Patent Publication Number: US-10307271-B2

Title: Control system and method for non-gait ankle and foot motion in human assistance device

Description:
CLAIM TO DOMESTIC PRIORITY 
     The present application is a continuation-in-part of U.S. patent application Ser. No. 14/210,331, filed Mar. 13, 2014, which is a continuation-in-part of U.S. patent application Ser. No. 13/767,945, filed Feb. 15, 2013, which claims the benefit of U.S. Provisional Application No. 61/600,141, filed Feb. 17, 2012, which applications are incorporated herein by reference. U.S. patent application Ser. No. 14/210,331 further claims the benefit of U.S. Provisional Application No. 61/790,259, filed Mar. 15, 2013, which application is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates in general to a human assistance device, and more particularly, to a control system and method for non-gait ankle and foot motion in the human assistance device. 
     BACKGROUND OF THE INVENTION 
     Prosthetic and orthotic devices help restore mobility to people who lack able-bodied motion. Prosthetic devices are intended to replace the appearance of a missing limb or portion of a limb and can return mobility to the wearer or user. Orthotic devices are intended to support or supplement an existing limb, by assisting with movement, reducing weight-bearing loads on the body, reducing pain, and increasing endurance. Prosthetic and orthotic devices are available to replace or support various portions of the body. Lower limb prosthetic devices include a prosthetic foot, foot-ankle prosthesis, prosthetic knee joint, and prosthetic hip joint. Lower limb orthotic devices include a foot orthoses, ankle-foot orthoses, knee-ankle-foot orthoses, and knee orthoses. People who require a lower limb prosthesis or orthosis often expend more metabolic power to walk or move at the same speed as able-bodied individuals. 
     Human locomotion, such as walking and running, is commonly described in terms of gait. Gait is a cyclical pattern of leg and foot movement that creates locomotion. A gait cycle is defined for a single leg and begins with the initial contact of the foot with the ground or heel strike. The conclusion of a gait cycle occurs when the same foot makes a second heel strike. The gait cycle can be divided into two phases, stance phase and swing phase. Stance phase begins with heel strike and ends when the toe of the same foot leaves the ground. Swing phase begins when the foot leaves contact with the ground and ends with the heel strike of the same foot. One goal of lower limb prosthetic and orthotic devices is to help the user achieve a normal gait, while reducing energy expended by the user. 
     Most if not all control systems for prosthetic and orthotic devices have focused on gait and other cyclical patterns of motion. Yet, humans spend a considerable portion of the day involved in non-gait activities, while wearing the prosthetic or orthotic device. For example, the person may slide foot position or cross legs while sitting in a chair, or change balance point while leaning against a bar or podium, or shift stance while standing in a social gathering. The person may be engaged in random, complex, non-cyclic activities, such as dancing, exercise routines, or sporting activities, while wearing the prosthetic or orthotic device. The person may be wearing long pants or long dress that covers the prosthetic or orthotic device. In any case, the person likely prefers the non-gait activity while wearing the prosthetic or orthotic device to appear as natural as possible, without indicating, revealing, or otherwise drawing attention to the presence of the prosthetic or orthotic device. The non-gait activity should appear as biological motion, without an artificial or mechanical appearance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a user wearing an active lower limb prosthesis; 
         FIG. 2  illustrates a coordinate system for measuring motion in the active lower limb prosthesis; 
         FIG. 3  illustrates further detail of the active lower limb prosthesis; 
         FIG. 4  illustrates a non-gait activity of shifting foot position with the active lower limb prosthesis; 
         FIG. 5  illustrates a block diagram of a control system for controlling non-gait activity with a human assistance device; 
         FIG. 6  illustrates a block diagram of a control system for controlling a non-gait activity of a lower limb prosthesis; 
         FIG. 7  illustrates a control surface of ankle angle and nut position with corresponding ankle moment; 
         FIG. 8  illustrates a plot of ankle moment versus nut position for zero ankle moment; and 
         FIGS. 9 a -9 b    illustrate a non-gait activity of shifting foot position while sitting. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention is described in one or more embodiments in the following description with reference to the figures, in which like numerals represent the same or similar elements. While the invention is described in terms of the best mode for achieving the invention&#39;s objectives, those skilled in the art will appreciate that the description is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims and their equivalents as supported by the following disclosure and drawings. 
       FIG. 1  shows an example of user  10  wearing an active lower limb prosthesis  12 , which includes a control system for controlling the operation of the prosthesis. Lower limb prosthesis  12  is an active prosthetic device or wearable robotic device, including active and passive components. In one embodiment, lower limb prosthesis  12  is a below-the-knee prosthesis, also known as a foot-ankle prosthesis. In another embodiment, lower limb prosthesis  12  includes a robotic or prosthetic joint, such as an ankle joint or knee joint. 
     Lower limb prosthesis  12  includes an ankle prosthesis  14 , shank portion  16 , and foot portion  18 . Ankle prosthesis  14  includes active components, such as one or more actuators, controlled by a computer system or microcontroller with local electronic memory. A sensor or sensor system  20  is worn by user  10 . In one embodiment, sensor  20  is worn on thigh  22 , tibia  24 , or other part of user  10 . In another embodiment, sensor  20  is disposed on ankle prosthesis  14 , shank portion  16 , or foot portion  18 . In yet another embodiment, a plurality of sensors  20  is disposed on user  10  and/or lower limb prosthesis  12 . Sensor  20  detects a kinematic state, loading state, or kinematic state and loading state of user  10 . Measurements from sensor  20  are used by the control system to control ankle prosthesis  14  and lower limb prosthesis  12 . 
       FIG. 2  shows a coordinate system for measuring non-gait activities involving ankle prosthesis  14  and lower limb prosthesis  12 . Sensor  20  measures a kinematic state or loading state of a mobile body of user  10 . A mobile body includes a limb segment or robotic segment. In one embodiment, the mobile body is ankle prosthesis  14 , shank portion  16 , foot portion  18 , or tibia  24 . A kinematic state includes an angular position, linear position, linear velocity, angular velocity, linear acceleration, or angular acceleration with an associated frame of reference to the mobile body. A loading state includes a moment or force on the mobile body. 
     Sensors  20  are configured to measure kinematic state, such as velocities, accelerations, angular positions, and linear positions in coordinate frames, as oriented with the limb segment or robotic segment. A limb segment includes thigh  22  or tibia  24  of user  10 . A robotic segment includes ankle prosthesis  14 , shank portion  16 , or foot portion  18  of lower limb prosthesis  12 . Sensor  20  determines the kinematic state of user  10  in linear coordinates, polar coordinates, or a combination of coordinate systems. The coordinate frames have three orthogonal axes: a sagittal axis (θ s , X s ), coronal axis (θ c , X c ), and transverse axis (θ T , X T ). The sagittal direction  30  is in the direction of sagittal axis (θ s , X s ) normal to the sagittal plane of the mobile body. The coronal direction  32  is in the direction of coronal axis (θ c , X c ) normal to the coronal plane of the mobile body. The transverse direction  34  is in the direction of transverse axis (θ T , X T ) normal to the transverse plane of the mobile body. Each sensor  20  is oriented so that the axis of measurement can be expressed as a linear combination of three unit vectors in the direction of the sagittal axis (θ s , X s ), coronal axis (θ c , X c ), and transverse axis (θ T , X T ). 
       FIG. 3  shows further detail of lower limb prosthesis  12 , including ankle prosthesis  14 , shank portion  16 , and foot portion  18 . Shank portion  16  of lower limb prosthesis  12  includes a socket or couples to a socket, which fits onto a residual limb of user  10 , such as tibia  24 . Ankle prosthesis  14  is coupled to shank portion  16 , and foot portion  18  is coupled to ankle prosthesis  14 . Ankle prosthesis  14  includes one or more active members or actuators  40 , such as a motor, and may include one or more compliant members, such as a spring or beam, disposed within housing  44  of ankle prosthesis  14 . A control system or controller  50  is disposed within housing  44  of ankle prosthesis  14  and coupled to actuator  40 . Control system  50  responds to an input from sensor  20  and outputs a reference command to control actuator  40  in non-gait movement of ankle prosthesis  14 . 
       FIG. 4  shows user  10  in the seated position on chair or bench  70  with a shifting motion of lower limb prosthesis  12  in a backward direction under the chair with the heel of foot portion  18  rises from the floor by height Hi, while the ball of foot portion  18  remains in contact with ground or floor  72 . The motion lower limb prosthesis  12  in  FIG. 4  is an exemplary non-gait activity and should be a natural, biological motion, without an artificial or mechanical appearance. To measure and control the non-gait foot shifting motion, sensor  20  includes an accelerometer, rate gyro, potentiometer, inclinometer, or other sensor to measure velocity, acceleration, angular position, linear position, or a combination thereof. In one embodiment, sensor  20  determines velocity, acceleration, angular position, or linear position of ankle prosthesis  14  with respect to the sagittal axis (θ s , X s ), coronal axis (θ c , X c ), and transverse axis (θ T , X T ). The kinematic state measurements from sensors  20  are used as inputs for control system  50 . Measurements from sensors  20  are ultimately used to control an actuator of ankle prosthesis  14 , lower limb prosthesis  12 , or other wearable robotic device. 
       FIG. 5  shows a block diagram of a method for controlling non-gait activities with a human assistance device using control system  50 . Control system  50  includes a computer system or microcontroller with local electronic memory to store and process data from sensor  20  and generate a reference command output signal to control actuator  40 . The method for controlling non-gait activity for lower limb prosthetic  12 , or other prosthetic, orthotic, and robotic devices, collectively referred to as a human assistance device, and includes a series of operations performed on kinematic or loading data. The operations performed by control system  50  relate kinematic motion to a requisite output of actuator  40  or other control function of lower limb prosthetic  12 . 
     The method for controlling non-gait activity for lower limb prosthetic  12 , or other prosthetic, orthotic, and robotic devices, using control system  50  involves one or more mobile bodies  80  under a physical condition of one or more kinematic states  82 , loading states  84 , or combination of kinematic states  82  and loading states  84 . Sensing block  86  detects or measures kinematic states  82  and/or loading states  84  of mobile body  80 . In particular, sensor  20  detects or measures one or more kinematic states  82 , loading states  84 , or combination of kinematic states  82  and loading states  84  of one or more mobile bodies  80 . Kinematic state  82  and loading state  84  comprise physical states of mobile body  80 . The output of sensing block  86  is a sensed state measurement representing kinematic states  82  and loading states  84  sensed by sensor  20 . 
     In conversion block  88 , the sensed state measurement is converted in control system  50  to a unit of measurement compatible with reference command block  94 . Conversion block  88  converts the sensor output, e.g. voltage or digital measurement, to a coordinate system compatible with reference command block  94 , e.g. radians, radians per second, or G-force. The output of conversion block  88  is the physical state measurement. 
     In conditioning block  90 , the state measurements are conditioned in control system  50  by various numeric processing operations, such as Kalman filtering, transfer function, integration, differentiation, and amplification. Conditioning block  90  can use any combination and order of the conditioning operations on the state measurements and repeated as necessary. In one embodiment, conditioning block  90  includes amplification, attenuation, or gain of any nonzero number, including unity gain, of the state measurements. Filtering is employed for multiple uses including noise reduction in the state measurements. For example, conditioning block  90  may implement a low pass filter. Other conditioning operations can use interpolation and substitution to reduce inaccuracies in the state measurements, and adjustment and alteration of the state measurements. Alteration of the state measurements is performed in a manner similar to integration or differentiation to reduce drift in numerical integration or noise in numerical differentiation. The output of conditioning block  90  is the conditioned state measurements. 
     In transformation block  92 , the conditioned state measurements are transformed in control system  50  to change coordinate system using isometric or non-isometric transformations. The types of transformations for changing coordinate systems include rotations and dilations. Other types of transformations include identity transformations, orthogonal projections, oblique projections, changes to other coordinate systems, and changes of scale. In addition, other coordinate systems include polar coordinate systems, barycentric coordinate systems, and similar types of coordinate systems. Changes of scale include log scale or any other function of scale. Moreover, the transformations may include any transformation as a mathematical function of the conditioned state measurements, or any combination in any order of transformations, projections, changes of coordinate system, changes of scale, or other mathematical function. The output of transformation block  92  is the transformed state measurements. 
     The transformed state measurement coordinate system may have the same number or a different number of dimensions as the conditioned state measurement coordinate system. In fact, there may be more or less transformed state measurements than conditioned state measurements. In one embodiment, transformation block  92  transforms state measurements to time independent data for the reference function, e.g. creating phase plane, and surfaces defining possible positions of angular velocity. In another embodiment, transformation block  92  convert time dependent measurements, e.g. angular velocity over time, to time independent measurements, a non-temporal based phase angle and polar radius in a phase plot or polar plot. 
     In an alternative embodiment, transforming block  92  is performed prior to conditioning block  90 . In this case, the state measurements are transformed in transformation block  92  of control system  50  to yield the transformed state measurements. The transformed state measurements are conditioned in conditioning bock  90  of control system  50  to yield the conditioned state measurements. In either embodiment, conditioning block  90  prior to transformation block  92 , or transformation block  92  prior to conditioning block  90 , the result is conditioned and transformed state measurements. 
     In calculate reference command block  94 , the transformed state measurements (or conditioned state measurements) are used as arguments in one or more reference command functions to calculate reference commands. The reference function is represented with a function that accepts inputs and that outputs a unique value for each combination of inputs. The reference function includes look up tables, mathematical functions, or combinations of tables and mathematical functions, or other suitable method stored in the electronic memory and executed by the computer system or microprocessor. 
     In one embodiment, the reference function is determined by recording data from similar non-gait activities in an able-bodied individual. One or more sensors, similar to sensors  20 , are coupled to an able-bodied test subject to detect physical states, such as kinematic or loading states, of biological activities. For example, the able-bodied test subject sits in a chair, similar to  FIG. 4 , and shifts position of his/her foot in a backward direction under the chair while remaining contact with the ground. The sensors on the able-bodied test subject monitor kinematic states and loading states, as well as sagittal axis, coronal axis, and transverse axis of motion, of the biological non-gait activity associated with shifting the foot in continuous contact with the ground while sitting. After sensing the physical states of the able-bodied test subject, the non-gait data is processed and used for the reference function. Able-bodied non-gait data can be processed using a series of operations, such as conversion block  88 , conditioning block  90 , and transformation block  92  to produce the reference function of natural, biological non-gait activity. The reference function is produced to match data from one or more non-gait activities, such as shifting position of lower limb prosthesis  12  while sitting, standing, or leaning. Other non-gait activities include random, complex, non-cyclic motions, such as dancing, exercise routines, sporting activities, or other similar activities. In another embodiment, the transformed state measurements are combined with a recording or a calculation of a desired reference command to achieve a natural, biological motion, without an artificial or mechanical appearance. The output of calculate reference command block  94  is the reference command. 
     In control block  96 , the reference command produced by reference command block  94  controls operation of lower limb prosthesis  12 , e.g. motion of actuator  40 . Control system  50  is a continuous function relating the operation of lower limb prosthesis  12  to a measured signal. The continuous nature of control system  50  eliminates decision making by the system, if-then logic, and changes in state. An invariant signal, such as tibia angle, is used to control the non-gait activity for the prosthetic, orthotic, or robotic device. By measuring kinematic or leading states, control system  50  adapts to changes in the non-gait activity. Control system  50  continuously calculates an output, rather than waiting on a non-gait event to trigger an output. The measured signal is phase locked to the user&#39;s non-gait motion, and thus, the output of control system  50  is phase locked to the user&#39;s non-gait motion rather than being time based. Because control system  50  is not time-based, control system  50  better adapts to changes in non-gait activity. 
       FIG. 6  shows a block diagram showing a control system  98  implemented in a computer system or microcontroller with local electronic memory to control non-gait activities of ankle prosthesis  14 . In particular, control system  98  controls foot positioning of ankle prosthesis  14 . A plurality of sensors, e.g. rate gyro and accelerometers, is disposed on mobile body  100 , e.g. residual tibia  68  of user  10  or ankle prosthesis  14 , to measure velocity, acceleration, angular position, or linear position with respect to the sagittal axis (θ s , X s ), coronal axis (θ c , X c ), and transverse axis (θ T , X T ). Rate gyro  102  measures an angular velocity θ s  as a kinematic state of residual tibia  68  of user  10  or ankle prosthesis  14  in sagittal direction  30 . 
     Accelerometer  102  measures acceleration {umlaut over (X)} as a kinematic state of residual tibia  68  of user  10  or ankle prosthesis  14  in coronal direction  32 . Accelerometer  104  measures acceleration Ÿ as a kinematic state of ankle prosthesis  14  in transverse direction  34 . Acceleration {umlaut over (X)} and acceleration Ÿ represent 2D acceleration of residual tibia  68  of user  10  or ankle prosthesis  14 . 
     Rate gyro  102 , accelerometers  104  and  106 , and ankle moment  130  correspond to sensing block  86  providing the sensed states in  FIG. 5 . The sensed states are converted to a unit of measurement compatible with reference command block  116 . Conversion block  108  converts the sensor outputs of rate gyro  102  and accelerometers  106  and  108  to a coordinate system compatible with reference command block  116 , e.g. digital measurement to radians or radians per second. 
     The acceleration {umlaut over (X)} and acceleration Ÿ are input arguments to ATAN2 block  110 . ATAN2 block  110  implements an arctangent function with two arguments and determines angle and magnitude. ATAN2 block  110  is implemented in the computer system or microcontroller with local electronic memory and determines the appropriate quadrant of the angle in radians between π and −π based on the signs of the input arguments. ATAN2 block  100  provides output angle β in response to acceleration Ÿ and acceleration Ÿ. Conversion block  108  may convert the output angle β of ATAN2 block  110  to a coordinate system compatible with reference command block  116 . 
     The output θ s  of rate gyro  102  and output angle β of ATAN2 block  110  is coupled to inputs of filter  112 . In one embodiment, filter  112  is implemented as a low pass filter in the computer system or microcontroller with local electronic memory according to equation (1):
 
θ T   =A   1 *(θ PREV +{dot over (θ)} s   *Δt )+(1− A   1 )*β  (1)
 
     where: θ T  is tibia angle
         A 1  is a calibration coefficient   θ PREV  is previous θ T      Δt is rate of time change       

     Filter  112  operates to remove sensor noise and combines the calibration coefficient A 1  weighted output {dot over (θ)} s  of rate gyro  102 , θ PREV , and angle β of ATAN2 block  110 . The output of filter  112  is the tibia angle θ T . The calibration coefficient A 1  can be a value between zero and one, typically close to one. In one embodiment, calibration coefficient A 1  is 0.995. When tibia  68  is moving quickly, output {dot over (θ)} s  of rate gyro  102  dominates tibia angle θ T . When tibia  68  is moving slowly, output angle β of ATAN2 block  110  dominates tibia angle θ T  to reduce drift. Filter  112  corresponds to conditioning block  90  providing the conditioned state measurements in  FIG. 5 . 
     Tibia angle θ T  is input to the reference function in reference command block  116 . Reference command block  116  is implemented in the computer system or microcontroller with local electronic memory and is represented as a continuous 3D control surface  118  in  FIG. 7  relating ankle angle and nut position on the x,y axis to ankle moment on the z-axis. Control surface  118  is a continuous function of ankle angle θ A  and nut position NP 0  with corresponding ankle moments. Nut position generally refers to an amount of extension or absolute position of actuator  40 . Nut position NP 0  is read from actuator  40  as the current vertical extension of the actuator. No extension of actuator  40  corresponds to zero nut position; maximum extension of the actuator is max nut position. In one embodiment, control surface  118  is determined by recording data from similar non-gait activities in an able-bodied individual. 
     For the scenario of user  10  seated in chair  70  in relaxed mode, ankle moment is zero and ankle angle θ A  for control surface  118  is made equal to tibia angle θ T . In particular, line  120  through control surface  118  in  FIG. 7  represents relaxed mode of ankle prosthesis  14  with zero ankle moment, shown as a 2D graph in  FIG. 8 . For a given ankle angle θ A , line  120  shows the corresponding nut position NP 1  with zero ankle moment. Given tibia angle θ T  from filter  112  and present nut position NP 0  from actuator  40 , with ankle angle θ A  made equal to tibia angle θ T , new nut position NP 1  is determined from line  120  of control surface  118  in reference command block  116 . 
     Consider the non-gait motion of ankle prosthesis  14  from  FIG. 4 . User  10  is seated on chair or bench  70 . Assume the left residual tibia  68  and ankle prosthesis  14  begin 90° with respect to the thigh of user  10 , i.e. foot portion  18  flat on ground or floor  72  directly under the knee, see  FIG. 9 a   . The position of ankle prosthesis  14  in  FIG. 9 a    is zero ankle angle and zero ankle moment, also referred to as relaxed mode with no loading on ankle prosthesis  14 . In relaxed mode, there is no loading or compression of the spring or extension of actuator  40  in ankle prosthesis  14 . User  10  decides to move tibia  68  to position ankle prosthesis  14  under chair  70 . The non-gait motion should a natural, biological motion, without an artificial or mechanical appearance. The muscles of user  10  act to move the left residual tibia  68 , and accordingly ankle prosthesis  14 , backward in a direction under chair  70  with an angular velocity {dot over (θ)} s  in sagittal direction  30 . 
     For an able-bodied person, the natural, biological motion in moving the tibia from zero ankle angle to position the foot under chair  70  involves sliding the biological foot backward across floor  72 . As the biological foot moves backward in the direction under chair  70 , the heel naturally rises off floor  72 , while the ball of the biological foot maintains contact with the floor. 
     In a similar manner, rate gyro  102  measures angular velocity {dot over (θ)} s  of residual tibia  68  or ankle prosthesis  14  in sagittal direction  30 . At the same time, accelerometer  102  measures acceleration {umlaut over (X)} of residual tibia  68  or ankle prosthesis  14  in coronal direction  32 , and accelerometer  104  measures acceleration Ÿ of residual tibia  68  or ankle prosthesis  14  in transverse direction  34  as an acceleration of residual tibia  68 . Acceleration {umlaut over (X)} and acceleration Ÿ are processed through ATAN2 block  110  to provide output angle β. Angular velocity {dot over (θ)} s  and angle β are processed through filter  112  to provide tibia angle θ T  during the slide of ankle prosthesis  14  across floor  72 . The movement of the left residual tibia  68  to slide ankle prosthesis  14  under chair  70  increases tibia angle θ T . Given that present ankle angle θ A  is made equal to tibia angle θ T  for zero ankle moment, reference command block  116  converts the increasing ankle angle θ A  and present nut position NP 0  to new nut position NP 1 , where NP 1  is greater than NP 0  due to the increasing tibia angle θ T  and ankle angle θ A  as per line  120  of control surface  118 . Summation block  134  has inputs NP 1  and NP 2  and provides output NP 3 =NP 1 +NP 2  to control the extension of actuator  40  in ankle prosthesis  14 . Nut position NP 2  is substantially zero while user  10  is seated in chair  70 , i.e. no-load in relaxed mode with zero ankle moment. Nut position NP 3  is approximately equal to the new nut position NP 1  to extend the length of actuator  40 . During the motion of positioning ankle prosthesis  14  under chair  70 , the extension of actuator  40  in response to NP 3 =NP 1 +NP 2 , where NP 2 =0, causes the heel of foot portion  18  of ankle prosthesis  14  to rise off ground  72 , while the ball of foot portion  18  remains in contact with the ground. As user  10  continues the slide of ankle prosthesis  14 , tibia angle θ T  and corresponding ankle angle θ A  continue to increase and NP 3  continues to increase as well from line  120  of control surface  118 , as shown in  FIG. 9 b   . Accordingly, reference command block  116  maps increasing tibia angle θ T  to increasing nut position NP 1 , and corresponding increasing NP 3  with NP 2 =0, to control actuator  40  to lift the heel of foot portion  18  off ground  72 , while the ball of foot portion  18  maintains contact with the ground. That is, reference command block  116  causes actuator  40  to dorsi flex ankle prosthesis  14  by same amount as the increasing tibia angle θ T  during the foot slide. The movement of ankle prosthesis  14  under chair  70 , in response to control system  98 , is a natural, biological motion involving a heel lift of foot portion  18  while the ball of foot portion  18  maintains contact with the ground, without an artificial or mechanical appearance. 
     Now consider the reverse non-gait motion of bringing ankle prosthesis  14  from the position of  FIG. 9 b    to the position of  9   a . User  10  decides to move tibia  68  to position ankle prosthesis  14  from a position under chair  70  to zero ankle angle, i.e. foot portion  18  flat on ground or floor  72  directly under the knee. Again, the non-gait motion should be a natural, biological motion, without an artificial or mechanical appearance. The muscles of user  10  act to move the left residual tibia  68 , and accordingly ankle prosthesis  14 , forward from under chair  70  with an angular velocity {dot over (θ)} s  in sagittal direction  30 , opposite the previous example of sliding ankle prosthesis  14  backward under chair  70 . 
     For an able-bodied person, the natural, biological motion in moving the tibia from a position of the foot under chair  70  to a position directly under the knee involves sliding the biological foot forward across floor  72 . As the biological foot moves forward, the heel naturally returns to rest on floor  72 . 
     In a similar manner, rate gyro  102  measures angular velocity {dot over (θ)} s  of residual tibia  68  or ankle prosthesis  14  in sagittal direction  30 . At the same time, accelerometer  102  measures acceleration {umlaut over (X)} of residual tibia  68  or ankle prosthesis  14  in coronal direction  32 , and accelerometer  104  measures acceleration Ÿ of residual tibia  68  or ankle prosthesis  14  in transverse direction  34  as an acceleration of residual tibia  68 . Acceleration {umlaut over (X)} and acceleration Ÿ are processed through ATAN2 block  110  to provide output angle β. Angular velocity {dot over (θ)} s  and angle β are processed through filter  112  to provide tibia angle θ T  during the slide of ankle prosthesis  14  across floor  72 . The movement of the left residual tibia  68  to slide ankle prosthesis  14  from under chair  70  decreases tibia angle θ T . Given that present ankle angle θ A  is made equal to tibia angle θ T  for zero ankle moment, reference command block  116  converts the decreasing ankle angle θ A  and present nut position NP 0  to new nut position NP 1 , where NP 1  is less than NP 0  due to the decreasing tibia angle θ T  and ankle angle θ A  as per line  120  of control surface  118 . Summation block  134  has inputs NP 1  and NP 2  and provides output NP 3 =NP 1 +NP 2  to control the extension of actuator  40  in ankle prosthesis  14 . Nut position NP 2  is substantially zero while user  10  is seated in chair  70 , i.e. no-load with zero ankle moment. Nut position NP 3  is approximately equal to the new nut position NP 1  to reduce the length of actuator  40 . During the motion of positioning ankle prosthesis  14  to a position under the knee, the reduction in extension of actuator  40  in response to NP 3 =NP 1 +NP 2 , where NP 2 =0, causes the heel of foot portion  18  of ankle prosthesis  14  to return to ground  72 . As user  10  continues the slide of ankle prosthesis  14 , tibia angle θ T  and corresponding ankle angle θ A  continue to decrease and NP 3  continues to decrease as well from line  120  of control surface  118 , as shown in  FIG. 9 a   . Accordingly, reference command block  116  maps decreasing tibia angle θ T  to decreasing nut position NP 1 , and corresponding decreasing NP 3  with NP 2 =0, to control actuator  40  to lower the heel of foot portion  18  to ground  72 . That is, reference command block  116  causes actuator  40  to relax ankle prosthesis  14  by same amount as the decreasing tibia angle θ T  during the forward foot slide. The movement of ankle prosthesis  14  from under chair  70  to zero ankle angle, in response to control system  98 , is a natural, biological motion involving lowering a heel lift of foot portion  18  to the ground, without an artificial or mechanical appearance. 
     Now consider the scenario where user  10  decides to stand up from the seated position. Assume user  10  is sitting in chair  70 , but has returned ankle prosthesis  14  to zero ankle angle, i.e. foot portion  18  is flat on ground or floor  72  directly under the knee as in  FIG. 9 a   . The standing action imposes a load on ankle prosthesis  14 . Returning to  FIG. 6 , ankle moment block  130  senses and measures moment from loading from the ankle angle and nut position, see  FIG. 7 . The output M A  of ankle moment block  130  is routed to gain block  132 . In one embodiment, gain block  132  is implemented in the computer system or microcontroller with local electronic memory according to equation (2): 
     
       
         
           
             
               
                 
                   
                     NP 
                     2 
                   
                   = 
                   
                     
                       
                         A 
                         2 
                       
                       ⁢ 
                       
                         cos 
                         ⁡ 
                         
                           ( 
                           
                             
                               
                                 M 
                                 A 
                               
                               
                                 M 
                                 T 
                               
                             
                             * 
                             π 
                           
                           ) 
                         
                       
                     
                     - 
                     
                       A 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where: NP 2  is nut position after gain  132 
         A 2  is a calibration coefficient   M T  is maximum ankle moment       

     The cos( ) function is unitless and calibration coefficient A 2  has units of length, e.g. centimeters or millimeters. The calibration coefficient A 2  corresponds to one-half the maximum extension of actuator  40 . In one embodiment, calibration coefficient A 2  is value 10. When ankle prosthesis  14  is loaded, e.g. by standing from a seated position, gain  132  provides nut position NP 2  to summation block  134 , i.e. NP 3 =NP 1 +NP 2 . The output NP 3  of summation block  134  controls the extension of actuator  40  in ankle prosthesis  14 . Nut position NP 2  is now a value greater than zero due to the loading on ankle prosthesis  14 , while nut position NP 1  is substantially zero with zero ankle angle θ A . Accordingly, nut position NP 2  causes an extension of actuator  40 , NP 3 =NP 1 +NP 2 , where NP 1 =0, to assist user  10  out of chair  70  upon loading of ankle prosthesis  14 . The standing motion of user  10  from chair  70 , in response to control system  98 , is a natural, biological motion, without an artificial or mechanical appearance. 
     Control system  50  can be applied to other non-gait activity, such as shifting position of ankle prosthesis  14  while standing or leaning, as well as other random, complex, non-cyclic motions, such as dancing, exercise routines, sporting activities, random play with children, or other similar activities. Assume a non-gait sporting activity involving a forward motion, followed by a sudden stop and change of lateral direction. Sensing block  86  implements sensing kinematic states  82  and/or loading states  84  of mobile body  80  associated with the non-gait activity. Sensors  20  detect or measure one or more kinematic states  82 , loading states  84 , or combination of kinematic states  82  and loading states  84  of one or more mobile bodies  80  for the specific non-gait sporting activity. Conversion block  88  converts sensor data to a unit of measurement compatible with reference command block  94 . Conditioning block  90  performs signal processing, to accentuate a relevant portion of the state measurements or provide sensor noise reduction. Transformation block  92  transforms the conditioned state measurements to be compatible with reference command block  94 . For example, transformation block  92  transforms the time dependent conditioned state measurements to time independent transformed state measurements. 
     Reference command block  94  implements a control surface, similar to  FIG. 7 , that maps attributes of the non-gait activity to control of actuator  40 . In the example of a sudden stop and change of lateral direction, the relevant attributes for the control surface may be lateral velocity and lateral position. The output of reference command block  94  controls ankle prosthesis  14  to respond in a natural, biological motion for the sudden stop and change of lateral direction, without an artificial or mechanical appearance. In the case of complex, multi-dimensional, non-gait activities, reference command block  94  may have a library of control surfaces each optimized to a particular phase of the overall activity. Control system  50  switches between the various implementations and control surfaces in response to sensor input for the particular phase of the overall non-gait activity. 
     While one or more embodiments of the present invention have been illustrated in detail, the skilled artisan will appreciate that modifications and adaptations to those embodiments may be made without departing from the scope of the present invention as set forth in the following claims.