IEEE Robotics & Automation Magazine - September 2018 - 26

important components and providing references to product
part numbers.
Many animals, including humans, have walking and run-
ning gaits that can be described by springy, mathematically
simple legs. A common spring-mass model is the spring-
loaded inverted pendulum (SLIP) model, with a point-mass
body, a massless point toe, and a massless linear spring con-
necting the two. When in contact with the ground, the toe is
assumed to be in perfect contact and completely fixed. When
leg forces drop to zero, the toe is no longer fixed to the ground
and moves rigidly with the point mass. This model is com-
pletely energy conservative, because the toe is massless and
there is a massless spring between the toe contact and the
point mass; it can walk or run continuously as long as the aver-
age ground height is consistent.
Observers can see the influence of spring-mass models
in the ATRIAS design: carbon-fiber legs for minimum iner-
tia connected by series springs to the concentrated mass at
the hips. Such construction gives the biped noticeably SLIP-
like dynamics (Figure 3). With series compliance, unfore-
seen effects are softened, and energy can be recycled from
step to step and released at higher rates than the motor
alone can deliver. These combined factors have the poten-
tial to improve the robustness of the mechanism as well as
energy efficiency.
Kinematically, ATRIAS has two planar legs comprising a
parallel mechanism, two actuators colocated at the hip, and a
distal toe. Each leg has an abduction degree of freedom
(DoF), with both sharing an axis in the sagittal plane of the

B

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Pla

A

Kinematics

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(a)

B
A

(b)

Figure 3. (a) A rendered view of the ATRIAS leg design. (b) A
schematic view showing the kinematics and compliant behavior of
the toe. The basis vectors of motors A and B are drawn at the toe
to illustrate the kinematics of this pose. The instantaneous motion
of the toe is the weighted sum of these basis vectors. Similarly, a
compliance ellipse shows the elastic behavior of the physical toe
around the neutral point [25]; the ellipse represents the deflection
of the toe for a unit force in all directions, so the major axis is soft
and the minor axis is stiff. (Figure courtesy of Andy Abate.)

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september 2018

torso. Six total actuators exist on the robot: two legs, each with
hip extension, knee extension, and hip abduction. (Note that
ATRIAS has 13 DoF and thus is heavily underactuated for a
bipedal robot.) The robot lacks long-axis rotation of the hip
and thus cannot actively turn.
A passive foot is attached at the ankle in a way that simu-
lates a point contact at the ground but restricts yaw rotation,
thereby removing this DoF from the dynamics of the robot.
The two-point line contacts at the bottom of the feet keep
ATRIAS pointed in roughly the same direction between steps
by providing frictional contact with the ground [23].
Mechanical fuses at the knees protect the robot from dam-
age due to excessive sideloads at the toe. This resistance to sig-
nificant damage makes rapid iteration and testing possible.
Expensive and difficult repairs to bearings, the transmission,
and the carbon-fiber legs would halt progress, but fuses are
easy to reattach.
Because ATRIAS is a prototype experimental platform, it is
fairly fragile and cannot withstand torso collisions or falls. A por-
table gantry system protects the robot from falls via a safety
line. During operation, the line is kept slack so as to not affect
the robot's dynamics unless it drops or goes wildly off course.
ATRIAS is otherwise entirely self-contained, and this connection
is meant to catch the robot only in the event of a malfunction.
Electrical/Software Overview
ATRIAS's electrical architecture is built around commodity
personal computer (PC) hardware, custom sensor interface
boards, and off-the-shelf motor drivers. An EtherCAT data
bus provides high-throughput, offers real-time communica-
tion between the system components, and interfaces directly
with the Simulink real-time operating system. Commercial
off-the-shelf (COTS) lithium polymer batteries power the
motor drivers as well as the computer and other components
through COTS dc-dc voltage regulation modules. Figure 4
shows the major components of the electrical system.
All control processing is done with an onboard miniature
desktop computer. This device is a commercially available
small-form-factor PC based on a modern Intel desktop pro-
cessor. The robot computer executes our control software,
developed in MATLAB and Simulink, on top of the Simulink
Real-Time kernel. The Simulink kernel ships with drivers for
using the EtherCAT protocol with standard Ethernet chipsets,
which are used to retrieve sensor data and send torque com-
mands to the motor drivers.
ATRIAS's six motors are driven with two different types
of motor amplifier. The hip extension and knee extension
motors use EtherCAT-enabled COTS servo drivers capable
of supplying a peak current of 200 A. The hip abduction
motors are driven by smaller COTS motor drivers able to
deliver a peak current of 60 A. All of these drive three-
phase brushless motors in current-control mode, using
Hall effect sensors and an incremental encoder for sinusoi-
dal commutation.
ATRIAS uses only proprioceptive sensing for control and
is otherwise blind to the environment. High-resolution
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