IEEE Robotics & Automation Magazine - March 2017 - 42

(a)

(b)

(c)

(d)

(e)
Figure 1. The design iterations of the BigANT PARF hexapod: (a)
chassis #1, (b) chassis #2, (c) chassis #3, (d) chassis #5, and (e)
chassis #7. Chassis #3 was the first design that was capable of
walking. Chassis #7 could successfully traverse outdoor terrain.

(a)

(b)

Figure 2. The PARF robots from a classroom. (a) A crawling robot
using reciprocating limb motions. (b) A robot arm drawing a square.

typically fiberglass [5], poster board [4], or carbon fiber [9],
and the flexible material is typically a polymer film. While
enabling the development of a rich collection of robot mechanisms, so far, the maximum size of robots manufactured by
SCM has been limited.
It is not surprising that scaling up SCM designs geometrically by an order of magnitude does not lead to functional
mechanisms, as structural properties do not scale in a simple
way. For example, according to Euler beam theory, a cantilever plate made twice as long and wide but no thicker becomes
four times softer in bending. Extended lengths also increase
the misalignment that comes from angular play at the joints
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IEEE ROBOTICS & AUTOMATION MAGAZINE

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March 2017

between the plates. A combination of new materials, components, and design heuristics must be made for larger scales.
To make low-cost, larger-scale robots, we developed PARF
mechanisms. In our designs, rigid plates are made from inexpensive materials such as foam board, corrugated plastic, and
cardboard; joints are constructed with fiber-reinforced tape.
The tooling required to work these materials is minimal: scissors, a hacksaw blade, and a straight-edge metal ruler will do.
For improved results, a laser cutter and/or hot-melt adhesive
may be used as well. We present several designs of structures
and joints possible with PARF and report quantitative results
of tests for strength and durability. Such variety, with wellunderstood properties, enables PARF designs to satisfy
diverse mechanical functional requirements.
PARF mechanisms are readily actuated with servomotors.
The servomotors and associated power electronics are by far
the most intricate and expensive parts of PARF robot systems.
We chose to encapsulate this complexity in modular, reusable
components [3], making it easy and inexpensive to modify
designs and adapt to changing needs.
The efficacy of our method is demonstrated using a
90-cm # 60 -cm walking robot (see Figure 1) with six individually actuated 1-degree-of-freedom (DoF) legs. We were
able to rapidly iterate through different mechanisms to
improve the design, often changing designs and testing them
on a daily basis. Through this rapid evolution, we reached a
platform that allows for walking in both indoor and outdoor
environments up to 30 ! 3 cm/s with a foot clearance of
approximately 4 cm and turning at 4.2°/s.
Furthermore, PARF was employed extensively in a robotics
design class (Hands on Robotics, a senior-level class taught
at the University of Michigan). Students were challenged
to use PARF to build functional walking robots and arms
(see Figure 2).
PARF as SCM for the Meter Scale
For PARF's joints and structural elements, we selected materials that were inexpensive, lightweight, and easy to work with
yet could effectively withstand the physical demands of
the application.
Material Selection: Rigid Plates
We considered candidate rigid materials of foam board
(Elmer's Products Inc. 50.8 -cm # 76.2-cm # 0.72-cm foam
board), corrugated plastic board (SABIC Polymershapes
Coroplast COR-2436 91.4 cm # 61.0 cm # 0.40 cm ), and
corrugated cardboard (Home Depot 55.9-cm # 53.3-cm
# 0.3-cm box). The price per square meter of foam board is
US$14.03 (OfficeMax), of corrugated plastic is US$22.21
(DisplayShops), and of corrugated cardboard is US$1.05
(Home Depot). Thus, all of the candidate materials are available at convenient sizes and low prices.
We quantified each material's resistance to bending
via its mass-specific flexural rigidity. This is the ratio
EI/M = (Pl 3) / (3dM) , which assumes a Euler cantilever
beam bending model [10] with load P, length l, deflection δ,
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