Touch/Force/Flex Sensing

Introduction

A force-sensing resistor is a material whose resistance changes when a force is applied. These resistors are made of a conductive polymer whose resistance changes in a predictable manner when a force is applied to its surface. These polymers are normally supplied as a sheet or ink that can be applied to a surface. 

Types of Sensors

1) Load Cells

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Basic Principle: This sensor records the pressure and changes in pressure on a pressurized fluid. For this sensor, the force measured is calculated with the equation F = PA, where P = measured hydraulic pressure (psi or N/mm2 ) and A = the cross-sectional area of the cylinder bore, A = p/4 D2 ( in2 or mm2 ). While this example is using an automotive brake cylinder, there are many uses of this sensor. In automobiles, the brake cylinder contains hydraulic fluid, which pushes apart two pistons that expand to put pressure on the brake drum. This, in turn, slows the vehicle. That is the conceptual explanation; in electronics, the pressure gauge would be replaced with a transducer that translates the force into a measurable electrical output. Strain gauge load cells are the most common type.

 

2) Transducer

 

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Basic Principle: A transducer converts energy from one form to another, usually a signal in one form of energy to another. Transducers are often employed for automation, measurement and control systems where electrical signals are converted to and from other physical quantities such as force or light. Here is the example, a surface transducer.

This transducer is capable of converting electrical signals into physical vibrations, effectively turning any surface into a speaker if so desired. At the same time, the electrical signals produced by transducers can be responses to the physical environment, in this case, vibrations.

This is the interior of a vibration speaker. Electrons are pushed through the voice coil, producing a magnetic field that pushes or attract the permanent magnet. This movement creates the vibrations that produce sound waves.

These transducers require an understanding of polarity and the specific input power for them to function. They can be hooked up to an audio device for use. For the purposes of measuring or signaling pressure or touch, this transducer should be straightforward. This probably lands on the rookie scale or maybe reaches the competent level according to Sparkfun.

 

3) Force Sensing Resistor

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Basic Principle: These sensors consist of conductive polymers that change resistance once a force is applied to their surface.

These resistors can be supplied as a polymer ink or a sheet. The sensing film is composed of both conductive and nonconductive particles. A force applied to the surface touches particles to the electrodes, changing the conductivity of the film. These sensors operate on a relatively simple interface.

This FSR will vary its resistance depending on how much pressure is being applied to the sensing area. The harder the force, the lower the resistance. When no pressure is being applied to the FSR its resistance will be larger than 1MΩ. This FSR can sense applied force anywhere in the range of 100g-10kg.

Skill level: Noob.

Limitation: These sensors are not very accurate for sensing the amount of force applied.

 

4) Touch Switch

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Basic Principle: A touch switch is easiest explained as the switch that turns a lamp on and off just by touching a nondescript part of the surface of a lamp. These touch switches are placed on nonconductive surfaces where the electrode is triggered by body capacitance, the electricity moving through a person. The electrode continuously charges and discharges its metal exterior to measure the electrical current and to sense changes in the capacitance that can be detected when a person touches the surface it is positioned on. These are relatively simple in their installment and operation.

Skill Level: Rookie to competent level. Can have limited scope of measurement or use cases.

5) Pneumatic Tube Detector

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Basic Principle: Pneumatic tubes are tubes of pressurized air that are often used to transport capsules or carriers, the objects that travel through the tubes such as carriers at a bank. As a sensor, these pressurized tubes are laid across roadways and send a high-pressure burst of air through the tube, which activates an air switch. This air switch, in turn, sends an electric signal to the receiver to record the instance. These are used for short term traffic counting and are relatively easy to use. These tubes are advantageous because they are cheap, easy to use, and self-contained. Unfortunately, they are not useful for high capacity roads and are not very durable.  

Skill Level: This sensor should be easy to use, noob to rookie ranking. 

 

6) Conductive Rubber Cord Stretch Sensor

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Basic Principal: This sensor measures stretch force. The resistance within the sensor will increase when an external force applied on (stretch or pull). The longer it is stretched, the larger the resistance.

Limitations:The rubber takes a few minutes to revert to its original length after the force is released. It’s not a linear sensor. Consider it a way to measure stretching motion instead of precise force.

 

7) Piezo Vibration Sensor

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Basic Principle: The piezoelectric sensor is used for flex, touch, vibration and shock measurement. It is most applicable in fast processes, such as explosion due to its quick responsiveness.

Basic principal:When structure moves, they experience acceleration. Piezoelectric shock sensor can generate a charge when physically accelerated (piezoelectric effect).

 

8) FlexiForce Pressure Sensor

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Basic Principle: The harder you press, the lower the sensor’s resistance. Pressing hard, the resistance changes from infinite to ~300k. The sensor itself is thin and flexible, but the resistance does not change while being flexed. Resistance changes only when pressure is applied to the round area at the end of the sensor.

Another form of piezoresistive force sensor. They can detect and measure:

  • Force from an applied load
  • Rate of change of a force load over time
  • Force thresholds to trigger appropriate action
  • Contact and/or touch

Used as a presence sensor, weight sensor, pressure sensor (impact testing), etc.

 

9) Pressure-Sensitive Conductive Sheet

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Basic Principle:  Squeezing it will reduce the resistance, so it’s handy for making flexible sensors. Since its resistance changes with pressure, it can be used to create a basic analog sensor.

This conductive material (also known as “Velostat” or “Linqstat”)can be a addition for wearable/sensor hacking toolkit. For example, it can be made into step sensor by pasting under shoes.

 

10) Round Force-Sensitive Resistor (FSR) – Interlink 402

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Basic Principle: FSRs are sensors that allow you to detect physical pressure, squeezing and weight. These sensors are fairly low cost, and easy to use but they’re rarely accurate. They also vary some from sensor to sensor perhaps 10%. So basically when you use FSR’s you should only expect to get ranges of response. While FSRs can detect weight, they’re a bad choice for detecting exactly how many pounds of weight are on them.

FSR’s are basically a resistor that changes its resistive value (in ohms Ω) depending on how much its pressed. FSRs are made of plastic and the connection tab is crimped on delicate material. The best way to connect to these is to simply plug them into a breadboard or use a clamp-style connector like alligator clips, female header, or a terminal block.

 

11)  FlexiForce Pressure Sen

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Basic principle: This is a piezoresistive force sensor. The sensor itself is thin and flexible, but the resistance does not change while being flexed. Used as a presence sensor (someone standing), weight sensor, pressure sensor (impact testing), etc. This sensor ranges from 0 to 25lbs of pressure.

The harder you press, the lower the sensor’s resistance. Pressing hard, the resistance changes from infinite to ~50k. Resistance changes only when pressure is applied to the round area at the end of the sensor.

Skill Level: Noob.

Core Skill: Electrical Prototyping. If it requires power, you need to know how much, what all the pins do, and how to hook it up.

Additional Resources:

 

12) Tilt Sensor

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Basic principle: This AT407 basic tilt switch can easily be used to detect orientation. Inside the can is a ball that make contact with the pins when the case is upright. Tilt the case over and the balls don’t touch, thus not making a connection.

There are numerous uses for these basic sensors, but keep in mind you might need to use some debouncing code, as the sensor isn’t immune to small vibrations and such.

Skill Level: Rookie

Core Skill: Electrical Prototyping. If it requires power, you need to know how much, what all the pins do, and how to hook it up.

Additional Resources:

13) Sensor Film Kit

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Basic Principle: The Sensor Film Kit is a great new way to make custom-sized sensors out of force sensitive material. This kit includes one piece of black plastic sensor film (it’s thicker and more durable than Velostat but similar in function), two pieces of heavy aluminum foil, and two pieces of clear adhesive polyester film. The sensor film is super durable and changes its electrical resistance when force is applied. It’s a lot like Velostat but a bit more responsive. The aluminum foil is for electrodes and the clear adhesive polyester film is given to keep the sensors together. It’s a great way to make a FSR of any size for your microcontroller project.

14) SoftPot Membrane Potentiometer

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Basic principle: These are very thin variable potentiometers. By pressing down on various parts of the strip, the resistance linearly changes from 100Ohms to 10,000Ohms allowing the user to very accurately calculate the relative position on the strip. Can be used as an accurate positional indicator for CNC head positioning, variable user input (volume level input for example), straight user input (multiple button areas translate to given resistance levels), and many other applications.

Unit comes with adhesive backing. Connector is 0.1″ spaced and bread board friendly. However the connector does not fit nicely into standard female headers because the pins are too small.

Skill Level: Noob

Core Skill: Electrical Prototyping. If it requires power, you need to know how much, what all the pins do, and how to hook it up.

Additional Resources:

15) Liquid Level Sensor

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Basic Principle: These sensors measure the level of a liquid. The sensors covering is sensitive to changes in pressure that would occur due to being immersed in a liquid.  This allows the sensor to detect resistance changes that are inversely proportional depending on the distance from the top of the sensor to the top of the liquid. However, liquid level sensors can be sensitive to bending, so it is necessary to keep the sensor straight.

Skill Level: Competent

Core Skill: Electrical Prototyping

Additional Resources: 

16) Flex Sensor

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Basic Principle: One side of the sensor has polymer ink embedded with conductive particles. As the sensor is flexed or bent, the resistance in the current is increased due to the conductive particles being forced away from each other.  This sensor is limited due to the fact that they can only measure flex in one direction.

Skill Level: Noob

Core Skill: Electrical Prototyping

Additional Resources: 

Human Presence Sensor

Each channel within the sensor has a different field of view, so not only can it detect a human, but it can also tell which direction the person is moving.

Technical Description

The onboard AK9753(sensor name) is a digital sensor giving you a 16-bit digital value over I2C. Each of the four sensors outputs the IR current in pico-amps. A PIR reading can vary from roughly -200 (no human present) to 1500 when a human is detected standing in front of a given channel, but it varies due to environmental factors and other heat sources in view.

Limitations/Features

  1. Voltage: 1.7V to 3.3V
  2. Extremely low current: 100µA

Skill

3- Competent

Links

https://github.com/sparkfun/Qwiic_Human_Presence_Sensor_Breakout_AK9753

https://www.sparkfun.com/products/14349

Examples

https://github.com/sparkfun/SparkFun_AK975x_Arduino_Library/tree/master/examples

Open Camera

The OpenMV M7 Camera is a small, low-power microcontroller board that allows you to easily implement applications using machine vision in the real world. The best part about the OpenMV is that it is not only capable of image capture, but also face detection, color tracking, QR code reading and plenty more. If you are looking for an economical camera module boasting multiple high-end features, look no further than the OpenMV M7!

Technical Description

The OpenMV can be programmed in high-level Python scripts (courtesy of the MicroPython Operating System) instead of C/C++. This makes it easier to deal with the complex outputs of machine vision algorithms and working with high-level data structures. You still have total control over your OpenMV M7 and its I/O pins in Python. We can easily trigger taking pictures and video on external events or execute machine vision algorithms to figure out how to control your I/O pins.

Parts

  1. 1x OpenMV M7 Camera
  2. 2x 8-pin Male Header
  3. 2x 8-pin Female Stackable Header

Limitations/Features

  1. Three I/O pins for servo control.
  2. Interrupts and PWM on all I/O pins (there are 10 I/O pins on the board).
  3. RGB LED and two high-power 850nm IR LEDs.
  4. A full-speed USB (12Mbs) interface to your computer; your OpenMV Cam will appear as a virtual COM port and a USB flash drive when plugged in.
  5. A μSD card socket capable of 100Mbs reads/writes, which allows your OpenMV Cam to record video and easily pull machine vision assets off of the μSD card.
  6. An SPI Bus that can run up to 54Mbs, allowing you to easily stream image data off the system to either the LCD shield, the WiFi shield or another microcontroller.

Skill

4-Experienced

Links

https://cdn.sparkfun.com/datasheets/Sensors/LightImaging/STM32F765VI.pdf

https://www.sparkfun.com/products/14985

 

Stereo Camera

Technical Description

The EX8029 Stereo Camera from eYs3D is a depth-map imager controller and patterned IR illuminator capable of both color and point map output modes. Thanks to its dual cameras, this Stereo Camera is ideally used in applications involving motion and gesture control, field mapping, and 3D scanning. The best part about this module is the fact that it has been designed to be as straight forward and easy to use as possible for a stereo camera. Simply plug in the included USB 3.0 cable download the required files and start seeing things through the eyes of your new camera.

Limitation/Features

  1. IR pattern Illuminator Wavelength: 850nm +/-10 nm
  2. IR pattern Illuminator Number of Features: >10,000
  3. Lens: 1LS1022G by AOET
  4. Image Sensor: OV9714 by OVT
  5. USB Bus Power: 5V/900mA by USB3.0; 5V/500mA by USB2.0
  6. PC Connection: USB3.0 & USB2.0
  7. Power Voltage: 5V
  8. Operating Current With Patterned IR Illuminator: ≦350mA
  9. Operating Current Without Patterned IR Illuminator: ≦280mA
  10. Suspend Current: ≦15mA

Skill

4-Experienced

Links

https://www.sparkfun.com/products/14726

 

Magnetic Imaging Tile

Cost 149.95$

Technical Description

The Magnetic Imaging Tile uses an array of 64 hall effect sensors to convert magnetic fields to the visual spectrum. This allows the user to see magnetic fields in real time. As is to be expected, there are caveats: the magnetic sensors used on the tile are some of the most sensitive on the market but being within 1 to 2 centimeters of the tile is required to get a good image.

Limitation/Features

  1. Powered with 3.3V to 5V
  2. Uses approximately 150mA during sensing at 3.3V
  3. Size: 52x34mm
  4. Boards can be tiled next to each other to increase field size with a one ‘pixel’ gap between boards.

Skill

4-Experienced

Links

https://www.sparkfun.com/products/14652

https://github.com/sparkfunX/Magnetic_Imaging_Tile

RFID Button

Technical Description

This is a simple, thumbnail-size RFID tag. These tags are great for sensing the presence, identification, etc., and they’re small so they can be easily sewn into clothing or inserted into enclosures. Each tag comes with a unique 32-bit ID code and is not reprogrammable. The carrier frequency of this tag is 125kHz, so it works great with our ID-2, ID-12, and ID-20 RFID readers.When this was tested this RFID tag with one of our ID-12 readers and measured a maximum read distance of about 32mm.

Limitation/Features

  1. 32-bit unique ID – non-reprogrammable
  2. 125kHz read frequency
  3. EM4001 ISO based RFID IC
  4. Manchester encoding
  5. Diameter: 16mm
  6. Thickness: 1.85mm

Skill

2-Rookie

Links

https://www.sparkfun.com/products/9417

 

GPS Receiver

Technical Description

The GP-20U7 is a compact GPS receiver with a built-in high performances all-in-one GPS chipset. The GP-20U7 accurately provides position, velocity, and time readings as well as possessing high sensitivity and tracking capabilities.

Limitation

The following are the features

  1. 56-Channel Receiver (22 Channel All-in-View)
  2. Sensitivity : -162dBm
  3. 2.5m Positional Accuracy
  4. Cold Start: 29s (Open Sky)
  5. 40mA @ 3.3V

Skill

2-Rookie

 

Links

https://www.sparkfun.com/products/13740

Infrared Array Breakout

“It’s like having a thermal camera, just in a lower resolution. To make it even easier to get your low-resolution infrared image, all communication is enacted exclusively via I2C, utilizing our handy Qwiic system”

Cost: $40.95

Technical Description

The on-board AMG8833 Grid-EYE possesses an accuracy rate of ±2.5°C (±4.5°F) with a temperature range of 0°C to 80°C (32°F to 176°F). Additionally, this IR “camera” board can detect human body heat at about 7 meters or less (that’s about 23 feet) and has a frame rate of 10 frames a second to one frame a second. It is important to point out that while this version of the Grid-EYE is the high-performance type with a high gain, it is only 3.3V tolerant. IE: Don’t try to operate it at 5V (you won’t have a very good time).

Applications

Can detect human body heat at about 7 meters

Limitations

It has very specific features which are the following :

  1. Operating Voltage(Startup): 1.6V – 3.6V
  2. Operating Voltage(Timekeeping): 1.5V – 3.6V
  3. Current Consumption: 4.5 mA
  4. 8×8 Thermopile Array
  5. Temperature Range: 0°C to 80°C (32°F to 176°F)
  6. Accuracy Rate: ±2.5°C (±4.5°F)
  7. Human Detection Distance: 7m or less (22.966ft)

Skill

3-Competent

Links

https://www.sparkfun.com/products/14607

https://github.com/sparkfun/Qwiic_GRIDEye

Sample

https://github.com/sparkfun/SparkFun_GridEYE_Arduino_Library

Infrared (IR) break-beam sensor

Infrared (IR) break-beam sensors are a simple way to detect motion. They work by having an emitter side that sends out a beam of human-invisible IR light, then a receiver across the way which is sensitive to that same light. When something passes between the two, and its not transparent to IR, then the ‘beam is broken’ and the receiver will let you know.

Technical Description: 

Compared to PIR sensors, breakbeams are faster and allow better control of where you want to detect the motion. Compared to Sonar modules, they’re less expensive. However, you do need both emitter and receiver on opposite sides of the area you want to monitor.

Limitations:
The 5mm IR version works up to 50cm / 20″, so we need a lot of them to measure open space.

Skill:
Easy

Sample Sensor:
IR Break Beam Sensor:
https://www.adafruit.com/product/2167

Sample Exercise:

https://learn.adafruit.com/ir-breakbeam-sensors

 

Pressure mats

 

Pressure sensor pads are electronic devices that capture physical force contact to generate some sort of a response. There is a truly endless amount of applications for pressure sensor pads, ranging from an input mechanism in a human-machine interface (HMI), a method to capture intruders in a force-sensitive security system, and beyond.

Technical Description: 

They have a force range of 0-222 N (0-50 lb), specified with Tekscan electronics. The model is linear through a much lower range of 0-22N (0-5 lb), and is capable of measuring loads up to 44,482 N (10,000 lb).*

The dynamic range of this square force sensor can be modified by changing the drive voltage and adjusting the resistance of the feedback resistor

Limitations:
Expensive

Skill:
Easy

Sample Sensor:
FlexiForce A502 Sensor:
https://www.tekscan.com/products-solutions/force-sensors/a502

Sample Exercise:
https://www.tekscan.com/blog/flexiforce/how-create-pressure-sensor-pad