Stanford Makes Giant Soft Robot From Inflatable Tubes

This large-scale isoperimetric soft robot can safely change size and shape without tethers or pumps

4 min read

Evan Ackerman is IEEE Spectrum’s robotics editor.

Stanford soft robot
Image: Farrin Abbott

As much as we love soft robots (and we really love soft robots), the vast majority of them operate pneumatically (or hydraulically) at larger scales, especially when they need to exert significant amounts of force. This causes complications, because pneumatics and hydraulics generally require a pump somewhere to move fluid around, so you often see soft robots tethered to external and decidedly non-soft power sources. There’s nothing wrong with this, really, because there are plenty of challenges that you can still tackle that way, and there are some up-and-coming technologies that might result in soft pumps or gas generators.

Researchers at Stanford have developed a new kind of (mostly) soft robot based around a series of compliant, air-filled tubes. It’s human scale, moves around, doesn’t require a pump or tether, is more or less as safe as large robots get, and even manages to play a little bit of basketball.

Stanford soft robotStanford’s soft robot consists of a set of identical robotic roller modules mounted onto inflated fabric tubes (A). The rollers pinch the fabric tube between rollers, creating an effective joint (B) that can be relocated by driving the rollers. The roller modules actuate the robot by driving along the tube, simultaneously lengthening one edge while shortening another (C). The roller modules connect to each other at nodes using three-degree-of-freedom universal joints that are composed of a clevis joint that couples two rods, each free to spin about its axis (D). The robot moves untethered outdoors using a rolling gait (E).Image: Stanford/Science Robotics

This thing looks a heck of a lot like the tensegrity robots that NASA Ames has been working on forever, and which are now being commercialized (hopefully?) by Squishy Robotics. Stanford’s model is not technically a tensegrity robot, though, because it doesn’t use structural components that are under tension (like cables). The researchers refer to this kind of robot as “isoperimetric,” which means while discrete parts of the structure may change length, the overall length of all the parts put together stays the same. This means it’s got a similar sort of inherent compliance across the structure to tensegrity robots, which is one of the things that makes them so appealing. 

While the compliance of Stanford’s robot comes from a truss-like structure made of air-filled tubes, its motion relies on powered movable modules. These modules pinch the tube that they’re located on through two cylindrical rollers (without creating a seal), and driving the rollers moves the module back and forth along the tube, effectively making one section of the tube longer and the other one shorter. Although this is just one degree of freedom, having a whole bunch of tubes each with an independently controlled roller module means that the robot as a whole can exhibit complex behaviors, like drastic shape changes, movement, and even manipulation.

There are numerous advantages to a design like this. You get all the advantages of pneumatic robots (compliance, flexibility, collapsibility, durability, high strength to weight ratio) without requiring some way of constantly moving air around, since the volume of air inside the robot stays constant. Each individual triangular module is self-contained (with one tube, two active roller modules, and one passive anchor module) and easy to combine with similar modules—the video shows an octahedron, but you can easily add or subtract modules to make a variety of differently shaped robots with different capabilities.

Since the robot is inherently so modular, there are all kinds of potential applications for this thing, as the researchers speculate in a paper published today in Science Robotics:

The compliance and shape change of the robot could make it suitable for several tasks involving humans. For example, the robot could work alongside workers, holding parts in place as the worker bolts them in place. In the classroom, the modularity and soft nature of the robotic system make it a potentially valuable educational tool. Students could create many different robots with a single collection of hardware and then physically interact with the robot. By including a much larger number of roller modules in a robot, the robot could function as a shape display, dynamically changing shape as a sort of high–refresh rate 3D printer. Incorporating touch-sensitive fabric into the structure could allow users to directly interact with the displayed shapes. More broadly, the modularity allows the same hardware to build a diverse family of robots—the same roller modules can be used with new tube routings to create new robots. If the user needed a robot to reach through a long, narrow passageway, they could assemble a chain-like robot; then, for a locomoting robot, they could reassemble into a spherical shape.

Stanford soft robotImage: Farrin Abbott

I’m having trouble picturing some of that stuff, but the rest of it sounds like fun.

We’re obligated to point out that because of the motorized roller modules, this soft robot is really only semi-soft, and you could argue that it’s not fundamentally all that much better than hydraulic or pneumatic soft robots with embedded rigid components like batteries and pumps. Calling this robot “inherently human-safe,” as the researchers do, might be overselling it slightly, in that it has hard edges, pokey bits, and what look to be some serious finger-munchers. It does sound like there might be some potential to replace the roller modules with something softer and more flexible, which will be a focus of future work.

An untethered isoperimetric soft robot,” by Nathan S. Usevitch, Zachary M. Hammond, Mac Schwager, Allison M. Okamura, Elliot W. Hawkes, and Sean Follmer from Stanford University and UCSB, was published in Science Robotics.

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