Closed-Loop Control and Disturbance Mitigation of an Underwater Multi-Segment Continuum Manipulator

TL;DR

The paper addresses the challenge of achieving precise control for long, compliant underwater manipulators by integrating a modular tendon-driven continuum arm with a PCC kinematic model, IMU-based state estimation, and closed-loop tendon-length control augmented by tension supervision. The authors validate configuration-space and task-space tracking in water, achieving end-effector positioning within about $3$ cm ($\sim3\%$ of length) and posture errors largely under $5^{\circ}$, while also demonstrating robustness to disturbances up to $300$ g. Key contributions include a fully integrated mechanical-sensing-control architecture, a practical tension-supervision scheme, and underwater experimental evidence of reliable disturbance rejection. The work advances large-scale compliant manipulation for subsea inspection and maintenance, offering safer, longer-reach alternatives to rigid-link designs and a foundation for real-world deployment with future robustness enhancements.

Abstract

The use of soft and compliant manipulators in marine environments represents a promising paradigm shift for subsea inspection, with devices better suited to tasks owing to their ability to safely conform to items during contact. However, limitations driven by material characteristics often restrict the reach of such devices, with the complexity of obtaining state estimations making control non-trivial. Here, a detailed analysis of a 1m long compliant manipulator prototype for subsea inspection tasks is presented, including its mechanical design, state estimation technique, closed-loop control strategies, and experimental performance evaluation in underwater conditions. Results indicate that both the configuration-space and task-space controllers implemented are capable of positioning the end effector to desired locations, with deviations of <5% of the manipulator length spatially and to within 5^{o} of the desired configuration angles. The manipulator was also tested when subjected to various disturbances, such as loads of up to 300g and random point disturbances, and was proven to be able to limit displacement and restore the desired configuration. This work is a significant step towards the implementation of compliant manipulators in real-world subsea environments, proving their potential as an alternative to classical rigid-link designs.

Figures (11)

• Figure 1: (a) Diagram of the assembled manipulator, showing: a TPU plug, secured at the bead centre for routing a 2mm diameter NiTi rod; TPU arches between beads; an IMU encapsulated in resin; compressible TPU hinges. Also, (b) an example of how neighbouring beads move in orthogonal planes and (c) the manipulator in operation underwater.
• Figure 2: Overview of the sensing system implemented within the manipulator design. (a) IMUs are daisy-chained to estimate the shape of the manipulator and (b) load cells are integrated within the motor housing to provide cable tension feedback.
• Figure 3: Piecewise Constant Curvature (PCC) kinematic model of a single spatial segment, where the dashed lines represent the bending plane.
• Figure 4: Block diagram of the control architecture, where the dashed elements indicate procedures only in use during task-space feedback control.
• Figure 5: The experimental arrangement, showing (a) the setup of the manipulator and cable routing to the base of the test rig, (b) the motor layout and peripheral connections on the base and (c) the manipulator during operation underwater.