IEEE Robotics & Automation Magazine - March 2020 - 57

Torque Generator Box
Torque generator boxes are the passive actuation units that
store and transform the elastic energy of a set of two parallel
springs into assistive action. The compensation system comprises two connected gears, of which one is coupled with the
shoulder joint and the other is not concentrically bonded to
the set of springs. The described mechanism is arranged in
such a way that, when the arm lays parallel to the trunk, the
spring force reaches its maximum value, and the moment
arm is null with respect to the spring gear axis, thus generating null torque. As the arm elevation increases, the elastic
force decreases but the moment arm increases: this mechanism generates a resulting torque that consistently varies
with the gravity torque profile of the arm in a physiological
shoulder elevation. Hence, the maximum assistive torque
value is reached when the arm's elevation angle is at 90°, and
it gradually goes to zero when the arm is relaxed. The protoMATE allows tuning the assistive torque over four discrete
levels [Figure 2(b)].
pHMI
The pHMI consists of the back-support structure and all
parts that connect the device with the user's body, such as the
shoulder straps, the arm cuffs, and the belt. The back-support structure is an aluminum frame molded in a T shape
and is designed to distribute the reaction forces produced by
the torque generator boxes to the user's pelvic area through a
custom belt.
Kinematic Chain of pDOFs
A kinematic pDOF chain was designed to enable selfalignment of the device's joint axes to the human joint
axes, minimizing the transfer of undesired forces to the
user's musculoskeletal system. The entire pDOF chain
enables the system to freely abduct/adduct the arm and
smoothly counteract the passive medial-lateral and
antero-posterior (AP) translational movements of the
glenohumeral joint that occur when abducting or elevating the arm [10], [11]. The passive mechanical
chain is symmetrical to the sagittal plane: on each side,
a horizontal slider is connected in series to two rotational joints with mutually perpendicular axes. The
first joint should be aligned with the shoulder abduction-adduction axis, while the second has a vertical
axis that is not coupled with any specific biological axis
and whose action, combined with the horizontal slider,
accounts for the passive AP movement of the glenohumeral joint. An additional passive slider is placed
between the active box and the arm cuff, to absorb the
undesired translational longitudinal reaction forces at
the arm-cuff interface.
Size Regulations
To fulfill the ergonomic requirements for safe and effective
human-machine fitting and interaction, additional size regulations have been designed to adjust the height of the back

frame and the width of the device, tune the orientation of the
actuation boxes on the frontal plane, and adjust the length of
the shoulder straps. Additional adjustments permit tightening the pelvic belt and adjusting the shoulder straps each
time the exoskeleton is worn.
Experimental Session
Participants
The experiments took place at the Villa Beretta Rehabilitation
Center, Valduce Hospital, in Lecco, Italy. Fifteen healthy subjects (of which 11 were
men) were recruited for
this study (32 ± 9 years,
Torque generator boxes are
72 ± 12 kg, 176 ± 6 cm).
All participants had no
the passive actuation units
prior experience with the
exoskeleton. Subjects who
that store and transform
had skin wounds in areas
on which the device is
the elastic energy of a set
applied or had a height
outside the required range
of two parallel springs into
(160-195 cm) were not enrolled in this study.
assistive action.
Experiments were
conducted in accordance
with the Declaration of
Helsinki, and the protocol was approved by the Institutional
Ethics Committee of Insubria (study 154 of 2017). Subjects
were informed about the study's general purpose and signed a
written informed consent.
Data Recordings
Muscular activity was monitored using the Free EMG
1000 from BTS Bioengineering. Surface EMG electrodes
(silver-silver chloride electrodes, Ambu WhiteSensor,
36 mm) were placed over the shoulder muscles following the Surface EMG for the Non-Invasive Assessment
of Muscles recommendations [12]. Electrodes were
placed unilaterally on the right limb and on the following muscles: anterior deltoid, medial deltoid, posterior
deltoid, pectoralis major, latissimus dorsi, biceps brachii
long head, triceps brachii long head, and trapezius
ascendens. Electrodes were secured to the user's skin
with medical tape, and signals were checked by the BTS
Free EMG graphical user interface to verify the consistency of their placement.
Shoulder and elbow kinematics were monitored using an
eight-camera optical motion capture system (BTS Smart DX
7000). Eight reflective markers were placed on suitable right
and left upper-body landmarks: the processus spinosus of
the seventh cervical vertebra, the processus spinosus of the
fifth dorsal vertebra, the acromion for the shoulder, the
most caudal point on the lateral epicondyle for the elbow,
and the most caudal-lateral point on the radial styloid for
the wrist.
MARCH 2020

IEEE ROBOTICS & AUTOMATION MAGAZINE

IEEE Robotics & Automation Magazine - March 2020

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