Effects of Control Parameters of Exoskeleton on Movement Accuracy During Assisted Elbow Flexion

To improve movement accuracy, exoskeleton control must consider natural human motion. This study investigates how stiffness (i.e., the persistence of exoskeleton in following the reference trajectory) and pre-emptive offset (i.e., how far ahead in time) affect movement accuracy in an impedance control the exoskeleton that provides assistance preemptively based on a predicted trajectory. Thirteen participants performed an elbow flexion from 40° to 135° within 1.12 seconds while holding a load equivalent to 15% of their maximum voluntary contraction under exoskeleton assistance. The experiment tested three stiffness levels (0.1, 0.2, and 0.31 N m deg-1) and two pre-emptive offset levels (0.1 and 0.4 s). The results indicate that lower stiffness combined with a less aggressive pre-emptive move causes positional undershoot, whereas increasing both parameters causes overshoot, which intensifies with higher stiffness and a more aggressive pre-emptive move. To enhance movement accuracy, we suggest avoiding simultaneous increases or decreases in both parameters.