Water hydraulic soft actuators for underwater autonomous robotic systems

Abstract Pneumatic soft actuators are widely studied in soft robotics. However, they are difficult to be used in underwater systems because of air supply and their large air compressor. The existing hydraulic soft actuators are usually actuated by pump-valve system and controlled with pressure feedback. This process is complex, and unsuitable for small autonomous underwater systems. This study presents the deployment of three new soft actuators in underwater robotic systems: (i) a strong water hydraulic soft bending actuator (SWBA) for robotic grippers operated underwater; (ii) a flexible water hydraulic soft bending actuator (FWBA) proposed for a biomimetic fish tail and (iii) a flexible water hydraulic extending soft actuator (FWEA) to implement the crawling motion as a sea cucumber. They can deform under water hydraulic energy rather than pneumatic power, and are actuated with only pumps and controlled on the basis of the supplied voltage of the pump rather than the pressure measured by a pressure sensor. SWBA and FWBA have different bending performance with different gaps in their structures, and FWEA can extend itself with the pressurized water. A simple fabricating method for the three types of water hydraulic soft actuators is explored: casting with molds for different parts and bonding to a whole. Their bending or extending characteristics are tested through experiments for the underwater robotic system. Results show the feasibility and good performance of the constructed water hydraulic soft actuators. The three water hydraulic soft actuators are simple and suitable for small autonomous underwater systems. This research provides new hydraulic soft actuators with simple drive and control for underwater robotic system. The findings will promote the development and offer a new idea for underwater soft actuators.