Modeling and Stiffness-Based Continuous Torque Control of Lightweight Quasi-Direct-Drive Knee Exoskeletons for Versatile Walking Assistance

State-of-the-art exoskeletons are typically limited by the low control bandwidth and small-range stiffness of actuators, which are based on high gear ratios and elastic components (e.g., series elastic actuators). Furthermore, most exoskeletons are based on discrete gait phase detection and/or discrete stiffness control, resulting in discontinuous torque profiles. To fill these two gaps, we developed a portable, lightweight knee exoskeleton using quasi-direct-drive (QDD) actuation that provides 14 N·m torque (36.8% biological joint moment for overground walking). This article presents 1) stiffness modeling of torque-controlled QDD exoskeletons and 2) stiffness-based continuous torque controller that estimates knee joint moment in real-time. Experimental tests found that the exoskeleton had a high bandwidth of stiffness control (16 Hz under 100 N·m/rad) and high torque tracking accuracy with 0.34 N·m root mean square error (6.22%) across 0–350 N·m/rad large-range stiffness. The continuous controller was able to estimate knee moments accurately and smoothly for three walking speeds and their transitions. Experimental results with eight able-bodied subjects demonstrated that our exoskeleton was able to reduce the muscle activities of all eight measured knee and ankle muscles by 8.60%–15.22% relative to the unpowered condition and two knee flexors and one ankle plantar flexor by 1.92%–10.24% relative to the baseline (no exoskeleton) condition.

I. Introduction

Portable lower-limb exoskeletons have great potential for mobility restoration and human augmentation [1]–[3]. Compliance, low mass, and the ability to assist a diverse array of movements are key requirements for an exoskeleton to be viable in daily life. There is a wealth of literature on ankle exoskeletons to augment human walking [4]–[7] and a substantial amount of literature on hip or ankle exoskeletons for walking augmentation [8]–[13]. Comparably, there is little research study on knee exoskeletons for augmentation [14], [15], likely because the positive work done at the knee in level walking is less than that at the ankle or hip [16]. However, the knee is crucial in locomotion, and there is a compelling need to investigate the benefits of a knee exoskeleton on human performance and understand the human–robot interaction.