Fiber-reinforced Elastomeric Enclosures Research Articles

Modeling and Fabrication of Soft Actuators Based on Fiber-Reinforced Elastomeric Enclosures

Unlike rigid actuators, soft actuators can easily adapt to complex environments. Understanding the relationship between the deformation of soft actuators and external factors such as pressure would enable rapid designs based on specific requirements, such as flexible, compliant endoscopes. An effective model is demonstrated that predicts the deformation of a soft actuator based on the virtual work principle and the geometrically exact Cosserat rod theory. The deformation process is analyzed for extension, bending, and twisting modules. A new manufacturing method is then introduced. Through any combination of modules, the soft actuator can have a greater workspace and more dexterity. The proposed model was verified for various fiber-reinforced elastomeric enclosures. There is good agreement between the model analysis and the experimental data, which indicates the effectiveness of the model.

Co-Optimization of Morphology and Actuation Parameters of Multi-Sectional FREEs for Trajectory Matching

Fiber Reinforced Elastomeric Enclosures (FREEs) have gained significant popularity as a form of soft artificial muscle for the diversified deformation behaviors upon pressurization. In particular, modular FREEs connected in series, demonstrate enhanced flexibility and reconfigurability, thus are adaptable to more complex trajectory tasks. Morphology and motion parameters are strongly coupled for soft manipulators to achieve desired motion behaviors. In contrast to the traditional way of alternating between design optimization in either morphology or actuation space, we seek to co-optimize over parameters in both spaces in an end-to-end fashion with gradient-free optimization. The co-optimization framework allows for faster convergence towards optimal performance given desired trajectory plans. We demonstrated the feasibility of the framework with two simulated tasks: the multiple shape matching task and the octopus reaching task. The former task was further evaluated with real-world experiments.

Force Generation by Parallel Combinations of Fiber-Reinforced Fluid-Driven Actuators

The compliant structure of soft robotic systems enables a variety of novel capabilities in comparison to traditional rigid-bodied robots. A subclass of soft fluid-driven actuators known as fiber reinforced elastomeric enclosures (FREEs) is particularly well suited as actuators for these types of systems. FREEs are inherently soft and can impart spatial forces without imposing a rigid structure. Furthermore, they can be configured to produce a large variety of force and moment combinations. In this paper we explore the potential of combining multiple differently configured FREEs in parallel to achieve fully controllable multi-dimensional soft actuation. To this end, we propose a novel methodology to represent and calculate the generalized forces generated by soft actuators as a function of their internal pressure. This methodology relies on the notion of a state dependent fluid Jacobian that yields a linear expression for force. We employ this concept to construct the set of all possible forces that can be generated by a soft system in a given state. This force zonotope can be used to inform the design and control of parallel combinations of soft actuators. The approach is verified experimentally with the parallel combination of three carefully designed actuators constrained to a 2DOF testing rig. The force predictions matched measured values with a root-mean-square error of less than 1.5 N force and 8 x 10^(-3)Nm moment, demonstrating the utility of the presented methodology.

Design and analysis of coiled fiber reinforced soft pneumatic actuator

Fiber reinforced elastomeric enclosures (FREEs) are soft pneumatic actuators that can contract and generate forces upon pressurization. Typical engineering applications utilize FREEs in their straight cylindrical configuration and derive actuation displacement and forces from their ends. However, there are several instances in nature, such as an elephant trunk, snakes and grapevine tendrils, where a spiral configuration of muscle systems is used for gripping, thereby establishing a mechanical connection with uniform force distribution. Inspired by these examples, this paper investigates the constricting behavior of a contracting FREE actuator deployed in a spiral or coiled configuration around a cylindrical object. Force balance is used to model the blocked force of the FREE, which is then related to the constriction force using a string model. The modeling and experimental findings reveal an attenuation in the blocked force, and thus the constriction force caused by the coupling of peripheral contact forces acting in the spiral configuration. The usefulness of the coiled FREE configuration is demonstrated in a soft arm orthosis for crutch users that provides a constriction force around the forearm. This design minimizes injury risk by reducing wrist load and improving wrist posture.

Modeling and Design of “Smart Braid” Inductance Sensors for Fiber-Reinforced Elastomeric Enclosures

Fiber-reinforced soft actuators use inextensible fibers to shape the expansion of elastomeric fluid-filled chambers. The classic example, McKibben muscles, is part of a broader class of cylindrical soft actuators known as fiber-reinforced elastomeric enclosures (FREEs). These actuators can be designed to twist while extending or contracting. Using a circuit of conductive, electrically insulated wire as the reinforcing fibers allows one to measure the motion of the actuator with the circuit inductance. These sensors, developed previously for McKibben muscles, are known as “Smart Braids.” This paper extends the concept of Smart Braids to the broader class of two-fiber-family cylindrical FREEs. A dimensionless model for the inductance of Smart Braid FREEs is presented that can be scaled to specific sensor geometries. This model depends only on the ratio of the fiber lengths in the two families and the angle of one of the fibers. The model was validated both numerically and experimentally, predicting the sensitivity of experimental sensors accurately, the largest errors being −11% and 9%. The model presented in this paper will enable high-level design decisions for Smart Braid FREES and optimization without computationally expensive direct numerical simulation.

A Continuum Model for Fiber-Reinforced Soft Robot Actuators

Fiber-reinforced elastomeric enclosures (FREEs) generate sophisticated motions, when pressurized, including axial rotation, extension, and compression, and serve as fundamental building blocks for soft robots in a variety of applications. However, most modeling techniques employed by researchers do not capture the key characteristics of FREEs to enable development of robust design and control schemes. Accurate and computationally efficient models that capture the nonlinearity of fibers and elastomeric components are needed. This paper presents a continuum model that captures the nonlinearities of the fiber and elastomer components as well as nonlinear relationship between applied pressure, deformation, and output forces and torque. One of the key attributes of this model is that it captures the behavior of FREEs in a computationally tractable manner with a minimum burden on experimental parameter determination. Without losing generality of the model, we validate it for a FREE with one fiber family, which is the simplest system exhibiting a combination of elongation and twist when pressurized. Experimental data in multiple kinematic configurations show agreement between our model prediction and the moments that the actuators generate. The model can be used to not only determine operational parameters but also to solve inverse problems, i.e., in design synthesis.

A Closed-Form Kinematic Model for Fiber-Reinforced Elastomeric Enclosures

In this work, we present a closed-form model, which describes the kinematics of fiber-reinforced elastomeric enclosures (FREEs). A FREE actuator consists of a thin elastomeric tube surrounded by reinforcing helical fibers. Previous models for the motion of FREEs have relied on the successive compositions of “instantaneous” kinematics or complex elastomer models. The model presented in this work classifies each FREE by the ratio of the length of its fibers. This ratio defines the behavior of the FREE regardless of the other parameters. With this ratio defined, the kinematic state of the FREE can then be completely described by one of the fiber angles. The simple, analytic nature of the model presented in this work facilitates the understanding and design of FREE actuators. We demonstrate the application of this model in an actuator design case study.

A constrained maximization formulation to analyze deformation of fiber reinforced elastomeric actuators

Fiber reinforced elastomeric enclosures (FREEs) are soft and smart pneumatic actuators that deform in a predetermined fashion upon inflation. This paper analyzes the deformation behavior of FREEs by formulating a simple calculus of variations problem that involves constrained maximization of the enclosed volume. The model accurately captures the deformed shape for FREEs with any general fiber angle orientation, and its relation with actuation pressure, material properties and applied load. First, the accuracy of the model is verified with existing literature and experiments for the popular McKibben pneumatic artificial muscle actuator with two equal and opposite families of helically wrapped fibers. Then, the model is used to predict and experimentally validate the deformation behavior of novel rotating-contracting FREEs, for which no prior literature exist. The generality of the model enables conceptualization of novel FREEs whose fiber orientations vary arbitrarily along the geometry. Furthermore, the model is deemed to be useful in the design synthesis of fiber reinforced elastomeric actuators for general axisymmetric desired motion and output force requirement.