Soft, stiffness-controllable robotics


Material shifting for soft, stiffness-controllable robots

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 low-melting-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.

Furthermore, we created a hybrid fluidic – hydraulic and pneumatic – actuation system for soft robotic systems. The concept and design of the hybrid actuation system as well as the fabrication of the soft actuator are presented: Polyvinyl Alcohol foam is embedded inside a casted, reinforced silicone chamber. A hydraulic and pneumatic robotic syringe pump are connected to the base and top of the soft actuator. We found that a higher percentage of hydraulics resulted in a higher output force. Hydraulic actuation further is able to change displacements at a higher rate compared to pneumatic actuation. Changing between Hydraulic:Pneumatic (HP) ratios shows how stiffness properties of a soft actuator can be varied.

Variable-Stiffness-Link (VSL) robots

Nowadays, the field of industrial robotics focuses particularly on collaborative robots that are able to work closely together with a human worker in an inherently safe way. To detect and prevent harmful collisions, a number of solutions both from the actuation and sensing sides have been suggested. However, due to the rigid body structures of the majority of systems, the risk of harmful collisions with human operators in a collaborative environment remains.
In this video, we propose a novel concept for a collaborative robot made of Variable Stiffness Links (VSLs). The idea is to use a combination of silicone based structures and fabric materials to create stiffness controllable links that are pneumatically actuated. According to the application, it is possible to change the stiffness of the links by varying the value of pressure inside their structure. Moreover, the pressure readings from the pressure sensors inside the regulators can be utilised to detect collisions between the manipulator body and a human worker, for instance.

Open-loop position control for VSL robots

We have worked on a hybrid, learning based kinematic modelling approach to improve the performance of traditional open-loop position controllers for a modular, collaborative VSL robot. 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 task-oriented 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.

Surgical manipulators

The team has been working on various soft, stiffness-controllable manipulators for surgical robotics. The video below shows a 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. Inspiration in taken 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. This 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.

Another soft and stiffness manipulator based on a silicone structure with pneumatic actuation is shown in the next video:

This system has been created as part of the STIFF-FLOP project and integrates two 3 DoF F/T sensors, 3 Aurora pose sensors, an cauterisation tool at the tip, and granular jamming. The hardware is integrated via RoNeX into a ROS System Map.

Within the STIFF-FLOP team, a miniaturised 2-segment soft robot has been build that is able to fit through a 15mm standard Trocar port. The STIFF-FLOP project put the soft robotic arm through a series of successful tests on a cadaver. Read the latest news here.