EuroWire July 2015

Technical article

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. Figure 6 , caught by a CCD matrix at the place of the wire, shows the size of the (white) light line perpendicular to the wire axis. The Gaussian shape of the energy density in the light line makes the efficient width at about 20μm. Then the spot size along the wire axis (Line Resolution: LR) is about constant, but on the circumference (Circumference Resolution: CR), it fluctuates proportionally to the wire diameter (r*α.). The line resolution on the wire depends only on the light source system, not on the sensor. To succeed in this development, one key point was the light sources. They must be small and fast, but generate very homogeneous light beams with uniform characteristics. They have been specially developed for this application. Another key point was the sensor technologies. For the smallest range, it was necessary to use a highly sensitive sensor but one that was also very fast. The movement of the wire with the rotation of the light source generates an elliptic scan of the surface and a continuous image on the sensor. Image computing The sensor must be able to characterise the size and the shape of the defect according to the requirements of the user. The SQM computes in real time the peri- meter (P) and the surface (S) of the defect. The ratio R = k*S/P 2 gives information on the shape of the defect. k = 4π. In this case, R = 1 for a circular defect. It tends toward zero when the defect elongates. Then R and S are two key detection parameters. In order to have homogeneous resolution, the line speed is measured by the SQM (pulse counting) and the diameter is a user parameter. Then the scanning frequency is adjusted automatically. Test results At the time of writing, the authors were just at the start of application and conducting industrial tests.

E = A*ie* r*α*cosβ

Sensor optical system

Polished

Diffusing

▲ ▲ Figure 3 : Roughness effect

Light source rays, incident energy 7 ie 8

Wire, radius 7 r 8

▲ ▲ Figure 2 : Lighting

▲ ▲ Figure 4 : S hape changing effect. Modelling image

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 source 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.

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

Photo-sensor A Ring of elementary light sources Measurement area

Zone 1

Zone 2

Zone 3

Photo-sensor C

Photo-sensor B

Source/Photo-sensor locations Zone 1/sensors A+C Zone 2/sensors B+A Zone 3/sensors C+B

▲ ▲ Figure 5 : Front view of the system

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.

▼ ▼ Figure 6

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July 2015