IEEE Robotics & Automation Magazine - December 2017 - 68

mechanics are tuned and optimized to produce the highest
torque and gear ratio for the desired range of motion. In the
current application, each linear linkage actuator produces
approximately 110-Nm peak torque and no-load velocity of
9 rad/s about the point of rotation, with a 130° range of motion,
powered by an Allied Motion HS02303 motor with a continuous and peak current of 6 A and 20 A, respectively.
To house the actuators, each link of the device is a custommanufactured composite piece. The link structure is first
designed and 3D-printed as an ABS plastic mold. The mold is
used to maintain internal tolerances while layers of carbon
fiber weave and unidirectional cloth are laid over it. Epoxy
resin is then vacuum-infused into the cloth. Once the epoxy
has cured, the joint structures are demolded and post-processed to fit. This utilization of carbon fiber components
keeps the total exoskeleton mass, including the power electronics and backpack, to 34 kg.

Figure 2. An image of the Mina v2 exoskeleton, shown without
the backpack.

with our flexible trajectory design was critical to our team's
success in the 2016 Cybathlon.
Design of Mina v2
Designed as our entry to the 2016 Cybathlon, Mina v2 is the
latest exoskeleton developed by IHMC. This design drew on
our experience with the design and manufacture of Mina v1
[8], the NASA X1 exoskeleton [9], and the Hopper exercise
exoskeleton [14]. A complementary work describing the hardware in more detail is being prepared.
Mechanical Design
Mina v2 features a fully custom carbon-composite design.
The device includes six electric actuators, integrated into the
structure as load-bearing components, and a protective backpack for the electronics. Mina v2 functions as a prototype
device, designed and built to custom dimensions specifically
to fit our pilot. Future modifications will include adjustable
links to fit other pilots, the design of which were not feasible
within the time constraints of this project.
The actuators themselves are custom linear linkage actuators that are modular in construction to allow for ease of
replacement, accessibility, and repair. They were designed inhouse, specifically for use with Mina v2, and feature a frameless
electric motor, integrated electronics, and an onboard motor
amplifier and controller for distributed joint-level control. The
structure of the actuator uses a slider-crank linkage mechanism
driven by a linear ball-screw transmission. The slider-crank
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IEEE ROBOTICS & AUTOMATION MAGAZINE

DECEMBER 2017

Electrical Design
Each actuator is equipped with a magnetic incremental
encoder on the motor, a magnetic absolute encoder on the
output, and a load cell on the output of the linkage. The
motor is controlled by a Twitter Gold motor drive (Elmo
Motion Control, Petach-Tikva, Israel) capable of performing
position, velocity, and current control, depending on the
desired control mode. The motor drive is mounted on a custom carrier board that breaks out the connections for the
encoders and the load cell.
All of the other electrical components are housed in the
7.5-kg backpack. Central control is performed on an embedded
COM Express Type 6 computer (ADLINK Technology, Inc.,
New Taipei City, Taiwan) running a custom Ubuntu kernel.
The control code runs on a Java virtual machine using POSIX
real-time threads using a custom library. The embedded computer communicates with the motor drivers over EtherCAT.
The EtherCAT line is split into two separate lines by an
Omron EtherCAT junction, allowing for more efficient wiring of the legs. This Omron junction also allows the connection of other EtherCAT-enabled sensors, such as an inertial
measurement unit.
Mina v2 is powered by a 48-V, 480-Wh lithium ion battery (designed for electric bicycles) and is capable of approximately 2.5 h of fully powered autonomous runtime. The
battery has an onboard battery-management system to protect from current overdraw and under-voltage conditions. A
wireless emergency stop and secondary wired emergency
stop are included to interrupt motor power in case of an
error. Watchdog timers monitor communications with the
motor controllers and can disable the controller if needed.
Trajectory Design
Mina v2 was designed to explore the effects of including powered ankle plantar flexion on an orthotic robotic exoskeleton.
As such, the trajectories are designed around the use of this
additional degree of freedom. The powered ankle plantar
flexion allows the trailing leg to apply greater forces to the

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - December 2017