WCA September 2015

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Wire & Cable ASIA – September/October 2015

The energy gleamed to the observer (sensor) is strongly

modulated by the surface quality; roughness, colour

(absorption) and flaw, but also the local shape of the cylinder.

Then, rotating the light around the wire axis will also rotate the

gleam on the surface referring to a fixed observer.

This generates a circumference image. When the wire

moves, it develops a complete surface image of the wire.

If the design is well made, any small surface defect,

colour or shape change will produce locally a significant

reduction of the gleamed energy to the sensor.

Figure 2

shows the key parameters of the principle:

On the section plan of the wire, the incident light rays are

almost parallels. Perpendicularly to the wire axis, each

source beam is focused in a narrow line.

2*α, comes from the angular aperture of the optical

system. It determines the spot size on the circumference of

the wire: r*α.

2*β, comes from the angular incidence of the light source.

If “A” is the surface absorption/diffusion factor of the wire,

the light energy “E” received by the sensor is:

E = A*ie* r*α*cosβ

The consequences of these relations are:

• Spot size (r*α) proportional to the wire diameter, which

is quite satisfactory, and of the angular aperture of the

optical system

• The energy received by the sensor fluctuates with the

angular incidence of the light source by cosβ. Using

three sensors, “β” fluctuates within ±60° per sensor,

generating a signal amplitude modulation by 50 per

cent. This is compensated by a correction factor in

order to display a flat response. With five sensors, the

direct fluctuation falls to 20 per cent

• The energy received by the sensor is also directly

proportional to the wire diameter. This means that

the incident light source energy “ei” must be adapted

accordingly, but also the sensor technology depending

of the range of diameters to check. The smallest

diameter able to have been checked properly was from

a tungsten wire (black colour) of 10μm

• The A factor has a significant impact either by diffusing

the energy (roughness) or absorbing the light ray at

850nm

Another important effect is the shape change along the

wire axis (lump, neck, flaw) that deflects the reflected rays

out of the angular aperture of the sensor.

Design

To rotate the lighting point a ring of light sources was

made around the wire axis, with only one source light on at

once.

Switching the lighting from source to source generates a

rotating light point around the wire. Three sensors at 120°

simultaneously check the gleamed energy on the surface

of the wire.

The light source system concentrates on each source

beam in a narrow line perpendicular to the wire axis. The

beam is about parallel to the other plan.

The thickness of the line determines the resolution

on the wire axis. Then the sizes of the sources must

be small and the optical system good enough for the

application.

Polished

Diffusing

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Figure 3

Roughness effect

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Figure 4

Shape changing effect. Modelling image

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Figure 2

Lighting

E = A*ie* r*α*cosβ

Sensor optical system

Wire, radius

Light source rays,

incident energy

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Figure 1