Introduction

The French company CERSA-MCI designs and produces high-performance diameter measurement instruments and accessories based on interferometry, diffraction, and shadow. The firm specialises in optical metrology, including laser diffraction sensors, laser projection, and stroboscopic instrumentation, and in laser interferometers for military, aeronautic, and nuclear applications and the fine wire and optical fibre industry. CERSA-MCI products, which are employed on production lines as well as in laboratories, offer sensitivity and repeatability for reliable and accurate quality control. Their use enables the operator to certify the entire production within close specifications.

There are CERSA-MCI laser interferometric measuring instruments for optical fibre qualification and certification of drawing towers, and compassing telecommunication fibres, special fibres, GGP, and single coating layer. The company’s range of instruments for fine wires and cables comprises laser diffraction or projection instruments, for either laboratory or plant use, for diameter and ovality measurement and detection of fault (olives, lumps, necks, etc.) in ultra-fine wires and cables.

This article reviews the scanners for large and fine wire diameters currently available on the market, and describes the CERSA-MCI line of recently developed diameter measurement instruments and accessories.

Large wire diameter measurement

Basic scanner principle
Figure 1 shows the main principle of a basic scanner. On the left side, a vibrating mirror reflects the laser beam toward a collimating lens. Then the laser beam sweeps the measurement window vertically within a calibrated aperture of height (W). A second lens focalises the sweeping beam on a mono photocell. The total signal is time-proportional to the aperture size (W). The “shadow time” (t) of the wire is proportional to the diameter of the wire (D) if the wire does not move vertically.

Figure 1: Scheme of a basic scanner for large wire diameter measurement

If the wire does move vertically, the vibrating speed (v) of the wire is subtracted from the laser beam speed (V). An error occurs in the shadow time (t), in the relation dD/D = v/(V-v), and then in the measured diameter. This equation shows that the error is reduced if the laser sweeping speed increases.

Advantages and limitations
This method is satisfactory and accurate for non-high frequency vibrating wires or cables. But even on static wires the reliability of the measures obtained is limited. An average must be taken to compensate the oscillating wire speed and to improve stability. Even granted the capability of measuring large diameters, a first limitation occurs when the diameter of the wire becomes smaller than the diameter of the laser beam (no shadow), below 100µm. A second limitation becomes apparent with high-frequency vibrating wires, requiring high averaging. A vibrating mirror or a rotating faced wheel constitutes a poor solution for industrial machinery in production.

Small wire diameter measurement

Scanner adaptation to fine wire diameters
One solution for measuring very small wire diameters focalises the laser beam on the wire plan in a small sweeping spot (Figure 2). Depending on the quality of the optical system, the laser spot size could be as small as few microns. Again, fine wires can produce a shadow on the sensor signal. The bold-face laser lines represent a parallel beam. The mirror axis, the wire, and the photocell must be at the focal plans of the lenses.

Figure 2: Principle of a scanner for small wire diameter measurement

Advantages and limitations
This principle presents essentially the same picture as the system described above: the advantage of measuring smaller wires, together with the constraint of maintaining the wire in the focal plan. Another point worth mentioning is that, here, the focus laser beam becomes very small on the wire plan. The measure is highly sensitive to any local dust, which under conditions of normal use could cause fluctuations.

Determined to overcome the difficulties inherent in these two main measurement principles, over the past 15 years CERSA-MCI has developed its own complete line of diameter measurement instruments and accessories for large and fine diameters.

Large wire diameter measurement by CERSA-MCI

Projection (shadow) principle
In the new CERSA-MCI measurement system (Figure 3), the divergent beam of the laser diode is short-pulsed at 20µs with a period of 4ms. The laser emission is collimated in a parallel beam, which is linearly focused by a cylindrical lens on a CCD sensor of 2,048 pixels. The CCD signal appears as shown in Figure 4 for 5mm tube diameter.

Figure 3: Scheme showing the principle of the CERSA-MCI system

Figure 4: CCD signal

Interferometry and diffraction effects from the laser beam appear on each edge of the shadow from the tube. The modelling of these effects points the way to the right threshold for application of the signal to establish the limit - neutrality: rays neither reflected nor diffracted - thus effectively eliminating the diffraction and interferometry effect. The measures are then independent of the position of the tube (cable) on the laser axis. Linearisation of the laser ray transfer through the whole optical system makes the instrument insensitive to the position of the cable perpendicular to the laser axis. In addition, the very short exposure time of the sensor (pulsed laser diode) within 20µs maximum, makes the measure independent of wire vibrations. The application to steel wires is obvious.

Advantages and limitations
There are no moving parts in the new CERSA measuring system, which means that all components are static and long-lasting. The measurements taken are vibration- and position-independent. Metallic wire may vibrate at high frequency; the measure is not at all disturbed. At 2mm the laser beam is large, rendering the instrument far less sensitive to local dust. A summary of advantages would include:

• All static parts (no vibrating mirror);
• Lifetime limited only by electronic components:
• Measurement frequency of 40 Hz. No averaging;
• Limitation at a measurement uncertainty of ±5µm over the full range of 20mm.

Extension to high-speed cable diameter measurement
This system is suitable for metallic wires, welding wires, cables, plastics, metals, glass tubes etc. For high-speed (1,200m/mn) production of cables, it is necessary to measure the diameter and detect any local defects. CERSA-MCI recommends 10KHz diameter measurement with uncertainty reduced to ±20µm on 3 axes. Such a system is based on the same principle, with the addition of real-time computer hardware.

Small wire diameter measurement by CERSA-MCI

Diffraction principle: LDSN system
Here, the presence of the wire in the laser beam deviates the laser rays at the tangential points. These symmetric effects produce fringes on the CCD sensor (Figure 5).

Figure 5: Diffraction principle

The signal shown in Figure 6 comes from CERSA’s LDSN0200 system for measurements ranging from 5 to 200µm. Frequency of the fringes is proportional to the diameter. The optical system has been specially designed to make sure that the signal on the CCD is independent of the position and vibration of the wire in the laser beam. Actual results obtained with the system are very close to the theoretical.

Figure 6: CCD signal coming from CERSA-MCI’s LDSN0200 system

Advantages and limitations
Optically, the LDSN measurement here is independent of wire position and vibration, making the system particularly suitable for production as well as for laboratory use. It can measure very fine wire diameters (5µm). Its only limitation derives from the optical system angular aperture. The laser beam is 9 x 9mm square. The measure integrates the diameter over 9mm wire length. The LDSN system provides very high stability and repeatability of the measurements taken, proportional to the diameter at ±0.03%. The system also offers accurate measurements of metallic wire diameters below 2mm and down to 5µm. It is suitable for die diameters and ovality measurements as well as for on-line measurement of wires vibrating at high frequency.

Summary

With all static components, without any moving parts, CERSA-MCI equipment features robust mechanical assembly offering easy access to the lenses for cleaning. All the electronic computing means are included in the instrument inside one single compact case, easy to carry and to maintain. Electrical consumption is very low (below 3W). The company also offers a portable instrument, battery powered, with six hours’ autonomy.