Flex

Flex Sensors, by Rebecca Bray, Spring 2006

Spectra Symbol Flex Sensor, Abrams-Gentile Flex Sensor, FlexPoint Bend Sensor (click names for datasheets)

Go here for a full report that includes movies and more images.

Flex sensors are sensors that change in resistance depending on the amount of bend on the sensor. They convert the change in bend to electrical resistance - the more the bend, the more the resistance value. They are usually in the form of a thin strip from 1"-5" long that vary in resistance from approximately 10 to 50 kilohms. They are often used in gloves to sense finger movement.

There are two main brands of flex sensors that are affordable and easily available: SpectraSymbol Flex sensors and the Gentile-Abrams sensor, available from Jameco (part #150551 for $12.95). There are variations on the typical flex sensor such as the FlexPoint Bend Sensor which measures force as well and has a wider resistance range. Interlink is another brand that senses force as well. The Flexpoint sensors range in price from $3-$7 depending on length (1"-3"). They also add on a plastic connector casing for $0.75.

The Flexpoint Bend Sensor was developed through testing a thin potentiometer which would show analog feedback from finger movement. The bend-sensitive carbon-based ink was developed at that point.

I tested the following for these experiments: 2 different Abrams-Gentile sensors, 1 SpectraSymbol, and 2 FlexPoint Bend Sensors - a 1" and a 3".

Applications

Flex sensors are used in gaming gloves, auto controls, fitness products, measuring devices, assistive technology, musical instruments, joysticks, and more.

The Flexpoint bend sensor was first developed for automotive airbags and now is also used in car horns, toys that detect different degrees of pressure or bending, robots, machine control, medical devices and assistive technology.

ITP-related projects using flex sensors:

Glove by Dennis Crowley

Flex Sensor Ping Pong Paddle

ASL Glove - Digital Sign Language using flex sensors on a glove to sense what sign is being made

Container for Sound - gets MIDI data into Max/MSP from a flex sensor

How They Work

Flex sensors are analog resistors. They work as variable analog voltage dividers. Inside the flex sensor are carbon resistive elements within a thin flexible substrate. More carbon means less resistance. When the substrate is bent the sensor produces a resistance output relative to the bend radius. With a typical flex sensor, a flex of 0 degrees will give 10K resistance will a flex of 90 will give 30-40 K ohms. The Bend Sensor lists resistance of 30-250 K ohms.

The Flexpoint Bend Sensor is somewhat different from the simple flex sensors. It has a much bigger range of resistance - see the chart below.

It might be useful to note that these companies will do custom jobs, including screen-printing these special carbon-based inks on various materials.

Electrical Characteristics

Size: approx 0.28" wide and 1"/3"/5" long
Resistance Range:1.5-40K ohms depending on sensor. Flexpoint claims a 0-250 resistance range.
Lifetime: Greater than 1 million life cycles
Temperature Range: -35 to +80 degrees Celcius
Hysteresis: 7%
Voltage: 5 to 12 V

Schematics

Flex sensors can be used in a variety of ways:

Basic Circuit: Flex Sensor as Voltage Divider - Output voltage increases with the bend.

The impedance buffer in the circuit above is a single sided operational amplifier, used with these sensors because the low bias current of the op amp reduces errer due to source impedance of the flex sensor as voltage divider. Suggested op amps are the LM358 or LM324.

You can also test your flex sensor using the simplest circut, and skip the op amp.

Adjustable Buffer - a potentiometer can be added to the circuit to adjust the sensitivity range.

Variable Deflection Threshold Switch - an op amp is used and outputs either high or low depending on the voltage of the inverting input. In this way you can use the flex sensor as a switch without going through a microcontroller.

Resistance to Voltage Converter - use the sensor as the input of a resistance to voltage converter using a dual sided supply op-amp. A negative reference voltage will give a positive output. Should be used in situations when you want output at a low degree of bending.

Code Samples

In order to measure data for various sensors, I used a very basic circut with a stable resistor and the PIC18F452 microcontroller. The PIC code outputs one value from the sensor and the Processing code reads it after call and response, and outputs the numbers and visualization on screen. PIC code for data visualization Processing code for data visualization

Typical Behavior

Go here to see movies of the various sensors in action, with the data visualization in processing.

The Flexpoint Bend Sensor gave the greatest range of values for the bend. The Gentile-Abrams brand was the noisiest. In fact, when connected to a 6' wire, it was virtually unreadable.

Resistors Here are number values based on my experiments with various resistors with the Bend Sensor.

Some things to know when working with flex sensors:

Most flex sensors give better readings went bent towards the printed side. Backwards bending usually results in undesirable results while forward bending can give a nice logarythmic curve.

• Resistors matter! See the FlexPoint resistor experiment data above.
• Mount them on a surface firmly and evenly for more reliable readings
• Connections should be sealed with hot glue, heat shrink or plastic encasings.
• Be aware that any irregular bending may produce irregular results. For example, the two bends shown below will give very different data - the one on the left is bending backwards first which will usually give unreliable results.

• On most sensors, but the Jameco ones in particular, a longer wire connection between microchip and sensor will dramatically increase noise in the readings.
• Most flex sensors seem to take some time to revert back to low numbers at rest. In other words, the original at rest/ no bend reading may be 65, but after bending, the low number at no bend will be 80 for at least 20 seconds.
• It may seem obvious, but the flex sensor reads bends best. In the example on the right, the tops of the sensors are in a similar location, but the one on the right will give a finer resultion and more reliable readings.
• Curving the flex sensor around an object, such as dowel or finger, increases the resistance.

Application Notes

My application:

I was thinking about using the flex sensor to determine when a cardboard figure on top of a long vertical rod is moving. I was thinking that the flex sensor could detect the bend in the rod, but after extensive testing it looks like a tilt sensor would work better. Go here for movies showing glove and cow applications

Flex sensors are great for projects wherein you need to detect from at 10 to 50 degree bend, and sense a change of a least 10 degrees. Some possible applications for projects here at ITP:

- detecting the opening of a lid

- wearables that detect finger movement, elbow or knee bending

- assistive technology - movement on a chair, using or gripping objects correctly

Rebecca Bray, Spring 2006