Comparison of electrode size

In order to identify the influence of the electrode size, we use our reference shape of a rectangle and create a few electrodes with different sizes: 2x2 cm, 3x3 cm, 4x4 cm, 6x6 cm, 8x8 cm, 10x10 cm. They are created from both materials: conductive textile and conductive thread. The textile material used is the same as the one used in Section 5.1,Zell-RS. It is not stretchable and is woven in a ripstop manner.

The thread used for sewing is conductive thread made of two conductive strands,Adafruits Stainless Thin Conductive Thread²(0.2 mm thick, 2 ply thread, 1.3 ohm per inch). It is widespread throughout the shops for electrical craftship. The created electrodes are shown in Figure 5.13 and 5.12.

The textile electrodes are created by cutting out pieces of conductive textile, in the desired size.

They are subsequently machine-sewed to the supporting fabric. The thread used to sew the conductive material to the support material is regular thread. The snap for the connection with the wire and through that to the sensor, is sewn by hand using conductive thread on the reverse side of the fabric. The used supported fabric is 100% cotton, which is tightly woven, thus is not significantly stretchable.

The electrodes made of conductive thread are created by sewing the perimeter of the desired rectangle size. For this, a sewing machine was used equipped with a regular and a conductive thread. The rectangle is sewn in straight stitch on the setting "2" in stitch length. For the utilized sewing machine this represents a step length of 1.75 mm. The snap is sewn in a similar fashion as with all electrodes using conductive thread.

2https://www.adafruit.com/product/640

5.2. Efficiency of e-textiles in capacitive sensing

Figure 5.13.: Electrodes made of conductive thread rectangle perimeters of different sizes: 2, 3, 4, 6, 8 and 10 cm

The expected performance, for both conductive textile as well as conductive thread is derived from the basic formula of the capacitance, where the capacitance is proportional to the electrode area and inversely proportional to the distance between the capacitor plates, see Section 2.4.1.3 for more details.

C=ε0εrS d

Thus, with the area of the textile electrodes rising from 2-10 cm the capacitance at a given distance should also be bigger as the electrode size increases.

Figure 5.14 shows the resulting values of the measurements of different sizes of conductive textile electrodes. The upper left diagram shows the raw shifted data. All curves were shifted to the common value they are converging to. The placebo electrode has the lowest values and then the values increase with increasing size of the electrode. This diagram confirms the expected behaviour in performance. By plotting the normalized curves, one can see that the slopes of the curves are very similar. Consulting the graph displaying the NR, it shows that the NR reaches values of 6 mm. Up until a distance of 200 mm the NR is low, and afterwards it increases. In contrast, looking at the graph displaying the SNR, the values of the noise are higher in the first 7 cm in relation to the sensor values, resulting in lower SNR.

After the 7 cm the SNR of most electrodes is higher, indicating that the noise, in this case the standard deviation is smaller as at the beginning. The contrast between the good NR at the beginning and the worse SNR at the beginning do not indicate conflicting measures, they show that the NR does not solely depend on the standard deviation - the noise - but also on the slope of the curve, which for all electrodes

5. Properties of flexible capacitive proximity sensing electrodes

Figure 5.14.: Comparative graphs of conductive textile size comparison: shifted raw sensor data and standard deviation (top left); normalized sensor data (top right); Noise Range (bottom left); Signal-to-Noise Ratio (bottom right).

5.2. Efficiency of e-textiles in capacitive sensing

Figure 5.15.: Comparative graphs of conductive thread size comparison: shifted raw sensor data and standard deviation (top left); normalized sensor data (top right); Noise Range (bottom left); Singal-to-Noise Ratio (bottom right).

5. Properties of flexible capacitive proximity sensing electrodes

is very steep, especially for the 8 cm and 10 cm. Thus, the noise is not significantly high to influence the ability to discriminate between different distances.

Figure 5.15 shows the comparative graphs of rectangular electrodes made from conductive thread in different sizes. The graph on the top left represents the raw sensor data which is shifted. Similarly to the conductive textile electrodes, the smallest values at a given distance is achieved by the placebo electrode, with increasing values as the size of the electrode increases. The normalized graph on the top right shows that the curves have similar slopes. What can be observed is, that the behaviour observed at the distances of 1 cm for the conductive textile electrodes is not present anymore. Nonetheless, the values of the electrodes of sizes 6, 8 and 10 cm are very close together. This could be explained by the fact, that they are bigger as the reference electrode used for the measurements. In the NR view on the bottom left it still can be observed that the 10 cm electrode performs best in terms of NR. In contrast to to the conductive textile electrodes, the NR up until 20 cm is very low for all electrode sizes, indicating a good discrimination of distances. From 20 cm up, the NR starts to vary more, however it reaches a maximum at 6 mm, which indicates that even at a distance of 29 cm the distance of the electrode can be discerned. In practice a 6 mm NR means that an object at a distance of 29 cm could be confused with an object at 28.7 cm or at 29.3 cm. The SNR at close distances is small, indicating a higher standard deviation. After the distance of 7 cm, the SNR varies, but settles down, similarly as the behaviour of the SNR observed for the conductive textile electrodes.

Concluding, the two Figures 5.14 and 5.15 give an indication which the best performing electrode is in this group of size comparing electrodes. For both cases, the conductive textile and conductive thread, the largest electrodes have the smallest overall NR, thus the best performance. The overall NR of the conductive textile electrode of size 10 cm is 0 mm and the overall NR of the conductive thread electrode of size 10 cm is 4 mm - as also the trendlines indicate. The 2 cm electrodes have in contrast the highest overall NR besides the placebo electrode. This confirms the expected behaviour, supporting that a larger surface results in a higher detection range. These graphs show that this behaviour can be observed not only for surfaces of conductive material, but also applies to perimeters of rectangles made of conductive thread.