IEEE Robotics & Automation Magazine - June 2017 - 27

Table 1. The functions of the four color-coded
cable systems for Figure 4.
Color

Function

DoF

Input
RoM

Output
RoM

Blue

Translation:
middle, inner
tube

Pitch

±170º

4.6 cm

Red

Rotation:
middle tube

Roll 1

±170º

±250º

Yellow

Rotation:
inner tube

Roll 2

±170º

±250º

Green

Actuate end
effector

Open/
close

±45º

5 mm

The input RoM is the rotation of the actuation disks, and the output
RoM is the resulting movement seen at the cart.

despite the varying displacement, maintains constant cable
path lengths for the 3 DoF that route to the lead cart. The
lead cart is directly driven by the blue (pitch) capstan,
while the follower cart passively maintains constant cable
path lengths.
Figure 5 depicts a simplified version of the cart system.
When the lead cart is pulled to the right by the blue cable, the
follower cart will also move to the right due to the green
route. By moving to the right, the follower cart will ensure
that the red cable remains taut. When the lead cart is pulled to
the left, the red route will cause the follower cart to move to
the left and the green cable will be kept taut. This routing
causes the follower cart to move at half the speed of the lead
cart and ensures the cable paths that route to the lead cart
maintain length regardless of the position of the lead cart.
Thus, the 4 DoF are decoupled.
The MACRO Tool
The MACRO tool is another miniature custom tool developed to decrease the limitations of the CTI. The CTI's
strengths include a significantly reduced shaft diameter with
high stiffness; however, it is challenging to achieve sharp
bends with the tubes. To maintain a 2-mm shaft diameter and
reduce the bending radius, a cable-actuated, notched-tube
wrist was designed. A comparison of the CTI and MACRO
tool alongside standard da Vinci EndoWrist instruments is
shown in Figure 6.
MACRO Tool Wrist Design
To construct the wrist, both an RPR design, like the CTI, and
a roll-pitch-yaw (RPY) design were considered. An RPR
wrist would have cuts in a single plane for bending about a
single axis, while the RPY configuration requires cuts in multiple planes to allow for bending about multiple axes. RPR
wrists have a singularity in the middle of the wrist's workspace, while RPY designs do not. Also, achieving the roll at
the end of a sharp bend can be challenging, whereas with
RPY, the roll can occur proximal to the pitch-yaw bending.
Because of the limitations of RPR, an RPY wrist was

Table 2. An example tube selection
for the CTI.
Feature

Geometry

Outer tube OD, ID

3 mm, 2.5 mm

Middle curved tube OD, ID

1.9 mm, 1.6 mm

Inner tube OD, ID

0.8 mm, 0.7 mm

Middle tube curvature,
arc length

0.025 mm−1,
62 mm

Max pitch angle achievable
with tube

70º

Bending stiffness ratio

900, 38, 1

OD: outer diameter; ID: inner diameter.

developed, requiring notches in the tube to be in multiple
planes similar to the design presented by Peirs et al. [18].
Our notched-tube solution employs a unique cutting
arrangement consisting of a series of asymmetric cuts
spaced 120° apart, helically patterned around the tube, as
seen in Figure 7. Three cables are attached to the distal end

Follower Cart

Lead Cart

Lead
Cart
Translation
Cable

Figure 5. The cart system routing schematic.

8-mm EndoWrist
5-mm EndoWrist
Large Needle Driver Round-Tip Scissors

MACRO
CTI
Tool

Figure 6. A comparison of instrument sizes.

Proximal

Distal

Spring

Actuation Cables

Figure 7. The MACRO tool wrist design and assembly.

June 2017

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Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - June 2017