IEEE Robotics & Automation Magazine - March 2017 - 45

1) crushing the foam board along a line or 2) partially cutting
through the foam board, leaving one surface as a hinge.

Mechanism Design
We attempted to use only two motors in early iterations of
the design, based on scaled-up versions of Berkeley's VelociRoACH robot [1]. However, we were unsuccessful in mak-

(b)

Figure 7. The fabrication of a PARF mechanism is made easier
through laser cutting by partially cutting through the foam board.
A pattern is used with (a) side A and (b) side B. The acrylic fixture
is used to align the foam board for cutting on both sides (a).

ing the drive train rigid enough (in structure and joint play)
to bear the load of the robot from a single motor on each
side. This led us to our current design, which consists of six
identical leg mechanisms mounted on a baseplate with a
motor for each.
For walking, we chose to utilize an alternating tripod gait
wherein anterior and posterior ipsilateral legs move in phase
with the contralateral middle leg. This gait is the one used by
most hexapedal animals when moving at moderately high
speeds within their speed range [11]. The motion of each leg
is theoretically determined by the 1-DoF kinematic constraints of its linkage (see Figure 9) to give a roll angle of over
60° in midswing and an unloaded clearance of over half a leg
length. The individual leg moves vertically in the sagittal
plane around midstance then rapidly swings sideways and
outward at the end of stance to recycle forward. This motion
was verified through testing (see Figure 8).
Actuation
To complete the legged locomotion platform required power
circuitry, actuation, and control-complex, yet well-understood
problems. We encapsulated the electronics in modular components that can easily be attached and released from the chassis,
allowing rapid replacement of faulty parts and rapid design iteration. Our actuators were high-end hobby servomotors (Dynamixel MX64, Robotis, Inc.). Each servomotor drove a plastic
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Building a Meter-Scale Hexapod with PARF
Using PARF techniques, we built a 90-cm-long hexapedal
robot (76-cm baseplate with legs extending 7 cm on both
ends). The chassis, including legs and drivetrain, cost less
than US$20 and required fewer than 6 h of labor to build.
PARF allowed us to rapidly iterate through 40 design changes,
of them 25 drive mechanism design changes and seven
complete chassis rebuilds (see Figure 1), improving robot performance significantly over a short time period. The current
robot can walk, steer, and turn in place on both indoor and
outdoor surfaces, testifying to its viability as a legged platform.

(a)

(cm)

Tooling
One of the great strengths of PARF is its minimal tooling
requirement. The rigid plates for all of the mechanisms we
present can be cut with a straight edge and hacksaw blade.
With the addition of scissors or a knife for cutting tape, the
tool kit is complete. Plates can be cut by laser cutter instead
(ours is a Universal Laser Systems PLS6.150D, 150-W CO 2
laser). A laser cutter improves precision, especially for intricate
patterns. It also lowers fabrication time of both the cutting
and assembly phases.
One key advantage is that the laser can be calibrated to
score the plate, cutting through one face and the intervening
foam while leaving the other face intact, allowing the intact
face to be used as a hinge provided it is reinforced with tape.
Aligning multiple joints this way reduces assembly time.
We scored both sides of the foam board during laser cutting by flipping the plate and using alignment tabs. Our
procedure was as follows: 1) cut alignment slots in foam
board, 2) affix foam board to alignment tab in the laser cutter,
3) score first side, 4) flip and reaffix plate to the alignment tab,
5) score second side, 6) release the part, and 7) fold and tape
the part. Cuts on both sides created the outline; cuts made on
only one side created joints (see Figure 7).
We successfully built a hexapod leg mechanism using both
manual and automatic tools, namely a hacksaw and a laser
cutter (see Figure 8). Using the laser cutter, we made the leg
in about 45 min; using the hacksaw it took about 150 min.
Note, these times were for a design with notches, which are
particularly time intensive with the hacksaw. The legs made
from both methods had comparable quality. We tested joint
samples (N = 3) in torsion with plates cut by hacksaw and did
not find any noteworthy difference in failure torque.
The fabrication time of a single-folded PARF mechanism
using the laser cutter is similar to that reported for SCM
mechanisms [4], proving our method allows for equivalently
rapid prototyping despite the vastly different scale.

(cm)
(a)

(b)

Figure 8. (a) Comparable quality mechanisms can be built from
either a hacksaw (left) or a laser cutter (right). Using the laser
cutter took less than half the time. (b) Foot trajectories of chassis
#6, collected while walking using a Qualisys motion tracking
system at 120 Hz, given relative to chassis frame.

March 2017

IEEE ROBOTICS & AUTOMATION MAGAZINE

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