Commercially available prosthetic feet are fabricated to have a fixed forefoot and hindfoot stiffness that cannot be changed in a clinical setting. This does not allow for patients to quickly compare multiple prosthetic foot stiffnesses to choose the stiffness they like the most while walking. In this paper, we present the Footropter, a passive prosthetic foot prescription tool that allows Certified Prosthetists (CPs) to rapidly change both the forefoot and hindfoot stiffnesses. The forefoot stiffness is changed by repositioning a spring clamp along a length of unbonded fiberglass layers and the hindfoot stiffness is changed by repositioning a single heel spring support. We introduce the design and working principles, characterize the ranges of available forefoot and hindfoot stiffnesses, and demonstrate the utility of the Footropter through two preference and perception studies with two unilateral transtibial prosthesis users. The Footropter, when paired with a preference optimization algorithm, can enable CPs to integrate patients’ experiential input into the clinical prescription process.

The biological ankle dorsiflexes several degrees during swing to provide adequate clearance between the foot and ground, but conventional energy storage and return (ESR) prosthetic feet remain in their neutral position, increasing the risk of toe scuffs and tripping. We present a new prosthetic ankle intended to reduce fall risk by dorsiflexing the ankle joint during swing, thereby increasing the minimum clearance between the foot and ground. Unlike previous approaches to providing swing dorsiflexion such as powered ankles or hydraulic systems with dissipative yielding in stance, our ankle device features a spring-loaded linkage that adopts a neutral angle during stance, allowing ESR, but adopts a dorsiflexed angle during swing. The ankle unit was designed, fabricated, and assessed in level ground walking trials on a unilateral transtibial prosthesis user to experimentally validate its stance and swing phase behaviors. The assessment consisted of three conditions: the ankle in an operational configuration, the ankle in a locked configuration (unable to dorsiflex), and the subject's daily use ESR prosthesis. When the ankle was operational, minimum foot clearance (MFC) increased by 13 mm relative to the locked configuration and 15 mm relative to his daily use prosthesis. Stance phase energy return was not significantly impacted in the operational configuration. The increase in MFC provided by the passive dorsiflexing ankle prosthesis may be sufficient to decrease the rate of falls experienced by prosthesis users in the real world.

This paper presents the design of a swing-assist prosthetic knee capable of providing stabilizing passive torques during the stance phase of walking and supplementing the passive swing phase behavior of the prosthetic knee with small active torques. The prosthesis design utilizes a novel actuator which integrates a symmetric multi-chamber hydraulic cylinder and a highly backdrivable linear electromechanical drive system into a single compact package. This actuator is implemented in a self-contained prosthesis design (including batteries and embedded system) which weighs 1.7 kg and is 28 cm in length, making this design comparable in size/mass to commercially available microprocessor-controlled prosthetic knees. The device is controlled using a finite state machine with a novel assist-as-needed controller operating during the swing phase. This assist-as-needed controller supplements a nominally passive swing phase behavior with small amounts of active power to enhance user safety. The device and controller were assessed on a single participant who took part in level ground walking experiments with both the swing-assist prosthesis and his daily-use device. Results indicate that the swing-assist prosthesis increases maximum swing phase knee flexion angle relative to the subject’s daily-use device. Furthermore, the swing-assist prosthesis was shown to automatically vary its assistance across walking speeds.

This paper describes a semi-powered ankle prosthesis and corresponding unified controller that provides biomimetic behavior for level and sloped walking without requiring identification of ground slope or modulation of control parameters. The controller is based on the observation that healthy individuals maintain an invariant external quasi-stiffness (spring like behavior between the shank and ground) when walking on level and sloped terrain. Emulating an invariant external quasi-stiffness requires an ankle that can vary the set-point (i.e., equilibrium angle) of the ankle stiffness. A semi-powered ankle prosthesis that incorporates a novel constant-volume power-asymmetric actuator was developed to provide this behavior, and the unified controller was implemented on it. The device and unified controller were assessed on three subjects with transtibial amputations while walking on inclines, level ground, and declines. Experimental results suggest that the prosthesis and accompanying controller can provide a consistent external quasi-stiffness similar to healthy subjects across all tested ground slopes.

This paper describes the design of a simple and low cost compliant low profile prosthetic foot based on a cantilevered beam of uniform strength. The prosthetic foot is developed such that the maximum stress experienced by the beam is distributed approximately evenly across the length of the beam. Due to this stress distribution, the prosthetic foot exhibits compliant behavior not achievable through standard design approaches (e.g. designs based on simple cantilevered beams). Additionally, due to its simplicity and use of flat structural members, the foot can be manufactured at low cost. An analytical model of the compliant behavior of the beam is developed that facilitates rapid design changes to vary foot size and stiffness. A characteristic prototype was designed and constructed to be used in both a benchtop quasistatic loading test as well as a dynamic walking test for validation. The model predicted the rotational stiffness of the prototype with 5% error. Furthermore, the prototype foot was tested alongside two commercially available prosthetic feet (a low profile foot and an energy storage and release foot) in level walking experiments with a single study participant. The prototype foot displayed the lowest stiffness of the three feet (6.0, 7.1, and 10.4 Nm/deg for the prototype foot, the commercial low profile foot, and the energy storage and release foot, respectively). This foot design approach and accompanying model may allow for compliant feet to be developed for individuals with long residual limbs.

This paper describes the design, control, and preliminary assessment of a novel microprocessor-controlled multifunction ankle prosthesis that provides three microprocessor-controlled behaviors-a selectable stiffness equilibrium angle, lockable conformal damping, and swing-phase repositioning. Following a description of the motivation for providing these behaviors, the authors provide a detailed description of the device and walking controller design. This device utilizes a power-asymmetric linear actuator to provide the desired functionality in a compact and lightweight package through a combination of both hydraulic and electromechanical actuation approaches. The device is controlled for level ground walking via a finite-state machine. The functionality of the prosthesis is demonstrated by a set of benchtop experiments that characterize the ability of the prosthesis to provide the three desired behaviors and by an experiment in which the prosthesis was worn by a transtibial amputee during walking. Both sets of experiments indicate that the prosthesis provides the functionality for which it was designed.

This paper presents a control approach for an over-ground lower limb exoskeleton that is intended to provide guidance and assistance to poorly ambulatory individuals during walking without unduly interfering with their ability to maintain balance. The control approach achieves these objectives by emulating a viscous flow field acting on the lower limb joints. The extent to which the control approach achieves the objectives was assessed in experiments, conducted on five healthy subjects, comparing guidance and disturbance characteristics of the velocity-based controller to a potential-field-based controller. Results show that the flow controller provides a combination of lower guidance error and lower disturbance to the user, relative to the potential-field-based controller. The paper also discusses various potentially beneficial characteristics of the flow controller, such as first-order homogeneous behavior, implicitly combined guidance and assistance behaviors, and improved directionality in error correction relative to a potential-field-based controller.

This paper presents a formulation for selecting the stage ratios and number of stages in a multistage transmission with a given desired total transmission ratio in a manner that maximizes efficiency, maximizes acceleration, or minimizes the mass of the transmission. The formulation is used to highlight several implications for gear train design, including the fact that minimizing rotational inertia and mass are competing objectives with respect to optimal selection of stage ratios, and that both rotational inertia and mass can often be minimized by increasing the total number of stages beyond a minimum realizable number. Additionally, a multistage transmission will generally provide maximum acceleration when the stage ratios increase monotonically from the motor to the load. The transmission will have minimum mass when the stage ratios decrease monotonically. The transmission will also provide maximum efficiency when the corresponding stages employ constant stage ratios. This paper aims to use this optimization formulation to elucidate tradeoffs between various common objectives in gear train design (efficiency, acceleration, and mass).

This paper describes a controller for guiding and assisting leg movement during walking with a lower limb exoskeleton with actuated hip and knee joints. The primary novel aspect of the controller is that it employs a virtual flow field to influence movement during swing, rather than a more typical potential-energy-based field. The controller was tested on a single stroke subject. The stroke subject's leg kinematics demonstrate that the controller is capable of appropriately influencing leg kinematics during overground walking.

This paper presents the design and control of a lightweight swing assist (SA) knee prosthesis. The SA knee relies on passive stability to provide support during the stance phase of walking and incorporates a small motor and battery to actively assist the knee motion during the swing phase. A prototype SA knee was constructed and experimentally evaluated on a single transfemoral amputee. The experiments consisted of treadmill walking at three speeds, first on a dailyuse passive prosthesis and subsequently on the SA prosthesis prototype, while recording motion capture and ground reaction force data from which prosthesis knee kinematics and affectedside hip torque were computed. A comparison of the passive daily-use prosthesis and the SA prosthesis indicates that the SA prosthesis provides more consistent and repeatable knee motion and reduces pre-swing peak hip torque across all walking speeds.

Regular use of prostheses is critical for individuals with lower limb amputations to achieve everyday mobility, maintain physical and physiological health, and achieve a better quality of life. Use of prostheses is influenced by numerous factors, with prosthetic design playing a critical role in facilitating mobility for an amputee. Thus, prostheses design can either promote biomechanically efficient or inefficient gait behavior. In addition to increased energy expenditure, inefficient gait behavior can expose prosthetic user to an increased risk of secondary musculoskeletal injuries and may eventually lead to rejection of the prosthesis. Consequently, researchers have utilized the technological advancements in various fields to improve prosthetic devices and customize them for user specific needs. One evolving technology is powered prosthetic components. Presently, an active area in lower limb prosthetic research is the design of novel controllers and components in order to enable the users of such powered devices to be able to reproduce gait biomechanics that are similar in behavior to a healthy limb. In this case series, we studied the impact of using a powered knee-ankle prostheses (PKA) on two transfemoral amputees who currently use advanced microprocessor controlled knee prostheses (MPK). We utilized outcomes pertaining to kinematics, kinetics, metabolics, and functional activities of daily living to compare the efficacy between the MPK and PKA devices. Our results suggests that the PKA allows the participants to walk with gait kinematics similar to normal gait patterns observed in a healthy limb. Additionally, it was observed that use of the PKA reduced the level of asymmetry in terms of mechanical loading and muscle activation, specifically in the low back spinae regions and lower extremity muscles. Further, the PKA allowed the participants to achieve a greater range of cadence than their predicate MPK, thus allowing them to safely ambulate in variable environments and dynamically control speed changes. Based on the results of this case series, it appears that there is considerable potential for powered prosthetic components to provide safe and efficient gait for individuals with above the knee amputation.

This paper presents the design and characterization of a power-asymmetric actuator for a transtibial prosthesis. The device is designed to provide the combination of: 1) joint locking, 2) high power dissipation, and 3) low power generation. This actuator functionality allows for a prosthesis to be designed with minimal mass and power consumption relative to a fully-powered robotic prosthesis while maintaining much of the functionality necessary for activities of daily living. The actuator achieves these design characteristics while maintaining a small form factor by leveraging a combination of electromechanical and hydraulic components. The design of the actuator is described herein, and results of an experimental characterization are provided that indicate that the actuator is capable of providing the functional capabilities required of an ankle prosthesis in a compact and lightweight package.

This paper describes a lower limb exoskeleton control approach that facilitates a desired movement coordination between the hip and knee joints during the swing phase of gait. An important feature of the proposed controller is that it provides movement guidance while allowing a user to control step time and step length. Specifically, control of step time is enabled by the use of time-invariant movement constraints, while control of step length is enabled by a real-time path-planning feature. As such, the controller provides movement coordination, but still enables a user to retain the step-to-step variability required to maintain balance during walking. The controller was implemented on a lower limb exoskeleton and tested on five healthy subjects. The subjects walked overground in the exoskeleton without a stability aid under two conditions: with the proposed coordination controller (i.e., with sagittal plane movement constraints) and with no control implemented (i.e., without sagittal plane movement constraints). Data from these assessments indicate that the controller provided substantial movement coordination while still allowing subjects substantive control of step time and length across a range of walking speeds.

This paper describes the design, implementation, and preliminary validation of a controller for a powered lower-limb exoskeleton that reshapes a user's leg movement during over-ground walking. The intended application of the controller is to facilitate gait training for individuals suffering from post-stroke hemiparesis. The controller mimics a kinematic constraint between the knee and hip joints during the swing phase of gait, such that movement is not dependent on time (i.e., step time is determined entirely by the user). The controller additionally incorporates real-time path planning adjustment that allows step length to be adjusted by the user. As such, the controller provides movement coordination, but still enables a user to retain the step-to-step variability required to maintain balance during walking. As a preliminary assessment of efficacy, the controller was implemented on a lower limb exoskeleton and tested on a healthy subject, who walked at varying speeds without the use of a stability aid, with and without the proposed controller. The data indicates that the exoskeleton with controller provided the intended extent of movement coordination, while still allowing the subject to maintain walking balance.

This paper presents a method for selecting the optimal transmission ratio for an electric motor for applications for which the desired torque and motion at the transmission output are known a priori. Representative applications for which the desired output torque and motion are periodic and known include robotic manipulation, robotic locomotion, powered prostheses, and exoskeletons. Optimal transmission ratios are presented in two senses: one that minimizes the root-mean-square (RMS) electrical current and one that minimizes the RMS electrical power. An example application is presented in order to demonstrate the method for optimal transmission ratio selection.

A controller for a powered transfemoral prosthesis is presented, which can coordinate power delivery at the knee with the motion of the crankshaft on a bicycle. The controller continuously estimates the lengths of a four-bar linkage model through the application of a recursive least squares algorithm. The link lengths are used to estimate the angle of the bicycle crankshaft. With this measure, the delivery of knee torque is coordinated with the motion of the user. The controller is implemented on a prosthesis prototype and assessed on a transfemoral amputee subject ( N = 1). The subject exhibited a bilateral work asymmetry of 83.5% when cycling with his daily use prosthesis in a free-swing mode. When 60-N·m peak assistance was provided by the powered prosthesis, the bilateral work asymmetry was reduced to 11.4%. The subject's metabolic energy rate was measured for speed and power-matched cycling while the powered prosthesis provided zero assistance (i.e., turned off and providing its back-drive resistance) or 30-N·m peak assistance. Subject's metabolic energy rate decreased by 16.5% when receiving powered assistance relative to the zero-assistance condition.

This paper describes a control approach that provides walking and standing functionality for a powered ankle prosthesis, and demonstrates the efficacy of the approach in experiments with a unilateral transtibial amputee subject. Both controllers incorporate a finite-state structure that emulates healthy ankle joint behavior via a series of piecewise passive impedance functions. The walking controller additionally modifies impedance parameters based on estimated cadence, while the standing controller modulates the ankle equilibrium angle in order to adapt to the ground slope and user posture, and the supervisory controller selects between the walking and standing controllers. The system is shown to reproduce several essential biomechanical features of the healthy joint during walking, particularly relative to a passive prosthesis, and is shown to adapt to various cadences. The system is also shown to adapt to slopes over a range of ±15°, providing support to the user, as validated by quasi-static stiffness measurements recorded by the prosthesis. The subject is shown to place more weight on the powered prosthesis than on his passive prosthesis when standing on sloped surfaces, particularly at angles of 10° or greater. The authors also demonstrated that the prosthesis typically began providing support within 1 s of initial ground contact. Further, the supervisory controller was shown to effectively switch between walking and standing, as well as determine ground slope just prior to the transition from the standing controller to the walking controller, where the estimated ground slope was accurate to within 1.25° for all trials.

A successful walking gait with a powered prosthesis depends heavily on proper timing of power delivery, or push-off. This paper describes a control approach which provides improved walking on uneven terrain relative to previous work intended for use on even (level) terrain. This approach is motivated by an initial healthy subject study which demonstrated less variation in sagittal plane shank angle than sagittal plane ankle angle when walking on uneven terrain relative to even terrain. The latter therefore replaces the former as the control signal used to initiate push-off in the powered prosthesis described herein. The authors demonstrate improvement in consistency for several gait characteristics, relative to healthy, as well as controller characteristics with the new control approach, including a 50% improvement in the consistency of the percentage of stride at which push-off is initiated.

This paper presents a running control architecture for a powered knee and ankle prosthesis that enables a transfemoral amputee to run with a biomechanically appropriate running gait and to intentionally transition between a walking and running gait. The control architecture consists firstly of a coordination level controller, which provides gait biomechanics representative of healthy running, and secondly of a gait selection controller that enables the user to intentionally transition between a running and walking gait. The running control architecture was implemented on a transfemoral prosthesis with powered knee and ankle joints, and the efficacy of the controller was assessed in a series of running trials with a transfemoral amputee subject. Specifically, treadmill trials were conducted to assess the extent to which the coordination controller provided a biomechanically appropriate running gait. Separate trials were conducted to assess the ability of the user to consistently and reliably transition between walking and running gaits.