Printable Hydraulics

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.

Publications:

Robert MacCurdy, Robert Katzschmann, Youbin Kim, Daniela Rus - Printable Hydraulics: A Method for Fabricating Robots by 3D Co-Printing Solids and Liquids 2016 IEEE International Conference on Robotics and Automation (ICRA)

Design Files:

Download the STL Files for the Demo Part here and all design files here.

Videos:

CSAIL Printed Hydraulics Video:

8-legged robot video:

Automated assembly of integrated robotic structures

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

This example was fabricated in a single-step, with the working fluid already embedded inside. No materials need to be added or purged

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 a single rotational degree 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, creates a sealed system that is printed in a single step.
Two fingers, connected in an antagonistic configuration make a soft gripper.	The gripper can manipulate fragile objects, like a raw egg.

Soft robots are usually fabricated via cast elastomers, and although casting soft robots is often faster than assembling conventional rigid robots, the mold-making process can be time consuming, and embedding multiple materials within a cast object via overmolding adds complexity. Additionally, producing complex, graded materials via casting is difficult. Additive manufacturing, combined with the printed hydraulics approach, provides an alternative fabrication method for soft robotics that is automated, flexible, and enables geometries that are infeasible with other production methods. As a demonstration of the utility of printable hydraulics to the soft robotics community, we designed and printed a two-finger soft gripper, shown above. The design process required four iterations. Each iteration required 3.5 hours to print and a short time to evaluate the performance of the part. This method is faster and more automated than soft robot fabrication approaches that rely on casting materials into molds. Additionally, the final gripper design incorporates thin channels and internal fluid routing that would be difficult to achieve via casting.

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

Gear pumps are low-flow, high-pressure devices, are commonly employed in hydraulic systems, and are capable of producing continuous flow. We designed and printed a gear pump to present an alternative to the bellows pump, which produces only reciprocating flow. These pumps employ a pair of enmeshed counter-rotating teeth enclosed in a tight-fitting housing. Fluid trapped between the teeth and the housing is moved from the low-pressure port to the high pressure port, and is prevented from moving back by the meshed teeth near the center of the pump. The gears have a pitch diameter of 17.5 mm, an outer diameter of 19.6 mm, a modulus of 1.25, and a gear height of 8 mm. We followed the common practice of using involute gears with a 20 degree pressure angle.