Design

Two types of designs for soft pneumatic actuators are discussed here. The scripts and models available on the design tool enable customized variations in geometry, material properties and performance for these two designs depending on the desired application of study. 

1. Single-Chamber Shell-Reinforced Actuators

The classical unconstrained multi-chamber actuator undergoes excessive inflation at high values of input pressure which can eventually lead to mechanical failure at the stress concentration regions such as the narrow connecting corridors or at the chamber wall peripheries. As a design improvement over the unconstrained actuators, novel shell-reinforced actuators have been developed. These actuators comprise two materials, one for the highly stretchable actuator core and the other for the unstretchable shell. The shell constrains excessive inflation of the actuator and guides it along the desired trajectory. 

Two types of single-chamber actuators are studied - bending actuators and linear actuators. The shell patterns for these two different types of actuators are shown below. The slit number and slit width on the shell surface influence the motion and stress profile obtained. The design parameters include the chamber dimensions, wall thickness, shell dimensions and pattern, and material properties for both the shell and the chamber. 

Furthermore, to remove the stress concentration regions at the chamber connection regions, only a single air chamber is created in this design.  The images for these actuators in motion are shown below. 

2. Classical Multi-Chamber Unconstrained Actuators

In this type of soft actuator, the actuator is comprised of multiple air corridors connected by narrow connecting passages for pressurization. The actuator is fabricated by attaching two identical halves with an adhesive. The entire unit is made of a single material. The material used in the present work is a highly stretchable silicone rubber material. The actuator is not constrained radially in this case. The design parameters include the individual chamber dimensions, air passage dimensions, wall thickness and material properties. 

Two types of multi-chamber actuators are studied - bending actuators and linear actuators. The images for these actuators in motion are shown below. 

1. Shell-reinforced SPA

Despite the established potential of SPAs and their diverse implementation towards robotic systems for meeting desired functional requirements in crucial applications such as the ones described above, the lack of repeatability in currently existing SPA design and fabrication procedures greatly limits their potential and performance in such systems. 

In an effort to circumvent some of these fabrication and repeatability issues, a new actuator design has been developed where the actuators are made in a single molding step. The presented two-part, shell-reinforced, SPA design shown below allows “fool-proof” prototyping of both bending and linear actuators and produces results in the desired performance range. 

The presented numerical models using FEA accurately predict the complex mechanical response and the performance obtained with the designed actuators while allowing rapid design iterations to optimize the design parameters. The following images show the simulation results obtained for bending and linear shell-reinforced actuators using the design tool along with the corresponding experimental images. The design and modeling procedures for these actuators are listed step-by-step and illustrated in detail in the Model Demo Section

The design tool and a complete set of models used in the current study are also available open-source on the Reconfigurable Robotic Laboratory website (http://rrl.epfl.ch) where it is possible to use the models developed in the present work as a starting point to modify and create different geometries and materials for any robotic application. 

2. Multi-Chamber SPA

The open-source downloadable scripts available with the design tool enable automation of the design and modeling procedure for the classical multi-chamber SPAs. The following design capabilities are supported by the tool:

1. Creation of desired geometry for the actuator. Currently, linear and bending motion profiles are supported by the tool. The user can specify the number of air chambers desired, dimensions of chambers, connecting passage dimensions, wall thickness etc. as input parameters and a ready to use Abaqus INP file with the specified geometry is generated. 

2. Characterization of hyperelastic and viscoelastic soft material effects. These effects are demonstrated here with a typically used silicone elastomer material to fabricate SPAs - Ecoex 00-30, which exhibits desirable mechanical behavior for soft robotic components. The tool fits experimentally gathered stress-strain data on the elastomer collected through multiple mechanical tests to a hyperelastic stress-strain constitutive law. Multiple commonly used hyperelastic stress-strain constitutive laws are supported. Time dependent effects can also be included by fitting experimental stress-relaxation data collected through prescribed tests to a viscoelastic Prony series describing stress relaxation effects within the material.

3. Finite element procedures for modeling SPAs. The scripts automate procedures such as generation of INP file, meshing the actuators, desired pressure load application, specifying the necessary boundary conditions depending on the application and testing conditions for the actuator and generating the mesh desired. The model results are plotted using another script and the results can be simulataneously validated against experimental prototype results, for linear and bending actuators. Optimization of actuator parameters to meet targeted metrics is also possible. 

These procedures are illustrated step-by-step in the Model Demo SectionThe following images show the simulation results obtained for bending and linear shell-reinforced actuators using the design tool.