A Positive Pressure Jamming Based Variable Stiffness Structure and its Application on Wearable Robots

Abstract

Variable stiffness mechanisms establish a bridge between rigid, precise robots and soft, compliant robots. In certain applications it is necessary to exert high forces while having compliance, such as in wearable robots, high payload soft grippers, and manipulators. Negative pressure based jamming approaches have been widely used to tune variable stiffness. However, this variation of stiffness and strength is limited to negative one atmospheric pressure. To enable a wider pressure range which potentially enable wider stiffness range, we proposed a Positive Pressure Jamming (PPJ) approach. The PPJ is fabricated by embedding a pneumatic actuator inside a cylindrical non-stretchable fabric sleeve which contains granules. Pressurizing air into the pneumatic actuator will jam the granules, thus stiffening the soft body. To add degrees of freedom for the PPJ, we place a pair of friction pads in parallel between two PPJ units which act as a self-locking revolute joint. Both the PPJs and self-locking joint can be stiffened at the same time thus enabling the structure to be jammed in multiple configurations. Experiments showed that approximately six-fold stiffness increment (0.69N/mm to 4.02N/mm) could be achieved by our PPJ when input pressure is changed from 68 kPa to 172 kPa with exerted force ranges from 4N to 23N. Furthermore, we apply the proposed PPJ method into limb support devices, demonstrating a promising potential that PPJ has in wearable robot field.