WO1996012575A1

WO1996012575A1 - Gauge control of a rolling mill

Info

Publication number: WO1996012575A1
Application number: PCT/GB1995/002463
Authority: WO
Inventors: Peter William Burnand
Original assignee: Davy Mckee (Sheffield) Limited
Priority date: 1994-10-19
Filing date: 1995-10-18
Publication date: 1996-05-02
Also published as: GB9421030D0; AU3670295A

Abstract

The gauge control of a rolling mill is dependent upon adjustment of the rolling load by a hydraulic operable device which is supplied with hydraulic fluid from an electro-hydraulic device. The device is supplied with a control signal of variable gain which represents the difference between (H+ΔH)/h and v/V where H + V represent the entry thickness and entry speed and h and v represent the exit thickness and the exit speed. The gain of the signal is adjusted so that the control signal is at a level just below that at which hydraulic saturation occurs.

Description

GAUGE CONTROL OF A ROLLING MILL This invention relates to the gauge control of a rolling mill rolling metal strip- Automatic gauge control (AGC) of a rolling mill rolling metal strip is well known and in such systems the rolling load applied to the strip by the rolls is controlled by the operation of hydraulically operated piston-cylinder assemblies acting between the mill housing and bearing chock assemblies at the ends of at least one of the rolls.

There are a number of techniques for controlling the piston-cylinder assemblies and one of them is based on the "Mass Flow" principle. This principle states that the mass of material entering a rolling mill per unit time is equal to the mass of rolled material leaving the rolling mill per unit time. Assuming that the material being rolled (A) does not change in density and (B) it does not change in width during the rolling process, then the "Mass Flow" principle can be expressed as follows:- HV = hv where H is the entry thickness of the material, V is the entry velocity of the material, h is the exit thickness and v is the exit velocity. The equation may be expanded to define the entry thickness of the material as -

Nominal entry thickness H + entry material thickness deviation AH and the exit thickness of the material as

Nominal exit thickness h + exit material thickness deviation Ah,

The object of the control system is to control the mill so that the output gauge is constant, i.e. h is zero.

To do this the mill must be controlled so that

(H+ AH)V = (h+o)v i.e. (H + AH) = y h V. v is controlled as closely as possible by V varying the load applied to the mill rolls by means of the hydraulically operated piston-cylinder devices in response to variations in AH.

The overall controller's gain includes components based on (C) the mechanical construction of the mill stand;

(D) the hydraulic component based on the servo controlled hydraulic piston-cylinder assemblies and

(E) the electronic component of the electronic circuitry.

The mechanical component (C) is fixed; the hydraulic component (D) is subject to variation due to changes in, for example, oil temperature, oil supply pressure, and flow saturation; and the electronic component E can readily be modified because it incorporates a multiplying element.

Accordingly the present invention resides in a method of gauge control of a rolling mill in which the rolling load is adjusted by hydraulically operable means in such a way that the effects of entry thickness variations AH on the exit thickness are substantially eliminated, saturation of the device controlling the flow to the hydraulically operable means is detected and the gain of the electronic circuitry is kept at a level just below that at which substantial hydraulic saturation is detected.

In order that the invention may be more readily understood it will now be described, by way of example only, with reference to the accompanying drawing which shows diagram atically a control system in accordance with the present invention.

A rolling mill illustrated as a four-high stand 1 has its rolling load controlled hydraulically by piston- cylinder assemblies, one of which is indicated by reference numeral 3. Metal strip 5 is fed into the rolling mill and its ingoing speed V is measured by a transducer 7 and its thickness deviation (AH) is measured by a gauge 9. After it has been rolled its outgoing speed v is measured by a transducer 11.

The flow of hydraulic fluid to the piston-cylinder assemblies is controlled by a servo-valve 15 with its driver 17.

The signals representing the velocities v and V are supplied as separate inputs of an electronic divider 19 to produce a reset term V. The gauge 9 measures the v thickness deviation upstream of the mill rolls and so a delay device (not shown) is employed to delay the signal AH by the time taken for the ingoing strip to pass from the gauge to the mill rolls. The delayed thickness deviation signal D A H is added to the nominal thickness signal H in an adder 21 and the resultant signal H + D A H is divided into the signal representing the desired output thickness in a divider 23. The signals representing h and V are

H+D A H v entered as negative and positive signals respectively in a comparator 25 to produce a difference signal. This signal from the comparator 25 passes through a variable gain (E) amplifier 27 and is applied to the driver 17. ^"The output of the driver is monitored by a detector 29 and a ramp generator 31 produces a signal which is dependent upon the output of detector 29. The ramp signal is supplied as the multiplier input to the variable gain amplifier 27.

To satisfy equation H+DAH = V it is desirable for h v the gain of the control circuit to be as high as possible so that a small difference output from the comparator 25 produces a large flow of hydraulic fluid from the servo valve to the hydraulic assemblies 3 to rapidly change the rolling load to reduce the signal to zero. If there is a small difference signal from comparator 25 and it exists for a substantial period of time then the level of the signal from the driver 17 is small and the detector 29 detects no saturation. The detector indicates to the ramp generator 31 that its output signal is to be gradually increased in order to increase the gain E of the amplifier 27. The hydraulic servo valve is most likely to exhibit saturation of fluid flow and thereafter an increase in gain E has no effect on the fluid flow. When saturation is detected by the detector 29, the gain of the amplifier 27 is reduced. To this end, the detector 29 monitors the voltage applied from the driver 17 to the servo valve 15. As soon as the servo valve is driven at full rate for periods longer than a few milliseconds it is assumed that the servo valve is in substantial flow saturation. When substantial saturation is detected the gain of the circuitry 27 starts to be reduced by reducing the ramp voltage applied to it from the generator 31 thus reducing the input to the driver 17. The optimum state is to control the gain of the circuitry 27 until the level of hydraulic flow is just less than saturation.

It is convenient to arrange for the gain E to increase slowly until saturation is detected and then for the gain E to be rapidly decreased (say by 8-12 times the rate of increase) when saturation is detected.

In a practical 'Mass Flow' controller of an Automatic Gauge Control System of a rolling mill, many things will modify the saturation level of the servo-valve, hence its unpredictability. One interesting effect is that for earlier passes of the rolling process where entry gauge deviations AH are large, then large flows of oil are needed for AH to be removed, consequently low gains would be required to prevent saturation.

However, as the rolling progresses and pass numbers increase then AH is small and as such would not demand large oil flows in the servo valves. This system in these later passes of small AH would then continue to raise the electronic gain of element Ε' thereby ensuring the small AH is clearly reduced.

This technique may be employed in multi stand mills as individual sets of controllers each setting the gain of related mass flow systems.

Claims

Claims :

1. A method of gauge control of a rolling mill in which the rolling load is adjusted by hydraulically operable means supplied with hydraulic fluid under the control of an electro-hydraulic device comprising the steps of obtaining an electrical signal representing

(H+AH) where h

H is the entry thickness of the material being rolled,

AH is thickness deviation h is exit thickness of the material being rolled, obtaining an electrical signal representing v where

V v = exit velocity of the material

V = entry velocity comparing said electrical signals to produce a difference signal, applying said difference signal to electronic circuit means having a variable gain to produce a control signal for said electro-hydraulic device to control the rolling load of the mill in the sense to reduce the difference signal substantially to zero, detecting when hydraulic saturation of said electro- hydraulic device occurs, and controlling the gain of said electronic circuit means to adjust the control signal to a level just below that at which hydraulic saturation occurs.

2. A method as claimed in claim 1 in which the control signal to the electro hydraulic device is detected and a ramp signal is applied to the circuit means to adjust the gain thereof in the sense to reduce the gain when hydraulic saturation occurs.

3. A method as claimed in claim 2 in which the ramp signal increases the gain of the circuit means until saturation is detected and thereafter the ramp signal is decreased to reduce the gain of the circuit means.

4. A method of gauge control of a rolling mill as claimed in claim 1, 2 or 3 , in which the gain of said circuit means is increased at a slower rate than the rate at which the gain is decreased.

5. A method as claimed in claim 4 in which the rate of decrease of the gain is between 8-12 times the rate of increase.

6. A method as claimed in any preceding claim in which a signal representing AH is obtained from a gauge upstream of the rolling mill and is delayed for a time interval equal to the ingoing strip to pass from the gauge to the mill.

7. A rolling mill having hydraulically operable means for adjusting the rolling load; an electro-hydraulic device for controlling the supply of hydraulic fluid to the operable means; means for obtaining an electrical signal representing

(H+AH) where h

H is the entry thickness of material being rolled,

AH is the thickness deviation h is the exit thickness of the material being rolled; means for obtaining an electrical signal representing v where

V v is the exit velocity of the material and

V is the entry velocity; means for comparing said electrical signals to produce a difference signal, an electronic circuit means having a variable gain to which said difference signal is applied and which produces a control signal for said electro-hydraulic device; means for detecting when hydraulic saturation of said electro hydraulic device occurs and means for controlling the gain of said electronic circuit means to adjust the control signal and wherein in use the control signal is adjusted to a level just below that at which hydraulic saturation occurs.

8. A method of gauge control of a rolling mill in which the rolling load is adjusted by hydraulically operable means in such a way that the effects of entry thickness variations AH on the exit thickness are substantially eliminated, saturation of the device controlling the flow to the hydraulically operable means is detected and the gain of the electronic circuitry is kept at a level just below that at which substantial hydraulic saturation is detected.