IEEE Robotics & Automation Magazine - September 2013 - 63

robot capable of building a 3-D structure composed of 102
truss elements, but the robot is stationary and cannot climb
to any point in the structure for inspection or repair. Galloway et al. [4] built a similar system that can create 3-D
structures layer by layer using a robot fixed to the floor.
Automatic modular assembly system (AMAS) [29] includes
an assembler robot and passive cube-shaped building
blocks. It showed the automatic construction capability for
3-D structures. The robot presented in this article is a further development of a design shown by Hjelle [27]. This
robot exhibited two identical halves connected via a hinge
joint. Each side had a translational and a rotational mechanism, similar to the robot presented in this article. The
robot did not exhibit any feedback and was open-loop controlled. The robot engaged onto a 1/8-in pinion wire with
16 threads per inch. Testing of the original design showed
the need for improvements of the hardware and electronics.
The pinion wire gearing/threading was not adequate and
needed bigger teeth to allow better engagement by the
robot mechanism. Additionally, it was determined that
position feedback from the motors would be required if significantly improved control was to be achieved. All of these
issues are addressed and resolved using the robots demonstrated in this article.

To create the female bidirectional gears, a contour of a
spur gear rack was revolved around a rod axis and was
merged with a spur gear thread in the orthogonal direction.
The different design stages of the female bidirectionally
geared rod can be seen in Figure 2(a). To create the male
bidirectional gear, the outside contour of a spur gear rack was
revolved around and subtracted from a spur gear. Figure 2(a)
shows the CAD drawings of the steps on sample gears and
pictures of the 3-D printed actual bidirectional gears.
Connector Design
To join the rods together at their vertices, two different types
of connectors were designed: a fixed connector and a robotlockable connector. A fixed connector, for traversal by robot
R1, was designed to securely connect structural elements
together, which could not be manipulated by the robot. In this
structural iteration, the angles between the rods are nearly
orthogonal, which required less flexibility from the robot to

(a)

(b)

(c)

(d)

Structural Truss Design
The structural components have to fulfill three primary criteria to ensure a reliable interaction with the robot. First, the
parts have to be strong enough to support the robot but light
enough to be handled by the robot. Second, the robot has to
lock and unlock the connectors, so the entire structure may
be reconfigured. Lastly, the robot has to reliably engage,
rotate, and translate on an individual rod.
Initial designs utilized cylindrical rods which the robot
engaged via rubber wheels. Using this method of attachment,
translation and rotation of the robot along and around the
rod resulted in slippage. To avoid the problem of slippage and
to increase the power transmitted from the robot to the rod,
bidirectional gearing was developed for the rods and the
robot [Figure 2(a)].

Figure 3. Assembly of connector: (a) rod approaching connector,
(b) rod in unlocked state on connector, (c) rod twisting into
locking position, (d) rod in locked position, and (e) photo of 3-D
printed connector node.

Rod Design
These novel bidirectional geared rods have gearing in the
longitudinal direction as well as in the rotational direction,
allowing for increased power transmission from the robot
to the rod regardless of the plane of travel. Uniquely, these
bidirectional gears allow motion in one direction while
inhibiting motion in the orthogonal direction, thus arresting slippage. This enables the robot to remain at the unstable position on top of the rod while successfully translating
along the rod.
The bidirectional gearing system consists of a pair of gears:
a female bidirectional gear and a male bidirectional gear. The
female bidirectional gearing is used on the rods, whereas the
robot uses the male bidirectional gearing with its servos to
effectively engage the structure.

Figure 4. Rod with female part of lockable connector.

(e)

september 2013

IEEE ROBOTICS & AUTOMATION MAGAZINE

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2013