The flexibility and compliance of Soft Robotic Actuators like the PneuFlex by Raphael Deimel offer several advantages over traditional, rigid mechanisms. They are inherently safe and light, robust to impact and collision, have a high degree of compliance without the need for explicit control, and can be designed and build quickly at low costs.
There are situations, however, in which softness and compliance become a disadvantage. The softness of the PneuFlex actuator (or any other soft actuator) limits the amount of force it can exert onto the environment, for example, when lifting a heavy object or when pressing a switch.
To alleviate this limitation we propose soft actuators capable of changing their stiffness by employing jamming.
By increasing the density of granular materials, such as sand or ground coffee, their physical state can be changed from flexible to solid-like. This process is called granular jamming. Similarly layers of flat sheets that are initially sliding across each other can be fixed in place by applying pressure to the layers, effectively jamming the sheets together. Hence this method has been coined layer jamming.
These two mechanisms were used to extend the design of the PneuFlex actuator by adding a jamming chamber on its back. This creates a sandwich layout that allows normal operation of the finger, while the jamming material is flexible, but stiffens in-place, when the jamming is activated.
Five prototypes were built and evaluated using a visual tracking setup, to identify the curvature, and therefore the stiffness, of the actuators under stress. Additionally another experiment was done employing the final prototype in a task inspired by the real-world scenario of pushing buttons. With it the increase in exercisable force gained from jamming was measured.
From the five prototypes that were analyzed, the second iteration of the layer jamming prototype (L2) has shown the greatest increase in stiffness. When the jamming chamber is activated the actuator's susceptibility to forces at the fingertip is reduced by the factor eight.
While the performance increase was not as great in the second experiment, the jammed actuator was still able to more than double the force that a straight finger can exert during a pushing motion.