Knee Joint Assist in Seated Motion Focusing on Variable Viscosity Property in Exoskeleton Assistive Device: Comparison of Variable and Constant Viscosity Coefficients

Abstract

As exoskeletal assistive devices are directly attached to a person, coordination must be improved. However, existing exoskeletal assistive devices have difficulty coordinating with human behavior. Existing exoskeletal assistive devices are driven by estimating the wearer’s movement intention based on predetermined movement time and device angle data. If the wearer does not move as expected, a gap in the command signal occurs, making it difficult to provide the appropriate assistance. Therefore, an assistive method is required to reduce such deviations. In a previous study, comparing the assist method using constant viscosity with other methods, it was confirmed that the viscous commands were less likely to cause command signal misalignment. However, even at constant viscosity, deviations from the target knee joint torque values obtained from the motion analysis were observed. Therefore, by focusing on the variable viscosity property, this study aims to output commands that are closer to the target torque value. In this study, the seated motion was used as the target motion for assistance and the viscosity coefficients of the variable viscosity were obtained for each of the three motion sections based on the motion analysis results. Subsequently, constant and variable viscosities were compared by measuring the command signal and amount of muscle activity. The results showed that the command signal was close to the target value and that the variable viscosity assist reduced the EMG potential in the majority of subjects. Even when the timing of the viscosity coefficient change deviated from the target, most actual deviation of subjects from the command peak timing was small.

Keywords

• Exoskeletal assistive device
• variable viscosity coefficient
• seated movement