Inhalt des Dokuments
How to create a PneuFlex actuator
Disclaimer
Linking, Citing and Contact
We welcome linking to this site, and we will maintain it under the URL http://www.robotics.tu-berlin.de/index.php?id=pneuflex_tutorial. If you want to copy or reuse all or parts of this content, please contact the RBO Lab for permission.
If you want to cite our work, please use:
Raphael Deimel and Oliver Brock. A Compliant Hand Based on a Novel Pneumatic Actuator. Proceedings of the IEEE International Conference on Robotics and Automation (ICRA), pp. 2047–2053, 2013.
or
Raphael Deimel and Oliver Brock. A Novel Type of Compliant and Underactuated Robotic Hand for Dexterous Grasping. The International Journal of Robotics Research 35(1-3):161-185, 2016.
You also can find additional information on the PneuFlex design principles there.
For any questions or errors found, please contact: Vincent Wall
1.1 Silicone rubber
1.2 Fiber reinforcement
1.3 Silicone rubber tubes
1.4 Molds
For a simple version of a PneuFlex actuator, the mold could be created manually, but 3D-printing is getting inexpensive enough to be readily accessible. Operating on CAD models also enables the easy customization of geometry. Our print shop provides us with molds made of epoxy-infiltrated plaster, but you can also use printers based on other technologies, e.g. ABS polymer.
Here are the links to two CAD models in STL format for printing:
Model 5 (shown in this tutorial): small size (9mm), recommended silicone: DragonSkin 20 (E=300kPa)
Model 7 (shown in the tutorial video) : bigger size (18mm), recommended silicone: DragonSkin 10 (E=150kPa)
We have also published the more recent version of the PneuFlex CAD models.
1.5 Vacuum pump & Vacuum chamber
- Vacuum chamber (with pressure gauge and valve) and vacuum pump. The chamber volume can be reduced by filling up with styrofoam elements to accelerate evacuation.
- © Robotics
The only specialized equipment you’ll need is a vacuum pump and a vacuum chamber. Thoroughly mixed pre-cure silicone contains many air bubbles. If they are not removed, they weaken the rubber’s ultimate tensile strength, and also create weak spots. Even more important, we use it during casting to avoid trapped air bubbles.
As uncured silicone is quite viscous, we need a good vacuum of about 5-10 kPa (50-100mbar). You may find suitable setups in certain labs (chemistry, mechanics) of universities. You can get suitable vacuum pumps for as low as 100$, and chambers for as low as 150$. A cheaper alternative to vacuum chambers are lab (vacuum) desiccators made of glass.
1.6 Shopping List
- [1] Vacuum chamber
- [2] Vacuum pump (or here)
- [3] Gram scale
1.7 Shoppping List Consumables
- [4] Silicone DragonSkin 10 or 20 (ca. 150 and 300kPa Young’s modulus respectively)
- [5] Mold sealant
- [6] Silkscreen
- [7] Sewing thread
- [8] Adhesive
- [9] Silicone tubes 0.5mm ID, 1.5mm OD
- [10] Adapters for silicone tubing
- [14] Mold release agent (making it easier to unmold and not break the mold)
- [29] Silicone tube, 2mm inner, 4mm outer diameter (for connecting to PE tubes)
- [30] 4mm PE pneumatic tubes (for connecting to industrial pneumatics)
- [33] Color pigments for silicone
1.8 Various helpful tools
- [11] Vinyl gloves (for handling liquid silicones)
- [12] big Polyethylene/Polypropylene cups (for mixing silicones)
- [13] chopsticks (for mixing small amounts of silicone)
- [15] clamps and wooden blocks for assembling casting setups and fixing molds
- [16] cutter, scissors, metal ruler, cutting mat
- [17] 3mm soft plywood for making spatulas, funnels and general prototyping
- [18] Fiberglass polyester resin (used in car repair) for sculpting and general prototyping
- [20] Aspiration cannula 2mm inner diameter (for inserting supply tubes into actuator)
- [21] small cable straps (to secure rubber tubes)
- [22] Sewing needles (to fix/add reinforcements, stuff glue into gaps, nudging threads)
- [23] 50ml syringe (for demonstrating/checking rubber actuators)
- [24] 1-2ml syringes and ca. 0.9 mm inner diameter cannulae (for fixing actuators)
- [25] Hot glue gun
- [27] Polystyrene blocks to fill vacuum chamber (speeds up evacuation)
- [28] metal pan fitting into the vacuum chamber (easier to clean spills)
- [29] brush (for applying sealant)
- [34] connectors for 4mm tubes
1.9 Built tools
- [19] Polypropylene/Polyethylene sheet pan (for casting sheets of rubber) (0.8mm)
- [35] funnel for molds, made of plywood plates, wood bar, and polyester resin
- [31] broad spatula (with two spacers) to create evenly thick sheets of silicone, made of plywood
- [32] narrow spatula (plywood) for mixing and applying silicone
2 Production of Silicone parts
2.1 Create and prepare molds
To aid the release of the cast, you can apply a so called release agent to the mold prior to each casting. Again, you may do it without if your mold is tough or flexible enough. It is mandatory though, if you use silicone as mold material.
2.2 Setup molds
2.3 Measure and mix silicone
Each mold needs about 10ccm silicone. Add about 10-20g for losses during handling. The density of silicone is similar to water (1.07g/cm³). Measure the amount of both components with a gram scale into a mixing cup that is at least 4 times as big. Mix vigorously (e.g. with a chopstick, or a narrow piece of plywood), and scrape off the walls and bottom several times to make sure the parts mix thoroughly.
2.4 Degas silicone
Figure 11: Stages of degassing. First, the silicone foams up to 3-5x its initial volume (from 60ccm to 275ccm in this example). When reaching a critical pressure, most bubbles collapse quickly within about 10-15 seconds.
2.5 Fill mold
2.6Cure
2.7 Unmold
To extract the casts, disassemble into the single molds. Cut off excess silicone at the top, and on the sides. Then, gently wiggle the two mold parts loose, and finally separate them. It helps to run a cutter along the seam to loosen the silicone. Finally, clean the seam on the cast.
3.1 Cast active layers
3.2 Winding the reinforcement helices
3.3 Cast passive layer
Take a sheet pan (PP/PE recommended) big enough to hold all active layers. Cut out a sheet of silkscreen slightly smaller than the pan, and place it inside. Calculate the amount of silicone for a layer 1.5mm thick (about 0.16g/cm²), mix, and pour on the pan. place the pan into the vacuum chamber and evacuate. wait until the foaming stops and no air is trapped below the silkscreen fabric.
Take the pan out of the vacuum chamber and scrape off excess silicone with a spatula to yield an evenly thick layer. The spatula is spaced from the bottom with two small round-headed screws on both edges.
3.4 Combine
3.5 Fixate reinforcement helix
Mix a small amount of silicone (ca. 0.8g/actuator) and glue the threads of the reinforcement helix in place. One can also use one-part silicone glues that cure more rapidly. Don’t use large amounts, as this only needs to fix the thread in place when its not under tension. Again, wait until silicone is cured.
- Spread a small amount of silicone over the actuators (e.g. with a wooden spatula [17]) to fix the thread in place.
- © Robotics
You’re now finished with assembling the actuator. Let the silicone cure according to the data sheet. Curing is a gradual process, so it takes up to a week for the silicone to develop its maximum strength.
3.6 Insert supply tubing
4 Working with PneuFlex actuators
4.1 Connecting to industrial pneumatics
4.2 Controlling PneuFlex actuators
The simplest way to control the actuator is to use solenoid valves, such as from Festo (MHE2 series) or The Lee Company. Using an integrated Darlington transistor circuit (TIP121), you can control these valves with the IO ports of an Arduino, Raspberry Pi, or any other embedded system.
4.3 Fixing PneuFlex actuators