Version du 11 juin 2016 à 16:47

Summary

This article deals with the benefits of using elastic soft Robots. For now, the approaches to create them are made by inspiration from nature but with their high elasticity it's really hard to control them.
So it's necessary to know how to modelize their movement and that's what the article explain with the Finite Element Method ( FEM ) in order to model the deformations that are the cause of a mechanical coupling.

So, in first the author insist on the difficulty to anticipate the movement because of the infinite number of degrees of freedom, and also on the several advances made on this subject by other scientists.
But to control a soft robot he explains that we need a real-time computation and thanks to algorithms adapted to GPU, the computation time can be reduced.

After that he explains the principle of the FEM. Before everything he needed to know the constitutive law of the soft material by making stress-strain response under tension experiments and he found with a linear regression that the strain/stress ratio is approximatively linear that will reduce the time of online execution of the FEM. But when he assembled the possible rotation in the model is no more linear but it's still possible to compute it with an C++ implementation thanks to the open-source framework SOFA. To have the FEM model we need to compute the following linearization of the internal forces.

But, in addition of the FEM we add a new method that refer new directions in the (J^T) matrix and after that we have to resolve the structure.
The next step is to find λ by using the constraint-based solver that allow us to find the final configuration :
xt = xfree + K^(−1) * J^T * λ.
Where plus definir
So it's possible to obtain actuator model by studying its physical chracteristic to finally knowing the final model.

The next step after knowing the model is to achieve to create the control algorithm. </br>For that we need to know the mechanical coupling between effector and actuator and between actuator to have kinematic equations to build the position control with Gauss Seidel in order to reduce the shift between between actual and desired position. The author also explains that we can add external contact constraints imposed by the nearby object in the Gauss Seidel algortihm to improve the accuracy.

In conclusion, he finishes by showing us two examples of this work : the first one is a mathematics example by simulate some deplacements and check the accuracy with and without collision. The second one is a test on a deformable robot to validate the approach.

Main contribution

This article has the objective to present a new method for the control of Soft Robots with elastic behavior.
The main contribution of this project is the use of the Finite Element Method with the computation in real time for the control of this robot and also by using Lagrange multipliers at the end of effector's position.
It's also showing that it's possible to modelize and to control the movement of a soft robot but also to try to prove that an accomplishable method is to use the Gauss Seidel method by giving a mathematical model and
by doing experiments with a 3D silicone made robot that obtain a maximum error of 2.9 mm that prove that the FEM method can be really accurate.
This work also open new possibilities for several future work with the objective to improve the control of this kind of robot especially when they are in contact with soft object because until now experiments are done with "hard" objects.

Applications

This work can be the opening of several applications for the future in the medical field especially in surgery.
Because for example when you're doing an operation you put some rigid tools in the patient's body and the surgeons spend their time to avoid any contact with organs and tissues. As soon as they touch them, they damage them.
So with the use of soft robot we can allow a more secure way to doing it.
But we can also imagine the creation of a soft exoskeleton to improve the comfort of disabled compare to today's solutions or also a new way to create better robotic organs for example.
Because when researchers in robotics want to find a way to make their robots indestructible to avoid damage when they can't avoid collision, this new kind of robots can engage the environment without damage the robot or anything around it
( or someone ) that is really important in a time when robots are way more present in our houses.

An other thing is because of their soft structure, elastic robot are way more resistant than hard robot and less sensitive to hot or cold temperature so they are more capable to resist to extreme conditions. We can for example create a new kind
of automatic vehicle to allow the travel in a hilly area in a more secure way. But with the use of elastic material the robot will have the possibility to change it's form in order to go into some place where a hard robot won't that could be an advantage
for the exploration of new place like underwater into some cave or also to secure people stuck into small place unreachable by first-aider.