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| "Mr
Digital " ahead in photofinish |
A loop is formed in the Finishing Mill stands of Merchant Mill to relieve the tension in
the bar being rolled and thereby achieve the desired dimensional tolerance. The loop is
formed by using the looper actuation system, which uses photocells to track the material
being rolled and give the command to operate the looper roll.
While using this system, the operation crew regularly faced
the following difficulties:
- Cobbles at the Finishing Mill because of flickering of the
photocells.
- Interruption in the viewing capability of the photocells
owing to the heavy smoke.
- Incorrect alignment of the photocells.
- Cobbles at the cooling bed arising out of the failure of the
photocells.
To get over these problems, the Team at Merchant Mill came up with the following:
- Change of the bar tracking method by a stand load tracking
system.
- The analog output (0-10 V) corresponding to full load
current that is generated in the digital drive of the Mill stand was fed to the cooling
bed PLC analog input card.
- A logic was provided so that whenever the stand load
increased beyond the No-load current value, a digital output is generated by the PLC.
- This digital output was used to actuate the pneumatic
cylinder of the looper roll in a manner similar to that in the photocell system.
These modifications resulted in the following benefits :
- Complete elimination of photocells and power packs.
- A completely maintenance-free system.
- Substantial reduction in looper delays.
- Elimination of cobbles arising out of photocell failure.
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