Abstract:
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.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a divisional of U.S. application Ser. No. 13/171,307, filed Jun. 28, 2011, which is a continuation of U.S. application Ser. No. 11/395,448, filed Mar. 31, 2006, now abandoned, which claims the benefit of U.S. Application No. 60/666,876, filed Mar. 31, 2005, and 60/704,517, filed Aug. 1, 2005. The entire teachings of the above applications are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to prosthetic devices and artificial limb systems, including robotic, orthotic, exoskeletal limbs, and more particularly, although in its broader aspects not exclusively, to artificial ankle, knee, and hip joints. 
       BACKGROUND 
       [0003]    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. 
         [0004]    As noted in {1}{2}{3}{4}, an artificial limb system that mimics a biological limb 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 actuator and limb architectures capable of achieving these many requirements. 
         [0005]    From recent biomechanical studies {1}{2}{3}, researchers have determined that biological joints have a number of features. Among these are:
       (a) The ability to vary stiffness and damping.   (b) The ability to generate large amounts of both positive and negative mechanical work.   (c) The ability to produce large amounts of nonconservative power and torque when needed.       
 
         [0009]    An example of the use of more than one control strategy in a single biological joint is the ankle {1}{2}. 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. 
         [0010]    In the discussion immediately below, the biomechanical properties of three human joints, the ankle, knee and hip, 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. 
         [0011]    Joint Biomechanics: The Human Ankle 
         [0012]    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. 
         [0013]    Level-Ground Walking 
         [0014]    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). 
         [0015]    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. 
         [0016]    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}{3}, 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. 
         [0017]    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}. 
         [0018]    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}. 
         [0019]    During the 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. 
         [0020]    Stair Ascent and Descent 
         [0021]    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 . 
         [0022]      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. 
         [0023]    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. 
         [0024]    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. 
         [0025]    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 . 
         [0026]      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). 
         [0027]    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 . 
         [0028]    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}. 
         [0029]    Joint Biomechanics: The Human Knee 
         [0030]    There are five distinct phases to knee operation throughout a level-ground walking cycle {8}. To further motivate the hybrid actuator design described herein, a description of these phases is included.
       1. Beginning at the time the heel strikes as indicated at  403  in  FIG. 4 , the stance knee begins to flex slightly. This flexion period, called the Stance Flexion phase, allows for shock absorption upon impact as well as to keep the body&#39;s center of mass at a more constant vertical level throughout the stance period. During this phase, the knee acts as a spring, storing energy in preparation for the Stance Extension phase.   2. After maximum flexion is reached in the stance knee as indicated at  404 , the joint begins to extend, until maximum extension is reached at  406 . This knee extension period is called the Stance Extension phase. Throughout approximately the first 60% of Stance Extension, the knee acts as a spring, releasing the stored energy from the Stance Flexion phase of gait. This first release of energy corresponds to power output indicated at  501  in  FIG. 5 . During approximately the last 30% of Stance Extension, the knee absorbs energy in a second spring and then that energy is released during the next gait phase, or Pre-Swing, that begins at  406 .   3. During late stance or Pre-Swing, the knee of the supporting leg begins its rapid flexion period in preparation for the swing phase. During early Pre-Swing, as the knee begins to flex in preparation for toe-off, the stored elastic energy from Stance Extension is released. This second release of energy corresponds to power output level indicated at  503  in  FIG. 5 .   4. As the hip is flexed, and the knee has reached a certain angle in Pre-Swing, the leg leaves the ground and the knee continues to flex as indicated at  407  in  FIG. 4 . At the time of toe-off at  407 , the Swing Flexion phase of gait begins. Throughout this period, knee power is generally negative where the knee&#39;s torque impedes knee rotational velocity. During early Swing Flexion, the knee behaves as a variable damper, and during terminal Swing Flexion, the knee can be modeled as a spring, storing energy in preparation for early Swing Extension.   5. After reaching a maximum flexion angle during swing, the knee begins to extend forward as indicated at  408 . During the early Swing Extension period, the spring energy stored during late Swing Flexion is then released, resulting in power output level indicated at  505  in  FIG. 5 . During the remainder of Swing Extension, the human knee outputs negative power (absorbing energy) to decelerate the swinging leg in preparation for the next stance period. After the knee has reached full extension, the foot once again is placed on the ground, and the next walking cycle begins at  410 .       
 
         [0036]    Joint Biomechanics: The Human Hip 
         [0037]      FIGS. 6 and 7  graphically depict human hip biomechanics for level ground walking Hip position (radians) is plotted in  FIG. 6  and hip power (watts) is shown in  FIG. 7 , and show the spring-like behavior of the hip in walking Maximum hip power absorption occurs as indicated at  702  during terminal hip extension, and maximum hip power output occurs during active hip flexion near toe-off as indicated at  704  to drive the lower leg from the walking surface. 
         [0038]    As discussed in more detail later, the hip can be modeled with a spring in parallel with a motor system. The parallel spring generally stores energy during hip extension and then releases that energy to power hip flexion. To the extent to which the desired joint behavior deviates from a conservative spring response, the hip model includes a parallel motor system designed to modulate stiffness, damping and power about the natural spring output. 
         [0039]    Prior Art Leg Systems 
         [0040]    The current state of the art in prosthetic leg systems include a knee joint that can vary its damping via magnetorheological fluid {5}, and a carbon fiber ankle which has no active control, but that can store energy in a spring structure for return at a later point in the gait cycle (e.g. the Flex-Foot or the Seattle-Lite) {4}{6}. None of these systems are able to add energy during the stride to help keep the body moving forward or to reduce impact losses at heel strike. In the case of legged robotic systems, the use of the Series Elastic Actuator (SEA) enables robotic joints to control their position and torque, such that energy may be added to the system as needed {7}. In addition, the SEA can emulate a physical spring or damper by applying torques based on the position or velocity of the joint. However, for most applications, the SEA requires a tremendous amount of electric power for its operation, resulting in a limited operational life or an overly large power supply. Robotic joint designs in general use purely active components and often do not conserve electrical power through the use of passive-elastic and variable-impedance devices. 
       SUMMARY OF THE INVENTION 
       [0041]    In the construction of a biologically realistic limb system that is high performance, light weight, quiet and energetically efficient, embodiments of the invention to be described below employ passive-elastic, variable-damping, and motor elements. Since it is desirable to minimize the overall weight of the limb design, the efficiency of the system is critical, especially given the poor energy density of current power supplies, e.g. lithium-ion battery technology. By understanding human biomechanics, the lightest, most energy efficient hybrid actuator design can be achieved. 
         [0042]    The illustrative embodiments to be described employ Biomimetic Hybrid Actuators in biologically-inspired musculoskeletal architectures and use an electric motor for supplying positive energy to and storing 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 stiffness to accommodate level ground walking as well as movement on stairs and surfaces having different slopes. 
         [0043]    These hybrid actuators manipulate first and second skeletal members connected at one or more joints for movement relative to one another. A motor applies a force to move one member with respect to the other. One or more passive elastic members are connected between the skeletal members for storing energy when the members move relative to one another in one direction and for releasing energy when the members relative to one another in the opposite direction, and one or more controllable variable damping elements dissipate mechanical energy to arrest the relative motion of the first and second members at controllable times. 
         [0044]    Some of the embodiments provide additional force using a catapult mechanism in which the motion of the members is arrested by a controllable damping element while the motor stores energy in one or more elastic members and the damping element thereafter releases the members which are then moved by the energy stored in the elastic member. 
         [0045]    One or more damping elements may be operatively connected in parallel with the motor to arrest its motion while energy is stored in one or more elastic members and thereafter the motor parallel damping element releases the motor to release the energy previously stored in the elastic member. 
         [0046]    The hybrid actuator may employ an elastic member operatively connected in series with a controllable damping member. When the controllable damping member exhibits a higher damping level, energy is stored in the series elastic member and thereafter, when the controllable damping member exhibits a lower damping level, energy is released from the series elastic member. 
         [0047]    The motor in the hybrid actuator may apply torque to a joint or joints through a gearbox and a first controllable variable damping element can be employed to arrest the motion of the motor at controllable times, and a further controllable variable damping element operatively connected between the motor and the gearbox can disconnect the motor and the gearbox at controllable times, such that the gearbox can be used as a damping element to arrest the motion of skeletal members at some times, and be used to apply force to move the members at other times. 
         [0048]    The hybrid actuator may be used to implement an artificial ankle joint which connects a foot member for rotation with respect to a shin member, and which includes a motor for applying torque to the ankle joint to rotate the foot member with respect to the shin member, one or more passive elastic members connected between the shin and foot members for storing energy when the foot member rotates about the ankle joint toward the shin member and for releasing energy to apply additional torque to rotate the foot member away from the shin member, and one or more controllable variable damping elements for dissipating mechanical energy to arrest the relative motion of the foot and shin members at controllable times. 
         [0049]    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. 
         [0050]    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. 
         [0051]    A hybrid actuator may also be used to implement an artificial knee in which a thigh and shin skeletal member are connected by a knee joint and a motor is used to apply torque to the knee joint to rotate the shin member with respect to the thigh member at controllable times and to absorb energy from the rotation of the shin member with respect to the thigh member at other times. One or more passive elastic members connected between the shin and thigh members may be used to store energy when the shin member rotates toward the thigh member and to thereafter release energy to apply additional torque to the knee joint to rotate the shin member away from the thigh member, and a controllable variable damping element may be used to dissipate mechanical energy to arrest the relative motion of the thigh and shin members at controllable times. 
         [0052]    An artificial knee may employ the motor to apply torque to the knee joint to rotate the shin member with respect to the thigh member at controllable times and to absorb energy from the rotation of the shin and thigh members at other times, and further employ an elastic member operatively connected in series with the motor to store energy when the shin and thigh members are forced toward one another, and to release energy and apply an additional torque to aid the motor when the shin member moves away from the thigh member. In addition, a controllable variable damping element 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. 
         [0053]    As artificial knee may further comprise a second elastic member operatively connected between the thigh member and the shin member in series with a second controllable variable damping element that supplies a high level of damping at controllable times to store energy in the second elastic member and a lower level of damping at other times to release the energy stored in the second elastic member. 
         [0054]    A further embodiment of the invention may take the form of an artificial hip that consists of a pelvis member, a thigh member and a hip joint that connects the thigh member for rotation with respect to the pelvis member, a motor for applying torque to the hip joint to rotate the thigh member with respect to the pelvis member, and an elastic member operatively connected in series with a controllable variable damping element between the pelvis member and the thigh member, with the controllable variable damping member providing a high level of damping at controllable times to store energy in the elastic member and a lower level of damping at other times to release the energy stored in the second elastic member. 
         [0055]    An artificial hip may include a controllable variable damping element for dissipating mechanical energy to arrest the relative motion of the thigh member with respect to the pelvis member at controllable times, and an elastic member operatively connected in series with the motor between the pelvis member and the thigh member to store energy from the motor when the relative motion of the thigh member and the pelvis member is being arrested by the controllable variable damping element and to thereafter apply an additional torque to the joint member when the relative motion of the thigh member with respect to the pelvis member is no longer arrested by the controllable variable damping element. 
         [0056]    An artificial hip may employ a motor connected for applying torque to the hip joint to rotate the thigh member with respect to the pelvis member at controllable times and to absorb energy from the rotation of the thigh member with respect to the pelvis member at other times, an elastic member operatively connected in series with the motor between the pelvis member and the thigh member to store energy when the thigh member is rotated upwardly member and to release energy and apply a torque to aid the motor when the thigh member rotates downwardly, and a controllable variable damping element for arresting 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. 
         [0057]    A Biomimetic Hybrid Actuator may span more than one joint to implement an artificial limb consisting of first, second and third elongated skeletal members, a first joint for connecting the first member for rotation with respect to the second member, a second joint for connecting the second member for rotation with respect to the third member, and a motor connected between the first and third member for applying a force to rotate the first member with respect to the second member about the first joint and to rotate the second member with respect to the third member about the second joint. One or more passive elastic members connected between the first and third members may be employed to store energy when the first and third members move toward one another and to release energy when the members move away from one another, and one or more controllable variable damping elements may be used for dissipating mechanical energy to arrest the motion of the motor or the relative motion of the first and third members at controllable times. 
         [0058]    In the detailed description to follow, several Biomimetic Hybrid Actuator variations are described which comprise motor, spring and variable damper components. These actuator embodiments combine active and passive elements in order to achieve high performance with minimal mass. In addition, the use of Hybrid Biomimetic Actuators as mono and poly-articular linear elements is described. In the development of low mass, efficient and quiet biomimetic artificial limbs, biologically-inspired musculoskeletal architectures and hybrid biomimetic actuation strategies comprising motor, spring and variable damper components are important design considerations. 
         [0059]    Advantages and Improvements Over Existing Methods:
       1. The Flex-Foot, made by Ossur of Reykjavik, Iceland, is a passive carbon-fiber energy storage device that replicates the ankle joint for amputees {7}. The Flex-Foot has an equilibrium position of 90 degrees and a single nominal stiffness value. Embodiments of the present invention which implement artificial ankle joints augment the Flex-Foot by allowing the equilibrium position to be set to an arbitrary angle by a motor and locking this configuration with a clutch or variable damper. The mechanism can also change the stiffness and damping of the prosthesis dynamically.   2. Embodiments of the present invention can provide adaptive stiffness control with respect to the walking speed.   3. Embodiments of the present invention provide adaptive damping control for stair descent and/or downhill walking.   4. Embodiments of the invention have the ability to provide substantial amount of plantar-flexion moment for forward progression during walking using a catapult mechanism.   5. Embodiments of the invention accommodate movement on stairs and surfaces having different slopes.       
 
         [0065]    These and other features and advantages of the present invention will be better understood by considering the following detailed description of twelve 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 
         [0066]      FIG. 1  illustrates the different phases of a walking cycle experienced by a human ankle and foot during level ground walking; 
           [0067]      FIG. 2  depicts the phases of a walking cycle experienced by a human ankle and foot when ascending stairs; 
           [0068]      FIG. 3  depicts the phases of a walking cycle experienced by a human ankle and foot during stair descent; 
           [0069]      FIGS. 4 and 5  depicts the phases of a walking cycle experienced by a human knee when walking on level ground; 
           [0070]      FIGS. 6 and 7  depict the phases of a walking cycle experienced by a human hip during level ground walking; 
           [0071]      FIG. 8  is a generalized lump parameter model of the components used to implement a hybrid actuator system employed to implement artificial ankles, knees and hips embodying the invention; 
           [0072]      FIG. 9  shows the substitutions that may be made for the series dampers and springs shown in  FIG. 8 ; 
           [0073]      FIG. 10  shows the substitutions that may be made for the motor and parallel damper seen in  FIG. 8 ; 
           [0074]      FIG. 11  depicts the substitutions that may be made for the global spring component shown in  FIG. 8 ; 
           [0075]      FIG. 12  is a lumped parameter model a first embodiment of the invention forming a biomimetic ankle; 
           [0076]      FIG. 13  is a schematic drawing of the first embodiment of the invention; 
           [0077]      FIG. 14  is a perspective drawing illustrating the physical implementation of the first embodiment of the invention; 
           [0078]      FIG. 15  is a lumped parameter model a second embodiment of the invention forming a biomimetic ankle; 
           [0079]      FIG. 16  is a schematic drawing of the second embodiment of the invention; 
           [0080]      FIGS. 17 and 18  are two perspective drawings illustrating the physical implementation of the second embodiment of the invention; 
           [0081]      FIG. 19  is a lumped parameter model a third embodiment of the invention, a biomimetic ankle; 
           [0082]      FIG. 20  is a schematic drawing of the third embodiment of the invention; 
           [0083]      FIG. 21  is a perspective drawing illustrating the physical implementation of the third embodiment of the invention; 
           [0084]      FIG. 22  is a lumped parameter model a fourth embodiment of the invention, a biomimetic ankle; 
           [0085]      FIG. 23  is a schematic drawing of the fourth embodiment of the invention; 
           [0086]      FIG. 24  is a perspective drawing illustrating the physical implementation of the fourth embodiment of the invention; 
           [0087]      FIG. 25  is a lumped parameter model a fifth embodiment of the invention, a biomimetic knee; 
           [0088]      FIG. 26  is a schematic drawing of the fifth embodiment of the invention; 
           [0089]      FIG. 27  is a perspective drawing illustrating the physical implementation of the fifth embodiment of the invention; 
           [0090]      FIG. 28  is a lumped parameter model a sixth embodiment of the invention, a biomimetic knee; 
           [0091]      FIG. 29  is a schematic drawing of the sixth embodiment of the invention; 
           [0092]      FIG. 30  is a perspective drawing illustrating the physical implementation of the sixth embodiment of the invention; 
           [0093]      FIG. 31  is a lumped parameter model a seventh embodiment of the invention, a biomimetic ankle; 
           [0094]      FIG. 32  is a schematic drawing of the seventh embodiment of the invention; 
           [0095]      FIG. 33  is a perspective drawing illustrating the physical implementation of the seventh embodiment of the invention; 
           [0096]      FIG. 34  is a lumped parameter model an eighth embodiment of the invention, a biomimetic hip; 
           [0097]      FIG. 35  is a schematic drawing of the eighth embodiment of the invention; 
           [0098]      FIG. 36  is a perspective drawing illustrating the physical implementation of the eighth embodiment of the invention; 
           [0099]      FIG. 37  is a lumped parameter model a ninth embodiment of the invention, a biomimetic hip; 
           [0100]      FIG. 38  is a schematic drawing of the ninth embodiment of the invention; 
           [0101]      FIG. 39  is a perspective drawing illustrating the physical implementation of the ninth embodiment of the invention; 
           [0102]      FIG. 40  is a lumped parameter model a tenth embodiment of the invention, a biomimetic ankle; 
           [0103]      FIG. 41  is a schematic drawing of the tenth embodiment of the invention; 
           [0104]      FIG. 42  is a perspective drawing illustrating the physical implementation of the tenth embodiment of the invention; 
           [0105]      FIGS. 43-49  illustrates a Biomimetic Hybrid Actuator that spans both the knee and ankle joints; and 
           [0106]      FIG. 50  is a schematic block diagram of a sensing and control mechanism used to control the operation of the motors and dampers in a Biomimetic Hybrid Actuator. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0107]    Eleven different embodiments of the invention are described which employ an arrangement here called a “Biomimetic Hybrid Actuator” (BHA) that is capable of providing biologically realistic dynamic behaviors. The key mechanical components of the actuator and their general functions are summarized below in Table 1. 
         [0000]    
       
         
               
             
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Mechanical components of the Biomimetic Hybrid Actuator System 
               
             
          
           
               
                 Component 
                 Function 
               
               
                   
               
               
                 Spring 
                 Store and release energy, absorb shock 
               
               
                 Motor 
                 Control positive and negative work 
               
               
                   
                 and power 
               
               
                 Efficient Variable-Damper or 
                 Dissipate mechanical energy, control 
               
               
                 Clutch 
                 damping, and clutch joint 
               
               
                   
               
             
          
         
       
     
         [0108]    As will be described, different combinations and configurations of these elements can provide a variety of biomimetic behaviors.  FIG. 8  shows a generalized lumped parameter model which summarizes the elements that make up the various embodiments, and the same symbols are used to in the lumped parameter model diagrams presented for each of the first ten embodiments. In  FIG. 8 , and in other drawings, the rectangular block labeled “motor” seen at  801  represents a power input such as an electric motor. A circle with the D represents a variable damper or clutch mechanism. In  FIG. 8 , MSD at  803  is the motor series damper, MPD at  807  is the motor parallel damper, and GD at  808  is the global damper. A jagged line represents a physical spring. MSS at  805  is the motor series spring, GDS at  811  is the global damper spring, and GS at  815  is the global spring. In the description that follows, the same terminology will be used to refer to like components. 
         [0109]    The parent and child links at  821  and  823  respectively represent the two segments being acted upon by the hybrid actuator and coupled at a rotary joint. For example, in the case of the ankle joint, the parent link is the shin and the child link is the foot. For knee and ankle joints, the vertical orientation is reversed so that, in the case of the knee joint, the parent link is the shin and the child link is the thigh, and in the case of the hip joint, the parent link is the thigh and the child link is the pelvis. 
         [0110]    By performing substitutions on the key elements of the master hybrid actuator depicted in  FIG. 8 , the lumped parameter models for the first ten embodiments described later can be derived. 
         [0111]      FIG. 9  shows the five substitutions that can be made for a damper/clutch and a series spring. The damper/clutch and spring may be replaced by a fixed link, or with nothing at all. The damper/clutch may be used alone or connected in series with a spring. Finally, a spring may be used alone without a damper/clutch. 
         [0112]      FIG. 10  shows two substitutions that can be made for the parallel motor and damper/clutch; that is, the motor alone may be used, or the motor may be combined with a parallel damper/clutch that brakes or arrests the motor at controlled times during a walking cycle. The motor is preferably and electric motor which can act as a source of power, or can act as a generator absorbing power, at different times during the walking cycle. 
         [0113]      FIG. 11  shows the three substitutions that may be made for a spring element. The spring may be eliminated, or may take the form of a one way spring that is engaged only when the joint moves into a particular position, or may take the form of a two way spring that has different stiffness properties in different joint positions. 
         [0114]    It should be understood that additional embodiments of the Biomimetic Hybrid Actuator beyond the variations specifically described below are possible. 
         [0115]    Component Implementations 
         [0116]    The variable damper or clutch mechanism illustrated in the parameter models by the circled D can be implemented using hydraulic, pneumatic (McKibben actuator), friction, electrorheological, magnetorhelogical, hysteresis brake, or magnetic particle brake damping/clutching strategies. The preferred method for damping control for the Motor Series Damper (MSD) and the Motor Parallel Damper (MPD) is a hysteresis brake because the zero power damping level is negligible. This feature is important because these particular variable damper elements are often behind a mechanical transmission thus low torque, high speed damping or clutching control is desirable. In distinction, the preferred method for damping control for the Global Damper (GD) is a magnetorheological (MR) variable damper since high torque, low speed damping control is desirable. More specifically, the MR fluid, as used in the shear mode, is positioned between a set of rotary plates that shear iron particles suspended in a carrier fluid. As a magnetic field is induced across the fluid layer, the iron particles form chains and increase the shear viscosity, which effectively increases joint dampening. Illustrative examples of such a magnetorheological (MR) variable damper are described in Sandrin et al. U.S. Pat. No. 6,202,806, the disclosure of which is incorporated herein by reference. 
         [0117]    The springs represented by jagged lines in the lumped parameter models can be implemented as linear or torsional spring elements. They may be metal die springs, carbon fiber leaf springs, elastomeric compression springs, or pneumatic springs. For this description, the springs are die compression springs. 
         [0118]    The motor element could be any electric motor, brushed or brushless. It could also be a hydraulic cylinder, pneumatic cylinder/McKibben system, or other power producing elements such as artificial muscle, piezoelectric or nitinol wire. In the specific embodiments described below, the motor component comprises an electric motor. 
         [0119]    It should be understood that the motor and variable damper/clutch functionalities could both be achieved using a single motor system if that system were capable of (1) generating isometric force or torque at low energy consumption and (2) dissipating mechanical energy (damping control) also at low energy consumption. Examples of such a motor system include a pneumatic system (McKibben actuator), hydraulic system or electroactive polymer (EAP) artificial muscle system. 
       EMBODIMENT DESCRIPTIONS 
       [0120]    In the description that follows, examples are provided which illustrate how the invention is employed at the ankle, knee or hip to provide specific ambulatory biomechanics. For each embodiment, a lumped parameter model, a schematic diagram, and a specific physical embodiment are presented. 
       Embodiment 1 
     Mechanical Design 
       [0121]    Embodiment 1 is depicted in  FIGS. 12-14 . As seen in the lumped parameter model of  FIG. 12 , the first embodiment implements an artificial ankle and comprises a motor  1201  and a global variable damper  1203  to provide control of joint position and mechanical energy absorption rate. In the description that follows, it will be shown how this first embodiment may be used to implement an artificial ankle, with a global one way spring  1205  being placed in parallel with the motor  1201  and the global variable damper  1203  between the parent link  1210  at the shin and a child link  1212  at the foot. 
         [0122]    As seen in the schematic diagram of  FIG. 13 , the first embodiment forms a joint  1300  between the parent link seen  1301  (at the shin shown at  1210  in  FIGS. 12 and 1402  in  FIG. 14 ) and a child link  1303  (at the foot link seen at  1212  in  FIG. 12  and at  1408  in  FIG. 14 ). An electric motor seen at  1305  (and at  1201  in  FIGS. 12 and 1401  in  FIG. 14 ) rotates the foot member  1303  with respect to the shin member  1301  about the joint  1300 . A one directional spring element  1304  (also seen at  1205  in  FIG. 12  and at  1406  in  FIG. 14 ) arrests the motion of the foot member  1303  when it rotates upwardly (ankle dorsiflexion) beyond a predetermined position toward the shin member  1301 . A brake member  1306  (corresponding to the global variable damper seen at  1203  in  FIG. 12  and at  1410  in  FIG. 14 ) can be controlled to arrest the rotation of the foot member with respect to the shin member. A gearbox at  1307  (also seen at  1405  in  FIG. 14 ) couples the motor to the foot member  1303  for rotation about the joint  1300 . 
         [0123]    The physical form of an artificial ankle employing the hybrid actuator is seen in  FIG. 14 . The electric motor is seen at  1401  attached to a parent link structure  1402  at the shin drives a bevel gear  1404  through a gearbox  1405 . A passive extension spring seen at  1406  attached to the parent link  1402  engages the child link attachment  1408  when it rotates upwardly past a predetermined position. A rotary MR damper seen at  1410  acts as a controllable brake. 
         [0124]    During level-ground walking, the global variable-damper is set at a high damping level to essentially lock the ankle joint during early to midstance, allowing spring structures within the artificial foot (not shown) to store and release elastic energy. Once body weight has transferred from the heel to the forefoot of the artificial foot, the ankle begins to dorsiflex and the passive extension spring is compressed. In PP, as the loading from the body weight decreases, the extension spring releases its stored elastic energy, rotating in a plantar flexion direction and propelling the body upwards and forwards. After toe-off, the variable damper minimizes joint damping, and the motor controls the position of the foot to achieve foot clearance during the swing phase and to maintain a proper landing orientation of the foot for the next stance period. 
         [0125]    From {1}{2}, it has been shown that the maximum dorsiflexion ankle torque during level-ground walking is in the range from 1.5 Nm/kg to 2 Nm/kg, i.e. around 150 Nm to 200 Nm for a 100 kg person. 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 Nm/kg. Because of these biomechanics, a uni-directional spring that engages at a small or zero dorsiflexion angle (90 degrees between foot and shank) would lower the peak torque requirements of the active ankle elements (global variable damper and motor) since the peak controlled plantar flexion torque is considerably smaller than the peak dorsiflexion torque. 
         [0126]    For ascending a stair or slope, the uni-directional extension spring is immediately engaged because the artificial toe is loaded at first ground contact. After the spring is compressed, the extension spring releases its energy, supplying forward propulsion to the body. The variable damper may be activated to control the process of energy release from the extension spring. After toe-off, the motor controls the equilibrium position of the ankle in preparation for the next step. For slope ascent, the ankle is dorsiflexed at first ground contact to accommodate the angle of the slope. The greater the slope angle or steepness, the more the ankle is dorsiflexed at first ground contact. Here the motor dorsiflexes the ankle during the swing phase, compressing the passive extension spring. Throughout the first half of ground contact, the spring is compressed farther, and then all the stored spring energy is released during powered plantar flexion throughout the latter half of ground contact, powering uphill progression. 
         [0127]    During stair descent, the body has to be lowered after forefoot contact until the heel makes contact with the stair tread {2}. Since the motor is in parallel with the variable damper, negative work can be performed by both the variable damper and the motor. Here the damper dissipates mechanical energy as heat, and the motor acts as a generator, converting mechanical energy into electrical energy. Once the foot becomes flat on the ground, the uni-directional extension spring becomes engaged, storing energy as the artificial ankle dorsiflexes. During PP, the extension spring releases its energy, propelling the body upwards and forwards. For slope descent, the ankle response is similar, except that mechanical energy is absorbed by the variable damper and motor during controlled plantar flexion instead of during controlled dorsiflexion. 
       Embodiment 2 
     Mechanical Design 
       [0128]    Embodiment 2 is shown in  FIGS. 15-18 . As seen by a comparison of the lumped parameter models seen in  FIGS. 12 and 15 , and also comparing the schematic drawings of  FIGS. 13 and 16 , it may be seen that the second embodiment includes an additional “motor series spring” element seen at  1501  in  FIG. 15 , at  1601  in  FIG. 16 , and at  1711  in  FIGS. 17 and 18 . In addition to the capabilities offered by Embodiment 1, Embodiment 2 provides for the control of hybrid actuator force by an active spring deflection control by the motor and an active damping control by the variable damper. In addition, Embodiment 2 includes the capacity to act as a catapult where a spring is slowly compressed and that stored potential energy is used all at once at a later time. For the catapult control, the global variable-damper  1605  will be able to control the damping of the joint in order to modulate how much energy is actually released from the stored catapult energy. In the section to follow, we provide an example of how the hybrid biomimetic actuator of Embodiment 2 can be employed as an artificial ankle. 
         [0129]    As seen in  FIGS. 17 and 18 , the second embodiment includes an electric motor seen at  1701  (and at  1603  in  FIG. 16 ) in parallel with a rotary magnetorheological (MR) variable-damper  1705  ( 1605  in  FIG. 16 ) where the MR fluid is used in the shear mode. Similar to Embodiment 1 as seen in  FIG. 14 , the uni-directional spring, the passive extension spring at  1706  ( 1607  in  FIG. 16 ), is engaged for ankle angles of 90 degrees or less (dorsiflexion). For angles greater than 90 degrees (plantar flexion), the spring is no longer engaged, and the ankle joint freely rotates without spring compression. As best seen in  FIG. 18 , a drive shaft  1803  links the motor  1701 , a gearbox  1710 , and motor series springs seen at  1711 . Torque is transmitted from motor  1701  through gearbox  1710 , to bevel pinion seen at  1810 . This gear transfers torque to the large bevel gear at  1812 . The rotational motion of the large bevel gear is converted to linear motion at the joint at  1820  by the spring pivot rod  1822  which compresses extension series springs at  1711 . The other end of the extension series spring pushes on the structure that is rigidly attached to the child link  1830 . For flexion, the direction of rotation of the motor is reversed, and the torque to the child link is transmitted via the flexion series springs seen at  1835 . 
         [0130]    The second embodiment, like the first embodiment described earlier, includes a uni-directional global spring (seen at  1205 ,  1304  and  1406  in the first embodiment and at  1706  in  FIG. 17 ) that provides passive spring operation throughout ankle dorsiflexion. However, in distinction to Embodiment 1, the Embodiment 2 artificial ankle further includes the flexion series spring  1835  to provide powered plantar flexion during the terminal stance period. 
         [0131]    One of the main challenges in the design of an artificial ankle is to have a relatively low-mass actuation system, which can provide a large instantaneous output power upwards of 200 watts during powered plantar flexion (PP) {1}{2}. 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. 
         [0132]    The catapult mechanism is mainly composed of three components: the motor  1701 , the variable damper  1705  and/or clutch and an energy storage element such as the springs  1711 . With the parallel damper activated to a high damping level or with the parallel clutch activated, the series elastic element (e.g. the motor spring seen at  1501 ,  1601  and  1711 ) can be compressed or stretched by the motor in series with the spring without the joint rotating. The spring will provide a large amount of instantaneous output power once the parallel damping device or clutch is deactivated, allowing the elastic element to release its energy. If the motor has a relatively long period of time to compress or stretch the elastic element, its mass can be kept relatively low, decreasing the overall weight of the artificial ankle device. In the ankle system of Embodiment 2, the catapult system comprises a magnetorheological variable damper (seen at  1203 ,  1306 , and  1410  in the drawings of the first embodiment and at  1705  in  FIG. 17 ) placed in parallel to a motor and a motor series spring. 
         [0133]    During the CP phase of level-ground walking, the motor controls the stiffness of the ankle by controlling the displacement of the series flexion springs seen at  1835  in  FIG. 18 . During CD, the series extension springs at  1711  then compress due to the loading of body weight, while the actuator additionally compresses the series springs to store additional elastic energy in the system. In this control scheme, inertia and body weight hold the joint in a dorsiflexed posture, enabling the motor to further compress the series extension springs. 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 springs. Independent of the catapult control approach, during PP, as the load from body weight decreases, the series extension springs begin releasing stored elastic energy, supplying high ankle output powers during PP. The variable damper is significant in the synchronization of the energy relaxation from the series extension springs. After toe-off, the actuator controls the position of the ankle to achieve foot clearance and a proper landing orientation for the next stance period. 
       Embodiment 3 
     Mechanical Design 
       [0134]    Embodiment 3 is shown in  FIGS. 19-21  and also comprises a motor, a motor series spring, and a variable damper in parallel with the motor as seen in  FIG. 19 . The third embodiment differs from the second in that the variable damper is connected to retard motor motion. In addition to the capabilities offered by Embodiment 1, Embodiment 3 provides for low-power spring stiffness and spring equilibrium point control. 
         [0135]    Usage Example for Embodiment 3 
         [0136]    The mechanical design and the corresponding schematic for Embodiment 3, as used for an artificial ankle application, are shown in  FIGS. 20 and 21 . Similar to the ankle designs corresponding to Embodiments 1 and 2, a uni-directional global spring (seen at  1205 ,  1304  and  1406  for the first embodiment) provides for a passive spring operation throughout ankle dorsiflexion. However, in distinction to Embodiments 1 and 2, the Embodiment 3 artificial ankle is capable of controlling ankle joint spring stiffness and equilibrium at low electrical power requirements. 
         [0137]      FIG. 21  provides a perspective view of Embodiment 3 used as an artificial ankle system. The ankle design includes an electric motor  2110 , a gearbox  2111  ( 2009  in  FIG. 20 ), a bevel gear  2109 , and a motor series spring  2120 . In parallel with the motor is a rotary hysteresis variable-damper  2125  in  FIG. 21 . Similar to the ankle of Embodiments 1 and 2, the uni-directional spring seen at  2140  in  FIG. 21  is engaged for ankle angles of 90 degrees or less (dorsiflexion). For angles greater than 90 degrees (plantar flexion), the spring  2140  is no longer engaged, and the ankle joint freely rotates without spring compression. The ankle joint seen at  2000  in  FIG. 20  is seen at  2142  in  FIG. 21  rotates child foot member  2155  ( 2013  in  FIG. 20 ) with respect to the parent shin member  2160  ( 2014  in  FIG. 20 ). 
         [0138]    There are separate series springs at  2120  and  2150  for extension and flexion respectively, and these two sets of springs can be selected to give distinct flexion and extension joint stiffnesses. If the motor changes ankle position when minimal torques are applied to the joint, such as during the swing phase of walking, very little electrical power is required to change the spring equilibrium position of the joint. Just before the joint is loaded by body weight at heel strike, the motor parallel variable damper can be locked, with relatively low electrical power required, so that the motor need not consume electrical power to hold the joint&#39;s position. Changing this spring joint set point can be useful, for example, when the wearer switches shoes with different heel heights, thus changing the natural angle of the ankle joint when the foot is resting on a flat ground surface. 
         [0139]    The variable damper and motor can also act to modulate the quasi-stiffness of the ankle joint at low electrical power requirements. Here quasi-stiffness refers to the slope of the ankle torque versus position curve. If the series springs  2120  and  2150  are set to maximal stiffness levels demanded by the application, and the damper and motor are controlled to absorb mechanical energy by backing off the opposite end of the spring as the spring is being compressed by torques applied to the joint, the effective stiffness of the ankle joint can be controlled. This system can directly control stiffness at low power, since the variable damper is attached before the motor&#39;s gear reduction, so that the damper rotates at high angular velocity but at low torque output relative to the joint being controlled. 
         [0140]    To generate high output mechanical powers during PP in walking, the body&#39;s weight and inertia can act as a “clutch” to essentially lock the ankle joint in a catapult mode control, so that as the body rotates above the stance foot, the motor can be steadily “winding up” its series extension springs in order to release that energy later during the PP phase. During this “winding up” control period, joint torque can be directly controlled by controlling series spring compression using feedback of series spring deflection. 
         [0141]    Similar to Embodiments 1 and 2, Embodiment 3 can also share the load of absorbing energy between the motor and the variable damper. This may cut down on heat generated by the variable damper under heavy use, and the electric motor can act regeneratively, generating electrical power and thus increasing overall efficiency. For example, in the case of walking down hill, it is important for the biomimetic ankle joint to absorb mechanical energy in order to smooth and cushion descent. This energy absorption can be achieved by allowing the motor to back drive and the variable damper to dissipate the energy in a controlled, modulated way, depending on the mass of the person, how fast they are walking, and how steep the descent may be. Here again, the motor can share the mechanical energy absorption with the parallel variable damper, generating electrical power in the process. It is noted here that back driving a motor of reasonable size and weight will not, by itself, absorb a sufficient amount of mechanical energy for this particular application, and that both motor and variable damper must therefore share in the power absorption. 
       Embodiment 4 
     Mechanical Design 
       [0142]    Embodiment 4, shown in  FIGS. 22-24 , comprises a motor, a motor series spring, a motor parallel variable damper seen at  2210  in  FIG. 22 , at  2310  in  FIG. 23 , and at  2412  in  FIG. 24 . A motor series variable damper is seen at  2220  in  FIG. 22 , at  2320  in  FIG. 23 , and at  2414  in  FIG. 24 . In addition to the capabilities offered by Embodiment 3, Embodiment 4 allows the actuator to be back driven very easily for tasks where hybrid actuator force needs to be minimized at minimal energy demands from the power supply. The addition of the motor series variable damper allows the gearbox to freewheel at high angular rates without the need for the motor to slew as well, lowering the minimal force output of the biomimetic actuator at minimal power input requirements. In the case of Embodiment 3 where no motor series variable damper exists, when the actuator is compressed passively, consuming zero energy from the power supply, the motor and the parallel variable damper both have to rotate. In distinction, with the Embodiment 4 architecture, when the motor parallel variable damper outputs high damping, locking the motor, only the motor series variable damper rotates when the actuator is compressed. Since the motor series variable damper is before the mechanical transmission, the damper can be relatively small with a negligible passive, zero-energy damping torque, and thus the mechanical transmission itself will be the only dominant source of passive actuator resistance or inertia under compression, resulting in a biomimetic joint that can go more limp or slack while requiring only minimal energy from the power supply. The child foot link at  2364  rotates with respect to the parent shin link at  2365  rotates about the joint is seen at  2370 . 
         [0143]    Embodiment 4, as seen in  FIG. 24 , includes an electric motor  2410  and motor series springs at  2420  ( 2335  in  FIG. 23 ) for joint flexion and extension. The motor  2410  drives the bevel gear  2418  to compress the series springs  2420 . In parallel and in series with the motor are rotary hysteresis variable-dampers. The hysteresis brake seen at  2412  in  FIG. 24  arrests the motion of the motor  2410 . The second hysteresis brake is seen at  2414  and is operatively connected between the motor  2410  and the gearbox  2430 . Similar to the three ankle embodiments described above, the uni-directional spring, shown at  2440 , and at  2330  in  FIG. 23 , is engaged for ankle angles of 90 degrees or less (dorsiflexion). For angles greater than 90 degrees (plantar flexion), the spring is no longer engaged, and the ankle joint freely rotates without spring compression. The parent shin link structure is seen at  2460  and the child foot structure is seen at  2470 . 
         [0144]    In addition to improving the low-energy, minimum force capabilities of the actuator, the actuator of Embodiment 4 can dissipate mechanical energy without back driving the motor by once again using the motor parallel variable damper  2412  to lock the motor rotor at low energy demands from the power supply. Although controlling the actuator in this manner eliminates the opportunity to employ the motor as a generator, it is beneficial in that it will result in a quieter biomimetic actuator operation. Since it is important that robots, prostheses and orthoses be quiet, this engineering tradeoff is often worthwhile. An example of the use of Embodiment 4 as an artificial ankle is provided in the next section. 
         [0145]    Usage Example for Embodiment 4 
         [0146]    In comparison with the previous ankle embodiments, the Embodiment 4 artificial ankle has a quieter operation and a lower output force while requiring minimal energy demands from the power supply. Since the motor will not be rotating while mechanical energy is being absorbed by the motor series damper, the force output of the system will be lowered, resulting in an ankle joint that can go more limp or slack while consuming only that energy required to output sufficiently high damping in the motor parallel variable damper to lock the rotor of the motor. In addition, this actuator feature reduces the level of noise from the actuator during mechanical energy absorption since no noise will result from back driving the motor. The motor series damper could also be used to modulate the force output of the series springs in a quiet and efficient manner as they discharge their energy after being “wound up” in a catapult mode. In addition to these distinct features, the ankle corresponding to Embodiment 4 offers the same capabilities as the ankle system of Embodiment 3. 
       Embodiment 5 
     Mechanical Design 
       [0147]    Embodiment 5 is a biomimetic hybrid knee shown  FIGS. 25-27 . As shown in the perspective drawing,  FIG. 27 , five elements are included in the design: a motor  2710 , a gearbox  2711 , a bevel gear  2719 , a motor series coil springs for joint flexion and extension seen at  2712 , a motor parallel variable-damper  2716 , a global variable damper  2718 , and a bi-directional spring that consists of a light extension spring  2720  and a stiff kneecap spring  2721 . The global damper  2718  and the motor-parallel variable damper  2716  are rotary magnetorheological (MR) and hysteresis variable-dampers, respectively. To resist knee hyperextension, the stiff kneecap flexion spring  2721  serves as an artificial knee cap stop. In addition, the light extension spring  2720  is included to bias the knee towards a fully extended posture. The structure rotates the child thigh member at  2730  with respect to the parent thigh structure at  2732  about the joint  2734 . 
         [0148]    In addition to the capabilities offered by Embodiment 3, the BHA of Embodiment 5 allows the joint to act as a “catapult” at any time in its operation. The addition of the global variable damper  2718  allows the joint to be locked while the motor  2710  slowly compresses the series springs  2712 , and that stored potential energy can then be used all at once at a later time. To release the stored elastic energy, the output damping from the global damper  2718  is minimized, unlocking the actuator and releasing the energy. Also, the global variable-damper  2718  of Embodiment 5 will be able to directly modulate the damping of the actuator in order to control how much energy is actually released to the external world from the stored catapult energy. An example of the use of Embodiment 5 as an artificial knee is provided in the next section. 
         [0149]    Usage Example for Embodiment 5 
         [0150]    State of the art commercially available knee prostheses employ a global variable damper and a global two-way spring {5}. Consequently, current knee prostheses cannot control knee position when the foot is off the ground, and are incapable of generating net positive work and power during stance or swing. As shown in  FIG. 5 , the human knee has three positive power contributions (seen at  501 ,  503  and  505 ) throughout the walking gait cycle. Because conventional prosthetic knees only have a global variable damper and a low stiffness global spring, these positive power contributions cannot be achieved. 
         [0151]    The artificial knee corresponding to Embodiment 5 improves upon these contemporary prosthetic knee designs by placing a motor, a motor parallel variable damper, and a motor series spring all in parallel with the conventional global damper/spring. During early stance knee flexion in level-ground walking, energy in the knee can be dissipated with the global variable damper as is typically done with conventional artificial knee systems. However, during stance knee extension, the motor parallel variable damper  2716  can be locked as the hip joint actively extends, rotating the thigh rearwardly. This movement allows energy from hip muscular work to be stored in the series flexion springs  2712  located in the knee assembly. The stored elastic energy can then be released during early pre-swing to help flex the knee during terminal stance in preparation for the swing phase. This positive power burst corresponds to  503  in  FIG. 5 . The global damper can be used to modulate the actual external power generated from the spring energy. It is worth noting again that this method allows the hip muscles to store energy in the knee as the stance leg is rotated rearwardly during hip extension, and that this energy can be reused at a later time to help flex the knee with very little energy required from the artificial knee&#39;s power supply. 
         [0152]    Once the elastic energy from the series flexion springs has been released and the artificial leg has entered the swing phase, the knee joint has to absorb mechanical energy to decelerate the swinging lower leg. To this end, during late swing flexion, the motor parallel variable damper  2716  can lock once again, causing the series extension springs  2712  in the knee assembly to deflect and store energy. This stored energy can then be using to create positive power burst at  505  ( FIG. 5 ) during the early swing extension period, requiring, once again, very little energy from the knee&#39;s power supply. In all cases, the global variable damper can be used to precisely modulate the amount of power delivered to the swinging artificial leg from energies storied in the series springs. Further, the global variable damper can dissipate kinetic energy from the swinging leg to achieve the large negative powers during the swing phase (see  FIG. 5 ). 
         [0153]    In summary, the artificial knee corresponding to Embodiment 5 is capable of reproducing the positive power contributions  503  and  505  shown in  FIG. 5 . Both positive power contributions  501  and  503  cannot be achieved by the architecture of Embodiment 5. However, Embodiment 7 described below is capable of capturing all three positive power contributions. 
         [0154]    For stair/slope descent, the global variable damper, motor and motor parallel variable damper can all be used to dampen the knee joint and to absorb mechanical energy for a prosthetic/orthotic knee wearer or humanoid robot. Although the variable dampers of the hybrid actuator dissipate mechanical energy as heat during the period of stance knee flexion, the motor can act as a generator, storing up electrical energy to be used at a later time. Through mid to terminal stance, the motor parallel variable damper  2716  can then output a high damping value that essentially locks the rotor of the motor, causing the motor series spring  2712  to store energy as the artificial knee undergoes terminal flexion. This stored energy can then be used during the swing phase to promote knee extension to prepare the artificial leg for the next stance period. 
         [0155]    For stair/slope ascent, during the swing phase the motor can actively control knee position to accurately locate the foot on the next stair tread or slope foothold. Once the artificial foot is securely positioned on the ground, the motor can then deflect and store energy in the motor series extension springs. This stored elastic energy can then assist the knee wearer or humanoid robot to actively straighten the knee during the stance period, lifting the body upwards. 
         [0156]    Finally, Embodiment 5 allows for the “windup” phase of the catapult style control to occur at any desired time, as opposed to embodiment 3, which requires an inertial clutch (body mass during stance phase for ankle joint for example). This means much greater flexibility as to when large amounts of power can be efficiently generated and used. This flexibility is critical when designing an artificial knee that can be used for jumping. For such a movement task, energy has to be stored prior to the jump, and then the elastic energy has to be released at a precise time to facilitate a jumping action. 
       Embodiment 6 
     Mechanical Design 
       [0157]    Embodiment 6, a biomimetic knee employing a Biomimetic Hybrid Actuator, is shown in  FIGS. 28-30 . The elements included in the design as shown in  FIG. 30 : a motor  3010 , a bevel gear  3011 , a motor series spring  3020  for joint flexion and extension, a motor series variable damper  3022 , a motor parallel variable-damper  3024 , a gearbox  3040 , a global variable damper  3026 , and a bi-directional spring assembly consisting of a stiff kneecap flexion spring  3032  and a light extension spring  3034 . The two dampers  3022  and  3024  are hysteresis brakes, and global damper  3026  is a rotary magnetorheological (MR) variable-damper technology in which the MR fluid is used in the shear mode. To resist knee hyperextension, the stiff kneecap flexion spring  3032  serves as an artificial knee cap stop. In addition, the light extension spring  3034  is included in the design to bias the knee towards a fully extended posture. The thigh (child link) structure at  3060  supports the axis of the joint at  3070  about which the shin (parent link) structure  3080  rotates. 
         [0158]    In addition to the capabilities offered by Embodiment 5, Embodiment 6 allows the actuator to be back driven very easily for tasks where the hybrid actuator force needs to be minimized at minimal energy demands from the power supply. The addition of the motor series variable damper  3022  allows the gearbox to freewheel at high angular rates without the need for the motor to slew as well, lowering the minimal force output of the biomimetic actuator at minimal power input requirements. In the case of Embodiment 5 where no motor series variable damper exists, when the actuator is compressed passively, consuming zero energy from the actuator power supply, the motor and the parallel variable damper both have to rotate or compress. In distinction, with the Embodiment 6 architecture, when the motor parallel variable damper  3024  outputs high damping, locking the motor, only the motor series variable damper  3022  rotates or compresses when the actuator is compressed. Since the motor series variable damper  3022  is placed before the mechanical transmission including a gearbox  3040 , the damper  3022  can be relatively small with a negligible passive, zero-energy damping torque, and thus the mechanical transmission including the gearbox  3040  and the global variable damper  3026  will be the only dominant sources of passive actuator resistance under compression, resulting in a biomimetic actuator that can go more limp or slack while requiring only minimal energy from the power supply. 
         [0159]    In addition to improving the low-energy, minimum force capabilities of the actuator, the actuator of Embodiment 6 can dissipate mechanical energy without back driving the motor by once again using the motor parallel variable damper to lock the motor rotor at low energy demands from the power supply. Although controlling the actuator in this manner eliminates the opportunity to employ the motor as a generator, it may be beneficial in that it will result in a quieter biomimetic actuator operation. Since it is often important that robots, prostheses and orthoses are quiet, this engineering tradeoff may be selected for many applications. An example of the use of Embodiment 6 of the Biomimetic Hybrid Actuator to implement an artificial knee (Embodiment 10) is provided in the next section. 
         [0160]    Usage Example for Embodiment 6 
         [0161]    Similar to the knee design corresponding to Embodiment 5, a bi-directional global springs  3032  and  3034  provides for a passive extension spring operation to bias the knee towards a fully extended posture (spring  3034 ), and a stiff flexion spring operation (spring  3032 ) to limit the knee&#39;s movement so that knee hyperextension cannot occur. However, in distinction to the previous knee embodiment, the Embodiment 6 artificial knee has a quieter operation and a lower output force while requiring minimal energy demands from the power supply. 
         [0162]    Since the motor will not be rotating while mechanical energy is being absorbed by the motor series and global variable dampers, the force output of the system will be lowered, resulting in a knee joint that can go more limp or slack while consuming only that energy required to output sufficiently high damping in the motor parallel variable damper  3024  to lock the rotor of the motor. In addition, this actuator feature reduces the level of noise from the actuator during mechanical energy absorption since no noise will result from back driving the motor. The motor series damper  3022  could also be used to modulate the force output of the series springs in a quiet and efficient manner as they discharge their energy after being “wound up” in a catapult mode. Further, the global variable damper can dissipate kinetic energy from the swinging leg to achieve the large negative powers during the swing phase (see  FIG. 5 ). In addition to these distinct features, the knee corresponding to Embodiment 6 offers the same capabilities as the knee system of Embodiment 5. 
       Embodiment 7 
     Mechanical Design 
       [0163]    A biomimetic hybrid knee design corresponding to Embodiment 7 is shown in  FIGS. 31-33 . Six elements are included in the design as seen in  FIG. 33 : a motor  3310 , a gearbox  3311 , a motor series spring  3320  for joint flexion and extension, a motor parallel variable-damper  3322 , a global variable damper  3334 , a bi-directional global spring comprising a stiff spring  3340  and a light extension spring  3342 , and a global damper spring  3336  also seen at  3136  in  FIG. 31 . The motor drives the joint via a bevel gear at  3388  to rotate the parent (shin) structure  3344  with respect to the child (thigh) structure  3346 . The global variable damper  3334  is a magnetorheological (MR) variable-damper, and the motor-parallel variable damper  3322  is a hysteresis brake. The global damper coil springs  3336  (Series Damper Springs) are included for joint flexion and extension. To resist knee hyperextension, the stiff kneecap flexion spring  3340  serves as an artificial kneecap stop. In addition, the light extension spring  3342  is included in the design to bias the knee towards a fully extended posture. 
         [0164]    Embodiment 7 allows for the engagement of a second series spring, the damper series spring  3336 , at any time during system operation. Further, the energy released from the damper series spring  3336  can be modulated using the global variable damper  3334 . An example of the use of Embodiment 7 as an artificial knee is provided in the next section. 
         [0165]    Usage Example for Embodiment 7 
         [0166]    As shown in  FIG. 5 , during level-ground walking the human knee has three positive power contributions (seen at  501 ,  503  and  505 ). Because conventional prosthetic knees only have a global variable damper and a low stiffness global spring, these positive power contributions cannot be achieved {5)}. 
         [0167]    The artificial knee corresponding to Embodiment 7 improves upon such contemporary knee designs. During early stance knee flexion in level-ground walking, the global variable damper  3334  can output a high damping value such that as the knee flexes, the global damper spring  3336  stores energy, and then that energy can be released during the stance extension period. This positive power burst corresponds to  501  in  FIG. 5 . As the stored energy from the global damper spring is being released during stance knee extension, the motor parallel variable damper  3322  can be locked, allowing the energy from hip muscular work and the stored energy in the global damper spring to be stored in the motor series flexion springs  3020  located in the knee assembly. The stored elastic energy can then be released during early pre-swing to help flex the knee during terminal stance in preparation for the swing phase. This positive power burst corresponds to  503  in  FIG. 5 . During this process, the global damper  3334  can be used to modulate the amount of stored elastic energy in the global damper spring  3336  that is actually released to power the knee joint. In addition, the global variable damper can dissipate kinetic energy from the swinging leg to achieve the large negative powers during the swing phase (see  FIG. 5 ). Still further, the motor parallel variable damper  3322  can be used to modulate the amount of stored elastic energy in the motor series springs  3020  that is actually released to power the knee joint. 
         [0168]    Once the elastic energy from the springs  3020  has been released and the artificial leg has entered the swing phase, the knee joint has to absorb mechanical energy to decelerate the swinging lower leg. To this end, during late swing flexion, the motor parallel variable damper  3320  can lock once again, causing the series extension springs  3020  in the knee assembly to deflect and store energy. This stored energy can then be using to create positive power burst seen at  505  in  FIG. 5  during the early swing extension period, requiring, once again, very little energy from the knee&#39;s power supply. 
         [0169]    In all cases, the variable dampers  3322  and  3334  can be used to precisely modulate the amount of power delivered to swinging artificial leg from stored elastic energies. In summary, the artificial knee corresponding to Embodiment 7 is capable of reproducing the three positive power contributions seen at  501 ,  503  and  505  in  FIG. 5  for level-ground walking For stair/slope descent and ascent and for catapult jumping actions, the artificial knee of Embodiment 7 is controlled in a similar manner to that of Embodiment 5. 
       Embodiment 8 
     Mechanical Design 
       [0170]    Embodiment 8 is an artificial hip employing a Biomimetic Hybrid Actuator and is shown in  FIGS. 34-36 . In the artificial hip, the “parent link” seen at  3601  corresponds to the thigh and the “child link” (which in the case of hip joint is above the parent link) at  3602  corresponds to the pelvis. The pelvis structure  3602  supports the axis of the joint at  3603  about which the thigh structure  3601  rotates. The elements included in the design as seen in  FIG. 36  are: an electric motor  3610 , a gearbox  3611 , a global variable MR shear mode damper  3620 , a global damper spring  3625 , and a bi-directional global spring consisting of a passive flexion spring  3640  and a passive extension spring  3642 . The global damper coil springs  3625  are included for joint flexion and extension. Finally, the two-way global springs  3640  and  3642  are included in the design to augment uphill locomotion. 
         [0171]    In addition to the capabilities offered by Embodiment 1, Embodiment 8 allows for the engagement of a second series spring, the damper series spring  3625 , at any time during system operation. Further, the amount of energy stored or released from the damper series spring  3625  can be modulated using the global variable damper  3620 . An example of the use of Embodiment 8 as an artificial hip is provided in the next section. 
         [0172]    Usage Example for Embodiment 8 
         [0173]    Basic hip biomechanics for level-ground walking, shown in  FIGS. 6-7 , can be modeled with a spring in parallel with a motor system. The Embodiment 8 hybrid architecture constitutes the least number of components to achieve basic hip functionality. During level-ground walking, the global variable damper  3620  outputs a large damping value such that the global damper flexion springs  3625  store elastic energy during terminal hip extension and release that stored energy during early hip flexion to promote knee flexion and toe-off throughout terminal stance. After this elastic energy is released, the global flexion springs  3640  and  3642  store energy throughout mid to terminal hip flexion and release that stored energy during early to mid hip extension to promote limb retraction and forward propulsion. To the extent to which the desired joint behavior deviates from a conservative spring response, the motor and global variable damper are controlled to generate or absorb mechanical power as needed. 
         [0174]    In addition to the motor  3610 , global variable damper  3620  and global damper springs  3625 , the hybrid biomimetic hip actuator seen in  FIGS. 34-36  also includes a two way global spring consisting of the passive flexion spring  3640  and the passive extension spring  3642 . The approximate equilibrium position of this global spring assembly is such that the spring exerts little to no force when standing with an erect posture. This global spring is important for uphill locomotion when the hip actuator is used for an orthosis or exoskeleton. During the swing phase as the hip flexes and the foot is placed on the next foothold, the global spring stores energy and then that energy is released to augment hip extension as the body is lifted upwardly during the stance phase. By adjusting the stiffness and equilibrium position of the global spring, muscles that flex the hip during the swing phase will fatigue at the same time as muscles that extend the hip during stance. Since the work required to ascent a hill is better distributed across a greater muscle volume as a result of the global spring, muscle fatigue can be effectively delayed. 
       Embodiment 9 
     Mechanical Design 
       [0175]    Embodiment 9 is a further biomimetic hybrid hip design seen in  FIGS. 37-39 . Five elements are included in the design as seen in  FIG. 39 : an electric motor  3910 , a gearbox  3911 , a bevel drive gear  3912 , a motor series spring  3915  for joint flexion and extension, a global variable damper  3925 , global series damper springs  3630 , and a bi-directional global spring consisting of a passive flexion spring  3940  and a passive extension spring  3942 . Additionally, global damper coil springs  3630  (Series Damper Springs) are included for joint flexion and extension. The global damper  3925  is a rotary magnetorheological (MR) variable-damper technology where MR fluid is used in the shear mode. The pelvis structure at  3970  supports the joint  3980  about which the thigh structure  3990  rotates. 
         [0176]    In addition to the capabilities offered by Embodiment 2, Embodiment 9 allows for the engagement of the second series damper spring, the damper series spring  3630 , at any time during system operation. Further, the amount of energy stored or released from the damper series spring can be modulated using the global variable damper. An example of the use of Embodiment 9 as an artificial hip is provided in the next section. 
         [0177]    Usage Example for Embodiment 9 
         [0178]    The functionality of the hybrid hip actuator corresponding to Embodiment 9 is similar to the Embodiment 8 hip system except that the addition of the motor series spring  3915  that allows the system to better able to augment the spring response from the global damper spring. Since the motor can perform a position control on the motor series spring, the force output from that spring can be effectively controlled, allowing for accurate modulation of impedance and motive force in parallel with the global variable damper/global damper spring components. Hence, the hip system of Embodiment 9 can more effectively absorb and generate mechanical power to augment the passive spring responses from the global damper spring and global two way spring. 
       Embodiment 10 
     Mechanical Design 
       [0179]    Embodiment 10 comprises a motor  4910 , a motor series spring  4913 , a motor series damper  4915 , a damper series spring at  4920 , a motor parallel damper  4914 , a gearbox  4922 , a bevel gear  4923 , a global damper  4916 , and a global damper springs at  4918  and  4919 . In addition to the capabilities offered by Embodiment 6, Embodiment 10 allows for the engagement of a second series spring, the damper series spring  4920 , at any time during system operation. Further, the amount of energy stored or released from the damper series spring  4920  can be modulated using the global variable damper  4916 . The pelvis structure at  4930  supports the joint axis  4945  about which the thigh structure  4950  rotates. An example of the use of Embodiment 10 as an artificial hip is provided in the next section. 
         [0180]    Usage Example for Embodiment 10 
         [0181]    The mechanical design and the corresponding schematic for Embodiment 10, as used for an artificial hip application, are shown in  FIGS. 42 and 41 . The functionality of the hybrid hip actuator corresponding to Embodiment 10 is similar to the Embodiment 9 hip system except for the functional capabilities resulting from the addition of the motor series and parallel variable dampers  4915  and  4914  respectively. These added components offer several advantages to the hybrid hip actuator of Embodiment 9. The motor series variable damper  4915  allows the Embodiment 10 hip system to be back driven very easily for tasks where hybrid actuator force needs to be minimized at minimal energy demands from the power supply. The addition of the motor series variable damper  4915  allows the gearbox to freewheel at high angular rates without the need for the motor to slew as well, lowering the minimal force output of the biomimetic actuator at minimal power input requirements. In addition to improving the low-energy, minimum force capabilities of the actuator, the actuator of Embodiment 10 can dissipate mechanical energy without back driving the motor by once again using the motor parallel variable damper  4914  to lock the motor rotor at low energy demands from the power supply. Although controlling the actuator in this manner eliminates the opportunity to employ the motor as a generator, it is beneficial in that it will result in a quieter biomimetic hip operation. 
         [0182]    Poly-Articular Actuation Using Biomimetic Hybrid Actuators 
         [0183]    In the previous sections, ten Biomimetic Hybrid Actuators were described and specific examples were provided as to their use for ankle, knee and hip actuation. For each of these descriptions, the hybrid actuator spanned a single joint. In this section, a Biomimetic Hybrid Actuator that spans more than one rotary joint is describe in connection with  FIGS. 43-49  of the drawings. The functional purpose of poly-articular muscle architectures in the human leg is to promote the transfer of mechanical energy from proximal muscular work to distal joint power generation {10}. To capture truly biomimetic limb function, both muscle-like actuators and mono, bi, and poly-articular artificial musculoskeletal architectures are critical. Hence, in this section we describe the use of Biomimetic Hybrid Actuators across two or more rotary joints. 
         [0184]    As a particular demonstration of Biomimetic Hybrid Actuator usage across more than one rotary joint, we describe the use of the Embodiment 3 actuator (see  FIGS. 19-21 ) as an artificial Gastrocnemius muscle that spans both knee and ankle artificial rotary joints. In  FIGS. 43-49 , seven leg postures are shown, depicting the movement of a normal human ankle joint  4302  and knee joint  4304  (joining the shin member  4305 ) during the stance period of level-ground walking A bi-articular Biomimetic Hybrid Actuator is attached posteriorly to the artificial leg, spanning ankle and knee joints. Here the child link corresponds to the artificial foot  4310 , and parent link  4303  corresponds to the artificial thigh or femur section of the biomimetic limb. The Embodiment 3 actuator, comprising motor, motor series spring and motor parallel variable damper, attaches between these two locations, and therefore acts in a bi-articular manner. 
         [0185]    The functionality of the Embodiment 3 system as a bi-articular actuator is described for level-ground walking using the leg postures sketched in  FIGS. 43-49 . Although we give only one example here, it should be understood that any of the ten hybrid actuator embodiments could be employed in a poly-articular manner where the hybrid actuator spans more than one rotary joint. During ankle controlled plantar flexion and knee early stance flexion (leg postures seen in  FIGS. 43-45 ), both the human ankle and knee exhibit a spring-like behavior where joint torque is a function of joint angular position. This spring-like behavior begins at approximately 5% gait cycle after the knee has begun flexing during early stance (see  FIGS. 4-5 ). In the artificial biomimetic leg design of  FIGS. 43-49 , immediately following heel strike as seen in  FIG. 43  the motor  4330  and motor parallel variable damper  4335  are controlled to absorb mechanical energy in a controlled manner. For this control behavior, the variable damper dissipates mechanical energy as heat, and the motor acts as a generator. After this initial period of about five degrees of knee flexion (mid flexion phase) the variable damper then outputs a much higher damping value such that elastic energy is stored in the motor series compression springs  4340 . Here the series compression springs are tuned to have a maximal stiffness required for this period of gait. To achieve lower effective stiffnesses from mid to terminal knee flexion, the motor and motor parallel variable damper absorbs mechanical energy as the motor series compression springs are compressed. Here again, for this control behavior, the variable damper dissipates mechanical energy as heat, and the motor acts as a generator. 
         [0186]    During ankle controlled dorsiflexion and knee stance extension (leg postures seen in  FIGS. 45 to 47 ), both the human ankle and knee exhibit a spring-like behavior where joint torque correlates with joint angular position. However, as walking speed increases, ankle stiffness during controlled dorsiflexion increases. In addition, as speed increases the ankle torque versus angle curve becomes increasingly nonlinear characterized by a hardening behavior where ankle torque increases with increasing angular deflection. It is also the case in walking that peak knee flexion becomes more pronounced with increasing walking speed, increasing the opportunity for energy to be transferred from hip muscular power exertion to ankle power generation. In the case of the bi-articular Biomimetic Hybrid Actuator shown in  FIGS. 43-49 , the elastic energy stored during mid to terminal early stance knee flexion/controlled is then released during early to mid knee extension. This positive power output corresponds to  502  in  FIG. 5 . After the elastic energy (leg postures seen in  FIGS. 43-45 ) is released, the motor series tension springs  4340  then begin to compress, storing energy from mid to terminal knee extension. Here the series tension springs are tuned to correspond to the minimal stiffness, slow walking human ankle stiffness value. To achieve higher stiffnesses as walking speed increases, the motor does work on the spring, slowly storing energy throughout the controlled dorsiflexion/terminal knee extension period, resulting in an ankle joint profile with increasing torque with increasing angular deflection. 
         [0187]    Throughout terminal stance (leg postures seen in  FIGS. 47-49 ), the human ankle undergoes powered plantar flexion, and the human knee begins rapid flexion in preparation for the swing phase. During this period of gait, the human ankle&#39;s positive power generation is quite large in comparison with the human knee&#39;s positive power generation (See 503,  FIG. 5 ), especially at fast walking speeds. To achieve these joint powers, the energy stored in the motor series tension springs during controlled dorsiflexion/knee extension is released, causing the knee to flex and the ankle to plantar flex. This positive power release at the knee corresponds to power output  502  in  FIG. 5 . To achieve a relatively higher power output through the ankle compared to the knee, the effective moment arm of the ankle joint could be significantly larger than that for the knee. During the swing phase, the motor, motor parallel variable damper and motor series springs are used to absorb and generate mechanical power as needed to reproduce a human-like swing phase trajectory. 
         [0188]    It should be understood that the bi-articular hybrid actuator described herein could be used in a variety of ways. For example, mono-articular motor, spring and/or damper components could act about the biomimetic ankle and/or knee joints to supplement the mechanical output resulting from the bi-articular hybrid actuator of  FIG. 17 . Still further, any of the ten embodiments described herein could be employed as part of an artificial musculoskeletal architecture comprising mono and poly-articular actuation strategies. 
         [0189]    Sensing and Control 
         [0190]    As described above in connection with  FIGS. 1-7 , investigations of the biomechanics of human limbs have revealed the functions performed by the ankle, knee and hip joints 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 joint or limb using motors to act as torque actuators and to position the skeletal members 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 in connection with  FIGS. 1-7  of the drawings. The specific mechanical structures, that is the combinations of motors, springs and controllable dampers used in the embodiments depicted in  FIGS. 8-49  and described above are specifically adapted to perform the functions needed by specific joint and limb structures. A variety of techniques may be employed to automatically control the motor and controllable dampers at the times needed to perform the functions illustrated in  FIGS. 1-7  and any suitable control mechanism may be employed.  FIG. 50  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  5000  which produces control outputs to operate the motor actuator and to control the variable dampers. 
         [0191]    The sensors used to enable general actuator operation and control can include:
       (1) Position sensors seen at  5002  in  FIG. 50  located at the biomimetic 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  5004 ) and additionally the motor&#39;s velocity (as indicated at  5006 ). A single shaft encoder may be employed to sense instantaneous position, from which motor displacement and velocity may be calculated by the processor  5000 .   (2) A force sensor (strain gauges) to measure the actual torque borne by the joint as indicated at  5008 .   (3) Velocity sensors on each of the dampers (rotary encoders) as indicated at  5010  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  5012  in order to measure the amount of energy stored.   (5) One or more Inertial Measurement Units (IMUs) seen at  5014  which can take the form of accelerometers positioned on skeletal members from which the processor  5000  can compute absolute orientations and displacements of the artificial joint system. For example, the IMU may sense the occurrence of events during the walking cycle such as heel strike and toe-off seen in  FIGS. 1-3 .   (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.       
 
         [0198]    The processor  5000  preferably comprises a microprocessor which is carried on the body and typically operated from the same battery power source  5020  used to power the motor  5030  and the controllable dampers  5032  and  5034 . A non-volatile program memory  5041  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  5042  may be used to supplement the available random access memory in the microprocessor  5000 . 
         [0199]    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 biomimetic 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. 
         [0200]    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 biomimetic joint itself. 
       REFERENCES 
       [0201]    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 curly 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 artificial 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, Jw. 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.