Tech Briefs Magazine - June 2021 - ST-13

TECHNOLOGY LEADERS Sensors
and converts it into a digital signal. The
software displays real-time activity across
the sensing area. This allows the user to
track the center of force, locate regions
of peak pressure, and identify exact
moments of pressure changes in a
frame-by-frame recording.
Additional features of matrix sensors
include:
* Sensors typically have about 2000 sensing
points, but some sensors can be
designed with over 16,000 .
* Sensing element spacing (pitch) can
as narrow as 0.64 mm (0.025 in.).
* Sensors can measure pressure ranges
up to 25,000 psi (172 MPa).
* High-temperature sensor options up
to 200 °C (400 °F).
* Scanning speeds available up to 20,000
Hz.
Single-Point Sensors for Embedded
Sensing Applications
Single-point piezoresistive force sensors
measure force feedback on a single
sensing area. This sensor type lends
itself well to integration within a product
or device not only because it is thin
and flexible, but because it can function
as a component of an op-amp circuit or
voltage divider. Depending on the
setup, the force range of the sensor can
be adjusted by changing drive voltage
and resistance of the feedback resistor.
This allows the user to have control over
parameters such as maximum force
range, and measurement resolution
within that range.
Since piezoresistive sensors are passive
components with linear conductance
response and have a wide dynamic
range of resistance, engineers integrating
them can use simple electronics
that do not require a lot of filtering.
An important benefit of this linearity is
that piezoresistive sensors only require
simple calibration. Force-sensing applications
that use load cells or strain gauges,
may need to be factory calibrated after
repeated use, while devices with piezoresistive
sensors can have their calibration
routine embedded into the device
firmware for on-the-fly recalibration.
Because of the flexibility of this technology,
along with its ability to function
with low-power electronics, piezoresistive
force sensors have been successful in
enhancing many different types of
Sensor Technology, June 2021
Cov
Figure 3. Example of a five-hour charge/discharge cycle test. The top two images show the battery placed in the
fixture, while the screenshot shows the pressure output. (Image courtesy of Tekscan)
www.techbriefs.com
ToC
13
RFEEDBACK
C1 = 47 pF
= 100kΩ
POTENTIOMETER
C1
Figure 2. Most piezoresistive sensors can be powered with simple circuitry, such as this inverted op-amp (dual
source) circuit. (Image courtesy of Tekscan)
devices, without adding complexity to
the design or difficulty for the user.
A Novel Method to Validate Li-ion
Battery Stack Stress
Although, battery manufacturers prioritize
maintaining constant battery
VREF
Options
Square Wave
Up to 5v, 50%
Max Duty Cycle
DC
0.25V - 1.25V
MCP6004
VREF
RSENSOR
V SS = Ground
RFEEDBACK
+
-
V OUT
stack construction during manufacturing,
many do not validate stack pressure
as part of their quality procedure, nor
do they quantify internal stresses on the
battery as it is charged or discharged.
However, research has found that
high stack pressures can have a strong
V DD = VSUPPLY

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Tech Briefs Magazine - June 2021

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