Abstract:
A novel actuator contains a housing having a first end, a second end, and an inner wall. A piston rod assembly is contained within the housing, wherein the assembly contains a piston rod, a first end cap fixed at one end of the piston rod, and a second end cap fixed at an opposing end of the piston rod. An energy management device is constrained about an outer diameter of the first end cap, wherein upon actuation of the actuator, the energy management device may under predetermined conditions be deformed upon movement of said piston rod.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application Ser. No. 61/884,967 filed on Sep. 30, 2013. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to an improved linear actuator that may be used to elevate an automotive hood or bonnet upon a pedestrian impact with the front of the vehicle. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to vehicle occupant protection systems or other safety systems employing linear actuators to elevate a vehicle surface such as a hood, in the event of collision or impact with a pedestrian. Exemplary linear actuators typically employ pyrotechnic means to activate a piston within an actuator. With regard to certain events, such as a bonfire event, or with regard to bonfire testing or mounted dry fire testing, the energy created at that time must be managed. Stated another way, in certain environments, pyrotechnic actuators may be activated under “no-load” conditions wherein an associated piston rod may be essentially unrestrained in the absence of a load from a hood of a vehicle or other load for example. When assembling various constituents of typical pyrotechnic actuators together, stress risers caused by manufacturing processes such as crimping, welding, press-fitting, or any other suitable assembly process may be an area(s) of weakness when exposed to physical shock upon actuation of the actuator. It would be an improvement to provide an improvement to the typical pyrotechnically actuated pistons, one that facilitates management of the energy attendant to a bonfire event for example. 
     SUMMARY OF THE INVENTION 
     The above-referenced concerns and others are reconciled by an actuator containing a housing having a first end, a second end, and a first housing inner wall. A piston rod assembly is contained within the housing wherein the piston rod assembly contains a piston rod having a piston rod third end proximate to the housing first end, a piston rod fourth end proximate to the housing second end, and a second or piston rod inner wall. A first piston end cap is fixed within the third end and includes an end cap outer wall. A second piston end cap may be fixed within the fourth end and also includes an end cap outer wall. The first piston end outer wall has a first portion with a relatively greater outer diameter, and a second intermediate portion with a relatively smaller outer diameter as compared to the outer diameter of the first portion, wherein the second portion is slidably engageable with the inner diameter of the second or piston rod inner wall. A plenum may be formed between the second portion outer diameter and the piston rod inner wall, and between a crimp formed from crimping the piston rod to the first piston end cap and a third portion. A crushable sleeve or energy management device may be positioned in the plenum, or about the second portion and within the second or piston rod inner wall, wherein upon actuation of the actuator, the resultant movement of the piston rod crushes the sleeve or stated another way, deforms the energy management device. Notably, if the actuator is under a load, such as the hood of the vehicle, the sleeve would not crush primarily because the piston rod would not extend to its full non-load potential. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an actuator in a pre-actuation state, in accordance with the invention, wherein the view is taken along the length of the actuator. 
         FIG. 2  is a cross-sectional view of an actuator in a non-load post-actuation state, in accordance with the invention, wherein the view is taken along the length of the actuator. 
         FIG. 3  is a cross-sectional view of an actuator in a post actuation state under a load, wherein the view is taken along the length of the actuator. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the Figures, a linear actuator  10  contains a housing  12 . The housing  12  contains a first end  14  and a second end  16 . An initiator or igniter  18  is fitted within the first end  14  and seals the first end  14  in a known manner. A piston or piston rod assembly  20  is positioned adjacent the igniter  18  prior to actuation of the actuator  10 , wherein the piston  20  contains a first end  22  and a second end  24 . A piston rod  26  connects the first piston end  22  with the second piston end  24 . As shown in the figures, a first crimp  22   a  fixes the piston rod  26  to piston end  22 . As indicated above, the first crimp  22   a  exemplifies any similar stress riser caused by manufacturing process such as welding, crimping, press-fitting and so forth. As shown in the drawings, the piston end  22  has a first portion  22   b  (proximate to the igniter  18 ) having a relatively wider diameter, and a third opposite portion  22   d  having a relatively smaller diameter. A second or intermediate portion  22   c  of piston end  22  has an even relatively smaller outer diameter as compared to first portion  22   b  and third portion  22   d . A seal  27  is fixed about the wider portion  22   b  of the piston end  22  thereby preventing blow-by of the ignition products into an annular region  32  defined between the housing  12  and the piston rod  26 . As a result, the piston efficacy is assured. 
     In accordance with the present invention, an energy management device such as a crushable sleeve  34  is positioned about the second or narrower intermediate portion  22   c , thereby covering at least part of the portion  22   c . As the piston rod  26  is crimped about the piston intermediate or second portion  22   c  to form crimp  22   a , the sleeve  34  is preferably slidably and/or loosely retained about the outer diameter of second portion  22   c , thereby permitting the slidable movement of the piston rod  26  along portion  22   c . A stop portion  22   d  is formed at an end of the piston end  22  and constrains the crushable sleeve  34  to a position located about portion  22   c , between crimp  22   a  and portion  22   d . As shown in  FIG. 1 , the crimp  22   a  attaches the piston rod about the outer diameter of narrower portion  22   c  thereby constraining the movement of sleeve  34  along a longitudinal axis Z. As also shown in FIG.  1 , a plenum  28  may be formed between the outer diameter of portion  22   c , the crimp  22   a , third portion  22   d , and the inner wall  26   a  of the piston rod  26 . Accordingly, sleeve  34  may then reside within the plenum  28  prior to actuation of the actuator  10 . 
     As with piston end  22 , the piston end  24  has a third portion  24   b  having a relatively wider diameter and a fourth portion  24   d  having a relatively smaller diameter. As the piston rod  26  is crimped about the piston portion  24   c  to form crimp  24   a , the piston end  24  is secured to the rod  26 . An annular collar  36  is welded or otherwise fixed about the exterior of the housing  12  and a portion thereof  36   a  radially and inwardly extends adjacent to piston end  24  and thereby positions and constrains the housing  12  and the piston rod  26  also adjacent to the second piston end  24 . The annular collar  36  also provides a stop for the piston rod  20  when first end  22  collides with the annular collar  36  upon activation of the actuator  10 . 
     As shown in  FIG. 1 , the actuator  10  is illustrated in a pre-activated state. Notably, the crush sleeve  34  is represented with a certain length L 1  prior to activation of the actuator  10 . Furthermore, the piston rod  26  is completely encased within the housing  12 . The piston end  24  is preferably flushly seated against the second end  16  of the housing  12  until piston  20  is propelled forward by ignition forces from igniter  18 , upon activation of the actuator  10 . 
     As shown in  FIG. 2 , the actuator  10  is illustrated in a non-load activated state. The term “non-load” is meant to convey a condition where the actuator  10  is activated without any load such as the weight of the hood and perhaps the weight of a pedestrian. In essence, the use of the crush sleeve  34  provides a safety dampening mechanism for those events or predetermined conditions wherein the actuator may be activated without a load. Notably, the crush sleeve  34  is represented with a shortened length L 2  whereby the sleeve  34  is compressed as the piston rod  26  traverses through housing  12  and out of end  16 . As also shown, upon activation of actuator  10 , the piston rod  26  has slidably traversed along portion  22   c  as the sleeve  34  is compressed to manage the energy of the piston rod  26  as it is propelled through and out of housing  12 . 
     As shown in  FIG. 3 , on the other hand, the actuator  10  is illustrated under a load in an activated state. The term “load” is meant to convey a condition where the actuator  10  is activated with the weight of the hood and perhaps the weight of a pedestrian as a load. In this “loaded” state, it can be seen that the crush sleeve  34  is essentially not crushed because the dampening mechanism is in fact the load that would occur when the actuator  10  is activated under normal operating conditions wherein a load is exerted against the movement of the piston rod  26  at some point as it slidably traverses through the housing  12 . Accordingly, the crush sleeve  34  would not typically function as a dampener during normal “load” conditions of an activated actuator  10 , such as when collision between a pedestrian and the vehicle is imminently indicated. 
     In operation, the present linear actuator  10  is activated when the igniter  18  receives a signal from a vehicle computer algorithm that responds to impact, deceleration, or other known appropriate sensor. As the pedestrian makes contact with the associated vehicle and/or vehicle hood, the algorithm senses the impact and signals the igniter  18  to activate. Upon activation, heat and pressure products from the igniter  18  propel the piston  20  forward or upward across the length of the body  12  to the second end  16 . As the piston  20  is propelled within and across the length of the housing  12 , the second piston end  24 , and piston portion  24   b , larger in diameter than the rest of the piston end  24 , functions to elevate or raise the hood to mitigate the harm or injury to the pedestrian in contact therewith. 
     It will be appreciated that the present actuator may be largely formed or manufactured as known in the art. For example, U.S. Pat. No. 6,568,184 exemplifies a pyrotechnic actuator and generally teaches the basic structure of the present invention, and is herein incorporated by reference in its entirety. Alternatively, U.S. Pat. No. 8,656,716 also incorporated by reference in its entirety exemplifies a hybrid (that is pyrotechnic and pressurized actuator) or a pressurized gas actuator (one containing a store of pressurized gas). The various constituents such as the housing and piston rod may be drawn or extruded, or otherwise formed as known in the art. The various constituents may be metallic, polymeric, or a combination of both, or made from any other tough and durable material useful in the present context. The igniter  18  may be formed as known in the art and may be purchased from any known supplier of igniters. The piston ends  22  and  24  may be stamped, extruded, or otherwise formed as will be apparent in the art. Notably, in accordance with the present invention, the piston end  22  combined with the sleeve  34  represents a solution to the concern of energy management, namely by the slidable engagement of the piston rod  26  across the narrowed portion  22   c . As shown in  FIG. 2 , the piston rod  26  actually departs or separates from the widened and first portion  22   b  as the piston rod assembly  20  is “stretched” to compress the energy management device or sleeve  34 . Accordingly, it will be appreciated that the piston rod assembly  20  has a relatively shorter or smaller pre-actuation length L 3 . Upon actuation of the actuator  10 , the piston rod assembly  20  attains a relatively longer or larger post-actuation length L 4 , as the piston rod  26  slidably engages the outer diameter of portion  22   c  to crush, compact, and/or deform energy management device or sleeve  34  and thereby “extend” the length of the assembly  20 . Not only does the present configuration as described above result in efficient energy management of the piston rod assembly  20  during a “non-load” firing event, but it also presents a packaging advantage. In essence, the package size of the actuator  10  is smaller prior to activation of the actuator  10 . 
     It should further be understood that the preceding is merely a detailed description of various embodiments of this invention and that numerous changes to the disclosed embodiments can be made in accordance with the disclosure herein without departing from the scope of the invention. The preceding description, therefore, is not meant to limit the scope of the invention. Rather, the scope of the invention is to be determined by the various equivalents as would be appreciated by those of ordinary skill in the art.