Rolling Mill Automatic Flatness Control (AFC)

The signal processing function decomposes the shape error to determine the correction that must be made by each control actuator. Calculated shape influence sensitivities for the various actuators are used to perform the signal decomposition. These influence sensitivities characterize the form and magnitude of strip shape changes associated with changes in actuator settings (roll bending, roll crown, HGC level) and mill disturbances (roll force, thermal camber). Shape influence sensitivities vary as functions of:

• Mill geometry (roll diameters, face lengths, bearing centerlines, etc.)
• Material properties (width, thickness, hardness)
• Actuator operating point

Since overall performance of the AFC system depends strongly on the accuracy of the sensitivities used, model-based actuator influence sensitivity vectors are used instead of polynomial or other approximations.

The following diagram shows examples of influence sensitivity vectors calculated for the work roll bending (WRB) actuators at various strip widths.

After the shape error has been decomposed and the required actuator corrections are determined, the symmetric and asymmetric control functions simultaneously move the mechanical actuators (roll bending, CVC roll side shifting, and HGC tilting). The actuator movements are coordinated to eliminate shape defects characterized by the shape influence sensitivities for these actuators.

The response characteristics of these controllers depend on actuator dynamics and mill speed. The TelePro/SSE AFC continuously calculates controller gains in order to optimize closed-loop response at every mill speed, taking into account changing material transport delay and CVC shifting rates.

The symmetric control function includes a feedforward mechanism to anticipate and reject shape disturbances associated with roll separating force changes occurring during acceleration and deceleration, or due to incoming material variations.

The zone cooling control function varies the individual zone cooling rates to remove residual shape errors that remain after the mechanical actuators have made their corrections.

The zone cooling control also monitors the overall spray pattern to maintain a specified average level of coolant flow and to reduce asymmetry in the coolant distribution.

• Product-dependent flatness actuator influence sensitivities calculated for each coil using fully-discretized roll stack deflection and strip shape models.
• Model-based decoupling of mechanical bending, tilt, and lateral roll shifting actuator controls from the multi-zone spray control.
• Multi-zone spray controls consisting of both residual shape and average spray level controllers.
• Delay-compensating feedback control algorithms continuously calculate controller gains to optimize system response at every rolling speed.
• Feedforward bending control based on separating force changes using fully discretized shape influence vectors to improve bending control during acceleration and deceleration and when changing gauge at coil heads and tails.
• Product-dependent selection of preset spray pattern, average spray target level, and exit stress distribution target.
• AFC control functions are performed at a 50-millisecond rate.
• Real-time process inputs, including shapemeter feedback, work roll bending pressures, and HGC cylinder positions and pressures, are acquired via high-speed digital communication link or by analog interface, depending on digital link performance, sensor capabilities and customer preference. Typical digital communication links include real-time ETHERNET and reflective memory interfaces to mill drive controls.
• Outputs to the work roll bending, HGC tilting, lateral roll shifting and work roll zone cooling device controls are via high-speed digital communication link or by analog interface, depending on control device capabilities, digital link performance and customer preference.
• Maintenance and diagnostic support tools are provided for monitoring the condition of key control functions. These include graphical controller monitoring displays showing the controller functional block diagrams with current values of important inputs, outputs, internal variables, and activation logic, updated in real time on the screen. Authorized personnel can modify certain system parameters on line through the screen.
• Additional system monitoring displays for actuator reference summing and product-dependent shape influence sensitivity calculation functions.
• A high-speed, real-time trend system, including tools for displaying and/or modifying any global system variable by name through an engineering workstation. Both graphical and tabular data display are provided.
• Secure, remote access to the diagnostic support displays via standard network connection.

The system is implemented as a stand-alone AFC system. However, the same computer is typically used to host automatic gauge control (AGC), interstand tension control (ITC) and model setup calculations. The computing platform is the TelePro Tsentry product. It consists of an off-the-shelf Intel Pentium-class PC running the Windows 2003 Server operating system with VenturCom RTX real time extensions. This system comes complete with all facilities required to support deterministic real-time program repetition rates as fast as 1 millisecond.

In addition to performing sheet flatness control, the system includes both real-time and historical trending. Tsentry also provides a general-purpose real-time computing platform that includes all facilities and libraries required to allow the end user to develop and implement custom process monitoring, control, signal processing, and human-machine interface (HMI) applications. This system interfaces to several varieties of commercially available process I/O hardware and is capable of performing all process control and human-machine interface functions required for mill control. It was developed specifically as a high-speed real-time process control system platform.

The associated HMI displays are implemented as standard Internet web pages hosted by the Tsentry system. The standard Tsentry product includes all facilities required to develop and support a human-machine interface system based on standard web pages. These facilities include custom controls for building animated displays with Microsoft Visual Basic .NET. Automated processes are included for building web screens to execute the controls and for publishing the controls to client workstations. Any network-connected PC workstation with a standard web browser and proper security credentials can be used as an HMI client.

All hardware employs industry-standard, open-architecture systems commercially available from multiple equipment suppliers. Communication is via industry-standard TCP/IP protocol over ETHERNET. Graphical HMI/Engineering workstation displays are via standard Internet web pages allowing display using standard web browsers and screen building using standard web page development tools.

In a typical configuration, the Tsentry host computer is interfaced to RTP I/O hardware from RTP Corporation via an ETHERNET link available commercially from the I/O equipment manufacturer. Device driver software already in wide commercial use is supplied to manage the data exchange.

A fully-redundant backup CPU can be included in the architecture, but is not required. If the backup CPU is available, switchover from the primary CPU to the backup consists of simply moving network cables and re-assigning network addresses. This switchover can be performed manually in less than 15 minutes.

The automation architecture permits remote technical support through a secure ETHERNET connection. To enable remote support, the customer provides an ETHERNET LAN connection available to a trusted external site and a bridge to isolate the process control computers from the external network.