Research on smart textiles represents a new model for generating creative and innovative solutions for integrating electronics into unusual environments and will lead to new discoveries that will push the boundaries of science. This technology is beginning to take hold in several sectors such as military, health, sports, etc.
State of the art
Several groups of researchers have studied the subject of textile sensors. One of the most important and striking is the Wealthy project.
- A Novel Textile Stitch-Based Strain Sensor forWearable End Users - Orathai Tangsirinaruenart and George Stylios
This article presents a survey of textile stretch sensors and evaluates their performance. The electrical resistance and mechanical properties of seven different textile sensors were measured and compared.
- Technical Embroidery for Smart Textiles: Review - Viktorija Mecnika, Melanie Hoerr, Ivars Krievins, Stefan Jockenhoevel, Thomas Gries
This article from Riga University of Technology, details the use of embroidery in intelligent textiles, especially in sensors and circuits.
In this project, I chose to play with the arrangement of the conductive wires to make stretch and crease sensors fully embroidered and washable.
Materials used :
- iron-coated wire : because this wire is very resistant over the length (500 Ohm/m) compared to the usual iron wires used to sew circuits (50 Ohm/m)
- Neoprene jersey fabric: this fabric is very elastic and resistant to tight and complex seams.
- Janome MB4S : digital embroidery machine
- Simple sewing machine
How the sensors work :
Stretch and crease sensors are very similar, as both have a resistance that will play out depending on the points of contact.
Stretch sensor :
Stretching the fabric increases the electrical resistance along the conducting wire. This is due to the opening of the mesh
and thus the breaking of the parallel contact points
, forcing the current to flow in series rather than parallel. This increase in the conductive path results in greater resistance as shown in the figure :
Crease sensor :
The conductive thread is sewn over an entire surface, like this :
When the fabric undergoes wrinkling, it means that it will be folded on itself, and create contact points on the surface, which will allow the current to short-circuit the pattern at this point, so this reduction in conductive path will result in less resistance.
How to make the sensors :
Stretch sensor :
For the stretch sensor we are going to use the sewing machine
- Place the conductive thread in the bottom spool and place the thread of the color of your choice above it
- Set the stitch mode to zigzag
- Set the stitch distance to 0.6 mm and the seam width to 3.0 mm.
- Position your fabric and start sewing to the length you want.
Your stitching should look like this:
and stretched should look like this:
The resistance of this sensor is taken at both ends of the seam.
Crease sensor :
For this sensor we are going to use the janome MB4S since we are going to make a complex diagram that would require a lot of time at the sewing machine.
- Choosing the sensor design
As the principle of the wrinkle sensor is to apply the conductive wire to a surface without it touching itself, we are quite free on how we will arrange the wire.
Let's take for example this one
2.Place the conductive wire in the bottom spool and the wire of the color of your choice on the top.
3. Place the fabric of your choice in the frame.
4. sew the design you created with the janome
If you don't know how to make janome work, I advise you to consult the tutorials dedicated to the subject that you will find at the bottom of this page.
Results achieved :
The stretch sensor was tested for different seam lengths and different forces ( modelled by the deformation length of the sensor) and by noting the evolution of the resistance according to the degree of stretching for different seam lengths, a linear evolution was observed.
So not only can you tell whether the fabric is stretched or not, but also to what degree it is stretched by measuring the initial length of the sensor.
The interesting thing about the crease sensor is that we have quite a lot of freedom on how to design the circuit according to its needs. And unlike the stretch sensor where the choice of fabric is important, with the wrinkle sensor the choice is quite free.
These sensors can be used in several fields, whether as potentiometers for an analog synthesizer, or as a measurement tool for motion capture, the uses of these sensors are very wide.
The interest of making these textile sensors was mainly to become familiar with and master the tools available to make textile circuits. In addition, the fact of doing this project will allow more freedom, understanding and especially efficiency on the projects around the smart textile.