Hello everybody, welcome to this CHI 2020
paper presentation, I'm Roland Aigner
and together with my co-authors Andreas Pointner
and Thomas Preindl I performed
a study on embroidered resistive
pressure sensors. So the objective here
is to manufacture force sensitive
resistors, or FSRs for short, on a
textile basis, using an embroidery
machine, and to get a better
understanding about their
characteristics and performance. Just as
a quick introduction to those who are
not familiar: FSRs are electrical
elements that change resistance whenever
stress is applied, which then can be
measured quite easily. They date back
to a patent by Franklin Eventoff from
1979 and they're quite widespread. They
are not particularly precise, but they
feature several advantages. They are easy
to fabricate, they're inherently low cost, durable
and very easy to use with basic driver
electronics. For instance you can use a
simple voltage divider which you hook
into an ADC of your Arduino
and you're good to go.
In a common implementation they consist
of sandwiched foils with resistive
polymer film in combination with interdigitated
conductive grids, which are
screen-print on top. Now when the
electrodes are pressed onto the polymer
sheet, that gives you variable resistance
depending on pressure. We replicate this
on textile accordingly, by using a
conductive yarn and a textile based
resistive sheet. These two electrode
yarns are then connected to measurement
electronics, which perform the readout
and the processing. Here you can see the
basic design of our sensor. We have a
backing material where the sensor is applied
upon we, have a carbon-based
semi-conductive material for a resistive
sheet and we have two electrodes made
from conductive yarn, which is looped
with a bobbin thread on the backside. In
our case the resistive material is a
CARBOTEX from Swiss company SEFAR;
it's a wave of PA monofilaments
which are covered with layers of carbon
particles and you can get them in
different mesh counts. There are also
other options for a resistive material -- a
very widespread one is
the Eeonyx, which then is not a wave, but a knit,
meaning it's stretchable. But we noticed
its behavior is kind of inconsistent, so
we decided to go with the CARBOTEX. In
terms of conductive thread we evaluated
a high number of candidate yarns.
You can find details in the
supplementary material but on our paper --
in the end we decided to use the
Madeira HC40. Note by the way that
the electrodes do not necessarily have
to be on top of the resistive sheet; one
or even both of them may also be below,
meaning you can create electrode traces
with intersections, which is quite useful,
as you will see later. Now it is often
stated that the change of resistance in 
an FSR is due to the compression
of particles in the piezo-resistive
material, when pressure is applied. 
Howeverl, according to the ICMA paper of
Weiss and Wörn, what's most contributing to the
functionality seems to be the so-called
interface effect. They state that the
polymer surface, which is kind of rough
on a microscopic level, is in little
contact with the electrodes at rest and
whenever pressure is applied,
the surfaces are compressed together.
This increases the contact area quite
dramatically and in turn decreases the
electrical resistance. This is also in line
with our observations of how our textile sensors
work, namely that the yarn is in loose
contact at rest, and when the yarn is
pressed closely to the resistive sheet,
the resistance decreases significantly. 
Now, there are several design parameters
influencing the characteristics of our
textile sensor, and the question is: can
we estimate them beforehand? What value
ranges will we get, and what are the
actual correlations to physics and
do they even hold for the rather
imprecise nature of textile
manufacturing? Well turns out that, yes,
they do, but let's first have a look at
what our parameters are in the first place.
We know that the resistance of an
electrical conductor depends on its
cross-section area, its length, and on a
resistivity constant that depends
on the actual material. So this is quite
intuitive. Translating this to our
textile sensor, we can see that the
contact area A is basically the traces
of our conductive yarn, which we
approximate with electrode length l
times sheet thickness t, while d is simply the
distance between the electrodes. Here we
see this mapped on our sensor patch.
Now we can go on and tell that the
material thickness is also constant, 
right? We can postulate we use the same
material for all of our sensors, hence we
can summarize material resistivity and
thickness to a single constants Rs which
is commonly termed the sheet resistance.
Now in our evaluation it turns out that,
yes, when we vary electrode distance d,
the resistance of the resulting sensor
patch scales directly proportional.
It's even strikingly linear. On the other
hand, when we vary electrode length, this
is also in line with theory, we can see
that the sensor resistance is inversely
proportional,
just as the formula suggests. So this
means, we can to some degree calculate
on this basis and approximate what the
resistance of the sensor will be beforehand,
and therefore, we can design it with
the capabilities of our hardware in mind.
Furthermore, we identified several
additional parameters that are also
highly influential to the sensor
characteristics, most of them are related
to the interface effect I mentioned
earlier, for example what we call "double
stitches", which is when you embroider
both ways along a single track, there
and back. This is necessary for some of our
pattern designs, as you will see later.
And expectedly it has a huge impact on
the amount of contact between yarn and
resistive sheet. Also mostly related to
the interface effect, there are several
more parameters, such as stitch length,
electrode layering, and intersections,
which we also evaluated. I skip over them
here due to the time limits, so please
refer to the paper for more details.
Instead, since our work is also about
space filling parents,
let's dive into our motivation for that.
Initially, we are coming from this angle
of fairly dense and high-resolution
sensing matrices that you get, when you
combine grids like in Cheng's work,
depicted here on the left. 
However, this isn't always
required or possible. Grau presents a
work dealing with mechanical force
redistribution for high accuracy
interpolation between sensor cells,
however this requires a rigid setup and
it's therefore not applicable
for textiles. Furthermore we don't want
to be stuck to square sensor cells and we
would like to combine also different
cell sizes. Suppose you have a layout
like this here. With a rather traditional
approach you would get something like
this. But this doesn't scale so well.
What if this is of a size of 10 by 10
centimeters and you want to sense
finger-touch. There are dead spots all
over the place, where you're not able to sense
anything. So what you can do to
compensate is obviously to increase
resolution, but this means, the minimum
size of your actuator dictates the
number of electrodes. And that's
unfavorable, since it also increases
complexity in electronics and processing,
it increases the amount of data, it
probably decreases your achievable
sample rate, and maybe you're not even
interested in this additional data. Then
it also causes unnecessary computational
load for data processing or downsampling.
And maybe you are limited
because you're running on an embedded
system -- you get the idea. For that reason
we propose to use space filling patterns
instead. So you use the same number of
electrodes but you guide them in a sort
of winding manner. This way, they cover
most of the space and you get a somewhat
uniform responsiveness all over the
sensor area. And in the textile field
this takes us to embroidery, because
embroidery machines are specifically
designed and tuned for stitching
arbitrary shapes and sizes. Also, the
workflow of designing or even
automatically generating shapes and
patterns
and then sending them to the machine
is quite useful for rapid prototyping. 
There you see an example of a resulting
sensor, front side and back side. Note that
the white bobbin thread on the back
side simply holds the electrode stitches
in place but, that's regular yarn, so only
the electrode traces on top are actual
conductive yarn. You can also fabricate
them in numerous scales due to the fractal
property of this particular pattern here,
without having to sacrifice resolution,
and without causing any dead spots. It
also gives you control about several
design options you can link multiple
patterns, you can combine different
scales depending on your required
granularity, however you like.
Note that for the Meander pattern, the
electrodes have to be on different
layers, thus separated by the resistive sheet;
one electrode is below, one above,
otherwise you would get obviously a
short at the center, where one trace
crosses the other. Some other patterns
don't require these intersections;
they're always beneficial. Now let's see
the actual space filling patterns that we
compared. We chose them along several
properties such as intersection count
and ratio of double visus single
stitches. First is the interdigitated
electrodes or "IDE" for short, which is
probably the most common for printed
FSRs. It has no intersections but
lots of double stitches along the forks
here. Second is the Boustrophedon, quite
the opposite, with lots of intersections
but no double stitches whatsoever. The
Meander, already mentioned, which we found
in related literature in context of
capacitive sensing. It has a single
intersection in the center, which causes
non-uniform response across the sensor
area, meaning it is most sensitive in the
center, which is in fact quite bad in
some use cases. This kind of Fermat spiral
avoids this intersection by just
stopping at the center, which means that,
when you want to link multiple of those,
you have to stitch inwards to the center
and then back out again.
So you would actually double stitch 100%
of your traces, which is probably a trade-off.
Lastly, the Hilbert Curve is an example
of a space-filling curve from
mathematical analysis, so we decided to
include one of those as well in our
evaluation, as they frequently show up. It
shows no intersections, however for
linking multiple of those, you also have
to double stitch most parts, just like
with the Fermat spiral.
Reason is there is dead-end for the
upper electrode right here; and to guide the
needle back out again, without
introducing an intersection, you would
have to stitch all the way back.
Also, most candidates of those space
filling curves, meaning Hilbert, Peano, Moore
curves, and so on are probably the worst in
terms of scaling, since their sizes
rise exponentially with curved order. For
the others presented here you are quite
flexible: you can for example always add
one more loop to the Meander when you
need it slightly bigger, but this doesn't
work for the Hilbert. Of course, there are
many many more possible patterns but
this should provide a good coverage for
lookup. So, we expect any possible pattern
will have a good equivalent within this
set. In terms of characteristics they all
perform similar, however in vastly
different ranges. Just to pick the most
outstanding one as an example, which is
the Boustrophedon: with its many
intersections it has a very low
resistance, starting at only 150 Ohms at
rests for the order 2 implementation, in
contrast to the Fermat with almost
ten times the resistance at the same
size. This will probably have
implications on the choice of your
electronics. I want to close with some
lessons learned in terms of
manufacturing, because this is highly
important in practice. First of all: don't
choose the stitch lengths too low. Not
only would you reduce the dynamic range
but you may also destroy or severely
harm the resistive sheet, which could even
rip in the worst case. Secondly, also
consider the backside of your patches.
Keep in mind that the bobbin thread
may pull conductive yarn out of place,
causing shorts between the two
electrodes.
Ideally, you already design your parents
with that in mind.
Lastly, whenever the machine trims the
conductive yarn, you unfortunately have
to double-check. There's always some
residue yarn, that you have to trim
manually, meaning with scissors. Tiny
fringes, hardly recognizable with bare eye,
will cause shorts and render your sensor
defective. So this was it thanks for
watching and if there are any questions
please don't hesitate to send me an
email or visit our website at mi-lab.org
