Interlink Electronics FSR 400 Series - Manual (Page 4)

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2
FSR
®
Integration Guide
2.0 Theory of Operation
The most basic FSR consists of two membranes separated by a thin air gap. The air gap
is maintained by a spacer around the edges and by the rigidity of the two membranes.
One of the membranes has two sets of interdigitated fingers that are electrically distinct,
with each set connecting to one trace on a tail. The other membrane is coated with FSR
ink. When pressed, the FSR ink shorts the two traces together with a resistance that
depends on applied force.
Figure 1: Basic FSR Construction
2.1 Basic Construction
Around the perimeter of the sensor is a spacer adhesive that serves both to separate the
two substrates and hold the sensor together. This spacer typically has a thickness
between 0.03mm and 0.15mm. This spacer may be screen printed of a pressure
sensitive adhesive, may be cut from a film pressure sensitive adhesive, or may be built
up using any combination of materials that can both separate and adhere to the two
substrates.
Both membranes are typically formed on flexible polymer sheets such as PET, polyimide,
or any other film material. In custom force sensors, the top substrate could be made with
a slightly less flexible material, such as polycarbonate, thin metal or very thin circuit board
material, as long as it is sufficiently deformable to allow a reasonable force to push the
top substrate against the bottom substrate to activate the sensor.
The inside surface of one substrate is coated with FSR® carbon-based ink. When the
two substrates are pressed together the FSR ink surface shorts across the interdigitated
fingers of the facing surface. The result is that the resistance between the conducting
fingers is inversely proportional to the applied force.