Cable, wire, tube, optical fibre and extruded profiles are made continuously, often at high speed, and their outside diameter has to stay within tight limits. A contact gauge would mark the surface or wear out. A laser scanning diameter gauge measures the product as it passes through the gauge head without touching it, many hundreds of times a second, so drift is seen immediately and can be corrected automatically.
How Laser Scanning Works
A laser beam is collimated and swept across the measuring field by a rotating scanner, producing a parallel beam that moves at a known speed. The product in the field interrupts the beam for part of each sweep. A photodetector on the far side sees a gap in the light, and the duration of that gap, multiplied by the known scan speed, gives the diameter. Because the beam is parallel, the reading does not depend strongly on where the product sits within the field.
Why Two Axes
A single scan measures diameter in one direction only. A product that vibrates in the field, or one that is slightly oval, can give misleading readings. Two-axis gauges scan in two directions at right angles. Comparing the two readings reveals ovality, and averaging them reduces errors from product movement. Both of MEWOI's gauges use two crossed scanning beams for this reason.
Calibration and Good Practice
- Two-rod calibration: the gauge is set against two reference rods of known diameter near the ends of the working range. MEWOI's FTB-06 procedure uses this method.
- Warm-up: MEWOI recommends 5 minutes before normal use and 30 minutes before precision measurement.
- Clean windows: dust, oil or water on the windows of the gauge head is the most common cause of error.
- Stable environment: large temperature swings, vibration and supply voltage fluctuations all affect readings.
From Measurement to Control
On an extrusion line, the gauge output can drive the process directly. A deviation from the target diameter adjusts line speed or extruder output through a PID loop, and relay alarms flag out-of-tolerance product. Data can also be logged to a PLC or PC for quality records.
MEWOI offers two gauges in its laser diameter meter range. The FTB-06 is a two-dimensional scanning gauge for round products such as communication, coaxial, optical and power cables. The 20XYT two-axis gauge measures 0.1–20 mm with ±1 µm accuracy and 1 µm sensitivity at 600 scans per second, with optional PID regulation, RS485/232 communication, a remote display up to 200 m away and relay alarm outputs.
Frequently Asked Questions
Can it measure transparent products? Many laser gauges can measure transparent tube and fibre, because the edges still deflect the beam, but performance depends on the material. Test samples first.
Does line speed matter? The scan rate sets how much product passes between readings. Fast lines need fast scanning to catch short defects, which is why scan rate is a key specification.