Design and Fabrication of SMT Automatic

Pick and Place Machine

Muhammad Asim Khan

Armughan Hameed

Adnan Ahmed

(Thesis submitted in partial fulfillment of requirements for the Degree of B.S.

Electrical Engineering)

Pakistan Institute of Engineering & Applied Sciences,

Nilore-45650, Islamabad

May, 2013

i

Declaration of Originality

We hereby declare that we are the sole author of this thesis named design and

fabrication of SMT automatic pick and place machine. We also certify that the

material in this thesis, to the best of our knowledge, neither violates any copyright

act nor any propriety published material of any other author, or otherwise, are

properly acknowledged and referenced with standard referencing practices.

We have neither allowed nor will anyone to copy our work with intention of

passing it on as his/her own work. We understand the Universitys policy about

plagiarism and will be held fully responsible for the consequence of such

violation.

Signature: ____________

Name: Adnan Ahmed

Signature:____________

Name: Armughan Hameed

Signature: ____________

Name: Muhammad Asim Khan

ii

Certificate of Approval

Certified that work contained in this thesis titled

Design and Fabrication of SMT Automatic Pick and Place Machine

was carried out by Adnan Ahmed, Armughan Hameed and Muhammad Asim

Khan under our supervision and that in our opinion, it is fully adequate, in scope

and quality, for the degree of BS in Electrical Engineering.

Approved by:

Signature: _______________

Supervisors Name: Mr. Nauman Masud

Verified by:

Signature: _______________

Head, Department of electrical Engineering

iii

Dedication

To our Beloved Parents

and Teachers

iv

Acknowledgement

First and foremost, we would thanks to Allah (Subhanahu Wa Taallah) for giving

us health, passion and knowledge to complete this project. Without His help we

would not have been able to carry out this project this far.

There are a number of people without whom help this project would not have been

success. First of all, we are thankful to our supervisor, Mr. Nauman Masud, for his

passion, motivation, immense knowledge, helping nature and practical approach.

We cannot imagine a better supervisor than him and we are really indebted to him.

We would like to place on record our thanks to coworkers with our supervisor like

Mr. Muhammad Shehzad and Mr. Noor Ahmed who helped us constantly with our

electrical and control portion of the project.

We express our gratitude to Mr. Mustafa Pasha for his help with control cards and

for arranging DC servo motors for us. We are also thankful to Mr. Inaam, Mr.

Khalid and our beloved Mr. Saeed for their help, support and guidance through

mechanical parts fabrication and assembly. We would always remember their

hope giving words regarding project progress.

We wish to express our sincere thanks to Dr. Nasir Rehman Jadoon for his help

with the molding and casting work in our project. We also thank Dr. Asloob and

our friends Masud-ur-Rehman, Khizer Siddique, Waqas Ali Joya. We are thankful

to our family for their moral and financial support through the project.

Finally, we put on record our gratitude for those who lent their helping hand

directly and indirectly in project.

v

Table of Contents

DeclarationofOriginality.....................................................................................................iCertificateofApproval........................................................................................................iiDedication..........................................................................................................................iiiAcknowledgement..............................................................................................................ivList of Figures....................................................................................................................ixList of Tables.....................................................................................................................xiiAbstract.............................................................................................................................xiiiChapter1: Introduction.................................................................................................21.1 Motivation...........................................................................................................21.2 Objective..............................................................................................................21.3 ToolsandSoftwares............................................................................................31.4 ThesisOverview...................................................................................................3

Chapter2: LiteratureReview........................................................................................42.1 XYAxisMovementAssembly..............................................................................52.1.1 Types............................................................................................................52.1.2 Limitations...................................................................................................6

2.2 Head(PickandPlaceAssembly)..........................................................................62.3.1 Types............................................................................................................62.3.2 Features.......................................................................................................82.3.3 Limitations...................................................................................................82.3.4 ImportantConsiderations...........................................................................8

2.3 Feeders................................................................................................................92.3.1 TapeFeeders...............................................................................................9

2.4 ProposedSpecsofMachine..............................................................................112.5 VisionFeedback.................................................................................................132.5.1 MachineVisionCameraControlCard.......................................................13

2.6 Automation........................................................................................................14

vi

2.6.1 DCMotors..................................................................................................142.6.2 SelectionofDCMotor...............................................................................162.6.3 RotatoryEncoder.......................................................................................162.6.4 LimitSwitches............................................................................................172.6.5 Relay.........................................................................................................18

2.7 Control...............................................................................................................182.7.1 MainControlUnit......................................................................................182.7.2 SlaveControlUnit......................................................................................212.7.3 PWMGenerator........................................................................................222.7.4 Microcontroller..........................................................................................232.7.5 ExtendedIndustryStandardArchitecture(EISA)......................................232.7.6 DriveUnit...................................................................................................24

Chapter3: MechanicalDesign.....................................................................................253.1 3AxisMovingAssembly....................................................................................253.1.1 BaseStructure...........................................................................................253.1.2 YAxisAssembly.........................................................................................273.1.3 Xaxisassembly..........................................................................................283.1.4 ZAxisAssembly........................................................................................29

3.2 StressAnalysis...................................................................................................333.2.1 Purpose......................................................................................................333.2.2 Observations..............................................................................................333.2.3 Results.......................................................................................................34

3.3 DesignofTapeFeeder.......................................................................................343.3.1 DesignConstraints.....................................................................................343.3.2 WorkingPrinciple......................................................................................37

Chapter4: MechanicalDesignFabrication.................................................................384.1 3AxisMovingAssembly....................................................................................384.1.1 BaseStructure...........................................................................................38

vii

4.1.2 YaxisAssembly.........................................................................................394.1.3 XaxisAssembly.........................................................................................404.1.4 ZAxisAssembly........................................................................................40

4.2 TapeFeederAssembly.......................................................................................41Chapter5: ControlSystemDesign...............................................................................435.1 ProposedDesign................................................................................................435.2 MainBoard........................................................................................................435.2.1 PowerModule...........................................................................................435.2.2 ProtectionModule....................................................................................445.2.3 LimitSwitchModule..................................................................................455.2.4 DataTransmissionModule........................................................................455.2.5 FeederRelayModule.................................................................................465.2.6 DecoderModule........................................................................................465.2.7 PWMGenerationModule.........................................................................47

5.3 MotorDrivers....................................................................................................47Chapter6: EISAControlCard......................................................................................49

6.1 EISAConnecter..............................................................................................496.2 AddressDecoding..........................................................................................496.3 DigitalOutputPorts.......................................................................................506.4 DigitalInputPorts..........................................................................................506.5 AnalogOutputPorts......................................................................................526.6 EncoderInputPorts.......................................................................................526.7 Software........................................................................................................53

Chapter7: PWMGeneration.......................................................................................547.1 UnipolarPWM...................................................................................................547.2 ADCModule.......................................................................................................547.3 PWMModule....................................................................................................567.4 SoftwareImplementation.................................................................................57

viii

Chapter8: ImplementationofCompleteSMTPickandPlaceMachine.....................598.1 PWMPCB...........................................................................................................598.2 Mainboard........................................................................................................59

Chapter9: ControlMechanism...................................................................................629.1 MasterSlaveProcessing....................................................................................629.2 SpeedandPositionControlAlgorithm..............................................................639.3 PositionLimiterandLimitingSwitchesAlgorithm.............................................639.4 HeadAssembly(Pickorplace)ControlAlgorithm.............................................649.5 PickandPlaceAlgorithm...................................................................................64

Chapter10: FutureRecommendationsandConclusion...............................................6610.1 FutureRecommendations.................................................................................6610.2 Conclusion.........................................................................................................66

Appendix............................................................................................................................67MechanicalDrawings....................................................................................................67

HeadAssembly..................................................................................................67 BaseAssembly...................................................................................................69 FeederAssembley.............................................................................................72

References.........................................................................................................................76

ix

List of Figures

Figure2.1:LPKFProtoPlaceS.............................................................................................4Figure2.2:SCARARoboticArm...........................................................................................5Figure2.3:BeltDriven3AxisPickandPlaceMachine.......................................................5Figure2.4:MultipleNozzleHead........................................................................................7Figure2.5:PneumaticDrivenHead.....................................................................................7Figure2.6:TapeFeeder.....................................................................................................10Figure2.7:BeltDrivenFeeders.........................................................................................10Figure2.8:PCBUnlimitedFeeder.....................................................................................11Figure2.9:BrushedDCMotors.........................................................................................14Figure2.10:BrushlessDCMotor.......................................................................................15Figure2.11:DCServoMotor.............................................................................................15Figure2.12:DCStepperMotor.........................................................................................16Figure2.13:Microswitch..................................................................................................17Figure2.14:InternalMechanismofProximitySwitch......................................................18Figure2.15:MachineControlUnits..................................................................................20Figure2.16:SlaveControl.................................................................................................22Figure2.17:PWMGenerationUsingMultivibrator.........................................................22Figure2.18:ExtendedIndustryStandardArchitecture(EISA)..........................................23Figure3.1:MechanicalDesignof3AxisAssembly...........................................................25Figure3.2:BaseFrame......................................................................................................26Figure3.3:BaseTopView.................................................................................................26Figure3.4:TopViewofYAxisAssembly(mm).................................................................27Figure3.5:PulleyandSideSupport...................................................................................28Figure3.6:FrontViewandIsometricViewofXAxisAssembly........................................28Figure3.7:ZaxisAssembly(isometricview).....................................................................29Figure3.8ZAxisassembly................................................................................................30Figure3.9:HeadAssembly(sideview)..............................................................................30Figure3.10:ZAxisAssembly(Frontview)........................................................................31Figure3.11:LinearBearingsInsideBearingBlock.............................................................31Figure3.12:NozzleDesign(Frontview)............................................................................32Figure3.13:NozzleChanger(Suctionpindetached)........................................................33Figure3.14:NozzleDesign................................................................................................33Figure3.15:StressAnalysisResult....................................................................................34

x

Figure3.16:CompleteFeederAssembly...........................................................................35Figure3.17:MainFeeder(isometricdrawing)..................................................................35Figure3.18:MainFeeder(Gunwithouttopcover)..........................................................35Figure3.19:TapeRemovalCloseUpView........................................................................36Figure3.20:TapeMovement............................................................................................36Figure3.21:BeltSlideSupport..........................................................................................37Figure3.22:TapeConveyer...............................................................................................37Figure4.1:Complete3AxisAssembly..............................................................................38Figure4.2:BaseTopView.................................................................................................38Figure4.3:MotorSupportedonMainFrame...................................................................39Figure4.4:MotorSupportIsoMetricView......................................................................39Figure4.5:YAxisSupportBlock........................................................................................39Figure4.6:XAxisAssembly...............................................................................................40Figure4.7:ZaxisAssembly...............................................................................................40Figure4.8:SuctionChamber.............................................................................................41Figure4.9:GearsforRotationofSuctionChamber..........................................................41Figure4.10:FeederBox.....................................................................................................41Figure4.11:FeederBoxFrontView..................................................................................42Figure4.12:FeederCassette.............................................................................................42Figure5.1:35Vand12Vsupplyvoltage............................................................................44Figure5.2:RelaySupplyVoltage15V................................................................................44Figure5.3:MainsSupplyRelays........................................................................................45Figure5.4:LimitSwitches..................................................................................................45Figure5.5:ReceivedDataPort..........................................................................................46Figure5.6:FeederRelays..................................................................................................46Figure5.7:DecoderCircuit................................................................................................47Figure5.8:HBridgeCircuit...............................................................................................47Figure6.1:EISAPortConfigurations.................................................................................49Figure6.2:Addressdecodingcircuit.................................................................................50Figure6.3:DigitalOutputPorts.........................................................................................51Figure6.4:DigitalInputPorts............................................................................................51Figure6.5:AnalogOutputPorts........................................................................................52Figure6.6:EncoderInputCircuit.......................................................................................53

xi

Figure7.1:PWMwithUnipolarVoltageSwitching(a)ComparisonBetweenReferenceWaveformandTriangularwaveform(b)GatePulsesforOneHalfBridge(c)GatePulsesforOtherHalfBridge(d)OutputWaveform.....................................................................54Figure7.2:PWMSimulation..............................................................................................57Figure7.3:SimulatedCircuitinProteus............................................................................58Figure8.1:PWMPCBCard................................................................................................59Figure8.2:MainBoard......................................................................................................59Figure8.3:MotorDriveCard.............................................................................................60Figure8.4:EISAControlCard............................................................................................60Figure8.5:Transformerfor35VDCPowerSupply...........................................................60Figure8.6:SMTPickandPlaceMachine(view1).............................................................61Figure8.7:SMTPickandPlaceMachine(view2).............................................................61Figure9.1:ProcessingFlowChart.....................................................................................62Figure9.2:SpeedandPositionAlgorithmFlowChart.......................................................63Figure9.3:LimitSwitchAlgorithm....................................................................................64Figure9.4:PickandPlaceAlgorithmFlowChart...............................................................65FigureA.1:HeadAssemblyPartsDrawing........................................................................67FigureA.2:HeadAssemblyPartsDrawing2.....................................................................68FigureA.3:BaseAssemblyPartsDrawings........................................................................69FigureA.4:YaxisAssemblyParts......................................................................................70FigureA.5:XAxisAssemblyParts.....................................................................................71FigureA.6:MainFeeder(Partsandisometricdrawings)..................................................72FigureA.7:MainfeederBasePlate...................................................................................73FigureA.8:GearassemblyforFeederBox(Partsandisometricview).............................74FigureA.9:FeederBox(Partsandisometricview)...........................................................75

xii

List of Tables

Table2.1:ProposedSpecs.................................................................................................11Table2.2:MotorsComparison..........................................................................................16Table7.1:ADCModuleRegisters......................................................................................55Table7.2:ConfigurationRegistersofPWMModule.........................................................56

xiii

Abstract

SMT (Surface Mount Technology) pick and place machine is a robotic

machine used to place electric components directly on PCB (Printed circuit

board). The electronic devices manufactured through this type of machine are

called surface mount devices and are used in many fields of engineering such as

telecom and robotics.

This thesis discusses the design and development of SMT pick and place

machine including mechanical design, fabrication, controller design and

programming. Three axis motions are carried out by DC servo motors and pick

and place phenomenon is done with switching of a vacuum cleaner providing the

required suction. Position is controlled through real time C language based

algorithm. Both the mechanical and controller design are discussed in detailed in

the thesis.

2

Chapter 1: Introduction

This chapter discusses the overview of the project by describing motivation and

objectives of the project.

1.1 Motivation

This project is specially designed to manufacture PCB with surface mount

technology. Surface mount technology contains very small electronic components,

for example size of 0201 SMT resistor is 0.25 0.125mm. Handling such small

component needs great care and is also time consuming. Human error with

placement of such components is also an added problem. This motivates for a

setup that automates the component placement process with precision and speed.

1.2 Objective

Pick and place machines are of great industrial importance. In this project, we

have to start from the gross root level. We have mechanically designed the whole

machine, keeping in mind all the requirements of a modern mechanical machine.

Design consists of three axis motion machine, done with the help of a belt driven

DC motors with a picking tool at front and feeders for feeding components.

On the other hand, controller design contains motor drive cards consisting of

bipolar PWM generating modules to drive heavy motors for axis motion. These

motors are controlled by a control card, which will provide a channel between

motors and PC for reading encoders and writing new positions to the machine.

Two crucial design parameters that should to be met are speed and precision.

Speed in terms of components per hour and precision to a level such that

component is exactly on its position on PCB. To add precision to the assembly,

we will be using visual feedback so that each component is at exact position of its

own. On whole, placing procedure consists of taking coordinates from PCB file

and driving the tool to these coordinates for placement and then visual feedback

will help to place the component precisely.

3

1.3 Tools and Softwares

Software used for development were:

Altiumdesigner6.6fordesigningPCB[1] Autodeskinventor12formechanicaldesign[2] MikroCforprogrammingmicrocontroller[3] BorlandCforalgorithmdevelopmentforcontrolcard[4]

1.4 Thesis Overview

Chapter 1 lays out general overview of the project by describing the motivation

and objective of the project and intended audience.

Chapter 2 reviews the research work related with the project. All paper work done

is constituted in this portion including the mechanical and electrical design.

Chapter 3 describes the mechanical design implementation of head, feeder and

base assembly in the Autodesk Inventor.

Chapter 4 briefly discusses the fabrication of different mechanical parts in the

machine where most of the parts were made on milling and lathe machines.

Chapter 5 describes the control portion of the project. It includes the motors

relays, limit switches and encoders.

Chapter 6 discusses briefly about the EISA card, its address decoding, input and

output ports.

Chapter 7 starts with PWM types and then briefly discusses its generation through

micro controller.

Chapter 8 gives pictorial overview of the project being implemented successfully.

Chapter 9 discusses the working algorithm of electrical and mechanical setup

integrated altogether with the help of flow charts.

Chapter 10 discusses the future recommendations and possible improvements in

the prototype.

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2.1 XY-Axis Movement Assembly

2.1.1 Types

Different approaches were considered for designing of XY axis movement. Some

of the considerations are given below:

SCARA robotic arm. In SCARA robotic arm the robotic arm is used for the placements of the SMT

components. Figure 2.2 shows a robotic arm pick and place machine

Figure2.2:SCARARoboticArm

3-Axis movement machine. In 3-axis machine a nozzle is attached to a plotter-like device to allow the nozzle

head to be accurately manipulated in three dimensions. Additionally, nozzle can

be rotated independently. Figure 2.3 shown a belt driven 3-Axis pick and place

machine. In some machines lead screws are also used instead of belts

Figure2.3:BeltDriven3AxisPickandPlaceMachine

6

2.1.2 Limitations

The length and breadth of the machine is limited due to the bending of the rail

rods. The rods get deflected due to the weight of head (pick and place nozzle

assembly). When the length and breadth of the machine is increased the deflection

of the structure in downward direction also increases. Therefore they are not

increased beyond a certain limit. For the same reason, the diameter of the rail rods

is also kept at a certain value and is not decreased as for larger diameter of the

rods weight also increases.

We choose belt driven mechanism to drive movement of assembly. Belt greatly

increases the speed of the machine but it decreases its precision. Increasing the

diameter of the pulleys do increases the speed of the machine but it decreases its

precision. Due to this reason, the diameter of the driving pulleys is also kept

small.

2.2 Head (Pick and Place Assembly)

It is most vital part of Pick and Place machine from speed and accuracy prospect.

Speed of machine is in direct relation with the speed of head. It is actually speed

with which machine can pick and place components and it is usually measured in

parts per hours (PPH).

2.3.1 Types

Pick and Place heads can be classified on the basis of following factor,

i. Number of nozzles on head

Most industrial machine has several nozzles on head. They pick up several

components during pick up run and place these components during place run. In

some kinds of machines components are feed into multiple heads (like bullets in a

chamber), these components are then fired one by one on PCB. Figure 2.4 shows

commercial multiple nozzle head.

ii. Speed

Speed can be increased by multiple factor by using multiple heads on a single

machine or multiple nozzles on single head. Small projects or research based Pick

7

and Place machines usually have single head with single nozzle. These machines

have speed around 1000pph to 4000pph.

Figure2.4:MultipleNozzleHead

iii. Drive mechanism(motor driven or pneumatic driven)

Most common mechanism for driving head up and down is motor driven. Motor

can be connected to lead screw system to drive nozzle up and down. In order to

attain high speed belt driven system is used. Figure 2.5 shows a pneumatic driven

head.

Figure2.5:PneumaticDrivenHead

8

Many industrial Pick and Place machines use pneumatic drives to move nozzles

up and down. Selection of drive mechanism depends on availability and price of

component. In our machine we will be using motion based on belt driven

mechanism due to the low price of DC servo motor and ease of control.

2.3.2 Features

Common features that typical Pick and Place head have are:

Shock absorption 180 degree component rotation angle Controlled suction Small pick place transient overshoots

2.3.3 Limitations

Most prominent limitations on Pick and Place head design are:

i. Maximum size of head

Increasing the size of head decreases the work area for Pick and Place machine.

Small head causes the designer to compromise on many quality factors.

Designers are carved to use small low torque motors, small diameter guiding rods

and small number of linear bearings.

ii. Maximum speed of head

Increasing the speed to pick and place components cause the danger of dropping

the component from vacuum nozzle. Head need a minimum amount of time for

settling before pulling up and putting down component. Also heads speed is

limited by control speed of vacuum pump and time delay of image processing

processor

2.3.4 Important Considerations

a. Shock Absorbing

There is always risk of slight shock to the nozzle while picking or placing

components on PCB. These shocks if not avoided can cause nozzle to bent from

9

top causing uneven suction distribution over component surface. This uneven

distribution of suction will cause frequent dropping of SMT components.

b. Nozzle Changer

SMT components are available in different sizes. Same nozzle cannot be used to

pick and place SMT components of different sizes. Nozzles of specific gauge are

used to pick place components of different sizes. In order to cope with this

limitation head needs to be designed such that nozzles can be changed either

manually or automatically

c. Availability of Nozzles

There is a large industry dealing with manufacturing nozzles for Pick and Place

machines. These companies make nozzles for almost every type of component.

But because of non-availability of nozzles in our region, nozzles are needed to be

fabricated.

2.3 Feeders

As SMDs are itself special components because of their size and packing. So

getting these component ready to be picked needs special devices called as

feeders. Feeders are specially designed to feed the components to the machine.

2.3.1 Tape Feeders

These are specially designed feeders for the components packed in the form of

long tapes. Following online links were examined:

Tapes are such that they have holes on their side. These holes are of standard size

and are distant equally. As the figure 2.6 below shows the driving gear for the tape

for its motion on the feeder as the gear moves it pulls the tape with its teeth.

10

Figure2.6:TapeFeeder

Consulting patent[6] of different feeders we see they are mostly belt driven with

driving element i.e. motor on side and driver on the other end. Figure 2.7 shows

the gear for reel movement at 55 and being driven by 57 at the other end which

can be driven by a motor.

Figure2.7:BeltDrivenFeeders

From the complexity of feeders coping with tapes inside them was bit difficult to

design so the having the tape completely isolated from the inside of feeder was

considered most practical design.

As components are covered with protective tape on the top so this tape should be

removed before component could be accessed. For this following simple design

figure 2.8 below was considered. It is a commercial feeder by PCB unlimited for

SMT components. It shows the tape removing pulley at the top and main

components being moved forward.

11

Figure2.8:PCBUnlimitedFeeder

For the position of each feeder on the machine we considered a product by LPKF

called proto place. It is a semi-automatic machine for SMT components pick and

place as shown in figure 1.1.

2.4 Proposed Specs of Machine

We list down the proposed specs that machine need to have to give a complete

setup belowTable2.1:ProposedSpecs

Placement specifications

No. of placement heads 1

Placement Rate 180 CPH

Placement Accuracy 0.00035mm

Min size of Component 0201 chip components

Minimum component lead pitch 0.6 mm

Alignment method Vision based

BGA/CSP/MBGA/MLF Placement

Capability

Yes

Tool specifications

Tool changer Yes

No. of tools 3

12

Vacuum( Integral or external) External

PCB Specifications

Max PCB size 1116

PCB loading method Manual

Fiducial Recognition Vision based

Feeders Specifications

Type of feeders 3(tape, IC, bin)

No. of feeders Tape(7),IC(1),Bin(1)

Feeder position Tape(left),IC(front),Bin(back)

Programming

Operating system Windows based(Match3,

EMC2)

Programming platform Matlab,Visual C, Open CV,

EMGU

Mechanical Specifications

X-Y-Z Axis drive mechanism Belt driven

X-Y-Z Axis motor DC motors

X-Y-Z Encoding Rotary

X-Y resolution 0.12 mm

Physical Specifications

Dimension(basic) 600mm900mm

Max dimension(with accessories) 800mm1000mm

Weight 20kg approx

13

Facility Requirement

Power supply 220-240V/50 Hz

Compressed air supply 80psi

2.5 Vision Feedback

On the basic this machine can be made without visual feedback i.e. machine

blindly places components on desired position according to the given coordinates.

As we are talking here with precision of about 15mils component to be on exact

position and this thus calls for the vision feedback.

The idea is simple we give position coordinates to the machine. Motors execute

that info into motion and at the end we use camera to take and feedback the visual

info of the current position of IC and position where it should be until both match.

This can be done from the following steps:

Cracking the code of the PCB file for coordinates Using camera for images at the placing end From the image exactly determine the current position of IC and its orientation

From the PCB determine the Pin place Convert the difference in both positions into coordinates Repeat the whole process until both exactly match

2.5.1 Machine Vision Camera Control Card

Image feedback data from camera is fed to computer through specially designed

control card. This control card takes mage data in analogue form through BNC

connector. OPENCV (open source computer vision) is a library of programming

functions which can be used to interact with machine camera. Programming is

done using Microsoft visual C sharp express EMGUCV. Visual C is used to

process vision data from camera and interact with program. EMGUCV enable

user to call OPENCV functions in C++.

14

2.6 Automation

2.6.1 DC Motors DC motor is an electric motor which is powered by dc current. It consists of stator

and rotor. It works on the principle of Faradays law, which states that force is

exerted on an electric charge passing through magnetic fields. There are several

types of dc motors. Type of motor used for a machine depends on speed and

torque requirements.

a. Brushed DC Motor Brushed dc motors use brushes or slip rings to make contact with commuter for

supplying power to the motor [8]. Brushed DC motor is shown in figure 2.9.

Figure2.9:BrushedDCMotors

b. Brushless DC Motor Brushless dc motor is an electrically commuted motor which has its stator winding

connected to DC source through an inverter or any waveform shaper. Figure 2.10

shows internal cross section of brushless motor [9].

15

Figure2.10:BrushlessDCMotor

c. DC Servo Motor DC servo motor is combination of dc motor, encoder for feedback, gearbox for

speed reduction and control unit. These components form closed loop feedback

path to control speed and angular position of motor. Control unit receive control

signals from master controller to change speed or position of rotor. Figure 2.11

shows block diagram of different components of servo motor.

Figure2.11:DCServoMotor

d. DC Stepper Motor Stepper motor is brushless motor in which one complete rotation of rotor can be

divided into number of equal steps. In this way position of motor rotor can be

controlled without feedback. Figure 2.12 shows rotor and stator of a stepper

motor.

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17

a. Absolute Encoder This type of encoder produces unique data signal in bits for specified number of

positions of shaft. Signal bits are changed one bit at a time when shaft rotates. One

important feature of this encoder is that it stores position of the shaft even in

unpowered state. When encoder is powered again it return most recent position of

shaft.

b. Incremental Encoder Incremental encoder has the additional feature of giving information about the

speed of shaft. These encoders usually produce three pulses. Out of these three

pulses two pulses are of same frequency and have phase difference of 90 degree.

Third pulse is produced at the end of each revolution. By counting number of

pulses per specified time period we can calculate speed of the shaft.

2.6.4 Limit Switches Limit switch is switch which is actuated whenever part of machine move beyond

safe limits. Their main purpose is to protect machines from self-destroying by

moving away from there working area.

There are mainly two types of limit switches:

a. Miniature Snap-Action Switch (Micro Switches)

This type of switch is actuated by external force applied by machine parts on

switch lever. This applied force causes switch to close its two terminals (normally

open NO) or open two terminals (normally closed NC). Figure 2.13 shows

external structure of a micro switch [7].

Figure2.13:Microswitch

18

b. Proximity Switch This type of switch is actuated by the presence of mechanical object near switch

without physical contact. Solenoid coil is used inside these switches. Magnetic

field produced by solenoid coil induces Eddie currents inside metallic objects as

they come close to proximity switch. Eddie current produce magnetic field of their

own which in turn disturbs the magnetic field of solenoid. This field disturbance is

detected by proximity switch as shown in figure 2.14.

Figure2.14:InternalMechanismofProximitySwitch

2.6.5 Relay Rely is electromagnetic switch which is actuated by electrical signal to

mechanically open or close contacts.

2.7 Control

Figure 2.15 shows the block diagram of complete machine control system.

Control units can be divided into three parts

Main control unit Slave control units Driving unit

2.7.1 Main Control Unit

a. Limitations

It is a master processor which stores all data regarding machine position and

executes program command. High speed microcontroller can be used as master

controller to serve our purpose. Some of the microcontrollers which fit our needs

19

are PIC18F4550 and PIC18F4520. But there are some disadvantages when

microcontrollers are for this purpose. These include:

Difficulty in debugging the code Low processing speed (PIC18F4520 can process up to 40MHz) Low data storage space

b. Proposed Solution

In order to eliminate these problems we used personal computer as our master

controller. Programs to be executed are written in visual C on windows based

operating system. These programs are then executed by computer processor.

Advantage of using pc as master controller is,

Easy to debug code Large data storage space Large processing speed

PC control Pick and Place machine by sending and receiving data through its I/O

ports. These ports can be parallel ports, COM ports, PCI slots etc.

20

Figure2.15:MachineControlUnits

21

2.7.2 Slave Control Unit

Data sent by computer still need to be processed for effective functioning of

machine. A breakout board is always used whenever some circuitry is interfaced

with computer. Breakout board separate computer I/O ports from external

environment by using opto-coupler junctions. Breakout boards have analogue to

digital converter and digital to analogue converters. Figure 2.16 shows the slave

control unit.

Three units fall in this category

Break out board Microcontroller Machine vision camera control card

a. Breakout Board

Computer generate digital signal for changing speed of a motor (x-axis, y-axis

motor etc.) and this digital data is received by breakout board. Breakout board

converts this digital signal to analogue signal by digital to analogue converter

(DAC). On the input side breakout board receive pulse from motor encoders.

Counters inside breakout board count pulse and send digital count value to

computer. This counter can be 4 bit, 8 bit, depending on resolution needed. By

using these count values pc compute new value for PWM and send it to controller.

Time required for this process to complete depend on speed of processor used

(Pentium I 100MHz, Pentium II 330MHz). Limit switches on axis are directly

connected to breakout board.

b. Microcontroller

This unit receive analogue signal from breakout board and convert it into pulse

width modulated PWM waveform. This PWM is then used to drive DC servo

motors. For example this microcontroller produces driving signals for feeders and

circular bin. Microcontroller receives a 4 bit data from breakout board. These 4

bits contains information regarding which tape feeder will ascend by one

component or which compartment of circular bin is needed. After receiving this

data mi

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23

Another method is to use microcontroller which has analogue to digital converter.

Analogue signal is applied to the microcontroller which is converted into digital

data and saved in a register. This register value is then used to change duty cycle

of PWM generated by microcontroller.

2.7.4 Microcontroller Microcontroller is used in our project to generate PWM signal from analogue

signal. After thorough survey we decided to use PIC18F2431 for PWM

generation. Main factors in choice of PIC18F2431 were

Availability of four ADC. High clock speed of up to 40MHz. Availability of internal PWM generation module.

2.7.5 Extended Industry Standard Architecture (EISA) EISA is standard bus originally developed for IBM brand. It is 32 bit wide bus

which is backward compatible to Industry Standard Architecture ISA (16bit) as

shown in figure 2.18. EISA provide direct access to 4GB of disc space and

support maximum processor clock speed of 8.33MHz. Control cards are

developed to interconnect external environment circuits to personal computers.

We used one such card for our project purpose. This card was provided us by the

university.

Figure2.18:ExtendedIndustryStandardArchitecture(EISA)

24

2.7.6 Drive Unit

Following are main driving units:

Power supply unit Motor drivers Solenoid plunger drivers Solenoid vacuum valve driver

a. Power Supply Unit

A power supply with several different DC voltages is used for Pick and Place

machines.

48v- motor drives 5V-microcoontroller 16v- solenoid plunger drive 24v-solenoid vacuum valve

b. Motor Drives

Motor drivers have three inputs. Two PWM signals and one enable signal for

drive card.

c. Solenoid Plunger Drive

NMOS is used to supply sufficient power to plunger so that it can operate

smoothly. NMOS is triggered by 5V signal from microcontroller. Signal from

microcontroller is separated from NMOS by using opto-coupler.

d. Solenoid Vacuum Valve Drive

This drive is same as that of solenoid plunger except voltage on drain of NMOS is

24V instead of 15V. Controller for vacuum pump is used to keep vacuum in

inches of mercury at times constant for perfect picking of components. A

potentiometer is placed on vacuum to increase decrease range of vacuum.

25

Chapter 3: Mechanical Design

3.1 3-Axis Moving Assembly

We start with the complete design of XY axis and pick and place assembly

(designed in Autodesk Inventor) shown in the figure 3.1 below.

Figure3.1:MechanicalDesignof3AxisAssembly

The assembly is divided into 4 parts which includes

Base structure Y-axis assembly X-axis assembly Z-axis assembly

3.1.1 Base Structure

The whole mechanical assembly is supported on a base frame which is made of

aluminum, shown in figure 3.2. For rigid support to y-axis assembly, blocks of y-

axis assembly are attached directly above this base frame. The driving motor of y-

axis assembly is also mounted on this base frame. Acrylic sheet of dimension

600mm x 810mm is attached above this frame. The main function of the acrylic

26

sheet is to provide support for the PCB placement assembly and reason for using

this as it is smooth and straight on its top surface as shown in figure 3.3.

Figure3.2:BaseFrame

Figure3.3:BaseTopView

27

3.1.2 Y-Axis Assembly

The y-axis assembly provides motion along the length of the machine. Figure 3.4

shows the top view of y-axis assembly. Driving motor is attached with a coupling

rod which is coupled with two driving pulleys. Two driving pulleys are used

because this provides an even force for the movement of x-axis assembly.

The rails rods are providing support for x-axis assembly; they are 20mm in

diameter and have length of 750mm. The diameter of the rail rods is reduced at

the ends for placement. They are attached with four side support blocks which are

directly attached to the base frame through acrylic sheet.

Belt is driven by aluminum pulleys. Pulleys are fixed on 12mm stainless steel rods

which are attached to side support blocks through bearings as shown in figure 3.5.

The belt used is of HTD type with 5mm pitch. These two belts are driving sliding

blocks which are moving in the y direction. The x-axis assembly is attached on

these sliding blocks.

Figure3.4:TopViewofYAxisAssembly(mm)

28

Figure3.5:PulleyandSideSupport

3.1.3 X-axis assembly

The x-axis assembly provides motion along the width of the machine. Figure 3.6

shows the front view and isometric view of y-axis assembly. The motor is

attached on a supporting block. This supporting block is directly attached to the

sliding block which is moving in the y-axis. Pulley which is driving belt is directly

attached to the motor. Driving belts is of HTD type with 5mm pitch. The rail rods

are attached between the sliding blocks. The diameter of the rail rods is 16mm

with length of 570mm. These rail rods are providing support for sliding block

which is moving in the x direction. The z-axis assembly is attached on this sliding

block

Figure3.6:FrontViewandIsometricViewofXAxisAssembly

29

3.1.4 Z- Axis Assembly

Here we go further deep into the z-axis assembly and have a closer look into it as

shown in figure 3.7 and figure 3.8.

Figure3.7:ZaxisAssembly(isometricview)

30

Figure3.8ZAxisassembly

a. Machine Head design

Belt driven mechanism is used to move head assembly up and down while picking

and placing components. Belts are permanently attached to linear bearing blocks.

Motor rotation causes the linear block hence nozzle to move up and down. Front

and side views of the machine are shown in the figure 3.9 and figure 3.10.

Figure3.9:HeadAssembly(sideview)

31

Figure3.10:ZAxisAssembly(Frontview)

b. Linear Bearing Block

In order to make picking and placing of components accurate and error free, z-axis

motion should be strictly straight line and without deflection. This purpose is

achieved by using linear bearings as shown in figure 3.11.

Figure3.11:LinearBearingsInsideBearingBlock

32

Two straight stain less steel rods are used as guiding rods. Four linear bearings,

two each SS rode, are derived over these rods. Motion of linear bearings over

guiding rods is accurate and in straight line. These linear bearings are enclosed in

aluminum block.

c. Shock Absorbing

As we discussed earlier shock absorber needs to be designed. One possible

solution can be that a spring is attached to upper side of guiding rode in order to

damp shock movement. A spring is at the base when bearing containing sleeve

moves downward it pushes the springs thus absorbing mechanical energy and

saving the suction needle at front.

d. Nozzle Changer

For this purpose nozzles are attached to the pipette by a using permanent magnet.

A rubber sealing is used between pipette nozzle junction to make junction air

tight. Figure 3.12 and figure 3.13 shows nozzles assembly.

Figure3.12:NozzleDesign(Frontview)

33

Figure3.13:NozzleChanger(Suctionpindetached)

e. Pick and Place Nozzle

Standard syringes which are provided with injections are used as nozzles in our

machines as shown in figure 3.14

Figure3.14:NozzleDesign

3.2 Stress Analysis To check the feasibility of the basic structure of the machine, stress analysis was

carried out in Autodesk inventor 12. Forces representing weights of z-axis

assembly and motor are applied on the x-axis assembly.

3.2.1 Purpose The main purpose of the analysis was to see the deflection of x-axis assembly

from its original position.

3.2.2 Observations Since all assembly is supported on rail rods therefore rail rods start deflecting in

downward direction. This deflection has provided a limitation on the diameter and

length of rods therefore these parameters are not changed beyond a certain limit.

This deflection has also provided a limitation on the width of the assembly

because deflection increases when width of assembly is increased.

34

3.2.3 Results

In the carried stress analysis the maximum deflection comes out to be 0.06753mm

which is mainly in the downward direction as shown in figure 3.15. From the

analysis the deflection is in safe limits so this design is feasible.

Figure3.15:StressAnalysisResult

3.3 Design of Tape Feeder Starting with tape feeder this design specifically copes with the 8mm paper tape.

From the reel packing peeling off up to component picking point this feeder

provides a drive unit.

3.3.1 Design Constraints Driving all the feeders with same motor. Uses of only fabricate able design in the lab i.e. use of aluminum sheets and simple design.

We start with the requirement of feeder. As we know SMD components like 0201

IC are very small and come in the bulk of tape reels. Our job is to peel that of reel

and get the component out for pickup. Design is simple as it works with single

motor for multiple feeders. After little playing in Autodesk Inventor we had the

design as shown in the fig 3.16.

35

Figure3.16:CompleteFeederAssembly

Now we individually examine each main part. Figure 3.17 and figure 3.18 shows

main feeder we call it tape gun where in the picture the plunger, disc and gear in

place can be seen.

Figure3.17:MainFeeder(isometricdrawing)

Figure3.18:MainFeeder(Gunwithouttopcover)

36

Figure 3.19 shows the close up of the tape removal attachment with the plate. As

there are groves we can adjust the position of tape removal part along with the

tightening and loosening of belt as required.

Figure3.19:TapeRemovalCloseUpView

Similar grove story is there for plunger attachment as this will help to adjust it

according to linear distance we need to move the gear for perfect mesh. As the

motor rotates but we need the tape to move linearly so we do this by a simple disc

having small pins on its periphery for locking into tape. As the disc rotates it

engages and disengages itself from the holes in the tape and drags it along the

path. Figure 3.20 shows this phenomenon.

Figure3.20:TapeMovement

As we know when we use belts they are required to tighten up for perfect fitting.

For this we have devised a method shown in the figure 3.21. We call them belt

slide supports. Belt at its any mean position(when feeder is meshed belt will be at

lower position then the position shown and when feeder will be free the belt will

be in slight upper position then the shown picture) along the path. So either mean

position belt will slide with support and thus gets a little bit tightened.

37

Figure3.21:BeltSlideSupport

For the tapes to be escorted into each feeder we had a small setup as shown in the

figure 3.22.

Figure3.22:TapeConveyer

Feeders(cassettes)areplacedintoaboxdesignedforholdingthemandalsostationforgearboxforthefeedergun.

3.3.2 Working Principle It has simple working principle that with the rotation of disc the tape is

being pulled forward by the movement of disc which is a belt driven

through a gear.

Gear can move up and down in circular track and this up locking is by the spring and down locking is by the plunger and by moving down it gets

meshed with the gear next to it which is on the rod coupled to the motor.

(Complete Mechanical Design with their dimensions are shown in the Appendix)

ChaFab

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39

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41

42

Figure4.11:FeederBoxFrontView

Figure4.12showsthefeeder(cassette)fordrivingthetape.

Figure4.12:FeederCassette

43

Chapter 5: Control System Design This chapter is reserved for discussing original design of control system of

complete SMT Pick and Place machine.

5.1 Proposed Design Our machine consists of four DC motors. Three of these motors are used to move

machine head along X-axis, Y-axis and Z-axis. Fourth motor is for rotating SMT

component clockwise and anticlockwise. Encoders are attached to shafts of all

four motors. These motors are connected to motor drivers through electric

connections. Encoder outputs routed to the EISA card mounted on EISA port.

Data bus from EISA card is attached to external main board. Motor drive cards

are mounted on main board. In addition to different signals for actuating different

relays, data bus also carry control voltage signals. These analogue voltage signals

vary in the range from +5V _-5V. Duty cycle of PWM signal vary with changing

magnitude of this analogue signal.

5.2 Main Board This is the part of our machines control system which acts as a bridge between

EISA card and machines motors and feeders. All data signals and control voltage

signals from EISA control card terminate at the main board. Depending on

purpose which it serves, main board has several main parts. These parts are

discussed here one by one.

5.2.1 Power Module This part of main board is associated with rectifying input AC voltage to DC

voltage as shown in figure 5.1. Main functions of this part are following:

Supply Power to motor drivers at 35v Supply 12V for powering operational amplifiers ICs. Supply 5V for powering TTL logic ICs. Supply 15V for actuating feeder plungers as shown in figure 5.2.

44

Figure5.1:35Vand12Vsupplyvoltage

Figure5.2:RelaySupplyVoltage15V

5.2.2 Protection Module This part contains relay switches to turn AC power on or off from software control

as shown in figure 5.3.

45

Figure5.3:MainsSupplyRelays

5.2.3 Limit Switch Module This part deals with providing power source to proximity switches and receiving

output of proximity or limit switches as shown in figure 5.4.

Figure5.4:LimitSwitches

5.2.4 Data Transmission Module This module receives data from some external data source through any

programmable protocol (RS232 etc.) and routs that data to EISA card as shown in

figure 5.5.

46

Figure5.5:ReceivedDataPort

5.2.5 Feeder Relay Module This module consists of 15 relays. These relays actuate respective feeder plunger

when decoded data is received. Each rely is connected with single plunger thus it

controls one feeder at time as shown in figure 5.6.

Figure5.6:FeederRelays

5.2.6 Decoder Module This module receives 4 bit data from EISA card and decodes this data to 16 bits.

15 bits of decoded data are used to actuate one feeder at time as show in figure

5.7.

47

Figure5.7:DecoderCircuit

5.2.7 PWM Generation Module This part consists of two PIC18F2431 microcontrollers. These microcontrollers

receive analogue control voltages and generate PWM. Microcontrollers are

programmed such a way that duty cycle of PWM is function of input control

voltage.

5.3 Motor Drivers Motor driver used in our project are H Bridge Bipolar type drivers. Input signal to

motor driver are either bipolar PWM or analogue control voltage as show in figure

5.8. Motor driver use two kinds of power sources. One source is for driving motor

when bridge is in conducting state. Motor driver receive 35V from main board for

this purpose. Motor driver receive isolation supply from external transformers

rated for 10V at output as shown in figure 5.9. This isolation supply voltage is

rectified to DC voltage.

Figure5.8:HBridgeCircuit

48

Figure5.9:IsolationSupplyforHBridge

49

Chapter 6: EISA Control Card

This machine is controlled by a specially designed EISA (Extended Industrial

Standard Architecture) control card.

This control card has following specifications

Two digital data input ports. Two digital data output pots. Four analog voltage outputs. Two HCTL2032 ICs for quadrature decoder, counter and bus interface function.

Following are the details of the control card.

6.1 EISA Connecter

The designed EISA card is connected by using EISA connector. It consists of 8

data lines and 20 address lines with various control signals. Figure 6.1 shows

EISA connecter with ports that are used in given EISA card.

Figure6.1:EISAPortConfigurations

6.2 Address Decoding

Address of EISA card is set by using DIP-8 switch. The logic level of switches is

than compared with address lines in two separate 74LS85 4-bit comparators. The

50

comparator enables the data lines by enabling a 74LS245 octal bus transceiver. It

also enables a 3X8 decoder which is used to select ICs for different functions.

Figure 6.2 shows the circuit used for address decoding.

Figure6.2:Addressdecodingcircuit

6.3 Digital Output Ports

There are 16 digital data output lines. The digital output ports are selected by

using 3 least significant bits of address lines and ARE pin. Figure 6.3 shows the

circuit for digital output ports where Y4 and Y5 are outputs of 3X8 decoder.

The primary function of data output ports is to provide control over different

relays which are used to connect supply voltages and vacuum compressor. These

ports provide control over bridge voltage and 12V voltage and can be turned off

and on by sending data to output register. Similarly vacuum compressor is also

controlled digitally and is used to pick and place required component by creating

vacuum. In addition to this, there are eight spare digital data outputs which can be

used for additional requirements.

6.4 Digital Input Ports

There are 16 digital data inputs lines. The digital input ports are selected by using

3 LSBs of address lines and AWE pin. Figure 6.4 shows the circuit for digital

input ports where Y4 and Y5 are outputs of 3X8 decoder. The primary function of

51

these ports is to receive data from limit switches which are installed at the end of

each axis to restrict movement beyond the specified point.

Figure6.3:DigitalOutputPorts

Figure6.4:DigitalInputPorts

When machine reaches its end point, limit switch is pushed providing a signal to

restrict any further movement. There signal are read from input register where this

52

data is used to generate an interrupt. There are eight additional input data ports

which can be used for additional requirements.

6.5 Analog Output Ports

The movement of the motors is control by analog voltages. Analog voltages are

generated by digital to analog convertors and it vary between +5Vand -5V. Figure

6.5 shows the one of the four circuits for analog voltage generation. The circuit is

selected by decoder output. The DAC convertor used is of 8 bits providing a

minimum change of 0.0390625Volts. There are four analog voltage outputs and

each analog voltage is used to generate PWM. These PWMs are then provided at

the inputs of H-bridge circuit to drive DC motor.

Figure6.5:AnalogOutputPorts

6.6 Encoder Input Ports

To detect outputs of encoder, two HCTL2032 ICs are used. HCTL2032 is

specially designed for quadrature decoder, counter and bus interface function with

53

dual axis support. Each IC provides support for two encoders. Figure 6.6 shows

one of the two circuits for encoder inputs.

Figure6.6:EncoderInputCircuit

6.7 Software

The control card is used with given software which is written in C++ language.

The software is used as a standalone application through DOS. By using this

software the various functions of the control card can be performed easily. The

functions which can be performed include reading and writing data values, writing

analog voltage of specific sampling time and reading encoder outputs.

54

Chapter 7: PWM Generation

7.1 Unipolar PWM

There are generally two PWM switching techniques which are PWM with

unipolar switching method and PWM with bipolar switching method. In unipolar

voltage switching the output voltage switches between 0 and Vbridge, or between 0

and Vbridge. While in bipolar switching the output voltage switches between

Vbridge and Vbridge. In unipolar PWM, the change in output voltage at each

switching event is halved. The effective switching frequency is seen by the load is

doubled and the voltage pulse amplitude is halved. Due to this, the harmonic

content of the output voltage waveform is reduced making the output closer to a

pure sine wave. The basic idea to produce PWM with unipolar voltage switching

is shown in figure 7.1[10]

Figure7.1:PWMwithUnipolarVoltageSwitching(a)ComparisonBetweenReferenceWaveformand

Triangularwaveform(b)GatePulsesforOneHalfBridge(c)GatePulsesforOtherHalfBridge(d)OutputWaveform

7.2 ADC Module

The unipolar PWM is generated by using PWM module and analog to digital

convertor of PIC18f2431 microcontroller. The requirement is to convert analog

voltage of control card into corresponding PWM. The analog voltage is first

55

converted in to digital voltage by using ADC of microcontroller which is then

used to set the duty cycle of PWM.

The 10-bit High-speed Analog to digital converter module of PIC18f2431

performs the task of converting analog signal to digital data. Following are the

features of this module:

Up to 200K samples per second Two sample and hold inputs for dual-channel simultaneous sampling Selectable simultaneous or sequential sampling 4-word data buffer for A/D results Selectable A/D event trigger Operation in Sleep using internal oscillator These features lead themselves to many applications including motor control,

sensor interfacing, data acquisition and process control. The ADC module is

controlled by 9 registers. These registers with their functions are described in table

7.1[14]

Table7.1:ADCModuleRegisters

Register Name Description

A/D Result High Register (ADRESH) The digital data is stored in these

registers. A/D Result Low Register (ADRESL)

A/D Control Register 0 (ADCON0) These registers are used to select

different ADC features which include

continuous or single shot mode

reference voltage selection, data

acquisition time and buffer address.

A/D Control Register 1 (ADCON1)

A/D Control Register 2 (ADCON2)

A/D Control Register 3 (ADCON3)

A/D Channel Select Register

(ADCHS)

It is used to select different channels

of ADC.

Analog I/O Select Register 0

(ANSEL0)

These registers are used to select a

specific input as analog input.

56

Analog I/O Select Register 1

(ANSEL1)

7.3 PWM Module

The Power Control PWM module performs the task of generating multiple

unipolar pulse width modulated (PWM) outputs. Following are the features of

PWM module:

Up to eight PWM I/O pins with four duty cycle generators. Pins can be paired to get a complete half-bridge control.

Up to 14-bit resolution, depending upon the PWM period. Edge- and Center-aligned output modes. Single-pulse Generation mode. Programmable dead time control between paired PWMs. Interrupt support for asymmetrical updates in Center-aligned mode. The PWM module is has 22 registers of which 8 register are used to configure

different features. The other 14 registers are used to configure features such as

duty cycle and PWM frequency. The configuration registers of PWM module with

their functions are described in Table 7.2[11]

Table7.2:ConfigurationRegistersofPWMModule

Register name Description

PWM Timer Control register 0

(PTCON0)

The PWM time base and modes are

configured through these registers.

PWM Timer Control register 1

(PTCON1)

PWM Control register 0 (PWMCON0) PWM output pins are enabled or

disabled through PWMCON0 register.

PWMCON1 is used to select

complementary or independent output

pins.

PWM Control register 1 (PWMCON1)

57

Dead Time Control register (DTCON) It is used to insert dead time between

going OFF of one PWM output pin to

going ON of complementary output

pin.

Output Override Control register

(OVDCOND)

These registers are used to override the

PWM output pins at any time in the

program. Output State register (OVDCONS)

Fault Configuration register

(FLTCONFIG)

This register is used when a hardware

fault occurs and PWM pins are put into

inactive state to save the power

devices.

7.4 Software Implementation

The code for programming PIC16F2431 is written in C language. First different

variables are defined and configuration registers are adjusted. Then analog to

digital converter was configured to take detect data continuously from the analog

input. This digital data is continuously used to adjusted duty cycle of PWM in for

loop. The resulting PWM is shown in figure 7.2.

Figure7.2:PWMSimulation

58

The PWM generated in center aligned mode which corresponds to unipolar PWM.

Dead time in PWM is also added which can be adjusted with the help of external

switches. Similarly, the frequency of PWM can be varied from 15 kHz to 25 kHz

with the help of external switches. The circuit is than simulated in Proteus ISIS 7

professional. Since output of control card varies from +5 to -5V and since

microcontroller does not accept negative voltages, therefore it is first scaled to 0 to

5 V by using op-amp based circuit. Simulation circuit is shown in figure 7.3.

Figure7.3:SimulatedCircuitinProteus

59

Chapter 8: Implementation of Complete SMT Pick and Place Machine

8.1 PWM PCB

Figure8.1:PWMPCBCard

8.2 Main board

Figure8.2:MainBoard

Figure8.5

Figure8.3:Mo

Figure8.4:EIS

5:Transformer

otorDriveCard

SAControlCard

for35VDCPow

d

d

werSupply

60

Figure8.6

Figure8.7

6:SMTPickand

7:SMTPickand

dPlaceMachin

dPlaceMachin

ne(view1)

ne(view2)

61

62

Chapter 9: Control Mechanism

Machine is mechanically fabricated and electrical control is added. Control

algorithm is explained for various tasks done by machine.

9.1 Master Slave Processing

Master pc with serial port on it is required. We connect this PC to another computer through serial port. This PC is connected to external hardware

through an EISA card.

Master PC performs the possible variables extractions such as position variables and passes them to the processing unit (pc with EISA card).

These variables are loaded through the EISA cards input port into the RAM.

Processing unit performs possible algorithms on it according to the information from the master PC.

Processing unit is running is single threaded mode (DOS) while master is running in windows mode so that the working of machine is uninterrupted

in the single threaded mode. A complete processing flow chart is shown in

figure 9.1.

Figure9.1:ProcessingFlowChart

Computerorlaptopwithaspareserielcommunicationport

ComputerwithEISAport

EISAcontrolcard

Breakoutboard

Serialcommunicationlink

Parallelcommunicationlink

63

9.2 Speed and Position Control Algorithm

As discussed earlier we have the position and speed variables from the master PC.

We define Counts per mm= Equation 9.1

We use this calculated values to have the encoder desired value.

Obtain difference of the desired and initial value called as error and

multiplied with a gain factor.

We manipulate this error as a binary value for the respective DAC of the

motor which generates respective PWM by comparing it with a DC level.

As the destination is reached the error variable defined earlier gets zero

and motor stops and ready for next position value.

Speed of motion is controlled by the gain factor. Gain is directly

proportional to the speed of motor.

Figure9.2:SpeedandPositionAlgorithmFlowChart

9.3 Position Limiter and Limiting Switches Algorithm

When the position value is fed and motion is being executed, if moving block

(mechanical structure) hits a limit switch on any of the axis and on any direction,

an interrupt is generated.

Encoder_desired_val

Diffrentiator

Error

Gainfactor(K)

DAC

PWM generator

Motor

64

Position error flag is toggled and DAC value to all motors is fed with value of 128 and thus motion is halted.

Figure9.3:LimitSwitchAlgorithm

9.4 Head Assembly (Pick or place) Control Algorithm

If machine is ready to pick a component following algorithm is executed:

DAC of z-axis is enabled. We calculated for a complete rotation of z-axis motor. Head position is

such that its at the pick position on the base frame and reverse complete

rotation takes it back to initial position.

Encoder of the respective motor is read and it is named as initial value and destination value is count per complete rotation of motor pulley.

When the destination is reached, vacuum generator is enabled. Now destination and initial values of encoders are swapped. When head is at desired position for placing the component, pick

mechanism is executed in reverse order.

9.5 Pick and Place Algorithm

Here we discuss a complete round trip of pick and place algorithm.

We have the destination and initial values of encoders. We enable the respective DAC i.e. either x, y or z.

Limitswitchenabled

Limit_switch_registerenable

MakeallDACvalueszero

65

Position and speed algorithm is executed for position of picking the component.

Pick algorithm is executed. Position and speed algorithm is executed again for the position of placing

the component.

Place algorithm is executed. One routine is complete and its ready for takingnext position variables.

Figure9.4:PickandPlaceAlgorithmFlowChart

Destination_flagenabled

DAC_Zenabled

Pickroutineexecution

Destination_flagdisabled

Loaddesired_encoder_val

Positionroutineexecution

Destinationflagenabled

Placeroutineexecution

Destination_flagdisabled

Loaddesired_encoder_val

66

Chapter 10: Future Recommendations and Conclusion

10.1 Future Recommendations

This project serves as prototype for SMT pick and place machine. To improve this

system several recommendations are listed below:

Placement head of z-axis assembly is attached magnetically therefore it can be changed according to the size of the components.

Components tray and strip feeders can also be attached with the machine for additional components support.

Number of placement of heads can also be more than one.

10.2 Conclusion

The aim of this project was to design and fabricate SMT (Surface Mount

Technology) pick and place machine that could work automatically from the PCB

loading up to placement of components on the PCB. Mechanical design is hand

built where most of the parts are lathed and milled to desired shape. Control

design was implemented on PCB and interfaced with EISA card.

The purpose of 3 axis motion machine along with its controller was successfully

implemented. Providing visual feedback to the system is in progress.

67

Appendix

Mechanical Drawings

Head Assembly

FigureA.1:HeadAssemblyPartsDrawing

68

FigureA.2:HeadAssemblyPartsDrawing2

69

Base Assembly

FigureA.3:BaseAssemblyPartsDrawings

70

FigureA.4:YaxisAssemblyParts

71

FigureA.5:XAxisAssemblyParts

72

Feeder Assembley

FigureA.6:MainFeeder(Partsandisometricdrawings)

73

FigureA.7:MainfeederBasePlate

74

FigureA.8:GearassemblyforFeederBox(Partsandisometricview)

75

FigureA.9:FeederBox(Partsandisometricview)

76

References

[1] Altium Limited(2013) Altium designer (Release 10)- [Online] Available

http://www.altium.com/en/products/altium-designer [2] Autodesk Inc.(2013) Autodesk Inventor 2012 [Online] Available

http://www.autodesk.com/suites/product-design-suite

[3] Mikroelectronica