Did You Know? How Force-Sensing Resistors Work

Aug 21, 2015

8435308101_69d0681cd1_zForce-sensing resistors live up to their namesake by detecting changes in applied force. They are commonly used in pressure-sensing keypads and buttons, as well as musical instruments, portable video gaming devices, car occupancy sensors, etc. When pressure is applied to the resistor, its resistance changes; thus, allowing different functions and commands based on the amount of force that’s being applied.

While the manufacturing specifications varies between the different types of force-sensing resistors, most are comprised of a conductive polymer that changes in resistance (to a fixed state) when pressure is applied to the surface. The force-sensing resistors are generally applied as a polymer ink, at which point screen printing is used during the device’s creation.

According to Wikipedia, the particles used in a force-sensing resistor are “sub-micrometre” in size, and they are “formulated to reduce the temperature dependence, improve mechanical properties, and increase surface durability.” When the operator touches the surface of a device featuring force-sensing resistors, these sub-micrometre-sized particles touch the nearby conducting electrodes, consequently causing a change in the film’s electrical resistance. Like all resistors, force-sensing resistors are capable of operating in vast range of different environments without any negative impact on their functionality.

Contrary to what some people may believe, force-sensing resistors aren’t a new invention, as they’ve been around for several decades now. Electrical engineer Franklin Eventoff is credited with pioneering the technology back in 1977, during which he received a patent for force-sensing resistors from the U.S. Patent and Trademark Office (PTO). Eventoff later founded a company to manufacture his new invention, with Interlink Electronics being the first company to sell force-sensing resistors. Fast forward to 1987 and Eventoff received the international IR 100 award for inventing the force-sensing resistor.

You might be wondering what (if any) benefits there are to using force-sensing resistors over standard resistors. Well, the most obvious benefit is the ability to create custom applications that change based on the amount of force applied to the surface. Rather than having a single command for each button, there can be several commands. A light touch may trigger one command; a medium touch triggers another; and a heavy touch triggers a third command.  Furthermore, force-sensing resistors are usually less than 0.5 mm, making them incredibly small and easy to implement. With that said, some people may argue that force-sensing resistors are less accurate than their standard counterpart.

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