Abstract:
The device for measuring the capacitance of electrical wires has a first and a second measuring tube both connected to a voltage source of high frequency. The first measuring tube has a conventional length while the second measuring tube is significantly shorter. With the first measuring tube the capacitance of the wire can be measured in a conventional manner. The second measuring tube allows the measurement of local changes of the capacitance.

Description:
BACKGROUND OF THE INVENTION 
     The invention refers to a device for measuring the capacitance electrical wires. 
     In the manufacture of electrical wires comprising a conductor and an insulating cover or sheathing it is important for the manufacturer to know the capacitance of the wire or cable. As known, the capacitance influences the resistance impedance of the cable. 
     From GB 2 003 613 it has become known to measure the capacitance of a cable in that a measuring tube of electrically conductive material is provided through which the cable is extend. Preferably, the measuring tube is located in a cooling path and thus is filled with water. A measuring voltage of high frequency is applied to the measuring tube while the conductor is connected to ground. Thereby, a current flows between the measuring tube and the conductor, the current depending upon the dielectric constant of the insulating material and the thickness thereof. 
     From EP 0 679 863 it has become know to use the mentioned measuring principle to measure the eccentric displacement of the color of a cable within the cover. To this purpose three arcuate electrode segments are arranged about the circumference of the cable. The segments are circumferentially spaced and electrically insulated relative to each other. Each segment or electrode is supplied with a measure voltage of high frequency. The current flowing to the individual electrode segments is a measure-for the thickness of the cover in the area of the electrode segments. In case the currents flowing to the individual electrode segments are different, this is an indication the conductor is not concentrically arranged in the cable cover. 
     In particular with electrical wires operated with high frequency, e.g. 2 GI-Iz also local changes of the capacitance along the wire may be important. Such change&#39;s an be caused by an inhomogeneous structure of the cable or by changes of the thickness. Such changes cause undesired reflections of electromagnetic waves. 
     It is an object of he invention to provide a device for measuring the capacitance of electrical wires which allows also the measurement of local changes of the capacitance. 
     SUMMARY OF THE INVENTION 
     In the device according to the invention a second measuring tube is co axially arranged with respect to a first measuring tube. The length of the second tube is significantly smaller than that of the first tube. The first tube may be comprised of a plurality of individual tube portions. The first tube has to have a minimum length if a precise capacitance measurement is desired. Such minimum length is for example 200 mm. Such a length, however, does not allow a satisfactory resolution in order to determine changes of the capacitance in smaller areas. Therefore, a second significantly short measuring tube is provided which has a length which is for example 10 to 25% of the total length of the first and second measuring tube. Thus, a second tube may have a length between 20 and 50 mm. By means of suitable capacitance measuring means the capacitance between the first measuring tube and the conductor and the capacitance between the second measuring tube and the conductor are measured. It is also possible, in view of the first capacitance values to take the fit and second measuring tubes together and to measure the capacitance between these tubes and the conductor. 
     The capacitance can be detained in a known manner by the measurement of current or voltage. For example the first and second measuring tube may be connected to separate current measuring devices, and a suitable evaluation device evaluates changes of the current to indicate them as changes of the capacitance. In case the changes reach critical values a corresponding indication can be carried out. Occasionally, a portion of the cable has to be eliminated 
     Another possibility for measuring the capacitance can be carried out by a bridge circuit in that the voltage drop of a known capacitor is measured which is series-connected to the capacitance to be measured, with the circuit being supplied with a known voltage. 
     In the general measuring of the capacitance the second measuring tube may be excluded. In an alternative case, it is included in that the currents flowing to the first and the second tube are added. In this case a current measuring device may measure the total current flowing to both measuring tubes. The inclusion of the second measuring has the advantage that the total length of a device for measuring the capacitance must not be larger than that of conventional measuring devices. 
     It is understood that also two or more than two measuring tubes may be provided which occasionally may have different length in order to achieve the desired degree of resolution. The local resolution increases with a decrease of the length of the measuring tube, however, the disturbing effects also increase with decreasing length so that the danger exist that with particular short second tubes no usable measuring results can be achieved. 
     In a particular embodiment of the invention the second measuring tube is located between at least two portions of the first measuring tube. Such an arrangement has the advantage that the disturbing fields effecting on the second measuring tubs are minimal so that changes of the capacitance can be particularly precisely measured. 
     Preferably, the second measuring tube has the same diameter as the first measuring tube, and both measuring tubes are preferably surround by a common screening tube. An insulation of a solid material can be provided between the screening tube and the measuring tubes which interconnects both parts forming an integral body. Additionally, a second screening tube connected to ground may surround the&#39; first screening tube and forms a unit with the other parts of the device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is hereinafter described in more detail by way of drawings. 
     FIG. 1 shows diagrammatically a second embodiment of a device according to the invention. 
     FIG. 2 shows extremely diagrammatically a second embodiment according to the invention capacitance. 
     FIG. 3 shows diagrammatically an electrical circuit for measuring capacitance. 
     FIG. 4 shows the currents I 1  I 2  being input to an evaluator which determines the capacitance of wire  10 . 
     FIG. 5 shows a schematic block diagram of the inventive system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1 a wire  10  is shown comprising a conductor  12  and a sheathing or cover  14  of electrically insulating material, the wire being extended through a water bath  16  of a cooling path in a cable production plant not shown. A measuring body  18  is located in the bath  16 . It includes a first measuring tube  20  which for example has a first length of 200 mm. A second measuring tube  22  having the same diameter as the first measuring tube is located with a small space with respect to the first measuring tube  20  and has for example a second length of 25 mm. First and second tube are coaxially arranged. The tubes  20 ,  22  made of electrically conductive material are surrounded by a screening tube  24  of conductive material at a radial distance. The screening tube  24  has medium portion  26  which surrounds the tubes  20 ,  22  at a radial distance and two end portions  28 ,  30  having a smaller diameter which substantially corresponds to the diameter of the measuring tubes  20 ,  22 . Measuring tubes  20 ,  22  and screening tube  24  are connected to a measuring voltage U of high frequency, e.g. 450 kHz. The peak voltage is for example 20 V. A first current measuring device  34  is provide in the line to the measuring tubes  20 ,  22 . In the line to the first measuring tube  20  a second current measuring device  36  is provided. A third current measuring device  38  is provided in the line to the second measuring tube. The conductor  12  is connected to ground, also a second screening tube  40  which surrounds the first screening tube  24  at a radial distance. An insulating layer  42 ,  44 , respectively, is located between the measuring tubes  20 ,  22  on the one side and the first tube  24  on the other side. This is not particularly indicated. It may consist of air. An insulating layer  46  is located between the screening tube  24  and the screening tube  40 . As to this, it can be compared with insulating layer  42 . The parts described are interconnected and form a uniform measuring body  18 . Annular insulating parts  48  are located between the insulating layers  42 ,  44  and between them and the insulating body  46 . 
     In case a measuring voltage is supplied, a current flows from the measuring tubes  20 ,  22  to conductor  12 . This is a measure of the capacitance of the wire  10 . In case the currents  11  and  12  of the measuring devices  36 ,  38  are summed a measure for the capacitance is achieved. The capacitance can be also measured in that the current measuring device  34  measures current Is. The current  12  measuring device  38  changes if during the forward movement of wire  10  in direction of arrow  48  the capacitance temporarily changes thus local changes of the capacitance exist. Measured changes of the current thus indicate changes of the capacitance which possibly may be critical for causing high reflections of electromagnetic waves in the wire produced. 
     In FIG. 2 a diagrammatically depicted arrangement of measuring tubes is indicated. One recognizes two tube portions  60 ,  62  which have a total length of 200 mm. They are somewhat spaced with respect to each other. A short measuring tube  64 , .eg. of 20 to 50 mm length is located in the space between the first tube portions. The tube portions  60  to  64  are supplied with a measuring voltage of high frequency as explained in connection with FIG.  1 . In the supply lines to the individual measuring tube portions  60 ,  62  a current measuring device is connected by which the current T 1 , T 2 , and T 3  are measured. The sum of these currents can be used for the measuring of the capacitance. The changes of current T 2  are an indicator for the changes of the capacitance. 
     It is understood that in the arrangement of FIG. 2 a similar structure can be choosen as indicated in FIG.  1 . Therefore, the screening are not shown. 
     The circuit of FIG. 3 shows a capacitor C of known capacitance which is series connected to the capacitor C. The latter is formed by the portion between the measuring tube portion, e.g. portion  20  or portion  22 , respectively, and conductor  12 . This capacitance is to be measured. The series connection is supplied with the voltage U of high frequency, the amount thereof is also known. At a point between the capacitors C and C x  the voltage U x  is measured. Thereafter, the capacitance C x  to be measured can be calculated along the following formula:          C   x     =         (     U   -     U   x       )        C       U   x                              
     Referring now to FIGS. 4 and 5, the measured values of currents T 1  and T 2 , as measured by current measuring devices  36  and  38  are shown input to evaluator  100 . The evaluator  100  determines the capacitance of the length of cable  12  inside measuring tube  20 , as is well known to one of ordinary skill in the art based on a known voltage, and measured current T 1 . The evaluator also determines the capacitance of the shorter length of cable  12  inside measuring tube  22 , as is well known to one of ordinary skill in the art based on the known voltage, and measured current  12 . As is well known to those of ordinary skill in the art, the measured capacitance of a length of cable per unit of length can be computed by dividing the measured capacitance of the length of cable by the length of the cable (i.e. pF/feet). The evaluator  100 , based on the known lengths of measuring tubes  20  and  22  computes the capacitance of cable  12  per unit length. A difference between the capacitance per unit length based on measuring tube  20  and the capacitance per unit length based on measuring tube  22  indicates local changes of the capacitance of the electrical cable. FIG. 5 schematically shows conductor  12  passing simultaneously through measuring tubes  20  and  22 . First and second capacitance measuring devices  102  and  104  measure the capacitance of the respective lengths of cable  12  within the measuring tubes  20  and  22 . The evaluator  100  then computes the capacitance per unit length of cable  12  for both measuring tubes  20  and  22 , and any differences between these values indicates local changes of the capacitance of the electrical cable. As is discussed above, the first and second capacitance measuring devices maybe based on the measured current The measured capacitance may also be based on a bridge circuit in tat the voltage drop of a known capacitor C is measured, which is series connected to the capacitance to be measured (C,), with the circuit being supplied with a known voltage (U) (see FIG.  3 ).