Abstract:
The actuator system has a external rotor motor having: (i) an internal armature configured as a stator having a set of coils wrapped around a set of arms, and (ii) an external permanent magnet rotor having a set of poles configured to rotate less than 90 degrees around the stator. The actuator system has a drive shaft configured to be rotated by the external rotor motor. The actuator system has a drive train connecting the external permanent magnet rotor to the drive shaft, and configured to provide gear ratio to the drive shaft.

Description:
BACKGROUND 
     Electro mechanical actuators (EMAs) are used to allow mechanical devices to achieve motion such as rotational motion and linear motion. Applications for EMAs that produce rotational motion include servoactuators, valve actuators, and flight controls. These types of EMAs are typically driven by electric motors and utilize power trains to deliver mechanical advantage. Typically a high gear ratio (e.g., 5:1 or higher) is required to deliver adequate performance at a reasonable cost for a particular application. The high gear ratio means that multiple rotations of the motor are required to achieve the desired range of motion. EMAs that produce linear motion include solenoids, linear motors, and voice coil motors. EMAs of this kind are often driven in a direct drive mode. 
     A typical rotational motion EMA used for flight control (e.g., a flight control EMA used to pivot the fins of a missile) consists of an internal rotor motor (an internal rotor is used to minimize system inertia) and a power train (to provide the desired gear ratio). 
     SUMMARY 
     Unfortunately there are deficiencies to the above-described conventional approaches to using a typical rotational motion EMA for achieving limited angle actuation. For example, EMAs with high gear ratios are relatively slow. This relatively slow speed is due to the fact that the motor must travel through a much larger input angle than is traveled by an output angle (e.g., an EMA with 10:1 gear ratio will have a motor rotate 600 degrees to produce only 60 degrees of motion for the system). 
     Another deficiency to the above-described conventional approaches to using a typical rotational motion EMA for achieving limited angle actuation is that the speed of response (for example, the frequency response) is necessarily slow because of the high gear ratio. 
     Yet another deficiency to the above-described conventional approaches to using a typical rotational motion EMA for achieving limited angle actuation is that typical rotational motion EMAs are complex devices. EMAs typically utilize commutation to switch the electric current running through the motor coils. Controlled commutation requires multiple additional wires and switches which make EMAs more complex and more expensive to produce. Furthermore the gear train (e.g., drive gears and screws) are typically required to be precise and have relatively complex configurations. 
     In contrast to the above-identified conventional approaches to using a typical rotational motion EMA for achieving limited angle actuation, an improved actuator design involves using an external rotor motor to rotate a shaft attached to the motor using a simple drive train made of pins and links. A motor with an external rotor can provide greater torque than an internal rotor motor of a similar size. This higher torque reduces the need for high gear ratios which tend to slow the effective speed of actuators. Due to the motor&#39;s limited travel, less than 90 degrees, and its magnet and stator design the motor requires no commutation. This makes the actuator less complicated, less expensive and more compact. 
     One embodiment is directed to an actuator system. The actuator system has an external rotor motor having: (i) an internal armature configured as a stator having a set of coils wrapped around a set of teeth, and (ii) an external permanent magnet rotor having a set of poles configured to rotate less than 90 degrees around the stator. The actuator system has a drive shaft configured to be rotated by the external rotor motor. The actuator system has a drive train connecting the external permanent magnet rotor to the drive shaft, and configured to allow transmission of a first component of rotational motion (for example, the tangential component) from the external permanent magnet rotor to the drive shaft and to prevent transmission of a second component of rotational motion (for example, the radial component) from the external permanent magnet rotor to the drive shaft, the first component of rotational motion being perpendicular to the second component of rotational motion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the invention. 
         FIG. 1  is a perspective view of a first embodiment of an actuator system having an external rotor motor, a drive shaft and a drive train. 
         FIGS. 2A ,  2 B, and  2 C are side views of the actuator system of  FIG. 1 . 
         FIG. 3  is a perspective view of a second embodiment of an actuator system having an external rotor motor, a drive shaft and a drive train. 
         FIGS. 4A ,  4 B, and  4 C are side views of the actuator system of  FIG. 3 . 
         FIG. 5  is a perspective view of a third embodiment of an actuator system having an external rotor motor, a drive shaft and a drive train. 
         FIGS. 6A ,  6 B, and  6 C are side views of the actuator system of  FIG. 5 . 
         FIG. 7  is a cross section of the external rotor motor of  FIG. 1 . 
         FIG. 8  is a block diagram showing a motor controller. 
     
    
    
     DETAILED DESCRIPTION 
     An improved device for achieving limited angle rotation in high speed and low torque environments employs an external rotor motor to oscillate quickly in a limited angle of the motor and to act through a simple drive train to rotate a drive shaft. Accordingly, the external rotor allows the motor to be smaller, cheaper, faster, and less complex than a motor with an internal rotor that achieves the same capabilities. 
       FIG. 1  shows a first embodiment of an actuator system  20  which includes an external rotor motor  22 , a drive train  24 , a drive shaft  26  and a chassis  28 . The external rotor motor  22  includes an external rotor  30 , an internal stator  32  and a motor pin  34 . The drive train  24  includes a crank arm  36  and a crank arm slot  38 . 
     In a general sense, the drive train  24  is configured to allow transmission of a first component of rotational motion from the external rotor  30  (i.e., external permanent magnet rotor) to the drive shaft  26  and to prevent transmission of a second component of rotational motion from the external rotor to  30  the drive shaft  26 . Relative to the axis of the drive shaft  26  the first component of rotational motion is perpendicular to the second component of rotational motion. 
     As seen in  FIG. 1 , the external rotor motor  22  is rigidly attached to the chassis  28  (e.g., structural member of a missile). This rigid attachment is made to the stationary internal stator  32  of the external rotor motor  22 . The external rotor  30  of the external rotor motor  22  rotates freely with respect to the chassis  28  along a first axis of rotation  56  which is parallel to the X axis. The rotation of the external rotor  30  is restricted to a limited angle rotation (e.g. less than 90 degrees). Due to the external rotor&#39;s  30  limited rotation and the 4-pole configuration of the stator and the magnets, commutation is not required to control operation of the motor. 
     The drive shaft  26  rotates about a second axis of rotation  58  that is parallel to the X axis. Other than this rotation, the motion of the drive shaft  26  restrained. The rotation of the drive shaft  26  provides motion for other devices not shown (e.g., fins of a missile). For example, a missile fin could be directly affixed to the drive shaft  26 . As the drive shaft  26  rotates, the missile fin also rotates. 
     The first embodiment of the actuator system  20  utilizes a single link, two dimensional (2D) version of the drive train  24 . The drive train  24  is a single link version, because the only link used is the crank arm  36 . The drive train  24  is considered 2D because the motion of the link (e.g., crank arm  36 ) is restricted to a 2D plane (e.g., Y-Z plane). 
     As seen in  FIG. 1 , the motor pin  34  extends from a location at some radius from the axis  56  on the end surface of the external rotor  30  (the end surface is the only portion of the external rotor  30  depicted in a plane perpendicular to the X direction). The motor pin  34  interacts with the drive train  24 . The single link 2D version of the drive train  24  incorporates the crank arm  36  and the crank arm slot  38  to transmit rotation of the external rotor  30  to the drive shaft  26 . The motor pin  34  inserts into the crank arm slot  38  on one end of the crank arm  36 . The drive shaft  26  affixes to the other end of the crank arm  36 . As will be discussed in further detail below with reference to  FIG. 2 , this single link 2D version of the drive train  24  transmits a portion of the motor&#39;s  22  rotation to the drive shaft  26  in the same direction and perpendicular to the X direction. 
       FIG. 2A  shows the external rotor  30  with the motor pin  34  in the maximum counterclockwise position (maximum Z position of  FIG. 2A , B, or C) and shows a range of motion  40  of the motor pin  34 .  FIG. 2B  shows the external rotor  30  with the motor pin  34  in the midpoint position (intermediate Z position of  FIG. 2A , B, or C) and shows the range of motion  40  of the motor pin  34 .  FIG. 2C  shows the external rotor  30  with the motor pin  34  in the maximum clockwise position (minimum Z position of  FIG. 2A , B, or C) and shows the range of motion  40  of the motor pin  34 . 
     Because the external rotor  30  and the drive shaft  26  are restrained except with respect to rotation about the X direction, as the motor pin  34  moves about the first axis of rotation  56 , the end of the crank arm  36  not attached to the drive shaft  26  is swept through an arc in a Y-Z plane. The motion of the crank arm  36  applies a torque to the drive shaft  26 , causing the drive shaft  26  to rotate. As shown, the motor pin  34  also moves radially across the crank arm slot  38  during the external motor rotor  22  rotation. Thus the motor pin  34  transmits motion to the drive shaft  26  that is solely perpendicular to the crank arm  36  (and not radially). An angular change in the external rotor  30  will result in an angular change in the drive shaft  26 , but since the range of angular displacement of the external rotor  30  is larger than the range of angular displacement of the drive shaft there will be a mechanical advantage essentially proportional to the ratio of these two angular displacement ranges. (i.e. the change in the angle of the drive shaft  26  about the second axis of rotation  58  will be less than the change in the angle of the external rotor  30  about the first axis of rotation  56 , and the difference of angular displacement will be a function of the radial location of the pin  34  and the length of the crank arm  36 ). 
     As seen in  FIG. 2A , since the motor pin  34  is in the maximum counterclockwise position of its range of motion  40 , the motor pin  34  occupies a minimum Y position of the crank arm slot  38 . As the motor pin  34  moves in the clockwise direction with respect to the first axis of rotation  56  to the midpoint position of the range of motion  40 , as seen in  FIG. 2B , the motor pin  34  drags the end of the crank arm  36  in a clockwise arc about the axis of the drive shaft  26  and moves to the maximum Y position of the crank arm slot  38 . Finally, as the motor pin  34  moves in the clockwise direction to the maximum clockwise position of the range of motion  40 , as seen in  FIG. 2C , the motor pin  34  drags the end of the crank arm  36  further to its most clockwise position and moves back to the minimum Y position of the crank arm slot  38 . 
       FIG. 3  shows an actuator system  20 ′ which includes the external rotor motor  22 , a drive train  24 ′, the drive shaft  26  and the chassis  28 . The external rotor motor  22  includes the external rotor  30 , the internal stator  32  and the motor pin  34 . The drive train  24 ′ includes a crank arm  46 , a drag link  42 , and a drag link pin  44 . 
     As seen in  FIG. 3 , the external rotor motor  22  is attached to the chassis  28  in the same way as was described with regard to actuator system  20  containing the single link 2D drive train  24  (as seen in  FIG. 1 ). Likewise, the drive shaft  26  is similarly restrained with the exception of rotation as was described with regard to actuator system  20  containing the single link 2D drive train  24  (as seen in  FIG. 1 ). 
     The second embodiment of the actuator system  20 ′ utilizes a double link 2D version of the drive train  24 ′. The drive train  24 ′ is a double link version, because it uses two links (the crank arm  46  and the drag link  42 ). The drive train  24 ′ is considered 2D because the motion of the links (e.g., crank arm  46  and drag link  42 ) is restricted to a 2D plane (e.g., Y-Z plane). 
     As seen in  FIG. 3 , the motor pin  34  extends from some radius of the end surface of the external rotor  30  (the end surface is the only portion of the external rotor  30  depicted in a plane perpendicular to the X direction). The motor pin  34  interacts with the drive train  24 ′. The double link 2D version of the drive train  24 ′ incorporates the crank arm  46 , the drag link  42 , and the drag link pin  44  to transmit rotation of the external rotor  30  to the drive shaft  26 . The motor pin  34  connects to one end of the drag link  42  (i.e. the drag link  42  is pinned to the external rotor  30 ). One end of the crank arm  46  connects to the other end of the drag link  42  using the drag link pin  44  (i.e. the crank arm  46  is pinned to the drag link  42 ). The drive shaft  26  affixes to the other end of the crank arm  46 . As will be discussed in further detail below with reference to  FIG. 4 , this double link 2D version of the drive train  24  transmits a portion of the motor&#39;s  22  rotation to the drive shaft  26  in the same plane and perpendicular to the X direction. 
       FIG. 4A  shows the external rotor  30  with the motor pin  34  in the maximum counterclockwise position (maximum Z position of  FIG. 4A , B, or C) and shows the range of motion  40  of the motor pin  34 .  FIG. 4B  shows the external rotor  30  with the motor pin  34  in the midrange position (intermediate Z position of  FIG. 4A , B, or C) and shows the range of motion  40  of the motor pin  34 .  FIG. 4C  shows the external rotor  30  with the motor pin  34  in the maximum clockwise position (minimum Z position of  FIG. 4A , B, or C) and shows the range of motion  40  of the motor pin  34 . 
     Because the external rotor  30  and the drive shaft  26  are restrained except with respect to rotation about axes in the X direction, as the motor pin  34  moves about the first axis of rotation  56 , the drag link  42  moves through the Z direction and the end of the crank arm  46  not attached to the drive shaft  26  is swept through the Z direction. The motion of the crank arm  46  applies a torque to the drive shaft  26 , causing the drive shaft  26  to rotate. The angle  60  between the drag link  42  and the crank arm  46  will change during the external motor rotor  22  rotation. Consequently, the motion of the drive shaft  26  is driven predominantly by displacement of the motor pin  34  in the Z direction. An angular change in the external rotor  30  will result in an angular change in the drive shaft  26 . This drive train  24 ′ configuration allows for small displacements of both the motor pin  34  and the drag link pin  44  while transferring the predominant motion in the Z direction. The mechanical advantage of this configuration is proportional to the change in the angle of the external rotor  30  about the first axis of rotation  56  relative to the change in the angle of the drive shaft  26  about the second axis of rotation  58 . This mechanical advantage will be a function of the relative positions of the external rotor motor  22  and the drive shaft  26 , the lengths of the crank arm  46  and the drag link  42 , and the position of the motor pin  34 . 
     As seen in  FIG. 4A , since the motor pin  34  is in the maximum counterclockwise position of its range of motion  40 , the motor pin  34  is at a maximum distance from the drive shaft  26 , resulting in a maximum angle  60  between the drag link  42  and the crank arm  46 . As the motor pin  34  moves in the clockwise direction with respect to the first axis of rotation  56  to the midpoint position of the range of motion  40 , as seen in  FIG. 4B , the drag link  42  pushes the end of the crank arm  46  in the negative Z direction and the distance between the motor pin  34  and the drive shaft  26  is reduced. The reduction in distance results in a reduction of the angle  60  between the drag link  42  and the crank arm  46 . Finally, as the motor pin  34  moves in the clockwise direction to the maximum clockwise position of the range of motion  40 , as seen in  FIG. 4C , the drag link  42  pushes the end of the crank arm  36  further in the negative Z direction and the distance between the motor pin  34  and the drive shaft  26  is reduced even further. The reduction in distance results in further reduction of the angle  60  between the drag link  42  and the crank arm  46 . 
       FIG. 5  shows an actuator system  20 ″ which includes the external rotor motor  22 , a drive train  24 ″, the drive shaft  26  and the chassis  28 . The external rotor motor includes the external rotor  30 , the internal stator  32  and the motor pin  34 . The drive train  24 ″ includes a crank arm  54 , a drag link  48 , a drag link pin  62 , a first ball socket joint  50 , and a second ball socket joint  52 . 
     As seen in  FIG. 5 , the external rotor motor  22  is rigidly attached to the chassis  28  (e.g., structural member of a missile) in the X-Y plane. This rigid attachment is made to the stationary internal stator  32  of the external rotor motor  22 . The external rotor  30  of the external rotor motor  22  rotates freely with respect to the chassis  28  along a first axis of rotation  56  which is parallel to the Z axis. The rotation of the external rotor  30  is restricted to a limited angle rotation (e.g. less than 90 degrees). Due to the external rotor&#39;s  30  limited rotation, commutation is not required to control operation of the motor. 
     The drive shaft  26  rotates about the second axis of rotation  58  that is parallel to the Y axis. Other than this rotation, the drive shaft  26  motion is restrained. The rotation of the drive shaft  26  provides motion for other external devices (e.g., fins of a missile). 
     The third embodiment of the actuator system  20 ″ utilizes a double link, three dimensional (3D) version of the drive train  24 ″. The drive train  24 ″ is a double link version, because it uses two links (the crank arm  54  and the drag link  48 ). The drive train  24 ″ is considered 3D because the motion of at least one of the links (e.g., drag link  48 ) is free to move in three dimensions. 
     As seen in  FIG. 5 , the motor pin  34  extends from the end surface of the external rotor  30  (the end surface is the only portion of the external rotor  30  depicted in the X-Y plane). The motor pin interacts with the drive train  24 ″. The double link 3D version of the drive train  24 ″ incorporates the crank arm  54 , the drag link  48 , the drag link pin  62 , the first ball socket joint  50 , and the second ball socket joint  52  to transmit rotation of the external rotor  30  to the drive shaft  26 . The motor pin  34  connects to one end of the drag link  48  at the first ball socket joint  50 . The first ball socket joint  50  is a joint capable of rotation about the Y direction and the Z direction. One end of the crank arm  54  connects to the other end of the drag link  48  using the second ball socket joint  52 . The second ball socket joint  52  is also a joint capable of rotation about the Y direction and the Z direction. The drive shaft  26  attaches to the other end of the crank arm  54 . As will be discussed in further detail below with reference to  FIG. 6 , this double link 3D version of the drive train  24  transmits a portion of the motor&#39;s  22  rotation to the drive shaft  26  in a different plane and in a different direction. 
       FIG. 6A  shows the external rotor  30  in the X-Y plane with the motor pin  34  in the maximum counterclockwise position (minimum Y position of  FIG. 6A , B, or C) and shows the range of motion  40  of the motor pin  34 .  FIG. 6A  also shows the drive shaft  26  in the X-Z plane with the crank arm  54  in the maximum counterclockwise position (minimum X position of  FIG. 6A , B, or C).  FIG. 6B  shows the external rotor  30  in the X-Y plane with the motor pin  34  in the midpoint position (intermediate Y position of  FIG. 6A , B, or C) and shows the range of motion  40  of the motor pin  34 .  FIG. 6B  also shows the drive shaft  26  in the X-Z plane with the crank arm  54  in the midpoint position (intermediate X position of  FIG. 6A , B, or C).  FIG. 6C  shows the external rotor  30  in the X-Y plane with the motor pin  34  in the maximum clockwise position (maximum Y position of  FIG. 6A , B, or C) and shows the range of motion  40  of the motor pin  34 .  FIG. 6C  also shows the drive shaft  26  in the X-Z plane with the crank arm  54  in the (maximum X position of  FIG. 6A , B, or C). 
     Because the external rotor  30  and the drive shaft  26  are restrained except with respect to rotation, as the motor pin  34  moves about the first axis of rotation  56 , the first ball socket joint  50  of drag link  48  is swept through an arc about the first axis of rotation  56  and thus drives the drag link  48  so that the second ball socket  52  and the end of the crank arm  54  not attached to the drive shaft  26  is sweep through an arc about the second axis of rotation  58 . The motion of the crank arm  54  applies a torque to the drive shaft  26 , causing the drive shaft  26  to have angular movement. Thus the angular displacement of the motor pin  34  about the first axis of rotation  56  transmits torque and angular displacement to the drive shaft  26  about the second axis of rotation  58 . An angular change in the external rotor  30  will result in an angular change in the drive shaft  26 . The change in the angle of the drive shaft  26  about the second axis of rotation  58  will be less than the change in the angle of the external rotor  30  about the first axis of rotation  56 , and the difference of angular displacement will be a function of the relative geometric positions of the external rotor motor  22  and the drive shaft  26 , and the length of the crank arm  54  and drag link  48 . 
     As seen in  FIG. 6A , since the motor pin  34  is in the maximum counterclockwise position of its range of motion  40 , the motor pin  34  is at a maximum distance from the drive shaft  26  and the end of crank arm  54  not attached to the drive shaft  26  is in its maximum counterclockwise position. As the motor pin  34  moves in the clockwise direction with respect to the first axis of rotation  56  to the midpoint position of the range of motion  40 , as seen in  FIG. 6B , the drag link  48  pushes the end of the crank arm  54  in the clockwise direction and the distance between the motor pin  34  and the drive shaft  26  is reduced. Finally, as the motor pin  34  moves in the clockwise direction to the maximum clockwise position of the range of motion  40 , as seen in  FIG. 6C , the drag link  48  pushes the end of the crank arm  54  further in the clockwise direction and the distance between the motor pin  34  and the drive shaft  26  is reduced even further. 
       FIG. 7  shows the external rotor motor  22  which includes the external rotor  30 , the internal stator  32 , a set of poles  64  (i.e., four or more poles  64 ), and a set of slots  66  (i.e., four or more slots  66 ). The stator windings have been omitted for clarity. 
     As shown in  FIG. 7  one possible configuration for the external rotor motor  22  is the brushless DC external rotor motor  22 . The brushless DC external rotor motor  22  is a four pole motor  22  that can rotate in both directions. The external rotor  30  has limited rotation (e.g., less than 90 degrees). Because of the limited rotation no commutation is required. Thus only two switches (e.g. field effect transistors) may be required to operate the motor  22  (as opposed to six switches typically required for typical brushless motors). This results in a less complicated, less expensive, and more compact motor  22 . 
       FIG. 8  shows a system having a motor controller  70  which accepts a command (e.g. an electronic signal) from a vehicle flight controller and converts that to the requisite motor signal to drive the external rotor motor  22 . 
     While various embodiments of the invention have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. 
     For example, the external rotor motor  22  can have more than four poles  64  and four slots  66  as long as the number of poles  64  and slots  66  are the same to ensure limited rotation.