Printable hydraulics, solid and liquid robots 3D-style

Hydraulically-powered 3D printed robot bodies with no assembly required

Hydraulically-powered 3D printed robot bodies with no assembly required

Hydraulically-powered 3D printed robot bodies with no assembly required

One reason we don’t yet have robot personal assistants buzzing around doing our chores is because making them is darn hard. Assembling robots by hand is time-consuming, while automation — robots building other robots — is not yet fine-tuned enough to make robots that can do complex tasks.

But if humans and robots can’t do the trick, what about 3D printers?

In a new paper, researchers at MIT’s Computer Science and Artificial Intelligence Laboratory (CSAIL) presenedt the first-ever technique for 3D printing robots that involves printing solid and liquid materials at the same time.

The new method allows the team to automatically 3D print dynamic robots in a single step, with no assembly required, using just a commercially-available 3D printer.

“Our approach, which we call ‘printable hydraulics,’ is a step towards the rapid fabrication of functional machines,” says CSAIL director Daniela Rus, who oversaw the project and co-wrote the paper. “All you have to do is stick in a battery and motor, and you have a robot that can practically walk right out of the printer.”

To demonstrate the concept, researchers 3D printed a tiny six-legged robot that can crawl via 12 hydraulic pumps embedded within its body. They also 3D printed robotic parts that can be used on existing platforms, such as a soft rubber hand for the Baxter research robot.

The printing process

For all of the progress in 3D printing, liquids continue to be a big hurdle says co-writer of the paper Robert MacCurdy (MIT postdoc). Printing liquids is a messy process, which means that most approaches require an additional post-printing step such as melting it away or having a human manually scrape it clean. That step makes it hard for liquid-based methods to be employed for factory-scale manufacturing.

With printable hydraulics, an inkjet printer deposits individual droplets of material that are each 20 to 30 microns in diameter, or less than half the width of a human hair. The printer proceeds layer-by-layer from the bottom up. For each layer, the printer deposits different materials in different parts, and then uses high-intensity UV light to solidify all of the materials (minus, of course, the liquids). The printer uses multiple materials, though at a more basic level each layer consists of a photopolymer, which is a solid, and a non-curing material, which is a liquid.

“Inkjet printing lets us have eight different print-heads deposit different materials adjacent to one another, all at the same time,” Mr MacCurdy says.

“It gives us very fine control of material placement, which is what allows us to print complex, pre-filled fluidic channels.”

Another challenge with 3D printing liquids is that they often interfere with the droplets that are supposed to solidify. To handle that issue, the team printed dozens of test geometries with different orientations to determine the proper resolutions for printing solids and liquids together.

While it’s a painstaking process, printing both liquids and solids is even more difficult with other 3D printing methods, such as fused-deposition modeling and laser-sintering.

He adds that inkjet-printing is currently the best way to print multiple materials.

The results

To demonstrate their method, researchers 3D printed a small hexapod robot that weighs about 250 grams pounds and about 15cm long. To move, a single DC motor spins a crankshaft that pumps fluid to the robot’s legs. Aside from its motor and power supply, every component is printed in a single step with no assembly required.

Among the robot’s key parts are several set of ‘bellows’ that are 3D printed directly into its body. To propel the robot, the bellows uses fluid pressure that is then translated into a mechanical force (as an alternative to the bellows, the team also demonstrated they could 3D print a gear pump that can produce continuous fluid flow).

Lastly, the team 3D printed a silicone-rubber robotic hand with fluid-actuated fingers, a “soft gripper” developed for Baxter.

“The CSAIL team has taken multi-material printing to the next level by printing not just a combination of different polymers or a mixture of metals, but essentially a self-contained working hydraulic system,” says Hod Lipson, a professor of engineering at Columbia University. “It’s an important step towards the next big phase of 3D printing — moving from printing passive parts to printing active integrated systems.”

MacCurdy envisions many potential applications, including disaster relief in dangerous environments. Many nuclear sites, for example, need to be remediated to reduce their radiation levels. Unfortunately, the sites are not only lethal to humans, but radioactive enough to destroy conventional electronics.

“Printable robots like these can be quickly, cheaply fabricated, with fewer electronic components than traditional robots,” MacCurdy says.

Looking ahead

The team is eager to further build on their work. While the hexapod’s 22-hour print-time is relatively short for its complexity, researchers say that future hardware advances would improve the speed.

“Accelerating the process depends less on the particulars of our technique, and more on the engineering and resolution of the printers themselves,” says Rus, the Viterbi Professor of Electrical Engineering and Computer Science at MIT. “Printing ultimately takes as long as the printer takes, so as printers improve, so will the manufacturing capabilities.”