Most lower-limb physical assistant robots are fixed to wearers using cuffs. Hence, skin injuries beneath the cuffs are one of the major concerns of the users. A model that describes the relationship between the body posture and the interaction forces at the cuff was developed for use in assessing the risk of injury and improving user comfort. We measured the motion and interaction force beneath cuffs during the sitting and standing motions of subjects and a physical assistant robot which has been hardly reported thus far. Because of slippage and biomechanical motion, a traditional spring-damper model was found to be insufficient to describe the interaction forces associated with the measured motion of the cuffs. A parameter representing the motion or the knee joint angle was added to take into account these factors. Our model for estimation of the interaction forces using a spring, a damper, and the attitude of the lower leg fits the measured data especially well for the thigh cuff and is better than the traditional model. The applicability of this model was verified for several assist modes and wearers. The model was found to describe approximately 90 of the burden on the wearer, which reached a peak of approximately 60 N, the most hazardous condition. Having been validated for a commercial assistant robot, the model can be used to estimate the skin burdens beneath the cuffs without any force-sensitive elements.
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