Potentials - January/February 2017 - 39

too difficult to control and prone to
overshoot, and too much makes it feel
sluggish and appear that it needs to be
pushed. The STRONGARM will only
permit movement of the patients in
the vertical direction above the seated
surface until the system weighs the
mass of the patients for manifold control (see the "Manifold Control of the
STRONGARM" section) as a further
safety measure. The mass of the
patients is measured each time that a
transfer is conducted, and results
must remain within 10% to avoid
recalibration.

FIg5 the handle for direct interaction.

Manifold control of
the STRONGARM
One of the notable benefits of using a
robot to assist with transfers is that
its internal machine intelligence can
help to simplify transfers and ensure
safe operation. A stability model of
the prototype STRONGARM was created (Wang, 2014). The STRONGARM
uses manifold control to ensure that
it does not violate the space of the
EPW occupant and that it remains
stable during the entire transfer (Fig. 6).
The control algorithm has three distinct features to allow for safe operation: first, there is a keep-out zone
that defines the user's personal space
within which the STRONGARM may
not enter; second, the range of
motion of the STRONGARM is limited
to remain within the systems quasistatic stability limits throughout the
transfer process; and, third, the
STRONGARM will not move unless a
clinician/caregiver is holding the
handle and directs the movement
(i.e., trained human in the loop). The
keep-out zone is defined by each
patient and clinician/caregiver pair
and then programmed by one of our
engineering team members. In a
future version, a therapist and manufacturer representative would perform this function. The keep-out zone
should only need to be programmed
once, unless the EPW user's bodymass changes by more than 10%, at
which time a prompt is provided to
recalibrate the STRONGARM. The
range of motion is restricted to
ensure stability at all times based on
a quasi-static model of the system on

Survivor Rescue
Dummy

HERL-RTD

Access Ramp
Force Plate for Wheelchair
and HERL-RTD
Force Plate
for Assistant

Force Plate for Target
Surface

FIg6 the transfer evaluation force-plate set-up for developing and evaluating constraint manifolds to prevent tipping. the upper left corner shows path planning with
constraint manifold.

a slope with cross-slope. When a
transfer is initiated, the STRONGARM restricts movement with the
patient connected to the vertical
direction to measure the weight of
the patient. The STRONGARM halts
movement for 10 s once the person is
lifted from the seat to measure the
user's weight using the load-cell
(AMTI, Omega) in the base. Inclinometers (US Digital) are used to calculate slope and cross-slope. Based
upon the user's weight, wheelchair
parameters, slope, and cross-slope,
our quasi-static model is used to calculate the safe range of motion. The
STRONGARM moves slowly, and a
factor of safety of 1.25 is used to
maintain dynamic stability.

Design review and evaluation
RE2 Inc. partnered with HERL and
received a small business federal
grant to further commercialize the
STRONGARM. Working with RE2, the
workspace of the STRONGARM has
been maintained while reducing the
production cost and complexity and
improving the reliability. Weight of
the system has been reduced from
70 lb to approximately 45 lb, and the
expected production cost has been
reduced from US$10,000-$12,000
down to around US$5,000.

Focus group testing
Multiple focus groups were conducted
with end users, clinicians, and caregivers. In all, 20 wheelchair users

IEEE PotEntIals

January/February 2017

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Table of Contents for the Digital Edition of Potentials - January/February 2017