Documents Final Report

8/13/2019 Documents Final Report

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Digital Wattmeter

Aaron Fogle & Pat Giles

May 3, 2012

Faculty Advisor:

Dr. Gregory Bucks

Ohio Northern University


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Summary

The goal of our project was to create a wattmeter and power factor reader that was

affordable and had a digital display. This proved to be a decently challenging task to accomplish

as several factors had to be considered.Such factors were how to interface with a circuit which

had around 30 volts across it to a device which could power an LCD display without

significantly modifying the behavior of the circuit. Other unforeseen issues did arise during

development causing problems, such as switching microcontrollers half way through

development and how to ensure the device would be as accurate as possible while still

maintaining the other goals of the project.

This required two components: a circuit to be designed to meet the specifications of the

project, and a program written for a microprocessor which would sample the input and compute

the results. All of this needed to be done on a small scale, designed for laboratory settings and

affordable to our client. Completing this while still providing the desired functionality proved to

be our biggest challenge in working on this project. This report will outline the problems

encountered over the course of the year and how we went about resolving them.


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Acknowledgement

The group would like to take this time to acknowledge the entire ECCS faculty at Ohio

Northern University for their various contributions towards our efforts to finish this project. We

would particularly like to thank our faculty advisor, Dr. Bucks, for many long afternoons talking

to us about the issues we encountered, as well as providing a third opinion on a matter of

discussion. We would also like to thank Dr. Vemuru, for assisting us in finding solutions to

problems we just did not see any answers for. We also owe many thanks to Brad Hummel for

assisting us in the building of the final prototype. Thanks all.


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L ist of Tables

Table 1. Marketing Requirements 5Table 2. Decision Matrix 9

Table 3. Level 0: System Block Diagram 10Table 4. Level 1: Voltage Requirements 11Table 5. Level 1: Current Requirements 11Table 6. Level 1: Converter 12Table 7. Level 1: Microcontroller 12Table 8. Level 1: LCD 12Table 9. Verification of Microcontroller Functionality 19Table 10. Development Budget 20Table 11. Cost Per Unit 20Table 12. Labor Costs 20Table 13. Project Cost 20

L ist of F igures

Figure 1Objective Tree 7Figure 2Level 0 Block Diagram 10Figure 3Level 1 Block Diagram 11Figure 4Final Design Overview 13Figure 5Device Schematic 15Figure 6Voltage Division Verification 18Figure 7Differential Op-amp Verification 19


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Needs Statement

The ECCS department within Ohio Northern Universitys College of Engineering is

currently using aged analog wattmeters to satisfy laboratory needs for its Circuits labs. This

device takes a considerable amount of time to set up and use to obtain the desired measurements.

In order to lighten the load on the students, there is need for a digital wattmeter which will allow

measurements to be taken faster, easier, and more accurately than they can with the current

equipment in use during their power factor experiment, as part of the Electronic Circuits course.

Objective Statement

The objective of this project is to design and prototype a small, cheap, and easily

reproducible digital wattmeter. This wattmeter will replace the analog wattmeters being used in

Circuits labs today. The device will have input and output terminals for connecting the device to

the system under analysis, as well as a digital display to more accurately convey the

measurements. The user will be able to incorporate this device within a circuit, between the

power source and the load, and the device will in turn digitally display the wattage absorbed and

power factor of the load. The meter will also display voltage and current. This design should be

affordable and simple to manufacture so that it can be replicated easily. If cost and time allow,

there will be a possibility of building 15 of these devices to be used by students in the Electronic

Circuits course for their lab assignments.


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Background Information

A wattmeter is a device which measures electrical power. The meter can be analog or

digital, and either option comes with very different designs. The basic theory behind these meters

is based on Ohms Law and Joules Law. In either case, the voltage and current from the

measured circuit are the sources to find the power absorbed by the circuit. Although the basis for

the wattmeter sounds simplistic, the design of such a device is not as intuitive.

As stated previously, both analog and digital wattmeters already exist. Most analog

wattmeters operate using the underlying principles of electrodynamics and magnetic fields.

Simply put, when a voltage is induced across a resistive load, current flows and a magnetic field

surrounds the device. This magnetic field can induce a current in a neighboring coil, causing a

piece of metal to move, namely a needle. The needle can be calibrated to read wattage, based on

the induced current. This analog setup is a very common design, and somewhat simple to

produce. However, it is not incredibly accurate and requires tedious calibration.

Newer technology has provided the ability to produce digital wattmeters. These meters

usually have a digital display and are portable, resulting in a more versatile and easy to use

product. Because these devices are digital, the analog approach is replaced with digital circuits to

read in and compute measurements. Digital devices offer the ability to read measurements at any

frequency (not just the typical 60Hz in America) and also compute many different quantities.

Assuming the microcontrollers read in the magnitude and phase of the voltage and current, the

device can compute peak, average, and apparent power, as well as power factor. Digital

wattmeters provide versatile capabilities and accurate readings in a very short period of time.

After calculations are complete, microcontrollers can output the data to a digital display, making

it easier to take measurements on the fly. As time progresses, digital circuits prove to be the most


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useful and effective method. With the falling price and improved capabilities of digital

electronics, they (Digital Wattmeters) have become popular for conveniently measuring power

consumption in household appliances with an eye toward saving energy and money.1

Simply put, power factor is a measure of how efficiently one utilizes power supplied.

Most loads are primarily resistive loads, including lighting and some electrical appliances, and

consume only real power. Devices which include a motor have an inductive component, and thus

have a power factor less than one. Power factor is a number ranging from zero to one, with one

being desirable; a power factor of one (unity) means all of the supplied power (real power) is

being absorbed. Power supplied includes two components, real power and reactive power. Power

factor is then the cosine of the angle difference between the voltage and current waveforms

within the circuit. Unless the load includes components that absorb reactive power, a power

factor less than one means the reactive power is not being utilized and is essentially returned to

the source; thus, obtaining a higher power factor is desirable. A purely resistive circuit has

voltage and current waveforms which are in phase, and thus power factor is unity. A lagging

power factor exists when the phase angle of the current lags behind that of the voltage. This

situation occurs with additional inductive loads. Accordingly, a leading power factor occurs

when the current waveform leads the voltage in phase. This situation occurs with additional

capacitive loads. Power factor can be corrected by placing reactive components in parallel with

the load.

As stated previously, digital wattmeters can be found commercially; however, the

groups device will function on a smaller scale not currently available and include additional

functionality. The available wattmeters on the market do not provide the ability to measure and

display power factor within the same design. The design in this project will be able to sense the


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phase difference between the voltage and current signals. Using this information, the wattmeter

will calculate the power factor (whether leading or lagging) and display it digitally in a simple to

read manner. This device will be more economical than competing devices. That is to say: it is

cheaper and easier to use than competing devices. Other existing designs can be incorporated

into our device; primarily, the ideas of how to allow a device to measure current and voltage

currently employed in digital multimeters. One option to measure current is by using very small

resistance in series with the load and measuring the voltage across that resistor. An alternative

solution for measuring current is using an integrated circuit known as a Hall Effect Sensor,

which converts a current signal into a voltage (readable by a microcontroller). To measure

voltage a very large resistance (on the order of k to M)is placed in parallel with the load. The

voltage across this resistance is equal to the voltage of the measured device. The design will also

implement a currently-existing digital display for output.


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Marketing Requirements

Table 1 lists the marketing and engineering requirements for the project. These

requirements were initially given to the group by the customer, and are the basis on which the

design will be researched and developed. In addition, justifications are listed to explain to the

reader why these requirements have been set.

Table 1. Marketing Requirements

Marketing

Requirements

Engineering

RequirementsJustification

1, 3, 4

The device should runon 120V AC or less.

The product should be able to be usedeasily around the laboratory, able to plug

into a 120V outlet.

1-4

The device should haveinput and outputterminals.

There will be input terminals and outputterminals possessing the ability to readvoltage and current from measureddevice.

1-3

Read up to 50Waccurately.

Device will read wattage accurately to50W and display power factor(leading/lagging) as well to suitlaboratory experiments.

1-4

Total device should notexceed $150.

This is based on component research andestimation of cost of circuitry and

mounting.

1, 3, 4

Possibility of producing15 units.

The goal is to have a sufficient number tooutfit the College of Engineering'scircuits laboratory.

Marketing Requirements

1. The system should be easy to use.

2. The system should read accurately.

3. The system should be inexpensive.

4. The system should be easy to reproduce.


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Reali stic Constraints

Performanceo The system should be able to calculate and display the voltage and the current for

a given loado The system should be able to calculate and display power factor for a given load,

indicating whether the power factor is leading or lagging

o The system should be able to measure and display up to 50 Watts accuratelyo The system should operate on 120V AC or less

Functionalityo The system will implement input and output terminals to be used for measuring

voltage and current for a given load

o The system should incorporate a digital display for ease of useo The product should interface with current measuring wires available in the Ohio

Northern University Analog Electronics lab

Economico The total parts and manufacturing costs cannot exceed $150 per unit

Health and Safetyo The system will not expose humans to unhealthy levels of electromagnetic

radiation

o The circuitry inside the system should be protected from voltage or current levelsthat are too high for normal operation

Manufacturabilityo The system must be manufactured on a circuit board, accompanied by a schematic

demonstrating exact location of all components

o The design should include only components that are readily available to anyonewho wishes to replicate this design

Operationalo The system should be able to operate at all temperatures within 20C of room

temperature in either direction

Reliabilityo The system should be able to be used quickly and read accurately


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Objective Tree

Figure 1 provides the objective tree. This figure begins the decision making process,

offering the weights of the marketing requirements as well as a more detailed breakdown. Each

criteria weight was assigned by the group members based off of research and logical reasoning.

Simply put, the group concluded that the most important criteria is cost in order to save the

department money and resources. To add, ease of use and accuracy are equally and nearly as

important, as this device will be used for teaching purposes in the Electronics Laboratory.

Finally, manufacturability is the final important category because multiple devices will

eventually be produced.

Figure 1Objective Tree showing project breakdown


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Al ternative Solutions and Decision Matri x

Table 2 provides the decision matrix which was used to select the best-fit design for the entire

project. The three considered products are as follows:

Modification of the analog wattmeters currently being used, retrofitting a digital displayand proper circuitry

Construction of a new device, using many integrated circuits for measurementcalculations, manipulations, etc. and an inexpensive microcontroller for the digital

display

Construction of a new device, using minimal amounts of circuitry and implementinglarger, more capable microcontrollers for measurement calculations, mathematical

operations, analog to digital conversion, etc., as well as operating the digital display

As one can observe from Table 2, the group decided that the cost and manufacturability

of the device weighed heavily on the selection of the most appropriate design. Thus, these factors

were given higher weights because of the general expectation that the final product is to be

reproduced and used within the universitys engineering department. Weights for each criterion

were chosen based on constraints given by the costumer and the expected outcome for the

project. Point values for cost were allocated from researching device components, and all other

points were discussed and agreed upon between group members. Point values for

manufacturability, safety and durability were a collective decision within the group. For these, a

higher point value is desired. Overall, the group felt that the first two listed options offer great

difficulty in modification or circuit layout to fit device requirements, as well as increasing risk of

component failure. The third option, based mostly off of larger microcontrollers, incorporates


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fewer parts and offers a significant amount of programmability, allowing the group to achieve

the desired operability through the direct input of mathematical equations, device measurements,

etc.

Table 2.Decision Matrix

Factor Weight RetrofittingIntegratedCircuits Microcontroller

Cost 35 150 150 100

Size 5 320 100 125

Manufacturability 25 4 4 3

Safety 15 9 9 9

Durability 20 7 8 9

Total: 100 48.9 53.6 64.4

Minimum Maximum

Cost 50 200

Size 75 512

Manufacturability 1 10

Safety 1 10

Durability 1 10


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Functional Decomposition of F inal Solution

This section outlines the details of the device operation and specifications, from high

level to low level. Level 0 corresponds to the overall basic functionality of the device

essentially a black box kind of description. The Level 1 sequence that follows breaks the

device down into its various sub-blocks, each with its corresponding description of the purpose

and operation of the sub-block.

Figure 2Level 0 Block Diagram showing system inputs and outputs

Table 3. Level 0: System Block Diagram

Module Digital Wattmeter

Inputs External Device: Outputs from device to measure current and voltageAC Power: 120V AC power

Outputs Digital Display: Display of MeasurementsOutput to External Device: A pass-through terminal to measure devicecurrent

Functionality Receive voltage and current inputs from external device. Operating on120V AC, must display power accurately up to 50W and power factor(leading/lagging) for the device.

Digital Wattmeter

External DeviceVoltage

AC Power

Digital Readout

Output to LoadExternal DeviceCurrent


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Figure 3

Level 1 Block Diagram showing the blocks for each system component

Table 4. Level 1: Voltage Requirements

Module Voltage Measurement

Input External Device Voltage: Device voltage signal

Output Stepped-down device voltage

Functionality Step down the voltage signal coming fromdevice to a smaller voltage that can be accepted

by microcontroller

Table 5. Level 1: Current Requirements

Module Current Measurement

Input Differential voltage across resistance

Output Amplified voltage signal

Functionality Differential op-amp circuit. This circuitamplifies a voltage measured across aresistance, to be sent to the microcontroller andconverted into a current measurement

LCDMicrocontroller

(Arduino Mega)

Converter

CurrentMeasurement

VoltageMeasurement

External DeviceVoltage

External DeviceCurrent

AC Power

Digital Reado

Output toExternal Devi(for CurrentMeasurement

Stepped-down Voltage

MeasuredCurrent

MeasuredCurrent

DC Voltage

MeasurementOutput


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Table 6. Level 1: Converter

Module Converter

Input 120V AC Power

Output Stepped-down DC voltage (12V DC)

Functionality Step down the voltage coming from outlet andconvert to 12V DC to supply power tomicrocontroller and op amp.

Table 7. Level 1: Microcontroller

Module Microcontroller

Input Stepped-down DC Voltage (12V DC) formodule powerStepped-down AC voltage from transformerMeasured current from device

Output Measurements

Functionality Compute and output desired measurements.These measurements include voltage, current,power (accurate up to 50W) and power factor(leading or lagging)

Table 8. Level 1: LCD

Module LCD

Input Measurements

Output Digital Display

Functionality Display measurement values received frommicrocontroller


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F inal Design and Integration

Figure 4Final design overview

Figure 4 offers a more detailed picture of the internal operation of the wattmeter. As

described in the Objective Statement and Marketing Requirements sections, the design includes

two input and two output terminals, allowing the device to be integrated between the source and

load. The microcontroller accepts analog waveforms as inputs and outputs digital signals to the

display. In order to measure voltage, the final design incorporates a simple voltage divider; this

branch of the circuit must step down the peak voltage of the incoming signal to less than 5V, as

the controllers analog inputs can safely accept values of 5V peak or less. For measuring current,

the device includes a differential op-amp circuit. In short, this component amplifies a differential

voltage measured across a resistance which is placed in series with the load. It is important to

note that this op-amp circuit serves a few purposesto isolate the microcontroller from a

potentially high input voltage, accept a small input voltage, and to amplify the input voltage for

increased accuracy in calculations. A Hall Effect Sensor, mentioned previously, was not


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incorporated in the design because a commercial device that can measure low currents (between

100-500 mA) was not found. An upper current limit of 500 mA (based on expected values during

laboratory experiments) has been set for this device, thus the maximum gain for the differential

op-amp circuit (across a resistance of 2-2.5) was designed to be approximately four times the

input voltage. More details on the internal circuitry are discussed later on in this section,

including descriptions as to why each component was chosen.

After the voltage and current waveforms are stepped-down and amplified respectively,

the signals are sampled and digitized by the microcontroller. Because the ADC inside the

controller samples much faster than the incoming 60 Hz signals, obtaining one full period of the

waveforms accumulates data quickly; thus, the microcontroller is programmed to save to

memory the peak of each waveform and the timestamp of when this peak occurred, all during

two full periods. Following this, the code recovers the loads voltage and current by utilizing

hard-coded resistor values from the surrounding circuitry. The magnitudes of both signals are

then multiplied to compute real power. Finally, the timestamps of each peak are utilized to

convert to a phase angle (using the time of the current peak minus the time of the voltage peak).

The cosine of this phase angle then represents the power factor of the load. If the power factor

angle is negative, the load current is said to be lagging the voltage. Alternatively, a positive

power factor angle results in the current leading the voltage. As a reference, the AC motors being

used in the circuits lab are inductive, resulting in the initial expected power factor to be lagging.

After all values are computed properly, results for load voltage, current, real power and power

factor are displayed simultaneously.


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Figure 5Device Schematic

Figure 5 depicts the actual schematic of the device, aside from the microcontroller.

Excluding the resistor labeled R10, all resistances are based off of components readily

available in the component stock in the circuits laboratory. These resistors are inexpensive, and

all are rated at .25W with tolerances of 10%. Due to such large tolerances, actual resistor values

can vary significantly compared to nominal. The last resistor, R10,was chosen separately

based on having a low resistance (thus causing minimal voltage drop between source and load)

and having a higher power rating (accounting for the maximum allowable current of 500 mA).

The two potentiometers, labeled R11 and R12, are in place to help create a nominal

differential op-amp circuit. To explain, resistors R4 and R7 are nominally equal, as well as

R5 and R6. The slide potentiometers in turn make up for the differences within each pair.

In addition, the basic diode labeled D1 cuts off any negative swing from the voltage

measurement; this is in place because the analog pins on the microcontroller board are rated at 0-

5V peak. Also, the zener diode located in parallel with R2 is designed to conduct when the


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voltage reaches 5V (caused by too high input terminal voltage), thus preventing damage to the

microcontroller. Furthermore, a fast-acting in-line fuse rated at .5A is incorporated in series with

the load, preventing current from reaching dangerous levels. If the current does reach a

dangerous level, the fuse will blow and the user must then remove the lid and replace the fuse.

The op-amp chosen for this design is the Texas Instruments TLV2401IP. This integrated

circuit is a rail-to-rail operational amplifier, meaning the output waveform can nearly reach the

minimum and maximum values of supply voltage. Fortunately, the op-amp and microcontroller

accept an identical supply voltage within a certain range; thus, both components are supplied

with power from a single 12V DC adapter, which plugs directly into a 120V AC outlet.

Finally, the microcontroller being used in the final design is the Arduino Mega. The

group acquired a smaller, older version, known as the Arduino Duemilanove for much testing

and development, but eventually decided the chip did not include a sufficient amount of memory

to execute the code successfully. A list of parts for a complete device, as well as instructions for

producing one unit, are provided in a secondary document. In addition, the information is

provided on the groups project web page and the data disc, which includes the necessary

electronic files to complete the project. Data sheets for the zener diode, op-amp, and

microcontroller are referenced with the construction materials.


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Test Plan

The testing of this system included verification of all designs of individual components

using PSPICE for simulation. Each circuit component was tested via repeated trials of the same

experiment for the desired output. This means the output voltage and current of each block was

measured using existing equipment in the lab to verify desired values were fed to the

microcontroller. The results of these experiments were consistent with calculated values and

values measured with other devices for verification. In addition, several iterations of the

microcontrollers code were initially tested for operation using test input signals.After the test

code was verified functional, additional iterations were executed in conjunction with the physical

circuitry. Values displayed on the device were compared to measurements taken with lab

equipment to verify accuracy. Finally, the device was tested by using it in the same lab

assignment that a student would. Output of the system was verified by comparing the results with

calculated values, as well as values measured using other existing devices found in the Analog

Electronics lab.


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Design Ver if ication

To verify operation of the design, results from two basic areas will be reported: circuit

board functionality and microcontroller functionality. To begin, it is important to prove that the

circuit board outputs signals between 0-5V peak to the microcontroller. The following figures

verify the desired outcome.

Figure 6Verification of the voltage division circuitry

Figure 6 depicts the operation of the voltage divider. Channel 1 in the picture shows a maximum

of 43.6V, which is the peak voltage of the transformer supplying power. Channel 2 demonstrates

the waveform being fed to the microcontroller for calculating input voltage; note that the

Channel 2 waveform peaks just below 4V.


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Figure 7Verification of the differential op-amp circuit

Additionally, Figure 7 demonstrates the operation of the differential op-amp circuit. Here, a

voltage of approximately 560mV is measured across the 2.2 resistor (Channel 1). Channel 2

then shows the output of the op-amp to be about 2V peak. One must note that the output voltage

will increase as the current is increased. The negative swing of the signal is eliminated due to the

characteristics of the op-amp. This, however, will not affect calculations.

To verify operation of the microcontroller, measured and calculated values within the

prototype are compared to actual values, as shown in Table 11 below.

Table 9. Verification of Microcontroller Operation

Arduino Actual

Voltage (V) 41.712 43.8

Current (A) 0.123 0.241

Power (W) 2.561 5.2779

PF .74 lag .739 lag

The group understands that the values are far from matching at the time the report is being

written. The accuracy of the device code is still in the process of being modified before the

design is completely finalized.


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Cost of the Project

The cost of this project can be divided into three different categories: device

development, cost per produced unit, and labor. Table 12 demonstrates the cost for physical

development of the product. The total for development is higher than that of a single unit due to

extra costs accrued for intermediate testing and integration. Table 13 highlights the cost to

produce a single unit. It is broken down into the cost of each basic component that will be used

in the design plus extra products purchased and used for device development and testing. Table

14 shows the labor costs associated with development of the product. Labor costs are based on a

$30/hour wage and hours worked per week are estimated based off of tasks to be completed (See

Gantt Chart). Finally, Table 15 provides the total cost of the entire project, development and

manufacturing of fifteen units.

Table 10. Development Budget

ComponentEstimated

Cost

Microcontroller $90.00CircuitElements $15.00

LCD Screen $15.00

Case $6.50

Total $126.50

Table 12. Labor Costs

Term Hours Cost

Fall 180 $5,400.00

Spring 300 $9,000.00Total 480 $14,400.00

Table 11.Cost Per Unit

ComponentEstimated

Cost

Microcontroller $60.00CircuitElements $6.00

LCD Screen $15.00

Case $6.50

PCB Board $3.00

DC Adapter $10.00

Total $100.50

Table 13. Project Budget

Labor $14,400.00Parts/Development $1,634.00

Total Cost $16,034.00


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Time Schedule

Appendix A contains the Gantt Chart demonstrating the time schedule for the entire

project, as well as the individual tasks associated with completion of the project.

Conclusion

While the first part of this project mostly involved research and spending time in the

requirements phase of the project, the second semester was primarily comprised of

implementation and testing phases. As time moved on, the group encountered a mix of problems

on a weekly basis. Whenever a problem arose, it was first discussed with the advisor, then

carried out to other faculty and resources as needed.

At this final juncture, the group considers the project mostly a success. The prototype

implemented on the bread board communicates successfully with the microcontroller. The case

is on its way to completion, and one PCB has been fully completed. The groups goal was to

have one fully working prototype finished by the end of the semester, but many accuracy issues

came about on a weekly basis.

In light of the previous paragraph, the current issues facing the group lie mainly within

the microcontroller. Several sources of inaccuracy could be involved, stemming from some of

the following sources (but not limited to):

Accuracy of measurement equipment in the lab. It is important to note that theoscilloscope displays voltages out to only two decimal places. To add, measuring exact

resistor values at values in the k and above range is difficult for current equipmentto

remain consistent.

The ADC of the microcontroller can only measure discrete values.


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Sampling and storing peak values and peak times may have mismatches. Possible rounding of values affecting the final outcome.

On top of current accuracy issues, the group acknowledges that future improvements

couldcertainly be added. One possible addition is using the microcontroller to detect resistances

instead of hard-coding values. Along with this, reproducing the unit would be less tedious

because hard-coding values is not required. However, constructing an ideal differential op-amp

will still be required. To add to reproduction improvements, more efficient ways of permanently

wiring components together could be addressed. Also, the group would like to see a plug-and-

play style port to supply power to the unit, instead of a fixed, permanent cable.Finally,

additional safety features may be added, such as a fuse in the return path from the load (in case

the user connects a large voltage across both negative terminals).

Overall, the group has gained sufficient knowledge in the engineering process, from

obtaining customer requirements, to testing, to completing a design. It has been a successful year

and the group is pleased to submit the design to the Engineering Department for further

verification and ultimate use.


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References

1. Arduino: Forum 2008.Arduino

2.

Barett, J.T. How Does a Wattmeter Work? eHow.com3. Electric Power 2011. Wikipedia

4. Grover, Sam Wattmeter Basics 2010. eHow.com

5. Kenneth, Joseph Digital Wattmeter 2008.

6. Linear scale analog watt meter


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Appendix A: Gantt Chart

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