Medical Design Briefs - July 2021 - 16

TECHNOLOGY LEADERS Motors/Motion Control
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Table 1. Selection of the 20DBM screw lead.
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Table 2. Power requirements for the handheld medical
device.
* They are usually available as standard
with no development required.
* Stepper technology makes it easy to
divide and deliver liquids in subvolumes.
*
Their preloaded ball bearing assembly
ensures no axial play in the DLA,
which allows good repeatability.
To select a motor, the following
process is recommended. Note that the
example provided uses a 20DBM motor
supplied by Portescap (refer to the company's
specification sheet).
* Dimension: Remove solution with diameter
of greater than 20 mm.
* Stroke length: The traveled distance is 50
mm, so the minimum stroke length is
50 mm. The captive version can be
removed because the stroke length is
smaller than 50 mm.
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Table 3. Calculation of the linear speed and torque, depending on the lead screw.
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* Power: Determine whether the motor
can work at the required force.
Calculate the frequency necessary to
reach the targeted linear speed. The
frequency depends on the lead screw
pitch. Refer to Columns 3 and 5 in
Table 1. Consult the pull-in force
graph to select the screw pitch. Refer
to Column 4 in Table 1.
* Coil: Be sure to choose a coil that is
adapted to the power supply. A coil
with a low number of turns has a low
resistance and is appropriate for a
power supply type of high current, low
voltage. A coil with a high number of
turns has a high resistance, and it is
appropriate for a power supply type for
low current and high voltage.
For this scenario, the motor 20DBM-L
with the screw 10 would be a good
option. Coil selection, as mentioned
above, depends on the power supply.
Example 2: A Custom Motor Assembly
Application: Another engineering
team is developing a medical device with
the following requirements: The tool will
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be handled by a doctor, and it will be battery
powered for better ergonomics. The
engineer can only accommodate a maximum
diameter of 13 mm, and the tool
must be optimized for efficiency. Table 2
shows the typical power requirements.
Power calculation: Since the tool is
battery powered, coreless brush DC
motors are suitable for achieving high
efficiency. The power requested by the
motor can be estimated by assuming a
gearbox efficiency of 75 percent and a
lead screw efficiency of 50 percent,
which results in 1.87 W of power.
By calculating the estimated power, the
typical motor size can be identified. For this
example, a small diameter can do the job.
Conversion of linear speed and force
into relative speed and torque: As the
motor produces a rotational motion, the
linear speed must be converted into
rotation speed and the force into
torque. The conversion depends on the
lead screw, which is specified by its lead.
Physical relations: If the screw, mounted
on the shaft, rotates one round (2 π),
the nut moves linearly on a distance
equal to the lead as shown in Figure 5.
Consequently, this presents the following
relation, so linear speed can be converted
into rotation speed:
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Looking at the power relation, the
force-torque relationship can be deduced:
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The physical conversion formula can
be applied in the example. As the torque
and speed depend on the lead screw, we
can make the calculation with two different
leads in order to guide the lead
selection. Note: The lead dimension
impacts the lead screw efficiency since
efficiency is related to the friction of the
material and the screw angle. The previous
relations are now used in the example
(see Table 3). A smaller lead requires
higher speed and lower torque than the
larger lead. In general, the smaller lead
Medical Design Briefs, July 2021
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Medical Design Briefs - July 2021

Table of Contents for the Digital Edition of Medical Design Briefs - July 2021

Medical Design Briefs - July 2021 - Intro
Medical Design Briefs - July 2021 - Cov4
Medical Design Briefs - July 2021 - Cov1
Medical Design Briefs - July 2021 - Cov2
Medical Design Briefs - July 2021 - 1
Medical Design Briefs - July 2021 - 2
Medical Design Briefs - July 2021 - 3
Medical Design Briefs - July 2021 - 4
Medical Design Briefs - July 2021 - 5
Medical Design Briefs - July 2021 - 6
Medical Design Briefs - July 2021 - 7
Medical Design Briefs - July 2021 - 8
Medical Design Briefs - July 2021 - 9
Medical Design Briefs - July 2021 - 10
Medical Design Briefs - July 2021 - 11
Medical Design Briefs - July 2021 - 12
Medical Design Briefs - July 2021 - 13
Medical Design Briefs - July 2021 - 14
Medical Design Briefs - July 2021 - 15
Medical Design Briefs - July 2021 - 16
Medical Design Briefs - July 2021 - 17
Medical Design Briefs - July 2021 - 18
Medical Design Briefs - July 2021 - 19
Medical Design Briefs - July 2021 - 20
Medical Design Briefs - July 2021 - 21
Medical Design Briefs - July 2021 - 22
Medical Design Briefs - July 2021 - 23
Medical Design Briefs - July 2021 - 24
Medical Design Briefs - July 2021 - 25
Medical Design Briefs - July 2021 - 26
Medical Design Briefs - July 2021 - 27
Medical Design Briefs - July 2021 - 28
Medical Design Briefs - July 2021 - 29
Medical Design Briefs - July 2021 - 30
Medical Design Briefs - July 2021 - 31
Medical Design Briefs - July 2021 - 32
Medical Design Briefs - July 2021 - 33
Medical Design Briefs - July 2021 - 34
Medical Design Briefs - July 2021 - 35
Medical Design Briefs - July 2021 - 36
Medical Design Briefs - July 2021 - 37
Medical Design Briefs - July 2021 - 38
Medical Design Briefs - July 2021 - 39
Medical Design Briefs - July 2021 - 40
Medical Design Briefs - July 2021 - Cov3
Medical Design Briefs - July 2021 - Cov4a
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