Patent Publication Number: US-11378169-B2

Title: Fluid rotary joint and method of using the same

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 15/612,578, filed on Jun. 2, 2017, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/345,425, filed Jun. 3, 2016, the contents of which are incorporated in their entireties herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under Contract No. N00014-15-P-1130 awarded by the Office of Naval Research. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to a strain wave actuator and, more particularly, to a fluid rotary joint using a strain wave actuator with an annular band instead of gears typically associated with a harmonic drive. 
     Description of Related Art 
     Directing attention to  FIG. 1 , a harmonic motor drive  10  typically utilizes three main components to reduce the speed of the input rotation of a wave generator and increase torque. Typical operation has a circular spline  20  fixed to the motor stator  30 , a wave generator  40  attached to the output of the motor rotor  50 , and the flex spline  70  as the output of a gearbox. The flex spline  70  has an oval shape where the major axis A of the oval is rotated by the wave generator  40 . It is commonly stated in harmonic drive trade literature that the difference in the number of teeth  22 , 72  between the outside  74  of the flex spline  70  and the inner surface  24  of the circular spline  20  generates the motion of the output. The flex spline  70  and circular spline  20  may have the same gear pitch and different circumferences so that the flex spline  70  must have fewer teeth  72  to mesh with the teeth  22  of the circular spline  20 . However, more fundamentally, it is the difference in the circumference of the flex spline  70  relative to the circular spline  20  that produces the gear reducer effect whether gear teeth are present or not. The gear ratio is a function of the difference in the circumference of the two gears  22 ,  72  and is entirely independent of the tooth size since the number of teeth in each gear is directly related to their pitch diameters. The teeth, therefore, could be made of infinitesimal size, or in fact there may be no teeth at all, with merely frictional contact engagement. The gear ratio will not be affected in the least by any such change in construction. The number of complete strain wave revolutions around the strain gear for one revolution of the output element is equal to the difference in pitch diameter of the driven element. This may also be presented in the following equation: Gear Ratio=(Ø Circular Spline)/(Ø Circular Spline−Ø Flex Spline). A traditional harmonic gearbox uses a mechanical gear wave generator to deform the flex spline into the circular spline. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a fluid rotary joint has a stator with a generally curved stator body and a flex spline with a flexible annular band disposed about and secured to the stator body. The annular band has an outer surface with an outer circumference and a high tensile strength in the direction of the circumference. The stator also has at least three radially extendable members disposed between the stator body and the annular band to deform the annular band away from the stator body. The rotary joint also has a generally cylindrical rotor surrounding the stator body, wherein the rotor has a wall with an inner surface having an inner circumference. The outer circumference of the annular band is less than the inner circumference of the rotor. A driver selectively expands the extendable members and brings the outer surface of the annular band of the stator into frictional driving engagement with the inner surface of the rotor for rotating the rotor. 
     Another embodiment is directed to a method for using the fluid rotary joint comprising the steps of:
         a) expanding at least one of the at least three radially extendable members at a pressure greater than the remaining members to deform the band against the inner surface of the rotor thereby providing friction between the band and the inner surface of the rotor; and   b) increasing pressure on an adjacent extendable member and relieving the pressure on the at least one extendable member to create relative motion between the annular band and the rotor.       

     These and other features and characteristics of a fluidic roll joint, as well as the methods of operation and functions of the related elements of structures and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only, and are not intended as a definition of the limits of the disclosure. As used in the specification and the claims, the singular form of “a”, “an”, and “the” include plural referents unless the context clearly dictates otherwise. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a prior art sketch showing the operation of a harmonic drive motor utilizing gears; 
         FIG. 2  shows the sequence (a)-(h) of inflations and deflations generating a clockwise wave pattern to create continuous rotation of the rotor in the opposite direction; 
         FIGS. 2A and 2B  are schematics depicting a fluidic harmonic actuator cell sequencing, from one pair of extendable members to two pairs being inflated, causing the rotor to turn slightly; 
         FIG. 3  is a cross-sectional view of a fluidic rotary joint according to the present disclosure; 
         FIG. 4  is a perspective view of a complete view of the cross-section illustration in  FIG. 3 ; 
         FIG. 5  is a perspective view of the fluidic rotary joint with a schematic of the associated hardware according to the present disclosure; 
         FIG. 6  is a perspective view of the fluidic rotary joint with a pneumatic drive ring assembly and circular spline removed; 
         FIG. 7  is a perspective view of the circular spline of the fluidic rotary joint with no friction treatment on the surface; 
         FIGS. 8A, 8B and 8C  show one extendable member in perspective, in top view and in cross-section, respectively; 
         FIG. 9  is a perspective view of a pneumatic drive ring with diaphragms installed; 
         FIGS. 10A and 10B  are perspective and side views of the annular band attached to the stator; 
         FIG. 11  is a perspective view of the stator body; 
         FIG. 12  is a side view of the annular band mounted within the stator body; 
         FIG. 13  is a schematic of the fluidic rotary joint with associated elements to operate the fluidic rotary joint; and 
         FIG. 14  is an expanded schematic of an overall system architecture plan according to the present disclosure. 
     
    
    
     DESCRIPTION OF THE DISCLOSURE 
     For purposes of the description hereinafter, the terms “upper’, “lower”, “right”, “left”, “vertical”, “horizontal”, “top”, “bottom”, “lateral”, “longitudinal”, and derivatives thereof, shall relate to the invention as it is oriented in the figures. However, it is to be understood that the invention may assume alternative variations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific systems and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary aspects of the disclosure. Hence, specific dimensions and other physical characteristics related to the aspects disclosed herein are not to be considered as limiting. 
     The present disclosure addresses the development of a lightweight fluidic rotary joint capable of continuous rotation. 
     The present disclosure provides a fluidic harmonic rotary joint capable of continuous motion, predictable small-step angles, and significant torque production. The demonstrated joint has a 0.22° step size enabling precise orientation control, and is capable of producing over 8 ft-lbs of torque. The design utilizes low-cost pneumatic actuators and an inexpensive friction material to transfer torque across the joint rather than the more typical, expensive, toothed surface. The low-cost design combined with the fluidic drive mechanism make this an improvement for producing roll motion for pneumatic or hydraulic robotic systems. 
     A fluidic rotary joint is a significant development for fluidic actuators because it has the ability to rotate with small precision angle steps continuously in either direction or hold static torque indefinitely in any orientation while energized only by a relatively low pressure fluidic source. Furthermore, the motion generated is rotation-only, eliminating the axial motion-coupling characteristic of some other fluidic torsional actuators. 
     Extensive analysis and experience has demonstrated the need for a torsional actuator at the end of a manipulator. The prior art acknowledges the need for torsional actuator capability for manipulators, but also concedes that many current actuators have not been torque tested, have limited ranges of motion, and couple axial motion with rotational motion. In one aspect, the fluidic rotary joint provided in the present disclosure is an actuator that may be made of lightweight materials and driven with non-proportional on-off valves for continuous rotation in either direction. In one aspect, the joint itself may not be inflatable, but rather, it may add continuous torque capability to an otherwise inflatable system. The concept for actuation is rooted in established strain wave gear reducer principles. However, there are two major points of departure for the fluidic rotary joint of the present disclosure that inspired the need to build hardware and demonstrate that the flex spline may be driven by multiple discrete pneumatic chambers instead of an oval shaped continuous wave generator and torque may be transmitted through friction instead of gear teeth. 
     The fluidic rotary joint of the present disclosure is a modification of the more typical motor driven harmonic drive that utilizes a toothed interface between the flex spline and the circular spline. The present disclosure takes advantage of the most fundamental principle of strain wave gearing to eliminate an expensive toothed interface and replace it with a friction/shear torque transfer interface. Along with removing the gear teeth, the present disclosure implements a simple fluidic rotational drive to remove the need for a motor at the joint. The hardware implementation of these improvements reduces cost, weight, and complexity. 
     The operation utilizes two adjacent sets of opposing diaphragms in order to press a rubber material attached to the flex spine into the circular spline. A design to actuate the harmonic drive was developed utilizing COTS pneumatic diaphragms to deform the flex spline as a departure from the traditional oval shaped wave generator. To generate rotational motion, one set out of two of the opposing diaphragms will remain pressurized as a pivot point while the pressurized adjacent set is vented and, simultaneously, the vented adjacent set is pressurized. The sequence is repeated to produce rotary motion in either the clockwise or counterclockwise direction, as shown in  FIGS. 2( a )-2( h ) . The joint will also resist static torques as the pressure is held constant in the set of opposing diaphragms. Unlike a typical harmonic drive, there is only one moving part in this design of the present disclosure. The wave motion required to generate the motion between the flex spline and the circular spline is generated by the diaphragms, which are fixed relative to the flex spline. With reference to  FIG. 2 , fluidic harmonic actuator cell sequencing is shown. 
     Overall,  FIG. 2A  illustrates a fluidic rotary joint  100  with a stator  110  having a generally curved stator body  114 . The stator  110  also has a flexible annular band  120  disposed about the stator body  114 . The annular band  120  has an outer surface  124  with an outer circumference C 1  and a high tensile strength in the direction of the circumference C 1 . The stator  110  further has a plurality of radially extendable members  130  disposed between the stator body  114  and the annular band  120  to deform the annular band  120  away from the stator body  114 .  FIG. 2A  illustrates eight radially extending members  130 . 
     A generally cylindrical rotor  140  surrounds the stator body  114  and has a wall  144  with an inner surface  146  having an inner circumference C 2 . The outer circumference C 1  of the annular band  120  is less than the inner circumference C 2  of the rotor  140 . As illustrated in  FIG. 2A , radially extendable members  130 A and  130 B are in an extended position whereby the annular band  120  contacts the inner surface  146  of the rotor  140  at contact regions R 1  and R 2 . As a result, a portion of the annular band  120  does not contact, or lightly contacts, the inner surface  146  of the rotor wall  144  in at least gap regions G 1 , G 2 . The gaps G 1 , G 2  are a function of the difference in the circumference C 1  of the outer surface  124  of the annular band  120  and C 2  of the inner surface  146  of the rotor  140 . Simply stated, by applying pressure by using the radially extendable members  130  and retracting and extending them in a pattern such as the sequences (a)-(h) illustrated in  FIG. 2 , there is relative rotational motion between the annular band  120  and the rotor  140 . Therefore, with the annular band  120  rotationally secured to the stator  110  and as the radially extending members  130  are retracted and extended sequentially, as illustrated in sequences (a)-(h) of  FIG. 2 , the rotor  140  travels in a counter-clockwise direction. A driver  160  ( FIG. 2A ) is connected to each of the extendable members  130  to extend and retract each extendable member  130 , as necessary. As illustrated in  FIG. 2B , in order to minimize slippage between the annular band  120  and the stator  110 , during the transition of extending adjacent expandable members  130 , in one embodiment, prior to releasing an extendable member  130 A, an adjacent extendable member  130 C is extended. This is also illustrated in  FIG. 2( b ) . Through continuous sequencing in such a manner as illustrated in  FIG. 2 , portions of the entire circumference C 1  of the annular band  120  may be radially extended thereby providing continuous counter-clockwise displacement of the rotor  140 . 
     As illustrated in  FIG. 2A , the stator body  114  may be cylindrical. 
     The ability of the deformation of the annular band  120  to drive the rotor  140  is based upon friction between the outer surface  124  of the annular band  120  and the inner surface  146  of the rotor wall  140 . Contact between the outer surface  124  of the annular band  120  and the inner surface  146  of the rotor  140  may have a coefficient or friction of between 0.01 and 2.0. Furthermore, the outer surface  124  of the annular band  120  may be selected from one of metal, plastic, rubber, and composites thereof while the inner surface  146  of the rotor wall  144  may be selected from one of metal, plastic, rubber, and composites thereof. In one embodiment, the outer surface  124  of the annular band  120  may be made from a liquid crystal aromatic polyester fiber. The inner surface  146  of the rotor wall  144  may be made of aluminum. Furthermore, the outer surface  124  of the annular band  120  may be textured to provide a friction surface. 
     Finally, the inner surface of the rotor wall  144  may have teeth (not shown) extending radially inwardly to engage the outer surface  124  of the annular band  120 , which would not have teeth. 
       FIGS. 3-12  illustrate one embodiment of the subject invention. The reference numbers used herein will be the same as those used to describe the elements of  FIGS. 2A and 2B  but will be incremented by 100. 
     A detailed model of the fluidic rotary joint is shown in  FIGS. 3-5 . This design consists of a fixed stator that may include twelve (12) fluidic diaphragms, or radially extending members  230  mounted in retainer plates  232  of a drive ring  233  inside the flex spline, which is an annular band  220 . The flex spline will then push against the circular spline that makes up the rotor  240  of the fluidic rotary joint  200 . A friction material is fixed to the outside of the flex spline annular band  220  to provide the shear contact area between the surfaces that will drive the rotor  240 . The pneumatic drive ring  233  is part of the rotor  240  and is fixed so that pneumatic air lines  234  ( FIG. 5 ) associated with the extendable members  230  are stationary. The air lines  234  are pressurized and depressurized by a controller  235 . The circular spline of the rotor  240  is the output of the gear box, enabling continuous rotation of the output of the drive. 
       FIG. 6  shows the thin wall flex spline  220  with 1.13″ wide elastomeric material (the friction material) bonded to the outer diameter of the flex spline adjacent to its open end.  FIG. 7  shows the rotor  240 . 
     The mechanical wave generator of the prior art has been replaced with a virtual wave generator by sequencing the inflation and deflation of pneumatic actuators arranged around the outside of a fluidic drive ring  233 . To drive a harmonic drivetrain, the force from the extendable members  230  on the inside of the flex spline  220  must be sufficient to deform the spline  220  and generate a normal force on the drive interface between the flex spline  220  and the circular spline of the rotor  240 . In one embodiment, fiber reinforced diaphragms may be used. Such a diaphragm is capable of supporting up to a 150 psi differential pressure without failure and will apply a force in the piston direction across the area of the face of the diaphragm.  FIGS. 8A-8C  show the size of the diaphragm, the direction of actuation, and the effective area that applies force to the flex spline. The diaphragm, or expandable member  230 , is shown in the deflated, or collapsed, configuration. The central region  231  is urged upwardly ( FIG. 8C ) when the diaphragm  230  is pressurized such that contact is made with the annular band  220  ( FIG. 6 ) to urge it radially outward against the inner surface  246  ( FIG. 7 ) of the wall  244  of the rotor  240 . Approximately 40 psi of pressure in the diaphragm, across two opposing diaphragms in the drive ring, will generate sufficient deflection in the flex spline to push out against the circular spline. Since the joint torque capability should increase linearly with pressure, the highest practical operating pressure up to 150 psi should be used. 
     The pneumatic drive ring  233  is designed to provide the tightest packaging possible of diaphragms used for the drivetrain. The drive ring is a single continuous ring to efficiently react to the pneumatic actuator forces in the plane of the actuators. The retainer plates  232  are assembled to the pneumatic ring  233  of the stator  210  and machined to slightly less than the anticipated inner minor diameter of the flex spline  220 . The precision contour supports the energized flex spline minor diameter for torque transmission. Also, maintaining a minimum gap between the drive ring outer diameter and the flex spline helps to minimize the portion of the diaphragm that is radially unsupported.  FIG. 9  shows the fabricated and assembled pneumatic drive ring  233  with the diaphragms  230  (extendable members) in the deflated configuration. As mentioned, pneumatic lines  234  ( FIG. 5 ) are used to inflate or deflate the diaphragms  230  as needed. The drive ring  233  is directly attached to the stator body  214  ( FIG. 3 ) and is therefore considered to be a part of the stator  210 . 
     Directing attention to  FIGS. 10A and 10B , the annular ring  220  is mounted to the rotor  240 . As seen in  FIGS. 11 and 12 , the expandable members  230  are mounted within pockets  216  of the stator body  214 . Only the outline of the stator body  214  is seen in  FIG. 12 . However, the mechanism by which the expandable members  230  urge the annular ring  220  against the rotor  240  is apparent. 
     Current harmonic drives utilize toothed surfaces to generate the driving torque for the mechanism. To reduce cost and complexity, the present disclosure uses a friction material in the interface between the flex spline  220  and the circular spline of the rotor  240 . 
     As applied to the embodiment illustrated in  FIGS. 3-12 , a generally cylindrical rotor  240  surrounds the stator body  214  and has a wall  244  with an inner surface  246  having an inner circumference C 2 . The outer circumference C 1  of the annular band  220  is less than the inner circumference C 2  of the rotor  240 . As illustrated in  FIG. 2 a   , radially extendable members  230 A and  230 B are in an extended position whereby the annular band  220  contacts the inner surface  246  of the rotor  240  at contact regions R 1  and R 2 . By applying pressure by using the radially extendable members  130  and retracting and extending them in a pattern such as the sequence illustrated in  FIGS. 2 a -2 h   , there is relative rotational motion between and annular band  220  and the rotor  240 . Therefore, with the annular band  220  rotationally secured to the stator  110  then as the radially extending members  130  are retracted and extended sequentially, the rotor  240  travels in a rotational direction. A driver (not shown) is connected to each of the extendable members  130  to extend and retract each extendable member  130 , as necessary. Through continuous sequencing of the extendable members  230 , portions of the entire circumference C 1  of the annular band  220  may be extended thereby providing continuous counter-clockwise displacement of the rotor  240 . 
     While the extendable member discussed herein has been directed to a diaphragm nested within a retainer plate, it is possible for the extendable member to be an independently inflatable bladder not nested within the retainer plate. 
     At least two other materials may also be utilized to provide friction between the deformable annular band and the rotor: Neoprene Rubber, 40A durometer and 3M Gripping Material, GM110, nylon backed. 
       FIG. 13  is the system for the operation of the fluidic rotary joint  200  while  FIG. 14  is an expansion of the details of  FIG. 13 . In particular, the overall system to provide the functionality of the fluidic harmonic drive is shown in  FIG. 14 . Operation of the system requires a regulated compressed air source, a bay of pneumatic valves to energize or vent the pneumatic actuator chambers, a power source for the pneumatic valves, electrical switches to sequence the actuator states, wiring to the pneumatic valves, hoses from the air source to the valves and pneumatic actuator chambers, and the rotary joint of the present disclosure. 
     The material utilized in the system of the present disclosure may be coated with a liquid crystal aromatic polyester fiber. This material is lightweight, strong, and flex-fold damage resistant. With the coating, it can be made watertight. A laminated, hermetically sealed version, may be 0.015″ thick and weigh 0.7 lbs/square yard. 
     The conceptual design for a fluidic harmonic drivetrain presented in the disclosure was adapted to available commercial off-the-shelf components combined with custom manufactured components. The objective goal of +/− 90° of rotation was achieved with a design capable of continuous roll motion and generating appreciable torque. The drivetrain is also capable of fine, deterministic motion, necessary for the precise positioning of objects called for in the program solicitation. The capability requirements for the inflatable structure and pitch joints were analyzed based on application requirements and the additional capability drivers imposed on the system by the fluidic harmonic drivetrain. The pressures required to resist buckling and provide more deterministic motion were calculated. 
     The fluidic harmonic drivetrain was successfully driven and shown to be capable of both precision stepping motion and large-scale continuous rotation. The resolution of the drivetrain, based on the design with 12 inflatable diaphragms distributed around the circumference of the drivetrain, was measured to be 0.22°. The resulting virtual gear ratio of the drivetrain is 134:1, and is driven by the ratio of the number of virtual revolutions of the deformation wave generated by the diaphragms to the output motion of the circular spline. The expected gear ratio is the ratio of the diameter of the circular spline divided by the difference in diameters of the circular and flex splines, as was shown in the section on Fluidic rotary joint Design: 
     
       
         
           
             
               Gear 
               ⁢ 
               
                   
               
               ⁢ 
               Ratio 
             
             = 
             
               
                 
                   5.620 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   in 
                 
                 
                   
                     5.620 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     in 
                   
                   - 
                   
                     ( 
                     
                       5.620 
                       - 
                       
                         2 
                         * 
                         .025 
                       
                     
                     ) 
                   
                 
               
               = 
               112.4 
             
           
         
       
     
     It can be seen that the gear ratio is sensitive to the radial gap that was nominally intended to be 0.025″ between the flex spline outer diameter and the circular spline inner diameter. Back-solving for the actual gap based on the measured 134:1 gear ratio yields an as-built gap of 0.021″. The small 0.004″ discrepancy can be explained by variations in actual part dimensions relative to the nominal values, and adhesive thickness variation between the neoprene rubber and the flex spline. 
     The harmonic drivetrain is also capable of resisting significant torque. The torque coupling features of the present disclosure were used to determine static holding torques with two sets of pneumatic diaphragms pressurized. 
     This harmonic drivetrain, utilizing simple fluidic actuators and inexpensive torque transfer material, enables motion that has not been possible with existing fluidic actuators. 
     The invention is also directed to a method for using the fluid rotary joint comprising method for using the fluid rotary joint of claim  1  comprising the steps of a) expanding at least one of the at least three radially extendable members  130  at a pressure greater than the remaining members  130  to deform the annular band  120  against the inner surface  146  of the rotor  140  thereby providing friction between the annular band  120  and the inner surface  146  of the rotor  140 ; and b) increasing pressure on an adjacent extendable member  130  and relieving the pressure on the at least one extendable member  130  to create relative motion between the annular band  120  and the rotor  140 . 
     By extending and releasing the extendable members  230  in a rotational sequence, the annular band  120  may be advanced in a single direction along the inner surface  146  of the rotor  140 . Furthermore, it is possible prior to releasing an extendable member  130  to extend an adjacent member  130  to prevent slippage between the annular band  120  and the rotor  140 . Additionally, two or more extendable members  130  may be extended simultaneously to provide greater contact area between the extendable members  130  and the inner surface  146  of the rotor  140 . 
     Furthermore, it is possible to relieve pressure among all of the members  130  such that friction between the annular band  120  and the rotor  140  is de minimus and the annular band  120  may move freely relative to the rotor  140  to produce a freewheeling configuration between the stator  110  and rotor  140 . 
     Finally, the pressure of the annular members  130  may be controlled such that friction between the annular band  120  and the rotor  140  is varied such that the torque transmission between the stator  110  and the rotor  140  may be controlled. 
     While several aspects of fluid rotary joint are shown in the accompanying figures and described hereinabove in detail, other aspects will be apparent to, and readily made by, those skilled in the art without departing from the scope and spirit of the disclosure. For example, it is to be understood that this disclosure contemplates that, to the extent possible, one or more features of any aspect can be combined with one or more features of any other aspect. Accordingly, the foregoing description is intended to be illustrative rather than restrictive.