Soft Robotic Arm Research Articles

A generalizable equilibrium model for bending soft arms with longitudinal actuators

Current models of bending in soft arms are formulated in terms of experimentally determined, arm-specific parameters, which cannot evaluate fundamental differences in soft robot arm design. Existing models are successful at improving control of individual arms but do not give insight into how the structure of the arm affects the arm’s capabilities. For example, omnidirectional soft robot arms most frequently have three parallel actuators, but may have four or more, while common biological arms, including octopuses, have tens of distinct longitudinal muscle bundles. This article presents a quasi-static analytical model of soft arms bent with longitudinal actuators, based on equilibrium principles and assuming an unknown neutral axis location. The model is presented as a generalizable framework and specifically implemented for an arm with fluid-driven actuators, a subset of which are pressurized to induce a bend with a certain curvature and direction. The presented implementation is validated experimentally using planar (2D) and spatial (3D) bends. The planar model is used to initially estimate pressure for a closed-loop curvature control system and to bound the accessible configurations for a rapidly-exploring random trees (RRT) motion planner. A three-segment planar arm is demonstrated to navigate along a planned trajectory through a gap in a wall. Finally, the model is used to explore how the arm morphology affects maximum curvature and directional resolution. This research analytically connects soft arm structure and actuator behavior to unloaded arm performance, and the results may be used to methodically design soft robot arms.

Design and Modeling of an Extensible Soft Robotic Arm

Soft robotic arms are receiving more and more attention for their intrinsic safety and natural compliance. Instead of traditional serialized rotary joints, soft robotic arms often have complex joints with coupled degrees of freedom like bending, rotation, and elongation, enabling them with more freedoms in achieving sophisticated movements. However, currently soft robotic arms are mostly focusing on bending and rotation, whereas elongation is either unavailable due to the inextensible backbone or only has a small range because of the limited contraction ratio of actuators. Furthermore, the coupling of actuators and complex structure makes the modeling and control hard. In this letter, we introduced an extensible soft robotic arm with a large elongation ratio of 400%, and give a novel modeling method from a new perspective of the force balance of the arm. The models were experimentally tested both in loaded and unloaded situations, showing the effectiveness of predicting the bending, rotation, and elongation of the arm.

Octopus‐Inspired Soft Arm with Suction Cups for Enhanced Grasping Tasks in Confined Environments

Octopuses have impressive manipulation capabilities empowered by the intrinsic adaptation of their soft arms combined with distributed suckers. Drawing inspiration from the arm morphology and abilities of the octopus, the integrated design of a conical soft robotic arm endowed with suction cups is presented herein. The proposed arm exploits soft robotic technologies as in materials (combining stiff and soft silicones) and actuation (tendons to drive motion and fluidic channels for suction cup actuation) for retrieving objects in confined spaces of unknown shapes and sizes. The soft arm is able to grasp varied and complex‐shaped objects, to move in air, water, and oil, and to retrieve items immersed in a 70 mm diameter pipe, under pressure (up to 18 bar). Suction cups enable the arm to retrieve objects that would be impossible to grasp without them and help in improving the grasping force up to ≈1.4 times in air, ≈2.4 times in water, and ≈12.5 times in oil. The enhanced holding force (up to 3.3 N, ≈3.9 times arm weight) and a wide spectrum of objects negotiable by the arm make it a universal tool for object retrieval in confined spaces and wet conditions.

Underwater Dynamic Visual Servoing for a Soft Robot Arm With Online Distortion Correction

Bioinspired soft robots have generated increasing attentions due to their optimal performance in specific environments stemming from their unique morphology and sensorimotor capabilities. This also raises new challenges in modeling and control due to their unconventional soft mechanism. In this paper, we aim to improve the controllability of an octopus tentacle-like soft robot arm operating in an underwater environment and to reproduce the perception-guided tracking performance of its biological counterpart when preying on and pursuing a target. To this end, we design an adaptive visual servoing controller for trajectory tracking tasks, taking into account system dynamics with environment interactions. Unlike on-the-ground visual servoing controller, the underwater environment exerts multirefraction effects on the image projection process, increasing difficulties in obtaining an accurate camera projection model. This paper thus proposes an adaptive method that can online correct image distortion caused by the deflection of light rays passing through different mediums to compensate for refraction effects. The adaptive algorithm can simultaneously estimate unknown camera parameters to completely avoid complex offline underwater camera calibration. The proposed adaptive controller validates the trajectory tracking performance experimentally and position and image velocity.

A Soft Master-Slave Robot Mimicking Octopus Arm Structure Using Thin Artificial Muscles and Wire Encoders

An octopus arm with a flexible structure and no rigid skeleton shows a high degree of freedom and flexibility. These excellent features are suitable for working in an environment having fragile and unknown-shaped objects. Therefore, a soft robot arm resembling an octopus arm can be useful as a harvesting machine without damaging crops in the agricultural field, as a rehabilitation apparatus in the welfare field, as a safe surgery tool in the medical field, and so on. Unlike industrial robots, to consider the applications of the soft robot arm, the instructions for it relating to a task cannot in many cases be given as a numerical value, and the motion according to an operator’s sense and intent is useful. This paper describes the design and feedback control of a soft master-slave robot system. The system is configured with two soft rubber machines; one is a slave machine that is the soft robot arm mimicking the muscle arrangement of the octopus arm by pneumatic artificial muscles, and the other is a master machine that gives the target motion to the slave machine. Both are configured with soft materials. The slave machine has an actuating part and a sensing part, it can perform bending and torsional motions, and these motions are estimated by the sensing part with threads that connect to wire encoders. The master machine is almost the same configuration, but it has no actuating part. The slave machine is driven according to the deformation of the master machine. We confirmed experimentally that the slave machine followed the master machine that was deformed by an operator.

Design and Implementation of a Soft Robotic Arm Driven by SMA Coils

Soft robots may potentially be used for human services and complex environmental locomotion. The bodies of the soft robots are made from soft materials to make them safe for human interactions and to enable them to deform enough to adapt to soft or rough terrains. However, their practical applications are limited because their body stiffness is too low and the deformation is continuous, which make it difficult to control their movements. In this paper, the design and implementation of a soft robotic arm driven by shape memory alloy (SMA) coils are reported. The arm is made from soft silicone, and there are three linear Hall sensors in certain slots on the body wall to measure the changes in height as the arm bends. With a compression compensation algorithm, a proportional-integral differential controller is used to precisely control the bending movement. This soft robotic arm can follow a two-dimensional motion with a relatively high accuracy; indeed, it displays quite a remarkable performance considering its low cost, simple structure, easy control, and small bending error. Since this soft arm can be used as the basic driving unit of a soft robot capable of manipulating and locomoting, it has shed light on the applications of SMA soft robots.

Utilizing the Nonlinearity of Tendon Elasticity for Compensation of Unknown Gravity of Payload

This paper discusses the positioning control of an elastic tendon-driven robot arm under gravity. The robot is driven by rubber string tendons and winding drums attached on the outside frames. Low-cost rubber strings that are available commercially are used as tendons. The goal is to utilize the nonlinear nature of the rubber materials to control a low-cost and soft robot arm. Theoretically, a mathematical model with accurate parameters and accurate measurement of the payload weight is necessary for rigorous gravity compensation. However, the necessity for the information of the robot parameters is hindering easy adaptability, versatility, and cost-efficiency. This paper presents an iterative estimation and compensation method for unknown payloads based on the steady-state position error and the nominal stiffness coefficient. Owing to the nonlinearity of the actual rubber strings, the position error remains after a single operation of the gravity compensation. However, experiments indicate that the error reduces by a simple iteration of the same compensation operation. Considering the nonlinearity in rubber strings, the mechanism of the error reduction is analyzed theoretically. Although the iterative process is time consuming, the method requires less prior information. In addition, it is cost effective because a sophisticated force sensor is not required. As the mechanism of error reduction applies to typical rubber string materials, it is useful for significant cost-reduction and reconfigurable robotics.

Underwater Dynamic Modeling for a Cable-Driven Soft Robot Arm

Soft robotics has attracted great attention because of its potential to overcome safety issues and produce a more harmonious cooperative environment for human beings and robots. One of the main challenges hindering its wider application to the daily routine is the dynamic modeling of its compliant mechanisms. In this paper, to further investigate the soft robot arm's performance and extend the utilization in water or other dense and viscous mediums, the dynamic model based on Kane's theory is proposed. This model takes complicated hydrodynamics into account with reasonable simplification, considering the physical conditions. Compared with the previous work, we adopt the Column friction model to compensate for the actuation force's loss in the transmission process. The proposed dynamic model is validated by comparing the theoretical results of both the dynamic responses and steady-state poses with the experimental results under different conditions. Given the modification method for the computed dynamic equation, the presented dynamics can adapt to variable environments and serve as the platform for the controller design for a soft robot working in a complex environment.

Novel Design and Position Control Strategy of a Soft Robot Arm

This article presents a novel design of a continuum arm, which has the ability to extend and bend efficiently. Numerous designs and experiments have been done to different dimensions on both types of McKibben pneumatic muscle actuators (PMA) in order to study their performances. The contraction and extension behaviour have been illustrated with single contractor actuators and single extensor actuators, respectively. The tensile force for the contractor actuator and the compressive force for the extensor PMA are thoroughly explained and compared. Furthermore, the bending behaviour has been explained for a single extensor PMA, multi extensor actuators and multi contractor actuators. A two-section continuum arm has been implemented from both types of actuators to achieve multiple operations. Then, a novel construction is proposed to achieve efficient bending behaviour of a single contraction PMA. This novel design of a bending-actuator has been used to modify the presented continuum arm. Two different position control strategies are presented, arising from the results of the modified soft robot arm experiment. A cascaded position control is applied to control the position of the end effector of the soft arm at no load by efficiently controlling the pressure of all the actuators in the continuum arm. A new algorithm is then proposed by distributing the x, y and z-axis to the actuators and applying an effective closed-loop position control to the proposed arm at different load conditions.

Design of Paired Pouch Motors for Robotic Applications

AbstractThe performance of soft linear actuators will determine the capabilities of future soft robots, and any actuator that can produce larger deformations and forces with a low weight and using lower pressures could potentially become ubiquitous in the field. In this work, the design of paired pouch motors that can produce a maximum contraction ratio of 41.9% and of 31% with a 10 kg payload is presented. The design of higher‐order pairings of pouch motors is demonstrated to produce even larger deformations, and this basic concept is also applied to serial pneumatic artificial muscles to produce a contraction ratio as high as 51.78%. This concept can be applied to other soft actuators that produce a lateral expansion to produce a longitudinal contraction. Paired pouch motors are also applied to a jointed soft robotic arm where a single pouch motor is sufficient to produce a joint deformation of 120° at a pressure of 100 kPa with a tip payload of 300 g. Paired pouch motors have a combination of characteristics including ease of manufacturing, low weight, and low cost paired with good actuation performance that could make it one of the leading actuator design in the field of soft robotics.

Pneumatically Actuated Soft Robotic Arm for Adaptable Grasping

Developing robotic manipulators capable of performing effective physical interaction tasks is a challenging topic. In this study, we design a soft robotic arm (SRA) with multiple degrees of freedom inspired by the flexible structures and the unique motion mechanism of the octopus arm. The SRA is fabricated with elastomeric materials, which consists of four series of integrated pneumatic chambers that play similar roles as the muscles in the octopus arm can achieve large bending in various directions with variable stiffness. This SRA displays specified movements via controlling pressure and selecting channels. Moreover, utilizing parallel control, the SRA demonstrates complicated three-dimensional motions. The force response and motion of the SRA are determined both experimentally and computationally. The applications of the present SRA include tightly coiling around the objects because of its large bending deformation (nearly $$360^{{\circ }})$$ , grasping multiple objects, and adjusting the grabbing mode in accordance with the shape of objects.

Design, Characterization, and Implementation of a Two-DOF Fabric-Based Soft Robotic Arm

Traditional rigid robotic arms have been widely used in various industries. They own the merits of high force output, precise control, and high efficiency. However, a rigid robotic arm is seldom used by people in daily life, as the cost is usually high and, most importantly, its rigidity, large weight, and noncompliance can cause safety concerns when it comes to human–robot interaction. Therefore, we want to build a robotic arm that is cheap, lightweight, and provides safe interaction with humans. This soft inflatable robotic arm, which is made from low-cost fabrics and flexible plastics, is a promising solution for a domestic robotic arm. The softness and natural compliance can guarantee safe interaction with people. In this letter, we present our soft robotic arm (SRA) made from nylon fabric. The SRA contains two joints that can bend the arm vertically and horizontally, and a gripper as an end-effector that can pick up objects of different shapes and sizes. The SRA has the advantages of low cost, good portability, and adequate force output. We characterized the bending modality and force output and demonstrated the functionality of the SRA in object manipulation. This fabric-based soft robotic arm is promising for use in assisting people's daily activities, for example, mounting on a wheelchair to enable a wheelchair bound person to reach objects from a distance.

Design and control of a novel variable stiffness soft arm

ABSTRACTSoft robot arms possess such characteristics as light weight, simple structure and good adaptability to the environment, among others. On the other hand, robust control of soft robot arms presents many difficulties. Based on these reasons, this paper presents a novel design and modeling of a fuzzy active disturbance rejection control (FADRC) controller for a soft PAM arm. The soft arm comprises three contractile and one extensor PAMs, which can vary its stiffness independently of its position in space. Force analysis for the soft arm is conducted, and stiffness model of the arm is established based on the relational model of contractile and extensor PAM. The accuracy of stiffness model for the soft arm was verified through experiments. Associated to this, a controller based on the fuzzy adaptive theory and active disturbance rejection control (ADRC), FADRC, has been designed to control the arm. The fuzzy adaptive theory is used to adjust the parameters of the ADRC, the control algorithm has the ability to control stiffness and position of the soft arm. In this paper, FADRC was further verified through comparative experiments on the soft arm. This paper reinforces the hypothesis that FADRC control, as an algorithm, indeed possesses good robustness and adaptive abilities.

Design, Kinematics and Controlling a Novel Soft Robot Arm with Parallel Motion

This article presents a novel design for a double bend pneumatic muscle actuator (DB-PMA) inspired by snake lateral undulation. The presented actuator has the ability to bend in opposite directions from its two halves. This behavior results in horizontal and vertical movements of the actuator distal ends. The kinematics for the proposed actuator are illustrated and experiments conducted to validate its unique features. Furthermore, a continuum robot arm with the ability to move in parallel (horizontal displacement) is designed with a single DB-PMA and a two-finger soft gripper. The performance of the soft robot arm presented is explained, then another design of the horizontal motion continuum robot arm is proposed, using two self-bending contraction actuators (SBCA) in series to overcome the payload effects on the upper half of the soft arm.

A Modular Soft Robotic Wrist for Underwater Manipulation.

This article presents the development of modular soft robotic wrist joint mechanisms for delicate and precise manipulation in the harsh deep-sea environment. The wrist consists of a rotary module and bending module, which can be combined with other actuators as part of a complete manipulator system. These mechanisms are part of a suite of soft robotic actuators being developed for deep-sea manipulation via submersibles and remotely operated vehicles, and are designed to be powered hydraulically with seawater. The wrist joint mechanisms can also be activated with pneumatic pressure for terrestrial-based applications, such as automated assembly and robotic locomotion. Here we report the development and characterization of a suite of rotary and bending modules by varying fiber number and silicone hardness. Performance of the complete soft robotic wrist is demonstrated in normal atmospheric conditions using both pneumatic and hydraulic pressures for actuation and under high ambient hydrostatic pressures equivalent to those found at least 2300 m deep in the ocean. This rugged modular wrist holds the potential to be utilized at full ocean depths (>10,000 m) and is a step forward in the development of jointed underwater soft robotic arms.

A Bio-inspired Soft Robotic Arm: Kinematic Modeling and Hydrodynamic Experiments

Soft robotics has several promising properties for aquatic applications, such as safe interaction with environments, lightweight, low cost, etc. In this paper, we proposed the kinematic modeling and hydrodynamics experiments of a soft robotic arm with 3D locomotion capacity. We developed a mathematical model that incorporates the angle correction, as well as the open-loop model-based motion control. The model could precisely predict the three-dimensional (3D) movement, and the location error is less than 5.7 mm in different attitudes. Furthermore, we performed the hydrodynamic investigations and simultaneously measured the hydrodynamic forces and the wake flows at different amplitudes (50 mm, 100 mm, 150 mm, 200 mm) and frequencies (0.3 Hz, 0.4 Hz, 0.5 Hz) of the soft arm. Surprisingly, we found that the magnitudes of the hydrodynamic force (<1 N) and the torques (<0.08 N·m) of dynamically moving soft arm were tiny, which leads to negligible inertial effect for the underwater vehicle than those of the traditional rigid underwater manipulator. Finally, we demonstrated underwater picking and placing tasks of the soft manipulator by using a computer program that controls the tip attitude and velocity. This study may inspire future underwater manipulators that have properties of low-inertial, low power cost and can safely interact with the aquatic environments.

Novel Design of a Soft Lightweight Pneumatic Continuum Robot Arm with Decoupled Variable Stiffness and Positioning.

Soft robot arms possess unique capabilities when it comes to adaptability, flexibility, and dexterity. In addition, soft systems that are pneumatically actuated can claim high power-to-weight ratio. One of the main drawbacks of pneumatically actuated soft arms is that their stiffness cannot be varied independently from their end-effector position in space. The novel robot arm physical design presented in this article successfully decouples its end-effector positioning from its stiffness. An experimental characterization of this ability is coupled with a mathematical analysis. The arm combines the light weight, high payload to weight ratio and robustness of pneumatic actuation with the adaptability and versatility of variable stiffness. Light weight is a vital component of the inherent safety approach to physical human-robot interaction. To characterize the arm, a neural network analysis of the curvature of the arm for different input pressures is performed. The curvature-pressure relationship is also characterized experimentally.

Multiobjective Optimization for Stiffness and Position Control in a Soft Robot Arm Module

The central concept of this letter is to develop an assistive manipulator that can automate the bathing task for elderly citizens. We propose to exploit principles of soft robotic technologies to design and control a compliant system to ensure safe human–robot interaction, a primary requirement for the task. The overall system is intended to be modular with a proximal segment that provides structural integrity to overcome gravitational challenges and a distal segment to perform the main bathing activities. The focus of this letter is on the design and control of the latter module. The design comprises of alternating tendons and pneumatics in a radial arrangement, which enables elongation, contraction, and omnidirectional bending. Additionally, a synergetic coactivation of cables and tendons in a given configuration allows for stiffness modulation, which is necessary to facilitate washing and scrubbing. The novelty of the work is twofold: 1) Three base cases of antagonistic actuation are identified that enable stiffness variation. Each category is then experimentally characterized by the application of an external force that imposes a linear displacement at the tip in both axial and lateral directions. 2) The development of a novel algorithm based on cooperative multiagent reinforcement learning that simultaneously optimizes stiffness and position. The results highlight the effectiveness of the design and control to contribute toward the development of the assistive device.

Morphological computation-based control of a modular, pneumatically driven, soft robotic arm

The dynamics of soft robotic bodies are typically complex and exhibit nonlinearities and a high-dimensional state space. As a result, such systems are difficult to model and, therefore, hard to con...

Design of a Lightweight Soft Robotic Arm Using Pneumatic Artificial Muscles and Inflatable Sleeves.

As robots begin to interact with humans and operate in human environments, safety becomes a major concern. Conventional robots, although reliable and consistent, can cause injury to anyone within its range of motion. Soft robotics, wherein systems are made to be soft and mechanically compliant, are thus a promising alternative due to their lightweight nature and ability to cushion impacts, but current designs often sacrifice accuracy and usefulness for safety. We, therefore, have developed a bioinspired robotic arm combining elements of rigid and soft robotics such that it exhibits the positive qualities of both, namely compliance and accuracy, while maintaining a low weight. This article describes the design of a robotic arm-wrist-hand system with seven degrees of freedom (DOFs). The shoulder and elbow each has two DOFs for two perpendicular rotational motions on each joint, and the hand has two DOFs for wrist rotations and one DOF for a grasp motion. The arm is pneumatically powered using custom-built McKibben type pneumatic artificial muscles, which are inflated and deflated using binary and proportional valves. The wrist and hand motions are actuated through servomotors. In addition to the actuators, the arm is equipped with a potentiometer in each joint for detecting joint angle changes. Simulation and experimental results for closed-loop position control are also presented in the article.