Design of automatic tracking control system for dish-type Stirling solar system

Introduction
A dish-type Stirling solar thermal power generation device is a device that generates high temperature by focusing sunlight, thereby driving the Stirling engine (heat engine) to output electrical energy. Figure 1 is a schematic diagram of a typical dish-type Stirling solar thermal power generation device system. It focuses sunlight through a dish-type concentrator, allowing it to enter the solar receiver and convert it into heat energy to drive the generator to operate and output electrical energy. The device is designed with a steering mechanism to ensure that the device can track the sun by adjusting the elevation and horizontal angles of the concentrator to improve the utilization rate of solar energy. In order to enable the steering mechanism to realize its function and maximize the use of solar energy, it is necessary to design a solar tracking system that can track the sun over a large range in real time, so that the sunlight is always concentrated on the solar receiver of the Stirling engine.

This paper adopts a hybrid control method, combining the advantages of open-loop tracking and closed-loop control. With ATmega128 single-chip microcomputer as the main controller, a dish-type Stirling solar tracking control system is designed to achieve solar tracking in a wide range of weather conditions, so that the focused light spot of the dish-type concentrator falls on the receiver of the Stirling engine, improving the safety and power generation efficiency of the system.

1 System Principle
Through the GPS receiver, the longitude, latitude and UTC time of the location of the dish-type Stirling solar thermal power generation device are obtained; through open-loop program tracking, the solar altitude angle and azimuth are calculated to determine the current position of the sun; PWM signals are output to drive the horizontal and pitch stepper motors of the steering mechanism to make the dish-type concentrator reach the expected position; four-quadrant silicon photovoltaic cells are used to
automatically position and correct the dish-type concentrator for error, and closed-loop control is performed to fine-tune the position of the concentrator to ensure that the sunlight focused by the dish-type concentrator falls on the receiver of the Stirling engine to maximize the efficiency of the engine. The system composition principle block diagram is shown in Figure 2.

2 System Hardware
2.1 Microcontroller ATrnega128
ATmega128 microcontroller is an 8-bit low-power CMOS microprocessor based on AVR RISC structure. It is a high-performance microcontroller in the AVR microcontroller series. ATmega128 microcontroller has 128 KB of in-system programmable Flash program memory, 4 KB of EEPROM, 4 KB of SRAM, 53 general I/O port lines, 32 general working registers, 8-channel 10-bit ADC, 53 programmable I/O port lines, 4 flexible timers/counters (T/C) with comparison mode and PWM function, which can easily control stepper motors.
2.2 Weather module
The weather module is mainly used for weather status detection, such as light intensity, wind speed, cloudy and rainy. When the wind speed reaches or exceeds the set value, the ATmega128 microcontroller issues a command to drive the steering mechanism through the interface circuit to keep the dish concentrator in a safe position, thereby ensuring the safety of the system. When the real-time ambient light intensity and real-time time collected by the system are both less than the set minimum economic power generation light intensity, the dish-type Stirling solar thermal power generation device will make corresponding processing and wait for the light intensity to reach the set value before running again.
2.3 GPS receiver
The GPS receiver uses the M12 module of Motorola. The M12 module receives satellite signals and outputs information through the serial port. The ATmega128 microcontroller decodes the local longitude, latitude and UTC time, and calculates the current solar altitude angle and azimuth according to the astronomical formula. Since the I/O operating voltage range of the ATmega128 microcontroller and the M12 module are different, the serial ports of the two need to be converted through a photoelectric coupler to maintain stable communication. The circuit is shown in Figure 3.

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2.4 Dish steering mechanism
At time t, the solar altitude angle h and the solar azimuth angle α are respectively:


where φ is the geographical latitude of the current dish installation location, δ is the solar declination angle, and ω is the solar hour angle.
The dish steering mechanism is mainly composed of a horizontal adjustment motor, a pitch adjustment motor and a reduction gearbox. When the ATmega128 microcontroller obtains the latitude and longitude and time of the current position according to the M12 module, it calculates the current solar altitude angle increment and azimuth angle increment according to formulas (1) and (2), outputs a certain number of PWM signals, and drives the motor to adjust in both the horizontal and pitch directions to achieve preliminary tracking of the sun.
2.5 Feedback adjustment part
As shown in Figure 4, the system uses a four-quadrant silicon photovoltaic cell as a sensor for solar tracking error correction. Assume that the sunlight passes through the optical system to form a circular spot on the four-quadrant silicon photovoltaic cell and is evenly distributed. Let its center coordinates be (X, Y). When the dish concentrator is aimed at the sun, the optical axis of the four-quadrant silicon photovoltaic cell is aimed at the sun, and the center of the circular spot is on the optical axis. The silicon photovoltaic cells in the four quadrants receive the same optical power and output the same voltage signal. After the signal is processed by the sum-difference operational amplifier, the output is zero. When the dish concentrator is not aimed at the sun, that is, the optical axis of the four-quadrant silicon photovoltaic cell is not aimed at the sun, the sunlight forms an angle θ with the optical axis. The light spot formed by the light irradiating the four-quadrant photovoltaic cell through the optical system will inevitably shift (X≠0, Y≠0), and the voltage generated by each quadrant photovoltaic cell is not the same. The output of the sum-difference operational amplifier also produces positive and negative changes in the relative displacement direction. The ATmega128 microcontroller calculates the angle that needs to be adjusted based on the change, outputs a PWM signal, and rotates the pitch angle motor and the azimuth angle motor until the sum-difference operational amplifier outputs zero, indicating that the system has been aimed at the sun. According to the above principle, the position error of the dish concentrator can be corrected.

2.6 Touch screen
The touch screen is mainly used to set parameters or control the operation of the disc Stirling system, and to output and display the real-time operation status of the system. It is an industrial tablet computer with touch function. Information is exchanged with the ATmega128 microcontroller via RS-232.

3 System software description
The ATmega128 microcontroller software development uses Atmel's AVR STUDIO3.53 integrated development environment, which provides AVR ASSEM-BLER compiler, AVR STUDIO debugging function, AVR PROG serial/parallel download function, and JTAG ICE simulation function. This design uses JTAG ICE simulation debugging. Software design includes microcontroller initialization design, GPS module interface program design, main program design, and host computer program design.
3.1 Microcontroller initialization design
The ATmega128 microcontroller needs to be initialized before working, mainly to set the working mode of the ATmega128 microcontroller and peripherals so that it can meet the system working requirements. The initialization work mainly includes I/O port initialization, ADC initialization, PWM initialization and serial port initialization. The program of serial port initialization usart0_init() is as follows:

3.2 GPS module interface program design
The system only needs the M12 module to provide longitude and latitude. The interface program flow between the microcontroller and M12 is shown in Figure 5. ATmega128 first requests the M12 module to output positioning data (sends the instruction "@@EqmC" to the M12 module), and then waits for the M12 module to send back data. When the M12 module sends back data, ATmega128 first makes a judgment to confirm whether the received positioning information is the expected one, and then intercepts the positioning information (longitude and latitude data), and finally clears the serial port receiving buffer to avoid errors when receiving information next time.

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3.3 Main program design
The main() function of the main program is an infinite loop program. It can adjust the position of the dish-type Stirling system according to the sun angle after running, make fine adjustments according to the feedback signal, generate pulse signals to drive the motor to rotate, and realize sun tracking. When an interrupt is detected, the main program stops running. The interrupt program design includes weather hazard detection and processing and stepper motor driver chip exception processing. After the interrupt program is completed, it returns to the main program. The main program flow is shown in Figure 6.

3.4 Host computer program design
The host computer program design adopts Visual C++6.0, and ActiveX controls are used to simplify the design difficulty of the program. ActiveX controls include a series of properties, methods and events. The working mode between the application using ActiveX controls and ActiveX controls is client/server mode, that is, the application accesses the ActiveX controls through the interface provided by ActiveX controls. The opening of the host computer serial port, the setting of serial port parameters and the operation of initializing variables can be realized through the function CTestDld::OnInitDialog(). The code is as follows:


4 Application results
The developed automatic tracking control system has been put into use in the dish-type Stirling solar thermal power generation device. In actual application, the solar ascension and longitude and hour angle are obtained through GPS, the solar azimuth and altitude angle are calculated, a certain number of pulses are output, the servo motor is driven to rotate, and the dish-type concentrator is controlled to track the sun. According to the feedback information from the sensor, the sunlight spot is accurately concentrated on the receiver of the Stirling engine, and the edge of the light spot of the dish-type concentrator almost completely overlaps with the edge of the receiver of the Stirling engine. As the solar azimuth changes, the light spot can always remain in the receiver of the Stirling engine, and the deviation error of the light spot is within 1cm, which meets the working needs of the dish Stirling solar thermal power generation; when the weather changes, the system can make corresponding processing to ensure the safety of the entire power generation device.

Conclusion
Although the open-loop program tracking can work normally in any weather, the cumulative error generated during the tracking process cannot be eliminated by itself. Closed-loop control can eliminate errors through feedback, but the photosensitive element as the feedback signal source cannot receive sunlight for a slightly longer period of time, which will cause the failure of the tracking system. The closed-loop tracking solar method based on GPS open-loop program coarse tracking and four-quadrant silicon photovoltaic cell tracking calibration designed in this paper not only makes up for the shortcomings of open-loop tracking control, but also overcomes the shortcomings of the photoelectric tracking method that is greatly affected by the environment, and finally forms a closed-loop control system with higher precision and less environmental impact. The actual application results prove that the effect is good and has a certain design reference value.