Non Destructive Examination Device

Final Year Design Project Report

Submitted by:

Ahmad Shamyl Akhlaq 2005010

Hamid Mahmood Pasha 2005076

Sheraz Ali Shah 2005904

Usama Imran Khan 2005283

Faculty of Engineering Sciences

Ghulam Ishaq Khan Institute of Engineering Sciences and Technology.

May 2009.

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Non Destructive Examination Device

Final Year Design Project Report

Submitted by:

Ahmed Shamyl Akhlaq 2005010

Hamid Mahmood Pasha 2005076

Sheraz Ali Shah 2005904

Usama Imran Khan 2005283

This Report is submitted in partial fulfillment for the degree of Bachelors of Science.

Dr. Rizwan Akram. Dr. Syed Ikram A Tirmizi

(Project Advisor) (Dean Faculty of Engineering Sciences)

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Dedicated to our families, teachers and friends.

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Acknowledgements

We will always remain deeply indebted to the efforts put in and support

provided to us by Dr. Rizwan Akram during the entire period of our FYP. He left

no stone unturned and it was his encouragement and brilliant leadership that

enabled us to complete the task to the fullest of our abilities. We would like to

thank the Mechanical and Metallurgical departmental staff (especially Mr. Afsar

Khan Sahib) for their help in constructing the major components of our project.

Last but not the least we wish to thank the Faculty and staff of the Engineering

Sciences for their full moral and technical support during the entire course of this

project.

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Abstract

In this project we have been able to construct a device which is capable of

determining the size and position of cracks and holes within a sample of

conducting material without destroying or damaging the specimen itself. We

started by constructing the prototype for the translation stage from a micrometer

screw gauge , which then evolved into a complete scanning stage, after detailed

analyses over its design and structure. We used stepper motors and their

respective controller boards for the desired micron level movement of the

specimen under the sensor. A hall probe sensor was used to measure the magnetic

field near gaps in cracks and holes. Data Acquisition was done using LabView

software by integrating it with a Lockin Amplifier and Source measuring Unit. A

2D (or 3D) graph can then be obtained from the sample readings determining the

size and place of the crack or hole.

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Table of Contents

Chapter 1 ....................................................................................................................................... 8

Section 1.1: Introduction .................................................................................................... 8

Section 1.2: Organization of the Report........................................................................ 8

Chapter 2 ....................................................................................................................................... 9

Introduction to Non destructive techniques .............................................................. 9

Section 2.1: Applications and Objectives of NDE ...................................................... 9

Section 2.2: Objectives of nondestructive testing methods ............................... 10

Section 2.3: Methods for NDE ........................................................................................ 12

Section 2.4: Methods Selected for Final Year Project. .......................................... 13

Section 2.4.1: Magnetic Flux Leakage ..................................................................... 13

Section 2.4.2: Current Induction .............................................................................. 16

Section 2.4.3: Eddy current ........................................................................................ 17

Section 2.4.4: Detection Method used in our FYP .................................................. 20

Chapter 3 .................................................................................................................................... 21

Experimental setup ........................................................................................................... 21

Section 3.1: Introduction ................................................................................................. 21

Section 3.2: Sensor types ................................................................................................. 22

Section 3.3: A Brief comparison of the Sensors ...................................................... 23

Section 3.4: Sensor Selection ......................................................................................... 23

Section 3.4.1: Hall Effect .................................................................................................. 24

Section 3.5: Scanning stage ............................................................................................ 25

Section 3.6: Motion Control Circuit ............................................................................. 27

Section 3.6.1: Stepper Motor Control ..................................................................... 28

Section 3.6.2: Parallel Port Interfacing .................................................................. 32

Section 3.6.3: Wiring Configuration ........................................................................ 32

Section 3.7: Electronically controlled ......................................................................... 34

Section 3.8: Lock-In Amplifiers Internal Workings ............................................... 35

Section 3.9: Source Measuring Unit (SMU) .............................................................. 36

Section 3.10: General Purpose Interface Bus (GPIB) ........................................... 36

Section 3.11: Software used for Data Acquisition ................................................. 37

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Chapter 4 ................................................................................................................................... 40

Results and Observations ................................................................................................... 40

Chapter 5 ................................................................................................................................... 41

Conclusion and References ................................................................................................ 41

Material Cost and Review ............................................................................................... 41

Future Scope and Conclusion ........................................................................................ 41

Appendix (A) ............................................................................................................................ 42

Atmel 89C051 Microcontroller ......................................................................................... 42

Appendix (B) ............................................................................................................................ 43

Bi-Polar Stepper Motor Control Circuit ......................................................................... 43

Pin Configuration and Block diagram of L-297/298. .......................................... 43

Appendix (C) ............................................................................................................................ 44

ULN-2003 .................................................................................................................................. 44

Appendix (D) ............................................................................................................................ 46

Parallel Port Configuration ................................................................................................. 46

Appendix (E) ............................................................................................................................ 47

Stepper Motor drivers .......................................................................................................... 47

Bibliography ............................................................................................................................. 57

References ............................................................................................................................. 57

Additional NDT Resources.............................................................................................. 59

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Table of Figures

Figure 2-1 Magnetic Flux Leakage 14

Figure 2-2 Direct Magnetization 15

Figure 2-3 Indirect Magnetization 15

Figure 2-4: Eddy Current Inspection Method 19

Figure 2-5 Diagram illustrating Eddy Currents created in a port 19

Figure 3-1 Complete setup of NDE 21

Figure 3-2 Hall Probe Sensor 24

Figure 3-3 Translation Stage (Top View) 26

Figure-3-4 Stepper Motor Control Unit 28

Figure 3-5 Unipolar Stepper Motor 29

Figure 3-6 bpolar stepper motor 31

Figure 3-7 circuit diagram for stepper motor controller boards 34

Figure-3-8 Lockin Amplifier (SR-830) 35

Figure 3-9 Source Measuring Unit- KE 236 36

Figure 4-1 Crack detection in the specimen 40

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Chapter 1

Section 1.1: Introduction

The main objective of this project was the construction of an NDE (Non-

Destructive Examination) Device which is a non invasive method for measuring

the size of holes and cracks inside a sample made of a conducting material.

Section 1.2: Organization of the Report

The project is constituted of the following stages which were then subdivided into

smaller units, namely:

1. Construction of the translation stage (which acts as a platform for the

sample),

§ Structural Design,

§ Material selection,

§ Control mechanism,

§ Electric circuitry and Instrumentation

2. Human Machine Interface(HMI) of the translation stage with the computer

through Parallel port,

§ Stepper motor controller mechanism and schematic diagram

§ Parallel port interface of the motors with the computer

§ development of the software in LabView

3. Formation of the Probe/Sensor and finally Data Acquisition.

4. Use of different invasive techniques in order to perform the actual

scanning of the sample for defects (holes and cracks)

5. Results, analyses and observation

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Chapter 2

Introduction to Non destructive techniques

By definition,

“Nondestructive techniques are the means by which materials and

structures may be inspected without disruption or impairment of serviceability. “

The method for the inspection of cracks and holes in a sample material is

known generally by the three names:

• Nondestructive testing (NDT), which is also called

• Nondestructive examination (NDE) and

• Nondestructive inspection (NDI),

NDE is vital for constructing and maintaining all types of components and

structures.

Section 2.1: Applications and Objectives of NDE

Many industrial components need regular non-destructive tests to detect

damage that may be difficult or expensive to find by everyday methods. Some of

the areas in the industry where the NDE is used are as follows:

§ Aircraft skins need regular checking to detect cracks;

§ Underground pipelines are subject to corrosion and stress corrosion

cracking;

§ Pipes in industrial plants may be subject to erosion and corrosion from the

products they carry;

§ Concrete structures may be weakened if the inner reinforcing steel is

corroded;

§ Pressure vessels may develop cracks in welds;

§ The wire ropes in suspension bridges are subject to weather, vibration, and

high loads, so testing for broken wires and other damage is important.

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Thousand of Manufactured products can benefit from this form of testing.

Section 2.2: Objectives of nondestructive testing methods

Objectives Attributes Measured or Detected

Discontinuities

Surface anomalies roughness, scratches, gouges, crazing, pitting, inclusions and

imbedded foreign material

Surface connected anomalies cracks, porosity, pinholes, laps, seams, folds, inclusions

Internal anomalies

cracks, separations, hot tears, cold shuts, shrinkage, voids, lack of fusion, pores, cavities, delaminations, disbonds, poor bonds, inclusions, segregations

Structure

Microstructure molecular structure, crystalline structure and/or strain, lattice structure, strain, dislocation, vacancy, deformation

Matrix structure grain structure, size, orientation and phase, sinter and porosity, impregnation, filler and/or reinforcement distribution, anisotropy, heterogeneity, segregation

Small structural anomalies

leaks (lack of seal or through-holes), poor fit, poor contact, loose parts, loose particles, foreign objects

Gross structural anomalies

assembly errors, misalignment, poor spacing or ordering, deformation, malformation, missing parts

Dimensions and metrology

Displacement, position

linear measurement, separation, gap size, discontinuity size, depth, location and orientation

Dimensional variations

unevenness, nonuniformity, eccentricity, shape and contour, size and mass variations

Thickness, density film, coating, layer, plating, wall and sheet thickness, density or

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thickness variations

Physical and mechanical properties

Electrical properties resistivity, conductivity, dielectric constant and dissipation factor

Magnetic properties

polarization, permeability, ferromagnetism, cohesive force

Thermal properties conductivity, thermal time constant and thermoelectric potential

Mechanical properties

compressive, shear and tensile strength (and moduli), Poisson's ratio, sonic velocity, hardness, temper and embrittlement

Surface properties color, reflectivity, refraction index, emissivity

Chemical composition and analysis

Elemental analysis detection, identification, distribution and/or profile

Impurity concentrations contamination, depletion, doping and diffusants

Metallurgical content variation, alloy identification, verification and sorting

Physiochemical state

moisture content, degree of cure, ion concentrations and corrosion, reaction products

Stress and dynamic response

Stress, strain, fatigue

heat-treatment, annealing and cold-work effects, residual stress and strain, fatigue damage and life (residual)

Mechanical damage wear, spalling, erosion, friction effects

Chemical damage corrosion, stress corrosion, phase transformation

Other damage radiation damage and high frequency voltage breakdown

Dynamic performance

crack initiation and propagation, plastic deformation, creep, excessive motion, vibration, damping, timing of events, any

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anomalous behavior

Signature analysis

Electromagnetic field potential, strength, field distribution and pattern

Thermal field isotherms, heat contours, temperatures, heat flow, temperature distribution, heat leaks, hot spots

Acoustic signature noise, vibration characteristics, frequency amplitude, harmonic spectrum and/or analysis, sonic and/or ultrasonic emissions

Radioactive signature

distribution and diffusion of isotopes and tracers

Signal or image analysis

Image enhancement and quantization, pattern recognition, densitometry, signal classification, separation and correlation, discontinuity identification.

Section 2.3: Methods for NDE

The categories for Nondestructive testing methods are given below:

Basic Categories Objectives

Mechanical and optical

Color, cracks, dimensions, film thickness, gauging, reflectivity, strain distribution and magnitude, surface finish, surface flaws, through-cracks

Penetrating radiation

Cracks, density and chemistry variations, elemental distribution, foreign objects, inclusions, micro porosity, misalignment, missing parts, segregation, service degradation, shrinkage, thickness, voids

Electromagnetic and electronic

Alloy content, anisotropy, cavities, cold work, local strain, hardness, composition, contamination, corrosion, cracks, crack depth, crystal structure, electrical and thermal conductivities, flakes, heat treatment, hot tears, inclusions, ion concentrations, laps, lattice strain, layer thickness, moisture content, polarization, seams, segregation, shrinkage, state of cure, tensile strength, thickness, disbonds

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Sonic and ultrasonic

Crack initiation and propagation, cracks, voids, damping factor, degree of cure, degree of impregnation, degree of sintering, delaminations, density, dimensions, elastic moduli, grain size, inclusions, mechanical degradation, misalignment, porosity, radiation degradation, structure of composites, surface stress, tensile, shear and compressive strength, disbonds, wear

Thermal and infrared

Bonding, composition, emissivity, heat contours, plating thickness, porosity, reflectivity, stress, thermal conductivity, thickness, voids

Chemical and analytical

Alloy identification, composition, cracks, elemental analysis and distribution, grain size, inclusions, macrostructure, porosity, segregation, surface anomalies

Auxiliary Categories Objectives

Image generation

Dimensional variations, dynamic performance, anomaly characterization and definition, anomaly distribution, anomaly propagation, magnetic field configurations

Signal image analysis

Data selection, processing and display, anomaly mapping, correlation and identification, image enhancement, separation of multiple variables, signature analysis

Section 2.4: Methods Selected for Final Year Project.

Out of the many methods listed in section2.3, we have chosen three methods that

reside in the Electromagnetic and Electric category. These three methods have

been considered profusely in our Final Year Project. These methods are listed

below and are going to be explained in the sequence mentioned below:

1. Magnetic Flux Leakage technique,

2. Current Injection method and

3. Eddy Current Technique

Section 2.4.1: Magnetic Flux Leakage

Magnetic flux leakage (MFL) techniques for non-destructive testing

of ferromagnetic materials use strong permanent magnets to magnetize the

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object of interest to near saturation flux density. Defects such as corrosion or

erosion damage result in magnetic flux leakage. The flux leakage is detected

by magnetic field sensors and is proportional to the volume of metal loss.

MFL is usually regarded as a qualitative technique, although some estimates

of defect size can be made. Thus, MFL is largely a screening tool which can

be followed by ultrasonic inspection for determination of defect size.

The magnetic lines of force (flux) much prefer to travel in the carbon steel

plate than in the surrounding air. In fact this flux is very reluctant to travel in air

unless it is forced to do so by the lack of another suitable medium. For the

purposes of this particular application a magnetic bridge (or magnetic yolk) is

used to introduce as near a saturation of flux as is possible in the inspection

material between the poles of the bridge. Any significant reduction in the

thickness of the plate will result in some of the magnetic flux being forced into the

air around the area of reduction. Sensors which can detect these flux leakages are

placed between the poles of the bridge. This is graphically illustrated in the figure

below:

Figure 2-1 Magnetic Flux Leakage

Magnetic Flux Leakage (MFL) is used to detect corrosion and pitting in

steel structures, most commonly pipelines and storage tanks. MFL detects changes

volumetrically. The disadvantage of Magnetic Flux Leakage is that no absolute

values but relative volumetrically changes are reported. However it is a very

suitable tool for detecting bad spots in the plates.

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In MFL technique, there are two basic types of Magnetization for ferromagnetic materials:

• Direct Magnetization (Magnetization using direct induction) • Indirect Magnetization (Magnetization using direct induction)

Direct Magnetization

In this type of magnetization we pass the current through the object. In this

way, a circular magnetic field will be created. When using direct magnetization, it

is very important to provide good contact between the test equipment and the test

component.

FIGURE 2-2 Direct Magnetization

Indirect magnetization

It is accomplished by using very strong external magnetic field to establish

magnetic field within the object (figure 6). External magnetic field can be

created using a permanent magnet or using a electro-magnet. Permanent magnet is

used rarely because it is very difficult to create field that will be strong enough

and it is very difficult to control it.

Figure 2-3 Indirect Magnetization

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Section 2.4.2: Current Induction

The current Injection method is the method used in our FYP for the detection of flaws in the sample. It involves the detection of a flaw by injecting current into the sample and then detecting the different current paths created outside the flaw. These lines of current are then scanned by the sensor placed above. The diagrammatical representation of the current injection setup is shown in the figure below:

FIGURE 2-4 Current Injection Method

A thin sheet with a small hole drilled through the centre was injected with

a uniform current. The hole perturbed the current flow and caused an aberration in

the normal component of the magnetic field Bz which was imaged by repeated

scans of the sheet beneath the sensor.

Figure 2-5 Flaw (shown in (a)) is detected and then represented in graphs both 3D (b) and 2D (c) with respect to magnetic field current in the sample

The plates are electrically connected in series at one end. At the opposite

end, a coaxial cable is used to inject current into one plate and retrieve it from the

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other plate. With this arrangement it was shown that the magnetic field due to the

edges of the sample plate are essentially cancelled and the interfering signal due to

the current cables is remarkably reduced.

Figure 2-6 Current Induction method using Cancellation Plate technique

Section 2.4.3: Eddy current

Eddy current testing is used to find surface and near surface defects in

conductive materials. It is used by the aviation industry for detection of defects

such as cracks, corrosion damage, thickness verification, and for materials

characterization such as metal sorting and heat treatment verification.

Applications range from fuselage and structural inspection, engines, landing gear,

and wheels. Eddy current inspection involves initial setup and calibration

procedures with known reference standards of the same material as the part.

Probes of appropriate design and frequency must be used.

Eddy current inspection is based on the principle of electromagnetic

induction. An electric coil in which an alternating current is flowing is placed

adjacent to the part. Since the method is based on induction of electromagnetic

fields, electrical contact is not required.

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Figure 2-4: Eddy Current Inspection Method

a) The Eddy Current flowing through the coil at a chosen frequency

generates a magnetic field around the coil.

b) When the coil is placed close to an electrically conducting material, eddy

current is induced in the material.

c) If a flaw in the conducting material disturbs he eddy current circulation,

the magnetic coupling with the probe is changed and a defect signal can be

read by measuring the coil impedance variation.

An alternating current flowing through the coil produces a primary

magnetic field that induces eddy currents in the part. Energy is needed to generate

the eddy currents, and this energy shows up as resistance losses in the coil.

Typical NDE applications are designed to measure these resistance losses. Eddy

currents flow within closed loops in the part.

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Figure 2-5 Diagram illustrating Eddy Currents created in a port

As a result of eddy currents, a second magnetic field is generated in the

material. The magnetic fields of the core interact with those in the part and

changes in the material being inspected affect the interaction of the magnetic

fields.

The interaction, in turn, affects the electrical characteristics of the coil.

Resistance and inductive reactance add up to the total impedance of the coil.

Changes in the electrical impedance of the coil are measured by commercial eddy

current instruments. The main method used in eddy current inspection is one in

which the response of the sensor depends on conductivity and permeability of the

test material and the frequency selected.

Advantages of Eddy Current Inspection (ECI)

• Sensitive to small cracks and other defects

• Detects surface and near surface defects

• Inspection gives immediate results

• Equipment is very portable

• Method can be used for much more than flaw detection

• Minimum part preparation is required

• Test probe does not need to contact the part

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• Inspects complex shapes and sizes of conductive materials

Limitations of Eddy Current Inspection:

Ø Only conductive materials can be inspected

Ø Surface must be accessible to the probe

Ø Skill and training required is more extensive than other techniques

Ø Surface finish and roughness may interfere

Ø Reference standards needed for setup

Ø Depth of penetration is limited

Ø Flaws such as delaminations that lie parallel to the probe coil winding

and probe scan direction are undetectable

Section 2.4.4: Detection Method used in our FYP

In our Project, the method used was of the current induction method. The

reason we preferred to use this specific technique instead of the other three

aforementioned techniques are the following:

1) It is easy to use and has relatively easy installation of setup

2) The sensor probe requirements or the construction of the setup are few as

compared to the other techniques involved;

3) The flaw (crack or hole) can be viewed with a greater resolution. 4) The change in the frequencies can be used to analyze the object ant do

depth profilation of the flaw.

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Chapter 3

Experimental setup

Section 3.1: Introduction

The objective of the Final Year Project was to construct a device that

could detect the flaws in a specimen using the Electric and Electromagnetic

method of Non Destructive Examination i.e. Current Injection technique. The

Overall setup of the whole FYP is given below:

FIGURE 3-1 Complete setup of NDE

The figure shows the whole setup of the FYP. The numbered parts are:

1) Hall probe Sensor: detects the cracks or holes in the specimen;

2) Transalation Stage: This stage works as the base for the whole FYP. It

serves as the scanning stage that performs the functions of moving or

positioning the specimen below the probe or sensor.

3) Motor Control Mechanism: The box contains three stepper motor control

boards stacked one on top of the other.(one for each in the XYZ directions)

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Section 3.2: Sensor types

There are basically two types of sensors currently in use. Coils, Hall

Effect Sensors and Squid sensors. They are both capable of detecting the flux

leakage fields caused by corrosion on the sample. There is a fundamental

difference, however, in the way that they respond to leakage fields and generate a

response.

a) Coils

Coils are passive devices and follow Faradays Law in the presence of a

magnetic field. As a coil is passed through a magnetic field a voltage is generated

in the coil and the level of this voltage is dependent on the number of turns in the

coil and the rate of change of the flux leakage. From this it can be seen that speed

will have some influence on the signals obtained from this type of sensor.

b) Hall Effect Sensors

Hall Effect sensors are solid state devices which form part of an electrical

circuit and, when passed through a magnetic field, the value of the voltage in the

circuit varies dependent on the absolute value of the flux density. It is necessary to

carry out some cross referencing and canceling with this type of sensor so that true

signals can be separated from other causes of large variations in voltage levels

generated by the inspection process.

c) SQUID Sensors

Superconducting Quantum Interference Devices (SQUIDs) are the most

sensitive magnetic field sensors known to date. With the discovery of High

Temperature Superconductors (HTS) ten years ago and the subsequent

development of HTS SQUIDs requiring only cooling down to liquid nitrogen

temperature, the greatest application barrier appears solvable. SQUID systems

offer a high sensitivity at low excitation frequencies, permitting the detection of

deeper flaws, and a high linearity, allowing quantitative evaluation of magnetic

field maps from the investigated structure

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Section 3.3: A Brief comparison of the Sensors

Section 3.4: Sensor Selection

For the purpose of our FYP the Hall Effect sensor was used. A Hall Effect

sensor is a transducer that varies its output voltage in response to changes in

magnetic field. Hall sensors are used for proximity switching, positioning, speed

detection, and current sensing applications. In its simplest form, the sensor

operates as an analogue transducer, directly returning a voltage. With a known

magnetic field, its distance from the Hall plate can be determined. Electricity

carried through a conductor will produce a magnetic field that varies with current,

and a Hall sensor can be used to measure the current without interrupting the

circuit. The sensor in our case is integrated with a wound core or permanent

magnet that surrounds the conductor to be measured.

Properties SQUID sensor Hall Probe Coils

1) Sensitivity Highest High Lowest

2) Spatial Resolution

Medium Lowest

Resolution Highest

Environmental factors

Needs special working temperature (normally below the critical temperature of the material)

Works in normal conditions ( at room temperatures)

Works in normal conditions

Size Larger Size as compared to the other two

Smallest size Comparatively is of smaller size

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FIGURE 3-2 Hall Probe Sensor

Section 3.4.1: Hall Effect

The Hall Effect comes about due to the nature of the current in a

conductor. Current consists of the movement of many small charge carriers,

typically electrons, holes, or both. Moving charges experience a force, called the

Lorentz Force, when a magnetic field is present that is not parallel to their motion.

When such a magnetic field is absent, the charges follow an approximately

straight, 'line of sight' path.

Figure 3-2.1 Hall Effect

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However, when a perpendicular magnetic field is applied, their path is

curved so that moving charges accumulate on one face of the material. This leaves

equal and opposite charges exposed on the other face, where there is a scarcity of

mobile charges. The result is an asymmetric distribution of charge density across

the Hall element that is perpendicular to both the 'line of sight' path and the

applied magnetic field. The separation of charge establishes an electric field that

opposes the migration of further charge, so a steady electrical potential builds up

for as long as the charge is flowing.

For a simple metal where there is only one type of charge carrier (electrons) the

Hall voltage VH is given by

Where I is the current across the plate length, B is the magnetic flux density, d or t

is the depth of the plate, e is the electron charge, and n is the charge carrier density

of the carrier electrons.

The Hall coefficient is defined as

Where j is the current density of the carrier electrons. In SI units, this becomes

As a result, the Hall Effect is very useful as a means to measure either the carrier

density or the magnetic field.

Section 3.5: Scanning stage

The basic idea behind the formation of the translation stage was to

construct a platform on which the specimen could be placed and then through the

use of actuators and computer controlled motors it (i.e. the specimen) could be

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positioned or moved under the sensor probe for the purpose of measurement of

cracks and discontinuities. We have ensured the stability of the scanning stage by

introducing two stainless rods that provided stable working of the Translation

Stage by minimizing the vibrations caused due to the working motor. Lead screw

were also introduced, which further contributed to the stability (acetone is used to

clean the screw, so as to remove any load produced due to friction or drag). Plastic

was used to for the outer body as it is both light weight and non magnetic. Bearing

was used for the smooth working of the micrometer screw in the translation stage,

improving stability and reducing friction. Interior Walls (also made of Plastic)

ware introduced in between to provide a more stable structure.

In the first translation stage we used a micrometer to control the precision

motion with manual motion control. The step size was calculated and fixed to

0.5mm for one complete revolution (i.e. for 1/100th movement on the radial scale

there is a linear displacement of 0.01mm).

Figure 3-3 Translation Stage (Top View)

The labeled pats in the above figure are described below:

1) Moving platform: The specimen is to be placed on top of this in such a

way that it does not fall off once the platform starts to move along the lead

screw.

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2) Stainless steel rods: These non magnetic rods provide stability to the

whole structure of the translation stage as well as restrict the vibrational

movements to a desirable minimum.

3) Lead Screw: It forms the basis for the graded movement of the specimen.

Its pitch and angle of screw is equal to that of a micrometer screw gauge.

Lead was chosen to be the material for its construction due to its non-

magnetic nature.

4) Stepper Motors: These are responsible for the movement of the platform

along the lead screw. The type of stepper motors used are Bipolar 6 wire

stepper motors

5) Parallel port Connection: The interface of the DB 25 to the 8 pin ports of

the stepper motor controllers (including the colour coding as well) is

illustrated in section 3.6.3.

The translation stages were constructed in such a way that they were placed (or

stacked on top of one another), in such a way that one is placed directly above the

centre of the other

Section 3.6: Motion Control Circuit

The translation or motion of the scannng stage is controlled through the

use of Bipolar stepper motors. The stepper motor controller boards used for the

motion control of the scanning stage use inverted logic. Such three stepper motor

controller boards were stacked on top of each pther and then enclosed in a plastic

box, each providing motion control for a single translation in each of the three

axis respectively i..e. X, Y and Z.

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FIGURE-3-4 Stepper Motor Control Unit

Section 3.6.1: Stepper Motor Control

Stepping motors come in two varieties, permanent magnet and variable

reluctance. Permanent magnet motors tend to "cog" when someone tries to twist

the rotor with their fingers, while variable reluctance motors almost spin freely.

An ohmmeter can also be used to distinguish between the two varieties. Variable

reluctance motors usually have three (sometimes four) windings, with a common

return, while permanent magnet motors usually have two independent windings,

with or without center taps. Center-tapped windings are used in unipolar

permanent magnet motors.

Stepping motors come in a wide range of angular resolution. The coarsest

motors typically turn 90 degrees per step, while high resolution permanent magnet

motors are commonly able to handle 1.8 or even 0.72 degrees per step. The

stepper motor used in our FYP has the step size of 200 steps, thus producing 1.8

degrees per step. With an appropriate controller, most permanent magnet and

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hybrid motors can be run in half-steps, and some controllers can handle smaller

fractional steps or microsteps.

a) Unipolar Motors

Unipolar stepping motors, both Permanent magnet and hybrid stepping motors

with 5 or 6 wires are usually wired as shown in the schematic in Figure 8, with a

center tap on each of two windings. In use, the center taps of the windings are

typically wired to the positive supply, and the two ends of each winding are

alternately grounded to reverse the direction of the field provided by that winding.

FIGURE 3-5 Unipolar Stepper Motor

Unipolar stepping motors, both Permanent magnet and hybrid stepping

motors with 5 or 6 wires are usually wired as shown in the schematic in Figure 8,

with a center tap on each of two windings. In use, the center taps of the windings

are typically wired to the positive supply, and the two ends of each winding are

alternately grounded to reverse the direction of the field provided by that winding.

The 30 degree per step motor in the figure is one of the most common

permanent magnet motor designs, although 15 and 7.5 degree per step motors are

widely available. Permanent magnet motors with resolutions as good as 1.8

degrees per step are made, and hybrid motors are routinely built with 3.6 and 1.8

degrees per step, with resolutions as fine as 0.72 degrees per step available.

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To rotate the motor continuously, we had to apply power to the two

windings in sequence. Assuming positive logic, where a 1 means turning on the

current through a motor winding, the following two control sequences will spin

the motor illustrated in Figure 8 clockwise 24 steps or 4 revolutions:

Winding 1a 1000 1000 1000 1000 1000 1000 1

Winding 1b 0010 0010 0010 0010 0010 0010 0

Winding 2a 0100 0100 0100 0100 0100 0100 0

Winding 2b 0001 0001 0001 0001 0001 0001 0

Time à

Winding 1a 1100 1100 1100 1100 1100 1100 1

Winding 1b 0011 0011 0011 0011 0011 0011 0

Winding 2a 0110 0110 0110 0110 0110 0110 0

Winding 2b 1001 1001 1001 1001 1001 1001 1

Time à

Note that the two halves of each winding are never energized at the same

time. Both sequences shown above will rotate a permanent magnet one

step at a time.

§ The top sequence only powers one winding at a time, as illustrated in the

figure above; thus, it uses less power.

§ The bottom sequence involves powering two windings at a time and

generally produces a torque about 1.4 times greater than the top sequence

while using twice as much power.

a) Bipolar Motors

Bipolar permanent magnet and hybrid motors are constructed with exactly the

same mechanism as is used on unipolar motors, but the two windings are wired

31

more simply, with no center taps. Thus, the motor itself is simpler but the drive

circuitry needed to reverse the polarity of each pair of motor poles is more

complex. The schematic in Figure 9 shows how such a motor is wired.

FIGURE 3-6 BPOLAR STEPPER MOTOR

The drive circuitry for such a motor requires an H-bridge control circuit

for each winding. an H-bridge allows the polarity of the power applied to each end

of each winding to be controlled independently. The control sequences for single

stepping such a motor are shown below, using + and - symbols to indicate the

polarity of the power applied to each motor terminal:

Terminal 1a +---+---+---+--- ++--++--++--++--

Terminal 1b --+---+---+---+- --++--++--++--++

Terminal 2a -+---+---+---+-- -++--++--++--++-

Terminal 2b ---+---+---+---+ +--++--++--++--+

Time à

Note that these sequences are identical to those for a unipolar permanent

magnet motor, at an abstract level, and that above the level of the H-bridge power

switching electronics, the control systems for the two types of motor can be

identical.

32

Note that many full H-bridge driver chips have one control input to enable the

output and another to control the direction. Given two such bridge chips, one per

winding, the following control sequences will spin the motor identically to the

control sequences given above:

Enable 1 1010 1010 1010 1010 1111 1111 1111 1111

Direction 1 1x0x 1x0x 1x0x 1x0x 1100 1100 1100 1100

Enable 2 0101 0101 0101 0101 1111 1111 1111 1111

Direction 2 x1x0 x1x0 x1x0 x1x0 0110 0110 0110 0110

Time à

To distinguish a bipolar permanent magnet motor from other 4 wire

motors, measure the resistances between the different terminals. It is worth noting

that some permanent magnet stepping motors have 4 independent windings,

organized as two sets of two. Within each set, if the two windings are wired in

series, the result can be used as a high voltage bipolar motor. If they are wired in

parallel, the result can be used as a low voltage bipolar motor. If they are wired in

series with a center tap, the result can be used as a low voltage unipolar motor.

Section 3.6.2: Parallel Port Interfacing

The Parallel Port is the most commonly used port for interfacing

homemade projects. This port will allow the input of up to 9 bits or the output of

12 bits at any one given time, thus requiring minimal external circuitry to

implement many simpler tasks. The port is composed of 4 control lines, 5 status

lines and 8 data lines. It's found commonly on the back of your PC as a D-Type 25

Pin female connector. There may also be a D-Type 25 pin male connector. This

will be a serial RS-232 port and thus, is a totally incompatible port.

Section 3.6.3: Wiring Configuration

The wiring configuration for the control of each motor through the motor controller boards is given below:

33

S. no Connector 1 (Y-Axis)

Connector 2 (X-Axis)

Connector 3 (Z-Axis)

Connector 4 (Power Supply)

Connection of 4 to the DB 25

1 PURPLE GREEN LIGHT

RED WHITE D0

2 YELLOW- GREEN

YELLOW LIGHT

GREEN GREEN D1

3 BLACK ORANGE GREEN LIGHT

ORANGE D2

4 BLUE RED WHITE

GREY MAROON D3

5 PINK WHITE BLACK WHITE

DARK BLUE

D4

6 YELLOW BLUE BROWN LIGHT BLUE

D5

7 WHITE BLACK

PINK BLUE RED Power (Adopter)

8 WHITE YELLOW YELLOW BLACK Ground (Adopter)

34

Section 3.7: Electronically controlled

The Internal Circuit diagram of the Stepper motor controller boards is given in the figure below.

FIGURE 3-7 circuit diagram for stepper motor controller boards

The circuit above shows the wiring configuration of the motor control mechanism.

The main features of the circuitry are as follows:

• Instead of giving a positive voltage (around 15 Volts), we have used an

inverted logic which prevents any damage to the wiring inside the motor. This

means that a ground input urns the motors to an ON state.

• A Voltage regulator is used to maintain a controlled voltage to the whole of

the circuit .Thus compensating for the need for a stabilizer.

• Speed of the motors can also be controlled manually

• The switches in the diagram allow the forward and reverse (clockwise or

anticlockwise rotation) of the motors.

35

• The circuit is constructed in such a way that it can work under both AC and

DC power supplies.

• The 8051 Microcontroller is used to control the Unipolar and Bipolar control

of the motor.

{For further information see references A to E}

Section 3.8: Lock-In Amplifiers Internal Workings

A lock-in amplifier (also known as a phase-sensitive detector) is a type of

amplifier that can extract a signal with a known carrier wave from extremely

noisy environment (S/N ratio can be as low as -60 dB or even less). Lock in

Amplifier was also used to generate the current inside the specimen. The response

of which has been detected from the Hall effect sensor’s signal by measureing the

the amplitude and phase of signal buried in noise.

Figure-3-8 Lockin Amplifier (SR-830)

In this project the FYP model no. “SR-830” lock in amplifier has been

used. The specifications of the lock in Amplifier are as follows:

Ø 1 MHz to 102.4 kHz frequency range

Ø >100 dB dynamic reserve

Ø 5 ppm/°C stability

Ø 0.01 degree phase resolution

Ø Time constants from 10 µs to 30 ks (up to 24 dB/oct rolloff)

Ø Auto-gain, -phase, -reserve and -offset

Ø Synthesized reference source

Ø GPIB and RS-232 interfaces

36

Section 3.9: Source Measuring Unit (SMU)

The SMU is a special kind of instrument that can work as a constant current

source or as a constant voltage source. It simultaneously sources to a pair of

terminals at the same time measuring the current or voltage across those terminals.

Typically when an SMU sources constant voltage it measures current through the

terminals. When it sources constant current through the terminals, it measures the

voltage built up across those terminals.

SMU used in our FYP has a model name KE 236 and it is interfaced with a

GPIB in order to connect it to a computer.

Figure 3-9 Source Measuring Unit- KE 236

Section 3.10: General Purpose Interface Bus (GPIB)

The GPIB is also known as Hewlett Packard Instrument Bus (HPIB). Its

standard name is IEEE-488. It allows up to 15 devices to share a single 8-bit

parallel electrical bus by daisy chaining. The speed of transaction is determined by

the slowest device that participates in control and data transfer handshakes.

There are three types of devices that can be connected to the IEEE-488

bus:

§ Listener

§ Talker and

§ Controller

In our FYP we have used the IEE-488 as a controller bus.

37

Figure 3-10 Arrangement of the Lock In Amplifier and the Source Measuring Unit (SMU) with the specimen sample

The main purpose of the source measuring unit in our FYP was to measure

the Voltage and Current. The SMU is used to bias the Hall Effect sensor onto a

fixed bias. Another reason for using the Source measuring Unit is that it is used to

read the Hall Voltage which is directly proportional to the applied field or the field

test.

In our FYP, the current supplied was 10mA and the frequency of the

current was 10 MHz. The data from the source measuring Unit was sent to the

computer software made in LabView which processed the information and plotted

its graph as shown in Chapter 4.

Section 3.11: Software used for Data Acquisition The program that we used to control the translation stage was made in the

software called LabView. The reasons for giving preference to using LabView

instead of any other software (like MatLab and Simulink) were its following

features:

• Labview and other NI software systems and hardware are very

well designed for discrete automation and sequential motion applications.

• Labview is better for control, MatLab is better for data manipulation.

• Labview is essentially a tool for instrumentation control.

38

• Labview is a better tool for Data Acquisition

Figure 3-11 Front panel of the software made in LabView

We have designed the Labview software ourselves in such a

way that it considers all the following aspects that need to be

considered while doing NDE of a material specimen. These are listed

below:

Mode:

We have an option for selecting between line scan and area scan. The

mode for the Line Scan shows the change in Amplitude with respect to time as it

detects the hole or cracks. Selecting the Area Scan mode we can perform scan of a

39

specific area of the specimen. (The maximum area that can be scanned is 8 x 8

cm2).

Sensitivity:

The sensitivity is the ability to respond to physical stimuli or to register

small physical amounts or differences. The greater the value of sensitivity the

greater is the

Hall Current and Voltage:

The reading of the Hall Voltage is obtained from the Lock In Amplifier.

No. of Averages:

After a particular time constant the measuring unit measures the value of

the Hall Voltage and Current. Then the average is calculated, the number of times

specified by the user i.e. this number specifies the quantity if reading that are to be

measured the then averaged to get a more precise reading.

Data File:

After running the program for one clock pulse, it saves data read from the

Source measuring unit and the Lockin Amplifier. The value of Hall Voltage and

Hall Current measured by these devices is recorded into a file whose path is

specified by the user at the start of the program.

40

Chapter 4 Results and Observations

The above graph in indiicates the size and position of the crack in a

specimen material. It is clear that from the reference point of ‘0’ in the graph,

there lie two holes: one ranging from ‘-2’ to ‘-4’ (on the x-axis) and the other from

the position ‘1.5’ to ‘4’ (again, on the x-axis).

Figure 4-1 Crack detection in the specimen

This means that through a hole or a crack, the Hall Voltage is very low,

since the lines of flux prefer to flow through the conducting material than through

an air gap.

The specimen used in the above experiment was shaped in such a way that it

contained two holes of 2mm diameter separated by 2mm, as shown in the figure

below:

Figure 4-2 An illustration of the specimen showing two holes of 2mm each spaced 2mm apart

41

Chapter 5 Conclusion and References

In our FYP we have constructed a Non Destructive Examination device

that is capable of measuring the size of flaws (cracks and holes) with a precision

of less than 3 micron scale size (3 x 10-6 m). The device uses Current injection

technique according to which current is injected into the sample piece using a

Lock in Amplifier, which is used to ‘lock’ a certain value of frequency injected

into the sample and then the Hall Probe Sensor is used in the detection of the Hall

Voltage and the reading are detected and the Source Measuring unit is used to

generate a voltage reading to the input current. The graph is then plotted of the

Hall Voltage reading with respect to the distance moved over the sample.

Material Cost and Review

The translation stages are an expensive equipment to buy. Their prices

vary according to their resolution. The smaller the resolution, the greater will be

the price. As far as our project was concerned, the price of a translation stage with

micron level precision is around Rs. 450,000/=, which is quite high. We managed

to prepare the translation stage exceeding just a little over the budget and still

managed to produce the translation stage costing a relatively small amount (i.e.

about 5,000).

Future Scope and Conclusion

The project offers a wide scope for further studies and development in the

design and development of the setup. This can be achieved by introducing another

translation stage that could move in the Z direction, thus making the scanning

stage ca[able of obtaining a 3D image of the specimen material.

42

Appendix (A) Atmel 89C051 Microcontroller

43

Appendix (B) Bi-Polar Stepper Motor Control Circuit

Pin Configuration and Block diagram of L-297/298.

44

Appendix (C) ULN-2003

45

46

Appendix (D) Parallel Port Configuration

Using Parallel ports to 8 input bits DB25

47

Appendix (E) Stepper Motor drivers

48

I. Stepper Motor Drive Modes

II. Stepper Motor Driver Configurations

49

III. Stepper Motor Switching Sequence a) Normal 4 Step Sequence

b) ½ Step – 8 Step Sequence

IV. Method used for Controlling Motors Via Parallel Port

In this section the operation principle of a Bipolar stepper motor is explained

which we implemented in our project.

50

BIPOLAR STEPPER MOTER

In the Bipolar stepper motor, a permanent magnet is used for rotor and coils

are put on stator. The stepper motor model which has 4-poles is shown in the

figure above. In case of this motor, step angle of the rotor is 90 degrees.

As for four poles, the top and the bottom and either side are a pair. coil,

coil and coil, coil corresponds respectively. For example, coil and coil

are put to the upper and lower pole. coil and coil are rolled up for the direction

of the pole to become opposite when applying an electric current to the coil and

applying an electric current to the coil. It is similar about and , too. The turn

of the motor is controlled by the electric current which pours into , , and .

The rotor rotational speed and the direction of the turn can be controlled by this

control.

51

A) Clockwise control

B) Counterclockwise control

, , and are controlled in

the following order.

Step angle

0 1 0 1 0°

0 1 1 0 -90°

1 0 1 0 -180°

1 0 0 1 -270°

, , and are controlled in the following order.

Step angle

0 1 0 1 0°

1 0 0 1 90°

1 0 1 0 180°

0 1 1 0 270°

CLOCK WISE CONTROL

COUNTERCLOCKWISE CONTROL

52

"0" means grounding.

From figure, it can be found that the rotor is stable in the middle of 2 poles of

stator. When one side of the stator polarity is changed, the bounce by the magnetism

occurs. As a result, the direction of rotor's turn is fixed. The characteristic of stepper

motor is the angle can be correctly controlled and to be stable rotates ( It is due to

the reliability of the control signal ). Moreover, because the rotor is fixed by the

magnetism in the stationary condition as shown in the principle, the stationary

power(Stationary torque) is large. It suits the use to make stop at some angle.

The motor which we used in our project is of 200 steps and the step angle is 1.8

degrees. The way of controlling is the same as the previous example completely. It

operates when controlling the electric current of coil, coil, coil and coil.

The case of the clockwise control is shown below. The combination of , , and

repeats four patterns.

Step angle

0 1 0 1 0

1 0 0 1 1.8

1 0 1 0 3.6

0 1 1 0 5.4

0 1 0 1 7.2

1 0 0 1 9

1 0 1 0 10.8

0 1 1 0 12.6

0 1 0 1 14.4

1 0 0 1 16.2

1 0 1 0 18

0 1 1 0 19.8

0 1 0 1 21.6

1 0 0 1 23.4

1 0 1 0 25.2

Step angle

0 1 0 1 43.2

1 0 0 1 45

1 0 1 0 46.8

0 1 1 0 48.6

0 1 0 1 50.4

1 0 0 1 52.2

1 0 1 0 54

0 1 1 0 55.8

0 1 0 1 57.6

1 0 0 1 59.4

1 0 1 0 61.2

0 1 1 0 63

0 1 0 1 64.8

1 0 0 1 66.6

1 0 1 0 68.4

53

0 1 1 0 27

0 1 0 1 28.8

1 0 0 1 30.6

1 0 1 0 32.4

0 1 1 0 34.2

0 1 0 1 36

1 0 0 1 37.8

1 0 1 0 39.6

0 1 1 0 41.4

0 1 1 0 70.2

0 1 0 1 72

1 0 0 1 73.8

1 0 1 0 75.6

0 1 1 0 77.4

0 1 0 1 79.2

1 0 0 1 81

1 0 1 0 82.8

0 1 1 0 84.6

The Following Pattern continues up to 360 degrees and then repeats in the same

fashion till the length specified by the user to scan some area. For Counter clock

wise motion of the motor the pulse sent through parallel port is inverted and the

same pattern is followed but in the reverse order.

V. Stepping Sequence used for controlling the Motors

Stepper Motor Drivers specify the amount of Current that they output to a

stepper motor in either RMS or Peak Current. Which of the two is better?

RMS, which stands for Root Mean Square, is a fundamental measurement

of the magnitude of an Alternating Current (AC) signal. The RMS value equals

the amount of Direct Current (DC) required to produce an equivalent amount of

heat in a same sized load. The shape of the alternating Current AC waveform is

not important. RMS values simplify the calculation of average power and energy.

However, Peak values, which only give the maximum value of the AC signal,

require more information and can only be compared to RMS values if the shape of

the waveform is known.

For example, when the AC waveform is an ideal sine wave, the

relationship between RMS and Peak Current is as follows:

RMS Current = Peak Current x 0.707 or Peak Current = RMS Current x 1.414.

54

The relationship between RMS and Peak stepper Current depends on the

driver’s configuration. The three common modes of operation for Stepper Motor

Drivers include Full-Stepping, Half-Stepping, and Micro-stepping. If you were to

view the Current waveform of these three modes on an oscilloscope, they would

all appear different representing the relationship between the RMS and Peak

Current:

• Ideal Full-stepping waveform (Figure A.)

• Ideal Half-stepping waveform (Figure B.)

• Ideal Micro-stepping waveform (Figure C.)

All stepper motors are rated in RMS Current, and when Full Stepping, about

200 steps/rev, there is no difference in stepper drivers that output RMS Current

and those that output Peak Current. The stepper driver simply outputs the Current

value that the motor requires. For the following example, assume 1.0 Amp RMS:

Figure A.

FULL STEPPING WAVEFORM: 1 AMP PEAK = 1 ARMS

However, when Half Stepping, 400 steps/rev, the RMS Current and Peak Current

are not equal, as shown in Figure B. Unless the peak driver raises its output

Current by 15% (as shown in figure B.) the motor will not receive its rated 1.0

Amp RMS Current, and is therefore not generating all of the torque that its

capable of providing.

55

Figure B.

HALF STEPPING WAVEFORM: 1.15 A PEAK = 1 ARMS

The Microstepping waveform illustrates an even larger difference between RMS

and Peak Current. In this case the Peak Current setting must be raised by

approximately 41% (as shown in Figure C.) to equal the required 1.0 Amp RMS

motor current.

Figure C.

MICRO STEPPING WAVEFORM: 1.414 A PEAK = 1 ARMS

RMS Current controlled drives will send the motor the selected RMS

value of the Current independent of the Current waveform. This may not be the

case with Peak Current controlled drives, which output different RMS Currents

dependent on the waveform.

56

In conclusion, because stepper motors only specify torque curves and

maximum ratings in RMS Current, using a stepper driver that is also specified in

RMS Current instead of one that is specified in Peak Current simplifies the

matching of a stepper motor to a stepper driver in any application.

57

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59

Additional NDT Resources

• American Society for ND Testing (www.asnt.org)

• British Institute of ND Testing (www.bindt.org)

• Canadian Society of ND Testing (www.csndt.org)

• European Federation for Nondestructive Testing (www.efndt.org)

• International Committee for ND Testing (www.aipnd.it/icndt.htm)

• International Foundation for the Advancement of ND Testing

(www.ifant.org)

• Nondestructive Management Association (www.ndtma.org)

• e-Journal of ND Testing and Ultrasonics (www.NDT.net)