Design of a Dynamic Waist Strap for Reducing Migration of Knee Exoskeletons

As a key element of knee exoskeleton, the stability of thigh cuff is essential to ensure the force acting on the motion of knee without failure. The challenge of its design is preventing the cuff downward sliding when external downward force exists. Generally, it is in virtue of the friction force between the cuff and skin, so the cuff being tighter means more reliable. However, for the thigh with the large cross area in the proximal part and the small cross area in the distal part, only depend on the tightness is not feasible, meanwhile it will limit the muscle activation and cause discomfort and pain. To address this problem, we propose a dynamic waist strap (DWS) where thigh cuff is connected to a waist belt though ropes dynamically. To verify the effectiveness of proposed DWS, a serious of contrast experiments based on a textile waist belt for rock climbing and a custom thigh cuff made of thermoplastic polyurethanes (TPU) are implemented. The results indicate this design can prevent the knee exoskeleton from downward sliding more effectively compared with three existing knee exoskeleton wearing forms. The thigh cuff supported by proposed DWS only slides down by 0.505 mm after the whole experimental process is finished, far less than the data in other conditions (10.525 mm, 19.234 mm and 30.538 mm respectively), when the interaction force between thigh cuff and waist belt is not large. This research highlights the importance of waist belt when attaching a knee exoskeleton to human and introduces a new dynamic interaction interface to improve the coupling from an exoskeleton to an individual.