User Guide for FSR Sensor

FSR Integration Guide and Evaluation Parts Catalog Page 5
with Suggested Electrical Interfaces
Force Sensing Resistors
An Overview of the Technology
Force Sensing Resistors (FSR) are a
polymer thick film (PTF) device which
exhibits a decrease in resistance with an
increase in the force applied to the
active surface. Its force sensitivity is
optimized for use in human touch
control of electronic devices. FSRs are
not a load cell or strain gauge, though
they have similar properties. FSRs are
not suitable for precision measurements.
Force vs. Resistance
The force vs. resistance characteristic
shown in Figure 2 provides an overview
of FSR typical response behavior. For
interpretational convenience, the force
vs. resistance data is plotted on a log/log
format. These data are representative of
our typical devices, with this particular
force-resistance characteristic being the response of evaluation part # 402 (0.5” [12.7 mm] diameter circular
active area). A stainless steel actuator with a 0.4” [10.0 mm] diameter hemispherical tip of 60 durometer
polyurethane rubber was used to actuate the FSR device. In general, FSR response approximately follows an
inverse power-law characteristic (roughly 1/R).
Referring to Figure 2, at the low force end of
the force-resistance characteristic, a switch-
like response is evident. This turn-on
threshold, or ‘break force”, that swings the
resistance from greater than 100 kΩ to about
10 kΩ (the beginning of the dynamic range
that follows a power-law) is determined by
the substrate and overlay thickness and
flexibility, size and shape of the actuator, and
spacer-adhesive thickness (the gap between
the facing conductive elements). Break force
increases with increasing substrate and
overlay rigidity, actuator size, and spacer-
adhesive thickness. Eliminating the adhesive,
or keeping it well away from the area where
the force is being applied, such as the center
of a large FSR device, will give it a lower rest
resistance (e.g. stand-off resistance).
Figure 1: FSR Construction
Figure 2: Resistance vs. Force