Patent Publication Number: US-10760657-B2

Title: Apparatus utilizing planetary gearset coupled to a constant torsion spring

Description:
This application is a Continuation of pending U.S. application Ser. No. 14/847,317, filed Sep. 8, 2015, now U.S. Pat. No. 9,791,027, entitled APPARATUS UTILIZING PLANETARY GEARSET COUPLED TO A CONSTANT TORSION SPRING, which is incorporated herein by reference. 
    
    
     SUMMARY 
     The present disclosure is directed to an apparatus utilizing a planetary gearset coupled to a constant torsion spring. In one embodiment, a gearbox includes a planetary gearset, an input shaft coupled to a sun gear of the planetary gearset, and an output shaft coupled to planet gears of the planetary gearset via a carrier. A constant torsion spring is coupled to a ring gear of the planetary gearset. The constant torsion spring is capable of preventing the ring gear from moving when a torque at the output shaft is below a threshold. The ring gear winds the constant torsion spring in response to the torque exceeding the threshold. 
     These and other features and aspects of various embodiments may be understood in view of the following detailed discussion and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The discussion below makes reference to the following figures, wherein the same reference number may be used to identify the similar/same component in multiple figures. 
         FIG. 1  is simplified perspective view of an output-torque-limiting gearbox according to an example embodiment; 
         FIG. 2  is a graph showing operation of an output-torque-limiting gearbox according to an example embodiment; 
         FIG. 3  is a perspective view of an output-torque-limiting gearbox according to an example embodiment; 
         FIG. 4  is a perspective view of a return-to-home-position apparatus according to another example embodiment; 
         FIG. 5  is a schematic view of a return-to-home-position apparatus according to another example embodiment; 
         FIGS. 6 and 7  are side and top views of an input shaft holding device of a return-to-home-position apparatus according to an example embodiment; and 
         FIGS. 8 and 9  are flowcharts illustrating methods according to example embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure generally relates to apparatuses utilizing planetary gearsets and constant torsion springs. A planetary gearset includes a sun gear located centrally within a ring gear. A set of planet gears couples the ring gear to the sun gear, and a carrier fixes the planet gears with respect to one another. By fixing any one of the carrier, the sun gear, and the ring gear, a gear ratio is set for the other two, and this gear ratio is different depending on which is fixed. In other configurations, all three of the carrier, the sun gear, and the ring gear can rotate at the same time. 
     In embodiments described herein, an apparatus (e.g., a gearbox or return-to-home device) is configured with an energy storage member, e.g., a constant force or constant torsion spring, that stores energy under various conditions. One advantage of employing a constant torsion spring verses a conventional rotary windup spring is that it that force or torque applied by the device to the load remains constant throughout it operation allowing engineers to optimize the design of the application around a single known torque level. This eliminates the need to oversize tooling, etc. to compensate for springs that would require higher forces the more that the spring is wound. n one embodiment, a spring reacts to an excessive load condition on the output by storing excess energy, thereby limiting torque applied to the output. Thereafter, when the excess load condition is removed, the energy storage member can be released causing it to retract/rewind. The resetting of the energy storage mechanism maintains a positional relationship between the input and the output before the excessive load condition. In other embodiments, the energy storage member can store energy at initialization, and then back drive the output in response to some occurrence, e.g., a loss in power. This can be used to reset the driven system to a default or home position (e.g., driving a valve or other device to a desired power-loss position) either automatically or in response to a command. 
     The simplified diagram of  FIG. 1  shows an output-torque-limiting gear assembly  100  according to an example embodiment. The gear assembly  100  may be used in a gearbox or other mechanical power-transmission apparatus. The gear assembly  100  includes an input shaft  102  affixed to a sun gear  104  of a planetary gearset  106 . The sun gear  104  meshes with planet gears  110  of the planetary gearset  106 . An output shaft  108  is affixed the planet gears  110  via a carrier  112 . The planet gears  110  mesh with inner teeth of a ring gear  114  of the planetary gearset  106 . 
     The illustrated gear assembly is designed to operate in a fixed ring gear configuration under most loading conditions. In a fixed ring gear configuration, the output shaft  108  turns in the same direction as the input shaft  102  (as indicated by arrows  116  and  117 ) due to the interaction between the sun gear  104  and the planet gears  110 . The fixed ring gear configuration with the input tied to the sun gear  104  results in reduction gearing, causing the output shaft  108  to move slower than the output shaft  102 . 
     While the illustrated arrangement has been described as operating in a fixed ring gear configuration under most load conditions, the ring gear  114  can move in some instances. In this arrangement, outer teeth of the ring gear  114  are meshed with an optional idler gear  118 , which meshes with a spring shaft gear  120 . The spring shaft gear  120  is affixed to a shaft  122 , the shaft  122  being affixed to part of a constant torsion spring assembly  124 . A constant torsion spring generally provides an approximately constant resistance to torsion. This is different than other torsional springs (e.g., helical springs) which provide a reactionary moment τ that is proportional to the amount of rotation θ, e.g., τ=−κθ, where κ is the spring constant. It will be understood that a constant force mechanism may also be used to form a constant torsion spring, e.g., by affixing a constant force mechanism (e.g., weight, hydraulic damper) to a cable that wraps around a spool. 
     The illustrated constant torsion spring assembly  124  includes spools  124   a - b  about which are wrapped a metal band  124   c . The metal band  124   c , via its bending action, imparts an approximately constant torque to a parallel shaft  122  and thus the ring gear, through its rotational range of movement between the two spools  124   a ,  124   b . When wound, the constant torsion spring assembly  124  stores energy that is later released when the constant torsion spring assembly  124  is unwound. 
     The spring assembly  124  and associated gearing  118 ,  120  act to prevent overloading the output shaft  108 . A motor driving the input shaft  102  may be able to exert a moment on the output shaft  108  that exceeds some desired limit of the system designer. It may be desirable to provide a motor with excess torque capacity for purposes such as efficiency, reliability, response speed, etc., but at the same time to prevent output torque from exceeding some value, e.g., to prevent breakage of mechanical parts driven by the output shaft  108 . 
     During normal operation (e.g., within expected torque limits of the output shaft), the ring gear  114  will exert a moment on the idler gear  118  and spring shaft gear  120  as represented by dashed arrows  126 ,  128 . The spring assembly  124  has enough holding torque to keep the gears  114 ,  118 , and  120  from moving under normal operational loads. Once the load on the output shaft  108  exceeds a particular amount, this will cause the spring assembly  124  to start winding in the direction indicated by arrow  128 . The movement of the spring assembly  128  will cause the ring gear  114  start rotating and the output shaft  108  will stop rotating. Once this happens, the planetary gearset  106  is operating in a fixed planet mode, where the ring gear  114  is driven in an opposite direction from the sun gear  104 . In the fixed planet mode, the planet gear carrier  112  does not rotate, although the individual planet gears  110  will rotate. 
     The effect of the output torque limiter shown in  FIG. 1  is illustrated in the graph of  FIG. 2 . The graph in  FIG. 2  represents torque measured at the output shaft (curve  200 ) for a gear assembly with a torque-limiting feature as shown in  FIG. 1 . Up until position θ 1 , the output shaft is applying torque τ 1 , which is an operational torque within specification. At position θ 1 , an obstruction or the like inhibits the motion of the output shaft, causing the torque applied to the output shaft to rise to τ 2  at position θ 2 . At position θ 2 , the ring gear overcomes resistance to movement offered by the spring assembly, and two events occur. First, the output shaft stops turning, being subject to torque no greater than τ 2  thereafter. Second, torque applied to the ring gear of the planetary gear assembly by the input shaft causes the spring to wind and thereby store energy. Winding of the spring prevents the output torque from exceeding the threshold τ 2 , thus preventing an overload condition within the limits of the springs windup capability. 
     One notable feature of the torque limiter shown in  FIG. 1  is that after the constriction on the output shaft is removed (e.g., the torque falls below the threshold τ 2  after winding), the constant torsion spring unwinds and attempts to restore the positional relationship between the input shaft and the output shaft to a fixed position via an internal stop relative rotation limiter. This retains the desired positional relationship between the output shaft and input shaft that existed before the load was exceeded. This can be useful, for example, where the output shaft orientation (e.g., absolute rotation degrees) is measured at the input shaft and/or via gears or other components of the gearbox. For example, an encoder may be used to determine a number of rotations of the input shaft or other gearbox component, and this measurement may be used to infer a position of a component driven by the output shaft, e.g., a linear actuator. The illustrated spring assembly returns the input and output shafts to their positions relative to one another after the overload condition is removed. This allows the system to continue operation without having to reset the system, e.g., move to a starting position and zero a position sensor. 
     In  FIG. 3 , a perspective view illustrates an output-torque-limiting gearbox  300  according to another example embodiment. The gearbox components are shown in a housing  301 . Similar to the gear assembly in  FIG. 1 , the gearbox  300  includes an input shaft  302  affixed to a sun gear (not shown) of a planetary gearset  306 . An output shaft  308  is affixed to planet gears of the planetary gearset  306  via a carrier (not shown). The planet gears mesh with inner teeth of a ring gear  314  of the planetary gearset  306 . The outer teeth of the ring gear  314  mesh with an optional idler gear  318 , which meshes with a spring shaft gear  320 . The spring shaft gear  320  is affixed to a constant torsion spring assembly  324  via a shaft (not shown). The constant torsion spring assembly  324  includes spools  324   a - b  about which are wrapped a metal strip  324   c . Unlike the arrangement shown in  FIG. 1 , the spool  324   a  is located around the input shaft  302 , about which the spool  324   a  may spin freely. Otherwise, the arrangement shown in  FIG. 3  may operate similar to the arrangement in  FIG. 1 . 
     The previously shown arrangements can include additional features to alter the behavior of the constant torsion spring and associated gearing such that a driven device (e.g., a valve, door) can return to home position upon loss of power. An example of this is shown in  FIG. 4 , which is a perspective view of a return-to-home-position apparatus  400  according to an example embodiment. This apparatus  400  facilitates returning a driven apparatus to a home position of in the event of power loss. The apparatus  400  includes an input shaft  402  and output shaft  408  affixed to a planetary gearset  406  as previously described. The outer teeth of a ring gear  414  of the planetary gearset  406  mesh with an idler gear  418 , which meshes with a spring shaft gear  420 . The spring shaft gear  420  is affixed to spool  424   b  of a constant torsion spring assembly  424  via a shaft. Spool  424   a  of the constant torsion spring assembly  424  is able to spin freely around the output shaft  408 . 
     The apparatus  400  further includes a first brake  430  (e.g., spring-holding brake) coupled to the shaft that ties together the spring shaft gear  420  and spool  424   b  of the constant torsion spring assembly  424 . The first brake  430  is power-on engaged, and is always engaged during operation of the apparatus  400 , except during an initialization procedure. During the initialization procedure, the output shaft  408  is prevented from moving (e.g., by driving an output device to a limit of movement at one end of its range of motion/home position, etc.) while the input shaft  402  is turned via a motor. This causes the input shaft  402  to turn the ring gear  414  and thereby wind the spring assembly  424 . Once the spring assembly  424  is sufficiently wound (by turning a shaft to a final, second position), the first brake  430  is engaged, holding both the spring assembly  424  and the ring gear  414  in place. The first brake  430  remains engaged thereafter during operation. During operation, the apparatus  400  moves through a defined range of motion between the end of travel home position and the opposite end of travel, such as the open and closed positions of a valve. 
     A second brake  432  (e.g., drive brake) is shown around the input shaft  402 . The second brake  432  is power-off engaged, meaning that it disengages from the input shaft  402  when power is applied and engages the input shaft  402  when power is removed. Generally, the first brake  430  and second brake  432  facilitate returning the output shaft  408  to the home position. The releasing of the first brake  430  allows the spring assembly  324  to unwind, thereby driving the ring gear  414 , which drives the output shaft  408  to the home position. This may be used, for example, where the apparatus  400  is used to open and close a device (e.g., valve, door, lock, etc.), and it is desired to automatically close or open the device upon loss of power. The engagement of the second brake  432  at or about the same time as the first brake  430  releases prevents back-driving of the input shaft  402  while the spring assembly  424  turns the ring gear  414 . The illustrated brakes  430 ,  432  are electromechanical, although other types of brakes may be used (e.g., mechanical, hydraulic, pneumatic, etc.) 
     While the function of the apparatus  400  can be achieved with a conventional torsional spring, the use of a constant torsion spring  424  may be useful in some applications. For example, the torque of a conventional torsional spring decreases as it unwinds due to the τ=−κθ behavior. As such, in order to apply the needed closing torque τ at the end of travel, the initial torque τ max  (e.g., immediately after power is lost when the spring begins to unwind) will be larger than this, in particular τ max , =τ+κθ T , where θ T  represents the maximum rotational angle needed to return to home position. In contrast, the constant torsion spring  424  can apply a torque that is sufficient to return to home position, but not significantly higher, through the full range of travel. 
     To utilize the apparatus  400  shown in  FIG. 4 , a controller (not shown) can activate the brakes  430 ,  432  as appropriate. The controller may include a microprocessor, memory and input/output busses, the latter interfacing with sensors and power control circuitry. The controller may include instructions that activate and deactivate brakes  430 ,  432 , e.g., based on signals from position sensors, accelerometers, strain gauges, limit switches, encoders, etc., located within or outside of the apparatus  400 . Controllers and electrically controllable devices are readily available and the system and components as described herein can be implemented by those of ordinary skill in the art. 
     Although electrically-controlled components such as the brakes  430 ,  432  may be readily available, in some cases it may be desirable to limit the number of such components. In some cases, there may be improvements in cost, reliability, robustness, etc., by utilizing purely mechanical devices in place of electrically-controlled components. In  FIG. 5 , a simplified diagram illustrates an alternate return-to-home-position apparatus  500  that reduces the number of electrically controlled components compared to the embodiment shown in  FIG. 4 . The apparatus  500  includes an input shaft  502  and output shaft  508  affixed to a planetary gearset  506  as previously described. The outer teeth of a ring gear  514  of the planetary gearset  506  mesh with an idler gear  518 , which meshes with a spring shaft gear  520 . The spring shaft gear  520  is affixed to a spool  524   b  of constant torsion spring assembly  524  via a shaft  525 , spool  524   a  of the constant torsion spring assembly  524  being able to spin freely. 
     The apparatus  500  includes an electrically-controllable spring-holding brake  530  that is operated similarly to brake  430  shown in  FIG. 4  (e.g., engaged to hold the spring shaft during normal operation). The releasing of the spring assembly  524  drives the output shaft  508 , via the ring gear  514 , to a home position upon power loss. Instead of using a brake on the input shaft  502 , a mechanical coupling device  540  is used that keeps the input shaft  502  from moving in response the brake  430  releasing the spring assembly  524  upon power loss, thereby preventing the spring assembly  524  from back-driving the input shaft  502 . 
     In order to better understand the operation of the mechanical coupling device  540 , arrows are shown in  FIG. 5  indicating a rotation of various components during winding (solid arrows) and unwinding (dashed arrows). Table 1 below shows how the various components are operating during modes of the apparatus  500 , with CW and CCW respectively indicating clockwise and counterclockwise. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Mode 
                 State of spring shaft 525 
                 State of input shaft 502 
               
               
                   
               
             
            
               
                 Normal CW Output 
                 No Movement 
                 Powered CW 
               
               
                   
                 (accomplished w/brake 
                   
               
               
                   
                 530) 
                   
               
               
                 Normal CCW Output 
                 No Movement 
                 Powered CCW 
               
               
                   
                 (accomplished w/brake 
                   
               
               
                   
                 530) 
                   
               
               
                 Winding Spring 524 
                 Turning CW 
                 Powered CCW 
               
               
                 (after output is stopped 
                 (Because of ring gear  
                   
               
               
                 during initialization) 
                 514 turning) 
                   
               
               
                 Unwinding Spring 524 
                 CCW 
                 No Movement 
               
               
                 (Brake 530 is released) 
                 (Torque of spring 524 
                 (Needed to prevent  
               
               
                   
                 turning) 
                 back-driving) 
               
               
                   
               
            
           
         
       
     
     The states of the spring shaft and input shaft in this table also apply to the embodiment shown in  FIG. 4 , where the input shaft  402  is held in the “No Movement” state (last row and last column of Table 1) via the brake  432  while the spring  424  unwinds after a power loss. In the embodiment of  FIG. 5 , the mechanical coupling device  540  keeps the input shaft  502  from moving backwards (or back driving) when spring  524  unwinds. In  FIGS. 6 and 7 , respective side and top views show a mechanical coupling device  540   a  according to one embodiment. The mechanical coupling device  540   a  includes a pawl  642  coupled to the spring shaft  525  via a slip clutch  644 . A top part  644  of the slip clutch  644  is affixed to the pawl  642  and a bottom part  644   b  of the slip clutch  644  is affixed to the spring shaft  525 . The two parts  644   a ,  644   b  of the slip clutch  644  have a breakaway interface (e.g., friction plates) that cause the parts  644   a ,  644   b  to move together until a certain amount of torque is transmitted through the clutch  644 , causing the parts  644   a ,  644   b  to slip with respect to one another. 
     An end of the pawl  646  is engageable with a ratchet  646  that is affixed to the input shaft  502 . As seen in  FIG. 7 , a stop  702  restricts clockwise movement of the pawl  642 , which will cause the clutch  644  to slip when the spring is being wound at initialization. Counterclockwise movement of the spring shaft  525  due to the release of the brake and unwinding of the spring will cause the pawl  642  to engage the ratchet  646 , as indicated by the dashed outline in  FIG. 7 . 
     Before running in operational mode, an initialization procedure causes the spring assembly  524  to be wound, e.g., by stopping the output shaft  508  (e.g., turning it to a mechanical limit of an output device) while turning the input shaft  502  counterclockwise. Counterclockwise movement of the input shaft  502  turns the spring shaft  525  in a clockwise direction, resulting in the pawl  642  being in the disengaged position against the stop  702  shown in  FIG. 7 . Once the pawl  642  hits the stop  702 , the slip clutch  644  overruns, allowing the spring shaft  525  to continue turning. After the spring  542  is wound, the brake  530  then locks the shaft  525 , and the pawl  642  is held in place by the slip clutch  644 . With the pawl  642  out of the way of the ratchet, input and output shafts  502 ,  508  of the apparatus  500  can turn in both clockwise and counterclockwise directions during operation. 
     In response to loss of power, the brake  530  releases, causing the spring shaft  525  to be powered counterclockwise by the spring  542  and driving the ring gear  514  of the planetary gearset  506 . The counterclockwise movement of the spring shaft  525  moves the pawl  642  into the ratchet  646 , stopping the input shaft  502  from moving in a clockwise direction. The input shaft  502  will be driven in a clockwise direction by the planetary gearset  506  when driven by the spring  524 . The clockwise movement of the input shaft  502  and counterclockwise movement of the spring shaft  525  ensures positive engagement between the pawl  642  and ratchet  646 . 
     In  FIG. 8 , a flowchart illustrates a method according to an example embodiment. The method involves driving an input shaft coupled to a sun gear of the planetary gearset is driven, resulting in movement of an output shaft coupled to planet gears of the planetary gearset via a carrier. Movement of a ring gear of a planetary gearset is prevented via a constant torsion spring, e.g., until a threshold torsion is applied to the constant torsion spring via the ring gear. In response to an output torque of the output shaft beginning to exceed an output threshold, the constant torsion spring is wound via the ring gear to prevent the output torque from exceeding the threshold. 
     In  FIG. 9 , a flowchart illustrates a method according to an example embodiment. The method involves holding an output shaft while driving an input shaft during an initialization procedure to wind a constant torsion spring coupled to a ring gear of a planetary gearset. The input shaft is coupled to a sun gear of the planetary gearset and the output shaft is coupled to planet gears of the planetary gearset via a carrier. After the initialization procedure and during subsequent operation, the constant torsion spring is prevented from unwinding. In response to a loss of power, the constant torsion spring is released and the input shaft is prevented from turning. The releasing of the constant torsion spring and the preventing of the input shaft from turning moves the output shaft to a home position. 
     The foregoing description of the example embodiments has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. For example, the illustrated gearboxes and apparatuses are shown and described using input and output shafts, however any input or output means may be used to couple rotational power into and out of the gearbox. These input and/or output means may include plates, flanges, pulleys, flexible joints, gears, splined hole, etc. Further, while the illustrated planetary gearset and other gears are shown as spur gears, other gearing means may be used such as helical gears, bevel gears, screw gears, etc. Any or all features of the disclosed embodiments can be applied individually or in any combination are not meant to be limiting, but purely illustrative. It is intended that the scope of the invention be limited not with this detailed description, but rather determined by the claims appended hereto.