Abstract:
A robotic finger that includes multiple phalanges, each phalange configured to be compliantly actuated. The robotic finger also includes compliant touch sensors that, in combination with the compliant actuation, provides the robotic finger with two levels of compliance. The two levels of compliance enable the robotic finger to gently conform to and manipulate objects.

Description:
[0001]    This invention was made with government support under Grant No.: FA8750-07-1-0033, awarded by the U.S. Air Force. The Government has certain rights in this invention. This application claims priority to U.S. provisional application Ser. No. 61/338,689 filed Feb. 23, 2010, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    A major problem in robotics is the lack of a general purpose hand or gripper with capability of fine manipulation. Available robotic hands are generally heavy and rigid and lack any type of touch feedback. Thus, the robotic hands can easily knock over or break the object they are supposed to pick up. 
       SUMMARY OF THE INVENTION 
       [0003]    In a first aspect, the invention is a robotic finger having two levels of compliance. The finger includes a proximal phalange having first and second joint ends and a distal phalange having a joint end and a tip end, wherein the joint end of the distal phalange is coupled to the second joint end of the proximal phalange in a hinged manner. A first compliant actuator is configured to exert a torque on the proximal phalange about the first joint end and a second compliant actuator configured to exert a torque on the proximal phalange about the first joint end, the first and second compliant actuators providing a first level of compliance. At least one compliant touch sensor is mounted on the distal phalange, the at least one compliant touch sensor configured to contact an object before the distal phalange and to compliantly conform to the object and to sense the object. The at least one compliant touch or tactile sensor provides a second level of compliance. In a preferred embodiment, the proximal phalange is coupled to a base at its first joint end in a hinged manner. This embodiment further includes at least one compliant touch sensor mounted on the proximal phalange, the at least one compliant touch sensor configured to contact an object before the distal phalange contacts the object and to compliantly conform to the object and to sense the object. 
         [0004]    In yet another aspect, the invention is a robotic finger including a mount and a proximal phalange coupled at a first end to the mount via a first joint. A distal phalange is coupled at a joint end to a second end of the proximal phalange via a second joint, the distal phalange including a tip end opposite the joint end. A first actuator is connected to the proximal phalange and configured to exert a torque on the proximal phalange about the first joint. A second actuator is connected to the distal phalange and configured to exert a torque on the distal phalange about the second joint. A first torque sensor detects the torque from the first actuator on the first joint and a second torque sensor detects the torque from the second actuator on the second joint. A controller is provided and is configured to actuate the first and second actuators to move the robot finger, to detect contact of the at least one of the proximal and distal phalanges with an object by sensing changes in the detected torque at the first and second joints and to cause at least one of the first and second actuators to exert a torque on the respective proximal and distal phalanges to exert a force on the object. 
         [0005]    In yet another aspect, the invention is a method of contacting an object using a robotic finger including moving at least one of a proximal and distal phalange of a robotic finger by applying a first compliant torque to at least one of the proximal and distal phalanges. Contact of at least one of the proximal and distal phalanges with an object is detected by sensing a change in the first compliant torque on at least one of the proximal and distal phalanges. A force is exerted on the object with at least one of the proximal and distal phalanges by exerting a second compliant torque to at least one of the proximal and distal phalanges. In a preferred embodiment of this aspect of the invention contact forces are sensed at the distal phalange. 
         [0006]    In yet another aspect, the invention is a robotic hand including a base, and a plurality of robotic fingers. Each robotic finger includes a proximal phalange coupled at a first end to the base via a first joint, a distal phalange coupled to a joint end to a second end of the proximal phalange via a second joint, the distal phalange including a tip end opposite the joint end. The finger further comprises a first actuator connected to the proximal phalange and configured to exert a compliant torque on the proximal phalange about the first joint, and a second actuator connected to the distal phalange and configured to exert a compliant torque on the distal phalange about the second joint. A first torque sensor detects the compliant torque from the first actuator on the first joint and a second torque sensor detects the compliant torque from the second actuator on the second joint. The robotic hand further includes a controller configured to actuate the first and second actuators of each of the plurality of robotic fingers and to detect contact of at least one of the plurality of robotic fingers with an object by sensing changes in the detected compliant torque at the first and second joints of the at least one of the plurality of robotic fingers. The controller is further configured to cause at least one of the actuators of the plurality of robotic fingers to exert a compliant torque on the respective ones of the plurality of robotic fingers to exert a force on the object. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
           [0008]      FIG. 1  is an isometric view of a robotic finger according to an embodiment of the present invention; 
           [0009]      FIGS. 2A and 2B  are side views of a series elastic actuator according to an embodiment of the present invention; 
           [0010]      FIG. 3  is an exploded isometric view of a series elastic actuator according to an embodiment of the present invention; 
           [0011]      FIG. 4  is an exploded isometric view of a robotic finger according to an embodiment of the present invention; 
           [0012]      FIG. 5  is a schematic diagram of wires routed in a robotic finger according to an embodiment of the present invention for series elastic actuators; 
           [0013]      FIG. 6  is a photograph of a robotic hand that includes two robotic fingers according to the present invention that detect contact and interaction between the fingers and an object. 
           [0014]      FIG. 7  shows sequential steps of a robotic hand that includes at least two robotic fingers according to the present invention for picking up a stone from a surface; and 
           [0015]      FIG. 8  shows sequential steps of a robotic hand that includes at least two robotic fingers according to the present invention for placing a stone on a surface. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0016]    A description of example embodiments of the invention follows. 
         [0017]      FIG. 1  illustrates a robotic finger  100  according to an embodiment of the present invention. The robotic finger  100  includes a base  102 , a proximal phalange  104  and a distal phalange  106 . The proximal phalange  104  is attached to the base  102  by a first joint  108 . The distal phalange  106  is attached to the proximal phalange  104  by a second joint  110 . The distal phalange  106  has an angled tip with an angled portion  112 . The distal phalange also has tactile touch sensors  116  and  118  that are compliant. 
         [0018]    Each of the proximal phalange  104  and the distal phalange  106  includes a series elastic actuator, such as the elastic actuator disclosed in U.S. Pat. No. 5,650,704, which is incorporated herein by reference in its entirety. An example of a series elastic actuator is shown in  FIGS. 2A and 2B .  FIG. 2A  shows a pulley  202  with a wire represented by  204  and  206  attached to it by a locking mechanism  203 . The pulley  202  pivots around its central axis to exert a pulling force on either end  208  of wire  204  or at end  210  of wire  206 , depending on the direction of rotation of the pulley  202 .  FIG. 2B  shows the pulley  202  and wire  204  and  206  in a series elastic actuator housing  214 . The series elastic actuator has a housing  214  and two chambers  220   a - b.  Each chamber  220   a - b  includes a spring  218   a - b  and an endcap  216 - b.  The wire portions  204  and  206  feed through holes  221 - b  at the bottom of chambers  220   a - b.  The wire portions  204  and  206  feed through the springs  218   a - b  and attach to the endcaps  216   a - b.  Rotation of the pulley  202  causes the wire portion, either  204  or  206 , under tension to compress its respective spring. For example, if pulley  202  is rotated clockwise as shown in  FIG. 2B , then wire portion  204  is pulled towards the pulley likewise pulling endcap  216   a  and compressing spring  218   a.  In an embodiment of a series elastic actuator, both springs  218   a  and  218   b  are maintained in a compressed state so that as spring  218   a  compresses as shown in  FIG. 2B , spring  218   b  expands. As spring  218   a  is compressed as shown in  FIG. 2B , a torque is exerted on housing  214  of the series elastic actuator about the axis of rotation of pulley  202 , the torque being equal to the spring force (caused by the compression of the spring) multiplied by the distance R  212  of the center of the spring  218   a  from the centerline of rotation of the pulley  202 . That torque causes the series elastic actuator housing  214  to rotate, in this case in a clockwise direction. 
         [0019]    Importantly, the series elastic actuator provides a compliant torque, which means that the torque applied by pulley  202  is not directly coupled to the series elastic actuator housing  214 . Instead, the torque load is transmitted through either spring  218   a  or spring  218   b,  depending on the direction of rotation of pulley  202 . Under no loading, when pulley  202  is rotated then series elastic actuator housing  214  will rotate at the same rate. However, if a load is applied to an exterior portion of series elastic actuator housing  214  then, as pulley  202  turns, one of springs  218   a  and  218   b  will compress, absorbing some of the load and enabling the torque about pulley  202  to increase gradually as the spring compression increases. If a spring  218   a  or  218   b  fully compressed, i.e., if the spring force transmitted as a torque about the pulley  202  is saturated, then the series elastic actuator can apply additional torque in a non-elastic manner. As shown in  FIG. 2B , spring  218   a  is nearly fully compressed. When spring  218   a  fully compresses, the force of endcap  216   a  is transmitted through the spring  218   a  directly to the bottom of chamber  220   a.  The force of endcap  218   a  can exceed the spring force of spring  218   a  when spring  218   a  is fully compressed. Therefore, the presence of the springs does not mean that the torque that can be applied is limited, in general. 
         [0020]      FIG. 3  illustrates a series elastic actuator being used in a phalange for a robotic finger such as that shown in  FIG. 1 . Series elastic actuator housing  314  is shown in a perspective view with chambers  320   a - b  visible. Springs  318   a - b  and endcaps  316   a - b  are shown in an exploded view above chambers  320   a - b.  Wire  304 ,  306  is shown in an exploded view beneath series elastic actuator housing  314 . The series elastic actuator housing  314  includes two walls  320 ,  322 , which attach to sides of the series elastic actuator housing  314 . A portion of walls  320 ,  322  include holes  321  and  323  through which a shaft  328  is threaded. The holes also support two bushings  324 ,  326  which enable the shaft  328  to rotate. The shaft also supports pulley  302  shown beneath the series elastic actuator housing  314 . Also shown is a potentiometer  330  which measures rotation of the shaft  328  relative to wall  322  in this case and thereby relative to series elastic actuator housing  314 . This rotation of the shaft  328  relative to wall  322  can be combined with the stiffness coefficients of the springs  318   a - b  to calculate a torque being applied to the series elastic actuator housing  314  about the shaft  328 , and thereby being applied to the phalange. 
         [0021]    As described in  FIG. 2B , the springs  218   a - b  and endcaps  216   a - b  enable the pulley  212  to rotate in a compliant manner with respect to series elastic actuator body  214 . Potentiometer  330  shown in  FIG. 3  enables measurement of the compliant rotation of pulley  302  and shaft  328  relative to series elastic actuator body  314  and walls  320 ,  322 . 
         [0022]      FIG. 4  shows an exploded view of a robotic finger  400  similar to the assembled finger  100  shown in  FIG. 1 .  FIG. 4  shows the base (or mount) portion  402 , the proximal phalange portion  404 , the tip portion  406 . Also shown in  FIG. 4  are joint portions  408  and  410 . The base portion includes two walls  414 ,  420  that support drive pulleys  416  and  418 , which are turned by motors (not shown), e.g., electric motors. The proximal phalange portion  404  includes a series elastic actuator housing  422  and end walls  428  and  430 . Walls  428  and  430  hold potentiometers  431  and  429 , respectively, in place. Walls  428  and  430  may also include printed circuit boards for the potentiometers  431  and  429 . Shaft  426  connects the proximal phalange portion  404  to the base portion  402  in a hinged manner. The shaft includes potentiometer portion  427  and wall  430  includes potentiometer portion  429 . Potentiometer portion  427  rotates inside of potentiometer portion  429 . Wall  428  carries a second potentiometer portion  431  and a second potentiometer portion  427  is attached to the base portion  402  at wall  414 . As described with respect to  FIG. 3 , potentiometer portion  427  on shaft  426  and potentiometer portion  429  on wall  430  measure rotation of pulley  425  relative to series elastic actuator housing  422  and walls  428 ,  430 . Potentiometer portions  433  and  431 , mounted to the base  414  and wall  428 , respectively, measure rotation of the proximal phalange portion  404  relative to the base portion  402 . The distal phalange portion  406  is coupled to the proximal phalange portion  404  in a similar manner as proximal phalange portion  404  is attached to base portion  402 . The distal phalange portion  406  includes a second series elastic actuator housing  432  and walls  438 ,  440 . The distal phalange portion  406  also includes a shaft  436  at second joint  410 . The shaft  436  carries potentiometer portion  443  and wall  440  carries potentiometer portion  439 . These potentiometer portions  443 ,  439  measure relative rotation of pulley  434  with respect to series elastic actuator housing  432 . Wall  438  carries potentiometer portion  441  and the first series elastic actuator housing  422  carries potentiometer portion  437 . Potentiometer portions  441  and  437  measure rotation of series elastic actuator housing  432  and walls  438  and  440  with respect to series elastic actuator housing  422 . 
         [0023]    The distal phalange portion  406  also carries a tip structure  442 . The tip structure  442  and series elastic actuator  432  carry sensor platforms  446  and  444 , respectively. A sleeve  412 , made of compliant material, fits over distal phalange portion  406 , covering sensor platforms  446  and  444 . The compliant cover  412  includes multiple surfaces, including surface  452  and angled surfaces  450  and  448 . 
         [0024]    The cover  412  also carries several compliant touch sensors  454  which are described in greater detail in U.S. Publication No. 2008/0106258, which is incorporated herein by reference in its entirety. The compliant touch sensors  454  deform when an external load or force is applied to its surface. For example, if a normal load, i.e., a load perpendicular to the surface of a touch sensor, is applied, then the touch sensor  454  will deform in an even manner. By contrast, if a sheer force or load, i.e., not parallel to the surface of the touch sensor is applied, then the surface of the touch sensor  454  will skew to one side. Sensors, not shown, on plates  446  and  444  detect deformation of compliant touch sensors  454 . By detecting deformations, the sensors (not shown) detect direction of forces or loads applied to touch sensors  454  and can also determine the magnitude of the force applied by detecting the amount of deformation of the touch sensors  454 . 
         [0025]    The compliant touch sensors  454  add a second level of compliance to the robotic finger in  FIG. 4  (the series elastic actuators in the proximal phalange portion  404  and distal phalange portion  406  providing the first level of compliance). When an object (not shown) is picked up by the robotic finger  400 , the touch sensors  454  are pressed between the underlying structure of the finger  400  and the object (not shown). The touch sensors  454  do not require structural strength to support the object (not shown); the strength is provided by the underlying structure of the finger  400  that backs the touch sensors  454 . Thus, the compliant touch sensors  454  can be made of a material that is much softer than the remainder of the finger and that deforms under very small applied forces. By detecting these tiny forces, the robotic finger  400  may apply delicate force to an object (not shown). 
         [0026]    The angled surface  450  is a polygonal approximation of a curved fingertip. A curvature of a human fingertip allows the contact with a gripped object to be shifted by rolling the fingertip over the object. The angled surface  450  approximating a curved fingertip, in combination with the touch sensors  454 , likewise allow an object grasped by the robotic finger  400  to be shifted to a different orientation. 
         [0027]      FIG. 5  shows a schematic representation of routing for wires of series elastic actuators in a robotic finger such as finger  100  in  FIG. 1  or finger  400  in  FIG. 4 . For the purposes of clarity,  FIG. 5  will be explained using reference to  FIG. 4 . However, it should be understood that  FIG. 4  may use different wiring configurations from that shown schematically in  FIG. 5 .  FIG. 5  shows a first motor  502  with wire  522  and  526  attached to the motor  502 . Wire  526  is shown in incomplete form in  FIG. 5 , but a person having ordinary skill in the art would understand that wire  526  is continued in a similar but opposite fashion as wire  522  which is to be explained. Wire  522  is attached to pulley  506 . With reference to  FIG. 4 , pulley  506  is similar to pulley  425 . Pulley  506  carries wire  522  up to series elastic actuator mechanism  514 , which is similar to series elastic actuator  422  in the proximal phalange portion  404  of the finger  400  shown in  FIG. 4 . The wire  522  can apply tension to endcap  519   a  to exert a compliant torque on series elastic actuator  514 .  FIG. 5  also shows a second motor  504  which is similar to motor  418  in  FIG. 4 . Motor  504  has wires  520  and  524  attached to it. Similarly to wire  526 , wire  524  is not shown in completion, but a person having ordinary skill in the art would understand that wire  524  is routed similarly but opposite to wire  520  as described following. Wire  520  wraps around an idler pulley  508  that is similar to idler pulley  524  shown in  FIG. 4 . Idler pulley  508  is coaxial with pulley  506  as indicated by center line of rotation  509 . This is similar to the coaxial relationship of idler pulley  424  to pulley  425  shown in  FIG. 4 . Wire  520  uses idler pulley merely as a relay and idler pulley  508  does not exert any torque on any portion of a robotic finger such as finger  400  shown in  FIG. 4 . Wire  520  continues to pulley  510 , which is similar to pulley  434  shown in  FIG. 4 . Wire  520  can apply a force onto endcap  517  in series elastic actuator  512  thereby compressing spring  516  and causing a compliant torque on series elastic actuator  512 . Note that wires  522  and  520  have been described as continuous from motor  504  or  502  up through series elastic actuators  512  and  514 . Alternatively, wires  520  and  522  may include multiple wire segments that are coupled together at pulleys  510 ,  506  and  508 . 
         [0028]      FIG. 6  shows a robotic hand  600  comprising two robotic fingers  602  and  604 . Robotic finger  602  includes a base portion  606 , a proximal phalange portion  616  and a distal phalange portion  612  having a compliant touch sensor cover. Likewise, finger  604  includes a base portion  608 , a proximal phalange portion  618  and a distal phalange portion  614 , also having a compliant touch sensor cover. Bases  606  and  618  are mounted in a common frame  610 . 
         [0029]      FIG. 7  is a schematic diagram showing how a robotic hand  760  having two opposing fingers  740 ,  750 , such as hand  600  in  FIG. 6 , may be used to pick up a small item such as a stone  712 , e.g., a biconvex GO stone, on a surface  732 .  FIG. 7  shows five steps for picking up the stone  712  (note that the reference numbers in  FIG. 7  are only shown on a single step, but apply to all five steps). In the first step  702 , the two fingers  740 ,  750  are positioned over the stone  712 . Each finger  740 ,  750  has a proximal phalange  714  and  718 , respectively, and a distal phalange  716  and  720 , respectively. In step  702 , finger  740  has its proximal phalange  714  and distal phalange  716  aligned substantially collinearly. Finger  750  has its distal phalange  720  aligned at an angle to the proximal phalange  718 . In step  702 , the hand  760  is moved down towards the surface  732  on which the stone  712  is resting. If tip  728  makes contact with the surface  732  before tip  726  or before compliant touch sensor  722  makes contact with the stone  712 , then the distal phalange  720  compliantly gives by pivoting with respect to proximal phalange  718 . As described above with respect to  FIG. 2B and 3 , the pivoting is detected by a controller, so the controller knows that finger  750  is in contact with surface  732 . If tip  726  contacts surface  732  or if compliant touch sensor  722  contacts the stone  712  before tip  728  of finger  750  contacts surface  732 , then later, in step  706 , finger  750  is extended to make contact between tip  728  and surface  732 . 
         [0030]    In step  704 , after finger  750  makes contact with the surface  732 , the hand  760  continues to move towards the surface  732 . Distal phalange  720  of finger  750 , if in contact, continues to compliantly give as the hand  760  continues to move. As finger  740  continues to move towards the surface  732 , compliant touch sensor  722  makes contact with the stone  712 . The contact with the stone  712  results in a contact force  730  being applied to the compliant touch sensor  722  that causes a measurable deformation of the compliant touch sensor  722 . As described in U.S. Publication No. 2008/0106258, the compliant touch sensor  722  provides a controller (not shown) that controls the fingers  740 ,  750  with a measurement of the contact force  730  being applied to it by the contact with the stone  712  and send a command to stop the motion of the hand. 
         [0031]    In step  706 , after compliant touch sensor  722  makes contact with the stone  712 , the hand  760  continues to move towards the surface  732  until it comes to a complete stop. If in contact with the surface, distal phalange  720  of finger  750  continues to compliantly give as the hand  760  is coming to a stop. As finger  740  moves towards the surface  732 , the contact force  730  between the compliant touch sensor  722  and the stone  712  may change in magnitude and also in direction, i.e., the vector of the contact force  730  may change. The compliant touch sensor  722  detects the change in the contact force  730  and provides the detected force to the controller (not shown). The contact force  730  also causes the stone  712  to lift an edge opposite that being contacted by the compliant touch sensor  722 . When the tip  726  of distal phalange  716  makes contact with the surface  732 , the distal phalange  716  does not compliantly give (or only compliantly gives by an insignificant amount) compared to the compliant give of distal phalange  720  because distal phalange  716  and proximal phalange  714  of finger  740  are substantially colinear to each other. The colinear alignment of the distal phalange  716  and the proximal phalange  714  of finger  740  results in forces from the hand  760  being transmitted on a vector that is almost colinear with the distal phalange  716  and proximal phalange  714 . Thus, there is negligible torque being applied about the hinge coupling the distal phalange  716  to the proximal phalange  714 . The series elastic actuator (not shown) in distal phalange  716  may apply an actuating force to counteract any torque about the hinge coupling the distal phalange  716  to the proximal phalange  714  to maintain the colinear relationship between the distal phalange  716  and the proximal phalange  714 . The finger  750  is extended to make sure that it is in contact with surface  732 . A small force is applied to avoid moving the hand  760  away from the surface  732 . This places finger  750  in a good position to approach stone  712  at the lowest point possible, which permits the distal phalange  720  of finger  750  to get beneath the stone  712 . This is an important step because if the distal phalange  720  cannot move beneath the stone  712 , the fingers  740 ,  750  cannot pick up the stone. 
         [0032]    In step  708 , finger  750  is moved towards finger  740 . The surface  732  prevents the finger  750  from fully moving toward finger  740 . If finger  750  was a non-compliant robotic finger, then moving finger  750  towards finger  740  while in contact with surface  732  could be dangerous because the finger  750  may damage the surface  732  or the actuators (not shown) operating finger  750  could be damaged by overloading. The compliant robotic finger  750  can be moved towards finger  740  safely because the compliance ensures that there will be no damage to the surface  732  or to the actuators (not shown) of the finger  750 . As finger  750  attempts to move toward finger  740 , its interference with surface  732  will result in increasing forces to actuators (not shown) controlling the proximal phalange  718 , the distal phalange  720 , and/or the hand  760 . Alternatively, the forces of actuators (not shown) controlling the proximal phalange  718 , the distal phalange  720 , and/or the hand  760  may be operated at constant levels predetermined to be sufficient to move the finger  750  towards finger  740  and to keep finger  750  in contact with surface  732 . 
         [0033]    In step  710 , when the increasing forces to actuators (not shown) controlling the proximal phalange  718 , the distal phalange  720 , and/or the hand  760  reach a predetermined limit, the hand  760  begins to move away from the surface  732 . As the hand  760  moves away from the surface  732 , fingers  740  and  750  also move away from the surface  732 . As finger  750  moves away from the surface  732 , the interference between the surface  732  and the tip  728  of distal phalange  720  will decrease, allowing the finger  750  to continue moving closer to finger  740 . Eventually, finger  750  will be able to move close enough to finger  740  that the stone  712  will be captured between the two fingers  740 ,  750  and the stone  712  can be lifted from the surface  732 . 
         [0034]      FIG. 8  shows how a stone  812 , similar to stone  712 , may be placed without simply dropping the stone. In step  802 , fingertips  816  and  820  are grasping stone  812  such that the stone  812  is in contact with touch sensors  822  and  824  on angled tip surfaces  826  and  828 . Step  802  shows fingertips  816  and  820  moving down towards a surface  830  as indicated by phantom lines. When stone  812  contacts the surface  830 , a sheer force will be produced with touch sensors  822  and  824  which is detected by sensors (not shown). In step  804 , fingertip  820  is moved away from stone  812  to allow the stone  812  to lower onto the surface  830 . At the same time, fingertip  816  moves up to help rotate the stone  812  into its resting position. Finally, step  806  shows finger  820  continuing to move away from the stone  812  and fingertip  816  moving up and away from the stone such that the stone is now resting completely on the surface  830 . 
         [0035]    The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety. 
         [0036]    While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.