A highly developed topic is the control of strip thickness during the aluminum rolling process. In order to meet the tighter tolerances demanded by the market, the third generation of control engineers and practitioners is now working to achieve this. In recent years, control theory has progressed, providing an arsenal of modelling toolboxes and control blocks that are designed to cope with uncertainty while also incorporating stability and robustness (and, inevitably, necessitating the development of new vocabulary). Higher-performance hardware allows virtual measurements to be incorporated into process descriptions without the computation of those measurements slowing down the overall system's responsiveness. The task of a process or maintenance engineer dispatched to a gauge control problem in the wee hours of the morning becomes more difficult with each additional layer of complexity.
Indicators of success
Measurement is the first issue to address. Although the method of measuring strip thickness is not important, it is important to understand why certain limitations exist and what needs to be done to overcome them. X-ray systems of the modern era are well understood, and suppliers are aware of the need to compensate for differences in exact composition, as well as differences in temperature between air columns and other factors that alter the apparent thickness. When robust and rigorous procedures to monitor strip thickness are implemented in the same way that our customers do, it should be possible to ensure that the discrepancies between the different measurement systems are never large enough to dominate the variation in strip thickness that our customers observe.
Center-line gauge control is not a problem from the customer's perspective. For the customer, the specification revolves around thickness tolerances, which means that if they use the material by blanking out parts, all of the parts must meet the specification regardless of where they originate in the sheet. Most sheet materials are thinner at the edges than they are in the center. This thickness variation across the width of the strip (strip profile) is determined during the hot mill stage of aluminum rolling, but it is now recognized to be at least as important as the thickness variation across the length of the strip (lengthwise thickness variation). The latter is controlled by the cold passes of aluminum rolling, which is where historically all of the effort in controlling thickness has been concentrated. . This can lead to a variety of issues, as illustrated in Figure 1, where the variation measured by a similar type of instrument near the center is compared to variation measured by a similar type of instrument near the edge. The variation is of a slightly different magnitude on either side of the edge, in addition to being thinner on one side than the other. All variations must fall within the tolerance of the customer.
Deviation in the aluminum rolling gauge
The deviation in gauge from the nominal (%) is shown in Figure 1. Red – near-edge; Blue – center-line. Observations and remarkingsIt has been decided to offset the edge signal vertically in order to prevent the signals from overlapping. As a result, there appears to be a time displacement between the measurements on the .
Also depicted in this figure is the inherently complex spectrum of frequencies contained within the thickness signal's frequency spectrum. Each component is generated by the rolls that deform the strip at each pass, and the frequency spectrum can be analyzed to determine which pass and which roll each component is generated at. When used in conjunction with feed-forward gauge control, incoming variation can be eliminated within a limited bandwidth; good design of control schemes prevents the variation from being amplified and the occurrence of resonant frequencies in coupled drive systems of aluminium rolling mills is prevented. The mass flow control system also takes care of issues with the time lag between mill bite and thickness measurement at the exit, allowing for stable control even when the mill speed changes. When rolling aluminum, it is important to pay attention to the details and follow good practice to avoid introducing too many new frequencies in the current pass.
The use of a comprehensive strategy
Due to the fact that some of the features in the gauge trace are caused by variations in material hardness that are interpreted as thickness variation, careful consideration must be given to the casting process in order to achieve a composition that is as homogeneous as possible throughout the strip length. Instead of using correlation techniques to identify features that persist through multiple passes and assuming they are due to compositional effects, this is a more effective approach.
coils of aluminum for rollingIn order to provide control robustness and stability in the face of moderate process perturbations over a wide frequency range, many of the more advanced control techniques have been designed to be flexible. The use of a holistic approach that incorporates an understanding of where thickness variation can occur, as well as knowledge of the measurement systems and what might cause them to give "false" thickness readings, eliminates the need for them altogether.
If the disturbances are small and dominated by the current pass, the controller's job becomes significantly less difficult. Using a gauge control scheme based on underlying feed-forward feed-back control loops for stability and mass flow control loops for accuracy and control of the strip from head to tail, the industry has come to terms with the challenges we now face in the production of aluminum as well as other metals. Of course, a thorough understanding of the material and the rolling processes from casting to finishing is required in order to eliminate features at every pass, which is the key to achieving ultimate control. This comes from a combination of experience and knowledge, and for those who are new to the industry, it may be necessary to undergo formal training.
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