 130 during
flight. With this arrangement, the rotors 134A-D may function as
one or more propulsion units, such as a propeller. Although the
rotors 134A-D are depicted as four rotors in this example, in other
examples the aerial vehicle 130 may include any number of rotors,
such as less than four rotors or more than four rotors that may be
spaced along main wing 131.
[0054] The tail boom 135 may connect the main wing 131 to the tail
wing 136. The tail boom 135 may have a variety of dimensions. For
example, the tail boom 135 may have a length of 2 meters. Moreover,
in some implementations, the tail boom 135 could take the form of a
body and/or fuselage of the aerial vehicle 130. And in such
implementations, the tail boom 135 may carry a payload.
[0055] The tail wing 136 and/or the vertical stabilizer 137 may be
used to stabilize the aerial vehicle and/or reduce drag on the
aerial vehicle 130 during hover flight, forward flight, and/or
crosswind flight. For example, the tail wing 136 and/or the
vertical stabilizer 137 may be used to maintain a pitch of the
aerial vehicle 130 during hover flight, forward flight, and/or
crosswind flight. In this example, the vertical stabilizer 137 is
attached to the tail boom 135, and the tail wing 136 is located on
top of the vertical stabilizer 137. The tail wing 136 may have a
variety of dimensions. For example, the tail wing 136 may have a
length of 2 meters. Moreover, in some examples, the tail wing 136
may have a surface area of 0.45 meters squared. Further, in some
examples, the tail wing 136 may be located 1 meter above a center
of mass of the aerial vehicle 130.
[0056] While the aerial vehicle 130 has been described above, it
should be understood that the methods and systems described herein
could involve any suitable aerial vehicle that is connected to a
tether, such as the tether 120.
[0057] b. Illustrative Components of an AWT System
[0058] FIG. 2 is a simplified block diagram illustrating components
of an AWT 200. The AWT 200 may take the form of or be similar in
form to the AWT 100. In particular, the AWT 200 includes a ground
station 210, a tether 220, and an aerial vehicle 230. The ground
station 210 may take the form of or be similar in form to the
ground station 110, the tether 220 may take the form of or be
similar in form to the tether 120, and the aerial vehicle 230 may
take the form of or be similar in form to the aerial vehicle
130.
[0059] As shown in FIG. 2, the ground station 210 may include one
or more processors 212, data storage 214, and program instructions
216. A processor 212 may be a general-purpose processor or a
special purpose processor (e.g., digital signal processors,
application specific integrated circuits, etc.). The one or more
processors 212 can be configured to execute computer-readable
program instructions 216 that are stored in data storage 214 and
are executable to provide at least part of the functionality
described herein.
[0060] The data storage 214 may include or take the form of one or
more computer-readable storage media that may be read or accessed
by at least one processor 212. The one or more computer-readable
storage media may include volatile and/or non-volatile storage
components, such as optical, magnetic, organic or other memory or
disc storage, which may be integrated in whole or in part with at
least one of the one or more processors 212. In some embodiments,
the data storage 214 may be implemented using a single physical
device (e.g., one optical, magnetic, organic or other memory or
disc storage unit), while in other embodiments, the data storage
214 can be implemented using two or more physical devices.
[0061] As noted, the data storage 214 may include computer-readable
program instructions 216 and perhaps additional data, such as
diagnostic data of the ground station 210. As such, the data
storage 214 may include program instructions to perform or
facilitate some or all of the functionality described herein.
[0062] In a further respect, the ground station 210 may include a
communication system 218. The communications system 218 may include
one or more wireless interfaces and/or one or more wireline
interfaces, which allow the ground station 210 to communicate via
one or more networks. Such wireless interfaces may provide for
communication under one or more wireless communication protocols,
such as Bluetooth, WiFi (e.g., an IEEE 802.11 protocol), Long-Term
Evolution (LTE), WiMAX (e.g., an IEEE 802.16 standard), a
radio-frequency ID (RFID) protocol, near-field communication (NFC),
and/or other wireless communication protocols. Such wireline
interfaces may include an Ethernet interface, a Universal Serial
Bus (USB) interface, or similar interface to communicate via a
wire, a twisted pair of wires, a coaxial cable, an optical link, a
fiber-optic link, or other physical connection to a wireline
network. The ground station 210 may communicate with the aerial
vehicle 230, other ground stations, and/or other entities (e.g., a
command center) via the communication system 218.
[0063] In an example embodiment, the ground station 210 may include
communication systems 218 that may allow for both short-range
communication and long-range communication. For example, ground
station 210 may be configured for short-range communications using
Bluetooth and may be configured for long-range communications under
a CDMA protocol. In such an embodiment, the ground station 210 may
be configured to function as a "hot spot"; or in other words, as a
gateway or proxy between a remote support device (e.g., the tether
220, the aerial vehicle 230, and other ground stations) and one or
more data networks, such as cellular network and/or the Internet.
Configured as such, the ground station 210 may facilitate data
communications that the remote support device would otherwise be
unable to perform by itself.
[0064] For example, the ground station 210 may provide a WiFi
connection to the remote device, and serve as a proxy or gateway to
a cellular service provider's data network, which the ground
station 210 might connect to under an LTE or a 3G protocol, for
instance. The ground station 210 could also serve as a proxy or
gateway to other ground stations or a command station, which the
remote device might not be able to otherwise access.
[0065] Moreover, as shown in FIG. 2, the tether 220 may include
transmission components 222 and a communication link 224. The
transmission components 222 may be configured to transmit
electrical energy from the aerial vehicle 230 to the ground station
210 and/or transmit electrical energy from the ground station 210
to the aerial vehicle 230. The transmission components 222 may take
various different forms in various different embodiments. For
example, the transmission components 222 may include one or more
insulated conductors that are configured to transmit electricity.
And in at least one such example, the one or more conductors may
include aluminum and/or any other material which may allow for the
conduction of electric current. Moreover, in some implementations,
the transmission components 222 may surround a core of the tether
220 (not shown).
[0066] The ground station 210 may communicate with the aerial
vehicle 230 via the communication link 224. The communication link
224 may be bidirectional and may include one or more wired and/or
wireless interfaces. Also, there could be one or more routers,
switches, and/or other devices or networks making up at least a
part of the communication link 224.
[0067] Further, as shown in FIG. 2, the aerial vehicle 230 may
include one or more sensors 232, a power system 234, power
generation/conversion components 236, a communication system 238,
one or more processors 242, data storage 244, and program
instructions 246, and a control system 248.
[0068] The sensors 232 could include various different sensors in
various different embodiments. For example, the sensors 232 may
include a global a global positioning system (GPS) receiver. The
GPS receiver may be configured to provide data that is typical of
well-known GPS systems (which may be referred to as a global
navigation satellite system (GNNS)), such as the GPS coordinates of
the aerial vehicle 230. Such GPS data may be utilized by the AWT
200 to provide various functions described herein.
[0069] As another example, the sensors 232 may include one or more
wind sensors, such as one or more pitot tubes. The one or more wind
sensors may be configured to detect apparent and/or relative wind.
Such wind data may be utilized by the AWT 200 to provide various
functions described herein.
[0070] Still as another example, the sensors 232 may include an
inertial measurement unit (IMU). The IMU may include both an
accelerometer and a gyroscope, which may be used together to
determine the orientation of the aerial vehicle 230. In particular,
the accelerometer can measure the orientation of the aerial vehicle
230 with respect to earth, while the gyroscope measures the rate of
rotation around an axis, such as a centerline of the aerial vehicle
230. Some IMUs may be commercially available in low-cost, low-power
packages. For instance, the IMU may take the form of or include a
miniaturized MicroElectroMechanical System (MEMS) or a
NanoElectroMechanical System (NEMS). Other types of IMUs may also
be utilized. The IMU may include other sensors, in addition to
accelerometers and gyroscopes, which may help to better determine
position. Two examples of such sensors are magnetometers and
pressure sensors. Other examples are also possible.
[0071] While an accelerometer and gyroscope may be effective at
determining the orientation of the aerial vehicle 230, slight
errors in measurement may compound over time and result in a more
significant error. However, an example aerial vehicle 230 may be
able mitigate or reduce such errors by using a magnetometer to
measure direction. For example, vehicle 230 may employ drift
mitigation through fault tolerant redundant position and velocity
estimations. One example of a magnetometer is a low-power, digital
3-axis magnetometer, which may be used to realize an orientation
independent electronic compass for accurate heading information.
However, other types of magnetometers may be utilized as well.
[0072] The aerial vehicle 230 may also include a pressure sensor or
barometer, which can be used to determine the altitude of the
aerial vehicle 230. Alternatively, other sensors, such as sonic
altimeters or radar altimeters, can be used to provide an
indication of altitude, which may help to improve the accuracy of
and/or prevent drift of the IMU.
[0073] As noted, the aerial vehicle 230 may include the power
system 234. The power system 234 could take various different forms
in various different embodiments. For example, the power system 234
may include one or more batteries for providing power to the aerial
vehicle 230. In some implementations, the one or more batteries may
be rechargeable and each battery may be recharged via a wired
connection between the battery and a power supply and/or via a
wireless charging system, such as an inductive charging system that
applies an external time-varying magnetic field to an internal
battery and/or charging system that uses energy collected from one
or more solar panels.
[0074] As another example, the power system 234 may include one or
more motors or engines for providing power to the aerial vehicle
230. In some implementations, the one or more motors or engines may
be powered by a fuel, such as a hydrocarbon-based fuel. And in such
implementations, the fuel could be stored on the aerial vehicle 230
and delivered to the one or more motors or engines via one or more
fluid conduits, such as piping. In some implementations, the power
system 234 may be implemented in whole or in part on the ground
station 210.
[0075] As noted, the aerial vehicle 230 may include the power
generation/conversion components 236. The power
generation/conversion components 326 could take various different
forms in various different embodiments. For example, the power
generation/conversion components 236 may include one or more
generators, such as high-speed, direct-drive generators. With this
arrangement, the one or more generators may be driven by one or
more rotors, such as the rotors 134A-D. And in at least one such
example, the one or more generators may operate at full rated power
in wind speeds of 11.5 meters per second at a capacity factor which
may exceed 60 percent, and the one or more generators may generate
electrical power from 40 kilowatts to 600 megawatts.
[0076] Moreover, as noted, the aerial vehicle 230 may include a
communication system 238. The communication system 238 may take the
form of or be similar in form to the communication system 218. The
aerial vehicle 230 may communicate with the ground station 210,
other aerial vehicles, and/or other entities (e.g., a command
center) via the communication system 238.
[0077] In some implementations, the aerial vehicle 230 may be
configured to function as a "hot spot"; or in other words, as a
gateway or proxy between a remote support device (e.g., the ground
station 210, the tether 220, other aerial vehicles) and one or more
data networks, such as cellular network and/or the Internet.
Configured as such, the aerial vehicle 230 may facilitate data
communications that the remote support device would otherwise be
unable to perform by itself.
[0078] For example, the aerial vehicle 230 may provide a WiFi
connection to the remote device, and serve as a proxy or gateway to
a cellular service provider's data network, which the aerial
vehicle 230 might connect to under an LTE or a 3G protocol, for
instance. The aerial vehicle 230 could also serve as a proxy or
gateway to other aerial vehicles or a command station, which the
remote device might not be able to otherwise access.
[0079] As noted, the aerial vehicle 230 may include the one or more
processors 242, the program instructions 244, and the data storage
246. The one or more processors 242 can be configured to execute
computer-readable program instructions 246 that are stored in the
data storage 244 and are executable to provide at least part of the
functionality described herein. The one or more processors 242 may
take the form of or be similar in form to the one or more
processors 212, the data storage 244 may take the form of or be
similar in form to the data storage 214, and the program
instructions 246 may take the form of or be similar in form to the
program instructions 216.
[0080] Moreover, as noted, the aerial vehicle 230 may include the
control system 248. In some implementations, the control system 248
may be configured to perform one or more functions described
herein. The control system 248 may be implemented with mechanical
systems and/or with hardware, firmware, and/or software. As one
example, the control system 248 may take the form of program
instructions stored on a non-transitory computer readable medium
and a processor that executes the instructions. The control system
248 may be implemented in whole or in part on the aerial vehicle
230 and/or at least one entity remotely located from the aerial
vehicle 230, such as the ground station 210. Generally, the manner
in which the control system 248 is implemented may vary, depending
upon the particular application.
[0081] While the aerial vehicle 230 has been described above, it
should be understood that the methods and systems described herein
could involve any suitable vehicle that is connected to a tether,
such as the tether 230 and/or the tether 110.
[0082] c. Illustrative Components of a Ground Station
[0083] FIG. 3A illustrates a first perspective view of an example
ground station for an aerial vehicle of an AWT, such as the aerial
vehicle 130 illustrated in FIG. 1. The ground station 300 may be
the same or similar to the ground station 210 of FIG. 2, or the
ground station 110 of FIG. 1. FIG. 3A is representational only and
not all components are necessarily shown. For example, additional
structural or restraining components may not be shown.
[0084] The ground station 300 may include a vertical tower 302,
platform 305, a tether termination mount system 350, and a perch
assembly 314. The tower 302 and platform 305 may be used to
facilitate the perch and launch of an aerial vehicle, such as the
aerial vehicle 130 of the AWT illustrated in FIG. 1. In some
embodiments, the platform 305 may be a perch platform upon which an
aerial vehicle, such as the aerial vehicle 130, can perch (e.g.,
when landing).
[0085] The ground station 300 may also include a winch drum 310 and
winch assembly (not shown), both of which may be coupled to the
platform 305. The platform 305 may be coupled to a rotating member
304 and thereby rotatably coupled to the vertical tower 302. Via
the rotating member 304, the winch drum 310, winch assembly, and
the platform 305 may rotate around an axis such as around a
vertical axis (e.g., rotate in an azimuthal plane). The rotating
member 304 may be a passive slewing ring or an active slewing ring
with a motor drive, for example. In this manner, the platform 305
may be rotated actively or passively around a vertical axis
(representatively shown in FIG. 3A by arrow 304a). While disclosed
embodiments make use of a slewing ring, any rotational bearing or
other configuration may be used that may allow the platform 305 to
rotate around the tower 302. The tower 302 may be a tubular steel
structure, although any structure that can resolve aerial flight
loads may be used.
[0086] The tether 320 may be connected to the ground station 300
via the tether termination mount system 350, and wound onto the
winch drum 310 when the winch drum rotates. In accordance with
example embodiments, the tether termination mount system 350 may
include a tether termination unit 351 that serves as a coupling
mechanism between the tether 320 and the tether termination mount
system 350, and is configured for rotation about two axes in
response to angular motion of tether 320 induced by flight motion
of the aerial vehicle (such as the aerial vehicle 130). As
described below, the tether termination unit 351 may take the form
of an assembly of, or include, other components, including a slip
ring and a spindle, for example. Also as described below, the two
axes of rotation can correspond to respective axes two gimbals.
[0087] The tether 320 may be describe as having a distal tether end
connected to an aerial vehicle of an AWT and a proximal tether end
coupled with the tether termination mount system 350 by way of the
tether termination unit 351. The tether 320 may include one or more
insulated conductors that have a proximal conductor end terminated
at the ground station 300 and a distal conductor end that is
coupled to the aerial vehicle of an AWT. The tether termination
mount system 350 may be coupled to the winch drum 310 in various
ways. For example, the tether termination mount system 350 may be
connected to a support structure 360, which in turn is connected to
the winch drum 310. The tether 320 may be guided onto winch drum
310 via a levelwind (not shown).
[0088] The platform 305 (and winch assembly, including winch drum
310) may rotate in response to bias pressure applied by the tether
320 to the tether termination mount system 350. For example, if an
AWT attached to the tether 320 changes its azimuth angle relative
to winch drum 310 (or the platform 305), the tether 320 may apply
bias pressure to the tether termination mount system 350, which may
in turn cause the platform 305 to rotate passively towards the bias
direction.
[0089] Alternatively or additionally, the tether termination mount
system 350 may include sensors to detect bias pressure applied by
the tether 320 to the tether termination mount system 350. The
sensors may be pressure sensors, force sensors, vibrational
sensors, or any other sensors configured to detect bias pressure.
In response to detecting bias pressure, a motor (not shown) or
other active component may then cause the platform 305 to rotate
relative to the tower 302 and in the direction of the bias.
Allowing or causing the platform 305 and winch assembly to rotate
around the tower 302 and to face a tension force applied by the
tether 320 can help reduce side loading forces and other
undesirable forces acting on components of the ground station
300.
[0090] d. Illustrative Components of a Tether Termination Mount
System
[0091] FIG. 3B illustrates a second perspective view of the ground
station 300, which, again, may be similar to the ground station 210
of FIG. 2, or the ground station 110 of FIG. 1. FIG. 3B is
representational only and not all components are shown. For
example, additional structural or restraining components may not be
shown.
[0092] As illustrated in FIG. 3B, the tether termination mount
system 350 may be coupled to the winch drum 310 and may include one
or more gimbals, for example the gimbals 352A and 352B, as well as
the tether termination unit 351. In accordance with example
embodiments, each of the gimbals 352A and 352B can serve as a
structural support for the tether termination unit 351 that allows
the tether termination unit 351 to rotate about a respective gimbal
axis. For example, the gimbal 352A may be configured to allow the
tether termination unit 351 to rotate about a primary axis, such as
an elevation (also referred to as altitude) axis or a z-axis with a
range of angles .eta. to -.eta. (representatively shown in FIG. 3B
as arrow 352z, for the current orientation of the tether
termination mount). Further, the gimbal 352A can be mounted in the
gimbal 352B in a manner that allows the tether termination unit 351
to rotate about a secondary axis, such as an azimuth axis or an
x-axis, with a range of angles .phi. to -.phi. (representatively
shown in FIG. 3B as arrow 352x, for the current orientation of the
tether termination mount). In FIG. 3B, the gimbals 352A and 352B
are shown with rotational axes oriented perpendicular (orthogonal)
to each other; however, the primary and secondary axes may be
aligned at some orientation other than perpendicular. Also, the
gimbals 352A and 352B may additionally or alternatively be
configured to rotate without limitation as to the range of angles
(i.e., a gimbal could rotate completely about its axis) or with
varying limits to the range of angles. Additionally, there may be
more or less than two gimbals in the system.
[0093] Each of the gimbals 352A and 352B may include a bearing
system. The bearing system may have a low overturning moment to
allow for easy rotation. In an example embodiment, the bearing
system can include two radial bearings, with one bearing located
near each end of gimbal system. Each the gimbals 352A and 352B may
further include one or more encoders, such as a rotary encoder. The
encoder may provide feedback related to speed, motion, and/or
angular position of the gimbal about its axis of rotation.
[0094] In accordance with example embodiments, the tether
termination unit 351 can be configured in the gimbals 352A and 352B
such that both of the two gimbal rotation axis are orthogonal to a
common pointing direction axis of the tether termination unit 351.
With this arrangement, the pointing direction axis--or just
"pointing direction" for short--will sweep through the range of
rotation angles of the two gimbal rotation axes as the tether
termination unit 351 rotates about the two axes. In particular, for
the example configuration of FIG. 3B in which the gimbals 352A and
352B provide rotation about elevation and azimuth angles, and the
pointing direction is perpendicular to both gimbal rotation axes,
the pointing direction will point in a direction given by the
elevation and azimuth angles of the gimbal axes at any given
instant. It will be appreciated that other configurations of
angular orientation between the pointing direction and the gimbal
rotation axes could be used in which the pointing direction axis
can sweep out range of elevation and azimuth angles. However, such
other configurations may not necessarily provide a direct,
one-to-one correspondence between gimbal rotation angle and
elevation or azimuth of the pointing direction.
[0095] In accordance with example embodiments, the tether
termination unit 351 can include a physical channel or tube through
which the tether 320 is "threaded," and out from which the tether
320 emerges or "emanates" and then extends toward its connection
point(s) on the aerial vehicle. More particularly, the physical
channel (or tube) of the tether termination unit 351 can be aligned
with the pointing direction. With this arrangement, the proximal
end of the tether 320 can be described as emerging or emanating
from the tether termination unit 351 along the pointing direction.
As discussed below, the path of the tether 320 from its proximal
end, where it emerges from the tether termination unit 351, to its
distal end, where it connects to the aerial vehicle, may not
necessarily remain aligned with the pointing direction, and may not
necessarily even be a straight line.
[0096] In accordance with example embodiments, the tether
termination unit 351 can be an assembly that includes a slip ring
354, and a spindle 356. The physical channel of the tether
termination unit can pass through both the slip ring 354 and the
spindle 356. The slip ring 354 can include a stationary portion
354A and a rotatable portion 354B. The slip ring 354 may be any
electromechanical device that allows transmission of power and
electrical signals from a rotating structure to a stationary
structure, and may be used during unrestrained, intermittent or
continuous rotation. In an example embodiment, the slip ring 354
can be constructed such that its rotatable portion 354B is
configured for rotation about the point direction axis.
[0097] As illustrated in FIG. 3B, the slip ring 354 may be directly
connected to the tether termination mount system 350. For example,
the stationary portion 354A of slip ring 354 may be mechanically
coupled to gimbal 352A. As such, the stationary portion 354A, being
an assembly component of the tether termination unit 351, can serve
as a physical coupling of the tether termination unit 351 to the
gimbal 352A. The rotatable portion 354B can be configured for
rotation about the pointing direction in response to rotation of
the tether 320 about its axial dimension. The rotatable portion
354B and the tether 320 may further be electrically coupled. The
stationary portion 354A and the rotatable portion 354B may also be
electrically coupled, which can then enable the tether 320 to
rotate while still communicating power and/or electrical signals to
the ground station 300. For example, the proximal conductor end of
one or more insulated conductors 358 included in tether 320 may be
electrically coupled to the rotatable portion 354B of the slip ring
354. Slip ring 354 may be rotated actively (e.g., with a
servomotor) or rotated passively (e.g., with potential energy
stored by the action of a rotating tether in a torsion spring).
[0098] As noted, the tether termination unit 351 may also include a
spindle 356, for example as an assembly component. The spindle 356
may be a length of housing that extends from the tether termination
unit 351. In accordance with example embodiments, the tether 320
can pass through the spindle 356 via the physical channel.
Alternatively the tether 356 could be fixed along the spindle
356.
[0099] For purposes of discussion the spindle 356 can be described
as having a proximal spindle end 356a, where the spindle attaches
to the tether termination unit 351, and a distal spindle end 356b,
from which the tether 320 emerges or emanates. In the example
embodiment illustrated in FIG. 3B, the proximal spindle end 356a
may be attached to the tether termination unit 351 by way of the
slip ring 354. The distal spindle end 356b may extend towards the
distal point of the tether for some length. In an example
embodiment, the distal spindle end 356b may include a bearing
system, for example, a radial bearing, which may be used to
accommodate rotation of the tether 320 within or about the spindle
356.
[0100] In further accordance with example embodiments, the spindle
356 may help to reduce bending loads on portions of the tether
termination mount system 350, such as on the gimbals 352A and 352 B
and/or slip ring 354. For example, if the tether 320 is connected
to an aerial vehicle of an AWT in cross-wind flight, the tether 320
may act as a bending load on those, and other, portions of the
tether termination mount system 350. The spindle 356 may extend the
point of contact of the tether 320 on the tether termination mount
system 350. For example, FIG. 3B illustrates a spindle 356
extending the point of contact of the tether 320 on the tether
termination mount system 350 by the distance from 356a to 356b.
Consequently, the spindle 356 may also increase the leverage the
tether 320 has on the tether termination mount system 350, which in
turn may help to increase the ability of the tether termination
mount system 350 to turn and follow the tether 320. As shown in
FIG. 3B, the spindle 356 can act as a lever for applying greater
leverage from the tether 320 to the tether termination mount system
350 than would be applied without the spindle 356.
[0101] Considering a tether termination unit 351 that, for purposes
of an example comparison, does not include a spindle 356, a bias
pressure or force from the tether 320 would apply a relatively low
level of rotational torque to the tether termination mount system
350 about one or more of the gimbal axes. For example, taking the
lever arm of a spindle-less tether termination unit 351 to the be
the vector A of length .parallel.A.parallel., and the force vector
applied by a biased tether 320 to be F, the torque, T.sub.1,
applied to a gimbal axis may be calculated as T.sub.1=F.times.A,
where .times. denotes the cross product.
[0102] In contrast, a tether termination unit 351 that does include
a spindle 356 can develop a larger torque for the same applied
tether force F at a distance greater than .parallel.A.parallel..
For example, taking a lever arm vector B of length
.parallel.B.parallel., where .parallel.B.parallel. is
.parallel.A.parallel. plus the length of the spindle 356, as shown
in FIG. 3B, then applying the tether force F at the distal tether
end 356b will result in a torque T.sub.2 given by
T.sub.2=F.times.B, where .times. again denotes the cross product.
For the same force F in both cases, and the same angle between F
and A as between F and B, the resulting torque T.sub.2 the applied
to the tether termination mount system 350 will have a larger
magnitude than the torque T.sub.1 that would be applied to the
tether termination mount system 350 where a spindle 356 is not
utilized.
[0103] In further accordance with example embodiments, the distal
spindle end 356b may include a sensor (e.g., a force sensor such as
a strain gauge or a load cell). For example, a force sensor may be
used to determine the force of the tether 320 against the distal
spindle end 356b. This force may be used to determine whether to
actively rotate a portion of the ground station 300. For example,
if the determined force of the tether 320 against the distal
spindle end 356b is above a threshold, the winch assembly may
actively rotate the winch drum 310 in a direction to reduce force.
To help accommodate elongation of the tether 320 when the tether
320 is loaded, the tether 320 may be allowed to slide within or
along the sensor, as noted.
[0104] FIG. 3C illustrates a cross-section view of a tether with
insulated electrical conductors, according to an example
embodiment. The tether 320 may have one or more insulated
electrical conductors 358 as previously described. In addition, the
tether 320 can include one or more fiber-optic cables or carriers
(e.g., optical fibers) configured for transmitting optical signals
between a ground station, such as the ground station 300, and an
aerial vehicle, such as the aerial vehicle 130.
[0105] FIG. 4 illustrates a perspective view of an alternative
embodiment of a tether termination mount system 450 for a ground
station of an aerial vehicle of an AWT, such as the aerial vehicle
130 illustrated in FIG. 1. The tether termination mount system 450
may be the same or similar to tether termination mount system 350
of FIG. 3B. FIG. 4 is representative only and not all components
are shown. For example, additional structural or restraining
components may not be shown.
[0106] The tether termination mount system 450 may include gimbals
452A and 452B, a slip ring 454, a spindle 456, a drive mechanism
458, encoders 460A and 460B, gimbal bearing systems 462A and 462B,
a spindle bearing system 464, and a spindle sensor 466. These
components may operate with a tether 420 and ground station in the
same or a similar manner to those already described. As shown, the
spindle 456 and slip ring 454 may be components of a tether
termination unit 451. Note that the tether termination unit 451 may
extend leftward of the gimbal 452B, although the identifying
bracket delineating the tether termination unit 451 stops short of
the gimbal 452B to avoid overcrowding of elements in FIG. 4.
[0107] As described above, the tether 420 may rotate, for example,
due to the cross-wind flight pattern of an aerial vehicle of an
AWT. In an example embodiment, the drive mechanism 458 may actively
or passively rotate a rotatable portion of the slip ring 454 and/or
the spindle 456. As illustrated in FIG. 4, a drive mechanism 458
may be directly connected to the slip ring 454, or it may be
coupled to the slip ring 454 through a drivetrain or other power
transmission system. The drive mechanism 458 may actively rotate
the rotatable portion of the slip ring 454 to follow the rotation
of the tether 420 about its axial dimension. For example, the drive
mechanism 458 may be a servomotor. In a further aspect, the drive
mechanism 458 may passively rotate the rotatable portion of the
slip ring 454 to follow the rotation of the tether 420. For
example, the drive mechanism 458 may be a torsion spring that
stores potential energy P from the action of a rotating tether
until the potential energy P is greater than an overturning moment
M required to turn the rotatable portion of the slip ring 454.
[0108] e. Illustrative Components and Operation of Enhanced Aerial
Vehicle Tracking
[0109] As described above, the tether termination unit (e.g.,
tether termination unit 351 or 451) may be configured for rotation
about the gimbal axes in response to motion of the tether (e.g.,
tether 320), as an aerial vehicle (e.g., aerial vehicle 130)
connected to the tether moves in flight. During stable flight of
the aerial vehicle, such as in cross wind flight, the tether may
tend to extend from the tether termination unit toward the aerial
vehicle generally along the pointing direction of the tether
termination unit. As a result, the angular motion of the tether
termination unit may tend to approximately track the motion of the
aerial vehicle. If the tether were perfectly rigid and straight,
angular tracking of the aerial vehicle by the tether termination
unit could be nearly exact. In practice, however, the tether,
describable as a flexible cable line, can be subject to forces that
may cause it to deviate from following a straight-line path to the
aerial vehicle. For example, the tether may tend to sag vertically
in a catenary droop under the force of gravity. In addition, the
tether may experience both horizontal and vertical drag forces as
the aerial vehicle moves about in flight. And angular motion of the
tether can also result in curvature due to inertial forces. As a
consequence of any or all of these forces, the pointing direction
of the tether termination unit may be rendered an approximation of
the angular position of the flying aerial vehicle. Correspondingly,
the gimbal axis angles, as measured by the encoders, for example,
may also then provide approximate rather than precise
determinations of the flying vehicle's angular position in the
sky.
[0110] Pointing direction of the tether termination unit and
angular motion of the tether termination unit induced by a tether
in response to flight motion of an aerial vehicle are illustrated
conceptually by way of example in FIG. 5. The right hand side of
FIG. 5 depicts a ground station 502 to which a flying aerial
vehicle 504 is tethered via a tether 506. The ground station 502
could be the same or similar to the ground stations 110, 210, or
300 described above, for example. However, for purposes of the
present illustration, detailed depiction of components of the
ground station 502 are not necessarily shown in FIG. 5.
[0111] By way of example, the aerial vehicle 504 is shown at two
locations, labeled "(a)" and "(b)," along a substantially circular
path 508 lying in a plane 509. In the example illustrated, the
aerial vehicle 504 may be considered as circling (at least
approximately so) about a rotation axis 510, which is depicted as
being perpendicular to the plane 509; the circular path 508 has a
radius 512 that lies in the plane 509, which is depicted as being
inclined to the vertical line 514. The vertical height of the
aerial vehicle can be referenced to the vertical line 514. It
should be noted that the concepts illustrated in this example do
not necessarily rely on the level of specificity of the path of the
aerial vehicle 504 is described herein, and that other paths of
flight motion of the aerial vehicle 504 are possible as well. For
example, the path could be a "figure-8" in a plane, such as the
plane 509; or the plane could be oriented vertically.
[0112] The depiction of the flight motion of the aerial vehicle 502
in FIG. 5 also illustrates curvature of the tether 506, which could
result from one or more of the forces described above. In
particular, the example shows how the pointing direction of the
tether termination unit can deviate from the angular position of
the aerial vehicle. As illustrated, when the aerial vehicle 504 is
at location (a), the tether termination unit points in pointing
direction 516-a; when the aerial vehicle 504 is at location (a),
the tether termination unit points in pointing direction 516-b.
Because of the curvature of the tether on its path from the ground
station to the aerial vehicle 504, the pointing directions 516-a
and 516-b each appear to point only generally (or approximately) in
the direction of the aerial vehicle 504 at the locations (a) and
(b), respectively. To the extent that the pointing direction can be
measured in terms of gimbal axis angles, the measured gimbal axis
angles may in turn yield only approximate locations of the aerial
vehicle during flight.
[0113] The left hand side of FIG. 5 is a schematic depiction of the
relation between the pointing direction of a tether termination
unit 551 and the tether 506 for the aerial vehicle 504 at locations
(a) and (b). By way of example, the tether termination unit 551 is
taken to be mounted in an elevation gimbal 520 and an azimuth
gimbal 518, which together enable the tether termination unit to
rotate simultaneously in elevation angle, about an elevation
rotation axis 521, and azimuth angle, about an azimuth rotation
axis 519. For each of the locations (a) and (b) of the aerial
vehicle 504, an elevation angle "El" of the tether termination unit
551 (and of the elevation gimbal 520) is shown in a side view (as
labeled), and an azimuth angle "Az" of the tether termination unit
551 (and of the azimuth gimbal 518) is in a top view (as labeled).
The side view corresponds to a view along a direction co-aligned
with (parallel to) the elevation rotation axis 521; the top view
corresponds to a view along a direction co-aligned with (parallel
to) the azimuth rotation axis 519. Note that the gimbals 518 and
520 are represented schematically in FIG. 5, and the structural
components are illustrative and not necessarily shown to scale.
[0114] In each view and for each of the locations (a) and (b), the
pointing direction of the tether termination unit 551 is
represented as a straight arrow aligned with the tether termination
unit 551. As shown by way of example for both of locations (a) and
(b) and in both the top an side views, the pointing direction
appears to be tangent to the tether 506 at the proximal end, where
the tether 506 emanates from the tether termination unit 551. But
beyond this point, where the tether 506 follows a path to the
aerial vehicle 504, the tether 506 appears to curve away from the
pointing direction. This again illustrates that the pointing
direction may provide an approximate sighting direction to the
aerial vehicle 504, but may not necessarily point directly at the
aerial vehicle 504 on a persistent basis. Consequently,
measurements of the gimbal axis angles, for example as provided by
angle encoders, may not necessarily yield persistently accurate
elevation and azimuth angles of the aerial vehicle 504.
[0115] For purposes of the discussion herein, the pointing
direction of a tether termination unit, such as the tether
termination unit 551, can be considered as providing (or
corresponding to) a "nominal pointing direction of the aerial
vehicle," or just "nominal pointing direction" for short. There may
be instances when the tethered flying aerial vehicle moves directly
across, or lingers direction in, the nominal pointing
direction--and for such an instances, the nominal pointing
direction may coincide with the true direction of the aerial
vehicle. However, because of the generally curved path of the
tether, the nominal pointing direction may not generally be a
persistent indicator of the aerial vehicle's true angular position
in the sky.
[0116] In accordance with example embodiments, one or more adaptive
measurement techniques can be applied to the nominal pointing
direction of a tethered flying aerial vehicle in order to determine
one or more corrections to the nominal pointing direction, and to
thereby derive a corrected pointing direction that more accurately
corresponds to a true direction to the aerial vehicle. More
specifically, an imaging device can be attached to the tether
termination unit with an image plane of the imaging device oriented
at a fixed angle to the pointing direction. In an example
embodiment, the fixed angle could be 90.degree., so that the image
plane is perpendicular to the pointing direction. In this
configuration, a line-of-sight (LOS) direction of the imaging
device that is normal to the image plane will be parallel to the
pointing direction, and thus also parallel to the nominal pointing
direction of the aerial vehicle.
[0117] In further accordance with example embodiments, the nominal
pointing direction can serve as a sufficiently accurate alignment
between the LOS direction and the true direction to the aerial
vehicle to accommodate sighting of the aerial vehicle within a
field-of-view (FOV) of the image plane. An image of the aerial
vehicle in the image plane can be analyzed to derive an angular
offset between the LOS in the image plane and the image of the
aerial vehicle. The derived angular offset can then be used to
determine corrections to one or more measured angles of the
pointing direction. The determined corrections can be applied to
the one or more measured angles in order to determine corrected
angles that more accurately correspond to the true angular position
of the aerial vehicle in the sky, as determined with respect to a
ground station 502.
[0118] FIG. 6 illustrates a perspective view of an example tether
termination mount system 600 that includes an imaging device 612,
according to an example embodiment. By way of example, the tether
termination mount system 600 includes a tether termination unit 651
mounted in a gimbal 608 with a rotation axis 609, and in a gimbal
610 with a rotation axis 611. In accordance with example
embodiments, the gimbal 608 could be an azimuth gimbal and the
rotation axis 609 could be an azimuth axis; and the gimbal 610
could be an elevation gimbal and the rotation axis 611 could be an
elevation axis. However, other axial orientations could be used as
well. Also in accordance with example embodiments, the tether
termination unit 651 can be an assembly including a slip ring 602
and a spindle 604, as shown. These components could the same or
similar to the slip rings and spindles discussed above.
[0119] In further accordance with example embodiments, a tether 606
can emerge from (or emanate from), or be coupled to, a distal end
of the spindle 604 (only a portion of the proximal end of the
tether 606 is shown). As illustrated, the tether termination unit
651 (and by extension the spindle 604) has a pointing direction 605
aligned with an axial dimension of the spindle 604. A depiction of
the tether 606 as curving away from the pointing direction 605 is
meant to represent the tether curvature, as discussed above, which
can result in a mis-alignment between the pointing direction 605
and a true angular position of a tethered flying aerial vehicle in
the sky.
[0120] In accordance with example embodiments, the imaging device
612 can be attached to the tether termination unit 651, with a LOS
direction 613 co-aligned with the pointing direction. The imaging
device 612 can be a camera including a lens or other focusing
element. Non-limiting examples of an imaging device 612 include a
digital camera, a line-scan camera, an imaging RADAR, and an
imaging LIDAR. For the LOS direction 613 oriented normally to an
image plane of the imaging device 612, this configuration orients
the image plane normally to the pointing direction as well. In
further accordance with example embodiments, a FOV of the image
plane can be sufficiently large in angular projection on the sky
(and in the LOS direction 613) to be able to capture the flying
aerial vehicle at least under circumstances of stable flight (such
as during cross wind flight).
[0121] FIG. 7 is schematic illustration of an example image plane
during example operation of a tether termination mount system,
according to an example embodiment. The left hand side of the
figure depicts a ground station 702 to which a flying aerial
vehicle 704 is tethered via a tether 706. The ground station 702
could be the same or similar to the ground stations 110, 210, or
300 described above, for example. However, for purposes of the
present illustration, detailed depiction of components of the
ground station 702 are not necessarily shown in FIG. 7. In
accordance with example embodiments, the ground station 702 can
also include an imaging device, such as the imaging device 612.
[0122] As shown, a pointing direction 708 of a tether termination
unit (not shown) of the ground station 702 points in an approximate
direction of the aerial vehicle 704. But because of curvature of
the tether 706, the pointing direction 708 may not align precisely
with a true angular position of the aerial vehicle 704 in the sky.
As described above, the pointing direction 708 can also be taken to
be a nominal pointing direction of the aerial vehicle 704.
[0123] The right hand side of FIG. 7 shows a representational
depiction of an image plane 710 of an imaging device (not shown),
such as imaging device 612, that could be attached to the tether
termination unit of the ground station 702. In accordance with
example embodiments, the image plane can be oriented
perpendicularly to the pointing direction 708, and aligned so that
the pointing direction 708 coincides with the center of the FOV of
the image plane 710. This coincidental alignment is indicated by a
bore sight 708-i marking the center of the FOV of the image plane
710. The bore sight 708-i can also be taken as the LOS of the
imaging device viewed in the image plane 710, as well as the
position of the nominal pointing direction in the FOV.
[0124] By way of example, a representation of an aerial-vehicle
image 704-i of the aerial vehicle 704 is displayed in the FOV of
the image plane 710, to the left and above the bore sight 708-i. A
tether image 706-i of a portion of the tether 706 near its distal
end is also displayed in the image plane 710. The apparent offset
between the position of the aerial-vehicle image 704-i and the bore
sight 708-i exemplifies a mis-alignment between the pointing
direction 708 and the true angular position of the aerial vehicle
704 in the sky. The appearance of an aerial-vehicle image 704-i of
the aerial vehicle 704 in the FOV of the image plane 710 also
exemplifies the functional aspect of the pointing direction 708
that, in accordance with example embodiments, enables the aerial
vehicle 704 to be captured visually within the FOV of the image
plane 710 by tracking (at least approximately) the tethered flying
aerial vehicle 704 via tether motion.
[0125] In accordance with example embodiments, the apparent offset
between the position of the aerial-vehicle image 704-i and the bore
sight 708-i can be used to determine one or more corrections to
angular measurements of the pointing direction 708, and to thereby
determine a more accurate measurement of the angular position of
the aerial vehicle 704 on the sky. The correction technique is
illustrated by way of example by an azimuth correction ".DELTA.Az"
and an elevation correction ".DELTA.El" indicated in the image
plane 710. As shown, these two corrections correspond to azimuth
and elevation components of the offset. In an example embodiment,
the FOV of the image plane 710 could have linear dimensions
measured in angular degrees. For example, the horizontal width of
the FOV, corresponding to azimuthal extent, could be 20.degree.;
and the vertical height of the FOV, corresponding to elevation
extent, could be 15.degree.. It will be appreciated that these are
example sizes of angular extent, and that other sizes could be used
as well.
[0126] In further accordance with example embodiments, the image
plane 710 could correspond to a two-dimensional pixel array. With
this arrangement, and ignoring possible FOV distortion due to the
lens, each pixel could correspond to an azimuthal angular
resolution given approximately by the azimuthal angular extent of
the FOV of the image plane 710 divided by the number of pixels in
the horizontal (azimuthal) direction. Similarly, each pixel could
correspond to an elevation angular resolution given approximately
by the elevation angular extent of the FOV of the image plane 710
divided by the number of pixels in the vertical (elevation)
direction. Analysis of the image plane 710 can then be used to
determine each of .DELTA.AZ and .DELTA.El in terms of a respective
number of pixels, which can be converted angular offset by
multiplying by the angular resolution a pixel in the each dimension
(azimuth and elevation). For some configurations, the lens of an
imaging device can introduce some distortion, such that angular
resolution and/or angular position in the image plane may not be a
simple linear function of pixel position. One example among others
is a fish-eye lens. In such cases, a mapping function can be
devised that accounts or compensates for one or another type of
distortion or nonlinear relation between pixel position and angular
resolution and/or position in the FOV. Analysis of the image plane
710 can then include application of the mapping function.
[0127] Considering again an example of a 20.degree. by 15.degree.
(azimuth by elevation) image plane of an example imaging device
with a linear mapping function (i.e., fixed angular size for all
pixels), and taking, also by way of example, the number of pixels
in the azimuth and elevation dimensions of the image plane 710 to
be 1,024 and 780, respectively, the angular resolution of each
pixel would then be approximately 0.02.degree. in each dimension.
If, in example operation, the aerial-vehicle image 704-i were
determined to be 300 azimuth pixels to the left of the bore sight
708-i and 250 elevation pixels above the bore sight 708-i, then the
angular offsets could be determined to be
.DELTA.Az.apprxeq.12.degree. and .DELTA.El.apprxeq.5.degree.. These
corrections could be added to the azimuth and elevation angles of
the pointing direction 708 to determine an accurate angular
position of the aerial vehicle 704 in the sky, as measured with
respect to the ground station 702.
[0128] In further accordance with example embodiments, the angular
orientation of the pointing direction can be measured with respect
to local reference angles at the ground station 702. For example,
the elevation angle could be measured with respect to a local
horizon at the ground station, and the azimuth angle could be
measured with respect a local meridian (line of geographic or
geodetic longitude). However, the local reference angles could be
other than a local horizon angle and/or local meridian.
[0129] In accordance with example embodiments, a ground station,
such as the ground station 702, for example, can determine a
distance to a tethered, flying aerial vehicle, such as aerial
vehicle 704. Distance determination is signified in the
illustration of FIG. 7 by a distance "D" shown between the ground
station 702 and the aerial vehicle 704. One technique can be to
determine the length of the tether during flight, and then take the
length to be at least an estimate of the distance. Tether length
can be determined, for example, by monitoring and measuring the
tether as it is unwound (e.g., from a drum or spool) during ascent
of the aerial vehicle.
[0130] In further accordance with example embodiments, a LOS
distance from the ground station 702 to the aerial vehicle 704 can
be determined using a distance ranging device. The distance ranging
device can also be attached to the tether termination unit. For
example, the distance ranging device can be separate from the
imaging device. Alternatively, the distance ranging device could be
a component of the imaging device, or the imaging device and the
distance ranging device could be integrated as a single device
configured for distance-range imaging.
[0131] Examples of a distance ranging device include a RADAR device
and a Light Detection and Ranging (LIDAR) device. A RADAR device
operates by detecting a back reflection from a distance object of
radio waves or signals emitted by the RADAR device. By measuring a
time delay between an emitted signal and the reflected, return
signal, a distance to the reflecting object can be determined by
dividing one half of the round-trip time delay by the speed of
light (possibly adjusted for a medium through which the radio
signal travels, such as air). A LIDAR device works in a similar
manner, except that it utilizes optical light instead of radio
radiation. Other non-limiting examples of a ranging device and/or
ranging technique include RF ranging. For example, the aerial
vehicle 704 can transmit a RF signal to the ground station 702,
which can then measure the LOS distance by determining a
propagation delay of the signal.
[0132] Both RADAR and LIDAR devices can be configured for imaging
by generating an effective spatial array of reflected, return
signals distributed across a FOV or a portion of a FOV. This can be
achieved by causing the device (RADAR or LIDAR) to scan across the
FOV in each of two dimensions, and then correlating return signals
with the scan positions in the FOV of the emitted signals. In this
way, the return signals can be associated with array positions, and
considered to effectively correspond to image pixels. In addition
to distance ranging, a resulting image can provide a
three-dimensional image of a distant object (or objects) from which
a spatial array of back-reflected signals is received. Each
back-reflected signal can be used to determine a distance to a
different point on the projected surface of the distant object, and
the array of determined distances can thereby yield a
three-dimensional relief map of the distant object. In practice, a
LIDAR device can provide higher spatial resolution than a RADAR
device, because the wavelength of optical radiation (light) is much
smaller than that of radio radiation (and spatial resolution is
inversely proportional to wavelength).
[0133] In accordance with example embodiments, the imaging device,
such as imaging device 612, can be an integrated distance-ranging
imaging device, such as a LIDAR imaging device. For such an
embodiment, the image of an aerial device in an image plane, such
as the aerial-vehicle image 704-i in the image plane 710, can be a
three-dimensional image. Analysis of the image plan data can be the
same or similar to that described above, such that a correction to
the pointing direction can be determined in terms of pixel offsets
in the azimuth and/or elevation directions.
[0134] In further accordance with example embodiments, a distance
from the ground station 702 to the aerial vehicle 704 (such as D in
FIG. 7) can be used to convert an angular offset correction, such
as .DELTA.Az and/or .DELTA.El, into an approximate linear distance.
For example, by estimating that .DELTA.Az and .DELTA.El correspond
to linear distance corrections .DELTA.x and .DELTA.y, respectively,
at a distance D to the aerial vehicle image 704, linear distance
corrections can be estimated as D.times.sin(.DELTA.Az) and .DELTA.y
D.times.sin(.DELTA.El). Note that for a small angle .DELTA..theta.,
sin(.DELTA..theta.).apprxeq..DELTA..theta..
[0135] In the schematic illustration of FIG. 7, the aerial-vehicle
image 704-i in the image plane 710 is represented as a likeness of
the aerial vehicle 704. This can be the case, for example, if the
imaging device is a digital optical camera or other optical imaging
device. A LIDAR device could similarly yield a three-dimensional
relief-map image of the aerial vehicle 704. In practice, automatic
analysis of imaging data, such as the data that might represent the
aerial-vehicle image 704-i in the image plane 710, may entail one
or more techniques for recognizing that the aerial vehicle 704 has
been imaged (as the aerial-vehicle image 704-i) in image plane 710.
For example, a pattern recognition technique utilizing a neural
network can be used to recognize the image as being that of the
aerial vehicle 704. Other techniques can be used as well. Once an
image in the image plane 710 is recognized or determined to be an
image of the aerial vehicle 704, angular offset analysis can be
applied as described above.
[0136] In further accordance with example embodiments, an aerial
vehicle, such as the aerial vehicle 704, can include one or more
identifying markings on its surface that could help enhance the
effectiveness and/or reliability of a vehicle image recognition
technique. For example, the aerial vehicle can be equipped with one
or more LEDs (or other lights) configured for emitting a specific
spatial and/or temporal (e.g. blinking) pattern of lights. Further,
the imaging device (or an associated processing component) can be
configured to recognize the specific pattern in the image plane of
the imaging device. In an example embodiment, the imaging device
can be a line-scan camera that is synchronized with the specific
pattern.
[0137] In an example embodiment, one or more reflective strips can
be applied to the surface of the aerial vehicle 704 that help
enhance reflection of a LIDAR or RADAR signal, such that a
back-reflected signal from the one or more reflective strips can be
reliably detected and distinguished from other signals that might
be detected by a LIDAR or RADAR device. Using such reflective
markers can also simplify the task of image recognition. For
example, instead of a possibly compute-intensive operation of full
pattern recognition for determining that imaging data includes or
contains an image of an aerial vehicle (e.g., the aerial-vehicle
image 704-i), recognition of one or more emitted and/or reflected
signals by one or more markings can, by comparison, be simpler
task.
[0138] In view of the possible use of LEDs and/or reflective
markings as described above, terminology such as "an image of an
aerial vehicle," or the like, shall be taken herein to include one
or more images of a specific pattern of lights (e.g. LEDs), a
specific optical (e.g. painted) pattern, and/or a back-reflected
signal from one or more reflective markings on an aerial vehicle.
More particularly, "an image of an aerial vehicle," or the like,
shall be taken to refer to a specific pattern of lights (e.g.
LEDs), a specific optical (e.g. painted) pattern, and/or
back-reflected signals, in addition to an optical or LIDAR image in
a form similar to the representation of the aerial-vehicle image
704-i in FIG. 7, or to just the back-reflected signal by
itself.
[0139] As described above, the pointing direction of a tether
termination unit can provide at least an approximate sighting
direction to a tethered aerial vehicle during stable flight, such
as in cross wind flight. There may be circumstances or instances in
which a deviation between the pointing direction and a true,
current angular position of an aerial vehicle in the sky exceeds a
boundary of the FOV of an image plane of an imaging device. For
example, if, during cross wind flight, the wind shifts direction by
a significant amount, the aerial vehicle might travel to a location
that takes it at least partially, and momentarily, out of the FOV
of the imaging device. If and when such a circumstance or instance
occurs, image analysis for determining one or more corrections to
the pointing direction might be impaired or inoperable, at least
temporarily. In order to help mitigate possible effects of such
circumstances or instances, the size of the FOV can be configured
to be large enough to maintain an aerial vehicle's image within the
FOV for a large range of angular deviations between pointing
direction and the true angular position of the aerial vehicle in
the sky, and over an expected range of operating conditions of the
aerial vehicle in tethered flight.
[0140] At the same time, for a given number of pixels, the larger
the FOV, the lower the angular resolution, and consequently, the
lower the precision with which angular corrections may be
determined. Conversely, for a given pixel angular resolution, the
larger the FOV, the larger the number of pixels, and consequently
the more compute-intensive the image analysis may be. In accordance
with example embodiments, the FOV size and pixel angular resolution
can be configured in manner aimed at optimizing image analysis
complexity/compute-intensiveness and precision of determined
angular correction.
[0141] In further accordance with example embodiments, one or more
techniques can be employed to help enhance the accuracy with which
the pointing direction tracks the true angular position of a
tethered aerial vehicle as it flies. By doing so, deviations
between the pointing direction and the true angular position of the
flying tethered aerial vehicle be reduced, and thereby reduce the
size of the FOV required to maintain the aerial vehicle within the
FOV. This, in turn, can help relax some constraints of design
optimization. For example, a smaller FOV of the imaging device
allows for a smaller number of high-resolution (small) pixels,
which, in turn, can reduce complexity and/or intensity of image
processing analysis.
[0142] In accordance with example embodiments, an imaging device,
such the imaging device 612, can be attached to a tether
termination unit, or a component of a tether termination unit, that
is configured to rotate about the pointing direction in response to
twisting and/or de-twisting of a tether along an axial dimension of
the tether. With such an arrangement, an image plane of the imaging
device can correspondingly rotate about the pointing direction,
while maintain a normal orientation to the pointing direction. By
such rotation of the image plane in response to twisting and/or
de-twisting of the tether, tracking by the pointing direction of
the true angular position of a tethered aerial vehicle as it flies
can be made more accurate.
[0143] Considering again the example ground station 300 of FIG. 3B,
an imaging device, such as the imaging device 612 of FIG. 6, could
be mounted on, or attached to, the rotatable portion 354B of the
slip ring 354. Alternatively, the spindle 356 can be connected to
the rotatable portion 354B such that the spindle 356 can rotate (in
common with the rotatable portion 354B) about the pointing
direction axis. In this configuration, the imaging device could be
attached to the spindle 356, as illustrated, for example, in FIG.
6. In either of these example configurations, the image plane of
the imaging device can be perpendicular to the pointing direction,
and can rotate about the pointing direction as the rotatable
portion 354B and/or the spindle 356 rotates about the pointing
direction in response to twisting and/or de-twisting of the tether.
As discussed above, this rotation of the image plane (FOV) can
improve tracking accuracy of the pointing direction, and thereby
facilitate use of a smaller FOV than might otherwise be needed to
keep a flying tethered vehicle within the FOV.
[0144] In accordance with example embodiments, the coordinated
functions of tracking an aerial vehicle, such as aerial vehicle
704, via the pointing direction (e.g., by measuring gimbal angles
and/or spindle rotation), detecting and/or recognizing an image of
the aerial vehicle in the image plane, determining one or more
angular corrections from analysis of the aerial vehicle's image in
the image plane, determining distance to the aerial vehicle, and
applying the angular corrections to the pointing direction in order
to obtain an accurate position of the aerial vehicle with respect
to a ground station, such as the ground station 702, can be carried
out by coordinated actions and/or processing steps of the imaging
device and one or more processors with access to various
observational data, such as gimbal angles and spindle rotation. In
further accordance with example embodiments, the coordinated
functions and physical components that carry them out (e.g., the
imaging device, processors, etc.) can be considered collectively as
a vehicle tracking subsystem of the ground station. It will be
appreciated that the vehicle tracking subsystem can take various
forms. In an example embodiment, the vehicle tracking subsystem
could be integrated as a single device, such as being incorporated
as part of the imaging device. In another example embodiment, the
vehicle tracking subsystem could be distributed among various
components of the ground station (e.g., the imaging device, gimbal
encoders, and one or more processors), and organized or configured
as an integrated subsystem by executable instructions carried out
by the one or more processors. Other architectures of a vehicle
tracking subsystem, in accordance with example embodiments, are
possible as well.
3. EXAMPLE METHOD
[0145] Operation of an example ground station, such as the ground
stations 300 or 702, for example, that include an imaging device,
such as the imaging device 612, to enhance the accuracy of tracking
a flying tethered aerial vehicle, such as aerial vehicles 130, 504,
or 704, for example, can be carried out in the form of a method,
such as a computer-implemented method. More particularly, in
accordance with example embodiments, a method can be implemented by
the example ground station (or other apparatus) that cause the
ground station to carry out tracking a flying tethered aerial
vehicle using the imaging device to enhance accuracy. In accordance
with example embodiments, the method could be a
computer-implemented method including executable instructions, that
when executed by one or more processors of the ground station,
cause the ground station to carry out the functional and/or
operational aspects of tracking a flying tethered aerial vehicle
using the imaging device to enhance accuracy.
[0146] In further accordance with example embodiments, the
executable instructions can be stored in a non-transient computer
readable storage medium. Non-limiting examples of a non-transient
computer readable storage medium include magnetic disk,
non-volatile solid state memory, DVD, and CDROM. A non-transient
computer readable storage medium with the executable instructions
store therein can be used, for example during a manufacture of the
ground station, to initialize a ground station, and/or as a means
for distributing the executable instructions for installation in
one or more ground stations.
[0147] FIG. 8 is a flow chart illustrating an example method 800 of
tracking an aerial vehicle, according to an example embodiment.
[0148] At step 802, a pointing direction of a pointing axis of a
tether termination unit is determined as a function of measured
rotation angles of the tether termination unit about a first axis
of rotation and a second axis of rotation. In accordance with
example embodiments, the tether termination unit can be configured
for terminating a proximal end of a tether that is coupled at a
distal end with an aerial vehicle. In further accordance with
example embodiments, the pointing axis can be configured for
tracking angular motion of the proximal end of the tether induced
by flight motion of the aerial vehicle.
[0149] At step 804 an angular offset between an image of the aerial
vehicle in flight in an image plane of an imaging device attached
to the tether termination unit and a reference point in the image
plane is determined. In accordance with example embodiments, the
image plane can be configured perpendicularly to the pointing axis,
and the reference point can correspond to the pointing axis
direction in a field-of-view (FOV) of the image plane.
[0150] Finally, at step 806, a location of the aerial vehicle in
flight with respect to the tether termination unit is determined by
applying the determined angular offset as a correction to the
determined pointing axis direction.
[0151] In accordance with example embodiments, the ground station
can include a distance ranging device, and the method can further
entail measuring a line-of-sight distance to the aerial vehicle
during tethered flight of the aerial vehicle with the distance
ranging device. By way of example, the distance ranging device
could be a LIDAR device or a RADAR device.
[0152] The first axis of rotation can be incorporated in a first
gimbal device, and the second axis of rotation can be incorporated
in a second gimbal device. In accordance with example embodiments,
determining the pointing direction of the pointing axis of the
tether termination unit as a function of measured rotation angles
of the tether termination unit about the first axis of rotation and
the second axis of rotation can then entail measuring a rotation
angle of the tether termination unit about the first axis of
rotation with a first encoder of the first gimbal device, and
measuring a rotation angle of the tether termination unit about the
second axis of rotation with a second encoder of the second gimbal
device. In further accordance with example embodiments, the first
axis of rotation can be orthogonal to the second axis of
rotation.
[0153] In accordance with example embodiments, determining the
pointing direction of the pointing axis of the tether termination
unit as a function of measured rotation angles of the tether
termination unit about the first axis of rotation and the second
axis of rotation can entail tracking angular motion of the proximal
end of the tether due to rotation of the tether termination unit
about the first and second axes of rotation.
[0154] As described above, the tether termination unit could
include a spindle through which the tether is physically routed or
threaded between the tether termination mount system and the aerial
vehicle.
[0155] Also as described above, the reference point can be at the
center of the image plane, and can correspond to the center of the
FOV. In accordance with example embodiments, determining the
pointing direction of the pointing axis of the tether termination
unit as a function of measured rotation angles of the tether
termination unit about the first axis of rotation and the second
axis of rotation can entail tracking the aerial vehicle during
flight with sufficient accuracy to locate the aerial vehicle within
the FOV of the image plane.
[0156] In further accordance with example embodiments, the method
can further entail rotating the image plane about the pointing axis
direction in response to a twisting motion of the proximal end of
the tether about the pointing axis.
[0157] It will be appreciated that the steps shown in FIG. 8 are
meant to illustrate a method in accordance with example
embodiments. As such, various steps could be altered or modified,
the ordering of certain steps could be changed, and additional
steps could be added, while still achieving the overall desired
operation.
4. CONCLUSION
[0158] While various aspects and embodiments have been disclosed
herein, other aspects and embodiments will be apparent to those
skilled in the art. The various aspects and embodiments disclosed
herein are for purposes of illustration and are not intended to be
limiting, with the true scope and spirit being indicated by the
following claims.
* * * * *