Cold Rolling Mills AGC improvements in 80's

Gauge control through Mechanical Screw Downs :

Prior to late 70's AGC for a Cold Mills were with Mechanical AGC system with DC motors driving screws in each housing posts. In older mills screws were controlled earlier as feeler control. In such mills operator used to move the screws manually depending upon his judgement based on the deviation of actual measured thickness as per a contact gauge from set or desired thickness. He used to continuously raise or lower the screws based on observed changes in out put thickness and the thickness to be achieved. He would also manually change back tension reference for the strip.  

Since point of measurement and material cross section in the bite are different direct feed back control from thickness gauge will result in erroneous control. Hence in later mills gauge control was achieved   automatically using mill stretch compensated feedback.  X-Ray Gauges were used for output thickness to act as finer control, Roll force measurement using two washer type load cells (may be from Kelk or ABB) and position of screws were measured via selsyn in earlier systems and pulse tachos in more modern system were used. This system gave tolerances may be of 5- 10 microns on the strip due to slower control of DC motors of screw downs. More closer controls of thinner strips were obtained using tension controls of Reel drives which were incidentally faster than mechanical screw downs.

Hydraulic Cylinders used for Gauge Controls:

Dramatic change took place in late seventies when Hydraulic AGC Cylinders - earlier short stroke ones and later larger stroke cylinders  were employed for the purpose of AGC controls and improved the time constant from say 200 mS to 20 mS and paving a way for accuracies in the range of 2-5 micro metres. There were some changes in the instruments used for AGC also such as use of 4 nos. Position Transducers such as Kelk make Absolute Position Transducers or Sony Magnascales (Only 2 nos Position transducers placed centrally in the cylinders are and 2 nos pressure transducers directly mounted on the load side of hydraulic cylinders are preferred now ). Thus actual  Another development was use of mass flow principle

In Tandem cold mills last stand was usually used as tempering pass and Siemens defined A, B, C mode of controls for Tandem Mill stands.

BBC which later became ABB also introduced various modes of AGC before it was developed as it stands today. These changes were needed due to technological improvements and due to the fact Hydraulic AGC were getting introduced as modernization of old rolling mills. Space constraints in existing Mill housing windows compelled use of short stroke cylinders along with mechanical screw downs for coarse correction earlier. In India one of the first introduction of Hyd AGC was in Rourkela's existing  Hot Strip Mill by MECON and Wean United with BHEL / Siemens Ag being electrical partner the modernization was   completed in record 40 days shut down saving 5 working days for the plant production against projected schedule.

I shall be putting other improvements in these aspects in future blogs..