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Optimising Cold Rolling Mills in Europe

Author : Christian Lequeux - C L Consulting SPRL

12 May 2010

Many outdated cold steel rolling mills exist in Europe, some of which were built more than 40 years ago and are no longer competitive because they are technologically lagging behind. Replacing these manufacturing lines is not an option, since it would costs billions of Euros. Therefore, the old production facilities need to be optimised and updated.

In a traditional, continuous, cold rolling mill throughput can often be increased by up to 20%. The existing mechanical parts of the plant are often in good shape, as these are usually over-dimensioned, so only the motors, control logic and motion control systems need to be replaced. This is a classical example of refurbishing. However the biggest challenge is at the start of the production line, where the end of a steel coil needs to be welded to the start of a new coil.

In order to permit continuous processing of the coils, the coils must be welded together. A metal sheet stock buffer installation on the processing lines gives us precisely enough time to weld each incoming coil to the preceding one, resulting in a single, endless steel strip. This takes place, for example, on the pickling line and in the cold rolling mill.

Welding strips using wrong parameters or insufficient control will result in bad quality and the risk that the metal strip will break at the weld during pickling and or rolling. When the strip breaks, it will spring back around the roll and coil around it. The strip then needs to be pulled back and recoiled as well as being fed through the machine. All in all this can lead to a downtime for up to 72 hours. Therefore a better control over welding quality is very important, and still be performed quickly and reliable.

Resistance Welding
The welding process is often done using resistance welding which involves the generation of heat by passing current through the resistance caused by the contact between the metal surfaces. Small pools of molten metal are formed at the weld area as high current is passed through the metal. Two types of resistance welding are applied often applied in rolling mills: seam welding and upset welding. In both cases two electrodes apply a high pressure and current to join the metal sheets.

The welding quality depend on several parameters, such as position of sheets and electrodes, pressure and the rate the pressure is applied as well as the current. These parameters should match and be synchronised all together within a very short time (typically within a single 50 / 60 Hz period). The pressure is controlled by a hydraulic system, capable of delivering 30 tons force, within 0,1 seconds to two cylinders. Since controlling the hydraulic system is a non-linear process - the stiffness of the system depends on the position of the cylinder – we needed a special control algorithm.

Increase Production Throughput by More Than Twenty Percent
Because there aren’t accurate enough hydraulic control systems we developed our own controller. In our controllers we account for non-linearity of the hydraulics, the thickness of the base material and the type of steel to be welded. In the past we used VME based systems, but 4 years we started to use LabVIEW, saving us a lot of time, trying to debug assembly and C code. The algorithm we deployed to CompactRIO, which offers the ruggedness, temperature range and industrial features necessary for this harsh environment.

The FPGA in the CompactRIO is used to implement the acquisition, I/O, data processing, and fast control, and the real-time processor, with solid state disk, is used for supervision and communication with other systems. All process parameters, including pressures are logged, and when out of tolerance the strip is slowed down, or even stopped, because the risk of breakage is too high.

The current, several thousand amperes, for the resistance weld should be controlled carefully as well. Thyristors are often used to control the flow of AC current. These thyristors can only cut off the current flow during the zero-crossing of the AC current. The required welding time is obtained by activating the thyristors at the right moment in time, within 50 µs accuracy. In the past this has always been a challenge, as well as turning the thyristors off, since it has to be synchronised with the power grid, where the large current draw also induces a disturbance.

Controlling the thyristor has become much better using the CompactRIO system. With the FPGA we can determine the correct moment to activate the thyristor within 10ns. This control loop was quite difficult, because the power grid contains many high frequency harmonics and disturbances. There is much more involved besides measuring the AC cycle period and then determining the starting time to open the thyristor. In our system we acquire samples at a very high rate and feed the signals through a digital filter implemented on the FPGA to remove the harmonics and determine the real zero crossing.

Conclusion
With this new control system we have refurbished several pickling and cold rolling mills. In old factories strip breakage occurs around 2%, and in modernised European factories around 0.5%. With the new CompactRIO based control system we have achieved 0.05%, a significant reduction. With the new system, we now have an improvement of 10 to 40 times and a huge economic saving by having less downtime and higher production throughput.

Christian Lequeux
C L Consulting SPRL

cl.consulting@skynet.be



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