Cables used for teleconmiunication and other high technology applications are required to be manufactured to high specifications since the way in which two ar more conductors are twisted together can effect attenuation and crosstalk.

Generally, cables which have two or more conductors twisted together rely on the apparatus generating the twist to ensure that the twisting takes place in a regular and uniform manner. However, in practice, the twist produced will vary and this in turn will vary the attenuation withir. the conductors and the crosstalk between them.

It is an object of the present invention to provide apparatus and method of measuring the twist in a twisted cable as it is being manufactured so that with the aid of feedback, the twisting action can be modified to reduce non-uniformity towards zero.

According to the present invention, there is provided apparatus for detecting the twist in a multistrand or multicore cable into which a nominal twist per unit length is introduced, the apparatus comprising means for measuring the speed at which the cable

travels and producing a reference signal having a frequency equal to the nominal twist rate of the cable, means for measuring the variation of a transverse dimension of the cable when viewed from a fixed point near which the cable passes, to produce an output signal including a frequency component equal to the twist frequency, an analyser for conducting an analysis on the output and conditioned by the reference signal to output only a measurement signal having said twist frequency.

The analysis may be a Fourier analysis, or a timing analysis, or other type of analysis.

According to the present invention, there is further provided a method of detecting twist in a multistrand or multicore cable comprising the steps of monitoring the variation in profile of the cable as it passes a predetermined location to produce a measurement signal having a frequency component equal to the actual frequency of twist, determining the nominal twist frequency of the cable, conducting an analysis of the measurement signal and with the aid of the nominal twist frequency separating out from the measurement signal the component having the actual frequency of twist.

According to the present invention there is still further provided apparatus for detecting the speed and twist rate in a cable having at least two twisted elongate elements and travelling along a predetermined path, the apparatus comprising first and second sensors spaced apart along said path by a predetermined distance, each sensor comprising a light source and detection means positioned about said path so that the cable interrupts the

light path from the source to the detector means to cast a varying shadow on the detector as the cable travels along the predetermined path, and means for processing the outputs of the two detector means to determine the actual speed and twist rate or the deviation, if any, from the actual speed and twist rate.

The outputs of the detector means may be used in conjunction with a nominal speed and twist rate for the cable, to determine the actual speed and twist rate or deviation.

Apparatus and methods for detecting and controlling the twists in multicore cables, will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a plan view of the apparatus; Figure 2 is a front elevation of the apparatus of Figure 1; Figure 3 is a view from one side of the optical detection system of the apparatus of Figures 1 and 2; and Figure 4 is a front elevation of another apparatus embodying the invention.

Figure 1 shows part of the twisted cable production line. Individual strands or conductors are taken from separate supply reels (not shown) and fed through a twisting assembly 2 in which orbital rotary components (not shown) produce a twist in the cable.

The twisted cable 4 emerging from the assembly 2 passes over a pair of spaced supporting wheels 6 and 8. A detection arrangement 10 straddles the cable as it passes between the wheels 6 and 8.

The shaft of the wheel 6 is coupled to a transducer 12 which provides an output proportional to the speed of the wheel which in turn is dependent upon the speed of travel of the cable 4.

The output of the transducer 12 is fed to a calibration unit 14. The calibration unit has an adjustable input which can be set to the nominal number of 360° twists that the twisting assembly induces per unit length of the cable. The frequency fref of the output signal of the calibration unit is thus arranged to equal nominal rate or frequency at which the conductors turn about each other (the twist frequency) as they pass over the wheel 6.

A detection assembly 10 downstream of the wheel 6 measures the variation in the lateral dimension of the cable as the conductors twist about each other and the resultant signal produced will include a number of frequency components including the actual twist frequency of the cable. The output of the detection assembly 10 is fed to an analyser 18 which conducts a Fourier analysis on the input signal. The analyser 18 also receives the reference frequency fref which it uses to establish a bandwidth to select only the actual twist frequency component ft from the multitude of different frequency components established by the Fourier analysis.

Other types of analysis may be used, eg timing analysis.

This twist frequency component ft is fed together with the reference frequency fref to comparator 20 which produces a difference signal fd. The difference signal of frequency fd

is fed back to the twisting assembly which responds by adjusting the twisting action in a sense to reduce the difference to zero.

Figure 3 shows the detection assembly 10 in more detail. As shown, a light emitter 22, on one side of the cable 4, is directed at a light receiver 24 in the opposite side of the cable. A first lens 26 located between the emitter 22 and the cable produces parallel rays of light, some of which are interrupted by the cable 4. Another lens 28 between the cable 4 and the receiver 24 receives the non-intercepted light and focuses the rays on the receiver 24.

As can be seen as the twist progresses, the amount of light intercepted by the cable will vary and so will the shadow cast by the light on the receiver 24. Hence, the output signal from the receiver will have a frequency component equal to the twist frequency.

The apparatus shown in Figure 4 is arranged to provide a first output indicative of the twist rate of a cable consisting of twin twisted strands or conductors and a second output indicative of the speed of the cable. Both of these parameters can be used in feedback systems to control the production of the cable.

As shown, the twisted cable 36, emerging from a twisting assembly 30, is supported by a downstream roller 32. A light shield 34 extending above the cable 36 is provided with two slots 34A and 34B spaced apart in the longitudinal direction of the cable 36 and extending tangential to the cable.

A light shield 38 extending below the cable 36 is also provided with two slots 38A and 38B spaced apart in the longitudinal direction of the cable and extending tangential to the cable. The slots 34A and 34B are in direct alignment with respective slots 38A and 38B.

A light source 40 projects a beam of light through slots 34A and 38A and a photo- detector 42 receives the light emerging from the slot 38A. Similarly, a light source 46 projects a beam of light through the slots 34B and 38B and a photo-detector 48 receives the light emerging from the slot 38B.

A filter 50 is connected to receive the output from the detector 44 and passes a signal having a frequency over a specific range.

A filter 52, similar to the filter 50, is connected to receive the output of the photo- detector 48. A phase comparator 54 is connected to the outputs of the two filters 44 and 48 and provides a phase difference or error signal at an output terminal 56.

A processor 58 receives the output of the filter 50 to provide a speed or speed error signal at output terminal 60.

In operation, as the twisted cable passes between respective pairs of slots 34A, 38A and 34B and 38B, it will present a varying profile and so the shadow it casts on respective photo-detectors 44 and 48, will vary in a generally sinusoidal manner. The output signal

from the detectors will thus include a selected frequency component related to the speed of the cable, assuming the twist rate remains constant. Any variation in the twist rate will manifest itself in a phase change in selected frequency components in the outputs of the two detectors 44 and 48.

The two filters 50 and 52 are arranged to have a relatively narrow passband having a centre frequency corresponding to the nominal twist frequency of the cable when run at nominal speed. The processor 58, upon receiving the output signal from the filter 50, coverts it into a speed signal which is then fed to the output 60. Instead, the processor 58 may compare the output signal from the filter 50 with a nominal value and then feed an error signal to the output 60.

The phase comparator 54 compares the phases of the two output signals from the filters 50 and 52 and provides a difference signal at output 56. Instead, the comparator may compare the phase difference with a nominal phase difference and feed an error signal to the terminal 56.

The signals at the outputs 60 and 56 can be fed back to the assembly 30 to maintain the speed and twist rate of the cable substantially constant.

It will be appreciated that while the detection assembly is described as an optical sensor, other sensors which can detect a change in the twist of the cable can equally be used, for example, a capacitive or ultrasonic detection system.

In some embodiments, it may not be necessary to determine the nominal twist frequency and twist rate for a cable.