Among other supportive measures active chatter damping has turned out to be an effective approach to counteract these problems by significantly reducing vibration issues of cold mills. Together with a partner, SMS group currently develops an active chatter damping system using piezo-electric actuators. A first installation of the new technology is scheduled for end of 2016.
In cold rolling mills vibrations of mechanical parts and/or strip can interfere with several quality parameters like thickness, shape, surface quality and others. Since vibrations occur at higher speeds in most of the cases, rolling speed of single- or multi-stand mills for steel and aluminium has to be limited quite often.
3rd octave chatter as a self-excited vibration form within the frequency range 100 Hz to 200 Hz causes the strips to break and impairs thickness quality. On the contrary, the 5th octave chatter produces shape issues and related surface defects (stripes) when resonant modes of the roll stack within the frequency range above 500 Hz are excited by rotating parts like gear wheels or roller bearings. Well known measures against chatter issues and their consequences are the following ones:
- online chatter monitoring with Auto-Slow-Down (ASD) functionality,
- mill design optimization,
- optimized maintenance and service practice,
- passive vibration damping.
As a new, promising approach active vibration damping is currently under development at SMS group. After comprehensive theoretical investigations, SMS group has decided to use piezo-electric elements as actuators which are generally capable of operating in a frequency range of up to 1,000 Hz. An analysis of design situations and efficiencies of piezo-actuators at different locations within the mill stand finally led to an optimum solution: a piezo-actuator box is placed underneath the bottom backup roll chocks (see figures).
Special control technology
Apart from two piezo-actuator boxes per mill stand, the active damping system comprises a special control technology which uses the signals of conventional accelerometers mounted on top of the mill stands. Using the accelerometer signals and adaptive filter technologies, the control system calculates actuator signals capable to induce additional damping into the mill stand. The additional damping reduces mill sensitivity against 3rd octave chatter vibrations and can be allocated to other vibrations in parallel in order to reduce the corresponding amplitudes as well.
For the realization of the active chatter damping system, SMS group is cooperating with a Germany based partner who contributes comprehensive, long-term experience regarding theoretical background, stability and industrial practice of the control technology mentioned above. The long-term experience of this partner and positive results of a pilot installation using active chatter damping at a cold rolling mill provided valuable input for development and implementation of the new concept.
A first installation using the new active damping technology is scheduled for end of 2016. The new technology will be implemented in the last mill stand of an operating aluminium tandem cold rolling mill built by SMS group.
After successful commissioning of the first pilot installation, the system can not only be used for new cold rolling mills but can as well be integrated into many existing mills currently facing speed limitations caused by 3rd octave chatter or other vibration issues.
