[Paper Review] Analysis and Control of Fiber-Reinforced Elastomeric Enclosures (FREEs)

Soheil Habibian|arXiv (Cornell University)|Dec 13, 2019

Soft Robotics and Applications12 references4 citations

TL;DR

This paper presents a dynamic lumped-parameter model and PID control framework for Fiber-Reinforced Elastomeric Enclosures (FREEs), a type of soft pneumatic actuator, enabling precise trajectory tracking via rotation angle control. Finite Element Analysis validates the model and determines the workspace of FREE modules, demonstrating accurate simulation of nonlinear material behavior and effective control for soft robotic manipulators.

ABSTRACT

While rigid robots are extensively used in various applications, they are limited in the tasks they can perform and can be unsafe in close human-robot interactions. Soft robots on the other hand surpass the capabilities of rigid robots in several ways, such as compatibility with the work environments, degrees of freedom, manufacturing costs, and safe interactions with the environment. This thesis studies the behavior of Fiber Reinforced Elastomeric Enclosures (FREEs) as a particular type of soft pneumatic actuator that can be used in soft manipulators. A dynamic lumped-parameter model is created to simulate the motion of a single FREE under various operating conditions and to inform the design of a controller. The proposed PID controller determines the response of the FREE to a defined step input or a trajectory following polynomial function, using rotation angle to control the orientation of the end-effector. Additionally, Finite Element Analysis method is employed, incorporating the inherently nonlinear material properties of FREEs, to precisely evaluate various parameters and configurations of FREEs. This tool is also used to determine the workspace of multiple FREEs in a module, which is essentially a building block of a soft robotic arm.

Motivation & Objective

• To develop a dynamic lumped-parameter model for simulating FREE actuator behavior under various operating conditions.
• To design a PID controller that enables trajectory tracking using rotation angle as the control input.
• To use Finite Element Analysis (FEA) to evaluate nonlinear material properties and optimize FREE configurations.
• To determine the effective workspace of multiple FREEs in a modular soft robotic arm configuration.
• To bridge the gap between theoretical modeling and practical control of soft pneumatic actuators for robotic applications.

Proposed method

• A lumped-parameter dynamic model is formulated to simulate the motion of a single FREE under different pressure and load conditions.
• A PID controller is implemented to regulate the actuator's rotation angle, achieving desired step responses or polynomial trajectory following.
• Nonlinear material properties of the elastomer and fiber reinforcement are incorporated into a Finite Element Analysis (FEA) model for high-fidelity simulation.
• FEA simulations are used to evaluate geometric and material parameters, including pre-stretch and fiber angle, to optimize actuator performance.
• The workspace of a multi-FREE module is computed using FEA to assess its potential as a building block in soft robotic arms.
• Model validation is performed by comparing simulation results with expected mechanical behavior under controlled inputs.

Experimental results

Research questions

• RQ1 How can a dynamic lumped-parameter model accurately represent the behavior of a FREE under varying pneumatic pressures and mechanical loads?
• RQ2 To what extent can a PID controller achieve precise trajectory tracking using rotation angle as the control variable?
• RQ3 How do nonlinear material properties of elastomers and fiber reinforcement affect the performance and deformation of FREEs?
• RQ4 What is the effective workspace of a modular FREE configuration when actuated in a coordinated manner?
• RQ5 Can FEA-based simulation reliably predict the performance and optimal design parameters of FREE-based soft actuators?

Key findings

• The lumped-parameter model successfully simulates the dynamic response of a FREE under various pressure inputs and load conditions, enabling accurate prediction of actuator motion.
• The PID controller achieves stable and accurate trajectory tracking for polynomial reference functions, with response times and overshoots quantitatively minimized through tuning.
• FEA simulations confirm that nonlinear material behavior significantly influences actuator deformation, and accurate modeling requires inclusion of hyperelastic material laws.
• The workspace of a multi-FREE module is computationally determined, showing that coordinated actuation enables a wide range of end-effector orientations suitable for robotic manipulation.
• Pre-stretch and fiber angle are identified as critical design parameters affecting actuation range and force output, with FEA providing optimal ranges for these variables.
• The integration of FEA with the control model enables reliable design optimization and performance prediction for FREE-based soft robotic systems.