Automatic Gauge And Interstand Tension Control (AGC/ITC)

The AGC/ITC functions utilize non-interactive actuator networks -- combinations of simultaneous stand speed and roll gap position (or force) changes which decouple strip thickness and tension changes. For multi-stand mills, an AGC load sharing mechanism is included to permit thickness corrections to be distributed over all available rolling stands according to a prescribed loading ratio. This load sharing mechanism reduces the amount of force variation at the final stand, thus reducing exit sheet flatness variation. Product-dependent sensitivities are computed by the mill setup models for each gauge and tension control actuator. These sensitivities determine the relative sizes and directions of the actuator movements required to make tension and gauge control corrections while reducing interactions between control loops.

The application software is implemented in the ANSI/ISO Standard C and C++ programming languages. The AGC/ITC functions are typically performed in the same PC-based computer system hardware used to implement the automatic sheet flatness control (AFC) and mill setup models.

Before a coil is processed, the AGC/ITC system receives rolling schedule information from the mill scheduling system or via manual data entry. This information includes:

• Coil identification number
• Material grade or alloy designation
• Thickness at which the material was last annealed (required if automatic calculation of control gains is desired)
• Scheduled entry, interstand, and exit thickness
• Scheduled entry, interstand, and exit tensions
• Strip width

During rolling the system receives the following process signals:

• Exit thickness deviation feedback
• Entry thickness deviation feedback (required for feedforward and mass-flow control)
• Interstand tension feedback
• Roll separating force (or HGC pressure) feedback
• Stand motor speed feedback
• Interstand and exit sheet speeds (if unavailable, these can be calculated from motor speeds, roll diameters, and nominal extrusion ratio)
• Entry sheet speed (required for mass-flow control)

Output corrections are passed to the HGC position (or pressure) and stand motor speed device controls for all rolling stands.

• A delay-compensating feedback AGC algorithm that produces the fastest possible closed-loop performance in the presence of material transportation delay in the feedback measurement. Controller gains are calculated continuously to optimize response at every rolling speed.
• This algorithm consists of a Smith Linear Predictor (SLP) with internal controller parameters calculated as functions of mill speed. A conventional proportional-integral algorithm is also provided as a temporary alternative for commissioning. The primary mode of control for basic feedback AGC is the SLP configuration.
• Additional feedback AGC control algorithms based on mass-flow and gaugemeter estimates of strip thickness. A bumpless transition mechanism is provided for switching between feedback AGC modes while rolling.
• A feedforward AGC control algorithm which employs entry thickness deviation and entry sheet speed measurements to improve AGC disturbance rejection performance for incoming thickness variations.
• A thread mode tension controller specifically designed to establish interstand tensions during initial threading. The output of the thread mode controller consists of changes to stand motor speeds.
• A steady-state run mode tension controller optimized for operation once all tensions have been established. The output of the run mode controller consists of simultaneous changes to roll gaps (or HGC pressures) and stand motor speeds to permit interstand tension corrections without causing thickness changes.
• Product-dependent actuator sensitivities calculated by process models are used to adapt the AGC/ITC systems for various product grades, widths and gauges.
• AGC/ITC control functions are performed at a 20 millisecond rate in order to take advantage of high-speed actuator dynamics.
• Real-time process inputs 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 HGC position or pressure regulation and stand motor drive 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 gain 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 AGC/ITC system. However, the same computer is typically used to host automatic sheet flatness control (AFC) 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 gauge and tension 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.