This bend sensor actually reacts (decreases in resistance) to pressure, not specifically to bend. But because it is sandwiched between two layers of neoprene (rather sturdy fabric), pressure is exerted while bending, thus allowing one to measure bend (angle) via pressure.
>> Fabric Bend Sensor Instructable
>> Neoprene Bend Sensor IMPROVED Instructable
We find that this construction method gives good results (sensitivity) for measuring the bend of human joints when attached to the body. It is sensitive enough to register even slight bend and has a large enough range to still get information when the limbs are fully bent. The neoprene is great for isolating the conductive thread stitches and keeps the sensor from wrinkling even when repeatedly being bend.
The resistance range of this bend sensor depends a lot on the initial pressure. Ideally you have above 2M ohm resistance between both contacts when the sensor is lying flat and unattached. But this can vary, depending on how the sensor is sewn and how big the overlap of the adjacent conductive surfaces are. This is why i choose to sew the contacts as diagonal stitches of conductive thread – to minimize the overlap of conductive surface. But only the slightest bend or touch of the finger will generally bring the resistance down to a few Kilo ohm and, when fully pressured, it goes down to about 200 ohm. The sensor still detects a difference, right down to about as hard as you can press with your fingers. The range is non-linear and gets smaller as the resistance decreases.
How to make a fabric bend sensor
The following stencil is a good guide for building your first bend sensor. But the great thing about this design is that it is highly customizable. You can easily change the shape, size and some of the materials in this sensor to better suit your own project. For a more sensitive sensor increase size of the conductive surfaces on either side of the Velostat. The piezoresistive (pressure sensing) properties of Velostat work through the material and not across it, so your conductive surfaces need to overlap adjacently on either side of the Velostat. Reduce or increase the overlapping conductive surface by removing or adding more stitches or even by replacing the conductive thread by a piece of conductive fabric. Take note that the conductive stitches in the following stencil are designed so that they only overlap in one point, reducing the conductive surface overlap to an absolute minimum. But the sensor is still very sensitive. To some degree you can also control the sensitivity of the sensor by adding multiple layers of Velostat. How tight you stitch together your sensor also impacts the range it will be able to detect, as pressure coming from these stitches will eliminate part of your sensing spectrum.
Materials and Tools
Materials: Neoprene, conductive thread, stretch
Tools: Fabric scissors, fabric pen, stencil, sewing needle, iron, pliers
Trace the stencil
After cutting out the neoprene pieces, trace the stencil to them. Fuse tabs of conductive fabric to either end.
Sew with conductive thread
Thread a needle with conductive thread and stitch along the stencil pattern until you reach the tab of conductive fabric. Stitch the conductive thread to the conductive fabric with a few stitches to make good contact. Repeat for both sides.
Layer the neoprene with the conductive traces inwards so that the stitches criss-cross. The Velostat goes in between.
Sew the sensor together around the edges making sure not to sew too tight.
To make sure your bend sensor is working. Connect either end to a multimeter and set to measure resistance (ohm). Your initial resistance should lie around 2K ohm and should sink to about 100 ohm when pressured by pushing down hard.
When attaching the sensor to the body one has to pay attention not to create too high initial pressure. Otherwise the sensor’s range is useless. We like to use metal poppers to connect the sensors to the rest of the wearable circuit.
Video: Neoprene Bend Sensor IMPROVED (Sheep demo)
Using Eeonyx stretch
Trying out some new things. Stitching the conductive thread traces next to each other works fine in combination with Eeonyx fabrics, but not with Velostat. Conductive fabric traces work even better in this case.
YouTube Fabric Bend Sensor Playlist
Patent for the Angular displacement sensor, patented by AEG in 1989, it is now open:
Wonderful tutorial from Images – Scientific Instruments on how to make your own bi-directional flex sensor:
>> Images bi-directional flex sensor Instructable
Great tutorial from the Pulsar project on how to make bend sensors using Velostat:
Summary of bend sensors on Sensorwiki: