Jonathon W. Sensinger

Adaptive Pattern Recognition of Myoelectric Signals: Exploration of Conceptual Framework and Practical Algorithms

Pattern recognition is a useful tool for deciphering movement intent from myoelectric signals. Recognition paradigms must adapt with the user in order to be clinically viable over time. Most existing paradigms are static, although two forms of adaptation have received limited attention. Supervised adaptation can achieve high accuracy since the intended class is known, but at the cost of repeated cumbersome training sessions. Unsupervised adaptation attempts to achieve high accuracy without knowledge of the intended class, thus achieving adaptation that is not cumbersome to the user, but at the cost of reduced accuracy. This study reports a novel adaptive experiment on eight subjects that allowed repeated measures post-hoc comparison of four supervised and three unsupervised adaptation paradigms. All supervised adaptation paradigms reduced error over time by at least 26% compared to the nonadapting classifier. Most unsupervised adaptation paradigms provided smaller reductions in error, due to frequent uncertainty of the correct class. One method that selected high-confidence samples showed the most practical implementation, although the other methods warrant future investigation. Supervised adaptation should be considered for incorporation into any clinically viable pattern recognition controller, and unsupervised adaptation should receive renewed interest in order to provide transparent adaptation.

Virtual Constraint Control of a Powered Prosthetic Leg: From Simulation to Experiments with Transfemoral Amputees.

Recent powered (or robotic) prosthetic legs independently control different joints and time periods of the gait cycle, resulting in control parameters and switching rules that can be difficult to tune by clinicians. This challenge might be addressed by a unifying control model used by recent bipedal robots, in which virtual constraints define joint patterns as functions of a monotonic variable that continuously represents the gait cycle phase. In the first application of virtual constraints to amputee locomotion, this paper derives exact and approximate control laws for a partial feedback linearization to enforce virtual constraints on a prosthetic leg. We then encode a human-inspired invariance property called effective shape into virtual constraints for the stance period. After simulating the robustness of the partial feedback linearization to clinically meaningful conditions, we experimentally implement this control strategy on a powered transfemoral leg. We report the results of three amputee subjects walking overground and at variable cadences on a treadmill, demonstrating the clinical viability of this novel control approach.

User-Modulated Impedance Control of a Prosthetic Elbow in Unconstrained, Perturbed Motion

Humans use the agonist-antagonist structure of their muscles to simultaneously determine both the motion and the stiffness of their joints. Designing this feature into an artificial limb may prove advantageous. To evaluate the performance of an artificial limb capable of modulating its impedance, we have created a compact series elastic actuator that has the same size and similar weight as commercially available electric prosthetic elbows. The inherent compliance in series elastic actuators ensure their safety to the user, even at high speeds, while creating a high-fidelity force actuator ideally suited for impedance control. This paper describes three serial studies that build on each other. The first study presents modeling of the actuator to ensure stability in the range of impedance modulation and empirically tests the actuator to validate its ability to modulate impedance. The actuator is found to be stable and accurate over a wide range of impedances. In the second study, four subjects are tested in a preliminary experiment to answer basic questions necessary to implement user-modulated impedance control. Findings include the superiority of velocity control over position control as the underlying motion paradigm and the preference for high stiffness and non-negative inertia. Based on the findings of the second study, the third study evaluates the performance of 15 able-bodied subjects for two tasks, using five different impedance paradigms. Impedance modulation, speed, and error were compared across paradigms. The results indicate that subjects do not actively modulate impedance if it is near a preferred baseline. Fixed impedance and viscosity modulation provide the most accurate control.

Improvements to Series Elastic Actuators

Actuators must be safe when interacting with humans, even in unexpected situations. Safety requires low impedance (low forces for a given perturbation) at all frequencies, not only in the actuators stable bandwidth. Series elastic actuators (SEAs) are capable of achieving low impedance across all frequencies, though their force-frequency saturation envelope is decreased as a result. This paper examines ways to increase the force fidelity of SEAs by introducing inner control feedback loops and using appropriate sensor location. Findings include the superiority of a low fidelity sensor distal to stiction sources over a high fidelity sensor proximal to stiction and the superiority of an inner position feedback loop over an inner velocity feedback loop

The Difference Between Stiffness and Quasi-Stiffness in the Context of Biomechanical Modeling

The ankle contributes the majority of mechanical power during walking and is a frequently studied joint in biomechanics. Specifically, researchers have extensively investigated the torque-angle relationship for the ankle during dynamic tasks, such as walking and running. The slope of this relationship has been termed the “quasi-stiffness.” However, over time, researchers have begun to interchange the concepts of quasi-stiffness and stiffness. This is an especially important distinction as researchers currently begin to investigate the appropriate control systems for recently developed powered prosthetic legs. The quasi-stiffness and stiffness are distinct concepts in the context of powered joints, and are equivalent in the context of passive joints. The purpose of this paper is to demonstrate the difference between the stiffness and quasi-stiffness using a simple impedance-controlled inverted pendulum model and a more sophisticated biped walking model, each with the ability to modify the trajectory of an impedance controllers equilibrium angle position. In both cases, stiffness values are specified by the controller and the quasi-stiffness are shown during a single step. Both models have widely varying quasi-stiffness but each have a single stiffness value. Therefore, from this simple modeling approach, the differences and similarities between these two concepts are elucidated.

Towards Biomimetic Virtual Constraint Control of a Powered Prosthetic Leg

This brief presents a novel control strategy for a powered prosthetic ankle based on a biomimetic virtual constraint. We first derive a kinematic constraint for the “effective shape” of the human ankle-foot complex during locomotion. This shape characterizes ankle motion as a function of the center of pressure (COP)-the point on the foot sole where the resultant ground reaction force is imparted. Since the COP moves monotonically from heel to toe during steady walking, we adopt the COP as a mechanical representation of the gait cycle phase in an autonomous feedback controller. We show that our kinematic constraint can be enforced as a virtual constraint by an output linearizing controller that uses only feedback available to sensors onboard a prosthetic leg. Using simulations of a passive walking model with feet, we show that this novel controller enforces exactly the desired effective shape, whereas a standard impedance (i.e., proportional-derivative) controller cannot. This brief provides a single, biomimetic control law for the entire single-support period during robot-assisted locomotion.

Design and analysis of a non-backdrivable series elastic actuator

The compliance of series elastic actuators makes them robust to environmental perturbations, and their ability to use impedance control may improve their performance in open-loop environments such as conventional prosthetic control. Series elastic actuators have historically required a backdrivable transmission to ensure adequate bandwidth. The authors have created a non-backdrivable series elastic actuator through the inclusion of a harmonic drive transmission, which contains no backlash; the dominant feature in limiting frequency resolution. The actuator has an acceptable speed and stall torque and an adequate frequency range, and would be used in the future to implement impedance control of prosthetics.

Evidence for a Time-Invariant Phase Variable in Human Ankle Control

Human locomotion is a rhythmic task in which patterns of muscle activity are modulated by state-dependent feedback to accommodate perturbations. Two popular theories have been proposed for the underlying embodiment of phase in the human pattern generator: a time-dependent internal representation or a time-invariant feedback representation (i.e., reflex mechanisms). In either case the neuromuscular system must update or represent the phase of locomotor patterns based on the system state, which can include measurements of hundreds of variables. However, a much simpler representation of phase has emerged in recent designs for legged robots, which control joint patterns as functions of a single monotonic mechanical variable, termed a phase variable. We propose that human joint patterns may similarly depend on a physical phase variable, specifically the heel-to-toe movement of the Center of Pressure under the foot. We found that when the ankle is unexpectedly rotated to a position it would have encountered later in the step, the Center of Pressure also shifts forward to the corresponding later position, and the remaining portion of the gait pattern ensues. This phase shift suggests that the progression of the stance ankle is controlled by a biomechanical phase variable, motivating future investigations of phase variables in human locomotor control.

Improved torque fidelity in harmonic drive sensors through the union of two existing strategies

The torsional compliance of harmonic drives lends itself to torque sensing, but the elliptical wave generator in harmonic drives introduces a spatially dependent local torque ripple on the flexspline of harmonic drives. It is difficult to extract the applied torque signal from this local torque ripple if the applied torque is sensed at the flexspline of the harmonic drive. Previous methods have used spatially offset Wheatstone bridges or gain tuning to attenuate torque ripple. The authors have found that combining both of these techniques through a different wiring configuration can attenuate local torque ripple to plusmn0.5 Nmiddotm, a better ripple reduction than either method alone. This local ripple reduction is adequate for large-range torque applications, but is not adequate for precision torque applications

Experimental effective shape control of a powered transfemoral prosthesis

This paper presents the design and experimental implementation of a novel feedback control strategy that regulates effective shape on a powered transfemoral prosthesis. The human effective shape is the effective geometry to which the biological leg conforms - through movement of ground reaction forces and leg joints - during the stance period of gait. Able-bodied humans regulate effective shapes to be invariant across conditions such as heel height, walking speed, and body weight, so this measure has proven to be a very useful tool for the alignment and design of passive prostheses. However, leg joints must be actively controlled to assume different effective shapes that are unique to tasks such as standing, walking, and stair climbing. Using our previous simulation studies as a starting point, we model and control the effective shape as a virtual kinematic constraint on the powered Vanderbilt prosthetic leg with a custom instrumented foot. An able-bodied subject used a by-pass adapter to walk on the controlled leg over ground and over a treadmill. These preliminary experiments demonstrate, for the first time, that effective shape (or virtual constraints in general) can be used to control a powered prosthetic leg.