Traditional robots often feature isolated mechanical joints. These discrete components limit a rover’s ability to traverse sand, stone, and other challenging environments.

A team at the University of California San Diego has demonstrated a more flexible option: a soft robot that lifts its legs over obstacles and operates on a variety of terrains. The 3D-printed quadrupedal technology may someday support search-and-rescue missions requiring intelligent navigation capabilities.

What is a Soft Robot?

UC San Diego Jacobs School of Engineering mechanical engineering graduate student Dylan Trotman with the 3D-printed, four-legged robot being presented at the 2017 IEEE International Conference on Robotics and Automation (ICRA). (Credit: UC San Diego Jacobs School of Engineering/David Baillot)

Dylan Drotman, a Ph.D. student at the Jacobs School of Engineering at UC San Diego, led the effort to design the soft robot, including its legs and control systems.

Each of the robot’s four legs contains three parallel, inflatable chambers, or actuators. The timing between the inflation of one chamber to the next determines the robot’s gait.

The bot has a programmed leg movement, and the “passive-compliant” actuators contain an elastic spring element that enable a more natural pace. Tethered to an open-source pneumatic board and air pump, the robot adapts to its environment and “walks” over terrain – all without intelligence.

“Based off the logic and the pressure inputs for each of the chambers, you control different gait sequences,” Drotman said.

The legs, connected to a rigid body, form an “X” shape. By sequencing the input of pressurized air into each of the soft pneumatic chambers, the researchers enable the robot to move forward or back, rotate in place, or travel sideways.

The researchers tested a variety of leg configurations, gait sequences, and terrains, including large rocks, small pebbles, and valleyed surfaces. The robot achieved walking speeds of 20 mm per second.

One of the better demonstrations, according to Drotman, showcased the technology’s ability to move under an inclined surface.

Because the inflation of actuators bends the system’s legs, the robot has a wider range of motion than a mechanically jointed system.

"A traditional robot wouldn’t be able to ‘squeeze’ in the same way that a soft robot would be able to,” Drotman said.

The Role of 3D Printing

Generally, soft robots today are made using a manufacturing method called soft lithography. The technique requires the curing of materials like silicon; additionally, the part must be printed and molded with an elastomeric stamp.

Drotman used a commercial printer, the Connex3 3D printer from the Eden Prairie, MN-based manufacturer Stratasys. By employing multiple materials – both soft and rigid – the team created robotic parts with much more complex geometries, including bellow-shaped actuators that extend farther than simple tubular designs.

Taking advantage of 3D printing is going to drastically change the approach for soft robotics, according to the Ph.D. student.

“As the 3D printing technology advances and new materials are developed, it’s going to greatly improve how we’re manufacturing and starting to think about soft robots in general,” Drotman said.

The Direction of Soft Robots

Drotman envisions the robots being deployed in search-and-rescue environments featuring unpredictable terrain. The soft systems could be used to someday monitor dangerous locations, such as areas impacted by a chemical spill or earthquake.

The next step for Drotman and his team, including Michael Tolley, a mechanical engineering professor at the University of California San Diego, will be to equip the robot to carry all the electronics on board.

The researchers are additionally working on miniaturizing both the board and the pump so that the robot can walk independently. The engineers are also looking to add vision and flexible sensing capabilities to enable smarter course-plotting capabilities.

“The main point of the newer board is to identify the obstacles in front of it,” Drotman said. “We’re looking at how do we actuate these legs to intelligently navigate over these obstacles.”

Drotman will present the robot at the IEEE International Conference on Robotics and Automation from May 29 to June 3 in Singapore.


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