Patent Publication Number: US-2018036148-A1

Title: Energy-harvesting mesofluidic impulse prosthesis

Description:
STATEMENT OF U.S. GOVERNMENT LICENSE RIGHTS 
     This invention was made with U.S. Government support under R43HD080309 awarded by National Institutes of Health. The U.S. Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Prosthesis users have adapted to the limitations of current prostheses by abnormal changes to gait which have been shown to result in reduced metabolic efficiency and increased loading at more proximal joints and on the sound limb. Long-term deleterious effects include skin breakdown on the residual limb, and overloading of the intact limb (gait asymmetry) with subsequent osteoarthritis. 
     A normal-functioning human ankle controls energy throughout the gait cycle, acting in turn as a dissipater, storage device, power producer, or energy neutral component. The ankle can produce as much as five times more work than is dissipated, but this is not required for many activities of daily living. Creating precisely timed impulses at the ankle joint is important to natural gait and motion, and active movement is particularly important for perceived effort, comfort, and stumble prevention. It would be advantageous for prostheses to similarly provide power output at specific moments in the gait cycle. 
     However, most current prosthetic ankles are either rigid bodies, transferring joint function to motion proximally and distally, or they generally function as springs. This latter type of flexible prosthetic feet have carbon fiber keels that initially bend in a plantarflexion direction, returning some energy as the tibial segment accelerates forward in early stance, then bend toward dorsiflexion, absorbing power during midstance, and then recoil toward plantarflexion again in late stance and pre-swing. Since these springs are passive, they do not return energy past the neutral position of the spring. 
     SUMMARY 
     Methods of harvesting and selectively reapplying energy to a prosthetic joint are disclosed. The methods may include providing a prosthetic joint. The prosthetic joint may include at least one chamber; at least one accumulator configured to store hydraulic fluid at a high pressure; at least one reservoir configured to store hydraulic fluid at a low pressure; one or more fluid flow paths connecting the chamber to the accumulator and the reservoir, flow controllers in the fluid flow paths, and fluid distributed throughout the chamber, accumulator and reservoir; a load-determining sensor; a displacement-determining sensor; a microprocessor configured to actuate one or more flow controllers based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof. The method further includes displacing fluid from the chamber to the accumulator during periods of a threshold negative work on the joint; and displacing fluid from the accumulator to the chamber to allow the joint to perform positive work. 
     In some embodiments, the method may further include displacing fluid from the chamber to the reservoir during periods below the threshold negative work. 
     In some embodiments, the prosthetic joint is an ankle joint, and the ankle joint is connected to a prosthetic foot and a pylon, wherein the ankle joint allows rotation of the prosthetic foot with respect to the pylon. 
     In some embodiments, the method may further include determining a flow controller state by determining a swing positioning state, a controlled plantarflexion state, a controlled dorsiflexion state, and a powered plantarflexion state. 
     In some embodiments, the method may further include storing energy in the accumulator during controlled dorsiflexion, or during controlled plantarflexion, or during both, and returning energy during powered plantarflexion. 
     In some embodiments, the method may further include returning energy during swing positioning. 
     In some embodiments, swing positioning includes dorsiflexing the foot and elevating the toe. 
     In some embodiments, the method may further include determining conditions to transition from the swing positioning state to the controlled plantarflexion state, conditions to transition from the controlled plantarflexion state to the controlled dorsiflexion state, conditions to transition from the controlled dorsiflexion state to the powered plantarflexion state, and conditions to transition from the powered plantarflexion state to the swing positioning state. 
     In some embodiments, in the swing positioning state, fluid is displaced from a posterior accumulator to a posterior chamber, and fluid is displaced from an anterior chamber to an anterior reservoir; in the controlled plantarflexion state, fluid is displaced from the posterior chamber to the posterior accumulator, and fluid is displaced from the anterior reservoir to the anterior chamber; in the controlled dorsiflexion state, fluid is displaced from the posterior reservoir to the posterior chamber, and fluid is displaced from the anterior chamber to the anterior accumulator; and in the powered plantarflexion state, fluid is displaced from the posterior chamber to the posterior reservoir, and fluid is displaced from the anterior accumulator to the anterior chamber. 
     In some embodiments, the threshold negative work is performed when a limb connected to the joint is applied on a ground surface to generate a ground reaction force greater than a pressure in the accumulator. 
     In some embodiments, the flow controllers include one or more automatically operated shut-off valves. 
     In some embodiments, the method further includes passing fluid through a restrictor when displacing fluid from the chamber to the reservoir. 
     In some embodiments, the method further includes producing the negative work above the threshold by contacting a limb connected to the joint on a surface to generate a ground reaction force. 
     In some embodiments, the displacement-determining sensor is an angle-determining sensor. 
     In some embodiments, the joint is a prosthetic knee joint. 
     In some embodiments, the method further includes storing energy in the accumulator when sitting from a standing position and returning energy when standing from a sitting position. 
     In some embodiments, the method further includes storing energy in the accumulator during descending and returning energy during ascending. 
     In some embodiments, the flow controllers are pulsed open during displacing fluid from the accumulator to the chamber. 
     Methods of harvesting energy from a first joint and selectively reapplying the energy to a second joint are disclosed. The methods include providing an energy-harvesting hydraulic system including at least one chamber; at least one accumulator configured to store hydraulic fluid at a high pressure; at least one reservoir configured to store hydraulic fluid at a low pressure; one or more fluid flow paths connecting the chamber to the accumulator and the reservoir, flow controllers in the fluid flow paths, and fluid distributed throughout the system; a load-determining sensor; a displacement-determining sensor; a microprocessor configured to actuate one or more flow controllers based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof. The methods include displacing fluid from the chamber to the accumulator during periods of a threshold negative work on a first joint; and displacing fluid from the accumulator to the chamber to allow a second joint to perform positive work. 
     In some embodiments, the first joint is an ankle and the second joint is a knee, or the first joint is the knee and the second joint is the ankle. 
     Prosthetic joints are disclosed. The joints may include a hydraulic system, including at least one chamber; at least one accumulator configured to store hydraulic fluid at a high pressure; at least one reservoir configured to store hydraulic fluid at a low pressure; one or more fluid flow paths connecting the chamber to the accumulator and the reservoir, flow controllers in the fluid flow paths, and fluid distributed throughout the hydraulic system; a load-determining sensor; a displacement-determining sensor; and a microprocessor configured to actuate one or more flow controllers based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof, wherein one or more flow controllers are configured to control displacing fluid from the chamber to the accumulator during periods of a threshold negative work, and one or more flow controllers are configured to control displacing fluid from the accumulator to the chamber to perform positive work. 
     In some embodiments, the joint may further include a piston in the chamber, wherein a limb is actuated by the piston during displacing fluid from the accumulator to the chamber. 
     In some embodiments, the limb actuates the piston during displacing fluid from the chamber to the accumulator. 
     In some embodiments, the joint may further include a cam and cam follower, wherein the cam follower is in contact with the cam, and the cam follower is connected to the piston. 
     In some embodiments, the cam includes an involute cam surface. 
     In some embodiments, the joint further includes a pivot, wherein the pivot rotates a first prosthetic limb with respect to a second prosthetic limb. 
     In some embodiments, the first prosthetic limb is a prosthetic foot, and the second prosthetic limb includes a pylon and socket. 
     In some embodiments, the joint further includes a first and second accumulator, a first and second reservoir, and a first and second chamber, wherein the first and second chambers are placed on opposite sides of a pivot, and the first chamber includes flow paths to the first accumulator and the first reservoir, and the second chamber includes flow paths to the second accumulator and the second reservoir. 
     In some embodiments, a fluid flow path from each chamber to the accumulator includes, in parallel, an automatically operated shut-off valve and a check valve, wherein the check valve is configured to allow flow from the chamber to the accumulator and obstruct flow from the accumulator to the chamber. 
     In some embodiments, a fluid flow path from each chamber to the reservoir includes an automatically operated shut-off valve and, in parallel, a restrictor and a check valve, wherein the check valve is configured to allow flow from the reservoir to the chamber and obstruct flow from the chamber to the reservoir. 
     In some embodiments, the load-determining sensor is a strain gauge. 
     In some embodiments, the load-determining sensor is a pressure transducer. 
     In some embodiments, the displacement-determining sensor is a potentiometer. 
     In some embodiments, the displacement-determining sensor is a hall effect sensor. 
     In some embodiments, the flow controllers include a solenoid valve. 
     Prosthetic joints are disclosed that may include a first and second connector and a pivot device that allows the first and second connector to rotate with respect to each other, wherein the first connector is configured to attach to a first prosthetic member and the second connector is configured to attach to a second prosthetic member; a first and second chamber, wherein the chambers are disposed on opposite sides of the pivot device; a first and second piston positioned in the first and second chamber, wherein the pistons are positioned to actuate the rotation of the joint; an accumulator configured to store hydraulic fluid at a high pressure, wherein the accumulator connects to each chamber through a flow path including, in parallel, a shut-off valve and a check valve, wherein the check valve is configured to allow flow from each respective chamber to the accumulator and obstruct flow from the accumulator to each respective chamber; a reservoir configured to store hydraulic fluid at a low pressure, wherein the reservoir connects to each chamber through a flow path including a shut-off valve; a load-determining sensor; a displacement-determining sensor; a microprocessor configured to actuate the shut-off valves based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof, for displacing fluid from one chamber at a time to the accumulator during periods of a threshold negative work on the joint, and displacing fluid from the accumulator to one chamber at a time to allow the joint to perform positive work. 
     Prosthetic joints are disclosed that may include a first and second connector and a pivot device that allows the first and second connector to rotate with respect to each other, wherein the first connector is configured to attach to a first prosthetic member and the second connector is configured to attach to a second prosthetic member; a first and second chamber, wherein the chambers are disposed on opposite sides of the pivot device; a first and second piston positioned in the first and second chamber, wherein the pistons are positioned to actuate the rotation of the joint; a first and second accumulator configured to store hydraulic fluid at a high pressure, wherein the first accumulator connects to the first chamber through a flow path including, in parallel, a shut-off valve and a check valve, wherein the check valve is configured to allow flow from the first chamber to the first accumulator and obstruct flow from the first accumulator to the first chamber, and the second accumulator connects to the second chamber through a flow path including, in parallel, a shut-off valve and a check valve, wherein the check valve is configured to allow flow from the second chamber to the second accumulator and obstruct flow from the second accumulator to the second chamber; a first and second reservoir configured to store hydraulic fluid at a low pressure, wherein the first reservoir connects to the first chamber through a flow path including, in parallel, shut-off valve and a check valve configured to allow flow from the first reservoir to the first chamber and obstruct flow from the first chamber to the first reservoir, and the second reservoir connects to the second chamber through a flow path including a shut-off valve and a check valve configured to allow flow from the second reservoir to the second chamber and obstruct flow from the second chamber to the second reservoir; a load-determining sensor; a displacement-determining sensor; a microprocessor configured to actuate the shut-off valves based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof, for displacing fluid from each chamber to the respective accumulator during periods of a threshold negative work on the joint, and for displacing fluid from each accumulator to the respective chamber to allow the joint to perform positive work. 
     Prosthetic joints are disclosed that may include a hydraulic system including: at least one chamber; at least one accumulator configured to store hydraulic fluid at a high pressure; at least one reservoir configured to store hydraulic fluid at a low pressure; one or more fluid flow paths connecting the chamber to the accumulator and reservoir, and flow controllers in the fluid flow paths; and hydraulic fluid in the system. The joints may further include a load-determining sensor; a displacement-determining sensor; a microprocessor to actuate the flow controllers based upon a load-determining sensor input, a displacement-determining sensor, any product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof, wherein the flow controllers are configured to displace fluid from the chamber to the accumulator during periods of a threshold negative work, and the flow controllers are configured to displace fluid from the accumulator to the chamber to perform positive work, and wherein the threshold negative work is performed on a first joint and the positive work is performed by a second joint different from the first joint. 
     Some embodiments of the prosthetic joints include flow controllers that are further configured to displace fluid from the chamber to the reservoir during periods below the threshold negative work. 
     The mechanical and hydraulic design of the energy-harvesting ankle is such that inherent mechanical properties are responsible for the bulk of the control intelligence, minimizing sensor requirements, electronic complexity, and cost. Furthermore, the inherent passive stability of the ankle joint and control system limits its potential to injure the user, providing clear benefits with respect to ensuring the safety of the amputee users for which it is intended. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a diagrammatical illustration of one embodiment of an energy-harvesting system for a prosthesis; 
         FIG. 2  is a diagrammatical illustration of one embodiment of an energy-harvesting system for a prosthesis; 
         FIG. 3  is a diagrammatical illustration of one embodiment of an energy-harvesting system for a prosthesis; 
         FIG. 4  is a diagrammatical illustration of one embodiment of an energy-harvesting system incorporated into a prosthetic joint and limbs; 
         FIG. 5  is a diagrammatical illustration of an energy-harvesting prosthesis in various phases of gait; 
         FIG. 6  is a finite state diagram of an energy-harvesting system; and 
         FIG. 7  is a diagrammatical illustration of an energy-harvesting system distributed in two prosthetic joints and limbs. 
     
    
    
     DETAILED DESCRIPTION 
     When people lose a limb due to illness or injury, a prosthesis may bring back some functionality and mobility to the person. With the loss of a limb comes the loss of muscle to move the limb. Accordingly, prosthesis can greatly benefit from having powered limbs. 
     A joint is any rotating element that can connect to two members or limbs. For example, an ankle joint connects the lower leg to the foot, a knee joint connects the lower leg to the upper leg, an elbow connects the lower arm to the upper arm, a hip joint connects the upper leg to the pelvis, and so on. 
     A prosthetic joint can move by application of an external force acting on one of the two limbs connected to the joint. Many prosthetic joints are only passive. That is, the joint is moved only when acted upon by external forces, such as when applying weight on the joint. Powered joints rely on batteries to power actuators that in turn power the limbs. In contrast to the joints that derive power from batteries, the joints disclosed herein derive power to move limbs from energy stored in a hydraulic system. The hydraulic systems described herein can allow joints to store energy during periods of negative work, and then, release the energy at selected periods when desired to power a limb. The hydraulic system also allows the joint to operate as a dampened joint during periods when the joint is neither storing energy nor being powered. 
     Work is the product of force and displacement. Thus, when a joint and limb are moved via the application of external forces, the joint experiences negative work, i.e., work done on the joint. The hydraulic systems described herein can use a fluid-filled system including a chamber with piston and a high pressure accumulator to store some of the energy during periods of negative work. The hydraulic systems described herein can then release the energy to the chamber from the high pressure accumulator, thus, causing the joint to do positive work and move a limb. During periods other than storing energy or releasing energy from the high pressure accumulator, fluid can be exchanged between the chamber and a reservoir. Periods during which negative work is produced so that energy may be harvested include periods during walking, particularly during periods in the stance phase. Other times for harvesting energy may include sitting from a standing position or vice versa. 
     A description of an ankle joint is used to describe several aspects of this disclosure. However, it is not meant to be limiting. It is intended that the energy-harvesting hydraulic systems can be used in other joints, such as the knee, the hip, the shoulder, the elbow, the wrist, or a finger joint. In some cases, the energy-harvesting system may use the work harvesting using one joint, and then, release the energy to another joint. For example, the ankle joint can be used to harvest energy during walking, but then, the energy is released at a different joint, such as the knee, or vice versa. 
     The behavior of an ankle during the stance phase of level walking is characterized by two major periods: power absorption from heel contact through full weight acceptance and power release as the ankle plantarflexes during the transition to toe-off. During swing, the ankle dorsiflexes so that the toe does not impact the ground as the leg swings to full extension in preparation for the next heel contact. The energy-harvesting system can store energy within one or more high pressure accumulators during the power absorption phase of support and then return a portion of this stored energy to plantarflex the ankle prior to toe-off while reserving the remainder for dorsiflexion of the ankle during swing. Alternatively, the energy can be used for powering a different joint. Control is achieved by first decomposing ankle torque during support and swing into passive impedances. Functional motion is then attained by switching the limb between the support and swing impedances as the subject progresses through the locomotive function. 
     Stance phase may be decomposed into three sub-phases: Controlled plantarflexion (CPF), controlled dorsiflexion (CDF), and powered plantarflexion (PPF). The healthy human ankle functions in a particular way during each of these phases and the energy-harvesting control system is intended to be biomimetic, directly emulating healthy ankle function at each stage. The swing phase (SW) is generally defined from toe-off to heel-strike (heel contact). 
     Use of an energy-harvesting system with an ankle joint can better replicate biofidelic loading and range of motion that may provide significant improvements in stability and locomotion efficiency. Initiation of the swing phase of gait is normally a propulsive moment in the gait cycle. With non-actuated prostheses, the user must overcome the dead-weight of the prosthesis by accelerating its mass through their prosthetic suspension. Pistoning is the displacement of the socket relative to the amputee&#39;s residual limb. Maximum pistoning is caused by initiation of swing phase, occurring at about 75% of the gait cycle, immediately following toe off. Many lower limb amputees report dissatisfaction with socket comfort, residual limb pain, and/or skin breakdown from exactly this kind of pistoning. Conversely, decreasing the displacement of the prosthesis to the amputated limb creates a more natural gait and the amputee is more likely to feel like the prosthesis is a part of their body. 
     The disclosed energy-harvesting systems provide harvesting of the energy normally dissipated in human locomotion, and subsequently can release the energy at an optimal timing. 
     With loss of a biological limb, lower limb amputees lack key features of efficient gait; such as push-off by their limb in late stance phase, dorsiflexion during early swing, and a nearly energy neutral profile over the gait cycle. The forces produced by the plantarflexors create joint moments that cause the ankle joint to rotate and produce net ankle power generation in late stance phase. Ankle power peaks in a powered-plantarflexion (PPF) event. The magnitude and timing of this powered plantarflexion impulse is part of efficient bipedal gait and is used for accelerating both the center of mass and the trailing limb into swing phase. 
     Referring to  FIG. 1 , a diagrammatical illustration of one embodiment of an energy-harvesting system is illustrated. The system includes a chamber  102 . A piston  104  resides in and is allowed to reciprocate within the chamber. The piston  104  can be connected to a rod  106 . The piston  104  and rod  106  can function as an actuator when coupled to a prosthetic limb. The space above the piston  104  is connected via line  116  to a first low pressure accumulator  112  (the reservoir) and a second high pressure accumulator  114 . Low pressure accumulators, such as  112 , can be referred to herein as reservoirs. 
     The line  116  branches into branch line  118  that connects to the reservoir  112  and the line  116  branches into branch line  120  that connects to the high pressure accumulator  114 . A first valve  108  is placed in branch line  116 , and a second valve  110  is placed in branch line  110 . The space above the piston  104  is thus connected to the reservoir  112  via line  116  and line  118 . The high pressure accumulator  114  is connected to the space above the piston  104  via line  116  and line  120 . The valves  108  and  110  can be remotely electrically opened and closed, or any amount in between, via the use of a microprocessor based on inputs from sensing instruments described herein. 
     The high pressure accumulators herein are any vessel for storing the hydraulic fluid under pressure. Hydraulic accumulators are known. The accumulator can include a floating piston that creates a variable volume within the accumulator. Such volume can be under pressure provided by a spring or compressed gas acting on the piston. In some embodiments, the reservoir  112  is also an accumulator. However, the reservoir  112  operates at a lower pressure than the high pressure accumulator  114 . The exact pressure of the accumulators and reservoirs can be adjusted based on the particular application. For example, the accumulator and reservoir pressures can be adjusted based on the weight of a person using the prosthetic joint, or based on the type of joint, for example. 
     The chamber  102 , low pressure reservoir  102 , high pressure accumulator  104 , and all lines connected thereto form a closed hydraulic system. That is, no hydraulic fluid enters or leaves the system under normal operation. The piston  104  and rod  106  can function as an actuator to do work. That is, the piston  104  is connected to a limb or other moving member. The piston  104  can also be moved by external forces acting on the limb. Thus, performing negative work on the system. When the piston  104  is compressed in the chamber  102 , the hydraulic fluid can be directed to either the low pressure reservoir  112  or the high pressure accumulator  114  depending on the amount of force. Sensors that can measure force, including pressure, torque, or displacement can be used to determine whether to open or close valves  108  and  110 . In some embodiments, other flow controllers, such as check valves and flow restrictors can be used. 
     As used in this application, work has the standard definition in physics meaning the product of a load (force) applied over a displacement. Displacement can be measured in angular displacement or linear displacement. Linear displacement can be converted to angular displacement, and vice versa, by applying a formula based on the geometry of the limb configuration. “Negative work” means net work done on the energy-harvesting system and “positive work” means net work done by the energy-harvesting system. Negative work means that the system gains energy. For example, negative work is performed when a gas (or spring) is compressed in the high pressure accumulator, thus, the energy-harvesting system gains energy. When the gas (or spring) is decompressed, the energy-harvesting systems losses energy by performing positive work. For a two chamber, two accumulator system, the contributions from both the high pressure accumulator and the reservoir would need to be considered. For example, net negative work is performed when a gas (or spring) is compressed in the high pressure accumulator minus the energy that that is used by decompressing the low pressure gas (or spring) in the reservoir. Overall, the net work done is negative, meaning the system gains energy. When the high pressure accumulator is decompressed (gas or spring) and the reservoir is compressed (gas or spring), overall, the net work is positive, meaning that the system loses energy. The energy-harvesting systems use flow controllers, such as shut-off valves, flow restrictors, check valves, for example, to modulate the hydraulic fluid into and out of the chambers, accumulators, and reservoirs. The systems further include sensors that can be used to calculate periods during which it is predicted there will be negative work and periods when to perform positive work. 
     Referring to  FIG. 1 , fluid is displaced from the chamber  102  to the accumulator  114  during periods of a threshold negative work, and fluid is displaced from the accumulator  114  to the chamber  102  to allow the performance of positive work. When the negative work is below a threshold, then, the fluid is displaced from the chamber  102  to the reservoir  112 . Practically, when the piston  104  is subjected to high external loads (the load exceeds the pressure of the high pressure accumulator), the valve  110  is open and the valve  108  is closed. This valve configuration allows fluid to enter the high pressure accumulator  114  through compression of the piston  104 , and energy is harvested and stored in the high pressure accumulator  114 . Valve  110  may be closed and valve  108  open when the piston is allowed to reciprocate and exchange fluid back and forth with the reservoir  112  under reduced or no external load. In such case, the energy-harvesting system is in passive impedance control. When the piston  104  is desired to perform work on an external limb or member, the valve  108  is closed and the valve  110  is open. This valve configuration allows the fluid in the high pressure accumulator  114  to do work on the piston  104  by expanding the piston  104  (when the external load is less than the pressure of the high pressure accumulator). Load (force) can be measured by strain gages or pressure in the chamber or elsewhere.  FIG. 1  shows an energy-harvesting system including a single chamber/piston unit, a high pressure accumulator, and a low pressure reservoir. Other energy harvesting systems may include multiples of the components of  FIG. 1 . For example, an energy harvesting system may include two chamber/piston units, each unit communicating with a high pressure accumulator and reservoir. Such two chamber energy-harvesting systems may be used in applications of one limb pivoting with respect to a second limb, where one chamber is placed on one side of the pivot and the second chamber is placed on the opposite side of the pivot. Alternatively, energy harvesting systems may include one or more chamber/piston units, one or more high pressure accumulators, one or more low pressure reservoirs, or any combinations thereof. The energy-harvesting systems disclosed herein are not limited to a particular number of chamber/piston units in the system, nor the number of reservoirs and high pressure accumulators. Furthermore, the energy-harvesting systems are not constrained to releasing the energy to the joint from which the energy is harvested. In some cases, the energy may be harvested using one limb or joint, and the energy is released to a second limb or joint that is different from the first. 
     Referring to  FIG. 2 , another embodiment of an energy-harvesting system is illustrated. In the system of  FIG. 2 , a first chamber  202  and a second chamber  204  are used. First chamber  202  includes a first piston  206  connected to a first piston rod  210 . Second chamber  204  includes a second piston  208  connected to a second piston rod  212 . When two chamber/piston units are used, the chamber/piston units may be placed to work in opposition to each other, such as on opposite sides of the pivot or on two different joints. 
     Each chamber  202 ,  204  can connect to the same accumulator  224  and the same reservoir  222 . The chambers connect to the accumulator with a flow path including, in parallel, an automatically operated shut-off valve and a check valve, wherein the check valve is configured to allow flow from the chamber to the accumulator and obstruct flow from the accumulator to the chamber. The chambers connect to the reservoir with a flow path including an automatically operated shut-off valve. The space above piston  206  connects to line  226  which branches into line  228 , line  230  and line  232 . Line  228  includes spring-loaded check valve  221 . Line  230  includes valve  220 . Line  232  includes valve  216 . Lines  228  and  230  reconnect and then enter the high pressure accumulator  224 . Spring-loaded check valve  221  permits flow into high pressure accumulator  224 , but obstructs flow therefrom. Line  232  enters low pressure reservoir  222 . The space above piston  208  connects to line  234  which branches into line  236 , line  238  and line  240 . Line  236  includes spring-loaded check valve  215 . Line  238  includes valve  214 . Line  240  includes valve  218 . Lines  236  and  238  reconnect and then enter the high pressure accumulator  224 . Spring-loaded check valve  215  permits flow into high pressure accumulator  224 , but obstructs flow therefrom. Line  240  enters low pressure reservoir  222 . High pressure accumulator  224  and low pressure reservoir  222  can be gas-charged at a high and low pressure respectively. The pistons  206  and  208  can be connected to limbs or other members to actuate the limbs or members. Depending on the placement of the chambers, the energy-harvesting system of  FIG. 2  can be configured to displace fluid from the one or both chambers at the same time or sequentially to the accumulator during periods of a threshold negative work, to displace fluid from the accumulator to one or both chambers at the same time or sequentially to allow the performance of positive work, to displace fluid from one or both chambers at the same time or sequentially to the reservoir during periods below the threshold negative work, or to displace fluid from the reservoir to one or both chambers at the same time or sequentially. In some cases, when the accumulator is displacing fluid to one chamber, the other chamber is displacing fluid to the reservoir, or when one chamber is displacing fluid to the accumulator, the reservoir is displacing fluid to the other chamber. This situation can arise when one chamber is placed in opposition to the second chamber, such as one on each side of a pivot. A microcontroller can be used to open and close the appropriate valves based on input from one or more sensors described herein. 
     Referring to  FIG. 3 , another embodiment of an energy-harvesting system is illustrated. The energy-harvesting system of  FIG. 3  is similar to the system of  FIG. 2  except for the addition of spring-loaded check valve  217  on line  242  that permits flow out of the low pressure reservoir  222  and spring-loaded check valve  219  on line  244  that permits flow out of the low pressure reservoir. In  FIG. 3 , each flow path from each respective chamber to the reservoir includes, in parallel, an automatically operated shut-off valve and a check valve, wherein the check valve is configured to allow flow from the reservoir to the chamber and obstruct flow from the chamber to the reservoir. Depending on the placement of the chambers, the energy-harvesting system of  FIG. 3  can be configured to displace fluid from the one or both chambers at the same time or sequentially to the accumulator during periods of a threshold negative work, to displace fluid from the accumulator to one or both chambers at the same time or sequentially to allow the performance of positive work, to displace fluid from one or both chambers at the same time or sequentially to the reservoir during periods below the threshold negative work, or to displace fluid from the reservoir to one or both chambers at the same time or sequentially. In some cases, when the accumulator is displacing fluid to one chamber, the other chamber is displacing fluid to the reservoir, or when one chamber is displacing fluid to the accumulator, the reservoir is displacing fluid to the other chamber. This situation can arise when one chamber is placed in opposition to the second chamber, such as one on each side of a pivot. A microcontroller can be used to open and close the appropriate valves based on input from one or more sensors described herein. 
     For an energy-harvesting system having two chamber/piston units, such as shown in  FIGS. 2 and 3 , the operation may generally include the following. When a piston is in compression by a load above a threshold (greater than the high pressure accumulator pressure), the hydraulic fluid is directed to the high pressure accumulator to store energy. One or both chamber/piston units can be capable of displacing hydraulic fluid to the high pressure accumulator for energy storage. Under some conditions, when a piston is in compression by a load below a threshold (less than the high pressure accumulator pressure), the hydraulic fluid is displaced to the reservoir (or a low pressure reservoir), which provides impedance control of movement of the limb (or other member). Under some conditions, when a piston is in expansion, the piston is being powered by the hydraulic fluid from the high pressure accumulator. Under some conditions, when a piston is in expansion, the piston is being moved by an external force as a consequence of the antagonistic piston being under load. 
     An energy-harvesting system incorporated into a prosthetic joint is diagrammatically illustrated in  FIG. 4 . It is to be appreciated that the energy-harvesting system of  FIG. 4  is the incorporation of two energy-harvesting systems, each one resembling the energy-harvesting system of  FIG. 1 . That is, two chamber/piston units are placed antagonistic to each other or in direct opposition with respect to a pivoting member. It is to be appreciated that  FIG. 4  is highly schematic such that the main components of the energy-harvesting system are illustrated. For purposes of illustration, the joint may be referred to as an ankle joint, however, it is not intended to limiting, as the energy-harvesting systems herein described can be incorporated into other joints. 
     The energy-harvesting system can be enclosed in a case defined by a broken line  307 . The case  307  can include a first connector  303  and a second connector  305  configured so that the first connector  303  pivots with respect to the second connector  305 . The first connector  303  can be connected to a limb or member and the second connector is connected to a second limb or member, such that there is movement of one limb with respect to the other. The joint can store energy under certain conditions and release the energy to move the limb under certain conditions as described herein. In some embodiments, the first connector  303  is further attached to a sensing device  301 . The sensing device  301  can use strain gauges  380 , accelerometers  382 , a magnetic hall-effect encoder  384 , potentiometers  386 , or any combination, to sense loads, axial force, joint angle, joint angle rate of change, joint torque, joint torque rate of change being experienced by the joint. Strain gauges  380 , potentiometers  386  can be used to measure load, for example. Accelerometers  382  and magnetic hall-effect encoders  384  can be used to measure displacement, including angular displacement (e.g. tilt angle, shank angle, etc.). The sensing device  301  can include a load-determining sensor or sensors and the displacement- or angle-determining sensor. A suitable sensing device can be the device known by the designation of EUROPA by Orthocare Innovations, of Mountlake Terrace in the state of Washington, USA. However, a sensing device can be assembled based on the description herein. The sensing device  301  in turn is connected to a pylon  309 , and the pylon  309  is connected to a prosthetic socket  313  for receiving a lower limb. While the sensing device  301  is shown directly attached to the case  307  of the energy-harvesting system, the sensing device can be placed at the base of a prosthesis socket  313 . The second connector  305  may be connected to one of a plurality of commercially available prosthetic feet  315 . As can be appreciated, the joint can pivot to rotate the prosthetic foot  315  with respect to the pylon  309 . 
     When the energy-harvesting system is used with an ankle joint, the energy harvesting system can incorporate antagonistic chambers/pistons, such as anterior and posterior with respect to the pivot  330  and foot  307 . The energy-harvesting system includes a first posterior chamber  332  and a second anterior chamber  334 . The posterior chamber  332  can be placed diametrically opposite the anterior chamber  334  with respect to a pivot  330 . Antagonistic means that the pistons move in direct opposition. That is, when one piston is in the compression stroke, the other is in the expansion stroke. The length of stroke need not be the same for both pistons, because the distance from the pivot  330  to each piston may be different for each piston. However, in some embodiments the distance from the pivot  330  to each piston is the same. 
     The posterior chamber  332  includes a piston  336  connected to a piston rod which in turn is connected to a cam follower  340 . The cam follower  340  is in contact with the cam surface  344 . The anterior chamber  334  includes piston and piston rod  338  which in turn is connected to the cam follower  342  which makes contact with the cam surface  346 . The cam surfaces or cams  344  and  346  are rigidly connected to a platform  348 . The platform  348  can pivot about the pivot  330 , such that plantarflexion will compress the posterior piston  336  and dorsiflexion will compress the anterior piston  338 . While a cam and cam follower are shown to convert the linear motion of the actuator pistons  336 ,  338  into rotational motion of the joint, other mechanisms can be used, including ratchets, rack and pinion gears, and cranks. When a cam is used, the cam can be an involute cam. Two involute cams placed on opposite sides of a pivot can offer advantages. 
     The posterior  332  and anterior  334  chamber include flow paths with flow controllers from the respective chamber to a respective one of a high pressure accumulator and a low pressure reservoir. Chambers  332  and  334  connect to the respective accumulator with a flow path including, in parallel, an automatically operated shut-off valve and a check valve, wherein the check valve is configured to allow flow from the chamber to the accumulator and obstruct flow from the accumulator to the chamber. Chambers  332  and  334  connect to the reservoir with a flow path including an automatically operated shut-off valve, followed by, in parallel, a flow restrictor and a check valve, wherein the check valve is configured to obstruct flow from the chamber to the reservoir and allow flow from the reservoir to the chamber. Flow restrictors can include orifices, for example. 
     Referring to the posterior chamber  332 , the space above the piston  336  connects to a line  346 . The line  346  has three branch lines  340 ,  342 , and  344 . Line  340  includes shut-off valve  322 . Line  344  includes shut-off valve  302 . Line  340  further branches into lines  348  and  350  which then reconnect and connect to the posterior reservoir  302 . Line  348  includes a restrictor  310  and line  350  includes a check valve  312 . The check valve  312  only allows fluid to flow out of the low pressure reservoir  302 . Line  342  includes a spring-loaded check valve  314 . The spring-loaded check valve  314  only allows fluid flowing into the high pressure accumulator  304 . Lines  342  and  344  connect before entering the high pressure accumulator  304 . Shut-off valves  322  and  344  can be electrically driven solenoid valves. 
     Referring to the anterior chamber  334 , the space above piston  338  connects to a line  352 . Line  352  branches into three separate lines  354 ,  356 , and  358 . Line  354  includes shut-off valve  326 . Line  358  includes shut-off valve  328 . Line  354  further branches into lines  360  and  362 . Line  360  includes a restrictor  316 . Line  362  includes a check valve  318 . Line  360  and  362  connect before anterior low pressure reservoir  306 . Check valve  318  only allows flow out of the low pressure reservoir  306 . Line  356  includes a spring-loaded check valve  320 . The spring-loaded check valve  320  only allows flow into the high pressure accumulator  308 . Lines  356  and  358  connect before the high pressure accumulator  308 . In this disclosure, “anterior” and “posterior” may be used to associate a reservoir and accumulator with the respective anterior chamber and posterior chamber as the case may be, and should not be interpreted to mean that any reservoir or accumulator is in an anterior or posterior position. Reservoirs and accumulators may be placed in any suitable location regardless whether they are fluidly connected to an anterior or posterior chamber. Shut-off valves can be electrically driven solenoid valves. 
     The joint may further include a battery  368 , a microprocessor  370 , a memory  364 , and an input/output device  366 . The battery  368  can power the microprocessor  370  and the shut-off valves  322 ,  302 ,  326 , and  328 . The memory  364  can store instructions that command the opening and closing of the shut-off valves based on inputs from the sensing device  301 . The input/output device  366  can be used to download instructions or retrieve data. The microprocessor  370  can be a Texas Instruments MSP430 microprocessor, for example. The battery  368  can be a 1200 mAh lithium polymer battery, for example, and can provide a day or more of operations per two-hour charge cycle. 
     Flow controllers include the shut-off valves  322 ,  302 ,  326 ,  328 , check valves  312 ,  314 ,  318 ,  320 , and flow restrictors  310  and  316 . The microprocessor  370  is configured to actuate one or more flow controllers, primarily the shut-off valves, based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof. The flow controllers are configured to displace fluid from each chamber to the respective accumulator during periods of a threshold negative work on the joint. The flow controllers are configured to displace fluid from each accumulator to the respective chamber to allow the joint to perform positive work. The flow controllers are configured to displace fluid from each chamber to the respective reservoir during periods below the threshold negative work. Because of the antagonistic nature of the chambers/pistons, when one piston is in expansion, the other is in compression. Because of the antagonistic nature of the chamber/pistons, the pistons store energy and release energy sequentially (under the right conditions). In some circumstances, the flow controllers allow for passive fluid exchange back and forth between each chamber/piston unit and the respective reservoir simultaneously for impedance control. For an energy-harvesting system having two chamber/piston units, wherein each unit has both a respective high pressure accumulator and low pressure reservoir, the operation may generally include the following. When either piston is in compression by a load, the hydraulic fluid is displaced to the respective high pressure accumulator to store energy. When a piston is in compression, the hydraulic fluid is displaced to the reservoir through a restrictor, which provides an impedance to movement of the limb (or other member). When a piston is in expansion, the piston can be powered by the hydraulic fluid from the respective high pressure accumulator. When a piston is in expansion, the piston can be moved by an external force as a consequence of the antagonistic piston being under load. The different states of operation are described herein using an ankle joint as a representative example. 
     Referring to  FIG. 5 , the different phases of gait for one leg is depicted with the corresponding actions of the hydraulic fluid of an ankle joint with an energy-harvesting system of  FIG. 4 . 
     With an energy-harvesting ankle joint of  FIG. 4 , the joint can go through the different phases and dorsiflex and plantarflex a prosthetic foot to mimic the natural muscular actions of a healthy foot. The stance phase begins with heel strike. Then, a phase of controlled plantarflexion follows. Controlled plantarflexion transitions to a phase of controlled dorsiflexion before, after, or about midstance. The transition is referred to as “rollover.” After controlled dorsiflexion, a phase of powered plantarflexion follows for push-off. From push-off to the next heel strike is the swing phase. Dorsiflexion during swing phase positions the toe by raising the toe to prevent stubbing. “Powered” as opposed to “controlled” in the context of dorsiflexion and plantarflexion refers to net work being performed by the energy-harvesting system to actively actuate the joint and foot, such as to dorsiflex or plantarflex the foot. “Controlled” can refer to net work being done on the energy-harvesting system (negative work) and rotation that encounters an amount of designed resistance. The resistance can be provided by the restrictors in the flow paths to the receivers, for example. 
     The high pressure accumulators  304  and  306  are capable of storing energy to perform work during the powered plantarflexion and swing phases as further described. The stored energy comes because of the ground reaction forces generated by the weight of the body being applied on the foot, which transfers the force to each piston/chamber unit sequentially through the gait cycle. The weight places the pistons under a high load which is sufficient to overcome the pressure in the high pressure accumulators  304 ,  308 . The reservoirs  302  and  306  coupled with the restrictors can provide resistance to rotation thus providing a controlled rotation about the ankle during controlled plantarflexion and controlled dorsiflexion. 
     As can be seen in  FIG. 5 , in the controlled plantarflexion phase and starting about heel strike, both high pressure posterior and anterior accumulators  304  and  308  are essentially empty and both posterior and anterior reservoirs  302  and  306  are full. Fluid displacement is generally not occurring about the time of heel strike. After heel strike, and still during controlled plantarflexion, it can be seen that fluid is transferred into the posterior high pressure accumulator  304  from the posterior chamber  332 , and fluid is transferred out of the anterior reservoir  306  to the anterior chamber  334 . Thus, resulting in net negative work on the joint and energy being stored. During controlled plantarflexion, a human ankle will behave as a linear spring. The energy-harvesting ankle implements a mechanical impedance control scheme wherein ankle angle throughout plantarflexion increases torque on the joint. Selecting the posterior valves  322  and  302  to closed causes the posterior piston  336  to charge the posterior high-pressure accumulator  304  during controlled plantarflexion. Valve  326  is open to allow the anterior piston  338  volume to compensate via the low pressure reservoir  306 . A spring-type accumulator can be used for the posterior high pressure accumulator  304  to serve linear-impedance behavior while the valves are in controlled plantarflexion. 
     About midstance, controlled plantarflexion ends and controlled dorsiflexion begins during “rollover.” During controlled dorsiflexion, it can be seen that fluid is transferred into the anterior high pressure accumulator  308  from the anterior chamber  334 , and fluid is transferred out of the posterior reservoir  302  into the posterior chamber  332 . Thus, resulting in net negative work on the joint and energy being stored. During controlled dorsiflexion, the human ankle functions as a nonlinear spring which stores energy in tendon structures to support powered plantarflexion later. The energy-harvesting ankle control scheme will select valves  326  and  328  closed to communicate the anterior piston  338  with the anterior high pressure accumulator  308 . Valve  322  is open to allow the posterior piston  336  volume to compensate via the low pressure reservoir  302 . The high pressure accumulator  308  can be designed as a spring or gas-charged accumulator with a nonlinear response. Therefore, the function of the human ankle during rollover (increasing stiffness and energy storage) is emulated by the energy-harvesting ankle by enabling the correct valve state. 
     After a period of controlled dorsiflexion, the heel begins to lift off the ground and powered plantarflexion begins about such time. In powered plantarflexion, the energy-harvesting system is performing work to plantarflex the foot to provide power to propel the body forward. During powered plantarflexion, it can be seen that the high pressure anterior accumulator  308  is transferring fluid to the anterior chamber  334 , thus, causing plantarflexion and the transfer of fluid from the posterior chamber  332  to the posterior reservoir  302 . Thus, resulting in net positive work performed by the joint. During powered plantarflexion, the human ankle behaves as a torque source, providing power (through muscle contraction and stored tendon energy) to the ankle and accelerating the body and leg upward and forward into swing phase. Therefore, the energy-harvesting ankle will behave as a torque source by pulse-width-modulating the solenoid-driven valve  328  of the anterior high pressure accumulator  308  into the anterior actuator piston  338 . Thus, providing the correct amount of power at the desired rate through a torque controller with feedforward friction and inertia terms. 
     About push-off, powered plantarflexion ends, and swing with dorsiflexion begins. During swing, the high pressure posterior accumulator  304  is transferring fluid to the posterior chamber  332 , thus, causing dorsiflexion and the transfer of fluid from the anterior chamber  334  to the low pressure anterior receiver  306 . Thus, resulting in net positive work performed by the joint. During swing phase, the human ankle is repositioned to prepare for heel-strike. The energy-harvesting ankle engages a position-controller to emulate this behavior by dorsiflexing the foot. Dorsiflexion of the foot provides toe clearance. Valves  302  and  328  are modulated by the controller to drive the ankle back to the neutral position. 
     After swing phase, both the posterior and anterior high pressure accumulators  304 ,  308  are essentially emptied and have released their energy, and both the posterior and anterior reservoirs  302 ,  306  are essentially full in preparation for the next cycle. The determination of the different phases to correctly time the automatic opening and closing of the shut-off valves is carried out via the microprocessor  370  based on inputs received from the sensing device  301 . As the control system is largely one of selecting appropriate valve timing relative to a gait cycle (low-level control is dominantly mechanical); at a higher level, a finite state machine accurately determines each phase of gait and appropriate control state. In one embodiment, the rate of change of the ankle torque is selected as the variable to define the timing of the pulse. 
     The sensing device can be programmed to use strain gauges to determine torque, rate of change of torque, and the axial force (load-determining sensor). A magnetic hall-effect encoder can be used to determine joint angle (displacement-determining sensor), and the rate of change of joint angle. The different phases of gait can be defined. Torque refers to the torque experienced at the pivot location  330 . Joint angle can be described as the angle created between a line parallel to the pylon  309  and a line parallel to the longitudinal axis of the foot  315 . Axial force is the vertical component of force passing through the pivot. 
     The logic instructions for determining the state can be implemented in a variety of hardware, software, and combined hardware/software configurations. In some embodiments, the control logic is implemented by the microprocessor  370  and memory  364 . The memory can include a random access memory (“RAM”) and an electronically erasable, programmable, read-only memory (“EEPROM”) or other non-volatile memory (e.g., flash memory) or persistent storage. The RAM may be a volatile form of memory for storing program instructions that are accessible by the microprocessor. The microprocessor is configured to operate in accordance with logic instructions. Hardware or software may implement logic instructions to 1) determine when the different phases of gait exist, 2) determine the transition between the different phases of gait, and 3) open or close certain valves depending on the phase of gait or when a transition is deemed to occur. 
     Referring to  FIG. 6 , a finite-state diagram is illustrated for defining the phases of gait and representative logic instructions for transitioning from state to state. The logic instructions for transitioning from state to state can be implemented in the form of hardware or software. Table 1 summarizes the energy-harvesting ankle sensory inputs. One or more load-determining sensors are used to measure the axial force and torque, while a displacement-determining sensor is used to measure the angle. Angular rate and torque rate are derived values. Time derivatives of variables are computed discreetly by the microprocessor. The energy-harvesting ankle can measure real-time dynamic load measurements in the prosthesis as inputs to the control algorithm described in  FIG. 6 . The transducer described in U.S. Pat. No. 7,886,618, incorporated herein expressly by reference in its entirety, can be adapted to be used in the energy-harvesting ankle. A suitable sensor device may be in a form of a pyramid adapter that incorporates silicon strain gauges to monitor moments (such as ±150 N-m in sagittal and coronal planes) and axial forces (such as ±510 N) in the prosthesis. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Parameter 
                 Variable 
                 Sensor 
               
               
                   
               
             
            
               
                 Joint Angle, Angular rate 
                 θ 
                 Magnetic hall-effect encoder 
               
               
                   
                 δθ/δt 
                   
               
               
                 Joint Torque, Torque rate 
                 τ 
                 Strain gauge sensors 
               
               
                   
                 δτ/δt 
                   
               
               
                 Axial Force 
                 α 
                 Strain gauge sensors 
               
               
                   
               
            
           
         
       
     
     Starting at the swing state  506 , to enter the controlled plantarflexion state  510  from the swing state  506  requires that the axial force α is greater than 0 and the joint torque τ is greater than 0, block  508 . To enter the swing state  506  from the controlled plantarflexion state  510  requires the axial force α equals 0 and the joint torque τ equals 0, block  522 . To enter the controlled dorsiflexion state  514  from the controlled plantarflexion state  510  requires a rate of change in joint angle greater than or equal to 0, block  512 . To enter the powered plantarflexion state  518  from the controlled dorsiflexion state  514  requires the rate of change in joint torque equal to 0, block  516 . To enter the free state  502  from the controlled dorsiflexion state  514  requires the rate of change in joint angle be less than or equal to 0, block  528 . Free state refers to a state where both chambers are open to the respective reservoir. To enter the swing state  506  from the powered plantarflexion state  518  requires the axial force be equal to 0 and the joint torque be equal to 0, block  520 . To enter the free state  502  from the powered plantarflexion state  518  requires the time in the powered plantarflexion state to be greater than a setpoint, block  526 . That is, the powered plantarflexion state  518  can time out when a timer is less than or equal to the setpoint, block  524 . To enter the swing state  506  from the free state  502  requires the axial force be equal to 0 and the joint torque equal to 0. The shut-off valves  322 ,  302 ,  326 , and  328  of  FIG. 4  can be programmed according to open or close automatically according to the state diagram. In one embodiment, the valve states (open or closed) of the energy-harvesting system of  FIG. 4  are shown in Table 2. When in the open position, the valves may be pulsed (cycled between open and closed). 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Valve 
                 CPF 
                 CDF 
                 PPF 
                 SW 
               
               
                   
               
             
            
               
                 V1 (322) 
                 Closed 
                 Open 
                 Open 
                 Closed 
               
               
                 V2 (302) 
                 Closed 
                 Closed 
                 Closed 
                 Open 
               
               
                 V3 (326) 
                 Open 
                 Closed 
                 Closed 
                 Open 
               
               
                 V4 (328) 
                 Closed 
                 Closed 
                 Open 
                 Closed 
               
               
                   
               
            
           
         
       
     
     The state diagram of  FIG. 6  is not limiting. It should be understood that  FIG. 6  applies to a particular joint, namely an ankle, with a particular energy-harvesting system including two antagonistic chamber/piston units. Furthermore, it should be understood that not all states may be programmed into the joint device, and fewer or more states may also be programmed. For example, a joint device can have one or more states selected from a swing state (positioning), a controlled plantarflexion state, a controlled dorsiflexion state, a powered plantarflexion state, and a free state or any combination thereof. A swing state for positioning the foot can be viewed as powered dorsiflexion. It should be understood that plantarflexion and dorsiflexion generally refer to the movement of the foot with respect to the ankle. However, in the context of other joints, movement is referred to as flexion and extension. Other joints may have powered flexion, powered extension, controlled flexion, controlled extension, or any combination thereof. The chamber/piston units can be configured to power any joint in either flexion or extension. In general, for the joints described herein, there are periods where the net negative work on the joint will be sufficiently negative (such as above a threshold) to trigger energy storage, periods where the net negative work on the joint will be below the negative work threshold, and periods where the joint performs net positive work. The flow controllers of an energy-harvesting system disclosed herein can be programmed to store energy when net negative work is above a certain threshold. When the net negative work is below the threshold, the hydraulic fluid is exchanged between the chamber and the reservoir. 
     The design criteria for the energy-harvesting systems can be determined on a case-by-case basis. For example, the high pressure and reservoir pressures, as well as damping factors, can be determined based on certain variable design criteria. As an example, the physical constraint defining the high pressure accumulator for powered plantarflexion accumulator may include an ankle acceleration in the range of 300 radians/s 2 . In order to accelerate a 2.5 kg prosthetic system (foot, ankle, pylon, and socket) around the metatarsal area of a 27 cm foot (15 cm from ankle mass to toe) at this rate 300 (radians/s 2 ), over an anthropometric distance of 28 degrees, requires 11 Joules of energy. Therefore, the high-pressure accumulator for powered plantarflexion can be capable of storing greater than 11 Joules. That energy should be applied over a period of approximately 80 ms; requiring a peak power of just under 140 W. 
     The volume needed in the accumulators can be based upon the knowledge of ankle position through stance phase, combined with piston area and distance to ankle pivot. For example, during controlled dorsiflexion, when the ankle should harvest 11 Joules, the ankle travels approximately 15 degrees. Assuming, a piston area of 3.6 cm 2  that acts at a distance of 2.4 cm from the pivot; therefore, the piston travels approximately 6 mm of linear displacement from a 15-degree sweep and 2.16 mL of fluid is displaced in the high pressure accumulator  308 . The pressure required for the high pressure accumulator  308  to store 11 J in 2.16 mL is: 11 Joules/2.16 mL=5 MPa. Storage of 25 Joules is estimating over a 50% application energy loss and results in an 11.5 MPa pressure. The remaining hydraulic features can be designed similarly, with the posterior hydraulic system driving dorsiflexion in swing phase. Further design criteria for an ankle joint may include a range of motion of about 10° of dorsiflexion to 20° of plantarflexion. The torque can be about 1.6 N*m/kg of body mass. The angular velocity can be about 1.5 rad/s. The power and energy can be about 11 joules with peak power of 140 W. 
     While a description of an energy-harvesting system has been shown to be incorporated into an ankle joint with respect to  FIG. 4 , other energy-harvesting system depicted in  FIGS. 1-3  can also be incorporated into an ankle joint or other joints. Also, the energy-harvesting system described for an ankle joint can also be incorporated into a hip joint, shoulder, elbow, wrist, or a legged nonhuman robot. In other embodiments, an energy-harvesting system for an ankle joint does not require the use of two opposed chambers. 
     The system of  FIG. 1  showing a single chamber  102  can be incorporated into an ankle joint to provide powered plantarflexion, powered dorsiflexion, or both. The single chamber  102  may be placed anteriorly, posteriorly, or in the center of the joint. The valves  108 ,  110  are appropriately controlled to store energy in the high pressure accumulator  114  during periods of net negative work above a threshold, and release the energy to perform positive work. Likewise, the two chamber/piston unit, two accumulator energy-harvesting systems of  FIGS. 2 and 3  may also be incorporated into an ankle joint. The valves in those systems are appropriately controlled to store energy in the high pressure accumulator during periods of net negative work above a threshold, and release the energy during periods when it is desired for the ankle to perform positive work. Further, the energy-harvesting system can be distributed across two or more joints. For example, the energy stored from the ground forces on the foot can be used to power other joints besides the ankle, including the knee, hip, shoulder, elbow, or wrist. 
       FIG. 7  shows a schematic illustration of an energy-harvesting system distributed across two joints. The energy-harvesting system is similar to the system of  FIG. 3  with the following modifications. A first chamber/piston unit  202  is placed at a first joint and a second chamber/piston unit  204  is placed at a second joint. The first joint has a limb  252  (or member) that can flex or extend with respect to another limb  254  (or member). The second joint has a limb  250  (or member) that can flex or extend with respect to another limb  256  (or member). The first chamber/piston unit  202  in contact with the first limb  252  is the recipient of the net negative work above a threshold. That is, the first chamber/piston unit  202  is used to harvest energy in the accumulator  224 , and the second chamber/piston unit  204  releases the energy from the accumulator to power the different limb  250 . 
     Based on the foregoing, methods and joints are disclosed for harvesting energy and reapplying the energy. The following are representative and not meant to be limiting. 
     Methods of harvesting and selectively reapplying energy to a prosthetic joint are disclosed. The methods may include providing a prosthetic joint. The prosthetic joint may include at least one chamber; at least one accumulator configured to store hydraulic fluid at a high pressure; at least one reservoir configured to store hydraulic fluid at a low pressure; one or more fluid flow paths connecting the chamber to the accumulator and the reservoir, flow controllers in the fluid flow paths, and fluid distributed throughout the chamber, accumulator and reservoir; a load-determining sensor; a displacement-determining sensor; a microprocessor configured to actuate one or more flow controllers based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof. The method further includes displacing fluid from the chamber to the accumulator during periods of a threshold negative work on the joint; and displacing fluid from the accumulator to the chamber to allow the joint to perform positive work. 
     In some embodiments, the method may further include displacing fluid from the chamber to the reservoir during periods below the threshold negative work. 
     In some embodiments, the prosthetic joint is an ankle joint, and the ankle joint is connected to a prosthetic foot and a pylon, wherein the ankle joint allows rotation of the prosthetic foot with respect to the pylon. 
     In some embodiments, the method may further include determining a flow controller state by determining a swing positioning state, a controlled plantarflexion state, a controlled dorsiflexion state, and a powered plantarflexion state. 
     In some embodiments, the method may further include storing energy in the accumulator during controlled dorsiflexion, or during controlled plantarflexion, or during both, and returning energy during powered plantarflexion. 
     In some embodiments, the method may further include returning energy during swing positioning. 
     In some embodiments, swing positioning includes dorsiflexing the foot and elevating the toe. 
     In some embodiments, the method may further include determining conditions to transition from the swing positioning state to the controlled plantarflexion state, conditions to transition from the controlled plantarflexion state to the controlled dorsiflexion state, conditions to transition from the controlled dorsiflexion state to the powered plantarflexion state, and conditions to transition from the powered plantarflexion state to the swing positioning state. 
     In some embodiments, in the swing positioning state, fluid is displaced from a posterior accumulator to a posterior chamber, and fluid is displaced from an anterior chamber to an anterior reservoir; in the controlled plantarflexion state, fluid is displaced from the posterior chamber to the posterior accumulator, and fluid is displaced from the anterior reservoir to the anterior chamber; in the controlled dorsiflexion state, fluid is displaced from the posterior reservoir to the posterior chamber, and fluid is displaced from the anterior chamber to the anterior accumulator; and in the powered plantarflexion state, fluid is displaced from the posterior chamber to the posterior reservoir, and fluid is displaced from the anterior accumulator to the anterior chamber. 
     In some embodiments, the threshold negative work is performed when a limb connected to the joint is applied on a ground surface to generate a ground reaction force greater than a pressure in the accumulator. 
     In some embodiments, the flow controllers include one or more automatically operated shut-off valves. 
     In some embodiments, the method further includes passing fluid through a restrictor when displacing fluid from the chamber to the reservoir. 
     In some embodiments, the method further includes producing the negative work above the threshold by contacting a limb connected to the joint on a surface to generate a ground reaction force. 
     In some embodiments, the displacement-determining sensor is an angle-determining sensor. 
     In some embodiments, the joint is a prosthetic knee joint. 
     In some embodiments, the method further includes storing energy in the accumulator when sitting from a standing position and returning energy when standing from a sitting position. 
     In some embodiments, the method further includes storing energy in the accumulator during descending and returning energy during ascending. 
     In some embodiments, the flow controllers are pulsed open during displacing fluid from the accumulator to the chamber. 
     Methods of harvesting energy from a first joint and selectively reapplying the energy to a second joint are disclosed. The methods include providing an energy-harvesting hydraulic system including at least one chamber; at least one accumulator configured to store hydraulic fluid at a high pressure; at least one reservoir configured to store hydraulic fluid at a low pressure; one or more fluid flow paths connecting the chamber to the accumulator and the reservoir, flow controllers in the fluid flow paths, and fluid distributed throughout the system; a load-determining sensor; a displacement-determining sensor; a microprocessor configured to actuate one or more flow controllers based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof. The methods include displacing fluid from the chamber to the accumulator during periods of a threshold negative work on a first joint; and displacing fluid from the accumulator to the chamber to allow a second joint to perform positive work. 
     In some embodiments, the first joint is an ankle and the second joint is a knee, or the first joint is the knee and the second joint is the ankle. 
     Prosthetic joints are disclosed. The joints may include a hydraulic system, including at least one chamber; at least one accumulator configured to store hydraulic fluid at a high pressure; at least one reservoir configured to store hydraulic fluid at a low pressure; one or more fluid flow paths connecting the chamber to the accumulator and the reservoir, flow controllers in the fluid flow paths, and fluid distributed throughout the hydraulic system; a load-determining sensor; a displacement-determining sensor; and a microprocessor configured to actuate one or more flow controllers based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof, wherein one or more flow controllers are configured to control displacing fluid from the chamber to the accumulator during periods of a threshold negative work, and one or more flow controllers are configured to control displacing fluid from the accumulator to the chamber to perform positive work. 
     In some embodiments, the joint may further include a piston in the chamber, wherein a limb is actuated by the piston during displacing fluid from the accumulator to the chamber. 
     In some embodiments, the limb actuates the piston during displacing fluid from the chamber to the accumulator. 
     In some embodiments, the joint may further include a cam and cam follower, wherein the cam follower is in contact with the cam, and the cam follower is connected to the piston. 
     In some embodiments, the cam includes an involute cam surface. 
     In some embodiments, the joint further includes a pivot, wherein the pivot rotates a first prosthetic limb with respect to a second prosthetic limb. 
     In some embodiments, the first prosthetic limb is a prosthetic foot, and the second prosthetic limb includes a pylon and socket. 
     In some embodiments, the joint further includes a first and second accumulator, a first and second reservoir, and a first and second chamber, wherein the first and second chambers are placed on opposite sides of a pivot, and the first chamber includes flow paths to the first accumulator and the first reservoir, and the second chamber includes flow paths to the second accumulator and the second reservoir. 
     In some embodiments, a fluid flow path from each chamber to the accumulator includes, in parallel, an automatically operated shut-off valve and a check valve, wherein the check valve is configured to allow flow from the chamber to the accumulator and obstruct flow from the accumulator to the chamber. 
     In some embodiments, a fluid flow path from each chamber to the reservoir includes an automatically operated shut-off valve and, in parallel, a restrictor and a check valve, wherein the check valve is configured to allow flow from the reservoir to the chamber and obstruct flow from the chamber to the reservoir. 
     In some embodiments, the load-determining sensor is a strain gauge. 
     In some embodiments, the load-determining sensor is a pressure transducer. 
     In some embodiments, the displacement-determining sensor is a potentiometer. 
     In some embodiments, the displacement-determining sensor is a hall effect sensor. 
     In some embodiments, the flow controllers include a solenoid valve. 
     Prosthetic joints are disclosed that may include a first and second connector and a pivot device that allows the first and second connector to rotate with respect to each other, wherein the first connector is configured to attach to a first prosthetic member and the second connector is configured to attach to a second prosthetic member; a first and second chamber, wherein the chambers are disposed on opposite sides of the pivot device; a first and second piston positioned in the first and second chamber, wherein the pistons are positioned to actuate the rotation of the joint; an accumulator configured to store hydraulic fluid at a high pressure, wherein the accumulator connects to each chamber through a flow path including, in parallel, a shut-off valve and a check valve, wherein the check valve is configured to allow flow from each respective chamber to the accumulator and obstruct flow from the accumulator to each respective chamber; a reservoir configured to store hydraulic fluid at a low pressure, wherein the reservoir connects to each chamber through a flow path including a shut-off valve; a load-determining sensor; a displacement-determining sensor; a microprocessor configured to actuate the shut-off valves based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof, for displacing fluid from one chamber at a time to the accumulator during periods of a threshold negative work on the joint, and displacing fluid from the accumulator to one chamber at a time to allow the joint to perform positive work. 
     Prosthetic joints are disclosed that may include a first and second connector and a pivot device that allows the first and second connector to rotate with respect to each other, wherein the first connector is configured to attach to a first prosthetic member and the second connector is configured to attach to a second prosthetic member; a first and second chamber, wherein the chambers are disposed on opposite sides of the pivot device; a first and second piston positioned in the first and second chamber, wherein the pistons are positioned to actuate the rotation of the joint; a first and second accumulator configured to store hydraulic fluid at a high pressure, wherein the first accumulator connects to the first chamber through a flow path including, in parallel, a shut-off valve and a check valve, wherein the check valve is configured to allow flow from the first chamber to the first accumulator and obstruct flow from the first accumulator to the first chamber, and the second accumulator connects to the second chamber through a flow path including, in parallel, a shut-off valve and a check valve, wherein the check valve is configured to allow flow from the second chamber to the second accumulator and obstruct flow from the second accumulator to the second chamber; a first and second reservoir configured to store hydraulic fluid at a low pressure, wherein the first reservoir connects to the first chamber through a flow path including, in parallel, shut-off valve and a check valve configured to allow flow from the first reservoir to the first chamber and obstruct flow from the first chamber to the first reservoir, and the second reservoir connects to the second chamber through a flow path including a shut-off valve and a check valve configured to allow flow from the second reservoir to the second chamber and obstruct flow from the second chamber to the second reservoir; a load-determining sensor; a displacement-determining sensor; a microprocessor configured to actuate the shut-off valves based upon a load-determining sensor input, a displacement-determining sensor input, a product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof, for displacing fluid from each chamber to the respective accumulator during periods of a threshold negative work on the joint, and for displacing fluid from each accumulator to the respective chamber to allow the joint to perform positive work. 
     Prosthetic joints are disclosed that may include a hydraulic system including: at least one chamber; at least one accumulator configured to store hydraulic fluid at a high pressure; at least one reservoir configured to store hydraulic fluid at a low pressure; one or more fluid flow paths connecting the chamber to the accumulator and reservoir, and flow controllers in the fluid flow paths; and hydraulic fluid in the system. The joints may further include a load-determining sensor; a displacement-determining sensor; a microprocessor to actuate the flow controllers based upon a load-determining sensor input, a displacement-determining sensor, any product of the load-determining sensor input and the displacement-determining sensor input, any time derivative thereof, or any combination thereof, wherein the flow controllers are configured to displace fluid from the chamber to the accumulator during periods of a threshold negative work, and the flow controllers are configured to displace fluid from the accumulator to the chamber to perform positive work, and wherein the threshold negative work is performed on a first joint and the positive work is performed by a second joint different from the first joint. 
     Some embodiments of the prosthetic joints include flow controllers that are further configured to displace fluid from the chamber to the reservoir during periods below the threshold negative work. 
     While illustrative embodiments have been illustrated and described, it will be appreciated that various changes can be made therein without departing from the spirit and scope of the invention.