## 4 Experimental Results
### Automatic Solar Tracking Results
#### 4.1.2 Reported Solar Tracking Accuracy  
RSTA is the statistical processing of a set of ISTEs measured by a standardized method. In this work, RSTA was calculated using the following considerations established in the IEC 62817 standard [1]:  
* The pointing error data set was measured in different weather conditions. * The tracker pointing error was recorded in 1-second instantaneous increments. * The data were filtered to remove possible data taken during low irradiance conditions. * The wind speed was measured with a digital anemometer, and the data were classified into a high and low wind speed bin based on a 4 m/s threshold. * Date, time and DNI parameters were recorded at all times. The RSTA was measured by the DSS-MEMS; the PPS could not be used because it is a visual device that cannot store data. Solar tracking experiments were performed on 8, 9 and 20 October 2021, under clear sky and partially and mostly cloudy conditions, respectively. In all cases, the STS started at 8:30 from its initial position, where the Sun can be detected because of the wide range of the ASS-MEMS. Solar tracking ended at sunset. Figure 10 depicts the typical solar tracking results under clear sky conditions. As seen in Figure 10a, when the STS starts (08:30), the Sun's rays hit with an angle greater than the FOV of the DSS-MEMS (\(\pm 5^{\circ}\)), so the sensor cannot measure the angle of incidence. When the solar tracker is oriented towards the Sun's rays (STE \(<5^{\circ}\)), the DSS-MEMS starts to measure the accuracy of the STS. Figure 10b shows the DNI during the experimentation. It can be observed that the solar irradiance exceeded 900 W/m\({}^{2}\), and there was no cloudiness during the day. Note that when DNI decreases at sunset (18:30), ISTEs increase because of the typical non-linear response of the photosensors under changes of radiation and temperatures levels [12]. During the solar tracking stage, the absolute mean azimuth and zenith axes errors and total error were 0.310\({}^{\circ}\), 0.237\({}^{\circ}\) and 0.390\({}^{\circ}\), respectively; see Table 3. Solar tracking was performed during other clear sky days, and an average variation of 7% was observed. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \multirow{2}{*}{**Time**} & **DNI** & \multirow{2}{*}{**Wind Speed**} & **ISTEs (\({}^{\circ}\)). ** & **DSS-MEMS** & \multicolumn{3}{c}{**ISTEs (\({}^{\circ}\)). PPS**} \\ \cline{3-8}  & **(W/m\({}^{2}\))** & & \(\mathbf{\alpha}\) & \(\mathbf{\beta}\) & \(\mathbf{\theta}\) & \(\mathbf{\alpha}\) & \(\mathbf{\beta}\) & \(\mathbf{\theta}\) \\ \hline
10:00 & 838 & Low & 0.15 & 0.42 & 0.45 & \(-\)0.19 & 0.29 & 0.35 \\ \hline
12:00 & 918 & Low & \(-\)0.33 & 0.17 & 0.37 & \(-Figure 11 represents the RSTA during a partly cloudy day. In Figure 11, it is observed that when there is a period of partially cloudy sky, without the DNI decreasing below 300 W/m\({}^{2}\) (from 8:30 to 10:30, as seen in Figure 11), the STS remains in a solar location stage and continually tries to locate the Sun's apparent position. Therefore, the tracking accuracy is low. Starting at 10:30, there is then a mostly clear sky stage, in which the tracking precision improves and the control algorithm compensates the disturbances caused by the diffuse radiation generated by the clouds. The RSTA for the azimuth and zenith axes was 0.466\({}^{\circ}\) and 0.297\({}^{\circ}\), respectively; therefore, the total tracking precision was 0.536\({}^{\circ}\), 37.5% higher than that achieved on a clear sky day; see Table 3. Similarly, Figure 12 shows the response of the STS to a period of the day that is mostly cloudy, from 16:00 to 18:00; see Figure 12. At that time, the DNI drops below 300 W/m\({}^{2}\), and the DSS-MEMS cannot measure the angle of incidence of the Sun's rays, so the precision of solar tracking cannot be plotted. In this situation, the STS control system does not update the position of the solar tracker in order to avoid unnecessary over consumption of electrical energy and the instability of the system.