Path based power generation control for an aerial vehicle

Methods and systems described herein relate to power generation control for an aerial vehicle. An example method may include operating an aerial vehicle in a crosswind-flight orientation substantially along a first flight path to generate power. The first flight path may include a substantially circular path that allows the aerial vehicle to generate the power. While the aerial vehicle is in the crosswind-flight orientation the method may include determining to reduce the power being generated by the aerial vehicle, and responsive to the determination, determining a second flight path that will reduce the power generated by the aerial vehicle when operating on the second flight path. Once determined, the aerial vehicle may operate substantially along the second flight path.

BACKGROUND

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

Methods and systems for managing power generation of an aerial vehicle operating in a crosswind-flight orientation are described herein. Beneficially, embodiments described herein may help mitigate overheating of components of the aerial vehicle by quickly reducing power generated by the aerial vehicle as it operates. Further, embodiments described herein may help to reduce fluctuations in power output as the aerial vehicle operates in crosswind flight.

In one aspect, a method may involve operating an aerial vehicle in a crosswind-flight orientation substantially along a first flight path to generate power. The first flight path may be constrained by a tether that defines a tether sphere having a radius based on a length of the tether. The aerial vehicle may be coupled to a ground station through the tether. The first flight path may be substantially on the tether sphere and may include a substantially circular path that allows the aerial vehicle to generate the power. The method may also involve, while the aerial vehicle is in the crosswind-flight orientation: determining to reduce the power being generated by the aerial vehicle and responsive to the determination, determining a different second flight path that will reduce the power generated by the aerial vehicle when operating on the second flight path. The second flight path may be substantially on the tether sphere. The method may further include operating the aerial vehicle substantially along the second flight path.

In another aspect, a system may include a tether coupled to a ground station, an aerial vehicle, and a control station. The aerial vehicle may be coupled to the tether. The aerial vehicle may be configured to operate in a crosswind-flight orientation substantially along a first flight path to generate power. The first flight path may be constrained by a tether that defines a tether sphere having a radius based on a length of the tether. The first flight path may be substantially on the tether sphere and may include a substantially circular path that allows the aerial vehicle to generate the power. The control system may be configured to: determine to reduce the power being generated by the aerial vehicle and responsive to the determination, determine a different second flight path that will reduce the power generated by the aerial vehicle when operating on the second flight path. The second flight path may be substantially on the tether sphere. The control system may further be configured to cause the aerial vehicle to operate substantially along the second flight path.

In another aspect, a system may include a tether coupled to a ground station, an aerial vehicle coupled to the tether, and a control system. The aerial vehicle may be configured to operate in a crosswind-flight orientation substantially along a first flight path to generate power. The first flight path may be constrained by a tether that defines a tether sphere having a radius based on a length of the tether. The first flight path may be substantially on the tether sphere and may include a substantially circular path that allows the aerial vehicle to generate the power. The control system may be configured to determine the power being generated by the aerial vehicle is greater than a rated power of the aerial vehicle. The rated power of the aerial vehicle may define a maximum power of the aerial vehicle. The control system may also be configured to, responsive to the determination, determine a different second flight path that will reduce the power generated by the aerial vehicle when operating on the second flight path. The second flight path may be substantially on the tether sphere.

In still another aspect, a system may include a means for operating an aerial vehicle in a crosswind-flight orientation substantially along a first flight path to generate power. The first flight path may be constrained by a tether that defines a tether sphere having a radius based on a length of the tether. The aerial vehicle may be coupled to a ground station through the tether. The first flight path may be substantially on the tether sphere and may include a substantially circular path that allows the aerial vehicle to generate the power. The system may also include a means for, while the aerial vehicle is in the crosswind-flight orientation: determining to reduce the power being generated by the aerial vehicle and responsive to the determination, determining a different second flight path that will reduce the power generated by the aerial vehicle when operating on the second flight path. The second flight path may be substantially on the tether sphere. The system may further include a means for operating the aerial vehicle substantially along the second flight path.

DETAILED DESCRIPTION

Exemplary methods and systems are described herein. It should be understood that the word “exemplary” is used herein to mean “serving as an example, instance, or illustration.” Any embodiment or feature described herein as “exemplary” or “illustrative” is not necessarily to be construed as preferred or advantageous over other embodiments or features. More generally, the embodiments described herein are not meant to be limiting. It will be readily understood that certain aspects of the disclosed methods systems and can be arranged and combined in a wide variety of different configurations, all of which are contemplated herein.

Illustrative embodiments relate to aerial vehicles, which may be used in a wind energy system, such as an Airborne Wind Turbine (AWT). In particular, illustrative embodiments may relate to or take the form of methods and systems for transitioning an aerial vehicle between certain flight modes that facilitate conversion of kinetic energy to electrical energy.

By way of background, an AWT may include an aerial vehicle that flies in a path, such as a substantially circular path, to convert kinetic wind energy to electrical energy. In an illustrative implementation, the aerial vehicle may be connected to a ground station via a tether. While tethered, the aerial vehicle can: (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.)

In an AWT, an aerial vehicle may rest in and/or on a ground station (or perch) 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 3.5 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.

Moreover, in an AWT, an aerial vehicle may be configured for hover flight and crosswind flight. Crosswind flight may be used to travel in a motion, such as a substantially circular motion, and thus may be the primary technique that is used to generate 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.

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.

In crosswind flight, the aerial vehicle may be propelled by the wind substantially along a path, which as noted above, 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.

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.

In some embodiments, a tension of the tether during crosswind flight may be greater than a tension of the tether during hover flight. For instance, the tension of the tether during crosswind flight may be 15 kilonewtons (KN), and the tension of the tether during hover flight may be 1 KN.

In line with the discussion above, the aerial vehicle may generate electrical energy in crosswind flight and may thereby allow the AWT to extract useful power from the wind. The aerial vehicle may generate electrical energy during various environmental conditions such as high wind speeds, large gusts, turbulent air, or variable wind conditions. Generally, the inertial speed of the aerial vehicle, the tension of the tether, and the power output of the AWT increase as the wind speed increases. However, at times, it may be desirable to balance power generation of the AWT during various environmental conditions such as those noted above. Additionally, at other times, it may be desirable to reduce power generation to prevent over heating of components of the aerial vehicle.

Considering this, disclosed embodiments may allow for operating an aerial vehicle, of an AWT, in crosswind-flight in a manner that may balance power generation in variable wind conditions (e.g., during increasing wind speeds) or reduce power generation during high wind speeds. In an example embodiment, a method may involve operating an aerial vehicle in a crosswind-flight orientation substantially along a first flight path to generate power. The aerial vehicle may be connected to a tether that defines a tether sphere having a radius based on a length of the tether and may be connected to a ground station via the tether. As the aerial vehicle operates along the first flight path that is substantially on the tether sphere, in the crosswind-flight orientation, the aerial vehicle may determine a need to reduce power generation. Responsively, the aerial vehicle may determine a different second flight path that will allow the aerial vehicle to continue to operate in a crosswind-flight orientation and operates in a manner such that power generated by the aerial vehicle may be reduced. Accordingly, as noted above, embodiments described herein may help mitigate overheating of components of the aerial vehicle by quickly reducing power generated by the aerial vehicle as it operates. Additionally, embodiments described herein may help to reduce fluctuations in power output as the aerial vehicle operates in crosswind flight.

FIG. 1depicts an AWT100, according to an example embodiment. In particular, the AWT100includes a ground station110, a tether120, and an aerial vehicle130. As shown inFIG. 1, the aerial vehicle130may be connected to the tether120, and the tether120may be connected to the ground station110. In this example, the tether120may be attached to the ground station110at one location on the ground station110, and attached to the aerial vehicle130at two locations on the aerial vehicle130. However, in other examples, the tether120may be attached at multiple locations to any part of the ground station110and/or the aerial vehicle130.

The ground station110may be used to hold and/or support the aerial vehicle130until it is in an operational mode. The ground station110may also be configured to allow for the repositioning of the aerial vehicle130such that deploying of the device is possible. Further, the ground station110may be further configured to receive the aerial vehicle130during a landing. The ground station110may be formed of any material that can suitably keep the aerial vehicle130attached and/or anchored to the ground while in hover flight, forward flight, crosswind flight.

In addition, the ground station110may include one or more components (not shown), such as a winch, that may vary a length of the tether120. For example, when the aerial vehicle130is deployed, the one or more components may be configured to pay out and/or reel out the tether120. In some implementations, the one or more components may be configured to pay out and/or reel out the tether120to a predetermined length. As examples, the predetermined length could be equal to or less than a maximum length of the tether120. Further, when the aerial vehicle130lands in the ground station110, the one or more components may be configured to reel in the tether120.

The tether120may transmit electrical energy generated by the aerial vehicle130to the ground station110. In addition, the tether120may transmit electricity to the aerial vehicle130in order to power the aerial vehicle130for takeoff, landing, hover flight, and/or forward flight. The tether120may 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 vehicle130and/or transmission of electricity to the aerial vehicle130. The tether120may also be configured to withstand one or more forces of the aerial vehicle130when the aerial vehicle130is in an operational mode. For example, the tether120may include a core configured to withstand one or more forces of the aerial vehicle130when the aerial vehicle130is in hover flight, forward flight, and/or crosswind flight. The core may be constructed of any high strength fibers. In some examples, the tether120may have a fixed length and/or a variable length. For instance, in at least one such example, the tether120may have a length of 140 meters.

The aerial vehicle130may be configured to fly substantially along a path150to generate 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.

The aerial vehicle130may 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 vehicle130may be formed of solid structures of metal, plastic and/or other polymers. The aerial vehicle130may be formed of any material which allows for a high thrust-to-weight ratio and generation of electrical energy which 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 possible as well.

The path150may be various different shapes in various different embodiments. For example, the path150may be substantially circular. And in at least one such example, the path150may 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 path150may be an oval, such as an ellipse, the shape of a jelly bean, the shape of the number of 8, etc.

As shown inFIG. 1, the aerial vehicle130may include a main wing131, a front section132, rotor connectors133A-B, rotors134A-D, a tail boom135, a tail wing136, and a vertical stabilizer137. Any of these components may be shaped in any form which allows for the use of components of lift to resist gravity and/or move the aerial vehicle130forward.

The main wing131may provide a primary lift for the aerial vehicle130. The main wing131may be one or more rigid or flexible airfoils, and may include various control surfaces, such as winglets, flaps, rudders, elevators, etc. The control surfaces may be used to stabilize the aerial vehicle130and/or reduce drag on the aerial vehicle130during hover flight, forward flight, and/or crosswind flight.

The main wing131may be any suitable material for the aerial vehicle130to engage in hover flight, forward flight, and/or crosswind flight. For example, the main wing131may include carbon fiber and/or e-glass. Moreover, the main wing131may have a variety dimensions. For example, the main wing131may have one or more dimensions that correspond with a conventional wind turbine blade. As another example, the main wing131may have a span of 8 meters, an area of 4 meters squared, and an aspect ratio of 15. The front section132may include one or more components, such as a nose, to reduce drag on the aerial vehicle130during flight.

The rotor connectors133A-B may connect the rotors134A-D to the main wing131. In some examples, the rotor connectors133A-B may take the form of or be similar in form to one or more pylons. In this example, the rotor connectors133A-B are arranged such that the rotors134A-D are spaced between the main wing131. In some examples, a vertical spacing between corresponding rotors (e.g., rotor134A and rotor134B or rotor134C and rotor134D) may be 0.9 meters.

The rotors134A-D may be configured to drive one or more generators for the purpose of generating electrical energy. In this example, the rotors134A-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 rotors134A-D may also be configured to provide a thrust to the aerial vehicle130during flight. With this arrangement, the rotors134A-D may function as one or more propulsion units, such as a propeller. Although the rotors134A-D are depicted as four rotors in this example, in other examples the aerial vehicle130may include any number of rotors, such as less than four rotors or more than four rotors.

The tail boom135may connect the main wing131to the tail wing136. The tail boom135may have a variety of dimensions. For example, the tail boom135may have a length of 2 meters. Moreover, in some implementations, the tail boom135could take the form of a body and/or fuselage of the aerial vehicle130. And in such implementations, the tail boom135may carry a payload.

The tail wing136and/or the vertical stabilizer137may be used to stabilize the aerial vehicle and/or reduce drag on the aerial vehicle130during hover flight, forward flight, and/or crosswind flight. For example, the tail wing136and/or the vertical stabilizer137may be used to maintain a pitch of the aerial vehicle130during hover flight, forward flight, and/or crosswind flight. In this example, the vertical stabilizer137is attached to the tail boom135, and the tail wing136is located on top of the vertical stabilizer137. The tail wing136may have a variety of dimensions. For example, the tail wing136may have a length of 2 meters. Moreover, in some examples, the tail wing136may have a surface area of 0.45 meters squared. Further, in some examples, the tail wing136may be located 1 meter above a center of mass of the aerial vehicle130.

While the aerial vehicle130has 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 tether120.

B. Illustrative Components of an AWT

FIG. 2is a simplified block diagram illustrating components of the AWT200. The AWT200may take the form of or be similar in form to the AWT100. In particular, the AWT200includes a ground station210, a tether220, and an aerial vehicle230. The ground station210may take the form of or be similar in form to the ground station110, the tether220may take the form of or be similar in form to the tether120, and the aerial vehicle230may take the form of or be similar in form to the aerial vehicle130.

As shown inFIG. 2, the ground station210may include one or more processors212, data storage214, and program instructions216. A processor212may be a general-purpose processor or a special purpose processor (e.g., digital signal processors, application specific integrated circuits, etc.). The one or more processors212can be configured to execute computer-readable program instructions216that are stored in data storage214and are executable to provide at least part of the functionality described herein.

The data storage214may include or take the form of one or more computer-readable storage media that may be read or accessed by at least one processor212. The one or more computer-readable storage media can 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 processors212. In some embodiments, the data storage214may 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 storage214can be implemented using two or more physical devices.

As noted, the data storage214may include computer-readable program instructions216and perhaps additional data, such as diagnostic data of the ground station210. As such, the data storage214may include program instructions to perform or facilitate some or all of the functionality described herein.

In a further respect, the ground station210may include a communication system218. The communications system218may include one or more wireless interfaces and/or one or more wireline interfaces, which allow the ground station210to 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 station210may communicate with the aerial vehicle230, other ground stations, and/or other entities (e.g., a command center) via the communication system218.

In an example embodiment, the ground station210may include communication systems218that allows for both short-range communication and long-range communication. For example, the ground station210may be configured for short-range communications using Bluetooth and for long-range communications under a CDMA protocol. In such an embodiment, the ground station210may 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 tether220, the aerial vehicle230, and other ground stations) and one or more data networks, such as cellular network and/or the Internet. Configured as such, the ground station210may facilitate data communications that the remote support device would otherwise be unable to perform by itself.

For example, the ground station210may 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 station210might connect to under an LTE or a 3G protocol, for instance. The ground station210could 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.

Moreover, as shown inFIG. 2, the tether220may include transmission components222and a communication link224. The transmission components222may be configured to transmit electrical energy from the aerial vehicle230to the ground station210and/or transmit electrical energy from the ground station210to the aerial vehicle230. The transmission components222may take various different forms in various different embodiments. For example, the transmission components222may include one or more 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 allows for the conduction of electric current. Moreover, in some implementations, the transmission components222may surround a core of the tether220(not shown).

The ground station210could communicate with the aerial vehicle230via the communication link224. The communication link224may 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 link224.

Further, as shown inFIG. 2, the aerial vehicle230may include one or more sensors232, a power system234, power generation/conversion components236, a communication system238, one or more processors242, data storage244, and program instructions246, and a control system248.

The sensors232could include various different sensors in various different embodiments. For example, the sensors232may 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 vehicle230. Such GPS data may be utilized by the AWT200to provide various functions described herein.

As another example, the sensors232may 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. The apparent/relative wind may be wind that is being applied to the aerial vehicle230, for example. Such wind data may be utilized by the AWT200to provide various functions described herein.

Still as another example, the sensors232may 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 vehicle230. In particular, the accelerometer can measure the orientation of the aerial vehicle230with respect to earth, while the gyroscope measures the rate of rotation around an axis, such as a centerline of the aerial vehicle230. IMUs are 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 of sensors are also possible.

While an accelerometer and gyroscope may be effective at determining the orientation of the aerial vehicle230, slight errors in measurement may compound over time and result in a more significant error. However, an example aerial vehicle230may be able mitigate or reduce such errors by using a magnetometer to measure direction. 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.

The aerial vehicle230may also include a pressure sensor or barometer, which can be used to determine the altitude of the aerial vehicle230. 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.

As noted, the aerial vehicle230may include the power system234. The power system234could take various different forms in various different embodiments. For example, the power system234may include one or more batteries for providing power to the aerial vehicle230. 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.

As another example, the power system234may include one or more motors or engines for providing power to the aerial vehicle230. 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 vehicle230and delivered to the one or more motors or engines via one or more fluid conduits, such as piping. In some implementations, the power system234may be implemented in whole or in part on the ground station210.

As noted, the aerial vehicle230may include the power generation/conversion components236. The power generation/conversion components326could take various different forms in various different embodiments. For example, the power generation/conversion components236may 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 rotors134A-D. And in at least one such example, the one or more generators may operate at full rated power 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.

Moreover, as noted, the aerial vehicle230may include a communication system238. The communication system238may take the form of or be similar in form to the communication system218. The aerial vehicle230may communicate with the ground station210, other aerial vehicles, and/or other entities (e.g., a command center) via the communication system238.

In some implementations, the aerial vehicle230may 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 station210, the tether220, other aerial vehicles) and one or more data networks, such as cellular network and/or the Internet. Configured as such, the aerial vehicle230may facilitate data communications that the remote support device would otherwise be unable to perform by itself.

For example, the aerial vehicle230may 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 vehicle230might connect to under an LTE or a 3G protocol, for instance. The aerial vehicle230could 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.

As noted, the aerial vehicle230may include the one or more processors242, the program instructions244, and the data storage246. The one or more processors242can be configured to execute computer-readable program instructions246that are stored in the data storage244and are executable to provide at least part of the functionality described herein. The one or more processors242may take the form of or be similar in form to the one or more processors212, the data storage244may take the form of or be similar in form to the data storage214, and the program instructions246may take the form of or be similar in form to the program instructions216.

Moreover, as noted, the aerial vehicle230may include the control system248. In some implementations, the control system248may be configured to perform one or more functions described herein. The control system248may be implemented with mechanical systems and/or with hardware, firmware, and/or software. As one example, the control system248may take the form of program instructions stored on a non-transitory computer readable medium and a processor that executes the instructions. The control system248may be implemented in whole or in part on the aerial vehicle230and/or at least one entity remotely located from the aerial vehicle230, such as the ground station210. Generally, the manner in which the control system248is implemented may vary, depending upon the particular application.

While the aerial vehicle230has 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 tether230and/or the tether110.

C. Transitioning an Aerial Vehicle from Hover Flight to Crosswind Flight to Generate Power

FIGS. 3A and 3Bdepict an example300of transitioning an aerial vehicle from hover flight to crosswind flight in a manner such that power may be generated, according to an example embodiment. Example300is generally described by way of example as being carried out by the aerial vehicle130described above in connection withFIG. 1. For illustrative purposes, example300is described in a series of actions as shown inFIGS. 3A and 3B, though example300could be carried out in any number of actions and/or combination of actions.

As shown inFIG. 3A, the aerial vehicle130may be connected to the tether120, and the tether120is connected to the ground station110. The ground station110is located on ground302. Moreover, as shown inFIG. 3A, the tether120defines a tether sphere304having a radius based on a length of the tether120, such as a length of the tether120when it is extended. Example300may be carried out in and/or substantially on a portion304A of the tether sphere304. The term “substantially on,” as used in this disclosure, refers to exactly on and/or one or more deviations from exactly on that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein.

Example300begins at a point306with deploying the aerial vehicle130from the ground station110in a hover-flight orientation. With this arrangement, the tether120may be paid out and/or reeled out. In some implementations, the aerial vehicle130may be deployed when wind speeds increase above a threshold speed (e.g., 3.5 m/s) at a threshold altitude (e.g., over 200 meters above the ground302).

Further, at point306the aerial vehicle130may be operated in the hover-flight orientation. When the aerial vehicle130is in the hover-flight orientation, the aerial vehicle130may engage in hover flight. For instance, when the aerial vehicle engages in hover flight, the aerial vehicle130may ascend, descend, and/or hover over the ground302. When the aerial vehicle130is in the hover-flight orientation, a span of the main wing131of the aerial vehicle130may be oriented substantially perpendicular to the ground302. The term “substantially perpendicular,” as used in this disclosure, refers to exactly perpendicular and/or one or more deviations from exactly perpendicular that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein.

Example300continues at a point308with while the aerial vehicle130is in the hover-flight orientation positioning the aerial vehicle130at a first location310that is substantially on the tether sphere304. As shown inFIG. 3A, the first location310may be in the air and substantially downwind of the ground station110.

The term “substantially downwind,” as used in this disclosure, refers to exactly downwind and/or one or more deviations from exactly downwind that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein.

For example, the first location310may be at a first angle from an axis extending from the ground station110that is substantially parallel to the ground302. In some implementations, the first angle may be 30 degrees from the axis. In some situations, the first angle may be referred to as azimuth, and the first angle may be between 30 degrees clockwise from the axis and 330 degrees clockwise from the axis, such as 15 degrees clockwise from the axis or 345 degrees clockwise from the axis.

As another example, the first location310may be at a second angle from the axis. In some implementations, the second angle may be 10 degrees from the axis. In some situations, the second angle may be referred to as elevation, and the second angle may be between 10 degrees in a direction above the axis and 10 degrees in a direction below the axis. The term “substantially parallel,” as used in this disclosure refers to exactly parallel and/or one or more deviations from exactly parallel that do not significantly impact transitioning an aerial vehicle between certain flight modes described herein.

At point308, the aerial vehicle130may accelerate in the hover-flight orientation. For example, at point308, the aerial vehicle130may accelerate up to a few meters per second. In addition, at point308, the tether120may take various different forms in various different embodiments. For example, as shown inFIG. 3A, at point308the tether120may be extended. With this arrangement, the tether120may be in a catenary configuration. Moreover, at point306and point308, a bottom of the tether120may be a predetermined altitude312above the ground302. With this arrangement, at point306and point308the tether120may not contact the ground302.

Example300continues at point314with transitioning the aerial vehicle130from the hover-flight orientation to a forward-flight orientation, such that the aerial vehicle130moves from the tether sphere304. As shown inFIG. 3B, the aerial vehicle130may move from the tether sphere304to a location toward the ground station110(which may be referred to as being inside the tether sphere304).

When the aerial vehicle130is in the forward-flight orientation, the aerial vehicle130may engage in forward flight (which may be referred to as airplane-like flight). For instance, when the aerial vehicle130engages in forward flight, the aerial vehicle130may ascend. The forward-flight orientation of the aerial vehicle130could take the form of an orientation of a fixed-wing aircraft (e.g., an airplane) in horizontal flight. In some examples, transitioning the aerial vehicle130from the hover-flight orientation to the forward-flight orientation may involve a flight maneuver, such as pitching forward. And in such an example, the flight maneuver may be executed within a time period, such as less than one second.

At point314, the aerial vehicle130may achieve attached flow. Further, at point314, a tension of the tether120may be reduced. With this arrangement, a curvature of the tether120at point314may be greater than a curvature of the tether120at point308. As one example, at point314, the tension of the tether120may be less than 1 KN, such as 500 newtons (N).

Example300continues at one or more points318with operating the aerial vehicle130in the forward-flight orientation to ascend at an angle of ascent to a second location320that is substantially on the tether sphere304. As shown inFIG. 3B, the aerial vehicle130may fly substantially along a path316during the ascent at one or more points318. In this example, one or more points318is shown as three points, a point318A, a point318B, and a point318C. However, in other examples, one or more points318may include less than three or more than three points.

In some examples, the angle of ascent may be an angle between the path316and the ground302. Further, the path316may take various different forms in various different embodiments. For instance, the path316may be a line segment, such as a chord of the tether sphere304.

As shown inFIG. 3B, the second location320may be in the air and substantially downwind of the ground station110. The second location320may be oriented with respect to the ground station110the similar way as the first location310may be oriented with respect to the ground station110.

For example, the second location320may be at a first angle from an axis extending from the ground station110that is substantially parallel to the ground302. In some implementations, the first angle may be 30 degrees from the axis. In some situations, the first angle may be referred to as azimuth, and the angle may be between 30 degrees clockwise from the axis and 330 degrees clockwise from the axis, such as 15 degrees clockwise from the axis or 345 degrees clockwise from the axis.

In addition, as shown inFIG. 3B, the second location320may be substantially upwind of the first location310. The term “substantially upwind,” as used in this disclosure, refers to exactly upwind and/or one or more deviations from exactly upwind that do not significantly impact transitioning an aerial vehicle between certain flight modes as described herein.

At one or more points318, a tension of the tether120may increase during the ascent. For example, a tension of the tether120at point318C may be greater than a tension of the tether120at point318B, a tension of the tether120at point318B may be greater than a tension of the tether120at point318A. Further, a tension of the tether120at point318A may be greater than a tension of the tether at point314.

With this arrangement, a curvature of the tether120may decrease during the ascent. For example, a curvature the tether120at point318C may be less than a curvature the tether at point318B, and a curvature of the tether120at point318B may be less than a curvature of the tether at point318A. Further, in some examples, a curvature of the tether120at point318A may be less than a curvature of the tether120at point314.

Example300continues at a point322with transitioning the aerial vehicle130from the forward-flight orientation to a crosswind-flight orientation. In some examples, transitioning the aerial vehicle130from the forward-flight orientation to the crosswind-flight orientation may involve a flight maneuver. When the aerial vehicle130is in the crosswind-flight orientation, the aerial vehicle130may engage in crosswind flight. For instance, when the aerial vehicle130engages in crosswind flight, the aerial vehicle130may fly substantially along a path, such as path150, to generate electrical energy. In some implementations, a natural roll and/or yaw of the aerial vehicle130may occur during crosswind flight.

FIG. 3Cdepicts example300from a three-dimensional (3D) perspective. Accordingly, like numerals may denote like entities. As noted above, tether sphere304has a radius based on a length of a tether120, such as a length of the tether120when it is extended. Also as noted above, inFIG. 3C, the tether120is connected to ground station310, and the ground station310is located on ground302. Further, relative wind303contacts the tether sphere304. Note, inFIG. 3C, only a portion of the tether sphere304that is above the ground302is depicted. The portion may be described as one half of the tether sphere304.

As shown inFIG. 3C, the first portion304A of the tether sphere304is substantially downwind of the ground station310. InFIG. 3C, the first portion304A may be described as one quarter of the tether sphere304.

LikeFIG. 3B,FIG. 3Cdepicts transitioning aerial vehicle130(not shown inFIG. 3Cto simply the Figure) between hover flight and crosswind flight. As shown inFIG. 3C, when the aerial vehicle130transitions from the hover-flight orientation to a forward-flight orientation, the aerial vehicle may be positioned at a point314that is inside the first portion304A of the tether sphere304. Further still, as shown inFIG. 3C, when aerial vehicle130ascends in the forward-flight orientation to a location320that is substantially on the first portion304A of the tether sphere304, the aerial vehicle may follow a path316. Yet even further, as shown inFIG. 3C, aerial vehicle130may then transition from location320in a forward-flight orientation to a crosswind-flight orientation at location322, for example.

FIG. 4is a flowchart illustrating a method400, according to an example embodiment. The method400may be used to control power generation of an aerial vehicle in a crosswind-flight orientation. Illustrative methods, such as method400, may be carried out in whole or in part by a component or components of an aerial vehicle, such as by the one or more components of the aerial vehicle130shown inFIG. 1, the aerial vehicle230shown inFIG. 2, the ground station110shown inFIG. 1, and the ground station210shown inFIG. 2. For instance, method400may be performed by the control system248. For simplicity, method400may be described generally as being carried out by an aerial vehicle, such as the aerial vehicle130and/or the aerial vehicle230. However, it should be understood that example methods, such as method400, may be carried out by other entities or combinations of entities without departing from the scope of the disclosure.

As shown by block402, method400involves operating an aerial vehicle in a crosswind-flight orientation substantially along a first flight path that may allow the aerial vehicle to generate power. The first flight path may be constrained by a tether such as tether120and, as noted above, the tether may define a tether sphere having a radius based on a length of the tether. For example, the tether sphere may be the same as or similar to tether sphere304ofFIGS. 3A-3C. The first flight path may be substantially on the tether sphere and may include a substantially circular path (e.g., path150) that allows the aerial vehicle to generate the power. For example, the first flight path may be located at a position of the tether sphere the same as or similar to that of322inFIGS. 3A and 3C.

Within this disclosure, the term “substantially circular” refers to exactly circular and/or one or more deviations from exactly circular that does not significantly impact the aerial vehicle from generating power. Substantially circular paths may include, for example, oval-shaped paths, balloon-shaped paths, and bowl-shaped paths to name a few. Other substantially circular paths are possible as well.

To begin operating along the first flight path, the aerial vehicle may be deployed, may engage in hover flight, may engage in forward flight, and may then transition to the first flight path on the tether sphere. For example, at block402, the aerial vehicle may be operated in the same or a similar way as the aerial vehicle130may be operated when transitioning from a hover flight orientation to a crosswind flight orientation as described with reference to example300ofFIGS. 3A-3C. Accordingly, when operating along the first flight path in the crosswind-flight orientation, the aerial vehicle may be oriented the same as or similar to aerial vehicle130at point322ofFIGS. 3B and 3C.

Note, in other examples, some of the above referenced flight maneuvers may be omitted. For instance, in some examples, the aerial vehicle may be deployed, engage in forward flight to a position on the tether sphere, and thereafter immediately transition to the first flight path. Thus, in such examples, the aerial vehicle may omit the hover flight maneuver.

In one example, as shown inFIGS. 5A and 5B, an aerial vehicle530may be operating in a crosswind-flight orientation along flight path502substantially on a tether sphere504.FIG. 5Aillustrates an isometric view of aerial vehicle530operating in crosswind-flight orientation along flight path502from a perspective that is above and behind ground station510. InFIG. 5A, flight path502may be the same as or similar to flight path150and tether sphere504may be the same as or similar to tether sphere304, for example.

InFIG. 5A, an axis503may extend from the ground station510to flight path502and may intersect with flight path502. Axis503may be oriented substantially parallel to the ground (not shown inFIG. 5A) and flight path502may be substantially downwind (in accordance with relative wind503) from ground station510. Aerial vehicle530may operate along flight path502in a manner such that it generates power. In some examples, flight path502may allow aerial vehicle530to generate a maximum power.

As shown by block404, method400involves while the aerial vehicle is in the crosswind-flight orientation, determining to reduce the power being generated by the aerial vehicle. The aerial vehicle may determine to reduce the power being generated based on a general desire to become less efficient in generating power. For instance, as noted throughout this disclosure, the aerial vehicle may desire to produce less power to prevent overheating various components of the aerial vehicle or to maintain a certain power level despite, for example, increasing wind conditions.

In one example, determining to reduce the power being generated by the aerial vehicle may be made for example, based on a temperature of a component of the aerial vehicle. For example, using a thermometer or other heat measuring mechanism of sensors232, the aerial vehicle may determine that a temperature or heat threshold of the component (e.g., a motor) is too high. Based on this determination, the aerial vehicle may determine that too much power is being generated and provided to the component. Responsively, the aerial vehicle may determine to reduce power generation.

In another example, block404may involve determining that the power being generated by the aerial vehicle is greater than a rated power of the aerial vehicle. The rated power of the vehicle may define a maximum power that may be generated by the aerial vehicle. Upon determining that the aerial vehicle is generating rated power, the aerial vehicle may determine to reduce power generation to, for example, prevent or mitigate the overheating of components, such as motors.

In some examples, determining the aerial vehicle is generating rated power may be performed by measuring the wind speed (e.g., using pitot tubes of sensors232) being applied to the aerial vehicle. The wind speed that may be measured may be the wind at speeds conducive to power generation (i.e., above a wind speed threshold), such as wind speeds of 3.5 meters per second. Because power generation is a function of wind speed, in examples in which the wind speed is equivalent to a rated power wind speed (e.g., speeds of 11.5 meters per second) then the aerial vehicle may determine it may be generating max power and may responsively determine to reduce power generation prior to encountering overheating, for example.

Returning to the example ofFIG. 5A, aerial vehicle530may determine to reduce power being generated by aerial vehicle530as it travels around flight path502. Aerial vehicle may employ any of the methods noted above using, for example, a control system the same as or similar to control system248. In other examples, the determination to reduce power may be made by the ground station510and communicated to aerial vehicle530. In further examples, the determination may be made using operations of both aerial vehicle530and ground station510.

In response to the determination made at block404, as shown by block406, method400involves determining a different second flight path that will reduce the power generated by the aerial vehicle when operating on the second flight path. Similar to block404, block406may be performed while the aerial vehicle is in a crosswind-flight orientation. The second flight path may be substantially on the same tether sphere as the first flight path and may be substantially circular in shape as well.

In some examples, determining the second flight path may include determining a wind speed of apparent wind-flow being applied to the aerial vehicle. Based on the wind speed of the apparent wind-flow being applied to the aerial vehicle the second flight path may be determined. Because power generated by the aerial vehicle is a function of wind speed, the power generation may be reduced based on the wind speed. More specifically, for example, the aerial vehicle may determine the relative wind speed of the apparent wind-flow being applied to the aerial vehicle. Based on the determination, the aerial vehicle may determine a variation angle and using the variation angle may vary the second flight path in a manner such that the second flight path is varied from the first flight path at the variation angle. Thus, the determined second flight path may be varied from being substantially downwind of the ground station.

Continuing with the example ofFIGS. 5A and 5B, after determining to reduce the amount of power being generated, aerial vehicle530may determine a second flight path506, as shown inFIG. 5B, that aerial vehicle may operate on to reduce power generation. Similar to flight path502, flight path506may include a second axis507that may extend from the ground station510to flight path506and may intersect with flight path506. Axis507may be oriented substantially parallel to the ground (not shown inFIG. 5B).

To determine flight path506, aerial vehicle530may measure the wind speed of relative wind505and based on the measurement determine a variation angle509at which to vary the axis507of flight path506. Once determined, axis507may be varied from axis503at the variation angle, as shown inFIG. 5B. Resultantly, the second flight path507may be varied from being substantially downwind of ground station510. InFIG. 5B, second axis507is varied from first axis503at angle509in a clockwise direction or to the right. However, this variation is intended to be an example only, in other examples the variation may be above, below, or to the left of the first axis503.

Similar to determining to reduce the power, aerial vehicle530may determine the second flight path using, for example, a control system the same as or similar to control system248. In other examples, the second flight path may be determined by the ground station510and communicated to aerial vehicle530. In further examples, the determination may be made using operations of both aerial vehicle530and ground station510.

As shown by block408, method400involves operating the aerial vehicle substantially along the second flight path. Like blocks404and406, block408may be performed while aerial vehicle530is in a crosswind-flight orientation. At block408, the aerial vehicle may be transitioned from first flight path502to second flight path506in the same way or a similar way as the aerial vehicle130may be transitioned from location320to location322as described with reference toFIGS. 3B and 3C.

Additionally, 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. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. 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 contemplated herein.