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sensors:force-sensitive_resistor [2018/11/01 18:55] – external edit 127.0.0.1sensors:force-sensitive_resistor [2022/03/20 20:13] (current) – [Devices] brady.boettcher
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 =====  Summary  ===== =====  Summary  =====
 ====  Introduction  ==== ====  Introduction  ====
-A **force-sensitive resistor** (alternatively called a **force-sensing resistor** or simply an **FSR**) has a variable resistance as a function of applied pressure.  In this sense, the term "force-sensitive" is misleading – a more appropriate one would be "pressure-sensitive", since the sensor's output is dependent on the area on the sensor's surface to which force is applied.+A **force-sensitive resistor** (alternatively called a **force-sensing resistor** or simply an **FSR**) has a variable resistance as a function of applied pressure.  In this sense, the term "force-sensitive" is misleading – a more appropriate one would be "pressure-sensitive", since the sensor's output is dependent on the area on the sensor's surface to which force is applied. Commercially fabricated FSRs are often made with fixed shapes and sizes, but offer reliable outputs and are relatively inexpensive.
  
 These devices are fabricated with elastic material in four layers, consisting of: These devices are fabricated with elastic material in four layers, consisting of:
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 When external force is applied to the sensor, the resistive element is deformed against the substrate.  Air from the spacer opening is pushed through the air vent in the tail, and the conductive material on the substrate comes into contact with parts of the active area.  The more of the active area that touches the conductive element, the lower the resistance.  All FSRs exhibit a "switchlike response", meaning some amount of force is necessary to break the sensor's resistance at rest (approximately 1 MΩ), and push it into the measurement range (beginning at approximately 100 KΩ) (Interlink Electronics 2005). When external force is applied to the sensor, the resistive element is deformed against the substrate.  Air from the spacer opening is pushed through the air vent in the tail, and the conductive material on the substrate comes into contact with parts of the active area.  The more of the active area that touches the conductive element, the lower the resistance.  All FSRs exhibit a "switchlike response", meaning some amount of force is necessary to break the sensor's resistance at rest (approximately 1 MΩ), and push it into the measurement range (beginning at approximately 100 KΩ) (Interlink Electronics 2005).
 +
 +There are several options for creating custom FSR shapes and configurations using pressure-resistive conductive materials such as conductive rubber, paper, and ink, or [[https://www.adafruit.com/product/1361
 +|Velostat]]. These flexible materials can be cut, shaped and stacked to better fit designs and resistance characteristics needed in a design. The pressure-resistive materials alone don't operate as sensors though, and must be configured with conductors to extract resistance changes from the material.
 +
 +{{:sensors:velostatconfigurations.png?400|Configurations for custom FSR constructions using pressure-resistive materials, taken from Boettcher et al. 2022.}}
 +
 +Utilizing the same voltage divider concept in commercial FSRs, both configurations should attach an additional fixed resistor in series with the sensor to avoid short circuits. The sandwich configuration shown above can also be extended to detect touch position by separating the bottom conductor into different shapes. With each conductive piece receiving its own voltage difference, relative touch position can be determined.
  
 Operationally, an FSR is very similar to a [[strain gauge]], the main difference being that a strain gauge's backing deforms with the resistive element, while an FSR's does not.  This fact is important to consider when mounting an FSR against a support, as discussed below. Operationally, an FSR is very similar to a [[strain gauge]], the main difference being that a strain gauge's backing deforms with the resistive element, while an FSR's does not.  This fact is important to consider when mounting an FSR against a support, as discussed below.
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 The same applied force will result in a wider output swing in a FSR than a strain gauge. Strain gauges, however, have higher accuracy than an FSR. Depending upon the particular needs of the application, one may choose one or the other. Ultimately, a major consideration in the choice of a sensor is cost; a major advantage of FSRs is their low cost. The same applied force will result in a wider output swing in a FSR than a strain gauge. Strain gauges, however, have higher accuracy than an FSR. Depending upon the particular needs of the application, one may choose one or the other. Ultimately, a major consideration in the choice of a sensor is cost; a major advantage of FSRs is their low cost.
 ==== Using an FSR ==== ==== Using an FSR ====
-One of the most common circuits implemented to utilize an FSR's output is the [[tutorials:Basic_sensor_interfacing_techniques#Dividing_a_voltage|voltage divider]].  A voltage (usually +5 V) is applied to one of the leads, while the other is grounded.  FSRs are not polar, meaning it does not matter which side receives the voltage.  One lead from a second resistor (with fixed value) is then connected to the voltage side, while the other lead of the second resistor is also connected to ground.  In this way the FSR is able to measure the "voltage drop across a resistor".  The resistance value of the second resistor determines the output range of the sensor.  Typically, 100 KΩ will yield a sensor output suitable for common [[ADC]]s used for musical applications.+One of the most common circuits implemented to utilize an FSR's output is the [[tutorials:Basic_sensor_interfacing_techniques#Dividing_a_voltage|voltage divider]].  A voltage (usually +5 V) is applied to one of the sensor's leads, while the other is attached to a second resistor with fixed value. The other end of the fixed resistor is grounded, dividing the voltage  and preventing a short circuit when the FSR's resistance drops to 0. In this way the FSR is able to measure the "voltage drop across a resistor".  The resistance value of the second resistor determines the output range of the sensor.  Typically, 100 KΩ will yield a sensor output suitable for common [[ADC]]s used for musical applications. FSRs are not polar, meaning it does not matter which side receives the voltage.  
  
 === Mounting === === Mounting ===
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 Resources:\\ Resources:\\
 Notes:The information on the website of the company are very vague about the technical characteristics of the sensors. No datasheet is available. \\ Notes:The information on the website of the company are very vague about the technical characteristics of the sensors. No datasheet is available. \\
 +Variants:\\
 +</box>
 +
 +<box 100% round blue|**Velostat**>
 +<box 30% round red right|Sources>
 +  * [[https://www.adafruit.com/product/1361|Adafruit]] US$4.95 per sheet
 +</box>
 +Description: 1mm flexible pressure-sensitive resistive material\\
 +Datasheet: N/A \\
 +Resources:\\
 +Notes: Material also known as Linqstat.\\
 Variants:\\ Variants:\\
 </box> </box>
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   * G. Burdea, //Force and Touch Feedback for Virtual Reality.// New York, NY: Wiley, 1996.   * G. Burdea, //Force and Touch Feedback for Virtual Reality.// New York, NY: Wiley, 1996.
   * Interlink Electronics, 2005, "[[http://www.sensorwiki.org/lib/exe/fetch.php?cache=cache&media=http%3A%2F%2Fwww.interlinkelec.com%2Fsites%2Fdefault%2Ffiles%2F94-00004A_FSR_Integration_Guide.pdf|FSR Integration Guide & Evaluation Parts Catalog]]", Company brochure, Camarillo, CA, 26 pp.   * Interlink Electronics, 2005, "[[http://www.sensorwiki.org/lib/exe/fetch.php?cache=cache&media=http%3A%2F%2Fwww.interlinkelec.com%2Fsites%2Fdefault%2Ffiles%2F94-00004A_FSR_Integration_Guide.pdf|FSR Integration Guide & Evaluation Parts Catalog]]", Company brochure, Camarillo, CA, 26 pp.
 +  * Koehly, Rodolphe, et al. "In-house development of paper force sensors for musical applications." Computer Music Journal 38.2 (2014): 22-35.
   * [[http://cnmat.berkeley.edu/sensor_module/interlink_force_sensing_resistor|FSR info at CNMAT]]   * [[http://cnmat.berkeley.edu/sensor_module/interlink_force_sensing_resistor|FSR info at CNMAT]]
  
 {{tag>Sensor FSR Force Resistance}} {{tag>Sensor FSR Force Resistance}}