Printable Hydraulics

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Printable Hydraulics: A new method for fabricating force-transmission elements within robots

Multi-material additive-manufacturing techniques offer a compelling alternative to conventional rigid and soft robot fabrication techniques, allowing materials with widely varying mechanical properties to be placed at arbitrary locations within a structure, and enabling design iterations to be rapidly fabricated with trivial effort. This capability enables complex composite materials with new bulk properties, and in contrast to virtually all other fabrication techniques, the incremental costs of additional design complexity when using additive manufacturing are zero. We show how commercial multi-material 3D printers can be adapted to co-fabricate solids and fluids within the same 3D-printed structure, demonstrating a new capability for transmitting force within 3D-printed assemblies: Printable Hydraulics.


ICRA 2016 Paper - Printable Hydraulics: A Method for Fabricating Robots by 3D Co-Printing Solids and Liquids

ICRA Video Submission
CSAIL Promo Video

Automated assembly of integrated robotic structures

Printed Hydraulics Overview
Printed Hydraulics Overview
Printed Hydraulics Overview
Printed Hydraulics parts are fabricated layer-by-layer, by depositing adjacent droplets of liquids and solids

This method, based on inkjet deposition of photopolymers and non-polymerizing fluids, can achieve resolutions better than 100μm, allowing the fabrication of complex channels for fluid routing and capillary structures for selectively distributing hydraulic pressure to regions of the assembly with precisely graded elasticity, enabling prescribed movements in response to pressure changes. Control of complex composite assemblies fabricated in this manner is simplified because the working fluid is incompressible; because the solid and fluid regions are fabricated together, there is no need to purge air bubbles or remove support material. The key idea of this approach to robot fabrication is to automate the assembly of complete robotic structures. By reducing or eliminating assembly steps, this method breaks the connection between design complexity and fabrication complexity, allowing complex designs to be fabricated with trivial effort.

Basic Transducer Unit: the Bellows

Basic Bellows
This example was fabricated in a single-step, with the working fluid already embedded inside. No materials need to be added or purged
Bellows FEA
Finite-element analysis modeling allows the material deformation and stress to be estimated

The achievable feature sizes of drop-on-demand inkjet printers are too coarse to print sliding seals; they would leak. As a result, conventional piston designs are not practical. An alternative design using a bellows avoids the need to have seals entirely. As the pressure inside the bellows increases the material deforms and the bellows extends. This deformation can be estimated using finite element modeling tools to ensure that the stress and strain in the printed material does not exceed allowable limits. The bellows design is inherently modular: if greater actuator extension is required additional folds can be added, and if larger force is required (for a given input fluid pressure) the cross-section of the bellows can be increased.

Fabricating a robot in a single step

The printer builds the hexapod robot layer-by-layer from bottom to top.
When the switch is closed, the battery is connected to the motor and the robot walks.
A single geared motor turns a crankshaft which is connected to a bank of bellows pumps. The pumps are connected hydraulically to the legs.
A sensor and controller allows the robot to respond to environmental stimuli, as well as communicate with a cellphone app.

We designed a tripod-gait hexapod with six rotational degrees of freedom (DOF), illustrated above. All mechanical components of this robot are printed in a single step with no assembly required. This robot weighs 690 g, is 14 cm long, 9 cm wide and 7 cm tall. The legs are designed with a neutral position that inclines their major axis 60 degrees above the floor and each leg is actuated by a bellows, causing the leg to rotate 10 degrees in either direction, relative to this neutral position. Three of the legs are inclined toward the front of the robot (bank A) and three are inclined toward the rear (bank B). Each driven bellows is internally connected to a corresponding driving bellows via a fluid channel that runs through the robot’s body; the fluid in each driving/driven bellows pair is isolated from the other bellows. The three driving bellows from bank A are kinematically linked and attached to a crankshaft via a connecting rod. The bellows from bank B are similarly connected to a separate section of the crankshaft that is 90 degrees out of phase. The crankshaft is turned at 30 RPM by a single geared DC motor consuming approximately 2 W (Pololu #3070), yielding a locomotion speed of 0.125 body-lengths per second. This arrangement moves the legs from the two banks 90 degrees out of phase with each other, enabling forward or backward locomotion without an additional DOF at each leg, and does not require the feet to slide on the floor.

Applications to Soft-Robotics

A single-sided bellows fabricated with soft rubbery material (Shore A28) creates a "finger"
The same finger, connected to a bellows, creating a sealed system that is printed in a single step
Two fingers, connected in an antagonistic configuration make a soft gripper

Printed Gear Pumps

A printed gearpump allows continuous fluid flow. Though shown in isolation, this gearpump module would be integrated into an assembly, leveraging the capability to deposit free-spinning components that do not require extensive post-processing
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