Page 188 - Flexible Robotics in Medicine

Tendon routing and anchoring for cableriven single-t surgical manipulators 175

Figure 7.3
(A) Guiding of tendons to ensure pure vertical forces; (B) avoiding tangling of tendons; (C)
spacing and constraints within the 3 mm diameter spring.

tendon-driven mechanism. First, the tendons must be guided to ensure that the force exerted
on the compression springs by the tendons is purely vertical since any horizontal force
exerting on the spring will cause unpredictable bending (Fig. 7.3A). Second, the tendons
must be fixed to spots that aligned with their vertical force direction to ensure that the
tendons do not entangle with each other (Fig. 7.3B). Entanglement between strings will
cause the forces exerted on each of the tendons to be distributed onto other tangled cables,
causing the spring to experience a combined force, resulting in unpredictable and
unintended bending. These two problems raise repeatability and reproducibility issues in the
control system and raise safety concerns as it fails to meet our oncological principles.
Third, the anchors must fit within the 3 mm diameter spring while ensuring enough room
for the surgical instrument’s actuation system (Fig. 7.3C). Finally, the anchors must ensure
that they do not compromise the flexibility and functionality of the device. The following
options were explored for the creation of tendon guides.

7.3.2.1 Lashing technique

To create guides that are flexible enough to accommodate spring manipulation while being
strong enough to anchor the tendons, various rope lashing techniques and knots were
explored, including (1) shear lashing, (2) square lashing, and (3) clove hitch knot. As shear
lashing is used to tighten two parallel poles, we move forward with testing the square
lashing method. Square lashing is a technique used to secure two mutually perpendicular
poles, in our case, one being the spring and the other being the tendons.
However, to fulfill our requirement, the tendons must be anchored to the spring
circumference while still being able to move freely in the vertical directions to transmit
vertical force exerted on the tendons to the compression springs. Thus slight modifications
were made to the square lashing technique. Those modifications include having tight lashes
on the circumference of the compression springs but a weak lashing on the tendons using
copper wires to ensure that lesser resistance is exerted by the copper wires onto the tendons,
while the tendons are moving in the vertical direction. Although this technique can meet the