IEEE Robotics & Automation Magazine - December 2017 - 70

motion is combined with significant toe-off push, so that
the center of mass will begin to be raised during transfer,
when both legs are on the ground. Including this motion
greatly decreases the required torque at the upcoming support knee. This, in turn, results in lessening the pilot's effort.
Similar to our toe-off motion during flat walking, an additional fast toe-off motion at the end of the transfer phase
adds an impulse to the system, as shown in Figure 5(c).
Instead of descending the stairs in the traditional forward
manner, we chose to descend backwards, similar to how a person
descends a ladder. We believed that this would provide greater
stability and control to our pilot, making it much less likely for
him to fall down the stairs. We followed the same approach as for
ascension, using only one foot for each step, without, however,
including toe-off; as the pilot is descending, this is unnecessary.
This approach is highlighted in Figure 5 (f)-(i).
Design of Ramp Trajectory
Similar to the other tasks, the trajectories we designed for
ascending and descending a ramp revolve around the use of
powered ankle plantar flexion. For ascending, we use the
same approach as in flat walking, except that the hips are only
slightly in front of the trailing foot at the start of transfer.
For descending, we similarly place the hips over or only
slightly in front of the trailing foot. Also, there is no additional
fast toe-off motion at the end of transfer when descending.
This approach is illustrated in Figure 6.
Pilot Interface
The human-machine interface for Mina v2 is based on manual
input by the pilot. It consists of a Raspberry Pi 3 computer with a

(b)

(a)

(f)

screen, a thumb joystick, and a momentary switch, all of which
are mounted on the right crutch. The joystick is integrated in the
front-facing part of the handle, and the switch is mounted as a
trigger on the bottom of the handle. The human-machine interface runs off a separate battery pack and communicates with the
embedded computer over transmission control protocol/Internet protocol (TCP/IP). The connection can be made using
WiFi, making the whole crutch wireless. However, during the
competition, a wired connection was used to avoid interference.
The joystick is employed to change behaviors; the legality
of these behavior changes was tailored to the competition.
The trigger button acts as a play cue and will initiate movement; every step is triggered separately. This gives the pilot
the ability to synchronize his weight shift and the start of a
step himself but also allows for a brief recovery after an unbalanced step. The next step can be triggered 0.25 s before the
end of the current step, allowing the pilot a continuous walking motion if desired.
Results and Discussion
In preparation for the Cybathlon, extensive training was undertaken so that our pilot could complete tasks in as little time as
possible. Prior to this event, our pilot had approximately 20 h of
experience in previous exoskeletons over the course of six years.
Our pilot was confident enough with the walking gait that, after
the competition, he used Mina v2 to navigate the streets of
Zürich, as shown in Figure 1. For the competition, the low-level motor controllers were set to track desired positions, but also
had the capability of controlling the joint impedance, which is
being investigated for future work as we explore the dynamic
effects of our toe-off motion in greater depth.

(c)

(g)

(d)

(h)

Figure 5. An illustration of stair ascent (a)-(e) and descent (f)-(i) with powered ankles.

IEEE ROBOTICS & AUTOMATION MAGAZINE

DECEMBER 2017

(e)

(i)

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