Abstract:
Embodiments discussed herein provide improved control of an unmanned aerial vehicle camera gimbal. In some embodiments, a linear control circuit is described that uses an operational amplifier that allows the system to retain its passive isolation of high frequency disturbances.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/141,141, filed Mar. 31, 2015 and entitled “MECHANICAL ROTARY STABILIZATION OF IMAGE SENSORS,” which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    Embodiments disclosed herein relate generally to the mechanical rotary stabilization of image sensors used for taking photographs or video from an aerial vehicle. While the embodiments disclosed herein may be applicable to a wide range of aircraft, certain embodiments may be particularly applicable to multi-rotor unmanned aerial vehicles (UAVs). Certain embodiments are applicable to a wide range of multi-rotor UAVs, but especially designed for quadrotor UAVs. 
         [0004]    2. Description of the Related Art 
         [0005]    UAVs are increasingly being used to capture videos and photographs during flight. When capturing videos or photographs (images) it is desirable that the images are free of any undesired motion. Any rotation of the image sensor creates large changes in what is captured within the field of view of the image sensor, resulting in distorted, shaky and blurred images. 
         [0006]    When capturing images from a UAV there are rotational movements of the platform, this case the main body of the UAV, to which the image sensor is attached. The UAV must change its angle to maintain its desired position or flight path; these rotational movements are generally large in amplitude and occur at low frequency. There is also vibration from mechanical and aerodynamic forces on the motors and propellers; these rotational movements are generally small in amplitude and occur at high frequency. 
         [0007]    A common solution to this problem is to suspend the image sensor on a gimbal. The gimbal platform to which the image sensor is attached is rotated relative to the outer stages of the gimbal which are attached to the UAV. 
         [0008]    Many attempts have been made at building low cost UAV gimbals using servo or brushless motors. The servo driven gimbals are generally very slow and inferior in performance to brushless motor gimbals. Brushless motor gimbals are generally driven by switched transistors to control the torque generated by the motor. 
         [0009]    In general a brushless motor gimbal does not have sufficient power to drive the system at high frequencies. It is also challenging to sense small amplitude and high frequency movements because of noise in the sensor. It is thus desired to have a system with a low natural frequency to provide passive isolation of the high frequency disturbances. To achieve a low natural frequency the stiffness of the system must be minimized. Adding mass to the stabilized platform also lowers the natural frequency but is undesirable especially for a UAV where minimizing weight and size is important to safety, flight endurance, and flight performance. 
       SUMMARY 
       [0010]    Some innovations relate to a gimbal with a plurality of axes actuated by brushless motors for stabilizing an image sensor, wherein the motors are driven by a high bandwidth operational amplifier circuit. 
         [0011]    Some innovations relate to a gimbal with a plurality of axes actuated by brushless motors for stability an image sensor, wherein the motors are driven by a high bandwidth linear drive circuit. 
         [0012]    The amplifier component can be an operational amplifier. The amplifier component can be a bi-polar junction transistor. The amplifying means can be a network of bi-polar junction transistors. 
         [0013]    Some innovations relate to a gimbal with a plurality of axes actuated by rotational motors for stabilizing a payload requiring precise directional precision, wherein the motors are driven by a high bandwidth linear drive circuit. 
         [0014]    Some innovations relate to a gimbal with a single axis actuated by rotational motors stabilizing an image sensor wherein the motor is driven by a high bandwidth linear drive circuit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is an isometric view of an exemplary unmanned aerial vehicle that includes a gimbal. 
           [0016]      FIG. 2  is a view of bottom side of the fuselage showing location and orientation of a gimbal. 
           [0017]      FIG. 3  is a front top isometric view of a 2-axis gimbal. 
           [0018]      FIG. 4  is a rear bottom isometric view of a 2-axis gimbal. 
           [0019]      FIG. 5  is an exploded assembly of a 2-axis gimbal. 
           [0020]      FIG. 6  is an exploded view of the pitch stage sub-assembly. 
           [0021]      FIG. 7  is an exploded view of the roll stage sub-assembly. 
           [0022]      FIG. 8  is a schematic diagram of a motor driver circuit. 
       
    
    
     DETAILED DESCRIPTION 
     Hardware 
       [0023]    Referring now to  FIG. 1  a UAV  4  is comprised of a fuselage  8 , a thrust pod assembly  12 , a battery  18 , and a gimbal  16 . Fuselage  8  and battery  18  each include electronic processing and flight control sub-systems that would be familiar to one skilled in the art of UAV design and will not be described in detail. Likewise the basic composition, components, and function of the thrust pods  12  will not be described in detail. 
         [0024]    Referring now to  FIG. 2 , the location and orientation of gimbal  16  in relation to fuselage  8  is shown. A gimbal cavity  34  is a molded concave form integral to fuselage  8  that substantially shields gimbal  16  from impacts with objects or the ground. In another embodiment, an optically clear dome-shaped shield is fastened to the front of fuselage to further protect gimbal  16 . 
         [0025]      FIG. 3  shows 2-axis gimbal  16 .  FIG. 4  shows an exploded assembly view of gimbal  16 . A pitch stage sub-assembly  24  is rotationally attached to a roll stage  20  sub-assembly, which is in turn rotationally attached to a gimbal mount  2 . The rotation of pitch stage  24  is driven by a pitch stage motor  28  and the rotation of roll stage  20  is driven by a roll stage motor  32 . 
         [0026]    Gimbal mount  2  is fixedly attached to fuselage  8  with two screws  14   a  and  14   b.  Pitch stage  24  rotates about an axis that is parallel to the UAV  4  pitch axis. Roll stage  20  rotates about an axis that is parallel to the UAV  4  roll axis. In one embodiment gimbal mount  2  is comprised of injection molded polycarbonate plastic. 
         [0027]      FIG. 6  shows an exploded assembly view of pitch stage  24 . A camera lens  22  includes a threaded portion that screws into a threaded portion of a camera mount  26 . A lens guard  30  is retained on camera mount by camera lens  22  and a set screw. Camera mount  26  also includes a circular mounting feature for a pitch stage motor magnet set  66 . Note that motor magnet set  66  fixed to camera mount  26  constitutes the function of a rotor in the conventional definition of a DC brushless motor. A pitch stage flux ring  74  is affixed to camera mount  26  surrounding motor magnet set  66 . An image sensor rigid printed circuit board  58  is a portion of gimbal flexible circuit board  6 , and functionally connects an image sensor  54 , and a pitch inertial measurement unit (IMU)  86 . 
         [0028]    In one embodiment camera lens  22  is part number 123038MPF, manufactured by CCOM Electronics Technology of Fuzhou City, China. Camera mount  26  is CNC machined 6061 aluminum. Lens guard  30  is made from injection molded polycarbonate plastic. 
         [0029]      FIG. 7  shows an exploded assembly view of roll stage  20 . A roll stator  72  is fixedly attached to a pitch stage insert  80 , which in turn is fixedly attached to stage link  42 . Bearing  96   a  and  96   b  are respectively pressed into stage link  42 . Note that the motor magnet set  70  fixed to stage link  42  constitutes the function of a rotor in the conventional definition of a DC brushless motor. In one embodiment stage link  42  is comprised of CNC machined 6061 aluminum. 
         [0030]      FIG. 6  shows that stage link  42  includes a pre-load slot  118  that is a machined feature adjacent to the cylindrical surface that mates with pitch stage insert  80 . The cylindrical hole is machined with a slightly larger diameter than pitch stage insert  80  mating surface. During assembly, pitch stage insert  80  accepts bearing  96   a  and is lightly pre-loaded against the mating surface on camera mount  26 . Stage link screw is then tightened and due to the slight flexing of stage link in the area of the re-load slot  118 . The result is there is no motion of pitch stage  24  along the axis of rotation. 
         [0031]    A separate flexible portion of gimbal flexible circuit  6  includes motor drive conductors is routed to and connected to roll stage motor  32 , and a separate portion of flexible circuit  6  includes motor drive conductors is routed to and connected to pitch stage motor  28 . 
         [0032]    Fuselage  8  includes a camera microprocessor  98 , a gimbal microcontroller  94 , and a platform IMU  90 . In one embodiment, camera microprocessor  98  is a model A9 microprocessor, manufactured by Ambarella, Inc., of Sunnyvale, Calif. In one embodiment gimbal controller MCU  94  is model STM32F373C8T6, manufactured by ST Micro of Geneva, Switzerland. In one embodiment pitch IMU  86  and platform IMU  90  are the same component, model number MPU-6050, manufactured by Invensense of San Jose, Calif. 
         [0033]    Gimbal flexible circuit board  6  electrically connects image sensor  54  to camera microprocessor  98 , and electrically connects pitch stage motor  28  and roll stage motor  32  to gimbal motor control circuit  102 . The electrical connections are made through gimbal receptacle connector  38  mated to a corresponding gimbal header connector  114  in fuselage  8 . Platform IMU  90  is electrically connected to gimbal MCU  94  inside the fuselage. 
       Gimbal Function 
       [0034]    During operation of UAV  4  when recording video, gimbal  16  is stabilized, that is, gimbal  16  rotates along the pitch axis and roll axis to compensate for UAV  4  platform motion to keep image sensor  54  view vector substantially stable or to steer the view vector. Pitch stage motor  28  and roll stage motor  32  are activated by commands from gimbal controller  94  based on changes in the position of pitch IMU  86  and from platform IMU  90 . 
       Motor Drive Circuit 
       [0035]    Referring now to  FIG. 8 , in one embodiment, gimbal motor phases are driven by a gimbal motor analog operational amplifier (op-amp) circuit  102  that controls the voltage across each of pitch motor  28  and roll motor  32  phase to produce a desired current in the motor phase  108   a, b,  and  c . The current is measured by the voltage difference across a current sensing resistor  110   a  in series with motor phase  108   a,  and across current sensing resistor  110   b  in series with motor phase  108   b.  The high bandwidth of each of op-amp  106   a  and  106   b  allows the system to retain its passive isolation of high frequency disturbances. For instance, if zero current is desired in pitch motor  28  and the relative motion of pitch motor  28  stator and rotor produce a back EMF voltage, op-amp  106   a  output is driven to the same voltage and zero current is maintained in motor  28  and neither stiffness nor damping is added to the gimbal  12 . Op-amp circuit  102  drives two of the three motor  28  and motor  32  phases. The current in the remaining phase is the sum of these two currents so the third phase does not require an active driver and is driven by a constant voltage. 
         [0036]    The following functional description references only pitch motor  28  for the purpose of brevity—it should be understood that the description relates to the function of roll motor  32  also. The desired proportion of currents in motor phases  108   a  and  108   b  are calculated by adding or subtracting a 90 degree offset in the electrical angle of motor  28  back-EMF waveform. These proportions of current produce the peak torque for any given current in motor phases  108   a  and  108   b.    
         [0037]    The electrical angle of motor  28  is related to the mechanical angle between the rotor and stator by the number of magnetic pole pairs. For a seven pole pair motor, there are seven electrical revolutions for one mechanical revolution. The mechanical angle of motors is calculated using both pitch IMU  86  located on pitch stage  24 , and platform IMU  90 , located in fuselage  8 . The angle may also be directly measured using an encoder, but the mechanical simplicity and absence of added stiffness or friction of the dual IMU solution is favored. Each of pitch IMU  86  and platform IMU  90  run an attitude filter by combining the gyroscope and accelerometer data. The currents in motor  32  are then commanded in the desired proportions based on electrical angle, with a combined magnitude that is proportional to the desired torque in the motor. A feedback control loop calculates the desired motor torque based on the angle and rotational rates of the stabilized IMU. 
         [0038]    This embodiment does not include encoders on the pitch and roll axes. Therefore the initial relationship between the initial mechanical and electrical angles is established by an open loop process as follows. An initial control signal corresponding to zero electrical angle is applied to each of motor  28  and motor  32 . The electrical angle is then stepped up or down until the desired mechanical angle is reached as measured by the pitch IMU  86 . The offset between the electrical and mechanical angle is stored in gimbal controller  94  RAM and used for subsequent control calculations. 
         [0039]    In some embodiments, only image sensor  54  and lens  22  are stabilized by gimbal  16  to reduce the size and weight of the system. 
         [0040]    Another embodiment includes a third motor with its axis aligned with the vertical axis of the UAV (yaw axis). 
         [0041]    In another embodiment the gimbal rotational actuators are voice coils. 
         [0042]    In another embodiment, the motor driver circuit includes bi-polar junction transistors. 
         [0043]    The gimbal is not required to stabilize an image sensor. In another embodiment, the gimbal stabilizes a laser pointing component. 
         [0044]    In the foregoing description, specific details are given to provide a thorough understanding of the examples. However, it will be understood by one of ordinary skill in the art that the examples may be practiced without these specific details. For example, electrical components/devices may be shown in block diagrams in order not to obscure the examples in unnecessary detail. In other instances, such components, other structures and techniques may be shown in detail to further explain the examples. 
         [0045]    Headings are included herein for reference and to aid in locating various sections. These headings are not intended to limit the scope of the concepts described with respect thereto. Such concepts may have applicability throughout the entire specification. 
         [0046]    The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.