Innovative measuring solutions using laser sensing technologies
– RDCIS interventions in steel plant applications
Archana Sharan, S Ilangovan, Deepak Kumar, Anup Prasad, Ramanuj Prasad, S Majumdar
and B K Prasad
RDCIS, SAIL, Ranchi-834002
e-mail: archana@sail-drcis.com
ABSTRACT
Use of Non-contact principles, in general and Laser technologies, in particular is
considered one of the major evolution in sensing systems for industrial applications.
Unique properties of monochromaticity, low divergence, and high intensity at low power
have made it possible to develop laser based measurement sensors with very high
accuracy, large sensing ranges, high reliability and long life. For steel plant applications,
these sensors are definitely a boon in view of the arduous ambient conditions of high
temperature and dust which make many other techniques of measurement ineffective. With
continuous advancements in Laser technologies, especially in design of laser diodes,
optics as well as signal processing, these systems promise enormous application potential
and are now being used increasingly for never before thought of applications.
SAIL R&D Centre for Iron and Steel has strategically designed, developed and
successfully implemented several laser based applications ranging from Steel making to
Rolling Mills enabling performance improvement of the process. This paper describes
some of the unique measurement and control innovations based on Laser sensors,
conceptualized and implemented in Steel plants of SAIL. These systems, range from mould
width control in casters, auto slowdown control and looper storage measurement in
continuous process lines such as pickling line to sheet shearing control of hot rolled steel
strips. The systems, based on variety of measuring principles from distance measurement
to Doppler velocimeters, demonstrate ingenious designs to customize for unique
applications and also interface with controls of the process lines and mills to derive true
benefit. The systems have enabled significant improvement in productivity of the
concerned shops and established the versatility of laser sensors for manufacturing industry
applications.
Keywords: Laser, Sensor, PLC, Measurement, Distance, Accuracy, Steel, Doppler,
Velocimeter
INTRODUCTION
Sensors can be categorized in many ways, based on the applications, measurement
principle, motion type, and so on. When based on measurement principle, first forms of
classification that comes to mind is contact type vs. non-contact type and discrete vs.
analog measurement. A variety of sensors both contact as well as non-contact type are
available for various applications and measurement of different parameters in steel plant.
The non-contact type sensors are based on measurement principles that do not require a
physical contact with any object or machine, such as opto-electronic, magnetic, ultrasonic,
eddy current and so on. Contact type sensors have limitations of mechanical slippage,
friction, breakage and limitation in hot zone applications, often restricting the accuracy of
measurement and trouble free use. By contrast, the non-contact methods have several
merits as far as these issues are concerned.
Advantages of laser based systems for measurement
Among all opto-electronic techniques, laser technology is most recent and promises high
level of accuracy, reliability and versatility in industrial applications. The unique
properties of laser source that help achieve such immense advantages are low divergence,
monochromaticity, coherence and high intensity at low power. Because of the vast scope
and promise in this field, use of laser technology has gained immense popularity and a lot
of research is going on in this field to develop commercially viable systems for
applications ranging from simple positioning to speed and dimension measurements to
non-destructive testing and processing.
In terms of applications, laser systems are broadly characterized into measurement and
power systems. Power laser applications include processing systems for welding, cutting,
drilling etc that utilize much higher power and therefore a much stringent system design as
well as precaution while handling. Sensing and measurement applications range from
positioning, surveying, alignment, speed and dimension measurement to ranging and
communication etc. In these systems, laser source utilized are very low powered (<5mW)
in the visible or infra red range. This low power, coupled with the unique properties of
lasers makes it ideal for highly accurate sensing and measurement applications. Low
divergence of the laser beam renders an extremely narrow beam of light over a large
distance. This reduces optical design problems like determination of focal length and
accuracy of detection with respect to centre line, as associated with ordinary optical
sensors. Being a monochromatic source, many common installation and tuning problems
associated with varying ambient light conditions is eliminated. Because of high intensity of
even very low powered sources, laser beam can penetrate through industrial atmosphere
having dust, fumes, oil mist etc. Since the output power of the laser does not diminish over
very long distances and divergence is also very low, the sensor unit can be mounted away
from the target, which is especially desirable for safety to maintenance personnel and
safety of equipment itself, especially in case of high speed moving objects and high
temperature targets. These render laser sensors extremely versatile for never before
thought of applications.
Laser classification and design consideration
Laser based systems are grouped into classes depending, not only on the total power of the
laser but also taking into account, maximum level of radiation to which a person could
have access. Class 1 lasers are extremely safe as the maximum exposure level can never
be exceeded either because of their power and wavelength or by virtue of their engineering
and design. Class 2 lasers are low powered devices in the visible range with a maximum
power output of 1mW. These are hazardous only if the person looks continuously into the
beam for a long time. Class 3A lasers are with output powers of up to 5mW in visible
range with power density not exceeding 2.5mW/cm2. Viewing of the direct beam using
ordinary means may cause damage. Class 3B laser product may have power up to 500mW
for CW lasers. Vision problems may be caused by direct exposure through reflections
also. Class 4 laser products have very high power output exceeding those of class 3B and
are very dangerous as direct as well as indirect radiation may cause both skin and eye
damage and can also ignite combustible material.
Class 1 systems should not present any hazard when installed, used and maintained
correctly. Class 2 systems should be provided with precautionary labels and should be
operated & maintained by trained and knowledgeable personnel. Class 3 and 4 systems
require more stringent precautions to be taken during installation, operation and servicing.
Some general safety precautions to be observed in case of low powered measurement and
sensing systems are that beam should not be looked at directly, system should not be
operated without proper electrical shielding, general illumination level should be kept high
and optical path of a laser beam should be at a level other than eye level.
APPLICATION AREAS OF LASER MEASUREMENT SYSTEMS IN
STEEL PLANTS
Laser based measurement systems are generally based on following three or four types of
sensors. The common and popular manifestations are positioning based on through beam,
retro-reflective and scanning sensors, distance sensing based on time of flight or
triangulation principle and speed measurement based on Doppler principle. Innumerable
applications can be thought of and implemented in steel plants using these sensors.
Examples include positioning of hot and cold steel stocks on roll tables, loop sensing in
long product mills, edge guide and centre guide detection, alignment of sheet, detection of
objects in reheating furnaces, coke oven alignment system for pusher and guide cars,
profile and dimension measurement, strip speed measurement, flatness measurement,
camber detection, shape detection in strip mills, weld detection in continuous lines,
measurement of degree of alloying in coating lines, crop measurement and so on.
Some of these solutions are today very commonly implemented, especially in green field
area or newly refurbished units. However, sometimes there are unusual applications that
require a customized and ingenuous thought process. Also, implementing such a system
successfully in an existing unit is not always straightforward and requires an innovative
and multi-pronged approach. R&D Centre for Iron and Steel has strategically designed,
developed and successfully implemented several laser based applications ranging from
Steel making to Rolling Mills enabling performance improvement of the process.
IMPROVISED LASER APPLICATIONS IN STEEL
Improvement in Productivity of continuous process lines of flat product plants
Continuous process line such as Pickling line, Coating lines, Continuous annealing
furnaces require that the strip processing is unhindered from one coil to another. For this
purpose, coils are welded end to end and then processed. At the exit coils are separated
out. At the continuous pickling line of Cold Rolling Mill, Bokaro Steel Plant, following
solution based on laser based sensors were designed and implemented to ensure this
uninterrupted strip movement.
Hot rolled coils from Hot Strip Mill at the steel plant has a layer of scale(mixture of
different iron oxides) with a thickness of 5-20µ. The strip is de-scaled with a combination
of mechanical and chemical dissolution in pickling line. Removal of scale is performed by
chemically treating the surface of hot rolled strip at continuous pickling line of Cold
Rolling Mill(CRM). This is done using H2SO4 at Pickling Line-1(P/L-1). Pickling line-1
is a long continuous processing line with three independent sub-units namely entry,
process and exit sections. In the continuous processing line, preceding coil back end is
welded with following coil front end. The line also has storage facility called loopers in the
entry as well as exit sections. The storages of coils in each looper are utilized to ensure that
the process section speed is maintained during Welding and Shearing at entry & Coiling at
exit sections.
Auto Slow Down Scheme for tail end of strip
After welding of the coils, entry section line speed is increased maximum upto 10 mps.
Before reaching end of the coil in the un-coiler, it is required that line speed is made slow
to take care the falling of strip in the loop pit and enable welding of the front end of the
next coil. Due to no proper scheme for predicting the correct requirement or time to initiate
slowdown, extra precaution was taken by operator to keep the line speed slow, adversely
affecting the productivity of line.
A LASER based auto slow down system
implemented with interfacing to the existing drive
system. The implemented system has digital
display for showing the linear distance in terms of
coil diameter; occurrence of the event of auto slow
down and stoppage of the line after the tail end
leaves the pay-off reel. The new system has
eliminated the effects of roll slippages. The
system also initiates Entry Line Stop command
once tail end of coil leaves Un-coiler to avoid falling of the coil in Wet Loop Pit, thus
preventing coiled end segment damage and also damage of mechanical equipment due to
impact.
Measurement and Display system for Horizontal Looper storage
The loopers in the pickling line store strip in horizontal fashion and the required speed of
pickling can be maintained only if loopers function properly and information regarding
amount of strip available as buffer is available to operators. The existing selsyn based
system for looper measurement was erratic, had limited accuracy and was difficult to
maintain due to obsolescence. The incorrect information about the storage in the loopers
was leading to reduced line speed and derailment of the looper trolley.
A new state of the art system was designed and
implemented based on laser distance sensor as the
main device.
Laser distance sensor was installed
having distance measurement capability of 200
meter
for
entry
and
exit
looper
trolleys
independently. The sensor was mounted on a vertical
mounting structure especially designed to take care
of vibration and with provision for adjustment in
horizontal as well as vertical axis to adjust the focusing. Due to high distance under range,
special pre-fabricated large size diamond reflectors were used to reflect the laser light back
to sensor. These reflecting tapes offered great flexibility of installation since they can be
fixed on any metal surface. The new system accurately measures the distance between
trolley and platform where pulley is located. The distance shown by sensor system and
actual distance showed a maximum error of 500mm, which is within tolerable limits
considering the maximum range of 80 meters.
The systems have enabled increase in the entry section line speed from 4.8 to 6.2 m/s
resulting in improved productivity by increasing the number of pickled coils per hour by
12 % (from 4 to 6 coils/Hr.).
An automatic cut to length system using Laser Doppler sensor
The coil coming from Hot Strip Mill(HSM) of Bokaro Steel Plant (BSL) is cut in a length
of sizes 4.5 to 10 mtrs at Shearing Line of Hot Rolled Coil Finishing shop(HRCF), packed
and dispatched to customers. In order to cut the coil into steel sheets of desired lengths, the
shear system was based on traditional servo roll, encoder and dedicated controller block to
measure the length and initiate the cut command at appropriate time. The measurement
and control system, being outlived, posed several problems in running and maintenance.
Other related issues were mis-cuts, length variation in first cut, intermediate cuts and last
cut. All these abnormalities lead to rejection of sheets and also sometimes jamming of the
sheets in the piler.
An
automatic
cut-to-length
system, based on state-of- theart laser based speed sensor
has been designed, developed
and implemented successfully
at shearing line-1 of the shop.
Accurate speed measurement
is used for calculation of
length by integrating speed
over short and fixed interval of time The output of speed sensor i.e. Laser Doppler
Velocimeter is a train of pulses corresponding to speed of sheet and these pulses are
counted by high speed counter module in PLC. All other logic and interlocks are also
interfaced to the PLC including tail end sensor. The cut command to flying shear drive and
brake command to stop the flying shear drive and thereafter stop the flying shear motor are
also programmed in the PLC.
The results achieved through the implementation of the auto cut –to-length system include
accuracy of 0.65% of set length consistently as against 1% of the same earlier and
elimination of effects of backlash in length measurement
is concerned w.r.t. earlier
system. The implemented control logics also eliminates large length variation w.r.t. first
cut, intermediate cuts and last cut in earlier control system.
Innovative Measurement and Control of mould plates for width control in Slab
caster
In the slab caster, it is essential that accurate measurement of mould plate separation is
done for maintaining the cast speed as well as control of tapered length. At the Slab Caster
of Bokaro Steel Plant, there was no on-line measurement and control system for mould
width. During changeover of the casting width, manually adjustment was carried out
which added more than 1.5 Hrs towards machine preparation time. An innovative on-line
measurement and control system was
conceptualized to measure the narrow plate
displacement,
based
on
non
contact
LASER sensor, PLC and interfacing with
existing control scheme to operate the
mould plates and achieve the set width of
the slab to be cast. Sensor installation in
this application was not a very easy task
and required lot of improvisation in view
of presence of grease, severe limitation of space and high temperature zone. Four nos. of
such sensors were installed on specially design mounting brackets retrofitted in the mould
cassette, to measure directly movements of the narrow plates. The analog signal from the
distance measurement sensors, was interfaced with existing PLC where the calculation for
mould plate separation was carried out and drive operated to move the mould plate
motors.
The system ensured an automatic cold adjustment of the mould width with an accuracy of
+ 2mm and also facilitates reduction in machine preparation time which was due to
manual measurement and adjustment of the mould plates.
CONCLUSION
Presently, whenever newer units are set up or existing units are revamped and modernized
in an integrated fashion, laser based measurement solutions available commercially are
normally included as a package. However, sometimes for a specific requirement in an
older unit, customized and ingenuous solution is required which is simple to use and
maintain, cheap and at the same time effective. This can best be carried out by in-house
expertise through customized development keeping in mind existing plant logistics.