A simple Soft Robot

The Tripod Robot has been developed *date* in the INRIA team Defrost in Lille (France). Composed of a soft silicone piece – the soft part of the robot – and actuated by three servomotors, the robot can achieve a nearly infinite number of shape configurations.

Ideal for the first steps into Soft Robotics

Its design is ideal for enriching first steps into the world of Soft Robotics: simple enough for confident first tries, and offering at the same time an important number of opportunities for applications, and additional developments. You will find on this page all the instructions and illustrations you will need to build you own without hassles.

An open source design

The Tripod has been designed with simple materials and common electronical components. As long as you can get access to a 3D printer, you can easily build your own Tripod, since its design is entirely open-source. We also provide the necessary files if you would rather laser-cut it. On the software side, it is controlled with the open source framework Sofa *link* and some Arduino coding.

Build your own Tripod

Fully open source
Overall dimensions: 15x15x9 cm
A cast silicone piece
All hard pieces can be 3D-printed
Most of them can also be laser-cut
Servomotors and microcontroller board from MakeBlock

In order to build your own Tripod, you will need the parts listed below. You have the choice between 3D-printing all the hard pieces of the robot – in which case you will need the .STL files that we provide for your 3D-printer – or laser-cutting most of these parts – see the .SVG files provided – and only 3D-printing the pieces that secure the servomotors. We don’t currently provide ready-to-assemble kits, so you will also need to purchase the MakeBlock MegaPi board and the three MakeBlock SG90 servomotors on your end, as well as the bolts, nuts and washers described below. The detailed instructions are presented hare after and can be downloaded here. You can also follow us building a Tripod in the video below. If you have any question, please head to our Contact *link* section.

We also love to hear about new Tripod owners, so don’t hesitate to send us pictures of your own robot!

Parts List

**g of ** in your favorite color if you 3D-print all the hard parts
**g of ** in your favorite color + **** of ** if you laser-cut most of the hard parts
40g of silicone for the soft part (we use Dragan Skin 10A) + silicone pigments (optional)
A MakeBlock MegaPi board
A 12VDC power supply
A USB cable for the board
3 MakeBlock SG90 servomotors & the associated servo horns
6 M2 7.8mm cross head screw (included with the servomotors)
6 M2 9.8mm cross head bolt (included with the servomotors)
6 M3 4.5cm threaded rod
3 M3 25mm flat head bolt
6 M3 20mm hex socket bolt
6 M3 11.8mm carriage hex bolt
6 M3 8.3mm carriage hex bolt
3 M2 11.8mm flat head bolt
42 M3 nuts
9 M3 washers
Cups of coffee (optional?)

Instructions

You can download the complete pdf file (parts list plus instructions) directly by clicking here or also from our Download page.

Tools

For the different screws and bolts, you will need a cross head, a Philips and an M3 Allen screwdrivers, an M3 Allen nut driver and/or an M3 Allen key.

Servomotor preparation

Before assembling the servos, make sure they all are rotated as close as possible to the middle of their span. To do so, you can upload this Arduino code to the MegaPi and connect the servos to ports A6, A7 and A8 making sure that the colors of the servo plug match those on the board. After the code is uploaded and the servos connected, simply plug the board and let the servos move freely for 3 seconds and then turn off the board.

Now you can attach the horn to the shaft of the servos horizontally and start the assembly.
Attach the servo base and the servo cover using M2 9.8mm cross head bolts and place the plastic horn onto the shaft of the motor (Figure 1). It is not necessary to add a bolt to the horn at this point.
Do the same for the other 2 servos.

Upper base assembly

Fix the assembled servo on top of the upper base using a M3 11.8mm carriage bolt from the bottom and tighten it using a nut on the top (Figure 2). Make sure that the inner face of the servo base is tangent to the center hole of the upper base.

Fix the arm support beside the servo base in the same manner (Figure 3).

With the arm support in place, attach the servo arm to the plastic horn (Figure 4) placing one M2 11.8mm flat head bolt to the center hole of the arm and one M2 7.8mm screw to one of the side holes of the arm.

Slide the 2 back arms into the square slots of the servo arm (Figure 5).

Slide the passive arm into the shaft of the arm support while fitting the back arms into its square slots (Figure 6). Fix the passive arm in place using a M3 8.3mm carriage bolt and a washer.

Slide 2 M3 Allen socket bolts through both back arms of each servo from the front and place a nut at the end of the bolts without tightening them (Figure 7). These two bolts will allow us to fix the silicon rubber in a ”sandwich” between the back arms.

Repeat for the other 2 servos. Your upper base assembly should look like Figure 8.

Lower base assembly

To fix the MegaPi to the lower base, first place 3 M3 25mm flat head bolt from the bottom of the base facing upwards and place a nut in each one of them. Then, slide the board on the screws on top the nuts (Figure 9). Place another set of washers and nuts on top of the board to fix it in place.

Place the threaded rods into the lower base attaching nuts to each side of it (Figure 10). At the top of the rod, screw another nut leaving 8mm of rod at the top.

Finishing the assembly

Pass the connectors of the servos through the center hole of the upper base and connect the servos to the MegaPi in ports A6, A7, and A8. The order of the connections does not matter, however, you should make sure that the color of the connector of the servo matches the one in the board.

With the servos connected to the board, just slide the upper base in place on top of the threaded rods and tighten it with nuts (Figure 11).

The assembly should look like this:

Silicone casting & placing

The silicone we used is bi-component. The .SVG files for the mold are included in the SVG files for the parts of the robot. Start by pouring 20g of any of the two components into a container. If you want a colored silicone piece, you can add some pigment now and mix it well. Then add 20g of the other component and mix the two with a stick. If some pigment has been added, the difference in color between the two components helps to ensure thorough mixing.

In order to remove the bubbles from the mixture, place the silicone into a vacuum chamber and lower the pressure down to -80kPa. Let at this pressure for 5-6 minutes, waiting for the bubbles to come out.

Pour the paste into the mold and let cure at room temperature for about 5 hours before demoulding.

Once demoulded, it is ready for assembly. If there has been any leakage on the edges, it can be trimmed with a sharp knife.

Insert 1cm of the extremities of the silicon ”Y” in the space between the arm backs and squeeze it using the socket bolts. Your Tripod is now ready for testing!

If you have never used Sofa, you can have a look at our page First Steps with Sofa and then try our Tripod tutorial.

An exemple of interactive simulation implemented with Sofa

Hands on Soft Robotics

Tripod