Method of swing assist control

Abstract

A knee prosthesis includes a shank link, a thigh link rotatably coupled to the shank link and at least one powered control element configured to apply a torque between the thigh link and the shank link. The shank motion is described by at least a shank angular velocity relative to an inertial reference frame. The knee prosthesis further comprises at least one sensor, and a controller. The controller is configured to: receive sensor measurements from the at least one sensor and determine a present state of a plurality of states including at least a swing state and a stance state; and provide torque control for the at least one powered control element if the present state is a swing state. The knee prosthesis measures the shank angular velocity. The torque control is a function of at least the shank angular velocity.

Description

METHOD OF SWING ASSIST CONTROLCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of and priority to U.S. Provisional Application No. 63 / 619,817, filed January 11, 2024, which is hereby incorporated by reference herein in its entirety.FIELD OF THE INVENTION

[0002] This application describes a control method to assist the swing-phase movement of a motorized prosthetic knee joint.BACKGROUND OF THE INVENTION

[0003] As prostheses and orthoses become increasingly electronically-controlled, they have an increasingly wide range of functional capabilities, and in particular are capable of adapting between multiple discrete activities, such as level walking, sloped walking, and stair ascent or descent.SUMMARY

[0004] According to one aspect of the present disclosure, a knee prosthesis comprises a shank link, a thigh link rotatably coupled to the shank link, and at least one powered control element configured to apply a torque between the thigh link and the shank link. A shank motion is described by at least a shank angular velocity relative to an inertial reference frame. The knee prosthesis further comprises at least one sensor, and a controller. The controller is configured to: receive sensor measurements from the at least one sensor and determine a present state of a plurality of states comprising at least a swing state and a stance state; and provide a torque control law for the at least one powered control element if the present state is a swing state. The knee prosthesis measures the shank angular velocity. The torque control is a function of at least the shank angular velocity.

[0005] According to a configuration of the above implementation, the torque control is an unstable function of the shank angular velocity such that the torque control drives the shank away from a given equilibrium shank angular velocity.

[0006] According to a further configuration of the above implementation, the unstable torque control is proportional to the negative shank angular velocity.

[0007] In a further aspect of the above implementation, the unstable torque control is proportional to the negative of the square of the shank angular velocity.

[0008] In yet a further aspect of the above implementation, the shank motion is further described by a shank angle relative to an inertial reference frame. The knee prosthesis further measures the shank angle and the torque control is an unstable function further including the shank angle.

[0009] According to a configuration of the above implementation, the torque control is in negative proportion to the difference between the shank angle and a shank equilibrium angle.

[0010] According to a configuration of the above implementation, the torque control is in negative proportion to the sine of the shank angle.

[0011] In yet a further aspect of the above implementation, a knee prosthesis comprises a shank link, a thigh link rotatably coupled to the shank link and at least one powered control element configured to apply a torque between the thigh link and the shank link. A relative motion is between the thigh link and shank link is described by at least a knee angular velocity. The knee prosthesis further comprises at least one sensor, and a controller. The controller is configured to: receive sensor measurements from the at least one sensor and determine a present state of a plurality of states comprising at least a swing state and a stance state; and provide a torque control for the at least one powered control element if the present state is a swing state. The knee prosthesis measures at least the knee angular velocity. The torque control is an unstable function of at least the knee angular velocity such that the torque control drives the knee away from a given equilibrium knee angular velocity.

[0012] According to another aspect of the present disclosure, the unstable torque control is proportional to the negative knee angular velocity.

[0013] According to a configuration of the above implementation, the unstable torque control is proportional to the negative of the square of the knee angular velocity.

[0014] According to a further configuration of the above implementation, the relative motion between the thigh link and shank link is further described by a knee angle. The knee prosthesis further measures the knee angle and the torque control is an unstable function further including the knee angle.

[0015] In a further aspect of the above implementation, the torque control is in negative proportion to the difference between the knee angle and a knee equilibrium angle.

[0016] According to a further aspect of the present disclosure, a knee prosthesis comprises a shank link, a thigh link rotatably coupled to the shank link, and at least one torque control element configured to apply a torque between the thigh link and the shank link. A shank motion is described by at least a shank angle relative to an inertial reference frame. The knee prosthesis further comprises at least one sensor, and a controller. The controller is configured to: receive sensor measurements from the at least one sensor and determine a present state of a plurality of states comprising at least a swing state and a stance state; and provide a torque control for at least one powered control element if the present state is a swing state. The knee prosthesis measures at least shank angle. The torque control is an unstable function of at least the shank angle. The torque control is in negative proportion to the difference between the shank angle and a shank equilibrium angle.

[0017] According to a further aspect of the present disclosure, a knee prosthesis comprises a shank link, a thigh link rotatably coupled to the shank link, and at least one torque control element configured to apply a torque between the thigh link and the shank link. The relative angle between the thigh link and shank link is described by at least a knee angle. The knee prosthesis further comprises at least one sensor, and a controller. The controller is configured to: receive sensor measurements from the at least one sensor and determine a present state of a plurality of states comprising at least a swing state and a stance state; and provide a torque control for at least one powered control element if the present state is a swing state. The knee prosthesis measures at least the knee angle. The torque control is an unstable function of at least the knee angle.

[0018] According to a configuration of the above implementation, the torque control is in negative proportion to the difference between the knee angle and a knee equilibrium angle.

[0019] The above summary is not intended to represent each embodiment or every aspect of the present invention. Additional features and benefits of the present invention are apparent from the detailed description and figures set forth below.DESCRIPTION OF THE DRAWINGS

[0020] Other advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:

[0021] FIG. 1A is a diagram of a prosthetic leg attached to a user’s thigh in one position according to one embodiment, defining links, angles, and accelerations.

[0022] FIG. IB is a diagram of a prosthetic leg attached to a user’s thigh, defining inertial and geometric parameters and knee torque direction.

[0023] FIG. 2 is a block diagram of an equation showing dynamics of a passive prosthesis.

[0024] FIG. 3 is a block diagram of an equation directed to extended passive dynamics of a passive prosthesis extended to a thigh angle input.

[0025] FIG. 4 is a block diagram of an equation showing dynamics of a passive prosthesis illustrating separate knee and shank angles within the knee / shank passive dynamics.

[0026] FIG. 5 is a block diagram of an equation representation of a passive thigh / knee / shank dynamics.

[0027] FIG. 6 is a block diagram of a simplified representation of a passive thigh / knee / shank dynamics.

[0028] FIG. 7 is a block diagram of a motorized prosthesis showing two distinct exogenous control inputs including one from the user (user source of motion) and another from the prosthesis (the prosthesis source of motion).

[0029] FIG. 8 is a block diagram of a motorized prosthesis dynamics that entails a single user- controlled exogenous input according to one embodiment.

[0030] FIG. 9 is a block diagram of a motorized prosthesis dynamics that entails a single user- controlled exogenous input according to another embodiment.

[0031] FIG. 10 is a block diagram of a simplified motorized prosthesis systems of FIGS. 8 and 9.

[0032] FIG. 11 is a block diagram of a simplified motorized prosthesis system of FIG. 9.

[0033] FIG. 12 is a motor torque and shank or knee angle velocity plane.

[0034] While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that it is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.DETAILED DESCRIPTION

[0035] This application describes a control method to assist the swing-phase movement of a motorized prosthetic knee joint.

[0036] This application describes a control method to assist the swing-phase movement of a motorized prosthetic knee joint. A knee prosthesis for individuals with transfemoral amputation provides two primary functions: (1) support the weight of a user (i.e., resist knee yielding) during the stance phase of walking, and (2) provide swing motion at the knee during the swing phase of walking. A conventional knee prosthesis provides both of these functions by resisting movement; typically a high level of resistance during the stance phase of walking, and a much lower level of resistance during swing phase. Although the function of the knee during swing phase is to provide appropriate knee movement (to pick up the foot and swing it forward), conventional knee prostheses conventionally do not internally generate movement or otherwise provide power for movement. Instead, swing-phase movement of the knee results indirectly from movement of the user’s (intact) hip joint, which drives the thigh segment, which is inertially coupled to the artificial shank segment. Specifically, the user generates an initial forward acceleration and subsequent deceleration of the thigh segment, which in turn generates an initial flexion and subsequent extension of the prosthetic knee joint via inertial coupling between the thigh and shank segments. Such knee movement, which results from (and is powered by) the user’s thigh movement, is referred to in this application as the user source of knee (angular) movement. It should be noted that this mechanism of knee movement is also the primary mechanism of swing-phase knee movement in healthy walking. As such, this mechanism is also referred to herein as the natural mechanism of knee movement.

[0037] Recently, motorized knee joints have started to emerge. A motorized knee joint as defined herein is one that contains at least one powered control element configured to apply a torque between the thigh link and the shank link. In a motorized knee joint, angular movement of the knee can result directly from the knee prosthesis motor (or more generally, from any powered actuator or control element). This mechanism of knee movement is herein referred to as the prothesis source of knee movement. Motorized knee joints can therefore potentially have two mechanisms for generating knee angular movement: the motor in the prosthesis (the prothesis source), and thigh movement generated by the user (the user source).

[0038] A conventional prosthesis that achieves angular movement solely through inertial coupling between the thigh and prosthesis must have a sufficiently low knee joint outputimpedance to enable such inertial coupling (i.e., the knee joint must be sufficiently free-swinging to enable inertially coupled motion). Many motorized prostheses have a much higher knee joint output impedance, either due to design factors (e.g., gear friction and reflected motor inertia) or control factors (e.g., use of trajectory control). The relatively high output impedance of motorized prostheses attenuates substantially the natural mechanism of knee movement, typically to the extent that almost all knee movement must be generated artificially by the knee motor. This approach essentially supplants the natural mechanism of knee movement in favor of an artificial mechanism of knee angular movement (i .e., the inertially coupled mechanism is essentially filtered out by the knee impedance). For motorized prosthetic knees, effectively replacing the natural inertially coupled mechanism of knee angular movement with the artificial one provides the advantage that knee motion results (almost) exclusively from a single control source, which avoids the potential for competing control objectives or undesirable interaction between user-sourced control of angular motion and the prosthesis-sourced control of angular motion. Specifically, the existence of two control inputs (from the user and motor, respectively) from two different sources provides the potential for each one to be a disturbance to the other, and further, the two controllers could energetically interact in undesirable ways, such as to cause oscillation or inefficiency in the movement. In cases of a high output impedance approach (by design and / or control), the dominant source of knee angular movement is from the motor, which essentially eliminates the potential for undesirable interaction or interference between the user and motor inputs. In such systems, some form of knee angle trajectory control is common, where the trajectory is often constructed based on a healthy knee angle trajectory, and / or as a function of thigh angle (e.g., one might form a virtual linkage between the thigh angle and knee angle). Such trajectory control approaches fundamentally override the natural dynamics by which the inertially coupled mechanism of knee movement occurs (i.e., the prescription of an angle in trajectory control specifies the output of the dynamic system, which therefore effectively eliminates the dynamic mapping from thigh acceleration to knee angle, and therefore effectively eliminates or substantially attenuates the user contribution to knee angular movement).

[0039] Although motorized knee prostheses that employ a high knee joint output impedance (either by means of design or control or both) eliminate the problem of undesirable interference between the user and prosthesis control systems, they also forego several prospective advantages associated with preserving the natural mechanism of knee movement. Specifically, retaining theuser-sourced mechanism of knee movement as a primary source of movement: (1) provides a natural source of power for knee movement (thereby reducing the power required by the prosthesis); (2) gives the user a physical means of control and agency over the swing-phase movement of the knee; and (3) is the primary mechanism by which the healthy limb achieves knee movement, and therefore is the most faithful means of replicating the behavior of the healthy joint. Therefore, there is strong motivation to retain the user-sourced (i.e., inertially coupled) mechanism of knee movement as a primary source of knee angular movement in a knee prosthesis.

[0040] To retain the natural user source of knee movement, while also having the ability to supplement or assist that movement with an artificial source of power at the knee, a motorized prosthesis can be designed with a low output impedance, in which case both the natural and artificial mechanisms of knee movement can contribute substantially to a resultant angular movement of the knee. Retaining the natural means of knee movement as the primary means of movement provides several advantages, as previously mentioned. This form of knee movement may at times, however, be energetically deficient, as such, there is also a need to supplement this form of knee movement with assistive power (i.e., with a motorized source of movement). Having two sources of angular knee motion, however, introduces a control problem in such prostheses; namely, how to control movement (i.e., power) contributions from the prothesis source of knee motion (i.e., the motor) such that it supplements the natural source of knee motion (i.e. thigh motion), without destructively interfering with it (i.e., without interfering with the movement or movement intent of the user). To address this problem, this application describes a method for controlling a motorized knee prosthesis that preserves the natural mechanism of knee movement as the primary source of knee movement, but also supplements this source of knee movement with an artificial source, in a manner that preserves the inertially-coupled natural dynamics and avoids potential destructive interference between the user and prosthesis sources of power.

[0041] This application represents the dynamics of the prosthesis in conventional ordinary differential equation form, where the homogeneous dynamics are given on the left-hand-side (LHS) of the equation and the forcing terms are given on the right-hand-side (RHS). The homogeneous dynamics are the portion of the differential equation written strictly as a function of the generalized coordinate or coordinates of the dynamic system, and their derivatives. Rather than describe the homogeneous portion of the dynamics as a function of the generalized coordinate(s) and their derivatives, one can equivalently describe the homogeneous portion of the dynamics asa function of the state of a system and the first derivative of the state, where the state of the system is the set of variables required to fully characterize the energy of the system. This set of variables is called the state vector.

[0042] The forcing term or terms of the dynamics is given on the RHS of the dynamic equation. The forcing term or terms are variables that affect the state of the system (i.e., affect the potential and kinetic energy content of the system), but are not required to describe the amount of energy in the system. A forcing term can also be regarded as an exogenous input to the system; specifically, an exogenous input is one that is externally imposed on a system. Alternately stated, an exogenous input is one that affects the state or output of the system, but is not affected by the state or output of the system. In contrast to an exogenous variable, a variable that is either a state variable or an output of the system is referred to as an endogenous variable.

[0043] A passive term is one that either stores energy in the system, or removes energy from the system. A strictly passive term is one that removes energy from the system. An element of a system that is not passive is an active element; such elements can supply net power over time (presumably sourcing power from a reservoir of energy, such as a battery).

[0044] Mechanism for knee movement in passive prostheses

[0045] The inertially coupled natural mechanism of knee angular movement, also called the user source of knee movement, is described in this section. FIG. 1 shows a diagram of a prosthetic leg attached to a user’s thigh with variable definition of thigh / knee / shank system. FIG. 1 A defines three angles: the thigh angle (9t), defined relative to the vertical, which is assumed to be under the direct control of the user; the shank angle (0S), defined relative to the vertical; and the knee angle (0fe), which is defined as the relative angle between the thigh and shank, defined as zero when the knee is fully extended (in accordance with biomechanical convention). The thigh and shank angles as defined here are positive when the thigh and shank are oriented posterior to the user’s body. FIG. IB defines the location of the center of mass of the shank from the knee center (Z); the mass of the shank (m); the rotational inertia of the shank ( / ) about the knee joint; the normal and tangential acceleration components of the knee joint (anand at, respectively), defined relative to the angle of the shank; and the torque imposed by the knee joint (Tfc).

[0046] For this this thigh / knee / shank system, the knee angle is a function of the thigh and shank angles:

[0047] It should be noted that the knee torque, Tk, can be regarded as having two distinct components: 1) a passive component that includes the passive behaviors of the knee joint (Tkp), and 2) in the case of a motorized knee prosthesis, a motorized component (Tkm) which include a torque command that can be active or passive, depending on the torque control law. The passive component of knee torque will typically include friction, potentially controllable forms of resistance (e.g., hydraulic resistance), and a spring to aid with extension. The knee torque can therefore be written as:

[0048] For both passive and motorized prosthesis, and the passive portion of torque can be represented as:wherein Tkpis a passive function of 9kand 9k. In the case of a passive (i.e., a non-motorized) prosthesis, the motorized component of knee torque is zero. Therefore, the equation of motion that determines the movement of the shank for a passive prosthesis is shown in FIG. 2.The equation of J9S+ mgl * s (9s) in FIG. 2 is the shank dynamics and Tkp(9k, 9k) is the passive knee behavior, making the LHS of equation 4 the combined knee / shank passive dynamics. The RHS of equation 4 is the user’s input, controllable exclusively by the user’s thigh motion (i.e., the sole forcing input is at, which is a function of user-controlled thigh motion). A block diagram depicting this equation is shown in FIG. 2, where the “knee / shank passive dynamics” block represents the left-hand-side (LHS) of equation 2, and Turepresents the input torque generated by the user (via the translational acceleration of the knee joint, which results primarily from thigh angular acceleration), where:

[0049] Assuming the linear (i.e., translational) acceleration at the hip joint is small relative to that at the knee (an assumption supported by biomechanical data of human walking), the tangential acceleration of the knee joint, oriented relative to the shank, at, can be expressed as a function of the user’s thigh angle (and its derivatives):

[0050] Incorporating equation 6 into the block diagram of FIG. 2, the dynamics of shank motion can be represented in block diagram form in FIG. 3, where it can be easily seen that the user’s thigh motion controls the prosthesis shank motion. As shown in the figure, the passive dynamics from thigh angle motion (i.e., angle and its derivatives) to shank angle is referred to as the “extended passive dynamics.” This is shown in FIG. 3 with dynamics of a passive prosthesis extended to thigh angle input.

[0051] FIG. 3, while conceptually useful, is oversimplified, since the passive dynamics represented within the knee / shank passive dynamics block (i.e., on the LHS of equation 4) are a function of both the shank angle and the knee angle. Since the knee angle is the difference between shank angle and thigh angle (equation 1), the terms in Tkpcan be decomposed into a homogeneous portion (a function of 0S) and a forcing portion (a function of 0t). For example, assume the knee torque is given by the passive function:which can be written in terms of 9Sand 9tas:after which the homogeneous terms can be grouped on the LHS and forcing terms (which are both from the user’s input via thigh motion) on the RHS as follows:

[0052] Equation 10 describes the dynamics of the shank motion during swing phase in the conventional ordinary differential equation form (i.e., homogeneous dynamics on the LHS, forcing terms on the RHS). The forcing terms in equation 10 are functions of the linear acceleration of the knee (i.e., ut) and the angular velocity of the thigh (i.e., 0t), both of which are directly associated with user thigh movement (i.e., see equation 6). Therefore, the user thigh motion is the sole forcing input in the passive prosthesis.

[0053] Equation 10 therefore illustrates the point that both 0Sand 0kterms can be included in the dynamics between 0tand 0S, where 0kterms can subsequently be decomposed into both 0Sterms (i.e., homogeneous terms that are strictly a function of 0Sand its derivatives) and 0tterms (i.e., forcing terms that are strictly a function of 0tand its derivatives). The decomposition of homogeneous terms expressed in terms of knee angle, and the thigh motion input associated with them, is shown explicitly in the block diagram of FIG. 4. The passive knee torque term is fed back into the shank dynamics with a negative sign, which is a result of representing it as a separate input into the shank homogeneous dynamics (i.e., it is temporary moved to the RHS of the dynamics in order to represent it as an input). This negative sign of course makes sense, particularly since these passive behaviors will be stabilizing, and therefore would have to entail negative feedback of the state (since positive feedback of the state would destabilize the system). Since both the user input associated with knee tangential acceleration and the user input associated with passive knee torque terms are a function of thigh angle and its derivatives (note that an assumption is made here that the linear acceleration of the hip joint is small), the diagram of FIG. 4 can be simplified to the diagram of FIG. 5, and subsequently to the diagram of FIG. 6, which explicitly illustrates that the user’s thigh motion (i.e., thigh angle and its derivatives) is the sole input powering and controlling the passive prosthesis shank motion during swing phase, which it does through the passive dynamics of the thigh / knee / shank system.

[0054] FIG. 4 shows the dynamics of a passive prosthesis, illustrating separate knee and shank angles within the knee / shank passive dynamics. The primary mechanism of shank motion is frominertial coupling (i.e., from Tu), assuming the knee output impedance remains sufficiently small (i.e., assuming Tkpremains small relative to Tu). FIG. 5 is a simplified representation of the passive thigh / knee / shank dynamics. FIG. 6 is a further simplified representation of the passive thigh / knee / shank dynamics, illustrating the fact that the shank motion is a filtered version of the user’s thigh motion. The shank motion output is therefore controlled strictly by the user’s thigh motion input. Further, the primary mechanism of shank motion is due to inertial coupling, assuming the output impedance of the knee is sufficiently low (i.e., Tkpis small relative to Tu).

[0055] Dynamics of a motorized knee prosthesis

[0056] As previously mentioned, powered knee prostheses capable of the high torques required to accommodate activities such as stair descent, often entail high amounts of reflected friction and inertia associated with the mechanisms required to provide sufficient torque from an electric motor. In this case, the magnitude of the Tkpterm in equation 4 dominates the other homogeneous terms for much of the swing phase motion, which suppresses or attenuates substantially the ability of user input to generate motion. I n cases for which a motorized knee prosthesis is designed with a low mechanical output impedance (which is the case considered here), the Tkpterm in equation 4 remains small relative to the inertial and gravitational terms in the homogeneous dynamics, or of the same order. In this case, the knee prosthesis system will be substantially influenced by two sources of knee movement: the natural mechanism of knee movement (i.e., equation 4), in addition to an artificial means of moving the knee through the prosthesis motor. Specifically, in addition to the passive knee torque Tkpin equation 2, the knee torque Tkin a motorized knee prosthesis may also include a motorized source of knee torque, Tkm, as given in equation 2. Incorporating this term into the shank dynamics yields:where the LHS of equation 11 are the passive dynamics, including the shank dynamics) and the passive knee behavior (Tkp(0k, 6k^ ), and the RHS are the user’s natural input to the dynamics (matl ), and the knee motor’s artificial input to the dynamics (Tkm).

[0057] To be consistent with the sign conventions of equations 2 and 4, the term Tfemappears in equation 11 on the RHS as a negative term. As previously mentioned, the system now has twosources of movement power - one from the user (i.e., thigh movement) and one from the prosthesis (i.e., the motor) - which must be coordinated such that the latter assists the former, without destructively interfering with it. Specifically, these sources of movement are under the control of two different controllers - the central nervous system (CNS) of the user controls natural movement of the knee (i.e., via thigh movement), while a controller on the knee prosthesis controls artificial movement of the knee (i.e., via the knee motor). It is important that the motor assist the user in performing a movement, without competing with or otherwise interfering with or supplanting the user’ s movement or movement intent.

[0058] Referring to FIG. 7, a block diagram of a motorized prosthesis is shown with two distinct exogenous control inputs: one from the user (the user source of motion) and one from the prosthesis (the prothesis source of motion). These two inputs (user and motor) are shown in the block diagram of FIG. 7, where now there exists two exogenous inputs (i.e., inputs externally imposed on a system, but not otherwise affected by the system): one from the user and one from the prosthesis. In contrast, the passive prosthesis system, depicted in FIG. 5, includes only a single exogenous input, the one from the human user.

[0059] The means of the prosthesis control (i.e., control of the motor input) of this application ensures that the motor input constructively assists but does not destructively interfere with the user input or associated dynamic interaction between the user and prosthesis, thereby preserving the user input as the primary means of knee movement, and preserving the nature of interaction between the user and prosthesis, but also enables powered assistance that makes such movements easier when appropriate. The essence of the approach is to the define a prosthesis control law such that the motor input becomes an endogenous rather than an exogenous term - specifically an endogenous input that adds power - thus leaving the user control input as the sole exogenous input to the prosthesis system. Specifically, if Tkmbecomes solely a function of 0S(and its derivatives), that term effectively becomes an endogenous term, and therefore can be moved to the left-hand- side of the equation (i.e., homogeneous dynamics of the system), thus leaving only 0t(and its derivatives) on the right-hand-side (i.e., thus leaving a single exogenous input, which is solely under the control of the user). The same principle can also be used if Tkmis also a function of 0k(and its derivatives), since 0kcan be decomposed into a 0Sand 0t, and therefore separated into homogeneous and forcing terms respectively. Thus the objective of the control approach is to usethe motor torque in a manner that alters the homogeneous dynamics, rather than use it as a separate forcing term (which is the convention). Therefore, Tkmcan take the general form:In the simplest embodiment, the motor controller is a function solely of 0Sand 0S,

[0060] Referring to FIG. 8, a block diagram of the prosthesis dynamics is shown. Specifically, FIG. 8 shows Motorized prosthesis dynamics with Tkma function of 0S(and derivatives). As in FIG. 5 (the passive system), this system entails only a single user-controlled exogenous input. In the more general case in which motor control torque is a function of both 0Sand 0kand their derivatives:

[0061] Referring to FIG. 9, a block diagram of prosthesis dynamics is shown. More specifically, FIG. 9 shows Motorized prosthesis dynamics with Tkma function of the measured 0Sand 0k. The system again entails only a single user-controlled exogenous input. In both cases (i.e., FIGS. 8 and 9), the exogenous input from the prosthesis motor has been replaced by a new artificial homogeneous dynamics, leaving a single forcing term, which is the physical motion of the thigh segment. Therefore, as with the dynamics of the passive prosthesis, the user input remains the sole exogenous input to the thigh / knee / shank prosthesis system. Accordingly, the (motorized) systems shown in FIGS. 8 and 9 can be simplified to the block diagram shown in FIG. 10, which can be further simplified into the diagram shown in FIG. 11, which are clearly power- assisted analogs to the passive versions shown in FIGS. 5 and 6. Thus, by converting the exogenous motor torque term into an endogenous term, the motorized knee prosthesis system is able to function for all intents and purposes as the passive system, but with a modified homogeneous dynamics rather than the original homogenous dynamics.

[0062] Referring to FIG. 10, a simplified depiction of the motorized knee prosthesis systems depicted in FIGS. 8 and 9 are shown. FIG. 11 shows a simplified depiction of the motorized knee prosthesis system shown in FIG. 9.

[0063] Destabilizing homogeneous control of a motorized knee prosthesis

[0064] One of the primary purposes of the knee motor is to provide assistive power to aid the shank motion during swing phase. Reformulating the prosthesis motor control law such that the motor torque is strictly a function of 6Sas described above converts the exogenous motor input into an artificial homogeneous term rather than a forcing term. Alternatively, if the prosthesis motor control torque is a function of 6k, the control law can be decomposed (as illustrated above) into a homogeneous term (i.e., function of 0S) and a forcing term, where the forcing term is strictly a function of 9t(and its derivatives); as such, the original forcing term (thigh-shank inertial coupling) remains an essential component of user-generated movement, although it is supplemented by an additional forcing term that is also a function of thigh motion, thus leaving the natural exogenous input from the user (i.e., thigh motion) as the sole exogenous input. The prosthesis control law for motor torque can alternatively be formulated as a function of both 9Sand 9k, with the same result. In all cases, the net effect is that the user remains the sole exogenous input, and therefore is assumed to have greater control and agency over the movement of the shank, since there is no potentially competing input to the system, and also more natural control of movement, since movement assistance is constructed upon the preservation of underlying passive dynamics (i.e., the natural mechanism of knee angular movement). As such, rendering the exogenous input from the knee prosthesis motor into a homogeneous dynamics term in the shank dynamics is an important element of the invention described here.

[0065] While the proposed control structure as described here both preserves the inertially coupled mechanism of knee movement and eliminates the potential for the motor input to compete with it, it does not necessarily guarantee that the motor provides movement assistance. The biomechanical function of the knee joint during locomotion varies across different activities. Prosthesis knee function may therefore at times benefit from the motor in a motorized knee prosthesis resisting movement, while at other times knee function may benefit from the motor assisting movement (i.e., by providing assistive power for movement). The latter is in fact an important motivation for a motorized knee prosthesis (although there is also merit in using a motor at times to resist knee movement).

[0066] Whether the motor is assistive or resistive in the control formulation described here is a function of whether the homogeneous terms rendered by the control law are passive or active terms, or alternatively whether they have a stabilizing or destabilizing effect on the homogeneous dynamics. That is, if the homogeneous terms rendered by the motor control law are either active, or provide a destabilizing effect on the homogeneous dynamics, the net effect of the control law will be to provide assistance to movement. I f, however, the terms are passive or stabilizing, they will provide resistance to movement.

[0067] Distinguishing what defines assistive versus resistive behaviors is made clearer by considering homogeneous terms of the form where motor torque is defined as a function of shank angle, angular velocity, or angular acceleration. In the case that the motor torque is defined as a function of shank angular velocity (or knee angular velocity), and given the sign conventions in equation 11, the motor torque control term will be active when the torque function lies in the second and / or fourth quadrants of the torque / velocity plane representing the function, as illustrated in FIG. 12. Specifically, the product of motor torque and angular velocity (which is power) in this case will always be negative; since this term is fed back into the knee / shank passive dynamics through a negative sign (i.e., the term is positive when appearing on the LHS), the product of motor torque and angular velocity (i.e., power) when appearing on the homogeneous side of the dynamics will always be positive. In other words, the motor will always add power to the homogeneous dynamics. Alternatively viewed, the term will show up on the RHS as a positive term in 6S, which will show up on the LHS as a negative term in 6S, which is well known in the study of second- order dynamic systems to be destabilizing. Therefore, any motor control torque function that exists strictly in the second and fourth quadrants of the torque / velocity plane will appear in the dynamics as an active homogenous term, and therefore will always provide assistance without introducing an independent exogenous input.

[0068] Referring to FIG. 12, a motor torque and shank or knee angular velocity plane is shown. Any function in the first and third quadrants will generate power. Three example functions are a linear function (1), a quadratic function (2), and a signum function (3). A large number of functions can satisfy the requirements illustrated by FIG. 12, some of which include:

[0069] Consider as an example the destabilizing control function given in equation 15. Substituting this control function into the general motorized knee dynamics of equation 11 yields:where the LHS of equation 20 is the passive dynamics, matl is again the user’s input (i.e., the natural forcing input) andthe contribution from the knee’s motor (i.e., the artificial forcing input). This knee motor contribution is an active control torque, and because it is exclusively a function of the measured state, it effectively becomes part of the homogeneous dynamics (even though it is generated artificially by the motor), and therefore can be moved to the LHS:

[0070] As is well known, negative functions of the state in a second-order homogeneous equation are de-stabilizing terms. The control term will therefore add power without introducing an exogeneous input, therefore leaving the thigh motion as the sole exogeneous input into the shank dynamics. The homogeneous destabilizing control functions, such as those listed in equations 15 through 19, can be functions of 0Sor 0k. Consider as an example the destabilizing control function given in equation 17, which is a function of 0k. Substituting this control function into the general motorized knee dynamics of equation 11 yields:Substituting and differentiation equation 1 yields:Moving the artificial homogeneous term (BS) to the LHS yields:

[0071] In this case, the motor adds a destabilizing term to the homogeneous dynamics, which adds supplemental power to the shank system, while leaving the RHS strictly a function of user thigh motion (i.e., user thigh motion remains the sole exogenous input to the shank dynamics), and while also leaving the natural mechanism of knee motion (i.e., the first term on the RHS) fully intact.

[0072] As indicated by equation 14, control terms that can be recast as homogeneous dynamics terms can alternatively be expressed as a function of knee angle. Destabilizing terms expressed as a function of angle can be regarded as negative stiffness terms, and may be written either as a function of 0Sor 0k. As with the velocity terms, a term that appears in the LHS of equation 11 that is a negative function of shank (or knee) angle will destabilize the homogeneous dynamics (i.e., the term will always push the system away from the minimum energy configuration), and as such, a negative spring for all intents and purposes guarantees that the term contributes power (i.e., provides assistance). Two simple versions of a negative stiffness include the functions below:where the term 0EQis an equilibrium angle (i.e., in this case an unstable equilibrium). Note that any function of angle that yields a negative definite energy function with respect to the equilibrium angle will destabilize the homogeneous dynamics, and therefore is a candidate function for thecontroller described here. Note that a stabilizing stiffness is defined by a positive definite energy function with respect to the equilibrium angle, the physical interpretation of which is that a perturbation must increasingly add energy to move the stiffness away from its equilibrium point. In the case of a negative stiffness defined by a negative definite energy function, a perturbation requires increasing energy to move the stiffness towards the equilibrium point; as such, the minimum energy state is away from rather than towards its equilibrium position. As such, although the examples in equations 25 and 26 are linear functions, a negative stiffness can be any function for which the energy is negative definite relative to the equilibrium angle.

[0073] It is noted that a control function can also be expressed as a function of the angular acceleration, with the same restriction as discussed for angle-based functions; namely, a destabilizing term requires the energy function to be negative definite with respect to an equilibrium (presumably the equilibrium would be at zero angular acceleration).

[0074] The knee motor torque command could consist of any of the aforementioned destabilizing components; could be comprised of a superposition of several of these components; or could be comprised of a combination of both stabilizing and destabilizing components.

[0075] In one preferred embodiment, the homogeneous destabilizing control described here would be employed such that the overall stability of the homogeneous shank system is bounded in time and / or in space. Specifically, the destabilizing homogeneous control functions described here can be used to add supplemental power to passive dynamics, where the collective system (i.e., natural homogeneous dynamics combined with the artificial homogeneous dynamics) remains passive. In this case, the controller will modify the passive dynamics to be less passive, but leave the overall system passive (i.e., stable). For example, the overall homogeneous dynamics will remain passive if a destabilizing control term of the form of equation 17 is used with the system described by homogeneous passive knee torque of the form of equation 7, as long as the artificial destabilizing parameter B is less than or equal to the passive intrinsic b of the mechanical system.

[0076] In another preferred embodiment, the destabilizing homogeneous control functions can be used to add sufficient supplemental power, such that the collective system (i.e., natural homogeneous dynamics combined with the artificial homogeneous dynamics) becomes active (i.e., unstable). In the case of an unstable dynamics, the instability can be bounded, either naturally or artificially, such that the instability provided by the motor control law results in a localinstability rather than a global instability. In such an embodiment, the controller can monitor the system state (e.g., knee angle or angular velocity), and can modify the control law to saturate or limit motor torque or power when the system exceeds a predetermined threshold (e.g., a knee angle threshold, knee angular velocity threshold, motor power threshold, etc.). The controller can alternatively monitor and limit the duration of the unstable state. Limits may also be due to mechanical means, such as a transmission ratio that attenuates or saturates the maximum motor torque as a function of knee angle, or a hard stop that prevents the knee angle from exceeding a given angle.

[0077] Another assumed restriction on the form of the controller, which is implicitly assumed here but was not previously explicitly stated, is that the control law cannot prescribe either a desired 0Sor desired 6kas a function of user input (i.e., as a function of the thigh angle or its derivatives). If the motor torque Tkmwere defined as such, it would override rather than modify the homogeneous dynamics, and importantly, it would alter substantially (and theoretically completely impede) the mechanism of natural knee movement (i.e., the mechanics by which the thigh generates knee movement, illustrated in FIG. 5 and described by equation 2). If either a desired knee motion (0fc) or desired shank motion (0S) were defined directly as a function of the thigh motion input (0t), the system would be reduced to a single exogenous input; however, the dynamic mechanism by which the user naturally attains knee motion would be fully supplanted by the kinematically-based rubric enforced by the knee controller. As such, the assistance provided by the knee motor would no longer employ the natural mechanism of knee movement as the primary mechanism of knee movement, which would defeat the advantages of retaining natural knee movement (previously mentioned). Therefore, it is important that Tkmmodify the homogenous dynamics of the system without supplanting or bypassing them by directly attempting to control the resulting motion (of either 9kor 0S). The mechanism of interaction between the user and device should remain dynamic if they are effectively bypassed by a form of trajectory control (even if that trajectory control is a function of 0t), then the natural mechanism of swing-phase control has effectively been eliminated, and the system becomes similar to a motorized prosthesis with a high mechanical output impedance (as previously mentioned), which effectively eliminates the natural mechanism of knee movement (and therefore fails to preserve and supplement the natural mechanism of knee movement, as is the objective here). Rather, the method described hereseeks to retain and assist the natural mechanism of knee movement, rather than supplant it with an alternate form of movement generation.

[0078] The proposed approach therefore eliminates the motor as an exogenous input, such that the only exogenous input into the prosthesis system is the natural channel of user input, as it exists in passive prostheses. Rather than attempt to coordinate an artificial exogenous input with the human user, the approach described here uses the motor simply to modify the homogenous dynamics in a way that makes the knee easier for the user to move, but still uses the same mechanism of interaction dynamics between the user and prosthesis that exists in passive prostheses and healthy movement. This effectively removes the challenge of coordination, since the user is the only entity providing control input; the motor is used to modify the homogeneous dynamics to make it easier for the user to generate movement, but otherwise does not by exogeneous command generate movement, nor directly impose a desired movement.

[0079] Although the control method described here has been discussed largely in the context of assisting the swing phase of walking, the method is similarly effective in assisting the swing phase of other locomotion activities, such as during slope ascent and stair ascent.

[0080] While the foregoing written description of the invention enables one of ordinary skill to make and use what is considered presently to be the best mode thereof, those of ordinary skill will understand and appreciate the existence of variations, combinations, and equivalents of the specific embodiment, method, and examples herein. The invention should therefore not be limited by the above-described embodiment, method, and examples, but by all embodiments and methods within the scope and spirit of the invention.

Claims

WHAT IS CLAIMED IS:

1. A knee prosthesis, comprising: a shank link; a thigh link rotatably coupled to the shank link; at least one powered control element configured to apply a torque between the thigh link and the shank link; wherein a shank motion is described by at least a shank angular velocity relative to an inertial reference frame; and wherein the knee prosthesis further comprises at least one sensor, and a controller, the controller configured to: receive sensor measurements from the at least one sensor and determine a present state of a plurality of states comprising at least a swing state and a stance state; provide a torque control law for the at least one powered control element if the present state is a swing state, wherein the knee prosthesis measures the shank angular velocity, and wherein the torque control is a function of at least the shank angular velocity.

2. The knee prosthesis of claim 1, wherein the torque control is an unstable function of the shank angular velocity such that the torque control drives the shank away from a given equilibrium shank angular velocity.

3. The knee prosthesis of claim 2, wherein the unstable torque control is proportional to the negative shank angular velocity.

4. The knee prosthesis of claim 2, wherein the unstable torque control is proportional to the negative of the square of the shank angular velocity.

5. The knee prosthesis of claim 1, wherein the shank motion is further described by a shank angle relative to an inertial reference frame; wherein the knee prosthesis further measures the shank angle; and wherein the torque control is an unstable function further including the shank angle.

6. The knee prosthesis of claim 5, wherein the torque control is in negative proportion to the difference between the shank angle and a shank equilibrium angle.

7. The knee prosthesis of claim 5, wherein the torque control is in negative proportion to the sine of the shank angle.

8. A knee prosthesi s compri sing : a shank link; a thigh link rotatably coupled to the shank link; at least one powered control element configured to apply a torque between the thigh link and the shank link; wherein a relative motion between the thigh link and shank link is described by at least a knee angular velocity; and wherein the knee prosthesis further comprises at least one sensor, and a controller, the controller configured to: receive sensor measurements from the at least one sensor and determine a present state of a plurality of states comprising at least a swing state and a stance state; provide a torque control for the at least one powered control element if the present state is a swing state. wherein the knee prosthesis measures at least the knee angular velocity; and wherein the torque control is an unstable function of at least the knee angular velocity such that the torque control drives the knee away from a given equilibrium knee angular velocity.

9. The knee prosthesis of claim 8, wherein the unstable torque control is proportional to the negative knee angular velocity.

10. The knee prosthesis of claim 8, wherein the unstable torque control is proportional to the negative of the square of the knee angular velocity.

11. The knee prosthesis of claim 8, wherein the relative motion between the thigh link and shank link is further described by a knee angle, wherein the knee prosthesis further measures the knee angle; and wherein the torque control is an unstable function further including the knee angle.

12. The knee prosthesis of claim 11, wherein the torque control is in negative proportion to the difference between the knee angle and a knee equilibrium angle.

13. A knee prosthesis, comprising: a shank link; a thigh link rotatably coupled to the shank link; at least one torque control element configured to apply a torque between the thigh link and the shank link;wherein a shank motion is described by at least a shank angle relative to an inertial reference frame; and wherein the knee prosthesis further comprises at least one sensor, and a controller, the controller configured to: receive sensor measurements from the at least one sensor and determine a present state of a plurality of states comprising at least a swing state and a stance state; provide a torque control for at least one powered control element if the present state is a swing state, wherein the knee prosthesis measures at least shank angle, and wherein the torque control is an unstable function of at least the shank angle, wherein the torque control is in negative proportion to the difference between the shank angle and a shank equilibrium angle.

14. A knee prosthesis, comprising: a shank link; a thigh link rotatably coupled to the shank link; at least one torque control element configured to apply a torque between the thigh link and the shank link; wherein the relative angle between the thigh link and shank link is described by at least a knee angle; and wherein the knee prosthesis further comprises at least one sensor, and a controller, the controller configured to: receive sensor measurements from the at least one sensor and determine a present state of a plurality of states comprising at least a swing state and a stance state; provide a torque control for at least one powered control element if the present state is a swing state, wherein the knee prosthesis measures at least the knee angle, and wherein the torque control is an unstable function of at least the knee angle.

15. The knee prosthesis of claim 14, wherein the torque control is in negative proportion to the difference between the knee angle and a knee equilibrium angle.

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