Automatic Sun Tracking System (ASTS)
Automatic Sun Tracking System (ASTS)
In remote areas the sun is a cheap source of electricity because instead of hydraulic generators it uses solar cells to produce electricity. While the output of solar cells depends on the intensity of sunlight and the angle of incidence. It means to get maximum efficiency; the solar panels1 must remain in front of sun during the whole day. But due to rotation of earth those panels canâ„¢t maintain their position always in front of sun. This problem results in decrease of their efficiency. Thus to get a constant output, an automated system is required which should be capable to constantly rotate the solar panel. The Automatic Sun Tracking System (ASTS) was made as a prototype to solve the problem, mentioned above. It is completely automatic and keeps the panel in front of sun until that is visible. The unique feature of this system is that instead of taking the earth as its reference, it takes the sun as a guiding source. Its active sensors constantly monitor the sunlight and rotate the panel towards the direction where the intensity of sunlight is maximum. In case the sun gets invisible e.g. in cloudy weather, then without tracking the sun the ASTS keeps rotating the solar panel in opposite direction to the rotation of earth. But its speed of rotation is same as that of earthâ„¢s rotation2. Due to this property when after some time e.g. half an hour when the sun again gets visible, the solar panel is exactly in front of sun. Moreover the system can manage the errors and also provides the error messages on the LCD display. In manual mode, through the software (GUI) at computer, the solar panel can be rotated at any desired angle.
Fig. 1: Automatic Sun Tracking Systemâ€Dishes and PV.
Fig. 2: Automatic Sun Tracking Systemâ€Heliostat and Parabolic Trough.
ASTS can be used for Parabolic Trough tracker, Dishes tracker, PV (Photovoltaic generator) tracker, Heliostat, Solar Furnace and so on. Even though the theory of the controller system is similar to all the applications, there are some differences: the precision requirement for dishes tracker is more strict than PV tracker, while the Heliostat and solar furnace need the strictest precision requirement, and more difficult to apply solar sensor to make a closed-loop control system. So, when design these systems, selection of motor type, controller type and tracking mode should be different.
STRUCTURE OF ASTS:
ASTS is a hybrid hardware/software project. Its general structural diagram is shown in figure-3.
The software includes:
Â¢ VB 6.0 based GUI.
Â¢ Microsoft Access Database.
Â¢ Embedded Software (written in C) for microcontroller AT89c52.
The hardware includes:
Â¢ Solar panel assembly structure containing six functional sensors, stepper motor and solar cells.
Â¢ System Control Unit containing LCD, Keypad, Error Indicators and Emergency Stop switch.
Â¢ Complete PCB containing two microcontrollers (89c52). First one is the Master Microcontroller which controls the automatic operation of ASTS. While second one, the Slave Microcontroller serially communicates (RS232) with VB software in computer.
Figure3. General Assembly of ASTS
There are many types of motor can be selected in ASTS design. Currently, several types of motors being used in the area of ASTS around the world are: Step-motor, Servo-motor, AC asynchronous motor, permanent magnetic DC servo motor, permanent magnetic brushless synchronous motor, etc.
Generally speaking, as the gear ratio is high for the transmission system, motor control precision has very small impact to the tracking precision. For example, for a system with the gear ratio of 20000:1, the tracker only covers an angle of 0.314mrad when a one complete circle is finished by the motor. Therefore, all kinds of the motor can satisfy the precision of the tracking system. However the feature of each type of motor is different.
Fig. 4: AC asynchronous motor with encoder
First of all, letâ„¢s take a look at the AC asynchronous motor. To use this kind of motor, we need an encoder to locate the position of the tracker. In our project, we install an encoder at the end of AC motor. The PLC accepts the pulse to locate the tracker, while the transducer is used to adjust the trackerâ„¢s speed. Certainly, we can install an encoder at the end of the transmission to ensure the position of the tracker.
The obvious advantage to use AC asynchronous motor is its price. Even though it can achieve all the needed functions, AC asynchronous motor is too heavy, too large, and too low in efficiency to be installed. Also, for this type of motor, the torque at low speed is very small. In addition to these disadvantages, it needs the work frequency be above 5Hz to function properly. The lowest work frequency in our project is 10Hz. As such, the possibility for us to use AC asynchronous motor in the future is eliminated.
Secondly, letâ„¢s compare the features of DC Servo-Motor and AC servo-motor.
There are two types of DC servo-motor: motor with brush and motor without brush. Motor with brush is low in cost, simple in structure, and high in start torque. Also it has wide range of speed adjustment, is easy to control. Though it needs maintenance from time to time, it is very convenient to repair (replace the brush). However it produces electro-magnetic interfere. Motor without brush is small in size, light in weight, high in output, fast in response, small in inertia, smooth in spinning, stable in output torque, low motor maintenance fee, high in efficiency, low in electro-magnetic radiation, long life, and can be applied in different working environments. However it has more complex control system.
AC servo-motor is also a type of motor without brush. There are two types of AC servo-motor: synchronous AC motor and asynchronous AC motor. Currently, synchronous AC motor is generally used in movement control. It can cover a wide power range, which could be up to a very high power level. Nowadays, with the fast development of semiconductor technology, the shift frequency of power assembly, and the processing speed of micro-computer have been increased significantly. As such, it is possible to put the AC motor controller into a two-axial coordinate system to control the directional current components, in order to achieve the performance similar to the DC motor.
Fourthly, the comparison between AC servo-motor and step motor is done.
AC servo-motor runs smoothly during low speed period; while step-motor is apt to have low-frequency vibration.
In terms of the frequency-torque Characteristics, the output torque of step-motor decreases with the increasing of rotation speed. Furthermore the decrease is steep in high-speed range. AC servo-motor has a comparably stable output torque, when the rotation speed is within the rated rotation speed. It gives the constant output power when the rotation speed is beyond the rated value.
Step-motor doesnâ„¢t have the overload capability; while AC servo-motor posses a satisfactory overload capability. The Panasonic AC servo-system is an example: The maximum
Output torque is three times big of the rated output torque, which can be used to overcome the inertia load during the start period. As the step-motor doesnâ„¢t have the overload capability, a much bigger size of step-motor is needed. Obviously the step-motor will be over-sized during normal operation.
Controllerâ„¢s type of Step-motor is open-loop type. It is easy to have the error of step loss or blockage when the start frequency is high or the load is heavy. Also it is easy to have the error of overshoot when it is stopped. So, to make sure the precision of control be achieved, designer needs to pay more attention to the speed-increase or speed-decrease periods. AC servo-motor system is a closed-loop system. It is possible for the driver-component to sample the signal from the motor encoder to complete a position cycle and speed cycle internally. As such, AC servo-motor system generally will not have the errors of step loss or overshooting, and is more reliable in terms of controlling performance.
Step-motor needs 200 to 400 mil-seconds to accelerate from still to a typical working speed of several hundred rpm. AC servo-motor is better in terms of acceleration performance. For example, Panasonic MSMA 400W needs only a few mil-seconds to accelerate from still to its rated speed of 3000RPM. So it is clear that step-motorâ„¢s performance is not so good. However itâ„¢s cheaper. Started from late 70s and early 80s, with the development of micro-process technology, high-power, high-performance semiconductor technology, and manufacturing technology of permanent magnetic material, the performance price ratio of AC servo-system has been improved significantly. Price of AC servo-system also is gradually deceasing in recent years. AC servo motor is becoming the dominant product.
The conclusion is that all the motors, step-motor, AC asynchronous motor, DC motor with/without brush, AC servo-motor, can be applied in ASTS. Asynchronous AC motor is the cheapest. But it is big in size, and low in technical specification. The step-motor has a simple controlling mode and is also low in price. AC servo-motor has the best performance and wide power range. Its price is also the highest. As for the performance and price for permanent magnetic DC brushless motor, they are both rated between step-motor and AC servo-motor. Its performance is close to servomotor. For the situations that the output torque is not very high (less than 2 NM ), permanent magnetic DC brushless motor is a good option.
Dish type tracking controller and PV tracking controller can be both applied as four- quadrants solar sensor to correct tracking bias. It is known that solar sensor will lose its functionality temporally when itâ„¢s cloudy. In the area of solar thermal generation, solar sensor system usually follows the equation based on the astronomic formula to locate the position of the sun. When a MPU (micro-processor unit) is applied to calculate the sunâ„¢s position, because of its low process speed and low precision, itâ„¢s necessary to include a solar sensor to make a closed loop system. If the tracking system uses a PC or a high-performance DSP as the controller, the bias for the calculated sun position will be within one percent of mrad (milliradian), when the system clock is precisely set (Direct time from GPS is an option). No solar sensor is needed to track the sun, especially when the slope error and the gear-diastema are all small. Exception happens when the motor is a step-motor and the output torque is not enough. The situation can lead to a blockage of the motor (For example, a windy weather), which will fail the tracking system to track the sun precisely. As such, a closed loop solar sensor is recommended in such system. There are many kinds of solar sensor. The four- quadrants sensor was used in our project as shown in Figure 5.
Fig. 5: Four quadrant Sensor
In four quadrant sensor system, the Photovoltaic current will be bigger for the quadrant of bigger solar facular area, which will indicate whether the sunâ„¢s incident ray is parallel with the axial direction of the sensor, then to adjust the angle by the motor. It should be stressed during the solar sensor design that the inside wall of the solar sensor needs to be blacken to avoid misjudgment by the reflection of sunray inside the solar sensor
WORKING OF ASTS:
The basic functional blocks of this system are six sensors1, and their operation depends upon the intensity of light falling on solar panel. All sensors (each with different functionality) send their output to microcontroller AT89c52. Then the microcontroller executes predefined task in its software. These sensors are being used with following names and functionality:
Sun Tracking Sensors (STS)
These two sensors are mounted in V shape (figure-6) exactly in the middle of the solar panel (figure-8). The automatic sun tracking is accomplished according to following 3-step diagram.
Figure6. Basic Automatic Sun Tracking Operation
Â¢ Step-1 shows that when the sun is in front of solar panel, both sensors i.e. STS-1 and STS-2 are getting same amount of light.
Â¢ In step-2, after some time as the earth rotates the solar panel gets repositioned with respect to sun and STS-1 obtains less amount of light. At this point the LDR i.e. STS-1 sends signal to the microcontroller (figure 3). Then the controller1 rotates motor, resulting the rotation of solar panel towards the sun.
Â¢ Finally step-3 shows the reorientation of solar panel. The process continues until the end of day.
Figure7. Interface of LDR with Microcontroller
Night Time Fault Detector (NTFD)
In routine work of the system if a general fault2 occurs during nighttime then the next morning it would not work. So at the next sunrise, this sensor detects whether the solar panel is ready for tracking or not. As shown in figure-8, the NTFD is mounted in east of the solar panel so in normal conditions it does not work because it gets lesser intense light (predefined) as compared to the middle sensors i.e. STS-1 and STS-2, but as the fault arises, it starts working.
Day Time Fault Detector (DTFD)
Except some special conditions e.g. cloudy weather etc, the ASTS is supposed to track the sun the whole day. If the panel stops rotation then DTFD detects this type of fault. The mounting strategy of this sensor is same as that of NTFD except that it is mounted in the west.
Night and Cloud Detection
In a cloudy day light intensity is less than a normal day. Similarly during night, light intensity is far less than a cloudy day. So the night and cloud sensors work on this principle to detect the event. To sense a smaller change in light intensity cloud sensor is more sensitive than the Night sensor. In case of Night event, the microcontroller stops all operations of the system and repositions the solar panel towards east to track the sun for next morning.
Figure8. Sensor mounting assembly on solar panel
CONTROL STRATEGY OF ASTS
For a successful operation, the ASTS has two types of control approach:
Â¢ Automatic Control
Â¢ Manual Control
With the help of an efficient algorithm (written in C) only one Master Microcontroller1 is being used to manage the automatic operation of ASTS. This controller has following functions:
Â¢ Senses all of six sensors.
Â¢ Drives stepper motor.
Â¢ Drives LCD.
Â¢ Controls the warning indicators e.g. buzzer, LEDâ„¢s etc.
Â¢ Communicates (by parallel port) with the slave microcontroller. The central driving components of automatic control are only six sensors. Their operation has been explained on the previous page.
As no human made system is so perfect so an unpredictable fault may occur in the any system. That is why a manual control option was also kept in ASTS. While designing this part of control two objectives were kept in mind:
Â¢ The manual control should work efficiently.
Â¢ It should be as user friendly as possible. Following two approaches have been used to accomplish the manual control.
Â¢ Stand Alone System Control Unit
Â¢ Computer based control unit
Stand Alone System Control Unit
It is a simple user interface, which includes onboard LCD, Keypad, Buzzer and a complete PCB of the system circuit. The LCD (Hitachi HD44780) displays different messages, which can help the user in manually operating the system. While the keypad includes keys of Numeric Digits, Emergency Stop, clock wise rotation and counter clockwise rotation. Using keypad a user can manually rotate the solar panel by entering angle from 0o to 180o. The angle value is limited to only 180 values because after sunrise, the earth almost rotates only 180 degrees and then the sun disappears. The advantage of this unit is that to run the system it does not need computer but its disadvantage is that at a time it controls only one solar panel. In figure-3 this unit shown in yellow color, in middle of solar panel and computer terminal.
Figure 9. Flow Chart showing automatic operation of ASTS
Computer Based Control Unit
This is completely software based control, written in VB 6.0. It incorporates a GUI (figure-10) and a Database, linked with Microsoft Access. Using this software the computer serially communicates (RS232) with the ASTS. The Slave microcontroller (89c51) of the system makes this communication successful. Due to some fault if the solar panel stops rotating, then with the help of this software a user can:
Â¢ rotate the solar panel manually.
Â¢ stop all operations of ASTS (in emergency case)
The database of this system can be used to keep records, which can be retrieved even after a long time. While saving the new data, the database automatically takes the date and time from the computer and keeps them saved along with the data, entered by the human being. The advantages of computer-based control include:
Â¢ Facility of Database.
Â¢ At a time this software can handle three ASTS systems.
Â¢ It has an attracting GUI
Figure10. VB based GUI of ASTS
Although ASTS is a prototype towards a real system, but still its software and hardware can be used to drive a real and very huge solar panel. A small portable battery can drive its control circuitry. Therefore by just replacing the sensing instrument, its algorithm and control system can be used in RADAR and moveable Dish Antennas
Many modifications especially in hardware of ASTS are planned:
Â¢ The computer and System Control Unit would have a wireless communication with the mechanical structure of solar panel.
Â¢ More sensitive sensors would be used.
Â¢ To make emergency control better more powerful microcontrollers e.g. PIC 16F877A would be used.
Â¢ To reprogram the PIC microcontroller a wireless boot loader would be used.