Abstract:
A two-stage linear actuator particularly applicable to aircraft control surfaces. The first stage comprises a rotary input shaft driven by an electric motor having a helical threaded zone in its external surface at its inner end and a plurality of first helical roller gears configured to engage with the rotary input shaft in its helical threaded zone for rotating together. The second stage comprises a plurality of second helical roller gears configured to engage with the first helical roller gears for rotating together and with an output shaft having a helical threaded zone in its external surface at its inner end for converting the rotation of the second helical roller gears in a linear movement of the output shaft.

Description:
RELATED APPLICATIONS 
       [0001]    The present application claims priority to European Patent Application filed Feb. 10, 2012 as application number 12382047.4, the entire contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an electro mechanical actuator and more in particular to an electro mechanical actuator able to survive to a jamming of its internal mechanical components. 
       BACKGROUND OF THE INVENTION 
       [0003]    Linear actuators are used in many industrial products. In the aeronautical industry in particular are used as actuators of aircraft control surfaces and other aircraft components. 
         [0004]    Generally, each aircraft control surface is actuated by multiple linear actuators in parallel, so that in case of power loss of one of them, the surface can be controlled with the remaining actuators. As this configuration has the disadvantage that the jamming of one of the actuators may produce a blocking of the surface, the aeronautic regulations require extremely low jamming probabilities (of the order of 10e-9) to said actuators. Hydraulic actuators are capable of meeting this requirement. 
         [0005]    The trend toward greater electrification of aircraft (“More Electrical Aircraft”, MEA), oriented toward a reduction of weight and maintenance cost of systems, has led to the introduction of new technologies in flight command systems, including primary flight command systems. 
         [0006]    Electro-hydrostatic actuators (EHAs) have been incorporated in new platforms (A380, A400, A350, F35, . . . ). This type of actuator has an integrated hydraulic system so that interconnection with the power system of the aircraft is purely electric, but its power transmission to the aircraft control surface is through an integrated hydraulic actuator. They meet the jamming probability target because the power transmitted to the surface is done by means of a hydraulic actuator and at the same time allows the elimination of the aircraft hydraulic system from the aircraft. This technology is considered an intermediate step in the progressive electrification of aircraft actuation systems. 
         [0007]    Electro Mechanical Actuators (EMA) have not yet been implemented in primary flight command systems (except for experimental applications), despite their potential advantages with respect to complexity, efficiency, weight and maintainability. The main reasons why EMAs have not been introduced in primary flight command systems controls are:
       The probability of jamming of current available actuators is not as low as required.   The reversibility of these actuators in case of loss of electrical power is not as good as in the case of actuators with a hydraulic output stage, especially if the mechanical advantage between the electric motor and the output is high.       
 
         [0010]    Commonly applied technologies in the output stage of EMA actuators (primarily ball screws and mechanical reduction gearbox) do not fully guarantee the above requirements because of:
       The mechanical gearbox usually connected between the electric motor and the screw has a higher jamming probability than required for the application. The ball screw has the same problem by incorporating re-circulating mechanical elements, which, when blocked, impede or degrade the movement of the screw up to a non-functionality level.   The reversibility of the ball screw in case of jamming is low, because small       
 
         [0013]    The planetary roller screws have advantages over ball screws with similar efficiency in terms of strength, life and load capacity among others. Their design is simpler and does not include circulation elements. However, they are not free of jamming in their moving parts (rollers, synchronism crown, gears, etc.) by the presence of external contamination, fractures, etc., thus jamming the output shaft of the actuator. 
         [0014]    U.S. Pat. No. 7,410,132 and U.S. Pat. No. 7,610,828 disclose ball screw linear actuators incorporating means for releasing the output shaft in case of jamming 
         [0015]    One disadvantage of these proposals is that the ball screw has recirculating elements susceptible to jamming in the recirculation channel. They involve therefore a relatively high probability of jamming. 
         [0016]    Another disadvantage is that in both proposals the unlocking of the output linear element is performed at the level of the nut of the screw. Then, after releasing the output linear element, a parallel actuator (in the above-mentioned case of a flight control surface actuated by a set of parallel actuators) should drag both the screw and the nut. This means that the actuator shall be designed leaving free the volume swept by the screw and the nut along the whole run of the parallel actuator. In addition, the inertia to be dragged by the parallel actuator is the inertia of the screw and the nut. 
         [0017]    US Patent Application US2007/295125 discloses a linear actuator comprising:
       a rotatory input shaft driven by an electric motor;   an output shaft having a helical threaded zone in its external surface at its inner end;   a first roller gear configured to rotate with respect to it axis when the input shaft rotates;   a plurality of second roller gears configured to engage with the first roller gear and with the output shaft in its helical threaded zone so that the rotation of the first roller gear is firstly transmitted to the second roller gears, which rotate with respect to their axis, and secondly converted in a linear movement of the output shaft.       
 
       SUMMARY OF THE INVENTION 
       [0022]    An object of the present invention is to provide a linear actuator driven by an electric motor with a lower probability of jamming than those known linear actuators with re-circulating elements. 
         [0023]    Another object of the present invention is to provide a linear actuator allowing decoupling the output shaft from other mechanical components of the transmission chain. 
         [0024]    These and another objects are met by a two-stage linear actuator, the first stage comprising a rotary input shaft driven by an electric motor having a helical threaded zone in its external surface at its inner end and a plurality of first helical roller gears configured to engage with the rotary input shaft in its helical threaded zone for rotating together; the second stage comprising a plurality of second helical roller gears configured to engage with an output shaft having a helical threaded zone in its external surface at its inner end for converting the rotation of the second helical roller gears in a linear movement of the output shaft, the second helical roller gears being also configured to engage with the first helical roller gears for rotating together. 
         [0025]    Advantageously, the input shaft is a hollow shaft and the output shaft is placed in an inner conduit comprising the inside of the input shaft. 
         [0026]    Advantageously, the second helical roller gears have two threaded zones at two different levels: a first threaded zone for engaging with the first helical roller gears and a second threaded zone for engaging with the helical threaded zone of the output shaft. 
         [0027]    In an embodiment of the invention, the cooperating pairs of first and second helical roller gears are mounted in gear carriers in a pivoting manner with respect to the axis of the first helical roller gears so that they can hold the second helical roller gears in an engaging or in a disengaging position with respect to the output shaft. Therefore the linear actuator is provided with a means for releasing the output shaft in a jamming event or in an event where a dangerous degradation of a mechanical component is detected. 
         [0028]    The releasing system is implemented by the interaction of the gear carriers with a disk mounted rotatably on the output shaft for keeping the gear carriers with the second helical roller gears engaged or disengaged with respect to the output shaft. 
         [0029]    A particular field of application of the linear actuator of this invention is the actuation of aircraft components and particularly aircraft control surfaces. 
         [0030]    Other desirable features and advantages of the linear actuator according to this invention will become apparent from the subsequent detailed description of the invention and the appended claims, in relation with the enclosed drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a perspective view of a linear actuator according to an embodiment of this invention. 
           [0032]      FIG. 2  is a partial cross sectional view of a linear actuator according to an embodiment of this invention. 
           [0033]      FIG. 3  is a perspective view of the main components of a linear actuator according to an embodiment of this invention. 
           [0034]      FIGS. 4   a  and  4   b  are perspective views of the main components of a linear actuator according to an embodiment of this invention incorporating releasing means of the output shaft in, respectively an engaged and a disengaged position. 
           [0035]      FIG. 5  is a perspective view of the main components of a linear actuator according to an embodiment of this invention incorporating releasing means of the output shaft showing the means used for driving the releasing means. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0036]      FIG. 1  shows an overview of a linear actuator  10  for moving linearly an actuating member  30  according to the embodiment of the invention that will be now described. 
         [0037]    A set of linear actuators  10  can be used, for example, for actuating a control surface of an aircraft. 
         [0038]    The linear actuator  10  converts the rotatory motion of an input shaft located inside the casing  11 , which is driven by an electric motor  7  through a gearbox  9 , in a controlled linear movement of an output shaft  29  to which the actuating member  30  is connected. The linear actuator  10  could also be directly driven by the electric motor. 
         [0039]      FIGS. 2 and 3  show the main components of the linear actuator  10 : an input shaft  13 , an output shaft  29 , three first helical roller gears  21 ,  21 ′,  21 ″ and three second helical roller gears  23 ,  23 ′,  23 ″ (although in the  FIG. 3  only the first roller gears  21 ,  21 ′ and the second roller gears  23 ,  23 ′ are clearly shown, the corresponding numerical references for the first roller gears  21 ″ and for the second roller gear  23 ″ are also included in the  FIG. 3  and will used in this specification). 
         [0040]    The input shaft  13  is rotatably mounted on the casing  11  by means of a pair of contact bearings  16  axially preloaded to achieve the required stiffness. The input shaft is configured as a hollow cylinder with a helical threaded zone  15  at its inner end in its external surface. At its outer end the input shaft is connected to the gearbox  9 . 
         [0041]    The output shaft  29  is placed in a longitudinal conduit delimited by the input shaft  13  and a tubular housing  12  inside the casing  11  so that it can be displaced longitudinally along said conduit. The output shaft  29  has a helical threaded zone  18  in its outer surface. The length of the helical threaded zone  18  is the maximum length foreseen for the displacement of the output shaft  29 . At its 
         [0042]    The first helical roller gears  21 ,  21 ′,  21 ″ are arranged for engaging with the input shaft  13  in the helical threaded zone  15 . They are arranged tangentially with respect to the input shaft  13  so that the rotation of the input shaft  13  produces a rotation of the three roller gears  21 ,  21 ′,  21 ″ around their axis  22 ,  22 ′,  22 ″. 
         [0043]    The second helical roller gears  23 ,  23 ′,  23 ″ are arranged with their axis  24 ,  24 ′,  24 ″ parallel to the axis  22 ,  22 ′,  22 ″ of the first helical roller gears  21 ,  21 ′,  21 ″ for engaging, on the one side, with the first helical roller gears  21 ,  21 ′,  21 ″ and, on the other side, with the output shaft  29  in its helical threaded zone  18 . The rotation of the first helical roller gears  21 ,  21 ′,  21 ″ is transmitted to the second helical roller gears  23 ,  23 ′,  23 ″ and the rotation of the second helical roller gears  23 ,  23 ′,  23 ″ is converted in a linear movement of the output shaft  29 . The engagement of the second helical roller gears  23 ,  23 ′,  23 ″ with the first helical roller gears  21 ,  21 ′,  21 ″ is done in first helical threaded zones  25 ,  25 ′,  25 ′ and the engagement of the second helical roller gears  23 ,  23 ′,  23 ″ with the helical threaded zone  18  of the output shaft  29  is done in second helical threaded zones  27 ,  27 ′,  27 ″. 
         [0044]    Said first and second helical threaded zones  25 ,  25 ′,  25 ′;  27 ,  27 ′,  27 ″ are arranged at a different level for allowing the simultaneous engagement of the second helical roller gears  23 ,  23 ′,  23 ″ to the first helical roller gears  21 ,  21 ′,  21 ″ and to the output shaft  29 . 
         [0045]      FIGS. 4   a ,  4   b  and  4   c  show the main components of an arrangement of the first helical roller gears  21 ,  21 ′,  21 ″ and the second helical roller gears  23 ,  23 ′,  23 ″ that allows a full release of the output shaft  29  when any component of the linear actuator  10  fails. 
         [0046]    The first helical roller gears  21 ,  21 ′,  21 ″ and the second helical roller gears  23 ,  23 ′,  23 ″ are mounted by pairs  21 ,  23 ;  21 ′,  23 ′;  21 ″,  23 ″ in gear carriers  31 ,  31 ′,  31 ″ that allow positioning the second helical roller gears  23 ,  23 ′,  23 ″ in an engaged or in a disengaged position with respect to the output shaft  29  in cooperation with a disk  41  rotatably mounted on the output shaft  29 . 
         [0047]    The gear carriers  31 ,  31 ′,  31 ″ are mounted pivoting around the axis  22 ,  22 ′,  22 ″ of the first helical roller gears  21 ,  21 ′,  21 ″ (that are rotatably mounted on the casing  11 ) by means of a spring  39  (see  FIG. 2 ) and comprise protruding tabs  33 ,  33 ′,  33 ″ in their border in front of the disk  41 . 
         [0048]    The disk  41  comprises an axial extension having configured its border in front of the gear carriers  31 ,  31 ′,  31 ″ by a series of alternating protrusions  43 ,  43 ′,  43 ″ and recesses  45 ,  45 ′,  45 ″. 
         [0049]    When the gear carriers  31 ,  31 ,  31 ′ are mounted with their protruding tabs  33 ,  33 ′,  33 ′ in contact with protrusions  43 ,  43 ′,  43 ″ of the disk  41  (see  FIG. 4   a ) the gear carriers  31 ,  31 ′,  31 ″ are arranged in an engaged position according to the predefined preload and adjustment conditions of the second helical roller gears  23 ,  23 ′,  23 ″ with respect to the output shaft  29  minimizing or preventing any axial movement of it. 
         [0050]    When the disk  41  is rotated and the protruding tabs  33 ,  33 ′,  33 ′ of the gear carriers  31 ,  31 ′,  31  are positioned in front of recessions  45 ,  45 ′,  45 ″ of the disk  41 , the gear carriers  31 ,  31 ′,  31  are pivoted to a disengaged position (see  FIG. 4   b ) by means of the spring  39 . 
         [0051]    The disk  41  comprises, as rotating means, a ring gear  47  coupled to a worm drive  49  driven by a suitable driving device  51 , for example, an electric motor or a solenoid. Other types of driving elements for the ring gear  47 , like helical gears (held in position with a brake when they are not operated) can be considered. 
         [0052]    The linear actuator  10  further comprises control means connected to monitoring means for detecting a blockage or a degradation of any component for activating the driving device  51  when a need of releasing the output shaft  29  is detected by the monitoring means. 
         [0053]    Said monitoring means comprise as detecting means dedicated sensors (acceleration, force) integrated into the linear actuator, or means using the control variables of the linear actuator (electric current, voltage, speed, position), or a combination of both, and a digital diagnostic system that can assess in real-time the evolution of selected parameters (in the time domain or in the frequency domain) and compare them with their expected evolution in the event of a linear actuator free of defects. 
         [0054]    One advantage of the present invention is that the linear actuator has no re-circulating elements that involve a high probability of jamming 
         [0055]    Another advantage of the present invention is that the releasing mechanism acts over the output shaft. Therefore, after releasing the output shaft, a parallel actuator (in the above-mentioned case of a flight control surface actuated by a set of parallel actuators) should drag only the output shaft allowing a more compact design of the linear actuator and facilitating the operation of the parallel actuator. 
         [0056]    As is apparent from the foregoing specification, the invention is susceptible of being embodied with various alterations and modifications which may differ particularly from those that have been described in the preceding specification and description. It should be understood that I wish to embody within the scope of the patent warranted hereon all such modifications as reasonably and properly come within the scope of my contribution to the art.