Abstract:
A harmonic drive assembly with selective disconnect includes a motor with a motor drive shaft, a harmonic drive operatively coupled to the motor drive shaft, and a wave generator operatively coupled to the motor drive shaft and a flex gear of the harmonic drive. Also included is a ring gear of the harmonic drive operatively coupled to the flex gear with a plurality of ring gear teeth configured to mesh with a plurality of flex gear teeth. Further included is an output shaft operatively coupled to the harmonic drive. Yet further included is a retracting mechanism configured to selectively retract the motor drive shaft and the wave generator to decouple the motor drive shaft and the wave generator from the harmonic drive, wherein a clearance between the ring gear teeth and the flex gear teeth is established upon retraction of the motor drive shaft and the wave generator.

Description:
BACKGROUND OF THE INVENTION 
     The embodiments herein relate to harmonic drives and, more particularly, to a harmonic drive assembly and method for disengaging a harmonic drive. 
     Aircraft typically include electro-mechanical actuators and other flight control systems that control flight control surfaces on aircraft wing and tail structures. These flight control surfaces are moved and positioned to alter the lift characteristics of the wing and tail structures. For safety, aircraft usually have redundancies in the electro-mechanical actuators and flight control systems that control the flight control surfaces to allow for controlled maneuverability of the aircraft in the event that the primary system malfunctions or fails. When the primary system malfunctions, the backup system takes over and controls the movable flight control surface. The primary system may have become stuck or jammed in one position, making it difficult for the backup system to overcome the primary system and control the flight control surface. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one embodiment, a harmonic drive assembly with selective disconnect includes a motor with a motor drive shaft. Also included is a harmonic drive operatively coupled to one end of the motor drive shaft. Further included is a wave generator of the harmonic drive operatively coupled to the motor drive shaft and a flex gear of the harmonic drive. Yet further included is a ring gear of the harmonic drive operatively coupled to the flex gear with a plurality of ring gear teeth configured to mesh with a plurality of flex gear teeth. Also included is an output shaft operatively coupled to the harmonic drive. Further included is a retracting mechanism configured to selectively retract the motor drive shaft and the wave generator axially to decouple the motor drive shaft from the harmonic drive and to decouple the wave generator from the flex gear, wherein a clearance between the ring gear teeth and the flex gear teeth is established upon retraction of the motor drive shaft and the wave generator. 
     According to another embodiment, a method of disconnecting a harmonic drive assembly is provided. The method includes retracting a wave generator of a harmonic drive and a motor drive shaft out of a flex gear of the harmonic drive. The method also includes disengaging a plurality of flex gear teeth of the flex gear from a plurality of ring gear teeth of a ring gear of the harmonic drive, wherein disengaging the flex gear teeth and the ring gear teeth provides a clearance therebetween. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a block diagram of a drive assembly with selective disconnect; 
         FIG. 2A  is a cross-section view of a drive assembly with selective disconnect in an engaged, locked position; 
         FIG. 2B  is a cross-section view of the drive assembly with selective disconnect system of  FIG. 2A  in an engaged, unlocked position; 
         FIG. 2C  is a cross-section view of the drive assembly with selective disconnect of  FIG. 2A  in a disengaged, unlocked position; 
         FIG. 2D  is a cross-section view of the drive assembly with selective disconnect of  FIG. 2A  in a disengaged, locked position; and 
         FIG. 3A  is an end view of the drive assembly with a wave generator disposed within a flex gear of the drive assembly; 
         FIG. 3B  is an end view of section A of  FIG. 3A ; 
         FIG. 4A  is an end view of the drive assembly with the wave generator withdrawn from the flex gear in a retracted condition; and 
         FIG. 4B  is an end view of section B of  FIG. 4A . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , a block diagram of a drive assembly with selective disconnect is illustrated. Drive assembly with selective disconnect  10  includes motor  12 , motor drive shaft  13 , harmonic drive  14 , output shaft  16 , actuator  18 , retracting mechanism  20 , and controller  22 . Controller  22  receives feedback signals  24 A- 24 C and gives instruction or command signals  26 A and  26 B. 
     Motor drive shaft  13  is at least partially within motor  12  and receives energy from motor  12 . The energy received by motor drive shaft  13  from motor  12  causes motor drive shaft  13  to rotate. One end of motor drive shaft  13 , when in an engaged position (as will be discussed below), is engaged/coupled to harmonic drive  14  which, in turn, is connected/coupled to output shaft  16 . Output shaft  16  is connected to and drives actuator  18 , which can be used for a variety of purposes, including in an aircraft to control a movable flight control surface, such as an aileron or an elevator. The other end of motor drive shaft  13  is adjacent to retracting mechanism  20 . Connected to output shaft  16 , motor  12 , and retracting mechanism  20  is controller  22 . Controller  22  receives feedback signals from motor  12 , output shaft  16 , and actuator  18 , and provides instruction or command signals to motor  12  and retracting mechanism  20 . 
     Motor  12  may be an electric motor, hydraulic motor, pneumatic motor, or fuel combustion motor, or other type of motor that is able to receive energy from another source and convert it to mechanical work in the form of rotating motor drive shaft  13 . Motor drive shaft  13  may be made from any suitable material, such as steel or another alloy, and is most commonly cylindrical in shape. Motor drive shaft  13  may have a solid core or may be hollow, depending on design considerations and if there is a need to reduce weight. The core of motor drive shaft  13  may also be a different material than the shell. Motor  12  should be configured such that motor drive shaft  13  is accessible from either end so as to allow for one end to be engaged/coupled to harmonic drive  14  while the other end is adjacent to retracting mechanism  20 . Motor  12  may contain sensors that monitor motor  12  to detect any malfunctions or failures. The sensors are configured to communicate with controller  22  and inform controller  22  of any malfunctions or failures in motor  12 . Motor  12  is configured to rotate motor drive shaft  13 , thereby enabling motor drive shaft  13  to perform work. 
     At one end of motor drive shaft  13  is harmonic drive  14 , which is a gear reduction that includes a wave generator, flex gear or flex spline, and a ring gear or ring spline (which will be discussed below in conjunction with  FIGS. 2A-2D ). The configuration and functionality of a harmonic drive is known in the art, but will be described in greater detail below. Harmonic drive  14  is coupled to motor drive shaft  13  and output shaft  16  and transfers energy between motor drive shaft  13  and output shaft  16 . 
     Output shaft  16  is a drive shaft that receives energy from motor drive shaft  13  through harmonic drive  14 . Output shaft  16  may be made from any suitable material, such as steel or another alloy, and is most commonly cylindrical. Output shaft  16  may have a solid core or may be hollow, depending on design considerations and if there is a need to reduce weight. The core of output shaft  16  may also be a different material than the shell. 
     Output shaft  16  is attached to actuator  18 . Actuator  18  may be any electro-mechanical actuator, hinged line actuator, or any other device equipped to receive rotational energy from output shaft  16  and convert it to a useful end, such as moving a flight control surface on an aircraft. Additionally, actuator  18  may be configured to convert rotational motion into linear motion. Actuator  18  as used in an aircraft wing or tail may be a hinged line actuator that works to control a movable control surface, such as an aileron or elevator, for a flight control system. 
     Retracting mechanism  20  is located opposite harmonic drive  14  at the other end of motor drive shaft  13 .  FIG. 1  shows retracting mechanism  20  attached to motor drive shaft  13 , but retracting mechanism  20  may have other configurations (as will be discussed with regards to  FIGS. 2A-2D ). Retracting mechanism  20  is configured to retract motor drive shaft  13  when provided instruction or command signal  26 B from controller  22 . When motor drive shaft  13  is retracted, motor drive shaft  13  disengages and decouples from harmonic drive  14 . Once disengaged, energy from motor drive shaft  13  is no longer transferred to output shaft  16 . Conversely, output shaft  16  can no longer transfer energy to motor drive shaft  13 . Retracting mechanism  20  may also be configured to move motor drive shaft  13  back into its original position so as to engage/couple motor drive shaft  13  to harmonic drive  14  and allow energy to be transferred between motor drive shaft  13  and output shaft  16 . 
     Controller  22  may be electrically connected to every other component in drive assembly with selective disconnect  10  to determine the functionality of the system as a whole and/or each component within drive assembly with selective disconnect  10 . In  FIG. 1 , controller  22  is connected to motor  12 , output shaft  16 , and actuator  18  to receive feedback signals  24 C,  24 B, and  24 A, respectively, and motor  12  and retracting mechanism  20  to provide instruction or command signals  26 A and  26 B, respectively. If in an aircraft, controller  22  may also be connected to other components of the aircraft so as to give and receive information regarding aircraft functionality. After receiving feedback signals  24 A- 24 C and determining that drive assembly with selective disconnect  10  is not properly functioning to control actuator  18  or other components, controller  22  will provide instruction or command signal  26 A to power off motor  12  as well as instruction or command signal  26 B to retracting mechanism  20  to retract motor drive shaft  13 , which disengages/decouples motor drive shaft  13  from harmonic drive  14  and prevents energy from being transferred between motor drive shaft  13  and output shaft  16 . Feedback signals  24 A- 24 C provided to controller  22  may result in controller  22  instructing motor  12  to power off and retracting mechanism  20  to disengage/decouple motor drive shaft  13 . Such a result may be caused by any number of issues, such as a notification that output shaft  16  is not moving or receiving energy from harmonic drive  14  or that actuator  18  is malfunctioning, or that motor  12  is not producing the proper amount of rotational energy based on the energy drawn of motor  12 , or that motor drive shaft  13  has failed or malfunctioned. 
     Upon feedback signals  24 A- 24 C or other inputs into controller  22  that show drive assembly with selective disconnect  10  will function properly once in use again, controller  22  will provide the following instructions or command signals: instruction or command signal  26 A to motor  12  to power on and rotate motor drive shaft  13 , and instruction or command signal  26 B to retracting mechanism  20  to move motor drive shaft  13  into its original position to engaged/couple motor drive shaft  13  to harmonic drive  14 . Thereby, configuring drive assembly with selective disconnect  10  to use energy from motor  12  to drive output shaft  16  and actuator  18 . 
     Drive assembly with selective disconnect  10  is advantageous because of the ability to disengage/decouple motor drive shaft  13  from harmonic drive  14 , output shaft  16 , and actuator  18 . It is advantageous to disengage/decouple motor drive shaft  13  from harmonic drive  14  because many machines, such as movable flight control systems in aircrafts, contain redundant systems that take over the duties of actuator  18  in the event of failure to motor  12 , motor drive shaft  13 , and/or output shaft  16 . When failure of the primary system occurs, a backup system takes over and can more easily function to move the control systems and perform the duties of actuator  18  if the backup system does not have to overcome the resistance on output shaft  16  caused by output shaft  16  being coupled to motor drive shaft  13  and motor  12  through harmonic drive  14 . Thus, disengaging motor drive shaft  13  from harmonic drive  14  and output shaft  16  prevents wasted energy from transferring from the backup system, through output shaft  16 , to motor drive shaft  13  and the other components of drive assembly with selective disconnect  10 . If motor drive shaft  13  did not decouple from harmonic drive  14  in the event of failure and when the backup system is functioning, the backup system would have to overcome the resistance of actuator  18 , output shaft  16 , harmonic drive  14 , motor drive shaft  13 , and motor  12  to function. Drive assembly with selective disconnect  10 , because it disengages/decouples motor drive shaft  13  from harmonic drive  14 , allows the backup system to be more efficient and reliable. Additionally, disengaging/decoupling motor drive shaft  13  from harmonic drive  14  prevents the system (output shaft  16  and actuator  18 ) from becoming jammed in place upon failure or malfunction. Such a jam could result in an inability to control an aircraft in flight if the actuator is used to control a movable flight control surface. 
       FIGS. 2A, 2B, 2C, and 2D  show a drive assembly with selective disconnect in various stages of engagement (coupled) and disengagement (decoupled).  FIG. 2A  is a cross-section view of the drive assembly with selective disconnect in an engaged, locked position;  FIG. 2B  is a cross-section view of the drive assembly with selective disconnect in an engaged, unlocked position;  FIG. 2C  is a cross-section view of the drive assembly with selective disconnect in a disengaged, unlocked position; and  FIG. 2D  is a cross-section view of the drive assembly with selective disconnect in a disengaged, locked position. 
     Drive assembly with selective disconnect  110  of  FIGS. 2A-2D  includes motor  112 , motor drive shaft  113 , harmonic drive  114 , output shaft  116 , and retracting mechanism  120 . Harmonic drive  114  includes wave generator  128 , flex gear  130 , and ring gear  132 . Retracting mechanism  120  includes solenoid  134 , locking mechanism  136 , lock ball retainer  138 , lock ball bearings  140 , spring  142 , and reconnect sleeve  143 . In motor drive shaft  113 , proximate retracting mechanism  120 , are engaged groove  144  and disengaged groove  146 . Drive assembly with selective disconnect  110  also includes stroke spline  148  and guide ball bearings  150 . 
     Motor  112  is at least partially radially outward from motor drive shaft  113 , which forms the centerline about which motor drive shaft  113 , harmonic drive  114 , and many other components of drive assembly with selective disconnect  110  are centered. Motor  112  is radially outward from an area near the middle of motor drive shaft  113  and allows motor drive shaft  113  to extend out from both sides of motor  112 . Motor  112  may be any motor that provides mechanical energy in the form of rotating motor drive shaft  113 , such as an electric motor, a fuel motor, or another type of motor. 
     Motor drive shaft  113  may be cylindrical and constructed from any suitable material, such as a metal, an alloy, or other material that is able to handle the stresses caused by rotation of motor drive shaft  113  at high speeds. Motor drive shaft  113  should be strong enough in the axial direction to transfer energy from motor  112  to harmonic drive  114  without substantial deformation. Motor drive shaft  113  should also be strong enough to be able to be pulled from one end to move axially without deformation. Additionally, the diameter of motor drive shaft  113  may decrease in a stair-step manner along the axial direction of motor drive shaft  113  the closer motor drive shaft is to harmonic drive  114 , which is adjacent to one end of motor drive shaft  113 . 
     Radially between motor  112  and motor drive shaft  113  is stroke spline  148  and guide ball bearings  150 , which keep motor drive shaft  113  from radial movement. Stroke spline  148  is annular with a flange extending radially outward at the end furthest from harmonic drive  114  and may be made from a variety of materials, including a metal or alloy. Stroke spline  148  has a plurality of holes aligned axially around the circumference to provide a space for guide ball bearings  150 , which sit in the holes and contact motor drive shaft  113  so as to provide a support surface that has a low coefficient of friction. Guide ball bearings  150  are spherical ball bearings that may be made from any material that is sufficiently hard to adequately provide support to motor drive shaft  113  while also having a low coefficient of friction with motor drive shaft  113  and stroke spline  148  to reduce wear and increase efficiency. While  FIGS. 2A-2D  show two rows of guide ball bearings  150  in stroke spline  148 , other embodiments may include a different configuration or may not even include stroke spline  148  or guide ball bearings  150  if such components are not needed to keep motor drive shaft  113  from moving radially. 
     Harmonic drive  114  is adjacent to one end and radially outward from motor drive shaft  113 . Harmonic drive  114  is a harmonic drive or strain wave gearing that is known to one of skill in the art and includes, going from radially inward to radially outward, wave generator  128 , flex gear  130 , and ring gear  132 . Motor drive shaft  113  is connected to wave generator  128  such that if motor drive shaft  113  moves axially, wave generator  128  also moves axially. Wave generator  128  has a cross section that is rectangular with semi-circles on each end. On the radially inner side of wave generator  128  is motor drive shaft  113  and on the radially outer side is flex gear  130 . Between wave generator  128  and motor drive shaft  113  may be a lubricant to reduce friction and wear between wave generator  128  and flex gear  130  so as to improve durability and efficiency. 
     Flex gear  130  has wave generator  128  on the radially inner side and ring gear  132  on the radially outer side. Flex gear  130  may be or have another cross section, but in  FIGS. 2A-2D  has a cross-section that is substantially oval or elliptical when wave generator  128  is radially within flex gear  130 . Flex gear  130  is smooth on the radially inner surface to allow for wave generator  128  to easily slide as wave generator  128  rotates and has teeth on the radially outer surface that, when rotated, fit into teeth on the inner surface of ring gear  132 . Flex gear  130  is made from a flexible material, such as spring steel or another suitable material, to allow for flex gear  130  to take a shape similar to an oval when wave generator  128  is radially within flex gear  130 . When wave generator  128  is radially within flex gear  130 , only the smooth inner surface of flex gear  130  near the semi-circular ends of wave generator  128  contact wave generator  128 . As a result, only the teeth of flex gear  130  radially outward from the surface that is in contact with wave generator  128  are in contact with ring gear  132  at any one time, for flex gear  130  takes on a cross section that is substantially oval, while ring gear  132  has a cross section that is circular. As wave generator  128  rotates, wave generator  128  slides within flex gear  130  so that flex gear  130  does not rotate at the same angular velocity as wave generator  128  (thus there is a gear reduction). Generally, while wave generator  128  rotates, flex gear  130  rotates at a slower angular velocity. 
     Ring gear  132  is annular and has flex gear  130  on the radially inner side. Because ring gear  132  is annular and flex gear  130  has a substantially oval cross-section when wave generator  28  is within flex gear  130 , not all of the teeth on the radially inner surface of ring gear  132  contact the teeth on flex gear  130  simultaneously. While  FIGS. 2A-2D  show flex gear  130  connected to output shaft  116  while ring gear  132  is anchored, either flex gear  130  or ring gear  132  may be connected to output shaft  116  while the other (flex gear  130  or ring gear  132 ) is anchored in place. 
     Referring to  FIGS. 3A and 3B , the wave generator  128  is shown in an engaged condition, where the flex gear  130  is engaged with the ring gear  132 . Specifically, as noted above, the flex gear  130  includes a plurality of flex gear teeth  152  along a radially outer location of the flex gear  130 . The ring gear  132  includes a plurality of ring gear teeth  154  along a radially inner location of the ring gear  132 . The flex gear teeth  152  and the ring gear teeth  154  are in a meshed, or engaged, condition when the wave generator  128  is in a non-retracted condition and disposed within the flex gear  130 . 
     Referring to  FIGS. 4A and 4B , the wave generator  128  is shown in a retracted condition, which corresponds to a removed position that disposes the wave generator out of the flex gear  130  (wave generator void referred to with reference numeral  129 ). In the retracted condition, the flex gear  130  takes on what is referred to herein as a “free state” due to the withdrawal of the wave generator  128 . In the free state, the flex gear teeth  152  and the ring gear teeth  154  are completely disengaged from each other to form a clearance therebetween. By establishing a complete clearance between the flex gear teeth  152  and the ring gear teeth  154 , it is ensured that ratcheting between the flex gear  130  and the ring gear  132  is avoided. To ensure such a condition, the flex gear teeth  152  and the ring gear teeth  154  are sized to have a specific relationship. In particular, the flex gear teeth  152  have a pitch diameter in the free state that is less than a pitch diameter of the ring gear teeth  154  minus the sum of a flex gear tooth height in the free state and a tooth height of the ring gear. This relationship is defined by the following equation:
 
PD FS &lt;PD RG −(TH FS +TH RG )
 
     where PD FS  is a pitch diameter of the flex gear teeth in the free state; PD RG  is the pitch diameter of the ring gear teeth, TH FS  is the flex gear tooth height in the free state; and TH RG  is the ring gear tooth height. 
     The use of harmonic drive  114  as a gear reduction is advantageous because it may be desired to disengage/decouple motor drive shaft  113  from harmonic drive  114  and output shaft  116  and later reengage/couple motor drive shaft  113  to harmonic drive  114  and output shaft  116 . As will be discussed in greater detail below, to disengage motor drive shaft  113 , motor drive shaft  113  and wave generator  128  are removed from harmonic drive  114  (as shown in  FIG. 2C ) so that wave generator  128  is no longer radially within flex gear  130  and ring gear  132 . This prevents the transfer of energy between motor drive shaft  113  and output shaft  116  through harmonic drive  114 . Conversely, to reengage motor drive shaft  113 , motor drive shaft  113  and wave generator  128  are moved back into harmonic drive  114  so as to position wave generator  128  radially within flex gear  130  and ring gear  132 . Because the outer surface of wave generator  128  and the inner surface of flex gear  130  are smooth and without teeth, wave generator  128  and flex gear  130  do not have to be specifically aligned, making the insertion of wave generator  128  into harmonic drive  114  substantially easier than with a conventional gear reduction that has teeth on these two surfaces and requires the teeth to be perfectly aligned. One system used to disengage and reengage motor drive shaft  113  and wave generator  128  with harmonic drive  114  is described below. 
     As mentioned above, output shaft  116  may be attached at one end to either flex gear  130  or ring gear  132  and extends as a cylinder or another configuration away from harmonic gear  114  in an opposite direction from motor drive shaft  113 . The other end of output shaft  116  may be attached to any drive or other device that is equipped to receive energy from output shaft  116  and convert it to a useful end, such as actuator  18  in  FIG. 1 . Output shaft  116  may be hollow or solid and made from various materials with sufficient properties to be able to receive rotational energy from harmonic drive  114  and convey it to an actuator or another device. The diameter of output shaft  116  is likely larger than that of motor drive shaft  113  due to output shaft  116  being connected to either flex gear  130  or ring gear  132 . 
     Adjacent to the other end of motor drive shaft  113  is retracting mechanism  120 . Retracting mechanism  120  include spring  142 , which is be radially outward from motor drive shaft  113  and adjacent to stroke spline  148 . One end of spring  142  should be attached to motor drive shaft  113  so as to cause motor drive shaft  113  to move axially when possible. The other end of spring  142  should be anchored. Spring  142  may be one helical spring that coils around motor drive shaft  113 , a number of helical springs that are arranged circumferentially around motor drive shaft  113 , or another device, mechanical or otherwise, that is able to push motor drive shaft  113  away from harmonic drive  114  when allowed or prompted. Spring  142  should be sufficiently strong to move motor drive shaft  113  and wave generator  128  out from harmonic drive  114  and overcome the resistance caused by friction between wave generator  128  and flex gear  130 . 
     Retracting mechanism  120  also includes solenoid  134 , which is be located the furthest from motor drive shaft  113  of all the components of retracting mechanism  120 . Solenoid  134  is centered axially along the same centerline that motor drive shaft  113  is centered and extends away from the end of motor drive shaft  113  that is opposite harmonic drive  114 . Solenoid  134  may be pneumatic, electro-mechanical, or another configuration able to retract and pull on locking mechanism  136  to move locking mechanism  136  away from motor drive shaft  113 . While  FIGS. 2A-2D  show solenoid  134  configured to pull locking mechanism  136  to unlock motor drive shaft  113  and allow spring  142  to remove motor drive shaft  113  and wave generator  128  from harmonic drive  114  (as will be discussed below), solenoid  134  may also be configured to directly retract and remove motor drive shaft  113  and wave generator  128  from harmonic drive  114  and, conversely, to extend and push motor drive shaft  113  and wave generator  128  back into harmonic drive  114 . 
     Between solenoid  134  and motor drive shaft  113  is locking mechanism  136 , which is annular with an open end that is radially outward from motor drive shaft  113  and a closed end that is attached to solenoid  134 . Locking mechanism  136  may have at least one flange on the outer surface to provide structural support. The inner surface of locking mechanism  136  has a tapered portion where the diameter of locking mechanism  136  increases as it gets closer to the end of locking mechanism  136  that is adjacent to motor drive shaft  113 . The inner surface of locking mechanism  136  also has a non-tapered portion with a consistent diameter near the closed end of locking mechanism  136 . When solenoid  134  retracts, it pulls on locking mechanism  136 , moving locking mechanism  136  away from motor drive shaft  113  and unlocking motor drive shaft  113  (allowing for axial movement of motor drive shaft  113  and wave generator  128 ). In this embodiment, spring  142  is used to remove motor drive shaft  113  and wave generator  128  from harmonic drive  114  and reconnect sleeve  143  is used to move motor drive shaft  113  and wave generator  128  back into harmonic drive  114 , but both of these tasks could be completed by a solenoid or other device. 
     Radially outward from solenoid  134 , locking mechanism  136 , and other components of retracting mechanism  120  is reconnect sleeve  143 , which may be one or a number of jack screws or another device able to move motor drive shaft  113  and wave generator  128  towards harmonic drive  114  to engage/couple motor drive shaft  113  and wave generator with harmonic drive  114 . Other embodiments may not include reconnect sleeve  143  and may include a solenoid (either solenoid  134  or another solenoid) capable of pushing motor drive shaft  113  towards harmonic drive  114 . 
     Radially within locking mechanism  136  at least partially between the annular part of locking mechanism  136  (as opposed to the closed end of locking mechanism  136 ) and motor drive shaft  113  is lock ball retainer  138 . Lock ball retainer  138  is be annular with one end abutting the inner surface of the closed end of locking mechanism  136  and the other end adjacent to motor drive shaft  113 . Lock ball retainer  138  is anchored so as to not move when motor drive shaft  113  or locking mechanism  136  move. The end of lock ball retainer  138  closest to motor drive shaft  113  may not be radially within locking mechanism  136  and may have at least one flange that extends radially outward to provide structural support and other functions. The diameter and thickness of lock ball retainer  138  is substantially consistent throughout the length of lock ball retainer  138  and is not tapered like locking mechanism  136 . Lock ball retainer  138  includes a plurality of holes aligned circumferentially around lock ball retainer  138  at a point near the middle of lock ball retainer  138  when measured along the length of lock ball retainer  138 . 
     Within the plurality of holes in lock ball retainer  138  are lock ball bearings  140 , which are spherical and have a diameter that is greater than the thickness of lock ball retainer  138  so that lock ball bearings  140  protrude from either the inner surface or outer surface of lock ball retainer  138 , depending on whether locking mechanism  136  is in the locked position ( FIGS. 2A and 2D ) or the unlocked position ( FIGS. 2B and 2C ). When lock ball bearings  140  are radially within the non-tapered portion of locking mechanism  136  and when motor drive shaft  113  is in the engaged position (as will be discussed below), lock ball bearings  140  are at least partially within engaged groove  144 , as is shown in  FIG. 2A . 
     Engaged groove  144  is an indentation aligned circumferentially around motor drive shaft  113  and, along with lock ball bearings  140 , prevents axial movement of motor drive shaft  113 . Engaged groove  144  is near the end of motor drive shaft  113  such that when lock ball bearings  140  are within engaged groove  144 , motor drive shaft  113  and wave generator  128  are within harmonic drive  114 . When lock ball bearings  140  are radially within the non-tapered portion of locking mechanism  136  (the inner surface of locking mechanism  136  is pushing on lock ball bearings  140 ) and when motor drive shaft  113  is in the disengaged position (as will be discussed below), lock ball bearings  140  are at least partially within disengaged groove  146 , as is shown in  FIG. 2D . Disengaged groove  146  is similar to engaged groove  144  but is closer to harmonic drive  114  such that when lock ball bearings  140  are within disengaged groove  144 , motor drive shaft  113  and wave generator  128  are not within harmonic drive  114 . 
     When lock ball bearings  140  are not being pushed into engaged groove  144  or disengaged groove  146  by the non-tapered portion of locking mechanism  136  (when lock ball bearings are radially within the tapered portion of locking mechanism  136 ), lock ball retainer  138  is configured to push (through the use of springs or other device) lock ball bearings  140  radially outward so that lock ball bearings are not within engaged groove  144  or disengaged groove  146 . In this situation, motor drive shaft  113  and wave generator  128  are not prevented from moving axially by lock ball bearings  140  and are able to be removed from or inserted into harmonic drive  114 . The various stages of drive assembly with selective disconnect  110  as motor drive shaft  113  is disengaged from harmonic drive  114  and output shaft  116  are described below. 
       FIG. 2A  shows drive assembly with selective disconnect  110  in an engaged, locked position. At this point, motor drive shaft  113  and wave generator  128  are within harmonic drive  114 , allowing energy to be transferred between harmonic drive  114  and output shaft  116 . Additionally, solenoid  134  is in a non-retracted position so locking mechanism  136  is in a position closer to harmonic drive  114 , meaning that the non-tapered portion of locking mechanism  136  is radially outward from lock ball bearings  140 . In this situation, the inner surface of the non-tapered portion of locking mechanism  136  pushes on lock ball bearings  140 , causing lock ball bearings  140  to be at least partially within engaged groove  144  and preventing motor drive shaft  113  from moving axially. This keeps motor drive shaft  113  and wave generator  128  in the engaged position within harmonic drive  114 . 
     Drive assembly with selective disconnect  110  is in the engaged, locked position during normal working conditions, allowing output  116  to drive an actuator or other device. When in this position, solenoid  134  has not retracted locking mechanism  136 , pushing lock ball bearings  140  into engaged groove  144 . Motor drive shaft  113  is in a forward, engaged position and spring  142  is compressed and prevented from pushing motor drive shaft  113  away from harmonic drive  114  by lock ball bearings in engaged groove  144 . 
       FIG. 2B  shows drive assembly with selective disconnect  110  in an engaged, unlocked position. At this point, motor drive shaft  113  and wave generator  128  are within harmonic drive  114 , allowing energy to be transferred between motor drive shaft  113  and output shaft  116  through harmonic drive  114 . Solenoid  134  has retracted locking mechanism  136 , putting locking mechanism  136  in a position further away from harmonic drive  114  than its position in  FIG. 2A . Because locking mechanism  136  is retracted by solenoid  134 , the tapered portion of locking mechanism  136  is radially outward from lock ball bearings  140 . In this situation, the inner surface of the tapered portion of locking mechanism  136  is not pushing on lock ball bearings  140 , allowing lock ball bearings  140  to move radially outward and not be at least partially within engaged groove  144 . Because lock ball bearings  140  are not within engaged groove  144 , motor drive shaft  113  is not axially locked in position. 
     Drive assembly with selective disconnect  110  is in the engaged, unlocked position for only a short period of time until spring  142  extends and pushes motor drive shaft  113  away from harmonic drive  114 . Solenoid  134  will be instructed to retract locking mechanism  136  by a control system, such as controller  22 , that is monitoring drive assembly with selective disconnect  110 . When it is determined that output shaft  116  is malfunctioning so that the actuator or other device driven by output shaft  116  is not functioning properly, solenoid  134  will be instructed to retract locking mechanism  136  and begin the process of disengaging/decoupling motor drive shaft  113  and wave generator  128  from harmonic drive  114  (by removing wave generator  128  from being in contact with flex gear  130 ). 
       FIG. 2C  shows drive assembly with selective disconnect  110  in a disengaged, unlocked position. At this point, motor drive shaft  113  and wave generator  128  are not within harmonic drive  114  so wave generator  128  is not in contact with flex gear  130 , preventing energy from being transferred between motor drive shaft  113  and output shaft  116 . Additionally, because motor drive shaft  113  and wave generator  128  are not within harmonic drive  114 , a backup system activated to take over control of the actuator or other device attached to output shaft  116  does not have to overcome the resistance on output shaft  116  provided by output shaft  116  being coupled to motor drive shaft  113 . 
     When drive assembly with selective disconnect  110  is in a disengaged, unlocked position, solenoid  134  and locking mechanism  136  are in a retracted position, as they were in  FIG. 2B . Because locking mechanism  136  is retracted by solenoid  134 , the tapered portion of locking mechanism  136  is radially outward from lock ball bearings  140 . In this situation, the inner surface of the tapered portion of locking mechanism  136  is not pushing on lock ball bearings  140 , allowing lock ball bearings  140  to be radially outward from motor drive shaft  113  and not be at least partially within engaged groove  144  or disengaged groove  146  (which would prevent motor drive shaft  113  from moving axially). While lock ball bearings  140  are not within disengaged groove  146  in  FIG. 2C , lock ball bearings  140  are aligned in a position radially outward from disengaged groove  146 . Lock ball bearings  140  are in this position because spring  142  has moved motor drive shaft  113  and caused disengaged groove  146  to move axially into alignment with lock ball bearings  140 . 
     Drive assembly with selective disconnect  110  is in the disengaged, unlocked position for only a short period of time until solenoid  134  extends and moves locking mechanism  136  towards harmonic drive  114 , which again locks motor drive shaft  113  in place as is shown in  FIG. 2D . As mentioned above, when motor drive shaft  113  is in the disengaged position, energy is not transferred between motor drive shaft  113  and output shaft  116  through harmonic drive  114  and output shaft  116  is not restrained or locked in place by motor drive shaft  113  but rather is free to rotate. 
       FIG. 2D  shows drive assembly with selective disconnect  110  in a disengaged, locked position. At this point, motor drive shaft  113  and wave generator  128  are not within harmonic drive  114  and are locked in place such that no axial movement is allowed. Motor drive shaft  113  is prevented from axial movement by lock ball bearings  140 , which are at least partially within disengaged groove  146  because locking mechanism  136  has be moved by solenoid  134  toward harmonic drive  114 , causing the non-tapered portion of locking mechanism  136  to be radially outward from lock ball bearings  140  and push lock ball bearings  140  into disengaged groove  146 . 
     Drive assembly with selective disconnect  110  is in the disengaged, locked position when it is determined that output shaft  116  is malfunctioning so that the actuator or other device driven by output shaft  116  is not functioning properly. Output shaft  116  and/or the actuator or other device connected to output shaft  116  may not be functioning properly because it or another component has failed. As mentioned before, once drive assembly with selective disconnect  110  is in the disengaged, locked position, it will remain there until the system is fixed or has been determined to be working properly. When the system is determined to be working properly, reconnect sleeve  143  may then be activated to move motor drive shaft  113  and wave generator  128  back into harmonic drive  114  and reengage motor drive shaft  113  (after solenoid  134  has retracted locking mechanism  136  to unlock motor drive shaft  113  and allow for axial movement). 
     Reengaging/inserting motor drive shaft  113  and wave generator  128  into harmonic drive  114  so that wave generator  128  is radially within flex gear  130  requires wave generator  128  to come into contact with the smooth inner surface of flex gear  130 . Because the location of axial movement within harmonic drive  114  is between the smooth outer surface of wave generator  128  and the smooth inner surface of flex gear  130 , wave generator  128  does not have to be substantially aligned with flex gear  130 , making reengagement easier than it would be with a conventional gear reduction that has teeth on these two surfaces and requires the teeth to be perfectly aligned. 
     Additionally, drive assembly with selective disconnect  110  is advantageous because it allows motor drive shaft  113  to be disengaged/decoupled from harmonic drive  114  and output shaft  116  so any backup system does not have to overcome the resistance that motor drive shaft  113  provides by being coupled to output shaft  116 . Because motor drive shaft  113  is decoupled from output shaft  116 , output shaft  116  does not become stuck or jammed in place, providing a safer and more easily fixed system. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.