Thickness measurement of heavy plates can be accomplished by using radiometric as well as laser-based measurement methods. Laser-based processes, such as the scanCONTROL laser scanners, offer clear advantages, which numerous users in the steel and metal processing industry appreciate.

Figure 1. Thickness measuring systems from Micro-Epsilon, which work with laser scanners, offer a precise economical solution for thickness, width and length measurements in the metal industry. They are also easy to install and do not require complex calibration. Source: Micro-Epsilon

In the past, thickness measurement of sheet metal was often carried out with radiometric methods using isotopic radiation or an X-ray source. Stability, material temperature, measuring range, offset distance and accuracy posed great challenges to alternative measurement methods at that time.

The laser technology further developed by Micro-Epsilon changes the status quo. In heavy plate applications, laser-based thickness measuring systems are superior to radiometric systems in several aspects. Micro-Epsilon's measuring systems use the company's own patented Blue Laser Technology. This laser technology delivers precise measurement results even on red-hot metals and enables measurements at temperatures up to 1,250° C (2,340° K). Accuracies of up to 20 μm are therefore achieved on slabs with a thickness of 400 mm. At these material thicknesses, systems with isotopic radiation reach their limits. Furthermore, with increasing material thicknesses, increased radiation exposure is inevitable.

The C-frame systems from Micro-Epsilon, which work with laser scanners, are equipped with protection and cooling systems for use in hot rolling mills. This enables continuous temperature monitoring and compensation, even at high object temperatures.

Figure 2. The patented Blue Laser Technology is used in the metal industry, especially for red-hot metals. The blue laser light uses a different wavelength range than red laser light, which means that the laser line is formed sharply on the measuring object and focused sharply on the sensor element accordingly. Source: Micro-Epsilon

Different types of lasers ensure optimal results

An essential technical detail of the laser scanners is their laser technology. The red laser is used for measurement tasks in the cold rolling area. The patented Blue Laser Technology is used in the metal processing industry, especially for red-hot metals. The blue laser light uses a different wavelength range than red laser light, which means that the laser line is formed sharply on the measuring object and focused specifically on the sensor element. Special filters prevent the sensor from being blinded by the radiation intensity of the glowing target. This results in much more stable and reproducible measurement signals.

Figure 3. An essential technical detail of the Micro-Epsilon laser scanners is the laser technology used. The red laser is used for measurement tasks in the cold rolling area. The patented Blue Laser Technology is used in the metal industry, especially for red-hot metals. Source: Micro Epsilon

Laser line for highly accurate results

Laser line technology offers several advantages. Gaps in the signal caused by steam or particles do not affect the measurement. Sheet tilting can also be detected and compensated for and, as a result, the thickness accuracy is identical for flat sheets and tilted sheets. The laser line can also detect sheet edges. The sheet width can therefore be measured using two C-frame systems.

Simple application, high time savings

Radiometric measuring systems are maintenance-intensive and require high calibration efforts. Laser-based systems are much easier to use. They perform the calibration automatically and quickly so that it can be done between two slabs. Alloy compensation is not necessary, as the thickness measurement can be performed independently of the material or surface. CGK tests can be easily performed via the software. Maintenance is simple, limited to intermittent cleaning of the protective windows, and is usually done via the air purge. The system issues a message in the event of heavy contamination. Since the lasers used are class 3R and can be easily switched off, maintenance work can be carried out quickly and safely.

Low-cost solution

Normally acquisition and operating costs for laser-based thickness measuring systems are significantly lower than for X-ray-based measuring systems, for example. A laser diode lasts for many years and the lasers can be turned off when not in active use to further extend their life.

C-frame systems from Micro-Epsilon

Micro-Epsilon has a wide range of thickness measuring systems for different requirements and applications. C-frames are designed for use in very difficult environments, including hot rolling mills. A three-layer stainless steel housing with active cooling protects against heat radiation. In addition, the temperature inside the C-frame is continuously measured at a number of points to monitor and compensate for the lingering effects of rapidly changing ambient temperatures.

The C-frame systems can measure material up to 2.8 m wide. Two C-frames can also be combined to double this width to 5.6 m. The C-frame can be positioned anywhere in the plate width direction and also move continuously to scan the complete width, so both transverse and longitudinal thickness profiles are possible. In traversing mode, the system also measures the sheet width. Compared to a radiometric gauge, the C-frame occupies much less space in plate transport direction. This makes integration into existing lines easier.

Where are laser scanners used?

Thickness, width and length measurements in the continuous casting plant

In the continuous caster, slab thickness, thickness profile, width and length can be measured by a C-frame. Since the slab moves slowly, the user can let the C-frame scan continuously to get a thickness map of the slab in two dimensions. The measured values can then be used for the continuous caster as well as for pre-setting of the down-stream hot rolling mill.

Laser scanners in the Quarto mill

A common application for thickness gauges occurs after the mill. Here they control the rolling mill, providing measured values for adjustments and ensuring quality. Since the mills are reversing, a single C-frame can measure centerline thickness in one direction, scan the plate for thickness profile and width at stand-still and then continue the scanning as the plate moves back into the mill. When the rolling is finished, a more detailed thickness measurement can be performed before the plate goes to hot leveling and the cooling bed. This information can be used for the subsequent cutting process. As a result, the sheets meet the required specifications, which increases the material yield.

Plate shearing measurements

The sheets passing through the shearing machine are cooled down and have a constant temperature. In the shearing plant, they are cut to the required format. Compared to a simple visual inspection, the thickness gauges allow a continuous and highly accurate inspection of specified tolerance values and measurement patterns. C-frames from Micro-Epsilon match different scan patterns and tolerances for each individual sheet. In this way, customer-specific test processes are possible. In the final process of sheet metal finishing, which acts as a final check, the thickness measuring systems can be used effectively and reliably to check the sheets for compliance with the correct dimensions and, at the same time, avoid any customer complaints.

Figure 4. The thickness CONTROL MTS 9202.LLT-400/xxx models are equipped with a special Blue Laser triangulation sensor, which provides two sequential measuring ranges. This perfectly covers the varying thicknesses of the rolled material, enabling increased measurement accuracy with thinner materials. In addition, this innovative approach ensures significantly increased process reliability due to a very large distance between the pass line and the upper belt. Source: Micro-Epsilon

Conclusion

Thickness measuring systems from Micro-Epsilon, which work with laser scanners, offer a precise economical solution for thickness, width and length measurements in the metal processing industry. They are easy to install and do not require complex calibration for different alloys. The measurement results are output at high speed, which means that these systems are designed for dynamic use in the production line. Thanks to high-quality protective measures, Micro-Epsilon laser scanners deliver precise results even in difficult environmental conditions where dust or dirt are present. The scanners, based on different technologies with red and blue laser diodes, some of which have been patented by Micro-Epsilon, deliver highly accurate results for both cold and red-hot metals. C-frame systems with laser scanners from the scanCONTROL series offer maximum precision, enormous flexibility and application diversity in automated industrial processes.

Visit the Micro-Epsilon website for more information.