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result in the waste energy in cloudy days. Passive tracking has higher accuracy and
low cost [7], but tracking system can do nothing and sometimes move the platform in
the wrong state in cloudy days. A compound tracking system, a combination of active
and passive tracking methods, is used to track sun in the study. It is composed of pho-
toelectric tracking and elevation angle-azimuth tracking, and takes advantages of two
methods in different weather conditions to make the system more accurate and stable.
The whole system is based on photosensitive sensors. According to the information
measured by photosensitive, MCU can control the solar panels to track sun. The sys-
tem includes tracking sensor system, control system, driving system, solar panels and
batteries. The diagram of the system composition is shown in Fig. 1.
There are five photosensitive sensors in the tracking sensor system. A photo-
sensitive sensor detects light intensity to judge the weather. The other photosensitive
sensors are responsible for tracking sun. Four photosensitive sensors can detect the
light intensity of four corners. Four sensors point to four directions, and tilted 45 de-
grees with the horizontal direction. Tracking sensor device is shown in Fig. 2. When
the east-west light intensity received by the sensor is inconsistent, the system will get
the value which voltage difference that sensors provide is converted to, and judge
which direction has strong light. Then the system can control the motor to run to the
direction. When sensors output voltage value is same, the motor is stopped. The
method of judge which the running direction of solar panels is north or south is same.
Control system is the core of the whole system, which is responsible for judging
and processing the information obtained from the other parts. AVR MCU is used for
the control system since it is high performance, low power consumption RISC struc-
ture, CMOS 8-bit microcontroller [8]. The AVR MCU used in the system is Atmega
16 produced by Atmel Corporation. The on-chip resources are rich and the confiden-
tiality is good. It has the high speed, high performance and low power consumption.
Its peripherals have 8-channel ADC which is 10-bit [9] and has A/D conversion func-
tion. Therefore, the information given by tracking sensor system can be used in the
MCU. The scheme of the tracking system is shown in Fig. 3.
Two stepper motors are used for the dual-axis drive device. They can make solar
panels turn along vertical and horizontal axes. Each axis has an absolute photoelectric
rotary encoder. The main characteristics of it are zero fixed and the angle of per revo-
lution is monotonic in 0-359°. Each location has a corresponding code. Encoder is
mainly to assure system to turn to the specified angle. When the weather is cloudy,
the system can automatically make solar panels point to the best location for this sea-
son at noon, which makes the sun tracking system into a fixed solar system. Thus, the
method deals with the problem properly when the system can not find the best loca-
tion in cloudy days, and not make system wrong movement [10]. The best location
can be computed by MCU which uses the time, the date and latitude and longitude.
The system uses DS12C887 as a clock chip, which has complete clock and calendar
functions. MCU can output eight different signals by three ports to control motors.
The system needs a decoder to encode, so it makes two stepper motors of the system
running in different directions. The drive control system is shown in Fig. 4.
Lead-acid battery is selected in the system since it has low price and easy mainte-
nance. And the raw materials can be easily obtained. In the battery part, the battery
must be protected. When the system is in the case of continuous cloudy days, the bat-