BASICS OF BASICS OF AUTO GAUGE CONTROLAUTO GAUGE CONTROL

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IntroductionIntroductionRoll ingRoll ing  is a  is a metal formingmetal forming  process in which metal stock is passed through a pair of rolls.  process in which metal stock is passed through a pair of rolls. Rolling is classified according to the temperature of the metal rolledRolling is classified according to the temperature of the metal rolled. If the temperature of the . If the temperature of the metal is above its metal is above its recrystall izationrecrystall ization  temperature (  temperature ( RecrystallizationRecrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility), then the process is termed as  then the process is termed as hot roll inghot roll ing . If the temperature of the metal is below . If the temperature of the metal is below its recrystallization temperature, the process is termed as its recrystallization temperature, the process is termed as cold roll ingcold roll ing

The term Cold Rolling is so called because it is working the metal below its recrystallization The term Cold Rolling is so called because it is working the metal below its recrystallization temperature is around 0.4 Tm – 0.5 Tm (where Tm is melting point in degrees Kelvin) in most temperature is around 0.4 Tm – 0.5 Tm (where Tm is melting point in degrees Kelvin) in most of pure metals. Usually Cold Rolling is done at ambient temperature. of pure metals. Usually Cold Rolling is done at ambient temperature.  Cold rolling is a process of reducing thickness of metallic strips by passing Cold rolling is a process of reducing thickness of metallic strips by passing it   through rolling mill. The rolling mill consists mainly of rolls drives, bearings, mill-spring it   through rolling mill. The rolling mill consists mainly of rolls drives, bearings, mill-spring and housing for these parts.and housing for these parts. The electric motors drive the work rolls and the torque imparted by the rolls The electric motors drive the work rolls and the torque imparted by the rolls on metal provides the load to bring about reduction in thickness on metal provides the load to bring about reduction in thickness .The reduction in thickness .The reduction in thickness goes into increase in length without much lateral spread. Flow of coolant is applied to goes into increase in length without much lateral spread. Flow of coolant is applied to prevent rolls from heating due to friction between rolls and strips. prevent rolls from heating due to friction between rolls and strips. Frictional force acts on the work piece and is opposite in direction Frictional force acts on the work piece and is opposite in direction to motion of strips .At entrance the frictional force acts in forward to motion of strips .At entrance the frictional force acts in forward   direction and draws a strip in to rolls. Tension from entrance as direction and draws a strip in to rolls. Tension from entrance as well as exit side helps to keep neutral point within the arc of well as exit side helps to keep neutral point within the arc of contact so that metal just does not slip. Neutral point is thecontact so that metal just does not slip. Neutral point is the point where opposite forces nullify each other.point where opposite forces nullify each other.  Fig 1Fig 1

TYPES OF MILLSTYPES OF MILLS1. Tandem1. Tandem

There is a payoff reel at the entry and a coiler at the exit. Between the stands are There is a payoff reel at the entry and a coiler at the exit. Between the stands are dancer rolls, and the speed of each stand is adjusted automatically .Tandems dancer rolls, and the speed of each stand is adjusted automatically .Tandems have two, three, or four mill stands, depending on the total reduction to perform. have two, three, or four mill stands, depending on the total reduction to perform. However, the total reduction is performed in one passHowever, the total reduction is performed in one pass..

2 .Reversing mil l2 .Reversing mil l

If the production schedule has many different kinds of material, a reversing mill is If the production schedule has many different kinds of material, a reversing mill is better .The rolling direction is alternatively from left to right and from right to left. better .The rolling direction is alternatively from left to right and from right to left. The payoff reel allows the operator to prepare the next strip during rolling. It is The payoff reel allows the operator to prepare the next strip during rolling. It is used as an uncoiler only during the first pass.used as an uncoiler only during the first pass.

There are two types of reversing 6-high mill:There are two types of reversing 6-high mill:

1. Two work rolls and four back-up rolls (type 1-2) . 1. Two work rolls and four back-up rolls (type 1-2) . The horizontal force due to the torque is absorbedThe horizontal force due to the torque is absorbed by the back-up rolls in both rolling directions. But by the back-up rolls in both rolling directions. But the distribution of the R.S.F. on the back-up rolls is the distribution of the R.S.F. on the back-up rolls is not favorable. This design is rarely used now.not favorable. This design is rarely used now.

2. Six rolls on the same vertical axis . For example, two2. Six rolls on the same vertical axis . For example, two work rolls (Dia=385), two intermediate rolls work rolls (Dia=385), two intermediate rolls (Dia= 440), and two back-up rolls (Dia=1200) (Dia= 440), and two back-up rolls (Dia=1200) ( This dia is used in our Mill)( This dia is used in our Mill)

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AUTOMATIC GAUGE CONTROL Many factors cause final thickness to vary. Therefore, in order to get a Many factors cause final thickness to vary. Therefore, in order to get a

thickness that falls within required tolerances, it is necessary to control it.thickness that falls within required tolerances, it is necessary to control it. Corrections may be carried out by action either on the screwdown Corrections may be carried out by action either on the screwdown or on the tension.or on the tension.

TensionTension Back tension (at the entry side) and front tension (at the exit side) are applied to the str ip. The back tension has the reverse action: force is moved downstream. The front tension increases the FWD SLP, and force is moved upstream.

NOTE : Mil ls having small work rolls (for example, 20-high mil ls) need high tensions.

USE OF TENSION· to make easier reduction· to improve f latness· to keep the str ip centered in the roll ing axis (to avoid shift ing)

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Manipulation of thickness through tension works as follows:Manipulation of thickness through tension works as follows:· Increase the tension (back and front) if the exit thickness is larger · Increase the tension (back and front) if the exit thickness is larger than required.than required.· Decrease the tension (back and front) if the exit thickness is smaller · Decrease the tension (back and front) if the exit thickness is smaller than required.than required.

Generally, tension variation is limited to 20%. If this limit is reached, an Generally, tension variation is limited to 20%. If this limit is reached, an action on the screwdown is triggered.action on the screwdown is triggered. Manipulation of tension in order to alter thickness is rarely used. Manipulation of tension in order to alter thickness is rarely used. It advantage is its high precision. But its drawbacks are many: only It advantage is its high precision. But its drawbacks are many: only small thickness variations can be corrected, especially on “hard” small thickness variations can be corrected, especially on “hard” materials, and the correction speed is often slow. This method can only materials, and the correction speed is often slow. This method can only be used for “soft” materials and to correct small and rare variations be used for “soft” materials and to correct small and rare variations (i.e., in very particular cases).(i.e., in very particular cases). Therefore, we shall focus on the method that acts on the roll Therefore, we shall focus on the method that acts on the roll gap, that is to say, alteration of the screwdown.gap, that is to say, alteration of the screwdown.

Roll Separating Force (R.S.F.) The basis of understanding rolling is knowing the forces created by rolling, the The basis of understanding rolling is knowing the forces created by rolling, the

main one being Roll Separating Force(R.S.F.). During rolling, the strip is main one being Roll Separating Force(R.S.F.). During rolling, the strip is deformed, but so is the mill housing. The difference is deformed, but so is the mill housing. The difference is

that the deformation of the strip is permanent that the deformation of the strip is permanent (plastic) while the deformation of the mill is temporary (plastic) while the deformation of the mill is temporary (elastic).(elastic). Roll Separating force is directly proportional Roll Separating force is directly proportional to width of the sheet. The strip acts as a wedge between to width of the sheet. The strip acts as a wedge between the two rolls and creates the forces the two rolls and creates the forces F F and and F F (Fig 2) . This (Fig 2) . This is the R.S.F.is the R.S.F. All parts of the housing experience R.S.F. For a given Fig 2All parts of the housing experience R.S.F. For a given Fig 2mill, the higher the reduction and the harder the material,mill, the higher the reduction and the harder the material,the higher the R.S.F. the higher the R.S.F. The R.S.F. is uniformly distributed along the width of the strip The R.S.F. is uniformly distributed along the width of the strip ( Fig 3 ) and is proportional to the strip width. ( Fig 3 ) and is proportional to the strip width.

Fig 3Fig 3

. .

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The consequences of R.S.F. are:· rol l bending (Fig 4).· a shearing effect on the necks of the rolls. (Fig 4). · rol l f lattening: the rol ls are f lattened where they make contact with the strip as well as with any other rol l.· deformation of the housing: some parts are in (Fig 4). compression, others in tension. . an axial force on the rolls ( Fig 5), i f the strip is thicker on one side than on the other.

Fig 5.Fig 5.

( Fig 5)

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With a constant passive gap, let’s say 1 mm, several strips thicker thanWith a constant passive gap, let’s say 1 mm, several strips thicker than1 mm are rolled. For each case, the separating force and the exit1 mm are rolled. For each case, the separating force and the exitthickness are plotted on a diagram. thickness are plotted on a diagram.

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Test aTest a::The exit thickness is 1.2 mm.The exit thickness is 1.2 mm.The separating force is 150 tons.The separating force is 150 tons.

Test b:Test b:The exit thickness is 1.4 mm.The exit thickness is 1.4 mm.The separating force is 300 tons.The separating force is 300 tons.

In test a, the rolling mill deformation was 0.2 mm (active gap – In test a, the rolling mill deformation was 0.2 mm (active gap – passive gap). In test b, the deformation was the double: 0.4 mm; the passive gap). In test b, the deformation was the double: 0.4 mm; the separating force was also the double: 300 tons.separating force was also the double: 300 tons. The rolling mill undergoes an elastic deformation since it comes The rolling mill undergoes an elastic deformation since it comes back to its original shape when the separating force is zero (otherwise back to its original shape when the separating force is zero (otherwise it would be destroyed). In all elastic deformation, the deformation is it would be destroyed). In all elastic deformation, the deformation is proportional to the force. Therefore, the 1 mm abscissa (passive gap) proportional to the force. Therefore, the 1 mm abscissa (passive gap) and points A and B are on a straight line.and points A and B are on a straight line. The same series of tests performed with a different gap, i.e., 2 The same series of tests performed with a different gap, i.e., 2 mm, gives a line parallel to the first line: always the same deformationmm, gives a line parallel to the first line: always the same deformationunder the same force.under the same force.

Gauge PLCMMS PCGauge PC

ABB PLC

HMI PC(Wonderware & Omni Server)

Profibus Cable

Com Port comm.

Ethernet Comm.

Ethernet Comm.

Gauge C-Frame & RIO Panel

Screen Cable

Special Signal Cable

GAUGE TRANSFER SYSTEM

Hydraulic Automatic GaugeHydraulic Automatic GaugeControl SystemsControl Systems

Conventional hydraulic gaugeConventional hydraulic gaugecontrol systems work on a complexcontrol systems work on a complexelectro-mechanical spindle principle.electro-mechanical spindle principle.However, this is associated with However, this is associated with relatively long response times,relatively long response times,extensive maintenance and inaccurateextensive maintenance and inaccurategauge (roll gap) settings. Manygauge (roll gap) settings. Manysystem operators are modernizingsystem operators are modernizingtheir installations and convertingtheir installations and convertingthem at the same time. Nowadaysthem at the same time. Nowadayspractically all modern systempractically all modern systembuilders rely on automatic hydraulicbuilders rely on automatic hydraulicgauge control systems.gauge control systems.

HAGC CylindersHAGC Cylinders

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What is Gauge? GaugeGauge - is a measuring instrument for measuring and indicating a quantity such as - is a measuring instrument for measuring and indicating a quantity such as

the thickness of strips or the amount of oil etc. the thickness of strips or the amount of oil etc. Thickness gages are used to make precise dimensional cross section Thickness gages are used to make precise dimensional cross section

measurements on a wide variety of coatings and materials including steel, plastic, measurements on a wide variety of coatings and materials including steel, plastic, glass, rubber, ceramics, paint, electroplated layers, enamels etc. There are many glass, rubber, ceramics, paint, electroplated layers, enamels etc. There are many mechanical gaging, nondestructive and destructive techniques available to mechanical gaging, nondestructive and destructive techniques available to accomplish this task: IR or nuclear gauges, eddy current, magnetic particle, laser, accomplish this task: IR or nuclear gauges, eddy current, magnetic particle, laser, ultrasonic, X-ray, are only a few of the many techniquesultrasonic, X-ray, are only a few of the many techniques

Automatic gauge control

Automatic Gauge Control (AGC) is a closed loop control function designed to Automatic Gauge Control (AGC) is a closed loop control function designed to regulate thickness at the exit of the rolling mill stand to increase the thickness regulate thickness at the exit of the rolling mill stand to increase the thickness accuracy. Auto gauge control is nothing but the gauge monitoring and the amount accuracy. Auto gauge control is nothing but the gauge monitoring and the amount of load to be applied by loading cylinder to get final thickness. To this process to of load to be applied by loading cylinder to get final thickness. To this process to happen by automation we need some factors to be measured during rolling happen by automation we need some factors to be measured during rolling process. AGC mainly controls the loading of cylinder with respect to the thickness process. AGC mainly controls the loading of cylinder with respect to the thickness to be obtained.to be obtained.

AGC depends on main four factors AGC depends on main four factors Thickness feed-forward controlThickness feed-forward control Thickness feedback controlThickness feedback control Mass flow control. Mass flow control. Speed feed-forward controlSpeed feed-forward control

Thickness feedback controlThickness feedback control : : The basic control strategy is thickness feedback (monitor control) based on the The basic control strategy is thickness feedback (monitor control) based on the

measured thickness deviation at the exit side of the mill.measured thickness deviation at the exit side of the mill. The delay time, transport of the strip from the roll gap to the The delay time, transport of the strip from the roll gap to the

thickness gauge, essentially determines the control system’s dynamic thickness gauge, essentially determines the control system’s dynamic response, particular at low rolling speed. The software package provides with a response, particular at low rolling speed. The software package provides with a predictive model based option to improve the dynamic behavior of the predictive model based option to improve the dynamic behavior of the thickness feedback controller significantly.thickness feedback controller significantly.

Thickness feed-forward controlThickness feed-forward control With a thickness gauge at the entry side of the roll gap, the With a thickness gauge at the entry side of the roll gap, the

thickness feed-forward control can be applied. It is able to compensate any thickness feed-forward control can be applied. It is able to compensate any thickness deviation caused by changing entry thickness. thickness deviation caused by changing entry thickness.

Feedforward control employing entry thickness deviation and Feedforward control employing entry thickness deviation and entry sheet speed measurements to improve AGC disturbance rejection entry sheet speed measurements to improve AGC disturbance rejection performance for incoming thickness variationsperformance for incoming thickness variations

Mass flow controlMass flow control Using Mass Flow for Automatic Gauge Control to achieve better gauge Using Mass Flow for Automatic Gauge Control to achieve better gauge

control during the rolling process in Cold Rolling mills, requires the speed and control during the rolling process in Cold Rolling mills, requires the speed and thickness of the strip to be measured at the entry and exit of each mill stand. thickness of the strip to be measured at the entry and exit of each mill stand. The mass of the steel strip flowing into the rolling stand equals the mass of the The mass of the steel strip flowing into the rolling stand equals the mass of the strip exiting the stand.strip exiting the stand.

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For Mass Flow Automatic Gauge control, a X-RAY gauge is used in conjunction For Mass Flow Automatic Gauge control, a X-RAY gauge is used in conjunction with a thickness gauge to perform Mass Flow Automatic Gauge control. The with a thickness gauge to perform Mass Flow Automatic Gauge control. The MFAGC is calculated by:MFAGC is calculated by:

Mass Entry = Mass ExitMass Entry = Mass Exit Mass = Thickness (T) X Width(W) X Density(D) X Length(L)Mass = Thickness (T) X Width(W) X Density(D) X Length(L) Length = Speed(S) X Time(T).Length = Speed(S) X Time(T). Width, Density, and Time are constant.Width, Density, and Time are constant. Therefore, substituting in the Mass formula;Therefore, substituting in the Mass formula;

TEntry X SEntry = TExit X SExit Or TEntry X (SEntry/SExit)= TexitTEntry X SEntry = TExit X SExit Or TEntry X (SEntry/SExit)= Texit

In summary, you can control the thickness out of a mil l stand if In summary, you can control the thickness out of a mil l stand if

you know the entry side thickness and speed and the exit speed.you know the entry side thickness and speed and the exit speed.

Speed feed-forward controlSpeed feed-forward control With the speed feed forward control solution velocity depending process With the speed feed forward control solution velocity depending process

variations are compensated, in particular in the acceleration and deceleration variations are compensated, in particular in the acceleration and deceleration phases.phases.

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Strip thickness controlStrip thickness control

The major difficulty in strip thickness control in cold rolling is the high The major difficulty in strip thickness control in cold rolling is the high speed combined with the high accuracy that is required from the speed combined with the high accuracy that is required from the control systemcontrol system. The control system will face a "new process" for each . The control system will face a "new process" for each new strip entering the mill. The strip thickness, which is a major quality new strip entering the mill. The strip thickness, which is a major quality variable, is disturbed by e.g. variations in mill speed, strip hardness, variable, is disturbed by e.g. variations in mill speed, strip hardness, strip tension and thickness of the incoming strip. Changes in process strip tension and thickness of the incoming strip. Changes in process parameters may mean that the process will react in different ways to parameters may mean that the process will react in different ways to control actions. The purpose of the adaptive functions is to compensate control actions. The purpose of the adaptive functions is to compensate for these disturbances automatically based on previous rolling for these disturbances automatically based on previous rolling experience. experience.

BLOCK DIAGRAM OF AGC

The control structure in the AGC system is shown belowThe control structure in the AGC system is shown below

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Roll gap preset functionRoll gap preset function

First Control has also added a special roll gap setup function based First Control has also added a special roll gap setup function based on an on-line learning network. The learning network is a form of neural network on an on-line learning network. The learning network is a form of neural network which is very fast and is capable of handling large input/output dimensions. The which is very fast and is capable of handling large input/output dimensions. The learning network will estimate the gap position and force in the next pass based learning network will estimate the gap position and force in the next pass based upon data obtained from previous rolling. After each passupon data obtained from previous rolling. After each pass of rolling, the learning of rolling, the learning network will be fed with setup data and data from the previous rolling pass. Based network will be fed with setup data and data from the previous rolling pass. Based on this data, a new roll gap position/roll force is predicted for the next pass or coil. on this data, a new roll gap position/roll force is predicted for the next pass or coil. At the same time, the network will slightly modify itself depending on the outcome At the same time, the network will slightly modify itself depending on the outcome in the previous pass. In this way, the network is made to ”remember” previous in the previous pass. In this way, the network is made to ”remember” previous rolling to be used the next time a similar mill setup situationrolling to be used the next time a similar mill setup situationarises. For the moment, the network is capable of estimating the roll force with an arises. For the moment, the network is capable of estimating the roll force with an average error of about 6 tons, according to a test run made on data stored in the average error of about 6 tons, according to a test run made on data stored in the production database. Work is still going on to improve the accuracy of prediction.production database. Work is still going on to improve the accuracy of prediction. The predicted gap/force value is used to preset the roll gap position at The predicted gap/force value is used to preset the roll gap position at a starting point for the AGC so that the thickness will be as close to target as a starting point for the AGC so that the thickness will be as close to target as possible when the rolling starts.possible when the rolling starts.

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The automatic AGC start up procedure The automatic AGC start up procedure

The AGC start-up sequenceThe AGC start-up sequence

In order to maximize the amount of strip within tolerance, First Control has developed a new In order to maximize the amount of strip within tolerance, First Control has developed a new AGC start-up procedure that will quickly take the thickness to the correct target. Normally AGC start-up procedure that will quickly take the thickness to the correct target. Normally the thickness will be within a few μm from the correct target after only 2-5 m rolling, see the the thickness will be within a few μm from the correct target after only 2-5 m rolling, see the recording in fig 8. It has been estimated that as much as 20-40 m strip can be gained in this recording in fig 8. It has been estimated that as much as 20-40 m strip can be gained in this way in the final pass. The adaptive control functions normally need about 2-5 m rolling to be way in the final pass. The adaptive control functions normally need about 2-5 m rolling to be fully adjusted to the process parameters in the new strip. The operator activates the fully adjusted to the process parameters in the new strip. The operator activates the automatic start-up sequence by switching on the AGC before the strip starts moving. The automatic start-up sequence by switching on the AGC before the strip starts moving. The control system will then activate the different control functions in a specified sequence to control system will then activate the different control functions in a specified sequence to minimize the setting time to correct output thickness. The control system also uses an minimize the setting time to correct output thickness. The control system also uses an adaptive mass flow preset function based on previous rolling, which essentially predicts the adaptive mass flow preset function based on previous rolling, which essentially predicts the mass flow thickness before it can be seen (and corrected) at exit gauge.mass flow thickness before it can be seen (and corrected) at exit gauge.

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The thick end disturbance reduction.The thick end disturbance reduction.

The AGC stop sequenceThe AGC stop sequence

At the strip ends there are normally fairly large thickness disturbances originating At the strip ends there are normally fairly large thickness disturbances originating from the thickness setting in the preceding passes (”thick end”) or from some from the thickness setting in the preceding passes (”thick end”) or from some problems in the preprocessing. The thick end disturbances are mainly controlled problems in the preprocessing. The thick end disturbances are mainly controlled by efficient feedforward control. Since the feedforward control is self-adaptive, it by efficient feedforward control. Since the feedforward control is self-adaptive, it can remove nearly 100% of the disturbance. Experience from rolling shows that can remove nearly 100% of the disturbance. Experience from rolling shows that the AGC is capable to reduce thickness deviations of an magnitude of 10-30μm the AGC is capable to reduce thickness deviations of an magnitude of 10-30μm down to 1-4μm. In this way more strip will be kept within tolerances. Normally, the down to 1-4μm. In this way more strip will be kept within tolerances. Normally, the operator will keep the AGC in operation until the very stop of the mill to remove as operator will keep the AGC in operation until the very stop of the mill to remove as much thick end disturbances as possible. much thick end disturbances as possible. The AGC will be automatically switched The AGC will be automatically switched off when the minimum speed is reachedoff when the minimum speed is reached..

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