南台科技大學
機械工程研究所
碩士學位論文
雙軸追日系統的設計與探討
Design and Investigation of
A Two-Axis Automatic Solar Tracking System
指導教授：林克默
Advisor: Assistant Prof. Lin, Keh Moh
研究生：範越雄
Graduate Student: Pham, Viet Hung
中華民國九十六年七月
ABSTRACT
English
The purpose of this research is to design and build an automatic solar tracking
system for PV panel so that the efficiency of PV modules can be enhanced and solar
energy can be used effectively.
First, the most suitable technical solution for the solar tracking system is
proposed. Then, the proposed solar tracking system was built and the performance of
this system was characterized. Finally, the affect of using the automatic solar tracking
system on output power was experimentally investigated.
The results indicated that our automatic solar tracking system which uses
proposed sensors, microcontroller and stepper motors is simple, low-cost, and
efficient. The measured variables of our automatic solar tracking system were
compared with those of a fixed PV panel. The output power of our automatic solar
tracking system has an overall increase of about 7 %.
Chinese
本研究之目的為設計並建構一太陽能追日系統以便提高太陽模組的轉換效率有
效率地運用太陽能源。首先對多種技術方案進行探討以便找出最適合的方案設
計出一追日系統。接著對所提方案建購成實體並對其性能進行測試。最後再以
實體探討此自動追日系統對功率輸出之影響。結果顯示，利用感測器及步進馬
達所建構之追日系統不只簡易、低成本且有效率。將其測量變數與固定軸的 PV
系統比較，發現自動追日系統之 PV 系的功率輸總夹高出 7%。
Keywords: Solar energy; Two-axis solar tracking system; Open loop control;
Microprocessor (AVR); Stepper motor; Photo-Sensor
i
ACKNOWLEDGEMENTS
There are many people I would like to thank for their help and support over my two
years. I may not mention them all here, but deserve it, thank you very much for all
you have done.
™ Firstly, I would like to thank to my advisor Professor Keh Moh Lin and his
wife for all their time and patience in helping me with my Research and Thesis. He
has endlessly and tirelessly mentored, taught and encouraged me since I was freshman.
I appreciate his advice and willingness to discuss any questions or ideas that I have
had. I am very grateful for having had such a wonderful supervisor to guide me
through my master studies.
™ I would like to thank my readers, Professor Chung-Jen Tseng and Professor
Jang for their helpful comments throughout this process.
™ Many thanks as well for the generous support of Southern Taiwan University
of Technology (STUT) and Hue Nong Lam University, both academic and finance aid.
™ Thank you to my friends at STUT that I have made in the two years, including
(Taiwanese and International Students) for making my stay and studies here
comfortable and enjoyable.
™ Last but not least, I would like to thank my parents, my wife and son for all
their love and for always supporting me. Their encouragement and willingness help
me to make this work enjoyable.
Thank you all for everything, I am truly grateful to each and every one of you for
everything you have done for me throughout my life.
Pham Viet Hung
STUT, July 20th 2007
ii
TABLE OF CONTENT
ABSTRACT...........................................................................................................i
ACKNOWLEDGEMENTS..................................................................................ii
TABLE OF CONTENT...................................................................................... iii
LIST OF FIGURES ..............................................................................................v
LIST OF TABLES..............................................................................................vii
1. INTRODUCTION................................................................................................1
1.1 Solar energy ....................................................................................................1
1.2 Automatic Solar Tracking System (ASTS).....................................................4
1.3 Purpose and main works of Research .............................................................5
2. NUMERICAL SIMULATION ...........................................................................6
2.1. Numerical Model ...........................................................................................6
2.2. Solution of Numerical Module ....................................................................11
2.3 Results of Matlab PV module model ............................................................13
3. DESIGN OF A TWO-AXIS ASTS ...................................................................18
3.1. Requirements of system...............................................................................18
3.2. The STS overview........................................................................................18
3.3 System Operation Principle ..........................................................................22
3.4 Structure of Solar Tracking System..............................................................29
3.4.1 The ME movement mechanism ...........................................................29
3.4.2 The sensors signal processing unit.......................................................50
3.4.3 ASTS Control program Installation .....................................................51
3.5 The control approach ....................................................................................54
iii
3.6 Guide & Note for using System....................................................................56
4. EXPERIMENTAL RESULTS AND DISCUSSION ......................................57
4.1. Experiments .................................................................................................57
4.1.1 Collection Data System........................................................................57
4.1.2 Experimental setup...............................................................................59
4.2. Result and Discussions ................................................................................61
5. CONCLUSION ..................................................................................................64
iv
LIST OF FIGURES
Figure1. 1: Schematic of a simple conventional solar cell [1].......................................2
Figure1. 2: Illustration of the concept of drift in a semiconductor [1] ..........................2
Figure2. 1: Simple solar cell circuit model....................................................................7
Figure2. 2: Sample I-V curve of a silicon solar cell ......................................................7
Figure2. 3: Characteristic of PV module model ..........................................................12
Figure3. 1: Illustration of the summer and winter solstices.........................................19
Figure3. 2: Tilt Angle θ of a PV panel.........................................................................20
Figure3. 3: Zenith and Altitude angle of sun ...............................................................21
Figure3. 4: The elevation angle varies throughout the day [5] ....................................21
Figure3. 5: The System control Block Diagram. .........................................................24
Figure3. 6: System Control Program Simplified flowchart .........................................25
Figure3. 7: The proposed photo-sensor .......................................................................26
Figure3. 8: Photo resistors set-up.................................................................................27
Figure3. 9: The proposed Two-Axis ASTS .................................................................29
Figure3. 10: The metal frame of ASTS .......................................................................30
Figure3. 11: The steel base of ASTS ...........................................................................30
Figure3. 12: Dimension of PV panel of ASTS ............................................................31
Figure3. 13: The worm gear mechanism. ....................................................................32
Figure3. 14: The Stepper Motor (57SH-52A9H-Japan) ..............................................34
Figure3. 15: Stepper and DC Motor Rotation .............................................................35
Figure3. 16: Cross-section and Wiring diagram of a Unipolar stepper motor.............37
Figure3. 17: The stepper motor driver .........................................................................41
Figure3. 18: Typical Stepper Motor Driver System ....................................................42
v
Figure3. 19: Stepper Motor drive elements .................................................................42
Figure3. 20: The Basic Unipolar Drive Method ..........................................................44
Figure3. 21: Interfacing with Microprocessor (AVR AT 90S8535)............................46
Figure3. 22: AVR AT 90S8535 Pin Configurations....................................................48
Figure3. 23: Sensor controller circuit ..........................................................................50
Figure3. 24: The AVR programmer wiring (AT90Sxxxx)..........................................52
Figure3. 25: The AVR programmer (connect LPT port).............................................53
Figure3. 26: The BASCOM-AVR Interface................................................................54
Figure3. 27: The detail control program flowchart......................................................55
Figure4. 1: The Collection Data System......................................................................58
Figure4. 2: The ASTS Control System ........................................................................59
Figure4. 3: The output current of tracking and non-tracking PV panel.......................62
vi
LIST OF TABLES
Table1. 1: The advantages and disadvantages of PV [1] ...............................................3
Table2. 1: The characteristic parameters of PV model................................................11
Table2. 2: Characteristic of PV module model (Day time) .........................................13
Table2. 3 Result of PV parameters calculation............................................................17
Table3. 1: Illuminations of light source.......................................................................23
Table3. 2: Step sequence for a Unipolar stepper motor...............................................38
Table3. 3: Excitation Methods.....................................................................................43
Table3. 4: Overview features of AVR AT90S8535....................................................45
Table3. 5: General Package Info ( AT90S8535) ........................................................45
Table4. 1: The experimental data................................................................................61
vii
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1. INTRODUCTION
The energy demands in the world are increasing day by day. However, the storage
of fossil fuels (coal, oil, normal gas…) on the earth is limited. Thus, renewable energy
sources are needed to satisfy these energy needs. One renewable energy source
available everywhere on the earth is the solar energy. This is the potential energy
source in the future.
1.1 Solar energy
The electrical methods what can harness the solar energy use semiconductors to
convert the incident photon flux into a current while simultaneously producing a
photo voltage. Solar electricity, also known as photovoltaic (PV), has existed since
the 1950s.
A solar cell is a semiconductor diode that has been carefully designed and
constructed to efficiently absorb and convert light energy from the sun into electrical
energy.
Semiconductors have the capacity to absorb light and to deliver a portion of the
energy of the absorbed photons to carriers of electrical current – electrons and holes.
A semiconductor diode separates and collects the carriers and conducts the generated
electrical current preferentially in a specific direction. A simple conventional solar
cell structure is depicted in Figure 1.1
Sunlight is incident from the top on the front of the solar cell. A metallic grid forms
one of the electrical contacts of the diode and allows light to fall on the semiconductor
between the grid lines and thus be absorbed and converted into electrical energy. An
antireflective layer between the grid lines increases the amount of light transmitted to
the semiconductor.
1
The semiconductor diode is fashioned when an n-type semiconductor and a p-type
semiconductor are brought together to form a metallurgical junction. This is typically
achieved through diffusion or implantation of specific impurities or via a deposition
process. The diode’s other electrical contact is formed by a metallic layer on the back
of the solar cell [1].
Figure1. 1: Schematic of a simple conventional solar cell [1]
Figure1. 2: Illustration of the concept of drift in a semiconductor [1]
2
Table1. 1: The advantages and disadvantages of PV
Advantages
Disadvantages
Fuel source is vast and essentially infinite
Fuel source is diffuse (sunlight is a
relatively low-density energy)
No emissions, no combustion or radioactive
fuel for disposal (does not contribute
perceptibly to global climate change or
pollution)
Low operating costs (no fuel)
High installation costs
No moving parts (no wear)
Ambient temperature operation (no high
temperature corrosion or safety issues)
High reliability in modules (>20 years)
Poorer reliability of auxiliary (balance
of system) elements including storage
Modular (small or large increments)
Quick installation
Can be integrated into new or existing
building structures
Can be installed at nearly any point-of-use
Lack of widespread commercially
available system integration and
installation so far
Daily output peak may match local demand
Lack of economical efficient energy
storage
High public acceptance
Excellent safety record
3
1.2 Automatic Solar Tracking System (ASTS)
The major problem of solar power systems is the poor efficiencies (approximately
14-16%). There are three methods to enhance the efficiency of a PV system as the
following:
™ To enhance the efficiency of solar cell
™ To use a MPPT system
™ To use a STS
The cost of a PV system that applies method 1 and 2 is quite high. The first
method is not only too easy to enhance the efficiency of solar cell but also of too high
cost. The second method is high cost too. Many expensive equipments are needed to
assistant its operation. Our researches focus on the ASTS. A low cost, efficient ASTS
is the most exact reason can explain our attention to the third method.
ASTS has been widely studied to improve the efficiency of PV modules. Many
methods are applied to control ASTS with different types of mechanisms. A tracking
mechanism must be reliable and able to follow the sun with a certain degree of
accuracy, return the collector to its original position at the end of the day or during the
night, and also be able to track during periods of cloudy.
Fixed PV panel producing electricity throughout the year are usually installed and
tilted at an angle equal to the latitude of the installation site facing directly to the sun.
In this case, the solar energy collected during both winter and summer is less due to
the sun’s changing altitude. The use of a tracking mechanism increases the amount of
solar energy received by the PV panel resulting to a higher output power.
There are two types of ASTS: one-axis and two-axis ASTS. Usually, the singleaxis tracker follows the Sun’s East–West movement while the two-axis tracker
follows also the Sun’s changing altitude angle.
4
In our earlier work, some technical solutions of ASTS were investigated with
focus on the following: The efficiency of the systems, the ability of the ASTS in
tracking the sun and the costs of the ASTS. This helps us to choose the suitable
technical solution for our system. The results indicated that the two-axis ASTS that is
controlled automatically by using the proposed sensor, stepper motors and
microprocessor is inexpensive, precisive and efficient.
1.3 Purpose and main works of Research
The purpose of this research is to enhance the efficiency of PV modules so that
solar energy can be used effectively.
This work is to present the installation of a two-axis ASTS which is based on the
combined use of the conventional photo-resistors and the programming method of
control which works efficiently in all weather conditions regardless of the presence of
clouds for long periods.
The main works are as follows:
™ The analysis of a PV numerical model
™ The design and construction of a Two-axis ASTS
™ Make a comparison of the operation of the PV panel with the two-axis ASTS
with a fixed PV panel and numerical simulation models
5
2. NUMERICAL SIMULATION
2.1. Numerical Model
A numerical model allows the investigator to examine the effects related directly
to changes in one parameter. Also, the numerical model permits a variable to be
changed over a wide range, possibly greater than what can be achieved in a laboratory
setting. Again, this provides insight into the effects of that one variable, even if these
parameters are unrealistic or impractical.
Numerical model is the ability to examine exactly what changed between each
case. It is easy to find out unexpected result, reexamine the input parameters and
deduce the causes of the result based on the changed parameters.
However, numerical models are the inability to exactly model the semiconductor
device. Therefore, any results obtained from numerical simulations must be
considered within the constraints of that model.
In many cases, looking at the relative changes shown by the model may be more
useful and universally applicable than looking at the absolute values obtained.
A simplest equivalent circuit of a solar cell is a photo current source in parallel
with a single diode and a series resistance, includes temperature dependences. The
output of the current source is directly proportional to the light falling on the cell.
The diode determines the I-V characteristics of the solar cell. In the dark, the I-V
output characteristic of a solar cell has an exponential characteristic similar to that of
a diode. When exposed to light, photons with energy greater than the band gap energy
of the semiconductor are absorbed and create an electron-hole pair. These carriers are
swept apart under the influence of the internal electric fields of the p-n junction and
create a current proportional to the incident radiation.
6
A solar cell equivalent circuit as Figure 2.1
Figure2. 1: Simple solar cell circuit model
A solar cell can be modeled by an ideal current source (Iph) in parallel with a
diode, as shown in Figure 2.1. The current–voltage (I –V curve) characteristic of a
typical silicon solar cell is plotted in Figure 2.2
Figure2. 2: Sample I-V curve of a silicon solar cell [1]
7
Of particular interest is the point on the I–V curve where the power produced is at
a maximum. This is referred to as the maximum power point with U = U Pmax
and I = I Pmax .
The fill factor, FF, is a measure of the squares of the I–V characteristic and is
always less than one. It is the ratio of the areas of the two rectangles shown in Figure
2.2
FF =
U
I
PMP
= P max P max
U OC I SC
U OC I SC
(2.0)
When the cell is short circuited, this current flows in the external circuit; when
open circuited, this current is shunted internally by the intrinsic p-n junction diode.
The characteristics of this diode therefore set the open circuit voltage characteristics
of the cell.
An accurate PV module electrical model is presented based on the Shockley diode
equation. The equations which describe the I-V characteristics of the solarcell are:
I = I ph − I 0 (e
U = U T ln(
U + IRPV
UT
− 1)
I ph − I + I 0
I0
or
(2.1a)
) − IRPV
(2.1b)
UT
dU
= −(
+ R PV )
dI
I ph − I + I 0
P = IU = IU T ln(
I ph − I + I 0
I0
(2.2)
) − I 2 R PV
(2.3)
8
I ph − I + I 0
UT
dP
= U T ln(
+ 2 R PV )
) − I(
dI
I0
I ph − I + I 0
(2.4)
Solar cell characteristic curve parameter：ISC、UOC、MPP(Pmax、UPmax、IPmax)
Asks to find: RPV, UT, I0, Iph.
Supposes the cell at Upmax，Then may result in the following system of equations:
dU
dI
dP
dI
=M
U
I =0
=0
U
I = I p max
I =0
= U OC
I = I SC
=0
Supposes the cell at M，Then may result in the following system of equations
U
I = I p max
dP
dI
= U p max
=0
I = I p max
U
U
I =0
= U OC
I = I SC
=0
Using above conditions to substitution in equations (2.1) ~ (2.4), we have result:
⎧ dU
=M
⎪
U T + (M + RPV )(I ph + I 0 ) = 0 , ⎨ dI
⎪⎩ I = 0
(2.5a)
⎛ U P max +UI P max .RPV
⎞
⎧U = U P max
T
I0 ⎜e
− 1⎟ − I ph + I P max = 0 , ⎨
⎜
⎟
⎩ I = I P max
⎝
⎠
(2.5b)
⎞
⎛ I P max ⎛⎜ U T
⎞
⎟
⎜ U T ⎜⎝ I ph − I max + I 0 + 2 RPV ⎟⎠ ⎟
− 1⎟ − I ph + I P max = 0 ,
I0 ⎜ e
⎜
⎟
⎝
⎠
⎛ UUOC
⎞
⎧U = U OC
I 0 ⎜ e T − 1⎟ − I ph = 0 , ⎨
⎜
⎟
⎩I = 0
⎝
⎠
⎧ dP
=0
⎪
⎨ dI
⎪⎩ I = I P max
(2.6)
(2.7)
9
⎛ I SCU.R PV
⎞
I 0 ⎜ e T − 1⎟ − I ph + I SC = 0 ,
⎜
⎟
⎝
⎠
⎧U = 0
⎨
⎩ I = I SC
(2.8)
Where:
Iph: Photo current
I0: The saturation current of the diode
ISC: Short-circuit current
UT: Temperature voltage
UOC: Open-circuit voltage
MPP: Pmax、Upmax、Ipmax
RPV: Solar cell resistance
RS: Series resistance
Set values: I P max , U P max , I SC , U OC ; Unknowns: I ph ,U T , RPV , I 0
Ipv is the light generated current inside the solar cell and is the correct term to use
in the solar cell equation. At short circuit conditions the externally measured current is
Isc. Since Isc is appropriately equal to Ipv, the two are used interchangeably and for
simplicity and the solar cell equation is written with Isc in place of Ipv. In the case of
very high series resistance (> 10 ohm cm) Isc is less than Ipv and writing the solar cell
equation with Isc is incorrect.
Another assumption is that the illumination current Ipv is solely dependent on the
incoming light and is independent of voltage across the cell. However, Ipv varies with
voltage in the case of drift-field solar cells and where carrier lifetime is a function of
injection level such as defected multi-crystalline materials.
At short circuit conditions: I ph ≈ I SC
The characteristic parameters (Isc、Uoc、Upmax、Ipmax) of PV model that we
measure at Pmax as below:
10
Table2. 1: The characteristic parameters of PV model
Voc
Vpmax
Pmax
Ipmax
Isc
57.0507
44.8853
13.3369
0.30
0.33
The method of parameter extraction and model evaluation are carried out in Matlab
Software.
2.2. Solution of Numerical Module
We use MATLAB software to solve the PV numerical module. MATLAB is a
high-performance language for technical computing. In university environments, it is
a good instructional tool for introductory and advanced courses in mathematics,
engineering, and science.
MATLAB features a family of add-on application-specific solutions called
toolboxes. Toolboxes allow you to learn and apply specialized technology. Toolboxes
are comprehensive collections of MATLAB functions (M-files) that extend the
MATLAB environment to solve particular classes of problems.
Areas in which toolboxes are available include signal processing, control systems,
neural networks, fuzzy logic, wavelets, simulation, and many others. In this work, we
use MATLAB to solve a nonlinear equations system to get the parameters: Iph, Ut,
Rpv, Io.
A typical 70W PV panel was chosen for modeling. The module has 1 panel
connected polycrystalline cells.
11
Figure2. 3: Characteristic of PV module model
The model was evaluated using Matlab® Software. The model parameters are
evaluated by using the equations 2-5b, 2-6, 2-7 and 2-8, listed in the previous section
using the above data. The current I is then evaluated using these parameters, and the
variables Voltage, Irradiation, and Temperature. If one of the input variables is a
vector, the output variable (current) is also a vector.
The solution of the non-linear equations system may result in the solar cell
parameters RPV、UT、I0、Iph. All of the constants in the above equations (ISC、UOC,
UPmax、IPmax) can be determined by examining the manufacturer’s ratings of the PV
array, and then measured I-V curves of the array.
Short-circuit current ISC is measured when the two terminals of the device are
connected together through an ideally zero-resistance connection. The open-circuit
voltage describes the performance of the illuminated cell with no electrical
connections.
12
2.3 Results of Matlab PV module model
Table2. 2: Characteristic of PV module model (Day time)
Voc
Vpmax
Pmax
Ipmax
Isc
58.5907
49.8973
3.3549
0.07
0.08
59.0587
49.9107
6.9591
0.14
0.16
57.8767
44.8647
10.4041
0.23
0.25
57.3813
44.8747
12.3616
0.28
0.30
57.0507
44.8853
13.3369
0.30
0.33
56.702
44.884
12.3207
0.27
0.32
56.9713
44.886
10.8833
0.24
0.27
57.3007
44.8853
7.6518
0.17
0.18
56.154
44.872
2.8883
0.06
0.07
List of I-V curve and Power of PV panel:
13
14
15
16
17
3. DESIGN OF A TWO-AXIS ASTS
3.1. Requirements of system
The requirement of the system can be proposed as the following:
™ It must determine when the sunlight intensity is adequate to turn the tracking
circuits to work.
™ It must track the sun while ignoring transient shadow and lights from fast
moving source such as clouds, shrubbery, birds, etc
™ It must recognize the end of day and the position in anticipation of the next
sunrise.
™ It must protect the system upon command by removing the array from focus
and returning to its home position.
™ It must be adaptable to the user choice of driver motors.
™ It should be capable of operating from and charging a battery if the user
choose this option.
3.2. The STS overview
The position of the sun with respect to that of the earth changes in a cyclic manner
during the course of a calendar year as Figure 3.1.
Tracking the position of the sun in order to expose a PV panel to maximum
radiation at any given time is the main purpose of a solar tracking PV system, 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 panels must remain in front
of sun during the whole day. But due to rotation of earth those panels can’t maintain
their position always in front of sun. This problem results in decrease of their
efficiency.
18
Thus to get a maximal output, an automated system is required which should be
capable to constantly rotate the solar panel.
Figure3. 1: Illustration of the summer and winter solstices [4]
The ASTS was made as a prototype to solve the problem that is mentioned
above. It is completely automatic and keeps the panel in front of sun until that is
visible. In case the sun gets invisible e.g. in cloudy weather, then without tracking the
sun the ASTS keeps rotating the PV panel in opposite direction to the rotation of earth.
But its speed of rotation is same as that of earth’s rotation. Due to this property when
after some time when the sun again gets visible, the solar panel is exactly in front of
sun.
A typical ASTS must be equipped with two essential features:
a) Azimuth tracking for adjusting the tilt angle of the surface of the PV panel
during changing seasons; and
b) Daily solar tracking for maximum solar radiation incidence to the PV array.
19
The Tilt Angle θ of a PV system (Figure 3.2) required at any given time in the year
can be expressed as a function of the seasonal Sun’s Altitude φ as follows:
Tilt Angle θ = 900 − φ
(3.1)
Figure3. 2: Tilt Angle θ of a PV panel
The elevation angle (used interchangeably with altitude angle) is the angular
height of the sun in the sky measured from the horizontal. Confusingly, both altitude
and elevation are also used to describe the height in meters above sea level. The
elevation is 0° at sunrise and 90° when the sun is directly overhead (which occurs for
example at the equator on the spring and fall equinoxes).
The zenith angle (Figure 3.3) is similar to the elevation angle but it is measured
from the vertical rather than from the horizontal, thus making the zenith angle = 90° elevation.
20
Figure3. 3: Zenith and Altitude angle of sun
The elevation angle varies throughout the day. It also depends on the latitude of a
particular location and the day of the year (Figure 3.4).
Figure3. 4: The elevation angle varies throughout the day [5]
An important parameter in the design of photovoltaic systems is the maximum
elevation angle, that is, the maximum height of the sun in the sky at a particular time
of year.
The elevation angle at solar noon can be determined according to the formula for
locations in the Northern Hemisphere:
α = 90 − φ + δ
(3.2)
21
And for the Southern Hemisphere:
α = 90 + φ − δ
(3.3)
Where φ is the latitude of the location of interest
In the equation for the Northern Hemisphere, it is positive for Northern Hemisphere
locations and negative for Southern Hemisphere.
In the equation for the Southern Hemisphere, δ is positive for Southern Hemisphere
locations and negative for Northern Hemisphere locations.
δ is the declination angle, which depends on the day of the year.
The zenith angle is the angle between the sun and the vertical:
zenith = 90° - elevation
(3.4)
3.3 System Operation Principle
In this work, the programming method of control with Open loop system is used,
which works efficiently in all weather conditions regardless of the presence of clouds.
To implement this method, we need to design the suitable photo-sensor, write a
control program base on function of the photo-sensor. The microprocessor will
control the work of the actuator so that it will track the sun’s position.
The STS uses a microprocessor, the proposed photo-sensor and two stepper
motors to control the position of PV panel follow the sun. One stepper motor rotates
the PV panel in the Left-Right direction. The other rotates the PV panel in the UpDown direction.
A controller program that is written in BASCOM-AVR is installed in the
microprocessor. The microprocessor controls the STS base on this program. Its
operation depends upon the intensity of light falling on PV panel. The main
component of sensor is two pairs of LDRs as Figure 3.7. LDRs are used as sensors for
22
generating an electric signal proportional to intensity of light falling on it. The LDRs
are mounted at the focus of reflectors which are directly mounted on PV panel.
All LDRs have different function. These LDRs send the voltage signal to the
microprocessor and then the controller program will control the stepper motor drivers
to drive the two stepper motors to rotate and adjust PV panel so that it can follow the
sun.
A photo-resistor is an electronic component whose resistance decreases with
increasing incident light intensity. It can also be referred to as a light-dependent
resistor (LDR), photoconductor, or photocell.
A photo-resistor is made of a high-resistance semiconductor. If light falling on the
device is of high enough frequency, photons absorbed by the semiconductor give
bound electrons enough energy to jump into the conduction band. The resulting free
electron (and its hole partner) conduct electricity, thereby lowering resistance.
Applications include smoke detection, automatic light control, batch counting and
burglar alarm systems…
Table3. 1: Illuminations of light source and Characteristics of the LDR
Light source
Illuminations (Lux)
Moonlight
0.1
60W bulb at 0.1m
50
1W MES bulb at 0.1m
100
Fluorescent lighting
500
Bright sunlight
Characteristics of the LDR
30.000
Module
PGM552-MP (TOREN)
Photo Resistance (10Lx)
8~20 (KΩ)
Dark Resistance
1.0 (MΩ)
Vmax (DC):
150 V
Temp (ºC):
-30~+70
23
Figure3. 5: The System control Block Diagram.
24
Figure3. 6: System Control Program Simplified flowchart
25
The proposed photo-sensor is a box consisting of four dark rooms I, II, III and IV.
Each LDR is fixed inside a dark room. LDR1, LDR2, LDR3 and LDR4 locate in the
dark room I, II, III and IV correspondingly (as Figure 3.7.b). A pair of them,
controlling the angle of azimuth, is positioned East-West direction and the two of
them, controlling the angle of tilt, are positioned Up-Down direction.
Figure3. 7: The proposed photo-sensor
26
For tracking in the Left-Right direction, we make holes (three holes for each dark
room) to allow the sun light enter dark room I and II. It’s similar to Up-Down
direction.
The STS uses two pairs of LDR. The each pairs of LDR are set in two opposite
dark room. Two resistance values of each pair of LDR are used to compare. The
automatic sun tracking is accomplished according to following 3-step diagram as the
follows (Figure3.8):
Step-1: shows that when the sun is in front of the solar panel, both sensors i.e. LRD-1
and LRD-2 are getting same amount of light.
Step-2: the earth rotates the solar panel gets repositioned with respect to sun and
LRD-1 and LDR-2 obtains difference amount of light. At this point the LDR sends
signal to the microprocessor. The controller program control motor to rotate the PV
panel towards the sun until the LDR-1 and LDR-2 getting the same amount of light.
Step-3: shows the reorientation of the solar panel. The process continues until the end
of day.
Figure3. 8: Photo resistors set-up
27
The microprocessor receive voltage signal from 4 LDRs. The controller programs
make a comparison of the resistance values of these LDRs and will decide to rotate
the stepper motors or not. We set P1, P2 as two parameters of rotational limits
positions. The microprocessor remembers these positions to rotate the PV panel to the
expectant positions (i.e. for sunrise position and sunset position).
In a cloudy day light intensity is less than a normal day. Similarly during night,
light intensity is far less than a cloudy day. The voltage values of any LDR are
depended on the light intensity. So sensor work on this principle to compare the
voltage level of voltage signal which are sent to microprocessor.
We divide the value of output voltage of LDRs into 4 levels: V1, V2, V3, and V4.
V1: Sunshine and LDR is illuminated; V2: Sunshine and LDR is shaded; V3: It’s
cloud; V4: Day/night
With each direction, we have two rotational limits positions which are set and
written in the controller program. The function of which is to stop the motor from
going beyond the rotational limits, which restrict the overall rotation of the PV panel
in both of directions.
In case of night event, the program stops all operations of the system and
repositions the solar panel towards east to track the sun for next morning. When the
Sun goes down, sensor determines that it is night (lower than V4 voltage level of any
LDR). The controller program rotates the PV panel to the Left/Right (East) rotational
limits positions.
At the next sunrise, this sensor detects whether the solar panel is ready for
tracking or not. As any time there is sun as sensor get different values between two
LDRs of each direction. Then controller program compare and control system to
rotate PV panel until two LDRs get the same resistance values.
28
3.4 Structure of Solar Tracking System
As the sun’s position changes hourly, the solar power devices should be adjusted
to produce the maximum output power. Single-axis-tracking systems are considerably
cheaper and easier to construct, but their efficiency is lower than the two axes suntracking systems.
The proposed Sun tracking system consists of the following components:
•
The Mechanical and Electronic (ME) movement mechanism
•
The sensors signal processing unit
•
The system software.
3.4.1 The ME movement mechanism
a. Description of the mechanism system
Figure3. 9: The proposed Two-Axis ASTS
29
“Two-Axis” means that the system can be able to tracking sun follow two axes,
Left-Right (East-West) and Up-Down (Tilt angle) direction.
The M-E mechanism consists of two parts: aluminum frame and steel base.
The first is aluminum frame (Figure 3.10). It has two movement mechanisms, a Y
shaped bar and an aluminum beam. The Y shaped bar is connected to beam by two
ball-bearing bases and has the ability to rotate the PV panel in the Left-Right direction.
The PV panel is fixed on this Y shaped bar.
Figure3. 10: The metal frame of ASTS
Figure3. 11: The steel base of ASTS
30
The aluminum beam is connected to the base by two ball-bearing bases and has
the ability to rotate the metal frame in the Up-Down direction.
The steel base can move by four wheels (Figure 3.11). It has to be as stable as
possible. Its height is 1.5m.
The size of PV panel is 45cm width and 50cm length. The frame has the same size
with PV panel (Figure 3.12).
Figure3. 12: Dimension of PV panel of ASTS
b. The worm gear mechanism
The PV panel movement consists of two movement directions: Let-Right and UpDown. The movement mechanism is the worm gear mechanism (Figure 3.13). A gear
consisting of a spirally threaded shaft and a wheel with marginal teeth those mesh into
it.
The major particularity of this mechanical system is self-lock. The worm can
easily turn the gear, but the gear cannot turn the worm. This is because the angle on
the worm is so shallow that when the gear tries to spin it. The friction between the
gear and the worm holds the worm in place. This mechanism only works when the
31
motors are supplied the power. A tracking mechanism must be able to rotate the PV
panel following the Sun accurately and return the PV panel to its original position at
the end of the day or during the night, and also track during periods of intermittent
cloud cover.
Figure3. 13: The worm gear mechanism.
This mechanism has many advantages as:
•
Higher torque capacity with no increase in size, or conversely, smaller, more
reliable speed reducers.
•
High shock resistance and the ability to withstand heavy starting and stopping
loads.
•
Low backlash due to the inherent precision of the double enveloping design.
•
Increased durability and longer gear life.
•
Design flexibility resulting from smaller and lighter envelopments.
The disadvantages of worm gears are not by the strength of the teeth but by the
heat generated by the low efficiency. It is necessary therefore to determine the heat
generated by the gears. The worm gear must have lubricant to remove the heat from
32
the teeth in contact and sufficient area on the external surfaces to distribute the
generated heat to the local environment. If the heat lost to the environment is
insufficient then the gears should be adjusted (more starts, larger gears) or the box
geometry should be adjusted, or the worm shaft could include a fan to induced forced
air flow heat loss.
The reduction ratio of a worm gear is worked out through the following formula:
z2
z1
n=
(3.5)
n = Reduction Ratio
z 1 = Number of threads (starts) on worm
z 2 = Number of teeth on worm wheel
Efficiency of Worm Gear (η)
η=
cos α n − μ tan γ
cosα n + μcotγ
(3.6)
Peripheral velocity of worm wheel
V p (m / s) = 0,00005236.d 2 .n2
(3.7)
Where:
n 2 = Rotational speed of worm wheel (revs /min)
d 2 = Ref dia of worm wheel (Pitch dia of worm wheel) =( p x.z/π ) = 2.a - d 1 (mm)
A worm gear is used when a large speed reduction ratio is required between
crossed axis shafts which do not intersect. As the worm is rotated the worm wheel is
caused to rotate due to the screw like action of the worm. The size of the worm gear
set is generally based on the centre distance between the worm and the worm wheel.
The accuracy of the tracking mechanism depends on the PV panel acceptance angle.
This angle is defined as the range of solar incidence angles, measured relative to the
33
normal to the tracking axis, over which the efficiency varies by less than 2% from that
associated with normal incidence.
c. The Stepper motor
We used two stepper motors for the solar tracking system. Two stepper motors are
controlled by two motor drivers and a microprocessor base on the sensors. They
adjust the PV panel reflecting two directions, one is for the up-down direction (tilt
angle tracking) and other is for the left – right direction (daily tracking).
Figure3. 14: The Stepper Motor (57SH-52A9H-Japan)
There are several factors to take into consideration when choosing a type of motor for
an application. Some of these factors are what type of motor to use, the torque
requirements of the system, the complexity of the controller, as well as the physical
characteristics of the motor. There are many types of available motors for ASTS as
stepper motor, DC motor,... We select stepper motor because it is very strong when
not rotating and very easy to control rotor’s position.
34
Figure3. 15: Stepper and DC Motor Rotation [11]
A stepper motor is an electromechanical device which converts electrical
pulses into discrete mechanical movements. The shaft or spindle of a stepper motor
rotates in discrete step increments when electrical command pulses are applied to it in
the proper sequence. The motors rotation has several direct relationships to these
applied input pulses. The sequence of the applied pulses is directly related to the
direction of motor shafts rotation. The speed of the motor shafts rotation is directly
related to the frequency of the input pulses and the length of rotation is directly
related to the number of input pulses applied.
One of the most significant advantages of a stepper motor is its ability to be
accurately controlled in an open loop system. Open loop control means no feedback
information about position is needed. This type of control eliminates the need for
expensive sensing and feedback devices such as optical encoders. Your position is
known simply by keeping track of the input step pulses.
™ The rotation angle of the motor is proportional to the input pulse.
™ The motor has full torque at standstill (if the windings are energized)
35
™ Precise positioning and repeatability of movement since good stepper motors
have an accuracy of 3 – 5% of a step and this error is non cumulative from one step to
the next.
™ Excellent response to starting/stopping/reversing.
™ Very reliable since there are no contact brushes in the motor. Therefore the life
of the motor is simply dependant on the life of the bearing.
™ The motors response to digital input pulses provides open-loop control,
making the motor simpler and less costly to control.
™ It is possible to achieve very low speed synchronous rotation with a load that
is directly coupled to the shaft.
™ A wide range of rotational speeds can be realized as the speed is proportional
to the frequency of the input pulses.
The Stepper Motor Disadvantages
™ 1. Resonances can occur if not properly controlled. They have high vibration
levels due to stepwise motion. Large errors and oscillations can result when a pulse is
missed under open-loop control.
™ 2. Not easy to operate at extremely high speeds (limited by torque capacity
and by pulse-missing problems due to faulty switching systems and drive circuits).
There are three basic types of stepping motors: permanent magnet (PM), variable
reluctance (VR) and hybrid (HB). The stator or stationary part of the stepping motor
holds multiple windings. The arrangement of these windings is the primary factor that
distinguishes different types of stepping motors from an electrical point of view.
The selected Stepper Motors for the ASTS is PM stepper motor (a unipolar stepper
motor). Unipolar stepping motors are composed of two windings, each with a center
tap 1, 2. The center taps are either brought outside the motor as two separate wires
36
Fig.3.16 or connected to each other internally and brought outside the motor as one
wire.
Figure3. 16: Cross-section and Wiring diagram of a Unipolar stepper motor.
The characteristics of selected stepper motor as the follows:
Motor Model: 57SH-52A9H
Voltage: 12V
Electric current: 0.6A
Impedance: 18 Omega
Precisions: the 0.9DEG
The wiring:
Black: A, Green: A , Yellow: B, Orange: B , White: V+
As a result, unipolar motors have 5 or 6 wires. Regardless of the number of wires,
unipolar motors are driven in the same way. The center tap wire(s) is tied to a power
supply and the ends of the coils are alternately grounded.
Fig.3.16 shows the cross section of a 30 degree per step unipolar motor. Motor
winding number 1 is distributed between the top and bottom stator poles, while motor
winding number 2 is distributed between the left and right motor poles. The rotor is a
permanent magnet with six poles, three North’s and three South’s.
37
The direction of this flux is determined by the “Right Hand Rule” which states:
“If the coil is grasped in the right hand with the fingers pointing in the direction of the
current in the winding (the thumb is extended at a 90°angle to the fingers), then the
thumb will point in the direction of the magnetic field.”
CW Rotation
W1-a
1
0
0
0
1
0
0
0
…
W1-b
0
0
1
0
0
0
1
0
…
W2-a
0
1
0
0
0
1
0
0
W2-b
0
0
0
1
0
0
0
1
…
CCW Rotation
Table3. 2: Step sequence for a Unipolar stepper motor
A stepper motor can be a good choice whenever controlled movement is required.
They can be used to advantage in applications where you need to control rotation
angle, speed, position and synchronism. Because of the inherent advantages listed
previously, stepper motors have found their place in many different applications.
™ The maximum power dissipation level or thermal limits of the motor are
seldom clearly stated in the motor manufacturer’s data. To determine this we must
apply the relationship as follow:
P = VxI
(3.8)
For example, a stepper motor may be rated at 6V and 1A per phase. Therefore,
with two phases energized the motor has rated power dissipation of 12 watts.
It is normal practice to rate a stepper motor at the power dissipation level where the
motor case rises 65 0 C above the ambient in still air. Therefore, if the motor can be
mounted to a heat sink it is often possible to increase the allowable power dissipation
level. This is important as the motor is designed to be and should be used at its
38
maximum power dissipation, to be efficient from a size/output power/cost point of
view.
™ The step angle, the rotor turns per step, is calculated as follows
Step angle =
360 0
360
=
N
N R xN ph
(3.9)
Where:
NR = Number of rotor poles = Number of equivalent poles per phase
Nph = Number of phases
N = Total number of poles for all phases together.
Usually stepper motors have two phases, but three and five-phase motors also
exist. The stator has three sets of windings. Each set has two coils connected in series.
A set of windings is called a “phase”. The motor above, using this designation, is a
three-phase motor. A unipolar motor has one winding, with a center tap per phase (Fig.
3.16). Sometimes the unipolar stepper motor is referred to as a “four phase motor”,
even though it only has two phases.
™ Torque is a critical consideration when choosing a stepping motor. Stepper
motors have different types of rated torque. The torque produced by a stepper motor
depends on several factors: the step rate; the current through the windings; the drive
design or type. Torque is the sum of the friction torque (Tf) and inertial torque (Ti).
T = T f + Ti
(3.10)
The frictional torque is the force (F), in ounces or grams, required to move a load
multiplied by the length, in inches or cm, of the lever arm used to drive the load (r).
T f = F .r
(3.11)
39
The inertial torque (Ti) is the torque required to accelerate the load (gram-cm2)
ω
Ti = I . .π .θ .K
t
(3.12)
Where:
I = the inertial load in g-cm2
ω = step rate in steps/second
t = time in seconds
θ = the step angle in degrees
K = a constant 97.73
™ In Micro-stepping Drive the currents in the windings are continuously varying
to be able to break up one full step into many smaller discrete steps. Micro-stepping is
a relatively new stepper motor technology that controls the current in the motor
winding to a degree that further subdivides the number of positions between poles.
AMS micro-steppers are capable of rotating at 1/256 of a step (per step), or over
50,000 steps per revolution.
Micro-stepping is typically used in applications that require accurate positioning
and a fine resolution over a wide range of speeds.
40
d. The motor driver
The motor driver board works as a programmer board for the microprocessor.
This operating voltage is 12V.
1: Connect Microprocessor
P-1: Power Supply (12V)
M-1: Connect Motor 1
Figure3. 17: The stepper motor driver
d.1 Steper motor drive technology overview
Stepper motors require some external electrical components in order to run. These
components typically include a Controller, Indexer, Motor Driver Fig. 3.23. The
Motor Driver accepts step pulses and direction signals and translates these signals into
appropriate phase currents in the motor. The Indexer creates step pulses and direction
41
signals. The Controller sends commands to the Indexer to control the Motor. Many
commercially available drives have integrated these components into a complete
package.
Figure3. 18: Typical Stepper Motor Driver System [9]
The logic section of the stepper drive is often referred to as the translator. Its
function is to translate the step and direction signals into control waveforms for the
switch set Fig. 3.25. The basic translator functions are common to most drive types,
although the translator is necessarily more complex in the case of a microstepping
drive. However, the design of the switch set is the prime factor in determining drive
performance.
Figure3. 19: Stepper Motor drive elements [9]
The operation of a step motor is dependent upon an indexer (pulse source) and
driver. The number and rate of pulses determines the speed, direction of rotation and
the amount of rotation of the motor output shaft. The selection of the proper driver is
critical to the optimum performance of a step motor.
42
The stepper motor drive circuit has two major tasks: The first is to change the
current and flux direction in the phase windings; The second is to drive a controllable
amount of current through the windings, and enabling as short current rise and fall
times as possible for good high speed performance.
Table3. 3: Excitation Methods
Single Phase
1-2 Phase
Dual Phase
Switching Sequence
Pulse
Phase A
Phase B
Phase A’
Phase B’
Torque reduced by 39%
High torque
Features
Poor step accuracy
Poor step accuracy
Good step
accuracy
Increased efficiency
Higher pulse rates.
To control the torque as well as to limit the power dissipation in the winding
resistance, the current must be controlled or limited. Furthermore, when half stepping
a zero current level is needed, while microstepping requires a continuously variable
current. Two principles to limit the current are described here, the resistance limited
drive and the chopper drive. Any of the methods may be realized as a bipolar or
unipolar driver.
43
d.2 The stepper motor drive methods: UNIPOLAR DRIVE
The name unipolar is derived from the fact that current flow is limited to one
direction. As such, the switch set of a unipolar drive is fairly simple and inexpensive.
The unipolar drive principle requires a winding with a center-tap (T1, T2), or two
separate windings per phase. Flux direction is reversed by moving the current from
one half of the winding to the other half. This method requires only two switches per
phase (Q1, Q2, Q3, Q4).
The basic control circuit for a unipolar motor, shown in Fig. 3.20. The extra
diodes across each of the transistors are necessary. These diodes prevent the voltage
fromfalling below ground across the MOSFETs.
Figure3. 20: The Basic Unipolar Drive Method
The drawback to using a unipolar drive however, is its limited capability to
energize all the windings at any one time. As a result, the number of amp turns
(torque) is reduced by nearly 40% compared to other driver technologies. Unipolar
drivers are good for applications that operate at relatively low step rates.
On the other hand, the unipolar drive utilizes only half the available copper
volume of the winding. The power loss in the winding is therefore twice the loss of a
bipolar drive at the same output power.
44
e. The microprocessor
Based on the AVR AT90S8535-High Performance and Low Power RISC
Architecture is designed as a CPU to control the ASTS illustrated as Figure 3.21.
Table 3.6: Overview features of AVR AT90S8535
Table3. 4: Overview features of AVR AT90S8535
Flash
EEPROOM
SRAM
Speed
Volts
512B
512B
0-8MHz
4-6V
8kB
Table3. 5: General Package Info ( AT90S8535)
44-pin TQFP
44-pin PLCC
40-pin PDIP
Package Lead Code
44
44
40
Carrier Type
TRAY
TUBE
TUBE
1.2
4.57
Body Thickness
1.00
3.81
3.81
Body Width
10.00
16.56
15.24
Body Length
10.00
16.56
52.32
JEDEC MSL
3
2
Units per Carrier
160
27
10
Carriers per Bag
10
40
20
Units per Bag
1600
1080
200
Quantity per Reel
2000
500
0
Tape Pitch
16
24
Tape Width
24
32
Max Package
Height
45
D-1: Connect Motor Driver 1
D-2: Connect Motor Driver 2
PC: Connect PC to Transfer the Controller Program
LCD: Connect the LCD (14x2)
S: Connect the Sensor
P-2: Power Supply (9V)
Figure3. 21: Interfacing with Microprocessor (AVR AT 90S8535)
46
The AT90S8535 is a low-power CMOS 8-bit microcontroller based on the AVR®
enhanced RISC architecture. By executing powerful instructions in a single clock
cycle, the AT90S8535 achieves throughputs approaching 1 MIPS per MHz allowing
the system designer to optimize power consumption versus processing speed.
The AT90S8535 provides the features as: 8K bytes of In-System Programmable
Flash, 512 bytes EEPROM, 512 bytes SRAM, 32 general purpose I/O lines, 32
general purpose working registers, RTC, three flexible timer/counters with compare
modes, internal and external interrupts, a programmable serial UART, 8-channel, 10bit ADC, programmable Watchdog Timer with internal oscillator, an SPI serial port
and three software selectable power saving modes. The Idle mode stops the CPU
while allowing the SRAM, counters, SPI port and interrupt system to continue
functioning. The Power Down mode saves the register contents but freezes the
oscillator, disabling all other chip functions until the next interrupt or hardware reset.
In Power Save mode, the timer oscillator continues to run, allowing the user to
maintain a timer base while the rest of the device is sleeping.
The device is manufactured using Atmel’s high density non-volatile memory
technology. The on-chip ISP Flash allows the program memory to be reprogrammed
in-system through an SPI serial interface or by a conventional nonvolatile memory
programmer. By combining an 8-bit RISC CPU with In-System Programmable Flash
on a monolithic chip, the Atmel AT90S8535 is a powerful microcontroller that
provides a highly flexible and cost effective solution to many embedded control
applications. The AT90S8535 AVR is supported with a full suite of program and
system development tools including: C compilers, macro assemblers, program
simulators, in-circuit emulators, and evaluation kits.
47
Port A (PA7…PA0) is an 8-bit bi-directional I/O port (Figure 3.22). Port pins can
provide internal pull-up resistors (selected for each bit). The Port A output buffers can
sink 20mA and can drive LED displays directly. When pins PA0 to PA7 are used as
inputs and are externally pulled low, they will source current if the internal pull-up
resistors are activated. Port A also serves as the analog inputs to the A/D Converter.
Figure3. 22: AVR AT 90S8535 Pin Configurations
Port B (PB7…PB0) is an 8-bit bi-directional I/O pins with internal pull-up
resistors. The Port B output buffers can sink 20 mA. As inputs, Port B pins that are
externally pulled low will source current if the pull-up resistors are activated. Port B
also serves the functions of various special features of the AT90S8535.
Port C (PC7...PC0) is an 8-bit bi-directional I/O port with internal pullup resistors.
The Port C output buffers can sink 20 mA. As inputs, Port C pins that are externally
48
pulled low will source current if the pull-up resistors are activated. Two Port C pins
can al ternat ively be used as oscil l a tor f or Timer/Counter2.
Port D (PD7…PD0) is an 8-bit bidirectional I/O port with internal pull-up
resistors. The Port D output buffers can sink 20 mA. As inputs, Port D pins that are
externally pulled low will source current if the pull-up resistors are activated. Port D
also serves the functions of various special features of the AT90S8535.
VCC: Digital supply voltage
GND: Digital ground
RESET: Reset input. A low on this pin for two machine cycles while the oscillator
is running resets the device.
XTAL1: Input to the inverting oscillator amplifier and input to the internal clock
operating circuit.
XTAL2: Output from the inverting oscillator amplifier
AVCC: This is the supply voltage pin for the A/D Converter. It should be
externally connected to VCC via a low-pass filter.
AREF: This is the analog reference input for the A/D Converter. For ADC
operations, a voltage in the range AGND to AVCC must be applied to this pin.
AGND:Analog ground. If the board has a separate analog ground plane, this pin
should be connected to this ground plane. Otherwise, connect to GND.
49
3.4.2 The sensors signal processing unit
The Sensor connection circuit as Figure 3.23
P-3: Power Supply
A-3: Connect Microprocessor
Figure3. 23: Sensor controller circuit
The sensors system consists of four photo-resistors (LDR1, LDR2, LDR3, LDR4,)
connected in an electronic circuit Fig. 3.23. We set the LDR-1, LDR-2 to control
system in the Left-Right direction and LDR-3, LDR-4 to control system in the UpDown direction.
Each LDR (LDR1, LDR2, LDR3, LDR4) are connected in series with a varies
resistor. Their rolls are adjusting the resistance values of LDR1, LDR2, LDR3, LDR4
so that each pair of them will get the same value (LDR1 is equal LDR1 and LDR3 is
equal LDR4).
50
The difference signals of each pair of LDRs representing the angular error of the
PV panel are employed to re-position the panel in such a way that the angular errors
are minimized. A microcontroller system with AVR AT90S8535 is used as a
controller of position. The signals, taken from voltage divider consisting of LDRs
(LDR1~LDR4), are applied to I/O port lines of AVR (PA0, PA1, PA2, RA3)
respectively. These analog signals are converted to digital signals and compared with
each others (LDR1-LDR2, LDR3-LDR4). If the difference between LDR1 and LDR2
(or LDR3 and LDR4), error signal, is bigger than a certain value (tolerance), the
micro-controller generates driving signals for stepper motors. If the error signals are
smaller than or equal to the value of tolerance, the micro-controller generates no
signal; which means that the solar panel is facing the sun and the light intensities
falling on the four LDRs are equal or slightly different.
3.4.3 ASTS Control program Installation
The system control program is written in BASCOM-AVR (of MCS Electronics)
programming language for AVR (AT90S8535). BASCOM-AVR is the original
Windows BASIC COMPLIER for the AVR family. It is designed to run on
W95/W98/NT/W2000/XP/VISTA
To program some AVR Micro-Controller Unit (MCU), we will need an AVR
programmer. To better way to do this is using some Development Kit like STK-500.
This have a lot of advantages as serial port, LCD connector, SRAM socket, 8 switches,
controllers for all types of MCU. But the solution for a low-cost AVR programmer is
able to making a suitable AVR programmer circuit for own system.
The AVR programmer that is a programming cable connects AVR and LPT port
for writing the system control program on AVR.
51
The hardware consist 1 male connector for PLT port (25pins), some resistors, 1
header with 8 pins as Fig. 3.24.
1. Connect the VCC pin from Pins 2,3,4 of PLT port to Pin 10 of AT90S8535
2. Connect the RESET pin from Pin 6 of PLT port to Pin 9 of AT90S8535
3. Connect the MOSI pin from Pin 7 of PLT port to Pin 6 of AT90S8535
4. Connect the SCK pin from Pin 8 of PLT port to Pin 8 of AT90S8535
5. Connect the MISO pin from Pin 10 of PLT port to Pin 7 of AT90S8535
6. Connect the GND pin from Pin 19, 20, 21, 22, 23, 24, 25 of PLT port to Pin 11
of AT90S8535
Figure3. 24: The AVR programmer wiring (AT90Sxxxx)
52
Figure3. 25: The AVR programmer (connect LPT port)
Steps transfer the ASTS control program from PC to Microprocessor. Be sure to
switch on the power supply before running BASCOM-AVR.
™ Step1. Start BASCOM-AVR software and create the Program in BASIC
™ Step2. Compile the Program
(F7). The program will be saving
automatically before being compiled.
™ Step3. Run Programmer (Send to Chip
F4)
™ Step4. Write the system control program to Chip
After step 3, the following window will be shown as Fig. 3.26. The program will
be sent to chip as:
+Select Chip (AT90S8535)
+Erase Chip (Chip must be erased before it can be programmed)
+Write the program to Chip
53
Figure3. 26: The BASCOM-AVR Interface
3.5 The control approach
The ASTS has two types of control approach, automatic control and manual
control. This can make the system to work successfully.
System operates with the help of the system controller program in the
microprocessor used to manage the automatic operation of ASTS. The system
controller program is written in MCS Electronics: Bascom-AVR programming
language. This controller has following functions:
™ Senses the proposed photo-sensor
™ Drives stepper motor.
The central driving component of automatic control is the proposed photo-sensor.
Their operation has been explained on the previous page. The controller program will
be written in PC and then it will be installed in the microprocessor via LPT port. The
program transfer cable is shown in Fig. 3.43.
The stepper motor controller has been powered by ATMEL AVR AT90S8535.
54
Figure3. 27: The detail control program flowchart
55
A manual control option was also kept in this system. Two objectives were kept in
mind. The manual control should work efficiently and should be as user friendly as
possible.
Manual operation of the reflector movement can be carried out activating the
“Left,” “Right”, “Up” and “Down”. Using the “Left End”, “Right End”, “Down End”,
and “Up End” buttons to set the movement limit forward 2 directions Left-Right, UpDown, and also to pre-set the initial position of the reflector during the beginning of
the operation.
The user has the ability to select a manual control option or not. When there is an
interruption of power supply, the tracking system is switched off, and when the
system restarts the reflector orientation procedure begins automatically.
3.6 Guide & Note for using System
About the ASTS mechanism: To keep rotation of the Y shaped bar be status of
stable position by adding a weight. When the centre of gravity of movement
component is out of vertical direction, the weight can generate a moment to keep
system be stable position. This is very useful for operation of the stepper motor.
Motors just have to working for skin friction between worm and gear.
The proposed photo-sensor is always to be checked for quality and positions of
LDRs inside sensor.
Power supply is 9V for Microprocessor and 12V for stepper motor and stepper
motor driver
56
4. EXPERIMENTAL RESULTS
AND DISCUSSION
4.1. Experiments
The experiments are carried out to investigate the performance and efficiency of
the two-axis ASTS. For evaluating the STS operation, the maximum amount of output
(power, I-V curve…) in PV modules is investigated. How changes in solar radiation,
temperature during the testing day… affect the output of PV modules, also they
examined the most important seasonal changes of PV module output. These are
showed up in the next part.
4.1.1 Collection Data System
The most important characteristics of individual solar cell technologies are their
current-voltage (I-V) curve. This curve is obtained by varying the load in incremental
steps, constantly logging voltage and current values from open-circuit to short-circuit
conditions, as described in earlier sections. Results will present significant differences
in efficiencies of the solar energy system.
The I-V curves reflect the performance of the PV panel during a particular instant
of the day. I-V curves vary significantly during the day mainly because of variations
in the sun radiation angle (cosine effect), influence of the atmosphere (changes in the
relative position of the earth respect to the sun) and variations in solar cell
temperature. Varying curves during the testing day for the horizontal position are
almost stationary along the day.
This system consist two main components, GPIB and PC with LabVIEW program.
The output data of PV panel (voltage) throughout GPIB is used to read in PC. The
LabVEW program will store, analysis and show up data (I-V curve.)
57
Figure4. 1: The Collection Data System
58
4.1.2 Experimental setup
In this work, the Microcontroller-controlled system for the STS was designed and
constructed. The system consists of an electro-mechanical setup, having low cost, and
easily installed and assembled. The programming was based on the comparison of
voltage signal of LDRs which works as a sensor.
Figure4. 2: The ASTS Control System
Two PV panels (Figure 4.1), one of which is Non-Tracking and the other is twoaxis automatic tracking, are employed in the experiments. The non-tracking PV panel
is tilted at a fixed elevation angle and is oriented in some azimuth angle. The
proposed photo-sensor is mounted on the tracking panel.
59
Data is collected from both of the non-tracking and tracking panel at the same
time. The stored data was recorded every 10 minutes. The collected data was
averaged to obtain the hourly data. Data is measured in the period 9:00 AM to
15:00PM. So, a total of measurement time is 6 hours. The data that was captured from
the stationary panel and the rotary panel will be analyzed.
b. Process of Experiments:
Step 01: The Experimental Design:
-
The experiment in case: sunshine day
-
The experiment on two type of PV panel: Non-tracking and tracking PV panel
-
Experimental condition: The input data of both tracking and non-tracking PV
panel are collected at the same time.
-
Position of Non-tracking PV panel: vertical and tilt at zero degree
Step 02: Experimental Preparation
™ The Experimental Equipments
-
Solar Equipments: Two PV panel (similar characteristic)
-
Data Collection: Load (DC electronic Load), 2000 Multi-Meter, LabVIEW
USB card, PC with LabVIEW Program, Power Supply (110V)
-
ME Control Equipments: ASTS (Mechanical and Electronic Equipments)
™ Program using in Experiments
-
LabView program for solar data collection
-
BASCOM-AVR program to control the ASTS
-
Excel program for analyzing the solar data
Step 03: Experiments (The results are showed in next part)
60
4.2. Result and Discussions
The experimental study is realized at Southern Taiwan University. The experiments
took place on 28th June 2007 from 08:51:47 to 15:00:40 PM. The results are shown in
Table 4.1 and Figure 4.3. During the experiments the weather conditions were very
good and there were no clouds in the sky.
Table4. 1: The experimental data
Date
Time
E
I-track
I-fixed
I-out
V
P-out
6/28/2007
8:51:47
660
0.38
0.27
0.65
23.96
15.62
6/28/2007
9:01:47
667
0.38
0.28
0.66
23.96
15.83
6/28/2007
9:11:47
674
0.37
0.29
0.66
23.96
15.92
6/28/2007
9:21:47
677
0.38
0.31
0.69
23.96
16.64
6/28/2007
9:31:47
760
0.39
0.32
0.71
23.96
17.13
6/28/2007
9:41:47
766
0.39
0.34
0.73
23.96
17.51
6/28/2007
9:51:47
788
0.39
0.34
0.74
23.96
17.68
6/28/2007
9:54:14
783
0.39
0.34
0.73
23.96
17.5
6/28/2007
10:04:14
0.39
0.35
0.75
23.96
17.86
6/28/2007
10:14:14
0.4
0.36
0.76
23.96
18.23
6/28/2007
10:24:14
0.4
0.37
0.77
23.96
18.51
6/28/2007
10:34:14
0.4
0.38
0.78
23.96
18.69
6/28/2007
10:44:14
0.4
0.39
0.79
23.96
18.97
6/28/2007
10:46:09
0.41
0.39
0.8
23.96
19.1
6/28/2007
10:56:09
0.41
0.4
0.82
23.96
19.56
6/28/2007
11:03:30
0.43
0.42
0.85
23.96
20.4
6/28/2007
11:13:29
0.43
0.42
0.84
23.96
20.23
6/28/2007
11:23:30
0.41
0.41
0.82
23.96
19.7
6/28/2007
11:33:30
0.39
0.39
0.78
23.96
18.68
6/28/2007
11:43:29
0.4
0.4
0.8
23.96
19.27
6/28/2007
11:53:29
0.41
0.42
0.83
23.96
19.86
6/28/2007
11:56:27
0.41
0.42
0.84
23.96
20.12
6/28/2007
12:06:27
0.42
0.43
0.84
23.96
20.19
6/28/2007
12:16:27
0.41
0.42
0.84
23.96
20.14
6/28/2007
12:26:27
0.42
0.43
0.85
23.96
20.25
6/28/2007
12:36:27
0.27
0.27
0.55
23.96
13.06
6/28/2007
12:46:27
0.42
0.43
0.85
23.96
20.31
6/28/2007
12:51:26
0.41
0.41
0.82
23.96
19.57
6/28/2007
13:01:26
0.39
0.39
0.79
23.96
18.83
6/28/2007
13:11:26
0.4
0.39
0.79
23.96
18.94
6/28/2007
13:21:26
0.39
0.39
0.78
23.96
18.78
786
808
788
754
61
6/28/2007
13:31:26
6/28/2007
758
0.39
0.38
0.77
23.96
18.53
13:41:26
0.39
0.37
0.77
23.96
18.4
6/28/2007
13:45:06
0.39
0.37
0.76
23.96
18.32
6/28/2007
13:55:06
0.39
0.36
0.75
23.96
18.09
6/28/2007
14:05:06
0.39
0.36
0.74
23.96
17.84
6/28/2007
14:25:06
0.38
0.34
0.72
23.96
17.31
6/28/2007
14:35:06
0.38
0.33
0.71
23.96
17.05
6/28/2007
14:40:40
0.38
0.32
0.7
23.96
16.84
6/28/2007
14:50:40
0.38
0.31
0.69
23.96
16.57
6/28/2007
15:00:40
0.38
0.3
0.67
23.96
16.13
786
673
Note:
I-track: output current of PV panel using ASTS
I-fixed: output current of PV panel with non-tracking
I-out: output current of two PV panels, tracking and non-tracking PV panel
C u r ren t [A]
V: output voltage of PV panel
0.5
0.5
0.45
0.45
0.4
0.4
0.35
0.35
0.3
0.3
0.25
0.25
0.2
0.2
I-tracking
I-non tracking
0.15
0.15
0.1
0.1
0.05
0.05
0
8:30
9:30
10:30
11:29
12:29
13:29
14:29
0
15:29
Day time
Figure4. 3: The output current of tracking and non-tracking PV panel
62
This result shows and verifies that the tracking PV panel containing ASTS
(tracking PV panel) takes more light density than the non-tracking PV panel.
In the Figure 4.3, the current curve of tracking PV panel (green colour) is above
the other of non-tracking PV panel (red colour). Thus, we can specify that there is an
increased efficiency of the PV panel using two-axis ASTS to compare with nontracking PV panel.
The efficiency of the PV panel using ASTS compared to non-tracking PV panel is
given in Table 4.1. From this data, it was found that there was an overall increase of
about 7% in the output power for the tracking system compared to the non-tracking
PV panel.
63
5. CONCLUSIONS
In this study, an experimental study is performed to investigate the effect of twoaxis tracking on the PV panel under normal weather conditions. The result and
discussions based conclusion as the follows.
™ M-E movement mechanism is flexible (180 deg in the east-west direction and
90 deg. In the up-down direction)
™ The proposed photo-sensor is not expensive but also efficient operation.
™ A new solar tracking technique based on microcontroller was implemented
and tested successfully in this study.
™ By using the proposed ASTS, PV panels is aligned orthogonally to the sun.
The result shows that there was an overall increase of about 7% in the output power
for the tracking system compared to the non-tracking PV panel.
™ The Two-axis ASTS is low cost system
The tracking system presented has the following advantages:
™ The tracking system is not constrained by the geographical location of
installation of the PV panel.
™ The solar tracking system is designed for automatically searching the
maximum solar irradiance in the whole azimuth (Left – Right direction) and tilt angle
(Up – Down direction) during any time.
™ The operator interference is minimal because of not needing to be adjusted
periodically.
™ The system control program is rewritten and installed in microprocessor easily
by LPT port.
64
™ The system can operate individually as an intelligent completed machine, may
not be employed. Any time there is sun, the solar tracking system operate
automatically.
Experimental results based on different modes of system operation are presented.
It is concluded that: The gain of the proposed two-axis tracking system is
considerable compared with the fixed surface for operation under normal weather
conditions.
65
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67