Soft Growing Robotics is a subset of soft robotics concerned with designing and building robots that use robot body expansion to move and interact with the environment. Soft growing robots are built from compliant materials and attempt to mimic how vines, plant shoots, and other organisms reach new locations through growth. While other forms of robots use locomotion to achieve their objectives, soft growing robots elongate their body through addition of new material, or expansion of material. This gives them the ability to travel through constricted areas and form a wide range of useful 3-D formations. Currently there are two main soft growing robot designs: additive manufacturing and tip extension. Some goals of soft growing robotics development are the creation of robots that can explore constricted areas and improve surgical procedures.
Additive manufacturing design One way of extending the robot body is through additive manufacturing. Additive manufacturing generally refers to 3-D printing, or the fabrication of three dimensional objects through the conjoining of many layers of material. Additive manufacturing design of a soft growing robot utilizes a modified 3-D printer at the tip of the robot to deposit thermoplastics (material that is rigid when cooled and flexible when heated) to extend the robot in the desired orientation.
Design characteristics The body of the robot consists of:
A base, where the power supply, circuit board, and spool of thermoplastic filament is stored. The tubular body of varying length created by additive manufacturing which extends outwards from the base. The tip where new material is deposited to lengthen the tubular body, and house sensors. The additive manufacturing process involves polylactic acid filament (a thermoplastic) being pulled through the tubular body of the robot by a motor in the tip. At the tip, the filament passes through a heating element, making it pliable. The filament is then turned perpendicular to the direction of robot growth and deposited onto the outer edge of a rotating disk facing the base of the robot. As the disk (known as the deposition head) rotates, new filament is deposited in spiraling layers. This filament solidifies in front of the previous layer of filament, pushing the tip of the robot forward. The interactions between the temperature of the heating element, the rotation of the deposition head, and the speed the filament is fed through the heating element is precisely controlled to ensure the robot grows in the desired manner.
Movement control The speed of the robot is controlled by changing the temperature of the heating element, the speed at which filament is fed through the heating element, and the speed the deposition head is spun. Speed can be defined as the function:
S = L d 1 ( tan α ) 2 + 1 {\displaystyle S={\frac {L_{d}}{\sqrt {{\frac {1}{(\tan \alpha )^{2}}}+1}}}}
Where L d {\displaystyle L_{d}} is the thickness of the deposited layer of filament, and α {\displaystyle \alpha } is the angle of the helix in which the filament material is deposited. Controlling the direction of growth (and thus the direction of robot "movement") can be done in two ways:
Changing the thickness of the filament deposited on one side of the deposition head (tilting the tip away from that side). Changing the number of layers of filament on one side of the deposition head by using partial rotation of the deposition disk to add extra material in that sector (tilting the tip away from the side with extra layers of filament). For example, the disk could normally rotate clockwise, rotate counter-clockwise for 1 radian, and then resume rotating clockwise. This would add two extra layers of material in the 1 radian section.
Capabilities One of the major advantages of soft growing robots is that minimal friction exists between the outside environment and the robot. This is because only the robot tip moves relative to the environment. Multiple robots using additive manufacturing for growth were designed for burrowing into the soil, as less friction with the environment reduces energy required to move through the environment.
Unsubmerged, one robot was able to grow at a speed of 1.8–4 mm/min. with a maximum bending speed of 1.28 degrees per minute and a growing force of up to 6 kg. Unsubmerged, a second prototype was able to grow at a speed of 3–4 mm/min. as well as passively turn 40 degrees with a 100% success rate and 50 degrees with a 60% success rate (where passively turning means the robot was grown into a slanted wall and the properties of the thermoplastic filament used to bend the robot in the desired direction).
Tip extension design A second form of soft growing robot design is tip extension. This design is characterized by a tube of material (common materials include nylon fabric, low density polyethylene, and silicone coated nylon) pressurized with air or water that is folded into itself. By letting out the folded material, the robot extends from the tip as the pressurized tube pushes out the inner folded material.
Design characteristics In contrast with additive manufacturing where new material is deposited behind the tip of the robot to push the tip forward, tip extension utilizes the internal pressure within the robot body to push out new material at the tip of the robot. Often, the tubing inside the robot body is stored on a reel to make it easier to control the release of tubing and thus robot growth. Multiple methods of turning a tip extension robot have been developed. They include:
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