Patent Application
20230195053
Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application are hereby incorporated by reference under 37 CFR 1.57.
The present disclosure relates to a magnetorheological actuator used, for example, in a prosthetic or orthotic device, and more particularly to a control system for regulating current circulating in a magnetic coil part of a magnetorheological (“MR”) actuator.
The majority of advanced lower-limb prosthetic devices deployed on lower-limb amputees use a microprocessor-controlled braking device. The braking device's technology and implementation vary significantly, but all provide a certain level of resistance to motion under load, which is leveraged to support the lower-limb amputee while standing on the prosthetic limb. Additionally, where the lower-limb prosthetic device is a knee, the braking device is typically controlled to allow the knee to move freely when the prosthetic limb is in swing phase, hence replicating typical lower limb kinematics observed during gait activities.
Common hydraulic technologies in lower-limb prosthetic devices use valves to provide damping or braking of the device by controlling and regulating the flow of hydraulic fluid. Such hydraulic technologies have a high capacity for quick transition between braking and non-braking states due to the limited displacement required by these components to significantly affect the flow properties, and in turn the hydraulic actuator behaviour. However, one disadvantage of these hydraulic technologies is that they present a rather high restriction to motion, even with the valves fully open, due to the need to move the hydraulic fluid around when the actuator is moving. While these systems are well suited to generating braking forces by creating restriction for the fluid to move through, they present significant inertia and damping when it is required to rapidly move the actuator around under low resistance levels.
MR actuators have also been used in lower-limb prosthetics (e.g., prosthetic knees). Use of MR actuators requires accurate control of the braking torque applied at any point in time because failure to generate the appropriate level of braking torque, and to do so in a timely manner, can be highly detrimental to system performance. On the other hand, when properly implemented, use of such actuator technology in a micro-processor controlled prosthetic device has been shown to provide significant benefits at functional, safety and consistency levels.
Shear-type MR actuators present different dynamic characteristics requiring more refined control approaches to optimize their performance range. One particular consideration comes from the time response of the MR actuator. As the braking is achieved through a change in the apparent viscosity of the MR fluid which results in increased friction between the thin discs mounted on the device rotor and stator, achieving a fast response time requires being particularly proactive in driving the current and the resulting magnetic field.
Another phenomenon at play here is the size of the magnetic circuit circulating the flux from the magnetic coil through the MR fluid chamber. Combined with the volume of MR fluid where the change of apparent viscosity needs to take place, this contributes to creating the latency typically observed between the point where the current change is affected and the point where the actuator properties are observed to change in a given set of circumstances.
Another consideration supporting the need for a more proactive current control strategy than what is normally used for controlling current in magnetic coil is the fact that the magnetic coil in a shear-type MR actuator does not behave like a pure theoretical inductor would do. Strongly coupled inductors as found in DC motors or solenoids are known to present well-defined current time-response. On the other hand, magnetic coils integrated in the MR actuators present different characteristics. Failure to properly account for these different characteristics results in the reduction of the actuator performance spectrum, either found as low responsiveness or poor torque generation capabilities.
In accordance with one aspect of the disclosure, a control system for regulating current circulation in a MR actuator used in a micro-processor controlled prosthetic device is provided.
MR actuators are controllable brakes or dampers, in which a MR fluid is subjected to a varying magnetic field, causing a change in the fluid properties, namely the apparent viscosity, which in turn affects the resistive or braking torque generated by the actuator. Varying the magnetic field is typically achieved by varying the amount of current circulating in a magnetic coil, functionally coupled with the actuator, such that the magnetic field variations directly influence the MR fluid state.
In accordance with one aspect of the disclosure, a MR actuator with a current control system is provided. The current control system employs gain scheduling to accelerate changes in current amplitudes. For applications where severe constraints on the MR actuator response time are present, this type of approach advantageously minimizes the time required for the current circulating in the magnetic coil to change, which in turn minimizes the time required for the magnetic field to change, affecting the MR fluid properties and, finally, the actuator braking torque. Such a MR actuator can advantageously be implemented in lower-limb prosthetic devices, like knee joints, which are known to present hard requirements for fast changes in the joint properties, allowing the actuator to properly meet the prosthetic device end-user requirements.
Lower-limb support devices (e.g., prosthetic devices, orthotic devices), such as prosthetic knees, have to support a wide variety of tasks in variable environments and a variety of use cases. An actuator of such lower-limb support devices associated with a particular joint advantageously can provide both static and dynamic levels of performance associated with the load and timing requirements of a given task to support the complete range of performance required by human locomotion.
The MR actuator devices, systems and methods herein disclosed make advantageous use of dynamically adjusting the current controller gains in real-time to minimize the time required to reach a set-point value, without having to deal with the negative aspects of running high gains at all times, including when the amplitude (e.g. magnitude) of the error found as the difference between the current set-point and the measured current circulating the magnetic coil is small. Running high gains at all times can cause oscillation of the current circulating in the coil, which can affect the actuator performance and increase energy expenditure (e.g., draining of the battery for the prosthetic device incorporating the MR actuator).
Advantageously, a closed loop current regulation is used at all times in the MR actuator and does not rely on heuristic decision making to decide whether to apply the control scheme or not. Current regulation under closed loop control was previously only used when the error between the current set-point and the measured current was found to be larger than a predetermined threshold. When the error was smaller than the predetermined threshold, closed loop control was not used, and the current set-point was directly fed through to the coil driver circuitry. Additionally, no particular attention was directed at controlling the current when the current set-point was observed to be reducing.
The embodiments herein described advantageously leverage the benefits of the approach while both increasing or decreasing the current set-point, resulting in a uniform response. Additionally, as mentioned above, the current is regulated under closed-loop control at all times, which also contributes to uniformizing actuator dynamic response in all circumstances. This implementation also advantageously makes possible the use of a recovery current pulse in order to reduce the fall time when the current set-point is reduced from a high current value to a negligible value. In addition to the fundamental benefits of the embodiments disclosed herein, additional benefits arise from the use of a new hardware and firmware platform. Particularly, in one implementation the use of a field programmable gate array (“FPGA”) to implement the current controller itself allows higher control bandwidth, independently without affecting the resources required to implement the remaining firmware functions. In that context, it is possible to execute the closed-loop current control scheme at a base frequency of 28 kHz, instead of the maximum of 5 kHz that was achieved in previous implementations of the MR actuator low-level control scheme. Increasing the controller frequency directly improves the performance of the current regulation, as the controller achieves higher bandwidth, which better matches the dynamic of the magnetic coil.
Finally, the embodiments disclosed herein also allow for the use of adjustable gains, as well as configurable duty cycle parameters. Adjustable gains can be used to customize each actuator in case the dynamic response is observed to vary between units in serial production. Similarly, gains or duty cycles values can be configured to uniformize actuator gain during production activities and ensure high repeatability, for example, of the knee joint braking performance from one device to another. Such high level of inter-device repeatability is important for the case of bilateral amputees or when users are replacing their worn unit with a new one, as the perceived consistency in performance is typically associated as a quality marker by the end users.
In addition to the benefits introduced above, configurable duty cycles can also advantageously be adjusted based on battery voltage, accounting for the progressive reduction of battery voltage with use of the prosthetic device. Hence, increasing the magnetic coil drive circuitry duty cycle while the battery voltage is reducing increases the average voltage applied on the magnetic coil, which in turn makes it possible to maintain the maximum magnetic coil current at a steady level and keep the knee joint actuator braking performance fairly uniform throughout the battery discharge cycle. Again, this advantageously improves the user experience through more consistent knee joint performance, which is perceived as a quality marker. Compensation of the battery voltage drop during discharge is particularly interesting when using a lithium battery to power the prosthetic device. As this particular battery chemistry is known to present a fairly large voltage drop between the fully charged voltage and the nominal discharge voltage, selecting a baseline magnetic coil drive circuitry duty cycle at the battery nominal voltage that ensures that sufficient braking torque is achieved, and scaling it down for voltages comprised between the nominal and fully charged voltage then ensures a consistent braking level.
While actuator braking performance and consistency is optimized using the duty cycle scaling scheme introduced above, it artificially reduces the peak braking torque that can be obtained from the system, as the maximum duty cycle would normally not be used at maximum battery voltage. While adequate selection of the required braking torque at nominal battery voltage should not require the use of additional braking torque, certain circumstances sometimes encountered during locomotion activities may call for a sudden increase of the braking torque over its typical maximal level. For example, in case of stumble, or in order to prevent a fall, or when the user is carrying an excessively heavy object, the duty cycle adjustment scheme introduced above advantageously provides peak braking torque that can exceed the typical maximum braking torque by deviating from the normal adjustment to battery voltage and using the maximum duty cycle with no respect to the actual battery voltage.
A cross-sectional view of a knee-axis mounted shear-type MR actuator 101 is detailed in
When current circulates in the magnetic coil 205, a magnetic field is created and induced in the core rod 204, the core side 206 and through the actuator cavity. Based on the magnetic field intensity across the actuator cavity, the MR fluid (not shown) changes viscosity and increases resistance to motion between the blade sets 208. Conversely, when the current is removed, the magnetic field disappears and the fluid viscosity returns to its original state, allowing the blade sets 208 to move relatively to each other with limited resistance to motion. In certain embodiments, it is also possible to use a recovery current pulse, which allows removing the magnetic field faster, by actively driving the current in the opposite direction for a finite amount of time. The direction in which the current circulates in the magnetic coil is irrelevant, as either direction will cause the desired change of apparent viscosity in the MR fluid, but for improved performance over time and usage, regular changes in the current direction are performed, preventing the onset of remanence and its subsequent build up. Onset and build up of remanence in the actuator are observed to negatively affect the actuator performance as it reduces the range of apparent viscosity possible to achieve with the MR fluid, preventing use of the low apparent viscosity properties of the MR fluid and leading to a permanent braking torque being generated by the actuator, even when no field is generated by the magnetic coil. In the case of a prosthetic knee joint, loss of the low apparent viscosity range provided by the MR fluid impacts the no-load speed of the joint, which is critical for proper operation in swing phase of locomotion gait.
MR brake 516 provides a controlled amount of resistance to motion based on the magnetic field strength going through the MR fluid. Magnetic field is induced by the magnetic coil 515 depending on the amount of current circulating in the magnetic coil. While MR brake 516 resistance is generated in open loop, magnetic coil is controlled in closed loop, through regulation of the current, which is performed through a current controller 513, such as a digital Proportional-Integral (PI) controller, implemented in either the device embedded microcontroller or any other digital processing platform available on the device. The current controller 513 compares the current measured in the magnetic coil (Im) 518 with the current set-point requested from the outer loop controller (Ir) 521 to determine how to adjust the command provided to the coil driver circuit 301 to ensure that the measured current Im 518 matches the requested current Ir 521. Adjustments to the command signal to coil driver circuit 301 are directly performed by the current controller 513, where the gains are adjusted to ensure that the correct dynamic response will be obtained from the current controller when the set-point is modified by the outer loop controller. In one non-limitative embodiment, the coil driver circuitry 301 is implemented using a pulse-width modulation scheme, where the duty cycle during which the magnetic coil is connected to the battery supply is proportionally adjusted based on the amplitude of the current command issued by the current controller 513, leading to an increased average current in the magnetic coil when the current command is increased.
The resulting combination of the MR actuator resistance to motion, user interaction and perturbation torques from the user and the environment result in the prosthetic knee joint instantaneous position Θ 505. Actuator position data Θ 505 is fed into a Luenberg Observer 504 to generate an actuator rotational velocity estimate ω 520, which is then fed through the velocity gain kω 507. Velocity term of the control loop is the main continuous control part, as a strong correlation exists between actuator velocity and the required actuator resistance level required for the user to successfully complete the locomotion task undertaken. To support the specific actuator behaviours, three discrete terms are added to the control loop and are executed based on specific conditions associated with the device usage (i.e., gait phases and subphases, 502 and 512). The Static Torque 503 block consists of a feedforward term which is fed through a static torque gain kTS 506 that allows forcing a minimum actuator resistive torque when the actuator velocity is null or below a certain threshold, while the knee prosthetic enters the stance phase following user weight transfer over the prosthetic foot. While the characteristics of the static torque profile can be varied based on the exact nature of the locomotion task, a general characteristic where amplitude is observed to decay in the time following triggering is considered optimum as it provides a better loading response to the user weight without preventing the user from quickly moving the knee if desired. In a situation where the actuator velocity would be sufficiently high, the velocity arm of the controller (Luenberg Observer 504 and velocity gain 507) would provide enough responsiveness and there is not hard need to add the contribution of the static torque 503 component. Static torque triggering is driven by the Phase detection engine 502, which is part of the intent management section of the control system of
On system initialization, the phase state can be defined as either Stance Phase state 452 or Swing Phase state 451. Once new data is issued by the sensors (e.g., of the knee prosthetic device 1), state will be re-evaluated and the state machine will directly transition to the correct state by comparing the sensor data to the pre-established transition conditions for stance to swing transition 454 or swing to stance transition 453. Due to the low latency required for the system to load new sensor data and the Phase state to be evaluated, there is not a hard functional limitation in having the phase considered stance or swing by default. In one non-limiting embodiment of the phase state machine, the swing to stance transition 453 is based on the comparison of the axial load applied on the lower-limb prosthetic device (e.g., of the knee prosthetic device 1) and measured by the on-board sensors with pre-defined threshold. For example, if the load cell measures an axial load superior to 8 kg, the state machine will make or identify the current state to be stance. Conversely, if the load applied on the lower-limb prosthetic device is measured to be less than 8 kg, the phase will transition to swing. A variety of sensor embodiments can be used to support the phase state machine operation. To name a few, loadcells, accelerometers, pressure sensors, displacement sensors, radar, inductive sensors, and resistive sensors can be leveraged to support the phase state machine. Additionally, stance to swing transition 454 and swing to stance transition 453 can be based on decision mechanisms other than the single thresholding comparison provided as example above. Decision making processes such as multiple thresholding or majority voting could also be applied to define the transition criteria. Additionally, the various methods for deciding if the conditions are met for allowing the system to transition from one state to the other in the phase state machine could also consider multiple data streams at the same time or use sensor integration or fusion to build data stream containing data at a higher level of abstraction.
While the stance and swing phase are characteristic of human locomotion, they do not provide a very detailed segmentation of the behavior required by a lower-limb prosthetic, as this phase segmentation only describes the general configuration of the lower-limb and not the behavior of the various joints in a single phase. To achieve a level of gait control refinement as required by the end user to achieve consistent, stable, and safe locomotion, additional granularity is required, such that the actuator behavior can also be adjusted in a more granular manner.
C1 subphase 400 represents the Contact 1 subphase of the gait cycle and consists of the level walking stance phase state starting from the occurrence of the contact of the prosthetic limb foot with the ground and where the knee is typically observed to flex under the weight of the end user. This flexion is typically observed to provide a certain level of shock absorption following the occurrence of the contact between the prosthetic foot and the ground surface. To properly support the end user weight transfer to the prosthetic limb, the prosthetic knee actuator (such as actuator 101 in
C2 subphase 402 represents the Contact 2 subphase of the gait cycle and consists in the level walking stance phase state where the knee is observed to extend, following the completion of the C1 subphase flexion motion. Extension motion occurs from a combination of the end-user forward momentum and the end-user residual limb hip extension. Proper support of the knee extension motion by the prosthetic knee actuator (such as actuator 101 in
KB subphase 403 represents the knee break or pre-swing subphase of the level walking gait cycle and consists of the stance phase state where the knee joint is prepared to transition to swing phase. In that part of the level walking stance cycle, the user starts unloading the prosthetic limb and the residual limb thigh segment starts moving into hip flexion, after having reached maximum extension. Proper care must be directed in this subphase actuator control to allow for a smooth transition to swing, without hindering the end user hip flexion, requiring the end-user to lift themself up on their sound foot (i.e., hip-hiking) or causing the prosthetic foot to stick to the floor and breaking the forward momentum of the end-user (late stance locking). To ensure proper synchronism with the user's motion and allow the user's motion to control the whole KB subphase, the prosthetic knee actuator (such as actuator 101 in
S1 subphase 404 represents the Swing 1 subphase of the level walking gait cycle and consists of the swing phase state where the knee joint is actively flexing while not in contact with the ground, allowing to clear the ground while the lower limb is brought back in the proper configuration for the following step. Knee flexion under the momentum imparted to the shank segment through the thigh segment acceleration requires the prosthetic knee actuator (such as actuator 101 in
S2 subphase 405 represents the Swing 2 subphase of the level walking gait cycle and consists in the swing phase state where the knee joint is actively extending while not in contact with the ground, allowing to fully extend the knee joint in preparation for the upcoming transition to stance phase. Again in this subphase, it is required for the knee joint to achieve sufficient velocity to ensure that the limb is fully extended ahead of the moment where the prosthetic foot would contact the ground surface, while also managing the joint velocity profile in such a way as to avoid the knee from hitting the extension stop, which is uncomfortable for the user and could negatively affect the end-user capacity to transfer weight to the prosthetic limb in a timely manner. Properly supporting the S2 subphase requires the prosthetic knee actuator (such as actuator 101 in
C1 to C2 state transition 410 occurs when the knee joint behavior is observed to transition from stance flexion to stance extension, under the influence of the end-user. This transition is typically triggered through monitoring of the knee joint velocity direction and typically occurs from C1 to C2 during level walking. It is possible however that transition in the opposite direction (i.e., C2 to C1) could be observed in atypical circumstances or in other activities where knee joint extension is followed by knee joint flexion while in stance phase. Interruption in the level walking stance phase gait cycle could cause the extension motion to stop and the knee to start flexing again. Similarly, sudden stopping due to the presence of an obstacle or losing balance over the prosthetic foot could also generate this type of transition. On the other hand, other activities can be managed through these subphases and provide a more visual example of the bidirectionality of this specific transition. For example, stand to sit transfer is typically supported directly through the C1 subphase, since the knee is flexing in stance phase. Conversely, sit to stand transfer is typically directly supported through the C2 subphase. Since these transfer activities can be performed in any sequence, with or without really reaching the standing and sitting end points, this illustrates the need to support this transition in a bi-directional manner.
C2 to KB state transition 409 occurs when the user is observed to be ready to initiate the transition to swing phase and is again triggered through monitoring of the embedded sensors. This corresponds to the mid-stance to late-stance part of the level walking gait cycle, where the user center of mass is observed to have moved anteriorly to the prosthetic foot and the hip is about to start flexing. Identification of this particular configuration of the user segments and body dynamics can be achieved by making sure that the knee is in an extended position and shows low velocity, indicative that the C2 subphase has really completed. Additionally, monitoring that the knee joint shows sufficient extension moment allows to estimate the end user center of mass to be located anteriorly to the prosthetic foot. Finally, monitoring that sufficient shank sagittal plane rotational velocity is observed allows to ensure that the user is actually showing forward progression momentum while rolling over the toe of the prosthetic foot and/or has started to flex the residual limb hip. As this transition is a rather dynamic one and actually requires the knee joint to be fully extended, it is typically considered as a one-way state transition. Exiting the KB state to return to an earlier stance phase state in level walking requires the knee to flex, which then matches the C1 subphase, and not the C2 subphase.
KB state 403 normal exit transition pattern during level walking gait is observed to be the KB to S1 state transition 408. This transition is observed to take place when the user has completed the weight transfer to the contralateral limb, leaving the prosthetic limb now in swing phase. This transition is managed through the combination of two conditions monitored from the sensor data stream. For one, the loadcell signal has to indicate that the load on the prosthetic limb has dropped below the threshold value used to determine the phase. In other words, the phase has to be detected as being swing. Additionally, to prevent the occurrence of a false positive detection of a KB to S1 transition, a minimum flexion angle is required for the state transition to be executed. In a non-limiting embodiment, an angle of 15° is used. Use of the additional minimum flexion angle condition over simply using the threshold on the loadcell measurement provides the added benefit of helping debounce the loadcell signal and makes sure the user has committed to the transition before actually implementing the state change (i.e., residual limb hip flexion has started to take place, causing the knee to flex). Debouncing is required for cases where the load transferred to the ground through the prosthetic limb would be moving across the threshold value with small amplitude, causing the system to quickly cycle through the stance and swing phases. Use of the minimum angle threshold minimizes that concern as selection of a correct threshold value allows to make sure that the user has started flexing the residual limb, which greatly removes the capacity to be loading the prosthetic limb.
KB to C1 and C1 to KB bidirectional state transition 401 integrate two specific cases that are not typically observed during level walking gait but are often encountered in the general user population based on the high variability of the environment in which human locomotion takes place, causing deviation in the normal occurrence of gait events. Firstly, C1 to KB state transition is in fact equivalent to the C2 to KB state transition introduced above and would typically be observed in cases where the C1 to C2 transition would not be observed, mainly caused by the knee extension not taking place as expected, but where KB transition is still observed to take place. For example, in the case where the user would enter C1 with a fully extended knee, the knee would not generate enough flexion torque to cause the knee to yield, or flex, under load and would not allow it to meet the conditions to enter C2 from C1 but could still meet the conditions for the KB transition in late stance. This type of situation is often observed when users were trained to use simpler mechanical prosthetic knee technology, where they have to force the knee in hyperextension during stance phase to compensate for the device's lack of capacity to provide support.
Secondly, KB to C1 state transition is required to provide a recovery mechanism to the end user and pre-empt some falls from being generated by the user failing at properly progressing through the normal gait events and subphases. Two main situations are observed in level walking where the KB to C1 transition is leveraged and are both associated with a general situation where the observation of the sensor stream indicates that it would be safer for the end-user to have the knee provide support instead of the no-support joint behavior associated with the KB state. In one case, this transition would be triggered by the observation of the knee extension moment being too small and the knee joint flexion angle being too large. This situation represents a case where the end-user center of mass progression has stopped while KB was previously entered, leaving the center of mass posterior to the prosthetic foot and the end-user in an unstable position. This could be observed if the user is required to abruptly stop, or get pushed back, for example. In the other case, if the user loading of the prosthetic device is observed to increase while KB was already entered, this is again indicative that the end-user center of mass is not progressing forward as expected and the user is not transferring their weight away from the prosthetic limb like normally observed in KB.
S1 to S2 state transition 407 occurs when the knee joint has completed the level walking swing phase flexion motion and is observed to start extending. This inversion of the knee rotation direction is normally synchronized with the residual limb hip flexion motion completion and will position the lower limb in a configuration that is ready to support the coming prosthetic foot contact with the ground. This state transition is managed through the monitoring of the knee joint velocity direction, which is implemented through using a null velocity threshold to decide whether the current direction of motion has reached the value allowing for the transition to take place.
S1 to C1 state transition 411 is observed to occur in specific circumstances where sufficient load would be observed on the prosthetic device, through monitoring of the loadcell sensor read out, while is the prosthetic device is expected to be operating in swing phase. Occurrence of stance phase detection during swing flexion can be related to a variety of factors but all indicate a disruption of the normal level walking gait cycle requiring providing support to the end-user and preventing the knee from collapsing under the user. For example, presence of an obstacle interrupting the swing flexion motion and/or interrupting the end-user forward progression momentum could easily result in this transition taking place. Obstacles such as curbs, shrubs, tall grass, or snow are typical examples of obstacles encountered in daily living. Similar to the KB to C1 case previously introduced, situations where the user would be pushed back (e.g., from pushing against a heavy door, or bumping into someone) are also cases where this transition would be observed to occur. Again, reverting the subphase to C1 ensures that the knee will be providing resistance to flexion load, preventing its collapse and subsequent user fall.
S2 to C1 state transition 406 is observed to occur as the level walking swing phase extension knee joint motion completes and the prosthetic foot enters in contact with the ground again, registering load on the prosthetic limb again. Detection of the loadcell sensor signal exceeding the predetermined threshold causes the phase to transition back to swing and the subphase to go back to the C1 subphase, allowing the whole cycle to start again.
With reference to
Finally, the Independent Component 509 is added to the three other arms of the controller. While the Independent Component 509 is a feedforward term which is fed through a gain kt 510 like the static torque 503 or the Angle Dependant Component 519, this one does not have an explicit dependency to a gait control parameter like the actuator angle 505 or the Phase detection engine 502, but is more heuristic in nature and accounts for specific actuator behaviours that are not properly accounted for by the three main branches of the controller structure. Typically, these behaviours are driven from specific user need and are not fundamentally part of physiological gait per se. For example, when maintaining the knee in full extension in swing phase is desired, the independent term is used to make the knee resistance increase to a level where the knee will remain in full extension, without any velocity term contribution. Similarly, it is found desirable to increase the actuator resistance prior to foot strike to mitigate any delays observed in ramping up the actuator resistance and user perceived knee buckling at initial loading, which is performed using the independent component 509, as velocity is also null in this case and the knee is operating in swing phase. Another example where the independent component 509 is used to directly affect the actuator behaviour is in stairs ascent foot placement management. As the knee flexion angle at foot strike is much higher in stairs ascent then in level ground or ramps ambulation, there is a need to stop the swing extension cycle to allow the user to step on the upcoming step, when climbing stairs step-over-step. When the appropriate actuator angle is reached, the high-level stairs ascent management will generate a short increase in actuator resistance to stop the extension motion, allowing the user to step on the flexed knee or kick-it to full extension in the case where transition back to walking is required.
The resulting sum of the four different types of contribution to the controller effort is then fed to the low-pass filter 511 block. The low-pass filter 511 characteristics are dynamically adjusted as a function of the Phase and Subphase information 512 provided by the intent management section of the control system of
Use of the low-pass filter on the setpoint allows getting rid of the high frequency components and smoothing out the input signal for control purposes. High-frequency components are typically generated by sensor noise, which are propagated and amplified through the different control scheme components. The high-frequency content also includes discontinuities found in the static torque (503), Angle Dependant Component (519) constitutive terms of the setpoint signal, as well as the independent component (509). While specific considerations can be taken to minimize the discontinuous parts generated by the independent contributors to the control signal, some are unavoidable in certain scenarios and use of the low-pass filter allows uniformizing the control signal as a whole, without requiring a heuristic approach. Finally, the Luenberg observer (504) also has a contribution to the high-frequency components since it essentially acts as a controller on the predicted and measured actuator positions with a fairly high gain. Attenuation of the aforementioned high-frequency noise and discontinuities removes audible noise generated in the actuator, contributes to attenuating possible chatter in the control loop, and provides a smoother experience to the user. Specific attention is required in compromising between having too small of a bandwidth and the delay of the filter, but the chosen design allows for minimal ringing artifacts in both the pass and stop-band, whilst still allowing for a steep drop-off.
Due to the nature of the plant at hand in this type of application and the use of the current as a control variable for controlling the magnetic field present in the MR actuator, use of a derivative term is known not to yield significant benefits and can optionally be removed from the current controller 7 to simplify the implementation. This embodiment of the inner control scheme is generally referred to as a Proportional-Integral controller 8 and its implementation in the current system is presented in
Unlike the static implementation of the PI controller illustrated in
As mentioned above, appropriate selection of the gains allows minimizing the current rise time and ensures that a MR actuator response is fast as possible, which is known to improve performance and user experience. On the other hand, attention must be directed at not using too high of gains when the error is small, as this will cause current oscillation or overshoot, which will negate the benefits of ensuring a fast rise time. In that context, the KP gains are populated in the look up table such that higher gain values are used when the error is large, forcing a more aggressive system response and increased controller action. When the error reduces, the gains are lowered to prevent excessive controller action, leading to poor tracking performance or instability. On the other hand, the KI gains follow an opposite trend, mainly showing lower values when the error is large and larger values when the error is small. These trends are based on the impact of the respective gains on the system response. Using large KI gain values when the error is larger would lead to reduced performance, as the time integration of the signal would result in a large value, which would then require the error to show the opposite sign for a significant amount of time to balance out its effect. In that context, when the error is small, a larger gain value allows the integrator to cause controller action to minimize the error over a smaller timeframe. The proportional gain KP does not act considering the timeframe, as it is not associated with a time integration process, just a direct correction of the control output. To be optimally effective, applying large corrections when the error is large, and conversely small corrections when the error is small is the logical approach to implement when the whole objective is to minimize the error amplitude at all instants.
Both theoretical and empirical approaches can be used to identify appropriate gains for a PI controller in this type of embodiment, where a gains schedule is established based on the amplitude of the error signal. Apart from the case where a single set of gains is required to meet all performance and convergence criteria for the controller application, use of a discrete gain schedule enables varying the performance targets of the gains for different values of the error amplitude. Gains targeting the optimal response based on the amplitude of the error can be empirically tuned by the submitting the system to step inputs of a size matching the amplitude of the error slot being investigated until the desired response is obtained. Alternatively, theoretical gains calculation methods, like Ziegler-Nichols, can be applied in a general fashion to the system under hand and then scaled across the range of error amplitude.
Similar to the gain schedule, division of the error range into slots must be carefully done, taking into consideration the type of dynamic response desired from the system for each slot, as well as the overall targeted system dynamic response. Use of discrete slots covering a certain range of error values limits the work required to establish a complete schedule, while also presenting a simpler correlation with the end application and its requirements (i.e., which type of dynamic response is desired based on the error amplitude). That being said, it would also be possible to create a continuous gain schedule by interpolating between discrete values previously established. In all cases, any of these strategies can be employed as long as it meets the end application requirements.
The exact number of error amplitude slots is also to be considered in view of the targeted overall system dynamic response and the impact of adding additional slots and/or gains set on the overall control loop performance. As the number of slots increases, gains definition overhead increases, while the benefit on the overall dynamic response is not necessarily increasing. Empirical testing shows that for a fixed overall performance target, improvement in dynamic response is no longer observed once a certain number of slots is reached, no matter how many additional slots are added. In a typical embodiment as the one herein described, eight slots of error amplitude were observed to be the optimal number, as addition of more slots did not yield further performance improvement.
In a similar fashion to the gain schedule where more refined gains are used for smaller error amplitudes, it is possible to divide the error slots such that smaller slots are used when the error is smaller, allowing for a more refined gains schedule to be implemented. While this type of embodiment can yield additional benefits in the form of flexibility, it is not a hard requirement and various other types of error slot patterns can be used without departing from the fundamental concept herein described. It is also possible to establish the error slots pattern using more advanced characterization techniques, to account for the combined operation of the control loop and the plant. For example, it is possible to measure the distribution of the error value while the control system is operating and use the resulting occurrence data to define the number of slots and their boundaries. Error amplitude slots such as the one provided in
At approximately time stamp 2015, it can be observed that the current rise rate is reduced as the amplitude of the current error is reducing, causing the controller to start using new gains values as per the gain schedule implemented. This trend of reducing current rise rate is continued to be observed as the current error is gradually reducing. Using 80% rise time as performance criteria allows quantifying a rise time characteristic of the performance of the current controller using a gain schedule as per herein described. Data point 1103 shows the 80% rise time point, which is found to happen approximately 9 msec after the system was subjected to the step input.
Aligning the current response plots for the gain scheduling current control scheme 1100 with the one not using the gain scheduling current control scheme herein disclosed 1104 allows for direct comparison between the performance of the two schemes. Bracket 1107 indicates the time difference required to reach the 80% current rise mark for both methods. As mentioned above, it takes approximately an additional 8 ms while not using the gain scheduling approach to reach 80% of the step input amplitude, which advantageously makes the gain scheduling current scheme to be about 47% faster ( 8/17) than the more traditional method.
The capacity to increase the current circulating in the magnetic coil of the MR actuator faster directly improves the performance of the lower-limb prosthetic device by allowing it to be more responsive to change of use context, where a sudden demand in braking torque is generated by the high-level controller. One such situation is known to happen when the user is standing at the top of a ramp or staircase and begins to walk downwards. As the user puts his or her prosthetic foot down, the combination of the forward momentum, user weight and increased leverage over the knee prosthesis actuator caused by the downward surface inclination will then create a large demand to brake the knee motion. Sudden increases in knee torque must be achieved through increase in magnetic field, which in turn is based on the current controller's capacity to quickly change the current circulating in the magnetic coil.
In that context, the current control scheme using gain scheduling herein described is observed to be almost 50% faster than other known implementations in achieving 80% of the set-point when the system is submitted to a large change in current set-point.
While certain embodiments have been described, these embodiments have been presented by way of example only and are not intended to limit the scope of the disclosure. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms. Furthermore, various omissions, substitutions and changes in the systems and methods described herein may be made without departing from the spirit of the disclosure. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure. Accordingly, the scope of the present disclosure is defined only by reference to the appended claims.
Features, materials, characteristics, or groups described in conjunction with a particular aspect, embodiment, or example are to be understood to be applicable to any other aspect, embodiment or example described in this section or elsewhere in this specification unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive. The protection is not restricted to the details of any foregoing embodiments. The protection extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.
Furthermore, certain features that are described in this disclosure in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features that are described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations, one or more features from a claimed combination can, in some cases, be excised from the combination, and the combination may be claimed as a sub-combination or variation of a sub-combination.
Moreover, while operations may be depicted in the drawings or described in the specification in a particular order, such operations need not be performed in the particular order shown or in sequential order, or that all operations be performed, to achieve desirable results. Other operations that are not depicted or described can be incorporated in the example methods and processes. For example, one or more additional operations can be performed before, after, simultaneously, or between any of the described operations. Further, the operations may be rearranged or reordered in other implementations. Those skilled in the art will appreciate that in some embodiments, the actual steps taken in the processes illustrated and/or disclosed may differ from those shown in the figures. Depending on the embodiment, certain of the steps described above may be removed, others may be added. Furthermore, the features and attributes of the specific embodiments disclosed above may be combined in different ways to form additional embodiments, all of which fall within the scope of the present disclosure. Also, the separation of various system components in the implementations described above should not be understood as requiring such separation in all implementations, and it should be understood that the described components and systems can generally be integrated together in a single product or packaged into multiple products.
For purposes of this disclosure, certain aspects, advantages, and novel features are described herein. Not necessarily all such advantages may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the disclosure may be embodied or carried out in a manner that achieves one advantage or a group of advantages as taught herein without necessarily achieving other advantages as may be taught or suggested herein.
Conditional language, such as “can,” “could,” “might,” or “may,” unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements, and/or steps. Thus, such conditional language is not generally intended to imply that features, elements, and/or steps are in any way required for one or more embodiments or that one or more embodiments necessarily include logic for deciding, with or without user input or prompting, whether these features, elements, and/or steps are included or are to be performed in any particular embodiment.
Conjunctive language such as the phrase “at least one of X, Y, and Z,” unless specifically stated otherwise, is otherwise understood with the context as used in general to convey that an item, term, etc. may be either X, Y, or Z. Thus, such conjunctive language is not generally intended to imply that certain embodiments require the presence of at least one of X, at least one of Y, and at least one of Z.
Language of degree used herein, such as the terms “approximately,” “about,” “generally,” and “substantially” as used herein represent a value, amount, or characteristic close to the stated value, amount, or characteristic that still performs a desired function or achieves a desired result. For example, the terms “approximately”, “about”, “generally,” and “substantially” may refer to an amount that is within less than 10% of, within less than 5% of, within less than 1% of, within less than 0.1% of, and within less than 0.01% of the stated amount. As another example, in certain embodiments, the terms “generally parallel” and “substantially parallel” refer to a value, amount, or characteristic that departs from exactly parallel by less than or equal to 15 degrees, 10 degrees, 5 degrees, 3 degrees, 1 degree, or 0.1 degree.
The scope of the present disclosure is not intended to be limited by the specific disclosures of preferred embodiments in this section or elsewhere in this specification, and may be defined by claims as presented in this section or elsewhere in this specification or as presented in the future. The language of the claims is to be interpreted broadly based on the language employed in the claims and not limited to the examples described in the present specification or during the prosecution of the application, which examples are to be construed as non-exclusive.
Of course, the foregoing description is that of certain features, aspects and advantages of the present invention, to which various changes and modifications can be made without departing from the spirit and scope of the present invention. Moreover, the devices described herein need not feature all of the objects, advantages, features and aspects discussed above. Thus, for example, those of skill in the art will recognize that the invention can be embodied or carried out in a manner that achieves or optimizes one advantage or a group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. In addition, while a number of variations of the invention have been shown and described in detail, other modifications and methods of use, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is contemplated that various combinations or sub-combinations of these specific features and aspects of embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the discussed devices.
| Number | Date | Country | |
|---|---|---|---|
| 63265544 | Dec 2021 | US |
