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ELE744 Electronics & Instrumentation

Electrical & Computer Engineering

MAJOR LABORATORY PROJECT

Developed & prepared by Dr. M.S. Kassam © Ryerson University

Instrumentation design - using PIC embedded controller - for real-time

Stress (cantilever based load-cell) and Deflection-Angle measurements.

1.0 PREAMBLE:

Although the evolution of the microcontroller began in early 1980s, its cost outweighed

the advantages at first for use in high-volume smart instrumentation. With the advent, and

rapid development, of high-density CMOS based Integrated Circuit (IC) technology, the

microcontroller has emerged as one of the fundamental building blocks in electronics

technology. In most modern instrumentation designs, digital signal processing and

control are increasingly replacing some of the traditional forms of analog signal

processing/conditioning. This has generally resulted in flexible product designs that tend

to be reliable and cost-effective for mass production, to have increased functionality on

significantly smaller packaging footprints, and to be easily scalable. In tandem with the

microcontroller evolution, the CMOS technology has also led to the emergence of a host

of “smart” IC based sensors and transducers (e.g. accelerometer, pressure, force,

temperature, humidity, position, proximity, flow, etc.) giving designers perhaps a broader

latitude to create sophisticated product designs.

For today’s engineers, the myriad of IC based analog and digital devices in the

marketplace has resulted in an increased blurring of the line between traditionally analog

and digital based instrumentation, thus demanding confidence and proficiency in their

abilities to apply knowledge to conjure up “neat” engineering solutions that would most

likely entail blending and integration of analog circuits and digital signal processing,

high-density and high-speed devices, smart transducers, effective PCB grounding/layout

techniques, and so on. In such instrumentation designs, optimization and trade-offs

between microcontroller (or DSP) resources and analog signal processing/ conditioning

circuitry to meet product requirement specifications can often be daunting.

Thus, the main thrust behind this Major Project is to expose senior-level students to make

effective design and implementation choices (and trade-offs) to create an intelligent

instrumentation that potentially embodies state-of-the-art digital (e.g. microcontroller,

smart sensors, etc.) and traditional analog (e.g. passive sensors, stable amplifiers, signal

conditioners, A/D, compensation networks, etc.) circuitry for a Cantilever Beam

application used in industries like Aerospace, Automotive, Construction, etc.

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2.0 BACKGROUND:

In certain small-winged manned or unmanned aircraft control application, it is often

desirable to continuously sense and monitor stress points along the wing span, and the

resultant deflection or inclination angle relative to its unstressed or resting longitudinal

axis. The principles behind such types of measurements can be demonstrated and

implemented using a simple cantilever beam setup as shown in Figure 1. For simplicity,

the measurement of stress on the beam is limited to a single point and the deflection

angle, θ as depicted in Figure 2. Brief summaries of stress and angle measurement

principles are given below, and the students are urged to review the lecture material,

suggested reference articles and sensor datasheets to gain further insights.

Strian-Gage

F

L

W

t

Upper Strain-Gage

Lower Strain-Gage

F

L

Figure 1 Simple Cantilever Beam schematic

Stress Measurement Technique: Figure 2 shows how an applied force, F can be

converted to strain (ε) and corresponding resistance change using dual strain-gages for

measurement. The cantilever beam assumes a semicircular shape because of the applied

force, the top surface of the beam elongates and the bottom compresses. Students should

investigate the design advantage of using two identical strain-gages mounted on either

side of the beam at a fixed point location, noting that either upward or downward

application of vertical force should cause one strain-gage to stretch and the other to

compress.

Upper Strain-Gage

Lower Strain-Gage

F

Figure 2 Cantilever Beam under Load

θ

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The equation below

o σ (Stress) = (6 x F x L)/(W x t2) N/m

2 (newtons/ square meter)

where applied force, F is measured in newtons (N) where 1N = 0.225 LBs.

o ε (Strain) = σ/E m/m (meters per meter)

where E is the Modulus of Elasticity of the beam material given in N/m2.

shows that the Stress (σ) at a given point is directly proportional to the applied force

magnitude, F and the force distance, L (or the bending moment F.L); and indirectly

proportional to the beam width, W and the square of the beam thickness, t. With all

distance measurements in meters and the force in newtons, the Stress (σ) will be found in

units of newtons per square meter. Since modulus of elasticity, E in newtons per square

meter is known for the beam material, this allows the Strain (ε) in meters per meter to be

calculated. Once this is accomplished, the resulting increase or decrease in the resistance

of the strain-gages can be determined. Conversely, if the strain-gage characteristics (e.g.

GF, Ro, etc.) and resistance changes are known, then the Strain (ε) can be easily

determined from the strain-gages resistance changes, and from which both the Stress (σ)

and F can be calculated using the beam’s E and dimensional values, respectively. As can

be noted from the equations, when the product (W x t2) related to the dimensional

Modulus of the beam is assumed relatively unchanged and the applied force, F is known,

then the Stress/Strain values do linearly increase from zero (at the force application point)

to their maximum values at the fixed pivot point, allowing extrapolation of the

Stress/Strain along the cantilever beam from just a single measurement. Students are

encouraged to investigate similar techniques used in designs of load cells or force

transducers. A robust circuitry incorporating bridge amplifiers, bridge balancing and

temperature compensation techniques, proper signal conditioning and interface need to

be designed to ensure accurate and repeatable measurements of very small changes in

the strain-gage resistances converted to d.c. voltages.

Tilt- Angle Measurement Technique: The Micro Electrical Mechanical Systems

(MEMS) IC technology has resulted in a new generation of MEMS devices to add a wide

variety of functionality to products such as cellphones, PDAs, Automobiles,

GPS/Compass, handheld gaming, pedometers, appliances, etc. These newer MEMS

devices are tilt and motion sensors, commonly known as accelerometers, and are

constructed with no moving parts. The basic principle of operation is based on

differential thermal sensing of a heated gas bubble in a hermetically sealed IC part

(referred to as MEMSic). Constructing of this MEMSic sensor in a standard CMOS

process has significantly lowered cost, thereby opening up a host of applications. With no

moving parts, this thermal-based MEMSic accelerometer sensor is capable of surviving

the high shocks experienced with consumer gadgets (e.g. cellphones), both in the field

and during mass production, since it eliminates the traditional problems of stiction and

particle issues with previous generation of capacitive-based MEMS IC accelerometers. In

addition to the acceleration sensing technology, the MEMSic IC offers “smarts” by

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incorporating signal processing and conditioning using built-in analog/digital ASIC

designs to deliver stable digital outputs in the form of Pulse-Width Modulated (PWM)

clock stream where the duty-cycles are directly proportional to induced accelerations.

Unlike passive sensors (e.g. strain-gage), these smart MEMS sensors do not require

additional signal conditioning circuits thereby significantly reducing the components

overhead. These transducers do provide for accurate and repeatable measurements, and

are mostly standalone devices directly interface-able to any type of microcontroller. Both

static (gravity and tilt) and dynamic (vibration and motion) accelerations can be reliably

detected with the MEMSic transducers, and the students are urged to research and review

references on MEMSic technology to grasp the principles of operation behind its thermal-

based acceleration sensing and signal conditioning.

The MEMS2125 device is a low-cost, dual-axis (x and y) accelerometer capable of

measuring static and dynamic accelerations with a range of +/- 2g. A common application

of it is in dual-axis tilt or angle sensing. When a tilt angle lies on a vertical plane defined

by the sensing axes and gravitational vector, the absolute inclination angle, θ (referenced

to horizontal) can be measured from any initial accelerometer orientation. Students are

encouraged to review the MEMS2125 specifications and references to properly

understand the geometry of the angle measurements depending on either a horizontal or

vertical position of the MEMSic device relative to the gravitational vector. For the angle

measurement requirement for this Cantilever project, students should confirm that single-

axis inclination is best measured and resolved from a vertical mounting of the MEMSic

device whereby the Ax and Ay accelerometer outputs can be effectively combined to

obtain a good resolution of angles through the full 360° arc range.

α

β

MEMSicAccelerometer

Device

MEMSicAccelerometer

Device

x

y

g

x

y

g

Figure 3 Inclination from Vertical orientation of Device

Students should also verify from Figure 3, the desired inclination angle, θ of the

Cantilever application can be calculated by applying the inverse of the tangent function:-

θ = tan-1

(Ay/Ax) Students should confirm the following:- (1) in addition to providing good inclination

resolution to any angle, the vertical mounting of the device offers other advantages, such

as, errors that are common to both outputs are removed in the signal process of dividing

Ay by Ax ; and (2) possibility of further increase in resolution (through use of tan-1

to

measure angles) by effectively “doubling” the size of the Look-Up-Table (LUT) in the

microcontroller making use the following trigonometric identity:

tan-1

(m) = π/2 - tan-1

(1/m) if m > 0.

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3.0 OBJECTIVE:

Analyze, simulate, design, implement and test a PIC microcontroller based

instrumentation for a Cantilever beam (Load-cell and Angle) sensing application to meet

the requirement specifications (given in Section 5.0). This design and development

exercise should embody the following:-

Robust analog signal processing and conditioning circuitry for the strain-gage

measurements and conversions to ensure accuracy, resolution and repeatability,

using available single-supply power supplies (+15, +5Vand +3.3V). Review the

PIC EXPLORER 16 specifications to determine appropriate power supply for

the main PIC board to avoid damaging the IC components.

Computational and resource efficient algorithms for the tilt-angle measurement

with desired resolution and update rate.

Well designed software structure and efficient use of the PIC architecture

resources to implement an integrated solution for simultaneous “real-time”

measurements and display of Stress, Force and Tilt-angle parameters.

Seamless execution of the required User functions in “real-time”, per the

specifications.

Properly conceived testing methodologies to validate the measurements.

Well documented source-code and schematics.

Formal technical report.

4.0 SUGGESTED REFERNCES:

ELE744 Lecture Material.

ELE744 Course Text: “Design with Operational Amplifiers & Analog Integrated Circuits”, 3rd Edition, by

Sergio Franco, McGraw Hill, 2002.

ELE744 Course Text: “Programming 16 bit Microcontrollers in C Learningg to fly the PIC”, by Lucio Di

Jasio, Newnew, 2007.

Microchip MPLAB ICD3 and EXPLORER 16 KIT documentation.

“Operational Amplifiers with Linear Integrated Circuits”, Stanley, Prentice-Hall., 2002.

“Transducers: Theory & Application”, Allocca & Stuart, Reston Publishing,

"Microelectronic Circuits", Sedra and Smith, 5th edition, Oxford University Press, 2003.

“Applications of Analog Integrated Circuits”, Soclof, Prentice Hall., 1996

“ A User’s Guide to IC Instrumentation Amplifiers”, App. Note AN-244, & “Error Budget Analysis in IA

Applications” AN-539, www.analog.com

www.parallax.com website for MEMSic technical information and applications.

Appl.-Note-#007 “Inclination Sensing with Thermal Accelerometers” & Appl.-Note-#001 “Accelerometer

Fundamentals” : www.memsic.com/memsic/products/product.asp?product=56

“It’s All About Angles”, Column #92, www.nutsvolts.com.

“Pulse Operations with the 16 bit Micro Experimenter”, Nuts and Volts August 2010 issue Page 46.

“Electronic Angle Measurement”, Circuit Cellar, Issue 179, June 2005, www.circuitcellar.com.

Explorer 16 (1-4), Elektor Jan, Feb, Mar, Apr, 2007, www.elektor-electronics.co.uk.

MEMS2125 Accelerometer datasheets

“Pieces of the Puzzle” & “Measuring Gage Factor”, The Mechanics

www.vishay.com/brands/measurements_group/guide/notebook/e5/e5.html “Cantilever Bending Beam Load Cell” Specifications from Futek Advanced Sensor Technology Inc.,

www.futek.com

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Datasheets for the strain-gage, IA amplifiers, voltage reference, IPB Board devices in your ELE744 kits, are

available on ELE744 Course website.

5.0 SPECIFICATIONS:

Angle Force Angle & Force

Indicator

LEDs

Push-button

Switches

2-Line

LCD Display

(Momentary action type)

A N G L E = X X X . X ( D E G )

System Block

Diagram

Explorer 162 Line by 16 Character LCD

Proto-Card

LED A LED B LED C

Push Button

Switches

A to D IC1 IC2

Bridge &

Balancing

Circuit

Bridge &

Balancing

Circuit

Instr.

Amplifier

Circuit

Signal

Conditioning

& Level Shift

Circuit

MEMS

2125

Strain

Gage

Voltage

Ref

Circuit

Power Supply

Sources

+5V, +3.3V from

Explorer 16

+15V from Bench

Supply

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Functional: (Sequence of Operation)

1. On Power-Up, the “Angle” LED should light-up & flash indicating angle

measurement in progress. The LCD display should display Angle in “real-time”,

together with the “live’ bar-chart display corresponding to current angle:-

A N G L E = X X X . X ( D E G )

2. For Force selection, the user momentarily presses the “Force” Push-button to

light-up & flash the “Force” LED, indicating Force measurement. The LCD

display should display Force in “real-time”, together with the “live’ bar-chart

display corresponding to current applied force:-

F O R C E = X X X . X (N o r gms )

3. When the user presses the “Angle & Force” Push-button, only the “Force &

Angle” LED is turned on and flashing, with “real-time” display of the parameters

as shown:

F O R C E = X X X . X (N o r gms )

A N G L E = X X X . X ( D E G )

4. When the user presses the “Angle” Push-button, the cycle reverts to Step 1

above.

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Electrical/Mechanical:

o Force: Should cover range (in a contiguous manner) to maximum weight of 1 LB or 455 grams

(maximum beam deflection of 0.28mm). Minimum accuracy of +/- 2 %, and maximum drift of <

1%/min at room temperature.

o Angle: Using a separate test-jig, angle range should cover tilt of 0° to 180° (in a contiguous

manner) from the horizontal, with minimum resolution of 0.2°.

o “real-time” Update Rate: Update or refresh rate for all measured values ≤ 12.5 msecs.

o LED “Flashing” Rate: 0.5 second ON, 0.5 second OFF.

o Cantilever Beam: Test jig on each lab bench has the following parameters (refer to the Load Cell

datasheets from Futek for the L, W and T parameters):-

E = 1.48 x 1011

N/M2. (for the steel-alloy beam material)

defl. = 0.30 mm (maximum deflection of the Beam permitted)

Strain-Gages: = (pre-mounted) 17-4PH Stainless Steel; GF = 2.0; Ro = 1000Ω.

o Voltage Reference: The stable bridge excitation voltage should be set anywhere in the range of

5V to 10V d.c. using the 10V Voltage Reference IC provided in your Kit.

o Accelerometer: MEMSIC2125

Refer to Datasheets.

o Microcontroller: PIC24FJ128GA010

Refer to Datasheets and Lab Manual.

o Power Supply: +15V and +5V only.

o Code Requirements: All hardware function for the PIC24FJ128GA010 must use .h and .c files.

For example, ADC initialization and polling functions must be stored in adc.h and adc.c file.

Hardware functions that require interrupts are exempt.

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6.0 Suggested SYSTEM SPECIFICATION BLOCK DIAGRAM:

Specification Block

Diagram

Explorer 16

2 Line by 16 Character LCD

Push Button

Switches

Angle = xxx.x (DEG)

[][][][][][]------

S3

8 LEDS

D10

S6 S5 S4

D3

Angle Force Angle & Force

PIC24JF128GA010

7.0 Recommended DESIGN PROCESS:

Undertake a thorough background research and analysis work on the sensor

technologies to understand their operating principles and interfacing/conversion

requirements. Analyze and properly understand the functionality and

specifications of the Cantilever Beam project, and then develop a high-level

conceptual design the engineering solution. Understand and analyze the design of

the Explorer 16 board, and make notes on the functionality, role and interfacing

of all the logic devices around the PIC24FJ128GA010 microcontroller.

Identify the critical areas or challenges that would require special attention in the

design and development, and then develop a Project Plan to guide you through

timely implementation per the milestones given.

Fully test the Explorer 16 board and ICD 3 programming interface, see the

tutorials and test procedures included with the kit. Understand the architecture

resources of the PIC24FJ128GA010 (e.g. ICR, OCR, FRC, Timers, Interrupt

structure, I/O, A/D, Output timers, etc.) to formulate a proper software structure

solution for the instrumentation requirements.

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Develop a detailed high-level flow chart or pseudo-code for your software

structure. Keep in mind that while it is prudent to test the Strain-gage and

accelerometer sensors independently, your software structure should take into

account the requirement of concurrent measurements in “real-time”. Also, use of

C's built-in functions (e.g. multiply, divide, trig., etc.) can consume exorbitant

amount of computational resources, and so dedicated in-code algorithm and

table-look-up schemes need to be used to meet the “real-time” specifications. Design, implement and test the push-button switch and the LEDs, together with

your source-code to monitor the switch and drive the LEDs,

Develop the interface and algorithms for the Tilt-Angle measurement. Use the

test-jig provided in the Lab to test your design, and make the appropriate

measurements to validate the specifications.

From your design analysis, use either pSPICE or MultiSIM software package to

capture and simulate your designs for the various analog signal

processing/conditioning circuitry required to accurately measure the Force (and

stress) on the Cantilever beam. Familiarize yourselves with the finer details of the

cantilever beam test jig supplied in the Lab. Compare the simulated results with

your theoretical analysis & predictions. Generate and plot all the relevant signals.

Once the simulation results are verified, draw proper detailed schematics of your

design, and then implement the physical hardware and thoroughly test your

design. Record all measurements.

Once the Angle and Force (or Stress) functions are independently tested, your

source-code should be evolved to integrate both sets of measurements to realize

the full functionality of the instrumentation. Look for opportunities where your

code can be optimized for seamless execution of the functions. Make appropriate

measurements to your integrated design to validate compliance to the desired

specifications.

Prepare a formal technical report as per the guidelines provided in Section 10.0.

One formal report per Lab group is required..

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8.0 MILESTONES, DELIVERABLES & GRADING SCHEME:

Week #

Scheduled Lab

Week of:

Suggested Activities/Milestones/Deliverables

Grade

weight

1 September 3rd

Background research on various technologies, and review of the specification requirements.

Analyze ICD 3 and Explorer 16 hardware. Familiarize self with the PIC MPLAB IDE, see included tutorials, write a C program that uses

the on board LED's, Switch, and LCD to demonstrate effective use of development tools.

Solder headers to Proto Board.

2 September 10th

3 September 17th

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September 24th

MILESTONE #1

=>

Oral & demo to Lab Instr. during scheduled lab session.:- Fully-functioning Explorer 16 board and proto board populated with

headers .

Sample Test program. Understanding of Explorer 16 board hardware design and use of

MPLAB IDE and debugging techniques (e.g. breakpoints, trace,

single-stepping program, etc.)

Submit at least 3 pages on Explorer 16 board design analysis, and

list of references reviewed on sensor technologies.

20%

5 October 1st

Analysis, design and implementation of accelerometer H/W & S/W to display Angle as per

specifications. Obtain test results. Analysis, design and simulations of strain-gauge circuitry.

6 October 8th

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October 15th

MILESTONE #2

=>

Oral & demo to Lab Instr. during scheduled lab session:- Demonstrate ANGLE measurement function as per specification. Submit test results, algorithms and source-code listing.

15%

8 October 22th

Implement and test analog circuitry design for strain-gauge, and then interface to

microcontroller. Implement S/W to provide FORCE function per specifications. Continue to work on integrating FORCE & ANGLE functions per specifications.

Continue to work on preparing formal technical report.

9 October 29th

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November 5th

MILESTONE #3

=>

Oral & demo to Lab Instr. during scheduled lab session:- Demonstrate FORCE measurement functions as per specifications.

Submit test results, algorithms and source-code listing.

25%

11 November 12th

Early-birds can demonstrate final Milestone #4 during this week.

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November 19nd

MILESTONE #4

=>

Oral & demo to Lab Instr. during scheduled lab session:- Complete demonstration of all functions per specifications.

Seamless execution “in real-time” of all functions, and any enhancements implemented.

Final source-code version to be compiled during the demo. A printed

copy to be submitted to Lab Instructor. Show final schematic, algorithms and test results.

20%

13 November 26th

MILESTONE #5

=>

Formal technical report to be submitted to your lab instructor by

4.30 p.m. on Friday, November 30th .

Only one formal report per group is required.

Reports will be graded as per the marking scheme presented in

Section 9.0.

20%

TOTAL MARK => 100%

Note:

1. A soft copy of all milestone files must be submitted to the TA via email. This includes the

MPLAB IDE project files and a PDF copy of all reports.

2. Support for the MPLAB X IDE will not be provided by the TA and all demonstrations must be

performed using the lab computers. 3. Failure to submit and/or demonstrate the above deliverables as scheduled will result in an

automatic zero mark for each of the missed milestone. No exceptions.

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9.0 FORMAL REPORT MARKING SCHEME:

There is no mark given for directly copying materials from the course notes (or text

books) or the descriptions from the component data sheets. For simplicity, the grading of

the report is based on total unit of 100.

Format: Title, date, index, page number (2)

Proper labelling of results (3)

General neatness and ease of reading (5) ______/20

Technical writing (grammer, spelling, etc.) (10)

Routine Content: Abstract & Objective (5)

Specification Summary (5)

Accurate schematics (5)

Experimental Procedures (5)

Conclusions & Recommendations (10) _______/30

Creative Content: Concise description of operating principles (10)

Theory and design analysis of all circuits,

software algorithms and code optimization. (25)

Measurements & observations (15) _______/50

(tables, waveforms, graphs, etc.)

_______/100

NOTE:-

FINAL REPORT should be limited to 12 PAGES, not including

Appendix section . Marks will be deducted for exceeding this maximum

page limit.

10.0 Suggested REPORT WRITING GUIDELINES:

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o TITLE PAGE

This page includes:- the title of the report; authors’ names; for whom and when the report

was prepared; course name; and Statement of Declaration claiming ownership of the

report content, signed by both authors.

o ABSTRACT

An abstract is a short paragraph summarizing the report. One or two sentences for each of

the following items would be appropriate:

Purpose

Methods

Observations (figures-of-merit)

Conclusions & Recommendations

An abstract can be thought of as an executive summary. The Corporate Executive or your

Engineering Manager may just want to read a short paragraph and to be able to put the

report into context with other related materials without spending much time reading the

whole report. Thus, an abstract requires careful preparation and is the LAST item to be

written in a report. It should be independent and the rest of the report should be written

as if the abstract does not exist.

o OBJECTIVES

A short paragraph states the purposes of the study, and the technical specifications.

o INTRODUCTION

This page (or 2 to 3 paragraphs) explains the initiation of the study (product design), the

problem to be investigated, the approach or the method to be employed for the study.

o THEORY

Provide the theory on the principle of operations of all relevant sensor/transducer

technologies; and the foundational basis for any algorithm created. Develop all

theoretical formulations or explanations for the expected performance of the system

under investigation. Use figures and/or graphs where appropriate.

o DESIGN ANALYSIS

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The proposed system overview of the product should be provided and explained. Explain

the design methodology used, provide detail design analysis and explanations with proper

circuit schematics, algorithms, software structure, etc.

o EXPERIMENTAL PROCEDURE

Describe the methodology and detail the procedures for performing various tests and/or

experiments, for both the Simulations and actual hardware implementation.

o RESULTS & OBSERVATIONS

Record all data in several tables and/or plots. Photographs, oscilloscope tracings and

waveform drawings should be reported in this section for both the Simulations and

hardware designs.

o CONCLUSIONS & RECOMMENDATIONS

It is important to make a precise conclusion of the project based on the Observations.

List the major results together with short explanations and comments. Recommendations

are also necessary in a report. In essence, the reader needs to know from your conclusions

about hoe the project worked and what limitations may be encountered.

o REFERENCES & BIBLIOGRAPHY

List all reference materials in this section.

o APPENDIX

In this section you may want to include a list of component parts used, cover page of any

specialized component, source-code; and whatever else you feel would be useful to the

reader.