Automatic Gauge Control (AGC)
The main target of the AGC control is to maintain a good thickness tolerance over the entire range of products in varying process conditions undependent of different materials, alloys, dimensions. A system is offered with an adaptive algorithm, different control strategies are available depending on the mill to realize the desired thickness tolerances. All functions necessary for the regulation, control and sequencing of mill actuators to realize the AGC function are provided. The immediate result of the AGC system is that the flat products rolled in the mill are of required thickness and within the required tolerance.
Using hydraulic roll force systems, steelmakers have the ability to control precisely their steel sheet’s gauge (thickness). Using feedback or feed-forward systems, a computer’s gap sensor adjusts the distance between the reduction rolls of the mill. These adjustments prevent the processing of any off-gauge steel sheet.
Roll gap control (RGC)
RGC is keeping the roll gap at its setpoint. By using gap sensors measuring the position of the hydralulc cylinders it is posible to adjusts the distance between the reduction rolls of the mill.
FB is regulating the exit thickness at its setpoint. Since the exit thickness is measured, some distance after the roll bite this mode is quite slow. The step response is app. 3-6 times the transportation time between roll bite and exit gauge meter.
The FB regulator is a I-regulator where the in signal is the exit gauge meter and the out signal is a gap correction signal to the RGC. The p&i gains are adapted to strip -hardness, and roll speed.
FB is normally correcting for:
- Offset in thickness at start.
- Roll diameter changes due to heat
- Tapered incoming strip
- Slow strip hardness variations
FF is correcting for entry strip thickness variations, measured by the entry thickness meter.
The FF regulator using the entry gauge meter and the out signal is a gap correction signal to the RGC. The FF regulator delays the correction signal to be précis in timing due to roll speed and responce time of the RGC.
The gain of the FF regulator adapts to strip –hardness.
FF corrects for variations due to roll eccentricity, speed changes and variations due to offset in the beginning of the strip introduced in previous passes.
MF is acting as a very fast feedback. Instead of using the exit gauge meter it is calculating the exit thickness by measuring the entry speed, exit speed and entry thickness. This means that the thickness is calculated directly in the roll bite.
Exit thickness is then calculated as:
Exit thickness = (entry thickness) * (entry speed) / (exit speed).
The MF regulator is PI-regulator where the in signal is the calculated exit thickness and the out signal is a gap correction signal to the RGC. The p&i gains adapts to strip hardness, and roll speed.
The calculated thickness should adapt to the measured exit thickness using a I-regulator where the in signal is the difference of the calculated and the actual exit thickness. The out signal correct the mass flow calculation.
- Incoming thickness variations
- Variations due to roll eccentricity
- Variations due to roll friction changes (speed effect).
- The advantage is a faster step response than in normal FB and it is not depending on the length to the out going gage meter. The accuracy of all four measuring devices has to be accurate to get full advantage of this method.
Speed Effect (SE)
SE compensates for variations in roll friction due to rolling speed.
The SE regulator is P-regulator where the in signal is the exit speed and the out signal is a gap correction signal to the RGC. The p gains adapts to strip hardness, width, roll force and roll speed. Observe that the p-gain is non-linear due to the parameters mentioned above.
The AGC gain has to be calculated for different strip width, thickness, hardness, roll force and rolling speed. This is done on-line while rolling.
Combinations of AGC modes
Normally the AGC modes are used in combinations.
The most common combinations are:
- FB/SE. Used in first passes when using pay off reel.
- FF/FB/SE. Used during normal rolling.
- MF/FF. Used during normal rolling.