Medical Design Briefs - July 2021 - 15

ing accuracy and motion repeatability.
The lead screw extends and retracts during
motion, and it can return to the
same starting position.
When considering the right linear
motion system, be sure to consider some
of the options generally available as standard
for an optimized transmission system.
They include:
* Stroke length.
* Lead screw pitches, which typically
involve two or three choices or references.
*
A bipolar or unipolar coil.
* Coil rated voltage.
* Rated current.
* Captive lead screw - antirotation integrated
- or noncaptive lead screws,
shown in Figure 4.
* Various thread tips - either metric or
imperial.
Linear actuators can be a very costeffective
option to deliver high linear
force and reliability for a device.
Option 2: Custom Motor Assembly
For applications requiring high performance
in a limited package, consider
a custom assembly. Most custom assemblies
are built with either a brush DC,
brushless DC, or stepper disc magnet
motor. Each technology has its own benefits
and advantages over can stack steppers.
For example, high-acceleration
applications are well-suited for a low
inertia motor such as a disc magnet stepper.
For higher power in a small package,
the best option may be a combination
brushless DC, gearbox, and lead
screw. And, for high efficiency, a coreless
brush DC motor can be especially desirable
for battery-powered applications.
Some accessories can also be mounted
on the motors, such as an encoder for
high-resolution positioning feedback.
A custom assembly also offers flexibility
when selecting the lead screw. The
R&D team can choose whether a ball
screw or regular lead screw is preferable,
suggest different pitches, adapt the material,
or even optimize the dimensions.
To design a motorized assembly, be sure
to understand both the power required
from the application and the power generated
at the motor level. There are some
physical relationships to keep in mind
when converting the desired output force
and linear speed into the required input
Medical Design Briefs, July 2021
Intro
Cov
Special ball bearing
assembly with preload to
reduce axial play
Permanent magnet,
Radially magnetized
Integrated Lead
Screw (sketch here
is captive)
Coil
Fig. 3 - Linear transmission system with an overmolded rotor.
Overmolded
rotor
Fig. 4 - Captive versus non-captive lead screws.
p
Motor
d
Fig. 5 - Motor and lead screw.
torque, as well as the rotational speed.
Here are some scenarios and solutions for
optimizing an assembly to achieve the
application's output requirements:
Example 1: Digital Linear Actuator
Application: A team is developing a laboratory
medical device that moves a tiny
amount of liquid in test tubes. One motor
controls a multi-pipette channel. The
motor package is limited to a maximum
diameter of 20 mm. The pipette must have
good repeatability and accuracy to consistently
deliver the same amount of liquid
with each operation. The working process
can be divided into two main steps:
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ToC
+
-
A
µ
* Step 1: Fill the pipettes in one step in
less than 4 seconds.
✧ Traveled distance of the pipette: 50
mm in 4 seconds. Speed = 12.5 mm
per second. Force for a viscous liquid:
20 N.
* Step 2: Empty the pipettes. The pipette
content is divided into tiny amounts
for several test tubes.
✧ Traveled distance: The pipette must
be able to divide the volume into 30
substeps - specifically, 50 mm divided
by 30 = 1.6 mm. Force: 15 N.
Solution: Digital linear actuators are
typical well-suited for this type of device
because:
15
m
v
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Medical Design Briefs - July 2021

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