Force Sensitive Resistors(FSRs): An introduction

1. Force Sensitive Resistors(FSRs):An introduction Ryan Chin, rchin@media.mit.edu

2. FSRs What are they? • Force Sensitive Resistors or Pressure Sensors • Type of resistor whose resistance changes when a force or pressure is applied • The resistance is inversely proportional to the force applied The threshold, or "break force", that swings the resistance from greater than 1M to about 50-100 K [ 1. Resistance (Ohms) vs. Force (g) for an Interlink FSR ]

3. FSRs How they work • Particle size at the scale of microns • Formulated to reduce temp. dependence, and improve mechanical properties • Force on the surface causes particles to touch conducting electrodes changing the resistance

4. FSRs How they work • FSRs consist of a piezoresistivity conductive polymer, which changes resistance in a predictable manner following application of force to its surface • Usually come in a sandwich sheet on sensing Film • Film: 1) electrically conducting and 2) non-conducting particles suspended in matrix.

5. FSRs Characteristics • Manufactured as single pads, linear pots, 2D arrays and in several other forms • Detect even very weak forces. Recognizing not only the magnitude of the force, but also the difference of the touching manner • 3 main components in a FSR assembly • Substrate to attach • FSR • Microcontroller • not sensitive to vibration or heat • No user feedback, except maybe the button can be induced to move slightly.

6. FSRs Characteristics • Usually come in a sandwich sheet on sensing Film • response is in the range of 0-2kg, which is common to human interface applications, is very linear force vs. conductance (1/resistance)

7. Simple FSRs and Arrays Parameter Value Conditions Size Range Max = 20" x 30" (51 x 76 cm)Min = 0.2" x 0.2" (0.5 x 0.5 cm) Any shape Device Thickness 0.008" to 0.050" (0.20 to 1.25 mm) Force Sensitivity Range 30g to 10kg Pressure Sensitivity Range 0.45psi to 150psi(0.03 kg/cm2 to 10 kg kg/cm2) 30g to 10kg1 cm2 actuator Part to Part Force Repeatability ± 15% full scale For typical part with consistent actuation Single Part Force Repeatability ± 2% full scale FSRs Characteristics

8. Force Resolution Better than 0.5% full scale Break Force 30 to 100g (1 to 3.5 oz) typical Dependent on probe size/shape Stand-Off Resistance > 1M Switch Characteristic Essentially zero travel Device Rise Time 1-2msec (mechanical) Lifetime >10 million actuations Use Temperature -30ºC to 170ºC High temperature adhesives Maximum Current 1 mA/cm2 of applied force Sensitivity to Noise/Vibration Not significantly affected EMI/ESD Passive device -- not damaged by EMI or ESD Lead Attachment Standard flex circuit techniques See TechNote FSRs Characteristics

9. FSR Accuracy • dynamic measurement, only qualitative results are generally obtainable.  • Force accuracy ranges from approximately ± 5% to ± 25% depending on the consistency of the measurement and actuation system • Repeatability tolerance held in manufacturing, and the use of part calibration. manufacturing ranges from ± 15% to ± 25% of an established nominal resistance. • The force resolution of FSR devices is better than ± 0.5% of full use force.

10. FSRs Types • Basic sensors – single point (usually circular) • Simple array - Two or more PS3s on a single substrate with one common conductor track plus an individual track for each sensor cell. • Matrix – rows and columns • Continuous Strip – up to 15 meters • Co-ordinates & Pressure Sensing Sensors

11. FSR Suppliers • Interlink electronics, http://www.interlinkelec.com/ • IEE (UK), http://www.steadlands.com/ • Tekscan, http://www.tekscan.com/index.html