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AGC Theory and new

UMAC Mass Flow Automatic Gage Control Description

by John J. Hryb 3rd AGC THEORY AND MASS FLOW ADVANTAGES THEORY INTRODUCTION

The classical problem of closed loop automatic gage control system on a cold reduction mill is that of the impossibility of measuring the strip gage at the point that the gage is generated, i.e. at the roll bite. The gage has to be measured at a downstream point, giving rise to a transport. SIMPLE FEEDBACK THEORY:

The problem is partially solved in the case of simple feedback systems by using a sampling technique or a phase lag (or integration) in the feedback path to ensure stability of the control loop in the presence of the transport lag.

However, simple feedback systems even when optimally designed, are limited in their response due to the presence of the transport lag. Another limiting factor is that of the roll gap transfer function; A given movement of the roll position system has a varying effect upon the roll gap, dependent upon the ratio of the elastic stiffness of the mill structure to the effective stiffness of the strip in the roll bite. When the strip is relatively soft (heavier gauges, narrower widths, softer alloys, low work hardening) most of the roll position system movement goes into changing the roll gap, and very little into stretching the mill structure.

When the strip is relatively hard (wider widths, lighter gauges, harder alloys, high work hardening) most of the roll position system movement goes into stretching the mill structure and very little into changing the roll gap. It is important to incorporate "load factor" compensation into feedback AGC systems to maintain performance throughout the range of materials, widths, and gages to be rolled.

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FEED-FORWARD/FEEDBACK THEORY:

Feed-forward/feedback systems attempt to overcome the limited response of the simple feedback systems by measuring the strip thickness upstream of the roll bite in addition to the downstream measurement. The problem of the transport lag between the upstream measurement point and the roll bite is overcome by using a sampling system together with a storage system such as a shift register or "FIFO" (first in first out) buffer to store successive upstream thickness samples and to read these samples out of the store when the portion of strip sampled approaches the roll bite. If there is any change in entry gage between one sample and the next, the roll position system is driven in the appropriate direction to compensate for the expected effect of this change upon the roll gap.

Feed-forward/feedback systems also require "load factor" compensation to work well, the feed-forward "load factor" being related to, but a little different from, the feedback "load factor".

While feed-forward/feedback systems give superior performance to simple feedback

systems, they are limited in that the feed-forward correction is "open loop" in that the roll position system movement made to correct for a change in incoming gage can only be checked "after the fact" i.e. by the downstream thickness gage. Furthermore, and perhaps more importantly, the feed-forward system cannot detect or compensate for changes in incoming strip hardness. CONSTANT MASS FLOW THEORY:

The constant mass flow AGC system that SEA-CAS (South East Asia Control And Automation) has perfected overcomes all of these problems. It retains the downstream thickness gage as a final arbiter of strip thickness, as any AGC system must do. The constant mass flow system obtains quick, accurate response by measuring the length of the strip at entry and exit sides of the mill, and also measuring entry gage. The entry gage is sampled every 10 millimeters of entry strip movement and stored in a memory buffer in the DSP processor. The entry gage samples are recalled from memory after that segment of the material reaches the roll bite. The gage control computer knows the exact instant when this measured segment reaches the roll bite due to the high resolution strip driven encoder arm that measures the entry strip length.

The AGC task executes every 0.001328 seconds. During every iteration of the AGC task

the entry and exit rotary strip encoders position is processed. The AGC computer keeps a running total of the entry (L1) to exit (L2) rotary encoder ratio.

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At this point, the actual entry gage H1, entry length L1 and exit length L2 are known with great precision. The exit gage H2 is then calculated using the formula;

H2 calc = H1 x L1/L2 which is a very accurate measurement, provided the following:

(a) The side spread is negligible (usually the case for cold rolling). (b) The length and gage measurements are very accurate.

The gage deviation is calculated using the formula O = Href - H2calc (the entry gage feed-

forward component is also added in) and the roll position system is immediately driven in the appropriate direction to bring the deviation to zero. It can clearly be seen that high speed and high accuracy is required when positioning the work rolls. A fast servo hydraulic response time of 0.01 seconds (or less) and position accuracy (that can be repeated) of 0.50 micron, is of paramount importance.

When the DSP processor completes the above move, a new roll position correction is

immediately issued. There is no time delay, the roll position systems never stops moving (unless the calculated gage error is zero), it continually moves the hydraulic cylinder(s) to remove any calculated gage error. This is a very important feature of the new AGC system.

Clearly the effect of the first roll position system movement upon elongation is immediately sensed due to its effect on L1 and L2, so the effect of each correction is sensed after last correction has passed through the mill, i.e. by the next sample and the next correction inherently is made based on the result of the previous correction. Thus SEA-CAS’s AGC system is essentially closed loop in concept, and it does not rely on the feedback signals to achieve its exceptional accuracy.

The reason that the downstream thickness gage measurement is used is that it makes absolutely certain that the calculated and actual measurements do not diverge. Two other major advantages result also from the use of the mass flow concept.

A. Firstly, because sample intervals are extremely short, it is possible to achieve the target gauge very close to the end of the coil at the start of a pass, usually within less than 0.5 meter from the end of the rolled portion of the strip.

B. Secondly, any variation in incoming hardness of the strip, even though the entry side thickness gauge does not sense it, is instantaneously detected because it will immediately affect the elongation - thus the mass flow system can detect it and correct the exit gauge at once. This is particularly important for customers rolling very hard material, where the incoming strip may have a substantial end-to-end gauge variation. If this gauge variation were "ironed out" by the AGC on pass 1, then the material coming to the mill on pass 2 would have little or no gauge variation, but would have a substantial hardness variation due to the differing amounts of work hardening applied to the strip on pass 2, corresponding to the different reductions applied to the strip during the pass, corresponding to the initial gauge variation.

In such a case it is necessary to use mass flow AGC on every pass, because variations

of incoming strip hardness can have just as serious an effect on exit gauge as variations of entry gauge!

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3rd GENERATION MASS FLOW AGC SYSTEM DESCRIPTION: SEA-CAS’s third-generation AGC system has been developed with the latest technology

using “off the shelf” components. The heart of the new AGC system is an advanced DSP (digital signal processor) motion control processor mounted in an industrial “3U” rack. This DSP processor performs all AGC calculations and provides the necessary analog and digital interfaces to the rolling mill. The DSP processor rack or commonly called the UMAC system is connected to a host computer via a USB or Ethernet interface. The basic AGC system utilizes the tried and proven mass flow concept. It is however greatly enhanced with digital filtering and the inclusion of an entry gage error feed-forward component. The AGC software also utilizes a rotary buffer for storing entry and exit length information. All AGC calculations are performed on the DSP processor board (described later). Due the DSP’s high processing throughput it is possible to calculate a gage error (incorrect thickness leaving the roll bite) every 0.001328 seconds. GENERAL AGC ENHANCEMENTS:

1. All analog channels have separate high voltage “analog isolators” to remove any potential problem related to ground loops. Ground loops are common when different computer systems share the same analog common ground.

2. All analog input signals utilize 16 bit high-speed 5-microsecond A/D converters.

3. Extremely fast transfer of data from the DSP processor to the host computer using USB

or Ethernet communications.

4. Color Graphic “Touch Screen” for the operator interface.

5. Any printers supported by Windows XP (including laser) may be used.

6. Direct network connection of the host computer.

7. All digital signals are multiplexed and de-bounced.

8. Customer can easily edit Microsoft Access Database to change any parameters pertaining to the control system. Example; if the left thickness gage is moved a new “bed-length value” is simply type into the configuration file.

9. The tasks on the DSP processor card execute extremely fast and are compiled into

DSP56303 assembly language.

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DSP PROCESSOR: The DSP processor (UMAC) performs all the data gathering and processing of the information required to implement AGC. A large motion control company manufactures this board in California. The central processor on this board is a Motorola DSP56303 operating at 80 MHz. A major advantage of having the AGC program running on the DSP processor is that there is no delay in issuing a roll position correction. Once the AGC task has calculated a gage error it simply sends this new value to the roll position system target register. The roll position system immediately starts moving to the new target position. The DSP processor performs the following actions;

1. Mass Flow AGC. 2. Feedback AGC. 3. Closes the position loop on the hydraulic cylinder(s). 4. Feed-forward component to the final AGC correction. 5. Low pass, high pass and Anti-alias filtering of AGC component signals. 6. Gathering of all quality control information required by the host PC. 7. Multi-step mill autostop algorithm. 8. Variable mill speed defect tracking (multi-point slowdown). 9. Left and right winder wrap counters. 10. Rolling force calculation. 11. Gather all analog channels and perform required unit scaling. 12. Gather all digital inputs and de-bounce signals. 13. Operator’s manual control of roll position system cylinders via the joystick. 14. Roll position system cylinders close to force calculations and operation. 15. All rotary encoder integrity tests including wrong encoder counts, missing A or B

channel, improper channel phasing, and over frequency. 16. “DSP processor” to “host computer” USB or Ethernet communication.

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HOST COMPUTER: The host computer is a standard Desktop PC (personal computer) running Windows XP and is used only for displaying and printing functions. There is no special hardware installed in the host computer. The software platform is Microsoft’s Visual Studio 2008, VB.NET running under Windows XP Professional version. Visual Basic is used to display and print related AGC information. Visual Basic does not perform any real time control. In fact Window’s XP can be “shutdown” and the DSP processor will still continue to perform it’s AGC tasks. Visual Basic was chosen because of it’s ease of use and wide spread acceptance as a programming language used for displaying information on a PC. The operator’s interface is a color graphic “touch screen” and presents a clear view of AGC operation. Numerical readouts of gage settings, speed, wrap counts, etc. is presented on the screen in large, clear characters. Bar graphs move to indicate rolling force and roll position. The most striking feature on the screen, however, is a scrolling strip chart window, which graphs the entry and exit gage readings in real time. The exit trace is offset from the entry trace, accounting for the distances between the gages and the reduction, so that adjacent points on the graph correspond to the same segment of material.

The combined power of Visual Basic and Windows XP make it possible to display gage readings at the same high sample rate as used by the mass flow AGC task (not a statistical sub-sampling like other systems), but also scroll up the screen in synch with the material on the mill. The display can be frozen at any time for closer inspection. We believe that this feature will give operators a better view than ever before of how the process is responding to their control.

The display is designed to show the most important information at any given time without being cluttered or confusing. Function keys select pop-up windows for data entry, fault analysis, calibration, etc. These appear overlaying part of the main screen but leaving important fields like wrap counters still visible. Soft key functions are always indicated at the bottom of the screen. Other message windows signaling interlocks and faults pop up automatically. For example, if the operator pushes a button to start feedback mode AGC when the exit side gage is not on the strip, a message will appear in a window, "Exit gage must be on strip to use feedback mode." Another example would be if the low pressure return filter became dirty, a message appears, "Time to change low pressure hydraulic filter".

At system start up Visual Basic downloads all required system variables (from a text file) to the DSP processor.

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SERVO HYDRAULIC SYSTEM DESCRIPTION

The operator will control the position of the hydraulic cylinders by means of a joy-stick. All information pertaining to the hydraulic roll position system will be displayed on our new “touch screen” operator interface. A brief description of the hydraulic roll position system that will be supplied follows:

A rugged steel enclosure housing a long stroke Sony Magnascale will be mounted on the roll position cylinder(s) for position feedback. The Sony Magnascale has a resolution of 1 micron (40 millionths of an inch), and unlike optical encoders it is impervious to oil and dust. The resolution and speed of our roll position system is at least twice that of any competing system.

On most new systems we will provide a new Moog or Vickers SM4 series high speed servo valve. The new Vickers Servo Valve will be controlled by our current feedback servo electronics. Current Feedback overcomes the lag in servo current caused by the inductance of the servo valve coil. This system provides reliable, high performance servo-control that is matched precisely to the Vickers valve. The servo electronics will receive a position signal(s) from the DSP processor card. COMPUTERIZED STRIP CHART With the added processing power of Windows XP we are able to print the exit linear strip thickness profile on a graphics printer. Using the graphics printer for linear strip thickness profile eliminates the need for a two-pen chart recorder. These obsolete recorders are typically slow, inaccurate, high maintenance items, which produce sloppy and cumbersome strip chart recordings. The response time of the computerized strip chart is 5 milliseconds. On the left side of the strip thickness profile are located exit meter increments. This data allows out of tolerance areas to be quickly located. The sample rate of these increments can be selected on the strip chart screen. SPC PRINTOUT The SPC printout will be metric and contains the following data: General data: Date, time, mill run time, average and maximum speed. Entry data: Nominal thickness, total meters, standard deviation, mean thickness, and a histogram with 60 different categories. Exit data: Nominal thickness, total meters, standard deviation, mean thickness, upper and lower spec limits, capability indexes, and a histogram with 60 different categories. The exit histogram has an auto-ranging feature that automatically adjusts the printout to obtain maximum resolution.

PASS SCHEDULE STORAGE Software Overview: The storing of the pass schedules for the rolled coils will be based on the “Shop Order Number” or any other identifying number that is entered on the operating parameters screen at the start of every coil. The storing of all the pass schedules will be in a database format supported by Microsoft Access. With the addition of a 250 megabyte zip drive, management personnel will be able to review this data off line. The database size will be limited to less than or equal to 250 megabytes (<=250 megabytes). The size of this database (<=250 megabytes) will be enough to store 25,000 pass schedules. When the operator enters the “Shop Order Number” (or other identifying number) the software will search the database to see if the number matches a coil that was rolled previously. If there is a match a small “pop-up” window will appear prompting the operator that this coil has been rolled before and ask him if we would like to review the pass schedule that was rolled before. If there is no match then no “pop-up” window will appear. The database will be updated when the operator has finished rolling all the passes of the coil. The indication to the software that this event has taken place is when the operator presses the print histogram button. At this time the software will search the database to see if a match occurs of the “Shop Order Number” that was entered before the coil was rolled. If there is no match this indicates that this coil has not been rolled before and that the pass schedule data will be written into the database based on the entered “Shop Order Number”. If there is a match this indicates that the same coil has been rolled previously and the operator will be prompted to he would like to update the database with the new pass schedule data or leave the old pass schedule stored in the database “as is”. At any time during rolling or during mill downtime the operator will be able to recall a pass schedule from the database, review it and print it out. Software Details:

Two operator screens are used to support this feature. One of these screens will allow the operators to view and printout previously rolled coils. The second screen will be for managing the database (editing, deleting and monitor disk space) stored data. The second screen will be intended for management personnel and can be protected with a password if the customer wishes. STORED DATABASE DATA (for every pass rolled for a given coil):

Pass Number: This value will be calculated every time that the mill direction changes and mill rolls at least 50 feet of material.

Entry Thickness: This value will be the entry thickness of the material for the given pass number.

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Exit Thickness: This value will be the exit thickness of the material for the given pass number.

Mill Direction: This value will be the mill direction for the given pass. Payoff Pass: This text field will indicate if the payoff tension is on and the left reel

tension is off (indicating a payoff pass). Entry Tension: This value will be the value of entry tension in pounds that used to

roll the coil. The value of entry tension will be sampled every minute and an average developed for the pass. If the operator should change the tension mid-way through the pass the averaged value will reflect the new tension setting.

Exit Tension: Same as entry tension described above. Average Speed: This value will be the average speed for the rolled coil. It will be

calculated by subtracting the coil start time from the coil end time and dividing this value into the length of the pass in feet.

Maximum Speed: This value will simply be the maximum speed that was run during the present pass.

Pass Start Time: This value will be the time at which the operator started rolling the pass.

Pass Stop Time: This value will be the time at which the operator stops the pass. If the operator should stop and start the mill in the middle of the pass (to inspect the strip, etc.) the software will not use the intermediate stop and start points because a direction change did not occur.

Bond Number: This is the value that the operator entered on the operating parameters screen.

Strip Width: This is the value that the operator entered on the operating parameters screen.

Upper Spec Limit: This is the value that the operator entered on the operating parameters screen.

Lower Spec Limit: This is the value that the operator entered on the operating parameters screen.

Standard Deviation: This value will be the standard deviation of the rolled strip that will be calculated for each and every pass.

Mill Autostop: Utilizing the rotary encoders mounted on the left and right winder motors will provide the AGC system with an isolated digital value that will be used for the “wrap counters”. The wrap counters will be accurate to 1/10 of a wrap and will be displayed and cleared on the AGC touch screen located on the main operator’s desk OS1. The mill autostop software will look at these wrap counters and mill speed to determine the stopping point. Mill autostop is very helpful in increasing productivity on high speed rolling mills. Our autostop algorithm looks at mill speed, the entry wrap counter, and entry nominal thickness to calculate the optimum location to start mill motor deceleration. To have good repeatability on autostops we utilize multiple dwell speeds during deceleration to slow the mill. These dwell speeds when plotted against time resemble a "staircase" effect. This staircase effect will compensate for any inconsistencies in the mill motor deceleration time. Our autostop system will stop within 0.1 wrap of zero every time.

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Multi-point Defect Slowdown: Our AGC system also has the ability to perform multi-point slowdown. Multi-point slowdown is a valuable tool for isolating strip defects and tracking them from pass to pass. As a strip defect approaches the roll bite the computer system will slow the mill down to thread speed, wait till the defect passes, then accelerate the mill back up to its previous speed. There will be a maximum of 64 multi-points that can be stored at one time in the AGC computer. There are three separate tasks associated with multi-point slowdown. Because of the complexity of this software we will describe these tasks in detail.

Multi-point Defect Slowdown Task #1: Storing the beginning and ending defect locations (with the pushbutton on the payoff

operator station labeled multi-point enter) as the operator winds a coil onto the wind-on solid block. The defect points will be stored in the AGC computer "wind-on" defect array.

Multi-point Defect Slowdown Task #2: (a). Tracking the various defect points and transferring them (as they are rolled) from one the defect array on one side of the mill to the defect array on the other side of the mill. Remember there are two points to each defect, a starting location and ending location. (b). Allow the operator to enter new defect points (while rolling) with the multi-point enter pushbutton located on the main desk. Although the mill will not slow down when the operator enters a new point, it will on the following pass. (c). Allow the operator to clear specific defect points as the defect is "rolled out" of the strip with the "clear multipoint" pushbutton. (d). Lengthen the "defect starting point" and shorten the "defect ending point" as each pass is rolled. The amount to lengthen or shorten is based on the reduction ratio. This is performed so that mill productivity does not suffer due to ever increasing defect points (strip getting longer). (e). When the right solid block lock clamps are disengaged (preparing to unwind the coil onto the recoiler), clear defect points for the left and right winder arrays.

Multi-point Defect Slowdown Task #3: (a). The AGC computer must monitor and store the present mill speed (as "current mill speed before multipoint slow down") when ever the multipoint slowdown system is in automatic. This is done so that after a multipoint slowdown defect passed through the mill “roll bite” the AGC computer can increase the speed of the mill to the speed that the mill was operating at before a multipoint slow down occurred. (b). Monitor the defect array on the entry side of the mill and slow the mill down to slow speed before the beginning defect point reaches the roll bite. The AGC computer will use the same algorithm as the autostop program uses to determine the slowing down point. With one exception. The autostop algorithm uses 500 mm (bare block) for its calculation of converting the

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number of wraps to "feet to stop". The multipoint algorithm must know the coil diameter to calculate the number of wraps to "feet to stop". The AGC computer must therefore receive the coil diameter from the PLC. (c). The AGC computer must maintain the slow mill speed throughout the entire defect location. After the ending defect point passes through the roll bite the PMAC computer can increase the mill speed to the value stored in "current mill speed before multipoint slow down". (d). If the operator presses any of the mill speed buttons (stop, slow, hold, fast, fast stop) while TASK 3 is controlling the mill speed through a defect point, the PLC multipoint automatic will be "dropped out of automatic" and the operator will regain control of the mill speed. It will be up to the operator if he would like to turn multipoint back on again. However, if the operator turns multipoint on in the middle of a coil and in the middle of a defect point the AGC computer will miss the proper slowdown point but do its best to slow the mill down for the current defect point. (e). The multipoint enter pushbutton on the main operators desk must be made inoperative by the AGC software during a defect slowdown point. (f). After a defect point has passed through the mill, and the AGC computer is accelerating the mill back up to the value "current mill speed before multipoint slow down", the AGC computer must monitor the location of the next multipoint through this acceleration phase. This is done is case two defect points are close together and the mill speed does not have time to reach the value "current mill speed before multipoint slow down" before another defect point comes along. It is up to the software to keep track of the complicated task timing.