Development of methods for monitoring of the moment of occurrence of self-oscillation of the cold rolling mill

In a cold rolling mill, self-oscillations occur when the rolling speed of a thin strip is manually increased. The thickness of the lining is usually no more than 2-2.5 mm. The oscillation spectrum is quite wide. The first harmonic is 120 Hz. During the control of vibration and technological parameters, a rigid relationship was revealed between the vibration of the elements of the mill stands with the vibration displacement of the hydraulic tensioned (GNU) of the working rolls and fluctuations in the tension level of the rolled strip. The regular control of the movement of wildebeest makes 200 measurements per second. In accordance with Kotelnikov's theorem, the frequency band of the spectral characteristic in this case is 100 Hz. Fluctuations in the tension level of the rolled strip allows you to control the frequency band in the range from 0 to 43 Hz, no more. This is clearly not enough to reliably control the moment of occurrence of self-oscillations. The existing control system for technological parameters has an internal resource for the frequency of polling when forming a database. Using software, it was possible to increase the frequency of the survey of vibration displacements of wildebeest to 1000 measurements per second. In this case, the frequency band of the spectral characteristic covers the range from 0 to 500 Hz. This allows you to control not only the first harmonic of 120 Hz, but also the higher harmonics up to 400-500 Hz. Fluctuations in the tension level of the rolled strip after switching to a faster controller resource made it possible to increase the polling frequency to 250 measurements per second. Thus, it is also possible to reliably control the first harmonic of the resulting self-oscillations through this channel. An analysis of the process of self-oscillation has shown that the cause of self-oscillation of the mill stands is a spontaneous fluctuation in the tension level of the rolled strip with a manual increase in the linear rolling speed. The vibrations of the mill stands are delayed relative to the fluctuations in the tension level from 2 to 3 seconds. Using discrete Fourier transform and short-range Fourier transform (STFT) algorithms, it is possible to reliably and timely recognize the moment of occurrence of self-oscillations.