Soft actuators are appealing for robotics because they’re cheap (made out of plastics or polymers and air), inherent compliant and relatively safe for humans to interact with, and able to adapt themselves to grip a wide range of objects. Being soft does tend to make them by definition bad at being hard, so for those times when you need an actuator with some stiffness, well, that’s just too bad.
Or is it?
Researchers at Technische Universitat Berlin led by Professor Oliver Brock have combined soft pneumatic actuators with a jamming system that results in a variable-stiffness actuator that’s soft when you want and hard when you want.
The researchers tested three different jamming systems, including the traditional coffee grounds, as well as two other designs based on scales and interleaved layers [see image below]. They eventually settled on the interleaved layers for the final design, because it requires far less pressure to jam, although it’s more complex to manufacture. In principle, it works in a similar manner: when the interleaved layers have air between them, they can slide against eachother, allowing the actuator to flex. When the air is pumped out, the layers compress against eachother, and the actuator is stiffened. You can do this by hand with a syringe, or using a pump or autonomous operation.
The three jamming systems tested by the researchers: granular jamming; layer jamming with overlapping fish-scale-like layers; and layer jamming with stacks of interleaved layers. The jamming chambers are indicated by dashed lines, and the Pneuflex actuator is shown on the bottom. (a) Unjammed chambers result in flexible actuators. (b) Jammed chambers result in stiff actuators. Image: TU Berlin
Overall, the jammed actuator exhibited a stiffness increase of 8x, resulting in an application of force increase of 2.3x, which is pretty significant. Incidentally, you can make these actuators yourself for free: they’re called Pneuflex actuators, and instructions are available here.
“Selective Stiffening of Soft Actuators Based on Jamming,” by Vincent Wall, Raphael Deimel, and Oliver Brock from the Robotics and Biology Laboratory at TU Berlin was presented at ICRA 2015 in Seattle, Wash.