JammJoint: A Variable Stiffness Device Based on Granular Jamming for Wearable Joint Support

Abstract

Abstract-In robotics, controlling the stiffness of the joints that contribute to the robots' degree of freedom dictates the adaptability, versatility, and safety of the whole system. We can achieve variable stiffness or impedance in a robotic system purely by the control or by introducing new material or mechanisms to address cases that require innate safety through system compliancy. This paper presents JammJoint, a compliant and flexible wearable robot, which uses jamming of granular media to vary its stiffness. It consists of a silicone sleeve with hollow sections that are filled with cubic rubber granules and subjected to different levels of vacuum pressure. Unlike contemporary vacuum-based actuators or systems, JammJoint is wearable, portable, and autonomous: It uses a powerful miniature vacuum pump, a small battery, and bluetoothenabled electronics. Experiments revolving around bending and torsional stiffness show that the system is able to achieve up to a fourfold increase in spring stiffness. Further measurements of individual variable stiffness structures indicate that for other modes of deformation, including simply supported bending or compression for alternative linear applications, higher changes in stiffness over a factor of seven are possible. These aspects make mobile jammingbased stiffness variation as wearable joint assistance promising for future applications such as rehabilitation after injuries and joint support in challenging working conditions. Index Terms-Wearable robots, soft material robotics, variable stiffness joint, vacuum jamming, hydraulic/pneumatic actuators.

Stiffness Control of Deformable Robots Using Finite Element Modeling

“…Della Santina et al [16] present an impedance controller for a soft continuum robot using a Piecewise Constant Curvature assumption to link the soft robot to an equivalent rigid robot representation. In the soft robot community, numerous works have focused on obtaining actuators with variable stiffness using, for example, particle jamming [17], shape memory polymers [18], magnetorheology [19], and antagonistic soft actuation [20]. These approaches allow local modification and control of stiffness, but do not necessarily take into account the compliance of the whole structure of the robot or consider the apparent end-effector stiffness.…”

Intelligent Liquid Integrated Functional Entity: A Basic Way to Innovate Future Advanced Biomimetic Soft Robotics

“…As with designs of soft robotics, although the bipedal or quadruped locomotion of animals is no longer the most necessary strategy to imitate, the variable stiffness structure is of positive potentials. The jamming effect, with an easy method to change the stiffness, has been widely used in various fields of variable stiffness devices, such as soft grippers, joints, pneumatic actuators, and soft manipulators . However, such devices are based on tiny granular materials with an inferior mobility, which must be constrained in finite space by specific membranes, so the jamming effect will prefer applications on some predetermined fixed positions of robots.…”

A Wearable Sensory Textile‐Based Clutch with High Blocking Force

“…[17] Replacing conventional electromagnetic clutches with custom designs tailored for wearable applications has helped in reducing size and weight of these components for unpowered exoskeletons. [22][23][24][25][26][27] However, several limitations are still present, such as the achieved blocking forces, the elongation ratio, and the low response time in the case of variable stiffness technologies. [14,[18][19][20] Electrostatic clutches, developed by flexible materials such as a polyethylene terephthalate film, demonstrate to be a promising soft robotics technology for the application in an unpowered ankle foot orthosis.…”

“…[21] Variable stiffness solutions, such as vacuum-activated granular jamming, textile layer jamming, low melting metals, and polymers, are other soft robotics technologies that can be used in wearable robotics to freeze body joints, instead of clutches. [22][23][24][25][26][27] However, several limitations are still present, such as the achieved blocking forces, the elongation ratio, and the low response time in the case of variable stiffness technologies. In particular, a major challenge is to produce high blocking forces with compact, thin, and wearable devices, to provide the capability to contrast the high torque generated by human joints (values can vary due to gender or specific training, however, for male subjects, e.g., for the knee, values up to %300 Nm are reported, whereas for the ankle, 200 Nm is reached).…”