Moving towards compliant robots Ways to make robots

Moving towards compliant robots

Ways to make robots compliant We can categorize softness into two levels: 1) Physical or Structural 2) Control • Hence softness can be achieved by either developing robot mechanisms that are soft in the structural sense or by introducing compliant control into robots based on traditional designs like ones with rigid links and motors. • Existing ideas explore both these aspects. Ex: Octor, RI-MAN We shall first explore how we can make robots structurally compliant

What does that mean? • Robots that do not have any rigid components Advantages • Safe interaction with humans • Being soft they can access convoluted spaces Interesting problems when robots get soft • How do we manipulate objects? • Locomotion – Bipeds, wheeled, undulatory(snake like)? Inflatable robots – A possible solution

Inflatable Robots • One important difference between traditional robot and soft robots: No single end effector, but whole body contact for manipulation • The end effector is the whole surface of robot • Therefore necessary to understand the and predict the shape of the surface of the inflatable robot

Predicting the shape of a the balloon • Material of the balloon : Mylar (Helium filled balloons are made of this) • The Material does not stretch (very stiff) • So the surface area of the balloon is preserved • The answer is then simple - at full inflation of the system the volume is maximized (from minimum Total potential energy principle) • So the shape must be a sphere! • But its not – this is due to crimping at the surface Initial Flat Deflated condition

What is the functional? c/s of the balloon • Assuming circular symmetry b • We are maximizing volume under the constraint that the length between a and b remains constant • Free boundary problem between b and a • Closed form solution not possible to the resulting E-L equation • Numerically find f(x) that maximizes J a

Transversality Conditions • Before we numerically find the surface we already have information regarding the shape of the surface at the end points • This is obtained from the transversality conditions

Results case(1)- Initial flat condition – Circular disk case(2)- Initial flat condition - boundary given by the polar equation • Observation: point discontinuities on the boundary map to parametric curves on the inflated surface • Useful to predict formation of creases on the surface

Balloons made of elastic material Initial Flat Deflated condition πD/2 • Given the flat initial circular conditions, will it map to a sphere when inflated? • If it were a sphere the balloon would not be uniformly strained D* D* Inflated condition D

Balloons made of elastic material By minimum potential energy principle we have the functional Again Numerically solving we get the surface of the balloon Used Nonlinear FEM as well. Modeled with beam elements of low bending stiffness. Cannot be modeled with bar elements – leads to singular stiffness matrix

Possible Soft Link Design • The bourdon tube type link The bourdon tube • As pressure within the tube increases the tube straightens as the elliptical c/s changes to a more circular one

• A similar concept could be used to make an inflatable link • For the inflatable link the cross section goes from being completely flat to the shape described earlier. • The shape change of the c/s results in shortening of the outer wall and lengthening of inner wall. To nullify the stresses caused by this, the tube curvature decreases • The resulting motion can be utilized for manipulation or other tasks. • This is an example where the actuator and the link are a single entity. Outer Wall Inner Wall

Other standard actuators Mckibben Actuator • The device consists of an expandable internal bladder (an elastic tube) surrounded by a braided shell. When the internal bladder is pressurized, it expands in a balloon-like manner against the braided shell. • The braided shell used to limit extension along the length of the actuator only. • Three Mckibben type actuators in the arrangement shown above used in the Oct. Arm

Conclusions • Predicted the shape of inflatable structures like balloons • Two types of materials explored – inelastic and elastic • The surface knowledge can be used for interaction between the physical world and the robot Future Work • Conduct tasks such as move a coffee mug around a table using an inflatable robot • Develop the bourdon tube type inflatable link and control its configuration • Develop compliant control for rigid link robots