Page 239 - Flexible Robotics in Medicine

Tendon-driven linkage for steerable guide of flexible bending manipulation 227

tendon that caused the bending movement along the y-axis increases the deflection in both
6 x directions. Furthermore, the current bendable joint only possesses a single bending
DOF along the y-axis, thus the deflection in y direction was depressed causing smaller
deflections in the 6 y directions.
From Fig. 9.9, the drill hole diameters, H n and H x , were not really affected by the increase
in speed or bending, with H x remaining generally lower than H n .

These could be explained as the initial drilling into the gelatin was affected by the
deflections as reflected in Table 9.1. This caused a bigger hole to be created due to the
unstable drill bit. However, as the drill bit entered the gelatin, the gelatin supported the drill
bit in all directions and absorbed the vibrations from the drill. This allowed the drill to pass
through with lesser deviations than when it entered.
Some holes further appeared to be smaller than the drill bit diameter. This could be due to
the elastic property of gelatin, which compressed to the sides as the drill moved through
and closed up slightly upon retraction of the drill. This would be similar to the property of
tissues but dissimilar to that of bones.
The support provided by the gelatin could further be used to explain the data shown in
Fig. 9.10, where small fluctuations occurred in the first two bending angles 0 and 80 degrees.
At 160-degree bending angle, the deviation maintained at about 20 degrees. The support
provided by the gelatin allowed the drill to continue in its direction from the entrance.

9.3.4.2 Possible force feedback for considerations

Possible force feedback for considerations includes using a force sensing sleeve configuration,
whereby two layers of supports are incorporated into the design. The external layer of support
can be similar to the design suggested in this study, with a softer material for the internal
layer, such as Nitinol tubes. By using the Nitinol tubes to push the drill via the drill bit, it is
possible to add in flexure and bush in between. With strain gauges equipped on the flexure,
reading can be taken from the force sensor and calibrated. The size of the external shaft, as
suggested in this project, can be maintained at a larger size of around 10 mm but reduced
thickness if a rigid material such as stainless steel is used. This will allow space for the
connection of wires from the strain gauge to the monitor, which should be outside the
patient’s body during surgery.

Another possible method is to obtain the bending of the support shaft, with the drill shaft
inside, under a given tension in the tendon wire. Using the information of the torsion,
coupling it with an angular displacement sensor for the pose of the shafts, if the torsion
increases while the pose maintains, given that the maximum bending is not reached yet, it
can be deduced that the drill is being stopped by the patient’s body and that increase in
torsion is now the acting force from the drill bit against the patient’s body.