Tailoring Artificial Muscles for the Design and Development of Soft Underwater Robots

Soft robots made out of highly deformable materials have many degrees of freedom similar to animals found in nature, and such robots are essential for numerous applications in harsh environments. Recently, soft robots are favored over rigid structures for their highly compliant material, high deformation properties at low forces and ability to operate in difficult (high pressure) environments. However, it is challenging to fabricate complex designs that satisfy application constraints due to the combined effects of material properties, actuation method, and structural geometry on the performance of the soft robot. Therefore, a robot must be comprised of both rigid and soft materials to achieve complex body morphologies. Specifically, underwater exploration or inspection requires suitable robotic systems capable of maneuvering, manipulating objects, and operating untethered in complex environmental conditions. Traditional robots have been used to perform many tasks underwater. However, they have limited degrees of freedom, limited manipulation capabilities, and have disruptive interactions with aquatic life. Research in biomimetic soft robotics seeks to incorporate the natural flexibility and agility of aquatic species into man-made technologies to improve its current capabilities. In this dissertation, we present different robotic structures for use in various exploratory and transportation missions. First, we start by characterizing unconventional artificial muscles used as the main actuators in the robots such as the twisted and coiled polymer fishing line (TCPFL) and NiTi shape memory alloy (SMA) actuators. As compared to current conventional actuators such as stepper motors and pnuematic systems, artificial muscles play a vital role in soft robotics due to their compliance, high strain, light weight, and low noise. Second, we show the design and testing of a fully functioning jellyfish like robot (Kryptojelly) that is capable of performing multidirectional swimming utilizing NiTi SMA actuators. Kryptojelly is a 260 mm bell diameter robot, constructed from a 3D printed rigid structure and a soft silicone bell that closely mimics the biological locomotion and appearance of a jellyfish species known as Chrysaora which has long tentacles. Third, an octopus-like robot (Kraken) having a 250 mm size dome, is presented that utilizes a hybrid actuation technology consisting of stepper motors and twisted and coiled polymer fishing line muscles (TCPFL). Kraken is equipped with interchangeable arm configurations that are actuated by both TCP muscles and stepper motors. We show Kraken operating wirelessly underwater in a swimming pool. Its soft arm structure helps grasp irregular objects underwater delicately. Fourth, we present a functionally graded (FGM) multidirectional 3D printed joint like soft robotic structure. The 3D printed structure is made of three rigid ball and socket joints connected in series, and actuated by twisted and coiled polymer fishing line (TCPFL) actuators, which are confined in the FGM accordion shaped channels. The FGM multidirectional joint is characterized and is shown performing different functionalities silently such as crawling, rolling, and bending while a camera is mounted at the tip of the structure. Moreover, experimental results of Kryptojelly performing underwater transportation tasks is demonstrated by picking a 70g object through electromagnet atatched to the robot. Swimming speeds of the robot while carrying trasporting tools are analyzed and presented, which is an effort to use soft robots in underwater repair tasks. Lastly, a simplified predictive model of the fishing line artificial muscles based on heat transfer is presented to estimate the actuation temperature and strain considering material properties, geometry and input parameters and validating the model results with experimental results. The work presented in this dissertation attempts to address the fundamental problems in actuation and design of soft robots and shows the great potential of employing artificial actuators in biomimetic soft robots, which can be deployed for eco-friendly exploratory missions or other applications.