Equipo

Another way to record biokinetics is not to measure the movement itself, but the resulting pressure of it. This can for example be done on the soles of the feet for step recognition or on the inside of the fingers to determine if one holds something in his hands. Therefore, one of the sensor developments was chosen to be a pressure sensor.

5.2.1

Sensing principle

It was tried to implement the idea of a pressure sensor with changing capacitance between two conductive layers. Some calculations, however, showed that the resulting capacitance is too small to be measured with a micro-controller directly and therefore the idea has been dropped. On another occasion left over silicone from a stretch sensor was made conductive using the carbon powder. The resulting material also changed its resistance when pressure was applied and this idea was developed further in several other iterations.

5.2.2

Ratio of carbon vs. silicone

Different ratios of carbon powder to silicone have been tried. If not enough carbon is added, the cured mixture ends up not being conductive. If too much carbon is used the cured mixuture ends up being brittle. A patent found sug- gested a ratio of 3:1 of silicone and carbon for it to be conductive. Upon checking such a uncured mixture with a multimeter it turned out to be not conductive. More carbon powder was added and a weight ration from 2:1 proved to be well conductive while still retaining a good amount of flexibility. A test with 1:1 ratio of silicone to carbon powder did not retain the flexibility but got to the point of being brittle. Therefore, a ration of 2:1 seems to be the a good trade-off between the flexibility and the conductivity.

5.2.3

Leads inside the silicone

In order to get a proper interfacing with the sensor to a micro-controller cables have to be used. The initial silicone mixture proved to be sturdy enough to keep simple flex-core wires embedded and also electrically connected. One small problem was that the wires could be pulled out straight of the silicone. Soldering a small hook-like U-turn at the end of the flex-core wire solved that issue.

5.2.4

Moulding principle

The first tries of the pressure sensor were moulded similarly to the stretch sensor. A laser-cut acrylic mould with the central moulding area cut out and filled with the mixture. To make it thinner, the acrylic piece of the moulding area was used to press the mixture down flat. This was a problem for the leads encapsulated in the mixture, since it was then encased in acrylic from all four sides. To get the leads still in there, wholes have been drilled into the sides of the mould to stick the wires through. This made the sensors thinner but, since the wholes cannot be too far off centre in the acrylic, not thin enough. The final solution was to get rid of the encasing mould at all, only a top piece and a bottom plate remain. This allows the wires to be placed freely on the sides. When the top part of the mould is pressed down, the mixture spreads below it perfectly and any excess material oozes out at the sides. This is not a problem, however, since the overflow can still be cut off when cured. This method allowed for sensors to be as thin as 2mm, which could potentially still be improved when thinner wires are used.

5.2.5

Hardness of the silicone used

Different hardnesses of the silicone have been tried to make the sensor as flexible as possible. The initial iteration was made with a ShA33 silicone which gets a bit harder by adding the carbon particles. The results still showed some flexibility, but not as much as desired. To achieve that, the softer silicone with a hardness of ShA0 was used in a later iteration. The result was very flexible, but this came at the cost of not being able to hold the leads with wires inside the carbon any more. They could be ripped out of the silicone easily which makes this not feasible. Another try with ShA10 was done to try to achieve better results. It also shows improved flexibility compared to the initial version, but the concern about the robustness is still there. That is why the Sha33 is the final choice, because it is reasonable regarding flexibility and very sturdy in regard to the electrodes.

5.2.6

Attachment

Since a sole disc of silicone is not easy to attach or integrate into clothing, the sensor has been cast on top of non-stretchable, non-woven perforated fabric. This allows for the sensor to be sewn inside clothing or to be quickly attached with safety pins. The bonding of the sensor and the fabric was good, so it could not be peeled off it. It has also been applied to a stretchable, woven fabric which worked as well. But at the spot the sensor sits on the stretchability of the fabric was gone.