The first gripper, shown in Figure 2, is a pneumatic gripper based on the one commercialized by Soft Robotics [43]. They developed several configuration solutions depending on the application. In this work, the four-finger solution, with fingers symmetrically spaced around the center of the gripper, has been adopted.

The most important aspect of the design of fingers is its asymmetry along the lengthwise plane. This is necessary to allow movements. The inner side presents a flat surface with small reliefs that stick out to improve gripping capacity. The outer side presents six chambers that inflate when pressurized air is introduced inside, generating the contraction movement. Vice versa, creating a vacuum inside, they contract and generate the extension movement.

At one end, the finger has a section that allows it to be held at the base of the gripper, as well as allowing air to enter and exit the chamber. On the other end, a section with greater rigidity and with a triangular profile has been added to the finger. This provides a relatively straight section at the end of the finger to perform a better grip with the extremity. Fingers measure 11 cm in length and 4.5 cm in width.

For the creation of this prototype, two parts can be distinguished: manufacturing silicone fingers and the 3D printing of the rest of the pieces that form the base of the gripping tool (all 3D printed parts are made with the thermoplastic polyester PLA).

To obtain a silicone finger, a mold has been realized with 3D printing, as shown in Figure 3. It comprises three different parts: a male that provides the internal shape of the finger, and two females, which provide the exterior. The silicone used is the Silastic^® 3483, with a cure time of one hour.

This silicone allows getting the desired characteristics with regard to stiffness, while other silicones such as Ecoflex^® 00-30 have proved too soft for this application.

The working pressure of the gripping tools is around 2 bars.

The second gripper, as shown in Figure 4, is also pneumatic and is based on the Versaball, which is a gripper produced by the company Empire Robotics Inc. [44]

Its operation is based on the jamming process by which granular materials increase their viscosity when the density of particles increases.

The gripper presents a spherical chamber made of an elastomeric plastic membrane filled with granular material. It is possible to get the liquid and solid pseudo-states when pressurized air and vacuum are applied, respectively.

To grab an object, a small amount of air has to be introduced in the chamber so that the granular material acts similar to a liquid. Then, the gripper has to be pressed against the object, adapting the contents of the chambers to the outer shape of the object. Finally, the vacuum is created inside the chamber, in order to solidify its interior and maintain the shape of the object. As long as the vacuum is maintained, the object is held by the gripper. Subsequently, it is enough to fill the chamber with air again, to return to the “liquid” state and release the object. This method of gripping requires that the object size be approximately half of the diameter of the pneumatic chamber.

This gripper is composed of the previously described pneumatic module, a piece to join this module with the claw, two pieces to keep the filter in place, and two final pieces that hold the pneumatic camera.

As in the previous case, the pneumatic chamber is the basic element of the gripper. To realize it, a latex balloon of approximately 1-mm thickness and about 8 cm in diameter has been chosen. The chosen thickness provides the chamber with resistance against the abrasive action of the granular material and ensures that it is not damaged when coming into contact with objects. At the same time, a smaller thickness adapts easier to the small irregularities of the objects, besides gaining flexibility and elasticity.

Several options of granular materials have been tested, such as ground coffee, fine sand, and breadcrumbs. After testing ground coffee and breadcrumbs, it has experimentally been observed that with ground coffee, the chamber adapts much better to objects. This is the same material used in the research previously mentioned.

A filter is necessary to allow air to pass in both directions and maintain the granular material isolated from the pneumatic system. The filter is composed of a felt circle with the same diameter as the upper section of the base and a thickness of approximately one millimeter. The working pressure is around 1 bar.

The first mechanical gripper, as shown in Figure 6, presents articulated fingers actuated by cables. When the cables are pulled by a motor connected to a pulley, the fingers are flexed.

The shape that fingers adopt is a function of the object to be grasped; the links of the finger will adapt to the outer surface of the object.

Here, the number of fingers selected to achieve optimum grip is three, which are arranged at 120 degrees. To work properly, the three fingers must perform their movements simultaneously.

To carry out this movement, the three cables (one for each finger) are wound on the same pulley that collects or releases the cables, generating thus the flexion or extension of the fingers on the basis of the rotating direction.

The pulley represents a crucial part of the design, since it is responsible for the synchronization of the movement of fingers. It has an inner section in which a nut is inserted to fasten the pulley to the flat face of the motor shaft. The pulley is designed with a diameter that is small enough to generate a good torque for the application but large enough to be printed in 3D and not too fragile. It also has a duct with a circular section that connects the outer face of the pulley, where the cables are wound, with the underside of the pulley.

The three fingers are held on a base, and an adapter is used for the connection to the robot. Each finger comprises three base modules, one end, and a mount. Modules are joined with screws that allow their relative rotation. Each finger measures 12 cm in length and 2.5 cm in width.

The fourth gripper, as shown in Figure 7, is based on the MultiChoice Gripper product manufactured by the FESTO^® Company [45]. This design is based on the mechanics of the human hand where the thumb serves to impose an effort in the same direction but opposite orientation as the rest of the fingers. To adapt to different object typologies, the tool developed by FESTO is able to reorient the fingers and obtain different configurations. For example, for objects with round perimeter are placed with ternary symmetry. However, for objects with parallel faces, one finger is placed in opposition to the other two.

Nevertheless, for the reproduction of this prototype, this feature has been obviated to focus attention on the design of the fingers: passive structures capable of adapting to the objects that deform them. This gripping tool comprises three fingers of a flexible material. The fingers are triangular with thin walls and, longitudinally, they present a nucleus formed by rigid parts that prevent the walls of the finger from approaching. With this morphology, the tip remains approximately in the same place when trying to deform the finger, but the body bends around the object. This deformation of the finger allows it to adapt to the shape of the surface of the object with which it will make contact, and, thanks to its flexibility, it will deform before damaging it. In this case, the choice of the number of fingers that form the gripper could have been two, three, or four. The number three has been selected because the spindle mechanism makes the movement perfectly synchronized, and it is unnecessary to complicate the design by adding a fourth finger, although in some scenarios, it might be useful to have it.

The gripping tool consists of three flexible fingers and a rigid base. The fingers are made from the elastomeric thermoplastic produced by the company Recreus [46] called FilaFlex, and they measure 11 cm in length and 1.5 cm in width. The base is formed by two pieces: the main one on which the fingers are mounted and to which the motor is coupled, and a secondary one that serves to join the gripping tool with the UR3. Furthermore, the mechanical actuation is composed of a direct current motor, a coupler flexible, a spindle and a nut, and a handle to which the nut, the finger connector as well as the motor support are attached.

To generate the grip movement, each finger can be held with rotational joints at both ends of its base. By maintaining the position of the outer ends of the finger bases and changing the height of the inner ends, the opening and closing movement of the hand is achieved. This movement, which can be generated in many ways, is done with a spindle nut mechanism. A motor rotates the spindle, which causes the nut to move up and down when its rotation is prevented. The inner ends of the fingers are connected to the nut by connectors, which allow the possibility of movement while preventing the rotation of the nut.

The rigid sections that form the internal structure of the finger have been made with a wire. This wire has enough rigidity to allow the deformation of the finger but at the same time allow it to return to its initial position.

3. Experiments