Field of invention
The present invention relates to a laser based position sensor for application in steel
rolling mills. The laser sensor of the present invention has a high level of accuracy yet is
economic and simple to maintain. Such sensors have wide applications in different
industries for detection of Hot Rolled (HR) product, broad sizing of objects, tracking of
material, counting of objects. The sensors are particularly useful in cold rolling mills in
steel plants.
Background of the invention
In cold rolling mills particularly in the pickling lines, accurate sensing of the steel strip is
required. In the pickling line, near the deflector roll just before the entry looper, the steel
strip occasionally loses tension and drops to the ground. If this is not immediately
detected, it leads to jamming of the line and ultimately results in stoppage of the
process. Since the strip thickness is in the range of 1.2mm to 5mm, detection accuracy
better than at least 1mm is required because of the beam diameter was more than at
least 1.2mm, the strip would go un-detected.
Conventionally different systems based on proximity switches and Infra Red (IR) based
sensors are employed. However, proximity switches are prone to mechanical breakage
due to very low sensing distance requirement from the object. IR based sensors do not
give the desired accuracy.
Therefore there is a need of a detection system which has a detection accuracy of less
than 1mm for such applications.
Laser based opto-electronic sensors which employ a laser source are very
effective for industrial applications requiring high degree of accuracy. The unique
properties of laser source that help achieve such high degree of accuracy are low
divergence, monochromaticity and high intensity, to name a few. Because of the
low divergence of the laser beam, it can be treated practically as a narrow
stream of light. This reduces problems like determination of focal length and
accuracy of detection with respect to centre-line, associated with ordinary visible or
infrared radiation based system. Because of being a monochromatic source, many
common design problems associated with ambient light conditions is eliminated.
Because of the high intensity of even very low powered sources, the beam can
penetrate through industrial atmosphere containing fumes, oil mist, steam
etc. making them very versatile as compared to other sensors. Since the output power of
the laser does not diminish over very long distances and divergence is also very low, the
source and detector can be mounted at large distance from each other and from the fast
moving object. This is specially desirable for detection of material in hazardous area
from the point of view of safety of operators and maintenance personnel and also safety
of the equipment itself. The unique properties, coupled with the fact that opto-electronic
sensors promise a better accuracy and reliability even in hot conditions, due to their non-
contact approach of measurement have made laser based sensors very versatile for
many industrial applications.
Few laser based systems are known in the art. GB 2236178 teaches a monitoring
arrangement for determining the position and angular velocity of a mirror through
production of interference pattern through 2 laser beams. The arrangement comprises
means for producing a moving inference pattern on the surface of the mirror by
combining two laser beams having slightly differing frequencies to produce the fringe
movement. A sensor detects the moving interference pattern after reflection and its
phase is compared with that of a moving reference signal at a phase meter. The
accumulated phase difference gives an indication of the angular position of the mirror
velocity. The difference in frequency as detected by the discriminator between the
reference beam and the reflected signal gives the angular velocity of the mirror. A third
beam enables independent detection in two planes. To extend the range of measurable
mirror angles a convex pattern reflector may be used.
Systems for automatic focussing of industrial laser beam relating to methodology for
tuning of laser source and novel designs of laser transmitter system are also known.
However, there remains a need for a dedicated laser positioning system which has high
accuracy yet economic and easy to maintain for applications in cold rolling mills of steel
plants.
Objects of the invention
Therefore, the basic object of the present invention is to provide a laser based position
sensor, which has high accuracy.
Another object of the present invention is to provide a laser based position sensor
having simple design of optical and electronic components thereby making the system
easy to maintain and cost-effective.
Further object of the present invention is to provide a laser based position sensor which
is free from laser related hazards.
Summary of the Invention
Accordingly, the present invention provides laser based position sensor for steel rolling
mills, said laser based sensor comprising :
i. a laser transmitter;
ii. laser detector comprising silicon detector, optical filter and amplifier-
comparator card;
wherein said amplifier-comparator card comprises:
amplifier circuit adapted to amplify the photo signal incident on said detector
and
comparator circuit adapted to process said photo signal received from said
amplifier block for further elimination of effect of ambient light on the output of the
sensor; and
iii. sensor controller comprising a digital logic processing and display unit and
sensor power supplies wherein said sensor controller comprises means to
eliminate effect of noise in the signal and prevent transients/surges in the signal
from triggering outputs and is adapted to provide logical outputs for detection of
the object or alignment of the sensor.
Detailed Description of the Invention
The sensor system of the present invention is a simple low power measurement sensor
which can be used for online as well as offline accurate positioning of steel stocks,
precise detection of the edge of the flat strip, broad sizing of objects, or just as a limit
switch. The sensor is a through beam laser based sensor with detection accuracy of
less than 1 mm.
The main components of the system are described below:
Laser Transmitter
The laser transmitter comprises of a helium-neon laser source, collimating optics and
mounts and laser power supply. The preferred particulars, of the helium-neon laser source
are 632.8nm, 0.5mW, TEMoo, Random polarization,- Class IIIa, Beam diameter 0.49mm,
Maximum beam divergence 1.7mRad.
Laser transmitter is housed in a suitable dust free enclosure with an aperture to output laser
beam. The power supply takes as input normal ac supply of 230Vac and generates the
necessary power for laser source operation. The He-Ne laser source emits a continuous red
light of 632.8nm wavelength. The collimating optics have been used to further reduce the
divergence of the beam. Both the laser source and its power supply incorporate suitable
safety measures to prevent any danger while operation and to conform to safety standards.
Laser Detector (Field mounted block)
The laser detector comprises a silicon detector and optical filter and an amplifier and
comparator card. The silicon detector preferably comprises circular active element of
active area 50mm , Low dark current, peak spectral sensitivity 850nm, BNC connector
and size 25.4mm diameter. The preferred particulars of optical filters are standard band
pass interference filter 632.8nm peak transmission, Band width llnm, 80% peak
transmission, size 25.4mm diameter for direct mounting with detector.
The detector comprising of a high performance silicon detector and filter mounted in a
suitable optical mount is housed in a suitable enclosure to prevent entry of dust and other
contaminants. The enclosure also houses a simply designed PCB, which has an amplifier
and comparator circuit. The amplifier block amplifies the photo signal when the laser
beam is incident on the detector. This signal is then fed to the comparator block for further
elimination of any effect of ambient light on the output of the sensor since the ambient
light also consists of 632.8nm component. A small amount of hysteresis is incorporated in
the comparator to prevent the problem of output hunting.
The output of the comparator is used to drive an open collector transistor for transmission
to the control room for further processing.
The alignment of the transmitter and detector block is visual by observing a light emitting
diode built into the amplifier board which glows when laser beam is incident on the
detector.
Sensor Controller (Control Room)
The sensor controller comprises a digital logic processing and display unit and sensor
power supplies.
The sensor controller is a remotely mounted signal processing and status-monitoring unit.
The signal received from the detector unit is further processed to drive field outputs. The
circuit is designed to eliminate effect of noise in the signal and to prevent transients/
surges in the signal from triggering the outputs. The circuit is designed for two modes of
operation through simple selection of jumper in the circuit. In one mode, the outputs are
enabled when the laser beam is incident on the detector. This mode of operation is useful
for alignment applications. In the second mode, output is enabled when the beam is
obstructed by object. This mode is useful for edge detection and limit switch applications.
Apart from relay and open collector outputs, the controller is provided with status display
for presence or absence of laser beam or object and for detecting internal faults of the
controller itself. The controller also provides stabilized 230Vac for operation of the laser
power supply.
All the connections between field mounted blocks and controller are through shielded
cables. Appropriate safety measures have been incorporated keeping in mind laser related
hazards and ease of operation and maintenance.
In order to reduce effective divergence of the source, piano convex lens with focal length
around 100 mm has been used. This coupled with the effective area of silicon detector
gives the desired accuracy of less than 1 mm.
The sensor configuration has been so selected so as to make the sensor system simple, to
minimise safety hazards and to ease maintenance. To reduce hazards a low power laser
source has been used. Since the incident light on the detector is small, the signal
generated in the detector is also vary small and therefore the amplifier portion of the
circuit has been specially designed to obtain an appreciable signal in order to drive the rest
of the circuit.
The sensor system of the present invention is free from hazards associated with laser
products, as it can be mounted away from the fast moving strip leading to safety of
maintenance and operation personal.
Description of Accompanying Figures
The details of the invention, its objects and advantages are explained hereunder in
greater detail in relation to non-limiting exemplary illustrations as detailed hereunder:
Figure 1 shows block diagram of the system,
Figure 2 shows the circuit diagram of amplifier-comparator card,
Figure 3 shows the circuit diagram of laser sensor controller, and
Figure 4 shows the block diagram of the sensor controller panel.
Description of preferred embodiment
Reference is first invited to figure 1 which shows the three major blocks of the system
namely the laser transmitter (1), laser detector (2) and sensor controller (3) and the inter-
connectivity between them. It also shows the installation locations for the transmitter and
the detector with respect to the object which is a moving strip (4). The laser beam is
incident on the detector (2) during normal movement of the strip (4). In this case, the
outputs are not energized for control. However, when the strip (4) loses tension and
starts falling to ground which could lead to jamming in line, the outputs are energized as
soon as the falling strip blocks the transmitted beam.
The output of the detector (2) is connected to status light emitting diodes (LEDs) (8) and
the signal processing unit (11) of the laser sensor controller (3). Input power 220 V is fed
from the main source (9) to AC DB (7) which distributes to the power supply (5) of the
laser transmitter (1) and to the DC power supply (6). The DC power supply is fed to the
laser detector (2).
The final outputs (10) are utilized for monitoring and stoppage of line to avoid
breakdown.
Figure 2 shows the details of the amplifier-comparator circuit built on a printed circuit
board which is mounted in the enclosure also housing the silicon detector and filter. As
shown in the figure, two wire output (1 & 2) from the silicon detector is connected to the
input of the preamplifier (PA). Another optional amplifier (OA) is also mounted which can
be optionally connected if output from the first stage is not sufficient to drive the rest
of the circuit. This can happen if the intensity of light falling on the detector is
poor in very stringent industrial atmosphere. However, during trials in pickling line, it was
found that this amplifier was not required and the intensity was sufficient to generate a
satisfactory output at the first stage amplifier itself. The output of the preamplifier (PA)
could then be directly connected to the input of the comparator circuit. The comparator
part (COMP) is set up in the following manner after installation and before start of
operation. In the absence of the laser beam, the voltage at the negative input of the
comparator is varied and increased more than the input at the positive input so that the
comparator output becomes low. This adjustment is required to eliminate effect of ambient
light which also contains constituent of same wavelength as that of the laser beam. This
makes the system absolutely full proof against spurious detection in fairly lighted
installation site. Now when the laser transmitter is switched on, laser beam is incident on
the detector the comparator is energized and drives the rest of the circuit. The field outputs
(FO) driven from the comparator output are configured for inverted operation through two
general purpose power transistors so that when the laser beam in incident, the open
collector output (OC) goes low and it goes high when the object is detected i.e. when the
beam is obstructed by the object. A light emitting diode is connected to one of the outputs
which glows when the transmitter and detector are in line and the beam is falling on the
silicon detector. This is helpful during installation of the sensor.
Figure 3 shows the details of the sensor controller circuit. The controller has two units, the
power supply unit (1) and the signal processing unit (2). The power supply unit comprises
a bridge rectifier circuit (la), filter circuit (lb) and regulation and protection circuit (lc).
The power supply unit provides dc power to the laser detector block and also for operation
of laser transmitter. The signal processing unit processes the open collector output (J)
received from the amplifier and comparator card in the field and drives the final control
outputs as well as indications on the controller panel. The circuit consists of an opto-
isolator (OPT01) which prevents noise and surges from triggering the controls. Thereafter,
a two stage op-amp circuit alongwith jumper selection is provided to generate logical
outputs for two modes of operation. In one mode, with jumper J l connected, the outputs
are enabled when the laser beam is incident on the detector. This mode of operation is
useful for alignment applications and thus provides versatility to the sensor. In the second
mode, with jumper J2 connected, outputs are enables when object is detected. Light
emitting diodes LD4 and LD5 are so connected that they indicate the status of operation.
Two types of outputs are provided which can be enabled through switches. These are relay
outputs (N, O, P, Q) and open collector outputs for connection to line control interlocks as
per site requirements.
Figure 4 shows the design of the sensor controller panel facia. The front side mounts the
various status indicators and selector switches for power supplies and outputs The
indicators are not only useful to know the operation but also helps to know instantaneously
if anything is wrong the sensor itself. All the connections are available on the rear side on
two terminal blocks. Power supply fuses are also mounted in rear side for easy
replacement. The rear side also has air vents to keep the controller properly ventilated.
Advantages of the Invention
The laser based sensor system of the present invention has the following distinct
advantages over other conventional sensors used for similar applications;
• The system is based on non-contact approach of measurement utilising sensors and
therefore better accuracy compared to contact type sensors, the accuracy of which is
affected by mechanical losses. Furthermore, contact type sensors are more prone to
mechanical damages. They are also more difficult to use in hot conditions.
• The laser sensor can be mounted at a large distance from the fast moving object
ensuring safety of maintenance and of operation personal.
• Similar imported sensors are well cost prohibitive and difficult to maintain because of
use of component is not easily available. In comparison, the electronics of developed
sensor is simple in design and uses low cost components.
We claim:
1. Laser based position sensor for steel rolling mills, said laser based sensor
comprising:
(i) a laser transmitter;
(ii) laser detector comprising silicon detector, optical filter and amplifier-
comparator card;
wherein said amplifier-comparator card comprises:
amplifier circuit adapted to amplify the photo signal incident on said detector
and
comparator circuit adapted to process said photo signal received from said
amplifier block for further elimination of effect of ambient light on the output of the
sensor; and
(iii) sensor controller comprising a digital logic processing and display unit and
sensor power supplies wherein said sensor controller comprises means to
eliminate effect of noise in the signal and prevent transients/surges in the
signal from triggering outputs and is adapted to provide logical outputs for
detection of the object or alignment of the sensor.
2. Laser based position sensor as claimed in claim 1 wherein said laser transmitter
comprises a helium-neon laser source, collimating optics and mounts.
3. Laser based position sensor as claimed in claim 2 wherein the preferred
particulars of said helium-neon laser source are:
632.8nm, 0.5mW, TEMoo, Random polarization, Class Illa, Beam diameter
0.49mm, Maximum beam divergence 1.7mRad.
4. Laser based position sensor as claimed in any preceding claim wherein said
laser transmitter is housed in a dust free enclosure with an aperture to output
laser beam.
5. Laser based position sensor as claimed in claim 1 wherein said silicon detector
comprises circular active element.
6. Laser based position sensor as claimed in claim 5 wherein said silicon detector
has the following preferred particulars:
circular active element of active area preferably 50 mm2, Low dark current, peak
spectral sensitivity 850nm, BNC connector and size 25.4mm diameter.
7. Laser based position sensor as claimed in claim 1 wherein the preferred particulars of
said optical filter are:
standard band pass interference filter 632.8nm peak transmission, Band width 11nm,
80% peak transmission, size 25 4mm diameter for direct mounting with detector.
8. Laser based position sensor as claimed in claim 1 wherein said amplifier block
comprises pre-amplifier connected to the output from said silicon detector.
9. Laser based position sensor as claimed in claim-8 wherein said amplifier block
optionally comprises an additional amplifier circuit.
10. Laser based position sensor as claimed in any preceding claim wherein the output of
said pre-amplifier is connected to the input of said comparator block/circuit.
11. Laser based position sensor as claimed in any preceding claim wherein field outputs
driven from output of said comparator block/circuit are adapted for inverted
operations through two general purpose power transistors.
12. Laser based position sensor as claimed in any preceding claim wherein a light
emitting diode is connected to one of said outputs of said comparator block/circuit
and is adapted to glow when the said transmitter and detector are in line and the laser
beam falls on said silicon detector.
13. Laser based position sensor as claimed in claim 1 wherein said sensor controller
circuit comprises:
an opto-isolator adapted to prevent noise and surges from triggering the controls and
two stage op-amp circuit alongwith jumper selection wherein said selection is adapted
to connect any one of two jumper circuits for two modes of operations as described
herein.

The present invention relates to a laser based position sensor for application in steel rolling
mills. The laser sensor has a high level of accuracy yet is economic and simple to
maintain. The sensor comprises a laser transmitter (1), a laser detector (2) comprising
silicon detector, optical filter and amplifier-comparator card. The amplifier-comparator
card comprises amplifier block/circuit adapted to amplify the photo signal incident on said
detector and comparator block/circuit adapted to process said photo signal received from
said amplifier block for further elimination of effect of ambient light on the output of the
sensor. The sensor controller (3) comprises a digital logic processing and display unit and
sensor power supplies (5) wherein the sensor controller comprises means to eliminate
effect of noise in the signal and prevent transients/surges in the signal from triggering
outputs and is adapted to provide logical outputs for detection of the object or alignment of
the sensor.