7 things you should know about ABB’s strip-position scanner for cold-rolling mills

Discover ABB’s Millmate Strip Scanner System, a solution for continuous, contact-free measurement of edge and off-center positions, as well as strip width in cold rolling mills and process lines.

The ability to precisely locate the position of strip relative to the machine center is fundamental to mill control and to product quality and yield.

The incorrect location of strip entering a mill stand or strip drift during the rolling sequence is likely to impact both the technical and economic success of the operation. Problems range from poor product flatness to bad coil ends and off-position entry into downstream processing stages such as slitters, trimmers, annealing lines, and so on. Mill operators who have experienced these problems will recognize not only the cost in downtime and reduced yield, but the difficulties in attempting to monitor strip position and rectify off-center running. Not least of the practical difficulties is the harshness of the general cold rolling mill environment, characterized by dirt, steam, coolant sprays and vibration.

Despite all the practical difficulties, the ability to precisely locate the position of strip relative to the mill rolls is fundamental to mill control. It is rarely the case that simple lateral alignment of the feedstock coil at the mill entry will guarantee that the strip remains centered in the mill. Transverse variations in hot mill coiling of the rolled feedstock can be of the order of 25 mm or more, and a non-trimmed strip from a hot mill may have a width variation of a similar magnitude over its length, with the middle of the strip being narrower than the front and back ends.

Understandably, most strip monitoring exercises are based on optical technology ranging from simple photocells to complex stereoscopic camera arrangements whose performance can quickly be impaired by so-called blinding. Such systems are also inherently fragile and at risk from impact. For these reasons, dependence on optical systems coupled with the need for frequent cleaning can prove to be counter productive. 

Thanks to ABB’s Pulsed Eddy Current technology, strip position can be monitored by using an electromagnetic scanner. The Millmate Strip Scanner system provides a continuous indication of the edge and off-center positions of the strip as well as its width. What’s more, it is unaffected by the harsh environment of the mill.

Here are seven things you should know about ABB’s Millmate Strip Scanner for strip position measurement in cold-rolling mills. 

1. Why the method used in flatness control matters

The importance of being able to measure and control strip flatness from edge to edge cannot be overemphasized. Poorly controlled edge flatness impairs not only overall strip quality but can also result in strip breakage. In the absence of precise knowledge of strip-edge position, overall monitoring for flatness and strip tension can be difficult. As a rule, flatness at the extreme edges of the strip is calculated from measurements taken from zones a short distance from the edge of the strip. Under ideal circumstances, this may be an acceptable approach to flatness control, however, in practice, several factors make this method inaccurate. These include: any edge thickness shortfall, the edge position in relation to the side shift roll, uneven wear of the working roll at the strip edge, and the temperature gradients occurring at the edge.

2. Operational and economic benefits

Apart from technical considerations, operational and economic benefits accrue from being able both to monitor and to use feedback control to achieve true centering of the strip. In threading, for example, the speed of the operation is an important factor in mill productivity, and threading time can be reduced dramatically where instrumentation can replace manual alignment (with a rule or against roller table markings). In the rolling operation itself, the use of dynamic feedback to control mill parameters enables flatness targets to be achieved early in the cycle. At offtake, uniform recoiling of the strip minimizes risks and the often-heavy cost of edge damage in subsequent handling and transportation.

3. Reliable and continuous strip position scanning

Accurate strip centering is clearly essential to optimum cold-rolling mill performance, but it is often difficult to achieve continuously and reliably. Because optical systems function on a silhouette of the strip edge, they are vulnerable to contamination and mechanical damage. The contactless position monitoring Millmate Strip Scanner systems from ABB use the electromechanical properties of the strip itself as part of the measurement circuit. This scanning principle couples high sensitivity with long-term reliability.

Millmate Strip Scanner systems use transducers, operating with a pulsed magnetic field to measure and analyze field fluctuations which occur as a function of the relative position of a sheet of metal as it passes over the transducers. The field is generated by several coils within the sensing head or measuring beam, which also houses electronic circuits for signal processing. Because there are no moving parts and no optics, measurement sensitivity is totally unaffected by dust, vibration or mill fluids.

Measurement is made by comparing the signal generated by coils fully covered by the strip with the signal from a coil overlapping the strip edge. Since the magnitude of the signal change depends on both the distance from the measuring beam to the strip and the degree of coverage of the outer coil by the strip edge, the signal difference can be used to display status or as the drive for other functions. Two sensors are used, one under each side of the strip, showing the individual strip edge positions and the signals are summed to provide a continuous indication of the width and off-center positions.

The strip scanner can be used with all types of metal to a maximum strip thickness of 20 mm and a minimum strip width of 220 mm. There is no maximum width limit. The minimum thickness which can be measured within specification is: steel, 0.1 mm; stainless steel, 0.3 mm; and aluminum and copper, 5 µm. The scanning system has an overall accuracy of ± 0.5 mm on set dimensions.

The solid-state nature of the scanner and rugged construction permit its use at any convenient point in the mill, irrespective of the environmental conditions. Orientation is arbitrary, except that there must be a metal-free zone of 250 mm in the operating direction. The housing is of corrosion-resistant aluminum bronze with a signal ­transparent cover of high-impact, and high temperature resistant  GRP (Glass Reinforced Polymer). In use, positive air pressure is maintained by a low­ level, compressed air supply within the sensor to prevent the ingress of dirt and mill fluids, as well as to remove any radiant heat that might penetrate the unit. Once installed, the sensors are virtually maintenance free.

4. Configuration and functions of the system

The system consists of two measuring sensors, an interface, and a control unit calculating output signals by adding mill geometry to the measured values and a touch panel monitor as HMI. The interface unit contains the power supply and a modem for communication between the sensors and the control unit.

Apart from the primary functions of presenting measurement values of strip-edge positions and off-center position and width, the control unit also provides other functions adaptable to differing mill requirements. These include systems testing, start-of ­run calibration, entry of nominal strip­ width and strip thickness. A direct link can be made to other plant systems, for example flatness control, or to a higher-level system.

5. Function monitoring

The system is so designed that it will not give spurious measurement values. Functions are monitored for faults affecting power supply, signal processing, memory, connector contacts, communications links, and sensor temperature. The controller also verifies that the strip being rolled is within its measurement range. If a fault is registered which appears to indicate that a measurement value could be in doubt, the system records nominal, rather than measured values. Step-by-step fault codes provide rapid location of the cause of any problem. Calibration is affected in-situ by a fully automatic procedure, requiring no manual trimming.

6. Single-edge operation

To cover events such as a cable break, or failure of one of the measuring beams, the system can be preset to switch automatically to measurement based on one measured edge position plus nominal strip width until the fault is rectified.

7. Field experience

Millmate Strip Scanner systems were first put on the market in 1992 and have since then been an appreciated instrument in the flat rolled metals industry. Many producers have experienced the reliability, accuracy, and low maintenance of the sensors throughout the years. One example is the stainless-steel company TISCO in Taiyuan, China. ABB has supplied in total 18 Millmate Strip scanner sensors to their two 5-stand tandem cold rolling mills. The Millmate Strip Scanner sensors have been running very well for several years in operation, according to the company.

*Millmate is a registered trademark of ABB.