“The project started back in 2004,” said Bentsman. “At that time Brian invited me for a visit to Nucor Steel Decatur in Alabama. A four-beam caster mold oscillator assembly at the plant experienced violent shaking at 3.4 Hz, precluding operation at higher frequencies. After much time spent on the problem, I came to the conclusion that the cause was the sinusoids that excite the beam resonance and are generated by a nonlinearity at the beam assembly input.”
Later, in 2009, Bentsman’s PhD student Vivek Natarajan found through plant measurements that this was the actuator nonlinearity and with the help of a MechSE Senior Capstone Design team, they put together an oscillator model. They proposed an elegant control solution, implemented it in collaboration with plant personnel in 2012, and developed full PDE-based control theory for it in 2016. Natarajan is now an Assistant Professor of Systems and Control Engineering at Indian Institute of Technology Bombay.
More recently, PhD students Oyuna Angatkina and Zhelin Chen helped develop a technique to tune the model to any mold oscillator; this development was completed in 2018. “There are still major steel casters in the U.S. and around the world that have this problem,” said Bentsman, “and we have an industry-proven solution for them.”
Rembold summarized the value for the company of the completed mold oscillator control project. “The success of the oscillator project – providing resonance-free frequency increase from 3.5 Hz to over 5 Hz at a smaller oscillation magnitude (shorter stroke) – was essential to Nucor’s current status as one of the country’s leading steel manufacturers. Since the implementation of a shorter stroke, we have decreased transverse cracking by 50 percent across all products. Another advantage has been the ability to add cooling water for quicker solidification. This has allowed us to increase the casting speeds on crack-sensitive and high-strength/low-alloy (HSLA) grades, and consistently run at 6 cm per second or higher, a level previously unattainable. The speed increase has also launched an equipment upgrade cycle, which is paving the way for running even faster in the near future.”