In this chapter the methodologies utilized will be listed, then the gripper fabri-cation process will be described in detail. Finally, the creation of an ad hoc testing bench for gripper force measurement will be described.
4.1 Methodologies
All the soft elements have been realized with a casting procedure of the commer-cial biocompatible silicone rubber ECOF LEX series 00−30, Smooth−On, Inc (Figure 4.1a). Eco ex rubbers are highly versatile since they show no volumetric restriction and low viscosity. These silicones have a platinum-based cure system, because platinum is added as a catalyst. Two separate components must be mixed to catalyze the polymers: one component contains a platinum complex which must be mixed with the second, a hydride- and a vinyl-functional siloxane polymer, cre-ating an ethyl bridge between the two.
Mold elements were CAD-designed with P T C Creo P arametric and printed in ABSplus material using a Dimension Elite 3D-printer and in V isiJet EX200 material using the 3D System P roJet HD 3000 3D-printer.
The actuator is obtained by using two shing materials: Berkley F ireLine 0.04 mm were used as wires (Figure 4.1b), coated by a polymer sheath to avoid friction gen-erated by the direct contact between the cable and the silicone body nger: Stonfo silicone tube of 0.2 mm inner diameter as sheaths (Figure 4.1c).
4.2. Manufacturing process Chapter 4. Materials and Methods
(a) (b) (c)
Figure 4.1: Equipment used for fabricating the gripper soft parts and its actuation mechanism: (a) commercial two-parts silicone, (b) and (c) shing braided line and siliconic tube.
4.2 Manufacturing process
3D-printing has provided an easy and rapid way of constructing mechanical pro-totypes which could have been iteratively improved until they were ready to be in-corporated in the nal design. Together with low-cost materials, the combined use of Computer-Assisted Design (CAD) and Computer-Assisted Manifacture (CAM) enables a rapid prototyping of new engineered solutions. The manufacturing pro-cess of the SCG is additive and uses printed molds. This makes the customization and combination of actuator shapes simple and supports the implementation of complex deformations. The gripper is obtained from a four-steps casting procedure of silicone in molds:
1. molds and lodging platform are designed and 3D-printed. The ngers are casted using molds (as shown in Figure 4.2a) to ensure a reproducible form. The mold is 3D-printed using Fused Deposition Model (FDM) and Multi-Jet Modeling technologies (MJM), using hard thermoplastic material;
2. three molds are prepared placing cables inside them. The preparation of the mold consists in the housing of the actuation cable permanent, which is performed using another sheathed cable xed to the top of the mold, to be removed at the end of the fabrication process. Both cables are put in tension and xed around a screw (Figure 4.2b);
3. the ngers are casted separately to carefully control the linearity of the path of the cables. The silicone consists of a P art A and a P art B, to mix in stoichiometric ratio: 1A : 1B by volume or weight, mixed and left to polymerize at ambient temperature. The mixing of the two parts of the
in the center of the frame. Finally, a second casting phase merges and xes in place the structures (Figure 4.2d).
Once the silicone casting is done, the prototype is ready (Figure 4.3)and weighs 3 g. The silicone sheath remains incorporated, so that the wires can slide with low friction.
(a) (b)
(c) (d)
Figure 4.2: The four-steps fabrication procedure: (a)CAD Design and 3D printing of molds and gripper frame, (b) cable arrangement inside the molds, (c) silicone casting and degasing, (d) nal merging casting.
4.2. Manufacturing process Chapter 4. Materials and Methods
Figure 4.3: Manufactured prototype.
Further than standardized prototypes, there is a wide range of solutions which can be implemented to enrich the behavior of the gripper. Mentioned below are three possible implementations that certainly deserve a further depth study later: Cable placing at dierent heights. The arrangement of the cables inside the mold determines the gripper physical behavior. The implemented choice carries actuation to the entire nipper's length, but dierent deformation be-haviors can be obtained via anchorages at dierent heights (Figure 4.4a); Fingernails. A bio-inspired implementation is the inclusion of nail-type elements
inside the nipper. Indeed, in the human hand, the nails provide stability during precision grasping and assist to the grip of small objects In Figure 4.4b is shown a variant of nipper embedding a Delrin rectangular (5×6×0.5 mm3)
ngernail in the terminal and dorsal part of it. However, rigid inclusion is a critical element, especially if it has sharp corners, which can lead to the failure of the elastomeric material under stress;
Grip increasing. To prevent the slippage of objects, an additionally milled layer on the nger ventral surface increases the contact area (Figure 4.4c) and so the tribological interaction between the digital surface and the object surface. The layer is highlighted adding SilcP ig (Smooth − on, Inc.) red pigment to the silicone. It is obtained by casting a thin layer of silicone on a 1.34 mm pitch milled grid, pressing above it the ventral part of the nger and then leaving the mixture to polymerize.
the frame of the gripper and bringing them out from the side, point at which the cables are inserted in a 2 mm diameter metal sheath. The metal sheath incorporates the cables up to a sliding base plate system mounted on the robot elbow. The cables pass through a double diameter hole of a wall integral with the base of the slide, that blocks the sheath, and are xed to the hook of the force gauge sensor. The optical slide system has a scrolling range of 2 cm. Alluris F MI − 210B Digital F orce Gauge sensor xed on the slide has been used for real-time evaluation of the cable tension during the gripper actuation (Figure 4.5b).
Two experimental tasks have been speci cally planned: grasping force and grasp stability measurements (Figure 4.6). The rst allows to derive the rela-tionship between cables tension and ngertip force. Quasi-static measures may be carried out by means of a force gauge sensor mounted on a micrometer slide system, to detect the cables tension.
4.3. Experimental test bench Chapter 4. Materials and Methods
(a)
(b) (c)
Figure 4.4: Additional implementations: (a) dierent placing of cables: prepara-tion of molds and resulting relative deformaprepara-tion at 2.5 N cables tension, (b) nipper with a Delrin ngernail and (c) nipper with a milled layer on the ventral surface.
(a)
(b)
Figure 4.5: Testing setup: (a) Robotic arm and (b) slide system with force gauge sensor.
4.3. Experimental test bench Chapter 4. Materials and Methods
(a)
(b)
Figure 4.6: Experimental planned tasks: (a) ngertip force measurement and (b) grasp stability measurement.
