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CHAPTER 3

EXPERIMENTAL DETAILS

Overview- This chapter deals the hardware and software implementation of

developed data acquisition system for monitoring the characteristics of Photovoltaic

cell. First section describes the overall procedural steps to implement this system. In

this section general layout and block diagram are explained. Second section describes

the complete operation of this as per its software. Third section explained the circuit

implementation using Proteus ISIS application package. Fourth section shows the GUI

interface unit for acquire the data from embedded system to digital computer system.

Finally, Simelectronics Model by using MATLAB Simulink is demonstrated for

single solar cell simulation.

3.1 Procedural steps

There are following stages which are required to develop realistic monitoring

system-

Stage I- In the first stage, we analyzed the various parameter, terms and affecting

factors of photovoltaic cell.

Stage II- After finding the required input and desired output, suitable, advanced and

easy to available microcontroller, sensors, and interface unit are chosen and studied.

In this research work PIC18F452 microcontroller, SHT71 digital sensor, PDC

photodiode and RS232 interface unit are selected. PIC is the Microchip

microcontroller.

Stage III- In this any one application package is selected to develop source code.

MPLAB IDE is chosen to develop applications for Microchip microcontroller [77]. It

is a Windows Operating System software program that runs on PC. It is called an

Integrated Development Environment, or IDE, because it provides a single integrated

“environment” to develop code for embedded microcontroller

with many Microchip and third party language tools

application code (written in assembly, C or BASIC language) and turn it into

executable code that may be programmed on your selected Microchip device.

system, CCS (Custom Computer Services, Inc.) C compiler (CCSC.exe) is used to

compile source code. The CCS C Compiler has complementary development tools

that work seamlessly with the compiler.

design functionality instead of having to become an MCU architecture expert

Stage IV- In this stage

MPLAB generates hex

application package PIC

“environment” to develop code for embedded microcontrollers. It is designed to work

with many Microchip and third party language tools [78]. These tools take your


ay be programmed on your selected Microchip device.



that work seamlessly with the compiler. C compiler frees developers to concentrate on

design functionality instead of having to become an MCU architecture expert

In this stage, generated hex file is tested and burned to microcontroller.

MPLAB generates hex file which is embedded into microcontroller by another

PIC ISP for 18x. Before implementing physical system, generated

Figure 3-1 Procedural Stages

46

s. It is designed to work

. These tools take your


ay be programmed on your selected Microchip device. In this



compiler frees developers to concentrate on

design functionality instead of having to become an MCU architecture expert [79].

, generated hex file is tested and burned to microcontroller.

embedded into microcontroller by another

Before implementing physical system, generated

hex file is simulated by Proteus ISIS application. Developed system is further

calibrated by standard system.

Stage V- Finally, data is recorded in varying parameters. Experimental values are

compared with simulated values. Simulated values are obtained b

Simelectronics model.

monitoring system.

3.2 General Layout

General layout of the

temperature and humidity is shown in figure

humidity (H), irradiance (I

characteristics of PV modules for each v

power available from the maximum power point (MPP) current and voltage. It is

equipped with an autonomous memory. The control of the data collection in the

acquisition system memory is done thr

with the specified communication software

Figure


calibrated by standard system.

Finally, data is recorded in varying parameters. Experimental values are

compared with simulated values. Simulated values are obtained b

Figure 3-1 shows the required stages to implement realistic

General layout of the acquisition system for measuring the effect of

temperature and humidity is shown in figure 3-2. This system is designed to acquire

, irradiance (Ir) and temperature (T); and records the current &

characteristics of PV modules for each value of H, Ir and T. It calculates the maximum



acquisition system memory is done through RS232 interface connection of the system

with the specified communication software [80].

Figure 3-2 General layout of measuring system

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Finally, data is recorded in varying parameters. Experimental values are

compared with simulated values. Simulated values are obtained by single solar cell

1 shows the required stages to implement realistic

acquisition system for measuring the effect of

This system is designed to acquire

T); and records the current & voltage

and T. It calculates the maximum



face connection of the system

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3.3 Block diagram and flow chart

Block diagram of this system which is shown in figure 3-3, has four major

blocks. Block 1 concern the current and voltage measurement, block 2 temperature

and humidity measurement, block 3 is the GUI interface and display unit and block 4

concerns the autonomous memory element [81] [82].

Procedure to acquire temperature and relative humidity and other related

photovoltaic values is explained by flow chart which is shown in figure 3-4. PIC

18F452 microcontroller is a major part which digitized the characteristics.

Microcontroller is a circuit or VLSI core that functions as a computer and consists of

Microprocessor, memory for the data (bits, bytes and words) and stack, memory for

storing the dedicated programs and subprograms, interfacing circuit between the

external memories and IO devices, interrupt handling circuit or unit to handle the

system interrupts, and timing circuit or unit for the system clock and the other timing

Figure 3-3 Block diagram of measuring system

related functions [83] [

record the value from digital sensor, process the command and instruction come from

computer, store the recorded value into EEPROM as well as hard disk of PC as a file

through graphical user interfa

software.

For demonstrating the effect of irradiance and various

is described through flowchart which is shown in figure

[83]. It receives analog signal and then converts into digital,



through graphical user interface application which is designed through commercial

For demonstrating the effect of irradiance and various wavelengths

is described through flowchart which is shown in figure 3-5.

Figure 3-4 Flowchart of the method

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It receives analog signal and then converts into digital,



ce application which is designed through commercial

wavelengths, procedure

Figure 3

3-5 Flow chart to capture various wavelength

50

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3.4 State diagram of monitoring system

Complete operation of the system can be understood from figure 3-5 which

shows the state machine diagram for the measurement system. The operation of this

system is similar to the one described by Mukaro, [Data Acquisition system] [17] and

other workers [84] [85]. Microcontroller was programmed to be in low-power mode,

except at specific time when monitoring or communication with computer in progress.

The first state of monitoring system is Initial state, entered / appeared after

pressing reset button or power ON. This state initializes all the internal, variables, I/O

ports and various modules like LCD, RS232, sensors etc. It ensures that all are defined

and configured properly. The system then goes into the second which is called WAIT

Figure 3-6 State diagram of monitoring system

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state. In this state, oscillator remains active to keep track of time but system does

nothing except to wait for interrupt. Instruction executed is stopped, power

consumption is decreased, however and internal RAM contents are preserved. The

program then starts the timer and reads particular channel to check if a computer is

connected to the data acquisition system. If the computer is not connected the timer

awakens the system from the WAIT mode to measure or acquisition mode where set

of readings is taken and stored, after which the data acquisition system goes back into

the WAIT state to wait for another data acquisition and storage cycle. If the computer

is connected, the system makes a transition into the LISTEN mode in which the

operator uses the computer keyboard to communicate with the data acquisition

system. The data acquisition system does not have a real-time clock so the operator

via the computer keyboard supplies the start time for the measurement before the

system is left to run [21].

If the data from sensor is less than ten (equivalent to an irradiance of 65 W/m2),

the system assumes it is night time and does not record this data. This was done to

save memory [86].

Capacitors are used to provide power supply filtering to the microprocessor, the

memory chip and the MAX232 respectively.

Agilent VeePro package [87] is used for data logging; it is facilitated between

this system and computer through RS-232 serial port. Through this program the

computer is able to receive measured values into file as well as for display into the

monitor. These recorded values are used for subsequent analysis. Measured values are

captured every 30 seconds.

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3.5 Schematic diagram

Figure 3-7 & 3-8 show the schematic diagram and screenshot of simulator,

respectively. As per the circuit layout digital sensor SHT71 is connected at pin RC0

and RC1. Voltage and current which are generated by Photovoltaic cell are measured

through ADC input pins AN0 and AN1. Data pins of output device 16x2 LCD module

is connected via pin RD0 to RD7, and its control lines RS, R/W & EN are connected

by RE0, RE1 & RE2 respectively [88]. Screenshot of simulator shows the various

connected components and output virtual terminal screen which displays the recorded

values which are stored into the excel file for further analysis.

Proteus from labcenter Electronics consists of two main parts- ISIS and ARES.

ARES is a layout package, which is used to create a PCB when the circuit has been

designed. ISIS is the schematic capture and interactive simulation software used to

create the circuit drawing and to test the circuit prior to building the real hardware.

SPICE is a mathematical circuit modeling system which has been developed over

many years- these models can now be used to bring the drawing to life. Onscreen

buttons and virtual signal sources, for example provide input to the circuit. Output can

be displayed on a voltage probe or a virtual oscilloscope. Now we have

microcontroller simulation as well. The Microcontoller can be dropped on the screen,

a program attached and debugged instantly [89] [90].

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Figure 3-7 Schematic diagram

D7

14D

613

D5

12D

411

D3

10D

29

D1

8D

07

E6

RW

5R

S4

VSS

1

VDD

2

VEE

3

LCD1LM016L

MCLR/VPP1

RA0/AN02

RA1/AN13

RA2/AN2/VREF-4

RA3/AN3/VREF+5

RA4/T0CKI6

RA5/AN4/SS/LVDIN7

RE0/RD/AN5 8

RE1/WR/AN6 9

RE2/CS/AN7 10

OSC1/CLKI13

RA6/OSC2/CLKO14

RC0/T1OSO/T1CKI 15

RC2/CCP1 17

RC3/SCK/SCL 18

RD0/PSP0 19

RD1/PSP1 20

RD2/PSP2 21

RD3/PSP3 22

RD4/PSP4 27

RD5/PSP5 28

RD6/PSP6 29

RD7/PSP7 30

RC4/SDI/SDA 23

RC5/SDO 24

RC6/TX/CK 25

RC7/RX/DT 26

RB0/INT033

RB1/INT134

RB2/INT235

RB3/CCP2B36

RB437

RB5/PGM38

RB6/PGC39

RB7/PGD40

RC1/T1OSI/CCP2A 16

U1

PIC18F452

+5V

+88.8Volts

3

21

84

+88.8mV

+5V

R11000R

R2

1k5

Solar cell(+)

+88.

8

mV

solar cell(-)

78%

RV4

1k

+88.8

mV

+88.8µA

3

21

84

R31000R

R4

1k5

+5v

+88.8mV

+88.8Volts

88.031.0

%RH

>

°C

DATA4SCK1

U2

SHT71R510k

LDR1TORCH_LDR

R6(1)

+88.8Volts

R61k

RXD

RTS

TXD

CTS

R81.3k

R9

3.9k

D11N4733A

+88.8mV

Figure 3-8 Screenshot of simulator

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3.5.1 Description of Data acquisition

The circuit description of various mentioned blocks are given as follows in

brief. Figure 3-9 shows the circuit detail of block 1 which is bas

voltage and current of PV cell. Operational amplifier is the core part to obtain this

feature.

Block 2 describes the measurement of relative humidity (%) and temperature

(OC) through SHT71 sensor which is manufactured by Sensirion

to choose it to their interface facility with microcontroller. It is a digital sensor.

10-bit ADC with Vref = 5V, resolution = 5 /1024 = 0.0049V. For input signal V

ADC output will be Vo x 0.0049V.

Block 3 permits the interfacing between the microcontroller and the computer

Figure 3-

Description of Data acquisition system


shows the circuit detail of block 1 which is bas



(OC) through SHT71 sensor which is manufactured by Sensirion

to choose it to their interface facility with microcontroller. It is a digital sensor.

= 5V, resolution = 5 /1024 = 0.0049V. For input signal V

x 0.0049V.


-9 Voltage and current measurement in block 1

Figure 3-10 Circuit layout of block 2 and 3

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shows the circuit detail of block 1 which is basically sense the



(OC) through SHT71 sensor which is manufactured by Sensirion. The main reason is

to choose it to their interface facility with microcontroller. It is a digital sensor. For

= 5V, resolution = 5 /1024 = 0.0049V. For input signal Vo, the


oltage and current measurement in block 1

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through EIA-232 driver IC. Figure 3-10 shows the block 2 and 3.

Graphical interface unit which is shown in figure 3-11 is required to visualize

the results of PV cells characteristics on the computer. This interface was

implemented and installed on the PC which provides directly communicates with the

data acquisition system. These recorded data is stored into the EEPROM of

microcontroller as well as in a computer as a file. The central element is the

microcontroller which performs various tasks like activate the ADC, sense to captured

data of digital sensor, transfer to PC through USART module, and store the recorded

data. Here, PIC18F452 microcontroller is chosen. It is a 40-pin RISC microcontroller

which is based on Harvard architecture. It has 16-bit wide instruction and 8-bit wide

data path. It can address program memory up to 32KB. It consumes little power, high

speed FLASH /EEPROM technology and wide operating voltage range (2.0V to 5.5V)

[91] [92]. Agilent VEE pro 9.0 is a powerful intuitive graphical language environment

that provides us the quick and easy path to measurement analysis. It is designed for

engineers and scientists who need to quickly create and automate measurements and

tests. Agilent VEE pro 9.0 can talk to any device from any vendor using GPIB, LAN,

USB, RS-232, VXI, LXI and other interfaces or buses, including PXI and SCXI data

acquisition and modular instrumentation devices from National Instruments. It can

control any standard instruments and many vendor’s PC plug-in cards with an

instrument driver, a vendor-supplied LL or via Agilent VEE pro’s easy and powerful

Direct IO capability [93] [53].

Figure 3-11 Graphical user interface and display sample data in a file

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3.6 Component detail

SHT71- It is a temperature

sensor. It is manufactured by Sensirion. The main

reason is to choose it to their interface facility with

microcontroller. Its main features are excellent long

term stability, no external components required, ultra

low power consumption, surface mountable or 4

fully interchangeable, small size, automatic power down

etc. Its accuracy in terms of humidity and temperature is

±3.5% RH and ±0.5

measuring range will be 0 to 100% RH for humidity a

-40 OC to +123.8 OC for temperature, it is more than existing system

from appendix III. This is seamlessly coupled to a 14

circuit on the same chip

polymer-sensing element for relative humidity and bandgap temperature sensor.

Humidity is an ambient parameter, which refracts, reflects and diffracts the direct

visible solar irradiation. This dispersion effect results in deterring the reception of th

direct component of solar

Figure 3-13 Digital Sensor

Component details

It is a temperature-humidity digital

It is manufactured by Sensirion. The main

reason is to choose it to their interface facility with

Its main features are excellent long-

term stability, no external components required, ultra

power consumption, surface mountable or 4-pin

fully interchangeable, small size, automatic power down

Its accuracy in terms of humidity and temperature is

OC @25OC, respectively and

measuring range will be 0 to 100% RH for humidity and

C for temperature, it is more than existing system

. This is seamlessly coupled to a 14-bit ADC and a serial interface

circuit on the same chip as referred in appendix III. The device includes a capacit

sensing element for relative humidity and bandgap temperature sensor.


. This dispersion effect results in deterring the reception of th

direct component of solar irradiation by water vapors present in the atmosphere

Figure

Digital Sensor Figure 3-14 Internal diagram of SHT7x

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C for temperature, it is more than existing system, it is referred

bit ADC and a serial interface

. The device includes a capacitive

sensing element for relative humidity and bandgap temperature sensor.


. This dispersion effect results in deterring the reception of the

by water vapors present in the atmosphere [94].

Figure 3-12 Pin diagram of PIC18F452

Internal diagram of SHT7x

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3.7 Reasons to choose PIC microcontroller

There are lots of PIC microcontrollers to pick from. The programmer I had

already purchased narrowed this selection down to 40-pin devices. At minimum 8

ADC channels, a UART and counter/timer were used. The first device found to meet

these requirement was the PIC 18F452. It contains 8 channels of 10-bit ADC, an

enhanced UART, two timer counter, 40MHz clock, and flash memory [95] [96]. The

PIC architecture is distinctively minimalist. It is characterized by the following

features (datasheet is given in appendix II)-

� Separate code and data spaces (Harvard architecture)

� A small number of fixed length instructions

� Most instructions are single cycle execution (4 clock cycles), with single delay

cycles upon branches and skips

� A single accumulator (W), the use of which (as source operand) is implied (i.e.

it is not encoded in the opcode)

� All RAM locations function as registers as both source and/or destination of

math and other functions.

� A hardware stack for storing return addresses.

� A fairly small amount of addressable data space (typically 256 bytes), extended

through banking.

� Data space mapped CPU, port, and peripheral registers.

� The program counter is also mapped into the data space and writable (this is

used to implement indirect jumps)

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� The program counter is also mapped into the data space and writable (this is

used to implement indirect jumps)

� Unlike most other CPUs, there is no distinction between "memory" and

"register" space because the RAM serves the job of both memory and registers,

and the RAM is usually just referred to as the register file or simply as the

registers.

� External data memory can be directly addressable by PIC18F452.

Here, PIC18F452 microcontroller is used, which is low power, high

performance CMOS 8-bit microcontroller with 16Kbyte of code space of Flash

Programmer and Erasable Read Only Memory with 10-bit multi channeled Analog to

Digital converter and 34 I/O pins. The on-chip Flash allows the programmable

memory to be reprogrammed in-system or by a conventional nonvolatile memory

programmer. It is a powerful microcontroller, which provides a highly flexible and

cost effective solution to many embedded control applications [91]. In order to use the

PIC, setting for all the internal registers needed to be determined or calculated via the

datasheet. The internal oscillator was used that set to 2MHz. Next the serial port was

configured. The enhanced UART (EUART) has an internal baud rate generated (no

external timer is needed). The EUART was setup to communicate at 9600 bps, 1 stop

bit, 8 data bits, and no parity. That was easily achieved with little error in bit-rate on

the PIC (about 0.16%). After the serial port was configured the analog to digital

converter was set up. A conversion-time of 4.0 µs as dictated by the datasheet was

selected. Then, registers were set up to use the positive supply as the ADC reference

voltage along with selecting the location of the most significant bit of the result [97].

Microchip provides an integrated development environment (IDE) called

MPLAB for coding, compiling, setting up and controlling programmer for the PIC

series of microcontroller. Included in the IDE is debugger and compiler. A simple

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com port programmer interfaced to the IDE and reprogrammed the flush program

memory in PIC microcontroller.

Microcontroller firmware- After configuration of peripherals the

microcontroller firmware consisted of a simple loop that acquire samples, convert

them to a format acceptable to Hyper terminal, and echo them to serial port, embedded

program is appended in appendix I. Programming code is attached to the end of this

document in appendix I. The capture event consists of setting ADC registers, then

waiting for the conversion to be completed. These events are repeated three times to

cover all input channels. The high integration of the PIC microcontroller leads to a

very simple hardware solution. On the digital side the PIC controller is connected to a

MAX232, RS-232 to logic-level converter. A 5V power supply and some supply

decoupling capacitor round out the digital section of the hardware. It is constructed on

the PIC kit development board. Highly integrated sensors reduced the difficulty of

implementing an analog interface board. A shunt resistor is chosen as an easy

integrated solution for current sensing [98].

Simelectronics Model- In order to obtain a realistic view of the photovoltaic

system, it is necessary to achieve computer simulations. It is impractical to achieve

complete experimental data due to continuously changing the parameters of

atmosphere. To experiment with PV cells in the laboratory is a time consuming and

costly task. So, to overcome this problem, simulation techniques are used to simulate

the behavior of PV cells under different conditions. For example, the only available

measurement result is often hourly or daily global irradiance on a horizontal plane. So,

modeling is necessary in order to deduce from the available partial data. In literature,

there are several mathematical models [99] [100] [101] that describe the operation and

behavior of the photovoltaic generator. These models differ in the calculation

procedure, accuracy and the number of parameters involved in the calculation of the

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current-voltage characteristic. The solar cell is modeled and simulated using Matlab

software. (Matlab R2012b (8.0.0.783) 32-bit, August 22, 2012 License number:

724504) [102]. SimElectronics provides component libraries for modeling and

simulating electronic and mechatronic systems. It includes models of semiconductor,

motor, drive, sensor, and actuator components. By using these components we can

perform system-level design of electromechanical actuation systems and to evaluate

analog circuit architectures using behavioral models. Models created from

SimElectronics components support control and algorithm design in electronic and

mechatronic system, such as vehicle body electronics, aircraft servoconductor models

include nonlinear and dynamic temperature effects, enabling us to select components

in amplifiers, analog-to-digital converters, phase-locked loops, and other circuits [103]

[104] [105]. We make validation through the experimental bench: we can use the

method of resistance variation or the charging and discharging capacitor method.

To put the simulation study on firm footing an experimental verification is also

carried out in the laboratory by developing a PC based data acquisition system.

Simulation model developed by many workers [53] [106] [107] for simulation

of a single solar cell has been developed using Simelectronics environment. A solar

cell block is available in simelectronics, which was used with many other blocks to

plot I-V and P-V characteristics under variations of parameters considering one

parameter variation at a time. Effect of two envrionmental parameters of temperature

and irradiance variation could be observed from simulated characteristics.

The solar cell bolck in SimElectronics, represent a single solar cell as a parallel

current source, IL and exponential diode, D and a shunt resistance Rsh that are

connected in series with a resistance Rs. The following SimElectronics Model used for

single solar cell simulation is shown below figure 3-15 [108]. This model was

developed using blocks of Simelectronics and simulink. Solar cell was connected with

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blocks such as current sensor and voltage sensor to measure the current and voltage

across solar cell [109]. Blocks for varying Irradiance level and temperature, were also

connected in the above model. Reset of the blocks were the interface between major

blocks and used for plotting I-V characteristics

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Figure 3-15 Single Photovoltaic cell Simelectronics Model

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