Most conventional sensor methods referred to references adopt four photo sensors mounted on east side and west side for sun tracking in azimuth direction, and mounted on south side and north side for tracking in elevation direction with reference to an installation position of sun tracking system. The sun tracking mission in azimuth direction is finished when the outputs of east and west sensor are equal each other. The mission in elevation direction is finished in the same manner using south and north sensor. In this method, sensors installed outside causes a problem at maintenance. And also, it may take much time to finish the exact sun tracking. However, if the installation position or orientation of the sun tracking system is changed, a decision for the directions such as east and west must be executed newly.

In order to avoid the above inconveniences and to give independence on the change of installation position and orientation, a sensor array with two DOF is adopted in this paper. Figure 2 presents a method to comprise the sensor array composed of 11 ⅹ 11 photo transistors.

Figure 2: 
Two DOF sensor array for the sun tracking system.

In order for the two DOF sensor array to have the sun tracking function, a sunlight focus on the sensor array must be made through a lens assembly.

Moreover, the lens assembly must have the function of thermal filter to eliminate the heat resulted from focusing the sunlight. The technology is one of the key solution for comprising the suggested sun tracking system. Figure 3 presents the structure of the sun tracking sensor module and Figure 4 shows a photo of the sensor module implemented. A sun tracking method using this two DOF sensor array will be explained later in section 3.

Figure 3: 
Structure of the sun tracking sensor module.

Figure 4: 
Sun tracking sensor module physically implemented.

The analog multiplexer is necessary to transfer sensor output signals from 11 x 11 sensor arrays into an A/D converter in successive order by using scan code method. The A/D converter converts analog sensor signals from the multiplexer into digital sensor data when a main controller executes interrupt routine for A/D conversion.

Figure 5 explains the signal flows in the signal processing and control module and Figure 6 shows the photograph of hardware physically implemented.

Figure 5: 
Functional diagram of the signal processing and control module for the sun tracking system.

Figure 6: 
Signal processing and control module physically implemented based on CPU.

The main controller executes the decision of focus location on the sensor array and calculates the rotation angles for azimuth and elevation axis in order to move the focus to the center on the sensor array. And then, it generates PWM signals corresponding to the rotation angles to drive two DC motors about azimuth and elevation axis. When the focus lies on the center, the sensor module exactly directs the sun and the sensor array panel is perpendicular to the sunlight.

When the power of the sun tracking control system is on initially, each port of the control system is initialized and a main CPU reads x and y position of the sunlight focus on the sensor array panel through A/D conversion. And then, CPU reads current angular positions of DC motors. If the focus is on the sensor panel, a series of routine to track the accurate sun position is executed to move the focus into the center of the sensor panel by generating control signals. If the focus cannot be found on the sensor panel, a series of searching mode must be accomplished to find the sun position.

Figure 7 describes the flowchart to operate the sun tracking control system mentioned in the above.

Figure 7: 
Flowchart of the software for the sun tracking control.

If a sunlight focus is not on the sensor array, a series of process to detect the sun position is accomplished so that a focus may resultantly lie on the sensor panel. The process is called the searching mode.

The searching mode is processed in the following manner. At first, the sun tracking system is moved at initial position for searching mode and then the CPU checks the current angular positions in the azimuth and elevation direction.

At second, under the current elevation position, azimuth motor is rotated from left-hand to right-hand limit angle position.

At third, if the focus is not found on the sensor array, the elevation angle is changed with defined increment by rotating elevation motor and then azimuth motor is rotated from left-hand to right-hand limit angle position.

The second and third step is repeated until the focus is found on the sensor array panel.

Figure 8 shows the flowchart of the process for the searching mode software mentioned in the above.

Figure 8: 
Flowchart of the searching mode to find the sun.

If a focus is on the sensor array by accomplishing the searching mode process, command inputs of two axis controllers for azimuth and elevation control must be generated in order for the sensor module to meet sunlight perpendicularly, that is, in order to track the sun exactly. Figure 9 explains a method to calculate the command inputs for motor controller. If the focus on an arbitrary sensor of the sensor array can be moved into the center of the sensor array, the sun tracking system can track the sun exactly.

Figure 9: 
A method to generate command inputs of two axis motor controllers to track the sun position exactly.

In order to move the current focus into the center of the sensor array, angle errors corresponding to the position errors in Figure 9 between current coordinates of the focus and center coordinates on the sensor panel, must be calculated:

where ݁e_a is an angle error along the azimuth axis, e_e is an angle error along the elevation axis, c_r, f_r, c_c and f_c are the row and column coordinates of the center of the sensor panel and the current focus, respectively. r_e and r_a are angle resolutions in azimuth and elevation, respectively.

The above equations expressed by unit [deg] can be converted into the equations expressed by unit [A/D digital value] for convenience,

where c_e and c_a are absolute reference values converted as A/D digital values and p_e and p_a are current elevation and azimuth angular position values of the sun tracking system read by A/D converter.

Conclusively, the sun tracking control system executes the sun tracking according to the functional block diagram as shown in Figure 10.

Figure10: 
Functional block diagram of the sun tracking system.

Figure 11 shows a photograph of the sun tracking control system used for experiments.

Figure 11: 
A Photograph of the sun tracking control system.

When a focus is nearly on a photo sensor of the sensor array, it is called the focus is in the holding mode. In the holding mode, only 9 sensor outputs from 0 to 8 as shown in Figure 12 are converted with priority. In this case, the only holding mode algorithm in Figure 7 is accomplished.

Figure 12: 
Holding mode in the two DOF sensor array.

This means that the searching time for the sun and the converting time for sensor arrays are saved at the same time.

Naturally, their A/D converted output values are larger than the other sensor values.

In order to verify this, an experiment was executed when a focus was nearly on the center in the holding mode as shown Figure 12. For experiment, all sensor outputs were acquired exceptionally beyond the holding mode. Figure 13 shows the measured sensor output values displayed in 3D graph. From the figure, it is apparently known that the output of the sensor in the center is much greater than other sensors.

Figure 13: 
Sensor outputs when a focus is nearly on the center of the x-y plane of the two DOF sensor array.

Figure 14 shows the sensor output results using digital data expressed as the A/D converted values. It is seen that the centered sensor located at (5, 5) position in x-y coordinates of the sensor array produces maximal output.

Figure 14: 
Sensor outputs expressed as A/D converted digital values when a focus is nearly on the center corresponding to Figure 13.

Therefore, it is concluded that whenever the output of the centered sensor of the two DOF sensor array is maximal, the sun tracking system always directs the sun almost normally.

Whenever a focus is not in the sensor array, a searching mode algorithm must be accomplished regardless of the initial angular positions of azimuth and elevation axis in the sun tracking control system.

Figure 15 shows the searching mode results for finding the sun position for various initial angular positions. According to the searching mode algorithm, the elevation angle was initialized at angle 90 degree and then incremented from 90 degree to 180 degree as soon as the azimuth searching was finished within -50 degree to 50 degree range or reverse range. It is seen that the last position is near the holding mode position from the result of sun tracking.

Figure 15: 
Searching mode results for various initial positions.

Figure 16 shows a sun tracking process using the GUI screen which was designed for monitoring sensor states during tracking control process. The initial focus on (2,1) coordinate in the sensor array moved to (5,5) coordinate, the center of the sensor array. It means that the face of the sensor module directs the sunlight perpendicularly and thus the sun tracking process has been finished.

Figure 16: 
A sun tracking control process from (2,1) to (5,5) corresponding to the center on the GUI screen for monitoring sensor states.

Figure 17 shows the tracking performance in the azimuth and elevation angle control for the case of Figure 16. In order to move the focus on (2,1) coordinate to (5,5) coordinate, the process must be operated in the direction that the azimuth angle is increased from right to left and also the elevation angle is decreased from top to down. Figure 17 explains the above mentioned process exactly.

Figure 17: 
Tracking control performance in the azimuth and elevation control system.