Hello, my name is Andreas Pointner and on behalf of my co-authors i'm going to present to you TexYZ:
Embroidering enameled wires for three degree-of-freedom mutual capacitive sensing.
In our paper we describe a method for rapid manufacturing of textile mutual capacitive
sensor grids using a commodity embroidery machine.
While the majority of related work in this field focuses on self-capacitance sensing,
there are a few notable exceptions.
I/O Braid, for instance, presents an interactive cord. By incorporating conductive yarns during the braiding process
a helical sensing matrix is created.
This matrix is scanned using the mutual capacitance approach to detect various gestures .
In the paper Sketch&Stitch a multi-layer approach is detailed for fabricating
two dimensional sensor grids.
As bare conductive yarns are used for traces, insulating layers are required
at electrode intersection points to prevent shorts.
The primary goal of our work is the augmentation of textiles with sensor grids
that use mutual capacitance sensing to achieve multi-touch detection capabilities
and high robustness to electromagnetic noise.
Additionally, we aim to simplify the manufacturing of such sensor matrices
by providing a single run and single layer machine embroidery solution.
Furthermore we present our findings regarding the scalability of this approach
to yield differently sized sensor grids and evaluate sensitivity and durability aspects.
Each sensor is fabricated with a polyester embroidery yarn as upper thread
and textilewire as bobbin thread.
This configuration is chosen to minimize stress on the enameled copper wire
during the embroidery process and thus preventing wire breaks.
When embroidering metal wires, thread tension settings for upper
and lower thread have to be particularly high and also well balanced.
When the upper thread tension is set to low the thread can get pulled out of place
by the bobbin, resulting in distorted electrode patterns.
Too little bobbin thread tension, on the other hand, can lead to undesirable wire loops.
However, applying high amount of tension to the bobbin wire causes pattern distortions.
Therefore we added additional stitches to support acute angles as they distribute
the exerted tension among them.
Our objective was to inspect different trace layouts to learn about
their advantages and downsides.
We chose commonly occurring patterns, prepared each one of them for embroidery
and implemented them on textile.
For selecting the two most promising candidates, our two main criteria were sensor capacitance
and manufacturing accuracy.
Small scale sensor matrices require patterns that still feature
adequately high capacitance at small sizes so we can operate them with commodity sensing hardware.
The manufacturing accuracy, on the other hand, controls the homogeneity and consistency
of individual sensor cells within the grid.
The chosen patterns to continue with the fabrication of embroidered sensor matrices
were the diamond spiral as well as the meander sensor layout.
Therefore, we implemented each one of them in two different sizes.
We measured signal to noise ratio for each sensor cell in all four
matrix variants to determine their sensitivity to touch.
For consistent results we used a stylus held in place by a custom built pmma frame.
This construction was aligned for each sensor before measurements were taken
with a grounded weight placed on top of the stylus to simulate a soft touch.
Peak to peak normalized SNR was measured using a Cypress PSoC 4.
A ratio of at least 5:1 is recommended to ensure reliable touch sensing performance
with this hardware platform.
As can be seen, the larger matrices yield significantly better results
than the smaller ones, easily exceeding the minimum requirement of 5:1.
The diamond spiral slightly outperforms the meander.
In contrast, smaller scale sensor matrices perform better when implemented with the meander pattern.
To demonstrate our sensing matrix for continuous three degree of freedom input we implemented
a wristband prototype for a smartwatch.
We use this extended interaction space for a basic text input application
and a basic map application, where panning is achieved by rolling the thumb on the sensor patch
and pressure information is used to control the zoom.
Furthermore, we augmented a pair of pants by sewing a sensor patch on them.
For this demonstration, the input was forwarded to a computer game built in unity 3d
for controlling an avatar in first person view.
We also evaluated the impact of washing on the sensor's performance.
The washing program was chosen specifically to put high amounts of mechanical stress
on the textile under test.
We noticed a considerable decline of sensor quality with each washing cycle.
Furthermore, wire breaks started to occur after the third washing cycle.
Locations of wire overlaps proved to be the weak spots.
Lastly, we also evaluated the effective sensing range of our sensor grids
by performing snr measurements at varying distances to the sensor.
Given these results, we estimate that robust sensing at a distance of 2 millimeters could still
be realistic for a large scale meander matrix.
Therefore, allowing for thin oblique textile covers on top of the sensor grid.
This concludes my presentation of our work about mutual capacitance sensing on textiles.
Thank you for your attention.