Manufacturing a silicone peristaltic robot by lost-wax casting
A soft pneumatic robot is an elastomer part with internal cavities, and the manufacturing problem is how to leave that void with the exact shape. This project solved the fabrication of the peristaltic robot previously designed in the lab with a lost-wax process, measured its repeatability and built the pneumatic system to drive it. The first test showed that the robot inflates in the intended order but deflates in the opposite order, and the work identifies the cause.
Context
The design is a chain of five spherical cavities connected by orifices, fed from one end. To advance, the cavities must inflate in sequence from the air inlet and deflate in the same sequence, so that the last one to empty acts as an anchor.
Inflation and deflation sequence the robot should follow
Process
The material is Ecoflex 00-30, a very soft platinum silicone. The process has two stages.
Paraffin core. Paraffin is melted and poured into a machined mold shaped like the cavities and channels. A wire spine stiffens the insert and allows it to be positioned.
Fabrication of the paraffin insert that defines the cavities
Silicone body. The core is centered inside a second mold, the silicone is mixed and degassed, poured and cured. The part is then heated to melt and remove the paraffin, the hose is connected and the holes are sealed.
Casting the silicone around the core, curing, and removing the paraffin
Dimensional control
Wall thicknesses and the outer, cavity and orifice diameters were measured on several robots, and a tolerance grade was assigned to the process from those data. The result lies between IT13 and IT15. The process is repeatable but coarse. The main sources of variation are the manual trimming of the paraffin core and the surface finish of the molds.
Actuation
The pneumatic system consists of a regulated air source and a valve driven by an Arduino through a relay. A MATLAB program sets the fill and vent times.
Actuation and air supply systems coupled to the robot
Results
Actual inflation (1 to 4) and deflation (5 to 8) sequence
The test showed two differences from the expected behavior:
- The first cavity is not the first to inflate. The wall separating it from the outside, where the hose enters, is 1 mm thicker than the walls between cavities, which makes it stiffer.
Thickness differences at the first cavity alter the inflation sequence
- Deflation happens in reverse order. The farthest cavities empty first, and net advance is negligible. As the cavities expand, the orifices connecting them dilate too, so flow resistance changes during the cycle. The design model assumed orifices of constant diameter.
The proposed solution is to space the cavities farther apart, so that the duct between them is longer and deformation does not reach its middle. A finite element simulation at 10 kPa indicates that deformation concentrates at the duct entrance and that the safety factor is 3.6.
Deformation of a cavity and its duct: deformation does not propagate along the duct
The thesis provides drawings of the new molds.
What is missing
The redesign was not built or tested in this project. The dimensional characterization was done on few parts. The work does not measure the pressure-volume curve of the cavities either, which is the datum that would allow the model to be corrected.
How it fits in Robiolab
This is the second project in the peristaltic-robot series and the one that confronts the model with the real part. It leaves two results the group kept using: a lost-wax process for closed cavities in silicone, and the realization that in a soft robot manufacturing tolerances are part of the controller, because one millimeter of wall changes the motion sequence. The next project addressed control with this same robot.