0 Introduction
With the rapid development of society and economy, the energy problem faced by mankind is becoming more and more prominent. As a clean energy source, solar energy has undoubtedly received widespread attention from all countries. How to improve the utilization efficiency of solar energy has become a research hotspot. Solar tracking is a way to improve the utilization rate. At present, there are many ways to track the sun, mainly photovoltaic and mechanical. The former is passive tracking, which is greatly affected by the environment, especially when it is cloudy or overcast. The latter is active, and its principle is to calculate the position of the sun through a program and control the stepper motor to track the sun. At present, most of the domestic methods use the latter [1]. However, this tracking method will have accumulated errors, mainly because the solar position coordinate model used is not accurate enough. Since it is an open-loop control, the deformation of the mechanical structure and the error generated by the motor during the execution process are difficult to eliminate, and the tracking accuracy decreases with the increase of running time. This paper adopts a tracking method with program-controlled tracking as the main method and photovoltaic tracking as the auxiliary method. At the same time, the mathematical model for calculating the position of the sun coordinate is updated, thereby improving the tracking accuracy and realizing fully automatic tracking, which is of great significance for realizing large-scale solar thermal power generation.
1. Functions and Structure of Solar Tracking System
1.1 System Composition and Structure
The intelligent solar tracking device mainly consists of a microprocessor control unit, a photoelectric detection unit, an LCD display module, a storage unit, a keyboard, and corresponding peripheral circuits and a manual control unit, as shown in Figure 1. The software part includes the porting of the operating system and the writing of application programs.
Figure 1 Hardware composition of a solar tracking system
1.2 System Functions
1) Keyboard and display screen. It is equipped with a 4×4 keyboard and a 320×240 dot matrix liquid crystal display (LCD), mainly used for manual control and human-machine interaction, so that users can set time and position parameters and monitor the system's operating status.
2) Detection module. The detection module is installed on the collector panel or on a panel parallel to the collector. It is mainly used to detect the environmental conditions during system operation [2]. Its functions can be divided into two aspects: ① Detecting the intensity of ambient light to distinguish between day and night; when the light intensity is insufficient on cloudy days, the system will automatically enter standby mode after detecting the signal to avoid unnecessary energy waste, thereby achieving automatic control of the system. ② Providing error signals in the horizontal and pitch directions. When the system is in normal tracking mode, the tracking mode is program-controlled tracking. Due to the existence of calculation error and mechanical error, the accumulated error cannot be eliminated after the system has been running for a long time. After the detection module detects the accumulated error, it sends an interrupt signal. The microcontroller chip responds to the interrupt and sends corresponding instructions to control the actuator to correct the error, thereby achieving closed-loop control.
3) Power Supply Circuit. The power supply circuit mainly provides operating power for the microcontroller, peripheral devices, and other chips used in the control system. Since the ARM microcontroller used is the LPC2290, which has independent analog and digital power supplies, these should be isolated to reduce the probability of errors. Therefore, the incoming 220V power supply is filtered and divided into two paths: one serves as the operating power for the stepper motor driver; the other, after rectification, provides +5V and +15V power to the system. The two power supplies for the microcontroller, based on the +5V, are regulated by a voltage regulator to output +3.3V and +1.8V respectively.
4) Storage Module. The LPC2290 chip only has 16kB of on-chip static random access memory (SRAM), and no on-chip read-only memory (ROM) or FLASH memory available. Therefore, it needs to be expanded to store the operating system and running programs to prevent program loss after power failure.
5) Actuator. The actuator mainly consists of the drive module, stepper motor, and corresponding support structure. The drive module receives the output pulses from the microcontroller, amplifies and cyclically outputs them after opto-isolation. The stepper motor can be directly digitally controlled, converting the pulse sequence into corresponding angular displacement; that is, receiving one pulse causes the stepper motor to rotate one angular displacement. The support structure reduces the power output of the stepper motor and increases the torque, converting it into low-speed rotational motion in the horizontal direction and pitch motion in the vertical direction to follow the sun.
2 Design and Implementation of Solar Tracking System
2.1 ARM Microprocessor
This paper selects the LPC2290 microcontroller. The LPC2290 is a 16/32-bit ARM7TDMI-S (ARM chip) CPU based on real-time simulation and embedded tracing. It has strict control over code size and can use 16-bit Thumb mode to reduce the code size by more than 30% with minimal performance loss [3]. Due to the LPC2290's 144-pin package, extremely low power consumption, two 32-bit timers, eight 10-bit analog-to-digital converters (ADCs), pulse width modulation output (PWM), and up to nine external interrupts, it is widely used. Through the external memory interface, the memory can be configured into four groups, each with a capacity of up to 16MB, for a total of 64MB. Therefore, the LPC2290's high-performance ARM7 CPU core and rich on-chip peripherals can simplify system design and significantly reduce system cost.
2.2 LCD Display Module
The LCD module uses the RT12864-M, which embeds the ST7920 Chinese character dot matrix LCD driver/controller. It can display letters, numbers, Chinese characters, and custom graphics. A key feature is that it provides three control interfaces: an 8-bit parallel microcontroller interface, a 4-bit parallel microcontroller interface, and a serial interface. The display RAM, character generator, LCD driver, and control circuitry are all contained within a single chip. The connection between the LPC2290 and the RT12864-M is shown in Figure 2.
Figure 2 Connection between LPC2290 and RT12864-M
2.3 Storage Module
The LPC2290 has 16kB of on-chip SRAM but no usable on-chip ROM or FLASH memory; the program is lost upon power failure and cannot be permanently stored. Therefore, a 2MB FLASH memory, model SST39VF160, is added to the system. This is a CMOS multi-function FLASH (MPF) device manufactured using SST's (silicon conservancy technology inc.) proprietary high-performance Super-FLASH technology. Super-FLASH technology provides fixed erase and program times, independent of the number of erase/program cycles. Figure 3 shows the connection diagram between the LPC2290 and SST39VF160. The SST39VF160's chip select signal is CS0, configured as Bank0, with its starting address being 0x80000000, which is the LPC2290's Bank0 memory space. The SST39VF160 has a 16-bit bus interface; pin A0 is unused, and only the LPC2290's address buses A1~A20 are used.
Figure 3 Connection between LPC2290 and SST39VF160
