Design and Evaluation of a Reaction-Force Series Elastic Actuator Configurable as Biomimetic Powered Ankle and Knee Prostheses
All commercial leg powered prostheses have been, up to this point, a one-size fits-all design. Yet, no human body is the same as the next. A configurable prosthesis potentially offers improvements in battery run-time, prosthesis mass, acoustic noise, user comfort, and even enables sport and economy modes within the same fundamental hardware. Further, of those existing systems, none has yet managed to fully achieve walking biological range of motion, torque and power. In this thesis, I present a reaction-force, series-elastic actuator (RFSEA) that is capable of achieving biomimetic ankle and knee kinetics and kinematics during level-ground walking across a range of body masses, heights and walking styles. The platform is configurable to inertial load by swapping a simple-to-manufacture flat-plate composite spring that allows tuning the actuator dynamics to match different user requirements. The RFSEA also comprises a high torque and pole-count drone motor that directly drives a ball screw with a tunable low-gear ratio lead. The design enables high dynamic range providing a closed-loop, torque-controlled joint that can demonstrate arbitrary levels of impedance. This control fidelity is important to support smooth control in free-space and high-inertial output conditions, such as the swing and late-stance phases of walking, respectively. A simulation framework is presented that defines mechatronic design specifications for the motor, spring, and gear-reduction components. The optimization procedure clamps output joint dynamics using subject-specific biological gait data, and searches for minimum electric energy solutions across the motor, gear-reduction and spring component space. A second optimization procedure then searches for optimal linkage and spring geometry to best approach the design targets as constrained by the availability of discrete drivetrain components. In this thesis, ankle and knee designs are presented with optimized components using biological joint data from a non-amputee subject walking at 1.5 m/sec with a body mass equal to 90Kg. For these designed biomimetic joints, system specifications are verified using bench test evaluations, and preliminary human gait studies. With a minimum viable actuator mass of 1.4Kg, the platform has a nominal torque control bandwidth of 6Hz at 82Nm, a repeated peak torque capacity of 175Nm, peak demonstrated power over 400W (with theoretical limits over 1kW), a 110 degree range of motion, as well as torque and power densities of 125 Nm/kg and 286 W/kg, respectively. Configured as an ankle-foot prosthesis, there are 35 degrees of dorsiflexion and 75 degrees of plantar flexion, and as a knee, 110 degrees of flexion are available to enable activities on varied terrain such as stairs and inclines. Walking dynamics are evaluated with a finite state-machine ankle controller piloted by N=3 subjects with below-knee amputation walking at 1.5 m/sec on an instrumented treadmill. In preliminary experiments, net positive work of 0.2 J/Kg, peak joint torque of 1.5 Nm/Kg, and peak mechanical power of 4.3 W/Kg all fall within one standard deviation of the intact-limb biological mean. Configured as an ankle-foot prosthesis, the system mass is 2.2Kg including battery and electronics, and as a knee the system mass is 1.6Kg, making the RFSEA platform the lightest and most biomimetic leg system yet published.
Professor Hugh Herr, Director Biomechatronics, MIT Media Lab, Director Center for Extreme Bionics
Associate Professor Sangbae Kim, Director Biomimetics Lab, MIT Mechanical Engineering
Associate Professor Amos Winter, Director GEAR Lab, MIT Mechanical Engineering