Influence of repeated switching of current through contacts made of electroconductive fabrics on their resistance

2.1 Textile materials used

The basic parameters of 11 electroconductive fabrics used for testing are presented in Table 1. The data presented in this table are taken from the manufacturer’s datasheet.

2.2 Measured parameters

All the measurements presented in the article were carried out under specific standard atmospheres at 23°C and 50% relative humidity according to standard atmospheres for conditioning and testing [20]. The research presented in the article consists of two stages. In the first stage of research, temperature measurements of electroconductive fabrics were performed as a function of the density of the current flowing through them. The tests were aimed at determining the maximum current density flowing through the cross-section of the tested fabrics, which will not result in exceeding their maximum permissible temperature. For further consideration, it was assumed that this temperature could not exceed 50°. This temperature protects the clothing user against possible burns. The results obtained at this research stage were to be used, among others, to determine the minimum width of contacts made of electroconductive fabrics used in the main studies. The measurement stand, used to carry out these tests, consists of two clamps where the tested electroconductive fabric is mounted, a DC power supply, and an FLIR ThermaCam E65 thermal cam. Before surface temperature measurements, samples of the tested fabrics were placed in an air-conditioned room to estimate their emission factor ε. Knowing this coefficient for each tested fabric enables a more accurate measurement of their surface temperature using a thermal imaging camera. After acclimatization, the fabric’s surface temperature was measured using a thermal imaging camera. If the surface temperature measured by the camera deviated from the temperature in the acclimatized room, the ε value in the camera was changed to obtain the sample surface temperature equal to the temperature in the room. After equalizing these readings, the ε obtained for the tested fabric was taken for further measurements. The process was repeated for each of the tested fabrics. The temperature in the room near the surface of fabrics was measured using a Tempmaster-100 thermometer which assures ±0.04°C basic accuracy.

Next, samples with the shape shown in Figure 2 were cut out of the fabrics whose parameters are shown in Table 1. A laser plotter was used to cut them to increase the repeatability of their dimensions. The cut samples were clamped between the clamps shown in Figure 3. The places of fabric where the measuring clamps are attached are marked in grey in Figure 2.

Figure 2

Shape of the fabrics used for studying changes in temperature under the influence of a flowing current.

Figure 3

Clamps (1) with tested fabric mounted (2).

The fabric shape shown in Figure 2 causes its cross-section to change linearly. This allows us to obtain the temperature distribution on the surface of the fabric in a single image from a thermal imaging camera. This facilitated the subsequent analysis of the obtained results and enabled easy presentation (in one photo) of the dependence between the temperature and the fabric width for the assumed current flowing through it. A DC power supply was connected to the clamps (1) shown in Figure 3 to ensure current flow through the tested fabric. This current was set using the current control knob of the DC power supply and measured using a built-in ammeter.

The tested fabric (2) was positioned with its surface oriented vertically during the measurements, as shown in Figure 3. This was done to simulate the working conditions in Smart clothing, where the surface of these fabrics is typically oriented perpendicular to the Earth’s surface.

In the second part of the research, the impact of repeated disconnection of current through rectangular contacts made of electroconductive fabrics on their electrical resistance was tested. The measurement stand shown in Figure 4 was constructed to investigate this.

Figure 4

Measurement stand.

The stand consists of two pneumatic actuators (1), an electrovalve (2), a force sensor (3), a computer acquisition board (4), a PC (5), an Agilent 34410A multimeter (6), two DC power sources (7), and a Tektronix digital oscilloscope (8) equipped with a current probe (9). An Agilent 34410A multimeter (6) operating in 4-wire mode was used to measure the resistance between two tested textile contacts. This instrument has an accuracy of ±0.0030% of reading ±0.003 ohm of range.

The PC (5) with a LabVIEW procedure controls all measurement processes. Pneumatic actuators are controlled by an electrovalve and used to connect and disconnect the tested electroconductive fabrics. The general appearance of the sample mounting holders and pneumatic actuators is shown in Figure 5.

Figure 5

Sample mounting holders and pneumatic actuators.

Each of the actuator piston rods (left and right) is equipped with a clamp (2) (Figure 5) with a rectangular strip of electroconductive fabric (5) (Figure 5) on it, which constitutes the tested electrical contact.

Each tested textile contact in the form of a strip of fabric (5) is placed on flexible rubber foam (4), which improves the contact of both tested fabrics when they are connected and pressed together. The ends of each strip of fabric are mounted between two plates (6) made of copper laminate usually used to produce printed circuits. The test leads of the Agilent 34410A multimeter are connected to the left and right clamp, assuring resistance measurement. A DC power supply (7) (Figure 4) is also connected to these clamps to ensure current flow through the tested contacts. This source is automatically disconnected from the tested contacts during the measurement to ensure the correct resistance measurement. The computer (5) (Figure 4) uses a computer acquisition board (4) (Figure 4) to control the electrovalve (2) (Figure 4) to ensure cyclic movement of the actuator piston rods, causing cyclic connection and disconnection of the tested fabric contacts. The arrows shown in Figure 5 indicate the directions of this movement. When the actuator piston rods are fully extended, the surfaces of electroconductive fabric contacts mounted in the left and right clamp are connected, allowing current to flow through them. When the piston rods retract, the tested fabrics are disconnected, interrupting this current.

One of the pneumatic actuators is equipped with a force sensor (3) (Figures 4 and 5) to ensure consistent compressive force control during the measurement. The signal from the force sensor is measured by Tektronix’s digital oscilloscope (8). This oscilloscope also has a current probe (9) to measure the current flowing through tested textile contacts.

Each connection–disconnection cycle of the tested fabrics consists of four steps, as shown in Figure 6.

Figure 6

Connection–disconnection cycle of the tested textile contacts.

During the T1 period, the piston rods of the pneumatic actuators (1) (Figure 5) extend, causing the connection and pressure of the tested fabrics with a set force. After this (during T2) (Figure 6), the voltage is disconnected from the tested fabric contacts, and their resistance is measured using an Agilent 34410A ohmmeter. The disconnection of the voltage is necessary to measure this resistance correctly. In the next stage, the voltage is switched on to both fabric contacts again, causing current to flow through them during T3. In the last stage, T4, the piston rods retract, and the fabric contacts are separated, interrupting the current flowing through them. Then, the entire connection disconnection cycle is repeated.

The measuring stand enables the study of changes in contact resistance between surfaces of two electroconductive fabrics caused by phenomena occurring when connecting and disconnecting the current flowing through them.

The oscillations visible in the force waveform (Figure 6) are caused by vibrations generated when the piston rods of the pneumatic actuators are moved in and out. These oscillations do not affect the clamping force measurement when measuring the resistance (time T2) because, during this time, the pressure force is constant.

3.1 Measurements of the dependence of fabric surface temperature on current density flowing through it

As mentioned earlier in the first stage of conducted research, the relation between the temperature of the fabric and the current flowing through it was found. Examples of results are shown in Figure 7. From Figure 7, it can be inferred that the surface temperature of the tested samples depends not only on the value of the current flowing through them but also on their geometric dimensions, especially the width. Therefore, to compare the influence of the flowing current on these temperatures, the average current density value was calculated from the formula:

where w and h are the fabric’s width and height, respectively.

Figure 7

Examples of the surface temperature of electroconductive fabrics carrying a current of 0.5 A.

Metallized fabric is a structure for which it is tough to determine the distribution of current flowing through its cross-section. This distribution depends on many factors, e.g. weave, metallization method, type of yarns, etc. Therefore, for simplicity, the average value of the current density calculated from equation (2) was adopted for further consideration. Surface temperatures in places of different widths were read from the thermograms, examples of which are shown in Figure 7. Then, the average current density value was calculated from equation (2).

The results obtained from these measurements and calculations are presented in Figure 8. The fabrics named Berlin and Bremen showed the least resistance to the flowing current. Compared to other samples, even low current density values cause a significant increase in their surface temperature. One possible reason is their highest surface resistivity and the smallest thickness among the tested fabrics.

Figure 8

Surface temperature of the textile line versus current density.

3.2 Measurements of the resistance of fabric contacts to repeated connection and disconnection of current

In the first step of this part of research, contact resistance was measured by repeatedly connecting and disconnecting fabric contacts in a current-free state. This was intended to detect a situation in which the change in resistance was caused by mechanical factors such as contact friction during the connection and disconnection process. These tests showed no significant change in the resistance of all tested fabrics after 10,000 connection–disconnection cycles.

Then, the connection–disconnection test of fabric contacts with the current flowing through them was performed. The test consisted of 10,000 connection–disconnection cycles for each tested fabric using 5 A current. Tylman et al. [21] found that the required electrical power of clothing with electric heating to ensure thermal comfort reached 60 W. Assuming a 12 V battery was used to power the heaters, this power corresponds to a current flowing through the heating elements and a connector of 5 A. Therefore, this current value was adopted in the conducted tests. At the same time, the width of the tested fabric was assumed to be 15 mm. This value is a compromise between the width ensuring no excessive heating of the sample and the required width of the textile connector contacts, which should be as small as possible.

The test described above was performed three times for each tested pair of contacts. Before each test repetition, the tested contacts were replaced with new ones. An example of changes in electrical resistance as a function of the number of connection–disconnection cycles is shown in Figure 9.

Figure 9

An example of changes in electrical resistance as a function of the number of connection–disconnection cycles for the 3050-525/Laird fabric.

As you can see from Figure 9, three stages were observed during these measurements. During the first stage, the contact resistance does not differ substantially from its initial resistance. In the second stage, the spread of resistance values increases. The third stage starts when resistance values appear many times higher than the average resistance.

This usually means a loss of contact between the tested contacts.

Figure 9 shows that after 7,200 connection cycles, the dispersion of the contact resistance starts to increase. This indicates the first signs of contact wear. Then, in the 8,846 cycle, a high resistance value of the connected contacts appears, indicating their further degradation. From this moment, the contacts do not ensure the correct connection with 100% certainty.

For further analysis, the length of the first stage expressed in the number of cycles was assumed. Figure 10 illustrates the duration of the first stage, i.e. the number of cycles needed for wear to be visible in the tested contacts.

Figure 10

Number of cycles until the first signs of tested contact wear appear (duration of the first stage).

Figure 10 shows that contacts made of Bremen RS and Berlin RS fabrics indicate the lowest durability to current switching. These fabrics are also the least resistant to continuous current flow. More detailed results of resistance changes of three pairs of contacts of the Bremen RS fabric are shown in Figure 11. For this fabric, a sharp increase in electrical resistance was recorded after several dozen cycles of connecting and disconnecting a current of 5 A.

Figure 11

Changes in the contact resistance made of Bremen RS fabric under the influence of 5 A current commutation.

As mentioned earlier, this is due to their low electrical conductivity, which causes the generation of significant amounts of heat. At the same time, the small thickness of these fabrics causes them to heat up significantly, resulting in rapid wear.

3.3 Statistical analysis of the obtained results

An analysis of the influence of the dominant metal on fabrics’ electrical conductivity was carried out. The term “dominant metal” refers to the metal used in the highest percentage during imparting electroconductivity of the tested fabrics. For example, the dominant metal for Zell RS fabric (Table 1) is copper (Cu). The dependence of the duration of the first stage on the type of dominant metal is shown in Figure 12.

Figure 12

Dependence of the duration of the first stage on the type of dominant metal.

Analysis of Figure 12 may lead to the conclusion that the dominant metal of the electroconductive fabric influences the durability of the contacts made.

To confirm this, a hypothesis H0 was assumed about the equality of the duration of the first stage for groups of fabrics characterized by different dominant metals. The non-parametric statistical Kruskal–Wallis test and Statistica software were used to verify this hypothesis. A non-parametric test was used instead of the multifactor analysis of variance due to the failure to meet the assumptions of variance analysis (lack of normality of distributions in groups determined by variables and lack of homogeneity of variance). In all tests, the significance level was assumed to be α = 0.05.

The probability p = 0.0000 obtained as a result of the test means that at the assumed α significance level, the H0 hypothesis should be rejected. Therefore, the effect of the dominant metal is statistically significant. Figure 12 shows that contacts made of fabrics with a coating containing most copper are characterized by the highest resistance to phenomena occurring during repeated switching on and off of the conducted current. In turn, fabrics with the highest silver content have low resistance to the above phenomena.

As part of the research, an analysis of the influence of other fabric parameters on the resistance to current commutation was also carried out. These parameters were thickness, surface mass, and surface resistance. The regression method was used to find a correlation between the duration of the first stage and each of these parameters. The analyses did not show any significant correlation between the duration of the first stage and the parameters of electroconductive fabrics such as thickness, surface mass, and surface resistance.