mat noh. m. liquid level sensing device a thesis submitted
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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
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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.
Liquid Level Sensing Device 3
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.
Liquid Level Sensing Device 4
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
Liquid Level Sensing Device 5
Chapter 2: Liquid Level Sensing Techniques Literature
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
Liquid Level Sensing Device 6
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
Liquid Level Sensing Device 7
Chapter 2: Liquid Level Sensing Techniques Literature
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).
Liquid Level Sensing Device 8
Chapter 2: Liquid Level Sensing Techniques Literature
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).
Liquid Level Sensing Device 9
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.
Liquid Level Sensing Device 10
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
Liquid Level Sensing Device 11
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
Liquid Level Sensing Device 12
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
Liquid Level Sensing Device 13
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
Liquid Level Sensing Device 14
!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).
Liquid Level Sensing Device 15
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,
Liquid Level Sensing Device 16
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