Researchers at MIT have hacked a 3D printer, allowing them to print a pneumatic robot, complete with pre-filled fluid chambers.
Building robots can be time consuming: they are usually sequentially assembled from many discrete parts, and because many robots are unique or specific to applications, there are relatively few opportunities to automate their assembly.
The MIT researchers Robert MacCurdy, Robert Katzschmann, Youbin Kim and Daniela Rus decided to look into multi-material 3D printing for creating robots. This printing technique allows materials with various mechanical properties to be placed at arbitrary locations within the printed structure, so complex multi-part designs, like robots, can be fabricated with low effort, and quickly.
To demonstrate the effectiveness of the technology, the researchers printed linear bellows actuators, gear pumps, soft grippers and a hexapod robot.
While multi-material prints are not particularly new, the innovation, however, lies in the research team's ability to create complex, pre-filled fluidic channels in their printed robot, using a commercially-available 3D printer. Unlike earlier attempts, which printed out the pneumatic parts, before manually filling the components with fluids and sealing them, this new approach simultaneously prints solid and liquid material, creating complex hydraulically actuated robots and robotic components, sans assembly.
The researchers utilised a Stratasys Objet260 Connex 3D printer, which uses an inkjet head to deposit three different photopolymers simultaneously and achieves finished-part resolutions better than 100µm.
The Objet260 uses eight print-heads with linear arrays of nozzles to deposit resins, layer by layer, onto the build surface. These resins rapidly cure when exposed to the high-intensity UV light source mounted on the print head.
The printer uses four heads to print removable support and allocates the remaining four heads to one or two model materials.
The researchers took advantage of Stratasys' model cleaning fluid material, a liquid composed primarily of polyethylene glycol, which is designed to be jetted by the printheads, but does not cure when exposed to the UV light.
While the printer does not, by default accept using cleaning fluid as a working material, the researchers spoofed the system by replacing the RFID chip in the cleaning fluid cartridge with one from a different model material.
The researchers could then instruct the printer to "print" the fluid out at the locations required, by specifying the use of the spoofed material at those points.
The result was a six-legged robot with 12 hydraulic pumps embedded within its body. A single DC motor spins a crankshaft that pumps fluid to the robot’s legs. Among the robot’s key parts are several sets of “bellows” that are 3-D printed directly into its body. To propel the robot, the bellows uses fluid pressure that is then translated into a mechanical force.
No assembly was required, since the force-transmitted fluid is built into the robot's solid body at the time of manufacturing. There is also no need to purge air bubbles from the fluid because the solid and fluid regions are fabricated together.
More complex, intricate actuated structures and geometries are now possible. It would be possible to eliminate or replace support materials with liquids, particularly in areas with highly complex miniature structures, where the removal of support material would be impossible.
While 3D printing has been used to create machines with kinematic linkages and gears, the researcher say printed hydraulics have the advantage thanks to low-friction, low-backlash, high force transmission elements.