Biomechatronics

Human walking neuromechanical models show how each muscle works during normal, level-ground walking. They are mainly modeled with clutches and linear springs, and are able to capture dominant normal walking behavior. This suggests to us to use a knee-flexor spring for below-knee amputees. We have developed the powered ankle prosthesis, which generates enough force to enable a user to walk "normally." However, amputees still have problems at the knee joint due to the lack of gastrocnemius, which works as an ankle-knee flexor and a plantar flexor. We hypothesize that metabolic cost and EMG patterns of an amputee with our powered ankle and virtual gastrocnemius will dramatically improve.

In contrast to traditional, purely dissipative prosthetic knees, we propose an active biomimetic knee prosthesis with two series-elastic actuators arranged in parallel in an agonist-antagonist architecture. This variable-impedance knee is capable of mimicking human knee mechanics during level-ground walking with a quasi-passive control strategy. This strategy reduces the overall electrical power requirements, allowing for an energetically economical powered knee system. The objective of this adaptive, powered prosthetic device is to improve gait and metabolic energy consumption of above-knee amputees on variant terrain conditions.

Motivated by applications in rehabilitation and robotics, we are developing methodologies to control muscle-actuated systems via electrical stimulation. As a demonstration of such potential, we are developing centimeter-scale robotic systems that utilize muscle for actuation and glucose as a primary source of fuel. This is an interesting control problem because muscles: a) are mechanical state-dependent actuators; b) exhibit strong nonlinearities; and c) have slow time-varying properties due to fatigue-recuperation, growth-atrophy, and damage-healing cycles. We are investigating a variety of adaptive and robust control techniques to enable us to achieve trajectory tracking, as well as mechanical power-output control under sustained oscillatory conditions. To implement and test our algorithms, we developed an experimental capability that allows us to characterize and control muscle in real time, while imposing a wide variety of dynamical boundary conditions.

We are studying the mechanical behavior of leg muscles and tendons during human walking in order to motivate the design of economical robotic legs. We hypothesize that quasi-passive, series-elastic clutch units spanning the knee joint in a musculoskeletal arrangement can capture the dominant mechanical behaviors of the human knee in level-ground walking. Biarticular elements necessarily need to transfer energy from the knee joint to hip and/or ankle joints, and this mechanism would reduce the necessary muscle work and improve the mechanical economy of a human-like walking robot.

This research aims to extract a potentially small set of underlying principles that govern human movement and to apply that set of principles to biomimetic control systems. Using a morphologically realistic human model and kinematic gait data, we find that spin angular momentum in human walking is highly regulated, and that there exists a nonlinear coupling between center of mass transverse forces, center of mass position, and center of pressure location. Using an open loop optimization strategy, we show that biologically realistic leg joint kinematics emerge through the minimization of spin angular momentum and the sum of the joint torques squared. This suggests that both angular momentum and energetic factors are important considerations for biomimetic controllers.

Towards the goal of developing stable humanoid robots and leg prostheses/othoses, we are developing a biologically motivated control system for walking where system angular momentum is explicitly controlled. Using human kinematic walking data, we find that spin angular momentum is highly regulated in walking. In addition, our analysis shows that the distribution of angular momentum throughout the human body, or the angular momentum partition, is invariant with walking speed. Motivated by these biomechanical results, we conduct numerical simulations of walking using a morphologically realistic human model. Our control system searches for joint reference trajectories that minimize the error between the model's angular momentum partition and the biologically determined partition. In order to understand motor control in humans, we are experimenting with biological time delays (order 100 ms) and correlating our simulation results with human behavior.

People on the autism spectrum face a number of challenges, including motor movement issues that can cause limbs to cease activity. Circumstantial evidence suggests that autonomic nervous system influences related to stress and overload may arise from and contribute to these problems. We propose allowing individuals to monitor several physiological parameters to see if there are patterns that recognize or predict the onset of their individual motor problems. We plan to develop new, wearable technology to treat these problems via the use of tiny, vibrotactile devices carefully placed at the joints. We hypothesize that some methods of touch-feedback and vibration at the joints may enable individuals to recover motor functioning during episodes of intermittent loss. We are also exploring the development of personally controlled devices that facilitate finer motor movement for augmenting communication as needed for assisting in typing or pointing.

The human ankle provides a significant amount of net positive work during the stance period of walking, especially at moderate to fast walking speeds. On the contrary, conventional ankle-foot prostheses are completely passive during stance, and consequently, cannot provide net positive work. Clinical studies indicate that transtibial amputees using conventional prostheses experience many problems during locomotion including a high gait metabolism, a low gait speed, and gait asymmetry. Researchers believe the main cause for the observed locomotion is due to the inability of conventional prostheses to provide net positive work during stance. The objective of this project is to develop a powered ankle-foot prosthesis that is capable of providing net positive work during the stance period of walking. To this end, we are investigating the mechanical design and control system architectures for the prosthesis. We also conduct a clinical evaluation of the proposed prosthesis on different amputee participants.

Augmentation of human locomotion has proven an elusive goal. Natural human walking is extremely efficient and the complex articulation of the human leg poses significant engineering difficulties. We present a wearable exoskeleton designed to reduce the metabolic cost of human walking by providing an upward force on the leg during swing phase. This design features a small size and low weight and is localized comfortably on the lower back with only lightweight nylon cables extending to the feet below.