Comparative analysis of harmonic drive and planetary gear actuation units for exoskeletons using load cell-based transparency control

Active exoskeletons rely critically on their actuation systems, yet significant challenges persist in balancing performance, cost, and intuitive human–robot interaction. A central issue is achieving transparency, where the device offers minimal resistance to the user’s natural movements. This study presents a comparative analysis of two widely used actuation systems for upper-limb exoskeletons—Harmonic Drive (HD) and Planetary Gear (PG)—and an evaluation of their performance using a transparency control algorithm. Intrinsic mechanical properties of HD- and PG-based actuators, including friction, backlash, inertia, and bandwidth, were characterized using a dedicated test bench. Performance was then assessed in a human-in-the-loop experiment employing a simple feedforward controller that infers user intent from load cell measurements. Results indicate that PG actuators offer superior technical performance, with lower friction, higher efficiency, and reduced user effort, whereas HD actuators exhibit higher power consumption and greater interaction torque. However, subjective evaluations reveal that HD actuators are perceived as safer and more controllable due to inherent viscous damping, while PG actuators, despite improved transparency, are often described as uncontrolled. These findings highlight the strong coupling between actuator mechanics and control design, demonstrating that effective transparency and good user perception cannot be achieved by designing the hardware or control strategies in isolation. • Case study comparison of Harmonic Drive- vs. Planetary Gear-based actuation units. • Evaluation of the intrinsic mechanical properties (friction, inertia) using objective and subjective metrics. • Subjective user tests reveal an apparent contradiction between efficiency and perceived safety. • High-friction actuator masked by the controller limitations is perceived as a more controllable system.