Medical Design Briefs - July 2022 - 22

TECHNOLOGY LEADERS Motors/Motion Control
Fig. 3 - maxon EC 60 Flat motor provides high torque
in a compact package.
Fig. 2 - maxon's ESCON controller provided a compact and powerful addition to the Harmony SHR exoskeleton.
Actuators used in the Harmony SHR
exoskeleton are either rotary actuators
with torque and angle sensing output
used to assist with patient motion, or
linear actuators with position feedback
used to resize the robot to fit a patient.
The exoskeleton is designed to provide
objective assessment of a patient's
movement capabilities. The system incorporates
more than 80 sensors that
record measurements 2,000 times per
second, allowing it to precisely measure
multiple parameters, including
range of motion, force generation, and
number of repetitions.
The motion system uses several motors
designed and manufactured by maxon,
which supplied motors to address a wide
variety of unique needs of the system. For
example, EC Flat motors provided the
high torque output needed in a compact
package that fit the exoskeleton's compact
size (see Figure 3). The flat motors could
also be supplied with angle sensors.
An EC 60 Flat motor was selected because
the outer diameter was similar to
that of the harmonic drive and because
this motor is axially compact. The shaft
of a maxon EC 60 DC Flat motor is attached
to a wave generator of a size 17
harmonic drive, which was selected for
its compact axial size and high torque
22
capacity. Harmonic drive gear reducers
offered the high reductions, low backlash,
and high torque capacity the company
needed for the design.
The stator of the motor is mounted to
the circular spline of the harmonic drive.
The flex-spline of the harmonic drive is
attached to an output shaft supported by
a crossed roller bearing, which supports
all loads that are not the torque output
of the actuator. Because the crossed roller
bearing supports the other loads, the
only load that acts on the load sensor is
the torque output of the load sensor.
The motor, output shaft, and harmonic
drive assembly are free to rotate in the
crossed roller bearing. A bracket is attached
to the circular spline of the harmonic
drive and to the stator of the motor.
A load sensor is attached to the
bracket so that output torque of the
overall assembly can be measured
through the load sensor.
The alignment of the harmonic drive
to the shaft of the motor is critical to the
life of the actuator. Even a minor misalignment
can cause premature wear
or failure of the mechanism. Therefore,
the machined components are manufactured
to be parallel to within about 20
µm while concentricity is maintained
within about 30 µm to align the circular
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spline to the motor. The overall footprint
of the actuators inside the robot
is approximately 80 mm in diameter
and 90 mm in length. The machined
components that go into each actuator
are made from various fatigue-resistant
aluminum alloys, and grade 5 titanium
where stresses are high.
The output torque of the overall actuator
is approximately 30 Nm, and the actuator
can output over 100 Nm peak. The actuator
is attached to an extremely sensitive
load cell, which has a resolution of less
than 10 mNm. Each actuator is designed
to have a life of over 3 million revolutions.
Conclusion
The actuators used in the Harmony
SHR are of a mature design and are expected
to need few improvements in the
future. According to the company, it is
unlikely that they will be changing any of
the COTS components during future revisions,
particularly the maxon components.
The maxon DC motors were highly
configurable, and low backlash versions
of gear reducers were used in the rotary
actuators. In addition, small diameter
brushed dc motor and planetary gearbox
combinations were used to create compact
rotary actuators. The leadscrew gear
reducers were also available to use in linear
actuator applications. The goal
achieved was to provide an intelligent,
data-driven treatment therapy for dealing
with neurological and musculoskeletal
movement impairments.
This article was written by Biren Patel,
Sales Engineer, maxon Group, Taunton, MA.
For more information, visit http://info.
hotims.com/82323-343.
Medical Design Briefs, July 2022
http://info.hotims.com/82323-343 http://www.medicaldesignbriefs.com

Medical Design Briefs - July 2022

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