Whole Arm Grasping Using Continuum Soft Robot

This technique is inspired by the octopus’s arm which involves wrapping the arm around the object to grasp instead of using a hand. This method of grasping was demonstrated using the Oct-Arm continuum soft robot arm. Figure (2-25) shows different-shaped objects that can be grasped using the Oct-Arm.

41 Figure 2-25: Grasping capabilities of the Oct-Arm continuum soft robot arm (Bartow,

A., Kapadia, A., & Walker, I., 2013).

Increasing the pneumatic pressure in the actuators will increase the stiffness of each section. This will reduce the compliance of the arm, but make object grasping more secure. While the objects differ in size and shape, one or more sections are required to achieve grasping.

Another whole arm grasping technique, as inspired by the elephant trunk, is the Air- Octor continuum soft robot arm. This continuum soft robot is constructed from only two sections. Figure (2-26) shows the grasping capability of the Air-Octor continuum soft robot arm.

Figure 2-26: Grasping capabilities of the Air-Octor continuum soft robot arm (McMahan, W., Jones, B., & Walker, I., 2005).

42 Giannaccini proposed a variable stiffness continuum soft arm (Giannaccini, M., et al., 2013). The proposed soft arm can grasp and hold cylindrical objects. It is constructed from an elastic outer shell filled with an incompressible liquid, and is actuated by two cables fitted on the side of the soft arm as shown in figure (2-27), which also shows the grasping capability of this arm.

Figure 2-27: Variable compliance soft arm grasping capabilities (Giannaccini, M., et al., 2013).

All these continuum soft arms are constructed to have a large contact area with any object being grasped. Hence, they have good capability in grasping large-sized objects. On the other hand, it is very hard to grasp small-sized objects using any of the above continuum soft robot arms. Finally, controlling the performance of the gripper is especially difficult as it deforms and bends in the working space whilst in contact with the grasped object.

Stilli introduced a hybrid actuation strategy to control the stiffness of soft manipulators by combining pneumatics and tendons in actuation (Stilli, A., Wurdemann, H., & Althoefer, K., 2014). The idea underpinning this type of actuation mechanism was inspired by nature, where one group of muscles works in opposition to another in order to change stiffness. Figure (2-28) shows the structure of the proposed pneumatic manipulator.

43 Figure 2-28: The structure of the pneumatic manipulator (Stilli, A., Wurdemann, H.,

& Althoefer, K., 2014).

This manipulator consists of inner airtight bladder covered by a stretchable latex bladder and then a polyester fabric sleeve. When pressure is applied to the manipulator, it will extend in length. The six surrounding tendons are used to control the bending, as well as the stiffness, of the manipulator by tightening the tendons while pressure is applied. Three of these tendons were fitted half way down the pneumatic manipulator whilst the other three were fitted to the end. The tendons will thus need six DC motors to actuate them. The resultant pneumatic actuator looks as though it has six DOF. The variable stiffness capability of the proposed arm was not investigated. As well as the use of six DC motors decreasing the efficiency of the manipulator, they will also add complications in controlling the performance of the proposed manipulator. Finally, the shape and the size of the objects to be grasped using the proposed manipulator are limited.

Maghooa proposed a soft robot arm actuated by a combination of pneumatic and tendon actuation, each one in opposition to the other (Maghooa, F., et al., 2015). The design imitates the structure of an octopus arm. By controlling the amount of pressure in the

44 inflatable manipulator and the tendon’s length, the stiffness of the entire arm can be controlled. Tendons are also used to control the bending of the inflatable manipulator by powering them in the opposite direction to that of the pneumatic pressure. Figure (2- 29) shows the conceptual system architecture of the manipulator.

Figure 2-29: Conceptual system architecture of the bio-inspired manipulator (Maghooa, F., et al., 2015).

It is clear from figure (2-29) that the manipulator is constructed from three pneumatic inflatable manipulator sections and 12 tendons. Hence, 12 stepper motors were required to spool the tendons to control the bending of the inflatable manipulator. Even though there is a variable stiffness capability in this manipulator, the use of a huge number of stepper motors in the construction of the proposed manipulator will decrease its efficiency. In addition to the complicated structure, it is not easy to control the performance of the proposed inflatable manipulator. Finally, using this sort of structure reduces the ability of the inflatable manipulator to grasp objects of different shapes and sizes.

45 Katzschmann introduced a soft planar grasping manipulator (Katzschmann, R., Marchese, A., & Rus, D., 2015). The soft manipulator is fabricated as one piece using soft materials and has a continuum bending ability. It is constructed as a homogeneous soft segment with fluidic cavities, which allows fluid to be pumped in and out during operation to control the bending of the manipulator during object grasping. Figure (2- 30) shows the proposed manipulator and the soft arm under actuation.

Figure 2-30: Soft planar grasping manipulator (Katzschmann, R., Marchese, A., & Rus, D., 2015).

The main characteristics of this manipulator are its complete compliance and the ability to interact safely with humans. However, this manipulator has a low payload. In addition, the objects being grasped must not be squeezed or broken easily. Finally, there are no sensors attached to this manipulator to give any feedback signal.

Mosadegh, introduced elastomeric actuators powered pneumatically (Mosadegh, B., et al., 2014). The basic idea underpinning the design of this actuator is to use a number of chambers and small air channels fabricated within the elastomeric materials and silicon to allow air pressure actuation. Applying a specific pressure to these small channels will produce sophisticated motions. These motions will result in a bending behaviour in the proposed actuator whose form is dependent on the design features of the air channels. The speed and range of bending depends on the actuation pressure. If the applied

46 pressure is removed, the actuator will return to its unactuated initial state. Figure (2-31) shows the shape and operation of the proposed elastomeric actuator.

Figure 2-31: Elastomeric actuator (Mosadegh, B., et al., 2014).

One of the main drawbacks of this actuator is that gravity itself can affect its ability to bend when the actuator’s bottom layer faces the ground. In addition, chambers can expand randomly in response to the applied pressure when the actuator is at its full deflection, fully actuated. Thus, the force applied to the grasped object by the actuator will not be equally distributed around the object. Finally, there is no possibility of being able to change the stiffness of the actuator, and it can only be used to grasp small or cylindrically shaped objects.