Force Sensitive Resistor

October 19th, 2009

Johnnie Chang

Force Sensitive Resistor （ FSR）

1

FSR is a type of resistor whose resistance changes when a force or pressure is applied.

2

Various Pressure/Force sensing principles

Piezoresistive

Capacitive

Electromagnetic

Piezoelectric

Optical

Potentiometric

3

The FSR is a force sensor that employs the Piezoresistive effect.

FSR

The sensitivity of piezoresistive devices is characterized by the gauge factor (or strain factor):

Where ε = strain = ΔL / L ΔR = change in resistance R = unstrained resistance α = temperature coefficient θ = temperature change

4

Basic constuction of FSR

A force sensing resistor is mainly made up of two parts.

A resistive material applied to a film

A set of contacts applied to another film

5

The resistive material serves to make an electrical path between the two sets of conductors on the other film.

Together they fit into a circuit that forms a simple voltage divider

When a force is applied to this sensor, a better connection is made between the contacts, hence the conductivity is increased.

Principle

Special resistive material

The resistive material applied to the sensing film consists of both electrically conducting and non-conducting particles suspended in matrix and are formulated to reduce the temperature dependence, improve mechanical properties and increase surface durability..

Force applied to the sensing film causes particles to touch the conducting electrodes, changing the resistance of the film.

6

The resistance of FCR is inversely proportional to the force applied.

Notice: The threshold, or "break force", that swings the resistance from greater than 1M to about 50-100 K.

Force vs. Resistance characteristics

7

Force vs Conductance

The area enclosed by two doted curves represents a typical part-to-part repeatability envelope.  This error band determines the maximum accuracy of any general force measurement.

8

Pro and Con

Pro

1. Small size, very thin, flexible application, low cost2. Not sensitive to vibration or heat 3. Linear response4. Detect even very weak forces.

Con

1. not very accurate, only qualitative results are generally obtainable.  The part-to-part repeatability tolerance held during manufacturing ranges from ±15% to ±25% of an established nominal resistance.

Ideal for control applications where accuracy isn’t critical

9

Application

Touch sensor in robotics, provide tactile force feedback for robotic arms to ensure safe, accurate, and independent handling of products and equipment

In sports, measure the actual grip forces on golf clubs, tennis racquets, baseball bats, etc. to better analyze the distinct forces involved, or diagnose a problem

Automotive Occupant detection

Common package and pricing

Price range from a few dollars to a few tens of dollars

Reference

http://en.wikipedia.org/wiki/Force_Sensing_Resistor http://en.wikipedia.org/wiki/Pressure_sensor http://www.trossenrobotics.com/c/robot-force-sensor-fsr.aspx http://en.wikipedia.org/wiki/Piezoresistive http://soundlab.cs.princeton.edu/learning/tutorials/sensors/node8.html http://www.interlinkelectronics.com/force_sensors/technologies/fsr.html http://blog.makezine.com/archive/2009/05/force_sensitive_resistor_fsr_tutori.html

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