Optimization of the multi modular manipulator

tube image

Scuola Superiore Sant’Anna (SSSA) worked on the optimization and performance evaluation of the multi-module STIFF‐FLOP manipulator.

The final aim was to develop a modular manipulator, which exploits its entire length to actively interact with the biological structures in the MIS (Minimally Invasive Surgery) theatre. The figure on the left proposes a schematic example of that objective: a generic multi-steerable manipulator (in black) can navigate between organs, orientate its tip to properly approach the target, and also modulate the stiffness along its body to withstand the weight of another organ and keep it aside. Such capability would be particularly helpful surgery since it offers the possibility of accomplishing two tasks in one go (for example the retraction of one soft-tissue organ and the manipulation of another organ) by using the same instrument.

The main focus of the research was firstly the assessment of the dexterity of a 2-module manipulator embedding the actuation and a separate stiffening mechanism in each module. The workspace of the 2-module manipulator was experimentally evaluated by tracking different points along the STIFF-FLOP manipulator using the AURORA magnetic tracking system.

optimization 2

The image on the right shows the extrapolated workspace of the manipulator. Two different types of tests were carried out to evaluate the forces exerted by the manipulator exploiting the selective stiffening of its body. One test measured the force exerted by the STIFF-FLOP manipulator’s tip on a F/T sensor in two different situations: firstly the stiffening was applied to the proximal module while the distal module was actuated against the F/T sensor, secondly the proximal module was actuated to push the stiffened distal module against the F/T sensor. For this test the vacuum level for inducing the stiffness of the related module was set to three different values (no stiffening: 0 MPa; partial stiffened: -0.052 MPa, fully stiffened: -0.098 MPa).

The second types of tests aimed at exploiting the stiffening capabilities together with the possibility to generate forces in a more surgery-like scenario. Although the previous described test provided a good overview of the two modules manipulator, it lacks thoroughly demonstrating the real capabilities of such a structure, in comparison with traditional rigid link surgical manipulators.

For that reason, scenarios as proposed in the schematic view have been taken as guidelines to build a more reliable, credible test setup for the 2-module manipulator. To reproduce the compliance, in terms of weight and shape of organs or anatomical parts, which the manipulator may encounter during a surgical laparoscopic procedure, water filled balloons have been employed and they have been placed around the manipulator to test its interaction with them. Among the variety of possible tasks, a few key movements were chosen to demonstrate the manipulation and stiffening capabilities. These are the wrapping and retraction of a water filled balloon (800 g), hung up to a load cell which revealed when the whole weight of the balloon was supported by the manipulator.

Another task is shown in the figure below where the manipulator navigates among compliant objects (water filled balloons), embraces one of them (270 g) and moves it aside. The last task demonstrates the manipulator supporting a weight of 500 g with the first module and applying a force on an F/T sensor. The same test was performed without stiffening activation and when the first module was fully stiffened. In this last experiment two F/T sensors were used. One F/T sensor was connected to the water filled balloon (5 N), while the other one was positioned in the proximity of the distal end of the manipulator. In this test, two pieces of information have been extracted: the first F/T sensor allowed verifying that the water filled balloon is completely supported and lifted, while the second F/T sensor measured the amount of force generated on the target.

 

 

optimization 3

Results from the testing on the combination of actuation and stiffening (highlighted with the dashed square). a) Stiffening of the base module and actuation of the distal one. b) Stiffening of the distal module and actuation of the base one. 

 

optimization 4

 Examples of the 2-module manipulator interacting with water filled balloons and exploiting combined stiffening, dexterity, and force capabilities to perform surgical-like tasks.

 

 
 
 
 
 

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