Non-contact linear speed measurement

Non-contact laser linear speed meters (Velocity Length Meter, VLM) are high-precision industrial instruments for the non-contact measurement of the linear speed and total length of moving products. They use laser Doppler or image cross-correlation principles to measure product speed non-invasively, with accuracies typically of the order of 0.05% of the reading. They replace measuring-wheel systems in applications where physical contact is not wanted because of wear, contamination, product temperature or the precision required.

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Laser Doppler principle

The classic Doppler principle (DGV — Doppler Global Velocimetry, or LDV — Laser Doppler Velocimetry) uses two coherent laser beams that intersect over the measuring area, forming an interference grid with fringes of known spacing. When a point on the moving product crosses the fringes, the scattered light modulates its intensity at a frequency equal to the product speed divided by the fringe spacing. A photoelectric sensor measures the modulation frequency and the electronics calculate the speed. The method is highly precise and independent of the characteristics of the product, of vibration in the installation and of the working distance (within the focal zone).

Cross-correlation principle

The cross-correlation method uses two optical sensors (cameras or pixel arrays) placed a known distance apart along the direction of travel. Both acquire the surface structure of the product in real time (the natural pattern of roughness, graininess, printing or marking). Cross-correlation analysis calculates the time shift between the two signals: τ = d/v, where d is the distance between the sensors and v the speed. The method is particularly reliable on products with a pronounced surface structure, and is robust against variations in the product (reflectivity, color, surface structure).

Technical characteristics

Industrial laser linear speed meters cover speed ranges from 0.1 m/min to 6000 m/min and beyond, with typical accuracy of 0.05% of the measured speed and 0.02% of the integrated length. Length resolution is typically 0.1 mm. The digital interfaces (Profinet, EtherCAT, Modbus, OPC UA) enable real-time integration with line systems. The typical working distance is 100-300 mm from the product, with a tolerance of ±15-30 mm on product position (important for the stability of an industrial setup). They are compact, with a rugged industrial design.

Advantages over measuring wheels

The main advantages over wheel-based systems are: no slip (the chronic problem of measuring wheels, which generates systematic errors in length); no contact with the product (eliminating marking, wear and contamination); applicability to hot products (rolling mill exit, hot extrusions); higher precision (0.05% for laser against a typical 0.5-1% for a wheel); reduced maintenance (no mechanical parts subject to wear); reliability over time (minimal calibration drift). The initial costs are higher, but the return on investment is positive for high-volume or high-precision applications.

Applications

The industrial applications are: rolling lines for steel, aluminum and copper (speed and length control for the cutting program and for batch certificates); extrusion lines for tubes and profiles (calculating lengths produced and cut-to-length); wire and cable production (length control in production and for sale); rotary printing lines (synchronization and speed control for print registration); the textile industry for webs and nonwovens; the paper industry (paper and board reels); and the production of panels and laminates.

Selection criteria

Selecting a laser VLM depends on the speed range, on the characteristics of the product (reflectivity, structure, temperature), on the width of the product (in order to size the measuring window), on integration with the line PLC and MES, on the IP protection rating for the industrial environment (typically IP65-IP67) and on maintenance requirements (cleaning of the optical windows). For lines with extreme speeds or precision, a feasibility study with preliminary trials on the real product is recommended.

Related products

The RODER instruments for this application. Go to the relevant catalogue pages for models, technical data and documentation.

Optical speed meters and encoders

VLM speed meters for non-contact measurement of speed and length.

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