Note: Descriptions are shown in the official language in which they were submitted.
lOSV092
This invention relates to electric arc furnaces,
and more particularly to an improved force sensing apparatus
and method for controlling travel of an electrode as it is
positioned in a furnace to prevent breakage of the electrode
and/or damage to components associated with the electrode.
In conventional operation of an electric arc furnace,
an electric arc is established between an electrode and a
conductive charge of material in the furnace to melt the
furnace charge. Electrode systems in arc furnaces include
an electrode, usually formed by a large rod like member which
projects downwardly into the furnace, and a movable electrode
support structure, formed by a mast and an electrode holder,
located outside the furnace. A drive motor lowers and raises
the electrode support structure to move the electrode toward
and away from the furnace charge. The electrode is connected
to an electrical power supply which provides power for estab-
lishing the arc.
An arc is established as the electrode is moved
toward the charge. After the arc is established, it is stabil-
ized and maintained by controlling the position of the elec-
trode relative to the charge in response to sensed arc con-
ditions. Arc condition responsive drive motor control cir-
cuitry has commonly been employed for governing positioning
of the electrode relative to the furnace charge in response
to sensed arc conditions after an arc is established.
In the absence of an arc between the electrode
and the furnace charge, properly functioning arc condition
responsive drive motor controls operate the drive motor to
advance the electrode toward the furnace charge. When an
arc fails to be properly established as the electrode approach-
es the furnace charge the electrode drive motor will drive
the electrode into the charge. In the absence of drive motor
1.
lO~iV09Z
controls responsive to the proximity of the electrode to the
furnace charge, damage to the electrode is virtually inevitable.
A common reason for failure to establish a stable
arc is that the furnace charge includes materials which will
not conduct a sufficient amount of current to establish an
arc. In recent years the quality of scrap used for arc fur-
nace charges has deteriorated in that increased amounts of
nonconductive materials, such as pit scrap, concrete, wood,
lime, coal, etc., are commonly present in furnace charges.
Accordingly, the possibility of electrode damage resulting
from electrodes being driven into nonconductive material
in the furnace charge occurs more frequently than in the
past.
Electrodes used in arc furnaces are of two basic
types which are commonly referred to as "consumable" and
"nonconsumableD" The so-called nonconsumable electrodes are
usually constructed of connected, rod-like sections formed
of graphite, or an equivalent material, and are actually
consumed relatively slowly during use due to arc erosion
and oxidation. Usage of nonconsumable electrodes is widespread
and these electrodes, because of the nature of the material
from which they are constructed, are relatively easily broken
by contact with nonconductive material in the furnace charge.
Nonconsumable electrodes are characterized by having
relatively high compressive strength and low flexural strength.
Thus these electrodes can usually withstand purely axial
loads resulting from vertical contact with a horizontally
disposed furnace charge during lowering of the electrode
with breaking. However, when such an electrode contacts
a nonhorizontal surface defined by the furnace charge, substan-
tial transverse loads can be applied to the electrode which
tend to flex it. The flexural strength of these electrodes
2.
lOSOO9Z
is so low that transverse loads of relatively small magnitudes
can break an electrode.
Consumable electrodes generally consist of a metallic
material which is consumed during usage at a much greater
rate than the nonconsumable electrodes. Consumable electrodes
are structurally more durable than the nonconsumable electrodes
in that they can withstand greater flexural loads, but these
electrodes can also be broken or otherwise damaged when driven
into engagement with the furnace charge.
Breakage of nonconsumable electrodes resulting
from the electrodes being driven into engagement with the
furnace charge in the absence of an arc has become a serious
problem in the industry not only because of the direct costs
incurred as a result of breaking the electrodes themselves,
but also as a result of consequent production losses and
repair and replacement costs. Electric arc furnaces are
normally "three-phase" furnaces in that each furnace includes
three separate electrode systems. When an electrode is broken
it normally breaks off at the juncture of electrode sections
nearest the electrode holder leaving a large broken-off portion
in the furnace. The broken-off portion may or may not be
salvagable but in any event furnace operation must be termin-
ated to enable replacement of the broken electrode. Extended
idle time of a three-phase furnace for repair and replacement
of one electrode results in substantial production losses
as well as exposing the unbroken electrodes of the remaining
phases to excessive oxidation. Accordingly, attempts have
been made to alleviate or avoid the problem of electrode
breakage.
An obvious technique for preventing the electrode
from being driven into the charge is to visually monitor
movement of the electrode towards the charge in the event
105009Z
an arc is not established. Unfortunately, this technique
is impractical because in order for an operator to visually
monitor the position of any one electrode system in a three-
phase furnace, a furnace door must be opened. In most circum-
stances, however, the geometry of the furnace, the charge,
and the electrodes in the furnace is such that an operator
cannot visually determine the distance between the electrode
tip and the charge. Furthermore, opening furnace doors during
operation of the furnace is a safety hazard. For these reasons
visual monitoring of electrode positîons is not a feasible
or desirable solution to the problem.
One known prior art control system has been proposed
which attempts to sense physical engagement of the electrode
with the furnace charge and to retract the electrode from
the charge. The proposed control system senses force varia-
tions on the electrode system which act along or parallel
to the electrode axis and which may be indicative of compres-
sion of the electrode due to engagement with the charge.
Ir this system an initial electrical signal level indicative
of forces produced by electrode system weight is established
and sensed by the control system. When the electrode engages
the furnace charge, an axial reaction force is applied to
the electrode and the signal level changes in accordance
with the magnitude of the sensed change in the axial force.
If the change in signal level is sufficiently great the control
system causes the electrode to be lifted from the charge.
After the electrode is retracted from the furnace
charge, in accordance with conventional arc furnace operation
the furnace is opened and conductive material, such as aluminum,
is placed beneath the electrode, or mechanical stirring of
the charge is effected to move conductive charge material
beneath the electrode. The electrode is then advanced toward
lOSO~9Z
the charge so that an arc can be established between the
conductive charge material and the electrode. This procedure
usually results in the nonconductive material, which was
originally engaged by the electrode, being melted or burned
up after an arc has been established.
While the prior art control system represents an
improvement over systems which do not employ any controls
to avoid electrode breakage, the proposed control system
exhibits seemingly irreconcilable sensitivity problems.
On one hand the system does not appear to be sufficiently
sensitive to prevent breakage of nonconsumable electrodes
in many instances where the electrode is driven into the
furnace charge. At the same time the system is sensitive
to sensed force changes resulting from electrode erosion
and electrode system position changes and as a consequence
there is a tendency for these sensed forces to falsely indicate
that the electrode has engaged the furnace charge. This
can result in needless withdrawal of the electrode from the
charge.
It should be appreciated that the materials forming
the furnace charge often provide an extremely irregular charge
surface. When an electrode engages the charge the direction
of the engaging force applied to the electrode by the charge
can range from a direction nearly at right angles to the
electrode axis to a direction along or parallel to the elec-
trode axis, depending on the angle of engagement between the
electrode and the charge. The magnitude of the axial component
of a given electrode engaging force varies according to the
angle at which the force is applied to the electrode. Hence
engaging forces applied to an electrode which have small
magnitude axial components can, in fact, have transverse
components which are sufficient to break the electrode. Since
5.
1050092
the prior art control system relies on sensing only the magni-
tude of changes in axial forces acting on the electrode, the
system inherently lacks sensitivity to actual breaking forces
applied to the electrode.
As indicated previously, electrodes are consumed
during use which results in electrode weight reductions~
The weight reductions reduce the level of the gravity forces
acting on the electrode systems thus changing the net axial
force applied to the electrode system. Over a period of
time the force changes on an electrode system due to electrode
consumption can be quite large.
~ach electrode system is connected to an electrical
power supply capable of producing the required arc via a
power cable which is quite massive (e.g., about eight inches
in diameter). Because the electrode systems must be capable
of substantial vertical motion relative to the furnaces,
the length of the power cables must be sufficient to accommo-
date the full travel of the electrode system. The cables
are commonly suspended between the power supply and the elec-
trode system and define catenary curves which vary according
to electrode system position. The axial component of the
force exerted on the electrode system by the power cable
can change appreciably as the electrode system changes position
relative to the furnace.
The magnitude of the changes in axial forces acting
on electrode systems which are attributable to electrode
consumption and electrode system position changes can be
relatively great over a period of time but these force changes
are nondetrimental in that they do not represent any threat
3~ of electrode breakage. The prior art system is undesirably
sensitive to these nondetrimental force changes. Since the
prior art control system detects magnitude changes of axial
1~5 DO9;~
forces applied to the electrode system, and since both elec-
trode breaking orces and nondetrimental forces have axial
lines of action, the prior art control system necessarily
responds to the total magnitude changes of these forces.
Consequently, the sensitivity of the prior art system to
actual breaking forces changes as the furnace operates and
relatively frequent manual compensation is required to re-
establish the desired sensitivity. As a result, while the
p~ior art system did reduce electrode breakage somewhat,
electrode breakage remained a serious problem because of the
lack of sensitivity to actual electrode breaking forces,
and "false tripping" of the control systems, i.e., withdrawal
of the electrode from the charge in the absence of any engage~
ment between the electrode and the charge, would become a
problem.
Electrode systems for arc furnaces have been subject
to damage from causes other than driving the electrode into
the furnace charge. When an electrode is withdrawn from
a furnace, the upward travel of the electrode support struc-
ture must be limited. Limit switches governing the extent
of withdrawal of electrodes from furnaces have commonly been
located near the uppermost position to which the electrode
support structure may be safely raised. The limit switches
frequently fail because of the hostile environment in which
they must be located. When the limit switches do fail the
electrode support structure moves beyond the limit switches
and engages mechanical stops which prevent further movement.
Engagement with the stops can result in damage to the electrode
support structure, the electrode system drive, and/or to
drive transmission components between the drive and the support
structure. The prior art control systems have not provided
fail-safe electrode system operation in the event of upp~r
limit switch failures.
lOS009Z
When an arc has been established and stabilized,
the electrode may be broken by furnace charge cave-ins during
the arcing process. The electrode support structure can
be damaged as a consequence of electrode breakage by a furnace
charge cave-in in the absence of suitable controls. Cave-
in breakage occurs when the electrode melts or burns the
charge immediately underneath its nose or tip leaving unmelted
charge nearby at a higher elevation than the electrode tip.
The elevated unmelted charge may be unstable and thus vibrations
encountered during operation of the furnace may cause some
or all of it to cave in and tumble against the electrode
causing breakage. Electrodes tend to break of~ at a location
adjacent the electrode holder so that a relatively short
electrode portion remains connected to the holder and a rela-
tively long, broken-off electrode portion remains in the fur~
nace.
Breakage of the electrode destroys the arc and,
in an effort to re-establish the arc, the arc condition respon-
sive control circuitry causes the remaining electrode portion
and its support structure to advance toward the charge to
re-establish the arc. If the broken-off electrode portion
is sufficlently long to stay upright due to the upper, broken
end resting against the furnace roof, an arc can be struck
between the broken-off electrode portion and the electrode
holder, causing damage to the electrode holder and the support
structure.
In accordance with the principles of the present
inven'ion movement of an electrode system in an arc furnace
is governed in part by a force sensing control which discrim-
inates between potentially damaging force changes applied
to the electrode system and nondetrimental force changes
applied to the system. By discriminating between potentially
~osoo9z
damaging force changes and those which are nondetrimentall
the sensitivity of the force sensing control to po~entially
damaging force changes is maximized. Still further, changes
in nondetrimental forces applied to the electrode system
are compensated for so that collective changes in detected
electrode system force levels attributable to nondetrimental
force changes are ineffective to alter the sensitivity of
the control.
In accordance with another important feature of the
invention, a method of controlling operation of an arc furnace
electrode system is provided wherein engagement between an
electrode and the furnace charge is sensed, the conductive con-
dition between the electrode and the furnace charge is deter-
mined and, if sufficient conduction exists between the elec-
trode and the furnace charge, the electrode is maintained in
engagement with the charge so that the electrode effectively
melts or burns through the charge material engaging it. A nor-
mal arc can thus be re-established between the electrode and
the charge without requiring withdrawal of the electrode from
the charge.
According to another important aspect of the inven-
tion, the electrode system control senses when movement of the
electrode system away from the furnace charge is obstructed and
automatically terminates operation of the electrode system drive.
This prevents supporting cables~ mechanisms, etc., associated
with the electrode drive from being overstressed or otherwise
damaged as a result of the electrode system withdrawal from the
furnace being obstructed.
Pote~tially damaging forces acting on electrode sys-
tems are characterized by having rates of change of magnitude
with respect to time which are great compared to the rates
of change of magnitude with respect to time of nondetrimental
lOSOO9Z
forces. For example, when an electrode is driven into engage-
ment with the furnace charge, the magnitude of the reaction
force between the electrode and the charge increases rapidly
regardless of the angle of engagement between the charge
and the electrode. Accordingly, the component of any such
force along the axis of the electrode increases rapidly even
if the actual change in force magnitude is small. Conversely,
nondetrimental force changes acting axially of the electrode
due to electrode erosion and/or position changes of the elec-
trode system occur slowly even though the magnitudes of these
force changes, over a period of time, may be great.
Even though these force changes are ~all applied
axially of the electrode and in the same direction of applica-
tion, the forces can be separately identified according to
their rates of change of magnitude with respect to time.
This enables the use of electrode system positioning controls
which are highly sensitive to the application of potentially
damaging forces to the electrode systems and which compensate
for nondetrimental force changes so that the electrode system
positioning controls are effectively insensitive to nondetri-
mental force changes. It should also be appreciated that
since the electrode system positioning controls respond pri-
marily to rates of change of applied force, the controls effec-
tively anticipake the actual application of damaging forces
to the electrode system and therefore are substantially more
sensitive than known prior art controls.
An arc furnace system constructed according to
the present invention includes an arc furnace provided with
an electrode system, a driving arrangement for the electrode
system and a control system for governing operation of the
electrode drive. The electrode syst_m includes an electrode
and an electrode support structure including a movable mast
10 .
10S0~92
and an electrode holder. The electrode system is connected
to an electrical power supply for establishing an arc between
the electrode and the furnace charge by a power cable which
is slung between the electrode system and the arc power supply.
In a preferred embodiment of the invention the driving arrange-
ment for the electrode system includes an electric drive motor
connected to the mast by a cable system so that movement
of the electrode system relative to the furnace is governed
by controlling operation of the drive motor.
The drive motor is controllable by an automatic
motor controller which is operated by an arc condition sensing
control and by an electrode system ~orce sensing control.
The arc condition sensing control detects the arc current
and voltage applied to the electrode and governs operation
of the drive motor to establish and maintain an optimum arc
between the electrode and the furnace charge~ Accordingly,
the arc condition sensing control operates the drive motor,
via the automatic controller, to advance the electrode towards
the charge until an optimum arc is established and continues
to control positioning of the electrode to maintaln an optimum
arc.
During normal operation of the furnace system the
force sensing control detects engagement of the electrode
system with an ohstruction to its movement and terminates
operation of the drive motor via the automatic controller,
regardless of the arc conditions sensed by the arc condition
sensing control~ If an arc is not established as the electrode
is lowered toward the charge the force sensing control detects
engagement of the electrode with the charge and terminates
operation of the motor.
When the electrode has engaged the furnace charge
and the motor is stopped, the electrode remains engaged with
lOSOO9~
the furnace charge. It can then be determined from the arc
condition sensing control whether current is flowing from
the arc power supply to the electrode. If so, the electrode
is maintained in position until the obstruction in the furnace
charge has been melted or burned away from the electrode
tip, after which normal operation of the arc furnace is re-
sumed. This method of operating the furnace effectively in-
creases production in that when the electrode engages the
charge the urnace need not always be shut down, the electrodes
withdrawn, and additional conductive material added to the
charge.
If a negligible amount of current flow through
the electrode is sensed, the electrode is withdra~n from
the furnace, conductive materials are located beneath the
electrode tip and normal furnace operation is resumed.
The force sensing control governs operation of the
motor during upward and downward movement of the electrode
and terminates operation of the motor when the electrode
system encounters an obstruction to its motion. The force
sensing control includes a force sensing circuit for producing
force signals which vary according to sensed changes in forces
applied to the electrode system, an upward motion force signal
responsive control and a downward motion force signal respon-
sive control, both of which are associated with the sensing
circuitry. The force signal responsive controls are individ-
ually effective to terminate operation of the drive motor.
During operation of the furnace and when the electrode
system is moved downwardly toward the furnace charge, the
electrode system may be subjected to force changes resulting
from electrode consumption, force changes caused by electrode
system position changes, and force changes resulting from
the electrode engaging the furnace charge. These force changes
lOS009Z
all alter the force signal produced by the force sensing
circuitry and such changes in the force signal are transmitted
to the downward motion force signal responsive control.
The downward motion force signal responsive control
includes force signal discriminator circuitry which effectively
discriminates between force signal changes depending on the
rate of change of the signal value with respect to time,
and a control signal circuit operable by the discriminator
circuitry for terminating operation of the drive motor.
In the preferred embodiment, when a detected force change
occurs relatively slowly~ the rate of change of the force
signal value with respect to time is low. When the detected
force change occurs relatively quickly, the rate of change
of the force signal value with respect to time is high. The
discriminator circuitry detects the high rate of signal value
change and causes termination of operation o the electrode
system drive motor via the control signal circuit.
Force signal value changes attributable to nondetri-
mental force changes on the electrode system are compensated
for by the downward motion force responsive control. In
the preferred embodiment of the invention, the discriminatorcircuitry includes a compensating signal generator which
detects the force signal changes caused by electrode consumption
and electrode system position changes and produces compensating
signals which are related to such force signal changes. The
compensating signal and the force signal are both fed to
a signal processor which has an output connected to the control
signal circuit. So long as the compensating signals and
the force signals bear a predetermined relationship with
each other, the signal processor output is ineffective to
cause the control signal circuit to terminate operation of
the drive motor.
13.
105~09Z
~s a result, collective changes in force signal
values due to electrode consumption and/or electrode system
position changes are compensated for by operation of the
compensating signal generator. Even though these force signal
value changes may be quite large over a period of time, the
force signal changes are, collectively or individually, in-
effective to terminate operation of the drive motor or to
change the sensitivity of the force responsive control to
force changes caused by engagement of the electrode with
the furnace charge.
The compensating signal generator includes circuit
elements which prevent a compensating signal from being pro-
duced in response to force signal value changes which exceed
a predetermined rate and which are indicative of the electrode
engaging the furnace charge. The circuit elements effectively
limit the compensating signal generator from responding to
force signal value changes which exceed the predetermined
rate. The resulting limited output from the compensating
signal generator is fed to the signal processing circuit
along with the force signal and since the force signal is
uncompensated the signal processor circuit renders the control
signal circuit effective to terminate operation of the drive
motor.
The upward motion force responsive control prevents
possible damage to the electrode system as a result of the
electrode system being engaged with mechanical stops at the
upper limit of its travel as well as in circumstances where
foreign materials become lodged between the mast and its
supports and strongly resist raising of the electrode system.
The upward motion force responsive control includes a compara-
tor for comparing force signal values representative of ob-
structions to upward motion of the electrode system with a
14.
lOCiO09Z
preset reference value an output signal from the comparator
terminates operation of the electrode system drive.
When the electrode system has engaged the upper
limit stops, or its upward motion is otherwise obstructed,
and the upward motion force responsive control has terminated
operation of the electrode system drive, the load imposed
on the electrode system by the upper limit stop remains rela-
tively high until the electrode system drive moves the elec-
trode system downwardly. When the electrode system moves
downwardly the load is abruptly removed from the electrode
system. The effect of the load reduction on the force signal
is the same as if the electrode had engaged the furnace charge
and the downward motion force responsive control tends to
terminate the downward motion of the electrode system. Such
operation of the downward motion force responsive control
is referred to as false tripping.
In the preferred embodiment of the inventi~n the
upward motion force responsive control is associated with
disabling circuitry which is effective to disable the downward
motion force responsive control whenever the electrode system
encounters an obstruction to its upward motion so that the
electrode system can be moved from the upper stops without
its motion being terminated by the downward motion control.
Other features and advantages of the invention
will become apparent from the fol~owing description of prefer-
red embodiments made with reference to the accompanying draw-
ings which form part of the specification.
FIGURE 1 is a schematic view of an electric arc
furnace system constructed according to a preferred embodiment
of the present invention;
FIGURES 2a-2c are perspective views of alternative
constructions of portions of the arc furnace system of FIGURE
1~500~2
1 which are effective to sense forces acting on components
of the arc furnace system;
FIGURE 3 is a side elevation of a sheave assembly
forming part of an arc furnace system embodying the invention
and which is associated with a force sensing element;
FIGURE 4a is a functional block diagram of a control
system embodying the present invention which is associated
with a drive motor for controlling positioning of an arc
furnace electrode system;
FIGURE 4b is a functional block dia~ram of portions
of the control system illustrated in FIGURE 4a;
FIGURE 5 is a circuit diagram of a portion of the
circuitry illustrated by FIGU~ES 4a and 4b; and,
FIGURE 6 is a graphic representation of forces
acting on an electrode in an arc furnace system constructed
according to the invention and forces acting on an electrode
of a prior art arc furnace system.
An arc furnace system 10 embodying the present
invention is illustrated in FIGURE 1 of the drawings and
includes an arc furnace 12, an electrode system including
an electrode 14 and an electrode support structure 16~ an
arc power supply unit 17 for providing arc producing electric
energy to the electrode 14, a drive unit 18 for positioning
the electrode system with respect to the furnace 12, and
a control system 20 for governing operation of the drive
unit 18 to control the positioning of the electrode system
with respect to the furnace.
The furnace system 10 is preferably a three-phase
furnace system in that it includes three electrode systems
and their associated components. The furnace system 10 is
illustrated with only one electrode system and its associated
16.
~105009Z
equipment for the sake of simplicity.
The furnace 12 includes a body 28 which defines
a floor 30. A top or roof section 32 extends over the body
2~ and defines a hole 34 through which the electrode 14 pro-
jects into the furnace. A charge of scrap material is illu-
strated disposed within the furnace body 28 which includes
a nonconductive charge portion 38 located directly below
the electrode 14.
The electrode 14 is preferably a nonconsumable
graphite electrode having a generally cylindrical rod-like
configuration and is formed by a series of joined electrode
sections (not illustrated). The electrode 14 projects gen-
erally vertically through the hole 34 into the furnace. The
electrode support structure 16 comprises a stationary guide
structure 40 situated near the furnace, a support column
42 which is supported by the guide 40 for generally vertical
movement relative to the furnace, a mast 44 which extends
from the support column 42 over the top of the furnace 12
and an electrode holder 46 which interconnects the mast 44
and the electrode 14. The components of the support structure
16 can be of any suitable or conventional construction and
are therefore not described in further detail.
The arc power supply unit 17 is schematically illus-
trated in FIGURE 1 and preferably includes a suitable power
transformer 50 which is positioned near the furnace 12.
A power cable 52 is slung between the transformer 50 and
the electrode system so that electrical power for establishing
an arc between the electrode 14 and the furnace charge 36
is supplied to the electrode from the transformer 50 via
the power cable 52. The cable 52 is preferably relatively
slack to enable vertical movement of the electrode system
relative to the furnace and the power supply unit 17 ~ithout
17.
l~soasz
overstressing the power cable. Accordingly the power cable
defines a catenary curve in its section extending between
the power transformer 50 and the electrode system. As the
electrode system moves upwardly and downwardly the curvature
of the catenary changes resulting in different reaction forces
being exerted axially on the electrode system by the cable.
For the purposes of illustration, the transformer 50 is station-
arily supported adjacent the uppermost level of the electrod~
system travel so that the cable force exerted axially on
the electrode system tends to b~ reduced as the electrode
system moves downwardly and vice versa.
The drive unit 18 preferably includes an electric
drive motor 54 which is connected to the electrode syste~
by a cable drive transmission so that the electrode system
is moved upwardly and downwardly, depending upon the direction
of rotation of the drive motor 54, via the drive transmission.
The cable drive transmission preferably includes a winch
58 connected to an output shaft of the motor 54 and a cable
60 which is wound on the winch 58 and has a dead end which
is fixed with respect to the anchor 62 on the guide structure
40. The cable 60 is reeved on a sheave 64 which is connected
to the guide 40 and a pair of sheaves 66 which are supported
at the lowermost end of the support column 42.
When the motor 54 is operated to drive the winch
58 in a clockwise direction, as viewed in FIGURE 1, the cable
60 is played out from the winch and the electrode system
is moved downwardly by gravity forces acting against the
cable drive transmission. When the motor 54 drives the winch
58 counterclockwise the cable 60 is taken up on the winch
58 to raise the electrode system relative to the furnace.
In the preferred embodiment, the winch 58 is pro-
vided with a suitable brake mechanism (not shown) which is
lOSOO9~
engaged to prevent downward drifting movement of the electrode
system.
The arc furnace system 10 also includes conventional
safety devices for preventing damage to components of the
system. These devices include: slack cable switches which
are effective to stop operation of the motor 54 when slackness
in the cable 60 is sensed in order to prevent the cable from
being detrained from the sheaves; upper limit switches for
sensing the approach of the electrode system towards its
upper limit of travel (defined by a mechanical stop which
is schematically illustrated at 70 in FIGURE 1), and stopping
operation of the motor 54 before the electrode system engages
the mechanical stop 70; and, lower limit switches which sense
the approach o~ the mast 44 towards the furnace top section
32 to stop operation of the motor 54 and prevent a collision
between the mast and the furnace top section. These devices
and their relationships to the drive motor 54 are conventional,
as noted, and therefore are not illustrated or described
in further detail.
The control system 20 is illustrated as housed
in part by a control panel 72 which is accessible to the
furnace operator. The control system 20 is effective to
govern operation of the drive motor 54 to control positioning
of the electrode system relative to the furnace. The system
20 is constructed and arranged so that the furnace operator
can manually control positioning of the electrode system
when desirable. The system 20 also controls the motor 54
automatically in response to sensed conditions of which the
operator may be unaware.
In the preferred embodiment of the invention the
control system 20 is provided with an electrical input signal
from the power transformer 50 by which arc conditions between
19.
lOS009Z
the electrode 14 and the charge 36 are sensed. O~eration
of the motor 54 is governed according to the sensed arc con-
ditions to maintain a stable arc between the electrode and
the charge. The control system 20 is also provided with
an electrical input signal from a force transducer element,
shown schematically at 74 in FIGURE 1, which indicates changes
in force levels acting on the electrode system in directions
parallel to the electrode axis. Characteristic changes in
these detected force levels indicate potentially damaging
forces acting on the electrode system and the control system
20 responds by terminating operation of the motor 54.
The transducer element 74 is illustrated in FI~URE
1 as disposed between the dead end of the cable 60 and the
anchor 62 and when so connected it should be apparent that
any change in the total force applied to the electrode system
which has a component acting axially of the electrode, or
parallel to the electrode axis, will change the level of
force applied to the element 74 between the cable 60 and
the anchor 62.
FIGURES 2a-c illustrate specific alternative con-
structions of transducer elements associated with the cable
60 and the anchor 62 for detecting axial force changes.
As shown in FIGURE 2a a transducer element in the form of
a pin-type load cell 74a is used to join a cable end eyelet
60a to a clevis 62a which is connected to the anchor 62
The load cell 74a is positioned to sense the reaction force
between the eyelet and the clevice. One suitable pin-type
load cell is manufactured by Strainsert of Bryn Mawr, Pa.,
and is disclosed in U.S. Patent No. 3,695,096.
In FIGURE 2b, the transducer is rigidly connected
to the anchor 62 and is acted on by the cable eyelet 60~.
IIere the transducer is a flat type load cell 74b, such as
20.
10,50'~9~
a cell manufactured by Strainsert of Bryn ~awr, Pa., and
~S
disclosed in~Patent No. 3,365,689.
In FIGURE 2c, the transducer 74c is connected to
the anchor 62 and is disposed in tension between the anchor
and the cable 60. The cell 74c is a tension type load cell
of the type manufactured by Lebow Associates, Inc., of Troy,
~lichigan, Model 3127.
FIGURE 3 illustrates an alternative location for
the force sensing transducer element. As illustrated by
FIGURE 3 the sheave 64 is attached to the mast guide 40 by
a support arm 40a. The arm 40a supports the sheave 64 and
its shaft on a pillow block 64a. The transducer 74d is posi-
tioned for compression between the pillow block 64a and the
support arm 40a and is responsive to changes in force applied
by the cable 60 to the guide 40. The transducer 74d is pre-
ferably a compression type load cell manufactured by Lebow
Associates, Inc., of Troy, Michigan, Model 360~.
The constructions described in reference to FIGURES
2 and 3 are illustrative of a few of the possible transducer
element constructions and arrangements which can be employed
to sense changes in axial forces acting on the electrode
system. Other transducer types and mounting arrangements
can be employed if desired.
Referring now to FIGU~E 4a, the control system
20 is schematically illustrated associated with the motor
54. As illustrated in FIGURE 4a the control system 20 includes
a controlled power source 80 for operating the motor 54,
a manual controller 82 by which the furnace operator can
govern operation of the motor 54 via the source 80, and an
automatic controller 84 by which operation of the motor 54
is governed via the power source 80 in response to sensed
conditions. The power source 80 is preferably a Metadyne
lO~OO9Z
~enerator, or equivalent device, and the controllers 82,
84 are suitably constructed circuits which effect operation
of the generator 80 to start and stop the motor 54 and to
drive the motor in opposite rotational directions.
The manual controller 82 includes a manual control
circuit 90 and a power supply 92 and can be operated manually
by the furnace operator to override control of the motor
54 by the auto~atic controller 84 whenever that is desirable.
The furnace operator commonly operates the furnace by con-
ditioning the automatic controller 84 to carry out desired
functions such as raising the electrodes and resuming normal
operation subsequent to electrode withdrawal from the furnace.
This is accomplished by actuation of suitable control switches
on the ~ontrol enclosure 72. In short, the manual controller
82 is used sparingly.
The automatic controller 84 is associated with
an arc condition sensing control circuit 96 and a force re-
sponsive control circuit generally indicated by the reference
numeral 100 which individually govern operation of the auto-
matic controller 84 in response to sensed arc conditions and
sensed electrode system force changes, respectively.
The arc condition control circuitry 96 controls
operation of the motor 54 via the controller 84 in response
to sensed arc current and voltage conditions in the furnace
so that the electrode system is properly positioned to main-
tain a stable, optimum arc current and arc voltage relationship
in the arc between the electrode 14 and the furnace charge
36. The circuitry 96 includes an arc voltage and current
sensing circuit 102 which i~ connected to the power trans-
former 50 via an input signal line 104. The sensing circuit
102 produces an output motor controlling signal w~ich is
transmitted to the con~:roller 84 via an output line 106.
92
An ammeter 108 is associated with the circuitry 96 to enable
the furnace operator to visually determine the current flow
to the electrode when such monitoring is desirable.
The character of the output signal on the 1ine
106 is such that the controller 84 can be conditioned to
operate the motor 54 in either direction at speeds depending
upon the sensed arc current and voltage conditions. When
automatic operation of the furnace system 10 is initiated
by the furnace operator with the electrode system raised,
the arc condition control circuitry 96 detects the absence
of an arc between the electxode and the furnace charge and
accordingly an output signal is delivered to the controller
84 which causes the motor to lower the electrode system rela-
tive to the furnace. As the tip of the electrode 14 approaches
conductive material in the furnace charge 36 immediately
below the electrode tip, an arc is established between the
electrode and the furnace charge. Establishment of the arc
is detected by the control circuitry 96 which in turn con-
ditions the controller 84 to substantially slow the operation
of the motor 54. When the optimum arc current and voltage
levels are established between the electrode and the furnace
charge, the control circuitry 96 is effective to stop the
motor 54 and the electrode system is maintained in position
with respect to the charge so long as the arc current and
voltage levels remain stable.
Conditions within the furnace affect the arc current
and voltage levels and the control circuitry 96 operates
to move the electrode 14 toward or away from the charge to
maintain the arc current and voltage at the optimum levels.
The force responsive control circuitry 100 is assoc-
iated with the controller 84 for stopping the motor 54 whenever
the electrode system encounters an obstruction to its movement
105009Z
which could otherwise damage the electrode system. The elec-
trode system can encounter obstructions when moving upwardly
relative to the furnace and when moving downwardly relative
to the furnace. The control circuitry 100 accordingly includes
a downward motion force responsi~e control 110 and an upward
motion force responsive control 112 each of which is connected
to a common force sensing circuit 114. Each control 11~,
112 is associated ~ith the controller ~4 and is effective,
via the controller ~4, to stop the motor 54 in response to
sensed forces resulting from the obstruction to the electrode
system movement.
The force sensing circuit 114 comprises a force
sensor circuit 115 including the transducer element 74, a
regulated voltage supply 116 and a zero adjusting circuit
117 which combine to produce a D.C. analog signal which varies
according to changes in forces applied to the transducer
element 74. The signal is fed to the input of a linear signal
amplifier 118. The amplifier output provides a D.C. analog
force signal on an output line 119 which varies according
to changes in forces applied to the transducer 74.
In the conv~ntion used in describing the illustrated
embodiment of the invention the force signal on the line
119 is positive with respect to circuit ground and the voltage
supply 116 and zero adjusting circuit 117 cooperate to enable
the force signal level to be ;nitially set within a desired
voltage range.
When the electrode 14 is lowered towards the furnace
charge either upon initiating operation of the furnace system
10 or at some time during its operation, the possibility
exists that nonconducti~e furnace charge material may be
located beneath the tip of the electrode. In th~se circum-
stances an opti~um arc is not established between the electrode
24.
1()50092
and the charge and the arc condition sensing control 96 con-
ditions the controller to operate the motor 54 in a direction
to drive the electrode into the charge.
~hen the electrode engages the charge the resultant
force acting on the electrode, to the extent that force is
not aligned with the electrode axis, has a component force
acting at right angles to the electrode axis, and a component
force acting along or parallel to the electrode axis. Since
the electrode 14 has a low flexural strength, a relatively
small component force acting at right angles to the electrode
axis can load the electrode sufficiently to break it. This
condition is illustrated in ~IGURE 1 with the resultant force
indicated by the vector 120, the axial component force indi-
cated by the vector 122, and the potential breaking force
component indicated by the vector 124.
The existence of potential breaking forces can
be sensed by sensing axial force levels applied to the elec-
trode system since the breaking forces are always accompanied
by an axial force component which acts towards the electrode
and tends to reduce the load of the electrode system on the
cable 60.
As noted previously, electrode weight loss due
to its consumption and changes in electrode system position
also cause changes in the axial forces acting on the electrode
system. It should be apparent that axial force changes caused
by electrode erosion and electrode system position changes,
even if they are of great magnitude, do not represent any
potential hazard to the electrode system while axial force
changes acting in the same effective direction of application
due to engagement of the electrode with the furnace charge
represent the existence of forces which can break the electrode.
25.
lO';iO09Z
The ~orce sensing circuit 114 (FIGURE 4a) reacts
to all of these axial force changes by producing a negative
going force signal on the line 119. The downward motion
force responsive control 110 is connected to the line 119
and is normally conditioned to sense and respond to negative
going force signals on the line 119.
As illustrated by ~IGURE 4a the downward force
responsive control comprises a discriminator circuit 130
and a control signal producing circuit 132 which coact to
terminate operation of the motor 54 via the controller 8g
when the electrode engages the furnace charge. Referring
to FI&URE 4b the discriminator circuit 130 includes a compensa-
ting signal generator 136 and a signal processor 138, prefer-
ably formed by a conventional adder. The input of the com-
pensating signal generator is connected to one input of the
adder by a line 142. The other input of the adder 138 is
connected to the force signal on the line 119 by a line 144.
The compensating signal generator 136 is constructed
and arranged so that its output signal can vary at a predeter-
mined, limited rate in response to negative going input signal.
Hence when the input force signal to the compensating signal
generator is slowly negative going, the compensating signal
generator output level can change at the same rate as the
input force signal. When the input force signal goes negative
at a rate greater than the limiting rate of the compensating
signal generator, the output from the signal generator con-
tinues to change at its limiting rate xegardless of the level
of the force signal relative to the output of the compensating
signal generator.
Nondetrimental force changes on the electrode system
cause the force signal level on the line 119 to shift gradually
in a negative sense and these gradually negative going signals
26.
10'~0092
are detected at the input of the compensating signal generator.
The signal generator 136 responds by producing a compensating
signal on its output line 142 corresponding to the force
signal, inverted. That is to say, if the instantaneous value
of the force signal is five volts positive, the instantaneous
value of the compensating signal output from the generator
136 is about five volts negative.
The compensating signal and the force signal itself
are fed to the adder 138 which functions to algebraically
add the signals at its inputs and produce an output signal
corresponding to the sum of the input signals. Since the
compensating signal is substantially the inverse of the force
signal the sum of these signals is substantially zero and
the adder output signal i5 substantially zero.
When the electrode engages the furnace charge a
rapidly negative going force signal is produced on the line
119. The compensating signal generator 136 is incapable
of producing an cutput signal which changes as rapidly as
the input force signal rate of change. The output signal
from the compensating signal generator 136 thus changes at
its limiting rate and lags the force signal. The output
from the signal generator 136 no longer corresponds to the
inverse of the force signal on the line 11~ and the adder
138 produces an output signal having a level which corresponds
to the difference ~etween the output from the signal generator
136 and the force signal on the line 119.
FIGURE 5 schematically illustrates the compensating
signal generator circuitry of the preferred embodiment.
The generator 136 includes an inverter 150 having its input
connected to the force signal on the line 119 through an
input line 152, normally closed relay contacts 154, a diode
156, an input resistor 158 and the input line 140. The
27.
OO9Z
output of the inverter 150 is connected to the output line
142. The diode 156 i5 poled to conduct forwardly from the
line 119 to the inverter input line 152 so that positive
going force signal voltage levels on the line 119 render
t7ne diode 156 conductive to provide an input signal to the
inverter which in turn produces a compensating signal on
the output line 142.
When negative going force signal voltage levels
appear on the line 119 the level of the input signal on the
inverter i~put does not see the change because of the diode
156. The level actually seen by the inverter input is the
voltage on the capacitor 160. The voltage level on the in-
verter input line 152 is permitted to decay towards the nega-
tive going force signal up to a predetermined, limited rate
of decay. Signal decay elements are associated with the
compensating signal generator for this purpose. The signal
decay elements include a capacitor 160 connected in parallel
with the inverter between the lines 142, 152, a resistor
162 connected around the capacitor from the line 142 to the
anode of the diode 156, and the diode 156 itself. The char-
acteristics of the diode 156 are such that a limited amount
of cathode to anode current flow can occur when its anode
is negative with respect to its cathode. This "leakage"
of the diode 156 enables the capacitor 160 to discharge through
the leakage path provided by the diode 156 and the resistor
162. The rate at which the capacitor 160 discharges is con-
trolled by the diode 156 and the resistor 162.
When the capacitor discharges, the inverter input
voltage on the line 152 decays towards the negative going
force signal on the line 119. As the electrode i5 consumed
and/or the electrode system changes position downwardly,
the force signal level on the line 119 is reduced. This
28.
~05~D09Z
negative going force signal has a low rate of chan~e with
respect to time and the capacitor 160 dischar~es at a rate
which effectively maintains the inverter input signal on
the line 152 equal to the force signal level. The output
signal from the inverter 150 at any instant of time is thus
a negative voltage level substantially corresponding to the
positive level of the force signal at that instant. The
change in the ~orce signal level is thus compensated for
by the compensating signal generator and the adder 138 pro-
duced essentially no output signal.
When the electrode engages the furnace charge the
sensed axial force change on the electrode system is quite
rapid and the resulting negatiye going force signal has a
high rate of change with respect to time. Since the discharge
rate of the capacitor 160 is limited, the inverter input
signal level is unable to follow the negative going force
signal level and the output signal from the inverter does
not correspond to the force signal. This results in the
adder 138 producing an output signal having a level corre-
sponding to the difference between the inverter output signal
on the line 142 and the force signal on the line 119.
The compensating signal generator of FIGURE 5 is
illustrative of one type of circuit usable in the downward
motion force responsive control. The illustrated circuit
~5 has been used in the past as a positive peak detector and
is described in more detail, as a positive peak detector,
in "Operational Amplifier Design and Application," McGraw-
~ill, 1971, p.357, Fig. 9.27.
The signal producing circuit 132 responds to the
output signal from the adder 138 by conditioning the controller
84 to terminate operation of the motor 54 and stop the down-
~ard motion of the electrode system. Referring again to FIGURE
29.
lOSOO9~
4a the signal producing circuit 132 preferably includes a
comparator 170, a reference level source 172, and a control
relay 174.
The comparator 170 has one input connected to the
output of the adder 138 by a line 176 and its other input
connected to the level source 172 by a line 178. When the
adder output signal level cn the line 176 exceeds the reference
level on the line 178 the comparator 170 produces an output
signal for operating the control relay 174 via an output
line 180.
The reference level source 172 can be of suitable
construction but is preferably constructed so that the refer-
ence level is adjustable. Adjusting the reference level pro-
duced by the source 172 changes the sensitivity of the control
110. In the preferred embodiment of the invention a meter
182 is connected in the line 176 between the adder 138 andthe comparator 170 for indicating the adder output level.
The meter 182 is usable in adjusting the reference source
level on the comparator input line 178 as desired.
The control relay 174 includes contact pairs which
are actuated, when the relay is energized, to condition the
controller 84 to stop the motor 54, operate an indicating
device, such as an annunciator horn 184, and to stabilize
the output signal level from the compensating signal generator
and thereby hold the relay 174 in its energized condition
until the electrode engaging force from the furnace charge
is relieved. Referring again to FIGURE 5 the normally closed
contacts 154 are operated by the control relay 174 so that
when the relay 174 is energized the contacts 154 open. This
interrupts the discharge circuit for the capacitor 160 and
further decay of the inverter input signal on the line 152
is prevented.
30.
1050092
The adder input signal on the line 142 is thus
maintained substantially constant and the adder output signal
continues to maintain the relay 174 energized so long as
the electrode remains firmly engaged with the charge. As
S the electrode engaging force is reduced, the force signal
level on the line 119 is positive going. This reduces the
output level from the adder 138 until the relay 174 is de-
energized via the comparator 170.
When the relay 174 is deenergized, the automatic
controller 84 is again enabled to operate the motor 54 to
lower the electrode, the horn 184 is silenced and the com-
pensating signal generator 136 is reset for normal operation,
i.e. the contacts 156 reclose.
There are other circumstances in which the downward
motion control llO terminates operation of the motor 54 to
avoid possible dama~e to the electrode system. In the event
the electrode 14 is broken off as a result of a furnace charge
cave-in the electrode holder 46 can be damaged as a result
of an arc being struck between the holder and the broken
off electrode portion. However, when the electrode 14 is
broken off, the electrode weight i5 abruptly reduced and
this weight reduction has the same affect on the force signal
level as engagement of the electrode with the furnace charge.
The downward motion control terminates operation of the motor
54 so that the electrode system can not be moved towards the
furnace.
Electrode consumption occurring over a period of
time can result in the dimension between the tip of the elec-
trode 14 and the electrode holder 46 being too short to enable
an optimum arc to be struck between the electrode and the
charge without the limit of downward travel of the support
structure 16 being reached. ~s noted previously lower limit
switches are normally provided for controlling the extent
1050092
of downward movement of electrode systems but these switches
are prone to failure. If the limit switches fail, the electrode
system can reach the physical limit of its travel. Should
the motor 54 continue operating to lower the electrode system,
the drive cable can be detrained from its sheaves. The down-
ward motion control 110 is effective to stop the motor 54
in the event of lower limit switch failures when the electrode
system reaches the physical limit of its downward travel.
~hen the electrode system reaches its travel limit, the force
signal is abruptly negative going and the motor 54 is stopped
before the drive cable is appreciably slackened.
It should also be noted that the compensating signal
generator 136 normally produces compensating signals for
any positive going force signal changes regardless of the
rate of change of these positi~e going signals. As a practical
matter the only forces which produce a positive going force
signal which should be compensated for are those forces exerted
on the electrode system as a result of adding additional
sections to the electrode 14 to replace electrode sections
which have been consumed. Electrode system force changes
resulting from the addition of electrode sections obviously
have no potential for damaging the electrode system and auto-
matic compensation for the force changes frees the furnace
operator from having to reindex the force signal level each
time an electrode section is added.
The up~ard motion force responsive control 112
is effective to stop the electrode drive motor 54 via the
automatic controller 84 when an obstruction to upward movement
is encountered by the electrode system and to prevent the
manual controller 82 from energizing the motor 54 to force
the electrode system upwardly against such an obstruction.
Obstructions to upward electrode system movement may result
lOSOO9Z
from objects becoming wedged between the support column 42
and the guide 4Q and resisting upward movement of the electrode
system sufficiently to unduly stress the cable 60. The ob-
struction may also be due to engagement of the electrode system
with the mechanical stop 70 in the event of failure of the
upper limit switch. The upward motion force responsive control
112 comprises a comparator 206, a reference level source
208 and a control relay 210.
The comparator 206 has one input connected to the
force signal on the line 119 via a line 200 and its other
input connected to the reference level source 208 via a line
209. The upward reference source 208 provides a pre-establish-
ed rèference level which is preferably the same as the force
signal level produced by the electrode system in its fully
raised position having an unconsumed electrode and an addi-
tional downward force acting on the system, e.g. 500 pounds.
When the electrode system encounters an obstruction to its
upward movement a positive going force signal is produced
on the input line 200 and when the magnitude of the force
signal exceeds the pre-established reference level the com-
parator 206 produces an output signal on a line 211 for engag-
ing the relay 210.
The relay 210 is associated with contacts actuated
when the relay is energize~ to condition the automatic con-
troller 84 to stop upward mode operation of the motor 54 via
a line 212; condition the manual controller 90, via a line
214, ~o prevent the manual controller from operating the
motor 54 in a direction to raise the electrode system; and
to operate an ann~nciator horn 204 via a line 216. It should
be appreciated that in the event the electrode system encoun-
ters an obstruction while being moved upwardly by operation
of the manual controller 90, operation of the relay 210 over-
lOS009Z
rides the manual controller to terminate operation of the
motor 54.
Since the upward motion control 112 is essentially
level sensitive, the absolute value of the force required
to terminate operation of the motor varies depending primarily
on the current weight of the electrode 14. If maintenance
of the sensitivity of the control 112 is desired, the furnace
operator can adjust the zero adjusting circuitry 117 relative-
ly frequently. However the force levels at which the control
112 terminates operation of the motor 54 are not generally
critical and frequent adjustment of the zero adjusting circuit
117 is not usually essential.
When the electrode system has encountered an obstruc-
tion to its upward movement and the drive motor 54 has stopped,
the load imposed by the obstruction on the electrode system
remains at or about the level at which the upward motion
force responsive control 112 stopped the motor. The force
signal on the line 119 is therefore maintained at a relatively
great positive level. The furnace operator normally conditions
the automatic controller 84 for normal furnace operation in
order to move the electrode system downwardly and release
the load imposed ~y the obstruction. When the motor 54 oper-
ates to move the electrode system downwardly the load imposed
by the obstruction is at least partially relieved quite
quickly resulting in a rapidly negative going change in the
level of the force signal on the line 119.
As noted previously the discriminator circuitry
130 normally responds to any rapidly negative going force
signal, regardless of the initial level of the force signal,
and tends to terminate operation of the drive motor 54.
If permitted to function normally as the load is relieved
the downward motion force responsive control could terminate
105C~09~
operation of the motor 54. A disabling circuit 220 (FIGURES
4a and 4b) coacts with the force responsive control 110,
112 to prevent false tripping of the drive motor 54 when
the load imposed by an obstruction to upward electrode system
movement is relieved.
In the preferred embodiment of the invention, the
disabling circuit 220 disables the upward motion force respon-
sive control 110 by altering the normal operation of the
compensating signal generator 136. To this end the disabling
circuit 220 includes relay contacts 222 which, as illustrated
in FIGURE 5, are connected in parallel with the diode 156.
When an obstruction to upward electrode system movement is
encountered the contacts 222 are closed to shunt the diode
156. The contacts 222 are maintained closed until the load
imposed by the obstruction is relieved. When the diode 156
is shunted, the signal generator operation is altered so
that the inverter input signal value on the line 152 is main-
tained substantially the same as the value of the force signal
on the line 119 regardless of the negative going change rate
of the force signal value. In essence, the signal generator
merely inverts the force signal and hence the output of the
adder remains substantially zero so long as the contacts
222 remain closed.
A preferred construction of the disabling circuit
220 is illustrated in FIGURE 4b and comprises a sh~rting
relay 224 for controlling the contacts 222, a relay 226,
a memory unit 228 for controlling operation of the relays
224, 226, and a memory resetting circuit including a comparator
230 and a compensating signal generator 232.
The memory unit 228 is preferably a flip-flop cir-
cuit, although a latching relay or other suitable memory device
could be employed, and provides output terminals Q, Q which
~o~oo9z
are connected to the respective rela~s 224, 226 by lines 234,
236. In the normal state of the memory 228 the relay 224
is deenergized and the relay 226 is energized. The memory
unit 228 has "SET" and "RESET" input terminals indicated
at S and R, respectively, which control the state of the
unit. The set terminal S is connected to the comparator
output line 211 of the upward motion force responsive control
112 via a line 240 while the reset terminal R is connected
to the output of the comparator 230 via a line 242.
The memory 228 is set by an output signal from
the upward motion force responsive control 112 via the line
240 and when set, the memory energi~es the shorting relay
224 and deenergizes the relay 226. So long as a load is
imposed on the electrode system by an obstruction to upward
movement the memory remains in its set state, and when the
load is relieved the resetting circuitry resets the memory
unit to its normal state, i.e. the shorting relay 224 is
deenergized and the relay 226 is energized.
The compensating signal generator 232 in the reset-
ting circuitry is connected to the force signal on the line
119 via a line 246 and is preferably substantially similar
in construction and general function to the compensating
signal generator 136, with some notable exceptions. The
signal generator 232 produces compensating signals which vary
according to variations in the force signal except for force
signals which are rapidly positive going and therefore char-
acteristic of an obstruction to upward electrode system move-
ment. The rate of change of the output from the signal gen-
erator 232 is limited and therefore does not correspond to
the rapidly positive going force signals. The diode in the
circuit 232 which corresponds to the diode 156 of the FIGURE
5 circuit is poled oppositely from the diode 156 in order
36.
10.~009'~
to enable the limit response to the posit~ve going signals.
The relay contacts (schematically illustrated at
250 in FIGURE 4b) in the signal generator 232 which correspond
to the contacts 154 of the FIGURE 5 circuit are controlled
by the relay 226 and are normally open contacts. The contacts
250 are maintained closed by the normally energized relay
226. Since the disabling circuit cannot falsely terminate
operation of the motor 54 there are no shorting contacts
around the diode in the generator 232.
The output from the signal generator 232 is fed
to one input terminal of the comparator 230 via an output
line 252. The other input terminal of the comparator is
connected to the force signal on the line 119 via an input
line 254.
When an obstruction to upward movement of the elec-
trode system is encountered the force signal goes positive
rapidly. The memory input terminal S is provided with a
set signal from the upward motion control 112 via the line
240 when the load created by the obstruction reaches a pre-
determined level. The memory changes state causing energiza-
tion of the shorting relay 224 and deenergization of the relay
226. Energization of the relay 224 disables the upward motion
control 110, as noted above, while deenergization of the
relay 226 opens the relay contacts 250 in the compensating
signal generator 232.
Opening of the relay contacts 250 prevents the
signal output from the generator 232 from decaying. The
output signal from the generator 232 i5 thus maintained close
to the force signal level which existed before the obstruction
was encountered and the input signals to the comparator 230
are at substantially different levels~ As the load created
by the obstruction is relieved during downward motion of
lOSOO9Z
the electrode system the force signal level on the comparator
input line 254 is reduced to the level of the signal generator
output on the line 252 which causes the comparator 230 to
change its conductive state and reset the memory 228 to its
normal condition.
The upward and downward motion controls 110, 112
are thus returned to their normal operating conditions and
automatic operation of the furnace system 10 ensues without
false tripping of the motor 54 having occurred.
FIGURE 6 graphically depicts the breaking force
levels applied to nonconsumable electrodes of electrode systems
-which do not have force responsive controls, versus the break-
ing force levels applied to an electrode of an electrode system
controlled according to the present invention.
The graph of FIGURE 6 assumes an electrode is ad-
vanced into engagement with the furnace charge with a ~WLXi~
force W which, for the purpose of the discussion is assumed
to be the weight of the electrode system. The ordinate of
the graph depicts electrode breaking force in terms of the
force W while the abscissa indicates the angle ~between the
electrode axis and the resultant furnace charge engaging
force.
The curve I of FIGURE 6 illustrates that an electrode
system without controls can engage the furnace charge with
a force W without exceeding the electrode ~reaking force
limit, indicated by the line 260, so long as the angle ~
is less than about 15 degrees. The broken line segment of
the curve I indicates the projected breaking force levels
which would be applied had the electrode remained unbroken.
Curve II illustrates the loci of breaking force
levels applied to an electrode when downward travel o~ the
electrode system is terminated by a downward motion force
38.
1050109Z
responsive control constructed according to the invention.
The total potential force with which the controlled
electrode system remains W but because of the operation of
the controlled electrode system the Eorce W is not actually
applied to the electrode. The controlled electrode is illus-
trated as being broken by engaging -the charge at an angle
of about 65. It should be appreciated however that by ad-
justing the level of the reference source 172 (FIGURE 4a),
the electrode could remain unbroken at contact angles greater
than 65.
As a practical matter, when the angle of contact
closely approaches 90 the axial force components are of
such small magnitude and duration that the control circuitry
itself becomes insensitive to them. This insensitivity to
small magnitude rapidly changing axial forces is not truly
a disadvantage of the new control system because electrode
systems are subjected to rapidly changing, small magnitude
vibrational forces during normal furnace operation. The
new force responsive control does not and should not respond
to such vibrational forces by terminating electrode movement.
In addition to stopping electrode motion when the
electrode engages the furnace charge, the control system
20 permits the furnace operator to determine the nature of
the obstruction which the electrode has engaged. If the
material engaging the electrode i5 somewhat conductive, the
meter 103 of the arc condition responsive control 96 will
indicate that current is flowing to the material even through
optimum arc current and voltage conditions do not exist.
The furnace operator can observe the meter 108 and if arc
current flow is substantial the operator allows the electrode
to remain engaged with the obstruction. The obstruction
is then melted or burned away by the current flowing through
39.
~05009Z
the electrode and the obstruction and normal furnace operation
automatically ensues.
If the meter 100 indicates little or no current
flow the obstruction cannot normally be melted or burned
away because it is not suf~iciently conductive. The operator
then raises the electrode a substantial distance away from
the charge and shuts down the power to the furnace. Additional
material, such as aluminum, is then introduced into the fur-
nace beneath the electrode, power is supplied to the furnace
and the operator conditions the controller 84 for automatic
operation of the furnace system.
These last mentioned procedures are time consuming
and reduce production. By melting or burning away electrode
obstructions which are somewhat conductive without having
to introduce additional conductive material into the furnace
needlessly the furnace production is increased.
In addition to minimizing electrode breakage due
to engagement with the furnace charge, the new force responsive
control may be used by the furnace operator in "slipping"
the electrode. In order to establish a proper length between
the electrode tip and the holder, the furnace operator cuts
off the arc power and lowers the electrode system until the
electrode contacts the furnace charge. The control system
20 indicates that the electrode has contacted the charge
and lowering of the electrode system is stopped with the
electrode engaging the furnace charge. The electrode holder
is then declamped from the electrode and the mast and the
holder are moved upwardly relative to the electrode, which
is now supported hy the charge, until the holder is located
3Q a desired distance from the electrode tip. The holder is
then reclamped to the electrode and the electrode system
is raised relative to the furnace preparatory to resumption
40.
lOSC~09Z
of normal furnace operation.
In the past, electrode slipping procedures were
such that electrode breakage sometimes occurred. In some
circumstances the electrode was engaged with the charge with
excessive force, causing the electrode to break off. In
other cases the holder was declamped from the electrode when
the electrode tip was located relatively far above the charge
and the electrode was dropped onto the charge, sometimes
resulting in breakage. The present invention substantially
reduces the possibility of electrode breakage during a "slip-
ping" procedure.
Although the invention has been described with parti-
cularity, in its preferred form, it should be understood
that the present disclosure of the preferred form has been
made only by way of example. Numerous adaptations, modifica-
tions and uses of the invention may occur to those skilled
in the ar~ and the intention is to cover hereby all such
adaptations, modifications and uses which fall within the
spirit and scope of the appended claims.
41.
