VTLG
laser thickness gauge
Non-Contact Strip Thickness Measurement for Rolling Mills
The VTLG is a laser-optical thickness gauge built specifically for cold rolling mills. It measures strip thickness without contact, independent of alloy composition, with accuracy down to ±0.5 µm and a scanning rate of up to 80 kHz. No other laser thickness gauge on the market is designed to operate inside the mill stand, in dense rolling oil mist, at strip temperatures up to 180 °C.
With more than 100 VTLG systems installed in cold rolling mills worldwide — including documented installations in 20-high mills under extreme conditions — the system has proven that non-contact laser measurement belongs in the mill, not just on the finishing line.
How the VTLG Measures Strip Thickness
The measurement principle is laser triangulation with dual synchronised sensors. Two compact laser measuring heads are mounted on the upper and lower beams of a rigid C-frame. Each sensor emits a laser beam toward the strip surface and receives the reflected light. The sensor measures the distance from the strip surface to the sensor by detecting where the reflected beam hits a position-sensitive detector inside the measuring head.
Strip thickness is calculated from a simple geometric relationship: the known, fixed distance between the two sensors minus the two measured air gaps (sensor-to-surface distances) equals the strip thickness. Because the measurement is based on physical distance — not on radiation absorption or electromagnetic interaction — the result is absolute and entirely independent of the strip's alloy composition, density, or magnetic properties.
The VTLG scans at an internal rate of up to 80 kHz, delivering a new thickness reading every 12.5 microseconds. After filtering with a selectable averaging time (Ti from 1 to 1,000 ms), the measurement signal is output as a scalable analogue and digital signal to the line's AGC system. The response is fast enough for closed-loop thickness control on the most demanding high-speed cold mills.
Automatic Self-Calibration with DAkkS-Certified Gauge Blocks
Accuracy means nothing without traceability. The VTLG calibrates itself automatically before every strip pass, without operator intervention.
As the C-frame moves from its parking position into the measurement line, it passes a permanent reference station mounted beside the strip path. Inside this station, four gauge blocks are installed — certified according to DAkkS (the German national accreditation body) — covering the full thickness range of the specific mill. The VTLG measures each gauge block in sequence, compares the measured values to the known certified values, and corrects its calibration accordingly.
This self-calibration cycle runs every time the gauge enters the line. There is no manual calibration, no calibration plate insertion, and no production delay. The result is a measurement system that maintains its accuracy over weeks and months of continuous operation, even as environmental conditions change.
Built for the Rolling Mill Environment
Most laser measurement systems are designed for clean-room or laboratory conditions. The VTLG is designed for the inside of a cold rolling mill.
Rolling Oil Mist and Emulsion
Cold rolling produces dense aerosol clouds of rolling oil and coolant. On other laser systems, this mist contaminates the optical windows and degrades signal quality within hours. The VTLG solves this with an automatic air purging system that continuously flushes the entry and exit windows of both measuring heads and the entire beam path with clean, filtered air. The system operates reliably at oil film levels up to approximately 500 mg/m² per side on the strip surface.
Temperature
Strip temperatures in cold rolling can reach 180 °C, and the ambient temperature inside a mill stand can vary significantly. The VTLG incorporates built-in temperature compensation that corrects for thermal expansion of the C-frame and sensor housings, maintaining measurement accuracy despite temperature fluctuations.
Vibration
Strip vibration is a constant in rolling — the strip is under tension, traveling at speed, and subject to periodic disturbances from the roll gap. The VTLG's 80 kHz scanning rate captures thickness data fast enough to average out the effect of strip vibration, delivering a stable and representative thickness value to the AGC.
Compact Design
The C-frame is available with air gaps of 125 mm and 285 mm, narrow enough to fit between closely spaced mill stands and roll assemblies. This compact footprint is what makes in-stand installation possible — the gauge fits where no other laser system can be placed.
VTLG Model Range
Vollmer offers the VTLG in several configurations, each optimised for a specific thickness range, strip width and installation scenario.
VTLG 101/1
Designed specifically for cold rolling of foils between 0.003 and 2.0 mm. This is the only laser measuring system available that can be installed in the immediate vicinity of the roll gap during foil rolling. It operates with an absolute measuring accuracy of ±0.5 µm.
| Parameter | Specification |
|---|---|
| Thickness range | 0,003–2,0 mm |
| Accuracy | ±0.5 µm (absolute) |
| Resolution | 0,05 µm |
| C-ramens luftspalt | 125 mm |
| Scanning rate | Up to 80 kHz |
| Interfaces | PROFINET, PROFIBUS DP, TCP/IP, analog/digital I/O |
VTLG xx6/6 Series
The general-purpose VTLG for cold rolling mills processing strip up to 6 mm. Available in three measurement depths (400, 800, 1,200 mm) for different strip widths.
| Parameter | VTLG 406/6 | VTLG 806/6 | VTLG 1206/6 |
|---|---|---|---|
| Thickness range | Up to 6 mm | Up to 6 mm | Up to 6 mm |
| Measuring depth | 400 mm | 800 mm | 1 200 mm |
| Resolution | 0,1 µm | 0,1 µm | 0,1 µm |
| C-ramens luftspalt | 125 or 285 mm | 125 or 285 mm | 125 or 285 mm |
| Scanning rate | Up to 80 kHz | Up to 80 kHz | Up to 80 kHz |
VTLG xx6/13 Series
For strip up to 13 mm. Same measurement depth options as the xx6/6 series.
| Parameter | VTLG 406/13 | VTLG 806/13 | VTLG 1206/13 |
|---|---|---|---|
| Thickness range | Up to 13 mm | Up to 13 mm | Up to 13 mm |
| Measuring depth | 400 mm | 800 mm | 1 200 mm |
| Resolution | 0,1 µm | 0,1 µm | 0,1 µm |
VTLG xx12/20 Series
For strip up to 20 mm. Available with measurement depths up to 1,200 mm.
| Parameter | VTLG 412/20 | VTLG 812/20 | VTLG 1212/20 |
|---|---|---|---|
| Thickness range | Up to 20 mm | Up to 20 mm | Up to 20 mm |
| Measuring depth | 400 mm | 800 mm | 1 200 mm |
| Resolution | 0,1 µm | 0,1 µm | 0,1 µm |
VTLG 1420/20
The newest addition to the VTLG range. Designed for wide strip up to 20 mm thick, with a measurement throat depth of 1,480 mm for full cross-profile scanning. Measures with ±5 µm accuracy at a resolution of 0.1 µm. C-frame jaw width is 215 mm.
General Performance Specifications
| Parameter | Specification |
|---|---|
| Accuracy | Up to ±0.5 µm (model-dependent) |
| Resolution | 0.05–0.1 µm (model-dependent) |
| Scanning rate | Up to 80 kHz |
| Strip temperature | Up to 180 °C |
| Strip speed | 0–2 000 m/min |
| Oil film tolerance | Up to approx. 500 mg/m² per side |
| Laser spot size | Approx. 0.1 mm |
| Averaging time (Ti) | Selectable, 1–1,000 ms |
| C-frame protection rating | IP65 |
| Controller protection rating | IP55 |
| Laser safety class | Class 3B (no additional protection measures required) |
| Self-calibration | Automatic, with 4 DAkkS-certified gauge blocks |
| Interfaces | PROFINET, PROFIBUS DP, TCP/IP, analog/digital I/O |
| Optional software | VRecoS (data logging), VGraph (statistical analysis) |
| Positionering | Motorised horizontal/vertical (±1 mm accuracy) |
| Cross-profile | Traversing mode available on all models |
| Air supply requirement | Per DIN ISO 8573-1 |
Request for quotation
Want the right model recommendation or a technical review of your line? Contact us and we’ll propose a configuration based on your measuring range, strip width, and installation space.