Characterization of Open-loop Impedance Control and Efficiency in Wearable Robots

Wearable robots, such as bionic prostheses and exoskeletons, have been conventionally designed with low-torque, high-speed motors and high transmission ratios; however, recently designers are increasingly implementing high-torque, low-speed motors with lower transmission ratios. These motors were popularized by the drone industry and have transitioned to general use in robotics for their improved output impedance, efficiency, and lower audible noise. Due to the relative newness of these motors, there is a lack of information regarding how transmission dynamics affect the desired output impedance. In this study, we developed system identification techniques to characterize the output impedance (stiffness and damping) of these actuators operating without torque feedback, termed "open-loop" impedance control, a common control strategy employed in wearable robotics. Open-loop stiffness errors reached up to 42%, but could be reduced to 2.9% using a linear model based on our characterization. Second, we characterized the total efficiency across various power regimes, during both positive and negative work, and measured an average positive efficiency of 65%. With these characterization experiments, we are able to better compensate for transmission losses, render more accurate impedance control, and operate actuators more efficiently. This work provides performance benchmarks and context for existing wearable robotic systems that implement similar open-loop control strategies.