Abstract:
Apparatus for providing mechanical feedback of a position of an actuator to a controller in a high vibration environment. The apparatus includes a member that contacts a cam surface of the actuator such that movement of the actuator and cam surface causes the member to move. The apparatus also includes a spring that exerts a force on the member toward the cam surface to maintain contact between the member and the cam surface. The apparatus also includes a damper that dampens motion of the member, thereby eliminating resonant movement of the spring and member caused by vibrations.

Description:
BACKGROUND 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to mechanical feedback actuators, and more specifically, to a spring and damper configuration for a scissor mechanism for a mechanical feedback actuator. 
         [0003]    2. Description of the Related Art 
         [0004]    Mechanical feedback actuators can be used in applications where control feedback needs to be provided even if electrical power is lost. For example, liquid fuel rocket engines are often mounted to a space craft by gimbals, which enable the engines to pivot and provide steering control to the space craft. In the event that electrical power is lost onboard the space craft, it may be desirable for the liquid engines to center themselves so the aircraft flies straight ahead. 
         [0005]    Mechanical feedback actuators can include a mechanical linkage between an actuator (e.g., a piston) and a controller for the actuator. When a first end of the mechanical linkage in contact with the actuator moves in response to the actuator moving, a second end of the mechanical linkage in contact with the controller can also move. The movement of the second end of the mechanical linkage can move a component of the controller to provide feedback for control of the actuator. For example, the controller may open a valve to send pressurized hydraulic fluid to displace an actuator by one inch. As the actuator reaches one inch of displacement, a corresponding motion of the second end of the mechanical linkage can exert a force that closes the valve, thereby stopping the actuator at one inch of displacement. 
         [0006]    To provide mechanical feedback, the mechanical linkage needs to maintain contact with the actuator. Springs are often used to provide a force that pushes the mechanical linkage into contact with a feedback surface (e.g., a cam surface) of the actuator. However, springs can be susceptible to resonance. More specifically, strong vibrations can cause the springs to vibrate at a resonant frequency, which could result in the force being applied to the mechanical linkage dropping such that the mechanical linkage loses contact with the actuator. In such instances, the mechanical linkage could transmit an erroneous actuator position to the controller. 
       SUMMARY 
       [0007]    According to an embodiment, a mechanical feedback actuator can include a movable actuator and a controller configured to control movement of the movable actuator. The movable actuator can include a cam surface that is movable with the movable actuator. The mechanical feedback actuator can also include a mechanical feedback linkage that is arranged in contact with the cam surface. The mechanical feedback linkage can move relative to the controller in response to movement of the movable actuator and cam surface. A position of the mechanical feedback linkage relative to the controller can indicate a position of the movable actuator to the controller. The mechanical feedback actuator can include at least one spring arranged in contact with the mechanical feedback linkage. The at least one spring can exert a biasing force on the mechanical feedback linkage toward the cam surface. The mechanical feedback actuator can include at least one damper arranged in contact with the mechanical feedback linkage to exert a damping force on the mechanical feedback linkage. 
         [0008]    According to an embodiment, a servo actuator can include a hydraulic actuator, a controller configured to output control signals, and an electrohydraulic servovalve in hydraulic communication with the hydraulic actuator. The electrohydraulic servovalve can be in communication with the controller, and the control signals to the servovalve direct hydraulic fluid to the hydraulic actuator to actuate the hydraulic actuator. The servo actuator can also include a mechanical feedback member arranged in contact with the hydraulic actuator. The mechanical feedback member moves relative to the electrohydraulic servovalve in response to movement of the hydraulic actuator. A position of the mechanical feedback member relative to the electrohydraulic servovalve can indicate a position of the hydraulic actuator to the electrohydraulic servovalve. The servo actuator can also include at least one spring arranged in contact with the mechanical feedback member to exert a biasing force on the mechanical feedback member toward the hydraulic actuator. The servo actuator can also include at least one damper arranged in contact with the mechanical feedback member to exert a damping force on the mechanical feedback member. 
         [0009]    According to an embodiment, a scissor linkage for providing mechanical feedback between an actuator and a controller can include a first elongate member that includes a first end, a second end, and a first pivot arranged between the first end and the second end. The scissor linkage can also include a second elongate member that includes a third end, a fourth end, and a second pivot arranged between the third end and the fourth end. The first pivot and the second pivot are coaxial with each other, and the first elongate member and the second elongate member pivot relative to each other about the respective pivots. The scissor linkage can include at least one spring arranged between the first elongate member and the second elongate member, wherein the at least one spring is arranged between the first end and the first pivot of the first elongate member and between the third end and the second pivot of the second elongate member, and wherein the at least one spring exerts a force to push the first end and the third end away from each other. The scissor linkage can also include at least one damper arranged between the first elongate member and the second elongate member, wherein the at least one damper is arranged between the first end and the first pivot of the first elongate member and between the third end and the second pivot of the second elongate member. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0010]      FIG. 1  is a schematic side view of a liquid fuel rocket engine; 
           [0011]      FIG. 2  is a schematic view of a hydromechanical servoactuator; 
           [0012]      FIG. 3A  is an illustration of the Space Shuttle; 
           [0013]      FIG. 3B  is an illustration of the Space Launch System; 
           [0014]      FIG. 4A  is a side view of a scissor linkage for use in a hydromechanical servoactuator, wherein the scissor linkage includes three springs and dampers shown in partial hidden view; and 
           [0015]      FIG. 4B  is a cross-sectional side view of a spring and damper for the scissor linkage of  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION 
       [0016]      FIG. 1  is a schematic view of a liquid rocket engine  102 . The engine  102  includes various mechanisms  108  (e.g., pumps and the like) that feed liquid fuel to a combustion chamber and nozzle  104 . The combusted fuel escapes from the engine  102  through the outlet  106  of the nozzle  104 . The engine  102  is connected to a frame  110  of a rocket by a gimbal and actuators  112  and  114 . The actuators  112  and  114  enable the engine  102  to pivot about two axes on the gimbal relative to the frame  110 . The first actuator  112  includes a piston  120  that can telescopically move relative to a cylinder  124 . The actuator  112  is connected to the engine  102  by a first pivot  118  and to the frame  110  by a second pivot  116 . Thus, movement of the piston  120  relative to the cylinder  124  causes the engine  102  to pivot about a first axis relative to the frame  102 . The second actuator  114  includes a piston  126  second telescopically move relative to a cylinder  128 . The actuator  114  is connected to the engine  102  by a first pivot  130  and the frame  110  by a second pivot  122 . Thus, movement of the piston  126  relative to the cylinder  128  causes the engine  102  to pivot about a second axis (perpendicular to the first axis) relative to the frame  110 . 
         [0017]    In environments in which high reliability is important, an actuator that can provide mechanical feedback for control may be preferred over a feedback system that relies on electrical power (e.g., that uses sensors to detect position). For example, the liquid fuel rocket engines may use actuators with mechanical feedback that enable the engines to center themselves in the event of a loss of electrical power.  FIG. 2  is a schematic illustration of a mechanical feedback actuator  200  that may be used on a liquid fuel rocket engine. The mechanical feedback actuator  200  includes a piston  210  inside a cylinder  208 . The piston  210  is connected to a connecting rod  206 , which is connected to a first pivot  202 . A second pivot  204  can be connected to the cylinder  208 . The piston  210  (and connecting rod  206 ) can move telescopically relative to the cylinder  208  by selectively pumping hydraulic fluid (or the like) into and out of chambers  212  and  214  in the cylinder. For example, to move the piston  210  and connecting rod  206  in the direction of arrow D, hydraulic fluid can be pumped into chamber  212  and out of chamber  214 . 
         [0018]    The pumping of hydraulic fluid into the chambers  212  and  214  of the cylinder  208  is controlled by an actuator control  220  that includes a power valve  232 . The power valve  232  can slide in the direction of arrow G (or in the opposite direction) to selectively enable hydraulic pressure source P to be in communication with chamber  212  or chamber  214 . Similarly, movement of the power valve  232  causes the other chamber  212  and  214  to be in communication with a hydraulic pressure return R. Movement of the power valve  232  is controlled by one or more servo valves  222 . Multiple servo valves  222  can be used to provide redundancy for control of the power valve  232 . 
         [0019]    Under normal operation, operation of each servo valve  222  is controlled by an electrical signal. Each servo valve  222  can include a torque motor  224 . An electrical current can be applied to cause an armature  226  in the torque motor  224  to twist relative to a magnet, as indicated by arrow A (or in the opposite direction). Twisting of the armature  226  causes a flexure sleeve  228  to shift laterally in the direction of arrow B (or in the opposite direction depending on the direction of current flow). Lateral shifts of the flexure sleeve  228  open valves  230 , which provide communication between the hydraulic pressure source P and hydraulic pressure return R and the servo valve  222 . The servo valve  222  can also move in the direction of arrow B (or in the opposite direction) to provide hydraulic pressure to faces of the power valve  232  to cause the power valve  232  to move the direction of arrow G. 
         [0020]    The mechanical feedback actuator  200  can provide mechanical feedback to the actuator controller  220 . The piston  210  can be coupled to an internal conical cam  260  that includes an inward-facing conical surface  262 . The conical cam  260  is movable (in the direction of arrow H) with the piston  210  relative to the cylinder  208 . A scissor linkage  242  can be arranged with a first end within the conical cam  260 . The scissor linkage  242  can include a first elongate member  246  and a second elongate member  248 . Rollers  250  and  252  on the first ends of the first elongate member  246  and the second elongate member  248 , respectively, of the scissor linkage  242  can enable the conical cam  260  to translate relative to first end of the scissor linkage  242 . Springs  254  push apart the first ends of the first elongate member  246  and the second elongate member  248 . A second end of the second elongate member  248  can be pivot about an anchor (e.g., anchored relative to the second pivot  204 ). A second end of the first elongate member  246  can be connected to a first feedback link  258 . 
         [0021]    As the piston  210  and conical cam  260  move relative to the scissor linkage  242 , the first ends of the first elongate member  246  and the second elongate member  248  will move toward or away from each other in the direction of arrow E. The second ends of the first elongate member  246  and the second elongate member  248  will move in an opposite direction. For example, if the piston  210  and the conical cam  260  move in the direction of arrows D and H, then the first ends of the first elongate member  246  and the second elongate member  248  will move away from each other in the direction of arrow E. At the same time, the second ends of the first elongate member  246  and second elongate member  248  will move toward each other. As discussed above, the second end of the second elongate member  248  can be fixed in place by anchor  256 . Put differently, the second end of the second elongate member  248  can pivot about the anchor  256 , but cannot translate relative to the anchor  256 . Thus, any movement between the second ends of the first elongate member  246  and the second elongate member  248  is transmitted to the first feedback link  258 . Continuing the example above, movement of the second ends of the first elongate member  246  and the second elongate member  248  toward one another results in the feedback link moving in the direction of arrow F. 
         [0022]    Movement of the first feedback link  258  can be communicated to a second feedback link  240 . The second feedback link  240  can be pivotably connected to anchors  272 , and movement of the second feedback link  240  can thereby be transmitted to feedback rods  265 . The feedback rods  265  can be connected to springs  264 , which can push on a feedback wire  266  of each servo valve  222 . An additional spring  268  can be connected to a fixed anchor  270 . The additional spring  268  can provide a biasing force that tends to move the feedback rods  265  toward a centered position. The feedback wire  266  can be connected to the flexure sleeve  228 . As discussed above, a current can be applied to an armature  226  of each servo valve  222  to cause the armature  226  to twist in the direction of arrow A. In various embodiments, a fixed amount of current or voltage can be applied to the armature  226  to results in a certain deflection (and ultimately movement of the piston  210 ). For example, one volt applied to the armature  226  may result in one inch of displacement of the piston  210  (from a centered position), two volts applied to the armature  226  may result in two inches of displacement of the piston  210 , etc. The springs  264  apply a force to the feedback wire  266  and ultimately to the flexure sleeves  228  that can cancel out the electromagnetic force acting on the armature  226 . Continuing the example, as the piston  210  achieves a 1 inch displacement, the resulting movements of the scissor linkage  242 , the first feedback link  258 , and the second feedback link  240  results in movement of the springs  264  and spring forces that cancel out electromagnetic forces from the armature  226  acting on the flexure sleeves  228 . As a result, the servo valves  222  will close, thereby stopping the flow of hydraulic fluid to and from the chambers  212  in  214  of the cylinder  208 . 
         [0023]    When the electrical signal that deflected the piston  210  is removed from the armatures  226  (e.g. when a master controller wants to center the piston  210  or if the controller loses power), the springs  264  will push the feedback wires  266  and the flexure sleeves  228  in an opposite direction (in the direction of arrow C), causing hydraulic fluid to flow in an opposite direction to move the piston  210  back to a centered position. 
         [0024]    As discussed above, the springs  254  push the first elongate member  246  and the second elongate member  248  outwardly such that the rollers  250  and  252  remain in contact with the conical surface  262  of the conical cam  260 . Referring now to  FIGS. 3A and 3B , in a relatively low-vibration environment, the springs  254  may be sufficient to provide contact between the rollers  250  and  252  and the conical surface  262  of the conical cam  260 . For example,  FIG. 3A  is a front view of the space shuttle  300  configured for liftoff. The space shuttle  300  includes three liquid rocket engines, similar to the engine  102  shown in  FIG. 1 , and two solid rocket boosters  304 . As can be seen in  FIG. 3A , the liquid fuel engines  302  of the space shuttle  300  are arranged significantly higher than the solid rocket booster engines  304 . As a result, the liquid fuel engines  302  are subject to a relatively small amount of vibration produced by exhaust gases leaving the solid rocket booster engines  304 . In other applications, such liquid fuel engines may be exposed to higher levels of vibration. For example,  FIG. 3B  illustrates the Space Launch System (SLS)  310  being developed by the Boeing Corporation. In the SLS  310 , the liquid fuel engines  302 ′ and solid rocket booster engines  304 ′ are aligned with one another. As a result, the liquid fuel engines  302 ′ may be subject to significantly higher levels of vibration from the solid rocket boosters  304 ′. Such increased levels of vibration may induce harmonic vibration in the springs  254  of the scissor linkage  242 . Such harmonic vibrations may cause the rollers  250  and  252  of the scissor linkage  242  to lose contact with the conical surface  262  of the conical cam  260 . As a result, the mechanical feedback actuator  200  would not receive feedback for control of the piston  210 , which could result in control excursions of the engine  102 . 
         [0025]      FIGS. 4A and 4B  illustrate an embodiment of a scissor linkage  400  for use in a high vibration environment, such as the environment for the liquid fuel engines  302 ′ for the SLS  310 . The scissor linkage includes a first elongate member  402  and a second elongate member  404 , which are pivotable relative to one another about a pivot  406 . The first elongate member  402  includes a roller  408  that can interact with the conical cam  260 . Similarly, the second elongate member  404  includes a roller  408  that can interact with the conical cam  260 . The first elongate member  402  and the second elongate member  404  can define an internal volume  410  that can house one or more spring/damper units  414 . The internal volume  410  can include recesses  412  that hold ends of the spring/damper units  414 . 
         [0026]      FIG. 4B  illustrates a partial cross-sectional view of a spring/damper unit  414  for use with the scissor linkage  400 . The spring/damper  414  can include a first body  416  and a second body  418 . The first body  416  can include an end  422  that can interface with a recess  412  in the first elongate member  402  or the second elongate member  404 . Similarly the second body  418  can include an end  428  that can interface with a recess  412  in the first elongate member  402  were the second elongate member  404 . The first body  416  can include a lip  424  and a seat  426  and the second body can include a lip  430  and a seat  432 . The spring  420  can rest against and be captured by the seats  426  and  432 . The connecting rod  434  can extend from the first body  416  and terminate with a piston  436 . The piston  436  and a portion of the connecting rod  434  can be arranged in the second body  418 . The second body  418  can define a cavity  438  and  440  in which the piston  436  can move. The cavity  438 ,  440  can be filled with a fluid (e.g., a damping oil) that resists movement of the piston  436 . The piston  436  can include one or more orifices  442  through which the damping fluid can pass as the piston moves within the cavity  438  and  440 . For example,  FIG. 4B  illustrates the orifice as an annular orifice between the piston  436  and walls of the cavity  438  and  440 . 
         [0027]    In one embodiment, the spring  420  can have an outer diameter of one half of an inch and the wire diameter can be 0.047 inches. The spring  420  can have a free length of 1.125 inches and, when installed between the seats  426  and  432 , and installed length of 1 inch. The spring rate for the spring  420  can be 7.46 pounds per inch. In various other embodiments, the spring  420  can have different dimensions and/or spring rates. 
         [0028]    In one embodiment, the piston  436  can have a diameter of 0.1875 inches. The piston  436  can define two apertures, each aperture having a diameter of 0.03125 inches. The piston  436  can have a total stroke in the cavities  438  and  440  of 1.12 inches. The cavities  438  and  440  can be filled with an 80 weight, silicon-based oil. The resulting damper can have a damping coefficient of 5.345 Lbf-second/inch. In various other embodiments, the damping coefficient can be between 5.3 Lbf-second/inch and 5.4 Lbf-second/inch. In various other embodiments, the damping coefficient can be between 5 Lbf-second/inch and 6 Lbf-second/inch. In various other embodiments, the damper can have different dimensions and/or damping coefficients. 
         [0029]    The combined spring/damper can be critically damped (i.e., have a damping ratio of 1), overdamped (i.e., have a damping ratio of greater than 1), or underdamped (i.e., have a damping ratio of less than 1). In various embodiments, the spring rate and damping coefficient can be chosen such that the damping ratio is as close to 1 as possible. 
         [0030]    The damper can dampen any resonant vibrations in the springs, thereby preventing the scissor linkage  400  from losing contact with the conical cam  260  in a high vibration environment. In the embodiment shown in  FIG. 4A , the dampers are co-located with the springs  420 . In various other embodiments, the dampers can be located next to (i.e., side-by-side with) the spring  420 . Also, a scissor linkage  400  can include any number of springs and dampers. For example, in certain embodiments, a scissor linkage may include a single spring and a single damper, two springs and two dampers, or other numbers of springs and dampers. 
         [0031]    Dampers can also be incorporated into other springs in such a mechanical feedback actuator, such as actuator  200  shown in  FIG. 2 . For example, dampers could be incorporated into the springs  264  and  268  in the feedback rods  265  to dampen any resonant vibrations of those springs. 
         [0032]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein. 
         [0033]    In the following, reference is made to embodiments presented in this disclosure. However, the scope of the present disclosure is not limited to specific described embodiments. Instead, any combination of the following features and elements, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the following aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s). Likewise, reference to “the invention” shall not be construed as a generalization of any inventive subject matter disclosed herein and shall not be considered to be an element or limitation of the appended claims except where explicitly recited in a claim(s). 
         [0034]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.