Title: Detection of roller damage and/or misalignment in continuous casting of metals
Abstract: A method for detecting roller irregularities during on-line continuous casting of a metal comprises; i) continuously monitoring the changes in the mould level over time; ii) identifying large periodic influences affecting the mould level versus time function and their frequency, iii) comparing the frequency of the periodic influences of step ii) with predicted frequency harmonics based on a normal operation of the casting process and highlighting by comparison of the predicted and actual frequencies characteristics indicative of irregularities in roller behavior.
Patent Number: 6,845,286 Issued on 01/18/2005 to Hewitt
| Inventors:
|
Hewitt; Philip Neill (Middlesbrough, GB)
|
| Assignee:
|
Corus UK Limited (London, GB)
|
| Appl. No.:
|
257380 |
| Filed:
|
October 18, 2002 |
| PCT Filed:
|
April 18, 2001
|
| PCT NO:
|
PCT/GB01/01739
|
| 371 Date:
|
October 18, 2002
|
| 102(e) Date:
|
October 18, 2002
|
| PCT PUB.NO.:
|
WO01/79588 |
| PCT PUB. Date:
|
October 25, 2001 |
Foreign Application Priority Data
| Current U.S. Class: |
700/146; 164/451 |
| Intern'l Class: |
B22D 011/16 |
| Field of Search: |
73/593
700/146
164/451,151.1,151.3,449.1
|
References Cited [Referenced By]
U.S. Patent Documents
| 6289971 | Sep., 2001 | Kagawa | 164/449.
|
| 6466001 | Oct., 2002 | Hanazaki et al. | 324/76.
|
| Foreign Patent Documents |
| 0 776 708 | Apr., 1997 | EP.
| |
Other References
Patent Abstracts of Japan, vol. 1998, No. 12, Oct. 31, 1998, & JP 10 193053
A, (Sumitomo Metal Ind Ltd), Jul. 28, 1998.
Patent Abstracts of Japan, vol. 1998, No. 05, Apr. 30, 1998, & JP 10 005957
A (NKK Corp), Jan. 13, 1998.
Patent Abstracts of Japan, vol. 1999, No. 08, Jun. 30, 1999, & JP 11 077268
A (NKK Corp), Mar. 23, 1999.
Patent Abstracts of Japan, vol. 014, No. 478, Oct. 18, 1990, & JP 02 192863
A (Sumitomo Metal Ind Ltd), Jul. 30, 1990.
DATABASE WPI Section Ch, Week 199832, Derwent Publications Ltd., London,
GB; AN 1998-370273, XP002181099 & JP 10 146658 A (Nippon Steel Corp), Jun.
2, 1998.
|
Primary Examiner: Chapman; John E.
Attorney, Agent or Firm: Bacon & Thomas, PLLC
Claims
What is claimed is:
1. A method for detecting roller irregularities during on-line continuous
casting of a metal comprising:
i) continuously monitoring the changes in the mould level over time;
ii) identifying large periodic influences affecting the mould level versus
time function and their frequency,
iii) comparing the frequency of the periodic influences of step ii) with
predicted frequency harmonics based on a normal operation of the casting
process and highlighting by comparison of the predicted and actual
frequencies characteristics indicative of irregularities in roller
behaviour.
2. A method as claimed in claim 1 wherein step ii) involves applying a Fast
Fourier Transform to the mould level versus time function of step I).
3. A method as claimed in claim 1 wherein predicted frequency harmonics of
step iii) are calculated from the equation:
##EQU3##
where:
.function..sub.d is the frequency of the harmonic in Hz
v.sub.c is the casting speed in m/s
d is the roll diameter in metres.
4. A method as claimed in claim 1 wherein predicted frequency harmonics of
step iii) are calculated from the equation:
##EQU4##
where
.function..sub.p is the frequency of the harmonic in Hz
v.sub.c is the casting speed in m/s
p is the roller pitch in metres.
5. A method as claimed in claim 1 further comprising monitoring the casting
speed and alerting the system user to significant variations in the
casting speed.
6. A method as claimed in claim 1 further comprising;
modeling the cast metal strand to determine the final point of
solidification and discounting any periodic influences arising from
segments in the casting machine which are before said final point of
solidification.
7. Apparatus for performing the method of claim 1 comprising:
a mould level sensor and a computer, the computer being provided with a
computer programme giving operating instructions to perform the method of
claim 1.
8. A computer programme for detecting roller irregularities during on-line
continuous casting of a metal, the computer program product comprising:
a computer readable storage medium having computer readable program code
means embodied in said medium, said code means comprising:
an algorithm which comprises the steps of:
analysing mould level data using Fast Fourier Transformation to create a
frequency spectrum;
determining whether casting speed is sufficiently stable during time span
of the mould level Fast Fourier Transform;
informing the user if the casting speed is too variable;
in the event that the casting speed is sufficiently stable plotting a graph
of the calculated roll diameter and roll pitch lines over the Fast Fourier
Transform; and
informing the user of any roll diameters and roll pitches which show up on
the Fast Fourier Transform graph.
Description
This invention relates to continuous casting of metals and to the detection
of roller malfunction or damage on-line.
The process of continuous casting is well known in the metal processing
industry. Basically, this process involves the use of a high level mould
for receiving the molten metal, the mould having an exit at its lower end
from which the cast strand emerges and is carried by a roller conveyor
from a vertical to a horizontal position, although some machines are
wholly vertical. Water sprays may be used to cool the metal strand in the
roller conveyor. The roller conveyor comprises a plurality of rollers
arranged in pairs at a set distance apart which defines the thickness
and/or depth of the cast strand. The process may run continuously for
weeks at a time at high temperature with large volumes of cast metal
running through the conveyor, thus there is considerable scope for damage,
wear or movement of the rollers from their starting condition.
Any change in the diameter, circularity, linearity, eccentricity, alignment
of a roll or failure of support bearings may lead to variations in the set
distance between pairs of the rollers resulting in consequent variations
in the thickness of the partially molten cast strand. Such changes in the
distance between pairs of rollers can squeeze or expand the cast strand
leading to distortions at the mould level which can lead to surface
defects in the final product formed at the initial point of
solidification. The pumping effect of intermittent squeezing and expansion
on liquid metal in the cast strand can also lead to segregation, internal
cracking and porosity problems in the centre of the strand.
Thus, it is desirable to monitor the condition of rollers and to maintain,
where possible, a continuity in the geometry and alignment of the rollers
both during and between castings. Existing methods for detecting
irregularities in the rollers of a continuous casting machine are based on
the use of a sensor head which is attached to the dummy bar and sent
through the machine when it is off-line or at the start of cast, These
sensors rely on contact with the surface of the rollers to provide
information as to the geometry and/or alignment of the rollers. Examples
of such methods and apparatus for performing these methods are known from
prior published patents and applications GB 2 097125 A, U.S. Pat. Nos.
4,344,232, 4,361,962, 3,983,631 and 3,962,794.
A disadvantage of the prior published methods and apparatus is that they
require the method to be carried out with the casting machine off-line and
cold. This can result in considerable down time in the casting process
thus increasing overhead costs. In addition, roller problems are often due
to effects such as adhesion of particles to the rollers at high
temperature or distortion at high temperature which cannot be detected
off-line. As sequence lengths increase to times in the order of weeks the
need for information during a sequence becomes more important.
The present invention seeks to alleviate these problems. In accordance with
the present invention there is provided a method for detecting roller
irregularities during on-line continuous casting of a metal comprising;
i) continuously monitoring the changes in the mould level over time;
ii) identifying large periodic influences affecting the mould level versus
time function; and their frequency
iii) comparing the frequency of the periodic influences of step ii) with
predicted frequency harmonics based on a normal operation of the casting
process and highlighting by comparison of the predicted and actual
frequencies characteristics indicative of irregularities in roller
behaviour.
The preferred means for identifying the large periodic influences in step
ii) is by applying a mathematical transformation, preferably a Fourier
transform, most preferably a Fast Fourier transform. This transform
separates the complex mould signal enabling highlighting of periodic
influences in the signal by separating out background noise, thus allowing
easier identification of periodic and unexpected influences due to the
asymmetric operation of a damaged or misaligned roller.
For the purposes of clarification, the invention will now be further
described with reference to the following figures in which:
FIG. 1 shows a typical signal from a mould level sensor illustrating the
function of mould level versus time.
FIG. 2 shows a Fast Fourier Transform of the function of FIG. 1, as
determined in step ii) of the method of the invention.
FIG. 3 shows a Fast Fourier Transform for a different mould level versus
time function on which has been superimposed predicted frequency harmonics
for rollers of known diameter and/or pitch for comparison as described in
step iii) of the method of the invention.
FIG. 4 shows a flow chart for an algorithm for use in performing the
method.
FIG. 1 shows a sample of mould levels recorded over a period of 512
seconds. The vertical axis of the graph shown depicts the mould level
measured and the horizontal axis depicts time elapsed over the monitored
period. As can be seen the signal has periodic components.
The inventors have found that a mathematical analysis of the function
produced by a plot of mould level against time reveals periodic influences
at frequencies which can be correlated with the activities of the rollers.
Any significant increase in amplitude of the transformed signal at a
particular frequency may be indicative of an irregularity in a roller's
behaviour which may be attributable to damage, misalignment or similar
problems with the casting machine. For example, a roller which has
sustained damage at a point on its circumference so as to affect its
rotational symmetry will impart a periodic variation to the strand width
passing between that roller and its pair. This periodic influence will be
highlighted in the transform generated in step ii) of the method.
The expected frequency of a harmonic for a particular roller at a
particular casting speed over the period sampled can be calculated from
simple formulae. Any significant increase in amplitude of the transformed
signal at a frequency harmonic can provide an indication of the type of
damage or other problem with the roller generating that harmonic.
Typically roller diameter and pitch of rollers on a casting machine are
designed to be different at different points along the length of the
machine to account for variations in the properties of the metal as it
cools. Rollers are generally grouped in multiples of similar size and
pitch across particular segment(s) of the casting machine. Thus, as well
as identifying the occurrence of a roller problem, the method can locate
the position of the problem roller to within an identifiable group of
rollers of known size and pitch.
The expected harmonic frequency associated with a roller of a particular
diameter can be calculated from the simple equation:
##EQU1##
where:
.function..sub.d is the frequency of the harmonic in Hz
.nu..sub.c is the casting speed in m/s
d is the roll diameter in metres.
It has been observed that the frequency harmonics associated with a
particular roll diameter will appear as multiples of the base frequency
determined from the above equation. For example, if a roll is
significantly warped the frequency may be twice or four times that
expected.
Similarly the harmonic frequency associated with a particular pitch between
roller centres can be calculated from the simple equation:
##EQU2##
where
.function..sub.p is the frequency of the harmonic in Hz
.nu..sub.c is the casting speed in m/s
p is the roller pitch in metres
It will be understood that since each of the above referenced formulae rely
on a continuous casting speed for accuracy it is desirable to monitor the
casting speed. Conveniently, the apparatus used to implement the method
may incorporate an alarm for alerting the system user to a variation in
casting speed. Optionally the apparatus may interpolate from periods of
constant speed to provide an estimate of roller properties.
The method is conveniently carried out by a computer programme which
receives as an input mould level data from a mould level sensor. The
sensor may be provided in any suitable form where the signal recorded can
be converted into computer readable form. Existing technologies include
electromagnetic sensors, radioactive sensors and light sensors. The
computer programme may also receive an input related to the casting speed.
When stable casting speed conditions are recognised, the programme applies
an appropriate mathematical transform to the mould level versus time
function to identify underlying periodic influences which relate to roll
behaviour. Once the periodic influences are identified the programme may
compare the recorded data against the predicted harmonics to locate
problem areas.
The method of the present invention is particularly suited to casting of
thin or narrow thickness strands where smaller diameter rollers and higher
casting speeds are used. A Fourier Transformer utilises binary numbers and
the period measured should consist of a binary number of seconds.
Typically mould level data taken over a period of 512 seconds of
continuous speed casting is sufficient for the method to provide an
accurate analysis of machine condition in these applications.
A Fast Fourier Transformation is applied to the mould level versus time
function and calculates the simplistic periodic waveforms which can be
summed up to obtain the original more complex waveform. Large periodic
influences on the mould level signal, such as that which may be caused by
damaged or misaligned rollers are highlighted as large peaks in the Fast
Fourier transform frequency distribution as shown in FIG. 2. As can be
seen a large peak has occurred around 0.1 Hz; this is indicative of an
irregularity with respect to a roller.
The Fast Fourier transform in FIG. 3 again shows a large peak at a
frequency of around 0.095 Hz. With the predicted frequencies superimposed
onto the transform, an association can be made between the magnitude of
the peak which indicates a problem with a roller, and the frequency at
which the peak occurs which locates the position of the problem. As can be
seen the peak at around 0.095 Hz occurs coincident with the frequency
harmonic calculated for the 140 mm roll diameter in Segment 1. Thus it can
be deduced that the problem is likely to be with a roller within that
segment or segments.
FIG. 4 shows a flow chart for an algorithm for use in performin the method.
Mould level and casting speed data is acquired from the plant. The mould
level data is then analysed using fast Fourier Transform to create a
frequency spectrum.
Next the casting speed is analysed to determine whether this is
sufficiently stable during the time span of the mould level Fast Fourier
Transform.
If the casting speed is sufficiently stable the fast Fourier transform data
is plotted on a graph. The roll diameter and roll pitch lies are then
calculated and the data plotted over the top of the Fast Fourier Transform
graph. Any roll diameters and roll pitches that show upon the Fast Fourier
Transform are determined and the user advised accordingly.
The algorithm then waits for the next sampling interval before returning to
the first step of acquiring data from the plant.
If the casting speed is too variable then the user is advised of this, the
algorithm then waiting for the next sampling interval before acquiring
fresh data from the plant.
The method may be further enhanced by modelling the strand to determine the
final point of solidification. As will be understood by the skilled
artisan, any segment(s) in the machine which have passed through the final
point of solidification are not able to influence the mould level signal
and can therefore be ignored in any analysis.
It is to be understood that the preferred techniques described for carrying
out the method are purely exemplary and other suitable techniques will
occur to the skilled reader without departing from the true scope of the
invention which is directed to the on-line detection and location of
roller irregularities during continuous casting through analysis of the
mould level signal.
*
