Flight control for an airborne wind turbine

An example method may include receiving data representing an initial position and an initial attitude of an aircraft. The method further includes determining a change to a first attribute and a second attribute of the position or the attitude of the aircraft to achieve a subsequent position and a subsequent attitude. The method also includes determining a priority sequence for changing the first attribute and the second attribute of the position or the attitude of the aircraft based on a first thrust of the actuator to achieve the change to the first attribute and a second thrust of the actuator to achieve the change to the second attribute. The priority sequence is configured to cause changes to the first attribute before causing changes to the second attribute where the actuator is unable to concurrently provide the first thrust and the second thrust.

BACKGROUND

Many techniques and systems exist for controlling a flight path of an aircraft. Generally, an ability to change a position or an attitude of the aircraft will depend on the location and functionality of actuators included as part of the aircraft.

SUMMARY

In one example, a method is provided that includes receiving data representing an initial position and an initial attitude of an aircraft configured to be coupled to a ground station via a tether. The aircraft includes an actuator configured to change a position and an attitude of the aircraft. The method also includes determining a change to a first attribute and a second attribute of the position or the attitude of the aircraft to achieve a subsequent position and a subsequent attitude. The method further includes determining a priority sequence for changing the first attribute or the second attribute of the position or the attitude of the aircraft based on a first thrust of the actuator to achieve the change to the first attribute and a second thrust of the actuator to achieve the change to the second attribute. The priority sequence is configured to cause changes to the first attribute before causing changes to the second attribute where the actuator is unable to concurrently provide the first thrust and the second thrust. The method also includes causing the actuator to change the first attribute and the second attribute according to the priority sequence.

In another example, a computer readable storage memory having stored therein instructions, that when executed by a computing device that includes one or more processors, cause the computing device to perform functions is provided. The functions comprise receiving data representing an initial position and an initial attitude of an aircraft configured to be coupled to a ground station via a tether. The aircraft includes an actuator configured to change a position and an attitude of the aircraft. The functions further comprise determining a change to a first attribute and a second attribute of the position or the attitude of the aircraft to achieve a subsequent position and a subsequent attitude. The functions further comprise determining a priority sequence for changing the first attribute and the second attribute of the position or the attitude of the aircraft based on a first thrust of the actuator to achieve the change to the first attribute and a second thrust of the actuator to achieve the change to the second attribute. The priority sequence is configured to cause changes to the first attribute before causing changes to the second attribute where the actuator is unable to concurrently provide the first thrust and the second thrust. The functions further comprise causing the actuator to change the first attribute and the second attribute according to the priority sequence.

In still another example, a system is provided that comprises one or more processors and memory configured to store instructions, that when executed by the one or more processors, cause the system to perform functions. The functions comprise receiving data representing an initial position and an initial attitude of an aircraft configured to be coupled to a ground station via a tether. The aircraft includes an actuator configured to change a position and an attitude of the aircraft. The functions further comprise determining a change to a first attribute and a second attribute of the position and the attitude of the aircraft to achieve a subsequent position and a subsequent attitude. The functions further comprise determining a priority sequence for changing the first attribute and the second attribute of the position and the attitude of the aircraft based on a first thrust of the actuator to achieve the change to the first attribute and a second thrust of the actuator to achieve the change to the second attribute. The priority sequence is configured to cause changes to the first attribute before causing changes to the second attribute where the actuator is unable to concurrently provide the first thrust and the second thrust. The functions further comprise causing the actuator to change the first attribute and the second attribute according to the priority sequence.

In yet another example, a system is provided that includes a means for receiving data representing an initial position and an initial attitude of an aircraft configured to be coupled to a ground station via a tether. The aircraft includes an actuator configured to change a position and an attitude of the aircraft. The system further comprises means for determining a change to a first attribute and a second attribute of the position or the attitude of the aircraft to achieve a subsequent position and a subsequent attitude. The system further comprises means for determining a priority sequence for changing the first attribute and the second attribute of the position or the attitude of the aircraft based on a first thrust of the actuator to achieve the change to the first attribute and a second thrust of the actuator to achieve the change to the second attribute. The priority sequence is configured to cause changes to the first attribute before causing changes to the second attribute where the actuator is unable to concurrently provide the first thrust and the second thrust. The system further comprises means for causing the actuator to change the first attribute and the second attribute according to the priority sequence.

DETAILED DESCRIPTION

Within examples, a processor may be configured to receive data representing an initial position and an initial attitude of an aircraft from sensors of the aircraft. (The processor may be part of a larger control system.) The aircraft may include at least one actuator configured to apply force to the aircraft to cause the aircraft to change position or attitude. Furthermore, the processor may be configured to determine a change to the position and the attitude of the aircraft to achieve a subsequent position and a subsequent attitude. In order to place the aircraft at the subsequent position and attitude, the processor may next determine a first attribute and a second attribute of the position or the attitude of the aircraft to be changed to achieve the subsequent position and the subsequent attitude. Such attributes of position may include an azimuth angle, an altitude, and a horizontal distance from a ground station. A horizontal distance may refer to a distance along the ground. Attributes of attitude may include pitch, roll, and yaw angles about axes of the aircraft. For example, the first attribute may include altitude and the second attribute may include yaw. In another example, the first attribute may include pitch and the second attribute may include roll. Attributes of position may also be expressed in rectangular or spherical coordinates.

By further example, the processor may be configured to determine a first thrust and a second thrust. The first thrust may be a maximum thrust that the actuator is configured to apply to the aircraft to change the first attribute towards the subsequent position or attitude, based on the current position and attitude of the aircraft. The second thrust may also be a maximum thrust that the actuator is configured to apply to the aircraft to change the second attribute towards the subsequent position or attitude, based on the current position or attitude of the aircraft. For example, based on the current position and attitude of the aircraft, changing the yaw angle of the aircraft as quickly as possible may require the actuator to be idle, while quickly changing the altitude of the aircraft may require the actuator to provide full thrust.

Based on the actuator being unable to simultaneously provide thrusts optimized to change both the first attribute and the second attribute, the processor may determine a priority sequence for changing the first and second attributes. The priority sequence may be determined based on a distance of the aircraft from the ground station, a speed of an apparent wind, or a tension on the tether. The processor may then cause the actuator to provide the first thrust, thereby committing the actuator's full actuating ability towards changing the first attribute.

Referring now to the figures,FIG. 1depicts a tethered flight system100, according to an example embodiment. The tethered flight system100may include a ground station110, a tether120, and an aircraft130. As shown inFIG. 1, the aircraft130may be connected to the tether120, and the tether120may be connected to the ground station110. The tether120may be attached to the ground station110at one location on the ground station110, and attached to the aircraft130at two locations on the aircraft130. However, in other examples, the tether120may be attached at multiple locations to any part of the ground station110or the aircraft130.

The ground station110may be used to hold or support the aircraft130until the aircraft130is in a flight mode. The ground station110may also be configured to reposition the aircraft130such that deploying the aircraft130is possible. Further, the ground station110may be further configured to receive the aircraft130during a landing. The ground station110may be formed of any material that can suitably keep the aircraft130attached or anchored to the ground while in hover flight, forward flight, or 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 aircraft130is deployed, the one or more components may be configured to pay out or reel out the tether120. In some implementations, the one or more components may be configured to pay out 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 aircraft130lands on the ground station110, the one or more components may be configured to reel in the tether120.

The tether120may transmit electrical energy generated by the aircraft130to the ground station110. In addition, the tether120may transmit electricity to the aircraft130to power the aircraft130for takeoff, landing, hover flight, or forward flight. The tether120may be constructed in any form and using any material which allows for the transmission, delivery, or harnessing of electrical energy generated by the aircraft130or transmission of electricity to the aircraft130. The tether120may also be configured to withstand one or more forces of the aircraft130when the aircraft130is in a flight mode. For example, the tether120may include a core configured to withstand one or more forces of the aircraft130when the aircraft130is in hover flight, forward flight, or crosswind flight. The core may be constructed of high strength fibers. In some examples, the tether120may have a fixed length or a variable length.

The aircraft130may include various types of devices, such as a kite, a helicopter, a wing, or an airplane, among other possibilities. The aircraft130may be formed of solid structures of metal, plastic, polymers, or 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 allow for a lightning hardened, redundant or fault tolerant design which may be capable of handling large or sudden shifts in wind speed and wind direction. Other materials may be possible as well.

As shown inFIG. 1, the aircraft130may include a main wing131, a front section132, actuator connectors133A-B, actuators134A-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 lift to resist gravity or move the aircraft130forward.

The main wing131may provide a primary lift for the aircraft130during forward flight, wherein the aircraft130may move through air in a direction substantially parallel to a direction of thrust provided by the actuators134A-D so that the main wing131provides a lift force substantially perpendicular to the apparent wind or to a ground. The main wing131may be one or more rigid or flexible airfoils, and may include various control surfaces or actuators, such as winglets, flaps, rudders, elevators, etc. The control surfaces may be used to steer or stabilize the aircraft130or reduce or increase drag on the aircraft130during hover flight, forward flight, or crosswind flight. The main wing131may be any suitable material for the aircraft130to engage in hover flight, forward flight, or crosswind flight. For example, the main wing131may include carbon fiber 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. The front section132may include one or more components, such as a nose, to reduce drag on the aircraft130during flight.

The actuator connectors133A-B may connect the actuators134A-D to the main wing131. In some examples, the actuator connectors133A-B may take the form of or be similar in form to one or more pylons. In the example depicted inFIG. 1, the actuator connectors133A-B are arranged such that the actuators134A and134B are located on opposite sides of the main wing131and actuators134C and134D are also located on opposite sides of the main wing131. The actuator134C may also be located on an end of the main wing131opposite of the actuator134A, and the actuator134D may be located on an end of main wing131opposite of the actuator134B.

In a power generating mode, the actuators134A-D may be configured to extract energy from an apparent wind using drag or lift forces to drive one or more generators for the purpose of generating electrical energy. As shown inFIG. 1, the actuators134A-D may each include one or more blades. The actuator blades may rotate via interactions with the wind and could be used to drive the one or more generators. In addition, the actuators134A-D may also be configured to provide a thrust to the aircraft130during flight. As shown inFIG. 1, the actuators134A-D may function as one or more propulsion units, such as a propeller. Although the actuators134A-D are depicted as four actuators inFIG. 1, in other examples the aircraft130may include any number of actuators.

In a forward flight mode, the actuators134A-D may be configured to generate a forward thrust substantially parallel to a velocity vector of the aircraft or to the tail boom135. Based on the position of the actuators134A-D relative to the main wing131depicted inFIG. 1, the actuators may be configured to provide a maximum forward thrust for the aircraft130when all of the actuators134A-D are operating at full power. The actuators134A-D may provide equal or about equal amounts of forward thrusts when the actuators134A-D are operating at full power, and a net rotational force applied to the aircraft by the actuators134A-D may be zero.

During flight, it may be useful to rotate the aircraft130to various orientations or attitudes. For example, it may be useful to rotate the aircraft130by a certain angle about an axis parallel to a span of the main wing131that passes through a center of mass of the aircraft130. Such an attitude adjustment may be referred to as a pitch adjustment. A pitch adjustment may be achieved by causing the actuators134A and134C to operate at full power while causing the actuators134B and134D to idle (or by causing the actuators134A and134C to operate at higher powers than the actuators134B and134D). As shown inFIG. 1, such a pitch adjustment may cause the front section132to rotate towards the ground. Such a pitch adjustment may be referred to as a negative pitch adjustment by convention. Alternatively the actuators134B and134D may be caused to operate at full power while the actuators134A and134C are caused to idle, causing the front section132to rotate away from the ground. By convention, such an adjustment may be referred to as a positive pitch adjustment. However, definitions of positive and negative pitch are arbitrary and included only as examples. The positive pitch adjustment may also be achieved by causing the actuators134B and134D to operate a higher power than the actuators134A and134C. Generally, pitch adjustments may be performed using other combinations of thrusts provided by the actuators134A-D. A further example of a pitch adjustment is depicted inFIG. 4B.

It may also be useful to rotate the aircraft130by a certain angle about an axis perpendicular to a plane defined by the tail boom135and the span of main wing131. The axis may pass through the center of mass of the aircraft130. Such an attitude adjustment may be referred to as a yaw adjustment. A yaw adjustment may be achieved by causing the actuators134A and134B to operate at full power while causing the actuators134C and134D to idle (or by causing the actuators134A and134B to operate at higher powers than the actuators134C and134D). During forward flight the aircraft may be travelling substantially parallel to the ground and the yaw adjustment may cause the aircraft to change a direction or heading of horizontal travel (substantially parallel to the ground). The yaw adjustment may be referred to as a positive yaw adjustment by convention. Alternatively the actuators134C and134D may be caused to operate at full power while the actuators134A and134B are caused to idle (or the actuators134C and134D may be caused to operate at higher powers than the actuators134A and134B). By convention such an adjustment may be referred to as a negative yaw adjustment. However, characterizations of positive and negative yaw are arbitrary and included only as examples. Generally, yaw adjustments may be performed using other combinations of thrusts provided by the actuators134A-D. A further example of a yaw adjustment is depicted inFIG. 4C.

Furthermore, it may be useful to cause the aircraft130to engage in a maximum rate of position change. The actuators134A-D may each be caused to provide (equal) maximum thrusts to the aircraft130, thereby causing the aircraft130to undergo the maximum rate of position change. It should be noted that in some embodiments, a certain position change may first require a change in the attitude of the aircraft such that the actuators134A-D are aligned to provide thrust in a direction that would move the aircraft toward the subsequent position.

The tail boom135may connect the main wing131to the tail wing136and the vertical stabilizer137. The tail boom135may have a variety of dimensions. Moreover, in some implementations, the tail boom135could take the form of a body or fuselage of the aircraft130. In such implementations, the tail boom135may carry a payload.

The tail wing136or the vertical stabilizer137may be used to steer or stabilize the aircraft130or reduce drag on the aircraft130during hover flight, forward flight, or crosswind flight. For example, the tail wing136or the vertical stabilizer137may be used to maintain a pitch or a yaw attitude of the aircraft130during hover flight, forward flight, or crosswind flight. InFIG. 1, 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.

While the aircraft130has been described above, it should be understood that the methods and systems described herein could involve any aircraft that is connected to a tether, such as the tether120.

FIG. 2is a simplified block diagram illustrating components of the tethered flight system200. The tethered flight system200may include the ground station210, the tether220, and the aircraft230. As shown inFIG. 2, the ground station210may include one or more processors212, data storage214, program instructions216, and a communication system218. 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 processors212may 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 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 the communication system218. The communications system218may include one or more wireless interfaces 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), or other wireless communication protocols. Such wireline interfaces may include an Ethernet interface, a Universal Serial Bus (USB) interface, or a 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 aircraft230, other ground stations, 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 as a gateway or proxy between a remote support device (e.g., the tether220, the aircraft230, and other ground stations) and one or more data networks, such as a cellular network 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 aircraft230to the ground station210or transmit electrical energy from the ground station210to the aircraft230. The transmission components222may take various different forms in 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 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 aircraft230via the communication link224. The communication link224may be bidirectional and may include one or more wired or wireless interfaces. Also, there could be one or more routers, switches, or other devices or networks making up at least a part of the communication link224.

Further, as shown inFIG. 2, the aircraft230may include one or more sensors232, a power system234, power generation/conversion components236, a communication system238, one or more processors242, data storage244, program instructions246, and a control system248.

The sensors232could include various different sensors in different embodiments. For example, the sensors232may include a global positioning system (GPS) receiver. The GPS receiver may be configured to provide data that is typical of GPS systems (which may be referred to as a global navigation satellite system (GNNS)), such as the GPS coordinates of the aircraft230. Such GPS data may be utilized by the tethered flight system200to 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 or relative wind. Such wind data may be utilized by the tethered flight system200to 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 or attitude of the aircraft230. In particular, the accelerometer can measure the orientation of the aircraft230with respect to earth, while the gyroscope measures the rate of rotation around an axis, such as a centerline of the aircraft230. 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 are also possible.

While an accelerometer and gyroscope may be effective at determining the orientation of the aircraft230, errors in measurement may compound over time. However, an example aircraft230may 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 aircraft230may also include a pressure sensor or barometer, which can be used to determine the altitude of the aircraft230. 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 or prevent drift of the IMU. The sensors232may also include a force sensor, spring scale, or other sensors configured to measure a tension force on the tether220. Such sensors may be included as part of the ground station210, the tether220, or the aircraft230.

As noted, the aircraft230may include the power system234. The power system234could take various different forms in different embodiments. For example, the power system234may include one or more batteries that provide power to the aircraft230. 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 or via a wireless charging system, such as an inductive charging system that applies an external time-varying magnetic field to an internal battery or a 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 aircraft230. In one embodiment, the power system234may provide power to the actuators134A-D of the aircraft130, as shown and described inFIG. 1. In some implementations, the one or more motors or engines may be powered by a fuel, such as a hydrocarbon-based fuel. In such implementations, the fuel could be stored on the aircraft230and 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 aircraft230may include the power generation/conversion components236. The power generation/conversion components236could take various different forms in different embodiments. For example, the power generation/conversion components236may include one or more generators, such as high-speed, direct-drive generators. The one or more generators may be driven by one or more rotors or actuators, such as the actuators134A-D as shown and described inFIG. 1.

Moreover, the aircraft230may include a communication system238. The communication system238may take the form of or be similar in form to the communication system218of the ground station210. The aircraft230may communicate with the ground station210, other aircrafts, or other entities (e.g., a command center) via the communication system238.

In some implementations, the aircraft230may be configured to function as a “hot spot” or as a gateway or proxy between a remote support device (e.g., the ground station210, the tether220, other aircrafts) and one or more data networks, such as cellular network or the Internet. Configured as such, the aircraft230may facilitate data communications that the remote support device would otherwise be unable to perform by itself.

For example, the aircraft230may 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 aircraft230might connect to under an LTE or a 3G protocol, for instance. The aircraft230could also serve as a proxy or gateway to other aircrafts or a command station, which the remote device might not be able to otherwise access.

As noted, the aircraft230may 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 aircraft230may 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 or with hardware, firmware, 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 aircraft230or at least one entity remotely located from the aircraft230, such as the ground station210. Generally, the manner in which the control system248is implemented may vary, depending upon the particular embodiment.

FIG. 3Adepicts a downward looking view of an example tethered flight system300which may include a ground station310, a tether320, and an aircraft330. Also depicted inFIG. 3Ais an azimuth angle340and an apparent wind350. As shown inFIG. 3A, the ground station310may be coupled to the tether320at a first end of the tether320while the tether320may be coupled to the aircraft330at a second end of the tether320. The aircraft330may be configured to freely fly in an azimuthal direction about the ground station310. A position of the aircraft330may be characterized in part by the azimuth angle340between a reference angle and the azimuthal position of the aircraft330. (The azimuth angle340may be designated by “φ”.) The ground station310may be rotated so as to deploy the aircraft330in a direction parallel to the apparent wind350. With respect to the ground station310, an “x” axis may be defined as parallel to the apparent wind350with a direction of decreasing “x” corresponding with a direction of the apparent wind350. Similarly, a “y” axis may be defined as perpendicular to the apparent wind350with a direction of increasing “y” parallel to a vector originating at a center of mass of the aircraft330and pointing towards an end of the main wing of the aircraft corresponding to the actuators134C-D, depicted and described inFIG. 1(i.e. a starboard, or right end of the main wing when facing in a direction of increasing “x”).

FIG. 3Bdepicts examples of the aircraft330engaging in hover flight at various horizontal positions and altitudes. The aircraft330may be tethered to the ground station310via the tether320.FIG. 3Balso depicts the apparent wind350, a ground360, a horizontal distance370, an altitude380of the aircraft, an elevation angle390of the aircraft, and a perch395. The horizontal distance370may refer to a distance between the aircraft330and the ground station310along the ground360, while the altitude380may refer to a distance the aircraft330is above the ground360.

Hover flight may be characterized by the aircraft330travelling at an attitude such that a primary force resisting a force of gravity on the aircraft330is provided by the thrust of the actuators of the aircraft330. The aircraft330may be deployed in a direction parallel to the apparent wind350. In such a configuration, the actuators may be oriented to provide thrust in a direction substantially perpendicular to the ground360and the main wing may be oriented so that the main wing is not configured to apply a lift force to the aircraft330in a direction perpendicular to the ground360. During hover flight, lift generating surfaces of the main wing, the tail wing, and the horizontal stabilizer may not be effective in generating lift as the lift generating surfaces may either be oriented to face substantially parallel to a direction of travel of the aircraft330, or may not be impacted with a sufficient apparent wind350to generate a lift force. In hover flight, forces causing the aircraft330to move along a flight path may include forces provided by the actuators and the apparent wind350.

Hover flight may begin with deploying the aircraft330from the ground station310in a hover-flight orientation. The ground station310may be rotated so as to deploy the aircraft330in an azimuthal direction parallel with the apparent wind350. Deploying the aircraft330in the direction of the apparent wind350may enable the aircraft330to travel the horizontal distance370from the ground station310while the actuators of aircraft330are thrusting in a substantially vertical direction. The tether320may be paid out or reeled out as the aircraft330achieves increasing horizontal distance370from the ground station310. Hover flight may include the aircraft330ascending, descending, or hovering over the ground360at an altitude380above the ground360.

A vertical position of the aircraft330may also be characterized by the elevation angle390, which may be an angle defined by the perch395of the ground station310and the aircraft330, with the perch395as a vertex. In other embodiments, the vertex of the elevation angle390may occur at the ground360and characterize an elevation of the aircraft330above the ground360.

With respect to the ground station310, an “x” axis, a “y” axis, and a “z” axis may be defined. The “x” axis may be defined as parallel to the apparent wind350with a direction of decreasing “x” corresponding with the direction of the apparent wind350. The “z” axis may be defined as parallel to a vertical support of the ground station310with increasing “z” corresponding with decreasing altitude. In one embodiment, as depicted inFIG. 3B, an origin of the axes may be located at a point where the perch395of the ground station310intersects the vertical support of ground station310. However, definitions of axes may be arbitrary and the origin may be located at any point.

FIG. 4Adepicts an example roll axis402of an aircraft430. In one embodiment, the aircraft430may include actuators positioned to apply a torque thrust to the aircraft430about the roll axis402of the aircraft430, causing the aircraft430to rotate about the roll axis402. To land and couple the aircraft430onto the ground station it may be useful for the aircraft430to assume a particular roll angle with respect to a reference roll angle. During forward flight, roll adjustments of aircraft430may be made by changing a position of flaps on the main wing of the aircraft430. It should be noted that the definition of the roll axis402is arbitrary and the roll axis402may constitute a different axis in another embodiment.

FIG. 4Bdepicts an example pitch axis404of the aircraft430. The aircraft430may include actuators434A-D positioned to apply a torque thrust about the pitch axis404of the aircraft430. To pitch the aircraft430in a negative direction, the actuators434A and434C may provide thrust while the actuators434B and434D are idle. Alternatively, the aircraft430may be pitched in a positive direction by causing the actuators434B and434D to provide thrust and causing the actuators434A and434C to be idle. Using the actuators434A-D to provide pitch control for the aircraft430may be useful during hover flight, during which the tail wing of the aircraft430may not be configured to provide a torque about the pitch axis404of the aircraft430. It should be noted that definitions of positive and negative pitch and the pitch axis404are arbitrary and not meant to be limiting. The pitch axis404may constitute a different axis in another embodiment.

FIG. 4Cdepicts an example yaw axis406of the aircraft430. The aircraft430may include the actuators434A-D positioned to apply a torque thrust about the yaw axis406of the aircraft430. To yaw the aircraft430in a negative direction, the actuators434C and434D may provide thrust while the actuators434A and434B are idle. Alternatively, the aircraft430may be yawed in a positive direction by causing the actuators434A and434B to provide thrust and causing the actuators434C and434D to be idle. Using the actuators434A-D to provide yaw control may be useful during hover flight during which the vertical stabilizer of the aircraft430may not be configured to provide a torque about the yaw axis406of the aircraft430. It should be noted that definitions of positive and negative yaw and the yaw axis406are arbitrary and not meant to be limiting. The yaw axis406may constitute a different axis in another embodiment.

FIG. 5depicts an example ability of actuators534A-D of an aircraft530to concurrently provide pitch, yaw, and thrust adjustments. For example, as shown at maximum pitch rate502, a maximum positive pitch adjustment rate for the aircraft530may require having the actuators543B and534D provide full powered thrust while the actuators534A and534C idle. However, as shown at maximum yaw rate504, a maximum positive yaw adjustment rate for the aircraft530may require the actuators534A and534B to provide full thrust while the actuators534C and534D idle. Therefore, in this configuration, the actuators534A-D may not be able to provide the maximum positive yaw rate and the maximum positive pitch rate concurrently because of conflicting demands of actuators534A and534C. Examples are also illustrated in the table ofFIG. 1.

Likewise, a thrust configured to provide a maximum rate of position change may require all four actuators534A-D to provide full thrust, as shown at maximum thrust point506. However, a maximum positive pitch adjustment rate508may require that the actuators534B and534D provide full thrust while the actuators534A and534C idle. Therefore, requests for maximum pitch rate may conflict with a request for a maximum thrust force. Similarly, a request for maximum thrust as shown at point510may conflict with a request for a maximum yaw adjustment rate as shown at point512. A maximized rate of position change may require all actuators534A-D to provide full thrust whereas a maximum positive yaw rate may require the actuators534A and534B to provide full thrust while the actuators534C and534D idle.

FIG. 6Adepicts example changes in amounts of thrust provided by the actuators over time in response to a request to provide the maximum yaw adjustment rate or the maximum pitch adjustment rate. The amount of thrust provided by an actuator may also correspond to a rotational speed of a motor driving the actuator. At point602, all actuators634A-D of an aircraft630are providing a substantially equal thrust force that is less than a maximum thrust level of the actuators634A-D. After a request to provide the maximum pitch rate, the actuators634A and634C may change the respective thrust provided to the maximum thrust level, as shown at point604, while actuators634B and634D may change the respective thrust provided to zero (or about zero), as shown at point606. At a time represented by points604and606, the aircraft630may be engaged in its maximum pitch rate, a zero yaw rate, and half of a total forward thrust available from the four actuators634A-D. Alternatively, points604and606may also represent the aircraft630engaged in its maximum yaw rate, a zero pitch rate, and half of a total forward thrust available from the four actuators634A-D. An example would include the actuators634A and634B providing full thrust while the actuators634C and634D idle. (Curves ofFIG. 6Arepresenting the thrust of the actuators634A and634C are separated at point604to illustrate that multiple actuators may be providing a maximum thrust level. The actuators634A and634C may provide a substantially equal amount of thrust at point604. Likewise, the actuators634B and634D may provide a substantially equal amount of thrust at point606. Other embodiments may include any number of actuators.)

FIG. 6Bdepicts example changes in amounts of thrust provided by the actuators over time in response to a request to provide the full forward thrust available to the aircraft630. The amount of thrust provided by an actuator may also correspond to a rotational speed of a motor driving the actuator. At point608, all actuators634A-D of the aircraft630are providing a substantially equal thrust force that is less than a maximum thrust level of the actuators634A-D. After a request is received for the aircraft to engage in maximum forward thrust, all four actuators may change the thrust provided to the maximum thrust level. At the point610, maximum forward thrust is provided with a zero pitch rate and a zero yaw rate. (Curves ofFIG. 6Brepresenting the thrust of the actuators634A-D may be separated at points608and610to illustrate that multiple actuators are providing substantially equal amounts of thrust at points608and610. Other embodiments may include any number of actuators.)

FIG. 7depicts an example ground station700of the tethered flight system, which may include a winch drum705, a levelwind706, a steel rail707, a top stop709, levelwind guides710, a gimbal711and perch buckets720. The winch drum705may be configured to wind the tether around the winch drum705as the tether is reeled in or out and may include grooves in which the tether rests as the tether is reeled in or out. The tether may be guided onto the winch drum705by the levelwind706. The levelwind706may include a single pulley mounted on a pivot, such that the levelwind706may follow vertical motions of the tether as the tether is reeled in or reeled out. The levelwind706may be supported by steel rails707, and driven by a leadscrew attached to a drivetrain that drives the winch drum705. The levelwind706further comprises levelwind guides710which may guide the tether into the levelwind pulley. The leadscrew and drivetrain may be oriented such that the levelwind moves upward as the tether is reeled out. As the levelwind reaches its maximum extent at the top of its range of motion, it may hit a top stop709, which prevents the levelwind from pivoting and holds the levelwind level. In a situation in which the levelwind is beginning to engage the tether during reel in, the top stop709holds the levelwind level such that the tether is guided into the levelwind706by the levelwind guides710.

The ground station may also include the gimbal711, which may be configured to attach an end of the tether to the ground station700so as to allow free movement of the tether in multiple axes. The ground station700may further include perch buckets720. The perch buckets720may include a cavity defined by a perch bottom stop and a perch engagement surface (shown inFIG. 8) configured to couple to landing pegs of the aircraft. The perch buckets720may be constructed to support the weight of the aircraft at rest. The aircraft may be configured to come to a stable rest on perch buckets720upon landing.

FIG. 8depicts a side view of the aircraft landing on a perch bucket of the ground station. A perch bucket801includes, a perch backstop804, a perch contact point805, a perch bottom stop806, and a perch engagement surface807. An aircraft830includes a landing peg831and a center of mass840.

During a landing of the aircraft830, the landing peg831of the aircraft830may contact the perch backstop804at any point between depicted perch contact point805and the top of perch backstop804. After the landing peg831engages the perch backstop804at the perch contact point805, an actuator of the aircraft802may reduce thrust, causing the aircraft830to decrease altitude, and the perch contact point805may slide down the perch backstop804until the landing peg831contacts the perch bottom stop806. Next, the actuator on the aircraft830may be idled and the landing peg831of the aircraft830may rest against the perch bottom stop806and the perch engagement surface807. The perch bottom stop806may be substantially offset on a horizontal axis from the center of mass840of the aircraft830, such that the gravitational force on the aircraft830pushes the landing peg831against the perch engagement surface807, so that the aircraft830is safely resting on the perch bucket801of the ground station.

FIG. 9depicts a portion of an example main wing901of the aircraft, including a landing peg930and a hook931. The landing peg930may configured to couple with the perch bucket of the ground station, and the hook931may be configured to secure the main wing to the perch bucket by coupling to a top of the perch bucket. The peg930and the hook931may be bonded to the main wing901. In some embodiments, the peg930is integrated into a fuselage of the aircraft or the hook931is integrated into the pylons holding the actuators on the aircraft. In another example, the hook931may not be included as part of the main wing901and the aircraft may rest on the ground station by the coupling of the landing peg930with the perch bucket.

FIG. 10is a block diagram of an example method1000for determining a priority sequence for changing a first attribute and a second attribute of the position and the attitude of the aircraft, in accordance with at least some embodiments described herein. Method1000shown inFIG. 10presents an embodiment of a method that, for example, could be used with a computing device. Method1000may include one or more operations, functions, or actions as illustrated by one or more blocks of1002-1008. Although the blocks are illustrated in a sequential order, these blocks may in some instances be performed in parallel, and/or in a different order than those described herein. Also, the various blocks may be combined into fewer blocks, divided into additional blocks, and/or removed based on the desired implementation.

In addition, for the method1000and other processes and methods disclosed herein, each block inFIG. 10may represent circuitry that is wired to perform the specific logical functions in the process.

Functions of the method1000may be fully performed by a processor of a computing device, by a computing device, or may be distributed across multiple processors or multiple computing devices and/or a server. In some examples, the computing device may receive information from sensors of the computing device, or where the computing device is a server the information can be received from another device that collects the information.

At block1002, the method1000includes receiving data representing an initial position and an initial attitude of an aircraft. The aircraft may be configured to be coupled to a ground station via a tether and may include an actuator, such as a propeller, configured to change the position and the attitude of the aircraft. A processor included in the ground station or the aircraft may receive the data from a sensor of the aircraft, the tether, or the ground station. Such positional data may include attributes of position such as a horizontal distance between the aircraft and the ground station, an azimuth angle of the aircraft, and an altitude of the aircraft. A horizontal distance may be a distance along the ground. Alternatively, the positional data may be expressed in rectangular or spherical coordinates and may be expressed without reference to a position of the ground station. The data representing the attitude of the aircraft may include attributes of attitude such as a yaw angle, a pitch angle, and a roll angle of the aircraft. The angles may represent angles of rotation about various axes of the aircraft. The position and attitude data may further include location coordinates as well.

At block1004, the method1000includes determining a change to a first attribute and a second attribute of the position or the attitude of the aircraft to achieve a subsequent position and a subsequent attitude. Determining a change to an attribute of the position of the aircraft to achieve a subsequent position may include determining a horizontal distance between the aircraft and the subsequent position, determining a vertical distance between the aircraft and the subsequent position, and determining an azimuth angle between the azimuth angle of the aircraft and an azimuth angle of the subsequent position. Determining a change to an attribute of the attitude of the aircraft to achieve a subsequent attitude may also include determining a yaw comparison angle between the yaw angle of the aircraft and a yaw angle of the subsequent attitude, determining a roll comparison angle between the roll angle of the aircraft and a roll angle of the subsequent attitude, and determining a pitch comparison angle between the pitch angle of the aircraft and a pitch angle of the subsequent attitude. The respective yaw, roll, and pitch comparison angles may represent a change in the respective yaw, pitch, and roll angles of the aircraft that are required to place the aircraft in the subsequent attitude.

In some embodiments, a change to an attribute of the position or the attitude of the aircraft may initially move the aircraft away from the subsequent position or the subsequent attitude to later return the aircraft to the subsequent position or the subsequent attitude. For example, if the aircraft is initially oriented so that the actuator is configured to provide a vertical thrust substantially perpendicular to the ground, a change in the yaw angle of the aircraft may orient the actuator away from the subsequent attitude to provide thrust in a direction to achieve a subsequent position located at a different azimuth angle. After the subsequent position is achieved, the attitude may be adjusted to the subsequent attitude which may be such that the actuator is positioned to provide a vertical thrust substantially perpendicular to the ground. By further example, if the aircraft is initially oriented so that the actuator is configured to provide a vertical thrust substantially perpendicular to the ground, a change in the pitch angle of the aircraft may orient the actuator away from the subsequent attitude to provide thrust in a direction to achieve a subsequent position located at a different horizontal distance from the ground station. After the subsequent position is achieved, the attitude may be adjusted to the subsequent attitude which may be such that the actuator is positioned to provide a vertical thrust substantially perpendicular to the ground.

The change to the subsequent position and the subsequent attitude may be determined based on the processor receiving a notification that the aircraft is moving away from the subsequent position and attitude, perhaps because of a high wind condition. While in the subsequent attitude, the aircraft may be configured for hover flight and be aligned to land and couple with a perch bucket of the ground station. Thus, within examples, the subsequent position and/or the subsequent attitude may be determined by the ground station or the aircraft and may correspond to a position and attitude desirable for landing the aircraft. The method may be used to control the flight path of the aircraft as the aircraft is deployed or as the aircraft is landing on the ground station.

At block1006, the method1000includes determining a priority sequence for changing the first attribute and the second attribute of the position or the attitude of the aircraft based on a first thrust of the actuator to achieve the change to the first attribute and a second thrust of the actuator to achieve the change to the second attribute. The priority sequence may be determined so that the first attribute is changed before the second attribute is changed in cases where the actuator is unable to concurrently provide thrusts that correspond to conflicting requests to maneuver the aircraft.

Determining the priority sequence may include receiving data representing a threshold distance and determining whether a distance of the aircraft from the ground station is greater than, less than, or equal to the threshold distance. The priority sequence may be determined based on whether the distance between the aircraft and the ground station is greater than, less than, or equal to the threshold distance. For example, the aircraft may have less space to freely maneuver in certain directions or rotate about certain axes while landing on or deploying from the ground station, for example, when the distance between the aircraft and the ground station is less than the threshold distance. The priority sequence may also be configured so that the aircraft avoids collisions with the ground station based on the position and attitude of the aircraft. In one embodiment, the threshold distance may be about 50 meters and the tether may be about 450 meters long.

By further example, when the distance between the aircraft and the ground station is less than the threshold distance, a yaw angle of the aircraft may be prioritized before a pitch angle of the aircraft in the priority sequence because docking between the ground station and the aircraft may require the yaw angle of the aircraft to remain within a smaller range of angles than may be required for the pitch angle of the aircraft. It may also be possible to control the pitch angle of the aircraft by the ground station controlling the tension of the tether. By using the tether to control the pitch angle of the aircraft, the actuator may be able to provide a thrust to change the yaw angle of the aircraft that may otherwise conflict with the pitch adjustment. Pitch may also be controlled via another designated actuator, leaving the original actuator to provide thrust for yaw adjustment.

For similar reasons, the yaw angle of the aircraft may be prioritized for change before the roll angle of the aircraft based on the distance of the aircraft from the ground station being less than the threshold distance. Docking of the aircraft with the ground station may require the yaw angle of the aircraft to remain within a smaller range of angles than may be required for the roll angle of the aircraft. Also, the roll angle of the aircraft may be controlled using flaps on a wing of the aircraft, leaving the actuator to provide a thrust to change the yaw angle of the aircraft that may otherwise conflict with the roll adjustment.

Likewise, the yaw angle of the aircraft may be prioritized for change before the azimuth angle of the aircraft based on the distance of the aircraft from the ground station being less than the threshold distance. Docking of the aircraft with the ground station may require the yaw angle of the aircraft to remain within a smaller range of angles than may be required for the azimuth angle of the aircraft because the ground station may be configured to freely swivel in an azimuth direction to follow the tether and the aircraft.

Also, the altitude of the aircraft may be prioritized for change before the pitch angle of the aircraft based on the distance of the aircraft from the ground station being less than the threshold distance. Increasing the altitude of the aircraft may be achieved via the actuator of the aircraft, whereas the pitch angle of the aircraft may be controlled by the ground station increasing or decreasing a tension of the tether. Such adjustments in tether tension may cause the aircraft to rotate about the pitch axis of the aircraft. The pitch angle of the aircraft may also be controlled by an additional actuator designated for pitch control, such that the original actuator may provide the thrust for altitude adjustment.

The altitude of the aircraft may also be prioritized for change before the roll angle and the azimuth angle of the aircraft based on the aircraft being within the threshold distance of the ground station. Flaps of a wing of the aircraft may be used to change the roll angle of the aircraft and the ground station may swivel to follow the aircraft in the azimuth direction, which may leave the actuator to provide a thrust to change the altitude of the aircraft that may otherwise conflict with the roll adjustment or the azimuth adjustment.

By further example, the pitch angle of the aircraft may be prioritized to be changed before the roll angle and the azimuth angle of the aircraft based on the aircraft being within the threshold distance of the ground station. Landing or deploying the aircraft may require the pitch angle of the aircraft to be within a smaller range of angles than is required for the roll angle of the aircraft. Flaps of the wing of the aircraft may be used to change the roll angle of the aircraft and the ground station may swivel to follow the aircraft in the azimuth direction, which may leave the actuator to provide a thrust to change the pitch angle of the aircraft that may otherwise conflict with the roll adjustment or the azimuth adjustment.

However, when the distance of the aircraft from the ground station is greater than or equal to the threshold distance, the yaw angle of the aircraft may be prioritized for change before the azimuth angle of the aircraft. When the aircraft is at least the threshold distance away from the ground station, the priority sequence may be configured so that the azimuthal angle of the aircraft is maintained directly downwind from the ground station (or a certain azimuthal angle offset from directly downwind). Although, in some embodiments, controlling the azimuth angle of the aircraft may have more value than controlling the yaw angle of the aircraft beyond the threshold distance, a change in the azimuth angle of the aircraft may require the aircraft to first change the yaw angle of the aircraft so that the actuator is positioned to provide a thrust in the azimuthal direction. The yaw angle of the aircraft may be changed back to the subsequent attitude after the azimuth angle of the aircraft has been changed toward the subsequent position. In one embodiment, the yaw angle of the subsequent attitude may be such that the actuator of the aircraft is positioned to provide a vertical thrust substantially perpendicular to the ground.

Similarly, the yaw angle of the aircraft may be prioritized for change before the altitude of the aircraft when the distance of the aircraft from the ground station is at least the threshold distance. For example, effective control of the altitude of the aircraft may require that the yaw angle of the aircraft is such that the actuator of the aircraft is in position to provide a vertical thrust substantially perpendicular to the ground to increase the altitude of the aircraft.

By further example, the pitch angle of the aircraft may be prioritized for change before the altitude of the aircraft when the distance of the aircraft from the ground station is at least the threshold distance. Effective control of the altitude of the aircraft may require that the pitch angle of the aircraft is such that the actuator of the aircraft is in position to provide a substantially vertical thrust perpendicular to the ground to increase the altitude of the aircraft.

When the distance of the aircraft from the ground station is at least the threshold distance, the azimuth angle of the aircraft may be prioritized for change before of the roll angle of the aircraft. Transitioning the aircraft between hover flight and cross-wind flight may require more precision in an azimuth angle of the aircraft than in the roll angle of the aircraft.

The method1000may also include receiving data representing a tension force on the tether and a predetermined tension force. The data representing the tension force on the tether may be received from a sensor of the aircraft, the tether, or the ground station. The data representing the predetermined tension force may be retrieved from memory or received as an input from an input device. The processor may receive the data and determine whether the tension force on the tether is substantially equal to the predetermined tension force. Based on the predetermined tension force and the tension force on the tether being unequal, the actuator may cause the aircraft to rotate the aircraft about the pitch axis of the aircraft. The rotation of the aircraft about the pitch axis may move a point at which the tether couples to the aircraft toward or away from the ground station, thereby reducing or increasing the tension force on the tether until the tension force is substantially equal to the threshold tension force.

The method1000may also include receiving data representing a speed of a wind within an ambient environment of the aircraft and a threshold wind speed. Data representing the speed of the wind within the ambient environment of the aircraft may be received from sensors of the aircraft or the ground station and data representing the threshold wind speed may be received from an input device. A processor may next determine whether the speed of the wind within the ambient environment of the aircraft is greater than the threshold wind speed. Additionally, the aircraft may include an additional actuator that, during hover flight in high winds, is configured to generate a drag force or a lift force from an impact of the wind within the ambient environment of the aircraft. The drag force or lift force may create a torque about the pitch axis of the aircraft. In this way, the priority sequence, based on the speed of the wind within the ambient environment of the aircraft being greater than the threshold wind speed, may be configured so that another actuator of the aircraft may provide a thrust that changes a yaw angle or an altitude of the aircraft before providing a thrust that changes a pitch angle of the aircraft.

At block1008, the method1000includes causing the actuator to change the first attribute and the second attribute according to the priority sequence. The processor may generate a request to be received by the control system of the aircraft and the control system may cause the actuator to move the aircraft. By further example, the aircraft may include an additional actuator configured to provide angular thrust for the aircraft about various axes of the aircraft. The control system may rotate the aircraft to the subsequent attitude by causing the actuator to provide an angular thrust about an axis of the aircraft and causing the additional actuator to idle. In this way, pairs of actuators may be configured to move the aircraft in opposite rotational directions with respect to an axis of the aircraft.