FORCE SENSOR

 

FSRs are sensors that allow you to detect physical pressure, squeezing and weight. They are simple to use and low cost. The 1/2″ diameter round part is the sensitive bit.

Working Principle:

Force Sensing Resistors (FSR) are a polymer thick film (PTF) device which exhibits a decrease in resistance with an increase in the force applied to the active surface.

 

The FSR is made of 2 layers separated by a spacer. The more one presses, the more of those Active Element dots touch the semiconductor and that makes the resistance go down.

 

Types of Force Sensors:

 

Part No. 400 (0.2” Circle)

Part No. 402(0.5” Circle)

Part No. 406(1.5” Square)

Part No. 408(24” Trimmable Strip)

 

Additional info about types:

Part No. 400 (0.2” Circle)

Active Area:              0.2” [5.0] diameter

Nominal Thickness:  0.012” [0.30 mm]

Material Build:         Semiconductive

layer                        0.004” [0.10] PES

Spacer adhesive               0.002” [0.05] Acrylic

Conductive layer              0.004” [0.10] PES

Rear adhesive          0.002” [0.05] Acrylic

Connector options   a. No connector

  1. Solder Tabs (not shown)
  2. AMP Female connector

Part No. 402(0.5” Circle)

Active Area:              0.5” [12.7] diameter

Nominal thickness:   0.018” [0.46 mm]

Material Build:         Semiconductive

Layer                                0.005” [0.13] Ultem

Spacer Adhesive               0.006” [0.15] Acrylic

Conductive Layer             0.005” [0.13] Ultem

Rear Adhesive          0.002” [0.05] Acrylic

Connector                        a. No connector

 

 

Part No. 406(1.5” Square)

Active Area:              1.5” [38.1] x 1.5” [38.1]

Nominal thickness:   0.018” [0.46 mm]

Material Build:         Semiconductive

Layer                                0.005” [0.13] Ultem

Spacer Adhesive               0.006” [0.15] Acrylic

Conductive Layer             0.005” [0.13] Ultem

Rear Adhesive          0.002” [0.05] Acrylic

Connectors                      a. No connector

  1. Solder Tabs (not shown)
  2. AMP Female connector

 

 

 Part No. 408(24” Trimmable Strip)

Active Area:              24” [609.6] x 0.25” [6.3]

Nominal thickness:   0.135” [0.34 mm]

Material Build:         Semiconductive

Layer                                0.004” [0.10] PES

Spacer Adhesive               0.0035” [0.089] Acrylic

Conductive Layer             0.004” [0.10] PES

Rear Adhesive          0.002” [0.05] Acrylic

Connectors                      a. No connector

  1. Solder Tabs (not shown)
  2. AMP Female connector

 

 

Features

  1. Actuation Force as low as 0.1N and sensitivity range to 10N.
  2. Easily customizable to a wide range of sizes
  3. Highly Repeatable Force Reading; As low as 2% of initial reading with repeatable actuation system.
  4. Cost effective
  5. Ultra thin; 0.45mm
  6. Robust; up to 10M actuations
  7. Simple and easy to integrate

 

Specifications

Actuation force as low as 0.2N

Sensitivity range up to 20N

Ultra thin

Life: 10 million

Operating temperature: -40ºC to +85ºC

Non actuated  Resistance: 10M Ω

Device rise time: < 3 Micro second

Bend Resistance Range: 100K to 1K Ohms(depending on force)

 

Link to datasheet :

  1. https://cdn-learn.adafruit.com/downloads/pdf/force-sensitive-resistor-fsr.pdf
  2. https://www.sparkfun.com/datasheets/Sensors/Pressure/fsrguide.pdf
  3. http://www.trossenrobotics.com/productdocs/2010-10-26-DataSheet-FSR402-Layout2.pdf

 

 

Do’s and Dont’s:

  • Do be careful if applying FSR devices to curved surfaces.
  • Do avoid air bubbles and contamination when laminating the FSR to any surface.
  • Do be careful of kinks or dents in active areas. They can cause false triggering of the sensors.
  • Do protect the device from sharp objects. Use an overlay, such as a polycarbonate film or an elastomer, to prevent gouging of the FSR device.
  • Do use soft rubber or a spring as part of the actuator in designs requiring some travel.

 

  • Do not kink or crease the tail of the FSR device if you are bending it; this can cause breaks in the printed silver traces.
  • Do not block the vent.
  • Do not solder directly to the exposed silver traces.
  • Do not use cyanoacrylate adhesives (e.g. Krazy Glue) and solder flux removing agents. These degrade the substrate and can lead to cracking.
  • Do not apply excessive shear force. This can cause delamination of the layers.
  • Do not exceed 1mA of current per square centimeter of applied force (actuator area). This can irreversibly damage the device.

 

Storing Instructions:

  1. Use a container that has a firm, flat and smooth mounting surface.
  2. Use some protective covering while storing.

 

Advantages:

  1. Low Cost
  2. Small size
  3. Highly sensitive to force

 

Disadvantages:

  1. Low accuracy
  2. Can break on excessive bending

 

Schematics:

Interfacing with Arduino

Arduino Code:

/* FSR simple testing sketch.

  • Connect one end of FSR to power, the other end to Analog 0.
  • Then connect one end of a 10K resistor from Analog 0 to ground
  • For more information see www.ladyada.net/learn/sensors/fsr.html */
  • int fsrPin = 0; // the FSR and 10K pulldown are connected to a0
  • int fsrReading; // the analog reading from the FSR resistor divider
  • void setup(void) {
  • // We’ll send debugging information via the Serial monitor
  • Serial.begin (9600);
  • }
  • void loop(void) {
  • fsrReading = analogRead(fsrPin);
  • Serial.print(“Analog reading = “);
  • Serial.print(fsrReading); // the raw analog reading
  • // We’ll have a few threshholds, qualitatively determined
  • if (fsrReading < 10) {
  • Serial.println(” – No pressure”);
  • } else if (fsrReading < 200) {
  • Serial.println(” – Light touch”);
  • } else if (fsrReading < 500) {
  • Serial.println(” – Light squeeze”);
  • } else if (fsrReading < 800) {
  • Serial.println(” – Medium squeeze”);
  • } else {
  • Serial.println(” – Big squeeze”);
  • }
  • delay(1000);
  • }

 

Applications:

Detect & qualify press Sense whether a touch is accidental or intended by reading force Use force for UI feedback

Detect more or less user force to make a more intuitive interface Enhance tool safety

Detect presence, position, or motion Of a person or patient in a bed, chair, or medical device 

Detect liquid blockage

Detect tube or pump occlusion or blockage by measuring back pressure

 Detect proper tube positioning Many other force measurement applications

  

FAQ’s:

Q1. What happens if 30lbs or more of force is exerted on the sensor? Would it indicate the max of 22lbs?

Ans. Any weight in excess of 22lbs will cause frailer of the device. 

Q2. What is resistance at zero pressure?

Ans. With no pressure applied the FSR is an open circuit. The range is 100ish kohm with almost no pressure to around 500 ohms with lots of pressure. I do not know if there is a linear or predictable response. It also responds the most to objects like the human finger that can compress a spread out covering max surface area.

Q3. What is the detectable pressure range?

Ans. 50 grams to 10 lbs.

Referral Links:

  1. https://en.wikipedia.org/wiki/Force-sensing_resistor
  2. http://www.npl.co.uk/reference/faqs/how-many-different-types-of-force-transducer-are-there-(faq-force)

 

 

Documentation links :

https://learn.adafruit.com/force-sensitive-resistor-fsr/using-an-fsr

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