WCA March 2016

A sag or bend of the conductor Even on the assumption that the conductor is stretched when running through an extrusion line, it may still show some bends, which have to be detected when entering the gauge head of an eccentricity system. Without any special measures, even invisible curve radii in the range of 5 to 20 metres can lead to eccentricity errors of up to 40 micrometres. Only in an ideal case, guide rollers can compensate this imperfect stretching of the conductor, or vice versa, even increase the error. Therefore, the aim should be to eliminate guide rollers completely. Measuring the concentricity of a conductor in the insulation ensures a high quality cable will be produced. During manufacturing of the cable, the measuring system should compensate completely the influence of production related variables that can affect the measuring result, such as angled cable positions and curve radii of the conductor. Concentricity systems, in combination with an integrated or external processor system, visualise short-term variations of the eccentricity in the form of a scatter plot. The system lays the foundation for the production of high quality cables, and ensures a reliable, flawless cable during the assembly process. Subsequently, it contributes to process reliability and additionally to cost effectiveness. ❍ ❍ Picture 1 : Devices for the concentricity measuring of a conductor in the insulation

Measuring system for concentricity measurement

The measuring system ( Picture 1 ) is based on an optical and inductive measuring technique. With the inductive measuring system, which is positioned between two optical measuring planes, the exact position of the conductor is determined. With the optical system, the position of the cable is measured. An eccentricity value occurs when both positions differ from each other. Simultaneously, the optical system captures precisely the diameter and the ovality of the cable. All necessary calculations and analysis are carried out in the measuring system. The measuring values are available from different interfaces for transferring data to a display and control unit or to a line computer. Inductive measuring circuit The cable passes through a toroidal transformer, which generates an alternating current of a few milliamps into the conductor. The alternating current creates a magnetic field, which ideally surrounds the conductor in a circular manner and the intensity decreases, exponentially, with the square of the distance. In the measuring system inductive sensors are placed at a determined distance, radially around the conductor. With the help of these sensors, the exact position of the conductor is calculated with high precision from the distribution of the intensity of the magnetic field strength. Due to the combination of multiple optical sensors and the special design of the inductive sensors, an angled position or a bend in the cable is detected and automatically compensated for. This ensures a precise eccentricity measurement. Due to the automatic centring of the gauge head to the cable position, the measuring system is able to provide precise measuring values at all times, even when the pulling forces in the cable vary. Guide rollers are unnecessary with automatic adjusting heads. The arrangement of the inductive measuring system for the determination of the conductor position in the centre of the gauge head, in combination with ferro-magnetic shielding, prevents environmental variables from having an influence on the eccentricity measuring value. For that reason, even a passing forklift, a moved cooling trough or changing grounding conditions have no influence on the measuring result. Optical measuring circuit The optical measurement is based on the principle of the diffraction analysis combined with pulse-driven laser diodes, the light beam from the diodes projects a shadow of the cable onto the CCD-line sensor in each measuring axis with an exposure time of 0.25 microseconds. The systems measure at four axes (at eight points) the position and the width of the shadow. From the position of the shadow in relation to the determined position of the conductor, signal processors calculate the exact value of the eccentricity and from the widths of the four

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Wire & Cable ASIA – March/April 2016