How FSR Works
Force-sensing resistors are an evolution of membrane switch technology, based on similar design principals and manufacturing techniques. The force sensor is essentially an analog, multi-position switch, while the membrane switch is simply ON/OFF. What defines a force-sensing resistor is its unique characteristic of dynamic conductance / resistance relative to the amount of pressure applied to the device. In general, the more pressure applied to the surface of the sensor, the greater the conductance / the lower the resistance. Force-sensing resistors are used for qualitative rather than quantitative or precision measurements.
Shunt Mode force sensors are the most frequently used design. They are constructed in two layers of flexible polymer. One layer is printed with a proprietary force-sensing ink and the other with conductive ink printed with interdigitating electrodes. The two substrates are then positioned facing each other and adhered using a spacer adhesive in the middle. The inter-digitating electrodes can be designed in various configurations depending on the application including, but not limited to: various patterns to influence sensitivity, various conductors including silver, copper, carbon and blends, or on a printed circuit board (PCB).
When a force is applied to the device, the shunt or shorting circuit is complete. The more force applied, the more conductive the output.
Thru Mode Force-sensing resistors are constructed with two layers of substrates, typically a polyester film. Conductive pads, which can be made of silver, or a silver / graphite blend ink, are printed on each of the two substrates. Proprietary force-sensing ink is printed on top of the lower conductive pad. These two printed substrates are then placed facing each other. Adhesive is used to laminate the two substrate layers together to form the sensor.
The more force applied to the Thru Mode device, the more conductive the output.
A typical Force vs. Resistance response (FR curve) is logarithmic and has three principal elements.
1. Break force - This is the force required for the two membrane layers to contact. In this region the resistance is infinite until enough force is applied to make contact.
2. Area Effect - In this region the size of the contact area between the two layers is increasing with increasing force and thereby reducing the resistance.
3. Surface Effect - In this region as the force is increased the contact of the two ink surfaces increases at a microscopic level.