Abstract:
An actuation system for a joint includes an actuator having a piston, an articulating element coupled to the actuator that is driven by the piston to match a gait cycle of an appendage, an energy source engaged with the piston that energizes working fluid within the actuator to drive the piston and generate exhaust gas, a plurality of valves coupled to the actuator, and an exhaust gas holding element coupled to the actuator that is sized to hold a volume of the exhaust gas contained within the actuator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a non-provisional of and claims priority to U.S. Provisional Application No. 61/851,750, filed Mar. 13, 2013, the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Some actuation systems for joints currently utilize battery power to actuate movement of one or more components in the system. However, these systems lose approximately 40% of the battery&#39;s stored chemical-potential energy into the atmosphere, primarily as heat, which can lead to faster system degradation and safety issues, particularly when operating in close proximity to humans. Also, due to their lower energy density, small batteries require frequent recharging, which is often impractical, and large, heavy batteries that provide longer operation times consume more of the available energy just to accelerate the additional mass of the battery. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an actuation system for a joint, and in particular to a high-efficiency biomimetic articulated joint actuation system. 
     In one construction, the invention provides an actuation system for a joint that includes an actuator having a piston, an articulating element coupled to the actuator that is driven by the piston to match a gait cycle of an appendage, an energy source engaged with the piston that energizes working fluid within the actuator to drive the piston and generate exhaust gas, a plurality of valves coupled to the actuator, and an exhaust gas holding element coupled to the actuator that is sized to hold a volume of the exhaust gas contained within the actuator. 
     In another construction, the invention provides a method of using an actuation system for a joint that includes coupling an actuation system to an appendage, the actuation system including a primary actuator and a secondary actuator, the primary and secondary actuators working in dual, opposing motion to one another as the appendage moves through a gait cycle. The method also includes moving a piston within the primary actuator to generate a first actuation movement of an articulating element coupled to the primary actuator, directing exhaust into the second actuator to power a second actuation movement of the articulating element, and matching the gait cycle of the appendage with a thermodynamic cycle of the actuation system. 
     In another construction, the invention provides a method of harvesting energy with an actuation system for a joint includes coupling an actuation system to an appendage, the actuation system including a primary actuator powered by a mixture of air and fuel. The method also includes pressing a portion of the appendage toward a surface to engage and activate a hydraulic bellows on the actuation system, and injecting air and fuel into the primary actuator with the activated hydraulic bellows. 
     Other aspects of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an actuation system for a joint according to one construction of the invention. 
         FIG. 2  is a schematic process diagram illustrating energy flow through the actuation system. 
         FIG. 3  is an ankle power versus time diagram illustrating an ideal power spike for a first actuation of the actuation system. 
         FIGS. 4-8  are schematic illustrations of the actuation system of  FIG. 1 , with pistons that allow for variable diameters and stroke lengths. 
         FIGS. 9-13  are schematic illustrations of an actuation system according to one construction with pistons that allow for variable diameters. 
         FIGS. 14-18  are schematic illustrations of an actuation system according to one construction with a single dual-acting piston. 
         FIGS. 19-24  are diagrams illustrating a thermodynamic cycle for the actuation system of  FIG. 1 , the thermodynamic cycles matching the gait cycle of human leg. 
         FIG. 25  is a pressure-volume diagram that relates work done by both pistons in the actuation system of  FIG. 1 . 
         FIG. 26  is a schematic illustration of an energy-flow diagram for the actuation system of  FIG. 1 . 
         FIG. 27  is a schematic illustration of an actuation system according to one construction with a lever arm and two pistons. 
         FIG. 28  is a schematic illustration of an actuation system according to one construction with a lever arm and a single dual-acting piston. 
         FIGS. 29-31  are schematic illustrations of the actuation system of  FIG. 28  in use, the actuation system having the same thermodynamic cycle as that in  FIGS. 19-24 . 
         FIGS. 32 and 33  are schematic illustrations of a plurality of the actuation systems of  FIG. 1  as used on a human or biomimetic-robot leg. 
         FIGS. 34 and 35  are schematic illustrations of a plurality of the actuation systems of  FIG. 1  as used on animal or animal-like (biomimetic) robot non-leg appendages (e.g., fins or wings). 
     
    
    
     DETAILED DESCRIPTION 
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. 
       FIG. 1  illustrates an actuation system  10  for a joint. The system  10  includes an articulating component  14 . In the illustrated construction, the articulating component  14  is a platform or shoe worn by a user. The articulating component  14  is form-fitted for at least a portion of a foot  18  of the user to fit within the articulating component  14 , and provides support for the foot  18  during a gait cycle of the user&#39;s leg. 
     With continued reference to  FIG. 1 , the system  10  also includes a set of independent pulleys  22 ,  26  coupled to the articulating component  14 , and first and second tension cables  30 ,  34  coupled to the pulleys  22 ,  26 . The pulleys  22 ,  26  mimic a pivoting, torsional action of a human ankle, and the tension cables  30 ,  34  mimic actions of tendons in the human body. 
     As illustrated in  FIG. 1 , the tension cable  30 , which includes a custom compliance shock and piston extension  36 , is coupled to the pulley  22 , and the tension cable  34  (which also includes a custom compliance shock and piston extension  36 ) is coupled to the pulley  26 , so that upward movement (i.e., in a direction away from the articulating component  14 ) of the tension cable  30  generates a first pivoting movement (i.e., plantarflexion) of the articulating component  14 , and upward movement of the tension cable  34  generates an opposite pivoting movement (i.e., dorsiflexion) of the articulating component  14 , mimicking the natural movement of a human foot during the gait cycle.  FIG. 1  further illustrates an example of an angle  38  through which one of the pulleys (i.e., pulley  26 ) moves. 
     With continued reference to  FIG. 1 , the pulleys  22 ,  26  convert linear force (corresponding to the up and down movement of the tension cables  30 ,  34 ) into torque (corresponding to the pivoting action of the articulating component  14 ). The pulley  26  is separate from the pulley  22 , and latches in only one direction. While the pulleys  22 ,  26  in the illustrated construction are circular, in some constructions the pulleys have radii that change about their axes to optimize torque output with respect to angular position. 
     With continued reference to  FIG. 1 , the system  10  also includes a first actuator  42  coupled to the first tension cable  26 . The actuator  42  includes a combustion chamber  46  that houses a reciprocating, shaped piston  50  that is coupled to the tension cable  30  (e.g., with the custom compliance shock and piston extension  36 ). Movement of the piston  50  generates tension within the tension cable  30 , which causes the articulating component  14  to swing. 
     The actuator  42  includes a fuel cartridge  54  coupled to the combustion chamber  46  to deliver fuel (e.g., butane) to the combustion chamber  46 . As illustrated in  FIG. 1 , the fuel passes through a valve  58  into the combustion chamber  46 . 
     The actuator  42  also includes an air injection component  62  coupled to the combustion chamber  46  to deliver air to the combustion chamber  46 . As illustrated in  FIG. 1 , air is drawn in through a valve  64  from the ambient environment, and passes out through a valve  66  into the combustion chamber  46 . 
     To deliver fuel and air into the combustion chamber  46  the illustrated system  10  utilizes harvested energy from movement of the foot  18  and the articulating component  14 . In particular, the system  10  includes a heel-strike-harvesting hydraulic bellows pump  70  coupled to a bottom, heel portion  74  of the articulating component  14 . The pump  70  is activated (i.e., compressed) when the heel portion  74  strikes a supporting surface (e.g., a floor or ground surface), generating a flow of fluid that moves through a set of conduits  78 . 
     With continued reference to  FIG. 1 , the conduits  78  deliver at least a portion of the flow of fluid to a hydraulic bellows (or bladder) fuel injector  82  coupled to the fuel cartridge  54 , where the energy transferred through the fluid is used to eject fuel from the fuel cartridge  54  through the valve  58  and into the combustion chamber  46 . 
     The conduits  78  also deliver at least a portion of the flow of fluid to a hydraulic bladder  86  in the air injection component  62 , where the bladder  86  is used to push (e.g., compress) air disposed within the air injection component  62  and force the air through the valve  66  and into the combustion chamber  46 . 
     With continued reference to  FIG. 1 , the system  10  includes a hydraulic reservoir  90  and a hydraulic solenoid valve or valves  94  that control whether the flow of fluid from the pump  70  is moved to the hydraulic reservoir  90 , or to the fuel injector  82  or bladder  86 . Control of the solenoid valve  94  is based on whether the system  10  is in a walking state or a standing state. For example, in the walking state, the valve or valves  94  are opened so that fluid may move to the fuel injector  82  and the bladder  86 . In other constructions, the delivery of fuel and air to the combustion chamber  46  is powered with an integrated power source (e.g., a battery), as opposed to being powered by harvested energy. 
     With continued reference to  FIG. 1 , the system  10  also includes a spark plug  98  that is coupled to the combustion chamber  46 . The spark plug  98  ignites the combined fuel and air mixture that is injected into the combustion chamber  46  by the fuel injector  82  and the air injection component  62 , so as to generate combustion within the chamber  46  and drive motion of the piston  50 . 
     In the illustrated construction the system  10  utilizes harvested energy from movement of the articulating component  14  (or the system  10  as a whole) to power the spark plug  98 . In particular, the system  10  includes a piezoelectric spark generator  102  that is coupled via a wire  104  to the spark plug  98 . As the articulating component  14  (or the system  10  as a whole) moves, the movement generates electricity in the piezoelectric spark generator  102 , which then provides power to the spark plug  98 . In other constructions, the spark plug  98  is powered with an integrated power source (e.g., a battery), as opposed to being powered by harvested energy. 
     With continued reference to  FIG. 1 , the system  10  also includes an exhaust-heat recuperator  106 , a thermoelectric generator (TEG)  110 , and a phase-change material (PCM)  114 , each coupled to the actuator  42 . The exhaust-heat recuperator  106 , thermoelectric generator  110 , and phase-change material  114  provide added power and energy conservation to the system  10 . For example, the phase-change material  114  recovers waste heat and supplies it to other regions for greater work production, while also maintaining a safe operating temperature for the user of the system  10 . 
     With continued reference to  FIG. 1 , the system  10  also includes a second actuator  118  that is coupled to the first actuator  42  and drives movement of the tension cable  34 , the pulley  26 , and the articulating component  14 . The second actuator  118  includes a reciprocating, shaped piston  122  disposed within a piston chamber  126 . The reciprocating piston  122  is coupled to the tension cable  34  (e.g., with the custom compliance shock and piston extension  36 ), such that when the piston  122  moves, the tension cable  34  and pulley  26  also move. 
     The second actuator  118  is an element that holds a volume of exhaust gas, is coupled to the first actuator  42  with an exhaust line  130 , and is driven by exhaust gas from the first actuator  42 . As illustrated in  FIG. 1 , the first actuator  42  includes an exhaust valve  134  that opens into the exhaust line  130 , as well as a solenoid pneumatic valve  138  that controls movement of exhaust from the first actuator  42 . When the pneumatic valve  138  is closed (e.g., during a walking state), the exhaust from the first actuator  42  is able to pass through the exhaust valve  134  and into the exhaust line  130 . When the pneumatic valve  138  is open, exhaust from the first actuator  42  may be expelled into the ambient environment or to one of the exhaust-heat recuperator  106 , thermoelectric generator (TEG)  110 , and/or phase-change material (PCM)  114 . 
     With continued reference to  FIG. 1 , the second actuator  118  includes a plurality of pneumatic valves  142  that control whether exhaust from the exhaust line  130  enters the piston chamber  126  or whether exhaust in the piston chamber  126  is expelled into the ambient environment. In the illustrated construction the valves  142  are powered by the thermoelectric generator  110 . 
     When the exhaust from the exhaust line  130  enters the piston chamber  126 , the exhaust has sufficient kinetic and/or thermal energy to power movement of the piston  122 , causing movement of the tension cable  34  and pulley  26 , as well as the articulating component  14 . Once the exhaust has powered the piston  122 , the exhaust is expelled from the piston chamber  126 . 
     With continued reference to both  FIGS. 1 and 2 , the system  10  overall is an antagonistic articulated joint actuation system that moves in correlation with a gait cycle of the foot  18 . The first actuator  42  provides a first, primary actuation (i.e., a “step off” or plantarflexion) and the second actuator  118  provides a secondary actuation (i.e., a “toe up” or dorsiflexion), the secondary actuation requiring less work than the primary actuation. The system  10  is also a bio-mimetic system, using a timed release of chemical energy for motion just as is performed by muscles, with the actuators  42 ,  118  taking the place of (i.e., mimicking) the muscles. 
     With reference to  FIGS. 1 and 2 , and as described above, the system  10  directs fluid from the bellows pump  70  to the air injection component  62 , where the air is compressed (referenced by element  144  in  FIG. 2 ) and then injected into the combustion chamber  46 . The fluid is also directed to the bellows or bladder  82  behind the fuel cartridge  54  opening valve  58  and injecting fuel into the combustion chamber  46 , thereby forming an air/fuel mixture (referenced by element  146  in  FIG. 2 ). With reference to element  148  in  FIG. 2 , in some constructions, the system  10  uses harvested energy (e.g., from the bellows pump  70 , or simply through movement of the foot  18  and articulating component  14 ), to compress the air/fuel mixture  146  within the chamber  46 . 
     With continued reference to  FIGS. 1 and 2 , the compressed air/fuel mixture  146  undergoes combustion (referenced by element  150  in  FIG. 2 ) in the chamber  46 . The combustion  150  moves the piston  50 , which moves the tension cable  30  and pulley  22 , resulting in the first, primary actuation or “step off” (referenced by element  154  in  FIG. 2 ). The combustion  150  also generates exhaust. Some of the exhaust (as represented by element  158  in  FIG. 2 ), is directed through the pneumatic valve  138  and to the ambient environment or to one of the exhaust-heat recuperator  106 , thermoelectric generator  110 , and/or phase-change material  114 , in order to store and recover energy from the exhaust  158 . As illustrated in  FIG. 2 , this energy is then re-directed to the air/fuel mixture  146 . Conduction (referenced by element  162  in  FIG. 2 ) also occurs, which directs energy to the exhaust-heat recuperator  106 , thermoelectric generator (TEG)  110 , and/or phase-change material (PCM)  114 , and in some constructions also back to the air injection component  62 . Some waste heat (referenced by element  166  in  FIG. 2 ) is generated from the exhaust  158  and the exhaust-heat recuperator  106 , thermoelectric generator (TEG)  110 , and/or phase-change material (PCM)  114 . 
     With continued reference to  FIGS. 1 and 2 , the rest of the exhaust from the combustion  150  is directed through one of the valves  142  to the piston chamber  126 , causing expansion in the chamber  126  (referenced as element  168  in  FIG. 2 ) and generating the secondary actuation or “toe up” (referenced as element  170  in  FIG. 2 ). The secondary actuation  170  is an example of exhaust energy recovery that is used in the system  10 . After the secondary actuation  170  occurs, exiting exhaust (referenced as element  174  in  FIG. 2 ) then passes through another one of the valves  142  and becomes part of the waste heat  166 . 
     In use, the system  10  produces engine noise that is less significant than conventional engine noise, such that the system  10  is not distracting or otherwise disruptive to the user. 
       FIG. 3  illustrates ankle power versus time and an ideal power spike  178  required of the actuator  42  to duplicate the torque (i.e., the “step off”  154 ) of an un-impaired human. This ideal power spike is best generated by a chemical release of energy, preferably within the chamber  46  of the actuator  42 . For a system  10  implemented as an orthosis, where weight is a premium, combustion of fuel provides, for example, a 500 psi minimum operating pressure to make the system  10  competitive with electric and electro-hydraulic systems that try to approximate this ideal power spike  178 . 
     With reference to  FIGS. 4-8 , and as described above, the illustrated system  10  includes two pistons  50 ,  122  disposed in separate actuators  42 ,  118 . The pistons  50 ,  122  may have variable diameters and stroke lengths (for pulley linkages like that in  FIG. 2 , as well as for lever-arm linkages as described further herein). 
     With reference to  FIGS. 9-13 , in some constructions the system  10  includes pistons  50 ,  122  that may have just variable diameters. 
     With reference to  FIGS. 14-18 , in some constructions the system  10  includes one dual-acting piston  50  (instead of two pistons) that generates both the first, primary actuation  154  and the secondary actuation  170  through re-routing of the exhaust. Multiple smaller chambers can also be used in parallel for compactness. 
     In some constructions, the system  10  is configured for rotary actuation (e.g., of the articulating component  14 ), whereas in other constructions the system  10  is configured for linear actuation of a component (e.g., a lever-arm linkage). 
     With reference to  FIGS. 19-24 , the system  10  includes a thermodynamic cycle  182 .  FIGS. 19-21  illustrate the first three stages of a preliminary portion of the cycle  182  and their relation to the gait cycle. With reference to  FIG. 19 , air and fuel are injected into the chamber  46 . This may be accomplished with energy recovered from some exhausted fluid, or harvested with the bellows pump  70  as described above. With reference to  FIG. 20 , the translation of a human body&#39;s center of mass with respect to the ankle then provides a torque that can be harvested to compress the air/fuel mixture  146  in the chamber  46 . There is a tradeoff here between potential energy production and hindering the gait cycle, as the chamber  46  pressure increases. The fluid does not reach the auto-ignition temperature until the end of this stage (like top-dead-center for a diesel). With reference to  FIG. 21 , compression or spark ignition (e.g., from the spark plug  98 ) initiates combustion, and subsequent expansion, of the air/fuel mixture  146  in the chamber  46 , providing the first actuation  154  (i.e., the “step-off” or plantarflexion) work required. 
       FIGS. 22 and 23  illustrate the last two stages of the preliminary portion, and their relation to the gait cycle. With reference to  FIG. 22 , a division of the combustion products occurs. With reference to  FIG. 23 , expansion of some combustion products for the secondary actuation  170  (i.e., the “toe-up” or dorsiflexion) actuation occurs, and a remainder of the exhaust is released to the atmosphere. 
       FIG. 24  illustrates a reset/external cool-down portion of the cycle  182 . 
     While reciprocating piston engines typically have peak thermodynamic efficiencies of only 30%, a gait-matched, recuperative (co-generation) thermodynamic cycle  182  like that illustrated in  FIGS. 19-24  for the system  10  provides a far greater system efficiency than current state-of-the-art pneumatic actuation systems, with a targeted minimal thermodynamic efficiency of 60%. Further, the high power-density of the system  10  leads to compact, light-weight powerplant design, increasing payload, and/or extending operation. 
     With reference to  FIG. 25 , a pressure-volume diagram is illustrated that relates an example of work done by both the piston  50  and the piston  122 . In some constructions, fifty percent of the combustion products in the primary actuator  42  are routed to the secondary actuator  118 , improving thermodynamic efficiency by eight percent. 
       FIG. 26  schematically illustrates an energy-flow diagram for the system  10 , displaying process efficiencies, and depicting energy-recovery capability. 
     With reference to  FIG. 27 , in some constructions the system  10  includes actuators  42 ,  118  that act on a lever arm  186  (as opposed to a pulley like in  FIG. 1 ) to rotate the lever arm  186  about a pivot point or axis  190  by an angle  194 , the lever arm  186  being coupled to the pistons  50 ,  122  at points  188  along the lever arm  186 . The system  10  produces torque about the axis  190 . At the end of the stroke for the lower, high-force actuator  42 , exhaust gas is routed via a fluid line(s)  198  to the secondary actuator  118  for the return stroke. Here, this secondary actuator  118  is smaller in diameter, but is longer and has greater mechanical advantage. In this construction exhaust is directed into the retracting piston  50 ,  122  until a difference in torque is zero, wherein exhaust is then released to the atmosphere while the piston  50 ,  122  continues to expand. 
     With reference to  FIG. 28 , in some constructions the actuators  42 ,  118  are embodied as a single actuator  202  with a single piston  206 , so the stroke and mechanical advantage is the same for both. A common piston diameter is illustrated, but two different diameters could be used. Exhaust is moved into a back end of the piston  206  with fluid line  198  until a pressure differential is zero, wherein exhaust is then released to the atmosphere while the piston  206  continues to expand. This system  10  may use a NiTiNOL wire, for example, to activate sliding exhaust vent(s). 
     With reference to  FIG. 29  (and to  FIG. 21 , which is the corresponding thermodynamic cycle operation), in the indicated range of the gait cycle the air/fuel mixture  146  is combusted, providing the primary torque to the articulated joint. 
     With reference to  FIG. 30  (and to  FIG. 22 , which is the corresponding thermodynamic cycle operation), in the indicated range of the gait cycle the cylinder has reached full extension. Here, the energized gas is released, no shaft work is being done, and some thermal energy is lost. The energized gas can either be fed directly into the other end of the actuator  202  until the pressure on both sides equalizes, or held in connective fluid lines  198  at an interim pressure. The fluid lines  198  function as an accumulator and in some constructions are modified to include a “traditional” accumulator. 
     For increased efficiency, some of the remaining gas on the primary side may be used for air and fuel injection, either for other actuators in the system, or stored for the next cycle of originating actuator. 
     With reference to  FIG. 31  (and to  FIG. 23 , which is the corresponding thermodynamic cycle operation), the energized gas has been fed directly to the other side of the actuator  202  until the pressure is equalized, and the actuator  202  has harvested a small amount of energy to move the piston  206  to an interim position. At this point, the remaining gas on the primary side is released to the atmosphere, while that on the secondary side is allowed to expand further, providing additional shaft work, in the opposite direction from the primary side. 
     The actuation systems described above provide asymmetric, antagonistic articulated-joint actuation. While the actuation system  10  is illustrated in the context of an orthosis device for a human foot  18 , in other constructions the articulation system  10  is used on other joints and appendages, including but not limited to arms, elbows, hips, etc., and/or is used in combination with other similar systems  10 . For example, and with reference to  FIGS. 32 and 33 , in some constructions multiple systems  10  (or a single system  10  with numerous actuators coupled together) are used on a human leg  210 . The systems  10  define a human-assistive exoskeleton, and assist in walking and/or running The fuel supply and exhaust routing in these systems  10  can be made similar to the vasculature of organic systems. Fuel may act as a thermal-energy transport (e.g., to transport out waste heat to prevent overheating, or to transfer in heat). In some constructions the fuel may be mixed with other fluid coolant, and separated locally for actuation. 
     In some constructions, the actuation system  10  is used not as an orthosis device, but rather as a full replacement for the joint or appendage (i.e., as a prosthetic). 
     With reference to  FIGS. 34 and 35 , in some constructions the actuation system  10  is used on non-human joints and appendages. For example, in some constructions the actuation system  10  is used on animal and animal-like (biomimetic) robot joints or appendages  214  (e.g., legs, wings, tails) to provide assistance and movement to animal joints and appendages, and to utilize cycles (e.g., gait cycles) associated with such joints and appendages. In some constructions the system  10  is used underwater, to assist in swimming gait cycles, or in the assistance of flying. In some constructions exhaust gases are used for ballast and buoyancy control, manipulating a center of gravity as desired. When surrounded by fluid and not in contact with ground, the balance between buoyancy and gravity can help to drive the system  10 . With reference to  FIG. 34 , for example, a main body of a wing appendage  214  can have its buoyancy reduced after primary actuation, and as it falls, the higher drag on the wing appendage  214  can be used to compress the air/fuel mixture  146  for the next downward propulsion stroke. In some constructions compressed-gas stores may provide an air/fuel-compression energy for takeoff, and other bursts of energy as needed. Exhaust gases from the system may replenish this source. 
     In yet other constructions the system  10  is used in conjunction with one or more other actuations systems  10  to form an entire robot or other mechanical structure that is capable of various degrees of articulated movement and gait cycles. 
     The system  10  described herein provides efficient, high-torque, light-weight, compact locomotion in the gait cycle. A portion of the residual thermal energy contained within the acting fluid for the “power stroke” (e.g., plantarflexion) of the gait cycle is recovered to drive the “return stroke” (e.g., dorsiflexion) and to provide asymmetric, antagonistic actuation of an articulated joint. Energy recovery (i.e., through using the exhaust from the primary actuator  42  to power the second actuator  118 ) significantly improves the efficiency of the system and the charging of the fluid through chemical-energy release affords higher torques (greater power density). 
     The highly-energized fluid to be exhausted at the end of the “step-off” (plantarflexion) phase of the gait cycle is used for the secondary actuation phase “toe-up” (dorsiflexion) that requires significantly less power. This energy recovery increases the efficiency of the actuator&#39;s operation. 
     The energy density of hydrocarbons in the system  10  remains much higher than that of battery cells, allowing for more compact and lighter-weight designs. Additionally, the energy is employed as completely as possible. As described above, butane is one type of fuel that may be used with the system  10 . Butane is advantageous because it burns cleanly, and is readily available in refillable cartridges  54 . The low fuel-consumption rate of the system  10  means that the emissions of CO 2  are also low, comparable to that produced by the muscles themselves. 
     Other energy sources aside from fuel combustion with butane or other fuels (e.g., natural methane gas) include compressed gas tanks and catalytic decomposition (e.g., H 2 O 2  (chemofluidics)). 
     Although the invention has been described in detail with reference to certain preferred embodiments, variations and modifications exist within the scope and spirit of one or more independent aspects of the invention as described.