Motivation and inspiration
Rigid manipulator robots have, as of this moment, a clear advantage over soft manipulators concerning the force generation and because of their stiff nature, the problem of position control is easily addressed. Nevertheless, the compliant structure of soft manipulators makes them suitable for tasks such as handling delicate objects in which the forces required are really minimal and where high inertial forces can damage the object being manipulated. Also, the contact energy on a soft manipulator is mostly transformed into deformation of the soft structure, which allows robots and humans to work safely and efficiently as a team. Still, because of the compliance of soft manipulators, some problems concerning vibrations and deviation from the desired position arise. Therefore, the implementation of some level of stiffness control is required in soft manipulators in order to operate efficiently.
If we take a look at nature, we can find some animals that achieve this rigidification of some of their limbs in an easy manner. The octopus tentacle and even the human tongue use antagonistic actuation to control their stiffness. What this means is that a muscle (or actuator in our case) is used to move the limb to a certain position, while a second muscle is used to counter-actuate this first one in order to produce the effect of rigidification or change in the apparent stiffness of the limb.
With the concept of antagonistic actuation in mind, we can think of a suitable actuation scheme that can emulate such actions. In the field of soft continuum robots, two main actuation classes can be identified: 1) Intrinsic actuation: The construction of intrinsic mechanisms combine the actuation and the supporting structure into a single unit. This actuation is mostly fluid operated and the operation relies on the elastic deformation of the actuator chambers. Internal pressures are controlled to generate extension forces and the structure deforms according to constraints provided by the end form [Robinson and Davies et al.]. 2) Extrinsic actuation: Extrinsic mechanisms use remote actuation, and transfer motion to the structure via groups of tendons or cables to achieve the bending of the backbone [Walker et al.]. For this project, a hybrid actuation scheme is implemented in our prototype, in which pneumatic and tendon actuators antagonize each other to accomplish the motion of the manipulator while allowing a certain level of stiffness control.
In the literature, we can find some examples of continuum manipulators that implement hybrid actuation. In McMahan et al., researchers from Clemson University, USA, present the design and implementation of a continuum robot based on hybrid actuation. In this example, the central cavity that functions as a backbone for the robot is pressurized to provide structural support and cable-tendons routed through cable guides along the backbone apply forces to it to make it bend. This prototype uses the pressurized central chamber to generate extension as well. The limitation of this design is the lack of strength due to high cable friction that cannot be overcome by low pressures in the central chamber.
Neppalli and Jones addressed the flaws of the previous design in Neppalli et al. by replacing the central hose by a rubber tube. The resulting design has more flexibility and strength capabilities. A noticeable limitation of the design remains in the cable friction that can cause binding and undesirable motions of the backbone.
It is interesting to notice that while this design use hybrid actuation, the pneumatic actuators are used to provide structural support or extension, and the cables are used to achieve bending.
"Giant squid tentacle club". Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Giant_squid_tentacle_club.jpg#/media/File:Giant_squid_tentacle_club.jpg
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"Gray1019" by Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 1019. Licensed under Public Domain via Wikimedia Commons - http://commons.wikimedia.org/wiki/File:Gray1019.png#/media/File:Gray1019.png
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