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Motion regulation in steel rolling mills

Steel rolling mills

Using two LSV Series 6000 Laser Surface Velocimeters to simultaneously measure the rotational and translational velocity of a rod at the exit of a planetary mill permits automatic control of rotational motion, thus minimizing (eliminating) tail swing exiting the mill.

Boschgotthardshütte (BGH) produces specialty steels and alloys to meet the toughest demands and effective process control is vital to efficient and profitable plant operation.

At BGH’s Edelstahl Freital facility, the existing control loop for the planetary mill introduced unwanted rotational motion into the end of the rod, causing the tail end to swing to the side upon exiting the mill. This created a significant problem when feeding the rod into the next process.

Laser surface velocimetry

Two Polytec LSV Series 6000 Laser Surface Velocimeters were selected to provide process feedback of the rod’s true rotational and translational motion.

Traditional tachometers on the drive rollers are prone to slippage and consequent inaccuracies. The picture on the left shows the two water-cooled LSV-026 sensor heads aligned to a moving hot steel rod.

In the outlet of the planetary mill, the rolled steel rods experience both a forward movement as well as a rotation, which causes undesirable effects in the next process. The second Laser Surface Velocimeter measures the rotational velocity of the rod. This measurement is fed back into the drive system of the planetary mill to eliminate rotation of the rod before the next process.

The ability to regulate the process to zero rotational velocity is dependent on sensing both positive and negative velocities. The heterodyne detection technique used in the Polytec LSV is uniquely sensitive to the direction of motion. Heterodyne detection begins with a reference frequency shift between the two intersecting laser beams on a stationary part generating a zero velocity interference beat frequency of 40 MHz (the so-called carrier frequency). Now, motion in one direction increases the frequency to greater than 40 MHz, while motion in the opposite direction decreases the frequency to less than 40 MHz. This makes it possible for the LSV to recognize and directly output both the direction and magnitude of motion. With this information a control loop can be built to regulate the rotational motion to standstill (v = 0).

Other non-contact measurement methods sense the magnitude of the velocity or the speed of the surface but do not resolve the direction of the motion. The closest LSV is aligned to measure the translational velocity and, thus, the length of the steel rod. The length measurement is used to achieve precision cutoff control.

laser scanning velocimeter software

The PC LSV Monitor Software enables acquisition, trending and recording of all relevant data for process development, system setup and diagnosis. The saved data can then be recalled and displayed for basic analysis with the LSV Software or exported to third party software for more detailed analysis and evaluation (left). After only a few hours of operation it was already apparent that the installed LSVs fulfilled all expectations. Pleased with the LSV success story, BGH has installed more LSV systems for cutoff control in their high-grade steel plant in Siegen.
LSV 6000 product page