IEEE Robotics & Automation Magazine - September 2023 - 136

adult-sized humanoid face Eva [67] is designed to communicate
using facial expressions and head movements and comprised
of 3D-printed skeletal structure and a molded exterior.
One of the most popular humanoid platforms that has
found application in several research domains such as locomotion,
cognition, haptics, grasping, vision, and learning is the
iCub humanoid robot [55] [see Figure 3(d)]. The hardware and
software for the robot were open sourced, along with relevant
accompanying documentation for both, and has fostered a
strong community of iCub users that help resolve issues, share
their work, and suggest improvements [68]. The iCub robot is
an example of an ORH that is not directly intended for the user
to fabricate and assemble in its entirety on his or her own, but
the access to documentation and hardware files in the form of
CAD drawings and circuit diagrams allows users to modify,
service, or upgrade the robot as necessary. Even though iCub
was one of the earliest open humanoid platforms and released
more than a decade ago, the open source nature of the project
has allowed researchers to continue using it as a development
platform in an evolving field like robotics, where hardware can
fall out of date quickly. The popularity of iCub may have also
paved the way for the other open humanoid robot platforms
mentioned previously and typifies the benefits of ORH for
both users and developers.
LEGGED ROBOTS
Evaluating complex control and learning strategies for
legged locomotion in robotics requires testing the algorithms
on physical legged robot hardware. But, developing an
advanced legged robot can require significant time and resources
and custom-fabricated components that might not be easily
and inexpensively accessible. Several open source quadruped
projects have recognized this challenge and tried to address it
by creating robots that use minimal custom-machined pieces,
inexpensive components, and off-the-shelf parts. The
Open Dynamic Robot Initiative [69] [see Figure 4(a)]
was built using low-cost, 3D-printed components and widely
available brushless dc motors for the actuator modules. This
actuator assembly allows for high-performance torque and
impedance control, previously limited to only complex legged
robots like Anymal [73] or the Massachusetts Institute of Technology
(MIT) Cheetah [74]. Moreover, this same actuation
module was implemented in the aforementioned TriFinger
robot hand [24]. The other morphologies of quadrupeds that
offer a low-cost entry into legged locomotion research include
the Stanford Doggo [75] and Pupper and Woofer [76] robots.
The Doggo boasts performance metrics comparable to state-ofthe-art
quadrupeds, while keeping the total cost of the robot
below US$3,000. Using a similar architecture, the Woofer and
Pupper robots incorporate high power and hobby actuators,
respectively, to offer even more options of robust, low-cost
legged robot hardware to researchers in this field.
The popularity of Boston Dynamics' quadrupeds has also
spawned smaller-scale open source versions such as SpotMicro
[77], which in turn inspired OpenQuadruped [78], an inexpensive
legged robot with an accompaniment in the PyBullet
simulator and OpenAI gym environment to test learned gait
policies [79]. Another quadruped platform for legged locomotion
study, Oncilla [70] [see Figure 4(b)], enables simplified
control in rough-terrain locomotion via compliant, springloaded
pantograph legs. In addition to releasing blueprints of
the robot, a simulated model of Oncilla was also created in
Webots for users to test control strategies before executing
them on the hardware.
Some quadruped platforms have been developed with
a specific application in mind. The Metabot robot [80] was
designed as an educational robot platform, enabling educators
and learners to experience building a legged robot, and subsequently
program it to walk or dance. The Aracna robot [81]
was developed to promote research in evolutionary robotic
algorithms that can generate robot behaviours automatically.
To this end, Aracna deliberately embodies unconventional
kinematics that require nonintuitive motor commands for generating
a successful gait. Similarly, the Dynamic Robot for
Embodied Testing [82] robot was designed to test the evolution
of both control as well as morphology of the robot in the
real world, and not just in simulation. This four-legged robot
is able to modify and reconfigure its legs to adapt to different
tasks and environments.
Smaller meter- or centimeter-scale legged robots might be
even more suitable than their larger counterparts for an open
source release as they are typically fabricated with simple
materials and processes and consequently cost less to build.
The OpenRoACH [83] hexapedal platform can be built using
(a)
(b)
(c)
(d)
FIGURE 4. Legged-robot ORH projects. (a) The Open Dynamic Robot Initiative quadruped [69]. (b) The Oncilla quadruped [70].
(c) The HOPPY kit [71]. (d) The OSL prosthesis [72].
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IEEE Robotics & Automation Magazine - September 2023

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