Medical Design Briefs - July 2023 - 17

TECHNOLOGY LEADERS Topic
Motors/Motion Control/Robotics
Linear Motion Design Considerations
for Medical Device OEMs
L
inear actuators - in particular,
electromechanical linear actuators
- have become integral components
of modern medical devices
because of their high precision, accuracy,
and ability to deliver repeatable motion
control. Patient comfort, positioning and
mobility, robotic surgery, imaging equipment,
infusion, and pumping are just a
few of the applications where the use of
linear actuators has revolutionized the
way medical devices are designed, improving
patient outcomes and enhancing
the overall quality of care.
Electric linear actuators offer significant
and game-changing advantages in
many medical device and equipment applications
over conventional manual or
fluid power driven solutions:
* They are more precise and provide
greater accuracy and control.
* In many cases they can offer a smaller
footprint to aid in creating more mobile,
more elegant design solutions.
* Electromechanical solutions are also
more reliable and less maintenance intensive,
reducing downtime and costs.
* They are quieter to support improved
patient experience.
* Finally, they can improve the efficiency
and speed of medical procedures,
reducing wait times and improving
patient flow.
For medical device and equipment designs
using electric linear actuators for
motion control, detailing exact requirements
for performance and reliability, as
well as environmental and human factors,
will help ensure successful outcomes.
Motion Design Considerations
In any new linear motion design exercise,
the first requirement set to be
evaluated typically, are the mass and
forces to be managed. Linear actuators
must meet the specific performance
requirements for each aspect of device
movement and often with multiple interactions,
including target accuracy,
motion precision, speed, force, and
linear travel.
Medical Design Briefs, July 2023
For example, in infusion pumps the speed
and force of the linear motion is needed to
ensure that system can provide the proper
the flow rate to reliably ensure the precise
and correct dosage is delivered to each patient.
In the case of surgical robots, the accuracy
and precision of the motion control system
determine the accuracy of the surgical
tool's placement, which is critical for achieving
surgical objectives while minimizing trauma
to surrounding tissue.
For design engineers, the use of application
worksheets and electromechanical
sizing software assists in identifying,
collecting, and calculating actuator specifications
that will perform successfully
in their application. It is critical that calculations
be performed correctly to deliver
the expected performance.
Rod-Style or Rodless Actuators
An application's requirements
will most often guide
the selection of either a rodstyle
or rodless actuator.
Rod style actuators are ideal
for situations where linear
thrust only is required, in applications
such as fluid compression,
dispensing, or injecting
procedures. Rod style
actuators offer limited stroke
length and loads, but support
higher forces designed for
pushing, pulling, or pressing
with force up to 50,000 lbf.
In contrast, rodless actuators
can support and carry loads,
eliminating the need, cost, and
design complexity for additional
load-bearing and guiding elements.
A rodless actuator's
stroke
lies completely within
the length of its body, resulting
in a smaller working footprint
and a more efficient design that
supports frequent cleaning, aesthetic
requirements, or even
decontamination processes.
The loads and forces carried
by a rodless cylinder makes it
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necessary to calculate the various moments
(torques) that are being placed
on the actuator's bearing system, based
on the position, size, and weight of the
load, including off-center or side loads.
Moment loading should also be calculated
for the Mx axis (roll) and the Mz axis
(yaw). The farther a load is from the center
of the load-carrying device, the larger
the resulting moment.
In addition, dynamic bending moments
are created by end-of-stroke acceleration
or deceleration. Some applications
contain compound moments
that involve two or more of the moments
described above. Each must be evaluated
and calculated to determine whether
the actuator can handle the combined
moment forces.
A rod-style electric actuator in this rehabilitation chair application
provides smooth, quiet vibration and controlled force to counter
muscle atrophy in recovering patients.
17
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Medical Design Briefs - July 2023

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