Automatic Gauge Control for Rolling Mills - AGC
How Automatic Gauge Control Works
AGC is a closed-loop control system. It continuously compares the actual strip thickness with the target thickness and corrects the roll gap to eliminate the difference.
The control loop has four elements:
1. Measurement — A thickness gauge measures the actual strip thickness. This can be a contact gauge (VBM, VBK, VBF), a laser gauge (VTLG) or an X-ray/isotope gauge, depending on the process and the material. The gauge delivers a thickness signal — typically updated every 1–10 ms — to the AGC controller.
2. Comparison — The AGC controller compares the measured thickness to the target thickness (the setpoint). The difference is the thickness error.
3. Computation — The controller calculates the correction needed to eliminate the error. This calculation accounts for the mill modulus (the elastic deformation of the mill stand under load), the strip hardness, the rolling speed, and the transport delay between the gauge and the roll gap. Different AGC modes use different correction strategies (see below).
4. Actuation — The correction signal is sent to the hydraulic screwdown system (HAGC), which adjusts the roll gap. In some configurations, the correction is applied through speed changes or tension adjustments instead of, or in addition to, gap changes.
The loop runs continuously — hundreds of times per second on a modern HAGC system — so the mill is constantly adjusting to keep the strip on gauge.
AGC Modes and Control Strategies
No single control strategy handles every situation in a rolling mill. Different phases of the rolling process — threading, acceleration, steady-state rolling, deceleration, tail-out — present different challenges. A well-configured AGC system combines multiple modes, each optimised for a specific aspect of the process.
Feedback AGC (Exit Gauge Control)
The most fundamental mode. An exit thickness gauge measures the strip after it leaves the roll gap. Any deviation from the target is fed back to the controller, which adjusts the roll gap to correct it.
Feedback AGC is inherently stable and self-correcting, but it has a limitation: there is always a transport delay between the roll gap and the exit gauge. At high rolling speeds, this delay means the strip has moved a significant distance before the error is detected and corrected. The result is a band of off-gauge material at the head and after every disturbance.
Feedforward AGC (Entry Gauge Control)
An entry thickness gauge measures the strip before it enters the roll gap. The controller uses this measurement to predict what the exit thickness will be and pre-adjusts the roll gap before the error occurs. This eliminates the transport delay problem of feedback control.
Feedforward AGC requires an accurate model of the rolling process (the relationship between entry thickness, roll gap, rolling force and exit thickness). When the model is accurate, feedforward control dramatically reduces head-end off-gauge and improves response to incoming thickness variations.
Mass Flow AGC
Mass flow control uses the principle of conservation of mass: the volume of metal entering the roll gap per unit time must equal the volume leaving it. By measuring both thickness and speed at the entry and exit of the stand, the controller can detect thickness errors faster than a gauge alone — because speed changes are detectable before the thickness gauge registers the corresponding thickness change.
Mass flow AGC is particularly valuable on tandem mills, where it provides rapid inter-stand thickness control without requiring additional thickness gauges between every stand.
HAGC — Hydraulic Automatic Gauge Control
HAGC refers specifically to the hydraulic actuation system that executes the AGC corrections. Servo-hydraulic cylinders adjust the roll gap with response times in the millisecond range, enabling the control system to make fast, precise corrections.
The key performance parameter of an HAGC system is its frequency response — how fast and how accurately it can follow the correction signal from the AGC controller. A well-designed HAGC system tracks corrections at frequencies up to 20–30 Hz, which is fast enough to compensate for roll eccentricity, incoming thickness variations and thermal effects.
Roll Eccentricity Compensation
Rolls are never perfectly round. Even small deviations from circularity (eccentricity) produce periodic thickness variations in the strip at a frequency determined by the rolling speed and the roll diameter. AGC systems with roll eccentricity compensation identify this periodic signal and generate a counter-correction synchronised to the roll rotation, effectively cancelling the eccentricity effect.
Direct Roll Gap Control (DRGC)
A Vollmer-specific enhancement. Instead of inferring the roll gap from the HAGC cylinder position (which is affected by mill stand elasticity, bearing play and housing deformation), DRGC measures the gap directly at the work roll necks on both drive side and operator side. This provides the AGC with a fast, unfiltered gap signal that is minimally affected by the mechanical compliance of the mill stand.
DRGC is particularly valuable during transients — strip head and tail, speed changes, load changes — where the indirect gap signal lags behind reality. By providing the AGC with a direct measurement of what is actually happening at the roll gap, DRGC reduces head-end off-gauge, stabilises tail-out behaviour and reduces wedge.
Request for quotation
If you have any questions about our products or if you need any consultation about for any kind of measuring task or questions about our strip processing lines. Don´t hesitate to contact us!
FAQ
What is an Automatic Gauge Control (AGC) System?
An Automatic Gauge Control (AGC) system is a technology used in various industrial processes to automatically regulate the thickness (or gauge) of materials, such as metal sheets or strips. The primary objective of AGC is to maintain a consistent target thickness throughout production by minimizing variations caused by equipment wear, temperature fluctuations, material properties, and process instabilities.
By ensuring precise thickness control, AGC enhances product quality, reduces material waste, and improves production efficiency.
What is AGC in a Rolling Mill?
In a rolling mill, AGC (Automatic Gauge Control) is a critical system that ensures precise control of the thickness of rolled metal sheets or strips.
The system continuously measures the material’s thickness using sensors, processes the data with control algorithms, and adjusts the mill’s screwdown mechanism via actuators to maintain the desired gauge.
AGC in rolling mills improves product uniformity, reduces material waste, and stabilizes the production process, enabling manufacturers to meet stringent quality standards.
What is HAGC?
HAGC (Hydraulic Automatic Gauge Control) is an advanced, servo-hydraulic system used in rolling mills for high-speed, precise thickness control.
It enhances AGC by utilizing hydraulic actuators that react instantly to thickness variations, ensuring rapid and accurate adjustments.
HAGC is particularly effective in high-speed rolling mills where quick and precise screwdown control is required. This system enables micrometer-level precision, optimizing both metal processing efficiency and product quality.
What is "Gauge" in Quality Control?
In quality control, "gauge" refers to the thickness measurement of a material, which is a critical parameter in industries such as metal processing, plastics, and paper manufacturing.
Maintaining an accurate gauge is essential for ensuring product consistency, durability, and compliance with industry standards.
AGC systems play a crucial role in quality control by continuously monitoring and adjusting material thickness, minimizing deviations, and maintaining precision throughout the production process.