What Is a Force-Sensing Resistor?

A force-sensing resistor is an electrical component that changes its resistance with changes in pressure on the resistive material. It is used in sensing touch, weight, and any entity characterized by change in mechanical force. This resistor has many applications in sensing, monitoring, and automation, such as robotics.

Early variable resistors for speed control made use of carbon disks that may be compressed or set loosened from each other by spring action. When the operator steps on a foot control to speed up a motor, the carbon disks are pressed together, resulting in a near-zero resistance. This resistance is in series with the motor, resulting in higher speed. When the operator releases the foot control, the pressure between the disks is decreased significantly, resulting in a high resistance that stops any significant electrical current going into the motor, which stops the motor.

The modern force-sensing resistor works in a similar principle as the early resistive speed control units. The difference is that the force-sensing resistor material is very thin and is not designed to dissipate more than a few milliwatts of electrical power. Any human-operated speed control using force-sensing resistor will be controlling analog or digital circuitry that controls higher-power devices.

The force-sensitive resistor makes use of a polymer between two plates with conductive materials suspended in a sort of a regular formation. When pressure is applied, the conductive material gets closer together, resulting in lower resistance. If pressure is removed, the polymer material’s ability to recover from the temporary deformation will return the force-sensing resistor back to its initial state. This results in the resting position being high resistance.

In general, all resistive components may be manufactured as force-sensing resistor. Some designs make use of polymers with small conductive particles, while some designs of force-sensing resistor make use of printed sensing films. Other force-sensing resistors come in arrays that can be used to detect two-dimensional tracking information such as in tracking pads for computing devices.

Force sensors are available in a variety of types and designs. Piezoresistive sensors are able to decrease their electrical resistance with increasing force of compression exerted to the sensor. The resistance increases with a decrease in force exerted to the sensor.

Applications for the force-sensing resistor include contact sensing for safety applications, digital weighing scales, water and air pressure sensing for pumps, and proximity detection by weight or contact. In automation, robotic equipment is able to partially emulate human touch or touch-like sensing. This feature makes it possible to apply tactile-like sensing and variable force control to automatically handle items that are considered too fragile to be handled by machines and robots alone.