An experimental comparison of four different structures for continuum manipulators

Continuum manipulators, inspired by the natural capabilities of elephant trunks and octopus tentacles, may find niche applications in areas like human-robot interaction, multiarm manipulation, and unknown environment exploration. However, their true capabilities will remain largely inaccessible without proper analytical tools to evaluate their unique properties. Ellipsoids have long served as one of the foremost analytical tools available to the robotics researcher, and the purpose of this paper is to first formulate, and then to examine, three types of ellipsoids for continuum robots: manipulability, force, and compliance.

Continuum manipulators are type of hyper-redundant robots that without traditional rigid-links, and their intrinsic properties assure continuum manipulators can work in highly unstructured environments. To extend the potential applications of continuum manipulators, this paper presents a novel biased channel retractable (BCR) continuum manipulator. BCR continuum manipulator possesses a hollow channel when reserving its main backbone, and this structure provides the manipulator 'swallow' functions without sacrificing the stiffness generated by the backbone. This paper also establishes its kinematics model based on constant curvature assumption, and experimental result validates its correctness.

Rigid instruments with limited flexibility utilized in the traditional endoscopic sinus surgery maybe damage the tissue when surgeon operates in the narrow space of nose specific area. Flexible continuum manipulator would solve this issue. However, current manipulators may not be able to address the intricate and varied surgical demands effectively, and they may not meet the surgical requirements adequately in terms of the flexibility, precision, and stiffness. This article introduced a novel modular continuum manipulator designed to offer exceptional adaptability, precision, and adjustable stiffness. The various sizes of the manipulators could be optimized according to different surgical scenario while taking its flexibility into account. Moreover, the model-based estimation and feedforward compensation control were applied to enhance its precision, taking into account the coupling effect of two sections and friction loss. Additionally, this study introduced a theoretical analysis on the feasibility of implementing variable stiffness for the manipulator, and the coupling was also lessened via increasing the stiffness of the first section. The experiments demonstrated that the maximum average deviation observed was 3.43 mm, which is equivalent to just 2.5% of the total length of the manipulator. Furthermore, the stiffness could be enhanced by up to 75%. Therefore, the proposed flexible manipulator has a potential for the nasal minimally invasive surgery.

Continuum manipulators coupled with magnetic actuation have great potential as steerable instruments for diverse surgical applications. They can be maneuvered inside the human body to reach difficult-to-access surgical sites with contactless actuation. This paper presents a new design of a compliant continuum manipulator of diameter 3 mm and length 70 mm, capable of spatial bending under magnetic actuation. A quasi-static model is developed to estimate the 3D motion of the manipulator. Experiments report an overall mean error in whole shape estimation of the manipulator between the model and the ground truth of 1.7 mm and 4.8 mm, when suspended vertically and horizontally from its base, respectively. Furthermore, fiber Bragg grating (FBG) sensors are integrated with the manipulator to enable shape sensing. Closed-loop control is demonstrated to trace different trajectories with the tip of the manipulator. A square trajectory and a straight line trajectory are generated with an average error in tip position of 4.1 mm between the desired and estimated positions. The potential of the manipulator as a steerable instrument is validated by maneuvering it inside phantoms of a bifurcating arterial system and a heart with visual guidance from a miniature camera.

Natural filaments, such as proteins, plant tendrils, octopus tentacles, and elephant trunks, can transform into arbitrary three-dimensional shapes that carry out vital functions. Their shape-morphing behavior arises from intricate patterning of active and passive regions, which are difficult to replicate in synthetic matter. Here, we introduce a filament-centric strategy for programmable shape morphing in which intrinsic curvature and twist are directly encoded within multimaterial elastomeric filaments during fabrication. By harnessing rotational multimaterial 3D printing, we directly prescribe the filament's natural curvature-twist field κ(s) through controlled material distribution and helical liquid crystal mesogen alignment. When heated above their nematic-to-isotropic transition temperature (TNI), the helically aligned liquid crystal elastomer regions contract along their local director field, while passive regions remain essentially unchanged. This approach enables independent control of bending and torsion at every cross-section along the filament centerline: the principal natural curvatures of the filament along two orthogonal axes as well as the local twist. Next, we printed architected lattices composed of unit cells formed by sinusoidal filaments that either reversibly contract, expand, or exhibit out-of-plane deformations. Discrete elastic rod simulations of Janus filaments with different natural curvatures and twist, which are interconnected within the printed lattices, allow accurate prediction of their observed shape-morphing behavior. By integrating active-passive elastomers, additive manufacturing, and computational modeling, we have created shape-morphing matter with complex programmable responses for applications that rely on adaptive, robotic, or deployable architectures.

As for the complex operational tasks in the unstructured environment with narrow workspace and numerous obstacles, the traditional robots cannot accomplish these mentioned complex operational tasks and meet the dexterity demands. The hyper-redundant bionic robots can complete complex tasks in the unstructured environments by simulating the motion characteristics of the elephant’s trunk and octopus tentacles. Compared with traditional robots, the hyper-redundant bionic robots can accomplish complex tasks because of their flexible structure. A hyper-redundant elephant’s trunk robot (HRETR) with an open structure is developed in this paper. The content includes mechanical structure design, kinematic analysis, virtual prototype simulation, control system design, and prototype building. This design is inspired by the flexible motion of an elephant’s trunk, which is expansible and is composed of six unit modules, namely, 3UPS-PS parallel in series. First, the mechanical design of the HRETR is completed according to the motion characteristics of an elephant’s trunk and based on the principle of mechanical bionic design. After that, the backbone mode method is used to establish the kinematic model of the robot. The simulation software SolidWorks and ADAMS are combined to analyze the kinematic characteristics when the trajectory of the end moving platform of the robot is assigned. With the help of ANSYS, the static stiffness of each component and the whole robot is analyzed. On this basis, the materials of the weak parts of the mechanical structure and the hardware are selected reasonably. Next, the extensible structures of software and hardware control system are constructed according to the modular and hierarchical design criteria. Finally, the prototype is built and its performance is tested. The proposed research provides a method for the design and development for the hyper-redundant bionic robot.

By combining the investigation of the biomechanics and behavior of elephant trunk in the performance of a wide range of dexterous manipulations, a novel approach in the design and kinematics modeling of a fruit harvesting continuum manipulator was proposed. By comparing the structure of two different species of elephant trunk, a new continuum structure which matched the key features of elephant trunk was designed. Based on analysis of the underlying elephant trunk’s grasping mode, a novel kinematics model was proposed. Contrast to traditional robot kinematics which focused on end effector’s position and posture, the proposed continuum manipulator kinematics focus on the center of manipulator’s position and posture, which is more effective when trunk robot realizing grasp and establishes the foundation for its application. Finally, three typical grasping experiments were implemented. The experiment results showed that the manipulator could conduct wrap/pinch manipulations effectively for both small objects and bigger ones. DOI: 10.3965/j.ijabe.20150801.008 Citation: Shao T, Zhang L B, Du M Y, Bao G J, Yang Q H. Fruit harvesting continuum manipulator inspired by elephant trunk. Int J Agric & Biol Eng, 2015; 8(1): 57－63.

Minimally Invasive Surgery represents the future of many types of medical inter- ventions such as keyhole neurosurgey or transluminal endoscopic surgery. These procedures involve insertion of surgical instruments such as needles and endoscopes into human body through small incision/ body cavity for biopsy and drug delivery. However, nearly all surgical instruments for these procedures are inserted manually and there is a long learning curve for surgeons to use them properly. Many research efforts have been made to design active instruments (endoscope, needles) to improve this procedure during last decades. New robot mechanisms have been designed and used to improve the dexterity of current endoscope. Usually these robots are flexible and can pass the constrained space for fine manipulations. In recent years, a con- tinuum robotic mechanism has been investigated and designed for medical surgery. Those robots are characterized by the fact that their mechanical components do not have rigid links and discrete joints in contrast with traditional robot manipula- tors. The design of these robots is inspired by movements of natural animals such as tongues, elephant trunks and tentacles. The unusual compliance and redundant degrees of freedom of these robots provide strong potential to achieve delicate tasks successfully even in cluttered and unstructured environments. This chapter will present a complete application of a continuum robot for Mini- mally Invasive Surgery of colonoscopy. This system is composed of a micro-robotic tip, a set of position sensors and a real-time control system for guiding the explo- ration of colon. Details will be described on the modeling of the used pneumatic actuators, the design of the mechanical component, the kinematic model analysis and the control strategy for automatically guiding the progression of the device inside the human colon. Experimental results will be presented to check the perfor- mances of the whole system within a transparent tube.

Cable-driven continuum robots with hyper-redundant deformable backbones show great promise in applications, such as inspection in unstructured environments, where traditional rigid robots with discrete links and joints fail to operate. However, the motion of existing continuum robots is still constrained by their homogeneous backbones, and limited to environments with modest geometrical complexity. In this study, inspired by highly deformable elephant trunks, we presented a modular tensegrity structure with preprogrammable stiffness for continuum robots. Then we derived a mechanical model based on a positional formulation finite element method for predicting the configuration of the structure in different deformation scenarios. Theoretical predictions revealed that the curvature of each segment could be regulated by preprogramming their spring stiffness. Hence, our customizable design could offer an effective route for efficient robotic interactions. We further fabricated a continuum robot consisting of 12 modules, and showcased its deformation patterns under multiple scenarios. By regulating the distribution of spring stiffness, our robot could move through channels with varying curvatures, exhibiting its potential for applications where varying curvature, and conformal and efficient interactions are needed. Leveraging the inherent intelligence, this robotic system could simplify the complexity of the required actuation and control systems.

Snake-like manipulators can navigate and perform manipulation in confined spaces. Their recent implementations in surgical robots attracted a lot of attentions. These slender manipulators usually possess either a hyper-redundant articulated vertebrate structure or a continuum one. Primary design considerations usually converge to a balance between proper workspace and acceptable stiffness. Efforts have hence been constantly made to achieve higher or adjustable stiffness for a manipulator to widen its applications. This paper presents a simple continuum manipulator design with variable stiffness based on redundantly arranged elastic backbones and continuously constrained bending curvature. The design concepts, kinematics, a preliminary formulation for stiffness adjustment, system construction and experimental characterizations are elaborated. The results showed that the manipulator's stiffness can be increased up to 4.71 times of the value without the curvature constraining rod, indicating the efficacy of the proposed idea.

Robotic fingers made of soft material and compliant structures usually lead to superior adaptation when interacting with the unstructured physical environment. In this paper, we present an embedded sensing solution using optical fibers for an omni-adaptive soft robotic finger with exceptional adaptation in all directions. In particular, we managed to insert a pair of optical fibers inside the finger's structural cavity without interfering with its adaptive performance. The resultant integration is scalable as a versatile, low-cost, and moisture-proof solution for physically safe human-robot interaction. In addition, we experimented with our finger design for an object sorting task and identified sectional diameters of 94\% objects within the $\pm$6mm error and measured 80\% of the structural strains within $\pm$0.1mm/mm error. The proposed sensor design opens many doors in future applications of soft robotics for scalable and adaptive physical interactions in the unstructured environment.

Due to their uniqueness, hyper-redundant manipulators are becoming increasingly relevant in robotics research. The abundance of degrees of freedom makes them adaptable to a variety of applications across various fields, including inspection, medical procedures, and space exploration. These manipulators are particularly effective where high maneuverability is needed, particularly in constrained environments, thanks to their surplus of degrees of freedom. While this hyper-redundancy offers significant benefits, it also introduces complexities in developing effective actuation strategies and innovative design solutions. This work is the first of two parts of a survey conducted on hyper-redundant manipulators, their design, and actuation strategies. This first part of the review provides an examination of the current state of tendon-driven hyper-redundant manipulator design and actuation, analyzing both continuum and rigid-link manipulators. The article synthesizes recent advancements, explores various designs, and identifies key challenges.

In this paper we describe the conceptual design and implementation of the SIMBA manipulator, designed and developed for participating in the RoboSoft Grand Challenge 2016. In our novel design we have proposed: 1) a modular continuum arm with independent actuation units for each module, to increase maintainability; 2) a soft reconfigurable hand, for a better adaptation of the fingers to objects of different shapes and size; 3) a moving base for increasing the workspace. We used a hybrid approach in designing and manufacturing by integrating soft and hard components, in both materials and actuation, providing high lateral stiffness in the arm through flat springs, soft joints in fingers for more compliancy and tendon-motor actuation mechanism providing flexibility but at the same time precision and speed. The SIMBA manipulator has demonstrated excellent grasping and manipulation capabilities by being able to grasp objects with different fragility, geometry and size; and by lifting objects with up to 2Kg of weight it demonstrate also to be robust and reliable. The experimental results pointed out that our design and approach can lead to the realization of robots able to act in unknown and unstructured environments in synergy with humans, for a variety of applications where compliancy is fundamental, preserving robustness and safety.

The paper focuses on design and control of a new anthropomorphic robot arm enabling the torque measurement in each joint to ensure safety while performing tasks of physical interaction with human and environment. When the contact of the arm with an object occurs, local impedance algorithm provides active compliance of corresponding robot arm joint. Thus, the whole structure of the manipulator can safely interact with unstructured environment. A novel variable control strategy was elaborated to increase the robot functionality and to achieve human-like dynamics of interaction. In the paper, we detail the design procedure of 4-DOF robot arm and optical torque sensors. The experimental results of variable joint impedance control show that proposed approach not only provides safe interaction of entire structure of robot arm with a person, but also improves the effectiveness of contact task performance, enables thus to contact with environment in a delicate manner.

A major goal of autonomous robot collectives is to robustly perform complex tasks in unstructured environments by leveraging hardware redundancy and the emergent ability to adapt to perturbations. In such collectives, large numbers is a major contributor to system-level robustness. Designing robot collectives, however, requires more than isolated development of hardware and software that supports large scales. Rather, to support scalability, we must also incorporate robust constituents and weigh interrelated design choices that span fabrication, operation, and control with an explicit focus on achieving system-level robustness. Following this philosophy, we present the first iteration of a new framework toward a scalable and robust, planar, modular robot collective capable of gradient tracking in cluttered environments. To support co-design, our framework consists of hardware, low-level motion primitives, and control algorithms validated through a kinematic simulation environment. We discuss how modules made primarily of flexible printed circuit boards enable inexpensive, rapid, low-precision manufacturing; safe interactions between modules and their environment; and large-scale lattice structures beyond what manufacturing tolerances allow using rigid parts. To support redundancy, our proposed modules have on-board processing, sensing, and communication. To lower wear and consequently maintenance, modules have no internally moving parts, and instead move collaboratively via switchable magnets on their perimeter. These magnets can be in any of three states enabling a large range of module configurations and motion primitives, in turn supporting higher system adaptability. We introduce and compare several controllers that can plan in the collective's configuration space without restricting motion to a discrete occupancy grid as has been done in many past planners. We show how we can incentively redundant connections to prevent single-module failures from causing collective-wide failure, explore bad configurations which impede progress as a result of the motion constraints, and discuss an alternative “naive” planner with improved performance in both clutter-free and cluttered environments. This dedicated focus on system-level robustness over all parts of a complete design cycle, advances the state-of-the-art robots capable of long-term exploration.