Patent Publication Number: US-11382773-B2

Title: Biomimetic joint actuators

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application to U.S. patent Application Ser. No. 15/877,680 filed Jan. 23, 2018, which is a continuation application to U.S. patent application Ser. No. 14/734,662, filed Jun. 9, 2015, which is a Divisional of U.S. patent application Ser. No. 13/417,949, filed Mar. 12, 2012, which claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/451,887, filed on Mar. 11, 2011, the contents of which are hereby incorporated by reference herein in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to powered human segmentation devices, such as lower-extremity prosthetic, orthotic, or exoskelton apparatus, and/or to humanoid robotic devices designed to emulate human biomechanics and to normalize function and, in particular, to device components that deliver mechanical power, and methods for controlling such components. 
     BACKGROUND 
     Superior biomimetic, lower-extremity augmentation systems and humanoid systems generally modulate mechanical impedance, joint equilibrium, and torque in accordance with gait cycle phase, walking speed, and/or terrain in a way that can emulate human behavior. In so doing, such systems can normalize or even augment metabolic cost-of-transport and self-selected walking speed with respect to average limb/joint function in a typical human. Some powered prosthetic, orthotic, and exoskeletal devices for providing and/or augmenting human joint function such that at least a biomimetic joint response is achieved have been described in co-pending U.S. patent application Ser. No. 12/157,727 “Powered Ankle-Foot Prosthesis” filed on Jun. 12, 2008 (Publication No. US2011/0257764 A1); co-pending U.S. patent application Ser. No. 12/552,013 “Hybrid Terrain-Adaptive Lower-Extremity Systems” filed on Sep. 1, 2009 (Publication No. US2010/0179668 A1); co-pending U.S. patent application Ser. No. 13/079,564 “Controlling Power in a Prosthesis or Orthosis Based on Predicted Walking Speed or Surrogate for Same” filed on Apr. 4, 2011; co-pending U.S. patent application Ser. No. 13/079,571 “Controlling Torque in a Prosthesis or Orthosis Based on a Deflection of Series Elastic Element” filed on Apr. 4, 2011; co-pending U.S. patent application Ser. No. 13/347,443 “Powered Joint Orthosis” filed on Jan. 10, 2012; co-pending U.S. patent application Ser. No. 13/356,230 “Terrain Adaptive Powered Joint Orthosis” filed on Jan. 23, 2012; and co-pending U.S. Provisional Patent Application Ser. No. 61/595,453 “Powered. Ankle Device” filed on Feb. 6, 2012, the disclosures of all of which are hereby incorporated herein in their entireties. 
     In these devices the torque, the impedance, and joint equilibrium are generally controlled in each joint to provide at least a biomimetic response to a wearer of the device. Specifically, these devices may provide torque in advance of toe off during a gait cycle to propel the joint. This can enable the wearer to walk faster and with less effort while at the same time improving gait mechanics, thereby mitigating the wearer&#39;s discomfort. 
     A series-elastic actuator (SEA), described in the above-referenced patent applications, can be used to create a backdrivable joint mechanism in prosthetic, orthotic, exoskeleton, and/or humanoid devices in which both force (torque) and impedance are controlled. Specifically, in various lower-extremity devices described in these patent applications, the SEA typically emulates the muscle-tendon unit response in an ankle, knee or hip device, specifically through implementation of a positive force or velocity feedback controller that mimics a characteristic reflex response of the joint. To this end, the SEA typically stores energy in one phase of a gait cycle (e.g., in the controlled dorsiflexion phase for an ankle device) and releases the stored energy later in the gait cycle (e.g., in the powered plantar flexion phase in the ankle device). Thus, the SEA may amplify the peak power of the actuator, thereby reducing the size and weight of the motor and the transmission. As such, devices employing an SEA may both require less battery power and produce less acoustic noise than other robotic systems that provide torque for propelling a joint, but that do not use an SEA. Nevertheless, the devices using an SEA (as well as those not using an SEA) can still require substantial battery power and may produce noise that is unacceptable to some users in certain situations. 
     One of the reasons for the high power consumption and noise is that conventional electric actuators in leg prosthetic, orthotic, and exoskeletal devices generally employ low-torque, high-speed (i.e., high revolutions per minute (RPM) motors that are light weight but are limited in their torque capability. For example, the EC-4Pole 30 Maxon Motor that may be employed in prosthetic and/or orthotic devices has a low-mass (about 300 grams), but has a rather modest torque capability of about 0.12 Newton-meter continuous tongue, and a relatively high speed (about 16,500 RPM zero-load speed). To achieve the high joint torque and low speed required to emulate the dynamics of a biological leg joint using a low torque, high RPM motor, a transmission having a large reduction ratio (e.g., greater than about 150:1) is generally needed. Transmissions having such high reduction ratios, when used in an actuator system to emulate the biological dynamics of ankle, knee, and/or hip joints, typically produce significant acoustic noise output. Such transmissions may also have large frictional losses and may have low backdrivability. 
     A high acoustic output may draw attention to the wearer of the device, and can thus be uncomfortable or embarrassing in certain social situations. Moreover, high friction and poor backdrivability can result in a relatively poor transmission efficiency, increasing the power consumption of the device. These two parameters can also adversely affect the overall control of the joint, whether for adjusting the joint position or for applying impedance and/or force/torque. In addition, high transmission ratios can be difficult to achieve and often require many functional pasts, which limits system cycle life and increases manufacturing complexities and associated costs. Therefore, there is a need for improved powered actuators for use in prosthetic, orthotic, exoskeleton, and/or humanoid devices. 
     SUMMARY 
     In various embodiments, the present invention provides powered actuator devices and methods for operating/controlling such actuators so that human augmentation devices using these actuators can accurately modulate the torque, joint equilibrium, and impedance applied to a human joint, while significantly reducing the power consumption and acoustic noise of the powered actuators. This is achieved, in part, by using a high torque, low RPM motor that is directly coupled to a highly backdrivable, low friction, low-reduction ratio transmission, and by using an elastic element, coupled in series with the transmission, coupling the joint to which torque/impedance is to be supplied with the transmission. 
     Conventional actuators typically employ high-rpm motors and high gear-ratio transmissions using timing belts and gears, which can generate acoustic noise and dissipate power. In various powered actuators described herein, an efficient, high-torque motor, such as a transverse flux motor, having low thermal dissipation is coupled directly to a low gear-ratio transmission, e.g., a transmission having a reduction ratio of about 80:1 or less. Examples of such transmissions include ball-screw and cable transmissions. These actuators are thus directly coupled to the robotic joint, via an elastic element coupled in series with the low-reduction transmission, delivering high-torque with low inertia and high efficiency to the joint. As these actuators eliminate the belts and gears used in conventional transmissions, they can be more durable, light-weight quiet, backdrivable, powerful, efficient, and scalable, compared to the conventional actuators. 
     These improved SEAs may be employed to emulate the behavior of human muscles and tendons. In general, low acoustic noise, force and impedance controllability, and high efficiency are important attributes of biological muscle-tendon units. An SEA including a high torque, low RPM motor (e.g., a transverse-flux motor), a low-reduction ratio transmission directly coupled to the motor, and tendon-like elastic element coupled in series with the transmission can provide many of these attributes of biological muscle-tendon units with greater efficacy compared to traditional actuator designs currently employed in wearable robotic systems. This particular combination of mechanical and electromechanical elements in the improved SEAs facilitates a biomimetic actuator platform capable of emulating the natural dynamics of biological leg joints in tasks such as walking, stair climbing/descending, and running, at high efficiency and controllability with relatively low acoustic noise output. 
     Accordingly, in one aspect, embodiments of the invention feature a powered actuator for supplying one or more of an augmentation torque, joint equilibrium, and an impedance to a joint augmented by a powered human augmentation device. The powered actuator includes a motor having a dissipation constant less than about 50 W/(Nm) 2 , and a transmission coupled directly to the motor. The powered actuator also includes an elastic element coupled to the joint that is also coupled, in series, to the transmission. The powered actuator, as adapted for use in an ankle, can generate a normalized joint torque in a range from about −2.8 to about 2.8 Nm/kg. 
     In some embodiments, the motor may include a high-torque motor supplying motor torque of at least about 0.06 Nm/kg. Alternatively, or in addition, the motor may include a low revolutions per minute (RPM) motor having an RPM less than about 1500, a transverse-flux motor, or both. The actuator may be adapted to be backdrivable. 
     In some embodiments, the transmission has a gear ratio less than about 80:1. The transmission may include a ball-screw transmission having a ball nut coupled to the elastic element. The ball-screw transmission may include a screw having a pitch in a range of about 2 mm up to about 10 mm, which can yield the gear ratio of less than about 80:1. 
     In some embodiments, the elastic element may include a spring, and the powered actuator may additionally include a cable and a joint output pulley. In those embodiments, the cable is coupled to both the spring and the joint output pulley. In some other embodiments, the transmission includes a ball-screw transmission having a ball nut coupled to the motor rotor and the screw is coupled to the elastic element. 
     In some embodiments, the motor of the powered actuator includes a motor having an external rotor, and the transmission includes a cable and a joint output pulley. The cable couples the external rotor and the joint output pulley. The cable may be any one of a synthetic cable, a steel cable, a belt, and a chain. 
     The powered actuator may also include a motor encoder adapted to measure angular displacement of a rotor of the motor with respect to a stator of the motor, and a joint encoder adapted to measure angular displacement of the joint about a joint pivot. The motor encoder, the joint encoder, or both may include an absolute encoder. The motor encoder and/or the joint encoder may also include a magnetic encoder having at least 13-bit resolution. 
     In another aspect, embodiments of the invention feature a method for augmenting joint function using a powered human augmentation device. The method includes modulating one or more of joint augmentation torque, joint impedance, and joint equilibrium during a phase of a gait cycle. The modulation is achieved, in part, using a motor having a dissipation constant less than about 50 W/(Nm) 2  and that is coupled directly to a transmission. The transmission is serially coupled to an elastic element that is coupled to the joint. The method includes energizing the motor to apply the augmentation torque to the joint during a phase of the gait cycle, such that the applied torque normalized by weight is in a range from about −2.8 Nm/kg up to about 2.8 Nm/kg. The transmission may have a gear ratio less than about 80:1. 
     In some embodiments, the method includes energizing the motor to apply stiffness (a component of the impedance) to the joint, so that energy is stored in the elastic element and power from release of the stored energy combines with the applied motor power to achieve a positive torque feedback response that approximates a muscle-tendon reflex. The method may also include energizing the motor to apply the torque to achieve a desired joint equilibrium, and subsequently shorting leads of the motor during a stance phase of the gait cycle to approximate a mechanical clutch, such that the joint equilibrium is substantially maintained during a portion of the stance phase. 
     In some embodiments, the method includes measuring angular displacement of a rotor of the motor with respect to a stator of the motor and measuring angular displacement of a structure with respect to the joint. The method also include determining a state of the elastic element based, at least in part, on both the angular displacement of the rotor and the angular displacement of the structure. Moreover, based at least in part on the state of the elastic element and the angular displacement on the motor, torque contribution of the motor is computed, and the modulation is adjusted, based at least in part on the computed contribution of the motor torque. 
     In another aspect, embodiments of the invention feature a method for supplying one or more of an augmentation torque, joint equilibrium, and an impedance to a joint augmented by a powered human augmentation device. The method includes directly coupling a motor having a dissipation constant less than about 50 W/(Nm) 2  to a transmission. The method also includes coupling an elastic element to both the joint and the transmission, whereby when the motor is energized to supply the augmentation torque, the torque applied to the joint normalized by weight is in a range from about −2.8 Nm/kg up to about 2.8 Nm/kg. 
     In some embodiments, the motor may include a high-torque motor supplying torque of at least about 0.06 Nm/kg. Alternatively, or in addition, the motor may include a low revolutions per minute (RPM) motor having an RPM less than about 1500, a transverse-flux motor, or both. The actuator may be adapted to be backdrivable. 
     In some embodiments, the transmission has a gear ratio less than about 80:1. The transmission may include a ball-screw transmission having a ball nut coupled to the elastic element. The ball-screw transmission may include a screw having a pitch in a range of about 2 mm up to about 10 mm, which can yield the gear ratio of less than about 80:1. 
     In some embodiments, the elastic element may include a spring, and the powered actuator may additionally include a cable and a joint output pulley. In those embodiments, the cable is coupled to both the spring and the joint output pulley. In some other embodiments, the transmission includes a ball-screw transmission having a ball nut coupled to the motor rotor and the screw is coupled to the elastic element. 
     In some embodiments, the motor of the powered actuator includes a motor having an external rotor, and the transmission includes a cable and a joint output pulley. The cable couples the external rotor and the joint output pulley. The cable may be any one of a synthetic cable, a steel cable, a belt, and a chain. The augmentation torque and/or the impedance may supplied to one or more of a hip joint, a knee joint, and an ankle joint. 
     These and other objects, along with advantages and features of the embodiments of the present invention herein disclosed, will become more apparent through reference to the following description, the accompanying drawings, and the claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. As used herein, the term “substantially” means about ±10% and, in some embodiments, about ±5%. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
         FIG. 1A-1C  illustrate a powered actuator employing a ball-screw transmission, according to one embodiments for use with ankle prostheses; 
         FIG. 2  illustrates a powered actuator employing a cable transmission, according to one embodiment, for use with ankle prostheses; 
         FIGS. 3A-3C  illustrate a powered actuator employing a ball-screw transmission, according to one embodiment, for use with knee prostheses; 
         FIG. 4  illustrates a powered actuator employing a cable transmission, according to one embodiment, for use with knee prostheses; and 
         FIG. 5  is a table of certain design and operating parameters of powered actuators according to various embodiments. 
     
    
    
     DESCRIPTION 
     The entire contents of each of U.S. patent application Ser. No. 12/157,727 “Powered Ankle-Foot Prosthesis” filed on Jun. 12, 2008 (Publication No. 2011/0257764 A1); U.S. patent application Ser. No. 12/552,013 “Hybrid Terrain-Adaptive Lower-Extremity Systems” filed on Sep. 1, 2009 (Publication No. US2010/0179668 A1); U.S. patent application Ser. No. 13/079,564 “Controlling Power in a Prosthesis or Orthosis Based on Predicted Walking Speed or Surrogate for Same” filed on Apr. 4, 2011; U.S. patent application Ser. No. 13/079,571. “Controlling Torque in a Prosthesis or Orthosis Based on a Deflection of Series Elastic Element” filed on Apr. 4, 2011; U.S. patent application Ser. No. 13/347,443 “Powered Joint Orthosis” filed on Jan. 10, 2012; co-pending U.S. patent application Ser. No. 13/356,230 “Terrain Adaptive Powered Joint Orthosis” filed on Jan. 23, 2012; and co-pending U.S. Provisional Patent Application Ser. No. 61/595,453 “Powered Ankle Device” filed on Feb. 6, 2012 are incorporated herein by reference. 
     In various embodiments described below, die use of an SEA in a biomimetic ankle device is described for the sake of convenience. That SEA mechanism can be readily adapted for use with biomimetic knee and/or hip devices also. A biomimetic ankle-foot prosthesis  100  depicted in  FIGS. 1A and 1B  includes an SEA  102 . The SEA  102  uses a direct-drive, ball-screw based transmission system in which an electric motor  104  is directly coupled to a ball-screw transmission  106 , which is serially coupled to an elastic element  108 , connecting the transmission  106  to a joint output, i.e., the ankle pivot  110 . Thus, the SEA  102  applies torque via a robotic joint, i.e., the ankle pivot  110 , to an output load, i.e., a carbon-fiber foot  112 . In some embodiments, another elastic element may be connected between the motor  104  and the foot  112 , in parallel to the serially connected elastic element  108 . 
     The motor  104  is a high-torque, low-speed (rpm) motor, e.g., a transverse-flux motor, an “external rotor” permanent magnet motor, etc. Modern transverse-flux motors employ a high-pole-count external rotor (internal stator) and circumferentially-applied stator windings to achieve high-torque density with low winding resistance, thereby mitigating many of the typical disadvantages of using other high-torque motors in portable devices. These transverse flux motors are particularly suited for prosthetic/orthotic/exoskeletal/humanoid devices because they have a high power-to-weight ratio. Transverse flux motors also have lower peak-to-continuous power ratings compared to those of other motors, thereby enabling a prosthetic/orthotic/exoskeletal device to operate at high power levels for longer periods without reaching thermal limits. Transverse flux motors can also provide a significant motor dissipation reduction as defined by the motor copper loss per square unit of torque as defined by R/k t   2 , whereby R is the stator winding resistance in ohms, and k t  is the motor torque constant measured in N-m/amp, thereby increasing motor efficiency. Lower frictional losses (generally due to the reduced number of motor revolutions per gait cycle) in the transmission further increase the overall efficiency of the prosthetic/orthotic/exoskeletal devices. The design life of the transmission can increase also, in part due to the reduction in motor revolutions per cycle. These benefits can be leveraged by the SEAs, such as the SEA  102 , using ball-screw and cable transmissions. 
     Specifically, the rotor of the motor  104  is attached to a ball-screw shaft  114  using a clamping nut  116 . The clamping nut applies a preload to the axial thrust bearings  118  that serve to align the ball-screw shaft  114  radially and support the thrust imparted by the ball nut  120  during actuation. The rotating screw  114  drives the ball-nut  120  longitudinally which in turn drives the series spring  108  about the ankle pivot beatings  110 , thus providing impedance and/or torque at the ankle to the foot  112 . Those skilled in the art appreciate that alternatively, in some embodiments, the rotor can be directly coupled to the ball-nut, thereby controlling the linear translation of the screw. 
     Typically in an ankle device, during the controlled dorsiflexion phase of the gait cycle, the SEA  102  delivers a programmable impedance and joint equilibrium at the ankle joint. It should be understood that in other devices, such as hip and/or knee devices, the SEA may deliver a programmable impedance, joint equilibrium, and/or torque in the controlled dorsiflexion and/or other phases of the gait cycle. In the ankle device  100 , the SEA  102  thus emulates a non-linear (hardening) torsional spring impedance of the ankle pivot  110 ; the associated torque is stored as potential energy in the series spring  108 . The hardening spring behavior can be accomplished through use of non-linear positive force or velocity feedback (as described in the various co-pending patent applications identified above) as a means of emulating the calf-muscle/Achilles tendon reflex response. At or near the end of the controlled dorsiflexion phase, the SEA  102  applies torque and, as the foot heel begins to lift off a surface on which the wearer is walking, the energy stored in the series spring  108  is released, like a catapult, combining with the motor applied torque to produce a positive force/torque feedback response to approximate a muscle-tendon reflex, thus producing at least a biomimetic response. The impedance and/or torque applied by the motor  104  may be normalized by the wearer&#39;s weight. 
     An absolute encoder may be used to measure angular displacement of the motor rotor in relation to the stator. Another absolute encoder may be used to measure angular displacement of the foot structure  112  about the ankle pivot bearings  110 . Instead of absolute encoders, magnetic field angle encoders, e.g., the RMB-20 having a 13-bit resolution, manufactured by Renishaw, may be used. The measured angular displacements can be used to determine the state of the motor  104 , for the purposes of commutation, torque, and/or joint equilibrium control, and of the output joint, i.e., ankle pivot  110 . These motor and ankle states can be used to estimate the state of the series spring  108 . (See for example, the co-pending U.S. patent application Ser. No. 13/079,564 “Controlling Power in a Prosthesis or Orthosis Based on Predicted Walking Speed or Surrogate for Same” filed on Apr. 4, 2011; U.S. patent application Ser. No. 13/079,571 “Controlling Torque in a Prosthesis or Orthosis Based on a Deflection of Series Elastic Element” filed on Apr. 4, 2011). In general, the motor position defines a joint equilibrium position through simple kinematics (e.g., the law of cosines). The difference between that joint equilibrium position and the actual joint position, when multiplied by a calibrated series spring constant, determines the series spring torque and, thereby, the energy stored in the spring. 
     In some embodiments, based on the determined series spring state and stiffness (i.e., spring constant) of the series spring  108 , force and joint torque contribution of the SEA  102  is determined. Furthermore, based on the determined contribution of the spring force and motor torque, the torque and impedance applied by the SEA  102  and equilibrium of the joint can be modulated. (See for example, the co-pending U.S. patent application Ser. No. 13/347,443 “Powered Joint Orthosis” filed on Jan. 10, 2012; co-pending U.S. patent application Ser. No. 13/356,230 “Terrain Adaptive Powered Joint Orthosis” filed on Jan. 23, 2012; and co-pending U.S. Provisional Patent Application Ser. No. 61/595,453 “Powered Ankle Device” filed on Feb. 6, 2012). 
     The SEA  102  can achieve a low gear ratio, i.e., a ratio of the motor rotor displacement and the output joint displacement that is less than about 30:1 or about 20:1. In one embodiment of the SEA  102 , the ball-Screw  114  typically delivers over about 2600 N of axial force at a screw pitch of 12 mm, delivering over 100 Nm of torque to the foot structure  112 . A dissipation constant of the motor  104  across a range of gear ratios (e.g., from about 15:1 up to about 80:1) is less than about 50 Watts/(Nm) 2 . The motor dissipation constant is a ratio of the total resistance R of the windings of the motor rotor and square of torque output by the motor per unit current supplied to the motor, denoted k t   2 . 
     In general, the torque output of a motor increases with the current drawn by the motor, which is related to the power supplied to the motor. However, the portion of the supplied power that is lost and dissipated as heat is proportional to the square of the current drawn by the motor. Therefore, as more power is supplied to a motor, the fraction of that power that increases the torque output of the motor can be less than the fraction that is wasted in the form of heat dissipation. Therefore, the motor  104 , which has a low dissipation constant, i.e., 
             R     k   t   2           
less than about 50 W/(Nm) 2 , can deliver high torque with low winding loss compared to other motors having a greater dissipation constant. As such, the motor  104  dissipates less heat, keeping the prosthesis  100  cool, and also requites less power, thereby increasing battery life.
 
     In operation, in addition to providing torque to the ankle pivot  110  (e.g., at or near the end of the controlled dorsiflexion phase and/or in the powered plantar flexion phase of the gait cycle) the motor  104  may also provide an impedance and joint equilibrium to the ankle pivot  110 , for example, to achieve an ankle (joint, in general) equilibrium trajectory during the swing phase of the gait cycle. Similarly, as in the application of torque as described above, the motor  104  can cause displacement of the ball nut  120 , applying a force to the series spring  108  which, in turn, provides the required impedance to the ankle pivot  110  with respect to the joint equilibrium trajectory. 
     In some embodiments, the motor leads are shorted, such that the motor draws substantially no current and operates as a dynamic mechanical clutch. This can enable an ankle or other augmentation device to provide stability during loss of battery or system malfunction. The shorted leads mode exerts a viscous damping torque on the motor, proportional to k t   2 /R. As measured at the output of the transmission, the viscous damping is amplified by the square of the gear ratio, kg, yielding a transmission damping, B, of kg 2 ×k t   2 /R. For an SEA with series stiffness, K SEA , the time constant of the dynamic (viscous) clutch is B/K SEA . In some embodiments that store energy in the series spring for rapid release later, it is useful to apply the viscous clutch at a time when the desired spring energy is achieved. Within a small time period in relation to the time constant above, the transmission is effectively a static brake, enabling the spring to release and deliver power to the joint. Such a mode of operation is useful in slow walking, where consistent and quiet power is desired, and in running, where the ankle functions primarily as a spring, and the series spring release occurs in less than 50 milliseconds. Such a mode is also useful in control of a knee in early stance, to deliver high torque through the series spring with no battery power. In all of the above embodiments, the clutch is used to apply high torque but without substantially drawing energy from the battery. 
     Thus, in general, in an SEA having a certain gear ratio and a certain series spring constant, the smaller the motor dissipation constant the longer the duration for which the applied stiffness (a component of the impedance) can be substantially maintained after shorting the motor leads. Thus, in an SEA using a motor having a large dissipation constant and, consequently, having a duration for which stiffness can be substantially maintained without drawing current that is shorter than the time period for which the equilibrium needs to be maintained, the power supplied to the motor cannot be turned off without adversely affecting the joint (e.g., ankle) equilibrium. In the SEA  102 , however, if the gear ratio of the transmission  106  is about 40:1, the motor dissipation constant is about 10, and the spring constant of the series spring  108  is about 400, the SEA  102  can maintain the applied stiffness for a time constant (i.e., holding time) of about 250 milliseconds. Typically during the stance phase of the gait cycle while walking or running, this holding time is sufficient to maintain a roughly fixed joint equilibrium for a required duration, typically about 50-100 milliseconds for walking and running. Accordingly, as the motor  104  draws substantially zero current after shorting the leads, a further reduction in the power consumption of the SEA  102  is achieved while simultaneously achieving ankle equilibrium. 
     With reference to  FIG. 1C , the three-phase stator assembly  122  of the motor  104  wraps around the rotor  124  to facilitate mounting of the motor to the prosthesis housing, e.g., using a needle bearing component. A bellows  126  protects the screw  114  from contamination. The ball-nut  120  employs an end-flexure  128  to isolate the thrust bearings  118  from out-of-plane moment loads as shown in Section A-A in  FIG. 1B . The end-flexure  128  can move side-to-side so as to eliminate side-loads, further isolating the thrust bearings  118  from moments applied by the series spring  108 . Typically, thrust loads on the end-flexure  128  are supported by needle bearings press fit into a end-flexure mounting hole  130 . 
       FIG. 2  depicts a biomimetic ankle-foot prosthesis that uses a direct-drive rotary actuator  200  with a cable transmission. The actuator  200  employs a high-torque, transverse-flux, external rotor motor  202  to directly drive the ankle output pulley  204  via a cable  206 . The cable can be a synthetic cable or a steel cable. In some embodiments, a belt of a chain drive may be used instead of a synthetic or steel cable. Motors other than transverse flux motors, but having an external rotor may also be used. A rotary series spring connects the ankle output pulley  204  to the ankle output joint  208 . The rotor of the motor  202  may be captivated by ankle shells using needle bearings. 
     Absolute angular displacement of the ankle output pulley  204  and of the rotor of the motor  202  may be used, as described above with reference to  FIGS. 1A and 1B  to determine the state of the actuator  200 . Magnetic field angle encoders may be used instead of absolute encoders. The flex in the cable  206  may be measured based on the span (length) of the cable, which is related to the difference between the output joint position and the motor position. With high resolution encoders, cable stretch can be sensed with sufficient bandwidth and resolution for closed-loop control. The flex in the cable  206  can then be compensated in an output torque feedback loop. The cable  206  in the actuator  200  can achieve a gear ratio, i.e., the ratio of the motor angle and the angle of the output joint, i.e., ankle pivot  208 , of about 20:1, in one embodiment. 
       FIGS. 3A-3C  illustrate a biomimetic knee prosthesis  300  that uses a direct-drive, ball-screw based system coupled to a series spring connecting the ball-screw transmission to the knee joint output. The device  300  controls the equilibrium position of the knee joint  302 , and applies torque or impedance to the knee joint  302  substantially along the centerline of the output pulley  304 . The knee prosthesis  300  can deliver about 200 Mm of torque over a range of about 120 degrees of angular displacement of the artificial knee joint  302  useful for stair and steep ramp ascent as well as for level ground walking. A transverse-flux, motor  306  (or other high-torque, external rotor motor) drives a screw  308  thereby driving a ball-nut  310  supported by a linear rail  312 . A retaining nut  314  preloads the angular contact bearings  316  inside the motor  306 . 
     A cable attachment device  318 , also supported by the linear rail  312 , connects to the ball-nut  310  via series springs  320 ,  322  and linearly drives the cable  324 . The cable  324  wraps around a light-weight pulley  326 , an idler pulley  328 , and the output pulley  304  to apply torque/impedance to the knee joint  302  about the knee axis  330 . The cable  324  can be a synthetic cable, a steel cable, a belt, or a chain drive. 
     The pitch of the ball screw  308  is in the range of approximately 6 mm up to about 10 mm so as to achieve a low gear ratio of less than about 30:1, of less than about 20:1. The gear ratio is a ratio of the respective angular displacements of the rotor of motor  306  and the knee joint  302 . An absolute magnetic encoder  330  having at least a 13-bit resolution (e.g. RMB-20 manufactured by Renishaw), is used to measure the angular displacement of the motor rotor in relation to the stator, and an absolute magnetic encoder  334 , which may also have at least a 13-bit resolution, is used to measure the angular displacement of the lower knee structure relative to the upper knee structure. The motor and knee joint angles are used, respectively, to determine the states of the motor  306  and the knee  302 , and can also be used to estimate the state of the series springs  320 ,  322 . For redundancy in sensing, a linear series-spring deflection potentiometer  336  is optionally included to measure series-spring deflections directly. Based on the series spring state and stiffness (i.e. spring constant), series-elastic actuator force and joint torque supplied by the SEA can be determined. (See for example, the co-pending U.S. patent application Ser. No. 13/079,564 “Controlling Power in a Prosthesis or Orthosis Based on Predicted Walking Speed or Surrogate for Same” filed on Apr. 4, 2011; U.S. patent application Ser. No. 13/079,571 “Controlling Torque in a Prosthesis or Orthosis Based on a Deflection of Series Elastic Element” filed on Apr. 4, 2011). 
       FIG. 4  depicts a biomimetic knee prosthesis that uses a direct-drive rotary actuator  400  using cable transmission, similar to that described above with reference to  FIG. 2 . A high-torque external rotor motor  402  (e.g., a transverse flux motor) is configured in a “pancake” arrangement to minimize stack height and to maximize torque-current gain. The motor  402  drives a pulley  404  and, through a direct-cable transmission  406 , drives the output pulley  408 . A rotary series spring couples the knee output pulley  408  to the knee output joint  410 . Thus, rotating the output pulley  408 , in turn, causes the knee joint  410  to rotate. Absolute encoders on the motor  403  and on the output pulley  408  may be used to measure the state of the actuator  400  similarly as described above with reference to  FIGS. 1A and 1B . The SEA  400  yields a low gear ratio, i.e., the ratio of the motor angle and the knee joint angle, of about 20:1. 
     Although various direct-drive SEAs are described above as components of wearable robot ankle and knee prosthetic devices, this is for illustrative purposes only. Hip prosthetic devices are also contemplated. To those skilled in the art, it should be apparent that these SEAs can be readily adapted for use in wearable robot ankle, knee, and hip orthotic devices, wearable robots for upper-extremity orthotic and prosthetic devices, and in humanoid robots. It should also be understood that although the powered actuators described herein take advantage of some of the key attributes of transverse flux motors, specifically high torque density and efficiency, these actuators can also leverage other high-torque motors, including hybrid stepping motors, induction motors, traditional radially-applied permanent magnet motors, and variable reluctance motors. The speed of these motors may be less than 5000 rpm, or less than 1500 rpm, or less than 300 rpm, depending on the optimum system design as defined by the motor, transmission gear ratio, series-spring stiffness, parallel elasticity, and battery power source; optimum generally referring to a tradeoff of battery economy per stride, design life of the transmission, and device weight. Table 1 in  FIG. 5  shows various design and operating parameters of the powered SEAs according to various embodiments. The minimum, typical, and maximum values of these parameters are also listed in Table 1. A typical wearer weighs in the range from about 190 lbs up to about 250 lbs. Wearer mass is used to normalize the actuator weight. Series Stiffness (i.e., spring constant of the serially connected elastic element) and Parallel Stiffness (i.e., spring constant of the optional elastic element connected in parallel with the serially connected elastic element). 
     While the invention has been particularly shown and described with reference to specific embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced.