ECE 4901, Fall 2020

Light intensity modulator design

Team members:Aaditya Sekar (Electrical Engineering)Adrian Gibson (Electrical Engineering)Lauren Boulay (Electrical Engineering)

Sponsoring Organization: UCONN ECE Dept

Faculty Advisor:Sung Yeul Park

sung_yeul.park@uconn.edu860-428-5647

May 6, 2021

Table of Contents

I. Abstract……………………………………………………………………………….......2II. Introduction……………………………………………………………………………….2

III. Problem Statement……………………………………………………………………......3IV. Proposed Approach and Design………………………………………………………......4

A. Signal Sensing Interface Board…………………………………………………...8B. Data Collection & Processing…………………………..………………………..10C. Optical Engineering……………………………………………………………...12

V. Project Management……………………………………………………………………..20VI. Summary and Next Steps………………………………………………………………...21

VII. References………………………………………………………………………………..22VIII. Glossary………………………………………………………………………………….23

List of Figures and Tables

I. Figure 1 - Buck converter simulated response…………………………………………....5II. Figure 2 - Voltage and current sensing circuit schematics………………………………..6

III. Figure 3 - PSpice voltage and current sensing circuit schematics………………………..7IV. Figure 4 - PSpice time domain simulation for voltage sensing circuit……………….......7V. Figure 5 - PCB schematic…………………………………………………………….......9

VI. Figure 6 - PV Panel equivalent circuit…………………………………………………...11VII. Figure 7 - Impedance plot of the equivalent circuit model……………………………...12

VIII. Figure 8 - Experimental setup…………………………………………………………...13IX. Figure 9 - Optical system using a collimating lens……………………………………...14X. Figure 10 - Photographs of collimating lenses…………………………………………..15

XI. Figure 11 - Photographs of aluminum reflectors………………………………………...16XII. Figure 12 - PV Panel Output Voltage, No Reflectors..………………………………….17

XIII. Figure 13 - PV Panel Output Voltage, Using Reflectors………………………………..18XIV. Figure 14 - PV Panel Output Voltage, All Trials………………………………………..19XV. Figure 15 - RACI chart.………………………………………………………………...20

XVI. Figure 16 - Gantt chart……………….………………………………………………....20

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Abstract

We are creating an LED driver circuit that controls an LED array to modulate the light incident

on a photovoltaic (PV) panel. This will allow us to perform impedance spectroscopy on the PV

panel so that we can determine the health of the module. To accomplish this, we have designed a

current-controlled buck converter to modulate the LED output, voltage and current sensing

circuits, a PCB for the sensing circuits, and LabVIEW programs for data processing. An

NI-DAW device was used for data collection and reference signal generation. We have tested our

designs using simulated data from PSpice and LabVIEW. In addition, we designed and tested a

collimating lens and reflector system to increase the output voltage of the PV panel.

Introduction

The purpose of this project is to design and test a LED driver circuit that controls the

current of an LED array in order to modulate the light incident on a photovoltaic (PV) panel.

This is done in order to perform impedance spectroscopy on the panel and determine equivalent

circuit parameters. Impedance spectroscopy is a testing technique where a small AC excitation

signal is applied to a material to determine equivalent circuit model resistances and capacitances.

This can be done using specialized equipment such as a frequency response analyzer; however,

the purpose of our project is to design a system that does not need this specialized equipment.

Our LED driver circuit will be able to modulate incident light and thus the voltage output of the

PV panel in order to attain impedance data over a range of frequencies that we use to derive

equivalent circuit parameters. This project is a continuation of research done at the University of

Connecticut Center for Clean Energy Engineering.

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Problem Statement

Photovoltaic (PV) panels are a growing technology in the sustainable energy industry,

especially grid-connected panels. To ensure the long term health and performance of PV panels,

it is crucial to estimate panel aging. AC characteristics of PV panels are essential for determining

health, and therefore efficiency, but the technology that is currently used in PV panel testbeds

cannot be used to assess these characteristics. We will need to create our own devices to allow

for the detection of panel deterioration.

In order to analyze the health and performance of the PV panels, we must perform

impedance spectroscopy (IS) to characterize the electrochemical structures in the solar cells. We

will need to create testing apparatus that can perform IS using an intensity-modulated light

source operated using an LED driver circuit.

We will be using impedance spectroscopy to analyze electrochemical structures within

photovoltaic panels, which will aid in the estimation of PV panel aging. The IS method will be to

modulate the intensity of an LED light source at a range of frequencies. We will apply this light

source to the PV panel and sweep the frequency of modulation, and then measure the impedance

at each frequency value. This will allow us to extract impedance information from the PV panel,

giving us components such as the recombination resistance and chemical capacitance. We

therefore must construct an LED driver circuit to provide the correct DC offset and AC ripple

current to the light source.

For this project, we will not be able to use MOSFETs in the power converter because

they do not have fast enough switching characteristics. The power converter will need to supply

a small-signal perturbation with frequencies up to 65 kHz. This will require a high switching

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frequency, which can lead to switching losses and power dissipation, and will require careful

planning to mitigate.

This project will be tested on a 250W monocrystalline rooftop PV panel. The frequency

of the light modulation will range between 1Hz and 65kHz. The LED driver circuit must be able

to handle 300W, with 20V output and 15 +/-1.5A. Hardware from dSPACE, NI DAQ, and

Opal-RT will be used to create a real-time hardware-in-the-loop controller. Measurement devices

must also be used to determine the voltage and current of the PV panel as well as the current

through the LED for current control. LabVIEW will be used to program the controller and collect

data from the PV panel. Bench power supplies, a function generator, and an oscilloscope will be

necessary for design and testing of our device.

Proposed Approach and Design

To begin, our team first did background research on impedance spectroscopy by studying

previous experiment reports. These reports were able to effectively show results using

specialized equipment to provide an AC signal. We were also introduced to another research

experiment done by researchers at the University of Connecticut which was also able to show the

type of results that we were to expect. Based on our background readings, the next step is to

perform similar experimentation using our designed driver circuit to push the AC signal.

Our team had previously worked on two design approaches for the LED driver circuit to

determine which will be simpler to implement and better suited to the application. We

determined that one of these, using an operational amplifier circuit, was not suited for our power

requirements. Instead, our approach for the LED driver circuit is to use a current-controlled buck

converter. The buck converter will convert a 50Vdc source voltage to a current-controlled output

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with the desired offset and sinusoidal perturbation. NI-DAQ will be used to provide the reference

current, which will be compared to the output current. The current control loop uses a PI

compensator designed to have a high bandwidth to avoid attenuation of the AC component at

frequencies up to 65kHz.

Figure 1: Simulated response of buck converter showing reference current (blue) and load

current (red) at 10kHz

A voltage and current sensing circuit will be used to measure the output of the PV panel.

The voltage sensing circuit will consist of two INA154 difference amplifiers, two LM741 op

amps, a voltage source, seven capacitors, and six resistors of various values as described below.

This is then passed through an analog-to-digital converter. The current sensing circuit contains

one INA154 difference amplifier, two LM741 op amps, a current source, six capacitors, and five

resistors of various values. The schematics for the voltage and current sensing circuits are shown

in the figure below.

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Figure 2: A circuit schematic showing the voltage sensing (top) and current sensing (bottom)

circuits.

Our input signal contains a DC voltage with a relatively small AC ripple. Therefore, our

AC value has a low resolution and is difficult to measure with precision. We want to bring the

resolution of the AC ripple up so that we can maximize its measurement. The INA154 amplifiers

are used to shift the output voltage of the signal downwards, and the op amps are used to amplify

the AC ripple of the signal without amplifying the DC component. This gives our AC value a

higher resolution so that we can easily measure it in relation to the DC value.

Using PSpice, we designed the voltage and current sensing circuits and ran a time domain

simulation to measure the voltage of the input and output waveforms over the span of 50ms. For

this simulation, our input voltage had a DC offset of 4V and a 40mV AC ripple. The output gives

an AC ripple that is four times greater than the amplitude of the input signal. The figure below

shows the voltage and current sensing circuits in PSpice respectively.

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Figure 3: Our PSpice-simulated voltage sensing (top) and current sensing (bottom) circuits.

Figure 4: A PSpice time domain simulation for the voltage sensing circuit. The input wave is

shown in the bottom graph in green, and the output wave is shown in the top graph in red.

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Signal Sensing Interface Board

To implement the design to PCB form, a schematic was designed using Altium Designer,

taking into account how to drive the various supply and reference voltages throughout the board.

The board was designed to sense the voltage and currents of the PV panel and the LED driver.

Each stage had three separate parts; the first part was to sense the voltage and current signals. For

the voltage signals, this was done through direct connection from the PV panel/LED driver to the

board. Both of these signals were connected to a voltage divider in order to meet signal

requirements for the NI-DAQ. The current signals were sensed through a hall effect sensor.

The second part was to clip DC offset voltage from our obtained signal. This was done by

supplying the signal to a differential amplifier along with a DC reference voltage that would have

to be clipped. To do this, we used amplifiers to modify a 2.5VDC signal in order to reach our

desired reference voltage. Through testing the board, we found that the sensed voltages did not

require any voltage to be clipped, but the sensed currents required clipping. Each of the current

signals required us to remove 14mVDC before proceeding to the next step. To do this, we had to

make our original 2.5V smaller using a dual inverting amplifier system, then provide it to the

differential amplifier to clip.

The final step is to amplify the signals in order to meet NI-DAQ requirements. Similarly

to the reference voltages, this was also done using dual inverting op-amps. The voltage signals

were already modified through a voltage divider during sensing, so these remained unchanged.

The current signals had to be amplified since the hall effect sensors we used only output

40mV/A. The PV panel current would have to be amplified with a gain of 200; we expect a gain

of about 8V between 0 and 1A. Testing resulted in a gain of about 7.9V. The LED driver current

would have to be amplified with a gain of 82, with an expected gain of 3.3V from 0 to 1A.

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Testing resulted in a gain of about 3.25V. Final signals are then sent to the NI-DAQ system for

processing.

Figure 5: Schematic and block diagram of voltage and current sensing circuits, to be used for

PCB.

While the PCB we developed was able to perform the necessary tasks we needed to

complete this project, further improvements can be made. The major change to be implemented

is to allow for faster alteration of the clipping reference voltage to match different currents. This

could be implemented using a potentiometer rather than replacing board components. In

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addition, the original board design had implemented a method of sensing currents that used a

shunt resistor rather than a hall effect sensor. This was not tested for this project due to the

unavailability of desired parts. Finally, the board mounted power supplies could be replaced by

different models that are rated for the power that this board uses.

Data Collection & Processing

The third stage of our design is data collection and signal processing. For this, we will

use NI-DAQ modules in LabVIEW, a programming environment suited to these applications. We

will collect voltage and current data at each frequency and use algorithms to preprocess the data,

estimate the peak voltage and current, estimate the phase difference, and calculate the impedance

from these values. Our methods are adapted from those presented in [1]. At present, our focus

has been on translating the math presented in the paper into LabView code. Our program is

currently able to subtract the DC component of voltage or current data to extract the AC ripple,

and then estimate the peak voltage. This algorithm is defined in the following equations [1].

Determining the phase difference requires performing a FFT, for which LabVIEW

provides an existing VI module. The VI outputs the magnitude and phase each as a cluster

containing the start frequency, the frequency interval between each point, and an array

containing the magnitude or phase data. The fundamental frequency of the waveform is found by

finding the index of the maximum value of the magnitude, then multiplying this index by the

frequency interval and adding the start frequency. The phase of the waveform is found by

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extracting the value of the phase array at that same index. The phase of the impedance is defined

in [1] as the phase of the voltage waveform minus the phase of the current waveform.

Another LabVIEW program was written to interface with the NI-DAQ device, provide

the reference signal for the current-controlled LED driver, and collect the frequency sweep data.

This program generates an array of frequencies to sweep based on user-inputed start and end

frequencies and number of steps. The program then used NI-DAQ modules to generate a sine

waveform and modify the frequency by changing the sample clock rate. This waveform is used

as the reference signal to the current controller. The program also collects current and voltage

data from the PV panel sensing circuit, and separates this data into chunks for each test

frequency. The impedance and frequency can then be calculated for each chunk, and this data is

used to calculate the equivalent circuit parameters of the PV panel.

The basic equivalent circuit model for a PV panel is a series resistance, a parallel

resistance, and a capacitance [5]. This diagram is shown below in Figure 6. The impedance

plane, which refers to the real impedance on the x-axis and the negative imaginary impedance on

the y-axis, is used to observe the frequency characteristics of an equivalent circuit model. The

model used here takes the shape of a semicircle as shown in Figure 7 [5].

Figure 6: The PV panel equivalent circuit model used in our project

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Figure 7: The impedance plot of the equivalent circuit model

As can be seen in the diagram, the points where the plot crosses the real axis represent the

series resistance Rs and the series resistance plus the parallel resistance, Rs+Rp, respectively.

The peak imaginary impedance represents the resonant frequency of the equivalent circuit and

can be used to find the capacitance. This is done according to the equation

𝐶 = 1𝑗2ω

𝑐 𝑝𝑒𝑎𝑘(𝑍

𝑖𝑚 )

The equivalent circuit calculation thus works by finding the frequency corresponding to

the peak imaginary impedance, and then calculating the capacitance from this information.

Optical Engineering

In order to optimize the output voltage being generated by the PV panel, we will want to

maximize the efficiency of our lab setup so as much light from the LED array as possible can be

collected by our panel. To do this, we want the light from the LED array to hit the PV panel at a

90 degree angle, we want to prevent light from spilling over the edges of the PV panel, and we

want to stop ambient light in the surrounding area from reaching the PV panel. Our lab setup is

shown in Figure 8 below.

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Figure 8: Our experiment setup, showing our microcontroller, driver, LED array with

collimating lens, and PV panel.

To mitigate some of these effects, we first implemented a 60 degree beam angle

collimating lens into our system. A collimating lens is able to align photons so that beams of

light are parallel to each other instead of spreading outwards from a source. This allows us to

more accurately control the direction of the beams of light, so we can position the LED array and

prevent any light from spilling over the edges of the PV panel. This also ensures light is spread

evenly over the effective area of the panel, whereas before light would be concentrated at the

center of the panel. The effects of a collimating lens on our system can be seen in Figure 9

below.

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Figure 9: A diagram showing the path of light beams in an optical system using a collimating

lens.

In later tests, we purchased a 120 degree beam angle collimating lens for our system. We did

this because we calculated that with a wider degree beam angle, we could move the LED array

closer to the PV panel while still allowing the light to spread evenly over the effective surface

area of the panel. We calculated that, using this 120 degree lens, we could position the LED array

4 inches away from the PV panel and get our optimal output voltage. The figure below shows

both our 60 degree and 120 degree collimating lenses.

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Figure 10: A photograph of our 60 degree beam angle collimating lens (left) and our 120 degree

beam angle collimating lens (right).

To stop light from spilling over the edges of the panel and to prevent ambient light from being

picked up by our PV panel, we created aluminum light reflectors to be placed around the LED

array and collimating lens. These reflectors allowed any stray beams of light to bounce off of the

reflectors and reflect back to the PV panel. These reflectors were constructed using cardboard,

aluminum foil, and duct tape.

Knowing the beam angle associated with each collimating lens, the effective area of the PV

panel, and the size of the LED array, we were able to perform basic trigonometry to determine

the optimal distance that the LED array must be from the PV panel so that the light can reach the

edges of the PV panel and spread across the effective area equally. If the array is too close to the

panel, light would be concentrated in the center of the panel and this would decrease the output

voltage, but if the array is too far away from the panel, light will spill over the edges of the panel

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and have a high amount of loss from reflecting off of the edges of the reflector and hitting the PV

panel at a non- 90 degree angle.

For our 60 degree collimating lens, we created one reflector that was fitted to encase the area

between the LED array and the PV panel at a distance of 11 inches between the two. For the 120

degree collimating lens, we created two reflectors. One of the reflectors, which we named the

‘short-range reflector’, we created to fit with a distance of 4 inches between the LED array and

the PV panel. The second reflector, which we named the ‘long-range reflector’, was created to fit

with a distance of 9 inches between the LED array and the PV panel. The three reflectors can be

seen in the Figure below.

Figure 11: A photograph showing the aluminum reflector for our 60 degree collimating lens and

the two reflectors for the 120 degree collimating lens.

To see if the addition of the collimating lens and reflector improved the output voltage of the

PV panel, we wanted to input a constant voltage into the LED array, but change the distance

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between the PV panel and the array, the beam angle of the collimating lens, and the type of

reflector used. We used distances in increments of 4 inches, starting with the LED array 4 inches

from the PV panel and increasing it until it was 24 inches from the PV panel. We tested both the

60 degree and 120 degree collimating lens at each distance, and used each lens with and without

its associated reflector(s). Then, we recorded the PV panel’s output voltage for each trial.

Without the use of any collimating lenses or reflectors, the PV panel gave a maximum

output of 15V at a distance of 16 inches from the LED array.

Testing the collimating lenses with no aluminum reflectors gave us the results shown in

the Figure below.

Figure 12: The PV panel output voltage for both collimating lenses without using any aluminum

reflectors.

As seen in the graph, the 120 degree lens works better at shorter distances, and the 60

degree lens works better over longer distances. This is because the 120 degree lens has a wider

beam angle, so at a shorter distance, the light is more evenly distributed, and at greater distances

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the light spills over the edges of the panel. At shorter distances, the 60 degree lens doesn’t reach

the entire surface area of the panel, and it is able to at greater distances.

Next, we tested each collimating lens with their reflectors. The results are seen in the

Figure below.

Figure 13: The PV panel output voltage for each collimating lens while using its associated

aluminum reflector(s).

As seen in Figure 13, the 120 degree lens with the long-range reflector consistently

produces the highest output voltage over long distances. The three trials all produced the same

maximum voltage of 20V at a distance of 8 inches. Despite the 60 degree lens working better at a

large distance without a reflector, using the long-range reflector gives the 120 degree lens a

higher output voltage.

We put the data from all trials into the Figure below, for comparison.

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Figure 14: A graph comparing the output voltage for each collimating lens, with and without

using its associated reflectors.

As seen in Figure 14, we can reach a maximum output voltage at a distance of 8 inches

using a reflector for any collimating lens, or using the 120 degree lens with no reflector. In

addition, the 120 degree lens with a long-range reflector gives the highest output voltage over

long distances.

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Project Management

Figure 15: The RACI chart displaying the responsibilities of each member for our project.

Figure 16: A Gantt chart showing the timeline for each task associated with our project.

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Summary/Recommendation for future work

This design project involves the creation of an LED driver circuit that will perturb the

light intensity incident on a PV panel. This signal perturbation will allow us to perform

impedance spectroscopy in order to determine the health of the panel. We have done background

research on this topic through various sources and have created an outline of the necessary tasks

performed by our design: signal generation, signal measurement, and signal processing. We have

developed approaches to realize all three of these objectives. Future work includes the testing of

the completed system and collection of real PV panel data to compare with the simulated models.

This also includes testing of the closed-loop controller. Future improvements could be made to

the equivalent circuit parameter calculation program by using more sophisticated curve fitting to

better characterize the impedance plot. It may be the case that other resistances, capacitances, or

inductances may be at play in the real PV panel, and the model may need to be refined after

future testing. Finally, the overall purpose of this project is to characterize PV panel aging,

meaning that the completed system will be used to find the equivalent circuit parameters of a

panel at different conditions of age.

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References

[1] S. M. R. Islam and S. Park, "Precise Online Electrochemical Impedance Spectroscopy

Strategies for Li-Ion Batteries," in IEEE Transactions on Industry Applications, vol. 56, no. 2,

pp. 1661-1669, March-April 2020, doi: 10.1109/TIA.2019.2958555.

[2] M.S. Suresh, Measurement of solar cell parameters using impedance spectroscopy, Solar

Energy Materials and Solar Cells, Volume 43, Issue 1, 1996, Pages 21-28, ISSN 0927-0248,

https://doi.org/10.1016/0927-0248(95)00153-0.

[3] D. Chenvidhya, K. Kirtikara, C. Jivacate, PV module dynamic impedance and its voltage

and frequency dependencies, Solar Energy Materials and Solar Cells, Volume 86, Issue 2, 2005,

Pages 243-251, ISSN 0927-0248, https://doi.org/10.1016/j.solmat.2004.07.005.

[4] Pankaj Yadav, Kavita Pandey, Vishwa Bhatt, Manoj Kumar, Joondong Kim, Critical

aspects of impedance spectroscopy in silicon solar cell characterization: A review, Renewable

and Sustainable Energy Reviews, Volume 76, 2017, Pages 1562-1578, ISSN 1364-0321,

https://doi.org/10.1016/j.rser.2016.11.205.

[5] O.I. Olayiwola, P.S. Barendse, “Photovoltaic Cell/Module Equivalent Electric Circuit

Modeling Using Impedance Spectroscopy,” IEEE Transactions on Industry Applications, vol. 56,

no. 2, pp. 1690-1701, 2020.

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Glossary

Altium - PCB and circuit design software developed by Altium Ltd.

Buck Converter - DC/DC converter that steps down voltage from input to output; switched-mode

power supply

Impedance spectroscopy - measures resistance and capacitance of material via injection of AC

signal

INA154 - difference amplifier IC

LabView - system programming & automation software developed by National Instruments

LED - Light-emitting diode

NI-DAQ - data acquisition system developed by National Instruments

PCB - printed circuit board

PV Panel - Photo-voltaic panel (also known as solar panel)

Schematic - symbolic representation of circuit and connections

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Senior Design Project Checklist

Project name:

2103 Light intensity modulator design

Sponsor:

UCONN ECE Department

Team members (majors/programs):

Aaditya Sekar, Lauren Boulay, Adrian Gibson (Electrical Engineering)

Faculty advisor(s):

Dr. Sung-Yeul Park

Skills, Constraints, and Standards: (Please check (√) all those that apply to your project.)

Skills: (√)Analog circuit design and troubleshooting √Digital circuit design and troubleshooting √Software development/programming √Embedded Systems/MicrocontrollersWeb designRF/wireless hardwareControl systems √Communication systemsPower systems √Signal processing √Machine shop/mechanical designOther (please specify):Constraints:Economic (budget)Health/safety √ManufacturabilityEnvironmental (e.g., toxic materials, fossil fuels)Social/legal (e.g., privacy)Standards:List standards/electric codes that you used (e.g.,IEEE 802.11, Bluetooth, RS-232, VHDL, etc.)

If applicable, list the name or # here:

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