Patent Publication Number: US-2017348176-A1

Title: High Performance Free Rolling Cable Transmission

Description:
RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 62/344,635, filed on Jun. 2, 2016. The entire teachings of the above application are incorporated herein by reference. 
    
    
     GOVERNMENT SUPPORT 
     This invention was made with government support under Grant No. NNX12AM16G from the National Aeronautics and Space Administration. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     Cables have long been regarded as one of the most flexible ways to transmit mechanical power and motion, especially, for long-distance actuation where the motor input is spatially separated from the output end effector. A non-exhaustive list of common cable driven devices include wearable robotic emulator devices for prosthetic, orthotic and exoskeletal devices; robotic therapy tools, robotic surgical tools, bicycle brakes, dental drills, hair shearing, and cranes. In order to transmit mechanical power and motion from an input to an output, cable housings or frames must be deployed to provide the required reaction forces to the actuation. Compared with other mechanical power transmissions, such as linkages and gears, cables are relatively lightweight and flexible but may suffer from friction losses and cable slackness, leading to poor control performance. 
     To achieve a high-performance cable transmission, typically engineers use a multiple-stage pulley system and a pretension mechanism, such as the devices taught in US 2007/0149328 A1 [1] and U.S. Pat. No. 7,736,254 B2 [2]. However, such devices are usually complex and bulky, limiting the size and flexibility of the systems. In order to avoid cable slackness, some devices, such as dental drills and bicycle brakes, make use of elastic bands and/or springs. This can introduce compliance to the systems, deteriorating the efficacy of the devices. Some high-performance cable transmission has been achieved by utilizing complicated pulley systems and pretension mechanisms, such as in a teleoperator system using a Whole-Arm Manipulation (WAM) robot [3], [4] and in laparoscopic surgery robots with multiple pulleys and pretension mechanisms [5], [6], [7]. However, cable housings or frames of these devices using multiple pulleys must be specifically designed and are usually bulky, limiting the size and flexibility of the systems, and inevitably leading to a difficulty in use in different forms. 
     As one type of flexible cable, Bowden cables are used to increase the flexibility of the transmission by deploying a hollow flexible outer cable conduit, which consists of an inner lining, a longitudinally incompressible layer, and a protective outer covering [8]. Typically, Bowden cables are commonly used in conjunction with aircraft control and bicycle brakes. They are now also frequently used with wearable devices, such as described in [9], [8], because they are lightweight and flexible, and because human movement is often unpredictable. Compared to Bowden cables, which are flexible, rigid mechanical transmission with limited degrees of freedom are usually intended to impede human motions by adding weights and/or restricting body motions. However, in order to provide large output power, the cable conduit has to be strong enough to provide required reaction forces, and consequently, the conduit becomes stiffer and heavier. When the cable is under tension, reaction forces on the conduit tend to straighten the cable conduit, causing external lateral impedance against the outside environment. Moreover, Bowden cables also often suffer from inefficiency and variations in cable tension due to bending of the cable housing and to friction losses [10]. An improved Bowden cable system has been designed to minimize friction resistance and to provide a better directional stability, as well as a much narrower and tension-free disposition in curves [11]. However, the friction losses are still significant when the cable is bending in a curvature due to changes of transmission angle between the input and output, and the forces between each cable segment of a conduit tend to lock the conduit in its current position. 
     Therefore, a need exists for a mechanical transmission and tethered actuation system that overcomes or minimizes the above-referenced problems. 
     SUMMARY OF THE INVENTION 
     The invention generally is directed to a mechanical transmission, a tethered actuation system, a method of actuating an end effector, and an autonomous ankle exoskeleton design. 
     In one embodiment, the mechanical transmission includes a ground link having first and second pivots defining parallel axes of rotation, a first pulley rotatable about the first pivot, and a second pulley rotatable about the second pivot. A first pulley housing rotates about the first pivot in response to a change in transmission angle of linear force of a cable at the first pulley, the cable being threaded across and between the pulleys. A second pulley housing rotates about the second pivot in response to a change in transmission angle of linear force of the cable at the second pulley. A transmission link is located between the pulley housings, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction, thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link, in response to a change in transmission angle of force across the mechanical transmission. 
     In another embodiment, a tethered actuation system of the invention includes an input mechanism, an output mechanism and a cable linking the input mechanism and the output mechanism. At least one mechanical transmission includes a ground link having first and second pivots that define parallel axes of rotation, a first pulley rotatable about the first pivot, and a second pulley rotatable about the second pivot. A first pulley housing rotates about the first pivot in response to a change in transmission angle of linear force of the cable at the first pulley, the cable being threaded across and between the pulleys. A second pulley housing rotates about the second pivot in response to a change in transmission angle of linear force of the cable at the second pulley. A transmission link is located between the pulley housings, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction, thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link, in response to a change in transmission angle of force across the mechanical transmission. A first cable housing extends about the cable between the input mechanism and the at least one mechanical transmission. A second cable housing extends about the cable between the at least one mechanical transmission and the output mechanism. 
     In yet another embodiment, the invention is a method of actuating an end effector, comprising the step of actuating an input mechanism, whereby force is transmitted from the input mechanism to an output mechanism through a cable that extends across at least one mechanical transmission, the at least one mechanical transmission including a ground link having first and second pivots that define parallel axes of rotation A first pulley is rotatable about the first pivot, and a second pulley is rotatable about the second pivot. A first pulley housing rotates about the first pivot in response to a change in transmission angle of linear force of the cable at the first pulley, the cable being threaded across and between the pulleys. A second pulley housing rotates about the second pivot in response to a change in transmission angle of linear force of the cable at the second pulley. A transmission link is located between the pulley housings, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction, thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link, in response to a change in transmission angle of force across the mechanical transmission. 
     In still another embodiment, the invention is an ankle exoskeleton system design, comprising an electric motor, an input mechanism, an output mechanism, a cable linking the input mechanism and the output mechanism, and at least one mechanical transmission, including a ground link having first and second pivots that define parallel axes of rotation, a first pulley rotatable about the first pivot, and a second pulley rotatable about the second pivot. A first pulley housing rotates about the first pivot in response to a change in transmission angle of linear force of the cable at the first pulley, the cable being threaded across and between the pulleys. A second pulley housing rotates about the second pivot in response to a change in transmission angle of linear force of the cable at the second pulley. A transmission link is located between the pulley housings, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction, thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link, in response to a change in transmission angle of force across the mechanical transmission. A first cable housing extends between the input mechanism and the at least one mechanical transmission, and a second cable housing extends between the at least one mechanical transmission and the output mechanism. 
     In one embodiment, the invention is a wearable device, including a distal member wearable by an individual distal to a skeletal joint of the individual, a proximal member, a link between the distal member and the proximal member, and at least one mechanical transmission. The proximal member includes a tube, an actuator and a harness wearable by the individual proximal to the joint, wherein one or the other of the distal members and the proximal member includes an elastic crossing member. The elastic crossing member and the link span an axis about which the distal member rotates, from one to the other of the distal member or the proximal member, and whereby actuation of the link is translated into a force at the distal or proximal member that is normal a major longitudinal axis extending through the distal and proximal members. A cable is connected to the crossing member and extends from the crossing member to the actuator. The mechanical transmission is between at least one of: the distal member and the proximal member; and the actuator and the tube, the mechanical transmission including a ground link defining parallel first and second pivots, a first pulley rotatable about the first pivot, and a second pulley rotatable about the second pivot. A first pulley housing rotates about the first pivot in response to a change in transmission angle of linear force of the cable at the first pulley, the cable being threaded across and between the pulleys. A second pulley housing rotates about the second pivot in response to a change in transmission angle of linear force of the cable at the second pulley. A transmission link is located between the pulley housings, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction, thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link, in response to a change in transmission angle of force across the mechanical transmission. 
     This invention includes many advantages. For example, the mechanical transmission, the tethered actuation system of the invention, and the method of the invention can efficiently transmit motion and mechanical power from an input device, such as a motor, to an output device, or end-effector, via a cable or its equivalent, such as a rope. With a high level of efficiency and minimal frictional forces in the transmission, angular transmission of linear force on the cable or rope can be effected without physical constraints on the location of the output relative to the input in three-dimensional space. Since the mechanical transmission significantly reduces friction resistance and significantly reduces cable slackness, independent of the location of the output relative to the input, it is highly backdrivable. Specifically, an embodiment of the mechanical transmission of the invention is compact, modular, lightweight, stiff, highly backdrivable and free to rotate in three-dimensional space with virtually zero backlash between the transmission&#39;s input and the output. Since the mechanical transmission of the invention is compact and modular, and since it can be used for both bidirectional and unidirectional actuation, it is useful for many applications. Moreover, the angular velocity ratio of two pulley housings need not be 1:1, and it can be programmable as a variable ratio by changing the shape or design of the coupling components. Such a design could be useful for some applications in which the tension of the cable should change when the transmission angle between the input and the output changes. When pulleys have the same angular velocity ratio as that of associated pulley housings, the force balance is still valid and there is no cable slack since the net changes in length of the cable wrapping around two pulleys is zero. It may also be beneficial to use more than two pairs of pulley housings and pulleys in the same transmission using the same principle. For instance, using multiple pairs of pulley housings and pulleys in one transmission eliminates the need of any cable housings. 
     Because the cable transmission is compact, modular, lightweight, stiff, highly backdrivable and free to rotate in three-dimensional space, it can easily be used as a general mechanical component for different applications, such as an emulator system for wearable devices, surgical robotics, therapy robotics, flexible dental drills, hair shearing and teleoperation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  is a topology of one embodiment a mechanical transmission of the invention. 
         FIGS. 2A and 2B  illustrate a topology of a mechanical transmission of  FIG. 1  and motion from one position ( FIG. 1 ) to a second position ( FIG. 2A ), wherein the transition angle has increased from 0° to 90°, and from the second position ( FIG. 2A ) to a third position ( FIG. 2B ), wherein the transmission angle has increased from 90° to 180°. 
         FIG. 3  is a perspective view of one embodiment of a mechanical transmission of the invention. 
         FIG. 4  is a side view of the mechanical transmission shown in  FIG. 3 . 
         FIG. 5  is a plan view of the mechanical transmission shown in  FIG. 3  and  FIG. 4 . 
         FIG. 6  is an exploded view of the mechanical transmission shown in  FIGS. 3-5 . 
         FIG. 7  is a perspective view of a tethered actuator employing a mechanical transmission of the invention. 
         FIG. 8  is a block diagram of one embodiment of an example control system for use with a tethered actuation system of the invention, employing two mechanical transmissions of the invention that are connected in parallel in a Bowden cable system. 
         FIG. 9  is a perspective view of another embodiment of a mechanical transmission of the invention. 
         FIG. 10  is an exploded view of the mechanical transmission shown in  FIG. 9 . 
         FIG. 11  is a perspective view of one embodiment of a bidirectional mechanical transmission of the invention. 
         FIG. 12  is an exploded view of the mechanical transmission shown in  FIG. 11 , wherein two cables, or two portions of a single cable, cross each other at a centerline between respective pulleys. 
         FIG. 13  is a perspective view of one two-degrees-of-freedom mechanical transmission consisting of two unidirectional mechanical transmissions shown in  FIGS. 10-11 . 
         FIG. 14  is a perspective view of an autonomous ankle exoskeleton device employing a mechanical transmission of the invention. 
         FIG. 15  is a frontal view of an autonomous ankle exoskeleton device shown in  FIG. 14 . 
         FIG. 16  is a perspective view of one embodiment of a bidirectional mechanical transmission of the invention wherein two cables, or two portions of a single cable, are essentially parallel to each other at a centerline between respective pulleys. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Generally, the invention is directed to a mechanical transmission that can efficiently transmit motion and power from an input to an output via a cable/rope essentially without physical constraints on the direction angles between the output motion and mechanical interface motion in three-dimensional space. 
     The invention generally is directed to a mechanical transmission, a tethered actuation system, a method of actuating an end effector, and to an autonomous ankle exoskeleton design. 
     In one embodiment, the invention is a mechanical transmission that includes a ground link having first and second pivots that define parallel axes of rotation. A first pulley is rotatable about the first pivot, and a second pulley is rotatable of the second pivot. A cable is threaded across and between the pulleys, whereby rotation of either the first pulley or the second pulley in one direction causes rotation of the other pulley in the opposite direction. In addition, a first pulley housing is rotatable about the first pivot in response to a change in a transmission angle of linear force of the cable at the first pulley. A second pulley housing is rotatable about the second pivot in response to a change in transmission angle of linear force of the cable at the second pulley. A transmission link is located between the pulley housings, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link in response to the change in transmission angle of force across the mechanical transmission. 
     In another embodiment, the mechanical transmission further includes an adapter fixed to the second pulley housing, wherein the adapter defines a first axis that is parallel to the axis of rotation of the first pivot, and a second axis transverse to the axis of rotation of the first pivot. A second ground link defines a third pivot and a fourth pivot, the third and fourth pivots define distinct axes of rotation parallel to the second axis. A third pulley is rotatable about the third pivot and the fourth pulley is rotatable about the fourth pivot. A third pulley housing rotates about the third pivot in response to a change in transmission angle of linear force of a cable threaded across and between the third and fourth pulleys. The third pulley housing is attached to the adapter. A fourth pulley housing rotates about the fourth pivot in response to a change in transmission angle of linear force of the cable at the fourth pulley. A transmission link is between the third and fourth pulley housings, whereby rotation of one of the third and fourth pulley housings in one direction causes rotation of the other of the third and fourth pulley housings in the opposite direction, thereby causing the third and fourth pulley housings to rotate about the third and fourth pivots, respectively, and about the second ground link, in response to a change in transmission angle of linear force across the third and fourth pivots. 
     In one embodiment, the mechanical transmission of the invention includes first and second pulleys that are of about equal diameter. In another embodiment, the first and second pulleys are of different diameters. The transmission link between pulley housings can be a pair of agonist and antagonist tendons wrapped in opposite directions about and between the housings. Alternatively, the link between the first and second pulley housings can include a pair of conjugated gears affixed to the pulley housings, wherein teeth of each gear are engaged with the teeth of the other gear, thereby causing rotation of one of the pulley housing in one direction to rotate the other pulley housing in the opposite direction. A cable housing can be coupled to each pulley housing and extends away from each respective pulley. A cable can extend within the housings and across and between the pulleys; and the cable can further extend through the cable housings. In one embodiment, each cable housing is rotatable about an axis coaxial to a major longitudinal axis of the cable through the associated cable housing. The transmission can further include a suspension handle at the ground link. 
     In one embodiment, shown in  FIG. 1 , the mechanical transmission  100  of the invention includes a pair of pulleys  100   a,    100   b,  a pair of pulley housings  200   a,    200   b,  a pair of agonist and antagonist tendons  300   a,    300   b,  inner cable  400 , and rotating arm  500 . Rotating arm  500  operates as a ground link, and all motions are defined with respect to rotating arm  500  as a ground link. Preferably, rotating arm  500  is rigid, but could be elastic as well. 
     Pulley housings  200   a,    200   b  and pulleys  100   a,    100   b  rotate with respect to centers (pivots)  501 ,  502  defined by rotating arm  500 . Pulley housings  200   a,    200   b,  having the same pitch diameter (the diameter of the standard pitch circle), are coupled by agonist and antagonist tendons  300   a,    300   b  so that the angular velocity ratio of pulley housings  200   a,    200   b  is 1:1. Pulleys  100   a,    100   b,  having the same pitch diameter, are free to rotate with respect to associated pulley housings  200   a,    200   b  but are driven by inner cable  400 . Inner cable  400  passes across and between pulleys  100   a,    100   b.  From input end  201  of housing  200   a,  inner cable  400  wraps around one pulley  100   a,  and crosses a line  503  between centers  501 ,  502 , between two pulleys  100   a ,  100   b,  and then wraps around the other pulley  100   b  in the opposite direction, going to output end  202  of housing  200   b.  Inner cable  400  can transmit motion and power from input end  201  to output end  202 , driving pulleys  100   a,    100   b  to rotate in opposite directions of rotation relative to each other. When cable  400  is pulled in the opposite direction, end  202  becomes the input end and end  201  becomes the output end. Since inner cable  400  is guided by pulleys  100   a,    100   b  essentially without resistance or slippage, the transmission has a very low friction loss and is highly backdrivable. 
     Herein, the “transmission angle θ trans ” is defined as the angle difference between the input and the output, and its domain is −180°&lt;θ trans ≦180°. It is zero when the input and output are collinear in the same direction and the sign of the transmission angle follows the right-hand rule by convention. The “wrap angle θ wrap ” is defined herein as the sum of the angles around pulleys  100   a,    100   b  wrapped by inner rope  400 , and thus it is always positive. The initial condition is defined as θ trans =0°, and accordingly, θ is the angle displacement of the tangential point of the cable on the pulley with respect to that in the initial condition. As shown in  FIGS. 2A-2B , when the transmission angle between the input and the output changes, it causes both pulley housings  200   a,    200   b  to rotate with respect to their respective centers of rotation  501 ,  502  on line  503  ( FIG. 1 ) at an equivalent angle θ but in opposite directions. Specifically, two pulley housings  200   a,    200   b  must be driven simultaneously at an equivalent angle but in opposite directions of rotation due to the coupling of agonist and antagonist tendons  300   a,    300   b.  As a result, the resultant change of the transmission angle is twice the angle change of one of the housings, namely, 2θ. However, the wrap angle of inner cable  400  around pulleys  100   a,    100   b  is always constant since the changes of the transmission angle are compensated by the resultant change of the angles of both pulley housings  200   a ,  200   b.  The relationship between θ trans  and θ wrap  can be described by the following equations: 
       θ wrap =θ o −θ+θ=θ o    (1)
 
       θ trans =270°−θ wrap +2·θ  (2)
 
     where θ o  is the preset wrap angle when running inner cable  400  through transmission  100  the first time, so it can be adjusted by changing the initial transmission angle. For example, as can be seen in  FIG. 1 , the preset wrap angle θ o  is 270°, while the component wrap angles, are θ 1 =135° and θ 2 =135° C., where θ 1 +θ 2 =270°. Whatever the change in transmission angle, the wrap angle remains constant, implying zero slackness of cable  400 ; to wit, there is effectively no backlash between the input and the output of cable  400  due to change in the transmission angle of linear force T on cable  400 .  FIGS. 2A-2B  illustrate a topology of a mechanical (or free rolling cable) transmission of  FIG. 1  in motion from one position ( FIG. 1 ) to a second position ( FIG. 2A ) where the transmission angle has increased from 0° to 90°, and from the second position ( FIG. 2A ) to a third position ( FIG. 2B ), where the transmission angle has increased from 90° to 180°. As can be seen in  FIG. 2A , the component wrap angles, θ 1  and θ 2 , have changed to 225° and 45°, respectively, and again total 270°. 
     When inner cable  400  is under tension T, it tends to push two pulleys  100   a,    100   b  away from each other, rotating the whole transmission mechanism. However, agonist and antagonist tendons  300   a,    300   b  and external cable housings contribute to the resultant reaction force on each housing  200   a  and  200   b,  balancing the resultant force on the associated pulleys  100   a  and  100   b,  respectively. Therefore, the total sum of forces, except the weight, on rotating arm  500 , is always zero. Accordingly, even if inner cable  400  is under great tension, transmission mechanism  100  is free to rotate and thus free to translate. Agonist and antagonist tendons  300   a,    300   b  can be substituted with alternative coupling components that keep a 1:1 angular velocity ratio between two pulley housings  200   a,    200   b,  such as a pair of gears, belts, linkages, etc. (See, e.g., gears  19  of  FIGS. 3-6 ). Moreover, the angular velocity ratio between two pulley housings  200   a,    200   b  can be other than 1:1, such as that the radius of pulley housing  200   a  could be 10 mm and the radius of pulley housing  200   b  could be 100 mm, so the angular velocity ratio between pulley housings  200   a,    200   b  would be  10 : 1 . When pulleys  100   a,    100   b  also have the same angular velocity ratio 10:1 as that of pulley housings  200   a,    200   b,  the force balance is still valid and there is no cable slack since the net changes in length of the cable wrapping around two pulleys is zero. In one embodiment, the angular velocity ratio of pulley housing  200   a,    200   b,  can be programmable as a variable ratio by changing the shape or design of the coupling components, such as using a pair of elliptical gears. It is also to be understood that the number of pulley housings and pulleys employed in one transmission can be more than two, such as embodiment  1400  shown in  FIG. 13 , which has one common pulley housing, three independent pulley housings and four pulleys. 
     Moreover, because of the flexible nature of cable  400 , the input end and the output end of the transmission can rotate with respect to the housing  200   a,    200   b  in directions orthogonal to the centers of the rotation of pulley housings  200   a,    200   b.  For example, the transmission can rotate about a major longitudinal axis of cable  400  extending from a point of contact with pulley  100   a  orthogonally to plane  600  ( FIG. 1 ) and about a major longitudinal axis of cable  400  extending from a point of contact with pulley  100   b  orthogonally to plane  690  ( FIG. 1 ). Accordingly, the transmission cannot only efficiently transmit motion and power, but also be free to rotate in three-dimensional space. 
       FIGS. 3, 4, 5 and 6  represent different views of one embodiment of a mechanical (or free rolling cable) transmission of the invention. As shown therein, mechanical transmission  110  includes a pair of pulleys  1   a,    1   b,  a pair of pulley housings  2   a,    2   b,  two pairs of conjugated gears  19 , and a pair of rotating arms  3 . Pivots at rotating arms  3  define axes I, II, about which pulleys  1   a,    1   b,  and pulley housings  2   a,    2   b,  rotate, respectively. Axes I, II are parallel to each other. In the example shown, each pulley housing is formed of two half parts. The half parts of pulley housing  2   a  and the half parts of pulley housing  2   b  are affixed by screws  8  and  11 , respectively (see  FIG. 6 ). Pulley housings  2   a,    2   b  have the same pitch diameter while pulleys  1   a,    1   b  have the same pitch diameter, but are smaller or equal to that of pulley housings  2   a,    2   b  to avoid interference. Aforementioned agonist and antagonist tendons  300   a,    300   b  are replaced by two pairs of conjugated gears  19 . 
     As illustrated in  FIG. 6 , two ball bearings  5 , are incorporated into pulleys  1   a,    1   b  and secured by two bearing caps  6  and screws  17 , so that the two pulleys are free to rotate with respect to the associated pulley housings  2   a,    2   b.  Two needle bearings  4  and four thrust bushings  9  are incorporated into pulley housings  2   a,    2   b.  Each of two axles  14  running through one needle bearing  4  and two thrust bushings  9  is fixed to the two rotating arms  3  by screws, so that pulley housings  2   a,    2   b  are free to rotate with respect to rotating arms  3  positioned on either side of the housings. Therefore, pulley housings  2   a,    2   b  and pulleys  1   a,    1   b  can independently rotate about two axles  14  with respect to rotating arms  3  with little friction. Two pairs of gears  19 , having the same pitch diameter, are affixed to pulley housings  2   a,    2   b  by screws  8 , so that the angular velocity ratio of two housings  2   a,    2   b  is 1:1. 
     Cable housings  12  are attached to pulley housings  2   a,    2   b  by threaded connectors  13  and nuts  10 . Thrust bearings  18  between cable housings  12  and pulley housing  2   a,    2   b  enable cable housings  12  to rotate with respect to the pulley housings about an axis orthogonal to the centers of rotation of the pulley housings. Accordingly, transmission  110  is free to rotate in three-dimensional space. Arrow  22  in  FIG. 3  illustrates rotation of pulley housings  2   a,    2   b  about axis  24  that passes through the center of rotation of pulley housing  2   a.  As illustrated by arrows  26   a,    26   b  in  FIG. 3 , each cable housings  12  can rotate about axes  28   a,    28   b  that are orthogonal to axis  24 . In the example shown, axes  28   a,    28   b  and two major longitudinal axes of cable  400  running through two cable housings  12  are coaxial, respectively. 
     Pulleys  1   a,    1   b  are free to rotate with respect to the associated housings  2   a,    2   b  but are driven by inner cable  20 . From the input end through one cable housing  12 , inner cable  20  wraps around one of two pulleys  1   a,    1   b,  crosses the line of the centers of the two pulleys, and then wraps around the other pulley  1  in the opposite direction, going to the output end through the other cable housing  12 . Inner cable  20  can transmit motion and power from the input to the output, driving the pulleys to rotate in the opposite directions of rotation. Since inner cable  20  is guided by the pulleys the transmission has a very low friction loss and thus is highly backdrivable. 
     The total sum of forces, disregarding mechanism weight, on rotating arms  3  is always zero; accordingly, even if inner cable  20  is under great tension, the transmission is free to rotate. To lift the transmission, suspension handle  15  is bolted to rotating arms  3  by shoulder screws  16 , so that suspension handle  15 , supported by a point force (such as tension force on a rope), can provide the force against the weight of the transmission while allowing the transmission to rotate in three-dimensional space. 
     Embodiments of the method of actuating and actuator system that are described herein can be used with the system and devices described in U.S. Published Application No.: 2013/0158444, entitled “A Robotic System for Simulating a Wearable Device and Method of Use,” by Herr et al., now U.S. Pat. No. 9,498,401, the relevant teachings of which are incorporated herein by reference. 
     In another embodiment, shown in  FIG. 7 , a tethered actuation system of the invention includes an input mechanism, an output mechanism, and a cable linking the input mechanism and the output mechanism. The at least one mechanical transmission includes a ground link having first and second pivots that define parallel axes of rotation. A first pulley is rotatable about the first pivot, and a second pulley is rotatable of the second pivot. A cable is threaded across and between the pulleys, whereby rotation of either the first pulley or the second pulley in one direction causes rotation of the other pulley in the opposite direction. In addition, a first pulley housing is rotatable about the first pivot in response to a change in a transmission angle of linear force of the cable at the second pulley. A second pulley housing is rotatable about the second pivot in response to a change in transmission angle of linear force of the cable at the second pulley. A transmission link is located between the pulley housings, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction, thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link in response to the change in transmission angle of force across the mechanical transmission. A first cable housing extends between the input mechanism and the mechanical transmission. A second cable housing extends between the mechanical transmission and the output mechanism. 
     More specifically,  FIG. 7  illustrates one embodiment of a tethered actuator system  700  that employs a mechanical transmission of the invention for one-degree-of-freedom tethered actuation. Tethered actuator is bidirectional and the system includes two free rolling cable transmissions  710 , input mechanism  720 , output mechanism  730 , and cable housings  740 . Input mechanism  720  transmits power to output mechanism  730  by movement of inner cable  20 ,  400  ( FIGS. 1-2B ) relative to cable housings  740  and transmissions  710 . A motor  725  at input mechanism  720  provides actuation to move cable  20 . Output mechanism  730  is coupled to end-effector  735  and can be configured to move, or otherwise actuate, the end-effector. 
     In one particular embodiment, the tethered actuation system of the invention further includes a control system that is in communication with the input mechanism and the output mechanism. The control system includes a host computer that includes a user interface, a master controller in communication with the host computer and that provides real-time control and sensor fusion. A local servo controller is in communication with the master controller and input mechanism, the local servo controller controlling the input mechanism. Sensors transmit measurements of output states from the output mechanism, and input/output modules convert signals from the sensors and transmit the converted signals to the master controller, whereby a torque command is produced and communicated to the input mechanism using measured feedback states from the sensors. 
     A method of actuating an end-effector, such as by use of a control system, as shown in  FIG. 8 , includes the step of actuating an input mechanism, whereby force is transmitted from the input mechanism to an output mechanism through a cable that extends across at least one mechanical transmission, the at least one mechanical transmission including a ground link that defines a plurality of pivots, a first pulley rotatable about one of the pivots, and a second pulley rotatable about another of the pivots and linked to the first pulley by the ground link having first and second pivots that define parallel axes of rotation. A first pulley is rotatable about the first pivot, and a second pulley is rotatable of the second pivot. A cable is threaded across and between the pulleys, whereby rotation of either the first pulley or the second pulley in one direction causes rotation of the other pulley in the opposite direction. In addition, a first pulley housing is rotatable about the first pivot in response to a change in a transmission angle of linear force of the cable at the second pulley. A second pulley housing is rotatable about the second pivot in response to a change in transmission angle of linear force of the cable at the second pulley. A transmission link is located between the pulley housings, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction, thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link in response to the change in transmission angle of force across the mechanical transmission. A first cable housing extends between the input mechanism and the mechanical transmission. A second cable housing extends between the mechanical transmission and the output mechanism. 
       FIG. 8  is a schematic representation of one embodiment of a general control system  800  of the invention for a system that employs a mechanical transmission of the invention. In order to ensure system performance and safety, the user interface and high-level control algorithms, e.g. biophysical control and virtual model control, are implemented in a host computer  810 ; real-time control, e.g. virtual model control, impedance control, output torque control, output position control and sensor fusion, (utilizing multiple sensors to assist the accuracy of the feedback system), are implemented in a standalone master controller  815 ; real-time servo control, e.g. current control, input position control, are implemented in a local servo controller  820 . An input/output (I/O) system  860 , which includes one or more I/O modules, is connected to master controller  815 . I/O system  860  has multiple digital inputs and multiple digital outputs, which can be used for gathering, via sensor  850 , output states  845 , such as encoder feedback signals, torque feedback signals, etc. Using these sensory data, master controller  815  can send torque commands to local servo controller  820 , to thereby enforce desired output performance. 
     As illustrated in  FIG. 8 , local servo controller  820  is coupled to input mechanism  830  and receives current and input angle feedback  825  from the input mechanism. In addition, input mechanism  830  provides emergency signals to local servo controller  820 , which can be used to inform the system that the output end is approaching the limitation of the range of motion, and thus the system can stop rapidly to avoid a dangerous event. Similarly, output mechanism  840  provides emergency signals  855  to I/O system  860  (I/O modules). The local servo controller controls input mechanism  830 , which is connected to the output mechanism  840  via the transmission  835 . 
       FIGS. 9 and 10  represent different views of another possible embodiment of a mechanical (or free rolling cable) transmission of the invention. As shown therein, mechanical transmission  120  includes a pair of pulleys  601   a,    601   b,  a pair of pulley housings  602   a,    602   b,  two pairs of gears  619 , and a pair of rotating arms  603 . Pulley housings  602   a,    602   b  have the same pitch diameter while pulleys  601   a,    601   b  have the same pitch diameter, but are smaller or equal to that of pulley housings  602   a,    602   b  to avoid interference. Aforementioned agonist and antagonist tendons  300   a,    300   b  are replaced by two pairs of conjugated gears  19 . 
     As illustrated in  FIG. 10 , two needle bearings  605 , are incorporated into pulleys  601   a ,  601   b,  and sandwiched between two thrust washers  604 , so that the two pulleys  601   a,    601   b  can rotate with respect to the associated pulley housings  602   a,    602   b.  Each of two axles  614  running through one needle bearing  605 , two thrust washers  604 , and two thrust bushings  609  is fixed to the two rotating arms  603  by screws  611 , so that pulley housings  602   a,    602   b  are free to rotate with respect to rotating arms  603  positioned on either side of the housings. Therefore, pulley housings  602   a,    602   b  and pulleys  601   a,    601   b  can independently rotate about two axles  614  with respect to rotating arms  603  with little friction. Two pairs of conjugated gears  619 , having the same pitch diameter, are affixed to pulley housings  602   a,    602   b  by screws  608 , so that the angular velocity ratio of two housings  602   a,    602   b  is 1:1. 
     Cable housings  612  are attached to pulley housings  602   a,    602   b  by adapters  613  and C-clips  610 . Thrust bushings  618  between cable housings  612  and pulley housing  602   a,    602   b enable cable housings  612  to rotate with respect to the pulley housings  602   a,    602   b  about an axis orthogonal to the centers of rotation of the pulley housings. Accordingly, transmission  120  is free to rotate in three-dimensional space, in the same way as the first embodiment shown in  FIG. 3 . 
     Pulleys  601   a,    601   b  are free to rotate with respect to the associated pulley housings  602   a,    602   b  but are driven by inner cable  620 . From the input end through one cable housing  612 , inner cable  620  wraps around one of two pulleys  601   a,    601   b,  crosses the line of the centers of pulleys  601   a,    601   b,  and then wraps around the other pulley  601  in the opposite direction, going to the output end through the other cable housing  612 . Inner cable  620  can transmit motion and power from the input to the output, driving the pulleys  601   a,    601   b  to rotate in the opposite directions. 
     Embodiment  120  has fewer mechanical components and less weight than that of embodiment  110 , so that no additional support structure may be needed. 
       FIGS. 11 and 12  show one possible embodiment of a bidirectional mechanical transmission  130  of the invention. As shown in  FIGS. 11 and 12 , a first length of cable and the second length of cable extend across and between the third fourth pulleys, respectively, and cross each other at a centerline A between the axes of rotation of the first and second pulleys, and the third and fourth pulleys, respectively. In one embodiment of the invention, where the two lengths of cable are both part of a common cable, the first and second pulleys each include two grooves. In an alternative embodiment of the invention the the two lengths of cable are two cables that operate independently. In this alternative embodiment (not shown), a length of cable is a first length of cable, and the mechanical transmission of the invention further includes a third pulley rotatable about the first pivot and a fourth pulley rotatable about the second pivot, whereby a second length of cable, such that the first and second lengths of cable are two lengths of the same cable or two lengths of different cables, extend across and between the third and fourth pulleys. The first and third pulleys, rotate independently, and the second and fourth pulleys rotate independently. 
     More specifically, in one embodiment, mechanical transmission  130  includes a pair of pulleys  901   a,    901   b,  a pair of pulley housings  902   a,    902   b,  two pairs of conjugated gears  919 , and a pair of rotating arms  903 . Pulley housings  902   a,    902   b  have the same pitch diameter while pulleys  901   a,    901   b  have the same pitch diameter, but are smaller or equal to that of pulley housings  902   a,    902   b  to avoid interference. 
     Mechanical transmission  130  shares a similar design to that of transmission  120  ( FIGS. 9-11 ), to wit, two rotating arms  903  are fixed to two axles  914  by screws  911 , and pulleys  901   a,    901   b  and pulley housings  902   a,    902   b  can rotate with respect to rotating arms  903  positioned on either side of the pulley housings. Each of two axles  914 , running through one needle bearing  905 , two thrust washers  904  and two thrust bushings,  909 , are fixed to the two rotating arms  903  by screws  611 . Two pairs of conjugated gears  619 , having the same pitch diameter, are affixed to pulley housings  902   a,    902   b  by screws  908 , forcing the two pulley housings to rotate in the 1:1 angular velocity ratio. However, pulley housings  902   a,    902   b  have built-in adapters to clamp cable housings  912   a,    912   b  by screws  910 . As a result, cable housings  912   a,    912   b  are fixed to the pulley housings. 
     Moreover, both pulleys  901   a,    901   b  have two grooves on each side to guide two inner cables  920   a,    920   b.  Pulleys  901   a,    901   b  are free to rotate with respect to the associated housings  902   a,    902   b  but are simultaneously driven by both inner cables  920   a,    920   b  on each side of the pulleys. From the input end through cable housing  912   a,  inner cable  920   a  wraps around one of two pulleys  901   a,    901   b,  crosses the line of the centers of the two pulleys, and then wraps around the other of the two pulleys  901   a,    901   b  in the opposite direction, going to the output end through the other cable housing  912   a.  Conversely, from the input end through cable housing  912   b,  inner cable  920   b  wraps around one of the two pulleys  901   a,    901   b,  crosses the line of the centers of the two pulleys (centerline A in  FIG. 12 ), and then wraps around the other of the pulleys  901   a  and  901   b  in the opposite direction, going to the output end through the other cable housing  912   b . Consequently, inner cable  920   b  and inner cable  920   a  wrap around the same pulleys in the opposite directions of rotation. As a result, inner cables  920   a,    920   b  can transmit bidirectional motion and power from the input to the output. Accordingly, transmission  130  is only free to rotate in one direction, unlike transmissions  110 ,  120  in the aforementioned embodiments  110 ,  120 . However, one can add more degrees of freedom by adding extra adapters to connect any two consecutive transmissions. As shown in  FIG. 13 , with an adapter  1300  coupling two transmissions  1301  and  1302 , transmission  1400  is free to rotate in two directions. Two inner cables  1320   a,    1320   b  should be actuated in an agonist-antagonist way so there is little or no sliding between pulleys  1310   a,    1310   b  and inner cables  1320   a,    1320   b,  such as when being driven by a rotational joint. Arrows in  FIG. 13  illustrates two centers of rotation of the embodiment. 
     In still another embodiment, the invention is an ankle exoskeleton system design, comprising an electric motor, an input mechanism, an output mechanism, a cable linking the input mechanism and the output mechanism, and at least one mechanical transmission, including a ground link having first and second pivots that define parallel axes of rotation, a first pulley rotatable about the first pivot, and a second pulley rotatable about the second pivot. A first pulley housing rotates about the first pivot in response to a change in transmission angle of linear force of the cable at the first pulley, the cable being threaded across and between the pulleys. A second pulley housing rotates about the second pivot in response to a change in transmission angle of linear force of the cable at the second pulley. A transmission link is located between the pulley housings, whereby rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction, thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link, in response to a change in transmission angle of force across the mechanical transmission. A first cable housing extends between the input mechanism and the at least one mechanical transmission, and a second cable housing extends between the at least one mechanical transmission and the output mechanism. 
     In one embodiment, the ankle exoskeleton system includes a first mechanical transmission and a second mechanical transmission connected in series. Another embodiment of an ankle exoskeleton system design of the invention includes a harness to which the first and second mechanical transmissions are connected, wherein the first mechanical transmission is fixed proximate to a human hip joint and the second mechanical transmission is fixed proximate to a knee of a human subject. 
     In another embodiment, shown in  FIGS. 14 and 15 , the invention is directed to a wearable device that includes a distal member  1207  wearable by an individual distal to a skeletal joint of the individual. A proximal member includes a tube  1203 , an actuator  1210  and a harness  1214  wearable by the individual proximal to the joint, wherein one or the other of the distal member and the proximal member includes an elastic crossing member  1205 . Links  1209 ,  1213  extend between the distal member and the proximal member, wherein the elastic crossing member and the link expand an axis about which the distal member rotates, from one to the other of the distal member or the proximal member. Actuation of the link is translated to a force at the distal or proximal member that is normal to a major longitudinal axis extending through the distal and proximal members. A cable is connected to the crossing member and extends from the crossing member to the actuator. At least one mechanical transmission is located between at least one of: the distal member and the proximal member; and the actuator and the tube. The mechanical transmission includes a ground link having first and second pivots that define parallel axes of rotation, a first pulley rotatable about the first pivot, a second pulley rotatable about the second pivot, a first pulley housing that rotates about the first pivot in response to a change in transmission angle of linear force at the first pulley of a cable threaded across and between the pulleys, a second pulley housing that rotates about the second pivot in response to a change of transmission angle of linear force at the second pulley of the cable, and a transmission link between the pulley housings. Rotation of one of the pulley housings in one direction causes rotation of the other pulley housing in the opposite direction, thereby causing the first and second pulley housings to rotate about the first and second pivots, respectively, of the ground link, in response to a change in angular transmission angle of linear force across the mechanical transmission. 
     As can be seen in  FIGS. 14 and 15 , the link includes strut  1219 , wherein the strut extends from the proximal member to the distal member. The strut is constrained at the proximal member normally and laterally to a major longitudinal axis of the proximal member extending from the proximal member to the distal member. The strut is not restricted along the major longitudinal axis of the crossing member. The link includes at least one roller at the proximal member that constrains the strut normally and laterally. As shown, the link includes a pair of rollers in opposition to each other, wherein the strut is normally constrained between the pair of rollers. As also shown in  FIGS. 14 and 15 , the strut is curved at the pair of rollers  1218 , whereby shear force between the strut and the pair of rollers during rotation of the distal member about the axis spanned by the crossing member and the strut is less than it would be if the strut were straight at the pair of rollers. Strut includes a guide tube at the pair of rollers, wherein the crossing member extends through the guide tube. The wearable device includes a pair of cross members and a pair of struts. Struts are essentially straight between the rollers and the distal member. Upon actuation, at least one of the struts deflects during eversion and inversion of the human foot secured to the distal member and a human calf secured to the proximal member. In an embodiment, the struts are rigid. In other embodiments, not shown, the struts are curved, whereby the struts operate as a series of springs during a normal walking cycle of a human foot secured to the distal member and a human calve secured to the proximal member. An example of such an embodiment is described in U.S. patent application Ser. No. 14/572,499, “Optimal Design of a Lower Limb Exoskeleton or Orthosis,” filed on Dec. 16, 2014 and published as US 2015/0209214 A1 on Jul. 30, 2015, the teachings of which are incorporated herein in their entirety. In still another embodiment, the link further includes a motor actuator assembly attached to a proximal end of the pair of crossing members, whereby actuation of the link will cause retraction of the crossing members, which causes rotation of the distal member and plantar flexion of a human foot secured to the distal member about a human ankle joint. As shown, the pair of crossing members is fixed to proximal end of the distal member. 
     More specifically, as shown in  FIGS. 14 and 15 , as one of applications of the embodiment, the configuration of transmissions for an autonomous ankle exoskeleton  1200  is proposed. The proposed invention mainly comprises an electric motor  1210 , a motor mount  1208 , two unidirectional transmission modules  1201 ,  1202 , a long carbon fiber tube  1203 , flexible conduit components  1204 , and ankle end-effector  1290  worn by a wearer. Ankle end-effector  1290  mainly consists of a shank guard component  1211 , a strut  1219 , a pair of rollers  1218 , guide tubes  1213 , a force sensor  1206 , an inner cable  1212 , an elastic cable  1205 , and an output shoe  1207 . Strut  1219 , rollers  1218 , and guide tubes  1213  can be regarded as an input mechanism  1209  of ankle end-effector  1290 . Motor  1210  is affixed to motor mount  1208  that is attached to harness  1214  around the waist of the wearer. Inner cable  1212  connects to motor  1210 , running through transmission  1201 , tube  1203 , transmission  1202 , flexible conduits  1204 , strut  1209 , and is affixed to proximal end of force sensor  1206 . The distal end of force sensor  1206  is affixed to one end of elastic cable  1205 , the other end of which is affixed to the ankle portion of shoe  1207 . Shank guard component  1211  is mounted on the anterior shank of the wearer. Transmission  1201  is located next to the hip joint, allowing free abduction, adduction, rotation motions of the hip, and transmission  1202  is located next to the knee joint, allowing free flexion and extension motions of the knee. Flexible conduits  1204  are used to compensate small differences in motions between the transmission and the wear. When motor  1210  pulls inner cable  1212 , the force is transmitted from motor  1210  through input mechanism  1209  to shoe  1207  while guide tubes  1213  provides the required reaction force also contributing the output force on  1207 . The output force can be measured directly by force sensor  1206  or indirectly by measuring the extension of elastic cable  1205 , since elastic cable  1205  serve as an artificial soleus that helps store the energy during walking or running. The other details of the ankle exoskeleton end-effector design can be found in [12]. It is to be understood that the proposed configuration of transmissions can be used with any cable-driven ankle end-effector. 
       FIG. 16  shows another possible embodiment of a bidirectional mechanical transmission of the invention. As shown therein, the mechanical transmission of the invention further includes a third pulley rotatable about the first pivot and a fourth pulley rotatable about the second pivot. First and second lengths of cable are two lengths of the same cable or two lengths of different cables. The first length of cable extends across and between the first and second pulleys, and the second length of cable extends across and between the third and fourth pulleys respectively. Between the respective pulleys (the first and second pulley for the first cable, and the third and fourth pulley for the second cable) the first and second lengths of cable are essentially parallel to each other at centerline B between the axes of rotation of the first and second pulleys, and the third and fourth pulleys, respectively. Where the two lengths of the cable are two lengths of the same cable, the first and third pulleys rotate independently, and the second and fourth pulleys rotate independently. Where the two lengths of cable are two lengths of different cables, then the first and third pulleys can rotate independently, or be rotationally locked, and the second and fourth pulleys can rotate independently, or be rotationally locked, depending on the device in which they are employed. 
     As shown in  FIG. 16 , mechanical transmission  1900  includes two pairs of pulleys  1901   a,    1901   b,    1901   c ,  1901   d , and a pair of pulley housings  1902   a,    1902   b,  two pairs of conjugated gears  1919 ,  1909 , and a pair of rotating arms  1903 . Pulley housings  1902   a,    1902   b  have the same pitch diameter while pulleys  1901   a,    1901   b,    1901   c ,  1901   d  have the same pitch diameter, but are smaller or equal to that of pulley housings  1902   a,    1902   b  to avoid interference. Two thrust washers  1904  are sandwiched between pulleys  1901   a,    1901   b,  and pulleys  1901   c ,  1901   d , respectively, to allow the independent rotation between any two adjacent pulleys. 
     Mechanical transmission  1900  shares a similar design to that of embodiment  130  ( FIGS. 11-12 ), except that, through tubes  1912 , inner cable  1920   a  and inner cable  1920   b  extend across and between pulleys  1901   a,    1901   b  and pulleys  1901   c ,  1901   d , respectively, in the same direction, whereby inner cables  1920   a,    1920   b  are essentially parallel to each other at the centerline (centerline B in  FIG. 16 ) between the first and second pulleys  1901   a,    1901   b,  and the third and fourth pulleys  1901   d ,  1901   c , respectively. 
     REFERENCES 
     
         
         [1] W. T. Townsend, “Automatic pretensioning mechanism for tension element drives,” US 2007/0149328 A1, 2007. 
         [2] B. M. Schena, “Compact cable tension tender device,” U.S. Pat. No. 7,736,254 B2, 2010. 
         [3] W. T. Townsend and J. a. Guertin, “Teleoperator slave—WAM design methodology,” Ind. Robot An Int. J., vol. 26, no. 3, pp. 167-177, 1999. 
         [4] J. K. Salisbury and W. T. Townsend, “Compact Cable Transmission with Cable Differential,” USO05207114A, 1993. 
         [5] S. P. Buerger, “Stable, high-force, low-impedance robotic actuators for human-interactive machines,” Massachusetts Institute of Technology, 2006. 
         [6] C. Y. Kim, M. C. Lee, R. B. Wicker, and S. M. Yoon, “Dynamic modeling of coupled tendon-driven system for surgical robot instrument,” Int. J. Precis. Eng. Manuf., vol. 15, no. 10, pp. 2077-2084, 2014. 
         [7] “Bowden cable,” Wikipedia. [Online]. Available at: https://en.wikipedia.org/wiki/Bowden_cable. (Downloaded Jun. 1, 2017) 
         [8] J. Kuan, K. A. Pasch, and H. M. Herr, “Design of a Knee Joint Mechanism that Adapts to Individual Physiology,” 36th Annu. Int. Conf. IEEE Eng. Med. Biol. Soc., pp. 2061-2064, 2014. 
         [9] J. F. Veneman, “A Series Elastic- and Bowden-Cable-Based Actuation System for Use as Torque Actuator in Exoskeleton-Type Robots,” Int. J. Rob. Res., vol. 25, no. 3, pp. 261-281, 2006. 
         [10] P. Letier, a Schiele, M. Avraam, M. Horodinca, and A. Preumont, “Bowden Cable Actuator for Torque-Feedback in Haptic Applications,” Eurohaptics, 2006. 
         [11] N. Noetzold, “Pull cable system,” U.S. Pat. No. 6,606,921 B2, 2003. 
         [12] Herr, Hugh M., et al. “Optimal design of a lower limb exoskeleton or orthosis.” U.S. patent application Ser. No. 14/572,499, 2014. 
       
    
     The relevant teachings of all references cited herein are incorporated herein in their entirety. 
     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.