FIELD OF THE INVENTION
The present invention relates to a closed loop automatic speed control system for desired
speed control of DC motors, driving rolling stands, for strip rolling mills to achieve precise
control of rolling speed to maintain optimum tension/looping of strip in the roughing group
and in the finishing group during rolling. More particularly, the invention is directed to
control the linear speed limits of strips by adjusting the speed of the rolls, by a closed loop
feed back control system to selectively control field excitation of driving DC motor such that
the speed ratio of successive group rolls in the strip mill is also favorably maintained for
desired tension limit avoiding undesired looping. Importantly also, the system of the
invention involves a closed loop speed control system for a set of group drive for driving
rolling stands e.g. the roughing stand or the finishing stand. The system according to the
present invention is thus directed to ensure that: any speed drop due to additional rolling
load, the field current is decreased and vice versa to maintain proper speed ratio between
successive stands. Maintaining a proper speed ratio favors minimum tension in the strip and
thereby improves the dimensional quality of the rolled products ensuring wide rolling mill
application with higher yield.
BACKGROUND ART
It is well known in the art that during strip rolling through roughing and finishing stands,
there is a need for accurate and precise speed control for the drive rolls such that proper
tension is maintained in the rolled strips in between the rolling stands in order to avoid
unwanted loop formation or cobbling/shearing and thus wasting some materials or invite
additional operation for its restoration attracting additional costs. The variation in rolling
load at any rolling stand or any drive roll motor in a stand may result in variation in tension
and thus resulting in non-uniform roll tension and resultant deformation of strip/dimensional
variation in strip section or formation of excess loop/cobble between groups of rolls causing
either wastage of partly processed material or involving additional work for
rectification/salvage of deformed strip/sheared loop. Usually in case of rolling with tension,
the final product thickness and width shall be less than the desired value. If the looping
tendency occurs in the strip, the final product thickness and width shall be more than the
desired value.

It is also well known in the conventional art of hot rolling that fine speed control to minimize
inter-stand tension or looping tendency is done manually. Under conventional operation, the
operator keeps continuous watch on the rolling process and the product formed. In case of
loop formation, the operator increases the speed of the next stand in the direction of rolling.
In case of tension, the operator decreases the speed of next stand in the direction of rolling.
When the rolling stands are driven by a common armature voltage, the speed control is
done by varying the field current. The control is usually done by manual adjustment of field
current.
Such conventional system for speed control of rolling stands suffers from the limitations
and/or disadvantages comprising, (a) error remains in sensing of speed mismatch by
manual observation of the strip being rolled and looping tendency or tension observed; (b)
The operator has to be very attentive as detection of any abnormality in operation depends
on the operator's personal attention to job and in case he fails to watch the rolling operation
even for a small time, a cobble may be created, which causes mill delay and loss of
material;(c) Adjustment of field current conventionally done by manual method cannot be
very precise and optimize desired strip tension; (d) The field circuit of the conventional
motor drive is having a large time constant and there by any change of field current shall be
reflected back in the rolling after a large time generally of the order of a few seconds,
making such manual speed control less efficient resulting in delayed response to rolling load
variation.
There has therefore been a persistent need in the related art to develop a closed loop feed
back control system having high degree of accuracy and quick response to change in load
for desired rolling speed variation by varying the field current in the motor circuit wherein
requirement of speed change is decided and activated automatically through sensing the
load dependent speed variation of the individual drive to thereby determine a reset value for
the field current the corresponding drive motor. The present invention is thus involving a
process of closed loop speed control that increases the yield of the rolling mill by accurate
speed control of drive motor/rolled strip in fast and precise manner so as to avoid loop
formation and wastage of processed materials.

OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide an automatic closed loop
control system for speed control of drive motors for the drive rolls in the roughing/finishing
stands of the strip rolling mills to achieve fast and accurate speed control to avoid loop
formation/cobbling between the stands, maintain desired tension in strip avoiding
dimensional variation of its section and save wastage of material or manhours for
rework/salvage of sheared loop.
Another object of the present invention directed to closed loop feed back control system for
automatic speed control of drive motors for the drive rolls in rolling stands is directed to
eliminating the human error by avoiding inaccuracy involved in the conventional manual
observation of strip speed or strip tension/or loop formation tendency from a distant
location from the rolling line, for desired rolling speed control manually.
A further object of the present invention is directed to an automatic feed back control loop
based speed control system for the drive motors for rollers in different strands using the
common bus voltage for all motors in group as reference and the actual speed of rollers as
the feed back signal measured involving hollow shaft encoder for desired adjustment of field
current through a separate circuit for independent motor drive in each stand.
A still further object of the present invention is directed to said automatic feed back control
loop based speed control system for the drive motors for rollers in different strands of strip
mills wherein the speed variation in drive motor is achieved automatically by controlling the
field current of each of drive motor independently depending on speed variation due to load
variation on roll or variation in strip tension, by sensing the corresponding feed back signal
fed to said control loop circuit used to decide on field current set value independently for
group drive motor.
A still further object of the present invention directed to said automatic feed back control
loop based speed control system for the drive motors which provides the required field
current to the DC motor for rolling stands such that full field is applied during starting and
the field current is set to nominal value after the required speed is built up.

A still further object of the present invention directed to said automatic feed back control
loop based speed control system for the drive motors wherein a human machine interface
(HMI) system enable logging in the speed of different rolling stands in the rolling line
through a HMI screen.
According to yet another object of the present invention directed to said automatic feed
back control loop based speed control system for the drive motors wherein the speed ratio
of successive stands is maintained to minimize inter stand tension/looping using appropriate
log in of stand speeds through said HMI screen.
A still further object of the present invention directed to said automatic feed back control
loop based speed control system for the drive motors wherein the response time of the DC
motor for signal input control circuit to effect variation in field current for speed control
through is much faster so that the loop tension or cobble formation or strip section
dimensions are efficiently, automatically and precisely controlled without delay and thus
avoiding wastage of rolled product and maintaining good quality of product as well as yield
of strip mill.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a closed loop
speed control system for a rolling mill comprising:
D.C. motor means adapted to drive plurality of rolling stands;
the armature of said D.C. motor being fed from a converter such that the speed of all
the stands can be changed by changing a common armature voltage (bus voltage) and
individual motor speed changed by changing of the field current;
said bus voltage being taken as speed reference and the motor speed as speed
feedback;
a hollow shaft encoder means providing for sensing of the motor speed as said speed
feedback and
a thyristor converter means adapted to generate the said field current of said D.C.
motor.

Another aspect of the present invention is directed to a closed loop speed control system
comprising
a circuit adapted to senses the bus voltage, the speed set point being derived from
the bus voltage;
said hollow shaft encoder being mounted in the motor shaft, and adapted to sense
the speed of the rolling stand, the output of the encoder being used as feedback
signal in the control loop;
a controller means a DC drive adapted to adjust the field current of the motor to
change the speed of the rolling stand.
A further aspect of the present invention is directed to a closed loop speed control system
adapted to minimize the inter stand tension and looping tendency during rolling.
A still further aspect of the present invention directed to said closed loop speed control
system wherein during starting full field is applied and after the speed is built up the field
current is set to nominal value.
A still further aspect of the present invention directed to a closed loop speed control system
which is adapted such that it logs the speed of different stands in the rolling line and the
speed ratio of successive stands is maintained to minimize inter stand tension/ looping.
According to yet another aspect of the present invention directed to said closed loop speed
control system wherein the speeds of all the rolling stands are logged in the human machine
interface screen.
A still further aspect of the present invention is directed to a closed loop speed control
system comprising two thyristor converters adapted for generating the bus voltages to feed
the armature of the DC motors, the first thyristor converter is for roughing group of drives
and the second converter is for finishing group of drives, a looping trough between roughing
and finishing stands, said looping tough adapted to de-link the tension between roughing
group and finishing group.

A still further aspect of the present invention directed to said closed loop speed control
system wherein the encoder generates pulses that are proportional to the speed of the shaft
and the encoder output signal is processed in the digital drive.
According to a further aspect of said closed loop speed control system of the present
invention wherein
the inputs to the field converter include Bus voltage, Speed feedback, Full field, Field
economy, Field reversal, Speed raise and Speed lower;
the output of the field converter is connected with the motor field,
the field circuit also comprising of a field failure relay and an ammeter, the field failure
relay drops whenever the field current is nearer to zero and when such condition occurs, the
motor is tripped, the speed reversal is done by changing the polarity of the field current.
The present invention and its objects and advantages are described in greater details with
reference to the accompanying non limiting illustrative figures.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: is the schematic layout of skelp mill showing the flow of raw material from
reheating furnace through the roughing and the finishing roll stands with cobble
shear/looping trough in between the two, for rolling into strips of desired dimension for
finally coiling in a coiler.
Figure 2: is the image of an embodiment of the hollow shaft encoder coupled with drive
motor of each stand used to generate the speed feedback signal for desired automatic feed
back loop based control of the speed of drive motor of rolling stands according to the
present invention.

Figure 3: is the schematic circuit diagram for the closed loop speed control system of the
present invention showing the DC motor and its protection features and the invented field
current control scheme.
Figure 4: is the schematic diagram for the control loop involving the common armature
voltage as the reference, actual rotation speed of the drive motor of rolling stand from
hollow shaft encoder coupled with stand motor used to generate the speed feedback signal
and the adjusted field current of group drive motor for desired speed control corresponding
to the error signal determined there from.
Figure 5: is the schematic diagram showing the power supply through Common bus for
armatures of DC motors in the roughing group.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE
ACCOMPANYING FIGURES
The present invention is directed to developing a closed loop speed control system for drive
motor for different rolling stands in strip mills comprising:
i) One thyristorised field converter which generates field current for the DC motor. The
field current is automatically adjusted for changing the motor speed thrpugh a feed
back loop arrangement;
ii) One hollow shaft encoder to sense the actual speed of rolling stand drive motor and
generate appropriate signal for said loop based control of field current;
Reference is first invited to the accompanying Figure 1, which illustrates the lay out of the
skelp mill showing the location of the rolling stand. E1, E2 and E3 are the three edgers
(vertical rolling stands), H1, H2, H3, H4, H5 and H6 are the six horizontal stands. The
edgers and the horizontal stands H1 to H6 constitute the roughing group.
The finishing group consists of five horizontal stands, H7 to H11. There are two Thyristor
converters for generating the bus voltages to feed the armature of the DC motors. The first
thyristor converter is for roughing group of drives and the second converter is for finishing

group of drives. There is a looping trough between roughing and finishing stands. The
looping trough de-links the tension between roughing group and finishing group.
Reference is now invited to the accompanying Figure 2 that illustrates an embodiment of
the system according to the present invention comprising the hollow shaft encoder which is
mounted in the motor shaft. The encoder generates pulses that are proportional to the
speed of the shaft. The encoder output signal is processed in the digital drive. The encoder
generates 1024 pulses per revolution. Any encoder however having pulse per revolution in
the range of 256 to 2048 can be selected to serve the purpose.
Reference is now invited to the accompanying Figure 3 that illustrates the closed loop
speed control scheme by varying the motor current. The field current is generated in a three
phase full wave dual converter of 7 k VA rating. The rating depends on the motor
parameters while the control philosophy remains the same. The following are the inputs to
the field converter;
Bus voltage,
Speed feedback,
Full field,
Field economy,
Field reversal,
Speed raise, and
Speed lower
The output of the field converter is connected with the motor field. The field circuit also
comprises of a field failure Relay (FFR) and an center zero ammeter. The field failure Relay
(FFR) drops whenever the field current is nearer to zero. When such condition occurs, the
motor is tripped. The speed reversal is done by changing the polarity of the field current.
This can be observed in the centre zero ammeter. The system provides for a speed raise
and speed lower push button in the control cabin for the operator. Normally the closed loop
speed control scheme maintains the rolling speed. In case operator wants to change the
speed, he can use the speed raise and speed lower push button as needed.
The armature circuit is capable of providing protection for over voltage and low voltage
corresponding to the 550 V common bus voltage.

Reference is now invited to the accompanying Figure 4 that illustrates control system block
diagram. The speed reference is derived from the bus voltage. If the bus voltage is raised,
the speed is also increased correspondingly. The speed feedback which is derived from the
hollow shaft encoder is fed to the control loop as feed back signal. The error signal derived
from these two is used to change the field current which in turn changes the speed. The
converter generates the field current depending on the error signal.
Reference is now invited to the accompanying Figure 5 that illustrates the connection of
armatures of DC motors that are driving the rolling stands E1, H1, H2, H3, E2, H4, H5, E3
and H6. The DC bus voltage is generated by a 3 phase full wave thyristor dual converter (7
KVA) from input from 3φ AC supply. Speed of all the stands as mentioned above can be
changed by changing the bus voltage. The speed of individual stand can be changed by
changing the field current of that motor. The electrical connection for field circuit is shown in
the accompanying Figure 3 explained earlier.
It is thus possible by way of the present invention to developing an automatic control
system involving a feed back control loop based speed control of the drive motor of the
rolling stands in strip mill in order to maintain the desired tension of strip, control on loop
formation/cobbling and shearing, favoring maintaining the dimension of strip section and
thus improving consistent performance and product quality in the hot rolling strip mills. A
human machine interface (HMI) console is further adapted to indicate and input control
variable optionally by the concerned operator for achieving desired precise control on rolling
stand drive motor speed for avoiding loop formation, control tension, and improved quality
and productivity and yield.

WE CLAIM:
1. A closed loop speed control system for a rolling mill comprising:
D.C. motor means adapted to drive plurality of rolling stands;
the armature of said D.C. motor being fed from a converter such that the speed of all
the stands can be changed by changing a common armature voltage (bus voltage)
and individual motor speed changed by changing of the field current;
said bus voltage being taken as speed reference and the motor speed as speed
feedback;
a hollow shaft encoder means providing for sensing of the motor speed as said
speed feedback and
a thyristor converter means adapted to generate the said field current of said D.C.
motor.
2. A closed loop speed control system as claimed in claim 1 comprising
a circuit adapted to senses the bus voltage, the speed set point being derived from
the bus voltage;
said hollow shaft encoder being mounted in the motor shaft, and adapted to sense
the speed of the rolling stand, the output of the encoder being used as feedback
signal in the control loop;
a controller means a DC drive adapted to adjust the field current of the motor to
change the speed of the rolling stand.
3. A closed loop speed control system as claimed in anyone of claims 1 to 2 adapted to
minimizes the inter stand tension and looping tendency during rolling.
4. A closed loop speed control system as claimed in anyone of claims 1 to 3 wherein during
starting full field is applied and after the speed is built up the field current is set to
nominal value.

5. A closed loop speed control system as claimed in anyone of claims 1 to 4 which is
adapted such that it logs the speed of different stands in the rolling line and the speed ratio
of successive stands is maintained to minimize inter stand tension/ looping.
6. A closed loop speed control system as claimed in claim 5 wherein the speeds of all the
rolling stands are logged in the human machine interface screen.
7. A closed loop speed control system as claimed in anyone of claims 1 to 6 comprising two
thyristor converters adapted for generating the bus voltages to feed the armature of the DC
motors, the first thyristor converter is for roughing group of drives and the second converter
is for finishing group of drives, a looping trough between roughing and finishing stands, said
looping tough adapted to de-links the tension between roughing group and finishing group.
8. A closed loop speed control system as claimed in anyone of claims 1 to 7 wherein the
encoder generates pulses that are proportional to the speed of the shaft and the encoder
output signal is processed in the digital drive.
9. A closed loop speed control system as claimed in anyone of claims 1 to 8 wherein
the inputs to the field converter include Bus voltage, Speed feedback, Full field, Field
economy, Field reversal, Speed raise and Speed lower;
the output of the field converter is connected with the motor field,
the field circuit also comprising of a field failure relay and an ammeter, the field failure
relay drops whenever the field current is nearer to zero and when such condition occurs, the
motor is tripped, the speed reversal is done by changing the polarity of the field current.
10. A closed loop speed control system for a rolling mill substantially as herein described
and illustrated with reference to the accompanying figures.

A close loop speed control system has been developed for a continuous strip rolling mill
directed to optimize inter stand tension / looping tendency for improved dimensional
accuracy of rolled section and reducing wastage. The system controls the speed of group
rolling stands driven by DC motors whose armatures receive supply from common bus from
a converter. Speed of all the stands are changed by changing the common armature
voltage/ bus voltage. Individual motor speed is changed by changing the field current
independently through a field circuit through the feed back response scheme. The invented
system takes the bus voltage as speed reference and the motor speed as speed feedback.
The motor speed is sensed through a hollow shaft encoder. A Thyristor converter is used to
generate the field current. The correct speed ratio between successive stands are
maintained and thereby minimizing the inter stand tension / looping tendency and thus
improving the dimensional quality of the rolled product and strip mill yield.