23-08-2014, 02:06 PM
Solar Tracker Seminar Report
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Abstract
Solar energy is rapidly gaining notoriety as an important means of expanding renewable
energy resources. As such, it is vital that those in engineering fields understand the
technologies associated with this area. My project will include the design and
construction of a microcontroller-based solar panel tracking system. Solar tracking
allows more energy to be produced because the solar array is able to remain aligned to
the sun. This system builds upon topics learned in this course. A working system will
ultimately be demonstrated to validate the design. Problems and possible improvements
will also be presented
Introduction
Renewable energy solutions are becoming increasingly popular. Photovoltaic (solar)
systems are but one example. Maximizing power output from a solar system is desirable
to increase efficiency. In order to maximize power output from the solar panels, one
needs to keep the panels aligned with the sun. As such, a means of tracking the sun is
required. This is a far more cost effective solution than purchasing additional solar
panels. It has been estimated that the yield from solar panels can be increased by 30 to
60 percent by utilizing a tracking system instead of a stationary array [1]. This project
develops an automatic tracking system which will keep the solar panels aligned with the
sun in order to maximize efficiency.
This paper begins with presenting background theory in light sensors and stepper motors
as they apply to the project. The paper continues with specific design methodologies
pertaining to photocells, stepper motors and drivers, microcontroller selection, voltage
regulation, physical construction, and a software/system operation explanation. The
paper concludes with a discussion of design results and future work.
Light Sensor Theory
Light sensors are among the most common sensor type. The simplest optical sensor is a
photoresistor which may be a cadmium sulfide (CdS) type or a gallium arsenide (GaAs)
type [2]. The next step up in complexity is the photodiode followed by the
phototransistor [2]. The sun tracker uses a cadmium sulfide (CdS) photocell for light sensing. This is the
least expensive and least complex type of light sensor [2]. The CdS photocell is a passive
component whose resistance in inversely proportional to the amount of light intensity
directed toward it. To utilize the photocell, it is placed in series with a resistor. A
voltage divider is thus formed and the output at the junction is determined by the two
resistances. Figure 1 illustrates the photocell circuit. In this project, it was desired for
the output voltage to increase as the light intensity increases, so the photocell was placed
in the top position
Stepper Motor and Driver Theory
Stepper motors are commonly used for precision positioning control applications. All
stepper motors possess five common characteristics which make them ideal for this
application. Namely, they are brushless, load independent; have open loop positioning
capability, good holding torque, and excellent response characteristics. [3].
There are three types of stepper motors: permanent magnet, variable reluctance, and
hybrid [3]. The arrangement of windings on the stator is the main distinguishing factor
between the three types [3]. Permanent magnet motors may be wound either with
unipolar or bipolar windings [3].
The sun tracker uses a unipolar step motor. As such, discussion will be limited to this
type of stepper motor. Unipolar motors have two windings with each having a center tap
as shown in Figure 2 from [4]
Project Design Methodology
This section will discuss the methodology involved in the design of the solar tracker. The
project was divided into parts to make the design process modular. The project consists of reading a series of light sensor values, comparing them, and then positioning a motor
to align with the greatest value which corresponds to the sun’s position. Follow-on
sections discuss hardware and software design considerations.
Microcontroller
Since the project’s focus is on embedded software control, the microcontroller is the heart
of the system. The microcontroller selected for this project had to be able to convert the
analog photocell voltage into digital values and also provide four output channels to
control motor rotation. The PIC16F877 was selected as it satisfies these requirements in
addition to already being provided with the class lab kit. Specifically, it possesses the
following three features to satisfy the specific project goals [5].
Motor Driver and Stepper Motor
A single unipolar stepper motor was chosen to position the tracking sensor. A stepper
motor was selected because of the precision it offers in positioning applications such as
this. Additionally, complicated drive circuitry is not required with the unipolar type
motor. The motor specifically used in the project was a 5 volt, 7.5 degree-per-step, 4
phase, unipolar motor. It was decided to half-step the motor in order to provide greater
positioning accuracy. This results in 3.75 degrees-per-step. The drive sequence used in
this design is shown in Figure 7.
Voltage Regulation
The PIC16F877 requires a regulated 5 volt supply voltage. The 7805 voltage regulator
was used to provide for that. The circuit shown in Figure 9 converts an unregulated
supply of 9 volts to 5 volts for use by the microcontroller
Construction
Ultimately the subparts of the project discussed in Sections 3.1 through 3.5 were
consolidated to construct a complete project. Figure 10 provides a block diagram of the
project while Figure 11 provides a complete hardware schematic of the project.
Some additional construction details worth mentioning deal with the motor and photocell.
The motor was mounted to a plastic perforated board using standoffs to provide a stable
base for it. The photocell was mounted on a small balsa wood platform which was
secured to the motor shaft.
Lastly, a reset switch was added to allow for the microcontroller to be reset after it enters
sleep mode.
Software/System Operation
As was fundamental to the course, the assembly language was utilized for the project. It
was more than adequate to satisfy design objectives while enhancing level of
understanding of the programming language.
Software operation can be divided into four main parts. The first part is initial
positioning. Prior to powering up the system, the photocell must be manually set to a
starting point (east). Once manually positioned, the tracking sensor will move one 3.75
degree step per second in the clockwise direction until a value of light intensity greater
than the preset threshold is measured. The threshold has been set as a constant in
program code to equal a voltage level of 4.60 volts. This level was selected to
correspond to what was measured with the shielded photocell pointed directly at the sun.
This level ensures that the tracker will seek out only an extremely bright source of light
(i.e. the sun or the flashlight used for testing).
The second part of the system code deals with light tracking. This is the heart of the
program. Once the tracker has set its initial position to a bright source of light (sun), it is
ready to align itself more precisely and continue tracking the light. The tracker first
measures light intensity at its present location. It then moves counterclockwise (left) by
one 3.75 degree step and takes another measurement. Next, it moves clockwise (right)
two 3.75 degree steps and takes a final measurement. The software comparison
subroutines compare these values and position the tracker at the point of greatest
measurement. If any of the values are equal, the tracker will return to the center position
and check again later. The tracker will wait four minutes (four seconds for classroom
Design Analysis and Results
Hardware and software portions of the project were separated into stages while
developing the overall system. The portions consisted of light detection, motor driving,
software tracking, and software enhancements. Building and testing smaller sections of
the system made the project more manageable and increased efficiency by decreasing
debugging time.
The project performs the required functions envisioned at the proposal phase. However,
while satisfied with software operation and simulation, less satisfaction was obtained
from two hardware areas. First, there is a potential for problems with motor/photocell
movement due to the photocell wires creating binding issues. There are two wires
attached to the photocell mounted on the motor shaft. Once the tracker has moved
approximately 30 to 45 degrees, the wires place a counter torque on the motor and the
motor slips. This creates positioning error. The present workaround for this is to hold
the photocell wires in a way as to keep them perpendicular to the rear of the photocell as
the tracker moves. This problem will be discussed further in Section 5.
Conclusion
This paper has presented a means of controlling a sun tracking array with an embedded
microprocessor system. Specifically, it demonstrates a working software solution for
maximizing solar cell output by positioning a solar array at the point of maximum light
intensity. This project presents a method of searching for and tracking the sun and
resetting itself for a new day. While the project has limitations, particularly in hardware
areas discussed in Section 4 and Section 5, this provides an opportunity for expansion of
the current project in future years.