Occasional use of commanded rate instead of sensed airspeed to inform flight control decisions

Abstract

A technique for controlling an unmanned aerial vehicle (UAV) includes: monitoring a sensed airspeed of the UAV; obtaining a commanded velocity for the UAV, wherein the commanded velocity represents a command to fly the UAV at a given velocity relative to the air mass or the earth; and when the commanded velocity is greater than the sensed airspeed, using the commanded velocity instead of the sensed airspeed to inform flight control decisions of the UAV.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of U.S. Application No. 16 / 597,338, filed October 9, 2019, the contents of which are incorporated herein by reference. Technical Field

[0003] The present disclosure relates generally to aircraft and, in particular, but not exclusively, to control systems for unmanned aerial vehicles (UAVs). Background Art

[0004] An unmanned vehicle, which may also be referred to as an autonomous vehicle, is a vehicle capable of traveling without a physically present human operator. An unmanned vehicle can operate in a remote-controlled mode, an autonomous mode, or a partially autonomous mode.

[0005] When an unmanned vehicle is operated in remote control mode, a pilot or driver at a remote location can control the unmanned vehicle via commands sent to the unmanned vehicle via a wireless link. When an unmanned vehicle is operated in autonomous mode, the unmanned vehicle typically moves based on pre-programmed navigation waypoints, a dynamic automation system, or a combination of these. In addition, some unmanned vehicles can operate in both remote control mode and autonomous mode, and in some instances can do so simultaneously. For example, a remote pilot or driver may wish to hand off navigation to an autonomous system while manually performing another task (such as operating a mechanical system for picking up an object, for example).

[0006] There are various types of unmanned vehicles for various environments. For example, there are unmanned vehicles for operation in the air, on land, underwater and in space. Unmanned aerial vehicles (UAVs) or drones in general are becoming more popular. As their designs are improved and their capabilities expand, their commercial applicability is expected to expand. Designs that improve the efficiency, stability, reliability and / or durability of UAVs will expand their mission capabilities. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. Not all instances of an element must be labeled so as not to clutter the drawings where appropriate. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles being described.

[0008] Figure 1 is a perspective view illustration of an unmanned aerial vehicle (UAV) according to an embodiment of the present disclosure.

[0009] Figure 2A UAV is shown navigating a drone mission along a flight path having hover and cruise segments in accordance with an embodiment of the present disclosure.

[0010] Figure 3 is a functional block diagram illustrating a control system for a UAV according to an embodiment of the present disclosure.

[0011] Figure 4A A UAV is shown with a forward axis aligned with a commanded path in accordance with an embodiment of the present disclosure.

[0012] Figure 4B A UAV is shown having a forward axis that is misaligned with a commanded path in accordance with an embodiment of the present disclosure.

[0013] Figure 5 is a flow chart illustrating a process for selectively informing flight control decisions of a UAV with commanded airspeed instead of sensed airspeed, according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0014] Embodiments of systems, apparatus, and methods for occasionally using a commanded airspeed instead of sensed airspeed to inform flight control decisions are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the embodiments. However, one skilled in the relevant art will recognize that the technology described herein can be practiced without one or more of the specific details or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring certain aspects.

[0015] Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0016] For aircraft such as unmanned aerial vehicles (UAVs), airspeed measured from onboard sensors (e.g., pitot tubes) is traditionally used to inform flight control decisions. These decisions may include scheduling gains by applying thrust using one or more propulsion units or allocating control effort applied via control surfaces (e.g., rudder, ailerons, etc.). However, in situations where airspeed cannot be reliably obtained because an airspeed sensor is unable to make an adequate measurement of airspeed, it may be beneficial to perform flight control decisions based on other information.

[0017] In exceptional cases, the inability to accurately measure airspeed may be due to a malfunctioning airspeed sensor. However, in a more common scenario, this may be due to a misalignment between the forward aerodynamic axis of the aircraft (also referred to herein as the vehicle's x-axis) and its flight vector (the direction of inertial motion relative to the Earth's reference frame). Typically, the airspeed sensor is mounted to the aircraft in an orientation that measures airspeed along the aircraft's forward aerodynamic axis. During normal flight orientation, aerodynamic surfaces on the aircraft cause a weathervaning effect that aligns the aircraft's forward aerodynamic axis with its flight vector (see Figure 4A ), resulting in accurate airspeed measurements. However, the aircraft may enter a flight mode in which the aircraft's forward aerodynamic axis is not well aligned with its flight vector (see Figure 4B ). This is particularly true for vertical take-off and landing (VTOL) vehicles when transitioning between hover mode and cruise mode. Such misalignment during the transition is typically short-lived. However, in certain scenarios, the aircraft may become unable to obtain sufficiently good airspeed measurements to facilitate exiting the misaligned state, resulting in a lack of control authority if control allocation is performed based on these airspeed measurements. As a result, the aircraft may become unable to accurately schedule gains and allocate control work to exit the trapped state. Therefore, embodiments described herein provide for the occasional use of "commanded rate" instead of "sensed airspeed" to inform flight control decisions. In one embodiment, the commanded rate is used as a reference point so that the amount of thrust and torque applied is appropriate to the airspeed the aircraft is recommended to travel, rather than its measured or estimated rate. The commanded rate can be specified relative to the air mass through which the aircraft is flying or relative to the earth above which the aircraft is flying.

[0018] Figure 1 1 is a perspective view illustration of an aircraft 100 according to an embodiment of the present disclosure. The embodiment of the aircraft 100 shown is a VTOL unmanned aerial vehicle (UAV) that includes separate propulsion units 106 and 112 for providing horizontal and vertical propulsion, respectively. The aircraft 100 is a fixed-wing aircraft and, as the name implies, has wing assemblies 102 that, when propelled horizontally by the propulsion units 106, can generate lift based on the wing shape and the forward airspeed of the vehicle.

[0019] The embodiment of the aircraft 100 shown includes an airframe comprising a fuselage 104, wing assemblies 102, and boom assemblies 110. In one embodiment, the fuselage 104 is modular and includes a battery module, an avionics module, and a mission payload module. These modules can be detached from one another and mechanically secured to one another to contiguously form at least a portion of the fuselage or body.

[0020] The battery module includes a cavity for accommodating one or more batteries that power the aircraft 100. The avionics module houses the flight control circuitry of the aircraft 100, which may include a processor and memory, communication electronics and antennas (e.g., cellular transceivers, Wi-Fi transceivers, etc.), and various sensors (e.g., global positioning sensors, inertial measurement units (IMUs), magnetic compasses, etc.). The mission payload module houses equipment associated with the mission of the aircraft 100. For example, the mission payload module may include a payload actuator for holding and releasing externally attached payloads. In another embodiment, the mission payload module may include a camera / sensor equipment holder for carrying camera / sensor equipment (e.g., cameras, lenses, radars, lidar (light detection and ranging), pollution monitoring sensors, weather monitoring sensors, etc.). Airspeed sensors (e.g., pitot tubes) may be mounted at various locations on the airframe to measure airspeed along the forward or x-axis of the airframe.

[0021] As shown, aircraft 100 includes a horizontal propulsion unit 106 positioned on wing assembly 102, which can each include a motor, a motor rotor with an axis, and propeller blades for horizontally propelling aircraft 100. The embodiment of aircraft 100 shown also includes two cantilever assemblies 110 fixed to wing assembly 102. A vertical propulsion unit 112 is mounted to cantilever assemblies 110. Vertical propulsion unit 112 can also each include a motor, a motor rotor with an axis, and propeller blades for providing vertical propulsion. Vertical propulsion unit 112 can be used during a hovering mode in which aircraft 100 descends (e.g., to a delivery position), rises (e.g., after delivery), or maintains a constant altitude. Aircraft 100 can include stabilizers 108 (or tail) to control pitch and stabilize the yaw (left or right turn) of the aircraft during cruising. In certain embodiments, vertical propulsion unit 112 is disabled during cruising, and horizontal propulsion unit 106 is disabled during hovering. In other embodiments, vertical propulsion units 112 are only powered at low power during the cruise mode and / or horizontal propulsion units 106 are only powered at low power during the hover mode.

[0022] During flight, the aircraft 100 can control the direction and / or rate of its motion by controlling its pitch, roll, yaw, and / or altitude. Thrust from the horizontal propulsion unit 106 is used to control airspeed as a scheduling gain. Stabilizer 108 may include one or more rudders 108a for controlling the yaw of the aircraft, and wing assembly 102 may include elevators for controlling the pitch of the aircraft and / or ailerons 102a for controlling the roll of the aircraft. Control work can be distributed across these control surfaces. As another example, increasing or decreasing the speed of all propeller blades simultaneously may cause the aircraft 100 to increase or decrease its altitude, respectively.

[0023] Many variations of the fixed-wing aircraft shown are possible. For example, an aircraft with more wings (e.g., an "x-wing" configuration with four wings) is also possible. Although Figure 1 One wing assembly 102, two boom assemblies 110, two horizontal propulsion units 106, and four vertical propulsion units 112 per boom assembly 110 are shown, but it should be understood that other variations of the aircraft 100 may be implemented with more or fewer of these components. Furthermore, the control techniques described herein are not limited to use with only VTOLs or UAVs, but rather these control techniques may also be applicable to other types of aircraft.

[0024] It should be understood that references herein to "unmanned" aerial vehicles, or UAVs, also apply to autonomous and semi-autonomous aerial vehicles. In fully autonomous embodiments, all functions of the aerial vehicle are automated; for example, they are pre-programmed or controlled via real-time computer functions that respond to input from various sensors and / or predetermined information. In semi-autonomous embodiments, some functions of the aerial vehicle may be controlled by a human operator, while other functions are performed autonomously. Furthermore, in some embodiments, the UAV may be configured to allow a remote operator to take over functions that would otherwise be controlled autonomously by the UAV. Furthermore, a given type of function may be remotely controlled at one level of abstraction and autonomously performed at another level of abstraction. For example, a remote operator may control high-level navigation decisions for the UAV, such as specifying that the UAV should travel from one location to another (e.g., from a warehouse in the suburbs to a delivery address in a nearby city), while the UAV's navigation system autonomously controls more fine-grained navigation decisions, such as the specific route to take between the two locations, the specific flight controls to achieve the route, and obstacle avoidance while navigating the route.

[0025] Figure 2A UAV 200 is shown navigating a drone mission over a neighborhood 205 along a flight path that includes hover segments 210A and 210C, and a cruise segment 210B. In one embodiment, UAV 200 may be implemented by aircraft 100. UAV 200 traverses the flight path in response to a command path generated by a mission manager. The command path is the path the control system commands UAV 200 to follow, while the flight path is the actual path executed. Ideally, these two paths are consistent, although in reality these paths may deviate slightly because the control system of UAV 200 strives to execute the command path despite real-world influences such as wind, turbulence, aerodynamic drag, and rain. The drone mission includes acceleration during hover segment 210A, a constant speed during cruise segment 210B, and deceleration to destination 215 during hover segment 210C.

[0026] The UAV 200 begins its drone mission from the staging area 220 in hover mode to traverse the hover segment 210A. During hover mode, the UAV 200 begins at ground level, ascends to a cruising altitude, and then accelerates toward a cruising speed before fully transitioning into cruise mode for the cruise segment 210B of the flight path. In some embodiments, the cruising speed is a fixed value (e.g., 50 mph). The fixed value may be selected based on the design / type of the UAV 200, the scope of the specific drone mission, the weight / drag of the payload, battery limitations, or other factors and considerations. When a fixed value is used for airspeed tracking, the fixed value is the airspeed at which the aircraft is expected to travel. The fixed value may be used to select gains and control distributions appropriate for such a speed.

[0027] In a typical scenario, as the UAV 200 accelerates toward cruising speed, wind resistance on the aerodynamic surfaces of the UAV 200 causes the forward axis 206 of the UAV 200 to move in a direction along the commanded path (see also Figure 4A However, in certain environmental situations, the UAV 200 may end up misaligned with the commanded path (e.g., see Figure 4B ), causing lateral axis 207 to tilt toward the commanded path. In this misaligned orientation, the onboard airspeed sensor may inaccurately reflect the true airspeed due to the misalignment of the sensor with the direction of travel through the air mass. Consequently, the flight control system may not receive sufficiently accurate feedback from the onboard sensor to regain control authority, and may become trapped in this misaligned and inefficient state.

[0028] Figure 3is a functional block diagram illustrating a control system 300 for a UAV 200 according to an embodiment of the present disclosure. The illustrated control system embodiment includes a mission manager 305 and a flight control system 310 that provide control commands to a propulsion unit 315 and a control surface 320. The propulsion unit 315 may include one or both of the horizontal propulsion unit 106 and the vertical propulsion unit 112. The control surface 320 may include any control surface, including the aileron 102a and the rudder 108a, or other actuated control surfaces.

[0029] Reference Figure 3 and Figure 4B , the task manager 305 generates a commanded inertial velocity 405, which is directed along a commanded path 410 (also generated by the task manager 305). The commanded inertial velocity 405 is a vector quantity (magnitude and direction) and should be interpreted as a commanded velocity relative to an inertial reference frame (e.g., the Earth reference frame).

[0030] The flight control system 310 decomposes the commanded inertial velocity 405 into two components: the forward commanded inertial velocity (V_xci) and the lateral commanded inertial velocity (V_yci). The "x" and "y" correspond to directions relative to the vehicle's body frame: the x-forward axis 206 and the y-lateral axis 207. The "c" refers to the fact that these quantities are commands rather than estimates of the current state. The "i" refers to "inertia."

[0031] A placeholder value called "forward command speed" (V_xc) is defined. In hover mode (including acceleration and deceleration), V_xc is set equal to the forward command inertial speed, V_xci. However, in cruise mode, V_xc is instead set to a fixed value associated with cruise mode. This fixed value can be thought of as the commanded airspeed for cruise (as opposed to a commanded speed that has directional information). In one embodiment, this fixed value is a preset, defined parameter of the aircraft. The subscript "i" is removed from V_xc to emphasize this distinction. Using the established values and terminology above, the command speed (cmd_speed) can be defined as:

[0032]

[0033] Using the above substitution, when the forward axis (x-axis) of the UAV is aligned with the commanded path 410 in cruise mode (see Figure 4A ), the UAV essentially ignores the commanded inertial speed 405 and simply tracks the commanded velocity (e.g., V_yci = 0; cmd_speed = V_xc = the current fixed value in cruise mode as the commanded velocity, not the measured or sensed airspeed). If the UAV is not well aligned with the direction of travel (see Figure 4B), the forward command speed V_xc remains a constant fixed value, but the magnitude of V_yci will increase, so the value cmd_speed will also increase according to Equation 1.

[0034] Conventionally, flight control systems reference sensed airspeed to schedule gains and distribute control effort. However, flight control system 310 instead occasionally uses cmd_speed to schedule gains and distribute control effort. In one embodiment, flight control system 310 occasionally references cmd_speed instead of sensed airspeed to inform flight control decisions according to Expression 2:

[0035] max(cmd_speed,sensed_airspeed) (expression 2) Wherein sensed_airspeed is the airspeed measured by the onboard sensors of the UAV.Thus, in one embodiment, the flight control system 310 uses the maximum value selected between cmd_speed and sensed_airspeed rather than using only the measured airspeed.

[0036] During acceleration (e.g., hover mode), when sensed_airspeed is artificially lowered due to poor sensor alignment with the direction of travel through the air mass, using expression 2 to inform flight control decisions increases robustness. Instead, a larger value of cmd_speed is used, which provides an appropriate value when scheduling gains. During constant rate cruise (e.g., cruise mode), when the vehicle becomes misaligned with its command path 410 due to disturbances (see Figure 4B ), using Expression 2 to inform flight control decisions increases robustness. In this scenario, Expression 2 provides a larger value for sensed_airspeed relative to the artificially suppressed sensed_airspeed. This larger value is more suitable for cruise flight.

[0037] Figure 5 is a flow chart illustrating a process 500 for selectively informing flight control decisions of the UAV 200 with a commanded speed (cmd_speed) instead of a sensed airspeed (sensed_airspeed) according to an embodiment of the present disclosure. The order in which some or all of the process blocks appear in the process 500 should not be considered limiting. Rather, one of ordinary skill in the art having the benefit of this disclosure will understand that some of the process blocks may be performed in various orders not shown, or even in parallel.

[0038] In process block 505, the flight control system 310 monitors the airspeed sensed from the onboard airspeed sensor (sensed_airspeed). In process block 510, the flight control system 310 obtains the commanded speed (cmd_speed). In one embodiment, the commanded speed is calculated based on Expression 1. If the flight control system 310 determines that sensed_airspeed is greater than cmd_speed, then the process 500 continues to process block 520, where the flight control system 310 uses sensed_airspeed to inform its flight control decisions to keep the UAV 100 tracking the commanded path 410.

[0039] However, if it is determined that cmd_speed is greater than sensed_airspeed, then process 500 continues to process block 525 and occasionally uses the command speed (see Equation 1) instead of sensed_airspeed to inform flight control decisions of the UAV 200. If the UAV 200 is operating in cruise mode (decision block 530), the forward command speed (V_xc) is set to a preset fixed value associated with cruising of the UAV 200 (process block 535). However, if the UAV 200 is operating in hover mode (decision block 530), the forward command speed (V_xc) is set to the forward command inertial speed (V_xci) calculated by the flight control system 310 as the forward component of the commanded inertial speed 405 provided by the mission manager 305 (process block 540). The forward component is the projection of the commanded inertial speed 405 onto the commanded path 410.

[0040] The processes explained above are described in terms of computer software and hardware. The described techniques may constitute machine-executable instructions embodied in a tangible or non-transitory machine (e.g., computer) readable storage medium, which, when executed by a machine, causes the machine to perform the described operations. In addition, the processes may be embodied in hardware, such as an application-specific integrated circuit ("ASIC") or other hardware.

[0041] A tangible machine-readable storage medium includes any mechanism that provides (i.e., stores) information in a non-transitory form accessible to a machine (e.g., a computer, a network device, a personal digital assistant, a manufacturing tool, any device having one or more processors, etc.). For example, a machine-readable storage medium includes recordable / non-recordable media (e.g., read-only memory (ROM), random access memory (RAM), magnetic disk storage media, optical storage media, flash memory devices, etc.).

[0042] The above description of the illustrated embodiments of the present invention, including that described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Although specific embodiments and examples of the invention have been described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the art will appreciate.

[0043] These modifications can be made to the present invention in light of the above detailed description. The terms used in the appended claims should not be construed to limit the invention to the specific embodiments disclosed in the specification. Instead, the scope of the invention is to be determined entirely by the appended claims, which are to be interpreted in accordance with established doctrines of claim interpretation.

Claims

1. A method for controlling an unmanned aerial vehicle (UAV), the method comprising: Monitor the UAV's sensed airspeed; Obtaining a commanded speed of the UAV, the commanded speed representing a command to fly the UAV at a given speed relative to an air mass through which the UAV is flying or the earth above which the UAV is flying, wherein obtaining the commanded speed of the UAV comprises calculating the commanded speed cmd_speed as: where V_xc represents the forward commanded velocity of the UAV and V_yci represents the lateral commanded inertial velocity of the UAV, which is calculated as the lateral component of the commanded inertial velocity directed along the commanded path of the UAV; When the commanded rate is greater than the sensed airspeed, using the commanded rate instead of the sensed airspeed to schedule thrust for propelling the UAV and / or to distribute control effort to control surfaces of the UAV; and When the sensed airspeed is greater than the commanded rate, the sensed airspeed is used instead of the commanded rate to schedule thrust for propelling the UAV and / or to distribute control effort to control surfaces of the UAV.

2. The method according to claim 1, wherein The sensed airspeed of the UAV includes a measurement of the airspeed sensed by onboard sensors of the UAV during flight.

3. The method of claim 1 , further comprising: When the UAV is operating in cruise mode, the forward command speed is set to a fixed value associated with the cruise mode of the UAV.

4. The method of claim 3, further comprising: When the UAV is operating in the hover mode, the forward command velocity is set to the forward command inertial velocity of the UAV, which is calculated as the forward component of the commanded inertial velocity directed along the commanded path of the UAV.

5. The method according to claim 4, wherein: The hovering mode includes accelerating toward a cruise mode and decelerating from a cruise mode.

6. The method of claim 1, wherein: The UAV comprises a vertical take-off and landing (VTOL) UAV, and the VTOL UAV comprises at least one vertical propulsion unit and at least one horizontal propulsion unit.

7. At least one machine accessible storage medium providing instructions which, when executed by a control system of an unmanned aerial vehicle (UAV), will cause the UAV to perform the method of any one of claims 1 to 6.

8. An unmanned aerial vehicle (UAV), comprising: control surfaces that affect one or more of the pitch, yaw, or roll of the UAV; Propulsion unit, which propels the UAV; a sensor that measures an airspeed of the UAV and responsively outputs a sensed airspeed of the UAV; as well as a control system coupled to the control surfaces, the propulsion unit, and the sensors, the control system including logic that, when executed by the control system, causes the UAV to perform operations comprising: Obtaining a commanded speed of the UAV, the commanded speed representing a command to fly the UAV at a given speed relative to an air mass through which the UAV is flying or the earth above which the UAV is flying, wherein obtaining the commanded speed of the UAV comprises calculating the commanded speed cmd_speed as: where V_xc represents the forward commanded velocity of the UAV and V_yci represents the lateral commanded inertial velocity of the UAV, which is calculated as the lateral component of the commanded inertial velocity directed along the commanded path of the UAV; When the commanded rate is greater than the sensed airspeed, using the commanded rate instead of the sensed airspeed to schedule thrust for propelling the UAV and / or to distribute control effort to control surfaces of the UAV, and When the sensed airspeed is greater than the commanded rate, the sensed airspeed is used instead of the commanded rate to schedule thrust for propelling the UAV and / or to distribute control effort to control surfaces of the UAV.

9. The UAV of claim 8, wherein: The control system includes further logic that, when executed by the control system, causes the UAV to perform further operations, the further operations including: When the UAV is operating in cruise mode, setting the forward command speed to a fixed value; and When the UAV is operating in a hovering mode including acceleration to and deceleration from a cruise mode, the forward command speed is set to a forward commanded inertial speed of the UAV, which is calculated as a forward component of the commanded inertial speed directed along a commanded path of the UAV.

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