Evolution of Automatic Gauge Control philosophy

Evolution  Control Philosophy for Single Stand Mills Chennai, 2017 Theme: Innovation in Engineering: Competitive Strategy Perspective

Evolution of Automatic Gauge Control philosophy for flat rolling in Single Stand Mills. Amitabh Kumar Sinha, B.Sc (Engg), FIE, Ranchi, India, amit1950@yahoo.co.in

Abstract

During the course of author’s working in Steel Industry, world of automatic gauge control changed from Mechanical Screw Down Control to Hydraulic AGC. This led to evolution of different control philosophies depending upon available sensors, required and possible response time as well as customer expectations and quality concerns. The control system also changed from available sophistication in automation e.g. from operational amplifiers and potted logic blocks of 70’s to Micro Processor and PLC based system including higher levels of process controls. The paper sums up the control strategies used from 1970’s till now for single stand steel mills, though some old methods may be still prevalent due to other considerations such as cost. The present study ignores effect of rolling speed, roll deformation, roll coolant etc on AGC.

Initially mechanical screw down with large size DC motors were used for AGC control with a selsyn or pulse encoder as position sensor and load cells as roll force sensors. Basic parameter for control was back tension which was a fastest controlled parameter at that time. Roll gap was changed only if permitted change in back tension was not sufficient to get a desired thickness control. Once hydraulic roll cylinders were available for Automatic Gauge Controls, the scenario changed from tension control to direct roll gap control since faster control of cylinder position was possible. With improvement of Gauge measurement and other sensors and advent of micro-processors and PLC the control philosophy constantly changed and methodology of mass flow were also used. In multi stand mills one stand was used as reference mill and other stands will have speeds as per reduction in each stand since speed ratio will correspond to thickness reduction as per constant mass flow. For constant mass flow, hinv1=houtv2, i.e. hout = hinv1/v2, for a given constant. These principles are explained in the following articles in some detail with electro mechanical explanations as per authors’ experience in design and commissioning of such mills. 

Keywords: AGC, Mass Flow, Screw Down, Mills

AGC through control of Back Tension

Before Hydraulic AGC Cylinders became a standard fixture in flat rolling mills, electrical DC motors were used to adjust top rolls for roll gap adjustment. Different systems from Ward Leonard to SCR’s controlled bridge rectifiers1 with or without circulating current were used to fasten the response of screw down motor controls.  However considering the slow response of such motorized screw downs roll gap adjustment was used only as coarse adjustment and alternative method was needed for fine gauge control. The fastest means was to control the back tension. Rolling tension moves the neutral plane of rolling forward or backward and thus changes the specific rolling pressure. This reduces rolling force for same reduction of the strip.   Tension Fz changes the roll force needed for same reduction in following manner (simplified formulae) :

P_m=Pm1 {1-(α₀+ α₁)/k} ……… (1)²

Where Pm = specific rolling pressure, Pm1 =mean specific pressure with zero tension, α0 & α1 are entry and exit stress due to tension on the strip in kg/mm2   for a given reduction, k= A constant.

This leads to following equation

R_a= R_f(1-k1*F_z) ………………. (1_a)⁴

Where Ra is actual roll force, Rf is roll force for same reduction with tension zero and k1 is a multiplication factor.

Now we know that
t2= s+Ra/mm   

( ie loaded gap = no load gap+ mill stretch)- Gage meter principle³

Where t2= output thickness, s= unloaded gap in the mill and mm= mill modulus.

Thus replacing Ra from first equation we get:

t2=s+(Rf*(1-k1*Fz))/mm ………(2)

Thus first fixing a calculated roll gap then manipulating back tension Fz as required up to a set limit for fine control, and then changing roll gap as coarse control allowing some more correction in Fz fast gauge control was achieved.

Earlier tension control of Pay off reel or tension reel was controlled using field control of DC motor since SCR for armature control of such powerful motors were not available then. This type of tension control was known as “Tension Control” as opposed to torque control now applied. This control works with following principle:

Fig-1

  n = reel rpm in MPM (measured by motor tacho / GR)

 ϑ = material speed in meter /min (measured by deflector roll tacho)

     ϑ= π*D*n                                 ie              D= ϑ/π/n............................. (3)

     Torque   T=   Fz * D/2                   ie                Fz= T*2/D..................................... (4)

     Since   T ά ф*Ia                  ie               T ά If*Ia .........................(5)

    Where Ia = motor armature current and If = motor field current. (фάIf)

  

    Putting value of T from (4) to (5) 

    we have  Fz=    2*k2*Ia*If/D     where k2  is a constant. ................(6)

For field control n  ά 1/ ф and ф ά If where ф= Field Flux, ie  n = k3/If….  (7)

where k3 is a constant. 

For a given fixed mill speed  ϑ   D*n = D*k3/If

Hence if    If/D is kept constant as per (6)   Fz ά Ia………………(8)

For keeping tension to a set value the motor has to run in constant power mode, where entire motor control is in field weakening range – keeping motor voltage constant. Base speed of motor is available at max OD of the coil and top speed at the ID of the coil. Motor current is adjusted as per tension required. We know motor power is proportional to tension x mill speed, Hence keeping motor power constant results in constant tension and if motor voltage is constant this means keeping motor current constant.

The above calculations and control block diagram are shown in the graph below (fig-2).

Fig-2

AGC through calculated Roll Gap:

During late 70’s after hydraulic AGC were introduced and became popular the faster controls directly of roll gap became possible and full dependence on tension control was not needed. However measuring out going thickness was not very fast and was not predictable accurately when material was in bite. This led to using the calculated roll gap for control of thickness using roll position control, roll force control & mill modulus to generate the calculated loaded roll gap and hence output gauge. Finer roll gap correction was done using feedback from X-Ray or isotope type thickness. Gauge Difference from set value was corrected using faster AGC position control and more accurate gap measurement using LVDT, relative and absolute position sensors or other position sensors. Use of hydraulic AGC cylinders with position sensors called for a procedure named calibration of the mill and position controller. This was required since the top level of bottom roll changed from campaign to campaign due to various factors including roll turn downs. 

Calibration of position control:

In this write up I assume that mechanical screw down for moving the rolls to compensate for roll turn downs for keeping pass line,  are mounted at the top of the mill where as hydraulic cylinders are mounted in the bottom. Process as per author’s experience is given below:

i)      Top motorized screw down is moved down to bring the rolls to pass line keeping it parallel as far as possible.

ii)    Bottom rolls are lifted under roll force mode with kissing roll force reference differential roll force reference as zero.

iii)   Rolls will rise to touch to top rolls and touch either on drive or operator side as per skew due to error in parallel position of top rolls.

iv)  Roll force will develop and rise up to the set value. Differential roll force also will rise since the rolls touches only on one side. Differential roll force controller comes into action to achieve the set value of zero, thus ensuring the rolls touch both side with equal force.

v)       The above roll position is registered as kissing position or unloaded zero gap position. Rolls are rotated at small speed.

vi)     Total roll force is then raised to another set value called the calibration roll force.

vii)    Though the rolls are still touching but position sensors will read another position due to mill stretch and this is also registered. The difference in the two registered position divided by roll force gives mill stretch coefficient ie mill modulus  “mm”

viii)  Mill calibrated signal lights up.

Fig-3

Mass Flow Principle

With the advent of faster non-contact thickness gauges, AGC suppliers introduced a another method of measuring loaded roll gap or output thickness by using a simple principle that mass remains constant. Thus if entry side thickness is h_in and entry material speed is ϑ1 , exit side thickness is h_out and exit side material speed is ϑ2  then considering that there is no change in width of material,

h_in *ϑ1= h_out ϑ2 thus    h_out=  h_in *ϑ1/ ϑ2…………………………………… (9)

Thus if we are able to measure entry and output material linear speed and entry side thickness we have quite accurate and fast value for exit thickness, thus accurate AGC was possible with this calculated output thickness with over all vernier correction from exit thickness gauge. See figure-4. 

Fig-4

Fig-5

Null Setting for Moog Servo Valves:

In the various mill stands commissioned by the author Moog Servo Valve of 72 series are used for control of Hydraulic Force Cylinders. These valves have certain amount of flow even with zero control current. This flow will either make to top and bottom roll come together or separate from each other and may move with a skew (un parallel movement) when the power to the Moog Valves are powered off. If the rolls do come together the chance is that they will move and raise the roll force to unsafe values. Hence to have safe operation it is imperative that top and bottom rolls move away from each other with near equal speed or in parallel motion. The Moog Valve are provided with a “Null flow screw” and the manufacturer has provided 2 methods to adjust in the catalogue⁵ namely mechanical and electro-magnetic. Author did come across a much simpler procedure which however needed the position control commissioned and working. The procedure is very simple where in the rolls are separated to a fixed gap and the control system will keep the rolls at this gap – not affected by the null flow. Now the null screws on both drive side and operator side servo valves are adjusted one by one such that direction of control current is for same as for gap “closing” and is 2-4 % of full flow. Care is taken that this null current is same for both operator and drive side. Thus once the power is off the roll gap will “open” with almost parallel movement of rolls.

Conclusion:

With the use of Hydraulic AGC cylinders and fast and accurate position sensors and thickness gauge Automatic Gauge Control for flat mills have undergone sea change from controls through back tension to actual position. Temper passing in Cold Mill with constant roll force has also become feasible. In India Hydraulic AGC were introduced in late seventies and now is a standard fixture in flat rolling mills.  

Acknowledgement / Bibliography

1)       G.Molten, Line Commuted Thyristor Convertors, Siemens Ag, 1972

2)       A.Teliskov, Stress and Strain in Metal Rolling. Mir Publishers, Moscow, 1967.

3)        Peter Kucsera & Zsolt Béres, Hot Rolling Mill Hydraulic Gap.., Published in Acta Polytechnica Hungarica, Vol 12 No 6, 2015,

4)       William L Roberts, Cold Rolling of Steel, 1978

5)       Moog Servo Valve 72 series Technical Catalogue Rev-2, 2013