EuroWire January 2016

Technical article

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. 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 shadows the outer diameter and the ovality are calculated. Optical measuring circuit

The measuring values of the outer diameter, if required in combination with the conductor diameter (the wall thickness), are suitable to amend the output capacity of the extruder or the haul-off speed of the cable in such a way that the measuring values are set to the respective nominal value. Moreover, measuring values with tight tolerances are of significant importance for the assembly. Each of these values influence the wave impedance (eg twisted LAN-cables) and consequently the value for the Structural Return Loss (SRL) of a data transfer cable, especially when deviations of these values occur periodically. With the help of the Fast Fourier- Transformation (FFT), the high scan rate of 2,500 measurements/second is suitable for creating a forecast for the SRL in dependence of the transmission frequency even at line speeds of 3,000 metres/minute for both current and future CAT-specifications. If a specification is given concerning the minimum wall thickness for the cable insulation, then any eccentricity leads inevitably to an increased consumption of insulation material. Therefore, eccentricities should be reduced for economic reasons. Recording of oscillating eccentricity values With a scan rate of 2,500 measurements per second, the measuring system records oscillating eccentricity values with high single value precision. These are visualised in form of a scatter plot ( Pictures 3, 4 and 5 ). The scatter plot provides an additional way to visualise the ongoing measure- ment at processor-based display and control devices, and with its help, the distribution of short-term variations of the eccentricity can be shown graphically. Each dot represents a single value of the eccentricity concerning value and direction. The overall distribution of the scatter plot highlights the standard deviation of the eccentricity. It is often sufficient to amend the guiding of the conductor close to the crosshead in order to avoid these oscillations, which usually occur within a certain speed range and/ or certain filling degrees of the coiler or decoiler, respectively. The standard way of representing eccentricity using a cross-section of the cable ( Picture 2 ) is additionally helpful for the operator when centring the crosshead. Picture 3 shows a random type distribution of the single values of the eccentricity, while Picture 4 shows a ring type distribution of the eccentricity values, which is often the result of a rotating

Measuring 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. the concentricity of 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. ▼ ▼ Picture 1 : Devices for the concentricity measuring of a conductor in the insulation Measuring system for concentricity measurement

83

www.read-eurowire.com

January 2016