There is an emerging trend toward 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 letter, 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 antagonistic 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.

This paper explores a new hybrid actuation principle combining pneumatic and tendon-driven actuators for a soft robotic manipulator. The fusion of these two actuation principles leads to an overall antagonistic actuation mechanism whereby pneumatic actuation opposes tendon actuation - a mechanism commonly found in animals where muscles can oppose each other to vary joint stiffness. We are taking especially inspiration from the octopus who belongs to the class of Cephalopoda; the octopus uses its longitudinal and transversal muscles in its arms to achieve varied motion patterns; activating both sets of muscles, the octopus can control the arm stiffness over a wide range. Our approach mimics this behavior and achieves comparable motion patterns, including bending, elongation and stiffening. The proposed method combines the advantages of tendon-driven and pneumatic actuated systems and goes beyond what current soft, flexible robots can achieve: because the new robot structure is effectively an inflatable, sleeve, it can be pumped up to its fully inflated volume and, also, completely deflated and shrunk. Since, in the deflated state, it comprises just its outer “skin” and tendons, the robot can be compressed to a very small size, many times smaller when compared to its fully-inflated state. In this paper, we describe the mechanical structure of the soft manipulator. Proof-of-concept experiments focus on the robot's ability to bend, to morph from completely shrunk to entirely inflated as well as to vary its stiffness.

This paper proposes a soft, inflatable manipulator that is antagonistically actuated by tendons and pneumatics. The combination of the two actuation mechanisms in this antagonistic robot structure is inspired by the octopus which uses its longitudinal and transversal muscles to steer, elongate, shrink and also stiffen its continuum arms. By “activating” its antagonistic muscle groups at the same time, the octopus can achieve multiple motion patterns as well as stiffen their arms. Being organized in a similar fashion, our robot manipulator uses, on the one hand, pneumatic actuation and, on the other hand, tendon-based actuation - one opposing the other, achieving an overall antagonistic actuation framework. Controlling the pressure inside the robot while at the same time controlling the tendons' displacements, the robot can be moved into a wide range of configurations while simultaneously controlling the arm's stiffness. This paper builds on earlier work by the authors: Here, we present a new conic-shaped manipulator structure and the control architecture. Using a constant curvature model, we have derived an approach suitable for controlling the robot manipulator. The manipulator's reachable workspace is analyzed and proof-of-concept experiments were conducted to show the robot's stiffness control and motion abilities.

The recent decade has seen an astounding increase of interest and advancement in a new field of robotics, aimed at creating structures specifically for the safe interaction with humans. Softness, flexibility, and variable stiffness in robotics have been recognized as highly desirable characteristics for many applications. A number of solutions were proposed ranging from entirely soft robots (such as those composed mainly from soft materials such as silicone), via flexible continuum and snake-like robots, to rigid-link robots enhanced by joints that exhibit an elastic behavior either implemented in hardware or achieved purely by means of intelligent control. Although these are very good solutions paving the path to safe human–robot interaction, we propose here a new approach that focuses on creating stiffness controllability for the linkages between the robot joints. This article proposes a replacement for the traditionally rigid robot link—the new link is equipped with an additional capability of stiffness controllability. With this added feature, a robot can accurately carry out manipulation tasks (high stiffness), but can virtually instantaneously reduce its stiffness when a human is nearby or in contact with the robot. The key point of the invention described here is a robot link made of an airtight chamber formed by a soft and flexible, but high-strain resistant combination of a plastic mesh and silicone wall. Inflated with air to a high pressure, the mesh silicone chamber behaves like a rigid link; reducing the air pressure, softens the link and rendering the robot structure safe. This article investigates a number of link prototypes and shows the feasibility of the new concept. Stiffness tests have been performed, showing that a significant level of stiffness can be achieved—up to 40 N reaction force along the axial direction, for a 25-mm-diameter sample at 60 kPa, at an axial deformation of 5 mm. The results confirm that this novel concept to linkages for robot manipulators exhibits the beam-like behavior of traditional rigid links when fully pressurized and significantly reduced stiffness at low pressure. The proposed concept has the potential to easily create safe robots, augmenting traditional robot designs.

Stroke is one of the leading causes of disability worldwide: post-stroke disabilities affect the upper and lower limbs, significantly undermining a subject's autonomy in the Activities of Daily Living (ADLs). Among post-stroke disabilities, one of the most impairing and widespread conditions is the clenched fist deformity: the subject experiences a permanent contraction of the hand, resulting in a closed hand rest pose. In this paper, the authors propose a novel light-weight inflatable soft exoskeleton device, called the AirExGlove, to deliver high-dosage, adaptive and gradual rehabilitation therapy to patients affected by clenched fist deformity. Our system is lightweight, low-cost, adaptable to any hand size and unobtrusive. The system has been extensively tested to assess the hand-opening range in which it can operate according to the severity of the patient condition, which is typically ranked on the Modified Ashworth Scale (MAS) scale. Experimental analysis demonstrates the suitability of the glove for patients affected by post-stroke muscle spasticity scoring up to 3 out of 4 in the MAS scale. Preliminary testing with clenched-fist patient confirmed a higher level of ergonomics of the system in comparison with rigid-linked robotic systems.

Collaborative robots (cobots) open up new avenues in the fields of industrial robotics and physical Human-Robot Interaction (pHRI) as they are suitable to work in close approximation and in collaboration with humans. The integration and control of variable stiffness elements allow inherently safe interaction. Apart from notable work on Variable Stiffness Actuators, the concept of Variable-Stiffness-Link (VSL) manipulators promises safety improvements in cases of unintentional physical collisions. However, position control of these type of robotic manipulators is challenging for critical task-oriented motions (e.g., pick and place). Hence, the study of open-loop position control for VSL robots is crucial to achieve high levels of safety, accuracy and hardware cost-efficiency in pHRI applications. In this paper, we propose a hybrid, learning based kinematic modelling approach to improve the performance of traditional open-loop position controllers for a modular, collaborative VSL robot. We show that our approach improves the performance of traditional open-loop position controllers for robots with VSL and compensates for position errors, in particular, for lower stiffness values inside the links: Using our upgraded and modular robot, two experiments have been carried out to evaluate the behaviour of the robot during taskoriented motions. Results show that traditional model-based kinematics are not able to accurately control the position of the end-effector: the position error increases with higher loads and lower pressures inside the VSLs. On the other hand, we demonstrate that, using our approach, the VSL robot can outperform the position control compared to a robotic manipulator with 3D printed rigid links.