## 2 Analysis and Design of a Sun Tracking Control System  
### Power Plant Tracking and Monitoring System  
A MW HCPV demonstration power plant system (Figure 1) consists of a monitoring and power generation unit. The monitoring unit includes a Closed-Circuit Television (CCTV), electrical characteristics of the driver and the receiver standard (RS485), RS485/Transmission Control Protocol Internet Protocol (IPTCP/IP) converter, console, and database server. The power generation unit includes HCPV modules, a tracker, a sun tracking control system, a grid inverter. A CCTV system was implemented for remote monitoring of the HCPV demonstration power plant system. The DC power output of the HCPV modules was connected to the inverter, which tracks the maximum peak power point to keep the HCPV modules output power in the optimum condition. There are two kinds of power measurement design implementation in the system. One is the measurement of DC current and voltage for the HCPV modules output power, and the other is the measurement of AC current and voltage for the consumption power of the load. The console collects those signals from power measurement devices through the RS485 communication, RS485/TCP/IP converter and Ethernet network, and stores the data on the database server.  
Figure 1: One MW High concentration photovoltaics (HCPV) demonstration power plant system architecture.  
This tracker is equipped with a sun position sensor and an azimuth-elevation tracker, consisting of two axes. One axis is a vertical pivot shaft that allows the device to be swung to a compass point. The other axis is a horizontal elevation pivot mounted upon the azimuth platform. The sun position sensor oriented to the sun direction is composed mainly of four photo detectors, located 90 degrees apart from each other and oriented to the cardinal points.  
The sun tracking control system, in Figure 2, includes the sun position sensor and the mechanism declined angle adjuster, the tracker position encoder, the analog and digital conversion module, the motor control module, the micro-processor, the azimuth and the elevation motor. The sun position sensor is mounted horizontally on the mechanism declined angle adjuster. The mechanism declined angle adjuster has four springs to adjust the declined angle in the west/east and south/north direction. It enables the sun position sensor mechanism to keep the same horizontal angle as the solar modules. The tracker position encoder uses the reed switch to detect the stroke length of the actuator in elevation and the displacement of motor in azimuth to calculate the solar tracker position. The elevation and azimuth angle of the solar tracker are stored as pulses count per minute. The analog and digital conversion module is for converting the signal of the sun position sensor to a digital signal. The motor control module receives the micro-processor command to start or stop the azimuth and elevation motor.  
Figure 3: Sun position sensor.  
Figure 2: Sun tracking control system.  
There are four photo detectors located in the sun position sensor (Figure 3), at 90 degrees apart from each other, and oriented to the four cardinal points. Two differential signals between east (E) and west (W) detectors, and south(S) and north (N) detectors are sent to the tracking controller. However, the sun position sensor is a simple iteration of the following equation:  
\[\alpha =E-W \tag{1}\] \[\beta =S-N \tag{2}\]  
where \(\alpha\) represents the difference of the sun position sensor in the west-east direction, and \(\beta\) represents the difference of the sun position sensor in the north-south direction axis. The E, W, S, N, are the voltages measured in each direction of the detectors. The intensities of E, W, S, and N are different, if the center of the light is not at the center of the sun position sensor.