Tech Briefs Magazine - June 2021 - ST-15

tion data the team acquired during the
design of the battery itself, they were
able to determine a relative force
threshold signifying the battery was
approaching failure. They then developed
a feature that would trigger an
alert to the user's screen before the
battery pressure reaches a critical
threshold.
Accounting for Sensor Drift in the
Relative Measurement Application
Because sensor drift will cause the
piezoresistive sensor's output voltage to
change gradually over time, using
absolute voltage outputs to determine
battery swelling becomes a challenge.
However, this battery safety application
only required measuring relative
changes in force, which is not affected
by drift since the slope of the Voltage vs
Force curve stays relatively constant
regardless of how much the output has
changed.
When the sensor is powered with the
circuit shown in Figure 2, our piezoresistive
force sensors typically have an
output drift rate <5% / logarithmic
time. Therefore, for relative measurement
applications, the design engineer
should look for the differential
voltage output as a function of force
(the slope of the V vs F curve) as
shown in Figure 5.
On the other hand, for an application
requiring measuring an absolute measure
of force to produce some sort of an
action (e.g., an actuator pressing on the
sensor at exactly 5 lbs would cause " X "
response, while 10 lbs would produce
" Y " response), then the engineer would
need to follow a different calibration
procedure.
Measurement Tools and Embedded
Components Help You Understand the
Full Scope of your Battery Design
Decisions
Satisfying consumer demands while
maintaining a safe, repeatable design, is
a difficult balance for battery designers.
Temperature increase during charging
causes lithiation, which leads to gassing.
Internal pressures from the additional
gasses in the battery can cause housing
or separator failures. This can lead to
thermal runaway reactions in extreme
cases.
Sensor Technology, June 2021
Cov
ToC
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Ultra-thin piezoresistive sensing
technology, whether as a test and
measurement tool in the design
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in the final product, helps measure
and identify regions of excessive pressure
that can signal complications or
potential battery failures. This, in
turn, helps battery designers develop
advanced energy technologies to
power our lives safely.
This article was written by Edward
Haidar, Applications Engineer & Embedded
Sensing Product Manager, Tekscan, Inc.
(South Boston, MA). For more information,
contact Mr. Haidar at EHaidar@
Tekscan.com or visit http://info.hotims.com/
79414-161.
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15
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Tech Briefs Magazine - June 2021

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