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What Are Force Sensitive Resistors (FSRs) and How Do They Work?

Table of Contents

Introduction

A type of variable resistor whose electrical resistance decreases with increasing applied force or pressure is called a force-sensitive resistors (FSRs).

A force sensitive resistors, blue polyester film and silver track.
A force sensitive resistors, blue polyester film and silver track.

They consist of a conductive polymer whose electrical conductivity varies with the amount of mechanical stress or pressure it experiences. It comes with a peel-and-stick rubber backing on the opposite side of the sensing area, which facilitates easy mounting of the FSR.

The Working Principles Of FSRs

Industrial membrane switches are constructed with robust and durable materials to ensure longevity and resistance to wear and tear. They are often built to withstand millions of actuations without degradation.

Technical Specifications

Thickness: <0.4mm
Range: 10kg
Response Point: 150g
Durability:> 1 Million Times
Initial Resistance:> > 10 MQ (No Load)
Test Voltage: Typical Value: DC 3.3V
Response Time: <1ms
Recovery Time: <15ms
Working Temperature: -20 to 60 degree
Electromagnetic Interference (EMl): Not Generated
Electrostatic Discharge ESD: Not Sensitive
Tolerance: Approx. 0.2mm / 0.01in
Pin Distance: Approx. 2.54mm / 0.1in

A force sensor, it has 2 wires.
A force sensor, it has 2 wires.

Sensitive Area Size

Force sensitive resistors come in various shapes and sizes, with some key features (such as size, shape, and sensing range) that set them apart. Most FSRs have circular or rectangular sensing areas. Square force sensitive resistors are great for broad-area sensing, while small circular sensors can provide higher accuracy for sensing range.

Here is one example for reference:

Length: Approx. 65mm / 2.6in
Width: Approx. 50mm / 2.0in
Tolerance: Approx. 0.2mm / 0.01in
Sensitive Area Size: Approx. 45x45mm / 1.8×1.8in
Sensor Size: Approx. 50x65mm / 2.0×2.6in
Pin Distance: Approx. 2.54mm / 0.1in

Force-sensing Range

The sensing range is another key characteristic of FSRs, which defines the minimum and maximum pressure the sensor can distinguish. The lower the rated force, the more sensitive the FSR is. Any pressure beyond the maximum range of the sensor cannot be measured and will also damage the sensor. For example, a smaller 1 kg-rated force sensitive resistors can provide more sensitive readings from 0 to 1kg but cannot differentiate between weights of 2kg and 5kg.

A force-sensing resistor is capable of detecting the force exerted on its sensing area within a range of 0g to 1000g. The resistance it offers changes based on the amount of pressure applied to the sensing area; the greater the force, the lesser the resistance.

Applications Of Force Sensitive Resistors

Healthcare: FSRs are used in medical devices such as digital blood pressure monitors and respiratory monitoring systems.

A force sensitive resistor, it's for foot wear and healthcare applications.
A force sensitive resistor, it's for foot wear and healthcare applications.

Consumer Electronics: In smartphones and tablets, FSRs are used to detect touch inputs and gestures. They can also be found in computer keyboards and mice.

Musical Instruments: Electronic drums and MIDI controllers often use force sensitive resistors to detect the intensity of a musician’s touch.

Automotive Industry: In cars, force sensitive resistors can be used in touch-sensitive controls for infotainment systems or climate controls.

A force sensing resistors (FSRs), co-molding with the wiring harness.
A force sensing resistors (FSRs), co-molding with the wiring harness.

Robotics: In robotics, FSRs can be used in grippers to detect if an object has been picked up.

Advantages Of FSRs

FSRs are uncomplicated and user-friendly.

They are economically efficient when compared to other sensor types.

Their slim and flexible design allows them to be incorporated into a wide variety of devices.

How To Calculate The Force

  1. Measure the resistance of your force sensitive resistors using a multimeter.
  2. Use Ohm’s Law (V = IR) to calculate the voltage across the force sensitive resistors .
  3. Use the voltage reading and the calibration curve to determine the force being applied.
  4. Using the calibration curve, you can determine that a voltage of 5 V corresponds to a force of 10 N.

Here’s an example calculation:

  1. Let’s say your FSR has a resistance of 10 kΩ.
  2. You’re using a 5 V power supply, so according to Ohm’s Law, I = V/R = 5/10000 = 0.0005 A.
  3. The voltage across the FSR is equal to I * R = 0.0005 * 10000 = 5 V.
A force sensor with many tiny circuit track.
A force sensor with many tiny circuit track.

Conclusion

Force sensitive resistors (FSRs) are resistive sensors that exhibit varying resistance in response to force applied to the sensing area. The resistance of an FSR changes with the amount of pressure applied to its sensing area.They are made of conductive polymer material that reacts to mechanical stress. If you want to calculate the force applied to an FSR, use a multimeter, Ohm’s Law, and a calibration curve.

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