Abstract:
An apparatus has a first motor for moving an actuator, a first brake for selectively braking the motor, a second brake that is engaged, the second brake dissipating torque spikes in the system, and a gear attaching the first motor to the actuator to cause the actuator to move and to the second brake whereby the torque spikes overcome stiction in the engaged brake so that the torque spike is dissipated in the engaged brake.

Description:
BACKGROUND 
     Throughout history, engineers have used actuators to move objects providing rotary or linear motion. A rotary actuator is simply a gearing system that either increases or decreases the rotational speed of a prime mover, typically a hydraulic motor, an internal combustion engine, a turbine engine, or an electric motor, to provide a desired level of rotational speed and torque at an output. Examples of rotary actuators include: gearboxes, transmissions, differentials, Rotac® actuators, and rotary electro-mechanical actuators. Linear actuators are machines designed to provide force and linear displacement to an object. Some examples of linear actuators include: rack &amp; pinion actuators, hydraulic rams, ball screw actuators, and crank arm actuators. 
     Historically, hydraulic/pneumatic motors and hydraulic/pneumatic rams have been the primary source of power for both linear and rotary actuators. Hydraulic systems offer many advantages to the designer including: high power density, accurate position control, low inertia (for high frequency response), and overload protection (via pressure relief valves). 
     More recently, engineers have replaced hydraulic/pneumatic actuation systems with electro-mechanical actuation systems. Electro-mechanical actuators (“EMA”), which typically include a motor, a gear box and an actuator, offer increased efficiency over their hydraulic and pneumatic counterparts and are less prone to leakage. 
     When designing small, high power density EMAs, a designer is faced with a problem caused by the rotational inertia associated with the EMAs electric motor. In order to create an EMA with a large force capability, the designer must create an electric motor that is capable of producing a large torque, or must create a gear train that reduces the motor&#39;s output torque requirement. If the designer chooses to create a motor with a large torque capability, its rotor will contain a significant amount of rotational inertia. If the designer chooses to utilize a gear reduction system to decrease the motor&#39;s output torque requirement, thereby reducing the motor&#39;s physical size and rotational inertia, the motor will be required to operate at a faster speed. The inertia of the motor, as felt by the output of the actuator, will be proportional to the motor&#39;s inertia multiplied by the gear reduction ratio squared. 
     The inertia of the EMA motor becomes extremely important when sizing the gear train and/or the actuator structure if, for instance, the actuator hits an internal stop at full speed, or if the actuated structure hits a stop at the end of its travel at full speed. In this scenario, the rotational inertia of the motor will tend to cause the actuator to continue driving through its stop, or through the structure&#39;s end stop, causing significant damage to the EMA, or its supporting structure. If the stops and structures are strong enough to maintain their integrity, the next weakest link, most likely the actuator or the gear train driving the actuator will be damaged. 
     Historically, the gear train and the EMA&#39;s stops are overbuilt to handle an intense torque spike associated with the rapid deceleration of the EMA&#39;s motor as the actuator hits its stops, and the internal shafting flexes as the motor spins down. This design approach tends to cause the actuator to become significantly larger and heavier than it would otherwise have to be. 
     Another method to handle the scenario described above is to incorporate a slip clutch in the driveline between the EMA&#39;s motor and the EMA&#39;s output. Incorporating a slip clutch in the driveline allows the EMA&#39;s output to nearly instantaneously stop, while the motor decelerates, with the stored energy of the rotating motor rotor being absorbed by the slip clutch&#39;s friction material. This type of system works well, however, it again adds components to the EMA that add size, cost, weight, and reduce the actuator&#39;s overall reliability. 
     SUMMARY 
     According to a non-limiting embodiment, an apparatus has a first motor for moving an actuator, an engaged brake for dissipating a torque spike in the EMA, and a gear attaching the motor to the actuator to cause the actuator to move and attaching to the engaged brake whereby the torque spike overcomes stiction in the engaged brake so that the torque spike is dissipated in the engaged brake. 
     According to a further non-limiting embodiment, an apparatus has a first motor for moving an actuator, a second motor for moving the actuator, the second motor moving the actuator upon a failure of the first motor, an engaged brake attaching to the second motor for dissipating torque spikes in the system, and a gear attaching the first and second motor to the actuator to cause the actuator to move and to the engaged brake whereby the torque spikes overcome stiction in the engaged brake so that the torque spike is dissipated in the engaged brake. 
     According to yet another non-limiting embodiment, an apparatus dissipates torque spike in therein and has a first motor for moving an actuator, a first brake for selectively braking the motor, a second brake that is engaged, the second brake dissipating torque spikes in the system, and a gear attaching the first motor to the actuator to cause the actuator to move and to the second brake whereby the torque spikes overcome stiction in the engaged brake so that the torque spike is dissipated in the engaged brake. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic depiction of an embodiment of a dual linear EMA with a stop compliant driveline. 
         FIG. 2  is a schematic depiction of a brake utilized in the EMA of  FIG. 1 . 
     
    
    
     DESCRIPTION 
     Referring to the Figure, a non-limiting schematic embodiment of an EMA  10  is shown. The EMA comprises a pair of motors  15 ,  20  a pair of brakes  25 ,  30  each brake being associated with a motor, a differential  35 , an impeller such as linear actuator  40 , such as a ball screw, a position sensor  45 , and an attachment  50  that attaches to a load (not shown). The linear actuator  40  has an internal stop  47  shown as a collar. 
     The EMA shows a pair of motors  15 ,  20  because some applications require redundancy. If one motor fails, the other may be used. In this application, one motor  15  operates while the other motor  20  does not. In the non-operating motor  20 , the brake  30  associated with it, is engaged in a non-energized state as will be discussed hereinbelow. If the motor  15  fails, motor  20  will then operate and the brake  30  will be disengaged in an energized state and does not provide braking torque on the motor unless desired. 
     Each motor is attached to the differential gear train as follows: motor output shaft  55  has a gear  60  mounted thereon that attaches to gear  65  that attaches to a brake gear  70 . Gear  65  is mounted on shaft  75  that has a reduction gear  80  mounted thereon. For motor  15 , the reduction gear  80  meshes to gear  85  on the differential  35 . For motor  20 , the reduction gear  80  meshes to gear  90  on the differential  35 . 
     Referring to  FIG. 2 , for each brake,  25 ,  30 , brake gear  70  is mounted on brake shaft  95 . A brake plate  100  having friction material  105  thereon is also mounted on the shaft  95 . A clapper plate  110  is urged into engagement with the friction material  105  by springs  115 . If the clapper plate  110  is to be disengaged from the brake plate  100 , an electromagnet  120  is actuated to overcome the force of the springs  115  to pull the clapper plate away from the brake plate  100  and the friction material  105  thereon thereby allowing shaft  95  to rotate. If a brake  25  or  30  is engaged, shaft  95  and gears  70 ,  65 ,  60  and shafts  55 ,  75 ,  95  do not usually rotate as will be discussed herein (see  FIG. 1 ). 
     Referring back to  FIG. 1 , the differential  35  is discussed further. Gear  85  is attached to a first input shaft  125  of the differential and gear  90  is attached to a second input shaft  130  of the differential. The first input shaft  125  is attached to a first sun gear  135  and the second input shaft is attached to a second sun gear  140 . The first sun gear  135  meshes with planetary gear  145  and the second sun gear  140  meshes with planetary gear  150 . Planetary gear  145  drives a differential output gear  155  via shaft  165  and planetary gear  150  drives the differential output gear  155  via shaft  160 . Differential output gear  155  meshes with a ball screw gear  170  to move the ball screw  40  via shaft  175  inwardly and outwardly. Also, planetary gear  145  meshes with planetary gear  150 . 
     During operation, operating motor  15  rotates to move motor output shaft  55 , gear,  60 , gear  65 , shaft  75 , reduction gear  80 , gear  85 , first input shaft  125 , first sun gear  135 , first planetary gear  145 , shaft  165 , differential output gear  155 , ball screw gear  170  and ball screw actuator  40 . The electromagnet  120  of the brake  25  is actuated so that the clapper plate  110  is drawn away from the brake plate  100  so that shaft  95  and brake gear  70  may rotate freely as the motor operates. 
     While the motor  15  operates, the electromagnet  120  of the brake  30  is not actuated so that the clapper plate  110  is pushed against the brake plate  100  thereby not allowing the shaft or the gear  70  mounted thereon to rotate. If the gear  70  does not rotate the gear  65  does not rotate and the motor  20  via gear  60  and shaft  55  do not rotate. Also, if gear  65  does not rotate, reduction gear  80 , gear  90 , shaft  130 , and second sun gear  140  do not rotate. However, because planetary gear  150  is attached to differential output gear  155  that rotates due to the motor  15  input as mentioned hereinabove, planetary gear  150  may still rotate around the second sun gear  140 . 
     If the actuator  40  hits its internal stop  47  with motor  15  spinning at full speed, a sudden torque spike, caused by the nearly instantaneous stopping of the actuator  40 , is absorbed by the EMA  10 . Because gear  170  can no longer rotate to extend the linear actuator beyond the stop and the drive path provided by the motor  15  can also not continue to rotate, the kinetic energy stored in the EMA passes through the second planetary gear  150 , the previously stationary second sun gear  140 , shaft  130 , reduction gear  80 , shaft  75 , gear  65 , gear  70  and shaft  95  to cause clapper plate  110  and brake plate  100  of brake  30  to exceed its maximum static torque rating, e.g., that force that causes the brake and clapper plates to stick together and not rotate or stiction. As brake  30  begins to slip, the stored energy associated with motor  15  rotational speed and rotational inertia will be dissipated by the friction material  105  on brake plate  100 . 
     The EMA illustrated in  FIG. 2 , utilizes the brake  30  to balance the input torque of motor  15  across the differential  35  in the event that actuator  40  hits its internal stop  47 . 
     This EMA allows for the dual use of the differential and brake system, associated with the dual redundant architecture of the EMA, to create a light weight, and mechanically simple mechanism that has the ability to dissipate the stored energy associated with the rotational inertia and rotational speed of the operating motor  15 . 
     The foregoing description is exemplary rather than defined by the limitations within. Various non-limiting embodiments are disclosed herein, however, one of ordinary skill in the art would recognize that various modifications and variations in light of the above teachings will fall within the scope of the appended claims. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described. For that reason the appended claims should be studied to determine true scope and content.