Tunable Soft Optical Sensors for Shape Sensing in Soft Continuum Robots and Multi-Modal Contact Sensing End Effectors

The design, control, and functionality of soft robotic manipulators and grippers are a growing area of research interest. Recent advancements are pushing toward closed-loop control via embedding proprioceptive and exteroceptive sensing into these soft platforms. Different soft sensing technologies have been explored, i.e., resistive, capacitive, and magnetic. Fully soft optical sensors are useful for both shape sensing and contact force recognition and have been utilized for a variety of soft gripper applications. However, these grippers are often attached to rigid robotic arms, which highlights the need for soft continuum robots with embedded sensing. Soft continuum robots can benefit from the integration of tunable soft optical sensors that provide the systems with real-time 3D shape sensing. The combination of shape sensing soft continuum robots and contact sensitive soft robotic grippers presents exciting application scenarios in minimally invasive surgery (MIS) and high-precision pick-and-place tasks.<br/><br/>Recently, we have developed a roughness tuning strategy for the fabrication of soft optical waveguide sensors (WG) to achieve shape sensing and contact recognition within a single multi-modal sensor. We integrated the sensors into a fully soft continuum robot consisting of a multi-directional bending module and a gripper. The robot module integrates two WGs for 3D shape sensing and three soft pneumatic actuators to steer the robot in all directions. The gripper embeds two soft pneumatic actuators to deploy itself, two soft pneumatic actuators to control grasping, and two WGs, one in each jaw of the gripper, with tuned roughness to monitor both gripper tip positions and subsequent occurrence of contact with an object. The deployment and grasping motions of the gripper cause measurable losses in optical power through the WGs. These losses are utilized to track the tip position of the gripper. Alternatively, when an object is contacted, we observe a distinct increase in the output power of the WG through the system, which allows us to distinguish the tip position tracking from contact recognition and thus create a multi-modal sensor. The proposed system simultaneously tracks the real-time shape of the robot body and the tip positions of the gripper via a graphical user interface (GUI). Furthermore, upon contact with an object, the GUI alerts the user of the successful grasping of an object. The robot module is calibrated so that the shape of the robot is mapped with an electromagnetic tracker throughout its entire workspace. This data is used to determine the true shape of the robot by using a constant curvature model, and then the WG signals are fit to a surface based on the data collected to predict the shape of the system in subsequent testing and controls. The surface fits implemented to use the WGs to predict the robot module shape had an average error of the tip position of less than 4 mm.<br/><br/>Our soft continuum robot can be utilized in a variety of pick-and-place applications where the integration of soft optical sensors can improve the accuracy of the control of soft robotic arms. Further, we have determined that proper tuning of the sensors can result in increased sensitivity across a large range of curvatures and enable a multi-modal sensor response, which allows for a reduced number of total sensors in the robot. These advancements in the tunability of soft optical sensors pave the way toward miniaturized devices and better closed-loop control of soft robots. Providing contact force recognition to continuum robots also pushes toward improving the capabilities of haptic feedback systems that can be integrated with soft sensors and soft continuum robots. Lastly, the low cost and scalable nature of the system presents opportunities for incorporation into a variety of MIS procedures where enhanced dexterity and sensor feedback is valuable for surgeons.