Manufacturing, Control, and Performance Evaluation of a Gecko-Inspired Soft Robot

1. Printing of molds

1. Download the *.stl data for molds from the Supplementary Data 1 “CAD/Moulds/”.
2. Use the printer-specific slicing software to convert the 3D models into a print job.
3. Print the molds using a 3D printer.
4. Clean the printed molds by putting them for 15 min in an ultrasonic bath.
5. Put the molds for at least 3 h in a UV chamber.

2. Preparing the elastomer

1. Gather the following before starting this step: elastomer (part A and part B), spatula, plastic cup, mold, weight scale, plastic syringe, screw clamps (or similar), acrylic glass plate with two corresponding holes, cutter knife.
2. Mix part A and part B of the elastomer in a 1:9 ratio in a cup. Place the cup on a weighing machine. First, add 5 g of part B (dark red). Then, using a spatula, add 45 g of part A (white and viscous).
   NOTE: Ensure the accuracy of weighing is to 1 g. 50 g is enough for one actuator. The best way for portioning part A is to take a spatula and let it drain. Approximately 6 g per draining operation is possible with the spatula used.
3. Keep stirring until no more white or red areas are visible at the edge of the cup.
4. Put the cup for 15 min in a vacuum chamber to remove the air that is trapped in the elastomer due to the stirring process.
5. Fill the mixed elastomer into a plastic syringe. This allows the elastomer to be positioned much more precisely.
   NOTE: Supplementary Figure 1 illustrates the processing steps described in this section.

3. Manufacturing of upper part (base part)

1. Clamp an acrylic glass plate with two corresponding holes onto the mold. Insert the syringe into the lower hole and press the elastomer into the mold.
2. Apply force to the syringe by pushing the plunger until the mixed elastomer emerges from the upper hole.
3. Loosen the screw clamps and pull off the acrylic glass plate sideways.
   NOTE: It is important to pull it off to the side and not upwards. Otherwise, the elastomer will be pulled out of the mold.
4. Puncture the rising air bubbles with a sharp tool. Do not puncture too deeply as this will create new air bubbles rather than remove the existing ones. It is especially important to pierce the larger bubbles as these will later significantly affect the functionality of the actuator.
   NOTE: Optionally, evacuate the filled mold in the vacuum chamber to remove any air still trapped. When doing so, however, it can happen that the rising air bubbles get stuck on the mold on their way to the surface and create holes in the casting at functionally relevant areas. Supplementary Figure 2 illustrates this phenomenon.
5. Put the mold into the oven at 65 °C for 30 min.
6. Check after 10 min if the level of the elastomer has fallen significantly. This happens if the mold is not completely tight or has bent slightly due to frequent use. If the level has dropped more than 1 mm, refill the elastomer. Then, continue curing.
7. After a total of 30 min in the oven, take out the mold and cut off the extruded elastomer with a cutter knife.
8. Open the mold by levering apart with a screwdriver. Be careful not to damage surfaces relevant for casting.
9. Remove the almost finished actuator from the part of the mold to which it had stuck in the previous step.
   NOTE: A first visual check can be made here to see if casting was successful. If irreparable defects are found (see Supplementary Figure 3), the manufacturing process is to be stopped here. Smaller holes can be repaired later. It is also important that the sealing lip is as pronounced as possible over its entire circumference.
10. Cut off any protruding burrs with a cutter knife. This is sometimes very laborious, but essential for a good final result.
    NOTE: Supplementary Figure 4 illustrates the processing steps described in this section. The described steps are valid for casting the four legs (the mold can be found in Supplementary File 1 “CAD/Moulds/small_leg_schwalbe*.stl”) and the two base parts of the torso (“CAD/Moulds/small_belly*.stl”). To cast the suction cups (feet of the robot, to be found in “CAD/Moulds/suctionCup*.stl”) or the bottom part of the torso (“CAD/Moulds/small_torso_base1*.stl”), carry out the same process steps, with the exception of steps 3.1 and 3.3 as these molds for casting have a built-in port for the syringe and therefore no additional acrylic glass plate is required. In total, build four base parts of the leg, two base parts of the torso, one bottom part of the torso, and four suction cups.

4. Manufacturing of lower part (bottom part)

1. Push a silicone tube through the holes provided for this purpose in the mold of the bottom part, see Supplementary Figure 5.
2. Fill the mold of the base part with elastomer and distribute it with the small spatula up to the corners.
   NOTE: The level of the elastomer should be no higher than 5 mm and no lower than 4 mm and must completely cover the embedded tube. The mold for the bottom part of the legs can be found in Supplementary File 1 “CAD/Moulds/small_base_schwalbe.stl”.
3. Put the mold into the oven for 15–20 min for curing. For the following steps, it is necessary that the bottom part remains in the mold for the time being joined with the top part.

5. Joining the base and bottom part

1. Fill the mold of the bottom part with elastomer so that the level is 1–1.5 mm above the already hardened elastomer.
2. Insert a butterfly cannula into the base part and mark the puncture site so that it can be found more easily later. This step is necessary to allow the expanding air in the oven to escape.
3. Place the base part into the bottom mold and press only the sides slightly into the elastomer bath.
4. Put the actuator into the oven for 10–15 min and remove the mold afterwards.
   NOTE: It should be easy to remove the actuator from the mold. If it fails to do so, either the elastomer is not yet fully cured (in this case, increase the curing time by 10 more min) or the bottom part is stuck in the mold (in this case, it should be pulled harder). But in general, it is a bad sign if the actuator cannot be released easily.
5. Connect a pressure source by using the puncture site from step 5.2 and perform the final leakage test, see Supplementary Figure 6.
   NOTE: If small leaks are present, they can be repaired. Application of a little elastomer with a small spatula and 10 min in the oven should fix the leak. If all leaks are fixed, the actuator is ready. Supplementary Figure 6 illustrates the processing steps described in this section and Supplementary Figure 7 illustrates the entire process described in Sections 3–5. For joining the base and bottom part of the torso, perform the same steps, with exception of step 5.1, where you do not fill the mold but the bottom part directly.

6. Joining of all limbs

1. Fix the parts to be joined with a pin needle on a wooden board so that they can be held together in the following process step.
2. Cover the joining surface with elastomer as shown in Supplementary Figure 8A. Make sure that the joining surface is clean and free of fat. Otherwise, the parts will delaminate at this point.
3. Put the assembly (see Supplementary Figure 8B) for 10–15 min into the oven.

7. Mounting of supply tube inlets

1. Widen the insertion point of the butterfly cannula from step 5.2 further by using a 1 mm Allen key.
2. Place the end of a silicone tube with a maximum outer diameter of 3 mm over the hole and press it in with the Allen key.
3. Seal the inlet with a little elastomer. This also protects against mechanical stress.
4. Put the assembly for 10 min into the oven.
   NOTE: Supplementary Figure 9 illustrates the processing steps described in this section.

8. Building the control box

1. Download the corresponding *.dxf drawings of the housing from Supplementary Data 1 “CAD/ControlBox/” and cut them out on a laser cutter.
2. Assemble the “User Interface Unit” on the front panel according to Supplementary Figure 10A and Supplementary Figure 11.
3. Build the six “Valve Units” according to Supplementary Figure 10B and Supplementary Figure 12.
4. Assemble the six “Valve Units” and the “User Interface Unit” on the bottom panel according to Supplementary Figure 10C, Supplementary Figure 13, and Supplementary Figure 14. Assemble the two side panels and the back panel. Last, assemble the top panel.
5. Configure the two single-board computers embedded in the control box according to Supplementary File 1 and upload the complete folder “Code” (including all subfolders) provided in the Supplementary Data 2 onto both boards.
6. Upload the script “Code/arduino_p_ctr.ino” provided in the Supplementary Data 2 onto the six micro-controllers embedded in the control box.

9. Building a test bench with embedded measurement system

1. Download the corresponding *.dxf drawing of the camera holder from Supplementary Data 1 “CAD/TestBench/” and cut it out on a laser cutter.
2. Download the corresponding *.stl files of the clamps from Supplementary Data 1 “CAD/TestBench” and print them on a 3D-printer.
3. Assemble the camera holder with the clamps on a DIN-A1 poster panel according to Supplementary Figure 15 and mount the camera and a single-board computer at the intended location.
4. Configure the ethernet interface and the SSH settings of the single-board computer according to Sections 4–5 of Supplementary File 1 and upload the complete folder “Code” (Supplementary Data 2) onto the board.

10. Setting up the entire system

1. Create a local network and assign the correct IP address from the script "Code/main.py" to all single-board computers and the computer used for monitoring – or rewrite the script accordingly.
2. Insert pin needles into both ends of the torso as shown in Supplementary Figure 16, so that the robot only contacts the walking plane with the pins and its feet (suction cups).
3. Print out the visual markers¹⁵ provided in Supplementary File 2 on a DIN-A4 sheet and cut them out using a scissor.
4. Attach the markers to the robot using pin needles according to Supplementary Figure 17.
5. Connect the robot to the control box.
   NOTE: Figure 1 illustrates the wiring of the entire system.

11. Running the control box

1. Power on the main switch of the control box and wait until everything is booted.
2. Log into the main single-board computer as “root” using SSH, browse to the folder “Code”, and start the control box by the command “root@beaglebone:~# python3 main.py”. At the same time, start the monitor on the personal computer by the command “user@pc:~ python2 monitor.py”.
   NOTE: Both programs must start more or less at the same time. The program “main.py” running on the single-board computer in the control box tries to connect to the personal computer used for monitoring. If there is no listening port at the personal computer (triggered by the script “monitor.py”), the monitor will not start. Except of “monitor.py”, all programs/scripts used in this protocol are intended to run with python3.
3. Connect a pressure source to the control box (max. 1.2 bar).
4. Connect a vacuum source to the control box.

12. Calibrating the robot

1. Place the robot on the walking plane of the test bench. For steep inclines, attach a string between the robot’s front and the top of the walking plane in order to hold the robot in place.
2. On the control box, activate the “pattern reference” mode by pushing the “mode 2” button as shown in Supplementary Figure 18.
3. Scroll through the menu displayed on the LCD by using the up and down buttons until you find the entry “clb”. Then, push the enter button.
4. Scroll through the next menu up to the entry “mode_4.csv” and push the "enter" button.
5. On the monitor, press the “record” button as shown in the Supplementary Figure 19.
   NOTE: Pressing the “record” button will automatically create a *.csv file on the monitoring computer at the location specified in “Code/Src/GUI/save.py:save_last_sample_as_csv()”, which is the folder “current_exp” (example measurements are provided in Supplementary Data 3).
6. On the control box, press the “function 1” button to start the calibration procedure.
7. After calibration, press the “record” button on the monitor to stop recording and the “function 1” button on the control box to stop the pressure controller.
8. Rename the automatically created “current_exp/*.csv” file so that it can be uniquely identified later.
9. Run the script “Calibration/eval_clb.py” provided in the Supplementary Data 4 and store the output (coefficients of the polynomial fit) in the file “Code/Src/Controller/calibration.py” as an entry with the keyword “[robot version]” within the existing dictionary.

13. Creating a gait pattern

1. Run the script “Code/Patterns/create_pattern.py” and store the outputted *.csv file(s) in the folder “Code/Patterns/[robot version]/”.
   NOTE: This script converts the predefined gait pattern for straight gait⁸ (see Supplementary Figure 20A or Supplementary Animation 1) formulated in angle references into robot-specific pressure references. To generate a gait pattern for steep inclines, modify the script by uncommenting line 222. This will generate a pattern according to Supplementary Figure 20B or Supplementary Animation 2. The interface for pattern references provided by the control box consists of *.csv files where each row defines a discrete set point for all actuators. Therein, the first eight columns define the reference pressures, the next four columns define the references for the direct acting valves, and the last column defines the time this set point should be held.
2. Synchronize the single-board computer in the control box with the personal computer, i.e., upload the folder “Code/Pattern/*” onto the board. For this purpose, the program “main.py” has to be interrupted (Ctrl+C).

14. Carrying out the climbing experiment

1. Perform steps 11–13 for each inclination to be tested.
2. Place the robot at the marked point on the walking plane.
3. Select a pattern reference as described in steps 12.2–12.4, but select in the first menu the desired “robot version” (instead of “clb”), and in the second menu the pattern reference according to the current inclination (instead of “mode_4.csv”).
4. Start recording as described in step 12.5.
5. Push the “function 1” button to activate the pressure controller.
6. Let the robot walk/climb for at least 6 cycles.
7. Stop recording by pushing the “record” button on the monitor (like in step 12.7).
8. Make sure the robot will not fall when executing the next step.
9. Stop the pressure controller by pushing again the “function 1” button. This will also stop the vacuum supply, and consequently the robot will fall.
10. Move the recorded *.csv file into the folder “ExpEvaluation/[robot version]/[pattern type]/[inclination]/”.
    NOTE: Repeat each run at least five times in order to have a solid base for the next step.

15. Evaluating the experiment

1. Run the script “ExpEvaluation/eval_vS11_adj_ptrn.py” provided in Supplementary Data 5 to automatically mean over all measurement data.
   NOTE: This script outputs the track of all feet, the applied pressure over time, the measured bending angle of all limbs over time, the velocity of the robot over time, the orientation of the robot over time, the mean velocity over inclination (cf. Figure 2A), and an approximation of the energy used over inclination (cf. Figure 2B).