Design and control of a continuum robot with switchable stiffness based on ball-and-socket joints

Multi-segment continuum robots (MSCRs) with switchable stiffness offer significant potential for advancing minimally invasive surgeries. Compliance is advantageous during navigation to prevent rigid collisions, while rigidity is required to ensure stability during surgical manipulation. However, most variable stiffness techniques involve additional structures, increasing the MSCRs' size beyond clinical applicability. This study proposes to leverage the inherent friction between the joints of discrete-jointed MSCRs to achieve stiffness switching, eliminating extra friction mechanisms. A ball-and-socket joint-based MSCR with a compact diameter of 6 mm is introduced. During operations, the proximal segment of the MSCR is rigidly locked, providing stable support for the distal segment and resisting force coupling between the segments. To control the MSCR, a friction model for the ball-and-socket joints is developed and integrated into the Cosserat rod framework to characterize its kineto-static behavior. A multicore fiber with included fiber Bragg gratings (MCF-FBGs) is integrated for shape sensing. Based on the shape feedback from MCF-FBGs, a general workflow for the proposed prototype is established, detailing the actuation strategies. Experiments are conducted to validate the robot's stiffness switching capability, quantify the modeling accuracy, and comprehensively evaluate the performance of the proposed prototype within the defined workflow.