Abstract:
A quadrotor or other vertical lift aerial vehicle measures an angle of a payload slung from the quadrotor relative to a body of the quadrotor. Using this measurement a signal may be generated that adjusts a flight characteristic of the quadrotor to counteract swing in the payload. A feedback function for generating the feedback signal may include proportional and derivative gain functions as well as non-linear signal processing functions. The feedback signal may be added to normal input control signals to cause acceleration in the direction of the payload angle that damp oscillation of the slung payload caused by wind or movements of the quadrotor.

Description:
FIELD 
       [0001]    This disclosure relates to a quadrotor aerial vehicle and more particularly to damping a slung payload being lifted by the quadrotor. 
       BACKGROUND 
       [0002]    Quadrotor aerial vehicles, also known as quadcopters, have been in use since the 1920&#39;s but were generally disregarded due to poor lifting performance and poor stability. In recent years, quadrotors have experienced a resurgence due to improvements in materials including high strength, light weight composites and improved battery technology. 
         [0003]    Application of quadrotors to seemingly simple tasks such as transporting materials in a sling can be limited by instabilities caused by wind, wind gusts, and the acceleration of the quadrotor. In similar applications, such as helicopter lifts, a human pilot can manually counter the effects of a swinging payload. However, even in a piloted vehicle controlling a swinging payload is not a simple task. The challenge is increased for an automatically piloted aerial vehicle when such visual and tactile inputs may not be available. Oscillatory motion in a payload can quickly develop into a situation where the quadrotor&#39;s efficiency is reduced or may even become uncontrollable. 
         [0004]    Attempts to predict and prevent slung payload oscillation have used a variety of techniques including predicting flight characteristics of the payload and feed-forward only predictions of trajectory of the payload based on a flight path of the lift vehicle. However, these techniques rely on predictive techniques and/or modeling of the load that may not always accurately portray the actual circumstances. Thus, there is a need for a way to quickly identify and correct oscillatory conditions in a slung payload of a quadrotor. 
       SUMMARY 
       [0005]    In one aspect of the disclosure, a method of controlling a payload angle of a payload that is slung from an aerial vehicle, such as a quadrotor may include determining an angle of the payload relative to a body of the aerial vehicle and generating an adjustment command corresponding to the angle of the payload. In an embodiment, the payload angle is measured as orthogonal angles in an x-direction aligned with a pitch of the aerial vehicle and a y-direction aligned with a roll of the aerial vehicle. The method may also include applying the adjustment command to cause a change of a pitch angle and/or a roll angle of the aerial vehicle so that the aerial vehicle accelerates in a direction of the angle of the payload. 
         [0006]    In another aspect of the disclosure, an aerial vehicle, such as a quadrotor is configured to damp oscillation of a payload that is slung from the aerial vehicle. The aerial vehicle may include a body having lift elements, such as rotors, a payload attachment configured to attach the payload and may also include a payload coupled to the payload attachment. The aerial vehicle may include a controller configured to measure an angle of the payload relative to the body and, responsive to the angle of the payload, adjust an orientation of the body in a direction of the angle of the payload. 
         [0007]    In yet another aspect of the disclosure, a quadrotor may be configured to automatically compensate for angle changes of a payload that is slung from the quadrotor. The quadrotor may include four lift rotors, a body coupling the four lift rotors, a payload attachment and a controller. The controller may be use computer executable instructions to measure an angle of the payload relative to a plane through the four lift rotors, and when a threshold condition is present, generate an adjustment command responsive to the angle of the payload. The application of the adjustment command to an input control signal of the quadrotor may cause the quadrotor to accelerate in a direction of the angle of the payload and damp oscillatory movement of the payload. 
         [0008]    The features, functions, and advantages that have been discussed can be achieved independently in various embodiments or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    For a more complete understanding of the disclosed methods and apparatuses, reference should be made to the embodiment illustrated in greater detail on the accompanying drawings, wherein: 
           [0010]      FIG. 1  is an illustration of an aerial vehicle with a payload slung from the aerial vehicle; 
           [0011]      FIGS. 2A ,  2 B and  2 C are illustrations of an aerial vehicle in various attitudes and respective payload angles; 
           [0012]      FIGS. 3A and 3B  are illustrations of an aerial vehicle in other attitudes with respective payload angles; 
           [0013]      FIG. 4  is an illustration of an exemplary processing unit for use in the illustrated aerial vehicle; 
           [0014]      FIG. 5  is a block diagram of an exemplary embodiment of a control process implemented on the controller of  FIG. 4 ; 
           [0015]      FIG. 6  is a block diagram of illustrating an embodiment of a portion of the control process of  FIG. 5 ; 
           [0016]      FIG. 7  is an illustration of operations performed by one embodiment of the control process; 
           [0017]      FIG. 8  is a detail of the operations illustrated in  FIG. 7 ; 
           [0018]      FIG. 9  is a detail of an intermediate signal of the embodiment of  FIG. 6 ; 
           [0019]      FIG. 10  is a detail of an output signal of the embodiment of  FIG. 6 ; 
           [0020]      FIG. 11  is an illustration of payload angle and input command angles using an undamped control scheme; and 
           [0021]      FIG. 12  is an illustration of payload angle and input command angles using an exemplary control process in accordance with the current disclosure. 
       
    
    
       [0022]    It should be understood that the drawings are not necessarily to scale and that the disclosed embodiments are sometimes illustrated diagrammatically and in partial views. In certain instances, details which are not necessary for an understanding of the disclosed methods and apparatuses or which render other details difficult to perceive may have been omitted. It should be understood, of course, that this disclosure is not limited to the particular embodiments illustrated herein. 
       DETAILED DESCRIPTION 
       [0023]    In order to address oscillatory motion in a slung payload, a quadrotor or other vertical lift platform may sense both an angle and rate of change of the angle in two dimensions to develop an adjustment command that is combined with regular motion control commands in order to damp the oscillatory motion. The use of actual angle eliminates the need to characterize the payload in terms of weight, aerodynamic shape, length of the tether, or the elasticity of the tether. Further, the use of the angle measurement in combination with feedback and feedforward loops in the control process eliminates the need to predict the effect of movement of the quadrotor on the payload. Thus, the effect of wind or wind gusts on a payload slung from a stationary quadrotor are dealt with as effectively as when the quadrotor is in motion. 
         [0024]      FIG. 1  illustrates an exemplary aerial lift environment  100  including a quadrotor  102  and a payload  116  that is slung from the quadrotor  102  by one or more tethers  114 , such as guy ropes or wires. The quadrotor  102  may include four rotors  104 ,  106 ,  108 ,  110  and may also include a control module  112 . The control module  112  may include a processing unit, radios for communication with a ground station, sensors including accelerometers, cameras, location devices such as a global positioning system (GPS) receiver, etc. The quadrotor  102  may also include one or more battery packs (not depicted). 
         [0025]    While a quadrotor  102  is used for illustration, the following discussion applies equally to other manned and unmanned aerial vehicles including, but not limited to, helicopters and other configurations of rotorcraft including aerial vehicles with fewer or more than four rotors. 
         [0026]      FIGS. 2A-2C  are illustrations of the quadrotor  102  in various attitudes and payload angles.  FIG. 2A  illustrates the quadrotor  102  oriented straight and level versus a horizon  118 . A long dimension of the quadrotor  102  is defined as a body dimension parallel to the horizon when the aerial vehicle is oriented straight and level with the horizon. The tether  114  coupled between the payload  116  and the quadrotor  102  is in alignment with a reference  120  perpendicular to the quadrotor  102 . The angle between the tether  114  and the reference  120  can be expressed in any of a number of ways, including both Cartesian and polar coordinates. For ease of illustration, the following discussion uses payload angles expressed in two terms, a representing a payload angle in one direction relative to the reference  120  and β representing a payload angle orthogonal to α. While not strictly necessary, coordinate systems may be defined for payload angle and for the quadrotor controls so that the angle a is aligned with a pitch, θ, of the quadrotor and is β is aligned with a roll, φ, of the quadrotor. By selecting the coordinate systems in this manner, any complex motion of the payload can be reduced to orthogonal components corresponding to either the pitch or the roll of the quadrotor  102 . 
         [0027]      FIG. 2B  illustrates the quadrotor  102  at an angle with respect to the horizon  118  such that the quadrotor  102  is in motion. The tether  114  coupling the payload  116  to the quadrotor  102  is at an angle a with respect to the reference  120 . The angle a may have been developed during an initial acceleration of the quadrotor  102 , as a result of wind, or both. The payload  116  may or may not have an oscillatory component such that the angle a is periodically increasing and decreasing. 
         [0028]      FIG. 2C  illustrates that a negative acceleration of the quadrotor  102 , for example, from 10 miles per hour (mph) to 9.7 mph will maintain the forward motion of the quadrotor  102  and payload  116  but can effectively reduce the angle a between the tether  114  and the reference  120  and reduce or eliminate oscillatory motion, as discussed more below. 
         [0029]      FIGS. 3A and 3B  illustrate the quadrotor  102  in other attitudes. In  FIG. 3A  the quadrotor  102  is shown flying straight and level with respect to the horizon  118 . However, an impulse, such as a wind gust, is shown to have created an angle a between the tether  114  in the reference  120 . When the gusts dies, the payload  116  will begin an oscillatory motion. Turning briefly to  FIG. 11 , a chart  350  illustrates an effect of an impulse on the stability of the quadrotor absent any damping. Motion in the payload  116  may cause the quadrotor to move out of position, causing the control module  112  of the quadrotor  102  to apply pitch  354  and roll  352  commands to maintain its position. The cumulative effect of the motion of the quadrotor  102  can be seen in the longitudinal (α) payload angle  356  that increases over time to, in this simulation, as much as +/−15 degrees or more. 
         [0030]      FIG. 3B  illustrates an acceleration of the quadrotor  102  in the direction of the angle of the payload resulting in a reduction of the angle a. Turning briefly to  FIG. 12 , a chart  360  illustrates another simulated impulse to the payload  116  as shown in  FIG. 11  in the a direction. (The scale of payload angle  366  is increased compared to  FIG. 11 .) Due to the damping, adjustments to the pitch, θ, of the quadrotor  102  responsive to the payload angle and rate of change of the payload angle effectively damps the payload swing in a few seconds with little or no impact on the roll, φ, of the quadrotor  102 . 
         [0031]      FIG. 4  is an illustration of an exemplary processing unit  130  that may be a component of the control module  112 . The processing unit  130  may include a processor  132  and a memory  134  coupled by a bus  136 . The memory  134  may include a random access memory (RAM) and a read-only memory (ROM) utilizing memory technologies including, but not limited, to silicon-based technologies, bubble memory, optical media, organic memory, nano technologies, etc. The memory  134  may also include a mass storage device, data memory, and/or on a removable storage medium such as a flash memory device. The memory  134  does not include propagated media such as a carrier wave. 
         [0032]    Also coupled to the bus  136  may be a flight input control  138  that receives signals via direct manipulation or via a radio receiver, an optical receiver, etc. Various sensor inputs  140  may also be coupled to the bus  136  and may include inputs coupled to location sensors, proximity sensors, accelerometers used for orientation sensing, angle encoders for determining the tether angles α and β relative to the reference  120 , cameras, battery sensors, etc. The processing unit  130  may also include flight output controls  142  used to manage lift at each rotor  104 ,  106 ,  108 ,  110  for use in controlling the pitch and roll of the quadrotor  102 . 
         [0033]    The memory  134  may include computer-executable instructions that implement an operating system  144  and utilities  146  used in a conventional manner for memory management, diagnostics, communications, etc. Additional computer-executable instructions may implement modules such as a forward control module  148  and a feedback module  150 , as well as constants, thresholds, and limits  152  that may be used for different models of quadrotor  102  or for various operating modes for a particular model of quadrotor  102 . While the exemplary embodiment illustrated is depicted as using computer instructions that are executed on the processor  132 , other implementations of the control process discussed below may be supported, for example, using programmable logic arrays (PLA), distributed processing, remote processing via high-speed data connections, neural networks, or other techniques. 
         [0034]      FIG. 5  is a block diagram of an exemplary embodiment of a control process  200  implemented via the processing unit  130 . The control process  200  is one of two separate, identical, control processes, one applied to pitch, θ, in the other applied to roll, φ using respective payload angles α and θ. A summing function  202  receives control input  201  and combines the control input  201  with feedback signals discussed below. A gain function  204  amplifies the resulting output of the summing function  202 . Another summing function  206  adds additional feedback and provides an output to another gain function  208  and an integrator function  210 . The output of the gain function  208  and integrator function  210  are combined at an adder  212  and provide a motor control output via flight output controls  142  to a motor associated with one of the rotors  104 - 110  that is external to the control module  112 , illustrated by functional block  214 . The functional block  214  may provide, via sensor inputs  140 , a payload angle signal  218  corresponding to a payload angle, an angle  224  of the quadrotor  102  and a rate (angle change rate)  222  of the quadrotor  102 . The loop gain settings at gain functions  204  and  208  set the control sensitivity while the angle and rate feedback signals  224  and  222  respectively are used as negative feedback to provide stability to the quadrotor  102 . A feedback function  216  takes the payload angle signal  218  and generates a correction signal  220  that is combined at the summing function  202  to accomplish acceleration of the quadrotor  102  in the direction of the payload angle, as discussed further below. 
         [0035]      FIG. 6  is a block diagram illustrating an embodiment of a portion of the control process of  FIG. 5 , more specifically, the feedback signal processing of feedback function  216 . Recalling that the control processes illustrated in  FIG. 5  and  FIG. 6  are duplicated for each payload angle measurement, a payload angle signal  218  may carry either the α payload angle or the β payload angle as supplied by functional block  214 . The payload angle signal  218  may be split and fed to a proportional gain function  234  and a derivative function  237 . At derivative function  237  the payload angle signal  218  may be fed to a derivative gain function  238 . The output of the derivative gain function  238  may be combined with an output of a delay filter  244  and fed to a filter coefficient function  242 . 
         [0036]    In an embodiment, the loop time may be in a range between 50 Hz and 150 Hz that is Ts may be in a range from about 0.006 seconds to about 0.02 seconds. In another embodiment, the loop time may be in a range between 90 Hz and 110 Hz, that is Ts may be in a range from about 0.009 seconds to about 0.01 seconds, although other loop times may be appropriate for different sizes of quadrotor  102  and/or different expected load ranges. The outputs of the proportional gain function  234  and the filter coefficient function  242  may be added at summing function  236 . 
         [0037]    The output of the summing function  236  may be sent to a sign function  246  with an output of either +1 or −1 based on the polarity of the payload angle signal  218 . The output of summing function  236  may also be sent to an absolute value function  248  and a relay function  250 . The relay function  250  creates a “null-control zone” and is set to allow its output to track the input, depending on two specified set points. When the relay is off, the output signal at line  251  is zero and when the relay is on, the output signal at line  251  is 1. In an embodiment, a first set point or ‘on’ threshold angle may be, for example, 2 degrees, while the ‘off’ threshold angle may be 1.9 degrees. These values are purely for the purpose of illustration and other threshold angles may be chosen depending on the type of quadrotor  102  and the expected load. Referring briefly to  FIG. 9 , the signal at line  251  is illustrated. It shows that the output of the relay function  250  is either a logical 1 or a logical zero. In a purely analog configuration, the output may be zero volts or one volt. 
         [0038]    A saturation function  246  is a function that caps the magnitude of the adjustment command to a output signal that is equivalent to a saturation threshold angle. In an embodiment, the saturation function  246  may have the positive saturation threshold angle set in a range of 35 degrees to 45 degrees and a negative saturation angle set in a range of −35 degrees to −45 degrees. To illustrate using a ±40 degree saturation threshold angle, if the payload angle a goes to +60 degrees, the output of the relay function  250  may be set to a value corresponding to a payload angle a of +40 degrees. All angles less than the saturation threshold angle will be passed through unaltered. This may help prevent overcorrection that might cause instability in the quadrotor  102  should the operating environment include, for example, large wind gusts or tampering with a payload. In an embodiment, the saturation function  246  may be set to unity so that any input angle is passed to the multiplication function  252 . Put another way, the saturation threshold angle may be set to ±90 degrees to pass all angle values to the multiplication function  252 . 
         [0039]    The output of the relay function  250 , a 1 or 0, may be multiplied by the output of the saturation function  246  at multiplication function  252  and the resulting correction signal  220  fed back to the summing function  202  as negative feedback to either a pitch command or a roll command, depending on to which angle the instance of the control process is directed, α or β. Turning briefly to  FIG. 10 , a representative signal on line  220  at the output of the multiplier  252  is illustrated. Whenever the relay function is off, the output is zero. When the relay is on, the output follows the signal at the output of the saturation function  246 , illustrated as a sine wave. 
         [0040]    In summary, the feedback function  216  takes a payload angle and processes it such that the control input  201  is adjusted to cause the quadrotor  102  to accelerate in the direction of the payload angle. The proportional nature of the feedback signal increases the feedback signal as the payload angle increases and the derivative nature of the feedback signal comprehends the changes in speed of the payload as it travels through an oscillation cycle. 
         [0041]    In an embodiment, the payload angle signal  218  may be half wave rectified before processing so that the quadrotor  102  only applies damping signals to payload angles on one side of the reference  120 . This may reduce the processing overhead required to calculate the correction signal  220 , potentially saving power and improving battery life and even allowing the use of a lower cost processor  132 , potentially saving on product cost. The time to damp an oscillation may be extended but depending on the application, it may be considered a worthwhile trade-off in light of the benefits. 
         [0042]      FIG. 7  is an illustration of operations  300  performed by one embodiment of the control process  200 . At a block  302 , an angle of the payload relative to the body of the aerial vehicle may be determined. As discussed above, the angle of the payload may be captured as a α angle aligned with a pitch direction of the quadrotor  102  and as a β angle aligned with a roll direction of the quadrotor  102 . 
         [0043]    At a block  304 , an adjustment command corresponding to the angle of the payload may be generated as discussed above and as will be discussed in more detail respect to  FIG. 8 . At a block  306 , the adjustment command may be applied to a control input command to cause a change in at least one of a pitch angle or a roll angle of the quadrotor  102  so that quadrotor  102  is accelerated in a direction of the angle of the payload. 
         [0044]      FIG. 8  is a detailed illustration  320  of the operation of block  304  illustrated in  FIG. 7 . To generate the adjustment command that is applied to the control input command, at a block  322 , a proportional gain function  234  is applied to the payload angle signal  218 . At a block  324 , a derivative gain function  238  is applied to the payload angle signal  218 . In an embodiment, the derivative function may also include a delay filter  244  fed back to an output of a derivative gain function  238 . 
         [0045]    At block  326 , a determination may be made as to whether the payload angle, represented by the absolute value of an intermediate signal at relay function  250  is below a threshold angle. As discussed above, in an embodiment, the threshold angle may be in a range of 2 degrees to 4 degrees. Other threshold angles may be applied depending on the operating environment. Because the signal at relay function  250  of  FIG. 6  incorporates not only an absolute angle component, but a rate of change component, there may be circumstances where even though the angle value itself is within the threshold range, a rate of change of payload angle may cause the intermediate signal to meet or exceed the threshold requirement. 
         [0046]    If, at block  326 , the magnitude of the payload angle is below the threshold value, the ‘yes’ branch may be taken to block  328  and the adjustment command may be set to zero, so that no changes are made to the input control signal. 
         [0047]    If, at block  326 , the magnitude of the payload angle is at or below the threshold value, the ‘no’ branch may be taken to block  330 . The correction signal  220  may be applied via the summing function  202 . 
         [0048]    A quadrotor  102  using the payload damping system and method described above benefit the operator of such a system by developing smoother flight paths and more predictable routes. The in-flight corrections reduce the control signal bandwidth requirements with a ground station by making the acceleration adjustments transparently to operator signals. Better payload stability may also result in more accurate drop placements and better use of target space. 
         [0049]    While only certain embodiments have been set forth, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.