In most rolling mills there are advanced AGC systems plus laser and X-ray thickness gauges. Yet you still see recurring issues with thickness peaks, wedge and unstable transient behavior. One important reason is that the control loop often lacks the most direct signal of all: the true roll gap.
With Direct Roll Gap Control from Vollmer, the gap is measured directly at the work roll necks. This gives a very short signal path, a cleaner measurement signal and therefore faster and more stable AGC.
Traditional thickness measurement only sees the result
Classical solutions for thickness control are based on measuring the outgoing thickness with, for example, X-ray or laser, supplemented with cylinder position and force signals.
The problem is that all of this is located downstream from where the thickness is actually created.
A lot happens between the roll gap and the measuring point:
- The mill stand deforms elastically under load
- Bearings and chocks have play and springiness
- The strip partially evens out variations
- Filtering in the measurement chain adds extra delay
In practice AGC often works with a delayed and partly filtered picture of what is happening in the gap, especially during fast events such as strip head and tail or rapid speed changes.
Direct measurement at the roll necks shortens the signal path
Direct Roll Gap Control moves the measuring point to the bite zone itself. Sensors are mounted at the work roll necks on both drive side (DS) and operator side (OS). There they measure the actual opening between the rolls locally.
This provides several advantages:
- The measurement is located inside the force flow of the stand
- Elasticity in housing, bearings and chocks affects the signal only minimally
- The mechanical signal path between measuring point and HAGC becomes very short
The result is that the control system sees changes in the gap immediately, before they have fully manifested themselves in the outgoing thickness.
How deformation and bearing play filter out information
If you only look at cylinder position or force as a proxy for the gap, you automatically get a mechanically filtered signal. Part of the movement is always taken up by:
- Stretching in piston rod, posts and housing
- Play and springiness in bearings and chocks
- Deformation in rolls and chocks
The result is that fast variations are damped before they reach the measuring point. The same thing happens when you rely solely on outgoing thickness. The strip has its own dynamics, and the gauge is located some distance away from the bite. Gap measurement directly at the roll necks captures the changes where they occur. You do not lose detail along the way.
Valsspaltsmätning direkt mellan arbetsvalsarna plockar upp förändringarna där de uppstår. Du förlorar inte signalvariationer på vägen.
A clean gap signal gives faster and more stable AGC
From a control perspective the gap signal does two things:
- Reduces dead time
- Increases information content
Less dead time means you can increase the gain in AGC without creating instability. That gives:
- Faster reaction to load and speed steps
- Less overshoot
- Shorter time to stable thickness after disturbances
The fact that the signal is local and unfiltered also means that AGC has already done a large part of the job when slower references like X-ray and laser report the outcome. They can then be used for supervision and slow trim, while the gap signal handles the fast part.
Typical problems in plate and strip mills that are solved by gap measurement
The impact of Direct Roll Gap Control is most visible where forces are high and dynamics are demanding.
Thickness peaks at head and tail
During biting and unbiting, load steps often create peaks and valleys in thickness. With direct gap measurement AGC sees the gap change immediately and can compensate before the peaks are fully built up. The result:
- More even thickness over strip head and tail
- Less need to scrap the first and last meters
Wedge and lateral spread in strip mills
With two sensors, one on DS and one on OS, the system can follow the gap side by side. This makes it possible to counteract wedge and reduce lateral spread by balancing the gap between the sides. The result:
- Better strip guiding on wedge shaped material
- Improved tracking and edge quality
Process security in the first stand and for heavy plate
In the first stand and for heavy gauges every small delay is costly. With a fast and local gap signal you get:
- More stable first passes
- Fewer re-rolling operations
- Lower scrap rate and better yield from each coil and plate
Conclusion
X-ray and laser are crucial for quality control and slow trim of the process. But for fast and robust thickness control the most central signal is often missing: the true roll gap.
By measuring directly at the work roll necks, Direct Roll Gap Control gives AGC the shortest and cleanest path into the process. The result is faster reactions, more stable control and fewer quality problems in both plate and strip mills.