Abstract:
Torsional loads can impart painful and potentially injurious shear stresses upon residual limb soft tissues in lower limb amputees. To protect the soft tissues, a controllable transverse rotation adapter (TRA) has been developed that permits rotation of the prosthetic socket relative to the prosthetic foot, relieving some of the rotational loads experienced by the residual limb, and controlling either the stiffness resisting the transverse rotational torque, or the torque. This TRA uses series elastic actuator (SEA) technology and includes a prime mover, a speed reduction device, and a torsion spring mounted in series. By measuring a displacement of the spring (e.g., using a strain gauge), motor current, and the motor shaft position, the load torque and position can be used to calculate actual stiffness or applied torque. The motor position is then adjusted relative to the load, to control the effective stiffness or torque.

Description:
RELATED APPLICATIONS  
       [0001]    This application is based on a prior copending provisional application, Ser. No. 61/219,271, filed on Jun. 22, 2009, the benefit of the filing date of which is hereby claimed under 35 U.S.C. §119(e). 
     
    
     BACKGROUND  
       [0002]    Turning is a ubiquitous task for human ambulation, and this task has been shown to be related to falling and fall-related injuries in Parkinson&#39;s Disease patients and for the elderly, and likely creates difficulties for lower limb amputees as well. Unlike an intact leg, which transfers loads to the skeletal system via fatty pads on the bottom of the foot, in amputees, as shown in a schematic diagram  20  in  FIG. 1 , loads are transferred from the ground to a prosthetic socket  26  via a prosthetic foot  22  and a rigid aluminum tube called a pylon  24 . The prosthetic socket then transfers the loads to the soft tissues of a stump  28 , or residual limb. When performing turning maneuvers, amputees experience increased transverse plane torques. These increased transverse plane torques result in increased shear stresses that are believed to be associated with pain, and the formation of epidermoid cysts and ulcers, which can require several visits to a physician to manage. Thus, new technology is needed to improve comfort and prevent injury related to transverse plane torques during amputee turning gait. 
         [0003]    The intact human ankle can serve as inspiration for a prosthetic device to reduce torsional loads while turning, since the actual ankle permits limited motion in the transverse plane. Previous research has investigated transverse plane ankle behavior and found that it behaves as a passive system with variable stiffness, both throughout the gait cycle and between straight and different turning steps. Despite this behavior, previous attempts to reduce transverse plane loading have focused on technologies with fixed stiffness. 
         [0004]    To protect the soft tissues from the effects of torsional loads, which can impart painful and potentially injurious shear stresses upon residual limb soft tissues in lower limb amputees, prosthetic manufacturers have developed transverse rotation adapters (TRAs) that are essentially torsional springs mounted in the pylon of the prosthesis, which permit the prosthetic socket to rotate relative to the prosthetic foot, relieving some of the load acting on the residual limb. A variety of stiffness values are available for TRAs, allowing prosthetists to choose linear and nonlinear options and even allowing different values to be chosen for external and internal rotation. But once installed in the TRA, the stiffness does not vary as it does in the human ankle While it would be advantageous to be able to adjust the stiffness of the torsional spring to suit different activities, once installed, the stiffness of conventional TRAs cannot be easily adjusted. Any adjustment requires removal of the prosthesis and either replacement of the torsion spring or adjustment of the spring force provided by the torsion spring. 
         [0005]    Another passive strategy that has been employed to reduce transverse loading is embodied in the Rotasafe™ device. This device is essentially a slip-clutch designed to prevent over-rotation of osseointegrated implants. Slip-clutches use static friction to maintain torsional rigidity until a certain torque is reached, at which point, the slip-clutch allows rotation, which, in the case of the Rotosafe™, acts to prevent damage to the bone-implant interface. Effectively, this device enables a binary variation selection between stiffness values (mainly, one very stiff and one soft), but cannot replicate the multitude and range of elastic behaviors exhibited by the human ankle. Furthermore, while the device can save an implant from excessive torques, the slipping rotations might induce falls that can cause other injuries. 
         [0006]    In addition to preserving residual limb health, another important challenge for lower limb prosthetic design is to improve the metabolic cost of walking for amputees. Walking with a prosthesis requires much more metabolic energy to walk than is expended by a person with intact limbs. Indeed, dysvascular transfemoral amputees require more than twice as much oxygen to walk a meter than intact individuals. Other amputee levels and etiology also require considerably more oxygen to walk than intact individuals. 
         [0007]    The cause of the elevated metabolic cost associated with amputee gait is largely unknown, and most of the research in the area has focused on how different prosthetic components affect metabolic cost. Prosthetic feet are some of the more popular components studied. With the advent of flexible energy storage and release feet, a number of researchers have investigated whether these feet can reduce the metabolic cost of walking Unfortunately, only three of the studies (of nine total) were able to detect differences in metabolic cost. Furthermore, one of those studies detected differences only for higher walking speeds, and the differences in the other studies were so small as to lack clinical significance, despite their statistical significance. Thus, it appears that energy storage and release feet have had limited success in reducing the metabolic cost of walking for amputees. 
         [0008]    With the advent of microprocessor-controlled prosthetic knees, researchers again raised the question of whether prosthetic technology could reduce the metabolic cost of walking and again had mixed results. One group of researchers compared the C-Leg to the Mauch SNS prosthetic knee with eighteen transfemoral amputees and was unable to detect a difference in metabolic cost. Others compared the C-Leg, Rheo, and Mauch SNS knees and found that amputees had 3% and 5% lower metabolic rates with the C-Leg and Rheo knees, respectively, than with the Mauch SNS. Still another research group found that the C-Leg reduced the metabolic cost by 6% compared to a mechanical knee. Accordingly, while some studies were able to detect a metabolic benefit of using microprocessor-controlled prosthetic knees, these benefits were small compared to the enormous metabolic losses associated with transfemoral amputee gait. Furthermore, it should be noted that the two studies that detected metabolic benefits were funded by manufacturers of microprocessor-controlled knees, while the study that did not detect a benefit was funded by the U.S. government. 
         [0009]    While innovations in energy storage and release feet and microprocessor-controlled knees have been unable to meaningfully decrease the metabolic cost of walking for lower limb amputees, recent research with inverted pendulum models of gait may hint at more fruitful interventions. Historically, walking has been believed to employ six kinematic features of gait to reduce the vertical displacement of the body center of mass (COM) in order to minimize metabolic cost. The inverted pendulum theory of gait proposes instead that the stance limb behaves like an inverted pendulum and that there are metabolic benefits associated with exploiting this natural dynamic behavior. With the inverted pendulum theory, step-to-step transitions are major sources of metabolic cost. More specifically, in order to redirect the COM along another pendular arc at the end of a step, the leading and trailing limbs perform negative and positive work simultaneously which exerts a metabolic cost. It has been found that transtibial amputees have difficulty generating positive work when the prosthetic leg trails, suggesting that a powered prosthetic ankle might decrease the step to step transitions and, consequently, the metabolic cost. Indeed, preliminary results with a powered sagittal ankle system have been able to reduce metabolic cost by an average of 14% with three subjects. In addition to the sagittal plane, considerable work must be performed to redirect the COM in the frontal plane, as well, suggesting that more metabolic gains can be achieved with active technology to propel the COM in this plane. 
         [0010]    Accordingly, a new generation of technology is needed that enables the transverse loading to be varied across a wide range of stiffness values and/or torque values. It would also be desirable to employ an active approach to control the effective stiffness of rotation in a prosthesis, to be more responsive to loading changes. It would also be desirable to enable the stiffness of the loading to be readily varied with a control to enable an amputee to more effectively engage in various activities that benefit from the application of different levels of torsional stiffness. As a further benefit, the use of such a prosthesis should substantially reduce the metabolic cost to the subject by providing a gait that more closely replicates that of an intact individual. 
       SUMMARY  
       [0011]    In order to allow lower limb amputees to adjust torsional stiffness, an exemplary controllable TRA has been developed. This adaptor uses a series elastic actuator (SEA) technology and includes a prime mover, such as an electric motor or other type of actuator, a gear reduction assembly, and a spring of known stiffness—all mounted in series along a longitudinal axis of the TRA. After measuring the displacement of the spring, the spring torque can be calculated. Then, by adjusting the prime mover position relative to the load, the effective stiffness of the adaptor can be controlled. 
         [0012]    Accordingly, an SEA and impedance control system has been developed to be used in a prosthetic limb. With a prosthesis that includes this novel TRA, amputees can set the impedance control system input to a desired effective stiffness or can choose a torque setting that is suitable for different activities. For example, an amputee might want a very stiff TRA in a prosthesis to maximize its performance during high intensity activities like playing tennis, but then desire a soft TRA in the prosthesis to maximize comfort during everyday activities, like walking. Control of torque might be better during high intensity activities, to minimize the metabolic effort on the user, while during daily activities of longer duration and lower intensity, the user may want to control the TRA to achieve the desired stiffness. With this new TRA, amputees can adjust the TRA stiffness or torque in a prosthesis simply by turning a dial or similarly providing a control input change. 
         [0013]    To design the control system, two levels of control were considered for an exemplary embodiment. The lowest level control system controls the effective stiffness or torque of the prosthesis through an impedance or moment control strategy, respectively. The next level control loop above the impedance system supplies a desired stiffness or torque at different periods of the gait cycle through a finite state strategy. 
         [0014]    An actuator for an exemplary TRA that implements this functionality comprises a compact actuator, such as a direct current (DC) motor (with brushes), and a lightweight harmonic drive transmission (or other type of gear reduction mechanism), used with a torsion spring or other type of elastic element. The appropriate stiffness for the elastic element used with the TRA can be chosen by comparing the effect of stiffness on motor power, spring power amplification, and control stability through simulation experiments. Once the stiffness is chosen, the appropriate physical elastic element can be designed and fabricated. A housing for the adaptor integrates the actuator and torsion spring into a prosthesis that can be worn on a residual lower limb. 
         [0015]    This application specifically incorporates herein by reference, the disclosure and drawings of the provisional patent application identified above as a 
       related application 
       [0016]    This Summary has been provided to introduce a few concepts in a simplified form that are further described in detail below in the Description. However, this Summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. 
     
    
     
       DRAWINGS  
         [0017]    Various aspects and attendant advantages of one or more exemplary embodiments and modifications thereto will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein: 
           [0018]      FIG. 1  (Prior Art) is a schematic diagram that illustrates an exemplary conventional transtibial prosthesis; 
           [0019]      FIG. 2  is a schematic diagram illustrating an exemplary series elastic actuator (SEA); 
           [0020]      FIG. 3  includes four graphs respectively illustrating from top to bottom, ankle angle, ankle velocity, ankle moment, and ankle power, for each of four different states, where the data were captured for ten subjects who were walking ahead in a straight line; 
           [0021]      FIG. 4  includes 12 graphs respectively illustrating the same parameters as in  FIG. 3 , for a 90 degree hallway turn, for the turn initiation, its apex, and its termination steps, again for data collected in regard to ten subjects; 
           [0022]      FIG. 5  is a schematic image of a cruciform hinge used for a torsional spring employed in an exemplary TRA in accord with the present novel approach; 
           [0023]      FIG. 6  is a schematic diagram of an impedance control system to control the effective stiffness in the exemplary TRA; 
           [0024]      FIG. 7  is a schematic block diagram illustrating the four types of steps, including straight, turn initiation, turn apex, and turn termination, which might be intended by a person using a prosthesis incorporating the TRA of the present novel approach; 
           [0025]      FIG. 8  is a flowchart of the logic employed by an exemplary finite state control system for implementing torque control during straight steps; 
           [0026]      FIG. 9  is a flowchart of the logic employed by an exemplary finite state control system for implementing torque control during initiation steps; 
           [0027]      FIG. 10  is a flowchart of the logic employed by an exemplary finite state control system for implementing torque control during apex steps; 
           [0028]      FIG. 11  is a flowchart of the logic employed by an exemplary finite state control system for implementing torque control during termination steps; 
           [0029]      FIG. 12  is an exploded view of an exemplary harmonic drive device that can be used for the gear reduction portion of the TRA in the present novel approach; 
           [0030]      FIG. 13  is an exemplary functional block diagram of the TRA system; 
           [0031]      FIG. 14  is an exemplary functional block diagram of a computing device suitable for use in the controller of the TRA in accord with the present novel approach; 
           [0032]      FIG. 15  is a schematic side view of an exemplary prosthesis that includes the TRA in accord with the present novel approach; 
           [0033]      FIG. 16  is a flowchart showing exemplary logical steps for controlling the TRA to achieve a desired stiffness in resisting the transverse rotational torque acting on a prosthesis; 
           [0034]      FIG. 17  is a flowchart of the logic employed by an exemplary finite state control system for implementing torque control when attempting to achieve a desired torque for the TRA in an alternative exemplary embodiment; and 
           [0035]      FIG. 18  is a schematic block diagram of an exemplary torque control system for torque control mode used in the alternative embodiment. 
       
    
    
     DESCRIPTION  
     Figures and Disclosed Embodiments Are Not Limiting 
       [0036]    Exemplary embodiments are illustrated in referenced Figures of the drawings. It is intended that the embodiments and Figures disclosed herein are to be considered illustrative rather than restrictive. No limitation on the scope of the technology and of the claims that follow is to be imputed to the examples shown in the drawings and discussed herein. Further, it should be understood that any feature of one embodiment disclosed herein can be combined with one or more features of any other embodiment that is disclosed, unless otherwise indicated. 
       Preliminary Studies 
       [0037]    To evaluate the hypothesis that turning is a requirement of activities of daily living, a study was performed using video data of the steps taken during several common activities. An investigator filmed the walking patterns of ten subjects below the waist while they performed the activities of walking from an office to a car, walking between two offices, purchasing goods at a convenience store, and purchasing a meal at a cafeteria. The study found that turning steps comprised up to 50% of the steps taken during a daily activity, that activities with more task demands like picking out items in a store or filling a drink cup at a cafeteria required more turning, and that shorter activities require more turning steps. Given these data and the fact that periods of walking by amputees tend to be of short duration, it can be concluded that turning is an important consideration for prosthetic design. 
         [0038]    In a second study, the biomechanical strategies used to perform transient turning maneuvers were investigated. Ten subjects were asked to walk straight ahead and navigate a 90 degree hallway corner while force plates recorded kinetic data and a 12 camera Vicon™ system recorded motion data. First, a data processing method was developed to express the ground reaction impulses (GRIs) during a transient turn in terms of a body frame, rather than a global reference frame. The body frame had an origin at the body center of mass (COM) and was aligned to the COM trajectory. Then, the GRIs for straight walking and turning were compared. It was found that the braking and propulsive GRIs during the initiation and termination steps of the turn resembled the GRIs for decelerating and accelerating during straight walking, suggesting that the body modulates these impulses to control walking speed during a turn. The medial-lateral impulses acted in the direction away from the turn. 
         [0039]    To determine the simplest system of mechanical elements that could mimic the intact human ankle, a mechanical model of the ankle in the transverse plane was created. First, the model investigated ankle behavior during straight walking. Motion capture data were collected of ten subjects walking straight ahead. The model divided the kinematic and kinetic data from the stance phase into four states, as shown in graphs  40  in  FIG. 3 , based on zero values in the power curve and investigated the elastic and viscous behavior of the ankle in each state. Viscous behavior was found to be negligible, and in the first state, the elastic behavior was found to resemble that of a quadratic torsional spring, while the remaining three states were found to resemble linear torsional springs. 
         [0040]    Next, the model was used to investigate the simplest system of mechanical elements that could mimic the ankle in the transverse plane during turning. 
         [0041]    Motion capture data were collected from ten subjects performing 90 degree hallway turns. As illustrated in graphs  50  in  FIG. 4 , data from turn initiation  52 , apex  54 , and termination  56  steps were investigated. States were separated for the kinematic and kinetic data, and elastic and viscous behaviors were investigated, yielding the graphical results shown in the graphs of rows  58 ,  60 ,  62 , and  64  for each of the three types of steps  52 ,  54 , and  56 . It was found that, in general, the ankle softens during initiation steps, stiffens to maintain constant position during apex steps, and stiffens during termination steps. 
         [0042]    Finally, as shown in  FIG. 2 , a dynamic actuator model was created to help determine the stiffness of a spring  38  for a series elastic actuator (SEA)  30  that would minimize power consumption while maximizing spring power amplification. The model was based on specifications for an RE40™ brushless DC motor  32  (such as available from Maxon Precision Motors, Fall River, Mass.) that is coupled via a drive shaft  34  to a 353:1 GP52C™ gear reduction box  36  (also available from Maxon Precision Motors), the inertial characteristics of an 80 kg man, and kinetic and kinematic data for ten subjects walking straight ahead at self-selected speeds. Based on the results of the model, a stiffness of 250 N-m/rad was chosen. As shown in a model  70  in  FIG. 5 , a cruciform hinge geometry  72  was chosen for the torsion spring, and it was fabricated from titanium in an initial exemplary embodiment. 
       Design of the Impedance Control System 
       [0043]    While the stiffness of the elastic element will not change, through an impedance control strategy, the effective stiffness, or the relationship between motor position and spring torque, can be controlled by dynamically varying the motor position with respect to the load position. Based on the dynamic actuator model, an exemplary control system  80 , which is shown in  FIG. 6 , was developed. 
         [0044]    As shown in  FIG. 6 , an input  82  to the proposed control system is the desired effective stiffness, k d . This desired effective stiffness can be input by the user of the TRA or can be automatically determined based on a sensor that detects the type of activity or a parameter such as a slippage of the residual limb in the socket of the prosthesis or a sensor of some other parameter that is beneficial in controlling the stiffness of the TRA. An actual effective stiffness  86  of the TRA, k l , is subtracted from k d  in a summing junction  84  to find a stiffness error  88 , k e , which is passed to a proportional-integral-derivative (PID) controller  90 . The error k e  is multiplied in a multiplier  92  by an angular load position θ l , to find a motor torque, T m . Transfer functions  94  and  98  respectively produce outputs  102  and  100  that are summed at a summing junction  104  to determine a load torque, T l . An input  96 , θ l  is supplied to transfer function  98 . Transfer functions  94  and  98  employ the gear ratio N, gear box efficiency η, a constant, K s , and polar moment of inertia J m . An operation  106  using the reciprocal of a is applied to T l  to obtain actual effective stiffness  86 . 
       Design of an Exemplary Finite State Control System 
       [0045]    Human ankle behavior in the transverse plane can be described as a variable stiffness spring that varies its stiffness throughout the gait cycle, with periods differentiated by separate states in the stance phase ( FIGS. 3-4 ). As such, a higher-level finite state control system is needed to switch between stiffness levels. Finite state control systems have been used in many prosthetic and orthotic systems, since the gait cycle is easily divisible into states. 
         [0046]    To achieve a long term goal of providing a neurally-controlled prosthesis, some sort of upper level control system will be required to determine an intended step type  112 , as shown in a block diagram  110  in  FIG. 7 . Each step type, straight  114 , turn initiation  116 , turn apex  118 , and turn termination  120 , will have its own corresponding finite state control system based on the states ( FIGS. 8-11 ) determined from a biomechanical model of the step type. In the finite state diagrams, the control system will remain in a previous state until one of the anticipated conditions is met. For example, as shown in a flow chart  130  for the straight step type, in  FIG. 8 , the control system will remain in an S-State  1 , as indicated in a block  138 , until either the axial force falls below a defined threshold, or the rotator moment and velocity both enter the internal range. The details of this logic are explained below. 
         [0047]    The flowchart in  FIG. 8  begins at an upper loop  132  for a straight step  134 . A decision step  136  determines if the axial force is greater than a threshold (indicating that the prosthetic foot is applying force against the ground or other adjacent surface), and if so, in a step  138 , the control system changes to (or remains in) an S-State  1 . If not, the logic loops until an affirmative result is returned. After entering S-State  1 , a decision step  142  determines if both the rotator moment and rotator velocity are in the internal range, and if so, in a step  144 , the control system changes to an S-State  2 . If not, the logic remains in S-State  1  at step  138 . While in step  138 , the logic also periodically determines if the axial force is below the threshold, in a decision step  140 . If the axial force becomes less than the threshold, as determined in decision step  140 , the prosthesis is apparently swinging and not in contact with the ground or other adjacent surface. If not, the control system remains in the current state, as indicated in a step  154 , but if so, the logic return to the upper loop in step  132 . The logic repeatedly checks the conditions of decision steps  140  and  142  until a positive result is achieved to either decision step. 
         [0048]    After entering S-State  2 , the control system remains in that state until the condition in decision step  140  is met, or until both the rotator moment enters the external range and the ankle angular position is below a threshold value, as determined in a decision step  146 . If both of these conditions in decision step  146  occur, the control system enters the S-State  3 , as indicated in a step  148 . If either condition is not met, the logic remains in S-State  2  and periodically checks to determine if the axial force is below the threshold in decision step  140 . After entering S-State  3 , a decision step  150  determines if both the rotator moment is in the external range and the ankle angular position is above the threshold. If so, the control system enters an S-State  4  in a step  152 , but if not, the control system remains in S-State  3 . While in S-State  4 , the logic periodically runs decision step  140 . The control system remains in S-State  4  until decision step  140  determines that the axial force is below the threshold. 
         [0049]    A flowchart  160  for a turn initiation step  164  is illustrated in  FIG. 9  and starts at an upper loop  162 . A decision step  166  determines if the axial force has exceeded a threshold and if so changes to (or remains in) an I-State  1 , as noted in a step  168 . If not, the logic loops until the condition in decision step  166  is met. After entering I-State  1 , the logic periodically executes a decision step  170  to determine if the axial force is below the threshold, indicating that the prosthesis is not in contact with the ground or other adjacent surface. If the condition of decision step  170  is met, the logic returns to upper loop  162 , but if not, the control system remains in the current state in a step  184 . After entering I-State  1 , the logic periodically also checks to determine if both the rotator moment and rotator velocity are in the internal range, as indicated in a decision step  172 . If not, the control system remains in I-State  1 . If these two conditions are met, the control system enters an I-State  2 , as noted in a step  174 . After entering I-State  2 , the logic periodically executes decision step  170 , which makes the determination discussed above. Also after entering I-State  2 , if the condition of decision step  170  is not met, the control system remains in I-State  2  until the rotator moment enters the external range and the ankle angular position is below a threshold, as indicated in a decision step  176 . If so, the control system enters I-State  3  in a step  178 . If both of these conditions are not met, the logic remains in I-State  2 , and periodically checks to determine if the axial force is below the threshold in decision step  170  or if the conditions of decision step  176  are met. After the control system enters I-State  3 , the control system periodically determines if the condition of decision step  170  is met, and if the rotator moment has entered the external range, and the ankle angular position is above the threshold, in a decision step  180 . If not, then the control system remains in I-State  3 . But, if the conditions of decision step  180  are met, the control system enters I-State  4  in a step  182 . Again, the logic periodically makes the determination in decision step  170 , as discussed above. 
         [0050]    The turn apex is illustrated at a step  194  in a flowchart  190  that is shown in  FIG. 10 . The turn apex begin at an upper loop in a step  192 . A decision step  196  determines if the axial force is exceeding a threshold, indicating that the prosthesis is in contact with the ground or other adjacent surface. The logic loops until this condition is met. Once the threshold is exceeded, the control system enters (or remains) in an A-State  1  in a step  198 . After entering A-State  1 , the logic periodically runs a decision step  200  to determine if the axial force is below the threshold and if so, returns to the upper loop. If not, the logic remains in the current state, in a step  206 . Also, after entering A-State  1 , a decision step  202  determines if the rotator moment has entered the external range, and if not, simply remains in A-State  1 . If so, the control system enters an A-State  2  in a step  204 . The logic remains in A-State  2 , until upon a periodic check in decision step  200 , the axial force is found to be less than the threshold, which resets the logic to the upper loop in step  192 . 
         [0051]    Finally, for a turn termination in a step  214 , a flowchart  210  shown in  FIG. 11 , starts at an upper loop in a step  212 . A decision step  216  determines if the axial force exceeds the threshold, and if not, simply loops until it does. Once the axial force is greater than the threshold in decision step  216 , the control system enters a T-State  1  in a step  218 . After entering T-State  1 , the logic periodically checks to determine if the axial force has dropped below the threshold in a decision step  220 . If so, the logic returns to upper loop  212 , but if not, the control system remains in the current state in a step  230 . Periodically, the logic determines if the rotator moment is in the external range and the ankle position is below a threshold, in a decision step  222 . If not, the control system remains in T-State  1 . But if so, the control system enters a T-State  2  in a step  224 . Once T-State  2  is achieved, the logic again periodically checks the axial force in decision step  220 , and also periodically determines if the external rotator moment has entered the external range and if the ankle position is above a threshold in a decision step  226 . Until both conditions are met, the control system remains in T-State  2 . But once both of these conditions are met, the control system enters a T-State  3  in a step  228 . The control system remains in T-State  3  until the axial force is less than the threshold in decision step  220 . 
         [0052]    Ambulating in the real-world environment is a highly uncertain task. Limiting the control system to an already occupied state until specific criteria are met can allow amputees to adjust steps with a predictable stiffness. For example, an amputee might start a turn initiation step, only to observe another person has stepped into the intended trajectory, forcing the amputee to react with kinematic and kinetic patterns that differ from the initiation steps. In this case, the control system should remain in I-State  1  to give the amputee predictable prosthesis behavior in order to complete the step before the control system resets itself during the swing phase and gets ready for the next step. 
         [0053]    A strain gauge on the prosthesis can be used to detect axial loading to determine if the prosthesis is in a stance phase (exerting a force on the ground or other adjacent surface), or in a swing phase. A separate strain gauge is employed to sense torque in the elastic element of the SEA and is used in determining the moment used for state transitions. An optical encoder on the motor shaft can be used to determine the angular position and velocity data for state transitions. 
       Control System Simulation 
       [0054]    As a preliminary validation of the control system design, simulation experiments were performed. Ankle torque and position data from previous experiments were used as T l  and θ l , respectively. The value k d  was provided from the ankle models developed previously. The effective actuator stiffness k d , or the quotient of T spring  and θ spring  are calculated throughout the gait cycle. The control elements can be tuned manually until k a  matches k d , within an error range of ±5%. 
         [0055]    The performance of the control system with a physical actuator was confirmed through bench top testing. The actuator used in this initial exemplary prototype was based on the design from the previous work and included a cruciform hinge made of titanium, an RE40™ brushless DC motor (Maxon) and a 353:1 GP52C™ gearbox (Maxon). The actuator was mounted in an MTS 858 Bionix™ system. Transverse plane ankle kinematics were simulated with the MTS&#39;s displacement control feature. The actuator was controlled in real time. Again, the control elements were tuned manually until k a  matched k d , within an error range of ±5% 
       Functional Block Diagram 
       [0056]      FIG. 13  is a functional block diagram of an exemplary TRA system  410 . A TRA  422  that is used in a prosthesis in place of a conventional pylon is controlled by a computing device (or other logic device)  416 . The logic device can be either hardwired or can be a microcontroller or an application specific integrated circuit (ASIC). Power for energizing a motor  424 , computing device  416 , and other components of the system is supplied by a power supply  412 , which includes a battery or other portable power source. A motor controller  414  is controlled by the computing/logic device to vary the current supplied to motor  424 . A current sensor  418  monitors the current being supplied to motor  424 , producing a current indicative signal that is supplied to a data acquisition circuit  420 . This analog signal can be converted to a digital signal by the data acquisition circuit, for input to computing/logic device  416 . An encoder  426  (e.g., an optical encoder) monitors the motor shaft position, producing a position signal that is input to motor controller  414 , to aid in controlling the current supplied to motor  424  to achieve a desired stiffness of the prosthesis (or in an alternative exemplary embodiment, a desired torque). 
         [0057]    Since the motor shaft turns at a relatively high speed compared to the driven torque applied by the TRA, a gear reduction module  428  is included to couple the motor to a torsional spring  430 . Attached to torsional spring  430  are a pair of strain gauges  432 , which produce strain indicative signals (one axial and one torsional) that are input to an amplifier  434 . The amplified strain signals are then input to data acquisition circuit  420 , for input as digital strain signals to computing device  416 . The user can manually adjust a set point dial  436  to vary the stiffness of the TRA (or a range of stiffness), as desired for specific activities in which the user wants to participate, as explained above. Accordingly, it is intended that the stiffness or torque resistance provided by the TRA might also be automatically and dynamically modified within a range set by the user using the set point dial or other user-manual control for input. It should be noted that instead of a dial (which might control a variable potentiometer), the desired stiffness or range of stiffness (or desired torque or range of torque) may instead be set by the user by an input to a digi-switch, a joystick, or other input device for a user-variable setting of a parameter. 
         [0058]      FIG. 14  illustrates further details of an exemplary computing device  416  (or other logic device), which is suitable for controlling the TRA as discussed herein. This exemplary computing device includes a processor  440 , which executes machine readable and executable software instructions that are stored in a memory  442 . Processor  440  receives input signals, such as from data acquisition circuit  420  or from set point dial  436  via input/output (I/O) ports  444 . The processor executes the machine software instructions to produce output signals that are supplied to motor controller  414  through the I/O ports. These software instructions cause the processor to execute the control functions indicated in the flowchart of  FIG. 16 , as noted below. Similar control functions are implemented when the alternative exemplary embodiment is employed to achieve a desired torque with the TRA. 
       Embodiment Appropriate for Prosthesis 
       [0059]    The experimental actuator and control system will be incorporated into a prosthesis. For this step, a new actuator, with smaller, lighter components more suitable for a prosthesis is being designed, and a mechanical interface is being developed to incorporate the actuator into a transtibial prosthesis. It should be understood that the general approach embodied in the exemplary embodiments disclosed herein are equally applicable to prosthetic devices for other portions of a patient&#39;s body and are not limited only to a transtibial prosthesis. 
         [0060]    When designing lightweight actuators, the limiting factor is often not the maximum motor torque, but instead, the maximum permissible torque for a particular planetary gear transmission system. To achieve biomimetic torques while maintaining an appropriate safety factor, the gearbox chosen in the initial exemplary prototype had a mass of 770 grams, which is far too great for a prosthesis. 
         [0061]    Fortunately, other types of transmission systems are available. One particularly promising technology is called a Harmonic Drive, an exploded isometric view  230  of which is illustrated in  FIG. 12 . In a Harmonic Drive, the motor shaft is drivingly coupled to an elliptical disc called a Wave Generator  236 . The Wave Generator is inserted into a thin-walled cup called a Flexspline  234 . The Flexspline has gear teeth machined into the outer edge and conforms to the Wave Generator&#39;s elliptical shape. The Flexspline acts as the output of the transmission and is inserted into a Circular Spline  232 . The Circular Spline is a rigid steel ring with internal gear teeth and is attached to the transmission housing and does not rotate. The teeth of the Flexspline interact with the teeth of the Circular Spline along the longitudinal axis of the Flexspline. And, since the Flexspline has fewer teeth than the Circular Spline, with every revolution of the Wave Generator, the Flexspline shifts relative to the Circular Spline as a function of the difference in the number of teeth on the two splines, creating an effective gear reduction as the Flexspline rotates slowly in the opposite direction from the Wave Generator. The main benefit of this technology is that high gear ratios can be achieved in compact, lightweight devices. It is noted that this technology could alternatively use a planetary gearbox (such as those made by Maxon Motors of Sachsein, Switzerland), or a spur gearbox (such as those made by Maxon), an Ikona Gear Drive (such as those made by Ikona, Port Coquitlam, Canada), or other types of gear (speed) reduction transmissions or mechanisms. 
         [0062]    In a more lightweight and compact embodiment for a prosthesis, a CSF-14™ Harmonic Drive Mini Gearhead (available from Harmonic Drive, Hauppauge, N.Y.) can be used. This particular model has a 100:1 gear ratio, can achieve peak torques of 28 N-m, and is only 295 grams in mass. The use of this Harmonic Drive should also enable a smaller motor to be used for the TRP than the one used in the initial exemplary prototype. For example, in this new embodiment, a RE30™ (Maxon) motor, which has a mass of 238 g, can be employed, providing a savings of over 242 g compared to the motor used in the prototype. The Maxon RE-40™ motor used in the initial prototype has a nominal voltage of 24 VDC, a maximum continuous torque of 170 N-mm, and a stall torque of 2280 N-mm. In contrast, the Maxon RE30™ motor that may be used in the TRA sized to fit in a prosthesis has a nominal voltage of 12 VDC, a maximum continuous torque of 51.7 N-mm, and a stall torque of 844 N-mm. It is likely that future embodiments of the novel TRA will be achieved that include even smaller (and perhaps more powerful motors), so long as a gear reduction mechanism with a suitable gear ratio/weight ratio, and load capability is employed. 
         [0063]    In the initial prototype, the elastic element stiffness of the experimental actuator was chosen based on simulations investigating motor power consumption and spring power amplification. For the new light and compact embodiment that will be appropriate for use in a prosthesis, a stiffness value for the prosthetic actuator can be selected based on both motor power consumption and spring power amplification. Additionally, the effect of stiffness values on control system stability can be empirically determined by measuring settling time through simulations. 
         [0064]    Similar to the initial prototype, a dynamic actuator model will be constructed using similar components to those described above in the prototype. Gait simulations will be performed with the actuator model to determine the effect of differing stiffness values on motor power consumption and spring power amplification. Then, using the control system that is designed, the effect of differing stiffness values on control system stability will be investigated. A stiffness value for the prosthetic actuator will be chosen qualitatively based on these three investigations. 
         [0065]    An elastic element for the new actuator can be made with the Alibre Design™ (Alibre, Richardson, Tex.) solid modeling software. A cruciform hinge geometry will likely be chosen again, for its resistance to compressive and bending loads and for its ability to deform torsionally. The elastic element will be fabricated from titanium for its high shear modulus-to-yield-strength ratio. The proximal end of the element can be designed to interface with the harmonic drive, while the distal end can be designed to interface with a standard female prosthetic pyramid adaptor. 
         [0066]    A housing can be designed to connect the TRA to a prosthetic socket. For example, the housing can also be created with the Alibre Design™ tool (Alibre) and can connect the Harmonic Drive to the socket in such a way that the motor is never loaded axially. 
         [0067]      FIG. 15  illustrates further details of this embodiment, showing an example of how it will be integrated into a prosthesis. TRA  410  extends between a socket  458  in which an amputee&#39;s residual limb (not shown) is inserted. Socket  458  is coupled to the TRA by brackets  454  (or sides or a housing), which extend downwardly from each side of the socket and are coupled to gear reduction mechanism  428  immediately above a proximal end  446  of torsion spring  430 . The components that control the TRA are miniaturized and are generally disposed within a controller  456  that is disposed immediately below the socket, although other convenient locations can instead be used. Details of the power supply are not shown in this view, but a battery pack  462  is shown attached to one side of brackets  454 . It will be appreciated that additional battery packs can be attached to the other sides of the brackets, or that the battery pack(s) can instead be disposed at other locations on the prosthesis, e.g., within an artificial foot  452 . 
         [0068]    Artificial foot  452  is coupled to a distal (lower) end  448  of torsion spring  430  and is secured by a clamp  450 . Other components of the TRA are identified by the reference numbers listed above. A user accessible knob  460  is provided to enable a user to adjust the desired stiffness (or range) or desired torque (or range) provided by TRA  410  and is coupled to set point dial  436  by an input shaft (not indicated). 
       Flow Chart Illustrating Exemplary Steps of Control Logic 
       [0069]      FIG. 16  illustrates exemplary steps  470  that can be implemented by the computing device or other logic device to control the operation of the TRA to achieve a desired stiffness. In a step  472 , the user can input or set the desired stiffness (or range of stiffness) for the TRA. The actual desired stiffness can be determined from the models discussed above, to correspond to that experienced by the limb of an intact person when ambulating. By setting the range of the desired stiffness, the user can shift the peaks of the modeled parameters as desired. Alternatively, the desired stiffness can be determined only from such models without any input from the user. 
         [0070]    A step  474  then subtracts the actual stiffness from the desired stiffness to determine an error signal. Control gains (i.e., amplification) are applied to the error signal in a step  476 . In a step  478 , the amplified error signal is divided by a load position (determined as explained below) to determine a desired gear shaft torque that should be applied by the electric motor of the TRA. This desired gear shaft torque is divided by a gear ratio of the gear reduction transmission to determine a desired motor torque, in a step  480 . The motor electrical current that is required to produce the desired motor torque is calculated in a step  482 . The magnitude of the electrical current is supplied as a current command to the motor controller in a step  484 , which then provide electrical current at that magnitude to energize the electric motor used as a prime mover. 
         [0071]    Returning back to step  474 , the actual stiffness must be determined by data calculated using two parallel logic paths. The first of these logic paths starts with a step  486 , which measures the actual electrical motor current being applied to drive the electrical motor. Based on this electrical current, a step  488  calculates the actual motor torque that is being produced by the motor. Using this actual motor torque, a step  490  calculates the gear shaft torque. In a step  492 , the gear shaft and spring torques are added to determine the load torque being applied. This value for load torque is supplied to a step  494 . 
         [0072]    The second logic path begins with a step  496 , which monitors the strain gauge mounted to the elastomeric torsion spring. The signal produced by the strain gauge is amplified in a step  498 , and in a step  500 , the amplified strain gauge signal is used to calculate the spring torque. The spring torque is supplied to step  492  for use in calculating the load torque. Also, the amplified strain gauge signal is used in a step  502  to calculate the torsion spring position. A step  506  determines the motor shaft position using an encoder, and a step  508  divides the motor shaft position by the gear ratio to determine the gear shaft position. A step  504  then adds the gear shaft and torsion spring positions to calculate the load position. The load position is then supplied to step  494 , and also to step  478 , for use as discussed above. Step  494  calculates the actual stiffness of the prosthesis (at the present time) by dividing the load torque by the load position. The resulting actual stiffness is provided to step  474 , for use as noted above. In the alternative embodiment of the TRA discussed below that controls load torque rather than stiffness, the desired load torque of the prosthesis can be compared to the actual load torque (instead of comparing the desired stiffness to the actual stiffness). It should be evident how to modify  FIG. 16  to achieve load torque control rather than stiffness control. 
       Torque Control Embodiment 
       [0073]    As noted in the background and significance section, lower limb amputees expend much more metabolic energy while walking than intact individuals. Research has shown that amputees are unable to effectively generate the simultaneous positive and negative work with the trailing and leading limbs required to redirect the COM to new pendular trajectories due to the passive nature of most prosthetic components. This result suggests that powered components could reduce the metabolic cost of walking and indeed, preliminary work with a powered sagittal ankle prosthesis has demonstrated a 14% decrease in metabolic cost in three transtibial amputees. However, substantial energy is also required to redirect the COM in the frontal plane. It is expected that active power generation in the transverse plane can help redirect the COM towards the contralateral limb and thus reduce the metabolic cost of this movement. 
         [0074]    Active transverse ankle torque generation is believed to influence step-to-step transitions and consequently, should reduce the metabolic cost of walking for lower limb amputees. In an alternative exemplary embodiment of the present novel TRA, a new control mode is used with the TRA to actively generate a desired torque, rather than controlling the TRA to achieve a desired stiffness. This alternative approach employs a state-based torque control system. State-based control systems are useful for prosthetic limb applications because they can enable the prosthesis to perform different functions in different parts of the gait cycle. The Rheo Knee™, which is produced by Ossur of Aliso Viejo, Calif., uses a state-based controller to adjust sagittal-plane mechanical resistance as needed throughout the stance and the swing phase of walking Similarly, the PowerFoot One™, available from iWalk™, of Cambridge, Mass., uses a state-based system to adjust mechanical resistance in early and mid-stance phase and then, actively generates torque in the sagittal plane in the late stance. However, for the TRA, a state-based system is employed to govern mechanical resistance and torque generation in the transverse plane of the prosthesis. 
         [0075]    It should be noted that if desired, a user may be provided a control to switch the TRA control system to either achieve a desired stiffness (which will typically provide enhanced comfort to the user), or a desired torque (which will typically reduce the metabolic load on the user—but may not be as comfortable). It is also contemplated that the selection of controlling the TRA to achieve a desired stiffness or a desired torque may be achieved automatically as the user of the prosthesis with the TRA engages in different types of activity, since the choice of the controlling parameter—i.e., either stiffness or torque, can automatically be optimized based on the type of activity in which the user is participating. For example, if the user is participating in a sports activity, the rapid changes in the parameters such as axial load and rotational movement can indicate that the user would be best served by employing the torque control embodiment rather than the stiffness control embodiment. Conversely, if the user is simply walking down the street, or engaging in less vigorous movements, the control system can automatically detect the lower level of activity and rotation movement to select the control based on stiffness. 
         [0076]    In this exemplary embodiment for controlling torque, an upper level state-based control system determines the stage of a gait cycle of the prosthesis at a given time and then, instructs a lower level impedance or torque control system to behave accordingly. States are determined based on information from local sensors on the active TRA, as shown for a flowchart  520 , in  FIG. 17 , and generally as discussed in connection with the first exemplary embodiment intended to achieve a desired stiffness. The flowchart start with a swing mode in a step  522 . When the load on the series elastic element is below a threshold of 20 N, for example, the system assumes that the active TRA is in the swing mode and sets itself in a neutral position in preparation for a heel strike, which is detected by monitoring the axial force. The logic repeatedly checks in a decision step  524  to determine if the axial force is greater than a threshold, and if not, remains in the swing mode. Once the axial force threshold is exceeded, as the heel of the prosthesis contacts the ground or other adjacent surface, the system switches to a State  1  in a step  526  and uses the impedance control system from the first exemplary embodiment (based on achieving a desired rotational stiffness of the TRA) in a decision step  530  to determine if the rotator moment and rotator velocity have entered the internal range. If so, the state system changes to a State  2  in a step  532 , and the stiffness input changes to the appropriate value for this state. Periodically, when in State  1  or State  2 , the control system determines if the axial force is less than the threshold in a decision step  528 . If not, the control system remains in the current state, in a step  542 . However, if the axial force becomes less than the threshold in decision step  528 , then the control system returns to the swing mode at step  522 . After switching to State  2 , in step  532 , a decision step  534  is periodically executed to determine if the rotator moment has entered the external range and if the ankle angular position is less than a threshold. If so, the system control switches into a State  3  at a step  536 . The lower level torque controller (see below) is employed to provide a torque burst to the system. Periodically, while in State  3 , the logic executes decision step  528  determines if the axial force has become less than the threshold, as discussed above. Also, a decision step  538  periodically determines if the rotator moment is in the external range and if the ankle angular velocity is greater than a threshold (for example, 20 deg/sec). If not, the control system remains in State  3 , but if both conditions in decision step  538  are met, then the control system switches to State  4  at a step  540 , supplying the appropriate torque trajectory to the torque control system. The control system remains in State  4  until the axial force slips below the threshold, as determined in decision step  528 , and the control system then again returns to the swing mode. 
         [0077]    The torque controller, which is shown in a schematic diagram  550  in  FIG. 18 , works as follows. A desired torque  552 , which is represented by T d , is input to the control system and the difference between this desired torque and an actual load torque  556 , which is indicated by T l , is determined by a summing junction  554 , yielding a torque error  558 . The torque error, T e , is then passed to a PID controller  560 . The PID controller produces a motor torque input, T m , which is applied to transfer functions  562  and  566  relating the load torque T l  to T m  and θ i . The load angle θ l  is provided on an input  564 . The transfer functions include the gear ratio, N, gear box efficiency, η, and polar moment of inertia, J m , as well as a constant, K s . (In the equations for the transfer functions, s is a state variable applied when using a Laplace transform to change from the time to the frequency domain.) An output  568  of transfer function  566  is subtracted from the output of transfer function  562  at a summing junction  570 , yielding the value for T l , which is provided to summing junction  554 , as noted above and is also used to control the current produce by the prime mover (i.e., the electric motor in this exemplary embodiment). 
       Neural Interface 
       [0078]    It is contemplated that future generations of the novel TRA may utilize a neural input to set the desired stiffness level in the impedance control system. In such an embodiment, a high level control system will determine the type of step (straight, turn initiation, stair descent, etc.) that the amputee is performing or intends to perform, and an intermediate level control system will determine the state of the step in which the prosthesis is and look up the appropriate stiffness for that state in a look-up table. Also, a lower level impedance control system will then use that stiffness as a set-point to control the actual stiffness of the TRA in the prosthesis. 
         [0079]    Several different types of neural inputs may be used in future generations of the present novel TRA to provide the desired torque or the desired stiffness instead of a direct user input or simply using the values derived from the model of steps by an intact individual. Without intending any limitation, examples of such neural inputs include surface electromyogram (EMG) electrodes (such as those made by Noraxon, in Scottsdale, Ariz.), which are placed on the skin of the residual limb or other areas of the amputee&#39;s body; implantable EMG sensors (such as the BIONS™ being developed by the Alfred E. Mann Foundation at the University of Southern California, Los Angeles, Calif.), which are injected into the muscle tissue, longitudinal intrafascicular electrodes (such as those being developed at the Department of Bioengineering at the University of Utah, Salt Lake City, Utah), which screw into peripheral nerves, and other types of neural interfaces. 
         [0080]    Although the concepts disclosed herein have been described in connection with the preferred form of practicing them and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made thereto within the scope of the claims that follow. Accordingly, it is not intended that the scope of these concepts in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.