This paper presents the design and modelling of a Transmissive Force Sensing Elastic Actuator (TFSEA) for the hip and knee joints of a gait assisting exoskeleton. Powered Lower Extremity Exoskeletons (pLEE) can serve as rehabilitation devices or as orthotic devices enabling paraplegics to walk. Even though several exoskeletons have been commercialized, the existing exoskeletons’ actuators are still not optimal, since they are facing significant challenges.
Some of these challenges are reducing the weight and size of the actuators, while still providing the required torque and power to produce desired joint trajectories. Also, they have limited performance and there are still torque sensing issues. Measuring output torque directly is quite difficult, thus it is measured indirectly, but often not so accurately.
Compliant actuators utilise springs for torque sensing and compliance with environment, but springs usually limits their torque bandwidth. Elastic elements increase safety by dampening impacts but can make control more difficult. Lastly, torque sensing usually has practical challenges, like determining the placement, attachment, and type of sensor.
The actuator’s components can be configured in many different ways, producing designs with various trade-offs that affect power output, size, weight, efficiency and mechanical robustness. Considering these trade-offs, this project focuses on developing an optimal configuration which provides a compact, lightweight, high performance exoskeleton actuator, while attempting to solve practical challenges involving torque sensing.
Towards this direction, the following approach was followed:
After reviewing existing Series Elastic Actuator (SEA) technologies, a new spring configuration was proposed. This configuration places the spring between the gear and the housing and measures the transmitted torque to the load.
A dynamic model was developed for the proposed design and its frequency response was compared with other common SEA configurations. The simulation showed good torque transmissibility and sensitivity bandwidths. Detailed CAD designs were made for the concept and a prototype was produced.
A combination of structural simulations, along with a careful component selection resulted in a very compact and lightweight design. Tests were designed to measure and evaluate the performance of the actual system. The spring stiffness calibration test showed excellent linearity, and the desired stiffness was achieved. Lastly, a good torque resolution and high torque density were achieved.