Key components for high-end thickness and flatness performance

Roll gap control
Two basic modes, either gap position or total roll force can be applied. Selection of mode depends on the type of superimposed Automatic Gauge Control (AGC) and the threading strategy. To ensure a constant response time over the whole working range, various process related adaptations and supervisions are included.

AGC for break down mills
Depending on the sensor concept based on customer needs, different thickness control strategies are possible:
o Thickness feedback control
o Thickness feed-forward control
o Mass flow control
o Speed feed-forward control
o Roll eccentricity control
o Tension feed-forward control

Thickness feedback control
The basic control strategy is thickness feedback (monitor control) based on the measured thickness deviation at the exit side of the mill.

The delay time, transport of the strip from the roll gap to the thickness gauge, essentially determines the control system's dynamic response, particular at a low rolling speed. The software package provides with a predictive modelbased option to improve the dynamic behaviour of the thickness feedback controller significantly.

Thickness feed-forward control
If a thickness gauge at the entry side of the roll gap is available, the thickness feed-forward control can be applied. It is able to compensate any thickness deviation caused by changing entry thickness.

A correction value is calculated according to a stored entry thickness deviation and forwarded to the roll gap control for interaction when the strip section reaches the roll gap.

Mass flow control
Using the mass flow principle, the outgoing strip thickness at the instant of rolling can be computed from the incoming strip thickness and the in-and-out coming strip speeds. By means of this mass flow control concept, high control accuracy can be achieved. Therefore it enhances clearly the concepts of thickness feedback and thickness feed forward concerning product quality.

Speed feed-forward control
With the speed feed-forward controlsolution, velocity dependent processvariations are compensated.

Roll eccentricity control
Irregularities in roll geometry cause periodic variations in the roll gap, which can lead to variations in thickness of the rolled product. The predominant sources of these variations are the backup rolls of the stand.

The function compensates for periodic disturbances from geometric asymmetries on the backup rolls. An automatic adaptation to compensate changes during rolling (e.g. due to thermal effects, wear, loading, etc.) is implemented.

Tension feed-forward control
The tension feed-forward control responds to entry thickness deviations and applies a suitable adjustment to the coiler torque in order to consider the interactions between roll gap and tension.

AGC for foil mills
Speed-tension optimization is used in foil mill applications, since the strip thickness depends very closely on tension and rolling speed. In order to achieve maximum material throughput, the speed of the stand is increased until the strip tension has reached its control limit or the speed has reached a pre-selected technological limit value.

AGC for temper mills
Based on the measured elongation and the actual strip dimensions, fed into an algorithm, the elongation control module calculates a correction for the reference value of total roll force and /or tensions.

Coil eccentricity control
The coil eccentricity compensation minimizes periodic tension oscillations generated by changes in the circularity of the coil. A major reason for it, is the strip head pressed in the slot of the mandrel or wrapped on a sleeve or mandrel. Tension torque corrections are applied at each rotation when the diameter change passes the contact zone of the strip on the coil.

Adaptive control concepts
The phenomena in the roll gap during rolling process are largely non-linear and time-variant. Adaptive controller is used to detect changes in the system and its disturbances to modify the parameters accordingly. This adaptation is achieved under consideration of quality and stability criteria for the entire control loop.

Flatness control
Correct strip flatness is essential to ensure an overall product quality, productivity and successful subsequent processing.

Homogeneity of stress distribution and material shape is controlled by modular flatness software. Example of a control concept for a single stand mill. The flatness error, given as difference between the measured strip flatness and the target curve, can be minimized by Example of a control concept for a single stand mill modifying the roll gap with various control functions, such as rollbending and skewing, shifting of rolls, cooling patterns and eccentric positioning control for multi-roll
stands. The influence of each separate type of control action is defined by evaluated action curves. A least square fit of these action curves to the flatness error results in the most efficient combination of control actions needed to reduce the flatness deviation.

Coordinated control
Given the complexity involved in interaction of the final control elements and the demanded dynamics, the desired results can be assured only by automated coordination of all screw-down reference values and control commands for strip thickness, tension and shape. At any time manual operations such as set-point changes or selection and de-selection of control loops can be performed with bump less transitions. To ensure best possible performance for a mill, latest generation Automation Systems are able to simulate the rolling process based on a non-linear simulation model. Comparison of different control strategies based on various sensor and actuator concepts allows the selection of the best technological solution.

- Andreas Vollmer