## 6 Performance Evaluation  
Although solar radiation recurs in a pattern every 24 h on Earth, factors such as weather changes, time of year, or PVC obstruction may cause changes in the amount of energy harvested each day. Performance of the developed controller to maintain a SOC by adjusting the fix rate was investigated compared to the current method of a fixed collection rate. Controller simulations were carried out to determine controller performance of stable fix rate and set point SOC tracking for constant and \begin{table}
\begin{tabular}{c c c} \hline
**Gain Type** & **Set 1** & **Set 2** \\ \hline Proportional & \(K_{P1}=0.3\) & \(K_{P2}=0.1\) \\ Integral & \(K_{I1}=0\) & \(K_{I2}=0\) \\ Derivative & \(K_{D1}=2000\) & \(K_{D2}=2000\) \\ \hline \end{tabular}
\end{table} 
Table 5: Gain values for a gain scheduled system. Figure 9: Selection of the derivative value using (**a**) PVC model with noise the variable PVC model; (b) battery SOC; and (c) GPS fix interval. variable PVC levels. The initial SOC, battery capacity, SOC set point, and PID gains were the same for each simulation. First, the PVC randomness model was used and the controller system was allowed to find a stable fix interval to ensure that the developed controller could arrive at a stable solution. Next, a peak PVC level of 10 or 5 \(mA-hr\) was chosen as a starting point and then changed, respectively to either higher or lower values, once a stable fix interval and SOC was detected. Changing the PVC level was done to simulate a distinct change in climate, location, or nesting either into the condition or away to normal sun exposure. Fixed PVC simulations indicated that for the given configuration (50 \(mA-hr\) battery, SOC starting at 100%, initial fix rate of 5 per minute, only collecting fixes during the day) it took 4 days of operation to reach a constant fix interval. During the first day, there was a steep increase in the fix interval using the first set of gains. The second set of gains then took over and slowly adjusted the fix rate to 45-min intervals. The PVC with randomness, SOC, and fix interval are shown in Figure 10 for a five day interval. A GPS fix interval increase of 1-3 s was observed at the beginning of each day. This was caused by a controller adjustment due to a decline in SOC as a result of energy consumed from sleeping overnight. A decrease in energy available for harvesting was simulated to determine controller performance, as shown in Figure 11. The battery and initial conditions were the same as the previous simulation setup. At first, the controller was allowed to settle at a constant fix interval before changing the PVC energy harvested, which took 6 days, plus some additional time to ensure stability and to highlight the constant fix method's drop in SOC. The fixed collection method, in this case, was set on the aggressive side to demonstrate the battery SOC drop over a 20 days where recovery was not possible. After the PVC levels changed, on day 20, the constant fix rate model entered an unrecoverable state where data collection was stopped to attempt a battery charge that was indicated to take 20 days. The developed controller was able to make adjustments over a few days to arrive at a new fix interval that was higher than the original, 1.5 fixes per minute, and never stopped collecting data. Figure 10: Controller performance evaluation using **(a)** the PVC randomness model; **(b)** battery and set point SOC; and **(c)** GPS fix interval. The opposite of diminishing energy harvesting, an increase in available energy, was also simulated, as shown in Figure 12. The battery and initial conditions were the same as the previous simulation setup. At first, the developed controller was allowed to settle at a fix rate along with the fixed collection rate, completely draining the battery. The controller first overshot and then settled after 13 days. The PVC harvesting was kept constant for 20 days at a 5 mA peak.