Investigation of pre-charged fluidic actuation for untethered soft robots

Abstract

Hard robots transmit electromotor torque directly through rigid links. In contrast, soft robots can undergo “large” deformation and actively interact with the environment by relying on structural compliance or inherent material elasticity. The “softness” brings advantages, such as less reliance on the sensory feedback and complex control algorithms when dealing with fragile objects, unknown environment, or interaction with human beings. However, “softness” also brings limitations. The controllability, accuracy, and load of soft robots are limited. What’s more, most soft robots are inevitably constrained by tethers to the power supply. It is still challenging to integrate the actuators, power supply, sensors, and controller in untethered soft robots for practical applications, such as wearable or field robotics. Therefore, developing untethered soft robotics with functionality and practicability has been a pivotal research gap for soft robotics. This thesis aims to investigate and develop new actuation methods for untethered soft robotics. First, an exploratory and experimental study of a soft actuator’s vibration-damping is conducted. Pneumatic soft actuators have highly elastic bodies that oscillate drastically once excited, which is undesirable in many applications. A simple and effective damping method based on passive and active particle damping is proposed and tested. Secondly, a new approach to untethered soft robot design based on Pre-Charged Pneumatics (PCP) is proposed. A PCP soft bending actuator is actuated by pre-charged air pressure and retracted by inextensible tendons. Complicated pneumatic systems used in traditional soft robotics are eliminated in a PCP based soft robot. A controllable PCP soft gripper is designed and fabricated. Closed-loop control of the soft gripper can be realized with sensor feedback. Thirdly, to demonstrate the practical applications of the proposed PCP, two untethered robotic dogs with PCP soft legs are designed, fabricated, and tested, which proves that PCP has great potential in developing untethered mobile robots. The first robotic dog achieves trotting gait with open-loop control. It also demonstrates excellent resilience under an unexpected and huge impact. The second robotic dog was developed for the amphibious locomotion. The quadrupedal paddling gait of canines is analyzed and implemented to the amphibian robotic dog. Hydrodynamics drag force analysis is performed for the paddling gait. Lastly, a new and effective approach to untethered fluidic actuation for soft and compliant robotics is proposed, namely twisting tube actuation (TTA). When twisting an elastic tube filled with fluid (gas or liquid), two effects are observed: 1. contraction along the twisting axial direction, 2. pressure output of the extruded fluid from the twisted tube. Various actuation TTA modes could be realized by utilizing these two effects. An anthropomorphic forearm based on the TTA is developed to showcase the effectiveness of the actuation modes. By adding braided nylon mesh sleeve to constrain the rubber tube, a special case of TTA, twisting pneumatic artificial muscle (TPAM), is designed and analyzed. A novel antagonistic and antagonistic robotic joint based on TPAM is presented with detailed design, analysis and experimental studies.