The dSPACE DS1103 controller board (manufactured
by dSPACE, Inc.) recorded the position and force data from
the motor’s encoder and force sensor respectively, in real-time
along with grabbing images at the rate of 13 frames/s. We have
implemented a proportional + derivative (PD) controller to enable precise movement of the motor (and, hence,
the cuttingblade during cutting tasks).
B. Experimental Procedure for Measuring Liver Cutting
Forces
Since the experiments were performed on ex vivo liver tissue,
the preparation of the tissue before the experiment helped main-
tain the properties of the tissue as close as possible to the in
vivo properties. We transported the liver from freshly slaughtered pigs to our laboratory within 2 h postmortem.
The livertissue sample was not preconditioned because in surgery the
cutting forces experienced by the surgeon are from nonpreconditioned tissues. Before starting the experiment,
we cut the pig’sliver into 8 cm 15 cm specimens. However the thickness of
the liver specimen was variable since we wanted to preserve the
outer capsule of the liver in the cutting experiments. The outer
surface of the specimen was covered with petroleum jelly to
minimize moisture loss during the experiment. A bar of rectangular shape made of machineable plastic
with an array of small nails clamped at the bottom end penetrated through one edge
of the liver specimen to simulate a single constrained boundary
surface. While this is not an exact replication of the boundary
conditions for a human liver (which is partially attached on one
end to the diaphragm) this is none-the-less a simplification for
our initial tests and model (based on our discussions with surgeon collaborators).
C. Determination of the Depth of Cut
All the liver samples had a natural bulge in the thickness direction, which lead to more liver tissue
being encountered bythe blade as the cutting progressed. The depth of cut played
a significant role in the magnitude of the cutting force acting on
the blade. In this paper, we used image-processing techniques
to determine the depth of the blade embedded inside the liver
specimen during each instant of the cutting process.We used the
Bumblebee system to capture the image of the cutting blade as
it cut the tissue. The images were analyzed offline usingMatlab
6.5 with the image processing toolbox.