Actuation of a soft-robot hand

Soft-robotics: creation and control of microfluidic soft-robots

As described in the review on soft robots, several modes of actuation exist, but the majority of soft-robots are currently pneumatically controlled. In this application note, the researches were directed towards this type of actuation.

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First soft-robot: Pneumatic Network bending actuator (PneuNet)

This PneuNet, also identified as a “finger”, was carried out using experimental protocols supplied by the Soft robotics Toolkit [1].

Design

This robot is composed of a main channel connecting a series of chambers, arranged in a row, where the thinnest walls are those between each chamber (Fig. 46). A stress layer is present at the base of the robot so that it bends when air is pressurized into the device. Without this stress limiting layer, the actuator would only extend in length.

In order to realize this robot, three molds were created using the SolidWorks CAD software and then realized in ABS by 3D printing. The ABS is very useful because it can withstand up to 90°C without deforming. The 3D printer used was anultimaker 3 (or a Prusa i3 Anet A8), with a nozzle of 0.4mm and a resolution in height of 0.2mm.

microfluidic soft robot mold 1

Figure 1 : Conception of the “finger” [1]

Realization and tests

a) REALIZATION

As presented in the review about soft-robots, the manufacturing process consists in soft lithographic molding, which relies on casting elastomer in molds obtained by soft lithography or 3D printing. Then, the construction of the robot is based on a gluing between two parts: the structural layer, containing the desired channel structures, and the stress layer, having an inextensible property (due to a higher rigidity) required for actuation. This production generally consists of three steps (Fig 2):

  • Each layer is molded by casting pourable silicone rubber in each mold, and cured. The bottom layer can be stiffened using a sheet of paper or a different polymer.
  • Then, the two cured layers are removed from their molds, joined using a thin layer of uncured elastomer as glue, and cured together.
  • Finally, once the curing done, the final actuator is unmolded and its end is pierced to insert the tubing enabling to inject the air needed for actuation

microfluidic soft robot mold 3

microfluidic soft robot mold 4

microfluidic soft robot mold 5

Figure 2 : General process of fabrication [1]

Two different prototypes of soft-robots were built in order to compare their rigidity:

  • One made of Ecoflex (+paper sheet). The Ecoflex [2] is one of the most used elastomer in the world of soft-robotics. This polymer is more soft and elastic than PDMS (E=82.7 kPA (12 psi)) and can deform up to 900%. Moreover, it cures in 3h at room temperature, or less than 1h at 80°C against 2h for the PDMS. Paper sheet is used in order to increase the rigidity of the constrain layer.
  • One made of Ecoflex and PDMS (+paper sheet). This second prototype exhibit two types of polymers: Ecoflex for the structural layer and PDMS for the constrain layer,in order to have a bigger rigidity.

b) TEST

microfluidic soft-robot setup

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The two prototypes were actuated using an OB1 MK3 pressure controller and their comportment were compared.

a) ECOFLEX (+PAPER SHEET)

Figure 3: Deformation of the « finger » made of Ecoflex under different pressure: a. p=0mbar, b. p=20mbar, c. p=40mbar, d. p=60mbar, e. p=80mbar, f. p=100mbar, g. p=120mbar

b) PDMS + ECOFLEX (+PAPER SHEET)

Figure 4:Deformation of the « finger » made of Ecoflex and PDMS under different pressure: a. p=0mbar, b. p=20mbar, c. p=40mbar, d. p=60mbar, e. p=80mbar, f. p=100mbar, g. p=120mbar, h. p=140mbar, i. p=160mbar, j. p=180mbar

c) RESULTS/DISCUSSION

From these results, the deformation of the two prototypes were modeled and represented in form of graphs, in order to compare them. (Fig. 5)

Figure 5: Measure of the deformation of the « fingers » a. in Ecoflex, b. in Ecoflex + PDMS, infunction of the applied pressure

These graphs witness the difference of rigidity between the two prototypes, because for a given pressure, the resulting flexions are not identical. Actually, 120mbar is enough tocurl the finger in Ecoflex, whereas it allows only to bend the fingers in Ecoflex + PDMS a 90 °. Unlike, the prototype combined of Ecoflex and PDMS curls under 200mbar.
The prototypes created using PDMS and Ecoflex were easier to control. This association was then used for all the others robots created.

SECOND SOFT-ROBOT: « HAND »

Design

From this previous robot, a « hand » composed of 3 fingers was designed. This latters are connected together with a structure composed by an assembly of 3D printed parts (Fig. 6). Each actuator possesses its own source of pressure, in order to enable an independent control of each finger.

microfluidic soft robot holder

Figure 6: Design of the support for the assembly of the « hand »

TESTS AND RESULTS

A first sequence was built to control each finger independently (Figure 7), one second to contract the three fingers at a time, and thus allow the grasping of objects (Fig. 8). Further information about the software capabilities on the dedicated software page.

Figure 7: Sequence allowing the independent control of the fingers

Figure 8: Sequence allowing the gripping by simultaneously controlling the three fingers

The advantage of such a device with respect to the same robot realized with solid materials is that this type of actuator allows the conformity matching. Thus, this robot can carry soft, fragile objects without damaging them and without any difficulty, whereas a rigid robot would need a much more complicated controlto carry out the same task. Similarly, a 5-finger hand orthosisprototypecould have been designed (see Soft-robots review), in order to be see a much more concrete application of such soft-robots.

THIRD SOFT-ROBOT: MULTIGAIT

This third soft-robot possessed amore complex fluidic circuit than the other designed robots.In addition, this robot shows another interesting application of flexible robotics, already mentioned in the review of : field exploration.

CONCEPTION

This robot is composed of 5 main channels, which connect a series of chambers, arranged in rows and which orientations depends on their position (Figure 9).The bottom layer is the stress layer, and allows the flexion of the different members when air is sent into the device.

This robot was designed based on the design of the multigait, a robot presented by Robert F. Shepherd et al. in 2011 [3]. Several types of designs were proposed for the molds without success of realization, before finally obtaining a functional prototype.

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Microfluidic soft robot Multigait 1

Microfluidic soft robot Multigait 2

Figure 9 : Design of the original multigait (lengths in mm) [3]

 

REALIZATION

Microfluidic soft robot Multigait 3

Microfluidic soft robot Multigait 6

 

Microfluidic soft robot Multigait 4Microfluidic soft robot Multigait 5

Figure 10: Design of the used molds

The structural layer was molded in the white mold and the red mold enable  to build the stress limiting layer (2mm) before to add a layer of liquid PDMS of controlled thickness (200µm), enabling to realize a perfect gluing between the two parts.

RESULTS AND DISCUSSION

Two displacement modes were developed based on the experiments conducted by Robert F. Shepherd et al. [3].

  • The « walking » mode

Figure 11: Sequence allowing the motion of the multigait in the « walking » mode

  • The « creeping » mode

Figure 12: Sequence allowing the motion of themultigaitin the « creeping » mode

FOURTH SOFT-ROBOT: TENTACULE

Design

The design of this fourth soft robot presents another manufacturing method, and provides a multitude of degrees of freedom to this robot in order to grasp and manipulate objects of complex shapes. This robot can also be improved by equipping it with functional components such as a suction cup to hold objects, a camera for video tracking in almost inaccessible locations, or a needle for fluid suction or distribution. Surgery would be one of the first applications of such robots.

Microfluidic soft robot tentacule mold

Figure 13: Design of the tentacle

REALIZATION AND TESTS

The different steps for the manufacturing of this new robot are shown Fig. 14:

Microfluidic soft robot tentacule mold 2

Figure 14: Manufacturing process of the tentacle [4]

a) and b) The different parts of the mold are assembled, before that c) the Ecoflex is cast inside the mold d) then the central cylinder is removed, the air supply pipes are placed and the PDMS is poured into the available enclosure, e) when baked, the other parts of the mold are removed and the ends are plugged with Ecoflex.

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Conclusion

This application note demonstrates that soft-robotics is a very interesting fields to study, and which present a lot of applications in the biomedical field. These soft actuators are easily controllable using a pressure controller, and the OB1 MK3 enables a very precise and automated control of these robots.

[1]         Site Sof- Robotics Toolkit : https://softroboticstoolkit.com (Avril 2017)

[2]         Site Smooth-on : https://www.smooth-on.com/product-line/ecoflex/ (Avril 2017)

[3]         Robert F. Shepherd  et al.,Multigait soft robot (2011)

[4]         Ramses V. Martinez et al., Robotic Tentacles with Three-Dimensional Mobility Based on Flexible Elastomers, Advanced Materials (2012)

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