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Robust Fabrication of a Soft Mechanosensor based onPneumatic Measurements

Stefan Escaida Navarro, Olivier Goury, Gang Zheng, Thor Morales Bieze,Christian Duriez

To cite this version:Stefan Escaida Navarro, Olivier Goury, Gang Zheng, Thor Morales Bieze, Christian Duriez. RobustFabrication of a Soft Mechanosensor based on Pneumatic Measurements. New Advances in TactileSensation, Perception, and Learning in Robotics: Emerging Materials and Technologies for Manipu-lation (RoboTac 2019), Nov 2019, Macao, China. �hal-02876688�

Robust Fabrication of a Soft Mechanosensor based on PneumaticMeasurements

Stefan Escaida Navarro, Olivier Goury, Gang Zheng, Thor Morales Biezeand Christian Duriez

Abstract— In this extended abstract, we complement thework presented in a previous paper where we have shown themodeling of a novel soft pneumatic mechanosensor. With theobjective of giving a demo at the RoboTac 2019 workshop,we discuss robust manufacturing techniques that enable usto fabricate such soft mechanosensors out of silicone withembedded cavities in a consistent manner.

I. INTRODUCTION

This work is a complement to a recent paper of our group,where we presented the modeling of novel soft mechanosen-sors [1]. For the modeling, we used the FEM, which inrecent years has been shown to enable the simulation of thebehavior of soft structures at interactive update rates. Thisis the key to transferring these results to real robots madeout of deformable materials. In the work of our team, thesemethods are implemented within the multiphysics simulationframework SOFA [2], [3].1 In this approach, we see anopportunity to advance towards generic models that canintegrate measurements from a diversity of sensors. For amore in depth discussion of the motivation and related work,please refer to the paper.

In this extended abstract, we present our work on fabricat-ing soft pneumatic mechanosensors made out of silicone. Wetarget to give a demo at the RoboTac 2019 with this newlyfabricated sensor and the designs shown in [1]. The specificchallenge we address here, is how to reliably fabricate asensor that has multiple airtight embedded cavities inside,suitable for air-flow and pressure sensing. Our casting tech-niques are based on 3D-printed molds, which makes themreproducible for Robotics laboratories around the world. Weprovide step by step instructions and discuss the experiencesthat lead to our design choices. In Fig. 1 (top) the sensor wefabricated is shown on a test bench. We have also modeledthis new design in SOFA (see Fig. 1 below). We aim to makeall the files needed for printing the molds and simulating thesensor in SOFA available to the public.

This work was supported by the Region Hauts-de-France, the projectCOMOROS (ANR-17-ERC2-0029), the European Regional DevelopmentFund (ERDF) and the project Inventor (I-SITE ULNE, le programmed’Investissements d’Avenir, Metropole Europeenne de Lille).

All authors are with team DEFROST at Inria Lille- Nord Europe and CRIStAL - Centre de Rechercheen Informatique Signal et Automatique de Lille, France{stefan.escaida-navarro, olivier.goury,gang.zheng, thor.morales-bieze,christian.duriez}@inria.fr.

1https://www.sofa-framework.org/

Fig. 1. In our recent line of work we have investigated the modeling ofsoft pneumatic mechanosensors. Our most recent sensor is depicted in thetop. The model in SOFA is shown in the bottom left and right panels.

II. FABRICATION

In [1] we presented a design of a soft pad sensor that hasfour symmetrical cavities. Several iterations were necessaryto fabricate a functioning sensor. Our first approach was toglue two symmetrical parts together, each one containingone half of the cavities’ structure. However, when usingnon-transparent silicone it is impossible to visually assessfrom the outside whether bubbles have been trapped insideduring the glueing process. We therefore face the odds of airbeing trapped somewhere near to the cavities on the commonsurface between the two sides. For this reason we had manynon-airtight cavities before succeeding with the fabrication.From the experience we gained we developed a new idea:A cavity is sealed not by a surface glueing process, but bya push-in process, similar to a cork. Here, only the seamneeds to be glued bubble free for airthightness. This processmakes the design of the 3D-printed pieces more complex,because the parts are not symmetric. However, it increasesthe robustness of the fabrication.

To sequentially cast the two silicone parts, a total offive distinct 3D-printed parts are necessary (four of themare depicted in Fig. 2). The mold-cap can be reused for

Fig. 2. 3D-printed parts that assemble to a mold in which the individualsilicone pieces making up the sensor, depicted in the top, can be casted. Theparts are a mold-wall, a mold-cap, a cavity cap and a cork cap. The secondmold-wall, which has the same contour but a different height, is omitted forbrevity in this figure.

both molds. We will make the CAD-files available on thehomepage of our team.2 The assembly for each mold consistsof three parts, as shown by Fig. 3. We leave the structuredpart of the mold in the bottom. In this way, when pouringthe silicone we can supervise that it adequately fills all theneeded structures. In addition, we use a paper-tape to coverthe flat cap, which helps at the de-molding stage. Siliconecan stick heavily to porous 3D-printed parts. The paper-tapeprevents silicone from entering the pores and unsticks moreeasily from the 3D-printed part or the silicone. In fact, thepaper might stick either to the mold-cap or to the siliconewhen de-molding.

We use the Dragon Skin A10 platinum cure silicon fromSmooth-On [4]. We take the one with a pot life of 20 min.in order to have enough time to degas (extract air trappedinside the silicone mixture) twice. After mixing the A and Bcomponents in a 1:1-ratio (as specified), we degas and pourthe silicone into the assembly (see Fig. 3) until it is about70% full. To prevent any air bubbles being trapped in thestructured part of the bottom cap, we degas the silicone atthis stage again. Then, we pour the rest of the silicone intothe mold and press the top cap with the paper-tape to closethe assembly. The mold should be filled up to the top with

2https://team.inria.fr/defrost/

Fig. 3. An assembly of one of the two molds.

Fig. 4. Top: The two casted pieces covered in glue. Bottom left: The jointpieces inside of the mold-wall, which helps in the alignment and preventsoozing of the glue. Bottom right: A weight is placed on top of the jointpieces.

silicone, so that a surplus oozes out when closing it with thecap. This surplus can easily be removed after curing. Aftereach part is cured, we make sure to cut away carefully anysurplus silicone that is attached to the part. When we have thetwo parts casted, we verify that they match together and closenicely. We then mix another small amount of silicone, whichwill be used as glue. We uniformly distribute the silicone onboth surfaces, taking special care that the seam of the cavitieshas a proper amount of glue (see Fig. 4). The amount shouldbe enough to cover everything uniformly, but not too mucheither, in order not to ooze into the cavities when both partsare pressed together. It is also important to remark that whilethe silicone is not yet cured, it can be easily removed fromthe piece. Therefore, the silicone used for glueing can beapplied coarsely at first and then any surplus can be wipedoff.

Before glueing the two parts together, we first put the partwith the cork structure inside the walls of mold assemblywithout any cap. We make sure it is flush with the floorby pressing on it until it reaches the bottom of the mold-wall piece. Then, we carefully press the other part onto it

0 21Time (s)

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Fig. 5. Left: A graph of the force reading during indentation. Right: Thepressure sensor readings for the same event.

inside the wall. In Fig. 4 (low left) the final result is shown.The seam is completely covered by the mold-wall piece.The mold-wall piece prevents the surplus glue silicone fromoozing on the outside of the sensor. It also helps in nicelysealing the seam on the exterior of the sensor. As seen inFig. 4 (low right), we place a weight on the sensor and welet the glue silicone cure. With this, we have finalized thesilicone fabrication part.

To equip the soft pad with tubes, we use a needle toperforate it from the outside, reaching just into the cavity. Ahollow (seringe) needle allows us to determine whether wehave reached the cavity or not, because we can detect theair coming in and out of the cavity through the needle whenpushing on the cavity. To reinforce the tunnel we created,we push a rigid rod (e. g. a small allen wrench) throughagain. We then push a tube through. We do not recommendcasting the channel for the tubing directly with the siliconeparts, at least not in final diameter of the tubing. A smallerdiameter for the channel makes the silicone press on thetubing, increasing the pressure a cavity is capable of handlingbefore it bursts (possibly through the channel). We make surethat the tubes have equal lengths to avoid different behaviorsdue to the difference in total volume (cavity + tubing). Thesensor in its final stage is shown in Fig. 1.

III. CHARACTERIZATION

To verify that we have created in fact four consistentcavities inside our sensor, we performed an experimentalvalidation. The setup is shown in Fig. 1. We place the sensoron a test bed that is equipped with a force sensor. We pressfive times on each cavity with two predefined indentationlevels. We record the pressure change in the cavity withthe sensor MPX5010DP by Freescale Semiconductor [5].Each cavity was connected by turns to the same sensor.Additionally, we left the x,y-position of the test bed identicaland reoriented the sensor for each measurement. Fig. 5 showsan example plot of one indentation procedure.

The results of our characterization are shown in Tables Iand II, where mean values, standard deviations and standarddeviations expressed in percent of the means are shown. First,a force of about 10N is applied (Table I). The pressure isaround 4kPa in this case. For the repeated indentation ofeach cavity (five times), the results are very consistent, with avariation of less than 0.3%. To compare the cavities betweenthem, we calculated the statistics for all the data collected(last row). Here it is shown that the standard deviation for the

Force (N) Pressure (kPa)Cavity mean std % mean std %

1 10.65 0.03 0.29 4.08 0.01 0.212 10.55 0.015 0.15 4.05 0.003 0.073 10.45 0.03 0.30 3.95 0.006 0.164 10.74 0.04 0.40 4.04 0.01 0.29

All 10.60 0.11 1.04 4.03 0.05 1.17

TABLE IRESULTS INDENTATION LEVEL 1

Force (N) Pressure (kPa)Cavity mean std % mean std %

1 16.16 0.05 0.31 6.69 0.006 0.092 15.96 0.05 0.29 6.72 0.004 0.063 15.90 0.04 0.26 6.54 0.004 0.064 16.11 0.04 0.27 6.64 0.004 0.06

All 16.03 0.12 0.74 6.65 0.07 1.01

TABLE IIRESULTS INDENTATION LEVEL 2

pressure over all four cavities is 1.17%. Similar results areobtained when indenting with a force of about 16N, wherethe pressure is around 6.6kPa for the cavities. The percentualdeviation between the cavities in this case is 1.01%. Theseresults indicate that the cavities are indeed fabricated in aconsistent manner, as the changes in pressure are repeatablebetween them under same conditions.

IV. CONCLUSIONS

In this extended abstract, we have shown the robustfabrication of a pneumatics-based soft mechanosensor. Thissensor has applications in Soft Robotics and can be modeledwith the help of our framework SOFA. We discussed thedesign and fabrication process of the sensor. The structurewith embedded air-tight cavities is obtained by casting twoparts in silicone, which are glued together. Here, we exploit acork-like design principle. Finally, we show that the varianceof the measurements between the cavities is low, indicatingthat the fabrication is indeed robust.

REFERENCES

[1] S. Escaida Navarro, O. Goury, G. Zheng, T. Morales Bieze, andC. Duriez, “Modeling Novel Soft Mechanosensors based on Air-Flow Measurements,” IEEE Robotics and Automation Letters, 2019.[Online]. Available: https://hal.inria.fr/hal-02239080

[2] C. Duriez, “Control of elastic soft robots based on real-time finiteelement method,” in Robotics and Automation (ICRA), 2013 IEEEInternational Conference on. IEEE, 2013, pp. 3982–3987.

[3] C. Duriez, E. Coevoet, F. Largilliere, T. Morales-Bieze, Z. Zhang,M. Sanz-Lopez, B. Carrez, D. Marchal, O. Goury, and J. Dequidt,“Framework for online simulation of soft robots with optimization-based inverse model,” in Simulation, Modeling, and Programming forAutonomous Robots (SIMPAR), IEEE International Conference on.IEEE, 2016, pp. 111–118.

[4] Dragon Skin 10 MEDIUM, Smooth-On. [Online]. Available: https://www.smooth-on.com/products/dragon-skin-10-medium/

[5] Integrated Silicon Pressure Sensor On-Chip Signal Conditioned,Temperature Compensated and Calibrated, Freescale Semicon-ductor. [Online]. Available: https://www.nxp.com/docs/en/data-sheet/MPX5010.pdf