Abstract:
An artificial foot and ankle joint consisting of a curved leaf spring foot member that defines a heel extremity and a toe extremity, and a flexible elastic ankle member that connects said foot member for rotation at the ankle joint. An actuator motor applies torque to the ankle joint to orient the foot when it is not in contact with the support surface and to store energy in a catapult spring that is released along with the energy stored in the leaf spring to propel the wearer forward. A ribbon clutch prevents the foot member from rotating in one direction beyond a predetermined limit position, and a controllable damper is employed to lock the ankle joint or to absorb mechanical energy as needed. The controller and a sensing mechanisms control both the actuator motor and the controllable damper at different times during the walking cycle for level walking, stair ascent and stair descent.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation in part of, and claims the benefit of the filing date of, U.S. patent application Ser. No. 11/395,448 filed on Mar. 31, 2006. application Ser. No. 11/395,448 was a non-provisional of, and claimed the benefit of the filing date of, U.S. Provisional Patent Application Ser. No. 60/666,876 filed on Mar. 3, 2005 and U.S. Provisional Patent Application Ser. No. 60/704,517 filed on Aug. 1. 2005.  
         [0002]     This application is a non-provisional of, and also claims the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/704,517 filed on Aug. 1, 2005.  
         [0003]     This application incorporates the disclosures of each of the foregoing application herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0004]     This invention relates generally to prosthetic devices and artificial limb and joint systems, including robotic, orthotic, exoskeletal limbs, and more particularly, although in its broader aspects not exclusively, to artificial feet and ankle joints.  
       BACKGROUND OF THE INVENTION  
       [0005]     In the course of the following description, reference will be made to the papers, patents and publications presented in a list of references at the conclusion of this specification. When cited, each listed reference will be identified by a numeral within curly-braces indicating its position within this list.  
         [0006]     As noted in {1} {2} {3}, an artificial ankle-foot system ideally needs to fulfill a diverse set of requirements. The artificial system must be a reasonable weight and have a natural morphological shape, but still have an operational time between refueling or battery recharges of at least one full day. The system must also be capable of varying its position, impedance, and motive power in a comparable manner to that of a normal, healthy biological limb. Still further, the system must be adaptive, changing its characteristics given such environmental disturbances as walking speed and terrain variation. The embodiments of the invention which are described in this specification employ novel architectures capable of achieving these many requirements.  
         [0007]     From recent biomechanical studies {1} {2} {3}, researchers have determined researchers have determined that early stance period ankle stiffness varies from step-to-step in wag. Furthermore, researchers have discovered that the human ankle performs more positive mechanical work than negative work, especially at moderate to fast wag speeds {1} {2}{3}. The added ankle power is important for providing adequate forward progression of the body at the end of each stance period. In distinction, for stair descent, the ankle behaves as a variable damper during the first half of stance, absorbing impact energies {2}. These biomechanical findings suggest that in order to mimic the actual behavior of the human ankle, joint stiffness, motive power, and damping must be actively controlled in the context of an efficient, high cycle-life, quiet and cosmetic ankle-foot artificial joint.  
         [0008]     For level ground ambulation, the ankle behaves as a variable stiffness device during the early to midstance period, storing and releasing impact energies. Throughout terminal stance, the ankle acts as a torque source to power the body forward. In distinction, the ankle varies damping rather than stiffness during the early stance period of stair descent. These biomechanical findings suggest that in order to mimic the actual behavior of a human joint or joints, stiffness, damping, and nonconservative, motive power must be actively controlled in the context of an efficient, high cycle-life, quiet and cosmetic biomimetic limb system, be it for a prosthetic or orthotic device. This is also the case for a biomimetic robotic limb since it will need to satisfy the same mechanical and physical laws as its biological counterpart, and will benefit from the same techniques for power and weight savings.  
         [0009]     In the discussion immediately below, the biomechanical properties of the ankle will be described in some detail to explain the insights that have guided the design and development of the specific embodiments of the invention and to define selected terms that will be used in this specification.  
         [0010]     Joint Biomechanics: The Human Ankle  
         [0011]     Understanding normal walking biomechanics provides the basis for the design and development of the artificial ankle joint and ankle-foot structures that embody the invention. Specifically, the function of human ankle under sagittal plane rotation is described below for different locomotor conditions including level-ground walking and stair/slope ascent and descent. From these biomechanical descriptions, the justifications for key mechanical components and configurations of the artificial ankle structures and functions embodying the invention may be better understood.  
         [0012]     Level-Ground Walking  
         [0013]     A level-ground walking gait cycle is typically defined as beginning with the heel strike of one foot and ending at the next heel strike of the same foot {8}. The main subdivisions of the gait cycle are the stance phase (about 60% of the cycle) and the subsequent swing phase (about 40% of the cycle) as shown in  FIG. 1 . The swing phase represents the portion of the gait cycle when the foot is off the ground. The stance phase begins at heel-strike when the heel touches the floor and ends at toe-off when the same foot rises from the ground surface. Additionally, we can further divide the stance phase into three sub-phases: Controlled Plantar flexion (CP), Controlled Dorsiflexion (CD), and Powered Plantar flexion (PP).  
         [0014]     Each phase and the corresponding ankle functions which occur when walking on level ground are illustrated in  FIG. 1 . The subdivisions of the stance phase of walking, in order from first to last, are: the Controlled Plantar flexion (CP) phase, the Controlled Dorsiflexion (CD) phase, and the Powered Plantar flexion (PP) phase.  
         [0015]     CP begins at heel-strike illustrated at  103  and ends at foot-flat at  105 . Simply speaking, CP describes the process by which the heel and forefoot initially make contact with the ground. In {1, 12}, researchers showed that CP ankle joint behavior was consistent with a linear spring response where joint torque is proportional to joint position. The spring behavior is, however, variable; joint stiffness is continuously modulated by the body from step to step.  
         [0016]     After the CP period, the CD phase continues until the ankle reaches a state of maximum dorsiflexion and begins powered plantarflexion PP as illustrated at  107 . Ankle torque versus position during the CD period can often be described as a nonlinear spring where stiffness increases with increasing ankle position. The main function of the ankle during CD is to store the elastic energy necessary to propel the body upwards and forwards during the PP phase {9} {3}.  
         [0017]     The PP phase begins after CD and ends at the instant of toe-off illustrated at  109 . During PP, the ankle can be modeled as a catapult in series or in parallel with the CD spring or springs. Here the catapult component includes a motor that does work on a series spring during the latter half of the CD phase and/or during the first half of the PP phase. The catapult energy is then released along with the spring energy stored during the CD phase to achieve the high plantar flexion power during late stance. This catapult behavior is necessary because the work generated during PP is more than the negative work absorbed during the CP and CD phases for moderate to fast walking speeds {1} {2} {3} {9}.  
         [0018]     During he swing phase, the final 40% of the gait cycle, which extends from toe-off at  109  until the next heel strike at  113 , the foot is lifted off the ground.  
         [0019]     Stair Ascent and Descent  
         [0020]     Because the kinematic and kinetic patterns at the ankle during stair ascent/descent are significantly different from that of level-ground walking {2}, a separate description of the ankle-foot biomechanics is presented in  FIGS. 2 and 3 .  
         [0021]      FIG. 2  shows the human ankle biomechanics during stair ascent. The first phase of stair ascent is called Controlled Dorsiflexion 1 (CD 1), which begins with foot strike in a dorsiflexed position seen at  201  and continues to dorsiflex until the heel contacts the step surface at  203 . In this phase, the ankle can be modeled as a linear spring.  
         [0022]     The second phase is Powered Plantar flexion 1 (PP 1), which begins at the instant of foot flat (when the ankle reaches its maximum dorsiflexion at  203 ) and ends when dorsiflexion begins once again at  205 . The human ankle behaves as a torque actuator to provide extra energy to support the body weight.  
         [0023]     The third phase is Controlled Dorsiflexion 2 (CD 2), in which the ankle dorsiflexes until heel-off at  207 . For the CD 2 phase, the ankle can be modeled as a linear spring.  
         [0024]     The fourth and final phase is Powered Plantar flexion 2 (PP 2) which begins at heel-off  207  and continues as the foot pushes off the step, acting as a torque actuator in parallel with the CD 2 spring to propel the body upwards and forwards, and ends when the toe leaves the surface at  209  to being the swing phase that ends at  213 .  
         [0025]      FIG. 3  shows the human ankle-foot biomechanics for stair descent. The stance phase of stair descent is divided into three sub-phases: Controlled Dorsiflexion 1 (CD1), Controlled Dorsiflexion 2 (CD2), and Powered Plantar flexion (PP).  
         [0026]     CD1 begins at foot strike illustrated at  303  and ends at foot-flat  305 . In this phase, the human ankle can be modeled as a variable damper. In CD2, the ankle continues to dorsiflex forward until it reaches a maximum dorsiflexion posture seen at  307 . Here the ankle acts as a linear spring, storing energy throughout CD2. During PP, which begins at  307 , the ankle plantar flexes until the foot lifts from the step at  309 . In this final PP phase, the ankle releases stored CD2 energy, propelling the body upwards and forwards. After toe-off at  309 , the foot is positioned controlled through the swing phase until the next foot strike at  313 .  
         [0027]     For stair ascent depicted in  FIG. 2 , the human ankle-foot can be effectively modeled using a combination of an actuator and a variable stiffness mechanism. However, for stair descent, depicted in  FIG. 3 , a variable damper needs also to be included for modeling the ankle-foot complex; the power absorbed by the human ankle is much greater during stair descent than the power released by 2.3 to 11.2 J/kg {2}. Hence, it is reasonable to model the ankle as a combination of a variable-damper and spring for stair descent {2}.  
       SUMMARY OF THE INVENTION  
       [0028]     The preferred embodiments of the present invention take the form of an artificial ankle system capable of providing biologically-realistic dynamic behaviors. The key mechanical components of these embodiments, and their general functions, may be summarized as follows: 
        1. One or more passive springs—to store and release elastic energy for propulsion;     2. One or more series-elastic actuators (muscle-tendon)—to control the position of the ankle, provide additional elastic energy storage for propulsion, and to control joint stiffness; and     3. One or more variable dampers—to absorb mechanical energy during stair and slope descent.        
 
         [0032]     The above-identified U.S. patent application Ser. No. 11/395,448 filed on Mar. 31, 2006 describes related artificial limbs and joints that employ passive and series-elastic elements and variable-damping elements, and in addition employ active motor elements in arrangements called “Biomimetic Hybrid Actuators” forming biologically-inspired musculoskeletal architectures. The electric motor used in the hybrid actuators supply positive energy to and store negative energy from one or more joints which connect skeletal members, as well as elastic elements such as springs, and controllable variable damper components, for passively storing and releasing energy and providing adaptive impedance to accommodate level ground walking as well as movement on stairs and surfaces having different slopes.  
         [0033]     As described in application Ser. No. 11/395,448, an artificial ankle may employ an elastic member operatively connected in series with the motor between the shin member and the foot member to store energy when the relative motion of the foot and shin members is being arrested by a controllable variable damping element and to thereafter apply an additional torque to the ankle joint when the variable damping element no longer arrests the relative motion of the two members.  
         [0034]     As further described in application Ser. No. 11/395,448, An artificial ankle may include an elastic member operatively connected in series with the motor between the shin and foot members to store energy when the foot member is moved toward the shin member and to release energy and apply an additional torque to the ankle joint that assists the motor to move the foot member away from the shin member. A controllable damping member may be employed to arrest the motion of the motor to control the amount of energy absorbed by the motor when the foot member is moved toward the shin member.  
         [0035]     The Flex-Foot, made by Össur of Reykjavik, Iceland, is a passive carbon-fiber energy storage device that replicates the ankle joint for amputees. The Flex-Foot is described in U.S. Pat. No. 6,071,313 issued to Van L. Phillips entitled “Split foot prosthesis” and in Phillips&#39; earlier U.S. Pat. Nos. 5,776,205, 5,514,185 and No. 5,181,933, the disclosures of which are incorporated herein by reference. The Flex-foot is a foot prosthesis for supporting an amputee relative to a support surface and consists of a leaf spring having multiple flexing portions configured to flex substantially independently of one another substantially completely along their length. The Flex-Foot has an equilibrium position of 90 degrees and a single nominal stiffness value. In the embodiments described below, a hybrid actuator mechanism of the kind described in the above-noted application Ser. No. 11/395,448 is used to augment a flexing foot member such as the Flex-Foot by allowing the equilibrium position to be set to an arbitrary angle by a motor and locking, or arresting the relative movement of, the foot member with respect to the shin member using a clutch or variable damper. Furthermore, the embodiment of the invention to be described can also change the stiffness and damping of the prosthesis dynamically.  
         [0036]     Preferred embodiments of the present invention take the form of an artificial ankle and foot system in which a foot and ankle structure is mounted for rotation with respect to a shin member at an ankle joint. The foot and ankle structure preferably comprises a curved flexible elastic foot member that defines an arch between a heel extremity and a toe extremity, and a flexible elastic ankle member that connects said foot member for rotation at the ankle joint. A variable damper is employed to arresting the motion of said foot and ankle structure with respect to said shin member under predetermined conditions, and preferably includes a stop mechanism that prevents the foot and ankle structure from rotating with respect to the shin member beyond a predetermined limit position. The variable damper may further include a controllable damper, such as a magnetorheological (MR) brake, which arrests the rotation of the ankle joint by controllable amount at controlled times during the walking cycle. Preferred embodiments of the ankle and foot system further include an actuator motor for applying torque to the ankle joint to rotate said foot and ankle structure with respect to said shin member.  
         [0037]     In addition, embodiments of the invention may include a catapult mechanism comprising a series elastic member operatively connected in series with the motor between the shin member and the foot and ankle structure. The series elastic member stores energy from the motor during a first portion of each walking cycle and then releases the stored energy to help propel the user forward over the walking surface at a later time in each walking cycle. The preferred embodiments of the invention may employ a controller for operating both the motor and the controllable damper such that the motor stores energy in the series elastic member as the shin member is being arrested by the controllable damper.  
         [0038]     The actuator motor which applies torque to the ankle joint may be employed to adjust the position of the foot and ankle structure relative to the shin member when the foot and ankle member is not in contact with a support surface. Inertial sensing means are preferably employed to determine the relative elevation of the foot and angle structure and to actuate the motor in response to changes in the relative elevation, thereby automatically positioning the foot member for toe first engagement if the wearer is descending stairs.  
         [0039]     These and other features and advantages of the present invention will be better understood by considering the following detailed description of two illustrative embodiments of the invention. In course of this description, frequent reference will be made to the attached drawings which are briefly described below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]      FIG. 1  illustrates the different phases of a walking cycle experienced by a human ankle and foot during level ground walking;  
         [0041]      FIG. 2  depicts the phases of a walking cycle experienced by a human ankle and foot when ascending stairs;  
         [0042]      FIG. 3  depicts the phases of a walking cycle experienced by a human ankle and foot during stair descent;  
         [0043]      FIG. 4  shows the mechanical design of an anterior view of embodiment 1;  
         [0044]      FIG. 5  shows a posterior view of embodiment 1;  
         [0045]      FIG. 6  shows a side elevational view of embodiment 1;  
         [0046]      FIG. 7  is a schematic depiction of embodiment 1;  
         [0047]      FIG. 8  depicts a lumped parameter model of embodiment 1;  
         [0048]      FIGS. 9-12  show the control sequence for embodiment 1 during ground level walking;  
         [0049]      FIGS. 13-15  show the control sequence for embodiment 1 during stair ascent;  
         [0050]      FIGS. 16-19  show the control sequence for embodiment 1 during stair descent;  
         [0051]      FIG. 20  shows the mechanical design of an anterior view of embodiment 2;  
         [0052]      FIG. 21  shows a posterior view of embodiment 2;  
         [0053]      FIG. 22  shows a side elevational view of embodiment 2;  
         [0054]      FIG. 23  is a schematic depiction of embodiment 2;  
         [0055]      FIG. 24  depicts a lumped parameter model of embodiment 2;  
         [0056]      FIGS. 25-28  show the control sequence for embodiment 1 during ground level walking;  
         [0057]      FIG. 29  is a schematic block diagram of a sensing and control mechanism used to control the operation of the motors and dampers in ankle foot systems embodying the invention. 
     
    
     DETAILED DESCRIPTION  
       [0058]     Two embodiments of an ankle-foot system contemplated by the present invention are described in detail below. The first embodiment (Embodiment 1) provides for elastic energy storage, variable-damping and a variable-orientation foot control. In addition to these capabilities, the second embodiment to be described includes a motor in series with a spring for providing joint spring stiffness control during the CP and CD phases, and a motive torque control during the PP phase of the walking cycle as described above.  
       Embodiment 1  
       [0059]     Mechanical Components  
         [0060]     The mechanical design of embodiment 1 is seen in  FIGS. 4-6  and the corresponding schematic and lumped parameter model of embodiment 1 are shown in  FIGS. 7 and 8 , respectively. As seen in the side elevation view of  FIG. 6 , there are four main mechanical elements in this embodiment: an elastic leaf spring structure  601 , a dorsiflexion clutch (Ribbon Stop) seen at  603 , a variable damper (MR brake) seen at  605 , and an actuator system comprising a small motor seen at  607 . As seen in the schematic of  FIG. 7 , these four main mechanical elements are shown as an elastic leaf spring structure  701 , a dorsiflexion clutch (Ribbon Stop)  703 , a variable damper  705 , and a motor actuator system  707 .  
         [0061]     The elastic leaf spring seen at  601  and  701  can be made from a lightweight, efficient spring material such as carbon composite, fiberglass or a material of similar properties. As seen in  FIG. 6 , and as described in Phillips&#39; U.S. Pat. No. 6,071,313 issued on Jun. 6, 2000, the elastic leaf spring structure includes a heel, portion seen at  609  and a toe portion seen at  660 . A curved, flexible ankle section  680  is attached at its upper end to a brake mount member  690  which is mounts the flexible foot for rotation about the axis of the ankle joint which, in  FIG. 6 , is located at the center of the MR brake  605 .  
         [0062]     The variable-damper mechanism seen at  605  and  705  can be implemented using magnetorheological (MR), electrorheological (ER), dry magnetic particles, hydraulic, pneumatic, friction, or any similar strategy to control joint damping. For embodiment 1, a MR system is employed. Here MR fluid is used in the shear mode where a set of rotary plates shear thin layers of MR fluid. When a magnetic field is induced across the MR layers, iron particles suspended in carrier fluid form chains, increasing the shear viscosity and joint damping.  
         [0063]     The ribbon stop seen at  603  and  703  prevents the ankle joint from dorsiflexing beyond a certain maximum dorsiflexion limit, ranging from 0 to 30 degrees depending on ankle performance requirements. The ribbon stop is uni-directional, preventing dorsiflexion but not impeding plantarflexion movements.  
         [0064]     The actuator motor seen at  607  and  707  is a small, low-power electromagnetic motor that provides foot orientation control. The motor can exert a torque about the ankle joint (indicated at  711 ) to re-position the foot (the elastic leaf spring  601 ,  701 ) relative to the shank depicted at  713  when the foot is not in contact with the ground. As seen in  FIGS. 4-6 , the shank frame for the ankle-foot assembly attaches to a shin member (not shown) using a standard pyramid mount seen at  613  which may be used to attach the shank frame to the shin portion of an artificial limb or the wearer&#39;s stump. As will be understood, both of the artificial foot and ankle joint embodiments described in this specification may be used in combination with artificial limb structures such as the artificial knees and hips described in the above-noted U.S. patent application Ser. No. 11/395,448.  
         [0065]     Control System  
         [0066]     For a better understanding of the control sequence of the artificial ankle, a simplified 1D lumped parameter model of embodiment 1 seen in  FIG. 8  is used to explain the behavior of the ankle-foot system under different walking conditions.  
         [0067]     From  FIG. 7 , it may be noted that the bending angle of the elastic leaf spring  701  is independent of the ankle angle of the pin joint, therefore the lumped parameter model includes two degrees of freedom: one for the displacement of the foot, X 1 , and the other for the displacement of the shank X 2  as shown in  FIG. 8 . The leaf spring structure, seen at  601  in  FIG. 6  and at  701  in  FIG. 7 , is modeled as a nonlinear spring shown at  801  in  FIG. 8  with a stiffness that varies with X 1 , the foot bending angle (displacement of the foot). The actuator motor seen at  807 , the variable-damper  805 , and the ribbon stop seen at  803  act between the mass of the shank at  820  and the mass of the foot at  830 . The loading force F load (t) due to body weight varies dynamically during the stance phase of each gait cycle.  
         [0068]     Level-Ground Walking  
         [0069]     The control sequence of Embodiment 1 for level-ground walking is depicted in  FIGS. 9-12 . During level-ground walking, the variable-damper is set at a high damping level to essentially lock the ankle joint during early to midstance, allowing the leaf spring structure to store and release elastic energy. Once a critical dorsiflexion angle is achieved (between 0 to 30 degrees), the ribbon stop becomes taught during the remainder of the CD phase. When the ribbon is engaged, the leaf spring and shank can be treated as one single component because the ribbon behaves as a clutch ( FIG. 10 ). From heel strike to maximum dorisflexion, the leaf spring structure stores elastic energy (Ax, 0, &amp; 2 =0). In PP, as the loading from the body weight decreases, the spring structure releases its stored elastic energy, rotating in a plantar flexion direction and propelling the body upwards and forwards ( FIG. 11 ). After toe-off, the actuator controls the equilibrium position of the foot to achieve foot clearance during the swing phase and to maintain a proper landing of the foot for the next gait cycle ( FIG. 12 ).  
         [0070]     The state of each element of the ankle-foot system during the four phases of a level ground walking cycle are listed below:  
         [0071]     Controlled Plantar Flexion ( FIG. 9 ) 
        1. Actuator motor is OFF     2. Ribbon clutch is OFF     3. Damper is ON     4. Leaf spring heel portion at  609  is being compressed        
 
         [0076]     Controlled Dorsiflexion ( FIG. 10 ) 
        1. Actuator motor is OFF     2. Ribbon clutch is ON     3. Damper is OFF     4. Leaf spring toe section  660  is being compressed        
 
         [0081]     Powered Plantar Flexion ( FIG. 11 ) 
        1. Actuator motor is OFF     2. Ribbon clutch is ON     3. Damper is OFF     4. Leaf spring ankle section  660  is releasing energy        
 
         [0086]     Swing Phase ( FIG. 12 ) 
        1. Actuator motor is ON (changing foot orientation)     2. Ribbon clutch is OFF     3. Damper is OFF     4. Foot leaf spring is slack        
 
         [0091]     The maximum dorsiflexion ankle torque during level-ground walking is in the range from 1.5 Ng to 2 Nm/kg, i.e. around 150 Nm for a 100 kg person {2}. With current technology, a variable-damper that can provide such high damping torque and additionally very low damping levels is difficult to build at a reasonable weight and size. Fortunately, the maximum controlled plantar flexion torque is small, typically in the range of 0.3 Nm/kg to 0.4 Ng. Because of these factors, a ribbon stop that engages at a small dorsiflexion angle such as 5 degrees would lower the peak torque requirements of the variable-damper since the peak controlled plantar flexion torque is considerably smaller than the peak dorsiflexion torque.  
         [0092]     During stair descent/downhill walking, the human ankle behaves like a damper from foot strike to 90° of dorsiflexion {11}. Beyond that, the ankle behaves like a non-linear spring, storing elastic energy during controlled dorsiflexion. Taking advantage of the biomechanics of the human ankle, it is reasonable to add a passive clutch for resisting dorsiflexion movements beyond 90°, thus allowing for a smaller sized variable damper. A ribbon stop is preferred as a unidirectional clutch because it is lightweight with considerable strength in tension.  
         [0093]     Stair Ascent  
         [0094]      FIGS. 13-15  depict the control sequence of embodiment 1 for stair ascent. It is noted here that there are only three control phases/modes for stair ascent, although the gait cycle for stair ascent can be divided into 5 sub-phases, including Controlled Dorsiflexion 1 (CD1), Powered Plantarflexion 1 (PP1), Controlled Dorsiflexion 2 (CD2), Powered Plantarflexion 1 (PP1), and Swing Phase. The main-reason is that in terms of control, we can combine phases PP1, CD2, and PP2 into one single phase since all three phases may be described using the same control law. For ascending a stair, the clutch is engaged and the leaf spring is compressed throughout ground contact ( FIG. 13 ) because the toe strikes the ground first, engaging the ribbon stop during CD (Ax, 0, &amp; 2 =0). After the heel strikes the ground and then lifts off the ground, the toe leaf spring begins releasing its energy, supplying forward propulsion to the body ( FIG. 14 ). The variable damper may be activated to control the process of energy release from the leaf spring, but in general, the damper is turned off so that all the stored elastic energy is used to propel the body upwards and forwards (Ax, 0, &amp; 2  0). After toe-off, the actuator controls the equilibrium position of the ankle in preparation for the next step ( FIG. 15 ).  
         [0095]     The state of each element of the ankle-foot system during these three phases of a stair ascent are listed below:  
         [0096]     Controlled Dorsiflexion ( FIG. 13 ) 
        1. Actuator motor is OFF     2. Ribbon clutch is ON     3. Damper is OFF     4. Leaf spring toe section  660  is being compressed        
 
         [0101]     Powered Plantar Flexion ( FIG. 14 ) 
        1. Actuator motor is OFF     2. Ribbon clutch is-ON     3. Damper is OFF     4. Leaf spring toe section  660  is releasing energy        
 
         [0106]     Swing Phase ( FIG. 15 ) 
        1. Actuator motor is ON (changing foot orientation)     2. Ribbon clutch is OFF     3. Damper is OFF     4. Foot leaf spring is slack        
 
         [0111]     Stair Descent  
         [0112]     The control sequence for embodiment 1 for stair descent is depicted in  FIGS. 16-19 . After forefoot contact, the body has to be lowered until the heel makes contact with the stair tread {11} ( FIG. 16 ). Therefore, the variable damper is activated as energy is dissipated during controlled dorsiflexion (ΔX 1 &lt;=0, ΔX 2 &lt;=0). As is shown in  FIG. 17 , when the foot becomes flat on the ground, the ribbon stop becomes taunt, compressing the toe leaf spring (ΔX 1 &lt;=0, ΔX 2 =0). During PP, the toe leaf spring releases its energy, propelling the body upwards and forwards ( FIG. 18 ).  
         [0113]     The state of each element of the ankle-foot system during the four phases of stair descent are listed below:  
         [0114]     Controlled Dorsiflexion 1 ( FIG. 16 ) 
        1. Actuator motor is OFF     2. Ribbon clutch is OFF     3. Damper is ON     4. Leaf spring toe section  660  is being compressed        
 
         [0119]     Controlled Dorsiflexion 2 ( FIG. 17 ) 
        1. Actuator motor is OFF     2. Ribbon clutch is ON     3. Damper is OFF     4. Leaf spring toe section  660  is being compressed        
 
         [0124]     Powered Plantar Flexion ( FIG. 18 ) 
        1. Actuator motor is OFF     2. Ribbon clutch is ON     3. Damper is OFF     4. Leaf spring toe section  660  is releasing energy        
 
         [0129]     Swing Phase ( FIG. 19 ) 
        1. Actuator motor is ON (changing foot orientation)     2. Ribbon clutch is OFF     3. Damper is OFF     4. Foot leaf spring is slack        
 
       Sensing for Embodiment 1  
       [0134]     The ankle foot system preferably employs an inertial navigation system (INS) for the control of an active artificial ankle joint to achieve a more natural gait and improved comfort over the range of human walking and climbing activities.  
         [0135]     To achieve these advantages, an artificial ankle joint must be controlled to behave like a normal human ankle. For instance, during normal level ground walking, the heel strikes the ground first; but when descending stairs, it is the toe which first touches the ground. Walking up or down an incline, either the toe or the heel may strike the ground first, depending upon the steepness of the incline.  
         [0136]     A difficult aspect of the artificial ankle control problem is that the ankle joint angle must be established before the foot reaches the ground, so that the heel or toe will strike first, as appropriate to the activity. Reliable determination of which activity is underway while the foot is still in the air presents implacable difficulties for sensor systems presently employed on lower leg artificial devices.  
         [0137]     The present invention addresses this difficulty by attaching an inertial navigation system below the knee joint, either on the lower leg segment or on the artificial foot. This system is then used to determine the foot&#39;s change in elevation since it last left the ground. This change in elevation may be used to discriminate between level ground walking and descending stairs or steep inclines. The ankle joint angle may then be controlled during the foot&#39;s aerial phase to provide heel strike for level ground walking or toe strike upon detection of negative elevation, as would be encountered descending stairs or walking down a steep incline.  
         [0138]     Inertial navigation systems rely upon accelerometers and gyroscopes jointly attached to a rigid assembly to detect the assembly&#39;s motion and change of orientation. In accordance with the laws of mechanics, these changes may be integrated to measure changes of the system&#39;s position and orientation, relative to its initial position and orientation. In practice, however, it is found that errors of the accelerometers and gyros produce ever-increasing errors in the system&#39;s estimated position. Inertial navigation systems can address this problem in one of two ways: by the use of expensive, high precision accelerometers and gyroscopes, and by incorporating other, external sources of information about position and orientation, for instance GPS, to augment the purely inertial information. But using either of these alternatives would make the resulting system unattractive for an artificial ankle device.  
         [0139]     However, we have found that an unaugmented, purely inertial system based on available low cost accelerometers and rate gyros can provide sufficiently accurate trajectory information to support proper control of the angle of an actuated artificial ankle system.  
         [0140]     An Illustrative Control Algorithm  
         [0141]     Control of an actuated artificial ankle joint may be implemented as follows: 
        A. During the foot flat (controlled dorsiflexion) phase of the walking cycle, reset and maintain the measured elevation to zero. When the foot is flat on the ground, its velocity and acceleration are zero. Thus, this particular foot posture serves as a reset point for the integration of angular and linear velocities in the estimation of absolute positions.     B. During the push off phase, when powered plantarflexion begins, measure the upward and downward movements to determine the current elevation relative to the initial zero elevation during the flat foot phase;     C. As long as the elevation remains above zero, maintain the foot orientation that will provide heelstrike; and     D. If the elevation decreases below zero, reorient the angle ankle to provide toe-first contact.        
 
         [0146]     The foot flat phase may be detected by the absence of non-centrifugal, non-gravitational, linear acceleration along the length axis,of the lower leg. Push off phase may be detected by the upward acceleration along the axis of the,lower leg. Elevation &gt;0 and elevation &lt;0 phases are recognized from the change in relative elevation computed by the INS since the end of foot flat phase.  
       Embodiment 2  
       [0147]     Mechanical Design  
         [0148]     The mechanical design of Embodiment 2 is shown in  FIGS. 20-23 . As seen in  FIG. 22 , the foot and ankle system includes an elastic leaf spring structure that provides a heel spring as seen at  2201  and a toe spring as seen at  2206 , the elastic leaf spring structure attaches to a brake mount member  2202  that rotates with respect to an ankle joint shank frame  2203  and a tibial side bracket  2204  about a pivot axis at the center of the MR brake seen at  2205 . The actuator motor  2207  is mounted within the tibial side bracket  2204  and its drive shaft is coupled through a drive gear (not shown) to rotate the elastic leaf spring structure  2201  and  2206  with respect to the shank frame  2203  and side bracket  2204  about the ankle joint. A catapult mechanism to provide powered plantar flexion during late stance is employed that consists of a series elastic spring element seen at  2210  having an internal slider  2212  that attaches to the brake mount  2202  at the lower actuator mount  2213 , and the spring element  2210  attaches to the upper actuator mount  2216  at the top of the tibial side bracket  2204 . A standard pyramid mount  2230  at the top of the tibial side bracket  2294  provides a connection to the shin member (not shown).  
         [0149]     The corresponding schematic of Embodiment 2 is seen in  FIG. 23  and is similar to that of Embodiment 1, including the heel and toe leaf spring  2301 , variable damper  2305 , and ribbon stop  2303 . The series elastic spring element is seen at  2310  connected in series with the actuator motor  2307  to form the catapult.  
         [0150]     One of the main challenges in the design of an artificial ankle is to have a relatively low-mass actuation system that can provide a large instantaneous output power upwards of 200 Watts during Powered Plantar Flexion (PP) {2,11} Fortunately, the duration of PP is only 15% of the entire gait cycle, and the average power output of the human ankle during the stance phase is much lower than the instantaneous output power during PP. Hence, a catapult mechanism is a compelling solution to this problem.  
         [0151]     The catapult mechanism is mainly composed of three components: an actuator motor, a variable damper and/or clutch and an energy storage element. The actuator can be any type of motor system, including electric, shape memory alloy, hydraulic or pneumatic devices, and the series energy storage element can be any elastic element capable of storing elastic energy when compressed or stretched. The damper can be any type of device including hydraulic, magnetorheological, pneumatic, or electrorheological.  
         [0152]     With the parallel damper seen at  2305  in  FIG. 23  activated to a high damping level or with the parallel clutch  2303  activated, the series elastic spring element  2310  can be compressed or stretched by the actuator  2307  in series to the spring  2310  without the joint rotating. The spring  2310  will provide a large amount of instantaneous output power once the parallel damping device  2305  or clutch  2303  is deactivated, allowing the elastic element  2310  to release its energy. If the actuator  2307  has a relatively long period of time to compress or stretch the elastic element  2310 , its mass can be kept relatively low, decreasing the overall weight of the artificial ankle device. In Embodiment 2, the catapult system comprises a magnetorheological variable damper  2305  placed in parallel to the series elastic electric motor system.  
         [0153]     Control System  
         [0154]     The lumped parameter model of Embodiment 2 is shown in  FIG. 24 . It is basically the same as the model of Embodiment 1 as depicted in  FIG. 8 , except that we now place a spring element  2410  in series with the actuator  2407  and the foot mass structure  2430 . The main idea here is that if the variable MR damper seen at  2405  outputs high damping, locking the ankle joint, the foot and the shank become one single component. Once the joint is locked, the actuator  2407  compresses or stretches the spring element  2310 . Once joint damping is minimized, the spring element  2410  will then push against the shank  2420  to provide forward propulsion during powered plantar flexion.  
         [0155]     The control sequence of Embodiment 2 for level-ground walking will be discussed in the next section. Stair ascent/descent can be deduced from the earlier descriptions for embodiment 1, and thus, will not be described herein.  
         [0156]     Level-Ground Walking  
         [0157]     The control sequence of Embodiment 2 for level-ground walking is depicted in  FIGS. 25-28 . During CP, the actuator controls the stiffness of the ankle by controlling the displacement of the series spring ( FIG. 25 ). During CD, the toe carbon fiber leaf spring  2206  is compressed due to the loading of body weight, while the actuator compresses the series spring to store additional elastic energy in the system ( FIG. 26 ). In this control scheme, inertia and body weight hold the joint in a dorsiflexed posture, enabling the motor to elongate the series spring. In a second control approach, where body weight and inertia are insufficient to lock the joint, the MR variable damper would output a high damping value to essentially lock the ankle joint while the motor stores elastic energy in the series spring. Independent of the catapult control approach, during PP as seen in  FIG. 27 , as the load from body weight decreases, both the leaf spring and the series catapult spring begin releasing stored elastic energy, supplying high ankle output powers. After toe-off, the actuator controls the position of the foot while both the series spring and the leaf springs are slack as depicted in  FIG. 28 .  
         [0158]     The state of each element of Embodiment  2  of the ankle foot system during the four phases of a level ground walking cycle are listed below:  
         [0159]     Controlled Plantar Flexion ( FIG. 25 ) 
        1. Actuator motor is ON     2. Ribbon clutch is OFF     3. Damper is OFF     4. Leaf spring heel portion at  2201  is being compressed        
 
         [0164]     Controlled Dorsiflexion ( FIG. 26 ) 
        1. Actuator motor is ON     2. Ribbon clutch is ON     3. Damper is OFF     4. Leaf spring toe section  2206  is being compressed        
 
         [0169]     Powered Plantar Flexion ( FIG. 27 ) 
        1. Actuator motor is ON     2. Ribbon clutch is OFF     3. Damper is OFF     4. Leaf spring toe section  2206  is releasing energy        
 
         [0174]     Swing Phase ( FIG. 28 ) 
        1. Actuator motor is ON (changing foot orientation)     2. Ribbon clutch is OFF     3. Damper is OFF     4. Foot leaf spring structure is slack        
 
       Sensing for Embodiment 2  
       [0179]     As with Embodiment 1, an inertial navigation system for the control of the active artificial ankle joint will be employed to achieve a more natural gait and improved comfort over the range of human walking and climbing activities. The manner in which these navigation sensors will be used is similar to that described for Embodiment 1.  
         [0180]     Sensing and Control  
         [0181]     As described above, investigations of the biomechanics of human limbs have revealed the functions performed by the ankle during normal walking over level ground, and when ascending or descending a slope or stairs. As discussed above, these functions may be performed in an artificial ankle joint using motors to act as torque actuators and to position the foot relative to the shin member during a specific times of walking cycle, using springs in combination with controllable dampers to act as linear springs and provide controllable damping at other times in the walking cycle. The timing of these different functions occurs during the walking cycle at times described in detail above. The specific mechanical structures, that is the combinations of motors, springs and controllable dampers used in these embodiments are specifically adapted to perform the functions needed, a variety of techniques may be employed to automatically control the motor and controllable dampers at the times needed to perform the functions illustrated, and any suitable control mechanism may be employed.  FIG. 29  depicts the general form of a typical control mechanism in which a multiple sensors are employed to determine the dynamic status of the skeletal structure and the components of the hybrid actuator and deliver data indicative of that status to a processor seen at  2900  which produces control outputs to operate the motor actuator and to control the variable dampers.  
         [0182]     The sensors used to enable general actuator operation and control can include: 
        (1) Position sensors seen at  2902  in  FIG. 29  located at the ankle joint axis to measure joint angle (a rotary potentiometer), and at the motor rotor to measure total displacement of the motor&#39;s drive shaft (as indicated at  2904 ) and additionally the motor&#39;s velocity (as indicated at  2906 ). A single shaft encoder may be employed to sense instantaneous position, from which motor displacement and velocity may be calculated by the processor  2900 .     (2) A force sensor (strain gauges) to measure the actual torque borne by the joint as indicated at  2908 .     (3) Velocity sensors on each of the dampers (rotary encoders) as indicated at  2910  in order to get a true reading of damper velocity.     (4) A displacement sensor on each spring (motor series spring and global damper spring) as indicated at  2912  in order to measure the amount of energy stored.     (5) One or more Inertial Measurement Units (IMUs) seen at  2914  which can take the form of accelerometers positioned on skeletal members from which the processor  2900  can compute absolute orientations and displacements of the artificial joint system. For example, the IMU may sense the relative vertical movement of the foot member relative to its foot flat position during the walking cycle to control foot orientation as discussed above.     (6) One or more control inputs manipulatable by a person, such a wearer of a prosthetic joint or the operator of a robotic system, to control such things as walking speed, terrain changes, etc.        
 
         [0189]     The processor  2900  preferably comprises a microprocessor which is carried on the ankle-foot system and typically operated from the same battery power source  2920  used to power the motor  2930  and the controllable dampers  2932  and  2934 . A non-volatile program memory  2941  stores the executable programs that control the processing of the data from the sensors and input controls to produce the timed control signals which govern the operation of the actuator motor and the dampers. An additional data memory seen at  2942  may be used to supplement the available random access memory in the microprocessor  2900 .  
         [0190]     Instead of directly measuring the deflection of the motor series springs as noted at (4) above, sensory information from the position sensors (1) can be employed. By subtracting the ankle joint angle from the motor output shaft angle, it is possible to calculate the amount of energy stored in the motor series spring. Also, the motor series spring displacement sensor can be used to measure the torque borne by the joint because joint torque can be calculated from the motor series output force.  
         [0191]     Many variations exist in the particular sensing methodologies employed in the measurement of the listed parameters. Although this specification describes preferred sensing methods, each has the goal of determining the energy state of the spring elements, the velocities of interior points, and the absolute movement pattern of the ankle joint itself.  
       REFERENCES  
       [0192]     The following published materials provide background information relating to the invention. Individual items are cited above by using the reference numerals which appear below and in the citations in curley brackets. 
        {1} Palmer, Michael. Sagittal Plane Characterization of Normal Human Ankle Function across a Range of Walking Gait Speeds. Massachusetts Institute of Technology Master&#39;s Thesis, 2002.     {2} Gates Deanna H., Characterizing ankle function during stair ascent, descent, and level walking for ankle prosthesis and orthosis design. Master thesis, Boston University, 2004.     {3} Hansen, A., Childress, D. Miff, S. Gard, S. and Mesplay, K., The human ankle during walking: implication for the design of biomimetric ankle prosthesis, Journal of Biomechanics (In Press).     {4} Koganezawa, K. and Kato, I., Control aspects of artifical leg, IFAC Control Aspects of Biomedical Engineering, 1987, pp. 71-85.     {5} Herr H, Wilkenfeld A. User-Adaptive Control of a Magnetorheological Prosthetic Knee. Industrial Robot: An International Journal 2003; 30: 42-55.     {6} Seymour Ron, Prosthetics and Orthotics: Lower limb and Spinal, Lippincott Williams &amp; Wilkins, 2002.     {7} G. A. Pratt and M. M. Williamson, “Series Elastic Actuators,” presented at 1995 IEEE/RSJ International Conference on Intelligent Robots and Systems, Pittsburgh, Pa.,     {8} Inman V T, Ralston H J, Todd F. Human walking. Baltimore: Williams and Wilkins; 1981.     {9} Hof. A. L. Geelen B. A., and Berg, J w. Van den, “Calf muscle moment, work and efficiency in level walking; role of series elasticity,” Journal of Biomechanics, Vol 16, No. 7, pp. 523-537, 1983.     {10} Gregoire, L., and et al, Role of mono- and bi-articular muscles in explosive movements, International Journal of Sports Medicine 5, 614-630.     {11} Koganezawa, K. and Kato, I., Control aspects of artifical leg, IFAC Control Aspects of Biomedical Engineering, 1987, pp. 71-85.     {12} U.S. Pat. No. 6,517,503 issued Feb. 11, 203.        
 
       CONCLUSION  
       [0205]     It is to be understood that the methods and apparatus which have been described above are merely illustrative applications of the principles of the invention. Numerous modifications may be made by those skilled in the art without departing from the true spirit and scope of the invention.