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Force sensitive film that can be easily cut into any shape to be used as a sensor or switch for Arduino and other electronic projects.

Description:

The Sensor Film Kit consists of a smart plastic film that can be cut to any shape to make force-sensitive switches and sensors. The black plastic sensor film is an ultra-durable material that changes its electrical resistance when force is applied to it.  The Sensor Film Kit contains everything needed to be able to quickly and easily make custom-shaped force-sensitive sensors and switches for Arduino based projects.

The Kit Idea:

After many years of marketing and manufacturing custom-shaped and calibrated sensor films, one thing stands out about this technology: how easily and quickly a custom size and shape sensor can be put together.  In recognizing the maker movement and how many people out there are commonly using Arduino and other microcontrollers in their projects and hobbies, I wanted to put together an affordable kit with the materials and instructions enabling people to create their own force sensors and switches and experience just how easily it can be done.

The kit comes with the following components:

  • one 4 inch x 6 inch sheet of smart polymer sensor film (0.010 inch thick)
  • two 4 in. x 6 in. sheets of industrial aluminum foil (0.003 in. thick) for electrodes
  • two 4 in. x 6 in. sheets of clear adhesive-backed polyester film with release liner for holding the sensors together
  • Two alligator clips to hook up a sensor to an Arduino or other micro controller with an A/D input.
  • Measurement Resistor
  • Instructions and tips for sensor assembly
  • Arduino code and connection diagrams for a variety of examples (including those shown below)
Sensor Film Kit Contents
Sensor Film Kit Contents

Making a Sensor

The assembly of a sensor is quite easy and involves four easy steps:

1) Cut the aluminum foil electrodes to shape leaving pigtails to attach alligator clips

Cut Aluminum Electrodes
Cut Aluminum Electrodes

2) Cut the sensor film slightly larger than the electrode pieces

Cut Polymer Sensor Film
Cut Polymer Sensor Film

3) Cut the clear, adhesive-backed cover layer pieces slightly larger than the sensor film such that the edges will adhere to hold the sensor together.   

Cut Protective Outer Layer
Cut Protective Outer Layer

4) Assemble the sensor by first sandwiching the polymer between the aluminum electrodes and then sandwiching the assembly with the adhesive-backed clear protective film to hold it all together.   

Assembled Sensor
Assembled Sensor
Exploded Side View of Sensor Layup
Exploded Side View of Sensor Layup

The above video is a fast motion (8x) recording of constructing a simple square-shaped sensor.

Examples:

One example of what can be done using the Sensor Film Kit is to use an Arduino Micro or Leonardo along with the Arduino keyboard and mouse libraries to control a Windows PC cursor (custom Arduino code will be provided to project backers).  The following video shows the use of five simple square shaped sensors to control cursor movement and left click functionality.

In addition to controlling a PC mouse, the Sensor Film Kit can also be used in conjunction with an Arduino Micro or Leonardo to control PC video games, as shown in the following video:

A single sensor from the Sensor Film Kit can also be used to control a tri-color LED.  The following video shows a simple single square shaped sensor connected to an Arduino Uno controlling a tri-color LED based on how hard the sensor is pushed - blue indicates the lightest force, green color indicates a moderate force, and red indicates the highest force.

The next example uses a simple round sensor as a switch to capture very fast impact events.  The Arduino monitors the output of a simple analog peak detection circuit that captures output spikes from the sensor that are too quick for the Arduino to capture through it's onboard A/D converter.  Upon detection of an impact, the LED will illuminate.

In another example, a tri-color LED can be variably controlled by letter-shaped sensors.  The following example shows the versatility of the Sensor Film Kit where the three colors of the LED are controlled by sensors shaped as the letter corresponding to the color.

A sensor film can also be used with the Arduino to variably control the frequency of a speaker according to force.  The following video shows a simple square shaped sensor film being used to control the frequency of a square wave being output to a small speaker with higher force corresponding to higher speaker frequency.

Another example shows how a sensor film can be used as a weight-sensitive switch, where lighter weights will not trigger an event (such as a lightbulb or alarm or even controlling a door), but heavier weights will.  In this example, a pet will not trigger the light bulb, but a person will.

Using the same example shown above, the force threshold programed in the Arduino can be decreased to make any weight trigger the event (a red light in this case).

In the following video, the durability of the sensor is shown.  An Arduino turns on a red light bulb through a solid state relay when the sensor readings are above a threshold, providing a much more exciting and conversation-provoking method of knowing how far to pull into the garage versus the tennis-ball-on-a-string method!

Sensors constructed with the Sensor Film Kit are capable of obtaining measurements much faster than what an Arduino is capable of.  The following video shows the output collected with a digital storage oscilloscope from a sensor that was shot.  As the video shows, the impact time was very brief - only 0.5 ms (200 times less than an eye blink).  The demonstration also shows just how durable the sensor film is - a sensor that has already been shot 6 times is still able to provide measurements.

How Does It Work?

The polymer film of the Sensor Film Kit differs from all other force sensing film technologies by the physics of how it obtains force measurements.  Other force sensing film technologies use rubber-like polymers or inks that change their electrical resistance due to compressive deformation of the material - i.e. those materials get thinner when force is applied to them.  In order to provide measurements, the material must be soft enough to deform.  This causes relatively poor long-term durability.

Conversely, the sensor polymer contained in the Sensor Film Kit has the highest impact and wear resistance of any thermoplastic (ultra-high molecular weight polyethylene) such that even when very high forces are applied, there is negligible compressive deformation of the polymer film.

The technology in use with the Sensor Film Kit relies on small changes in the nano-scale surface roughness of the polymer film when force is applied.  Because these changes are at the nano-scale, they multiply by orders of magnitude for a nominal sized film sensor, causing dramatic changes in the electrical resistance that are easy to measure with an Arduino or other microcontroller or any data acquisition system.

Sensor Film Resistance (red) and Output (blue) vs. Force
Sensor Film Resistance (red) and Output (blue) vs. Force

In order to most easily get measurements from the Sensor Film Kit, the Arduino's regulated 5V supply is fed through the sensor and a fixed-value measurement resistor to ground.  In this constant-voltage configuration, the amount of current that flows through the sensor indicates the force applied to it.  This is measured by reading the voltage across the fixed-value measurement resistor with one of the Arduino's analog input pins.

Circuit Schematic for Sensor Film
Circuit Schematic for Sensor Film
Sensor Film Connected to Arduino
Sensor Film Connected to Arduino

Measurement Range:

Sensors from the Sensor Film Kit are able to detect minimal forces of a few ounces all the way up to maximum forces of a few thousand pounds without changing the measurement resistor when using the constant-voltage method recommended here.  More advanced circuits, such as an op-amp current-to-voltage converter or constant current circuits, can be used to increase the accuracy and sensitivity of sensor films. Sensors made using the Sensor Film Kit are quite durable, being able to take extreme overload forces up to and typically beyond the polymer sensor film’s mechanical yield strength of 2,000 pounds per square inch of sensor area.  Even when overloaded severely, the zero point (voltage output at no force) rarely changes due to the fact that when no force is applied to the sensor, it acts as an open circuit, not allowing any electrical current to flow.

The graphs below show the sensor film's resistance vs. force for the range of 0 to 10 pounds of force (lbf) and from 10 to over 350 pounds of force.

Sensor Film Resistance vs. Force
Sensor Film Resistance vs. Force

 In order to better view the sensor's output graphically, the conductance of the sensor can be plotted against force.  Conductance is merely the inverse of resistance.  The plot below shows the Conductance of the sensor film from 0 to 4000 pounds of force, showing how wide the sensor film's range of measurement can be.

Sensor Film Conductance vs. Force
Sensor Film Conductance vs. Force

Why is Funding Needed?

In order to offer the Sensor Film Kit as described here, there are minimum order quantities needed for the kit materials.  Additionally, the Sensor Film Kit needs to obtain clear instructions and a web site allowing users to post their ideas and unique designs to help spark ideas for new and exciting uses of the Sensor Film Kit.  Although regular aluminum foil from a grocery store and clear packaging tape can be used with the black sensor film to construct sensors that work very well, these materials are much more difficult to work with and handle.  The 3 mil (0.003 in.) thick aluminum foil that will be supplied with the kit is thick enough to allow for it to be easily handled without creasing or tearing during assembly.  Additionally, the clear polyester protective outer layer film has a release liner and adhesive combination that makes the release liner easy to remove after the film is cut.  This allows for the release liner to remain in place while handling and cutting which keeps the film sticking to fingers and scissors.  The release liner can then be removed just prior to final assembly of the sensor.  This combination of materials will eliminate all the difficulties and frustrations than can be encountered during sensor assembly and allow the user to focus on what's important - creating force sensors and switches of any shape imaginable for new and exciting applications!

We have shown a few ideas and examples on this site to help inspire your imagination.  What will you create?

Risks and challenges Learn about accountability on Kickstarter

The biggest risk facing this project is potential delay in shipments due to inventory of the materials that will be included in the kit. The adhesive-backed protective outer layer material has a small lead time and minimum order sizes. Although the distributor assures us that we can get the product cut to size quickly, there is always potential for delays. Fortunately, we have already identified alternate suppliers for all the components of the kit, so if any delays are anticipated from the primary suppliers, alternate ones can be used.

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    Early Adopter Plastic-Only package. Supporters of this level will receive the smart plastic only (one 4" x 6" sheet) and will be able to get their hands on the key component of the Sensor Film Kit before the full kit is ready to ship.

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    Early adopter package. Backers of this level will be the first backers to get a Sensor Film Kit. The kit includes one 4" x 6" sheet of the smart plastic, two 4" x 6" sheets of industrial grade aluminum foil (0.003" thick), two 4" x 6" sheets of clear adhesive-backed envelope material, two alligator clip leads, a shunt resistor, instructions for sensor construction, and many different schematics and example programs for using the kit with an Arduino micro controller.

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    The Sensor Film Kit. The kit includes one 4" x 6" sheet of the smart plastic, two 4" x 6" sheets of industrial grade aluminum foil (0.003" thick), two 4" x 6" sheets of clear adhesive-backed envelope material, two alligator clip leads, a shunt resistor, instructions for sensor construction, and many different schematics and example programs for using the kit with an Arduino micro controller.

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    Double Deal: Get double the amount of Sensor Film Kit components all in one package. Backers of this level will receive two 4" by 6" sheets of the sensor film, four sheets of 4" by 6" sheets of industrial grade aluminum foil (0.003" thick), four sheets of clear, adhesive-backed envelope material, two alligator clip leads, a shunt resistor, instructions for sensor construction, and many different schematics and example programs for using the kit with an Arduino micro controller.

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    The Full Package: Backers of this level will receive one Sensor Film Kit and one Arduino Uno. This is the complete package needed to use custom-made Sensor Film Kit sensors to create any of the examples shown here or create your own project.

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    Large Sensor Film Kit: for those interested in making much larger sensors or for those who want more sensor film material at a better price per square inch. Backers of this level will receive a 6" wide by 12" long roll of sensor film, two 6" wide by 12" long rolls of industrial grade aluminum foil (0.003" thick), six 4" by 6" sheets of clear, adhesive-backed envelope material, two alligator clip leads, a shunt resistor, instructions for sensor construction, and many different schematics and example programs for using the kit with an Arduino micro controller.

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    Educational Five-Pack: For Institutions, Educators, or just those who want to have more Sensor Film to play with. This is the equivalent of 5 Sensor Film Kits, and comes with five (5) 4" x 6" sheets of smart plastic, ten (10) 4" x 6" sheets of industrial grade aluminum foil (0.003" thick), ten (10) 4" x 6" sheets of clear adhesive-backed envelope material, ten (10) alligator clip leads, five (5) shunt resistors, instructions for sensor construction, and many different schematics and example programs for using the kit with an Arduino micro controller.

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