Interaction forces beneath cuffs of physical assistant robots and their motion-based estimation

Abstract

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.

Graphical Abstract

Keywords:

• physical assistant robot
• safety
• sit–to–stand
• human–robot interaction

Notes

No potential conflict of interest was reported by the authors.

1 Because of contractual obligations, the name, manufacturer, and details of the assist algorithm cannot be mentioned, to avoid the evaluation of this particular robot.

Additional information

Funding

This study was conducted as a part of the ‘Practical Applications of Service Robot Project,’ which is directed by the New Energy and Industrial Technology Development Organization (NEDO); JSPS KAKENHI [grant number 26750121].