WCA September 2015

• 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

❍ ❍ Figure 1

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:

Polished

Diffusing

❍ ❍ Figure 3 : Roughness effect

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.

E = A*ie* r*α*cosβ

Sensor optical system

❍ ❍ Figure 4 : Shape changing effect. Modelling image

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.

Light source rays, incident energy 7 ie 8

Wire, radius 7 r 8

❍ ❍ Figure 2 : Lighting

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

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Wire & Cable ASIA – September/October 2015