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Let an experienced engineer guide you through the Siemens S7-400 rolling mill control system upgrade 🛠️
“Hey, Lao Wang, I heard your factory recently upgraded to S7-400 for rolling mill control? How’s it working?”
“Don’t mention it, Lao Li, the equipment is in place, but the debugging time was tight, and our team worked non-stop for a week! Now it’s finally running stably.”
“Our factory is also planning an upgrade; do you have any experiences to share? How about I treat you to hot pot!”
As a veteran in industrial automation with 15 years of experience, today I will share the blood and tears story of upgrading our metallurgical rolling mill control system from S5-115U to S7-400 with all of you!
👨🔧 Project Background and Pain Points
We are a medium-sized steel enterprise, originally using the Siemens S5-115U control system from the 1990s. Spare parts were hard to find, and failures were frequent, especially during the hot summer months when CPU faults would occur, leading to significant production losses! After extensive investigation, we ultimately decided to adopt the S7-400 as the control platform for the new system, primarily targeting a comprehensive technical upgrade of the roughing mill in the hot rolling production line.
To be honest, this project was under immense pressure! We were given only 72 hours of downtime to complete all hardware replacements, software debugging, and trial production. During that time, we had takeout for every meal, and it was a non-stop grind; I doubt the younger generation could handle it! (I’ll let you in on a secret, my back couldn’t take it the next day, haha)
🖥️ Hardware Configuration Details
Based on the site conditions and budget, we ultimately selected the following:
CPU Module: CPU 416-3 DP (Friendly reminder: for those with a sufficient budget, consider the 417-5H redundant configuration)
Power Supply Module: PS 407 10A (Make sure to leave some margin; we’ve learned this the hard way)
Communication Processor: CP443-1 for Ethernet communication, CP443-5 for Profibus-DP expansion
I/O Configuration: Approximately 300 points DI, 200 points DO, 80 points AI, 60 points AO
These configurations were derived from hard-earned lessons! A special reminder to everyone: always consider power supply margin and expandability! In our first design, we only used PS 407 5A, and as the system expanded, it frequently rebooted for no reason; it took us a long time to discover that it was due to insufficient power supply.
🔄 System Design Considerations
As a rolling mill control system, we adopted a typical three-layer control architecture:
L1 Layer: S7-400 PLC responsible for basic automation control
L2 Layer: Industrial computer + database server responsible for production management
L3 Layer: Enterprise MES system responsible for order management
Remember, the key control strategies are: rolling force PID control and rolling mill synchronization control. These two aspects directly affect product quality! I insisted on writing an adaptive PID algorithm on-site that automatically adjusts parameters for different materials, and the results were outstanding!
We used SCL programming to write the PID algorithm, which is much more flexible than the traditional FB41:
// This is just the algorithm concept; the actual code is much more complex
PIDOut := Kp * (Error + 1/Ti * ErrorSum + Td * (Error - LastError));💡 Implementation Pitfalls
To be honest, the program structure directly affects the difficulty of future maintenance. I divided the entire system into several major parts:
Main Control Program (OB1 cyclic scan)
Communication Processing (OB35 timed interrupt, 20ms cycle)
Rolling Mill Control Algorithm (OB32 timed interrupt, 10ms cycle)
Fault Diagnosis (OB82-OB86 error interrupts)
The most challenging part is the rolling mill synchronization control, which must ensure precision. I manually wrote a position compensation algorithm using STL instructions; although it was tedious, the precision was excellent!
All bosses, don’t forget to back up the data blocks! We once lost process parameters due to a power outage, which caused us to spend three days recovering. That lesson was too painful!
🔍 Debugging Insights
The debugging phase is the most exhausting! We adopted a “five-step” strategy:
Test I/O points one by one (tedious but necessary)
Debug single devices (drive parts first)
Conduct online no-load tests (focus on checking safety interlocks)
Perform low-speed load tests (low pressure, manageable)
Full-load production trial (everyone on high alert, ready to respond)
The biggest pitfall during this period was communication anomalies. Due to strong interference on-site, the Profibus network frequently dropped at first, and we later discovered it was a grounding issue. Remember: grounding issues in industrial environments are always the top priority!
🎯 Project Benefits and Insights
After the upgrade, the benefits were significant: equipment failure rate reduced by 85%, product qualification rate increased by 6.2%, and annual maintenance cost savings of about 1.2 million. But money isn’t the most important; the best part is not being woken up in the middle of the night to handle faults anymore!
That’s all for today; I’ll leave you with a few questions to ponder:
In rolling mill control, why is it recommended to use timed interrupts instead of free-running methods?
How to handle communication timeout issues between PLCs and inverters?
I hope this experience sharing can help all of you comrades! Remember, automation upgrades are not achieved overnight; the process may be painful, but the results will definitely be beautiful. Time to clock out; my wife is urging me to come home for dinner, so I have to run! If you have questions, feel free to message me, and I’ll answer them while we eat together 😄
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