mat noh. m. liquid level sensing device a thesis submitted

00

Mat Noh. M.

Liquid Level Sensing Device

A THESIS SUBMITTED [N PARTIAL FULFILMENT OF THE DEGREE OF MASTER OF SCIENCE IN MSC AUTOMATION & CONTROL

2003

lame OJ

TABLE OF CONTENT

AcknoWlegment............................................................................................ 1

Abstract...................... ................................................................................ ii

Chapter1: Introduction ................................................................................... 1

1.1 Introduction .............................................................................. 1

1.2 Problem Specifications..................................................................1

1.3 Project Objective .......................................................................... 2

1.4 The Organisation of the Theses .. ...................................................... 2

Chapter 2: The Liquid Level Techniques Literature..................................................4

2.1 Introduction ............................................................................... 4

2.2 The Blast Furnace ................................... .... .................................... 4

2.3 The Liquid Level Sensing techniques .................................................. 8

2.3.1 Fibre Optic......................................................................8

2.3.2 Ultrasonic (Ultrasound) ....................................................... 10

2.3.3 Capacitive ....................................................................... 12

2.3.4 Time Domain Reflectometry 13

2.4 Discussion ...............................................................................14

2.5 Conclusion..............................................................................15

Chapter3: Theory .......................................................................................... 16

3.1 The Permanent Magnet Brushed DC Motor ........................................ 16

3.2 Electrical and Mechanical Paramaters..............................................19

3.3 Controlling a Permanent Magnet Brushed DC Motor............................ 20

3.3.1 MOSFET ........................................................................ .22

3.3.2 Pulse Width Modulation (PWM). . .. . 2

3.3.3 MOSFET Driver ....................... ........................................... 27

3.4 Hall Effect Transducer or Current Sensor ......................................... 28

3.5 Circuitry to determine the liquid level .............................................. 29

3.5.1 Infra-red LED's ............................................................... 29

* 3.5.2 Phototransistor............................................................... 30

3.5.3 Voltage comparator..........................................................30

3.6 Display the Level ....................................................................... 31

3.6.1 Seven-segment-di splay ....................................................... 32

3.6.2 Seven-Segment-Display Driver ......................................... ....33

Liquid Level Sensing Device . iii

Table of Content

3.7 The Microcontroller................................................................... 35

Chapter 4: Mechanical Design ........................................................................ 37

Chapter 5: Hardware Design and Constructions....................................................39

5.1 Design of Power Circuit..............................................................39

5.1.1 Circuit Design.................................................................39

5.1.1.1 MOSFET.........................................................40

5.1.1.2 MOSFET Driver................................................ 40

5.1.1.3 Dead Zone Delay Circuit ...................................... 41

5.1.1.4 The Buffer ........................................................ 41

5.1.1.5 The Logic Inverter............................................... 42

5.1.2 Construction of the Power Circuit.......................................... 42

5.1.2.1 Circuit Construction on Breadboard...........................42

5.1.2.2 Power circuit PCB ...............................................43

5.1.3 Current Sensor................................................................. 44

5.2 Rotary Encoder Circuit................................................................. 45

5.3 Level Display Circuit..................................................................46

5.4 Control Board ........................................................................... 47

Chapter 6: Software Development..................................................................... 50

6.1 PlC Microcontroller................................................................... 50

6. 1.1 PICAXE 28X.................................................................. 51

6.2 System Description.....................................................................52

6.3 The Program............................................................................ 54

6.3.1 Main Program................................................................. 54

6.3.2 Sensor Program ................................................................ 56

6.3.3 Encoder Program............................................................. 56

6.3.4 Level Display Program ................................................ ....... 57

6.3.5 Complete Program............................................................ 57

Chapter 7: Result and Discussion...................................................................... 58

7.1 Electronic Hardware Design and Construction Stage............................. 58

7. 1.1 Power Circuit.................................................................. 58

7.1.2 Rotary Encoder Circuit ........................................................ 68

7.1.3 Level Display Circuit......................................................... 69

7.2 The Software Development Stage................................................... 69

7.2.1 Main Program.................................................................. 70

Liquid Level Sensing Device iv

Table of Content

7.2.2 Sensor Program...............................................................72

7.2.3 Encoder Program..............................................................72

7.2.4 Level Display Program ..................................................... ..73

7.2.5 Complete Program............................................................74

Chapter 8: Conclusion...................................................................................75

8.1 Conclusion......................................................................75

8.2 Further Improvements........................................................75

References................................................................................................77

Liquid Level Sensing Device v

List of Figures

LIST OF FIGURES

Figure 2.1: Schematic cross-section of a blast furnace............................................... 5

Figure 2.2: The structure of the blast furnace.........................................................6

Figure 2.3 : Representative materials balance for a large blast furnace............................7

Figure 2.4 : Schematic liquid level sensing system...................................................8

Figure 2.5: Principle of operation: (a) The sensor in air, (b) The sensor in liquid................9

Figure 2.6: The Probe Shapes: (a) "Shorter face" type, (b) "Hypotenuse face" type...........10

Figure 2.7: Principle elements of the transducer ..................................................... 11

Figure 2.8: Sketch of fluid filling rig..................................................................12

Figure 2.9: Sensor mounting positions...............................................................12

Figure 2.10: The tunnel diode diode based LC oscillator circuit schematic.....................13

of the capacitive liquid helium level sensor instrument

Figure 2.11: The schematic of tunnel bias circuit...................................................13

Figure 2.12: Isometric drawing of water level probe................................................14

Figure 3.1: General Purpose Permanent Magnet Brushed D.0 Motor............................17

Figure 3.2: Speed versus Torque Characteristic......................................................19

Figure 3.3: An example of H-bridge circuit using MOSFETs.....................................20

Figure 3.4: An example of H-bridge using BJTs....................................................21

Figure 3.5: (a) MOSFET cell internal structure (b) MOSFET Transfer Characteristic.........22

Figure 3.6: The MOSFET TO-220 package, symbol and equivalent.............................23

circuit of a MOSFET

Figure 3.7: (a) MOSFET Turn-ON (b) MOSFET Turn-OFF......................................23

Figure 3.8: Variation of tc fl with the same time period.............................................. 26

Figure 3.9: The waveforms seen for a simple PWM system........................................27

Figure 3.10: Simple Gate Driver Formed From CMOS Inverters .................................28

Figure 3.11: High Performance push-pull gate driver.............................................. 28

Figure 3.12: Arrangement for long-range remote control.......................................... 29

communication, with both lens and collimator

Figure 3.13: Circuit symbol of (a) LDR, (b) Photodiode, and (c) Phototransistor.............30

Figure 3.14: (a) LM339 comparator, (b) Transfer Characteristic .................................. 31

Figure 3.15: Truth table for a Seven-Segment Display.............................................32

Figure 3.16: A Common Anode Seven-Segment LED Display....................................33

Figure 3.17: A Common Cathode Seven-Segment LED Display..................................33

Liquid Level Sensing Device vi

List of Figures

Figure 3.18: BCD-to-Seven-Segment Decoder/Driver Driving ..................................... 34

A Common-Anode Seven-Segment LED Display

Figure 3.19: Truth Table of A BCD-to-Seven-Segment Decoder/Driver........................34

Figure 3.20: A Method of Driving A Common Cathode Seven-Segment Display ..............35

Figure 3.21: The P1C16F873A ......................................................................... 36

Figure 3.22: The P1C16F873A Features.............................................................. 36

Figure 4.1: The Mechanical Design................................................................... 38

Figure 5.1: Typical Connection 1R2106............................................................... 41

Figure 5.2: The power board ........................ ..................................................... 44

Figure 5.3: The Current Sensor Board................................................................ 45

Figure 5.4: The Rotary Encoder Board................................................................ 46

Figure 5.6: The Display Board......................................................................... 47

Figure 5.7: The Serial Download Circuit............................................................. 47

Figure 5.8: The Control Board......................................................................... 48

Figure 5.9: The Complete Boards......................................................................49

Figure 6.1: The PICAXE 28X..........................................................................51

Figure 6.2: The Feature of PICAXE 28X.............................................................. 52

Figure 6.3: A System Block Diagram................................................................. 53

Figure 6.4: A Four Quadrant Drive.................................................................... 53

Figure 6.5: The Flowchart of the main program..................................................... 55

Figure 7.1: The PWM Signal........................................................................... 58

Figure 7.2: The Waveform of Input/Output of a Logic Inverter..................................59

Figure 7.3: The Output of Delay Circuit ............................................................. 60

Figure 7.4: The Waveform of MOSFET's Driver ................................................... 61

Figure 7.5: The Output Waveform of MOSFET's Drive (Leg 1) ................................ 64

Figure 7.6: The Output Waveform of MOSFET's Drive (Leg 2) ................................ 66

Figure 7.7: The Current Waveform with Resistor = 10, 25W................................... 67

Figure 7.8: The Current Waveform Across the Motor.............................................. 68

Figure 7.9: The PWM Current Waveforms........................................................... 71

Figure 7.10: The Rotary Encoder Output Pulse...................................................... 73

Liquid Level Sensing Device vii

APPENDICES

APPENDIX A

APPENDIX B

APPENDIX C

APPENDIX D

APPENDIX E

APPENDIX F

APPENDIX G

APPENDIX H

Liquid Level Sensing Device viii

Abstract

ABSTRACT

The project designed to be implemented to measure the level of molten iron liquid inside a

tank. The design is based on a torque-controlled drive where the level of the liquid in the tank

is measured using a suspended float. The float is lowered from the disc drilled around its

perimeter and a lightweight rope, using torque sensitive electric drive and rotary encoder

circuit to produce the pulse that is used to calculate the level of the liquid inside a tank. When

the float makes contact with the liquid level surface, the torque reduces and the number of

revolutions made by the disc is counted and the level is calculated using an appropriate

formula and displayed.

Liquid Level Sensing Device ii

Chapter]. Introduction

CHAPTER 1: INTRODUCTION

1 . 1 Introduction

The determination of liquid level applications has been developed for many years especially

for process industries. There are many ways or methods to determine the level of a liquid.

Whatever method we chosen, much depend on the nature of the liquid itself and sometimes

the environment also might limit our choice of the methods available. For example, to

determine the level o f molten iron in a blast furnace it i s not a straightforward and many

delicate sensing devices cannot operate in this corrosive and extremely high temperature.

Therefore before choosing any sensing method, we have to investigate what are the methods

available and should be operable in very hazardous environment and extremely high

temperature.

After consideration of the specification and the conditions where the method will be applied,

it was decided to design a liquid level sensing device using torque controlled drive. Therefore

a small permanent magnet brushes dc motor is used to lower a float with a disc and cable at

the liquid, using a torque sensitive drive. When the float makes contact with liquid, the torque

reduces due to up-thrust force exerted from the liquid to the float and the number of

revolution made by the disc is used to calculate the level, which is displayed according to the

appropriate formula has been set in the program. The overall system is controlled using PlC.

1.2 Problem Specifications

Design of a liquid level sensing device not a straightforward. In order to determine the level,

we need to make the motor rotate and control it, otherwise we far from what we are targeting

for this project. Many factors need to put into account to avoid any unnecessary problem a

rise after completing the project. The work performed in this project incorporates a number of

different fields of work:

a) Power electronic circuit: This is the most important part of the project. The correct

switching current needed in order to make the motor rotate and produce the torque.

This basic operation is common for most types of motors.

Liquid Level Sensing Device

Chapter 1. introauction

b) Encoder circuit: The encoder circuit is used to provide feedback to the software

therefore can update the microcontroller how much the motor has travelled by

counting the number of revolution made by the disc.

C) Sensor Technology: The motor controller has a number of sensors, which provide

feedback to the software control loops. The sensor used in the motor controller is

current transducer for measuring the current and voltage measurement.

d) Display circuit: The display circuit is used to display the level of a liquid using

seven-segment-display. This is very common requirement in modern electronics is

that displaying alphanumeric characters.

e) Software development: This is the brain of the project. The control algorithm is

maintained by PlC microcontroller. The BASIC language is used to perform the

control algorithm.

1.3 Project objectives

The primary and most important goal of my project was:

To design and construct a liquid level sensing device which will determine the level of liquid inside a tank

To make this become reality, there are a few factors need to be considered:

i) To control the torque by controlling the armature current

ii) Motor speed is set to be constant

iii) To detect the motor shaft position

1.4 Organisation of the Theses Document

The remainder of the theses describes all work completed, problems encountered

and how these problems were overcome. Detailed descriptions including theory are

Liquid Level Sensing Device 2

Chapter 1: Introduction

presented to support practical design choices. The following chapters form the body of the

thesis document, and may be summarised as follows:

Chapter 2 : Liquid level technique literature, presents a literature review of all relevant works

in the field of liquid level sensing techniques

Chapter 3: Theory, presents the background material necessary to understand how a

permanent magnet brushed DC motor operates, and gives an insight of how to control such a

motor and the theory of all key components used in the project.

Chapter 4: Mechanical design, presents a mechanical design aspects where later the encoder

circuit had to be fitted in.

Chapter 5: Hardware design and construction, analyses the circuits designed and describes

their operation down to component level. Design formulas indicate how component values

were obtained.

Chapter 6: Software development, describes the control algorithms implemented in software,

which control the motor. There is a full listing of the code completed to date in Appendix B.

Chapter 7: Result and discussion, presents a discussion of the liquid level sensing device, the

result and the issues that emerged from such a project.

Chapter 8: Conclusion, concludes the document with a short summary of the findings

throughout the thesis project. Some possible future work is given as suggestions to

improve the liquid level sensing algorithm. A final note is then given to the overall picture of

liquid level sensing device and where the future of such a technology is headed.


Chapter 2: Liquid Level Sensing Techniques Literature

CHAPTER 2: LIQUID LEVEL SENSING TECHNIQUES LITERATURE

2.1 Introduction

An extensive literature search was carried out to review work completed previously. A list of

keywords relating to the topic for searching databases: e.g. liquid level, liquid depth gauge

was drawn up. A general WWW search resulted in a number of results however I found many

of the web sites were usually a company trying to sell their product, and offer little or no

technical information. The WWW is a very convenient way of obtaining product data sheets.

A comprehensive search using

the networked databases Inspec, Compendex, Engineering & Applied Science. This search

resulted in some journal and magazine articles relevant to aspects on liquid level sensing

technique.

It was discovered there are a few techniques being used to determine a liquid level. However

requirement of this project is to design the liquid level sensing device that can be used to

determine the molten iron liquid level inside a tank. Therefore before discuss all the

techniques available; we should look into the characteristics of the blast furnace, which is

used to produce molten iron.

2.2 The Blast Furnace

The blast furnace is used to produce molten iron. Figure 2.2.1 shows the cross-section of a

blast furnace plant showing the material handling, charging, tapping, gas handling and hot-

blast equipment"1.


WASTE GAS + COMBUSTION STACK CHAMBER CHEaTERWORK

CHUTES - ORE. COKE B LIMESTONE

BLAST FURNACE

)iapter 2. Liquid Level Sensing Techniques Literature

HOT METAL SLAG CAR LADLE

SLAG MOLTEN IRON CLEANED GAS

ON ON GAS BLAST

Figure 2.1: Schematic cross-section of a blast furnace plant showing materials handling, charging, tapping, gas handling and hot-blast equipmentt11.

The blast furnace working temperature is always greater than 1700°K 111 (14270C). Since we

know that the melting point of molten iron itself is aroundl600°K, therefore it is very

important to ensure that the temperature of the blast furnace should be kept higher than the

molten iron melting point.

Figure 2.2 shows the structure of a blast furnace tower '21 . The blast furnace tower comprises

of 6 sections that starts from the top to the bottom, which are the bell, throat, shaft or stack,

cylindrical belly, bosh, and hearth. The bell consists of two bells in different sizes (sometimes

four or even less), which are small bell on the top and large bell at the bottom. The solid

materials are charged through the bell. The throat is attached with a movable deflector to get

a uniform burden across the furnace. The stack or shaft is the largest section of the blast

furnace, which is about 3/5 of the total height 121 where its diameter increases from top to

bottom of the section to facilitate a uniform flow of the gases and the materials. The

cylindrical belly has largest diameter where the process of getting metal and slag starts at this

region. The diameter is decreases at bosh that connects with the most bottom section of

furnace, which is hearth where the molten iron and slag are accumulated. Other than these six



sections, there are two openings at the top of the furnace (left and right) for releasing the

output gases from reaction process in the furnace.

The molten iron made up from the combination of solid materials and gases. The materials

balance is shown in figure 2.3111. The solid materials are charged through the top of the

furnace consists of iron oxide (ore) usually hematite, sometimes magnetite, metallurgical

coke that is produced by heating mixtures of the powdered coking coal in absence of air"

and flux which reduce the melting point of coke and ore. There are materials that are charged

via the tuye'res which are the hot-blast air (sometimes also enriched with oxygen) and

hydrocarbons (could be solid, liquid or gas). The hot-blast air burns the coke in front of

tuye're which is used for heating and melting the charge and products. Therefore it is

important to maintain the temperature of the hot-blast air above the melting point of the

molten iron and slag.

. GCtL

4RM:)UR

I SHA I OR I

3 TACK I

3 ,_4 COCLFIl I PIAIS

STEEL

I II.

llAFT r-1SUPF'ORT

BOSH

I COOLG

I! .j \PI1SILE

Y'-VEAD

P 1

TJE V 7C L(wpIpE LAC r,OTCf

I I lkuj. l::I

Figure 2.2: The structure of the blast furnace121.

As mentioned before the main product of the blast furnace is molten iron. However there are

two other by-products are formed by the blast furnace, which are slag, which contains very

little iron oxide, which indicate the efficiency of the furnace. The second by-product is gas


Chapter 2: Liquid Level sensing i eci'zniques Literature

which leaves through the gas-collection System at the top of the furnace as shown in figure

2.3.

INPUTS (kg OUTPUTS (kg)

lPELlLTlS - SINTER /1600 TOP GAS flIo

IR 300 l Wit BLAST C Ga

HYDROCARBON F UEL

rjTEN 'METAL I-%S1, O.5%Mi

!0.05 P

Figure 2.3 : Representative materials balance for a large blast furnac&11



2.3 The Liquid Level Sensing Techniques

2.3.1 Fibre Optic

There are many works done using this method. Pekka Raatikainen and team, using this

method in their project named "Fibre-Optic Liquid Level Sensor" was published in October

1996 131 , The project was based on total reflection of light that is distributed by a contact of a

sensor tip with a liquid. There are four different tip of shapes being fabricated which are

conical, rounded conical, second order polynomial and third order polynomial131.

Figure 2.4 shows the sensor system, which comprises of a control unit, a transmitter, a

receiver, two fibres that is coupled together and the sensor is attached at the end of the

coupled fibres. The coupled fibres are used for forward path (transmitter) and return path

(receiver). LED is used as a transmitter, which is the source of the light, and photodiode is

used as a receiver as to response to the sensor signal.

Tan.srniter

ibers

ReLvr

Serisor

Figure 2.4 : Schematic liquid level sensing system 131

The basic operation of this system, the light from the LED (transmitter) is transferred to the

sensor through the fibre (first fibre - forward path). When the sensor in air, we know that, in

air the light reflection is 100% or also called total internal reflection (TIR) 141 whereas when

the sensor immersed in liquid, the light reflection is decreased because some of the light

being refracted into a liquid. The light then passes into the second fibre (return path) that is

connected to receiver (photodiode), which detects the optical power ' 31 . The value of optical

power is decreased when the sensor immersed in liquid as the light also being refracted into a

liquid. The condition of the light reflection when sensor is in air and in liquid is shown in

figure 2.5 (a) and (b).



Receiifl TransmktThg fiber fibcr

Sensitive ement

() (b)

Figure 2.5: Principle of operation: (a) The sensor in air, (b) The sensor in liquid 131

Another application of fibre optic as liquid level sensor is in cryogenic environment"'. In this

project optical time domain reflectometer (OTDR) was used to display the relative power of

the returned signal of probe versus distance. Two types of probes were designed which are

shorter face and hypotenuse face types that illustrated in figure 2.6 (a) and (b).

The principle operation of this system is based on angle of incident is compared to the critical

angle of the media. As both designs using right angle prism [41 as a sensor, therefore the

incident angle would be 450141• When the sensor in vapour which its critical angle is 410141

therefore the incident angle is greater than the critical, as a result light will be reflected into a

fibre (total internal reflection) and when the sensor in a liquid, the critical angle of liquid

(nitrogen) is 51°' is greater than the angle of incident, therefore most of the energy will be

refracted into a liquid. Thus, we can detect whether the probe is in vapour or in liquid, by

monitoring the intensity of light being reflected.

The different between these two types of sensor, in shorter face, the light reflects from the

surface of prism's shorter face is around the tip whereas in hypotenuse face, the light reflects

in the middle of the hypotenuse surface of a prism which can seen in figure 2.6 (a) and (b).


Chapter LLlqU1a Level ensing I ecnniques Literature

i Optical Fiber

Collimator

, Right-Angle prism

7.. N Light paths Light paths if inside liquid if inside liquid

Optical Fiber

Collimator

Right-Angte PiiinReflecting

/ coat 7 : Light paths Light paths It inside liquid If Inside liquid

Figure 2.6: The Probe Shapes: (a) "Shorter face" type, (b) "Hypotenuse face" type 141

2.3.2 Ultrasonic (Ultrasound)

This method can be used in many applications. Using of acoustic impedance as based on

measuring attenuation of longitudinal ultrasonic wave in waveguide sensing element to

control the level of coolant industrial power-generating equipment 151 . This ultrasonic

attenuation is determined when the sensing element is immersed in liquid.

The system comprises of an acoustic converter, waveguide communication, and the sensing

element as shown in figure 2.7151. The acoustic converter used is piezo-crystal converter,

which its operating frequency is 500KHz 151 . The principle operation of the system, the

impulses from the converter travel through the waveguide communication to the sensing

element. Therefore it is very important to use a good waveguide communication to minimise

losses of the signal when impulses travels through it to reach the sensing element, in order to

get a desired output. In addition, it is important to ensure the waveguide communication is

protected from ambient factors.


Chapter 2.Liquid Level Sensing Techniques Literature

-- - converter

wavequide line .>

A2 sensitive element

Figure 2.7: Principle elements of the transducer 151

Another application of ultrasonic is in monitoring system for container filling 161 . The sensing

system is based on ultrasonic impulses from a transducer. The system set-up is shown in

figure 2.8161. The system consists of fluid tank, dispensing valve, a control unit, a container

and a transducer to detect the fluid at the optimum level wanted (set-up).

To determine the level of a liquid in the container, the sensor was mounted at 3 different

positions, which are side, bottom and top of the container as shown in figure 2.9161. The

sensor positions require different ultrasound monitoring techniques. Consequently, three

monitoring techniques used to ensure the interpretation of liquid level is successful. The

techniques are wall resonance approach 161 (for side and bottom positions), far wall echo

approach 161 (for side position) and air transmission approach 161 (for top position).

In wall resonance technique, a transducer generates a short ultrasonic pulse that causes a local

resonance in the wall. When the pulse ends, the resonance will dies away whereby the ringing

time will depend on whether the container already fill with fluid or not. If the container is

empty, logically the ringing time will be longer as compared to the one when container is full.

The transducer now react as a receiver immediately detects this ringing time (as sensor is

located just behind the wall) and generates an empty or full signal as appropriate 161 . This


smE TOP

ROTTOM

pter 2: Liquid Level Sensing techniques Literature

technique detects the level of liquid without relying on the far reflection. In second technique

which is far wall echo used the principle in wall resonance technique, however now, a contact

of the transducer is acoustically coupled to the side of a container, instead of coupled to the

wall exited with a pulse, an echo will be detected a short time denoting the first wall. If the

liquid is present between the transducer and the far wall of the container, echo will be

detected after a longer time (dependent on the diameter or width of the container).

Figure 2.8: Sketch of fluid filling rig 161 Figure 2.9: Sensor mounting positions161.

Lastly, the third technique is air transmission approach. Although air is a reasonable good

sound carrier in the frequency of interest, it is poses a major obstacle. The acoustic

impedance of air is much differs from the acoustic impedance of transmitter and test parts,

the most acoustic energy being reflected, and only a very small fraction of that energy

penetrates in and out of the part and transducers. One solution to overcome this problem

without introducing an additional sound transmitting medium consists in generating high

enough sound levels and to use high-gain, low amplification.

2.3.3 CAPACITIVE

A capacitive instrument also can used to determine the level of liquid helium 171 . The sensor

system consists of two concentric cylindrical plates that are an inner and an outer rod of

stainless steel. The level of liquid helium can be determine by partially immerse the plates in

the liquid helium. By doing this, as plates immerse in liquid, the capacitance of the plates is


160 pF

1000 pF 1000 pE C I

SIC OUT PWR th

Chapter 2 . Liquid Level Sensing Techniques Literature

increased as the liquid level is increased. The capacitance of the plates can be measured using

LC oscillator that is based on a tunnel diode.

Figure 2.10 shows the schematic of the liquid helium temperature tunnel diode based on LC

oscillator [' ] . The tunnel diode is used to eliminate parasitic resistance introduced by the

inductor. The tunnel diode biased circuit is shown in figure 2.11171 These two circuits are

connected together by using a coaxial cable (RG-178B1U)'71 . The output frequency of the bias

circuit indirectly measure the capacitance value, hence level of helium liquid is detected.

Figure 2.10: The tunnel diode diode based LC oscillator circuit schematic of the capacitive liquid helium level sensor instrument 171

STRT/RST

L

6 VDC

F

100 U

—F 1000 PF

100

I cwFRO OUT

SG IN PWR OUT

BIAS A0J

Figure 2.11: The schematic of tunnel bias circuit171.

2.3.4 TIME DOMAIN REFLECTOMETRY (TDR)

A TDR method is widely used in fields of hydrology and soil science. A TDR can be used to

measure water level in tank that collecting surface run-of1 81 . The probes are designed to

measure the water level. Figure 2.12181 shows the isometric drawing of water level probe181.

The probe then connected to 50 coaxial cable, which both the probe and coaxial cable made

from stainless steel and connected using 50/200 (1 4) balun transformer 181 . The balun


pter2. Liquid Level Sensing Techniques Literature

transformer is used to match the impedance of the probe and coaxial cable and convert from

unbalanced to balanced transmission line and thus, reducing the signal losses.

--

Figure 2.12: Isometric drawing of water level probe 8'

The water level is determined by measuring distance between the probe head that is placed at

a known distance above the bottom level and the water level in the tank. The distance of the

probe above water level is estimated from the TDR trace using suitable software.

2.4 Discussion

Four techniques of determination of liquid level were presented. The techniques presented

were fibre optic, ultrasonic (ultrasound), capacitive and time domain reflectometry (TDR).

Fibre optic method used in many applications especially in area of chemical and process

industries. Fibre optic is a good candidate in this area since it is electronically passive and

inherently spark free, thus very suitable used in flammable environments such as hydrogen or

oxygen. Consequently, fibre optic is frequently used in low temperature environment because

temperature will gives effect on the light generated by the transmitter such as in [3] and [4] as

it change the behaviour and characteristics of the light as temperature is increased. Therefore

fibre optic will not survive in corrosive and extremely high temperature environment of

molten iron.

Ultrasonic (ultrasound) method demonstrated used in power plant steam generator [5] and in

food and drink industries [6]. Both applications presented being used in room temperature


!Wiapter 2: Liquid Level sensing lechniques Literature

and in temperature up to 350°C environments. Although this method was successfully

working in temperature up to 350°C, however there is insufficient prove to ensure it is

working or survive in temperature over 1000°C. For this reason, this method is not suitable to

be used in corrosive and extremely high temperature environment of molten iron.

Capacitive method is known by name that is very sensitive to heat and therefore capacitive

method definitely cannot be used to measure a molten iron level liquid.

TDR is widely used in area of hydrology and soil science, which is frequently dealing with

low temperature range, and therefore this method would not be used in determining of molten

iron liquid level.

2.5 Conclusion

Various methods of liquid level sensor were presented. All the methods presented were used

in many temperature ranges such as extremely low temperature (cryogenic environment),

room temperature and even in high temperature, which not exceed 500°C.

As mentioned in introduction section, in this project, we are dealing with molten iron which

known it temperature is extremely high. However all the techniques presented only

applicable for temperature ranging from extremely low temperature (cryogenic environment)

to high temperature which is not exceed 500°C and therefore none of the methods presented

will survive in this corrosive and extremely high temperature (over 1000°C).


Chapter 3: 1 neory

CHAPTER 3: THEORY

3.1 Permanent Magnet Brushed D.0 Motor

In a Permanent Magnet motor a coil of wire (called the an-nature) is arranged in the magnetic

field of a permanent magnet in such a way that it rotates when a current is passed through it.

Now, when a coil of wire is moving in a magnetic field a voltage is induced in the coil - so

the current (which is caused by applying a voltage to the coil) causes the armature to rotate

and so generate a voltage. It is the nature of cause and effect in physics that the effect tends to

cancel the cause, so the induced voltage tends to cancel out the applied voltage (indeed were

the effects to add, we should have a perpetual motion machine!).

Voltage is electrical pressure. Current is electrical flow. Pressure tends to cause movement, or

flow so an electrical pressure is a force, which moves electricity - or an 'electromotive force'

(EMF). The induced voltage caused by the armature's movement is a 'back EMF - 'back'

because it tends to cancel out the applied voltage so that the actual voltage (pressure) across

the armature is the difference between the applied voltage and the back EMF.

The value of the back emf is determined by the speed of rotation and the strength of the

magnet(s) such that if the magnet is strong the back emf increases and if the speed increases,

so too does the back emf. It follows from this that if a weaker magnet is used to make a

particular motor, we will get a higher speed motor.

If we now apply the load to the armature, it will slow down. The back emf will decrease so

the difference between applied voltage and back emf will increase. It is this difference that

causes the current in the armature to flow; therefore the current will increase as we increase

the mechanical loading. It should be apparent therefore that an unloaded motor would take

little current. It should also be clear that if we apply more voltage, the motor will speed up,

apply less and it will slow. This is what the motor speed controller does; it varies the voltage

applied to the motor.

Many small dc motors use a permanent magnet like the one shown in figure 3.1 to establish

the required flux. The reason using permanent magnet rather than an excited field winding is

mostly concerned with space, efficiency and the avoidance of having to provide a field

supply. The flux densities obtainable with permanent magnets are less than with exited poles,


mat noh. m. liquid level sensing device a thesis submitted

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