Automation in Steel Process Industry.

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Control philosophy of Elongation control in Skin Pass Mill
Skin pass mill is generally used after the annealing process of the cold rolled steel strip. After annealing the strip is not flattened and having dull finish. So with this Skin pass, flattened the strip by applying some roll force as well as strip elongation. Skin pass mill also give the brighter strip surface and hardened upper layer of the strip.
Elongation Function

The Elongation Function consists of two main modules :
-The Elongation Measurement Module monitors the speed of the strip before and after
the Mill stand. With this information calculates the actual elongation.
-The Elongation Control Module continuously calculates the material stiffness of the strip.
This is, together with the measured elongation and the actual strip dimensions, fed into an algorithm which, depending on the configured set-up, produces correction values for the reference total roll force and reference entry bridle tension.

Description of Technology

This function measures the actual elongation of strip passing through Mill Stand, calculates the deviation from the elongation reference and generates correction values for the control references Roll Force and Entry Tension.
The result is a closed loop for elongation control.
Measurement of actual elongation

Incremental pulse generators have to be installed on the end-shaft of the Entry and Exit bridles rolls of the Skin Pass (to avoid any false measurement generated by backlash and vibration occurring on motor end-shaft due to gearboxes).
The rolls should be chosen with small roll radius and close to the Skin Pass stand in order to avoid slippage between the roll and the strip.
A pulse generator with high resolution (e.g. 8192 p.p.r.) and Aquad B signals is used.
Calculation of Elongation

For entry & exit section Skin Pass following calculation will be done:
When a segment of material passes through the mill, its length is increased:
lex = len + *l
By definition elongation is represented by the formula:
L = *l [%/100]
len
The two pulse generators counts converted into an entry and an exit length when the circumference of the rolls and the pulses per revolution are taken into account:
l =C * D * ?
PpR
Thus the actual elongation can be calculated:
Lact =Cex * (Dex / PpRex) _ 1 [%/100]
Cen * (Den / PpRen)
Where: L is the elongation PpR= Pulses per revolution
C = pulse count ex = exit side
D = roll diameter en = entry side

Calculation of Roll Force & Tension Correction Values
To correct measured deviations of strip elongation from its reference, correction values are added to the roll force reference used in the Roll Gap Control function and / or the tension references used in the Bridle functions.
Either force or tension can be configured as the primary control loop. Up to a configured force or tension limit only this control value will be used to correct all deviation in elongation. When the primary control limit is exceeded the secondary control loop is also activated.
In some of the skin pass, two anti-crimping rolls are in connected in entry and exit side. These rolls lift /down the strip in mill area by the motorize action. With this up/down movement of the strip also change the wrap angle of force measuring roll in the entry and exit side. This is also compensated in the tension control. These rolls help to avoid effect of strip waviness in the skin pass mill.

Relationship between strip elongation and roll force / bridle tension
To produce an algorithm to calculate the correction value to the roll force and bridles tension references it is necessary to establish a relationship between force / tension and elongation:
The relationship between force and thickness is:
DF = Cm * Dh [kN]
This can be converted, using equations, to a function of elongation:
DF = Cm * hen * ? 1 _ 1 ) [kN]
1 + Lact 1 + Lref
The reference value for the elongation is an input of the standard function and the actual elongation can be calculated from equation above. Cm is the material stiffness of the strip.
Calculation of material stiffness
Equation above shows that the relationship between a physical change of the strip and force applied to it is a function of the material stiffness Cm. The material stiffness module is calculated as follows:
Cm = ? Fi [kN]
2* (Hen.ref - Hex.ref)
where: Fi = roll forces
H = strip thickness
The value Cm is passed through a low pass filter so any erratic force measurement will be filtered out. Additionally the resulting force and tension correction values (calculated by equation above ) are passed through integrators to ensure a bumpless performance.

Control Logic.
One control loop (roll force or bridle tension) is configured as the primary controller for elongation, (default is roll force as primary controller, bridle tension as secondary controller.) With elongation control mode activated a correction value is calculated according to equation above and added to the reference value of the primary controller. When this reference value exceeds a pre-defined limit the additional elongation deviation is corrected by an additional correction value added to the reference value of the secondary controller. When both control reference values have reached their limits the elongation is allowed to vary from set point and an error flag is set.
With the disabling of elongation control mode the force and tension correction values are fixed at their current levels until the current process is complete or the elongation control is reactivated.
With the reactivation of elongation control the correction value begins at this frozen level, thus ensuring bumpless transitions during rolling. The correction values can, through a reset flag, be reset to zero. This is done automatically, when the Weld Point is passing through the mill stand.

Rajeev Goyal
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