Screw theory-based stiffness analysis for a fluidic-driven soft robotic manipulator (original) (raw)
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Finite Element Modeling of Soft Fluidic Actuators: Overview and Recent Developments
Advanced Intelligent Systems, 2020
Soft robotics has experienced an exponential growth in publications in the last two decades. [1] Unlike rigid conventional manipulators, [2,3] soft robots based on hydrogels, [4,5] electroactive polymers, [6,7] and elastomers [7-9] are physically resilient and can adapt to delicate objects and environments due to their conformal deformation. [10,11] They also show increased safety and dexterity can be lightweight and used within constrained environments with restricted access. [12,13] Many soft robots have a biologically inspired design coming from snakes, [14-17] worms, [18-20] fishes, [21-24] manta rays, [25,26] and tentacles. [27-29] The scope of applications includes minimally invasive surgery, [30,31] rehabilitation, [32,33] elderly assistance, [34] safe human-robot interaction, [35,36] and handling of fragile materials. [37,38] Important features of soft robotics design, fabrication, modeling, and control are covered in the soft robotics toolkit. [39,40] The building blocks of soft robots are the soft actuators. The most popular category of soft actuator is the soft fluidic actuator (SFA), where actuation is achieved using hydraulics or pneumatics. [8,41] These actuators are usually fabricated with silicone rubbers following a 3D molding process, [42] although directly 3D printing the soft actuators is also possible. [43,44] Silicone rubber is a highly flexible/extensible elastomer with high-temperature resistance, lowtemperature flexibility, and good biocompatibility. [45] Elastomers can withstand very large strains over 500% with no permanent deformation or fracture. [46] For relatively small strains, simple linear stress-strain relationships can be used, and two of the following parameters can be used to describe the elastic properties: bulk compressibility, shear modulus, tensile modulus (Young's modulus of elasticity), or Poisson's ratio. [45] For large deformations, nonlinear solid mechanic models using hyperelasticity should be considered. [8,32,47-50] Due to the strong nonlinearities in SFAs and their complex geometries, analytical modeling is challenging. [51] A brief review of the analytical methods for modeling of soft robotic structures is provided in the following. 1.1. Analytical Modeling of Soft Actuators The majority of soft/continuum robots with bending motion can be approximated as a series of mutually tangent constant curvature sections, i.e., piecewise constant curvature. [52] This is a result of the fact that the internal potential energy is uniformly distributed along each section for pressure-driven robots. [53] This approach has also been validated using Hamilton's principle by Gravagne et al. [54] As discussed by Webster and Jones, [52] the kinematics of continuum robots can be separated into robotspecific and robot-independent components in this approach. The robot specific mapping transforms the input pressures P or actuator space q to the configuration space κ, ϕ, l, and the robot-independent mapping goes from the configuration space to the task space x. The actuator space contains the length of tubes or bellows. The configuration space consists of the curvature κ, the angle of the plane containing the arc ϕ (also called
IEEE Robotics and Automation Letters, 2022
Robotic structures based on variable stiffness enable high-performance and flexible motion systems that are inherently safe and thus allow safe Human-Robot Collaboration. This letter presents the design of a robotic structure based on variable stiffness. A robotic manipulator is developed using three variable stiff segments based on particle jamming with a backbone architecture and two tendons for an underactuated motion control of the whole structure. By switching the structural stiffness, the manipulator is able to perform complex planar motion with only one pair of tendons, reducing the number of actuators required. A kinematic modeling approach for the calculation of the forward kinematics of this soft continuum structure is presented, and the validation on the real system is explained. The kinematic simulation is performed with a multibody simulation model (MBS) using rigid body elements in combination with rotational springs. The validation of the model is carried out with visual measurements of the real system using defined target shapes. Simulation and experimental results are discussed and compared also with a common constant curvature model. The developed MBS-model demonstrates a promising modeling approach with a position error lower 3% for the calculation of the presented manipulator under gravity. Index Terms-Soft robot materials and design, compliant joints and mechanisms, modeling, control, and learning for soft robots, tendon/wire mechanism, kinematics. I. INTRODUCTION M ULTI-SEGMENTED soft continuum robots, unlike conventional rigid-link robots, are characterized by a continuous deformation, compliant structures and thus high dexterity and flexibility. These features make them suitable for humanrobot collaboration (HRC) or minimal invasive surgeries (MIS).
Most devices for single-site or natural orifice transluminal surgery are very application specific and, hence, capable of effectively carrying out specific surgical tasks only. However, most of these instruments are rigid, lack a sufficient number of degrees of freedom (DOFs), and/or are incapable of modifying their mechanical properties based on the tasks to be performed. The current philosophy in commercial instrument design is mainly focused on creating minimally invasive surgical systems using rigid tools equipped with dexterous tips. Only few research efforts are aimed at developing flexible surgical systems, with many DOFs or even continuum kinematics. The authors propose a radical change in surgical instrument design: away from rigid tools toward a new concept of soft and stiffness-controllable instruments. Inspired by biology, we envision creating such soft and stiffnesscontrollable medical devices using the octopus as a model. The octopus presents all the capabilities requested and can be viewed as a precious source of inspiration. Several soft technologies are suitable for meeting the aforementioned capabilities, and in this article a brief review of the most promising ones is presented. Then we illustrate how specific technologies can be applied in the design of a novel manipulator for flexible surgery by discussing its potential and by presenting feasibility tests of a prototype responding to this new design philosophy. Our aim is to investigate the feasibility of applying these technologies in the field of minimally invasive surgery and at the same time to stimulate the creativeness of others who could take the proposed concepts further to achieve novel solutions and generate specific application scenarios for the devised technologies.
Soft fluidic rotary actuator with improved actuation properties
2017 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), 2017
The constantly increasing amount of machines operating in the vicinity of humans makes it necessary to rethink the design approach for such machines to ensure that they are safe when interacting with humans. Traditional mechanisms are rigid and heavy and as such considered unsuitable, even dangerous when a controlled physical contact with humans is desired. A huge improvement in terms of safe human-robot interaction has been achieved by a radically new approach to robotics-soft material robotics. These new robots are made of compliant materials that render them safe when compared to the conventional rigid-link robots. This undeniable advantage of compliance and softness is paired with a number of drawbacks. One of them is that a complex and sophisticated controller is required to move a soft robot into the desired positions or along a desired trajectory, especially with external forces being present. In this paper we propose an improved soft fluidic rotary actuator composed of silicone rubber and fiber-based reinforcement. The actuator is cheap and easily manufactured providing near linear actuation properties when compared to pneumatic actuators presented elsewhere. The paper presents the actuator design, manufacturing process and a mathematical model of the actuator behavior as well as an experimental validation of the model. Four different actuator types are compared including a square-shaped and three differently reinforced cylindrical actuators.
Actuation and stiffening in fluid-driven soft robots using low-melting-point material
2019 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), 2019
Soft material robots offer a number of advantages over traditional rigid robots in applications including humanrobot interaction, rehabilitation and surgery. These robots can navigate around obstacles, elongate, squeeze through narrow openings or be squeezed-and they are considered to be inherently safe. The ability to stiffen compliant soft actuators has been achieved by embedding various mechanisms that are generally decoupled from the actuation principle. Miniaturisation becomes challenging due to space limitations which can in turn result in diminution of stiffening effects. Here, we propose to hydraulically actuate soft manipulators with lowmelting-point material and, at the same time, be able to switch between a soft and stiff state. Instead of allocating an additional stiffening chamber within the soft robot, one chamber only is used for actuation and stiffening. Low Melting Point Alloy is integrated into the actuation chamber of a single-compartment soft robotic manipulator and the interfaced robotic syringe pump. Temperature change is enabled through embedded nichrome wires. Our experimental results show higher stiffness factors, from 9 − 12 opposing the motion of curvature, than those previously found for jamming mechanisms incorporated in separate additional chambers, in the range of 2 − 8 for the same motion.
Hyperelastic Modeling and Validation of Hybrid-Actuated Soft Robot with Pressure-Stiffening
Micromachines
Soft robots have gained popularity, especially in intraluminal applications, because their soft bodies make them safer for surgical interventions than flexures with rigid backbones. This study investigates a pressure-regulating stiffness tendon-driven soft robot and provides a continuum mechanics model for it towards using that in adaptive stiffness applications. To this end, first, a central single-chamber pneumatic and tri-tendon-driven soft robot was designed and fabricated. Afterward, the classic Cosserat’s rod model was adopted and augmented with the hyperelastic material model. The model was then formulated as a boundary-value problem and was solved using the shooting method. To identify the pressure-stiffening effect, a parameter-identification problem was formulated to identify the relationship between the flexural rigidity of the soft robot and internal pressure. The flexural rigidity of the robot at various pressures was optimized to match theoretical deformation and exper...
Continuum and soft robotics showed many applications in medicine from surgery to health care where their compliant nature is advantageous in minimal invasive interaction with organs. Stiffness control is necessary for challenges with soft robots such as minimalistic actuation, less invasive interaction, and precise control and sensing. This paper presents an idea of scale jamming inspired by fish and snake scales to control the stiffness of continuum manipulators by controlling the Coulomb friction force between rigid scales. A low stiffness spring is used as the backbone for a set of round curved scales to maintain an initial helix formation while two thin fishing steel wires are used to control the friction force by tensioning. The effectiveness of the design is showed for simple elongation and bending through mathematical modelling, experiments and in comparison to similar research. The model is tested to control the bending stiffness of a STIFF-FLOP continuum manipulator module designed for surgery.
Advanced Materials Technologies, 2021
Compared with traditional rigid robotic manipulators, [8] these soft counterparts are more compliant, more adaptable, and easier to control, thereby more suitable for safe machine-human and machine-environment interactions, such as grasping fragile objects, picking apples, and assisting elderly people and children. Moreover, soft manipulators are unique for physical and distributed computations through their compliance feature, reducing the needs for sophisticated algorithms and mechanisms in planning and control for real-world applications. The inherent compliant feature of the soft robots, on the other hand, poses critical challenges for achieving high structural stability and high loading capacity for specific applications, such as manipulation of heavy objects [9] and surgical operations. [10] To overcome this, one potential solution is to tune the stiffness of soft robots so that they can sustain external loads (including self-weight) or exert large forces for robust interactions when needed. A variety of methodologies and materials have been proposed to adjust the stiffness of soft materials and structures, including electrorheological (ER) and magnetorheological (MR) fluids, [11] particle jamming, [12] thermoplastics, [13] shape memory polymers (SMPs), [14] and low melting point alloys (LMPAs). [15] However, ER/MR fluids generally have limitations of requiring proper leakproof packaging. Although particle jamming is a fast-responsive method for stiffness change, it requires a bulky setup for pumping and vacuum operations. The application of SMP is somewhat difficult for soft robots due to its high modulus in the martensite phase. Thermoplastic materials are based on localized heating to change stiffness, which is difficult for large area/volume materials as they would need many heating elements and complicated control designs. Compared to SMPs and thermoplastics, LMPAs can provide a large stiffness difference between solid and liquid states via Joule heating. Furthermore, self-healing can be intrinsically realized by the recrystallization of alloys. [15] Therefore, LMPA is a promising candidate for achieving stiffness variation for soft robotics applications. For many applications involving hazardous or dangerous environments (e.g., factory workspace or deep ocean), remote control of robots (including soft manipulators [16]) is desired. Human-machine interface (HMI) can achieve two-way information transmission between human and machines and has Soft robots have attracted great attention in the past decades owing to their unique flexibility and adaptability for accomplishing tasks via simple control strategies, as well as their inherent safety for interactions with humans and environments. Here, a soft robotic manipulation system capable of stiffness variation and dexterous operations through a remotely controlled manner is reported. The smart manipulation system consists of a soft omnidirectional arm, a dexterous multimaterial gripper, and a self-powered human-machine interface (HMI) for teleoperation. The cable-driven soft arm is made of soft elastomers and embedded with low melting point alloy as a stiffness-tuning mechanism. The self-powered HMI patches are designed based on the triboelectric nanogenerator that utilizes a sliding mode of tribo-layers made of copper and polytetrafluoroethylene. The novel soft manipulation system has great potential for soft and remote manipulation and human machine interactions in a variety of applications from elderly care to surgical operation to agriculture harvesting.
Tendon-based stiffening for a pneumatically actuated soft manipulator
— There is an emerging trend towards soft robotics due to its extended manipulation capabilities compared to traditionally rigid robot links, showing promise for an extended applicability to new areas. However, as a result of the inherent property of soft robotics being less rigid, the ability to control/obtain higher overall stiffness when required is yet to be further explored. In this paper, an innovative design is introduced which allows varying the stiffness of a continuum silicon-based manipulator and proves to have potential for applications in Minimally Invasive Surgery. Inspired by muscular structures occurring in animals such as the octopus, we propose a hybrid and inherently antagonstic actuation scheme. In particular, the octopus makes use of this principle activating two sets of muscles-longitudinal and transverse muscles-thus, being capable of controlling the stiffness of parts of its arm in an antagonistic fashion. Our designed manipulator is pneumatically actuated employing chambers embedded within the robot's silicone structure. Tendons incorporated in the structure complement the pneumatic actuation placed inside the manipulator's wall to allow variation of overall stiffness. Experiments are carried out by applying an external force in different configurations while changing the stiffness by means of the two actuation mechanisms. Our test results show that dual, antagonistic actuation increases the load bearing capabilities for soft continuum manipulators and thus their range of applications.