Online Dimensional Measurement: Diameter Gauge, Confocal Sensor, or Vision

Measuring a part while it is moving down a line is a different problem from measuring it on a bench. There is one pass, the part does not stop, and the environment is not controlled. Three sensing principles cover almost all of it, and they are chosen by what dimension is being measured rather than by accuracy ranking: transmitted-light gauging for outer diameter, confocal displacement sensing for height and thickness, and vision for 2D geometry.

Principle 1: Transmitted-light diameter gauging

A collimated beam passes the part and the shadow is projected onto a receiver array. The part's outer diameter is read from the shadow width. Because the measurement is a projection rather than an image, it is fast, insensitive to surface finish, and independent of the part's colour or reflectivity.

POMEAS LM-1030 is specified for 0.3–30 mm non-contact measurement at 2,400 samples per second, using a GaN green LED source, telecentric optics, and HL-CCD continuous exposure.

The telecentric optical path is what makes the reading stable: as with telecentric imaging, the magnification does not change when the part shifts within the beam, so a part vibrating on the line does not change its measured diameter.

Use it for: wire, cable, tube, rod, and shaft outer diameter measured continuously. Not for: internal dimensions, height, flatness, or anything measured along the axis rather than across it.

Principle 2: Confocal displacement sensing

A chromatic confocal sensor focuses different wavelengths at different distances; the wavelength returning in focus encodes the distance. The output is a distance to the surface, which becomes thickness, height, step, or position depending on how the sensor is referenced.

POMEAS SFS series spans a range of measuring ranges and resolutions:

Controllers are specified by sampling rate and channel count: SFS-C8004L (10 kHz, single channel), SFS-C8007 (20 kHz, multi-channel), SFS-C8008L (32 kHz, single channel), SFS-C8008-2L (32 kHz, dual channel), SFS-C8008-4 (32 kHz, four channel).

The range-versus-resolution trade is visible in the table: ±0.05 mm at 3 nm at one end, 150 mm reference distance at 1,550 nm at the other. Selecting on range first and checking whether the resolution still meets the tolerance is the practical order.

Use it for: thickness, height, step, position, displacement — any single-point distance measured on a moving surface. Not for: diameter across a part, or 2D geometry.

Principle 3: Vision

Vision measures 2D geometry: distances between features, diameters of holes, angles,轮廓 (contour), and presence. Its advantage is measuring many features at once; its constraint is that it needs the part presented to the camera, illumination controlled, and the feature edges defined.

For batch 2D work offline, the one-press image measuring machine and the conventional VMM are the two established paths. Inline, vision handles feature geometry where a gauge cannot reach — hole positions, lead spacing, contour deviation.

Use it for: multiple 2D features in one acquisition, contour and position. Not for: a single diameter at line speed when a gauge can do it, or a height dimension without an additional sensor.

The decision table

Three selection checks that get missed

Sampling rate against line speed. A sensor's sampling rate sets how many readings are taken per unit of travel. At 2,400 samples per second (LM-1030) and a line speed of 2 m/s, the part is sampled every 0.83 mm of travel. If the defect or variation being caught is shorter than that spacing, the sampling rate is the limiting factor, not the resolution.

Resolution against tolerance, not against the other sensor. A 3 nm resolution figure is meaningless if the tolerance is 50 µm and the real error sources are part vibration, temperature, and datum drift. Select the sensor whose resolution is comfortably finer than the tolerance, then spend the remaining effort on fixturing.

Standoff against the mechanics. Confocal sensors have a specified reference distance — 12 mm, 100 mm, 150 mm in the SFS series. The mounting has to hold the surface within the measuring range around that distance, which on a vibrating line usually means a mechanical reference, not a fixed bracket.

What online measurement does not give you

An inline gauge measures one dimension continuously and reports a trend. It does not replace a first-article inspection, and it does not tell you which feature is out of tolerance on a complex part. Inline and offline measurement are complementary: the gauge catches drift on the line, the VMM or image measuring machine characterises the part.

Measurement accuracy in any of the three principles is not equal to the sensor's resolution figure. It additionally depends on fixturing, thermal drift, surface condition, and — for vision — illumination stability and edge definition.

FAQ

What is the difference between a diameter gauge and a confocal sensor?

The gauge measures across the part (outer diameter) by projection. The confocal sensor measures distance to a surface, which becomes height, thickness or position. They measure different things.

Why is a telecentric path used in a diameter gauge?

So the measured diameter does not change when the part moves within the beam. Magnification stays constant, the same reason telecentric imaging lenses are used for parts with height variation.

How do I choose a confocal range?

Select the range that covers the expected variation with margin, then check the resolution at that range. Range and resolution trade against each other across the SFS series.

Does a higher sampling rate always help?

It helps up to the point where spacing between samples is shorter than the variation being detected. Beyond that, fixturing and drift dominate.

Can vision replace an inline gauge?

Not for a single diameter at line speed. Vision is the better choice when several 2D features must be measured in one acquisition.

When is white-light interferometry the answer?

When the output is roughness or step height at nanometre scale — a measurement none of the three principles above provides.

Next Step

Contact POMEAS with your requirements.