The FSR (Force Sensing Resistor) was originally patented over 30 years ago and was a pioneering product in the sector that became known as "Printed Electronics". Designers and Engineers now had an alternate to traditional mechanical switching, and it came with a host of new features and benefits.
The basic principles of Resistive sensing have been applied to a wide variety of solutions since, including single point sensors, linear potentiometers, micro joysticks and early generation track-pads. Applications were and are wide ranging, from occupancy sensing in vehicle seats through to laptop computing, industrial controls and medical devices.
The widespread adoption of Capacitive touch sensing around a decade ago in smartphones, mobile computing and other applications should have consigned the FSR to legacy status. But in an ironic twist of fate driven by the demand for next generation tactile HMI (Human Machine Interface) solutions, Resistive touch sensors are now more relevant than ever.
Fast Forward to Today
Today, Resistive “force touch” is enabling Designers and Engineers to differentiate their products with new and intuitive tactile HMI solutions, simplifying User Interfaces (UI), improving User Experience (UX), and enhancing safety in many applications.
• Capacitive sensing isn't “true” touch. It’s essentially a "presence detection" not force sensing technology. So where it’s important to use variable force, something that's intuitive to every human, Capacitive isn't the solution of choice. It’s also power hungry, susceptible to interference from EMI and water, and struggles with simple real-world issues like a user wearing gloves.
• FSR technology offers significant benefits over mechanical switching. These include improved durability, reduced packaging space and weight, reduced tooling costs & enhanced styling themes, to name just a few.
• Tangio has invested heavily to improve the performance of the basic FSR technology, addressing the variability and environmental problems suffered by previous generations of the technology. And by creating new structures (like our matrix sensors) resistive sensing can deliver next-generation HMI possibilities like 3D multi-touch, not possible with Capacitive.
• Integration, customisation & and rapid time to market. Tangio's multi-decade experience in Printed Electronics enables customers to realise their touch sensing requirements quickly and affordably.
How it 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.