Optimized Metal Strip Measurement

Metal strip is employed as a raw material in the manufacture of a number of products, and the metal strip is directly related to the final quality of the finished product. Therefore, before further processing, the strip has to be examined for compliance with the various quality criteria.

In addition to the physical properties, an important feature of the metal strip is its thickness profile. The thickness profile can be measured by many different techniques. Micro-Epsilon, a manufacturer of sensors and systems from Ortenburg, offers an interesting solution for this purpose.

Coils are the raw material for many other products. The thickness is an important factor.

Coils are the raw material for many other products. The thickness is an important factor.

It is not very easy to produce a metal with a constant thickness. Roll bending creates deviations from the desired thickness in hot- and cold-rolled metal strips. Therefore, various techniques such as roll cambering or using support rollers are adopted for optimizing the thickness profile.

The thickness profile over the width and length, the geometry, and the documentation of measured values are the important quality criteria of a metal strip. Measuring systems checking these and providing correction variables for regulation are essential to satisfy the quality criteria.

Thickness Measurement Methods

Traditional mechanical thickness measurement systems calculate the metal sheet thickness via contact using a caliper-type arrangement at set measurement points. The thickness values are then interpolated to provide an approximate thickness measurement.

As this method is quite slow, it is not suitable to obtain an in-process detailed thickness profile across or down the length of the strip. Additionally, these types of measurement methods are usually affected by wear and are not automated, so they are likely to interfere with the flow of production.

Alternatively, radiometric techniques can be used to measure metal sheet thickness. The radiation from an isotope source is dampened by the sheet, and the balance radiation is measured on the opposite side. The difference between the transmitted and the measured radiation is converted to an area-weighted value, and later, to a thickness measurement.

However, the radiometric method depends on the condition of the metal sheet and the alloy. Although this type of thickness measurement provides sufficient information about the thickness profile of a known alloy, it requires costly, complex safety provisions due to the radiation intensity.

Across three shifts, the cost of radiation protection, non-stop safety monitoring systems, and supervisors mean that there are high variable costs associated with these methods. Although the use of capacitive sensor technology provides a solution, the systems using the technology have a relatively large spot size.

The sensors can only provide averaged thickness profile information around the frontal areas of the sensors as they measure over the complete face. Better local resolution is needed for the edges of the products.

Different variants are available from Micro-Epsilon for this purpose. Using two-sided thickness measurement and laser sensors, a simple C-clamp measuring device measures the thickness on a selectable track in the direction of production.

A closed O-frame is used in another variant. Here, a sensor is placed on each side of the metal strip in an identical position. The sensors then continuously move perpendicular to the production flow and the thickness profile can be measured over the complete width of the product.

The new high-end system for measuring the dimensions and profile of strip metal from Micro- Epsilon.

The new high-end system for measuring the dimensions and profile of strip metal from Micro- Epsilon.

New Method for Profile Measurement

In this new high-end solution, the sensor system traverses along the measuring gap across the complete width of the strip. In contrast to traditional solutions, two laser line scanners are used to provide significant improvements in accurancy and base distance to the metal strip.

The new O-frame innovation from Micro-Epsilon Messtechnik uses specially adapted laser line scanners. These scanners provide higher precision thickness measurements in comparison with laser point sensors when there is a larger distance to the target, and therefore, a larger measuring gap.

The system possesses a measuring gap of 200 mm, it is quite robust and can handle large fluctuations in strip guidance. Long-term operation and extended life can be ensured by simple, effective tweaks such as an open structure at the bottom so that dirt and scale easily drop through the system, not obstructing the sensors.

Possible collisions are avoided by the 200 mm measuring gap that protects the sensors. Additional mechanical protection is provided by completely safeguarding the measuring system, as metal strip vibration or curved/bent strip ends are always a risk for the installed sensor system.

Better measurements on many different strip materials, irrespective of the alloy type, can be enabled using profile sensors rather than point sensors to increase the density of profile information. Laser line scanners instead of point lasers can be used to significantly improve the measurement accuracy; an accuracy of 0.01 mm for a strip of maximum width of 4 m can be achieved with laser line scanners.

The profile sensors are supported by high-tech light barriers, which accomplish the task of width measurement, and if necessary, the edge detection of each individual strip after splitting. All measured data can be used to document the metal strip. The “profile” and “thickness” data are assigned online to a precise position on the strip, and the system is used in service centers for flat metal strips, and after individual metal strips are separated from the coils.

The system is a high-end solution for the measurement of metal strip geometry. The new system has effectively replaced the popular, traditional measurement systems. The return on investment related to laser scanner measurement comes from the in-depth knowledge of real strip tolerances, including the fact that each individual strip is documented and traceable for end customers.

Laser scanners are used in the system. These provide very precise data, particularly for metals.

Laser scanners are used in the system. These provide very precise data, particularly for metals.

scanCONTROL Functioning Principle

During measurement, a highly sensitive CMOS matrix records the reflected light of the laser line and generates a precise image of the surface profile. The displayed line is changed by changes in the profile, and a changed image is formed on the matrix.

A 3D image of the object is generated by opposing the individual line profiles as the scanner or measurement object is usually moving. In this manner, a so-called “point cloud” is created because the image consists of several thousands individual measurement points.

The system in use for the cut-to-length shear. The geometries of the individual strips are inspected.

The system in use for the cut-to-length shear. The geometries of the individual strips are inspected.

This information has been sourced, reviewed and adapted from materials provided by Micro Epsilon.

For more information on this source, please visit Micro Epsilon.

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