Method for the automation of the horizontal continuous casting installation with programmable logical automata.
Tufoi, Marius ; Marta, Constantin ; Vela, Ion 等
1. INTRODUCTION
The present article aims at briefly presenting a method of
automatic control of industrial processes and implicitly of the
automatic controlling system through which processes are synthesised,
with the help of programmable automata (PLC-Programmable Logic
Controller). The method is new, modern, and has been implemented in the
world technique for a relatively short time. Nowadays the method is
gaining more and more terrain compared to the classical methods. The
method is the most rapid implementation and almost the cheapest method
for automatic controls of industrial processes of small and medium
complexity. The notion of system always refers to a multitude of
objects, actions or ideas in interaction for a certain purpose. In this
article we shall use this denomination only in connection with a
multitude of physical objects. If we refer to the technical objects in
an installation, which, together, provide certain services, the system
is also known under the name of process. The systems of automatic
control, in short the automatic systems, are a particular type of
systems whose goals are to operate well, without man's
intervention, reaching certain levels of performance in different
operation regimes: monitoring of a programme or of the variation of a
reference value, adjustment for perturbations elimination, filtering of
noises or insensitivity to the variation of the controlled process from
the installation. Preference is granted to projects promoting continuous
casting and casting techniques as close to the final shape as possible
with or without direct rolling. In the present case we refer to the
automation of processes of horizontal continuous casting (HCC).
2. THEORETICAL CONSIDERATIONS
Energy crises marked a turning point for the steel industry, i.e.
the energy saving in the processes of steel manufacture have become an
issue of intense interest. The goals for the saving and efficient use of
energy and labour force have drawn the attention upon the HCC as the
next generation of HCC facilities with simple structure, enabling the
direct rolling of the cast steel.
[FIGURE 1 OMITTED]
The diagram of an installation of continuous horizontal casting is
shown in figure 1. The process of continuos casting, essentially
consists in the direct transformation of the liquid metal into solid
metal in different geometric shapes: bars, slabs, billets, pipes, wires.
The liquid metal alloy in the canal induction maintenance furnace (1),
under its own weight (metal-static pressure), flows into a crystallising
die (2) and is cooled in cold water, and thus solidification takes
place. The result of this solidification is the obtaining of a section
of the resulted profile identical with that of the die. The resulted
profiles pass then to an intermediate support (3) and after that through
the intermediate cooling system (4), where it is cooled and then
evacuated through the installation of profile drawing (5). The process
is continuous, as the metal solidified in the die is evacuated, the
molten alloy comes under the metal-static pressure into the die and the
process goes on. After the drawing of the profile with the help of the
drawing installation, the profile is transferred to the synchronised
cutting system (6) where it is cut at the intended length and cooled in
the final cooling installation (7). In the horizontal continuous casting
it is necessary to form a primary shell solidified to a constant
position of the casting mould in order to remove the strand without
interruptions. The continuos elimination of the cast stand can lead to
the interruption induced by soldering at the exit from the mould.
Unfortunately, in order to prevent the interruption during the
horizontal continuos casting, it is important to intermittently remove
the cast stand. The intermittent removal of the cast stand normally
leads to the formation of a shell solidified from the piercing ring in
the casting mould / die and enables a steady operation. One has recently
adopted not only the intermittent removal, but also the removal with a
slight compression towards the direction opposite to the removal
direction (Push Back--back push) in order to facilitate the steady
removal of the solidified shell. (Marta, 2005). The removal of the stand
is generally done with the help of cylinders. The pressing cylinder can
cause considerable deformations at the surface of circular section
ingots. The removal unit using tightening sabots offers a larger contact
surface with the stand does not lead to important deformations like in
the case of the cylinder removal. The intermittent elimination and the
use of a piercing ring result in a structure of discontinuous solidification at the stand surface. This leads to separation or
cracking, the so-called cold interruption cracking, which results from
the insufficient cohesion at the discontinuous solidification border.
The measures for preventing these cracks include the reduction of the
cold cracks depths and the decrease of the cracking sensitivity; they
can be obtained through the increase of the intermittent removal rate,
the reduction of the height of the step between the mould and the
piercing ring, the electromagnetic stirring of the molten steel in the
casting mould and the increase of the casting temperature. On the other
hand, HCC eliminates the levelling cracks. The automation modality for
the continuos casting process is shown below. The automation of
industrial processes and their control by means of programmable automata
brings about a series of advantages, both from the viewpoint of quality
or products or services, and from the perspective of manufacture and
maintenance cost. At present, older automations of the horizontal
continuous casting installations are executed with contact schemes and
relays, and the most modern ones with process computers (Margineanu,
2005). The authors propose the achievement of the automation of the HCC
installation with programmable automata, which bring the following
advantages: low cost of automation through the replacement of process
computers by programmable logical automata, the outputs of the
programmable logical automata are directly compatibly with the execution
elements of the automated installation, one may dynamically change the
installation parameters in accordance with the type and dimensions of
the cast material, easy programming of the programmable logical automata
by medium-qualified personnel in the field, the dimensions of automation
are small, reduction of the energy consumption necessary to automation,
obtaining quality products in the process of HCC.
3. AUTOMATION OF THE INSTALLATION
The automation of the horizontal continuous casting installation
(HCC) implies the automation of each process in the system. These
processes are: the process of constant maintenance of temperature in the
molten metal mass; the process of profile drawing; the process of
profile cooling; the process of profile elimination; the process of
final cooling and cutting of the resulted profile. For the programming
of programmable logical automata one uses the firm software VersaPro
V2.03, from the manufacturer GE Fanuc. For programming one uses the LAD
(Ladder Diagram) language, which is a graphic language. The Ladder
language or the Ladder Diagrams are usual specialised diagrams in the
documentation of the industrial logical programmable logical control
systems. They are called the Ladder diagram (<< ladder >>),
as they resemble a ladder with two vertical lines (feeding) and several
"steps", horizontal lines which are the control circuits to be
plotted (Hughes, 2002).
[FIGURE 2 OMITTED]
The Ladder diagrams are used for applications with programmable
logical automata by programmers with previous experience in automations
with contacts and relays. The graphic symbols of the language are placed
in the diagram similarly to the placement of the contacts and relays in
the electrical diagram. Figure 2 presents the diagram with all the
processes intervening in the automated installation of horizontal
continuous casting.
The automation of a control system with programmable automata has
at least the following stages (Parr, 1999):
1-identification of the process;
2-establishment of the control algorithm and of the required
performances;
3-PLC programming;
4-PLC configuration.
Au automatic process is characterised by the presence of the memory
or feed back connection (feedback, response) fig. 3.
[FIGURE 3 OMITTED]
4. CONCLUSION
The programmable automata are simple microcomputers, specially
built for solving, through the programme, the problems of sequential
logic, and replacing the control of sequential automations with cable
logic done with logical circuits or relays. This equipment generally
offer less opportunities than the informatics or process computers, but
they can be used very easily by a personnel less specialised in computer
sciences, due to the simpler programming languages, such as the relay
language, Boolean equations language or graphic languages. The running a
programme on a programmable automaton is generally of the synchronous
type, which eliminates the complications occurring in the case of
multitasking. In the conventional HCC installations the intermittent
removal and the use of a piercing ring give a structure of discontinuous
solidification at the stand surface which leads to the so called cold
interruption cracking, resulted from the insufficient cohesion at the
discontinuous solidification border. It is important that in the
proposed automation variant with PLC, the abnormal structure is
eliminated, structure caused by the traces of the cold breaking, due to
the fact that is assures a continuous operation of the installation,
i.e. the strand removal is done continually, through the strict control
of temperature, eliminating the frequent intermittent removals of the
strand. This method can be improved by further research, possibly by
replacing PLC with programmable microcontrollers.
5. REFERENCES
Hughes, T. (2002). Programmable controllers, TWT Press Inc., ISBN:
978-1-55617-899-3
Margineanu, I. (2005). Programmable automata, Blue Publishing
House, ISBN: 973-650-156-6, Cluj-Napoca Marta, C. (2005) Numerical
methods of simulation of casting and solidification of metal alloys,
<< Eftimie Murgu>>, Publishing House, ISBN:973-8286-66-2,
Resita
Parr, E.A. (1999). Programmable controllers, Butterworth-Heinemann,
ISBN 075-063-935-0
***Versa Pro V2.03., programming PLC software
